Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/02—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/28—Treatment of water, waste water, or sewage by sorption
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
  • B01J20/0211—Compounds of Ti, Zr, Hf
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/0203—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of metals not provided for in B01J20/04
  • B01J20/0225—Compounds of Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt
  • B01J20/0229—Compounds of Fe
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/04—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising compounds of alkali metals, alkaline earth metals or magnesium
  • B01J20/041—Oxides or hydroxides
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/06—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising oxides or hydroxides of metals not provided for in group B01J20/04
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
  • B01J20/06—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising oxides or hydroxides of metals not provided for in group B01J20/04
  • B01J20/08—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising oxides or hydroxides of metals not provided for in group B01J20/04 comprising aluminium oxide or hydroxide; comprising bauxite
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/0018—Mixed oxides or hydroxides
  • C01G49/0036—Mixed oxides or hydroxides containing one alkaline earth metal, magnesium or lead
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/0018—Mixed oxides or hydroxides
  • C01G49/0045—Mixed oxides or hydroxides containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/02—Oxides; Hydroxides
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/02—Oxides; Hydroxides
  • C01G49/06—Ferric oxide [Fe2O3]
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/28—Treatment of water, waste water, or sewage by sorption
  • C02F1/281—Treatment of water, waste water, or sewage by sorption using inorganic sorbents
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/28—Treatment of water, waste water, or sewage by sorption
  • C02F1/288—Treatment of water, waste water, or sewage by sorption using composite sorbents, e.g. coated, impregnated, multi-layered
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D2253/00—Adsorbents used in seperation treatment of gases and vapours
  • B01D2253/10—Inorganic adsorbents
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/10—Particle morphology extending in one dimension, e.g. needle-like
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/50—Agglomerated particles
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2004/00—Particle morphology
  • C01P2004/60—Particles characterised by their size
  • C01P2004/64—Nanometer sized, i.e. from 1-100 nanometer
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
  • C01P2006/00—Physical properties of inorganic compounds
  • C01P2006/12—Surface area
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2101/00—Nature of the contaminant
  • C02F2101/10—Inorganic compounds
  • C02F2101/20—Heavy metals or heavy metal compounds

Definitions

• 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 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.
• 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).
• 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 2 S, SO x , NO x can also be adsorbed.
• 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.
• 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.
• 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° C.
• 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) 2 with additions of clays, stone dust, entrained dust and fly ashes, made porous and having a surface area of approx. 200 m 2 /g, are described here.
• 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.
• JP-A 55 132 633 describes granulated red mud, a by-product of aluminium production, as an adsorbent for arsenic. This consists of Fe 2 O 3 , Al 2 O 3 and SiO 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.
• 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.
• 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.
• 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.
• 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° C.
• This production process is extremely energy-intensive and leads to heavily salt-contaminated waste waters.
• this production process only very small granules with low mechanical resistance are obtained.
• 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.
• Continuous adsorbers which are commonly grouped together in parallel for operation, are preferably used for water treatment.
• such adsorbers are filled with activated carbon.
• the available adsorbers are then operated in parallel to prevent the flow rate from rising above the maximum permitted by the particular arrangement.
• 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.
• 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.
• 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.
• 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.
• 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) 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
• 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.
• 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.
• 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.
• the invention therefore also concerns a process for the production of an iron oxide/iron hydroxide-containing adsorbent/catalyst in pellet form.
• the material according to the invention can be obtained by mixing diverse phases of iron oxides and/or iron oxyhydroxides, including Fe(OH) 2 , in pure form or in any mixture in solid, semisolid or suspended form by the addition of AI(OH) 3 and/or Mg(OH) 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.
• the iron oxide and/or oxyhydroxide is firmly embedded in the foreign oxide or hydroxide matrix.
• 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 3+ , Mg 2+ , Ti 4+ or mixtures thereof until sufficiently poorly soluble deposits of Al(OH) 3 , Mg(OH) 2 , TiO(OH) 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) 3 , Mg(OH) 2 , TiO(OH) 2 or ageing products and secondary products thereof onto the iron oxide or iron (oxy)hydroxide particles suspended in Al 3+ , Mg 2+ , Ti 4+ solutions by the addition of alkalis, such as e.g.
• aluminium oxide or aluminium (oxy)hydroxide can also be precipitated from an aluminate suspension (e.g. NaAlO 2 ) onto the iron oxide and/or iron (oxy)hydroxide particles.
• an aluminate suspension e.g. NaAlO 2
• 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.
• 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/Flüssig-Filtration, H. Gasper, D. ⁇ chsle, E. Pongratz, 2nd edition 2000, Wiley-VCH Weinheim. Coagulants can thus be added to the suspensions, for example.
• the dispersions to be dehydrated can also contain iron carbonates.
• 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° C.
• the material can also be freeze dried.
• the products according to the invention preferably have a residual water content of less than 20 wt. %.
• 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.
• 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.
• the iron oxyhydroxides or iron oxides treated with Fe(OH) 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.
• Iron oxyhydroxide pigments e.g. goethite
• iron oxide pigments e.g. haematite, magnetite
• 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 ⁇ , ⁇ , ⁇ , ⁇ , ⁇ ′, ⁇ phases and/or Fe(OH) 2 and mixed and intermediate phases thereof.
• Yellow iron oxyhydroxides can be calcined to red iron oxides.
• the product displays BET surface areas of 50 to 500 m 2 /g, preferably 80 to 200 m 2 /g.
• 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 ⁇ -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 ⁇ -FeOOH particles. ⁇ -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 ⁇ -Fe 2 O 3 , ⁇ -Fe 2 O 3 , Fe 3 O 4 phases for example, the particle diameters can quite easily also be below 20 nm.
• 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 4 , FeCl 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 2 O 2 also leads to iron oxyhydroxides. NaOH is preferably used as alkaline precipitant.
• precipitants such as KOH, Na 2 CO 3 , K 2 CO 3 , CaO, Ca(OH) 2 , CaCO 3 , NH 3 , NH 4 OH, MgO and/or MgCO 3 , can also be used, however.
• nanoparticle ⁇ , ⁇ , ⁇ , ⁇ 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.
• the precipitations e.g. of yellow ⁇ -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 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.
• nucleus modifiers One of the advantages of nucleus modifiers, however, is that an adequate fine-particle character can be obtained even at elevated reaction temperatures.
• the products described, the process for their production and their use represent an improvement over 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.
• 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.
• Granulation of a semi-wet paste has proven effective as another method of producing granules.
• 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.
• 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.
• 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.
• 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.
• use in continuous plants such as continuous-flow adsorbers is preferred.
• 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.
• 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.
• the material is comparatively easy to dispose of after use.
• 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.
• 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.
• 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.
• the sample is baked for 1 h at 140° C. in a stream of dry nitrogen before measurement.
• 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.
• ICP-MS mass spectrometry
• ICP-OES optical emission spectroscopy
• 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 (Kühner 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 %.
• 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% ⁇ -FeOOH with very short needles.
• 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 2 /g.
• the abrasion value after 30 minutes was only 6%.
• Adsorption performance The adsorption rate for NaAsO 2 with an original concentration of 2.5 mg/l was 1.8 mg(As 3+ )/g(FeOOH)•h, for Na 2 HAsO 4 with an original concentration of 2.9 mg/l it was 1.5 mg(As 5+ )/g(FeOOH)•h.
• An X-ray diffractogram shows that the product consists of ⁇ -FeOOH and Mg(OH) 2 .
• the scanning electron micrograph e.g. at a magnification of 60000:1, shows that the ⁇ -FeOOH type needles are agglomerated or glued together by amorphous layers.
• the BET specific surface area was 43 m 2 /g and therefore, compared with Bayferrox® 920 (BET approx. 15 m 2 /g).
• the abrasion value after 30 minutes was only 11%.
• the X-ray diffractogram of the product indicated only ⁇ -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 2 /g.
• the abrasion value after 30 minutes was only 5%.
• the adsorption rate for an aqueous NaAsO 2 solution with an original concentration of 2.6 mg/l (As 3+ ) was 2.0 mg(As 3+ )/g(FeOOH)•h, for an Na 2 HAsO 4 solution with an original concentration of 2.1 mg/l (As 5+ ) it was 1.5 mg(As 5+ )/g(FeOOH)•h.

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