U.S. patent application number 09/741579 was filed with the patent office on 2001-06-28 for method for reducing the formation of zn(nh4)4cl2 from zno/nh4cl solutions.
Invention is credited to Myerson, Allan S., Robinson, Peter.
Application Number | 20010005496 09/741579 |
Document ID | / |
Family ID | 23323526 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005496 |
Kind Code |
A1 |
Myerson, Allan S. ; et
al. |
June 28, 2001 |
Method for reducing the formation of Zn(NH4)4Cl2 from ZnO/NH4Cl
solutions
Abstract
A method for reducing the formation of
Zn(NH.sub.4).sub.4Cl.sub.2 from ZnO/NH.sub.4Cl solutions formed
during an industrial waste stream recycling method useful for the
recovery of high purity zinc oxide products and other chemical and
metal values from industrial waste streams.
Inventors: |
Myerson, Allan S.; (Chicago,
IL) ; Robinson, Peter; (Fergus, CA) |
Correspondence
Address: |
TECHNOPROP COLTON, L.L.C.
P O BOX 567685
ATLANTA
GA
311567685
|
Family ID: |
23323526 |
Appl. No.: |
09/741579 |
Filed: |
December 19, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09741579 |
Dec 19, 2000 |
|
|
|
09338129 |
Jun 22, 1999 |
|
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Current U.S.
Class: |
423/100 ;
423/101 |
Current CPC
Class: |
C22B 19/26 20130101;
C22B 7/02 20130101; Y02P 10/20 20151101; C22B 7/007 20130101; C22B
19/24 20130101; Y02P 10/234 20151101; C22B 19/34 20130101; Y10S
75/961 20130101 |
Class at
Publication: |
423/100 ;
423/101 |
International
Class: |
C01G 009/02 |
Claims
What is claimed is:
1. A method for reducing the formation of
Zn(NH.sub.4).sub.4Cl.sub.2 from ZnO/NH.sub.4Cl solutions comprising
the steps of: (a) providing a quantity of water sufficient to
dilute a ZnO/NH.sub.4Cl solution to a concentration of between
about 2% and 10% ZnO/NH.sub.4Cl; (b) adding a first fraction of the
ZnO/NH.sub.4Cl solution to the quantity of water to form a first
intermediate solution and a ZnO precipitate; and (c) adding a
second fraction of the ZnO/NH.sub.4Cl solution to the first
intermediate solution to form a second intermediate solution.
2. A method as claimed in claim 1, wherein the first fraction is
between 20% and 45% of the ZnO/NH.sub.4Cl solution.
3. A method as claimed in claim 2, wherein the first intermediate
solution has a concentration of ZnO/NH.sub.4Cl to water of between
2% and 6%.
4. A method as claimed in claim 3, wherein prior to step (c) the
first intermediate solution is maintained at steady state at a
concentration of ZnO/NH.sub.4Cl to water of between 2% and 6%.
5. A method as claimed in claim 4, wherein the first intermediate
solution is maintained at steady state at a concentration of
ZnO/NH.sub.4Cl to water of 3.3%.
6. A method as claimed in claim 2, wherein the second intermediate
solution has a concentration of ZnO/NH.sub.4Cl to water of between
5% and 9%.
7. A method as claimed in claim 6, wherein the second intermediate
solution is maintained at steady state at a concentration of
ZnO/NH.sub.4Cl to water of between 5% and 9%.
8. A method as claimed in claim 7, wherein the second intermediate
solution is maintained at steady state at a concentration of
ZnO/NH.sub.4Cl to water of 6.6%.
9. A method as claimed in claim 2, further comprising the step of:
(d) adding a third fraction of the ZnO/NH.sub.4Cl solution to the
second intermediate solution to form a third intermediate
solution.
10. A method as claimed in claim 9, wherein the third intermediate
solution has a concentration of ZnO/NH.sub.4Cl to water of between
8% and 12%.
11. A method as claimed in claim 9, wherein the third intermediate
solution has a concentration of ZnO/NH.sub.4Cl to water of 10%.
12. A method for reducing the formation of
Zn(NH.sub.4).sub.4Cl.sub.2 from ZnO/NH.sub.4Cl solutions formed
during a method for recycling industrial waste streams, comprising
the steps of: (a) providing a waste stream containing zinc
compounds; (b) leaching said waste stream with an ammonium chloride
solution, resulting in a first product solution and undissolved
materials; (c) adding zinc metal to said first product solution,
whereby zinc-displaceable metal ions contained in said first
product solution are displaced by said zinc metal and precipitate
out of said first product solution as metals, leaving a second
product solution comprising ZnO/NH.sub.4Cl; and (d) diluting said
second product solution with water by adding a first fraction of
said second product solution to the water until the first fraction
of said second product solution has a concentration of between
about 2% and 10% ZnO/NH.sub.4Cl, resulting in the formation of a
first intermediate solution and a ZnO precipitate.
13. A method as claimed in claim 12, wherein said ammonium chloride
solution has a concentration of 23% or greater by weight ammonium
chloride.
14. A method as claimed in claim 12, wherein said ammonium chloride
solution has a concentration of 30% or greater by weight ammonium
chloride.
15. A method as claimed in claim 13, further comprising the step
of: (e) adding a second fraction of said second product solution to
the first intermediate solution to form a second intermediate
solution.
16. A method as claimed in claim 13, wherein said ammonium chloride
solution is maintained at a temperature above about 70.degree.
C.
17. A method as claimed in claim 14, wherein said ammonium chloride
solution is maintained at a temperature above about 70.degree.
C.
18. A method as claimed in claim 12, wherein said waste stream is
heated in a reducing atmosphere prior to leaching, resulting in an
iron-containing residue and a waste stream comprising oxides of
zinc.
19. A method as claimed in claim 12, further comprising the step of
adjusting the pH of said second product solution to between about 5
and about 8 with NH.sub.4OH prior to step (d).
20. A method as claimed in claim 16, wherein said ammonium chloride
solution is maintained at a temperature of between about 70.degree.
C. and 100.degree. C.
21. A method as claimed in claim 17, wherein said ammonium chloride
solution is maintained at a temperature of between about 70.degree.
C. and 100.degree. C.
22. A method as claimed in claim 12, further comprising the step of
washing the ZnO precipitate in water at a temperature of at least
70.degree. C.
23. A method as claimed in claim 12, wherein the method is a
continuous process.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present patent application is a divisional of
application Ser. No. 09/338,129 filed on Jun. 22, 1999, currently
pending.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a process for recovery of zinc
products including essentially pure metal oxides and metals. The
present invention more specifically relates to an improved method
for reducing the formation of Zn(NH.sub.4).sub.4Cl.sub.2 from
ZnO/NH.sub.4Cl intermediate solutions during a recycling process
for the recovery of essentially pure zinc oxide from EAF dust.
[0004] 2. Description of Related Art
[0005] Zinc oxide typically is a fine white or grayish powder which
has a variety of uses including as a rubber accelerator, as a
pigment, as a dietary supplement and in the semiconductor field.
Zinc oxide is found in commercial by-products including waste
material streams such as fly ash and flue dust. Methods for
recovering zinc oxides are known in the art, including recovering
zinc oxide from industrial waste materials. Such previous methods
have included leaching with mineral acid, caustic soda, ammonium
hydroxide, and ammonium carbonate solutions. However, these methods
have low yields of zinc oxide and typically do not recover pure
zinc oxide, the recovered zinc oxide being contaminated with other
metal salts. Therefore, in order to obtain pure zinc oxide,
subsequent reduction and washing processes were necessary.
[0006] U.S. Pat. No. 3,849,121 to Burrows discloses a method for
the selective recovery of zinc oxide from industrial waste. The
Burrows method comprises leaching a waste material with an ammonium
chloride solution at elevated temperatures, separating iron from
solution, treating the solution with zinc metal and cooling the
solution to precipitate zinc oxide. The Burrows patent discloses a
method to take EAF dust which is mainly a mixture of iron and zinc
oxides and, in a series of steps, to separate out the iron oxides
and waste metals. However, the material obtained in the last step
is a mixture of a small amount of zinc oxide, hydrated zinc phases
which can include hydrates of zinc oxide and zinc hydroxide, as
well as other phases and a large amount of diamino zinc dichloride
Zn(NH.sub.3).sub.2Cl.sub.2 or other similar compounds containing
zinc and chlorine ions. Currently, the Burrows method is not
economically viable because of Environmental Protection Agency
guidelines established subsequent to the issuance of the Burrows
patent. Additionally, the Burrows method is not a continuous method
and, therefore, is not economical as a continuous process.
[0007] U.S. Pat. No. 4,071,357 to Peters discloses a method for
recovering metal values which includes a steam distillation step
and a calcining step to precipitate zinc carbonate and to convert
the zinc carbonate to zinc oxide, respectively. Peters further
discloses the use of a solution containing approximately equal
amounts of ammonia and carbon dioxide to leach the flue dust at
room temperature, resulting in the extraction of only about half of
the zinc in the dust, almost 7% of the iron, less than 5% of the
lead, and less than half of the cadmium.
[0008] Steam distillation is contrary to dilution. Steam
distillation precipitates zinc carbonate, other carbonates and iron
impurities. Steam distillation also disadvantageously results in an
increase in temperature which drives off ammonia and carbon
dioxide, resulting in the precipitation of iron impurities and then
zinc carbonate and other dissolved metals. The purity of the zinc
carbonate obtained depends on the rate of steam distillation and
the efficiency of solids separation as a function of time.
Calcining converts the zinc carbonate to zinc oxide, whereas
washing and drying at temperatures between 100.degree. C. and
200.degree. C. converts the zinc compounds to zinc oxide.
[0009] U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with
the present application, discloses a method for the recovery of
zinc oxide by treating a waste stream with a 23% ammonium chloride
at 90.degree. C., separating undissolved components from the
solution, displacing undesired metal ions from the solution using
zinc metal, cooling the solution to precipitated out zinc
compounds, washing the precipitate to remove various soluble zinc
compounds, leaving zinc oxide of greater than 99%. The '596 patent
teaches that ammonium chloride solutions must be at least
90.degree. C. to sufficiently dissolve the zinc compounds. Heating
an aqueous solution to such a temperature requires the expenditure
of large amounts of energy. It is further taught that while
NH.sub.4Cl concentrations below 23% do not dissolve the maximum
amount of zinc oxide from the waste material, concentrations
greater than 23% result in an impure zinc oxide due to the tendency
of the NH.sub.4Cl to precipitate out of solution with the zinc
compounds at such high concentrations. Furthermore, the cooling of
the product solution results in the precipitation of various zinc
species, resulting in crystallization of some species. Because of
this, using the cooling step disclosed in Myerson '596, one cannot
use concentrations of ammonium chloride solutions above about 23%,
limiting the usefulness of the process disclosed in Myerson '596.
Further, contaminates of the zinc oxide must be removed by an
additional washing step.
[0010] U.S. Pat. No. 5,759,503 to Myerson, et al., commonly
assigned with the present application, discloses a method for the
recovery of zinc oxide by dissolving zinc oxide in an intermediate,
diluting the intermediate by a factor of 3 to 30 by adding
70-100.degree. C. water, and filtering out the resultant zinc oxide
crystals. The '503 patent, along with its family of patents,
disclose using ammonium chloride solutions of 23% and teach that
using higher concentration ammonium chloride solutions will produce
undesired results. This was the understanding at the time of
invention of the processes disclosed and claimed in the Myerson
'503 patent and its family of patents. These undesired results
include the precipitation of various zinc species, resulting in
crystallization of some species. Because of this, using the cooling
step disclosed in Myerson '596, one cannot use concentrations of
ammonium chloride solutions above about 23%, limiting the
usefulness of the process disclosed in Myerson '503 Although the
processes disclosed and claimed in Myerson '503 patent are valuable
and perform admirably, it has now been discovered that by altering
the process steps, including the addition of a dilution step,
ammonium chloride solutions of 30% or greater can be used to
recycle industrial waste streams.
[0011] Therefore, there exists a need for an alternative method
that will recover essentially pure zinc oxide from industrial waste
materials that is economical, quick, and efficient and, optionally
also will allow the recovery of elemental lead, cadmium, and copper
from industrial waste materials, at the lower end of the
temperature range and at higher leach solution concentrations than
previously thought possible.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention satisfies these needs in a method
which recovers essentially pure zinc oxide from waste material
containing zinc or zinc oxide. Along with the essentially pure zinc
oxide, zinc metal also can be recovered, as well as values of other
metallic elements originally contained in the waste material, such
as lead, silver, and cadmium. The solutions used in the process are
recycled such that liquid wastes are kept to a minimum and,
ideally, eliminated. The solids recovered from the process, namely,
the zinc oxide, zinc metal, other chemical and metal values, and
other residues all can be used in other processes. One such
residue, an iron oxide cake, is of such a quality that it can be
used directly as the feedstock for the typical steel production
process.
[0013] Briefly, the present invention provides a method for
recovery of high purity zinc oxide products from industrial waste
streams such as electric arc furnace effluents (dusts, fumes, and
vapors) containing zinc compounds comprising the steps of: (a)
leaching the waste stream with a solution of 30% or greater by
weight ammonium chloride at at least about 70.degree. C., resulting
in a first product solution and undissolved materials; (b) adding
zinc metal to the first product solution in a cementation step,
whereby zinc-displaceable metal ions contained in the first product
solution are displaced by the zinc metal and precipitate out of the
first product solution as metals, leaving a second product
solution; and (c) diluting the second product solution with water,
resulting in the precipitation of zinc oxide and a third product
solution. The dilution step circumvents the crystallization of
various species, allowing the use of the higher concentration
ammonium chloride solution of approximately 30% or greater by
weight. The various undissolved precipitates produced during the
process comprise both waste products and chemical and metal values
that can be recovered and sold, used in subsequent processes, or
added to the feed to various industrial processes such as the iron
and steel making processes.
[0014] The third product solution is concentrated after removing
the zinc oxide, resulting in a fourth product solution comprising
greater than 30% ammonium chloride. The fourth product solution
then is combined with the original ammonium chloride solution of
Step (a) to leach the industrial waste stream in a continuous
process.
[0015] If the industrial waste material stream contains significant
amounts of iron, the waste stream preferably is heated in a
reducing atmosphere prior to leaching, resulting in an
iron-containing residue and a combustion product (dust, fumes,
and/or vapors) waste stream comprising oxides of zinc. The dust,
fumes and/or vapors then are subjected to the process disclosed in
this specification.
[0016] In yet another embodiment of the present invention, using a
waste stream such as typical electric arc furnace dust (which
typically contains zinc, cadmium, copper, lead, and iron metals and
compounds) the precipitated metals will comprise zinc, lead,
cadmium and copper compounds. This alternate embodiment further
comprises the additional steps of: (1) treating the precipitated
metals with an aqueous solution of either H.sub.2SO.sub.4 or
NH.sub.4SO.sub.4, whereby zinc, cadmium, and copper compounds go
into solution and lead compounds do not, resulting in a fifth
product solution comprising zinc, cadmium and copper compounds and
a second undissolved precipitate comprising lead compounds; (2)
adding zinc metal to the fifth product solution, whereby cadmium
and copper compounds are displaced by the zinc metal and
precipitate out of the fifth product solution as a third
precipitate, leaving a sixth product solution; and (3) treating the
sixth product solution with calcium chloride, resulting in the
precipitation of CaSO.sub.4 from the sixth product solution. Again,
the undissolved precipitates produced during the process comprise
both waste products and chemical and metal values that can be
recovered and sold, used in subsequent processes, or added to the
feed to various industrial processes such as the iron and steel
making processes.
[0017] If it is desired to control sodium or potassium chloride
concentrations during the process, additional steps of: (i)
adjusting the pH of the second product solution to between about 5
and about 8 with NH.sub.4OH prior to step (c); (ii) cooling at
least a portion of the second product solution to precipitate
diamino zinc dichloride; (iii) evaporating the second product
solution to precipitate NaCl and KCl; and (iv) combining the second
product solution with the ammonium chloride solution to leach the
waste stream.
[0018] Therefore, it is an object of the present invention to
provide a method for recovering zinc oxide from waste materials,
such as fly ash or flue dust, which contain other metals, such as
iron oxide, lead oxide, cadmium, copper and other materials.
[0019] Yet another object of the present invention is to provide a
method for recovering zinc oxide in which all leaching and washing
solutions are recycled for further use, and no leaching or washing
solutions are disposed of into the sewers or the environment.
[0020] Still another object of the present invention is to provide
a method for recovering zinc oxide which also results in the
precipitation in elemental form of any lead, cadmium and copper
metals contained in the starting materials.
[0021] Another object of the present invention is to provide a
method for recovering zinc metal, zinc oxide and/or iron oxide
which is economical, quick and efficient.
[0022] These objects and other objects, features and advantages of
the present invention will become apparent to one skilled in the
art when the following Detailed Description of the Preferred
Embodiments is read in conjunction with the attached figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a flow chart of the purification steps executed in
the preferred embodiment of the present invention.
[0024] FIG. 2 is a flow chart of the purification steps executed
with the optional reduction step, in accordance with an alternative
embodiment of the present invention.
[0025] FIG. 3 is a flow chart of the purification steps executed to
further isolate precipitated metals, in accordance with an
alternative embodiment of the presented invention.
[0026] FIG. 4 is a flow chart of the purification steps executed to
control sodium or potassium chloride levels, in accordance with an
alternative embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Referring to FIG. 1, a preferred embodiment of the process
is shown. The preferred waste materials such as fly ash or flue
dust from electric arc furnaces, as well as other waste materials
streams, including other metal producing and altering processes,
contain economically valuable amounts of zinc compounds. However,
these waste materials also contain oxides of iron, lead, calcium,
potassium, cadmium, and copper. The preferred waste is dust,
solids, vapors and/or fumes from a steel making process.
[0028] In step 100, a waste material is leached in an ammonium
chloride solution resulting in a first product solution and
undissolved materials. The ammonium chloride solution is greater
than 30% ammonium chloride in water at a temperature above about
70.degree. C. Prior art concentrations were typically not above 23%
due to the tendency of ammonium chloride to precipitate out with
zinc oxide. As explained in more detail below, the present process
has overcome such limitations. The majority of the waste materials,
including any zinc and/or zinc oxide, lead oxide, cadmium oxide,
copper oxide, and other metal oxides, dissolves in the ammonium
chloride solution forming a first product solution. Iron oxide,
also typically present in the preferred waste materials, does not
dissolve in the ammonium chloride solution, and remains as an
undissolved material 102. Any other constituents in the waste
materials not soluble in ammonium chloride also remain as
undissolved materials 102. The product solution is filtered and the
undissolved materials 102 are removed 102.
[0029] If iron oxide is present in the waste materials, the
undissolved materials 102 separated from the first product solution
typically is rich in iron oxide, and also typically has some
impurities such as zinc ferrite. The undissolved materials 102 can
be used as a feedstock for steel mills so long as the quantity of
impurities is not too great. It is preferable to remove the
impurities from the iron oxide prior to using the iron oxide as a
feedstock. Reducing the iron oxide in the undissolved materials 102
to direct-reduced iron (DRI) also is desired as DRI can be used to
replace part or all of the steel scrap charge. This is a first
example of how even the waste from the present process is usable,
resulting in an environmentally friendly process.
[0030] The iron oxide in the undissolved materials 102 can be
reduced to DRI in two manners. First, carbon, in the form of
activated carbon, carbon dust, carbon pellets or the like, can be
introduced to the ammonium chloride and waste material mixture
during the leaching process. The carbon reduces the iron oxide
resulting in DRI upon heating. Second, the carbon can be introduced
to the dried undissolved material cake using a ribbon blender. The
carbon will react with the iron oxide, reducing the iron oxide to
DRI upon heating. Adding heat to this process assists in the
reduction.
[0031] The remaining first product solution contains zinc oxide and
other dissolved metal oxides; namely, oxides of lead, cadmium, and
copper. While this first product solution is still hot, finely
powdered zinc metal is added to the first product solution in a
cementation step 104. Through an electrochemical reaction, any
lead, cadmium, and copper metal in solution plates out onto the
surfaces of the zinc metal particles. The addition of sufficient
powdered zinc metal results in the removal of most of the lead,
cadmium, and copper of the first product solution by precipitation.
After cementation, there is now a second product solution and a
precipitate 108.
[0032] The second product solution is rich in zinc compounds, while
the precipitate 108 is rich in oxides of lead, cadmium, and copper.
The precipitate 108 is separated from the second product solution
and, as will be described in more detail below, can optionally be
treated further to recover the economically valuable metals. This
is a second example of how the waste from the present process is
usable, reducing the amount of waste form the process.
[0033] Purified zinc may be recovered as either zinc oxide by
diluting the second product solution in a dilution step 106 or as
elemental zinc by electrowinning (not shown). Recovery of zinc
oxide by dilution is preferred, as it has been discovered that this
allows the use of higher concentrations of ammonium chloride
solutions of 30% or greater by weight. To recover zinc oxide in the
dilution step 106, the second product solution is diluted to a
concentration between about 2% and about 10% ammonium chloride. As
the concentration of ammonium chloride in the second product
solution drops, the solubility of zinc oxide decreases, thereby
resulting in the precipitation of zinc oxide from the second
product solution. When diluting the second product solution, it is
preferable to add the second product solution to the water, rather
than adding water to the second product solution. The resultant
zinc oxide has significantly lower chloride content if the second
product solution is added to the water. After dilution, there is
now a third product solution and precipitated zinc oxide 112.
[0034] The preferred method of dilution involves adding the second
product solution to the water, and not adding the water to the
second product solution. Preferably, the water should be at
60.degree. C. or higher during the entire dilution step. The second
product solution should be added to the water in a gradual fashion.
For example, in a batch system, one-third of the second product
solution should be added to the total required amount of water and
stirred to form a first intermediate solution. The remainder of the
second product solution, either in one large batch or in two or
more smaller batches, should then be added to the first
intermediate solution to form the third product solution. This
allows the ammonium concentration to be lowered to a relatively low
level in the first intermediate solution, forcing the dilution to
proceed along a defined path. The zinc oxide precipitates out of
the first intermediate solution such that the concentration of
ammonium chloride in the first intermediate solution is even lower
when the remainder of the ammonium chloride solution is added to
the first intermediate solution.
[0035] In another example, in a continuous system, a first fraction
of the second product solution is added to water in a first vessel
to form the first intermediate solution. The first intermediate
solution should have a concentration of ZnO/NH.sub.4Cl to water of
between about 2% and 6%, preferably about 3.3%. Any zinc oxide
precipitating out of the first intermediate solution is removed
from the first vessel. The first vessel is maintained at steady
state at the desired concentration by removing any zinc oxide
precipitate, adding additional water or second product solution,
and/or removing first intermediate solution. The removed first
intermediate solution is transferred to a second vessel where a
second fraction of the second product solution is added to the
first intermediate solution to form a second intermediate solution.
The second intermediate solution should have a concentration of
ZnO/NH.sub.4Cl to water of between about 5% and 9%, preferably
6.6%. The second vessel is maintained at steady state at the
desired concentration by removing any zinc oxide precipitate,
adding additional second product solution, and/or removing second
intermediate solution. The removed second intermediate solution is
transferred to a third vessel where a third fraction of the second
product solution is added to the second intermediate solution to
form a third intermediate or final solution. The third intermediate
or final solution should have a concentration of ZnO/NH.sub.4Cl to
water of between about 8% and 12%, preferably 10%. This also allows
the ammonium concentration to be lowered to a relatively low level
in the first intermediate solution, forcing the dilution to proceed
along a defined path. The zinc oxide precipitates out of the first
intermediate solution such that the concentration of ammonium
chloride in the first intermediate solution is even lower when the
remainder of the ammonium chloride solution is added to the first
intermediate solution.
[0036] The use of thirds as the fractions of second solution added
to each vessel is preferred, but it should be understood that any
fraction of the second product solution can be added to the total
required amount of water at a time. The important criteria is that
the amount of the second product solution added to the total
required amount of water be kept to a level allowing for the
greatest amount of zinc oxide to precipitate out of the first
intermediate solution. This also prevents the formation of
tetra-amino-zinc-tetrachloride [Zn(NH.sub.4).sub.4Cl.sub.4], a low
zinc content, fine needle crystal that is difficult to filter
separate or wash. The amount of water necessary, and the addition
rates of second product solution to the water, can be determined by
one of ordinary skill in the basic chemical arts without undue
experimentation. Further, determining the total required amount of
water is simple chemistry and well within the skill of the person
of ordinary skill in the art of this invention. It simply is enough
water to lower the concentration of ammonium chloride in the second
product solution, which can be determined by simple chemical
analysis, to a theoretical concentration of from about 2% to about
10% ammonium chloride.
[0037] The third product solution is filtered to remove the
precipitated zinc oxide 112, which is placed in a drying oven at a
temperature of over 100.degree. C. After a sufficient drying
period, the resultant dry white powder is essentially pure zinc
oxide.
[0038] If it is desired to recover elemental zinc, the second
product solution is subjected to electrolysis in an electrolytic
cell containing an anode and a cathode. The second product solution
comprises zinc ions in solution as Zn.sup.2+, which will be
electrodeposited on the cathode. Although it is preferable to have
the cathode made from zinc metal, cathodes of other materials will
allow the electrodeposition of zinc metal from the second product
solution.
[0039] Any of the electrolysis cells discussed in the literature
are suitable, as long as such cells are configured for the
electrolysis of zinc ion containing solutions. The two electrodes
of the electrolysis cells are connected externally to a power
supply capable of impressing a suitable voltage across the
electrodes. The zinc ions, being positive in nature, migrate toward
the negative electrode, or cathode, where they combine with
electrons supplied by the external circuit to form neutral zinc
metal atoms. When this happens, the zinc metal, in effect,
electroplates onto the cathode. By using a zinc cathode, the entire
cathode can be removed and used as necessary as a source of zinc.
Alternatively, a cathode on which electroplated zinc metal can be
easily removed can be used.
[0040] If zinc oxide is precipitated from the second product
solution by dilution, the diluted third product solution contains
ammonium chloride and other compounds. Rather than dispose of this
solution, it is preferable to produce a more concentrated (>30%)
ammonium chloride solution 110 that is recycled back to the
leaching step 100. This can be accomplished using evaporators or
reverse osmosis membrane technology. This is a third example of how
the waste from the present process is usable, reducing the amount
of waste produced by the present process to a minimum.
[0041] From an economically competitive situation, the use of
reverse osmosis membrane technology to concentrate the third
product solution to obtain a concentrated ammonium chloride
solution on one side of the membrane will save feed costs. Every so
often, it will be necessary to back flush the salts off the
membrane to recover them for makeup use in the future. In essence,
reverse osmosis membrane technology is using a pump to pump a
solution through a membrane, which is significantly lower in cost
than burning natural gas in an evaporator condenser to evaporate
water from a solution. This technology is used to filter out sodium
chloride and the minerals out of seawater to make distilled
water.
[0042] Referring to now FIG. 2, if there is significant iron
content in the waste material, it is preferable to first heat the
waste material in a reducing atmosphere in a reduction step 200,
prior to the leaching step 100. The waste material is typically
reduced at a temperature greater than 420.degree. C., and
preferably between 700.degree. C. and 1300.degree. C. The reducing
atmosphere can be created by using hydrogen gas, simple carbon
species gases, such as carbon dioxide, or by heating the material
in an oxygen containing gas in the presence of elemental carbon.
The carbon, which may be in the form of powdered coke or coal, is
mixed with the iron and zinc containing waste prior to the
reduction step. Examples of reduction processes include rotary
hearth and rotary kiln furnaces. In these processes, the powdered
mixture is formed into briquettes or pellets before feeding to the
furnace. The iron oxide is reduced to metallic iron and remains
within the briquette or pellet. Zinc, lead and cadmium are reduced
and leave the briquette or pellet in vapor form which is
subsequently oxidized on contact with oxygen. Alternatively, a
mixture containing electric arc furnace dust may be added back to
the electric arc furnace which operates under reducing conditions.
Mixtures of waste materials such as electric arc furnace dust, oily
mill scale and other flue dusts can be used to improve the value of
the iron product. The carbon is preferably in the form of dust or
pellets. Typically heating times are from 4 minutes to 4 hours,
depending on the method used.
[0043] The heating and reduction step 200 results in the reduction
of the iron compounds in the waste material into direct reduced
iron (DRI), and the generation of volatile materials and combustion
products. The DRI can be fed directly into a steel mill as a feed
source. The combustion products, in the form of exhaust dusts,
vapors, and fumes, are recovered by a filter process, such as a bag
house or a wet scrubber. The zinc, lead, cadmium, and copper in the
waster material are vaporized in the heating and reduction step 200
and comprise a majority of the exhaust dusts, vapors, and fumes.
The heating and reduction step 200 is not necessary if there is no
significant iron content in the waste. The heating and reduction
step 200 is a fourth example of how waste from the present process
(in this case iron oxides) is usable, further reducing the amount
of waste produced by the present process.
[0044] Once the waste material is reduced, the process as
represented in FIG. 2 continues just as the preferred embodiment
represented in FIG. 1. The combustion product (the exhaust dusts,
vapors, and fumes) are leached in leaching step 100, undissolved
materials 102 are separated from the leachant, and the leachant
(first product solution) is subjected to cementation step 104 to
remove Pb, Cd, and Cu, for further processing 108. The remaining
solution (second product solution) is diluted with water in a
dilution step 106 to 2-10% NH.sub.4Cl, thus prompting ZnO to
precipitate out of the solution. The now remaining solution (third
product solution) is then concentrated back to >30% NH.sub.4Cl
in a concentration step 110 and is recycled back into the process
as a leaching solution.
[0045] Waste materials typically have varying amounts of lead,
cadmium, and copper metals contained therein. For various reasons,
it is desirable to remove such metals from the waste materials, for
example, to recycle the lead, cadmium, and copper, or to prevent
their release into the environment. Referring to FIG. 3, the lead,
cadmium, and copper compounds removed from the first product
solution during the cementation step 104 as precipitates 108 can be
recovered by first treating the compounds with H.sub.2SO.sub.4 or
(NH.sub.4).sub.2SO.sub.4 in a dissolution step 300. The zinc,
cadmium, and copper compounds will go into solution while the lead
compounds will not. The lead compounds are filtered out 304,
leaving a solution of zinc, cadmium and copper compounds 302.
[0046] The Zn, Cd, and Cu--containing solution 302 is then treated
with zinc powder in a second cementation step 306 to result in the
electrodeposition of Zn and Cd on the zinc, which is then filtered
out 310, leaving a solution of zinc compounds 308. To this
solution, calcium chloride is added. The calcium react with the
sulfate present in the solution from the H.sub.2SO.sub.4 or
(NH.sub.4).sub.2SO.sub.4 to form CaSO.sub.4, when precipitates out.
The CaSO.sub.4 is filtered out 314 and the remaining solution can
be recycled back to the leaching step to further recover zinc
312.
[0047] Referring to FIG. 4, sodium and potassium chlorides,
introduced by the initial waste materials stream, may accumulate in
the various product solutions, thus adversely affecting process
performance. The chloride level may optionally be controlled by
periodic removal of chloride salts. This is preferably accomplished
by adjusting the pH of the second product solution in a pH
adjustment step 400 prior to the dilution step 106. This will
neutralize the acidic effects of the chloride ions by forming
reacting therewith to form ammonium chloride and water.
[0048] At least a portion of the pH-adjusted second product
solution may be removed through a purge stream prior to the
dilution step 106 and allowed to cool in a cooling step 404. As the
purged pH-adjusted second product solution cools, a portion of the
zinc compounds will precipitate out of the purged pH-adjusted
solution as diamino zinc dichloride, which is then filtered out in
a filtration step 408. The use of a purge stream assists in the
control of the concentration of sodium chloride and/or potassium
chloride in the second product solution. It is preferred to keep
the concentrations of sodium chloride and potassium chloride down
and if these concentrations build up, a larger purge stream is
used.
[0049] The remaining product solution (third product solution) is
evaporated or otherwise concentrated in concentration step 110 to
precipitate sodium chloride and potassium chloride, which are
filtered out in a second filtration step 410. The remaining
concentrated solution can then be reintroduced into the process at
the leaching step 100. In this way, chloride levels are maintained
as to not adversely effect the formation of substantially pure zinc
oxide.
[0050] The above description sets forth the best mode of the
invention as known to the inventor at this time, and is for
illustrative purposes only, as it is obvious to one skilled in the
art to make modifications to this process without departing from
the spirit and scope of the invention and its equivalents as set
forth in the appended claims.
* * * * *