Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/69—Sulfur trioxide; Sulfuric acid
  • C01B17/90—Separation; Purification
  • C01B17/901—Recovery from spent acids containing metallic ions, e.g. hydrolysis acids, pickling acids
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/69—Sulfur trioxide; Sulfuric acid
  • C01B17/90—Separation; Purification
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B17/00—Sulfur; Compounds thereof
  • C01B17/69—Sulfur trioxide; Sulfuric acid
  • C01B17/88—Concentration of sulfuric acid
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G23/00—Compounds of titanium
  • C01G23/04—Oxides; Hydroxides
  • C01G23/047—Titanium dioxide
  • C01G23/053—Producing by wet processes, e.g. hydrolysing titanium salts
  • C01G23/0532—Producing by wet processes, e.g. hydrolysing titanium salts by hydrolysing sulfate-containing salts
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
  • C01G49/00—Compounds of iron
  • C01G49/14—Sulfates
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B22/00—Use of inorganic materials as active ingredients for mortars, concrete or artificial stone, e.g. accelerators, shrinkage compensating agents
  • C04B22/08—Acids or salts thereof
  • C04B22/14—Acids or salts thereof containing sulfur in the anion, e.g. sulfides
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B22/00—Use of inorganic materials as active ingredients for mortars, concrete or artificial stone, e.g. accelerators, shrinkage compensating agents
  • C04B22/08—Acids or salts thereof
  • C04B22/14—Acids or salts thereof containing sulfur in the anion, e.g. sulfides
  • C04B22/142—Sulfates
  • C04B22/149—Iron-sulfates
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B28/00—Compositions of mortars, concrete or artificial stone, containing inorganic binders or the reaction product of an inorganic and an organic binder, e.g. polycarboxylate cements
  • C04B28/02—Compositions of mortars, concrete or artificial stone, containing inorganic binders or the reaction product of an inorganic and an organic binder, e.g. polycarboxylate cements containing hydraulic cements other than calcium sulfates
• • 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
  • C01P2004/61—Micrometer sized, i.e. from 1-100 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
  • C01P2004/62—Submicrometer sized, i.e. from 0.1-1 micrometer
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
  • C04B2111/10—Compositions or ingredients thereof characterised by the absence or the very low content of a specific material
  • C04B2111/1075—Chromium-free or very low chromium-content materials
  • C04B2111/1081—Chromium VI, e.g. for avoiding chromium eczema
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
  • C22B7/04—Working-up slag
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/20—Recycling

Definitions

• the invention relates to a reducing agent for the soluble chromate content in cement and to methods for producing the same.
• the chromium content of cements is normally between 20 ppm and 100 ppm depending on the raw material basis used.
• the chromium contained in cement can dissolve when mixed with water as chromium (VI) and, if there is frequent contact, can sensitise the skin and cause a chromium allergy, so-called contact dermatitis from which bricklayers suffer.
• VI chromium
• contact dermatitis from which bricklayers suffer.
• Iron (II) sulphate (as a heptahydrate or monohydrate) is principally used as the reducing agent in the cement industry in order to achieve a chromium (VI) content of less than 2 ppm (cf. Locher, Friedrich Wilhelm: Zement: Klan der compassion undbericht [Cement: Basics of Production and Use], Verlag Bau+Technik GmbH, Düsseldorf 2000).
• iron sulphate to ground cement is described in EP 54314, 160746 and 160747 A1, wherein the iron sulphate is added in dry form prior to the cement storage silo.
• the iron sulphate is provided with a coating in order to increase resistance to oxidation.
• Iron (II) sulphate predominantly comes from the process for manufacturing titanium dioxide using the sulphate method, where it accumulates as a by-product. Iron (II) sulphate can thereby be obtained by crystallisation out of the titanium and iron-containing sulphuric acid solution, which is obtained when digesting the titanium and iron-containing ore or synthetic raw materials (black liquor). Part, however not all, of the iron is hereby removed from the solution. Crystallisation of the iron (II) sulphate thereby occurs by cooling the hot solution, e.g. by means of vacuum cooling, and, optionally, additional concentration by evaporation.
• the titanium oxide hydrate hereby obtained is separated from the residual so-called dilute acid by means of filtration. Whilst the titanium oxide hydrate is further processed into titanium dioxide, the dilute acid must be made available for further use or converted into harmless compounds in a suitable manner.
• Iron (II) sulphate heptahydrate can also be obtained from the dilute acid by means of crystallisation in a manner similar to that described above for the obtainment of iron (II) sulphate heptahydrate from the so-called black liquor.
• the possibility of separating iron (II) sulphate heptahydrate from the dilute acid before concentration is described in EP 132820.
• EP 132820 therefore suggests concentrating the dilute acid by evaporation, optionally separating metal sulphates and reacting the remaining metal sulphates with CaO, Ca(OH) 2 and/or CaCO 3 to form gypsum and poorly soluble metal compounds.
• the solids obtained in this manner are light brown in colour, which is due to the oxidation of iron (II) hydroxide to iron (III) hydroxide.
• the use of this mixture of solids as an iron-containing additive when calcining cement is, inter alia, described.
• EP 160 747 A1 describes that the chromate-reducing effect of iron (II) sulphate mixed into the cement decreases over time during storage.
• the amount of iron (II) sulphate to be added therefore has to be determined depending on the storage time in order to ensure that the chromate in the cement preparation is completely reduced at a specific point in time.
• the object of the invention is to provide an iron (II) sulphate-containing reducing agent for the soluble chromate content in cement as well as suitable methods for producing the same.
• This object is solved according to the invention by means of a method for producing an iron (II) sulphate-containing reducing agent, which includes concentrating an iron (II) sulphate-containing used sulphuric acid and separating the sulphuric acid from the iron (II) sulphate-containing precipitate obtained.
• An effective reducing agent for chromate in cement is obtained in this manner.
• subsequent reduction of the amount of sulphuric acid adhering to the separated precipitate preferably takes place by means of further separation, partial neutralisation or neutralisation of this sulphuric acid.
• An iron (II)-containing used sulphuric acid accumulates in various different industrial processes. For example, an approximately 25% aqueous sulphuric acid (dilute acid) is formed as a by-product when producing titanium dioxide according to the sulphate process.
• the iron (II) sulphate-containing reducing agents according to the invention can be obtained from this dilute acid following concentration, precipitation of iron (II) sulphate-containing salt mixtures and separation of the sulphuric acid from the obtained precipitate.
• a subsequent reduction of the amount of sulphuric acid adhering to the separated precipitate preferably occurs therein following separation of the sulphuric acid from the obtained precipitate.
• iron (II) sulphate-containing used sulphuric acids besides the dilute acid from the production of titanium dioxide can also be used.
• iron (II) sulphate-containing used sulphuric acids from metal pickling plants are suitable herefor.
• iron (II) sulphate-containing used sulphuric acids It is also possible to use mixtures of iron (II) sulphate-containing used sulphuric acids. Finally, it is furthermore possible to use iron (III) sulphate-containing used sulphuric acids provided that they are reduced beforehand with metallic iron or other reducing agents.
• the used sulphuric acids employed preferably have a titanium content of less than 1.5% by weight, particularly preferred less than 0.8% by weight.
• the iron (II)-containing used sulphuric acid is concentrated to a sulphuric acid content of more that 50%, preferably 60 to 80%, with the salts dissolved therein—predominantly iron (II) sulphate monohydrate—crystallising out to a large extent as a fine-crystalline precipitate.
• Concentration can occur either continuously or non-continuously in evaporation systems by evaporating or vaporising the water at normal pressure or under vacuum. Continuously operated forced-circulation evaporation systems under vacuum are preferably used.
• the subsequent crystallisation can take place in evaporating systems with subsequent salt ripening (cooling).
• the present metal sulphates can thereby crystallise out as sulphates, hydrogen sulphates, oxysulphates or as complex mixtures hereof.
• the iron thereby preferably crystallises as iron (II) sulphate monohydrate.
• the separated precipitate also called a filter salt if the used sulphuric acid is obtained from titanium dioxide production
• filter aggregates such as, for example, chamber filter presses, press belt filters, rotary filters, candle pressure filters. It is particularly preferred for separation to occur by means of filtration, for example using candle pressure filters or chamber filter presses.
• the separated precipitate e.g. filter salt
• the separated precipitate preferably contains between 40 and 60% iron (II) sulphate monohydrate, between 3% and 10% further metal sulphates, between 15% and 30% free sulphuric acid and approximately 10% to 13% water.
• the amount of sulphuric acid adhering to the separated precipitate is preferably further reduced, for example by displacing the sulphuric acid using compressed air or washing with steam, by washing with a washing medium such as dilute acid, saturated FeSO 4 solution, diluted FeSO 4 -containing aqueous solutions or water, by reaction with iron or an alkaline iron (II) compound and water, or by adding powdered alkali compounds, in particular CaCO 3 , CaO, Ca(OH) 2 , MgO and/or Mg(OH) 2 or elutriations thereof, such as lime water.
• Washing of the separated precipitate with a washing medium is preferably carried out with 40 to 500% by weight of washing medium based on the separated precipitate (e.g. filter salt). Washing is preferably carried out at a temperature of 55 to 100° C. Particularly preferred are temperatures between 55 and 75° C.
• Dilute acid has proven to be an ideal washing medium, particularly if washing is carried out at an elevated temperature.
• Washing with warm dilute acid can be carried out in the same filtration apparatus without having to remove the precipitate (filter cake) in the meantime.
• the filtration process preferably comprises the following steps in the given order: loading of the filtering apparatus, pressing out, optionally blowing through or out, washing with dilute acid, blowing through or out and discarding.
• the separated precipitate can also be suspended in warm dilute acid in a receiver and filtered off again.
• a saturated FeSO 4 solution or diluted FeSO 4 -containing aqueous solutions can also be used in addition to dilute acid as washing mediums for reducing the amount of sulphuric acid adhering to the separated precipitate.
• reduction of the amount of sulphuric acid adhering to the separated precipitate following concentration of the used sulphuric acid and separation of the precipitate from the concentrated used sulphuric acid can also occur by means of washing with steam, preferably at a temperature of greater than 100° C. It is thereby to be noted that any additionally used liquid or condensate must later be removed again, using energy, from the accumulating acid by means of distillation.
• Washing with steam has the advantage that only very little water in the form of steam is used since the acid is firstly diluted by the steam, which simultaneously blows off the adhering acid.
• the elevated temperatures used according to the invention no further condensation of steam occurs following the separation of the sulphuric acid from the separated precipitate carried out in this manner.
• Use is therefore made of the hygroscopic properties of the sulphuric acid; steam is namely only condensed in the separated precipitate for as long as highly concentrated sulphuric acid is still present.
• iron (II) sulphate monohydrate is poorly soluble in diluted sulphuric acid at an elevated temperature and is furthermore blown out in situ such that a solution equilibrium cannot be achieved. Therefore, only a small proportion of the iron sulphate contained in the separated precipitate is washed out by means of washing with steam.
• washing with steam is particularly preferred for washing with steam to be carried out at a temperature of 105 to 130° C.
• reduction of the amount of sulphuric acid adhering to the separated precipitate following concentration of the used sulphuric acid and separation of the precipitate from the concentrated used sulphuric acid can also occur by reaction with water and metallic iron or an alkaline iron (II) compound above 60° C.
• the separated precipitate does not react with metallic iron and only very slightly with alkaline iron (II) compounds. Therefore, the proportion of sulphuric acid in the adhering acid can technically only be partially neutralised or neutralised with difficulty by means of a direct reaction with iron or the alkaline iron (II) compound.
• iron (II) sulphate monohydrate is converted, at a normal ambient temperature, into iron (II) sulphate heptahydrate.
• iron (II) sulphate heptahydrate cleaves water to form iron (II) sulphate monohydrate. If the separated precipitate is used above this temperature with little water, the monohydrate is maintained. A free-flowing paste is obtained, in which sulphuric acid is diluted to such an extent that a reaction with iron or the alkaline iron (II) compound, forming additional iron (II) sulphate monohydrate, is possible.
• reaction between the separated precipitate and metallic iron or the alkaline iron (II) compound is preferred for the reaction between the separated precipitate and metallic iron or the alkaline iron (II) compound to take place at a temperature of 60 to 110° C., particularly preferred at a temperature of 75 to 85° C.
• the separated precipitate is preferably reacted with 80 to 98 mol-% of metallic iron or alkaline iron (II) compound, such as iron (II) carbonate or iron (II) hydroxide or iron (II) oxide, based on the amount of sulphuric acid adhering to the separated precipitate (e.g. filter salt), with as much water being added so that the molar ratio of water to iron sulphate is 6.5 to 7.
• metallic iron or alkaline iron (II) compound such as iron (II) carbonate or iron (II) hydroxide or iron (II) oxide
• iron (II) compounds When reacting the separated precipitate with an alkaline iron (II) compound, this compound can also be reacted in the form of a natural ore such as siderite.
• iron (II) compounds which are the component of an industrial by-product, can also be used for this purpose.
• metallic iron is used in the reaction with the separated precipitate, metallic iron having an average particle size of 5 mm or less is preferably used, particularly preferred iron (powder) having an average particle size of 100 ⁇ m or less.
• the start of the reaction can be accelerated by heating, for example with (direct) steam.
• the reaction itself is exothermic.
• the reaction takes longer if coarse iron is used.
• water loss as a result of evaporation and heat loss must, where necessary, be compensated by supplying steam.
• the reaction with siderite is slower and without apparent reaction heat. Heat and water losses must also be compensated here by supplying heat, for example, by direct steam.
• Regulation of the steam and/or water supply can be controlled by means of the temperature and/or viscosity (for example by measuring the current consumption of the stirring device).
• the reaction time can range from a few minutes (use of iron powder: 10 ⁇ m) to several hours (use of iron granules: 3 mm, or siderite fraction: ⁇ 1 mm).
• the reaction is generally carried out at normal pressure.
• the reaction of the separated precipitate with water and metallic iron or an alkaline iron (II) compound and water can technically take place in a mixing vessel into which the components of the separated precipitate, iron or alkaline iron (II) compound and water are continuously or non-continuously introduced.
• a paste-like mass results, which can be placed, for example, on a cooling section as pellets for solidification. Another possibility is blowing with cold air.
• a kneader double-arm kneader or the like
• a kneader double-arm kneader or the like
• the reaction between the separated precipitate and the metallic iron or an alkaline iron (II) compound takes place continuously, the non-reacted proportion of iron or alkaline iron compound can be removed from the overflow and returned. If metallic iron is hereby used for the reaction, the iron can be removed by means of a magnetic separator.
• reduction of the amount of sulphuric acid adhering to the separated precipitate following concentration of the used sulphuric acid and separation of the precipitate from the concentrated used sulphuric acid can also occur by displacing the sulphuric acid with compressed air.
• partial neutralisation or neutralisation by adding powdered alkali compounds, in particular CaCO 3 , CaO, Ca(OH) 2 , MgO and/or Mg(OH) 2 or elutriations thereof, such as lime water can also occur in order to reduce the amount of sulphuric acid adhering to the separated precipitate following concentration of the used sulphuric acid and separation of the precipitate from the concentrated used sulphuric acid.
• powdered alkali compounds in particular CaCO 3 , CaO, Ca(OH) 2 , MgO and/or Mg(OH) 2 or elutriations thereof, such as lime water
• these powdered alkali compounds can also occur according to one of the method steps described above for reducing the amount of sulphuric acid adhering to the separated precipitate, such as washing with a washing medium, for instance dilute acid, saturated FeSO 4 solution, diluted FeSO 4 -containing aqueous solutions or water, or displacing the sulphuric acid with compressed air or washing with steam or reaction with iron or an alkaline iron (II) compound and water, in order to partially neutralise or neutralise the residual acid in the separated precipitate.
• a washing medium for instance dilute acid, saturated FeSO 4 solution, diluted FeSO 4 -containing aqueous solutions or water
• iron or an alkaline iron (II) compound and water in order to partially neutralise or neutralise the residual acid in the separated precipitate.
• a defined amount of water, aqueous saline solution or diluted sulphuric acid is preferably added so that granulation can take place.
• This addition can occur particularly advantageously in the filer aggregate by displacement washing of the dilute acid with water, which then additionally increases the amount of acid regained without notably increasing the amount of water to be evaporated.
• the required amount of water can optionally also come from the addition of moist green salt. This variant has the advantage that drying of the green salt then occurs without requiring energy or chemical expenditure and a granular or powdered product is obtained.
• the amount of water thereby added can be 100 to 550 mol-% based on the iron (II) sulphate monohydrate contained in the separated precipitate. In a particularly preferred embodiment, the amount of water added is 250 to 350 mol-% based on the iron (II) sulphate monohydrate contained in the separated precipitate.
• Granulation and control of the granule size preferably occurs by means of mechanical formation or by blowing with air or by spraying with a nozzle or a rotary disk or by cooling, e.g. by means of a cooling roller or a cooling conveyer or by falling or swirling in cold air.
• cooling e.g. by means of a cooling roller or a cooling conveyer or by falling or swirling in cold air.
• solid blocks, coarse or fine granules or powdered bulk materials are thereby obtained.
• the iron (II) sulphate-containing reducing agent which is obtained according to the above methods and which is industrially particularly inexpensive, energy-saving and of a constant quality, can be used to reduce chromate in cement.
• the iron (II) sulphate-containing reducing agent according to the invention has an average crystallite size of less than 2 ⁇ m, preferably between 0.1 and 1.0 ⁇ m. In several particular embodiments, the average crystallite size is in the range of 0.2 and 0.5 ⁇ m.
• the average crystallite size is determined as follows: the samples are measured under a Kapton foil (in order to exclude moisture) on a Philips PW 1800 diffractometer. Determination of the crystallite size occurs by means of the Philips Fit program from the 100% reflection of the measured spectrum.
• iron (II) sulphate-containing reducing agent iron (II) sulphate monohydrate
• 100% reflection hkl 200 at 25.879° 2theta from the measuring range 25° to 28° 2theta was used to determine the crystallite size.
• the crystallite size is thereby not identical to the size of the primary particles as is recognisable from electron micrographs. However, clear differences can also be seen in the electron micrographs: the average primary particle size for the iron (II) sulphate-containing reducing agent according to the invention is approximately 5 ⁇ m; the average primary particle size of the reducing agent according to the prior art (i.e. green salt of the firm KRONOS) is approximately 50 ⁇ m.
• the iron (II) sulphate-containing reducing agent according to the invention preferably contains 5 to 15% by weight, particularly preferred 7 to 13% by weight, of titanium, based on iron, and/or preferably 1.5 to 4.0% by weight, particularly preferred 2.0 to 3.5% by weight, of manganese, based on iron.
• An advantage of the method according to the invention is that in addition to iron sulphate, all the metal sulphates crystallising out of the concentrated sulphuric acid, e.g. manganese (II) sulphate, can be recycled.
• the further metal sulphates contained in small amounts in the iron (II) sulphate-containing reducing agent according to the invention do not display any disadvantageous effects and are permanently bound in the cement matrix following hardening.
• the content of undesired soluble chromium can be effectively reduced to a sufficient extent despite the additional introduction of chromium into the cement.
• the effectiveness of the iron (II) sulphate-containing reducing agent according to the invention is comparable to that of conventionally used green salt (see Example 2). 0.01 to 5.0% by weight, particularly preferred 0.2 to 1.5% by weight of the iron (II) sulphate-containing reducing agent according to the invention is used.
• the addition of the iron (II) sulphate-containing reducing agent can thereby also take place as a solution or suspension.
• the iron (II) sulphate-containing reducing agent according to the invention When used, a lower effect as compared to the prior art can be determined for a low added amount after a short storage period (see Example 3: addition of 0.3% by weight or 0.6% by weight).
• the iron (II) sulphate-containing reducing agent according to the invention surprisingly does not display the generally known continuously decreasing reducing effect as the storage time increases, but rather a reducing effect that increases again.
• the reducing agent according to the invention therefore does not demonstrate a significant decrease in the reducing effect as the storage time increases, in particular not after 1 month.
• iron (II) sulphate-containing reducing agent or a mixture of the iron (II) sulphate-containing reducing agent with green salt with further inert inorganic and/or organic compounds so as to set favourable transportation and/or storage properties in a targeted manner.
• the iron (II) sulphate-containing reducing agent according to the invention can be mixed either
• the invention also provides a preparation comprising a mixture of cement and the reducing agent according to the invention, the preparation containing 0.01 to 5.0% by weight, particularly preferred 0.2 to 1.5% by weight, of the iron (II) sulphate-containing reducing agent.
• a further preparation according to the invention contains cement, water and the iron (II) sulphate-containing reducing agent according to the invention, the preparation containing 0.01 to 5.0% by weight, preferably 0.2 to 1.5% by weight, of the iron (II) sulphate-containing reducing agent, based on the cement.
• the dilute acid obtained during production of titanium dioxide according to the sulphate process which has a sulphuric acid content of 23.5% and an iron content of 3.8%, was concentrated by evaporation in a three-stage forced-circulation evaporation system with a gradually increasing vacuum until a sulphuric acid content of 48% was achieved (which corresponds to a sulphuric acid concentration in the liquid phase of 70%).
• a large part of the iron sulphate crystallises out as monohydrate during concentration by evaporation.
• the obtained slurry was subjected to ripening, whereby its temperature was reduced from approximately 90° C. to 60° C. in a stirring cascade.
• the slurry was then filtered under pressure in a candle filter system and the filter cake was partially freed from adhering sulphuric acid using compressed air.
• a crumbly, dry, easy-to-handle filter cake (filter salt) is obtained, which can be used as an iron (II) sulphate-containing reducing agent.
• the crystallite size was determined by means of radiography.
• the crystallite size of conventional green salt (KRONOS) is >>3 ⁇ m (determination of the crystallite size for green salt cannot be clearly ascertained using this technology; however, it is in any case far greater than 3 ⁇ m).
• the dilute acid obtained during production of titanium dioxide according to the sulphate process which has a sulphuric acid content of 23.5% and an iron content of 3.8%, was concentrated by evaporation in a three-stage forced-circulation evaporation system with a gradually increasing vacuum until a sulphuric acid content of 48% was achieved (which corresponds to a sulphuric acid concentration in the liquid phase of 70%).
• a large part of the iron sulphate crystallises out as monohydrate during concentration by evaporation.
• the obtained slurry was subjected to ripening, whereby its temperature was reduced from approximately 90° C. to 60° C. in a stirring cascade.
• a material having a pH value of 2.2 is obtained by adding 6% by weight of powdered Ca(OH) 2 .
• the iron (II) sulphate-containing reducing agent obtained has good rheological properties and is not a hazardous substance owing to its pH of value >2.
• Determination of the pH value is carried out in the eluate of 10 g of salt in 1000 g of H 2 O by means of a pH single rod measuring element with an Ag/AgCl reference system of the firm Schott, type H6580, on a pH meter of the firm Knick, type 765 Calimatic.
• a free-flowing iron (II) sulphate-containing reducing agent having a pH value of 1.5 is obtained.
• a material having a pH value of 2.6 is obtained by adding 15% by weight of powdered Ca(OH) 2 .
• the iron (II) sulphate-containing reducing agent obtained has good Theological properties and is not a hazardous substance owing to its pH value of >2.
• the pH value is determined in the same manner as in Example 3 a).
• Example 2 Part of the washed filter salt obtained according to Example 2, test 3, was mixed with two parts iron (II) sulphate heptahydrate and kept moving for several hours. A powder having no thixotropic properties is obtained. Mixing in the ratio described above can alternatively be carried out at 80° C. A viscous melt is obtained, which solidifies during cooling to form a hard cake.
• Table 4 shows an overview. TABLE 4 Filter Fe Reaction Reaction % Fe in Prod. % H 2 SO 4 salt (g) (met) (g) Water (g) time (min) temp. (° C.) Product (g) Analysis Theor. Analysis Theor. Raw material 16.20 20.30 30 3 (*1 7.5 15 70 (*3 38.9 20.24 20.21 2.33 2.11 30 3 (*1 7.5 15 80 38.6 20.39 20.36 2.18 2.13 30 2.5 (*1 7.5 15 80 39.5 19.05 18.63 4.09 4.30 30 2 (*1 7.5 15 80 38.8 17.65 17.68 6.43 6.64 30 3 (*2 7.5 360 80 38.5 not 20.58 not 2.15 determined determined (*1 iron powder, reinst. (particle size: 10 ⁇ m of the firm Merck) (*2 iron granules, techn. (particle size: approximately 1 to 2 mm) (*3 after 15 mins even smaller amounts of iron powder were recognisable
• Example 7 The reaction was carried out in the same manner as in Example 7. A product having a residual acid content of 4.09% was pulverised and the residual acid in the filter salt was neutralised by adding 5% by weight of CaCO 3 . The aqueous extract of the powder obtained in this manner does not demonstrate any acidic properties.
• Deviating from TRGS 613 five times the amount (however the same ratio of cement to water) was eluted in order to determine the soluble chromium, and the dissolved chromium was determined by means of ICP-OES.
• the limiting value of 0.5 mg/l of chromium in the eluate is not exceeded when 0.30% by weight of conventional green salt (KRONOS) is added or when 0.60% by weight or more of the iron (II) sulphate-containing reducing agent is added. It can furthermore be seen that when 0.60% by weight of the iron (II) sulphate-containing reducing agent is added, the content of soluble chromium is below the detection limit after a storage period of 4 weeks, even though higher contents had been detected in the meantime.
• KRONOS conventional green salt
• a sample of the iron (II) sulphate-containing reducing agent (filter salt) produced according to Example 1 was mixed with 10% by weight of CaCO 3 .
• the content of water-soluble chromate was only below the limiting value of 2 ppm cited in the TRGS 613 at a dose of 0.7%.
• a dose of 1.0% of iron (II) sulphate-containing reducing agent was added to the test cement, the content of water-soluble chromate was below the detection limit.
• test cement described above was again used with and without iron (II) sulphate-containing reducing agent.
• the times for the start of solidification and the end of solidification were each reduced by a third owing to the addition of 1.0% of iron (II) sulphate-containing reducing agent.
• the reason for this is the significant increase of the sulphate content in the cement that is already optimised for sulphate carriers.
• Example 11 10% by weight of CaCO 3 was added to the filter salt.
• the partially neutralised filter salt was mixed with green salt in a ratio of 1:1 and 2:1, and 0.5 and 0.7% of these mixtures were added to a test cement as described in Example 11.
• the obtained mixture displays good rheological properties.
• the content of water-soluble chromate was below the limiting value of 2 ppm as specified in TRGS 613 for both doses (0.5% and 0.7%)

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