FI91270B - Process for making titanium dioxide pigments - Google Patents

Description

91270

Process for the preparation of 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, c) the titanium compound is precipitated and by pressure, and e) the pure titanium dioxide is separated from the acid and, if necessary, post-treated to give 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 developed slag from ilmenite containing 72-87% TiO 2) is reacted with sulfuric acid 25 to give mainly titanyl sulphate ( TiOSO 4) and ferrous sulfate (FeSO 4), which is then dissolved in a mixture of water and acid solutions recovered from later stages of the manufacturing process. The solution is treated with a reducing agent such as iron scrap to reduce the present Ferri ions to ferro ions. The excess 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 rutile or anatase type titanium dioxide. 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 10 chlorine gases. 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 through an evaporator and a preheater to an oxidation plant where it is burned by means of oxygen or oxygen-enriched air to titanium dioxide, which is mainly of the rutile type. The fuel gas fed to the oxidation plant is preheated to about 1,000 ° C and sand and smaller amounts of hydrocarbons are also fed to the plant to promote the reaction. The sand is separated, dried and graded before being returned to its incineration system. The resulting rutile-type titanium dioxide is post-treated in appropriate grinding, washing, drying and coating steps before it is a finished 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 art 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, iron impurities are separated from the resulting solution, titrated titanium dioxide is precipitated from the iron-free solution and ammonium hydroxide is precipitated with ammonium hydroxide. Japanese Patent No. 3,91270 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 at 100 ° C to form a gel, followed by hydrolysis of the gel at 100 ° C. . 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 hydroxide was mixed into the gel and then hydrothermally treated at 160 ° C. The result was a titanium dioxide salt containing anatase-type crystals.

It appears from the above two publications that the titanium compound which can be further processed into pigment can be precipitated from solution 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 current commercial methods, which e.g. related to the use of expensive and energy-intensive calcination and incineration steps 25.

It is an object of the present invention to provide a method and 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. It is thus realized 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 obtain 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 sulphate, chloride and hydrogen fluoride processes mentioned above. In this case, the improved sulphate and chloride processes are particularly preferred. In the sulphate process, the production of rutile requires that the titanium hydrate mass precipitated in the non-embryonic form be 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 dilute water with water.

The process of the present invention involves replacing the cumbersome and expensive incineration or calcination step with a better step in all titanium dioxide pigment manufacturing processes having such steps. The invention thus covers all variants of the titanium dioxide production processes, preferably the sulphate and chloride processes, so that the conventional steps of 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 feedstock is mixed with ground, high purity coke and the chlorination is typically performed 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, ore impurities 5 mainly form ferric chloride. The effluent from the chlorination reactor is passed to a solids 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 passed to a condensing system in which the titanium tetrachloride condenses into a liquid form and from which the chlorine gases are passed on to a recovery and purification unit. 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 by dilution 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, e.g. 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 between 6 to 16 hours. The hydrothermal crystallization is carried out in a pressure vessel to which a solid titanium compound and a dilute acid 6 are added. The dilute acid can be easily recovered and reused substantially as such, making hydrothermal crystallization not only an advantageous but also an environmentally friendly process step. The method 5 described above gives a titanium dioxide crystal size of about 100 to 300 nm, preferably 150 to 250 nm, so that the coarse grinding step necessary in a 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%. Hydrochloric and / or nitric acid is preferably used in the preparation of rutile, while sulfuric and hydrofluoric acid can also be used in the preparation of anatase. It is now common for the base pigment obtained from the titanium dioxide manufacturing process to be modified by a surface treatment process to improve its pigment properties. In this alternative, prior art step, compounds of aluminum, silicon, titanium and other substances are deposited 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 by superheated steam. 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 later stages 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 scrap iron 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-value titanium in the solution is made 3-value, which ensures that 3-value iron does not precipitate later in the process. Undissolved impurities are removed by clarification in large tanks, preferably using clarification aids.

10 The alite or waste sludge is filtered and washed to recover soluble titanium. The excess clarification 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 vacuum evaporation. The solution thus obtained is transferred to precipitation tanks in which titanium is solidified by hydrolysis.

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

Up to this point, the methods for preparing rutile and anatase are identical, but in the future the methods will be different. If necessary, small amounts of additives can be added to the pulp, which, if desired, produce anatase or rutile in the hydrothermal crystallization. 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 hydrothermally by 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 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, 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 hydrothermal crystallization. The method 15 described above gives titanium dioxide of almost pigment quality, so that the coarse grinding step necessary in the 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 chloride and sulfate processes according to some embodiments of the invention using block diagrams that roughly show the different stages of the processes. However, the methods of the invention may include additional steps and / or replacement steps, or may lack steps.

91270 9

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)

Handling of raw materials Handling of raw materials 4 4 10 Chlorination or TiCl4 Chlorination or TiCl4 production (1000-1040 ° C) production (1000-1040 ° C) 4 4

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

Cleaning Cleaning 4 4

Vapor titration and preheating of titanium tetrachloride (150 ° C <T <500 ° C) with alkali to give a solid titanium compound 4 4

Oxygen preheating <1800 ° C Oxidation of the washed solid titanium-titanium tetrachloride for hydrothermal crystallization in acid 4 4

Combustion of titanium tetrachloride After-treatment at temperatures above 1000 ° C in a 30 "sand dust" burner 4 After-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 that sulphate process production method

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 4 4

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

Filtration and washing Filtration and washing 4 4

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

Diluted titanium tetrachloride was precipitated below 25 ° C with 25% ammonium hydroxide to form 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.

91270 11

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 class of finished pigment.

Example 2 16 g of a titanium compound mass prepared as above and 1.5 g of residual 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 complete recycling of the residual acid, which makes the method 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 was formed in the lower temperature range and rutile was 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 35 or nitric acid or hydrofluoric acid. The pressure capsule was kept at 160-260 ° C for 16 h, after which the reactor discharge was treated as described above. The result was a pigmented anatase.

Claims (25)

A process for preparing a titanium dioxide pigment, wherein 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, c) the titanium compound is precipitated by acidification, , 10 and e) the pure titanium dioxide is separated from the acid and, if necessary, treated to obtain the final titanium dioxide pigment, characterized in that the precipitation of step c) is carried out 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 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. Process according to Claim 1, characterized in that the titanium compound obtained by precipitation from step c) is washed before it is subjected to hydrothermal crystallization. Process according to Claim 1 or 2, characterized in that, after hydrothermal crystallization, the titanium dioxide is optionally washed. Process according to one of the preceding claims, characterized in that the acid used in the hydrothermal crystallization of step d) is an inorganic acid, e.g. hydrochloric acid, nitric acid, sulfuric acid or hydrofluoric acid, preferably 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 hydrothermal crystallization of step d) is about less than about 15% by weight, preferably between about 3 and 7% by weight. 5 Process according to one of the preceding claims, characterized in that the pressure of the hydrothermal crystallization of step d) is between 5 and 100 bar, most preferably between 5 and 50 bar. 10 Process according to one of the preceding claims, characterized in that the temperature of the hydrothermal crystallization of step d) is below 300 ° C, preferably between 180 and 280 ° C. 15 Process according to one of the preceding claims, characterized in that the hydrothermal crystallization of step d) takes 1 to 24 hours, preferably 6 to 16 hours. Process according to one of the preceding claims, characterized in that the hydrothermal crystallization of step d) is carried out in a pressure vessel, the pressure of which after the step is preferably used for recycling the dilute acid. 25 Method according to one of the preceding claims, characterized in that the post-treatment step e) comprises washing, grinding, drying and / or surface treatment steps. 30 Process according to any one of the preceding claims, characterized in that step a) comprises chlorinating the raw material to titanium tetrachloride with chlorine gas in the presence of coke, step b) comprises separating impurities from titanium tetrachloride, step c) comprises precipitating purified titanium tetrachloride as a solid with alkali or weak organic acid by diluting with water and optionally washing the precipitate, and step d) comprises hydrothermal crystallization of the precipitate. Process according to Claim 11, characterized in that rutile is used as raw material. Process according to Claim 11 or 12, characterized in that the chlorination step a) is carried out at a temperature of 850 to 1100 ° C. 10 Process according to Claim 11, 12 or 13, characterized in that the precipitation of step c) is carried out with alkali metal hydroxide, alkali metal carbonate or ammonium hydroxide, the concentration of which is preferably between 10 and 40% by weight, more preferably between 20 and 30% by weight. Process according to Claim 11 or 12, characterized in that the precipitation of step c) is carried out with a salt of a weak organic acid, preferably sodium acetate or sodium oxalate. Process according to one of Claims 11 to 15, characterized in that the precipitation of step c) is carried out at a temperature below 30 ° C, preferably at room temperature. Process according to one of Claims 11 to 16, characterized in that the dilute acid of the hydrothermal crystallization of step d) is hydrochloric acid or nitric acid when rutile is prepared. Process according to one of Claims 11 to 16, characterized in that step d) of the chloride process is carried out in such a way that the crystal size 35 of pure titanium dioxide is obtained to be about 150 to 250 nm. Process according to any one of claims 1 to 11, characterized in that step a) comprises dissolving the raw material 91270 with sulfuric acid and water, step b) comprises purifying and concentrating the solution obtained from step a), step c) comprises hydrolysing the concentrated solution to titanium. to a dihydroxide precipitate, recovering and washing the precipitate, and step d) comprises hydrothermal crystallization of the precipitate. Process according to Claim 19, characterized in that ilmenite is used as raw material. Process according to Claim 19 or 20, characterized in that the dissolution reaction a) is carried out at about 150 to 180 ° C. Process according to one of Claims 19 to 21, characterized in that the solution is reduced at the beginning of step b) if the raw material is ilmenite or the like. Process according to one of Claims 19 to 22, characterized in that a crystallization initiator is added in the precipitation of step c) in order to facilitate the precipitation. Process according to one of Claims 18 to 23, characterized in that the precipitate obtained from the precipitation of step c) is neutralized before the hydrothermal crystallization of step d), if rutile is produced by the process. Process according to Claim 24, characterized in that the precipitate is neutralized with ammonia (ammonium hydroxide).