U.S. patent application number 10/995829 was filed with the patent office on 2006-05-25 for method and apparatus for decontaminating molten metal compositions.
This patent application is currently assigned to Bechtel BWXT Idaho, LLC. Invention is credited to Eric P. Loewen, Larry D. Phelps.
Application Number | 20060107794 10/995829 |
Document ID | / |
Family ID | 36459739 |
Filed Date | 2006-05-25 |
United States Patent
Application |
20060107794 |
Kind Code |
A1 |
Loewen; Eric P. ; et
al. |
May 25, 2006 |
Method and apparatus for decontaminating molten metal
compositions
Abstract
A method and apparatus for decontaminating molten metal
compositions. A molten metal composition (typically containing
elemental lead or a lead alloy) is initially provided which also
includes various inorganic contaminants. The composition is placed
in contact with a specialized decontamination member which actively
allows diffusion of the contaminants therein. As a result, the
contaminants are removed from the molten metal composition.
Contaminants of particular interest in lead-based molten metal
compositions include arsenic, tin, antimony, tellurium, and
combinations thereof. A reducing agent is optimally combined with
the molten metal composition to prevent oxide formation on the
decontamination member. The decontamination member preferably
contains iron in the form of an iron alloy (for example, steel).
Additional preferred components in the decontamination system
include an iron trap for removing iron-containing contaminants from
the molten metal composition. As a result, the composition is
rapidly and effectively decontaminated.
Inventors: |
Loewen; Eric P.; (Idaho
Falls, ID) ; Phelps; Larry D.; (Pocatello,
ID) |
Correspondence
Address: |
Alan D. Kirsch
P.O. Box 1625
Idaho Falls
ID
83415-3899
US
|
Assignee: |
Bechtel BWXT Idaho, LLC
|
Family ID: |
36459739 |
Appl. No.: |
10/995829 |
Filed: |
November 22, 2004 |
Current U.S.
Class: |
75/406 ;
75/699 |
Current CPC
Class: |
Y02P 10/20 20151101;
C22B 9/02 20130101; Y02P 10/234 20151101; C22B 13/06 20130101; C22B
13/08 20130101 |
Class at
Publication: |
075/406 ;
075/699 |
International
Class: |
C22B 13/06 20060101
C22B013/06 |
Goverment Interests
CONTRACTUAL ORIGIN OF THE INVENTION
[0001] This invention was made with United States Government
support under contract number DE-AC07-99ID13727, awarded by the
United States Department of Energy. The United States has certain
rights in this invention.
Claims
1. A method for decontaminating a molten metal composition
comprising: providing a supply of a molten metal composition
comprising lead therein, said molten metal composition further
comprising at least one inorganic contaminant in said composition;
placing said molten metal composition in contact with at least one
decontamination member that will allow said inorganic contaminant
to diffuse into said decontamination member, said decontamination
member comprising iron therein; and allowing said inorganic
contaminant to diffuse into said decontamination member for removal
thereof from said molten metal composition.
2. The method of claim 1 wherein said molten metal composition is
comprised of a material selected from the group consisting of
elemental lead, a lead-containing alloy, and combinations
thereof.
3. The method of claim 2 wherein said lead-containing alloy
comprises a lead-bismuth alloy.
4. The method of claim 1 wherein said inorganic contaminant is
comprised of a material selected from the group consisting of
arsenic, tin, antimony, tellurium, and combinations thereof.
5. The method of claim 1 wherein said decontamination member is
comprised of an iron-containing alloy.
6. The method of claim 5 wherein said iron-containing alloy
comprises steel.
7. The method of claim 1 further comprising maintaining said molten
metal composition at a temperature of about 400-600.degree. C.
during said placing of said molten metal composition in contact
with said decontamination member.
8. The method of claim 1 wherein said placing of said molten metal
composition in contact with said decontamination member causes at
least one iron-containing contaminant to be introduced into said
molten metal composition, said method further comprising removing
at least some of said iron-containing contaminant from said molten
metal composition.
9. The method of claim 8 wherein said removing of said
iron-containing contaminant from said molten metal composition
comprises placing said molten metal composition having said
iron-containing contaminant therein within a magnetic field in
order to draw said iron-containing contaminant out of said molten
metal composition.
10. A method for decontaminating a molten metal composition
comprising: providing a supply of a molten metal composition
comprising lead therein, said molten metal composition further
comprising at least one inorganic contaminant in said composition;
introducing at least one reducing agent into said molten metal
composition; placing said molten metal composition in contact with
at least one decontamination member that will allow said inorganic
contaminant to diffuse into said decontamination member, said
decontamination member comprising iron therein; and allowing said
inorganic contaminant to diffuse into said decontamination member
for removal thereof from said molten metal composition.
11. The method of claim 10 wherein said molten metal composition is
comprised of a material selected from the group consisting of
elemental lead, a lead-containing alloy, and combinations
thereof.
12. The method of claim 10 wherein said reducing agent is comprised
of a material selected from the group consisting of C.sub.(s),
H.sub.2(g), CH.sub.4(g), C.sub.2H.sub.2(g), C.sub.3H.sub.8(g), and
combinations thereof.
13. The method of claim 10 wherein, after said placing of said
molten metal composition in contact with said decontamination
member, said molten metal composition comprises at least some of
said reducing agent therein which remains in an unreacted state,
said method further comprising removing at least some of said
reducing agent in said unreacted state from said molten metal
composition.
14. The method of claim 13 wherein said placing of said molten
metal composition in contact with said decontamination member
causes at least one iron-containing contaminant to be introduced
into said molten metal composition, said method further comprising
removing at least some of said iron-containing contaminant from
said molten metal composition.
15. A method for decontaminating a molten metal composition
comprising: providing a supply of a molten metal composition
comprised of a material selected from the group consisting of
elemental lead, a lead-bismuth alloy, and combinations thereof,
said molten metal composition further comprising at least one
inorganic contaminant therein, said inorganic contaminant
comprising a material selected from the group consisting of
arsenic, tin, antimony, tellurium, and combinations thereof;
introducing at least one reducing agent into said molten metal
composition, said reducing agent comprising a material selected
from the group consisting of C.sub.(s), H.sub.2(g), CH.sub.4(g),
C.sub.2H.sub.2(g), C.sub.3H.sub.8(g), and combinations thereof;
placing said molten metal composition in contact with at least one
decontamination member that will allow said inorganic contaminant
to diffuse into said decontamination member, said decontamination
member being comprised of steel; allowing said inorganic
contaminant to diffuse into said decontamination member for removal
thereof from said molten metal composition, said placing of said
molten metal composition in contact with said decontamination
member further causing at least one iron-containing contaminant to
be introduced into said molten metal composition, said molten metal
composition further comprising at least some of said reducing agent
therein which remains in an unreacted state within said molten
metal composition after said placing of said molten metal
composition in contact with said decontamination member; removing
at least some of said reducing agent in said unreacted state from
said molten metal composition; and removing at least some of said
iron-containing contaminant from said molten metal composition.
16. An apparatus for decontaminating a molten metal composition
comprising: a supply of a molten metal composition comprising lead
therein, said molten metal composition further comprising at least
one inorganic contaminant in said composition; and a containment
vessel in fluid communication with said supply of said molten metal
composition so that said molten metal composition can enter into
said containment vessel, said containment vessel comprising: at
least one decontamination member therein that will allow said
inorganic contaminant to diffuse into said decontamination member
when said molten metal composition comes in contact with said
decontamination member so that said contaminant can be removed from
said molten metal composition, said decontamination member
comprising iron therein; and at least one outlet port in said
containment vessel for passage of said molten metal composition out
of said containment vessel after said composition comes in contact
with said decontamination member.
17. The apparatus of claim 16 further comprising at least one
heater which is used to provide heat to said molten metal
composition.
18. The apparatus of claim 16 wherein said molten metal composition
is comprised of a material selected from the group consisting of
elemental lead, a lead-containing alloy, and combinations
thereof.
19. The apparatus of claim 18 wherein said lead-containing alloy
comprises a lead-bismuth alloy.
20. The apparatus of claim 16 wherein said inorganic contaminant is
comprised of a material selected from the group consisting of
arsenic, tin, antimony, tellurium, and combinations thereof.
21. The apparatus of claim 16 wherein said containment vessel is
produced from a composition which comprises zirconium therein.
22. The apparatus of claim 16 further comprising at least one iron
trap which receives said molten metal composition after contact
thereof with said decontamination member.
23. The apparatus of claim 22 wherein said iron trap comprises at
least one magnet.
24. An apparatus for decontaminating a molten metal composition
comprising: a supply of a molten metal composition comprising lead
therein, said molten metal composition further comprising at least
one inorganic contaminant in said composition; a supply of at least
one reducing agent which is in fluid communication with said supply
of said molten metal composition so that said reducing agent can be
introduced into said molten metal composition; and a containment
vessel in fluid communication with said supply of said molten metal
composition so that said molten metal composition can enter into
said containment vessel, said containment vessel comprising: at
least one decontamination member therein that will allow said
inorganic contaminant to diffuse into said decontamination member
when said molten metal composition comes in contact with said
decontamination member so that said contaminant can be removed from
said molten metal composition, said decontamination member
comprising iron therein; and at least one outlet port in said
containment vessel for passage of said molten metal composition out
of said containment vessel after said composition comes in contact
with said decontamination member.
25. The apparatus of claim 24 wherein said containment vessel
further comprises at least one additional outlet port therein for
passage of unreacted quantities of said reducing agent out of said
containment vessel.
26. The apparatus of claim 25 further comprising at least one iron
trap which receives said molten metal composition after contact
thereof with said decontamination member.
27. An apparatus for decontaminating a molten metal composition
comprising: a supply of a molten metal composition comprised of a
material selected from the group consisting of elemental lead, a
lead-bismuth alloy, and combinations thereof, said molten metal
composition further comprising at least one inorganic contaminant
therein, said inorganic contaminant comprising a material selected
from the group consisting of arsenic, tin, antimony, tellurium, and
combinations thereof; a supply of at least one reducing agent which
is in fluid communication with said supply of said molten metal
composition so that said reducing agent can be introduced into said
molten metal composition, said reducing agent comprising a material
selected from the group consisting of C.sub.(s), H.sub.2(g),
CH.sub.4(g), C.sub.2H.sub.2(g), C.sub.3H.sub.8(g), and combinations
thereof; a containment vessel in fluid communication with said
supply of said molten metal composition so that said molten metal
composition can enter into said containment vessel, said
containment vessel comprising: at least one decontamination member
therein that will allow said inorganic contaminant to diffuse into
said decontamination member when said molten metal composition
comes in contact with said decontamination member so that said
contaminant can be removed from said molten metal composition, said
decontamination member being comprised of steel; at least one
outlet port in said containment vessel for passage of said molten
metal composition out of said containment vessel after said
composition comes in contact with said decontamination member; and
at least one additional outlet port in said containment vessel for
passage of unreacted quantities of said reducing agent out of said
containment vessel; and at least one iron trap which receives said
molten metal composition after contact thereof with said
decontamination member, said iron trap comprising at least one
magnet.
28. The apparatus of claim 27 further comprising at least one
heater which is used to provide heat to said molten metal
composition.
29. The apparatus of claim 27 wherein said containment vessel is
produced from a composition which comprises zirconium therein.
Description
FIELD OF THE INVENTION
[0002] The present invention generally relates to the
decontamination and purification of molten metal compositions and,
more particularly, to the processing and treatment of
lead-containing molten metal compositions in order to remove
contaminants therefrom. The molten metal compositions being treated
are particularly useful in cooling systems for nuclear power
generating units and related applications, with the claimed
decontamination apparatus and method facilitating the safe,
continuous, and efficient operation of these systems in a rapid and
effective manner.
BACKGROUND OF THE INVENTION
[0003] Most modern power generation systems produce significant
quantities of heat as a by-product. Efficient heat removal is
therefore a high priority in order to extract useful energy to
ensure safe and continuous operation of these systems. Heat removal
is of particular concern in nuclear reactor-based power generating
facilities which have extensive cooling requirements. Various
methods have been developed for removing and otherwise dissipating
heat from nuclear reactors. A heat removal method of particular
interest has involved the use of molten heavy metal compositions
(for example, those which contain elemental lead [Pb] or
lead-containing alloys with particular reference to, for example,
lead-bismuth [Pb--Bi] alloys). Lead-containing molten metal
compositions are characterized by high levels of heat conductivity
and heat transfer efficiency. They therefore continue to be of
significant interest in the nuclear power generating industry.
Additional background information concerning the use of
lead-containing molten metal compositions in nuclear reactor
cooling systems is available from numerous sources and literature
articles including but not limited to, for example, Gromov, B., et
al., "Inherently Safe Lead-Bismuth-Cooled Reactors", Atomic Energy,
76(4):323-330 (1994) which is incorporated herein by reference.
[0004] Notwithstanding the usefulness of lead-containing molten
metal compositions as efficient coolants in nuclear reactor
systems, various difficulties have likewise been encountered when
these materials are employed which will now be discussed.
Specifically,- lead-containing molten metal compositions
(especially lead-bismuth alloys) have demonstrated an ability to
significantly corrode various metal components (conduits,
containment vessels, heat-transfer surfaces, and the like) in
reactor cooling systems and related structures. This corrosion
(caused by a variety of complex chemical and physical interactions)
can significantly degrade the operating components of the cooling
systems, thereby leading to costly damage, leaks, and general
system failures (including interruptions in reactor operation). The
corrosion problems discussed above are primarily caused by various
inorganic contaminants in the lead-containing molten metal
compositions including but not limited to the following materials
in elemental form and/or various combinations thereof (alloys,
compounds, complexes, and the like without limitation): antimony
[Sb], arsenic [As], tin [Sn], and tellurium [Te]. The presence of
these materials (in even relatively small quantities, namely, 0.1%
by weight or less) can cause significant corrosion of the cooling
system under consideration and its operating components.
[0005] The corrosion problems discussed above have been extensively
investigated and, in view of these difficulties, alternative
cooling systems using different coolant media have been studied
and, in some cases, implemented. These alternative cooling systems
involve, for instance, the use of liquid sodium [Na] which is
characterized by significantly lower corrosive activity compared
with lead-based molten metal cooling compositions. Nonetheless,
lead-based cooling systems continue to have numerous advantages
over other cooling methods and, for this reason, they continue to
be of interest. These advantages include but are not limited to:
(1) a high degree of thermal conductivity (and heat removal
efficiency); (2) a favorable level of thermal stability; (3) low
neutron capture cross-section (resulting in relatively uniform
power distributions); (4) self-shielding from reactor gamma-rays;
(5) high boiling points (which enable low-pressure operation at
high temperature levels without boiling); (6) a fast neutron
spectrum which makes it possible to burn radioactive wastes; (7)
low chemical reactivity at high temperatures compared with, for
example, liquid sodium-based cooling systems; and (8) a high
specific heat value which allows the cooling systems of interest to
be significantly smaller than conventional systems that employ
water, various gases, air, and the like. Accordingly, a number of
important benefits are provided by lead-based molten metal cooling
systems. A continued interest in this technology therefore exists
for cooling nuclear reactors and other related devices (including
but not limited to accelerator-driven radioactive waste
transmutators and the like) notwithstanding the corrosion problems
discussed above.
[0006] In accordance with the many beneficial features and
characteristics of lead-based molten metal cooling systems in
nuclear applications, various attempts have been made to control or
otherwise mitigate corrosion problems. For example, prior research
has demonstrated that a stable oxide film on the metallic
components employed in the cooling systems will control the
corrosion of ferrous metals. However, if the lead-containing molten
metal compositions being used in the cooling systems are too
oxidizing (which is characterized by the production of, for
example, lead oxide [PbO] within the systems), reduced flow rates
will occur as the oxide materials form. Even under highly-oxidizing
conditions, if one or more of the above-mentioned contaminants are
present (namely, arsenic, antimony, tin, tellurium, and/or
combinations thereof), corrosion will still occur. If the reverse
condition is implemented within the systems (namely, if a reducing
environment is created therein which is characterized by a lack of
oxygen [O.sub.2]), corrosion will nonetheless occur in the presence
of the foregoing contaminants since any previously-formed
protective oxide layers (e.g. lead oxide) will be removed from the
metal surfaces in the cooling systems. Thus, while the active
control of the overall environment in the cooling systems (from an
oxidation-reduction perspective) remains a potentially viable
approach for mitigating corrosion-based damage, the amount of
oxygen in the systems must be precisely controlled in order to
achieve this goal which can be difficult and complex.
[0007] From a historical standpoint, the purification of lead to
remove contaminants therefrom (including but not limited to
antimony, arsenic, sulfur, and tin) has been addressed in various
patents including U.S. Pat. Nos. 50,800; 786,581, 1,640,486;
1,640,487, 1,950,388; 2,062,838; and 3,335,569. U.S. Pat. Nos.
3,300,043; 3,393,876; and 3,689,253 disclose hydrometallugical
processes for purifying lead compositions. U.S. Pat. No. 4,194,904
involves the purification of lead and antimony oxide by partial
oxidation (namely, via the introduction of air into molten alloy
materials at controlled temperatures). As a result, the antimony is
preferentially oxidized to form antimony trioxide. U.S. Pat. No.
4,496,394 involves a method for removing tin from a molten
lead-containing composition by introducing chlorine and oxygen into
the composition (which contains tin therein as an impurity) in
order to form a tin-containing "dross" (e.g. an oxide composition
typically located on the surface of the molten metal composition).
Thereafter, the lead is physically separated from the dross.
Finally, U.S. Pat. No. 5,100,466 discloses a method wherein lead is
purified using a reactive mixture comprised of sodium and calcium.
In accordance with this process, the resulting mixture is allowed
to cool which yields three equilibrium phases, one of which
(located on the bottom of the product) involves refined lead.
[0008] Notwithstanding the processes and techniques generally
discussed above, the present invention offers a considerable
advance in the art of metal purification with particular reference
to the decontamination of lead-containing molten metal
compositions. The claimed invention provides numerous benefits
which, particularly from a collective standpoint, have not been
achieved prior to the present invention. Accordingly, the processes
and systems described below satisfy a long-felt need for a
decontamination method and apparatus which accomplishes the
following benefits and goals simultaneously (with the foregoing
list not necessarily being considered exhaustive): (1) the ability
to remove inorganic compositions (particularly arsenic, antimony,
tin, and tellurium) in a highly efficient manner from
lead-containing molten metal compositions; (2) rapid and highly
effective decontamination rates; (3) the implementation of an
efficient decontamination process using a minimal amount of
operating equipment and materials; (4) the ability to remove
contaminants without the need to employ hazardous, caustic, or
expensive chemical reagents; (5) a high level of versatility with
particular reference to the types of lead-containing molten metal
compositions which can be treated; (6) improved decontamination
efficiency resulting from the ability of the system to operate in a
substantially continuous fashion; (7) compatibility with a
considerable number of heat generating devices including but not
limited to a wide variety of nuclear power generating systems,
accelerator-driven radioactive waste transmutators, and the like
which employ lead-containing molten metal compositions as coolants;
(8) the ability to achieve decontamination without requiring highly
oxidizing conditions (which avoids the problems associated
therewith as discussed above); (9) a considerable degree of
versatility regarding the types of contaminants which may be
removed from the lead-containing molten metal compositions; (10)
the overall implementation of a procedure which is cost effective,
readily controllable (e.g. customizable on-demand to various
cooling systems and devices), easily scaled up or down as needed,
and capable of rapid integration into the cooling systems of
interest; (11) the capacity to decontaminate lead-containing molten
metal compositions in a manner whereby destructive corrosion of the
cooling systems is eliminated, thereby avoiding excessive
maintenance requirements, system failures, and other operational
problems; and (12) an accomplishment of the above-listed goals in a
manner which is superior to prior decontamination techniques and
represents a considerable advance in molten metal processing
technology.
[0009] As outlined above, the claimed invention is characterized by
a multitude of specific benefits in combination, with the foregoing
list not necessarily being exhaustive. These benefits include but
are not limited to items (1)-(12) recited above both on an
individual and simultaneous basis which are attainable in a
substantially automatic manner (with the simultaneous achievement
of such goals being of particular importance and novelty). The
decontamination method and apparatus described herein perform all
of the functions mentioned above in a uniquely effective and
simultaneous fashion while using a minimal quantity of reactants,
reagents, equipment, labor, and operational requirements. As a
result, a decontamination system of minimal complexity and high
effectiveness is created that nonetheless exhibits a substantial
number of beneficial attributes in an unexpectedly efficient
manner. In this regard, the developments disclosed herein represent
an important advance in molten metal decontamination technology
(with particular reference to lead-containing molten metal
compositions). Specific information concerning the novel process
steps, reaction conditions, operating components, equipment
configurations, and other elements associated therewith will be
presented below in the following Summary, Brief Description of the
Drawing, and Detailed Description sections.
SUMMARY
[0010] The following discussion shall constitute a brief and
non-limiting general overview. More specific details concerning
particular embodiments and other important features (including a
recitation of preferred reaction conditions, operational
parameters, processing equipment, and other aspects of the claimed
technologies) will again be recited in the Detailed Description
section set forth below.
[0011] In accordance with the present invention, a highly efficient
method and apparatus are disclosed for removing inorganic
contaminants from molten metal compositions, particularly those
which contain in whole or in part lead. While the claimed invention
shall not be restricted to the treatment of any particular
lead-containing molten metal compositions, representative
compositions of particular interest include those which are made
from elemental lead [Pb], lead-bismuth [Pb--Bi] alloys, or
combinations thereof in a variety of proportions without
limitation. Furthermore, the present invention shall not be
restricted regarding the particular inorganic contaminants which
can be removed from the lead-containing molten metal compositions
discussed above. However, in a preferred embodiment, the following
contaminants shall be considered of primary interest in the claimed
invention: antimony, arsenic, tin, tellurium, or combinations
thereof. As outlined further below in the Detailed Description
section, the term "combinations thereof" as employed herein and as
claimed shall be construed (wherever it appears) to encompass any
combination of two or more of the materials recited in connection
therewith and possibly others, with "combinations" being further
defined to encompass mixtures, alloys, compounds, and complexes of
the listed materials in any amounts, arrangements, or proportions
without limitation.
[0012] With continued reference to the claimed method, it is
optional (but preferred) to introduce at least one reducing agent
into the lead-containing molten metal composition in order to
substantially avoid and otherwise prevent an oxidation layer (e.g.
comprised of lead oxide and related compounds) from forming on the
decontamination member used in the claimed treatment process as
discussed further below. In a preferred and non-limiting
embodiment, the reducing agent will comprise a material selected
from the group consisting of solid particulate carbon [C.sub.(s)],
hydrogen [H.sub.2(g)], methane [CH.sub.4(g)], acetylene
[C.sub.2H.sub.2(g)], propane [C.sub.3H.sub.8(g)], or combinations
thereof without limitation. The use of at least one reducing agent
shall be considered a "default" process step employed in order to
obtain optimum results and, in this regard, shall be used unless
countervailing circumstances exist which would make it unnecessary
as determined by routine preliminary pilot tests. These tests would
take into account the particular chemical nature of the
lead-containing molten metal composition being treated, the
contaminant content thereof, and the environmental conditions which
exist within the decontamination system (along with other related
factors). It should likewise be noted that the introduction of the
reducing agent into the molten metal composition can occur at a
variety of intervals or locations in the claimed process and system
including before and during decontamination using the
decontamination member discussed below.
[0013] Next, the molten metal composition having the inorganic
contaminants therein is placed in contact with at least one
decontamination member which is comprised of a composition that
will allow the inorganic contaminants in the molten metal
composition to diffuse into the decontamination member. In a
preferred embodiment, the decontamination member will comprise iron
[Fe] therein, with optimum results being achieved when an
iron-containing alloy is employed (preferably steel). As
extensively discussed below, the term "diffuse" shall be construed
in the broadest possible sense to involve: (1) entry of the
inorganic contaminants into and beneath the surface of the
decontamination member to various depths without limitation; (2)
interaction of the inorganic contaminants with the decontamination
member at the surface thereof without necessarily passing beneath
the surface; and/or (3) a combination of [1] and [2] above.
Irrespective of the manner in which diffusion occurs, the
decontamination member is of a type that will have a selective
affinity for the inorganic contaminants of interest while avoiding
affinity (and diffusion therein as defined above) of the various
lead-containing materials (e.g. elemental lead, alloys, compounds,
or complexes thereof) that are associated with the lead-containing
molten metal composition. As a result (and as more fully described
in the Detailed Description section), the inorganic contaminants
can be efficiently removed from the lead-containing molten metal
composition in order to effectively decontaminate it.
[0014] With continued reference to the decontamination process,
specific operating parameters associated therewith (including
preferred residence times and the like) will be presented in detail
below. However, in order to obtain optimal results, it is preferred
that the lead-containing molten metal composition be maintained at
a temperature of about 400-600.degree. C. during placement of the
composition in contact with the decontamination member. This
temperature level promotes favorable reaction kinetics and
otherwise facilitates the decontamination process.
[0015] In accordance with the procedure discussed above wherein
direct physical contact occurs between the decontamination member
and the lead-containing molten metal composition, the inorganic
contaminants in the molten metal composition are allowed to diffuse
into the decontamination member and consequently be removed from
the molten metal composition. In this manner, rapid and effective
decontamination of the molten metal composition takes place as
stated above. Further information will again be provided in the
Detailed Description section regarding other operational parameters
associated with the decontamination procedure including residence
times, material quantities, and the like.
[0016] As stated above and in accordance with the claimed process,
the inorganic contaminants of concern will diffuse into the
decontamination member for removal thereof from the lead-containing
molten metal composition. However and during this procedure, it is
possible under some (but not necessarily all) circumstances that
placement of the molten metal composition in contact with the
decontamination member will cause at least one iron-containing
contaminant (e.g. elemental iron [Fe], alloys, mixtures, compounds,
and/or complexes containing iron) to be introduced into the molten
metal composition. Removal of at least some (and preferably all) of
the iron-containing contaminant is desired in order to preserve and
maintain the overall purity, cooling efficiency, and
non-corrosivity of the lead-containing molten metal composition and
to likewise avoid undesired precipitation of the iron within the
cooling system (which can cause flow restrictions and related
problems).
[0017] Removal of the iron-containing contaminant may be
accomplished in various ways without limitation. However, in a
preferred and representative embodiment, elimination of the
iron-containing contaminant is achieved using at least one iron
trap. In particular, effective results are attained through the use
of an iron trap system which supplies a magnetic field. The molten
metal composition is placed within this magnetic field in order to
draw the iron-containing contaminant out of the lead-containing
molten metal composition. It should be noted that a decision to
employ an iron trap in a given operational environment shall be
determined in accordance with routine preliminary pilot tests
taking into account the chemical and physical nature of the molten
metal compositions being treated, the structural configuration of
the decontamination member (and the materials from which it is
made), and other operational parameters associated with the
decontamination system. However, in a preferred embodiment, this
step will be employed as a "default" procedure unless
countervailing circumstances indicate otherwise. Additional
information concerning this particular aspect of the invention will
likewise be outlined in greater detail below.
[0018] It should likewise be recognized that in some (but not
necessarily all) circumstances where a reducing agent is employed,
the lead-containing molten metal composition will contain (after
decontamination) at least some of the reducing agent therein which
remains in an unreacted state. In particular, this reducing agent
will be present (at least temporarily) within the molten metal
composition after placement of the molten metal composition in
contact with the decontamination member. This situation typically
results in accordance with the use of excess quantities of reducing
agent within the system as a "default" procedure in order to ensure
that oxide formation on the decontamination member does not occur.
In a preferred embodiment to be discussed in greater depth below,
an additional feature of the claimed process can involve the step
of removing at least some of the unreacted reducing agent from the
molten metal composition (and the system as a whole). This step
enables maximum operating efficiency to be maintained within the
decontamination and cooling systems (namely, the minimization of
corrosion and improved economic performance by the recycling of
recovered quantities of reducing agent). As previously stated,
additional information concerning the process discussed above and
its various embodiments will be provided in the Detailed
Description section below.
[0019] Regarding the apparatus to be used in implementing the
claimed process, all of the information, definitions, and other
data set forth above in connection with the claimed method shall be
incorporated by reference in the current discussion of the
apparatus. Specifically, a supply of the lead-containing molten
metal composition described above is initially provided which
includes the previously-listed contaminants therein. Should the use
of a reducing agent within the system be needed and desired as
indicated above (with representative and preferred reducing agents
being discussed earlier in this section), a supply of the reducing
agent is also provided which is in fluid communication with the
supply of the molten metal composition. As a result, the reducing
agent can be introduced into the molten metal composition on
demand.
[0020] A containment vessel is also provided which is in fluid
communication with the supply of the molten metal composition so
that the composition can enter the vessel when decontamination is
desired. In an exemplary and preferred embodiment, the containment
vessel comprises therein the decontamination member outlined above.
As previously stated, the decontamination member comprises iron
therein (preferably an iron-containing alloy with optimum results
being achieved when steel is used for this purpose). In accordance
with the general information provided above, the decontamination
member is of a type that will allow the inorganic contaminants of
concern within the lead-containing molten metal composition to
diffuse into the decontamination member when the molten metal
composition comes in contact with the decontamination member. In
this manner, the contaminants can be removed rapidly and
effectively from the molten metal composition.
[0021] The containment vessel will further comprise at least one
outlet port therein for passage of the molten metal composition out
of the vessel after the composition comes in contact with the
decontamination member. Likewise and in an exemplary embodiment, at
least one additional outlet port is provided in the containment
vessel for the passage of unreacted quantities of the reducing
agent out of the vessel (with such quantities being previously
combined with the molten metal composition as outlined above). In
order to avoid corrosion and maintain structural stability, the
containment vessel (and other components associated with the
claimed decontamination apparatus) are optimally produced from a
composition which is highly resistant to corrosion, chemical
degradation, and the like. In a representative embodiment designed
to provide optimum results, the containment vessel (along with the
various conduits and other components of the decontamination
system) are produced from a composition which comprises zirconium
[Zr] therein (e.g. elemental zirconium or alloys, compounds,
mixtures, and complexes which contain at least some zirconium).
[0022] If it is desired that iron-containing contaminants be
removed from the system as previously described (which should again
be considered a "default" procedure unless countervailing
circumstances exist which would indicate otherwise), at least one
iron trap is provided which is able to receive the molten metal
composition after contact with the decontamination member. The iron
trap will again remove iron-containing contaminants from the molten
metal composition which were introduced into the composition during
contact thereof with the decontamination member. Furthermore and in
a preferred embodiment, the iron trap will comprise at least one
magnet which is able to generate a magnetic field in order to draw
the iron-containing contaminants out of the molten metal
composition.
[0023] Finally, in an exemplary and preferred (e.g. non-limiting)
version of the invention, the decontamination apparatus may
likewise include at least one heater which is used to provide heat
to the molten metal composition so that optimum temperature levels
can be maintained therein during decontamination. The heater can be
located in a variety of positions inside and outside the apparatus
without limitation although it is preferred that it be positioned
on or adjacent the exterior surface of the containment vessel so
that direct heating thereof can be accomplished.
[0024] It shall be recognized that the apparatus which may be used
to implement the claimed methods and processes shall not be
restricted by the description provided herein. Various additional
components, systems, and sub-systems may be employed in connection
with the containment vessel and the other sections of the claimed
decontamination apparatus without limitation provided that the main
functional capabilities of the system can be implemented in an
effective and cost-efficient manner. In this regard, the apparatus
associated with the claimed invention shall not be restricted to
any particular equipment types, arrangements, capacities,
materials, operational parameters, and the like unless otherwise
expressly stated herein.
[0025] As previously indicated, the Summary provided above shall
not limit the invention in any respect and is instead being
presented as a brief overview of the claimed technology from a
general standpoint. The Detailed Description section set forth
below will offer explicit and enabling information regarding the
foregoing subject matter including data involving the materials
being used, the reaction conditions of interest, and the operating
components associated with the claimed invention which achieve the
goals outlined above.
BRIEF DESCRIPTION OF THE DRAWING
[0026] The drawing figure provided herein is schematic and not
necessarily drawn to scale. It shall not limit the scope of the
invention in any respect. Any physical components or structures
shown in the drawing are representative only and are not intended
to restrict the invention or its implementation. In particular, the
claimed treatment methods are not limited to any specific hardware,
processing equipment, arrangements of components, orders and
sequences in which processing steps occur, and the like, with the
invention being useful in a variety of applications (including the
incorporation thereof in various nuclear reactors and cooling
systems without limitation). Accordingly, the claimed technology
shall not be considered "environment-specific" or
"application-specific" in any fashion. Likewise and as previously
noted, the current invention is not restricted to any particular
order or sequence in which the desired operating procedures are
implemented and is also not limited to any specific equipment
arrangements or configurations unless otherwise expressly stated
herein, with any representations of the same in the drawing figure
being presented for example purposes only. The use of any symbolic
elements in FIG. 1 regarding various materials, reactants,
structures, components, and the like which are employed in the
claimed invention shall also be considered exemplary and
non-restrictive.
[0027] FIG. 1 is a schematically-illustrated view of a
representative decontamination system for lead-containing molten
metal compositions which may be used to implement the process of
the claimed invention. No scale or size relationships shall be
construed from the drawing or other limitations implied
therefrom.
DETAILED DESCRIPTION
[0028] As described above, the present invention involves a highly
efficient method and apparatus for removing inorganic contaminants
from lead-containing molten metal compositions. The technology
discussed herein represents a significant advance in the field of
molten metal decontamination technology. Likewise, the claimed
method and apparatus are further characterized by an unexpectedly
high degree of operational efficiency as previously noted.
[0029] At this point, the claimed techniques and devices will be
discussed in depth with particular reference to the preferred
materials, components, equipment, quantities, operational
parameters, equipment configurations, reaction conditions, and the
like. All of the various embodiments disclosed herein shall not be
limited to any specific equipment, components, material quantities,
reactants, starting materials, and the like unless otherwise
expressly stated herein. Likewise, all scientific terms used
throughout this discussion shall be construed in accordance with
the traditional meanings attributed thereto by individuals skilled
in the art to which this invention pertains unless a special
definition is provided below. The numerical values listed in this
section and in the other sections of the present description
constitute preferred embodiments designed to offer optimum results
and shall not limit the invention in any respect. In particular, it
shall be understood that the specific embodiments, components, and
methods disclosed herein and illustrated in the drawing figure
constitute special versions of the claimed method and apparatus
which, while non-limiting in nature, can offer excellent results
and are highly distinctive. All recitations of chemical formulae,
structures, alloys, compounds, mixtures, complexes, and the like in
the following discussion are intended to generally indicate the
types of materials which may be used. The listing of specific
chemical compositions which fall within the general formulae and
classifications presented below are offered for example purposes
only and shall be considered non-limiting unless explicitly stated
otherwise.
[0030] The invention discussed herein and all of its various
embodiments shall likewise not be restricted with particular
reference to the order in which the claimed process steps are
implemented unless otherwise expressly indicated below.
[0031] Finally, any and all recitations of structures, materials,
chemicals, and components in the singular throughout the Claims,
Summary, and Detailed Description sections (for example, by using
"a", "an", or other comparable words) shall also be construed to
encompass a plurality of such items unless otherwise explicitly
noted herein. Employment of the phrase "at least one" shall be
construed in a conventional fashion to involve "one or more" of the
listed items, with the term "at least about" being defined to
encompass the listed numerical value and values in excess thereof.
Use of the word "about" in connection with any numerical terms or
ranges shall be interpreted to offer at least some latitude both
above and below the listed parameter(s) with the magnitude of such
latitude being construed in accordance with current and applicable
legal decisions pertaining to this terminology. Furthermore, all of
the definitions, terms, and other information recited above in the
Background and Summary sections are applicable to and incorporated
by reference in the current Detailed Description section. In order
to facilitate a full and complete explanation of the invention and
its various embodiments, the best mode associated with the method
and apparatus that are claimed herein will be described in a
sequential fashion beginning with the starting materials under
consideration, followed by an explanation as to how these materials
are decontaminated on a step-by-step basis along with detailed
technical information and definitions where needed.
I. The Lead-Containing Molten Metal Composition
[0032] With reference to FIG. 1, a system for decontaminating the
lead-containing molten metal composition of interest in the present
invention is generally shown at reference number 10 which will now
be discussed in substantial detail. Operatively associated with the
system 10 is a nuclear reactor 12 which is being cooled using the
lead-containing molten metal composition. It shall be understood
that, while a nuclear power-generating reactor 12 is shown in FIG.
1 and described herein, the lead-containing molten metal
composition to be decontaminated may be associated with any
heat-generating apparatus or facility without limitation that is
capable of being cooled using materials of this nature. For
example, instead of the nuclear reactor 12 discussed above, an
accelerator-driven radioactive waste transmutator may likewise be
cooled using the lead-containing molten metal composition being
described herein. Accordingly, the present invention shall not be
limited regarding the particular apparatus, device, or system being
cooled, or the cooling system in general which would employ the
lead-containing molten metal composition of interest.
[0033] As shown in FIG. 1, the nuclear reactor 12 includes a
cooling system 14 of conventional design which is associated
therewith. The cooling system 14 includes a variety of conduits,
components, and structures (not shown) which enable the
lead-containing molten metal composition to be effectively
circulated throughout the heat-generating regions of the nuclear
reactor 12 in order to remove fission energy in the form of heat
for use in electrical generation, H.sub.2 production, etc.
Lead-containing molten metal cooling systems for nuclear
applications are again known in the art to which this invention
pertains and have been used for decades, with general information
involving these systems being disclosed in a variety of articles
and references including but not limited to Gromov, B., et al.,
"Inherently Safe Lead-Bismuth-Cooled Reactors", Atomic Energy,
76(4):332-330 (1994) which is incorporated herein by reference.
Accordingly, the lead-containing molten metal composition being
discussed herein (as well as the processes and devices which are
used for decontamination purposes as disclosed below) shall not be
considered "reactor-specific" or "cooling system-specific" and may
be employed in connection with a number of conventional and
non-conventional heat-generating devices and cooling systems
without restriction.
[0034] Within the cooling system 14 associated with the nuclear
reactor 12 is a supply 16 of a lead-containing molten metal
composition (also characterized in an equivalent fashion as a
molten metal composition comprising lead therein). The terms
"lead-containing molten metal composition" and "molten metal
composition comprising lead therein" shall be broadly construed and
defined to encompass alloys, compounds, mixtures, complexes, and
other combinations of materials (including the use of pure
elemental lead [Pb]) which contain in part or in whole at least
some lead therein. As previously explained, a significant number of
benefits are achieved through the use of lead-based materials in
connection with the cooling system 14 and other comparable cooling
systems for nuclear applications and like. These benefits
specifically include but are not limited to: (1) a high degree of
thermal conductivity (and heat removal efficiency); (2) a favorable
level of thermal stability; (3) low neutron capture cross-section
(resulting in relatively uniform power distributions); (4)
self-shielding from reactor gamma-rays; (5) high boiling points
(which enable low-pressure operation at high temperature levels
without boiling); (6) a fast neutron spectrum which makes it
possible to burn radioactive wastes; (7) low chemical reactivity at
high temperatures compared with, for example, liquid sodium-based
cooling systems; and (8) a high specific heat value which allows
the cooling systems of interest to be significantly smaller than
conventional systems that employ water, various gases, air, and the
like. Accordingly, a number of important benefits are attributable
to lead-based molten metal cooling systems which have resulted in a
continued interest in this technology for cooling nuclear reactors
and related systems as outlined herein.
[0035] At this point, the lead-based molten metal composition
associated with supply 16 will be discussed in greater detail. In
accordance with the definition of this material provided above, a
number of different lead-based compositions (in molten/liquid form)
can be employed in connection with the cooling system 14 and the
present invention in general. However, in a preferred embodiment,
the lead-based molten metal composition will be produced from: (1)
elemental lead, (2) a lead-containing alloy; or (3) mixtures of [1]
and [2]. Regarding the lead-containing alloy, this composition will
contain at least some lead therein which is alloyed with one or
more other metals or non-metals. For example, some representative
examples of metals and non-metals which may be alloyed with lead in
the lead-containing molten metal composition include but are not
limited to bismuth [Bi], tin [Sn], zinc [Zn], or combinations of
two or more of the above elements. It should likewise be noted
that, within the lead-containing alloys associated with the supply
16 of the molten metal composition, the various individual
materials therein can be used in a wide variety of proportions
without limitation provided that at least some lead (optimally at
least about 45% by weight or more) is present in the alloys so that
the beneficial cooling effects associated with lead can be
achieved.
[0036] One composition of particular interest is the use of a
lead-bismuth [Pb--Bi] alloy which is characterized by a high degree
of cooling capacity and is therefore of considerable interest in
nuclear applications. A number of different lead-bismuth alloys can
be employed wherein differing amounts of lead and bismuth are
present therein. For example, two representative lead-bismuth
alloys which are suitable for use in the cooling system 14 are as
follows:
[0037] (1) Pb (89% by weight)+Bi (10% by weight) [with the balance
involving various impurities as discussed in greater detail below];
and
[0038] (2) Pb (45% by weight) and Bi (54% by weight) [with the
balance involving various impurities as likewise discussed in
further depth below].
[0039] While the lead-bismuth alloys of interest in the present
situation can involve many different lead and bismuth quantity
values without limitation, preferred lead-bismuth alloys which are
suitable for cooling purposes will include therein about 45-89% by
weight Pb (optimum sub-range=about 45-55% by weight) and about
10-54% by weight Bi (optimum sub-range=about 40-54% by weight).
Again, the claimed invention shall not be restricted to these or
any other numerical parameters unless otherwise expressly stated
herein.
II. The Inorganic Contaminants
[0040] As indicated above, the supply 16 of the lead-containing
molten metal composition will likewise contain at least some
inorganic contaminants therein which are naturally present in the
raw ore materials associated with the lead. The presence of these
contaminants contributes to increased corrosion problems in the
cooling system 14 which can adversely impair the operating
efficiency and safety of the nuclear reactor 12 or other
heat-generating devices as previously discussed. Regarding the
particular inorganic contaminants which reside within the
lead-containing molten metal composition, a variety of metals,
non-metals, or combinations thereof may be involved. Accordingly,
the term "combinations" as employed in connection with the
inorganic contaminants (and other materials associated with the
claimed invention) shall be broadly construed to encompass
mixtures, compounds, alloys, and complexes of two or more of the
listed materials and possibly others without limitation. Regarding
the inorganic contaminants, a typical supply 16 of the
lead-containing molten metal composition can include therein the
following metals and/or non-metals alone or combined (typically in
elemental form but possibly in other forms in accordance with the
definition of "combinations" recited above): arsenic [As], antimony
[Sb], tin [Sn], tellurium [Te], or combinations of two or more of
the above-mentioned elements (or with other materials). These
compositions are, for the most part, naturally occurring impurities
in lead-containing ores that will need to be removed in order to
avoid the problems discussed above including those associated with
corrosion and the like.
[0041] Regarding typical quantities of the above-mentioned and
other inorganic impurities in the supply 16 of the lead-containing
molten metal composition, these quantities will vary depending on
the type of ore from which the lead was derived, the geographic
location and grade associated with the ore, and other extrinsic
factors. However, in a representative and non-limiting embodiment,
typical amounts of the above-listed inorganic contaminants (in
elemental form in this example) are as follows: (A) arsenic (about
0.1-0.2% by weight); (B) antimony (about 0.5-2% by weight); (C) tin
(about 0.1-1% by weight); and (D) tellurium (about 0.1-0.5% by
weight). However, these numbers may again vary and otherwise
fluctuate without limitation and should therefore be considered
representative only. For example purposes, TABLE I below lists some
representative molten metal compositions associated with the supply
16 (and contaminants therein) which may be effectively treated in
accordance with the claimed invention: TABLE-US-00001 TABLE I
Composition Type Components (% by weight) Pb--Bi alloy Pb (89%) Bi
(10%) As (0.2%) Sb (0.8%) Pb--Bi alloy Pb (45%) Bi (54%) As (0.2%)
Sb (0.8%) Pb (elemental) Pb (98%) Bi (0%) As (0.2%) Sb (1.8%)
[0042] Again, the above-listed compositions (which do not include
appreciable amounts of tin and tellurium, with such materials and
possibly others being present in other lead-containing molten metal
compositions) constitute representative examples and shall not
restrict the invention in any manner. In this regard, the invention
as claimed shall not be limited to the treatment of any particular
lead-containing molten metal compositions, with a wide variety of
such materials being subject to rapid and effective decontamination
as discussed below.
[0043] It should also be noted that, for general information
purposes, the supply 16 of lead-containing molten metal composition
is typically maintained at a temperature of about 125-1000.degree.
C. during use within the cooling system 14 (for example, about
125-1000.degree. C. for lead-bismuth alloy cooling systems and
about 320-1000.degree. C. for cooling systems 14 which employ
molten elemental lead). Regarding the optimum temperature levels
which are maintained during the decontamination process, the higher
the temperature of the lead-containing molten metal composition,
the more efficient the decontamination process will be (with faster
processing times) as outlined further below. This
temperature-efficiency relationship is the result of improved
reaction kinetics at higher temperatures (although such
temperatures will likewise need to be balanced against energy costs
and the structural materials that are employed within the
decontamination system 10 as likewise discussed later in this
section). A preferred operating temperature range in the
decontamination system 10 which facilitates rapid and effective
treatment of the lead-containing molten metal composition will
likewise be presented below.
III. Representative Decontamination System Construction Materials
and Components
[0044] With continued reference to the schematic illustration of
FIG. 1, the supply 16 of the molten metal composition is then
routed into and through conduit 20 (using one or more conventional
pumps 22) and into a containment vessel 24. In particular, the
conduit 20 includes a first end 26 operatively connected to the
cooling system 14 and a second end 30 which is operatively
connected to an inlet port or opening 32 in the containment vessel
24. The pump 22 will involve, for example, a standard centrifugal
type that is known in the art for molten metal transfer or other
comparable pump devices which are suitable for this purpose. At
this point, however, some additional discussion is warranted
concerning the construction materials that are employed in
connection with the conduits, vessels, and other structures which
contain or allow the passage therethrough of the molten-metal
composition before, during, and after decontamination. While these
structures (including conduit 20, containment vessel 24, and the
other components described herein) may be produced from any
materials which are sufficiently durable to resist corrosion,
thermal deterioration, or other potentially-damaging effects caused
by the molten metal composition, certain construction materials are
preferred. In particular, effective results may be achieved through
the use of a composition which comprises at least some zirconium
[Zr] therein (including but not limited to elemental zirconium or
zirconium-containing alloys). Representative zirconium-containing
alloys that are suitable for this purpose include but are not
limited to the following materials: (A) "Zircaloy-2" (with the
approximate content of this alloy being [in % by weight]: tin
[1.5%], iron [0.12%], chromium [0.01%], and nickel [0.05%]) with
the balance being zirconium; and (B) "Zircaloy-4" (with the
approximate content of this alloy being [in % by weight]: tin
[1.5%], iron [0.18%], and chromium [0.01%] with the balance being
zirconium) wherein the foregoing values for both alloys are subject
to a certain degree of variance. Zirconium-containing compositions
of the types listed above (and others) are particularly useful in
that they form a self-protective zirconium oxide [ZrO.sub.2] layer
on the internal surfaces of the components discussed above. This
oxide layer can assist in avoiding corrosion and other related
problems (especially at operating temperatures within the
decontamination system 10 of about 550.degree. C. or less).
[0045] While the foregoing zirconium-containing compositions are
preferred as previously discussed, various other materials can
likewise be employed in connection with the conduits, vessels, etc.
of the claimed decontamination system 10 including but not limited
to molybdenum [Mo], tantalum [Ta], tungsten [W], and alloys or
other combinations of two or more of the above-listed materials (or
with other compositions). These alternative materials are
particularly effective at temperatures greater than 550.degree. C.
They will likewise form a protective oxide layer on the internal
surfaces of the structural components of the decontamination system
10 and (like zirconium) will have an insignificant degree of
solubility within the lead-containing molten metal compositions of
interest (including those made from elemental lead or a
lead-bismuth alloy). As a result, the supply 16 of the
lead-containing molten metal composition will not be further
contaminated by the above-mentioned materials.
[0046] Additional compositions which can be used effectively as
construction materials in connection with the operating components
of the decontamination system 10 (namely, the conduits and vessels
associated therewith) include iron-chromium-silicon alloys (with
these materials being present in varying proportions without
limitation) and a Russian alloy known as "EP-823". It should be
recognized that a number of different construction materials may be
used in connection with the decontamination system 10, with all of
the above-mentioned compositions being effective and suitable at
the temperature ranges and operational conditions associated with
the claimed apparatus and method. The selection of any given
structural materials in connection with the conduits, vessels, and
the like of the present invention shall therefore be undertaken in
accordance with routine preliminary pilot tests taking into account
the type of lead-containing molten metal composition being treated
(including the particular chemical nature thereof), the operating
temperatures of the decontamination system 10, and other related
factors.
IV. The Reducing Agent
[0047] Next and with continued reference to FIG. 1, it is preferred
that a supply 40 of a reducing agent (optimally in gaseous form as
indicated below) be provided which is in fluid communication with
the supply 16 of the lead-containing molten metal composition so
that the reducing agent can be introduced into the molten metal
composition on demand. In the representative embodiment of FIG. 1,
the supply 40 of the reducing agent is retained within a storage
vessel 42 having an outlet 44 therein to which the first end 46 of
a conduit 50 is operatively attached. In a preferred and
non-limiting embodiment, the second end 52 of the conduit 50 will
include multiple distributor portions 54 (e.g. in the form of, for
example, "tuyeres" or lances) which are in operative connection
with openings 56 in the conduit 20. In this manner, the reducing
agent can be effectively distributed or otherwise delivered into
the lead-containing molten metal composition within the conduit
20.
[0048] As noted above, it is preferred that the supply 40 of the
reducing agent be in a gaseous form which facilitates the delivery
thereof into the supply 16 of the lead-containing molten metal
composition in a rapid, cost-effective, and efficient manner.
Delivery of the reducing agent to the molten metal composition may
be achieved in a number of different ways without limitation. For
example, the reducing agent 40 within the storage vessel 42 can be
maintained in a pressurized state which will allow the reducing
agent to be spontaneously and automatically transferred into the
conduits 20, 50 (and the molten metal composition) in an effective
fashion. Use of the reducing agent in a pressurized state is, in
fact, preferred in that this delivery approach is highly efficient
and rapid, especially since the cooling system 14 and the claimed
decontamination system 10 operate at relatively low pressure levels
(e.g. about 1-1400 torr). Alternatively, an in-line gas flow pump
60 of a conventional type (for example, of a vacuum/diaphragm
variety) that is known in the art for gas transfer can be used to
deliver the supply 40 of the reducing agent into the molten metal
composition. It should therefore be recognized that the claimed
invention shall not be restricted to any materials, devices, or
components which may be used to deliver or transfer the various
compositions associated with the invention into, through, and out
of the decontamination system 10. A wide variety of different
transfer devices and equipment may therefore be employed for these
and other purposes without limitation.
[0049] Regarding the reducing agent, its purpose will now be
generally discussed. The employment of a reducing agent within the
decontamination system 10 should be considered preferred in that it
can provide a number of important benefits. Specifically, by
creating a reducing environment within the system 10, the formation
of oxide layers (for example, one or more layers comprised of lead
oxide [PbO] or other oxide materials) on the internal operating
surfaces of the decontamination system 10 (especially the
decontamination member discussed below) is effectively prevented.
If not prevented, oxide layers of this type can coat the
decontamination member and thereby prevent access to the surface of
this important structure by the lead-containing molten metal
composition. As a result, the decontamination process will be
blocked and otherwise substantially impeded, with the overall
decontamination procedure being discussed in extensive detail
below.
[0050] For the reasons given above (including the overall
maintenance of a high level of decontamination efficiency), it is
therefore preferred that the reducing agent be employed. While the
use of a reducing agent should nonetheless be considered "optional"
(since, under certain circumstances as determined by routine
preliminary pilot testing, the decontamination system 10 may
operate without it), it should nonetheless be employed as a
"default" procedure unless compelling reasons exist to the
contrary. It should also be recognized that the claimed invention
shall not be restricted to any location, interval, or point at
which the supply 40 of reducing agent is added or otherwise
introduced into the supply 16 of lead-containing molten metal
composition. While the point-of-introduction shown in FIG. 1 is
preferred, the reducing agent can be added into the decontamination
system 10 at any point upstream or downstream thereof provided that
the reducing agent is introduced into the lead-containing molten
metal composition in a manner which prevents oxide layer formation
as previously discussed.
[0051] Regarding the materials which can be employed in connection
with the supply 40 of the reducing agent, a number of different
compositions can be used for this purpose without limitation.
However, in a preferred embodiment designed to provide optimum
results, the reducing agent will be in gaseous form (in order to
facilitate rapid and efficient introduction into the molten metal
composition) and will involve the following exemplary materials:
hydrogen [H.sub.2(g)], methane [CH.sub.4(g)], acetylene
[C.sub.2H.sub.2(g)], propane [C.sub.3H.sub.8(g)], or combinations
of two or more of the above without limitation. Within this group
of materials, hydrogen is preferred in accordance with its ability
to significantly reduce the oxygen potential of the molten metal
composition. With respect to the ability of hydrogen to function as
an effective reducing agent in the present invention, the following
chemical reactions occur when hydrogen is combined with the
lead-containing molten metal composition:
O.sub.2+H.sub.2.apprxeq.H.sub.2O (1)
PbO+H.sub.2.apprxeq.Pb+H.sub.2O (2) (In general:
MxOy+H.sub.2.apprxeq.xM+yH.sub.2O) (3)
[0052] Regarding the overall quantity of the reducing agent to be
employed in the claimed decontamination apparatus and method, the
present invention shall not be restricted to any particular amount
for this purpose. The exact quantity of the reducing agent to be
used in a given application or situation is again determined in
accordance with routine preliminary pilot tests taking into account
numerous parameters including the overall size and capacity of the
decontamination system 10, the amount of lead-containing molten
metal composition being treated, and other related parameters.
However, in a representative, preferred, and non-limiting
embodiment designed to provide optimum results, the reducing agent
(namely, one or more of the gaseous compositions listed above) will
be used in an amount equal to about 0.1-10% by weight of the mass
flow rate of the decontamination system 10. By way of example, if
100 lb./hour of lead-containing molten metal composition was
flowing through the system 10, the addition of approximately 1
lb./hour of the chosen reducing agent would provide effective
results. However, it should again be emphasized that the
above-listed data (and the foregoing example) are representative
only and, in this regard, the claimed invention shall not be
limited to any particular quantities of reducing agent which may
vary in accordance with a variety of parameters as outlined
above.
[0053] It should likewise be recognized that, in a preferred
embodiment, an excess amount of reducing agent should be used over
and above the level which would theoretically be needed to prevent
oxide layer formation. This approach should specifically be
implemented as a "default" measure in order to be certain that the
above-listed goal is effectively achieved. Regarding the flow rate
associated with the reducing agent, this may likewise be determined
using routine preliminary pilot studies, with the claimed invention
not being restricted in this regard. The particular flow rate to be
selected should be sufficient to introduce the chosen amount of
reducing agent into the decontamination system 10 over a desired
time period, and is therefore readily determined once the desired
reducing agent quantity is selected (again taking into account
system size and other related parameters). Furthermore and in view
of the relatively high temperature of the lead-containing molten
metal composition as it leaves the cooling system 14 (and the
fairly large volumes thereof which are employed in most reactor
applications), pre-heating of the supply 40 of reducing agent is
typically not necessary (unless otherwise indicated by routine
preliminary tests).
[0054] It should likewise be noted that, in an alternative
embodiment, solid particulate carbon [C.sub.(s)] can be employed in
connection with the supply 40 of reducing agent (although gaseous
materials are again preferred for the reasons given above). Solid
carbon compositions will function effectively in the claimed
decontamination system 10, especially if temperatures above about
600.degree. C. exist. This material would be physically combined
(e.g. mixed) with the lead-containing molten metal composition
preferably within conduit 20 (or at any point upstream or
downstream therefrom in the same fashion as the gaseous reducing
agents discussed above). Regarding the quantity of this material to
be used, the amount thereof would again be determined by routine
preliminary pilot tests taking a number of factors into account
including the overall size of the decontamination system 10, the
metallurgical nature and content of the molten metal composition,
and the like. While this alternative embodiment is not restricted
to any particular amount of solid carbon reducing agent, a
representative and non-limiting preferred quantity would involve an
amount of solid carbon which would be sufficient to produce an
exposed carbon surface area in the range of about 5-15% of the
cross-sectional flow area of the conduit 20 to ensure contact
between the lead-containing molten metal composition and the solid
carbon reducing agent. Individuals skilled in the art will
recognize that the internal flow configuration of the solid carbon
reducing agent (if used) can be modified as needed and desired in
order to enhance the contact between the solid carbon reducing
agent and the lead-containing molten metal composition.
V. The Decontamination Process
[0055] At this point, the decontamination process associated with
the claimed invention will now be discussed in detail. With
continued reference to FIG. 1, the supply 16 of the lead-containing
molten metal composition is thereafter routed through the second
end 30 of conduit 20 and into the containment vessel 24 via the
opening 32 therein. Transfer of the molten metal composition will
occur using the pump 22 or possibly other auxiliary or supplemental
pumping devices (not shown), the need for which will be determined
by the overall size and configuration of the decontamination system
10 under consideration. Likewise, transfer of the molten metal
composition can take place using the differential pressure across
the core of the reactor 12. The containment vessel 24 includes a
side wall 62 which is produced from the materials discussed above
(optimally a material which comprises at least some zirconium
therein including but not limited to elemental zirconium, a
zirconium-containing alloy, or combinations thereof).
[0056] Once the lead-containing molten metal composition is present
within the interior region 64 of the containment vessel 24, it
comes in direct physical contact with at least one decontamination
member 70 which will now be discussed in detail. The central
location of the decontamination member 70 within the interior
region 64 of the containment vessel 24 as illustrated in FIG. 1
(namely, in the direct flow path of the incoming lead-containing
molten metal composition) ensures direct physical contact between
the molten metal composition and the decontamination member 70.
This process (which generally involves the intentional placement of
the lead-containing molten metal composition in contact with the
decontamination member 70) constitutes an important development
which facilitates the effective removal of the inorganic
contaminants (as defined above) from the molten metal
composition.
[0057] The decontamination member 70 involves a structure of
varying overall configuration which is separate and distinct from
any other structures within the cooling system 14 and
decontamination system 10. In particular, it is separate from the
conduits, walls, vessels, and other components associated with the
foregoing systems 10, 14 and is an independently-functioning
structure. The decontamination member 70 again resides in a central
location within the interior region 64 of the containment vessel 24
and is surrounded by the side wall 62 thereof. It is particularly
positioned within the flow path of the molten lead-containing
composition which enters the interior region 64 of the containment
vessel 24 so that the molten metal composition may directly contact
the decontamination member 70.
[0058] The decontamination member 70 can involve many different
structural configurations, shapes, sizes, surface areas, and the
like without limitation. The present invention shall therefore not
be limited to any particular dimensions, sizes, and designs in
connection with the decontamination member 70. As long as the
decontamination member 70 is present in some form (irrespective of
size, shape, etc.), it will remove at least some of the inorganic
contaminants from the lead-containing molten metal composition and
will therefore accomplish the goals of the present invention. The
exact size, shape, and structural configuration of the
decontamination member 70 will depend on the overall size and
capacity of decontamination system 10 in general (and the amount of
molten metal composition to be treated) which can be determined in
accordance with routine preliminary pilot tests.
[0059] With reference to FIG. 1, the particular decontamination
member 70 shown therein is comprised of an upper cap-like retaining
structure 72 having secured thereto a plurality of individual rod
or plate-like elements 74. The elements 74 are produced from the
particular materials that accomplish the actual decontamination of
the molten metal composition as discussed extensively below. In
accordance with FIG. 1, the elements 74 are elongate in character,
arranged in an annular (e.g. circular) configuration, and are
further retained in position using a bottom-mounted retaining
structure 76. In the exemplary configuration presented in FIG. 1,
the molten metal composition can flow around and between the
elements 74 in order to achieve a maximum degree of contact
therebetween. It should likewise be noted that, in a preferred
embodiment, the elements 74 which are produced from the chosen
decontamination material are readily removable from the system 10
once they become sufficiently "loaded" with contaminants that they
are no longer operationally effective as discussed further
below.
[0060] It should therefore be recognized that the structure set
forth in FIG. 1 in connection with the decontamination member 70
constitutes a single representative example thereof, with a number
of other structures and overall configurations being possible
without limitation. Likewise, the number of decontamination members
70 in the system 10 may vary from a single unit to multiple units
in combination. These units may be elongate, spherical, round,
square, or in any other configuration as determined in accordance
with the overall configuration of the entire decontamination system
10 and its capacity (with a maximum degree of surface area being
desired as a "default" condition). However and in general, removal
of the inorganic contaminants from the lead-containing molten metal
composition using the decontamination member 70 is dependent on the
following variables: (1) temperature; (2) oxygen potential; (3)
surface area; and (4) the types of materials associated with the
decontamination member 70 and the lead-containing molten metal
composition. The following mathematical correlation is provided in
order to explain and otherwise quantify the degree of impurity
removal relative to the physical characteristics of the
decontamination member 70 (e.g. size, shape, surface area, etc.)
and may therefore be used to produce a decontamination member 70
having desired characteristics: I=BmAe.sup.(RT/[PO.sup.2.sup.AL])
[0061] [I=Degree of contaminant removal (mass [kgs]/hour); [0062]
B=A constant dependant on the substrate of the decontamination
member 70 (1/meters.sup.2); [0063] T=Temperature of the molten
metal composition (K); [0064] PO.sub.2=Partial pressure of oxygen
[O.sub.2] in the molten metal composition (atm or force/area);
[0065] R=Gas constant (joules/mole/K); [0066] L=Length of the
decontamination member 70 (meters); [0067] A=Surface area of the
decontamination member 70 (meters.sup.2); [0068] m=mass flow rate
of the lead-containing molten metal composition through the
decontamination system 10 (kgs/hour); and [0069] e=natural
log].
[0070] The above-listed formula can generally be employed to
determine the overall structural characteristics of the
decontamination member 70 with particular reference to surface area
and the like. However, it should again be recognized that routine
preliminary pilot testing can likewise be employed to determine
these characteristics (and other features thereof) without
limitation. At this point, the materials which are used to produce
the decontamination member 70 will be discussed in detail. As
previously stated, the decontamination member 70 is produced from a
composition that will allow the above-mentioned inorganic
contaminants to diffuse into the decontamination member 70 (while
allowing lead in the molten metal composition to remain unaffected
so that it does not diffuse into the decontamination member 70 or
otherwise react therewith). As previously stated, the term
"diffuse" shall be construed in the broadest possible sense to
involve: (1) entry of the inorganic contaminants into and beneath
the surface of decontamination member 70 to various depths without
limitation; (2) interaction of the inorganic contaminants with the
decontamination member 70 at the surface thereof without passing
beneath the surface; and/or (3) a combination of [1] and [2] above.
Irrespective of the manner in which diffusion occurs, the
decontamination member 70 is again of a type that will have an
affinity for the inorganic contaminants of interest while avoiding
an affinity for the various lead-containing materials (e.g.
elemental lead, alloys, compounds, or complexes thereof) which are
associated with the lead-containing molten metal composition. As a
result, the inorganic contaminants can be removed in a selective
manner from the lead-containing molten metal composition in order
to effectively decontaminate it.
[0071] To accomplish the goals outlined above, the decontamination
member 70 will be made from a material which comprises or otherwise
contains at least some iron therein (e.g. elemental iron or
iron-containing alloys, compounds, complexes, or combinations
thereof). However, in a preferred and inventive embodiment designed
to yield unexpectedly superior results, the decontamination member
will be comprised entirely or partially of steel (namely, an
iron-based alloy). A number of different steel materials can be
employed for this purpose without limitation including stainless
steels (for example, both austenitic and ferritic stainless steels)
and carbon-based steels. Representative steel compositions which
can be used to produce the decontamination member 70 (for example,
the elements 74 as shown in FIG. 1) are as follows:
[0072] 1. "310-stainless steel" (with the approximate content of
this material in % by weight being as follows: C=0.25%; Cr=26%;
Mn=2%; Ni=22%; P=0.045%; Si=1.5%; and S=0.03%, with the balance
involving Fe).
[0073] 2. "316L-stainless steel" (with the approximate content of
this material in % by weight being as follows: C=0.01%; Cr=16.3%;
Cu=0.34%; Mn=1.5%; Mo=2.1%; Ni=10.1%; and Si=0.6%, with the balance
involving Fe).
[0074] 3. "410-stainless steel" (with the approximate content of
this material in % by weight being as follows: Cr=12.5%; Mn=0.7%;
and Si=0.8%, with the balance involving Fe).
[0075] 4. "F-22 carbon steel" (with the approximate content of this
material in % by weight being as follows: C=0.1%; Cr=2.1%; Cu=0.1%;
Mn=0.4%; and Mo=0.9%, with the balance involving Fe).
[0076] It should again be noted that the steel materials recited
above constitute representative examples which shall not restrict
the invention in any respect since various other steel and
iron-containing compositions can likewise be employed. For example,
a wide variety of other steel materials may be used including but
not limited to the group of austenitic stainless steels in the
"300-series", the group of ferritic stainless steels in the
"400-series", and "mild" carbon steels in general.
[0077] In accordance with the direct physical contact which is made
between the lead-containing molten metal composition and the
decontamination member 70, the inorganic contaminants recited above
(and possibly others not expressly set forth herein) will diffuse
into or onto (as previously defined) the decontamination member 70.
As a result, the inorganic contaminants are effectively removed
from the lead-containing molten metal composition while allowing
lead materials within the molten metal composition to remain
therein and not be removed (by diffusion into the decontamination
member 70 or otherwise). While the exact physical and chemical
mechanisms associated with the decontamination process are not
fully understood, it is theorized that a number of specialized
reaction processes take place which will now be generally discussed
with primary reference to arsenic-based impurities for example
purposes.
[0078] Under normal or oxidizing conditions, the dissolution of
oxygen into the iron-containing decontamination member 70 (e.g made
from steel or the like) forms a protective layer of metal oxide
that prevents the dissolution of metals (e.g. iron, nickel, and/or
chromium) from the decontamination member 70 into the
lead-containing molten metal composition. This situation likewise
prevents the above-listed inorganic contaminants from being
"exchanged" with the metals set forth above so that they can
diffuse into the decontamination member 70. The oxide material
discussed above primarily consists of spinelles of iron-chrome
oxides with a magnetite layer on the surface of the decontamination
member 70. A reducing environment (which may be induced by the
addition of a reducing agent as discussed herein) removes these
oxides, thereby allowing elements from the decontamination member
70 (e.g. iron, nickel, and/or chromium) to diffuse and dissolve
into the molten metal composition. As a result, inorganic
contaminants (for example, arsenic) can diffuse into the surface of
the decontamination member 70 as previously discussed and react
with iron therein (which becomes more "available" in accordance
with the diffusion of other materials such as chromium and nickel
out of the member 70). Diffusion of the inorganic contaminants into
the decontamination member 70 in the manner described above forms
metallic combinations (e.g. alloys) within the surface of the
decontamination member 70. For example, elemental iron and arsenic
react to form an iron-arsenic [Fe--As] alloy (e.g. iron arsenide).
It should also be noted that iron from the decontamination member
70 likewise diffuses into the lead-containing molten metal
composition, but at a much lower level than, for example, nickel
and chromium which are typical elements that reside within
steel-based decontamination members 70 of the type discussed above.
This situation occurs in accordance with the much lower solubility
of iron in the molten metal composition compared with, for
instance, chromium and nickel. Specifically and for general
information purposes, the solubility limit for iron in molten lead
is approximately 1 ppm at 500.degree. C. In contrast, solubility
limits for arsenic, nickel, and chromium in molten lead are
approximately 31,000 ppm, 32,000 ppm, and 16 ppm, respectively.
[0079] The high temperature of the lead-containing molten metal
composition (with particular but not necessarily exclusive
reference to the preferred operating temperature range listed
below) serves to anneal the decontamination member 70. It is
theorized that iron migrates during these elevated temperatures,
especially within gaps in the crystalline structure of the
decontamination member 70 caused by the dissolution of other
components from the member 70 (including chromium, nickel, and/or
possibly other elements). Arsenic, when present as an inorganic
contaminant in the lead-containing molten metal composition, has a
lower mobility compared with iron, thereby allowing a layer of
iron-arsenide to be formed at the surface of the steel-based
decontamination member 70 as the iron migrates to the surface and
becomes exposed to the slowly, inwardly-diffusing contaminants
(e.g. arsenic in this example). The relative purity of the
resulting iron-arsenide layer decreases as the distance from the
surface of the decontamination member 70 increases. This situation
is caused by the diminished migration of, for example, chromium
and/or nickel from the decontamination member 70 as the distance
from the surface increases. At these levels (e.g. depths),
materials such as nickel and chromium are unable to exchange
positions with the contaminants (for example, arsenic). This
situation generally reduces the purity of the resulting alloys
(e.g. iron-arsenide) as the distance from the surface of the
decontamination member 70 increases. It should also be noted that
cracks will typically form in the structures associated with the
newly-formed contaminant-based layers in the decontamination member
70 (iron-arsenide in the present example). Cracks form for many
reasons including the relatively high activity of the
layer-creation mechanism, tensile layer-substrate stresses caused
by lattice mismatches, high thermal stress gradients which result
from rapid cooling (at the point at which cooling is permitted to
occur), and the preparation process that is typically associated
with analysis of the decontamination member 70 using scanning
electron microscope ("SEM") techniques and the like. Furthermore,
the thickness of the resulting layer which occurs when contaminants
diffuse into the decontamination member 70 is expected to increase
as the time-of-contact between the molten metal composition and the
decontamination member 70 increases, and will likewise increase
when higher temperatures are employed.
[0080] In a representative decontamination system 10 which employs,
for example, (1) a decontamination member 70 that is made from
316L, 410, or F-22 steel materials; and (2) a lead-containing
molten metal composition of the type discussed above which includes
arsenic and antimony as inorganic contaminants, the following layer
(e.g. film) structures are typically produced in connection with
the member 70: [A] a first (e.g. outermost) layer which approaches
the stoichiometric composition of iron-arsenide [Fe--As]; and [B] a
second (e.g. inner) layer which involves a mixture (e.g. alloy) of
iron, arsenic, antimony, and lead [Fe--As--Sb--Pb]. It is believed
that these layers result in accordance with the migration of
chromium and/or nickel from the decontamination member 70 into the
lead-containing molten metal composition, thereby allowing the
contaminants (e.g. arsenic) to come in contact with exposed iron as
previously discussed. As chromium and/or nickel continue to diffuse
from the decontamination member 70 into the molten metal
composition, an "exchange" occurs in connection with the arsenic,
thereby permitting it to diffuse (along with antimony and possibly
other contaminants) into the member 70. With particular reference
to arsenic (which is of primary concern in this example), the
resulting iron-arsenide layer or layers in the decontamination
member 70 are characterized by reduced levels of chromium and/or
nickel compared with the quantities of these materials that were
initially present in the member 70.
[0081] In the above-listed example, the diffusion/decontamination
process produces a relatively pure layer (e.g. film) of
iron-arsenide at the surface of the decontamination member 70, with
the purity of this material again decreasing as the distance from
the surface of the member 70 increases (characterized by inner
layers of iron-arsenic-antimony-lead as previously indicated).
However, it should also be recognized that, notwithstanding the
formation of these layers, there is negligible dimensional change
in the decontamination member 70 in most cases. Structural defects
in the foregoing layers are normally attributed to the relatively
favorable production of these layers from a chemical and physical
standpoint and the fact that a "pure" iron (e.g. iron-only)
decontamination member 70 is not being used. As indicated earlier
in the current discussion, a highly reducing environment (produced,
for example, through the combination of at least one reducing agent
with the lead-containing molten metal composition) is beneficial in
the production of an iron-contaminant layer (for example,
iron-arsenide) on the steel-based decontamination member 70. Such a
reducing environment will typically involve an oxygen partial
pressure of about 10-40 atm in a representative embodiment. In
contrast and when oxidizing conditions are present, there is a
higher chemical affinity of various components in the steel (e.g.
iron, chromium, and/or nickel) to oxygen compared with lead. This
situation typically results in surface passivation that prevents
the "exchange" of inorganic contaminants (e.g. arsenic and
antimony) into the steel associated with the decontamination member
70. Thus, as a "default" condition in the present invention, the
introduction of a reducing agent into the lead-containing molten
metal composition should be employed unless special reaction
conditions exist which would dictate otherwise.
[0082] It should again be recognized that the description of the
reaction mechanisms set forth above represents a current
understanding of the manner in which they function to achieve
decontamination of the lead-containing molten metal composition. In
this regard, the explanations presented herein concerning these
mechanisms shall not limit or otherwise restrict the invention in
any manner and are being presented for information purposes
only.
[0083] It should likewise be noted that, in a representative
embodiment designed to provide optimum results, the decontamination
system 10 (preferably the containment vessel 24) will include at
least one heater 80 associated therewith as schematically
illustrated in FIG. 1. The heater 80 may involve a number of
different types, structures, and configurations without limitation
including but not limited to those that employ at least one or more
electric resistive heating elements, as well as heating systems
powered by other fuel sources including natural gas, and the like.
Furthermore, the claimed invention shall not be restricted to any
particular locations in connection with the heater 80 which may be
placed at any position on or within the decontamination system 10
provided that the desired degree of heat is imparted to the
lead-containing molten metal composition as discussed further
below. In a representative and non-limiting embodiment, the heater
80 (optimally comprising at least one or more electrically
resistive elements) will at least partially surround the exterior
surface 82 of the side wall 62 associated with the containment
vessel 24 as schematically illustrated in FIG. 1. Alternatively, an
internal heating system can be provided within the containment
vessel 24. For example, one or more laminate layers of graphite
(not shown) can be provided on at least a portion of the
decontamination member 70. Heat is then applied using a chosen
heating source (e.g. an induction coil) positioned outside of the
containment vessel 24. The graphite then becomes inductively heated
(in accordance with its favorable thermal susception
characteristics) which, in turn, heats the decontamination member
70.
[0084] It should therefore be understood that: (1) many different
type of systems and components may be used in connection with the
heater 80; and (2) the heater 80 can be positioned in a variety of
locations. Furthermore, use of the heater 80 should be considered
"optional" in that the additional heat generated by the heater 80
may not be necessary depending on a variety of factors as
determined by routine preliminary pilot testing (including the
overall size associated with the decontamination system 10, the
temperature of the incoming molten metal composition, and other
related factors). Nonetheless, the use of at least one heater 80
should be considered preferred and employed as a "default"
component in the claimed decontamination system 10 unless operating
conditions specifically indicate otherwise.
[0085] Regarding the temperatures to be maintained during
decontamination of the lead-containing molten metal composition
using the decontamination member 70, the claimed invention shall
not be restricted to any particular values. However, in a preferred
embodiment designed to obtain optimum results, the molten metal
composition will be maintained at a temperature of about
400-600.degree. C. during contact thereof with the decontamination
member 70. This temperature level is designed to promote favorable
reaction kinetics and maximum operating efficiency. The temperature
conditions set forth above may be maintained and otherwise achieved
using the heater 80 as previously described or, alternatively, the
molten metal composition will have a temperature within the
foregoing range as a natural consequence of the heat transfer
process which occurs in the cooling system 14. Thus, a number of
different approaches may be employed in order to achieve the
preferred temperature characteristics recited above without
limitation. Likewise, in certain circumstances, different (e.g.
higher or lower) temperatures may be desired which would be
determined on a case-by-case basis using routine preliminary pilot
tests taking into account a number of factors including the
quantity of lead-containing molten metal composition being treated,
the nature and chemical make-up of the composition, the type and
configuration of the decontamination system 10, and other
factors.
[0086] Regarding the "residence time" in the claimed
decontamination system 10 (namely, the amount of time during which
the lead-containing molten metal composition is maintained in
direct physical contact with the decontamination member 70), this
time period may be varied as needed and desired. In particular, it
may be determined in accordance with routine preliminary pilot
tests taking into account the particular size, surface area, and
configuration of the decontamination member 70, the compositional
characteristics of the molten metal composition, and other related
factors. Accordingly, the claimed invention shall not be restricted
to any particular residence time parameters. However, it should be
generally understood that the overall residence time as defined
herein is a function of temperature and the oxygen potential of the
molten metal composition. For example purposes, TABLE II below
provides some representative and non-limiting estimated residence
times for a decontamination member 70 made from any of the specific
steel compositions recited above. The decontamination member 70
associated with the data in TABLE II will have a single elongate
bar-like configuration that is approximately 1 meter long with a
surface area of about 1000 cm.sup.2 (wherein a primary goal is to
remove arsenic as a contaminant): TABLE-US-00002 TABLE II Molten
Metal O.sub.2 Potential Temp. Residence Time Pb 10.sup.-20 to
10.sup.-40 atm 400-600.degree. C. 2 min. to 20 hr. Pb--Bi
10.sup.-20 to 10.sup.-40 atm 400-600.degree. C. 1 min. to 10
hr.
Again, the values recited above are provided for example purposes
only and shall not limit the invention in any respect.
[0087] Finally and in an exemplary embodiment designed to achieve
optimum decontamination efficiency, the decontamination member 70
should be of a type that is readily removable from the system when
it becomes saturated (e.g. "loaded") with the inorganic
contaminants to a point where it is of diminished operational
effectiveness. As to when this point is reached will vary depending
on many factors including but not limited to the overall size,
surface area, and shape of the decontamination member 70, the level
of contamination within the lead-containing molten metal
composition, and other related factors. One method for deciding
when to remove the decontamination member 70 from the system 10
would be to conduct analytical tests on the member 70 which would
generally involve a periodic analysis of the member 70 during
system operation using SEM analysis and other related techniques.
These tests (and possibly others as discussed below) could then be
used to determine the amount of time that it takes for the
decontamination member 70 to become unable to form any additional
layers (through diffusion and the like) of the desired contaminants
therein. Once this time period is determined for a given type and
quantity of the molten metal composition and decontamination member
70, it may then be applied as a "standard" for subsequent use in
the decontamination of additional quantities of the molten metal
composition. Alternatively, other methods for determining when the
decontamination member 70 is fully "loaded" with contaminants
include diffraction pressure measurements across the member 70,
ultrasonic probe tests, resistive measurements, and the like.
[0088] In any event, the claimed invention shall not be restricted
to any particular methods or time intervals in connection with the
saturation and removal of the decontamination member 70 from the
system 10, with a number of different options being available.
VI. Other Features and Sub-Systems
[0089] Having discussed the basic operational capabilities of the
decontamination member 70 and other parameters of the
decontamination system 10, some additional features thereof will
now be discussed. While these additional items should be considered
"optional" and not mandatory in all cases (as determined by routine
preliminary pilot investigations), it is preferred that they be
employed as "default" measures in the methods and systems of the
present invention in order to achieve maximum operating efficiency.
First and with reference to FIG. 1, the side wall 62 of the
containment vessel 24 includes at least one main or primary outlet
port 90 therein for passage of the decontaminated lead-containing
molten metal composition as discussed further below. Furthermore
and in a preferred embodiment, the side wall 62 of the containment
vessel 24 will likewise include at least one additional or
secondary outlet port 92 therein, the function of which will now be
discussed.
[0090] Should a reducing agent be used in the decontamination
system 10 and if the molten metal composition contains excess (e.g.
unreacted) quantities of the reducing agent therein after contact
between the molten metal composition and the decontamination member
70, it is usually desirable to remove this excess reducing agent
from the molten metal composition and containment vessel 24. This
is particularly important when the gaseous reducing agents listed
above are employed, with the current discussion being primarily
directed to these particular reducing agents. In most cases
(namely, as a "default" condition), excess gaseous reducing agents
will be used in the claimed process for the reasons given above
(e.g. to ensure complete, continuous, and maximum operational
efficiency and to likewise avoid the oxidation problems discussed
herein). Typically, the excess/residual reducing agent will involve
about 1-5% more than is consumed or otherwise needed in the
process. Removal of the excess reducing agent is particularly
desired since, if allowed to remain in the lead-containing molten
metal composition, it can contribute to additional corrosion of the
cooling system 14 once the decontaminated molten metal composition
is recycled back into the cooling system 14 for reuse. Furthermore,
removal, recovery, and reuse of the excess reducing agent can
significantly improve the overall cost-efficiency and economic
performance of the entire decontamination process.
[0091] To accomplish the removal (and recovery if desired) of the
excess gaseous reducing agent, it is preferred that the
decontamination system 10 (with particular reference to the
containment vessel 24) be configured and operated so that an open
region 94 exists above the supply 16 of molten metal composition.
As a result, an exposed (e.g. top) surface 96 associated with the
molten metal composition exists within the interior region 64 of
the containment vessel 24. This exposed surface 96 represents an
"interface" between the molten metal composition and the open
region 94. In accordance with the particular chemical and physical
nature of the gaseous reducing agents discussed above, the limited
solubility thereof in the molten metal composition, and the high
temperature conditions within the containment vessel 24, the
unreacted (e.g. excess/residual) quantities of reducing agent will
spontaneously diffuse out of the molten metal composition and
reside (in gaseous form) in the open region 94. In order to remove
this material from the containment vessel 24 (for reuse or
otherwise), the vessel 24 will include the outlet port 92 through
the upper wall 100 which optimally resides at the top of the vessel
24. Operatively connected to the outlet port 92 is the first end
102 of a conduit 104 which, in a representative embodiment, will
contain an in-line vacuum pump 106 of conventional design (or other
comparable device) which will draw the excess reducing agent out of
the containment vessel 24 and through the conduit 104. The conduit
104 will have a second end 110 that is operatively connected to an
opening 112 in a storage vessel 114 which can be used to retain the
excess (e.g. withdrawn) reducing agent therein (shown at reference
number 115 in FIG. 1). This reducing agent may then be used for any
purpose, discarded, or (optimally) recycled back into the
decontamination system 10 for reuse.
[0092] With continued reference to FIG. 1, the storage vessel 114
preferably has an additional opening 116 therein which is
operatively connected to the first end 120 of a conduit 122 which,
in a representative embodiment, will have another in-line vacuum
pump 124 of conventional design (or other comparable device)
therein. The conduit 122 further includes a second end 126 that is
operatively connected to an opening 130 within the storage vessel
42 which contained the initial supply 40 of the reducing agent. In
this manner, effective recycling of the reducing agent can occur in
order to achieve the significant benefits listed above.
[0093] It should be recognized that the reducing agent removal
sub-system discussed herein and shown schematically in FIG. 1 is
being presented for example purposes only and shall not limit the
invention in any respect. Instead, a variety of different
components, conduits, and other structures may be used to remove
excess (e.g. residual) quantities of the reducing agent from the
molten metal composition and the containment vessel 24.
Nonetheless, the methods and procedures outlined above represent a
viable and practical approach by which the removal of
excess/residual quantities of reducing agent from the molten metal
composition and the containment vessel 24 can occur.
[0094] It should also be noted that at least some residual water
(e.g. in the form of steam--not shown) may be generated and
spontaneously released from the molten metal composition in
accordance with the decontamination procedures discussed herein.
This water may be eliminated from the containment vessel 24 in a
number of different ways without limitation. For example, it is
possible that the water (e.g. steam) can be withdrawn along with
the excess reducing agent discussed above (e.g. using the same
components and techniques) so that it is routed through the conduit
104 into the storage vessel 114. The storage vessel 114 would then
include a water trap/separatory system of conventional design (not
shown) that could be used to collect water from the materials in
the vessel 114. Again, however, a number of different techniques
and components can be used to accomplish water removal from the
containment vessel 24 without limitation, with the techniques
outlined above being representative only. It should also be noted
that, while the removal of water from the molten metal composition
and the containment vessel 24 may be considered "optional" in
nature, it is preferably employed as a "default" procedure unless
countervailing circumstances indicate otherwise. Water removal is
generally considered to be desirable so that the overall oxygen
potential in the decontamination and cooling systems 10, 14 is not
adversely affected.
[0095] Finally and as previously discussed, placement of the molten
metal composition in contact with the decontamination member 70
will typically cause at least one iron-containing contaminant to be
introduced into the molten metal composition. The iron-containing
contaminant can involve, for instance, elemental iron and
iron-containing alloys, compounds, complexes, or combinations
thereof without limitation. In a preferred embodiment, a process
step will be initiated in which at least some of the
iron-containing contaminants will be removed from the
lead-containing molten metal composition after decontamination.
While this step (and the components associated therewith) should
nonetheless be considered "optional" (with the need thereof
ultimately being determined by routine preliminary pilot studies),
it should be implemented as a "default" procedure unless compelling
reasons exist to do otherwise.
[0096] Many different methods and techniques can be employed in
order to remove the iron-containing contaminant materials from the
molten metal composition without limitation. Since the majority of
these materials will be in the form of solid particulate
compositions, a representative removal method and apparatus will
involve the use of one or more magnetic iron trap systems which
will now be explained in greater depth. With reference to FIG. 1,
an exemplary and non-limiting iron trap 140 is schematically
illustrated (with a number of other configurations and types also
being possible). The iron trap 140 involves a tubular member 142
having a first end 144 and a second end 146, with the tubular
member 142 optimally being designed so that it is readily removable
from the decontamination system 10. The first end 144 in the
embodiment of FIG. 1 is operatively connected to the main outlet
port 90 in the side wall 62 of the containment vessel 24.
Furthermore, the tubular member 142 may include a pump 150
associated therewith of conventional design (e.g. the same type as
pump 22 or otherwise). As a result, the decontaminated
lead-containing molten metal composition can readily pass from the
interior region 64 of the containment vessel 24 into the tubular
member 142 associated with the iron trap 140.
[0097] The tubular member 142 will preferably be produced from an
iron-containing composition (e.g. an iron alloy including but not
limited to stainless steel) and will have at least one magnet 152
preferably located on the exterior surface 154 of the tubular
member 142. A single magnet 152 can be used as shown in FIG. 1 or
multiple magnetic elements can instead be employed (not shown)
without limitation. Likewise, the size, shape, capacity, and other
characteristics of the tubular member 142, the magnet 152, and the
iron trap 140 in general can be varied as needed and desired in
accordance with routine preliminary pilot testing and shall not
restrict the invention in any respect.
[0098] Regarding the magnet 152, preferred and non-limiting
magnetic strength values associated therewith will be about 0.1-10
gauss. As the molten metal composition enters the interior region
156 of the tubular member 142, the magnetic field generated by the
magnet 152 will cause the solid iron-containing contaminants in the
molten metal composition to be drawn out of the composition and
become magnetically adhered to the interior surface 160 of the
tubular member 142. In this manner, the iron-containing
contaminants are effectively removed from the molten metal
composition in a rapid and efficient manner. It should be
understood that removal of the iron-containing contaminant
materials from the molten metal composition is desirable as a
"default" procedure for various reasons. For example, if the
iron-containing contaminants are not removed from the molten metal
composition, they can precipitate within lower-temperature regions
of the cooling system 14 when the molten metal composition is
recirculated for use therein. This precipitation process can, in
fact, cause significant flow restrictions in the cooling system 14
and substantially degrade its performance.
[0099] As needed and desired, the tubular member 142 associated
with the iron trap 140 can be removed for cleaning or replacement
at any desired interval. One method for determining when to remove
the tubular member 142 from the decontamination system 10 would be
to conduct pilot tests on the member 142 which would involve a
periodic analysis of the member 142 during system operation using
manual inspection techniques, flow pressure measurements, and other
related procedures. These techniques could then be used to
determine when the interior surface 160 of the tubular member 142
has become sufficiently "loaded" with iron-containing contaminants
to no longer be optimally effective. Once this time period is
determined for a given type and quantity of the molten metal
composition, it may then be applied as a "standard" for subsequent
use in the overall operation of the iron trap 140. Regarding flow
pressure measurements, these measurements will generally involve a
determination of the pressure levels of the molten metal
composition moving through the tubular member 142, with diminished
pressure levels indicating that the tubular member 142 has become
sufficiently "loaded" to warrant its replacement or cleaning. It
should also be noted that, aside from the approach outlined above,
an on-line "real-time" flow pressure measurement system of a type
which is conventional and known in the art may be used in
connection with the tubular member 142. Once the flow pressure in
the tubular member 142 decreases to a predetermined level, the
member 142 can be removed from the decontamination system 10 for
replacement or cleaning. It shall therefore be understood that a
number of different methods and components may be used to monitor
the activity of the iron trap 140 without limitation.
[0100] In the exemplary embodiment of FIG. 1, the second end 146 of
the tubular member 142 associated with the iron trap 140 is
operatively connected to the first end 162 of a conduit 164 which
includes, for example, an in-line pump 166 of conventional design
therein (e.g. of the same type as pump 22 or otherwise). The second
end 170 of the conduit 164 is operatively connected to the cooling
system 14 so that the decontaminated lead-containing molten metal
composition may be transferred through the conduit 164 (using, for
example, pump 166) for delivery into cooling system 14 to be reused
therein as desired.
[0101] As previously stated, the claimed invention provides many
key benefits in a simultaneous fashion. In particular, it is able
to effectively and economically decontaminate a wide variety of
lead-containing molten metal compositions and can likewise remove a
significant number of metallic and non-metallic contaminants with a
high level of efficiency. For example, it is expected that
implementation of the claimed invention using the processes and
equipment discussed above can remove a given contaminant (e.g.
arsenic, antimony, etc.) down to 10 ppm levels or below depending
on the manner in which the overall process is implemented, the
particular materials being decontaminated, and the like.
Accordingly, the present invention is capable of a significant
level of decontamination and, in this regard, can provide the
benefits listed above. These benefits again include, without
limitation: (1) the ability to remove inorganic compositions
(particularly arsenic, antimony, tin, and tellurium) in a highly
efficient manner from lead-containing molten metal compositions;
(2) rapid and highly effective decontamination rates; (3) the
implementation of an efficient decontamination process using a
minimal amount of operating equipment and materials; (4) the
ability to remove contaminants without the need to employ
hazardous, caustic, or expensive chemical reagents; (5) a high
level of versatility with particular reference to the types of
lead-containing molten metal compositions which can be treated; (6)
improved decontamination efficiency resulting from the ability of
the system to operate in a substantially continuous fashion; (7)
compatibility with a considerable number of heat generating devices
including but not limited to a wide variety of nuclear power
generating systems, accelerator-driven radioactive waste
transmutators, and the like which employ lead-containing molten
metal compositions as coolants; (8) the ability to achieve
decontamination without requiring highly oxidizing conditions
(which avoids the problems associated therewith as discussed
above); (9) a considerable degree of versatility regarding the
types of contaminants which may be removed from the lead-containing
molten metal compositions; (10) the overall implementation of a
procedure which is cost effective, readily controllable (e.g.
customizable on-demand to various cooling systems and devices),
easily scaled up or down as needed, and capable of rapid
integration into the cooling systems of interest; (11) the capacity
to decontaminate lead-containing molten metal compositions in a
manner whereby destructive corrosion of the cooling systems is
eliminated, thereby avoiding excessive maintenance requirements,
system failures, and other operational problems; and (12) an
accomplishment of the above-listed goals in a manner which is
superior to prior decontamination techniques and represents a
considerable advance in molten metal processing technology.
[0102] Having set forth herein preferred embodiments of the
invention, it is anticipated that various modifications may be made
thereto by individuals skilled in the relevant art to which this
invention pertains which nonetheless remain within the scope of the
invention. For example, the invention shall not be limited to any
particular equipment, operating components, reactant types and
quantities, contaminants to be removed, lead-containing molten
metal composition types and quantities, operating conditions and
parameters, decontamination system sizes and capacities, and other
related items unless otherwise expressly stated herein. The present
invention shall therefore only be construed in accordance with the
following claims:
* * * * *