FI91270C - Process for the preparation of titanium dioxide pigment - Google Patents

Description

i 91270

A process for preparing a titanium dioxide pigment

The invention relates to a process for the preparation of a titanium dioxide pigment, in which a) the solid titanium dioxide contained in the raw material is converted into a titanium compound in a non-solid state, b) the impurities are separated from the titanium compound into the titanium compound, c) the titanium compound is precipitated in the acid by heat and pressure, and e) the pure titanium dioxide is separated from the acid and further treated, if necessary, to obtain the final titanium dioxide pigment ai-15.

Pigment titanium dioxide (TiO 2) in rutile and anatase crystal forms is prepared commercially by two methods; by the sulphate method and the chloride method.

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In the sulphate process, finely ground, dried ilmenite (FeTiO 3 containing 40-60% TiO 2) and / or titanium dioxide slag (synthetically enriched slag from ilmenite containing 72-87% TiO 2) is reacted with sulfuric acid (mainly sulphate 25). TiSO 4) and ferrous sulfate (FeSO 4), which is then dissolved in a mixture of water and acid solutions recovered from the mydhem steps of the manufacturing process. The solution is treated with a reducing agent such as ferrous scrap to reduce the Ferri ions present to ferro ions. The excess from the clarification following the reduction is passed to a crystallization step in which the solution is cooled to such an extent that a considerable part of the iron contained in the solution crystallizes as ferrous sulphate heptahydrate, which is removed from the solution.

The solution is concentrated by evaporation and hydrolyzed to convert titanyl sulfate to insoluble titanium oxohydrate, which is filtered and washed. The insoluble compound is calcined at a temperature of about 900-1000 ° C to titanium dioxide of the rutile or 2 anatase type. The calcination product is post-treated in suitable grinding, washing, drying and coating steps to a finely divided and homogeneous rutile or anatase type commercial titanium dioxide-5 pigment.

The chloride process uses a purer raw material with a TiO2 content of generally 80-95% by weight. The raw material is mixed with pure, finely divided coke and chlorinated with chlorine gas. As a by-product, the impurities in the ore form mainly solid ferric chloride, which is separated from the titanium tetrachloride, e.g. by means of a cyclone-like separator. Titanium tetrachloride is separated from chlorine gas by condensation and purified by distillation and chemical treatment. The purified titanium tetrachloride is fed via an evaporator and a preheater to an oxidizer where it is burned with oxygen or oxygen-enriched air to titanium dioxide, which is mainly of the rutile type. The fuel gas injected into the oxidation plant is preheated to about 1,000 ° C and fed to the plant myds sand as well as smaller amounts of hydrocarbons to promote the reaction. The sand is separated, dried and graded before being returned to the incineration system. The resulting rutile-type titanium dioxide is post-treated in appropriate grinding, washing, drying and coating steps before being finished into a commercial product.

Because the aforementioned methods of making titanium dioxide pigments are multi-step and are among the most difficult processes in the chemical industry, there is great interest in the field for new processes and improvements to existing processes. U.S. Patent No. 4,107,264 relates to an improved process for separating titanium dioxide from rhodium-type ores, in which the ore is extracted into aqueous hydrofluoric acid, the resulting solution is separated from iron solution to precipitate iron oxide, and the hydrated titanium dioxide is precipitated from a non-ferrous solution of ammonium. Japanese Patent No. 3,912,701 discloses the preparation of alkaline ion stabilized crystalline anatase-type titanium dioxide sols by treating water-soluble titanium compounds with alkali metal hydroxides, carbonates and / or ammonium salts over a gel to form a gel. n låmpoti-lassa. According to an example of the publication, an aqueous solution of titanium tetrachloride was treated with ammonium hydroxide to form a gel. After washing, additional ammonium-10 hydroxide was mixed into the gel and then hydrothermally treated at 160 ° C. As a result, titanium dioxide diol containing anatase-type crystals was obtained.

It appears from the above two publications that the pigment -15 can be further precipitated from a solution of titanium compound with ammonia or ammonium hydroxide. However, U.S. Pat. No. 4,092,607 discloses the use of such a precipitate to recover titanium extracted into hydrofluoric acid, whereby the precipitated product is calcined, and JP Patent Publication No. 20 is directed solely to the preparation of alkali ion-stabilized titanium dioxide sols. Thus, these publications are not able to solve the problems encountered in the current commercial methods, which e.g. related to the use of expensive and energy-intensive calcination and combustion steps 25.

The object of the present invention is to provide a method and an apparatus by means of which said problems can be solved. The invention is then mainly characterized by what is stated in the preamble of the claims. Thus, it has been found that the titanium dioxide production can be improved by carrying out the precipitation of step (c) at a temperature below 30 ° C; in step (d) the hydrothermal crystallization is carried out at a temperature below 300 ° C, at a pressure below 100 bar and at an acid-35 concentration of less than about 15% by weight; and the acid separated from the titanium dioxide in step (e) is recycled back to the hydrothermal crystallization step (d) to give titanium dioxide crystals suitable for pigment preparation.

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The hydrothermal crystallization step is in principle applicable to all processes for the preparation of titanium dioxide pigments in which the phase change of the titanium raw material, its washing operation and its conversion into titanium dioxide are carried out. Such methods include e.g. the aforementioned sulfate, chloride and hydrogen fluoride processes. Particularly preferred are the improved sulfate and chloride methods of the general. In the sulphate process, the preparation of rutile requires that the titanium hydrate mass precipitated in the non-embryonic form is neutralized e.g.

10 with ammonia before its hydrothermal crystallization. In the chloride process, the hydrothermal crystallization requires that the precipitation step preceding it be carried out with an alkali such as alkali hydroxide, alkali carbonate, ammonium hydroxide and / or a salt of a weak organic acid, or diluted with water.

The process of the present invention involves the replacement of the cumbersome and expensive incineration or calcination step with a better step in all titanium dioxide pigment production processes with such steps. The invention thus covers all variants of the titanium dioxide production processes, preferably the sulphate and chloride processes, such that the conventional steps for calcination and combustion of the titanium compound have been replaced by a hydrothermal crystallization step.

The chloride process according to a preferred embodiment of the invention comprises, as a first step, the chlorination of a high-quality raw material by means of chlorine gas in the presence of coke. The raw material can be any pure titanium dioxide raw material, such as synthetic or naturally occurring rutile (95% by weight TiO 2) and / or leukoxene / very pure ilmenite and / or very pure titanium oxide slag. In general, it can be said that a suitable titanium dioxide content of the raw material of the chloride process is between 80 and 95% by weight.

The raw material is mixed with ground, very pure coke and the chlorination is typically carried out in a fluidized bed reactor, preferably at a temperature of 850-1,100 ° C.

5,91270

The chlorination results in the exothermic reaction to give crude titanium tetrachloride, which is normally a liquid titanium compound, but under the reaction conditions in gaseous form. As a by-product, mainly ferrochloride is formed from the ore impurities. The removal of the chlorination reactor is carried out to a solid separator, which is e.g. a cyclone, where solid impurities are separated from the gases. By cooling the gas to a temperature just above the boiling point of titanium tetrachloride, most of the other volatile chlorides, such as iron, manganese and chromium chlorides, can be condensed off. The reaction gas, which consists mainly of titanium tetrachloride and chlorine gas, is then introduced into a condensing system in which the tetrane tetrachloride condenses into a liquid form and from which the chlorine gases are passed on to their recovery and purification units. Titanium tetrachloride is purified by column distillation and chemical treatment, whereby e.g. vanadium-like metals stand out.

The purified titanium tetrachloride is then precipitated according to the present invention, e.g. with ammonia (ammonium hydroxide), sodium acetate, sodium oxalate, or diluted with water to a solid product. The precipitation is preferably carried out with ammonium hydroxide in a concentration of 10 to 40% by weight, preferably about 20 to 30% by weight. The temperature does not need to be raised or lowered, but is below 30 ° C, eg room temperature. The precipitate obtained is subjected to hydrothermal crystallization at elevated temperature and pressure with a dilute, preferably inorganic acid, which may be e.g.

30 hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid. The acid concentration is less than about 15% by weight, preferably between 3 and 7% by weight. The temperature is below 300 ° C, preferably between 180-280 ° C. The pressure is less than 100 bar, preferably 5-100 bar, even more preferably 5-50 bar. Under these conditions, the growth time of the crystals is preferably 1 to 24 hours, more preferably 6 to 16 hours. The hydrothermal crystallization is carried out in a pressure vessel to which a solid titanium compound and a dilute 6 acid are added. The dilute acid can be easily recovered and reused substantially as such, which makes hydrothermal crystallization not only a preferred but also an environmentally friendly process step. By the method 5 described above, the crystal size of the titanium dioxide is about 100-300 nm, preferably 150-250 nm, so that the coarse grinding step necessary in the conventional process could be omitted.

Depending on the crystallization conditions of the chloride process described above and the acid used, pure rutile can be obtained. The TiO2 content of rutile obtained with hydrochloric acid is almost 100%. In the preparation of rutile, hydrochloric and / or nitric acid is preferably used, while sulfuric and hydrofluoric acid can also be used in the preparation of anatase. It is now common for the basic pigment obtained from the titanium dioxide production process to be xnodified by a surface treatment process to improve its pigment properties. In this alternative step according to the prior art, compounds of aluminum, silicon, titanium and other substances are precipitated on the surface of the titanium dioxide particles. The proportions of the additives and the total amount of coating are adjusted for each use of the pigment.

At the end of the chloride process, the pigment is dried and finely ground before packing in sacks.

In the first step of the improved sulphate process according to another even more preferred embodiment of the invention, finely ground, dried ilmenite or titanium oxide slag 30 is charged to large reactors in which an exothermic reaction with sulfuric acid is initiated by raising the temperature of the mixture to superheated heat. The reaction temperature is between about 150-180 ° C, depending on the titanium raw material. The reaction product is obtained in the form of a solid, porous cake which is dissolved in a mixture of water and acid solutions recovered from the milder steps of the manufacturing process. Dissolution gives a solution of titanyl sulphate (TiSO 4) and ferrous sulphates.

7 91270

If ilmenite is used as a raw material, the solution obtained from the first step is preferably reduced by means of ferrous scrap to convert the 3-valued iron to the 2-valued. The reduction is carried out to such an extent that a small part of the 4-valent titanium in the solution becomes 3-valent, which ensures that the 3-valent iron does not precipitate later in the process. Undissolved impurities are removed by clarification in large scavengers, preferably using clarification aids.

10 The alite, or waste sludge, is filtered and washed to recover soluble titanium. The clarification excess is subjected to a crystallization step in which the solution is frozen to such an extent that a considerable part of the iron contained in the solution crystallizes as ferrous sulfate heptahydrate. The crystallized solution is filtered if necessary and, if necessary, concentrated by evaporation. The solution thus obtained is transferred to precipitation tanks in which titanium is solidified by hydrolysis.

- The hydrolyzed titanium hydroxide precipitate is separated, washed and, if necessary, extracted in sulfuric acid under reducing conditions to remove the final impurities.

Up to this stage, the methods for preparing rutile and anatase are identical, but in the future the methods will be different. If necessary, a small amount of additives can be added to the pulp, which in hydrothermal crystallization provide anatase or rutile if desired. In the process according to the invention, rutile is preferably prepared by neutralizing the titanium hydrate mass with ammonia (ammonium hydroxide), after which the completely neutralized titanium hydrate suspension is crystallized hydrothermally. In the production of anatase, such neutralization with ammonia is not necessary.

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The purified and optionally treated titanium hydroxide mass is then crystallized by hydrothermal treatment at elevated but moderate temperature and pressure with a dilute, preferably inorganic acid, which may be e.g. hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid. The concentration of the acid is less than about 15% by weight, preferably between 3 and 7% by weight. The temperature is below 300 ° C, preferably between 180-280 ° C. The pressure is less than 100 bar, preferably 5-100 bar, most preferably 5-50 bar. Under these conditions, the growth time of the crystals is 1 to 24 hours, preferably 6 to 16 hours. The hydrothermal crystallization is carried out in a pressure vessel to which titanium hydroxide and dilute acid are added. The dilute acid is recovered and recycled as such to the hydrothermal crystallization. The process 15 described above gives almost pigment-grade titanium dioxide, so that the coarse grinding step which is essential in a conventional process can be omitted.

At the end of the sulphate process, the dilute acid used in the hydrothermal crystallization is decanted and / or filtered off and the precipitate is washed with water if necessary. The product is subjected to suitable grinding, washing, drying and coating steps and the finished commercial titanium dioxide pigment is packaged.

The following is a distinction between conventional titanium dioxide chloride and sulfate processes and between the chloride and sulfate processes of some embodiments of the invention using block diagrams that roughly show the different stages of the processes. However, the methods according to the invention may include additional steps and / or replacement steps or may lack steps.

9 91270

Preparation of titanium dioxide by the chloride method; Comparison of methods 1. Production of titanium oxide (rutile) 2. Production of titanium dioxide 5 by the current chloride-chloride hydrothermal diprocessing process (Kemira Inc)

Processing of raw materials Processing of raw materials i i 10 Chlorination or TiCl4 Chlorination or TiCl4 production (1000-1040 ° C) production (1000-1040 ° C) i i

Condensation Condensation i 4 15 Crude titanium tetrachloride Crude titanium tetrachloride 4 4

Cleaning Cleaning 4 i

Titanium tetrachloride h5y- Liquidation of water-diluted titrate and preheating of tetrachloride precipitation (150 ° C <T <500 ° C) with alkali to a solid titanium compound 4 i

Oxygen preheating <1800 ° C Oxidation of washed solid titanium tetrachloride hydrothermal crystals of the compound in acid i i

Combustion of titanium tetrachloride Post-treatment at temperatures above 1000 ° C in a 30 "sand burner" i Post-treatment 10

Preparation of titanium dioxide by the sulphate process; comparison of methods 1. Preparation of titanium oxide (rutile or 2. titanium dioxide 5 anatase) current sulphate hydrothermal by the sulphate process

Handling of raw materials Handling of raw materials 4 4 10 Extraction into sulfuric acid Extraction into sulfuric acid 4 4

Reduction of the solution Reduction of the solution 4 4

Solution clarification and Solution clarification and 15 purification purification i 4

Concentration of the solution Possible concentration of the solution 4 i 20 Hydrolytic precipitation Hydrolytic precipitation 4 4

Filtration and washing Filtration and washing 4 4

Calcination Hydrotemic crystallization in 25 dilute acids 4 4 Post-treatment Post-treatment 30 Example 1

Diluted titanium tetrachloride was precipitated at 25 ° C with 25% ammonium hydroxide to give a solid titanium compound.

35 g of this mass and 1.5 g of hydrochloric acid were metered into a pressure capsule where they were mixed into a homogeneous suspension.

The pressure capsule was transferred to an oven at a temperature of about 250 ° C, where it was kept for 4-24 h.

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After crystallization, the pressure capsule was cooled to room temperature and the phases were separated by centrifugation. The solid was washed and centrifuged and analyzed.

5 Results: rutile content 100% by weight of dry matter, crystal size pigment size (150-200 nm), grindability in the order of the finished pigment.

Example 2 10 g of a titanium compound mass prepared as described above and 1.5 g of glacial acid (hydrochloric acid) were metered into a pressure capsule. The pressure capsule was kept in an oven at about 250 ° C for about 16 h, after which the phases were post-treated as mentioned above.

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Results: pigment-sized rutile, rutile% by weight of dry matter: 100.

This allows for the complete recycling of residual acid, which makes the process environmentally friendly.

Example 3

A total of 19.2 g of Ti hydrate and nitric acid were added to the pressure capsule in a ratio of 5/1. The pressure capsule was kept at 200-260 ° C for 6-24 h. After crystallization, the post-treatment was performed as described above.

Anatase formed in the lower temperature range and rutile formed in the higher temperature range.

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Example 4

The titanium hydrate mass of the sulphate process (non-precipitated and precipitated) was metered into a pressure capsule of about 20 g. The solution concentration was adjusted to 4-20% with either sulfuric acid or hydrochloric acid or nitric acid or hydrofluoric acid. The pressure capsule was kept at 160-260 ° C for 16 h, after which the removal of the reactor was treated as described above. The result was a pigmented anatase.

Claims (25)

A process for producing a titanium dioxide pigment in which a) in a solid titanium dioxide present in a raw material is converted to a titanium drain in a non-solid state, b) the impurities are separated from the titanium compound, c) the titanium compound is precipitated, d) the resulting titanium compound is crystallized and (e) the pure titanium dioxide is separated from the acid and post-treated if necessary to produce the final titanium dioxide pigment, characterized by the precipitation in step c) being carried out at a temperature below 30 ° C; in step d) the hydro-thermal crystallization is carried out at a temperature below 300 ° C, at a pressure below 100 bar and at an acid concentration below about 15% by weight; and the acid separated from the titanium dioxide in step e) is recycled to the hydrothermal crystallization step d) to obtain suitable titanium dioxide crystals for pigment production. 2. A process according to claim 1, characterized in that the titanium compound obtained from step c) is washed prior to its hydrothermal crystallization. A process according to claim 1 or 2, characterized in that the titanium dioxide is optionally washed after the hydrothermal crystallization. 4. A process according to any one of the preceding claims, characterized in that the acid used in the hydrothermal crystallization of step d) is an inorganic acid, for example hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid, sheep wire hydrochloric acid or nitric acid. 91270 Process according to one of the preceding claims, characterized in that the concentration of the acid used in the step d) hydrothermal crystallization is below about 15% by weight, preferably between about 3 and 7% by weight. 5 Process according to any of the preceding claims, characterized in that the pressure in the hydrothermal crystallization step d) is between 5 and 100 bar, preferably between 5 and 50 bar. 10 7. A process according to any one of the preceding claims, characterized in that the temperature of the hydrothermal crystallization step d) is below 300 ° C, preferably between 180 and 280 ° C. 15 Method according to one of the preceding claims, characterized in that the hydrothermal crystallization in step d) lasts between 1 and 24 hours, preferably between 5 and 16 hours. 20 9. A process according to any one of the preceding claims, characterized in that the hydrothermal crystallization in step d) is carried out in a pressure cooker. The process according to any of the preceding claims, characterized in that the finishing step d) comprises washing, grinding, drying and / or finishing steps. A process according to any one of the preceding claims, characterized in that step a) comprises chlorination of the raw material to titanium tetrachloride by chlorine gas in the presence of coke, step b) comprises separation of sheep impurities from the titanium tetrachloride, step c) comprises precipitation of the purified the titanium tetrachloride to a solid product by an alkali or a salt of a weak organic acid or by dilution with water and any washing of the precipitate, and step d) includes a hydrothermal crystallization of the precipitate. 12. Process according to claim 11, characterized in that the raw material is rutile. 13. Process according to claim 11 or 12, characterized in that the chlorination step a) is carried out in the temperature 850 - 1100 ° C. Process according to claim 11, 12 or 13, characterized in that the precipitate in step c) is carried out by means of alkali metal hydroxide, alkali metal carbonate or ammonium hydroxide, our concentration is preferably between 10 and 40% by weight, more preferably between 20 and 30%. weight-%. 15. A process according to claim 11 or 12, characterized in that the precipitate in step c) is carried out by a salt of a weak organic acid, preferably with sodium acetate or sodium oxalate. Process according to any one of claims 11-15, characterized in that the precipitation in step c) is carried out below 30 ° C, preferably at room temperature. Process according to any of claims 11-16, characterized in that the diluted acid in the step d) hydrothermal crystallization is hydrochloric or nitric acid when rutile is produced. Process according to any one of claims 11-16, characterized in that step d) in the chloride process is carried out such that the obtained crystal size of the pure titanium dioxide is about 150-250 nm. 91270 A method according to any one of claims 1-11, characterized in that step a) comprises locking the raw material by sulfuric acid and water, step b) comprises washing and concentrating the locking obtained from step a), step c) comprises hydrolyzing the concentrated lock to a titanium hydroxide precipitate, collecting and washing the precipitate, and step d) comprises hydrothermal crystallization of the precipitate. 10 20. Method according to claim 19, characterized in that ilmenite is used as raw material. 21. Process according to claim 19 or 20, characterized in that the locking reaction a) is carried out at about 150-180 ° C. 22. A method according to any one of claims 19-21, characterized in that the locking is reduced at the beginning of step b), if the raw material is ilmenite or the equivalent. 20 Process according to any one of claims 19-22, characterized in that the precipitation is failed by the addition of a crystallization initiator in the precipitation step c). 25 24. A process according to any one of claims 18-23, characterized in that the precipitate obtained from the precipitation in step c) neutralizes the lower step d) hydrothermal crystallization of the step, if rutile is prepared by the dyeing process. 25. Process according to claim 24, characterized in that the precipitate is neutralized with ammonia (ammonium hydroxide).