U.S. patent application number 09/962887 was filed with the patent office on 2002-05-09 for contact and adsorbent granules.
Invention is credited to Bailly, Petet, Kischkewitz, Jurgen, Rohbock, Klaus, Schlegel, Andreas.
Application Number | 20020053547 09/962887 |
Document ID | / |
Family ID | 27214085 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020053547 |
Kind Code |
A1 |
Schlegel, Andreas ; et
al. |
May 9, 2002 |
Contact and adsorbent granules
Abstract
The present invention relates to pellets or granules based on
iron oxides and/or iron oxyhydroxides and iron(III) hydroxide, a
unit suitable for the through-flow of a fluid medium at least
partially filled with an adsorbent/catalyst in pellet form
consisting essentially of iron oxide and/or iron oxyhydroxide
embedded in an iron hydroxide matrix, processes for their
production comprising filtering, drying and shaping steps and their
processes of their use.
Inventors: |
Schlegel, Andreas; (Krefeld,
DE) ; Bailly, Petet; (Odenthal, DE) ;
Kischkewitz, Jurgen; (Ratingen, DE) ; Rohbock,
Klaus; (Kempen, DE) |
Correspondence
Address: |
BAYER CORPORATION
PATENT DEPARTMENT
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Family ID: |
27214085 |
Appl. No.: |
09/962887 |
Filed: |
September 25, 2001 |
Current U.S.
Class: |
210/688 |
Current CPC
Class: |
B01J 20/06 20130101;
B01J 20/2803 20130101; B01J 20/0229 20130101; C01P 2004/62
20130101; C01G 49/02 20130101; C02F 2101/20 20130101; C01P 2006/12
20130101; B82Y 30/00 20130101; C01P 2004/64 20130101; C01G 49/06
20130101; Y10T 428/2982 20150115; C01P 2004/50 20130101; C01G
49/0036 20130101; C01P 2004/10 20130101; C01G 49/0045 20130101;
C02F 1/281 20130101; B01D 53/02 20130101 |
Class at
Publication: |
210/688 |
International
Class: |
B01D 015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2000 |
DE |
10047996.0 |
Mar 29, 2001 |
DE |
10115417.8 |
Jun 18, 2001 |
DE |
10129306.2 |
Claims
What is claimed is:
1. A unit suitable for the through-flow of a fluid medium at least
partially filled with an adsorbent/catalyst in pellet form
consisting essentially of iron oxide and/or iron oxyhydroxide
embedded in an iron hydroxide matrix.
2. The unit of claim 1 wherein the medium is a gas.
3. The unit of claim 1 wherein the medium is a liquid.
4. The unit of claim 3 wherein the medium comprises water.
5. A water treatment plant comprising the unit of claim 1.
6. A waterwork comprising the water treatment plant of claim 5.
7. A process for the production of an adsorbent/catalyst comprising
the steps of (a) mixing an aqueous iron(III) hydroxide suspension
into an aqueous suspension of iron oxide and/or iron oxyhydroxide,
comprising Fe(OH).sub.2 and then (b) either (b1) drying the
suspension until it reaches a solid state and then mechanically
comminuting the solid material to the desired shape and/or size, or
(b2) mechanically shaping the suspension, optionally in the
semisolid state after predrying, followed by additional drying
until a solid state is achieved.
8. The process of claim 7 wherein the iron oxides and/or iron
oxyhydroxides comprises structures based on .alpha., .beta.,
.gamma., .delta., .delta.', .epsilon. phases and/or Fe(OH).sub.2,
ferric hydrite and mixed and intermediate phases thereof.
9. The process of claim 7 wherein iron carbonates are used in
addition to or instead of the iron oxides and/or iron
oxyhydroxides.
10. The process of claim 7 wherein commercial pigments are used as
iron oxides and/or iron (oxy)hydroxides.
11. The process of claim 7 wherein transparent pigments are used as
iron oxides and/or iron oxyhydroxides.
12. The process of claim 7 further comprising the step of
precipitating the Fe(OH).sub.3 with Fe.sup.3+ salts from the series
of carbonates, chlorides, fluorides, nitrates, sulfates and
sulfites.
13. A process comprising treating a fluid medium in a unit
according to claim 1 by contacting the fluid medium with an
absorbent/catalyst obtained by a process for the production of an
adsorbent/catalyst comprising the steps of (b) mixing an aqueous
iron(ill) hydroxide suspension into an aqueous suspension of iron
oxide and/or iron oxyhydroxide, comprising Fe(OH).sub.2 and then
(b) either (b1) drying the suspension until it reaches a solid
state and then mechanically comminuting the solid material to the
desired shape and/or size, or (b2) mechanically shaping the
suspension, optionally in the semisolid state after predrying,
followed by additional drying until a solid state is achieved.
14. The process of claim 13 wherein the fluid medium comprises
water.
15. The process of claim 13 comprising removing a heavy metal,
phosphorus, antimony, beryllium, selenium, tellurium or cyano
compound from water.
16. The process of claim 13 comprising removing an arsenic compound
from water.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to particles, pellets or
granules based on iron oxides and/or iron oxyhydroxides having a
large specific surface area (50 to over 200 m.sup.2/g according to
BET), processes for their production and their conversion to pellet
form with high mechanical resistance, and their use as a contact
and/or adsorbent/catalyst for the catalysis of chemical reactions,
for the removal of impurities from liquids and/or for gas
purification.
[0002] Contact and adsorbent granules, including those based on
iron oxides and/or iron oxyhydroxides, have already been described.
They are predominantly used in continuous processes, whereby they
are conventionally found in tower or column-type units through
which the medium to be treated flows, and the chemical or physical
reaction or adsorption processes take place at the outer and inner
surface of the granules. Powdered materials cannot be used for this
purpose because they compact in the direction of flow of the
medium, thereby increasing the flow resistance until the unit
becomes blocked. If a unit is cleaned by back-flushing (see below),
large amounts of the powder are discharged and lost or cause an
unacceptable contamination of the waste water.
[0003] The flowing media also exert forces on the granules,
however, which can lead to abrasion and/or movement through to
violent agitation of the granules. Consequently the granules
collide, leading to undesirable abrasion. This results in loss of
contact or adsorbent material and contamination of the medium to be
treated.
[0004] Adsorbents/catalysts containing iron oxides and hydroxides
can advantageously be used e.g. in the area of water purification
or gas purification. In water purification this agent is used in
horizontal- or vertical-flow filters or adsorber columns or added
to the water to be treated in order to remove dissolved, suspended
or emulsified organic or inorganic phosphorus, arsenic, antimony,
sulfur, selenium, tellurium, beryllium, cyano and heavy metal
compounds from, for example, drinking water, process water,
industrial and municipal waste water, mineral, holy and medicinal
water as well as river, garden pond and agricultural water. It can
also be used in so-called reactive walls to separate the cited
contaminants from ground water and seepage water aquifers from
contaminated sites (waste disposal sites).
[0005] In gas purification the agent is used in adsorbers for
binding undesirable components such as hydrogen sulfide, mercaptans
and hydrogen cyanide, as well as other phosphorus, arsenic,
antimony, sulfur, selenium, tellurium, cyano and heavy metal
compounds in waste gases. Gases such as HF, HCI, H.sub.2S,
SO.sub.x, NO.sub.x can also be adsorbed.
[0006] The removal of phosphorus, arsenic, antimony, selenium,
tellurium, cyano and heavy metal compounds from waste oils and
other contaminated organic solvents is also possible.
[0007] Contact and adsorbent granules based on iron oxides and/or
iron oxyhydroxides are also used for the catalysis of chemical
reactions in the gas phase or in the liquid phase.
[0008] Various methods of removing trace constituents and
contaminants from aqueous systems with the aid of adsorbents are
also known.
[0009] For example, DE-A 3 120 891 describes a process in which a
filtration is performed using activated alumina with a grain size
of 1 to 3 mm for the separation principally of phosphates from
surface water.
[0010] DE-A 3 800 873 describes an adsorbent based on porous
materials such as e.g. hydrophobed chalk with a fine to medium
grain size to remove contaminants from water.
[0011] DE-A 3 703 169 discloses a process for the production of a
granulated filter medium to treat natural water. The adsorbent is
produced by granulating an aqueous suspension of kaolin with
addition of powdered dolomite in a fluidised bed. The granules are
then baked at 900.degree. to 950.degree. C.
[0012] A process for the production and use of highly reactive
reagents for waste gas and waste water purification is known from
DE-A 40 34 417. Mixtures consisting of Ca(OH).sub.2 with additions
of clays, stone dust, entrained dust and fly ashes, made porous and
having a surface area of approx. 200 m.sup.2/g, are described
here.
[0013] The cited processes and the contacts used therein have the
shared disadvantage that the component responsible in each case for
the selective adsorption of constituents of the media to be
cleaned, in other words the actual adsorbent, must be supplemented
with large quantities of additives to enable it to be shaped into
granules. This significantly reduces the binding capacity for the
water contaminants to be removed. Moreover, subsequent reprocessing
or reuse of the material is problematic since the foreign
substances used as binders first have to be separated out.
[0014] DE-A 4 214 487 describes a process and a reactor for the
removal of impurities from water. The medium flows horizontally
through a funnel-shaped reactor, in which finely divided iron
hydroxide in flocculent form is used as a sorption agent for water
impurities. The disadvantage of this process lies in the use of the
iron hydroxide in flocculent form, which means that because there
is little difference in density between water and iron hydroxide, a
reactor of this type can be operated at only very low flow rates
and there is a risk of the sorption agent, which is possibly
already loaded with contaminants, being discharged from the reactor
along with the water.
[0015] JP-A 55 132 633 describes granulated red mud, a by-product
of aluminium production, as an adsorbent for arsenic. This consists
of Fe.sub.2O.sub.3, Al.sub.2O.sub.3 and SiO.sub.2. No mention is
made of the stability of the granules or of the granulation
process. A further disadvantage of this adsorbent is the lack of
consistency in the composition of the product, its unreliable
availability and the possible contamination of the drinking water
with aluminium. Since aluminium is suspected of encouraging the
development of Alzheimer's disease, contamination with this
substance in particular is to be avoided.
[0016] DE-A 19 826 186 describes a process for the production of an
adsorbent containing iron hydroxide. An aqueous polymer dispersion
is incorporated into iron hydroxide in water-dispersible form. This
mixture is then either dried until it reaches a solid state and the
solid material then comminuted mechanically to the desired shape
and/or size or the mixture is shaped, optionally after a
preliminary drying stage, and a final drying stage then performed,
during which a solid state is achieved. In this way a material is
obtained in which the iron hydroxide is firmly embedded in the
polymer and which is said to display a high binding capacity for
the contaminants conventionally contained in waste waters or waste
gases.
[0017] The disadvantage of this process lies in the use of organic
binders, which further contaminate the water to be treated due to
leaching and/or abrasion of organic substances. Furthermore, the
stability of the adsorbent composite is not guaranteed in extended
use. Bacteria and other microorganisms can also serve as a nutrient
medium for an organic binder, presenting a risk that microorganisms
may populate the contact and thereby contaminate the medium.
[0018] The presence of foreign auxiliary substances, which are
required for the manufacture of the adsorbents, during
reprocessing, recycling or reuse of used adsorbents is
disadvantageous in principle because the reuse of pure substances
is less problematic than is the case with mixed substances. For
example, polymeric binders are disadvantageous when iron
oxide-based adsorption materials are reused as pigments for
concrete coloration because these binders can inhibit dispersion of
the pigment in liquid concrete.
[0019] DE-A 4 320 003 describes a process for the removal of
dissolved arsenic from ground water with the aid of colloidal or
granulated iron hydroxide. Where fine, suspended iron(III)
hydroxide products are used, it is recommended here that the iron
hydroxide suspension be placed in fixed-bed filters filled with
granular material or other supports having a high external or
internal porosity. This process likewise has the disadvantage that,
relative to the adsorbent "substrate+iron hydroxide", only low
specific loading capacities are achievable. Furthermore, there is
only a weak bond between substrate and iron hydroxide, which means
that there is a risk of iron hydroxide or iron arsenate being
discharged during subsequent treatment with arsenic-containing
water. This publication also cites the use of granulated iron
hydroxide as an adsorption material for a fixed-bed reactor. The
granulated iron hydroxide is produced by freeze conditioning
(freeze drying) of iron hydroxide obtained by neutralisation of
acid iron(III) salt solutions at temperatures of below minus
5.degree. C. This production process is extremely energy-intensive
and leads to heavily salt-contaminated waste waters. Moreover, as a
result of this production process only very small granules with low
mechanical resistance are obtained. When used in a fixed-bed
reactor, this means that the size spectrum is significantly reduced
by mechanical abrasion of the particles during operation, which in
turn results in finely dispersed particles of contaminated or
uncontaminated adsorption agent being discharged from the reactor.
A further disadvantage of these granules lies in the fact that the
adsorption capacity in respect of arsenic compounds is reduced
considerably if the granules lose water, by being stored dry for
extended periods for example.
[0020] Adsorbent/binder systems obtained by removing a sufficiently
large amount of water from a mixture of (a) a crosslinkable binder
consisting of colloidal metal or non-metal oxides, (b) oxidic
adsorbents such as metal oxides and (c) an acid such that
components (a) and (b) crosslink to form an adsorbent/binder
system, are known from U.S. Pat. No. 5,948,726. According to the
embodiments, colloidal alumina or aluminium oxide is used as
binder.
[0021] The disadvantage of these compositions lies in the need to
use acid in their production (column 9, line 4) and in the fact
that they are not pure but heterogeneous substances, which is
undesirable both for the production, regeneration, removal and
permanent disposal of such adsorbents, e.g. on a waste disposal
site. The scope of disclosure of this publication is also intended
to include adsorbents that are suitable for the adsorption of
arsenic; specific examples are not provided, however. Aluminium
oxide is known to be significantly inferior to iron oxides in
regard to force of adsorption for arsenic.
[0022] Continuous adsorbers, which are commonly grouped together in
parallel for operation, are preferably used for water treatment. To
free drinking water from organic impurities, for example, such
adsorbers are filled with activated carbon. At peak consumption
times, the available adsorbers are then operated in parallel to
prevent the flow rate from rising above the maximum permitted by
the particular arrangement. At times of lower water consumption,
individual adsorbers are taken out of operation and can be
serviced, for example, whereby the adsorption material is subjected
to special loads, as described in greater detail below.
[0023] The use of granules, which can be produced by compacting
e.g. powdered iron oxide using high linear forces, has also already
been considered. Such granules have already been described as a
means of homogeneously colouring liquid concrete. The use of high
linear forces for compacting is extremely expensive and
energy-intensive, and the stability of the compacted materials is
inadequate for extended use in adsorbers. The use of such materials
in adsorbers, for example, particularly continuous models, for
water purification is therefore of only limited interest. During
maintenance or cleaning of adsorber plants by back-flushing in
particular (see below), such granules lose large amounts of
substance due to the associated agitation. The abraded material
renders the waste water from back-flushing extremely turbid. This
is unacceptable for a number of reasons: firstly, adsorption
material, which is heavily laden with impurities and therefore
toxic after extended use, is lost. Secondly, the stream of waste
water is laden with abraded material, which can sediment, damaging
piping systems and ultimately subjecting the waste treatment plant
to undesirable physical and toxicological stresses, to name but a
few reasons. Preferably the abrasion should be below 20% by weight,
more preferably below 15% by weight, 10% by weight or most
preferably below 5% by weight according to the method described in
the examples of the present invention.
[0024] An object underlying the present invention was therefore to
provide a particle like a contact or an adsorbent/catalyst based on
iron-oxygen compounds in pellet form, exhibiting high mechanical
resistance in conjunction with a good binding capacity for
contaminants contained in liquids and gases without the need to use
organic binders or inorganic foreign binders to achieve adequate
mechanical resistance, and plants operated with such media. This
complex object is achieved by the contacts or adsorbents/catalysts
according to the invention, their preparation, their use and units
filled therewith.
SUMMARY OF THE INVENTION
[0025] The invention relates to a unit suitable for the
through-flow of a fluid medium at least partially filled with an
adsorbent/catalyst in pellet form consisting essentially of iron
oxide and/or iron oxyhydroxide embedded in an iron hydroxide
matrix.
DETAILED DESCRIPTION OF THE INVENTION
[0026] The material in question consists essentially of iron oxide
and/or oxyhydroxide firmly embedded in Fe(OH).sub.3 polymer,
which--as experiments have shown--has a high binding capacity for
the contaminants conventionally contained in waste water or waste
gases and exhibits an already adequate mechanical and hydraulic
resistance without addition of organic binders or inorganic foreign
substances.
[0027] Since this material is free from foreign binders, it has the
further advantage in comparison to adsorbents from the prior art
that, after stripping or removal of the adsorbed contaminants where
necessary, it can be disposed of completely or supplied to other
applications, for instance after grinding it can be used for
colouring concrete and other building materials and for
conventional pigment applications in plastics, paints and varnishes
or for colouring other substrates such as bark mulch or shredded
wood.
[0028] To prepare adsorbents of this type, an aqueous suspension of
iron oxyhydroxide and/or iron oxide and iron hydroxide is first
prepared, which is either dried until it is solid and the solid
material then optionally comminuted mechanically to the desired
shape and/or size, or alternatively the dispersion undergoes
mechanical shaping, optionally in the semisolid state after
predrying, and is then (further) dried until it reaches a solid
state.
[0029] The products obtainable in this way can then be comminuted
further, for example by rough grinding or grinding. However, since
the products reduce in size autogenously on first coming into
contact with water, for example when a freshly charged adsorber
unit is first filled with water, this will generally be
unnecessary.
[0030] The invention therefore also concerns a process for the
production of an iron oxide/iron hydroxide-containing
adsorbent/catalyst in pellet form.
[0031] The material according to the invention can be obtained by
mixing diverse phases of iron oxides and/or iron oxyhydroxides,
including Fe(OH).sub.2, in pure form or in any mixture in solid,
semisolid or suspended form by the addition of Fe(OH).sub.3 in
suspension or in gelatinous form with variable water content, and
then dehydrating this mixture completely or with retention of a
certain water content, for example by filtration or evaporation,
and then mechanically comminuting the solid or semisolid material
to the desired shape and/or size, or subjecting the dispersion to
mechanical shaping, optionally in the semisolid state after
predrying, followed by (additional) drying until a solid state is
achieved. In this way the iron oxide and/or oxyhydroxide is firmly
embedded in the Fe(OH).sub.3 polymer. The Fe(OH).sub.3 can also be
produced in situ from Fe(III) salt solutions and neutralisation or
from iron(II) salt solutions by oxidation and neutralisation. The
residual alkali from the production process for the suspended
pigment is preferably reacted with an equivalent amount of Fe(III)
salt for this purpose.
[0032] The iron hydroxide Fe(OH).sub.3 is preferably aqueous to
pasty in its original state, whereby the paste can exhibit almost
any water content, generally between 10 and 90 wt. %, preferably
between 40 and 70 wt. %. Freshly prepared iron hydroxide
Fe(OH).sub.3, obtained by precipitation from iron(III) salt
solutions or from iron(II) salt solutions by oxidation and
neutralisation, can also be used, however.
[0033] Dehydration by evaporation is preferably used if the
suspensions to be dehydrated are largely salt-free and/or if lower
demands are made of the mechanical strength of the resultant end
products in operation.
[0034] Dehydration can alternatively be performed by filtration.
The filtration performance of the suspensions can be improved by
the use of conventional filtration-improving measures, such as are
described for example in Solid-Liquid Filtration and Separation
Technology, A. Rushton, A. S. Ward, R. G. Holdich, 2nd edition
2000, Wiley-VCH, Weinheim, and in Handbuch der Industriellen
Fest/Flussig-Filtration, H. Gasper, D. chsle, E. Pongratz, 2nd
edition 2000, Wiley-VCH Weinheim. Coagulants can thus be added to
the suspensions, for example.
[0035] The suspensions to be dehydrated can also contain iron
carbonates.
[0036] The products according to the invention can undergo drying
in air, and/or in vacuo, and/or in a drying oven and/or on belt
dryers or in spray dryers at temperatures in the range from
5.degree. to 300.degree. C. The material can also be freeze
dried.
[0037] The products according to the invention preferably have a
residual water content of less than 20 wt. %.
[0038] The material is preferably comminuted by grinding to grain
sizes in the range between 0.5 and 20 mm. The semisolid material is
preferably shaped mechanically in a granulation or pelletising
plant or in an extruder, whereby shaped bodies whose size is in the
range from 0.5 to 20 mm in diameter or length can be obtained.
[0039] It was found that the pellets or granules obtained in this
way have a high binding capacity for contaminants contained in
water, liquids or gases and they additionally have an adequately
high resistance to flowing media in terms of mechanical or
hydraulic stressing.
[0040] It is particularly surprising that during drying, the iron
oxyhydroxides or iron oxides treated with Fe(OH).sub.3 solidify
into very hard agglomerates, which without the addition of binders
have a high mechanical abrasion resistance and high hydraulic
resistance to contact with flowing water, and which have a high
binding capacity for the contaminants and trace constituents
contained in the water.
[0041] Iron oxyhydroxide pigments (e.g. goethite) and iron oxide
pigments (e.g. haematite, magnetite) and/or iron carbonates are
suitable for use according to the invention. The production of iron
oxide pigments is prior art, they are obtained by precipitation and
oxidation or Penniman reactions from iron(II) salt solutions and
iron hydroxide by precipitation from iron(III) salt solutions. Such
pigments can contain structures based on .alpha., .beta., .gamma.,
.delta., .delta.', .epsilon. phases and/or Fe(OH).sub.2 and mixed
and intermediate phases thereof. Yellow iron oxyhydroxides can be
calcined to red iron oxides.
[0042] The product displays BET surface areas of 50 to 500
m.sup.2/g, preferably 80 to 200 m.sup.2g.
[0043] The primary particle size was determined by measurement from
scanning electron micrographs, e.g. at a magnification of 60000:1
(instrument: XL30 ESEM FEG, Philips). If the primary particles are
needle-shaped, as in the .alpha.-FeOOH phase for example, the
needle width can be given as a measurement for the particle size.
Needle widths of up to 100 nm, but mainly between 4 and 50 nm, are
observed in the case of nanoparticle .alpha.-FeOOH particles.
.alpha.-FeOOH primary particles conventionally have a length:width
ratio of 5:1 to 50:1, typically of 5:1 to 20:1. The length:width
ratio of the needle shapes can be varied, however, by doping or by
special reaction processes. If the primary particles are isometric,
as in the .alpha.-Fe.sub.2O.sub.3, .gamma.-Fe.sub.2O.sub.3,
Fe.sub.3O.sub.4 phases for example, the particle diameters can
quite easily also be below 20 nm.
[0044] By mixing Fe(OH).sub.3 with pigments and/or nanoparticle
iron oxides or iron (oxy)hydroxides, the presence of the cited
pigment or nucleus particles in their known particle morphology,
held or glued together by the amorphous Fe(OH).sub.3 polymer, can
be detected on the scanning electron micrographs.
[0045] Yellow iron oxyhydroxide pigments are generally synthesised
by precipitating iron(II) hydroxides or carbonates from
corresponding iron(II) salt solutions such as e.g. FeSO.sub.4,
FeCl.sub.2 in pure form or as pickling solutions in the acid or
alkaline pH range, followed by oxidation to iron(III) oxyhydroxides
(see inter alia G. Buxbaum, Industrial Inorganic Pigments, VCH
Weinheim, 2nd edition, 1998, p. 231ff). Oxidation of the divalent
to the trivalent iron is preferably performed with air, whereby
intensive aeration is advantageous. Oxidation with H.sub.2O.sub.2
also leads to iron oxyhydroxides. NaOH is preferably used as
alkaline precipitant. Other precipitants, such as KOH,
Na.sub.2CO.sub.3, K.sub.2CO.sub.3, CaO, Ca(OH).sub.2, CaCO.sub.3,
NH.sub.3, NH.sub.4OH, MgO and/or MgCO.sub.3, can also be used,
however.
[0046] By choosing suitable precipitation and oxidation conditions,
nanoparticle .alpha., .beta., .gamma., .delta. phases and mixed
phases of iron oxyhydroxides displaying a large specific surface
area can be prepared, such that the nanoparticles agglomerate in
the dry state and possess a high resistance to mechanical and
fluid-mechanical abrasion in comminuted form. To steer the
precipitated pigments in the direction of the extremely
fine-particle character that is required, the precipitations, e.g.
of yellow .alpha.-FeOOH as described in patents U.S. Pat. No.
2,558,303 and U.S. Pat. No. 2,558,304, are performed in the
alkaline pH range with alkali carbonates as precipitants, and
modifiers such as SiO.sub.2, zinc, aluminium or magnesium salts,
hydroxycarbonic acids, phosphates and metaphosphates are generally
added. Products produced in this way are described in U.S. Pat. No.
2,558,302. Such nucleus modifiers do not inhibit the subsequent
reprocessing, recycling or any other use of the adsorbents
according to the invention. In the case of precipitation processes
in an aqueous medium, it is known that precipitations in an
alkaline environment lead to less solidly agglomerated powders than
those in an acid environment.
[0047] One of the advantages of nucleus modifiers, however, is that
an adequate fine-particle character can be obtained even at
elevated reaction temperatures.
[0048] The products described, the process for their production and
their use represent an improvement over the prior art. In contrast
to those of the prior art, the granules according to the invention
can be subjected to considerably higher stresses and therefore
display a much greater abrasion resistance to mechanical and
hydraulic stressing. They can be used directly as such. When used
in adsorber plants for water purification, for example, there is no
need even for comminution or rough grinding of the crude dry
substance initially obtained from filter cakes or extruders, since
the coarse pellets break down independently on contact with water.
This results in a random particle-size distribution, but no
particles of such a size that they are discharged from the adsorber
to any significant extent by the flowing medium.
[0049] There is absolutely no need for a separate granulation
process, such as would be necessary when using conventional iron
oxyhydroxides in the form of (flowable) powders, either with the
aid of foreign binders or using extremely high linear forces during
compacting.
[0050] Granulation of a semi-wet paste has proven effective as
another method of producing granules. Here pellets or strands are
formed from a semi-wet paste, e.g. using a simple perforated metal
sheet, a roll press or an extruder, and either dried immediately or
additionally shaped into a spherical or granular form by means of a
spheroniser. The still wet spherules or granules can subsequently
be dried to any moisture content whatsoever. A residual moisture
content of <50% is recommended to prevent the granules from
agglomerating. A spherical shape of this type can be advantageous
for use in fixed-bed adsorbers due to the improved packing in the
adsorber vessel that is obtained in comparison with rough-ground
granules or pellets in strand form.
[0051] The quantities of iron oxyhydroxides or iron oxides on the
one hand and iron hydroxide on the other to be used according to
the invention are determined by the requirements of the product
according to the invention in terms of its mechanical stability and
abrasion resistance. Although a higher content of (powdered)
pigments will generally reduce the mechanical strength of the
products according to the invention, filtration of the suspensions
is possibly made easier. The person skilled in the art and
practising in the particular field of application will be able to
determine the optimum mixing ratio for the intended application by
means of a few orienting experiments.
[0052] The granules according to the invention are particularly
preferably used in the cleaning of liquids, especially for the
removal of heavy metals. A preferred application in this industrial
field is the decontamination of water, particularly of drinking
water. Particular attention has recently been paid to the removal
of arsenic from drinking water. The granules according to the
invention are extremely suitable for this purpose, since levels
that not only meet but actually fall below even the lowest limiting
values set by the US authority the EPA can be achieved using the
granules according to the invention.
[0053] To this end the granules can be used in conventional
adsorber units, such as are already used with a charge of activated
carbon, for example, to remove other types of contaminants.
Batchwise operation, in cisterns or similar containers for example,
optionally fitted with agitators, is also possible. However, use in
continuous plants such as continuous-flow adsorbers is
preferred.
[0054] Since untreated water to be processed into drinking water
conventionally also contains organic impurities such as algae and
similar organisms, the surface of adsorbents, especially the outer
surface of granular adsorbents, becomes coated during use with
mostly slimy deposits, which impede or even prevent the inflow of
water and hence the adsorption of constituents to be removed. For
this reason adsorber units are periodically back-flushed with
water, a process which is preferably performed at times of low
water consumption (see above) on individual units that have been
taken out of service. The adsorbent is whirled up and the
associated mechanical stress to which the surface is subjected
causes the undesirable coating to be removed and discharged against
the direction of flow during active operation. The wash water is
conventionally sent to a sewage treatment plant. The adsorbents
according to the invention have proven to be particularly effective
in this process, since their high strength enables them to be
cleaned quickly without suffering any significant losses of
adsorption material and without the back-flush water sent for waste
treatment being rich in discharged adsorption material, which is
possibly already highly contaminated with heavy metals.
[0055] Since the granules according to the invention are free from
foreign binders, the material is comparatively easy to dispose of
after use. For instance, the adsorbed arsenic can be removed by
thermal or chemical means in special units, for example, resulting
in an iron oxide pigment as a pure substance which can either be
recycled for use in the same application or supplied for
conventional pigment applications. Depending on the application and
legal regulations, the content of the adsorber can also be used
without prior removal of the heavy metals, for example as a pigment
for colouring durable construction materials such as concrete,
since the heavy metals removed from the drinking water are
permanently immobilised in this way and taken out of the
hydrological cycle.
[0056] The invention therefore also provides water treatment plants
or waterworks in which units filled with the granules according to
the invention are operated, and processes for the decontamination
of water by means of such units, as well as such units
themselves.
[0057] The BET specific surface area of the products according to
the invention is determined by the carrier gas process
(He:N.sub.2=90:10) using the single-point method, according to DIN
66131 (1993). The sample is baked for 1 h at 140.degree. C. in a
stream of dry nitrogen before measurement.
[0058] In order to measure the adsorption of arsenic(III) and
arsenic(V), 3 liters of an aqueous solution of NaAsO.sub.2 or
Na.sub.2HAsO.sub.4, each with the specified original concentration
of approx. 2-3 mg/l arsenic, are treated with 3 g of the sample to
be tested in a 5 liter PE flask for a specific period and the flask
moved on rotating rollers. The adsorption rate of As ions on iron
hydroxide over this specific period, e.g. one hour, is stated as
mg(As.sup.3+/5+)/g(FeOOH).h, calculated from the balance of the
As.sup.3+/5+ ions remaining in solution.
[0059] The adsorption of Sb.sup.3+, Sb.sup.5+, Hg.sup.2+,
Pb.sup.2+, Cr.sup.6+ or Cd.sup.2+ ions is measured in the same way,
whereby the desired concentrations are established by dissolving
appropriate amounts of Sb.sub.2O.sub.3, KSb(OH).sub.6, PbCl.sub.2,
NaCrO.sub.4 or CdCl.sub.2 in H.sub.2O and adjusting the pH value to
7-9.
[0060] The As, Sb, Cd, Cr, Hg or Pb contents of the contaminated
iron oxyhydroxide or of the solutions are determined using mass
spectrometry (ICP-MS) according to DIN 38406-29 (1999) or by
optical emission spectroscopy (ICP-OES) according to EN-ISO 11885
(1998), with inductively coupled plasma as excitation agent in each
case.
[0061] The mechanical and hydraulic abrasion resistance was
assessed using the following method: 150 ml of demineralised water
were added to 10 g of the granules to be tested, having particle
sizes>0.1 mm, in a 500 ml Erlenmeyer flask, which was rotated on
a LabShaker shaking machine (Kuhner model from Braun) for a period
of 30 minutes at 250 rpm. The >0.1 mm fraction was then isolated
from the suspension using a screen, dried and weighed. The weight
ratio between the amount weighed out and the amount weighed in
determines the abrasion value in %.
[0062] The invention is described in greater detail below by means
of examples. The examples are intended to illustrate the process
and do not constitute a limitation.
EXAMPLES
Example 1
[0063] 124 l of an aqueous NaOH solution (114 g/l) were measured
out at 24.degree. C. and 171 l of an aqueous solution of FeSO.sub.4
(100 g/l) quickly added with stirring, and oxidation was then
performed with 10 l air per hour and per mol Fe. Immediately upon
completion of oxidation, 56 l of an aqueous solution of
Fe.sub.2(SO.sub.4).sub.3 (100 g/l) were added and stirred for 30
minutes. The yellowish brown suspension thus obtained was processed
in the same way as in example 2.
[0064] An X-ray diffractogram showed that the product consisted of
b 100% .alpha.-FeOOH. Using a scanning electron micrograph e.g. at
a magnification of 60000:1, the needle widths were measured at
between 15 and 35 nm, the needle lengths between 70 and 180 nm. The
needles were extremely agglomerated. The BET specific surface area
was 131 m.sup.2/g. The abrasion value after 30 minutes was only 7
wt. %.
[0065] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 2.3 mg/l (As.sup.3+) was 1.7
mg(As.sup.3+)/g(FeOOH).h, the adsorption for an Na.sub.2HAsO.sub.4
solution with an original concentration of 2.7 mg/l (As.sup.5+) was
1.2 mg(As.sup.5+)/g(FeOOH).h.
Example 2
[0066] 7.4 l of an aqueous solution of Fe.sub.2(SO.sub.4).sub.3
(100 g/l) were added to 7.5 l of an aqueous solution of FeSO.sub.4
(150 g/l) and the mixture quickly treated with 2.9 l of an aqueous
NaOH solution (200 g/l) at 34.degree. C. with stirring. The
reaction mixture was then pre-oxidised for 10 minutes with 290 l of
air per hour and then precipitated further with 2.2 l of an aqueous
NaOH solution (200 g/l) with stirring. The reaction mixture was
then oxidised for a further 15 minutes with 290 l of air per hour.
The yellowish brown suspension was filtered off at a nutsch filter
and the deposit washed to obtain a residual filtrate conductivity
of 1 mS/cm.
[0067] An X-ray diffractogram showed that the product consisted of
100% .alpha.-FeOOH. Small particles as well as needles can be seen
in the scanning electron micrograph, e.g. at a magnification of
60000:1. In the case of the small particles the needle widths were
measured at between 15 and 35 nm, the needle lengths between 30 and
70 nm. In the case of the larger needles, needle widths of up to 50
nm and needle lengths of up to 350 nm were determined. The needles
and particles were extremely agglomerated. The BET specific surface
area was 177 m.sup.2/g. The abrasion value after 30 minutes was
only 3 wt. %.
[0068] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 2.3 mg/l (As.sup.3+) was 1.3
mg(As.sup.3+)/g(FeOOH).h, for an Na.sub.2HAsO.sub.4 solution with
an original concentration of 2.7 mg/l (As.sup.5+) it was 0.7
mg(As.sup.5+)/g(FeOOH).h.
Example 3
[0069] 470 ml of an FeCl.sub.3 solution (0.1 N) were added to 45 g
of a needle-shaped .alpha.-FeOOH pigment powder (Bayferrox.RTM.
930, Bayer AG, Leverkusen, Del.) and mixing performed for 5 minutes
at 500 rpm. 141 ml of an aqueous NaOH solution (1 N) were then
slowly added dropwise and the suspension stirred for 15
minutes.
[0070] The suspension was filtered through a nutsch filter, rinsed
with 1000 ml demineralised H.sub.2O and then dried for 15 h at
105.degree. C. 47.6 g of the dried product was redispersed in 2300
ml 0.1 M FeCl.sub.3 solution and 690 ml of an aqueous NaOH solution
(1 N) were then quickly added. The suspension was filtered through
a nutsch filter, rinsed with 2000 ml demineralised H.sub.2O and
then dried for 15 h at 105.degree. C. The dried product was very
hard, it was roughly ground and the screen fraction from 1 to 5 mm
isolated.
[0071] An X-ray diffractogram showed that the product consisted of
100% .alpha.-FeOOH. The BET specific surface area was 99 m.sup.2/g.
When shaken with water in a beaker, the granules displayed a high
abrasion resistance, indicated by the fact that the water was not
coloured with pigment, as happens with untreated .alpha.-FeOOH
pigment powder (Bayferrox.RTM. 930), for example.
[0072] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 23 .mu.g/l As.sup.3+, as can
conventionally occur in natural water, for example, was 17
.mu.g(As.sup.3+)/g(FeOOH).h after 30 minutes, corresponding to 84%
adsorption.
Example 4
[0073] 4096 kg NaOH (as solution with approx. 300 g/l) were
measured out and diluted with water to 40 m.sup.3. 4950 kg
FeSO.sub.4 were dissolved with water to form 48.5 m.sup.3 solution,
cooled to 15.degree. C. and then pumped into the prepared NaOH over
1 h. The suspension was then oxidised with 1500 m.sup.3/h air in
approx. 2 h. 14.4 m.sup.3 FeClSO.sub.4 solution (113.4 g/l) were
added to approx. 87 m.sup.3 of this suspension with stirring, and
stirred for a further 30 min. The batch was washed on a filter
press until the residual filtrate conductivity was <1000
.mu.S/cm and the paste pushed through a perforated metal plate with
hole diameters of 7 mm and formed into strands. The strands were
dried on a belt dryer to a residual moisture of approx. 5%. The dry
pellets were roughly ground to obtain a particle size of 2 mm. The
material thus obtained had a BET specific surface area of 142
m.sup.2/g and consisted of 100% .alpha.-FeOOH. Using a scanning
electron micrograph e.g. at a magnification of 60000:1, the needle
widths were measured at between 15 and 50 nm, the needle lengths
between 10 and 150 nm. The needles were extremely agglomerated.
[0074] Adsorption performance: The adsorption rate for NaAsO.sub.2
with an original concentration of 2.7 mg/l (As.sup.3+) was 2.1
mg(As.sup.3+)/g(FeOOH).h, for Na.sub.2HAsO.sub.4 with an original
concentration of 2.8 mg/l (As.sup.5+) it was 2.0
mg(As.sup.5+)/g(FeOOH).h- , for CdCl.sub.2 (original concentration
2.7 mg (Cd.sup.2+)II) the adsorption was 1.1 mg
(Cd.sup.2+)/g(FeOOH).h, for KSb(OH).sub.6 (original concentration
2.6 mg (Sb.sup.5+)/l) it was 1.9 mg (Sb.sup.5+)/g(FeOOH).h, for
Sb.sub.2O.sub.3 (original concentration 2.3 mg (Sb.sup.3+)/l) it
was 2.0 mg (Sb.sup.3+)/g(FeOOH).h, for Na.sub.2CrO.sub.4 (original
concentration 2.6 mg (Cr.sup.6+)/l) it was 1.1 mg (Cr.sup.6+), for
PbCl.sub.2 (original concentration 1.6 mg (Pb.sup.2+)/l) it was
1.57 mg (Pb.sup.2+)/g(FeOOH).h.
Example 5
[0075] 800 g of an aqueous solution of FeCl.sub.3 (100 g/l) were
added to 525 g of a suspension of a needle-shaped a-FeOOH pigment
powder (50 g/l FeOOH, Bayferrox.RTM. 920, Bayer AG, Leverkusen,
Del.) and an iron hydroxide precipitated on the pigment by addition
of 247 g of an aqueous NaOH solution (24%). The suspension was
filtered through a nutsch filter, the filter cake rinsed to obtain
a residual filtrate conductivity of <1 mS/cm and the filter cake
then dried in a drying oven at 75.degree. C. The very hard material
was then roughly ground to form granules having a particle size of
between 0.2 and 2 mm. The BET specific surface area was 64
m.sup.2/g. The scanning electron micrograph, e.g. at a
magnification of 60000:1, shows that the .alpha.-FeOOH type needles
are agglomerated or glued together by amorphous layers.
[0076] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 2.9 mg/l (As.sup.3+) was 1.8
mg(As.sup.3+)/g(FeOOH).h, for an Na.sub.2HAsO.sub.4 solution with
an original concentration of 2.8 mg/l (As.sup.5+) it was 1.6
mg(As.sup.5+)/g(FeOOH).h.
[0077] Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
* * * * *