U.S. patent application number 09/962971 was filed with the patent office on 2002-06-20 for contact and adsorbent granules.
Invention is credited to Kischkewitz, Jurgen, Schlegel, Andreas.
Application Number | 20020077249 09/962971 |
Document ID | / |
Family ID | 27214084 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020077249 |
Kind Code |
A1 |
Schlegel, Andreas ; et
al. |
June 20, 2002 |
Contact and adsorbent granules
Abstract
The present 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 oxyhydroxides, solidified with oxides and/or
(oxy)hydroxides of the elements Al, Mg and Ti, the pellets or
granules based on iron oxides and/or iron oxyhydroxides and
iron(III) hydroxide for the absorbent/catalyst, and processes for
their production and their use.
Inventors: |
Schlegel, Andreas; (Krefeld,
DE) ; Kischkewitz, Jurgen; (Ratingen, DE) |
Correspondence
Address: |
BAYER CORPORATION
PATENT DEPARTMENT
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Family ID: |
27214084 |
Appl. No.: |
09/962971 |
Filed: |
September 25, 2001 |
Current U.S.
Class: |
502/328 ;
502/330; 502/336 |
Current CPC
Class: |
B01J 20/041 20130101;
C01P 2004/50 20130101; C01G 49/0045 20130101; C01G 49/02 20130101;
B01J 20/08 20130101; C01P 2004/10 20130101; C01G 49/06 20130101;
C02F 1/281 20130101; B01D 53/02 20130101; C01P 2004/64 20130101;
B01D 2253/10 20130101; C01G 49/0036 20130101; B01J 20/0229
20130101; B82Y 30/00 20130101; B01J 20/0211 20130101; C02F 2101/20
20130101; C01P 2006/12 20130101; C02F 1/288 20130101; C01P 2004/62
20130101; B01J 20/06 20130101 |
Class at
Publication: |
502/328 ;
502/330; 502/336 |
International
Class: |
B01J 023/745 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2000 |
DE |
10047996.0 |
Sep 26, 2000 |
DE |
10047997.9 |
Mar 29, 2001 |
DE |
10115414.3 |
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 oxyhydroxides,
solidified with oxides and/or (oxy)hydroxides of the elements Al,
Mg and Ti.
2. The unit of claim 1 wherein the fluid medium is a gas.
3. The unit of claim 1 wherein the fluid medium is a liquid.
4. The unit of claim 3 wherein the liquid 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 step of (a) incorporating aluminium, magnesium
and/or titanium oxides or (oxy)hydroxides or ageing products and
dehydrated secondary products thereof into an aqueous suspension of
iron oxide and/or iron oxyhydroxide, including Fe(OH).sub.2 and
then (b) either (b1) drying the suspension until it reaches a solid
state and 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., 67 ', .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 transparent pigments are used as
iron oxides and/or iron oxyhydroxides.
11. The process of claim 7 wherein salts from the series of
carbonates, chlorides, fluorides, nitrates, sulfates and sulfites
are used to precipitate the Al, Mg or Ti oxides and/or
(oxy)hydroxides.
12. A process comprising treating a fluid medium in a unit
according to claim 1 by contacting the fluid medium with particles
obtained by a process for the production of an adsorbent/catalyst,
comprising the step of (b) incorporating aluminium, magnesium
and/or titanium oxides or (oxy)hydroxides or ageing products and
dehydrated secondary products thereof into an aqueous suspension of
iron oxide and/or iron oxyhydroxide, including Fe(OH).sub.2 and
then (b) either (b2) drying the suspension until it reaches a solid
state and 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.
13. The process of claim 12 wherein the fluid medium comprises
water.
14. The process of claim 12 comprising removing a heavy metal,
phosphorus, antimony, beryllium, selenium, tellurium and cyano
compound from water.
15. The process of claim 12 comprising removing arsenic compounds
from water.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to absorbents, catalysts,
pellets or granules based on iron oxides and/or iron oxyhydroxides
containing a small quantity of inorganic Al and/or Mg oxides or
(oxy)hydroxides as binders, having a large specific surface area
(50 to greater than 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 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 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 possible
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 and its unreliable
availability.
[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 embodied 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 organic 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.
[0020] 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.
[0021] 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.
[0022] An object underlying the present invention was therefore to
provide 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 to
achieve adequate mechanical resistance, and plants operated with
such media.
[0023] 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
[0024] 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 oxyhydroxides, solidified with oxides and/or
(oxy)hydroxides of the elements Al, Mg and Ti.
[0025] The invention also relates to a process for the production
of an adsorbent/catalyst, comprising the step of (a) incorporating
aluminium, magnesium and/or titanium oxides or (oxy)hydroxides or
ageing products and dehydrated secondary products thereof into an
aqueous suspension of iron oxide and/or iron oxyhydroxide,
including Fe(OH).sub.2 and then (b) either (b1) drying the
suspension until it reaches a solid state and 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
DETAILED DESCRIPTION OF THE INVENTION
[0026] The material in question consists of iron oxides and/or iron
(oxy)hydroxides bonded with a small quantity of magnesium or
aluminium oxides and/or (oxy)hydroxides, 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 displays only a small content of 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, since the quantity of additives does not have too
disadvantageous an influence on coloration.
[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 AI(OH).sub.3 and/or
Mg(OH).sub.2 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 foreign oxide or hydroxide
matrix.
[0032] The iron oxide and/or iron (oxy)hydroxide particles can also
be solidified in situ: either by preparing an alkaline suspension
of the iron oxides and/or iron (oxy)hydroxides, adding aqueous
salts of Al.sup.3+, Mg.sup.2+, Ti.sup.4+ or mixtures thereof until
sufficiently poorly soluble deposits of Al(OH).sub.3, Mg(OH).sub.2,
TiO(OH).sub.2 or ageing products and dehydrated secondary products
thereof are precipitated onto the suspended iron oxide and/or iron
(oxy)hydroxide particles, or, conversely, by precipitating the
poorly soluble deposits such as Al(OH).sub.3, Mg(OH).sub.2,
TiO(OH).sub.2 or ageing products and secondary products thereof
onto the iron oxide or iron (oxy)hydroxide particles suspended in
Al.sup.3+, Mg.sup.2+, Ti.sup.4+ solutions by the addition of
alkalis, such as e.g. NaOH, Ca(OH).sub.2, KOH, CaCO.sub.3,
Na.sub.2CO.sub.3, K.sub.2CO.sub.3, NH.sub.40H. The aluminium oxide
or aluminium (oxy)hydroxide can also be precipitated from an
aluminate suspension (e.g. NaAlO.sub.2) onto the iron oxide and/or
iron (oxy)hydroxide particles.
[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 dispersions 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 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(lI) 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.2/g.
[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 nanoparticle iron oxides or iron (oxy)hydroxides
with pigments and/or aluminium, magnesium or titanium (hydr)oxides,
the presence of the cited pigment or nucleus particles in their
known particle morphology, held or glued together by the
nanoparticle nucleus particles or the aluminium, magnesium or
titanium (hydr)oxides, can be detected on the scanning electron
micrographs.
[0045] Yellow iron oxyhydroxide pigments are generally synthesised
by precipitating iron(ll) 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 U.S. Pat. Nos. 2,558,303
and 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 litres 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 litre 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).cndot.h, calculated from the balance of
the As.sup.3+/5+ ions remaining in solution.
[0059] The adsorbed quantities of Hg or Pb are determined in the
same way.
[0060] The As, 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] 7905 kg FeSO.sub.4 were measured out, dissolved with water
to a volume of 53.3 m.sup.3, the solution cooled to 14.degree. C.
and 1000 kg MgSO.sub.4.multidot.7 H.sub.2O added to this solution.
The prepared solution was then diluted at 14.degree. C. with 5056
kg NaOH as a solution with approx. 300 g/l and then oxidised with
4000 m.sup.3/h air to a precipitation degree of >99.5%. 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, causing it to
be formed into strands. The strands were dried on a belt dryer to a
residual moisture of approx. 3%. An X-ray diffractogram showed that
the product consisted of 100% .alpha.-FeOOH with very short
needles. Using a scanning electron micrograph e.g. at a
magnification of 60000:1, the needle widths were measured at
between 30 and 50 nm. The needle lengths could not be clearly
determined as the needles were too greatly agglomerated. The BET
specific surface area was 145 m.sup.2/g. The abrasion value after
30 minutes was only 6%.
[0064] Adsorption performance: The adsorption rate for NaAsO.sub.2
with an original concentration of 2.5 mg/l was 1.8
mg(As.sup.3+)/g(FeOOH).cndot.h- , for Na.sub.2HAsO.sub.4 with an
original concentration of 2.9 mg/l it was 1.5
mg(As.sup.5+)/g(FeOOH).cndot.h.
Example 2
[0065] 569 ml of an MgSO.sub.4 solution (100 g/l) were added to 1 l
of a suspension of Bayferrox.RTM. 920 with a solids content of 50
g/l FeOOH, then 173 g of a 24% NaOH solution were added with
stirring, and stirring was continued for a further 15 min. The
yellow suspension is washed at a nutsch filter to obtain a residual
filtrate conductivity of 1 mS/cm, and the filter cake dried to a
residual moisture of <2% in a drying oven at 75.degree. C. The
product was granulated to particle sizes of between 0.5 and 2 mm
and the granules used for arsenic adsorption.
[0066] An X-ray diffractogram shows that the product consists of
.alpha.-FeOOH and Mg(OH).sub.2. 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. The BET specific surface area was 43 m.sup.2/g and
therefore, compared with Bayferrox.RTM. 920 (BET approx. 15
m.sup.2/g). The abrasion value after 30 minutes was only 11%.
[0067] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 2.6 mg/l (As.sup.3+) was 1.2
mg(As.sup.3+)/g(FeOOH).cndot.h, for an Na.sub.2HAsO.sub.4 solution
with an original concentration of 2.7 mg/I (As.sup.5+) it was 1.5
mg(As.sup.5+)/g(FeOOH).cndot.h.
Example 3
[0068] 46 ml of an Al.sub.2(SO.sub.4).sub.3 solution (100 g/l
Al.sub.2O.sub.3) were added to 950 g of a suspension of an alkaline
nanoparticle nucleus of .alpha.-FeOOH (solids content: 5.26 g/l
FeOOH, 1.14% NaOH) with stirring, and stirring was continued for a
further 15 min. The brown suspension is washed at a nutsch filter
to obtain a residual filtrate conductivity of 1 mS/cm, and the
filter cake dried to a residual moisture of <2% in a drying oven
at 75.degree. C. The product was granulated to particle sizes of
between 0.5 and 2 mm and the granules used for arsenic
adsorption.
[0069] The X-ray diffractogram of the product indicated only
.alpha.-FeOOH, which, as can be seen from the scanning electron
microgram, is present as very short and extremely agglomerated
needles. The BET specific surface area was 102 m.sup.2/g. The
abrasion value after 30 minutes was only 5%.
[0070] The adsorption rate for an aqueous NaAsO.sub.2 solution with
an original concentration of 2.6 mg/l (As.sup.3+) was 2.0
mg(As.sup.3+)/g(FeOOH).cndot.h, for an Na.sub.2HAsO.sub.4 solution
with an original concentration of 2.1 mg/l (As.sup.5+) it was 1.5
mg(As.sup.5+)/g(FeOOH).cndot.h.
* * * * *