WO2012163350A1 - Process for preparing titanium dioxide - Google Patents

Abstract

The invention relates to a process for preparing titanium dioxide, which includes a special treatment of the digestion residues which form in the course of preparation of titanium dioxide by the sulfate process, and to the digestion residues thus obtainable.

Description

 Process for the production of titanium dioxide

The invention relates to a process for the production of titanium dioxide, which includes a special treatment of the digestion residues which are formed in the production of titanium dioxide by the sulfate process, as well as the pulping residue thus obtainable and the use of the pulping residue as an aggregate in metallurgical processes.

In the production of titanium dioxide by the sulphate process, titania-containing raw materials (slag, ilmenite) are dried and ground and then digested with concentrated sulfuric acid. The reaction between the raw materials and the concentrated sulfuric acid is carried out batchwise in bricked reaction vessels. In the digestion reaction, almost all metal oxides present in the raw materials that react with sulfuric acid are converted into the corresponding metal sulfates. After the reaction remains a solid mass (digestion cake) which is dissolved with water and / or dilute sulfuric acid.

This digestion solution, the so-called black liquor or black liquor, is completely freed from the undissolved constituents (digestion residues, gangue) by sedimentation and / or filtration processes. In the further process, a metatitanic acid suspension is prepared from the solids-free digestion solution by hydrolysis. The metatitanic acid is calcined after washing, bleaching and optionally salt treatment and filtration in a rotary kiln.

The digestion residues which, depending on the raw materials used, essentially consist of titanium dioxide, silicon dioxide, aluminum and iron oxide and adhering metal sulfates, eg titanyl sulfate, iron sulfate, magnesium sulfate, aluminum sulfate and adhering sulfuric acid, are separated off by conventional solid / liquid separation processes such as sedimentation and filtration , These process steps remove largely, but not completely, the soluble TiO 2 components still adhering to the digestion residues and the adherent metal sulfates and sulfuric acid.

The resulting in the solid / liquid separation processes as sediment or filter cake digestion residues are mashed with water and / or dilute sulfuric acid and deposited after neutralization with usually calcium hydroxide suspension and re-filtration.

The disadvantage of this approach is in economic terms, the variety of apparatus and process steps and in ecological terms, the increase in the volume of waste or landfill by the resulting in the neutralization of calcium sulfate (gypsum). Another disadvantage is the increase in consumption

Neutralizing agent, which is due to the residue of digestion adhering and not washed out sulfuric acid. Also problematic are the adhering to the digestion residues metal sulfates. In addition, the mixture of digestion residues and gypsum can only be insufficiently dewatered. This complicates the handling and transport, since this mixture has residual moisture of well over 25% and also behaves thixotropic. Furthermore, filtrates of several filtration and washing steps, the different

Compositions and different pHs (acidic to slightly alkaline) have to be treated and treated for proper disposal. This treatment and workup is complex in terms of apparatus,

costly and burdened by the disposal of the environment.

Depending on the raw materials used and the yield of the digestion reaction, the digestion residue may still contain from 20 to 60% of titanium dioxide. Instead of depositing this residue, it is desirable to make the remaining Ti0 ₂ content available.

Thus, DE 29 51 749 C2 describes a process in which 5 to 95% of a titanium dioxide-containing digestion residue, which was obtained via rotary filtration with subsequent washing, digested together with 95 to 5% finely divided slag in sulfuric acid with a content of> 86% becomes. In DE 40 27 105 A1 a method is described in which the Digestion residue is concentrated with concentrated sulfuric acid under the supply of energy, for example in screws, rotary tubes or similar apparatus.

According to the processes described, the digestion residues not further pretreated after rotary filtration and washing are difficult to handle because of their high residual moisture (for example 30%), require higher sulfuric acid concentrations and the supply of energy for the digestion reaction and have a strongly corrosive effect due to the content of adhering sulfuric acid.

DE 19725018 and DE 19725018 21 B4 disclose processes for processing the digestion residues which, however, are still capable of being optimized with regard to the process steps and material flows despite improvements aimed at over the state of the art.

According to EP 1443121 A1, the digestion residues obtained in the digestion of titania-containing raw materials with sulfuric acid are already filtered in a diaphragm chamber filter press and the filtercake containing the digestion residues is neutralized with a basic solution or suspension, however, the process continues with respect to the material flows and their use improvement.

The object of the invention is to overcome the disadvantages of the prior art and in particular to provide a process for the preparation of resulting in the production of titanium dioxide by the sulfate process digestion residues, which is less expensive to carry out.

On the part of the inventors it was found that the filter cake in the membrane chamber filter press, here used synonymously to membrane filter press, wash completely and reproducibly and then neutralize in the membrane chamber filter press partially to completely and all accumulating wash filtrate of different composition and different pH in the process for Production of titanium dioxide can continue to be used. Accordingly, according to the invention, at least one wash and / or neutralization are preferably carried out in a filtration apparatus after the actual filtration to separate the black solution. Accordingly, according to the invention, at least one wash and / or neutralization and then preferably a further wash are also carried out in a filtration apparatus after the actual filtration to remove the black solution. The resulting wash filtrates can be completely recycled in the process. According to the invention, it is thus possible to provide a wastewater-free process, as well as to make the digestion residues treated according to the invention accessible to different utilizations and thus to optimize the material flows.

Thus, the inventors have taken it into account to make the Ti0 ₂ content still present in the digestion residues usable, for example by renewed reaction with conc. Sulfuric acid to obtain titanium dioxide or by using the residues as an additive to increase the strength and resistance of refractory linings of containers / furnaces in metallurgical and other high-temperature processes.

In one embodiment, the invention relates to a process for the production of titanium dioxide by the sulphate process in which titanium-containing raw materials are digested with sulfuric acid to form a digestion cake, the digestion cake is dissolved with the addition of sulfuric acid and / or water, the resulting solids-containing solution in a solids-free filter solution , which is processed for the production of metatitanic acid and further to titanium dioxide, and digestion residues is separated, wherein the digestion residue-containing filter cake is washed with dilute sulfuric acid and / or water and the resulting filtrate is preferably completely added to dissolve the digestion cake. The sulfuric acid used for washing the filter cake is preferably a weakly acidic filtrate from the washing of the metatitanic acid. "Schwachsauer" means in the context of the invention, a solution having a sulfuric acid concentration of about 20 g / l to about 300 g / l, preferably with a sulfuric acid concentration of 80 to 200 g / l.

The digestion solution from the process according to the invention is preferably completely removed from the undissolved constituents (digestion residues, gangue) by sedimentation and filtration processes. According to the invention, these digestion residues are aftertreated as described above, and the resulting wash filtrates can be reused in the process. In a further process step, a metatitanic acid suspension is prepared from the solids-free digestion solution, by hydrolysis. The metatitanic acid is calcined after washing, bleaching and optionally salt treatment and filtration in a rotary kiln.

Advantageously, the dilute sulfuric acid obtained as a filtrate during the washing of the digestion residues may be added to the sulfuric acid used to dissolve the digestion cake.

Further, after washing the filter cake from digestion residues with dilute sulfuric acid, the filter cake may be washed with water, and the resulting filtrate may be added to the sulfuric acid used to dissolve the digestion cake.

By combining the individual washing steps with washing media, each having a distinctly different pH (dil. Sulfuric acid from the washing of the metatitanic acid with a pH of 1, process water with a pH of about 7 and dilute sodium hydroxide solution with a pH value of about 10-12), it is surprisingly possible, the consumption of process water to 0.2 to 3 m ³ / t filter cake (dry), but in particular to 1, 0 to 2.6 m ³ / t To restrict filter cake (dry) and the consumption of eg sodium hydroxide solution at neutralization of the filter cake to 5 to 100 kg NaOH (indicated as 100%) / t filter cake (dry) but in particular to 15 to 60 kg NaOH (indicated as 100%) / t Filter cake (dry) to restrict. In a further embodiment of the method, the filter cake after washing with dilute sulfuric acid and / or water in the membrane chamber filter press can be neutralized with a basic solution. Surprisingly, this filtrate, which reacts alkaline, can be completely added to the solution for dissolving the digestion cake, without resulting in a possible premature hydrolysis of the Titanylsulfatlösung formed upon dissolution of the digestion cake. Preferably, the neutralization is carried out with an alkali metal hydroxide solution or an alkaline earth metal hydroxide suspension. Particularly preferred are NaOH or Ca (OH) ₂ . The concentration of the base-reacting solution or suspension is preferably 0.1 to 10 wt .-%, particularly preferably 0.1 to 4 wt .-%.

Since it is for the reaction of titanium dioxide-containing raw materials with conc. Sulfuric acid is required to increase the raw materials to an optimal particle size for the reaction (usually 90% <45 μηπ) are also the digestion residues already in a required for a re-digestion particle size. This particle size can advantageously be reduced by further grinding with the titanium-containing raw materials, but is already suitable for another use e.g. as a filler, in the refractory industry or as an additive for metallurgical processes very well suited.

Therefore, in a further embodiment of the process according to the invention, the titanium dioxide-containing raw materials (eg slag or ilmenite) are admixed with the neutralized digestion residues and subsequently dried, ground and digested with sulfuric acid in a known manner. The digestion residues are preferably admixed with the raw materials in a proportion of from 1 to 10% by weight, based on the mixture of raw materials and residues. Particularly preferred is a proportion of 2 to 5 wt .-%.

Thus, according to the invention, various embodiments of the method according to the invention are possible, which include the following steps after digestion with sulfuric acid: 1 . Filtration of the digestion solution

 dehumidifying the filter cake in the membrane filter press (so-called.

 Membrane presses)

 o washing the filter cake with dilute sulfuric acid as recirculated filtrates from the filtration and from the washing of the metatitanic acid

 o optional dehumidifying of the filter cake in the membrane filter press

2. Wash the filter cake with water

 o optional dehumidifying of the filter cake in the membrane filter press

3. Neutralize (partial or full neutralization) of the filter cake with dilute sodium hydroxide solution

 o optional dehumidifying of the filter cake in the membrane filter press o optional washing of the filter cake after neutralization

 o optional dehumidifying of the filter cake in the membrane filter press

4. optional dry-blowing of the filter cake in the membrane filter press with air having up to 200 ° C preheated air;

 5. Opening and emptying the membrane filter press.

An embodiment of the method according to the invention comprises steps 1, 2, 4 and 5, another steps 1, 3, 4 and 5, a further steps 1 to 5. It is advantageous if between the washing steps with water and dehumidification steps be interposed. A final dry-blowing of the filter cake is also advantageous.

The steps for washing and / or neutralizing the filter cake are preferably also carried out in the membrane filter press according to the invention. It is particularly advantageous if, after the filtration step and the subsequent so-called membrane pressing, a membrane pressure of 1.5 to 10 bar, preferably 2-7 bar, is maintained during the subsequent washing steps. As a result, the filter cake formed during the filtration is stabilized, and it is prevented during the subsequent washing steps leaching caused by the washing medium in the filter cake (channeling), which lead to non-reproducible wash results. The washing with water after neutralization has the advantage that the excess of alkali or unreacted alkali metal salts such. Example NaOH NaS0 ₄ and others can be removed. In this way, especially a low-alkali aggregate can be produced. Thus, according to the invention, such processes with steps 1 and 3, if necessary with washing step 2 between steps 1 and 3, followed by a further washing step with water to reduce the alkali content are advantageous.

In all the process variants according to the invention, the combined filtrates of the individual washing steps can be added to the sulfuric acid used to dissolve the digestion cake, without adversely affecting the dissolution of the digestion cake and the quality of the resulting digestion solution due to these additions. Preferably, the filtrates of the individual process steps 1 to 5 are derived independently of their individual combinations into a single buffer vessel. This container also serves to absorb sulfuric acid-containing filtrates from the filtration of metatitanic acid.

By combining and controlling the flow rates of the acidic filtrates from the filtration of metatitanic acid with the flow rates of the acidic, neutral and weakly basic filtrates of process steps 1-5 in this container ensures that the pH of the filtrate collected in this container always in acidic area lies. Only this allows the complete use of all o.a. Filtrate streams ("dissolving water") for dissolving the digestion cake, which is formed in the reaction of titanium dioxide-containing raw materials with concentrated sulfuric acid. The direct use of alkaline wash filtrates to dissolve the digestion cake could already result in the dissolution process to an undesirable premature hydrolysis of existing in the digestion cake titanyl sulfate and significantly affect further processing of the black solution.

Thus, the process works wastewater-free, which also means that no costs incurred for a wastewater treatment. The embodiment of the process with steps 1, 2, 4 and 5 is preferred. Even in this embodiment, the content of the sulfuric acid still adhering to the filter cake is <0.05% by weight and the residual moisture is about 20% by weight. The filter cake is usually firm to crumbly and is not thixotropic. The pH value of the filter cake is, for example, about 3. This thus optionally slightly acidic filter cake can then after emptying the filter press by mechanical mixing with small amounts of a neutralizing agent such as alkaline earth metal oxides, alkaline earth metal hydroxides and / or alkaline earth carbonates targeted to pH values between 3 and 1 1, preferably 5-9, mix. Calcium and / or magnesium carbonates, hydroxides or oxides are preferably used. The use of alkaline reactive fly ash or hydraulic binder is also possible. This procedure reduces the cycle time of the filtration and washing process and thus increases the capacity of this part of the plant in an economical manner. At the same time neutralized digestion residues for applications such as eg as an aggregate for metallurgical processes are provided in an economically advantageous manner.

Furthermore, the execution of the method with steps 1, 3, 4 and 5 is preferred. In this embodiment, the pH of the filter cake is 8-10 and the residual moisture content is also about 20 wt .-%. The filter cake is usually also firm to crumbly and not thixotropic.

Thus, the process of the invention allows the provision of digestion residues as a filter cake, which then due to the low content of alkali (alkali and / or alkaline earth metal salts) very well as a titanium-containing additive to increase the fire resistance in high temperature equipment is used.

The mode of action of the titanium-containing additives when used in metallurgical vessels or in refractory products to increase the durability of the wall containers and refractory materials used is based on the formation of high-temperature resistant and wear-resistant Titanium compounds such as. B. titanium carbonitrides. These titanium compounds form under the action of high temperatures.

The use of titanium-containing aggregates in metallurgy and in refractory systems therefore requires that the titanium dioxide-containing aggregate meet certain requirements. Even small proportions of accompanying substances can exert harmful influence on the hot strength and corrosion resistance of the refractory products.

These refractory products are used for the lining of furnace vessels and furnace spaces. At an operating temperature of 600 to 1,600 ° C, these masonries are exposed by the volatilization of alkali salts high chemical corrosive loads. The alkalis settle on the refractory linings, infiltrating them and causing corrosion.

Especially with the addition of titanium-containing materials in a furnace such. B blast furnace, by injection or otherwise shall be protected by the formation of titanium carbonitrides and deposition of these highly wear-resistant compounds on the surface of the masonry, the durability of refractory linings against infiltration of slag and pig iron and chemical attack (eg alkalis) and thus increase the durability , However, if the aggregate contains alkalis as a titanium-containing material, the formation and deposition of the titanium carbonitrides on the furnace linings is adversely affected and the protective effect is significantly minimized. The chemical-corrosive alkali salts are essentially u. a. oxides, hydroxides, sulfates and carbonates of alkali metals such as sodium and potassium.

Another important parameter in the provision of the aggregate for the applications mentioned is the filter-moist state. Depending on the moisture of the filter cake this can be considerable difficulties (bonding, liquefaction, etc.) during transport, storage and procedural problems in the further processing in the various facilities such. B. on conveyor belt and delivery units, Siloaustrag, Generate mixing plant and others. In particular, the filter cake tends to thixotropic behavior with increasing humidity. This thixotropic property is the more pronounced the finer the product.

In addition, it is necessary for sea transport that the filter-moist bulk materials meet the international requirements regarding Flow Moisture Points (FMP) and Transportable Moisture Limits (LMT).

Surprisingly, according to the inventive method, such an additive is available, which meets the aforementioned requirements.

The invention therefore also provides a low-alkali aggregate which is obtainable from the digestion residues obtained in the production of titanium dioxide by sulphate processes, with a content of alkali, calculated as Na ₂ O + K ₂ O of less than 3% by weight, preferably from less than 2% by weight and especially less than 1% by weight, with a residual moisture content of less than 26% by weight, preferably less than 20% by weight, and having a pH between 3 and 10, preferably between 4 and 9 and especially between 5 and 8.

The data in% by weight are in each case based on the total weight of the aggregate, as taken from the membrane filter press.

In summary, the advantages of the method described compared to conventional methods in:

 • a significantly reduced equipment of the process

 • A complete leaching of the digestion residues adhering metal sulfates and thus to a minimization of eluierbaren shares in the filter cake

 • a significantly reduced consumption of neutralizing agent

 • a partial to complete neutralization of the digestion residues adhering sulfuric acid

• a reduction in the mass of the resulting filter cake • an improvement in the handling characteristics of the filter cake

• additional utilization possibilities of the neutralized filter cake, such as as a filler, or in the refractory industry or as an aggregate for metallurgical processes.

 • a full use of all accumulating filtrates in the process.

 • Reduction of the cycle time of the filtration and washing process and thus increase the capacity of the plant part

 • Reduction of the alkali content of the filter cake

 • Reduction of the moisture content of the filter cake

By way of example, the process according to the invention can thus be carried out as follows: The titania-containing raw materials (slag, ilmenite) are dried and ground and then digested with concentrated sulfuric acid. The remaining after the reaction solid mass (digestion cake) is dissolved with water and / or dilute sulfuric acid. The digestion solution is separated from the gait by conventional solid / liquid separation processes such as sedimentation and filtration and further processed in the usual way. The separated, solids-containing fraction, ie the digestion residue can be added to improve the pumpability with dilute sulfuric acid. This step can be omitted if a diaphragm chamber filter press is already used in the separation of the digestion solution from the digestion residues. The digestion residue is then filtered in a membrane chamber filter press, optionally washed with sulfuric acid and / or water, optionally neutralized with dilute sodium hydroxide solution and optionally washed with water, and if desired blown dry. The concentration of the dilute sodium hydroxide solution is preferably 0.1 to 10 wt .-%, particularly preferably 0.1 to 4 wt .-%. The resulting optionally neutralized (dried or undried) digestion residue can then be added back to the raw material. All accumulating filtrates can - as described above - reused to dissolve the digestion cake.

Claims

claims 1 . A process for the production of titanium dioxide by the sulphate process, in which titanium-containing raw materials are digested with sulfuric acid to form a digestion cake, the digestion cake is dissolved with the addition of sulfuric acid and / or water, the resulting solids-containing solution in a solids-free filter solution, which is used for the production of metatitanic acid and is further processed to titanium dioxide, and digestion residue is separated, wherein the digestion residue-containing filter cake is washed with dilute sulfuric acid or water and the resulting filtrate is added to dissolve the digestion cake. 2. The method of claim 1, wherein the digestion residue-containing filter cake is washed after washing with sulfuric acid with water and the resulting filtrate is added to the sulfuric acid to dissolve the digestion cake. 3. The method of claim 1 or 2, wherein the digestion residue-containing filter cake is washed after washing with sulfuric acid or water with a basic solution and the resulting filtrate is added to the sulfuric acid to dissolve the digestion cake 4. The method according to claim 3, wherein the alkaline solution used is an alkali metal hydroxide solution or alkaline earth metal hydroxide solution. 5. The method of claim 3 or 4, wherein the digestion residue-containing filter cake after washing with the basic solution is washed with water. 6. The method according to any one of the preceding claims, wherein at least a filtrate is obtained in a filter press, preferably a membrane chamber filter press. 7. The method according to any one of the preceding claims, wherein the resulting filtrates are combined prior to addition to the digestion cake. 8. The method according to any one of the preceding claims, wherein the filter cake is subjected to a drying step prior to treatment with washing solution selected from dilute sulfuric acid, water or a basic solution. The method of claim 8, wherein the drying step is pressure dehumidification in the membrane chamber filter press. 10. The method according to any one of the preceding claims, characterized in that the digestion residue-containing filter cake is mixed with the titanium-containing raw materials and this mixture, if necessary dried in a known manner and ground, is digested with sulfuric acid. 1 1. A method according to claim 10, characterized in that the digestion residues are the titanium dioxide-containing raw materials in a proportion of 1 to 10 wt .-%, preferably from 2 to 5 wt .-%, based on the total weight of the mixture of raw materials and residues admixed. 12. Low-alkali digestion residue, obtainable by the process according to any one of claims 1 to 1 1, with a content of alkali, calculated as Na ₂ 0 and / or K ₂ 0, of less than 3 wt .-%, preferably less than 2 wt % and especially less than 1% by weight, with a residual moisture of less than 26% by weight, preferably less than 20% by weight, and having a pH between 3 and 10, preferably between 4 and 9 and especially between 5 and 8. 13. Use of the digestion residue according to claim 12 as an inert inorganic filler, as an additive in the production of refractory materials and as an aggregate for metallurgical processes. 14. Use of at least one in the process according to any one of claims 1 to 1 1 resulting wash filtrate, preferably all wash filtrates, for dissolving the digestion cake.