A new method of preparing titanium dioxide

New method of manufacturing titanium dioxide

The invention relates to a new process for the preparation of microcrystalline titanium dioxide with a crystal size <100 nm, which process comprises the steps of:

(a) solid titanium dioxide hydrate is treated with a base;

(b) the alkali - treated precipitate is treated with hydrochloric acid; and

(c) the precipitate treated with base and hydrochloric acid is neutralized.

The properties of microcrystalline titanium dioxide differ from those of conventional titanium dioxide known as a white pigment. The differences are due to the difference in crystal size, as the crystal size of microcrystalline titanium dioxide (10-100 nm) is about 5-10 times smaller than the crystal size of normal titanium dioxide (160-250 nm). As the crystal size decreases, the opacity of titanium dioxide in the visible light area disappears and the pigment becomes transparent. On the other hand, the transmittance of UV radiation decreases. Microcrystalline titanium dioxide is therefore suitable as a UV protection agent. Due to the small crystal size and large specific surface area, microcrystalline titanium dioxide has uses e.g. in catalysts, ceramics and paints as an effect pigment.

The production of microcrystalline titanium dioxide requires its own process, with the difficulty of creating, adjusting and maintaining a small particle size throughout the process. Controlling the purity and crystal size distribution of the product are important factors in evaluating different manufacturing methods. In addition, manufacturing processes must be economically advantageous and environmentally friendly. Different methods provide different crystal forms. Rutile is known as a more durable crystalline form and has lower UV transmittance than anatase.

Microcrystalline titanium dioxide can be prepared by a number of different methods, either by gas phase or precipitation techniques. The source of titanium, i.e. the titanium-containing substance, can be e.g. titanium tetrachloride, titanium alkoxide or titanium hydrate made from ilmenite. Precipitation methods for the production of rutile have been patented. JP patent 86/049250 discloses a process for the preparation of microcrystalline titanium dioxide aftertreated with aluminum and / or silica from titanium hydrate made from ilmenite and ammonia. The use of ammonia avoids the introduction of foreign metal cations into the pigment, but on the other hand has to deal with a cumbersome ammonia reagent. The patent does not mention the crystalline form of the product, but it is not necessarily rutile. Subsequent patent application JP-57/67681 by the same Japanese applicant discloses a similar production method for producing finely divided anatase. Rutile in the same patent is prepared by a recipe starting from titanium tetrachloride. Neutralization is performed with NaOH and water additions. The precipitation concentration is 30 g / l TiO2 ·

DE-3817909 discloses the preparation of microcrystalline titanium dioxide from both ilmenite and titanium tetrachloride.

The titanium hydrate mass made from ilmenite is treated with sodium hydroxide to form a sodium-containing titanium hydrate cake. The pH is initially adjusted to 2.8-3.0 by the addition of hydrochloric acid and at a later stage of cooking the ratio of acid to titanium dioxide is adjusted to 0.26. At the end of the cooking, the slurry is neutralized to pH 7.5, after which the titanium dioxide is filtered and washed. The pigment is then sand milled and post-treated with alumina and / or silica. The crystals of microcrystalline titanium dioxide prepared in this way become needle-like. The weakness of the method is that for many purposes e.g. the fact that the pigment is not calcined, making the crystals rounder and the crystal size adjustable. A disadvantage of this known method is also that the amount of hydrochloric acid to be added to generate rutile crystals depends on the diconcentration of titanium dioxide. Washing titanium dioxide free of precipitation salts is quite cumbersome when the final neutralization is performed to pH 7.5.

The same drawbacks also apply to the preparation of microcrystalline titanium dioxide from titanium tetrachloride in the same DE application.

CA patent 962142 discloses a very similar preparation method as in the above-mentioned DE publication, which starts from ilmenite, but after neutralization of the precipitation (with ammonia to pH 6.5) the mixture is boiled before filtration and washing and the so-called calcination chemicals (K2O, P2O5) j^a titanium dioxide is calcined. This method no longer produces enough microcrystalline titanium dioxide, but gives a crystal size of 50-150 nm.

The process disclosed in JP patent application 59223231, starting from a titanium hydrate mass, produces needle-like rutile crystals coated with organoaluminum compounds. Titanium dioxide is not calcined, so crystal size control is cumbersome. Filtration difficulties are overcome by organic post-treatment. In this case, however, no traditional inorganic treatment recipes for titanium dioxide can be applied.

In a corresponding production method starting from titanium hydrate pulp, JP 62/235215 discloses the solution of the filtration and drying problem by means of a 100 Å filter film and freeze-drying. The method is laborious, prone to interference and unnecessarily expensive.

It is an object of the present invention to produce microcrystalline titanium dioxide particles in rutile crystal form, to obtain the most favorable crystal size and size distribution of the product, to use the cheapest and most easy-to-handle chemicals and equipment, and to save as much process chemicals as possible.

The above objects have been achieved in the present invention by said four-step process, which is mainly characterized in that the base treatment of step a) is carried out at alkaline pH and the final hydrochloric acid content of step b) is adjusted to 8-25 g / l to precipitate microcrystalline rutile titanium dioxide.

The invention thus differs from the prior art in at least two respects.

First, it has been found that rutile crystals are formed in a dispersion with a measured HCl concentration of about 8-25 g / l, 10 regardless of the titanium dioxide concentration. Thus, microcrystalline titanium dioxide with a more favorable crystalline form is formed when a lower than usual hydrochloric acid concentration is used. Also, it is no longer necessary to know the exact titanium dioxide concentration and it is not necessary to increase the acid addition as a function of the titanium dioxide concentration. At the same time, chemicals are saved, as higher consistencies are also possible.

Second, it has been found that the pH value of the lop20 rune neutralization in step c) has a decisive effect on the filterability and salt-free washing of the precipitate. Optimal pH values ​​for final neutralization were found, ranging from about 4.0 to 6.0 and thus lower than before. Since the hydrochloric acid 25 used in the precipitation is neutralized in the just-mentioned final neutralization step, it can be seen that there is also a synergy between the two mentioned steps in terms of product quality and chemical savings. The invention thus contributes decisively to the technical implementation of microcrystalline titanium dioxide and is very inexpensive.

In the first step of the process of the present invention, the titanium-containing material is contacted with the first base in an aqueous medium.

The titanium-containing material can be a titanium compound capable of precipitating or recrystallizing any commercial process. In one embodiment, it is a washed titanium dioxide hydrate precipitate from a sulfate process, wherein it is prepared

i) reacting ilmenite, its concentrate or other impure titanium dioxide raw material with sulfuric acid, ii) dissolving the resulting solid reaction product with water and e.g. process waste acids, iii) reducing and clarifying the dissolved reaction mixture, iv) concentrating the solution obtained by dissolving the reaction mixture,

v) precipitating the titanium-containing substance from the solution by hydrolysis, and vi) washing the precipitated mass obtained to a titanium-containing substance for use in step a).

When the titanium-containing material is such an intermediate in the sulfate process, the process is preferably carried out as follows:

The solid titanium dioxide hydrate is treated with a first base, preferably aqueous sodium hydroxide solution, at an elevated temperature, after which the base-treated titanium mass obtained is washed, preferably hot, filtered and reslurried. The temperature of the slurry is then raised to about 60 ° C. In this case, it is preferred that the pH be adjusted to n with hydrochloric acid before raising the temperature.

1.5-2.0, and preferably to about 1.8. Finally, the hydrochloric acid concentration is adjusted to said final hydrochloric acid content of step b) to precipitate microcrystalline titanium dioxide.

In this case, it is preferred that the base treatment, i.e. step a), is carried out at a temperature of about 95 ° C. The base treatment of step a) is preferably carried out so that the base concentration corresponds to about 300-350 g NaOH / 1 H2O. The concentration of step b) is preferably about 95-180 g / l at the beginning. As already mentioned, the phase

b) the final hydrochloric acid content is adjusted to about 8-25 g / l. It is more preferable to adjust the concentration to 8-15 g / l and most preferably to 8-12 g / l, i.e. about 10 g / l.

After said final hydrochloric acid content of step b) is adjusted, the mixture is preferably heated before neutralizing step c). When a washed titanium dioxide hydrate precipitate from the sulphate process is used, heating preferably comprises slow heating to boiling point and boiling for about two hours.

The precipitate to be treated in step b) is in the form of a slurry having a concentration of at least 70 g / l, preferably 70-180 g / l.

In step c), the mixture obtained from step b) is neutralized. Neutralization is performed by raising the pH to above about 4.0, but below about 6.0, and preferably to about 4.45.0. The neutralization is preferably carried out with sodium hydroxide or sodium carbonate.

After neutralization of step c), the reaction mixture is optionally worked up in step d). After step c), filtration and washing of the microcrystalline titanium dioxide precipitate are generally always carried out. When the neutralization according to the present invention is carefully carried out to a pH of less than 6 and even more preferably less than 5, filtration and washing are particularly successful and almost all sodium and chloride ions which interfere with the further processing of the product can be removed from the precipitate.

After the filtration and washing step, the further processing step d) preferably comprises at least calcining the purified neutralized mixture, preferably at a temperature

350-800 ° C. In this case, it is particularly advantageous to adjust the crystal size of the microcrystalline titanium dioxide by means of the calcination temperature. Namely, the calcination parameters of the salt-free rutile titanium dioxide mass can be used to conveniently and accurately control the crystal size and distribution of microcrystalline titanium dioxide. The crystals of the calcined pigment are ellipsoidal (oval) in shape.

Further treatment d) of the process according to the invention may also comprise steps similar to the post-treatment steps of a conventional titanium dioxide pigment. In this case, dispersion and weathering properties similar to those of titanium dioxide pigment are achieved. The product obtained from the calcination can be coarsely ground, e.g. with a hammer grinder, and finely ground, e.g. with a sand grinder.

After grinding, the microcrystalline titanium dioxide is treated with existing titanium dioxide pigment treatment recipes so that the surface of the titanium dioxide contains aluminum, silicon and / or zirconia hydrate or oxide.

Alumina may be present on the surface of the pigment 0-10%, silica 0-10% and zirconia calculated as zirconium 15-5-5%. The treated pigment is dried before being treated with an organic excipient such as trimethylolethane or silicone (see FI patent publication 57124).

The microcrystalline TiÖ2 pigment can finally be ground with an efficient cabbage mill. The finely ground microcrystalline TiO2 pigment treated with various coatings is suitable for use e.g. in cars as an effect pigment, in cosmetics as UVA and UVB protection, in wood protection as UV protection or in food packaging plastics to protect UV-sensitive food.

Example 1

The production of titanium dioxide by the sulphate process is started by the reaction of the ilmenite concentrate with sulfuric acid. The resulting solid reaction cake is dissolved with water and waste acids. The impurities are removed and the ferrous sulfate is crystallized off. The titanium-containing solution is concentrated and the titanium dioxide hydrate is precipitated by hydrolysis. This precipitate is washed free of salts in several steps. Take 1,400 g of this washed filter cake and slurry it with 1,200 ml of distilled water. The slurry is made strongly basic by adding 1,070 ml of NaOH solution (700 g / l) at 60 ° C. The temperature of the slurry is raised to 95 ° C and the slurry is mixed in two

89900 hours at this temperature. The sulphate ions are then removed from the titanium hydroxide mass by washing the slurry with hot distilled water until no more sulphates are found in the filtrates when mixed with barium chloride.

The sulphate-free sodium cake is slurried in distilled water so that the concentration of the slurry, expressed as titanium dioxide, is about 180 g / l. The pH of the slurry is adjusted to 1.8 by adding 37% hydrochloric acid to the slurry. With constant stirring, the temperature of the slurry is raised to 60 ° C. At this temperature, the slurry is stirred for 30 min, after which the acid content of the slurry is adjusted to 10 g HCl / l by the addition of 37% hydrochloric acid solution to give rutile crystals. The temperature of this dispersion is slowly raised to 90 ° C with constant stirring. The slurry is boiled with stirring at this temperature for 120 min.

Finally, the slurry is neutralized with sodium carbonate or sodium hydroxide so that the pH is adjusted to 4.7-4.8. Neutralization makes it more difficult to wash off sodium ions and more neutralization makes it more difficult to wash off chloride ions. The neutralized slurry is filtered and washed (4 liters) with distilled water. The dry matter content of the filter cake is about 30%. An X-ray diffraction pattern is run on the dried filter cake and the crystalline titanium dioxide is found to be in the rutile form.

The titanium dioxide filter cake is calcined at 500 ° C for one hour. The electron micrograph of the calcined rutile product is measured to have an average crystal size of about 25 nm with a distribution of 10 to 50 nanometers. The calcined product has a sodium content of less than 0.1% and a chloride content of less than 0.05%. An electron micrograph of the resulting product is shown in Appendix 1 along with the crystal size distribution curve.

The salt-free titanium dioxide is slurried in distilled water with a dispersant to give a thick slurry. Grinding

9900 carried out in a sand mill. The crystals ground apart are post-treated with aluminum, silicon and / or zirconium compounds, depending on the application. Known recipes for the treatment of titanium dioxide pigments are used as a recipe, such as, for example, FI patent publication 62130.

The treated microcrystalline TiO2 is dried. Before drying, an organic substance such as trimethylolethane (TME) or silicone is added (see FI patent publication 57124). The dried microcrystalline TiO 2 is jet milled to a fine powder having a particle size of less than 200 nm.

Example 2

Microcrystalline titanium dioxide with a crystal size of 25 nm was precipitated and calcined as in Example 1. The aqueous dispersion of calcined microcrystalline titanium dioxide was ground in a sand mill as in Example 1. The pigment slurry was diluted to a TiO 2 content of 225 g / l and heated to 40 ° C. To the slurry was added an amount of acidic aluminum sulfate solution equivalent to 0.5% Al 2 O 3 based on the TiCl 2 in the slurry. During this addition, the pH of the slurry decreased to 2.5. The slurry was neutralized with sodium carbonate solution to

6.4. After neutralization, the treated titanium dioxide pigment was recovered by filtration. The filter cake was washed with desalted water using an ion exchanger. To the washed filter cake was added 4% dimethylpolysiloxane based on Tiit. The pigment was dried and ground in a jet mill. The dried and spray-milled pigment has a particle size of less than 200 nm. LDPE plastic films were prepared to which various amounts of microcrystalline titanium dioxide were added. The films were 25 m thick and had microcrystalline titanium dioxide concentrations of 0, 0.5, 1.0 and 2.0%. Light and UV transmittances were measured from these films. Annex 2.

Example 3

The base-treated titanium hydrate mass of Example 1, washed and filtered, is slurried in water so that the concentration of the dispersion, expressed as titanium dioxide, is about 180 g / l. The temperature of the slurry is raised to 60 ° C with constant stirring. The acid content of the slurry is adjusted to 10 g HCl / l by the addition of 37% hydrochloric acid solution. The temperature of this dispersion is slowly raised to 90 ° C with constant stirring. The slurry is boiled with stirring at this temperature for 120 min.

The slurry is neutralized, filtered and washed as in Example One. The titanium dioxide filter cake is calcined at 500 ° C for one hour. The resulting product is rutile microcrystalline titanium dioxide based on an X-ray diffraction pattern. Of the chemicals used in the preparation, titanium dioxide contains less than 0.05% sodium based on atomic absorption spectrophotometer assays and less than 0.15% chloride based on X-ray fluorescence assays.

Example 4

The precipitated mixed mass according to Example 1 is finally neutralized to pH 6.0. The slurry is washed and filtered as in Example 1 and calcined at 500 ° C for one hour. The analysis gives a Na content of the pigment of 7.1% and an Cl content of 282 ppm by X-ray fluorescence analysis on an atomic absorption spectrophotometer. When this pigment is dispersed in water as in Example 1, the slurry pellets into a thick mass so that it is particularly difficult to grind finely.

Example 5

The precipitated mixed mass according to Example 1 is finally neutralized to pH 4.5. The slurry is washed and filtered as in Example 1 and calcined at 500 ° C for one hour. The analysis gives an Na content of 0.01% of the pigment by atomic absorption spectrophotometry and a Cl content of 0.23% by X-ray fluorescence analysis. When this pigment is dispersed in water as in Example 1, the slurry is pulverized into a thick mass in a mill so that it

9 900 as such cannot grind in a sand mill. The only way is to wash off the salts with a very rich water wash after calcination.

Example 6

The sulphate-free filter cake prepared according to Example 1 is slurried in distilled water so that the concentration of the slurry, expressed as titanium dioxide, is about 95-100 g / l. The pH of the slurry is adjusted to 1.8 by adding 37% hydrochloric acid to the slurry. With constant stirring, the temperature of the dispersion is raised to 60 ° C. At this temperature, the solution is stirred for 30 minutes, after which the acid content of the slurry is adjusted to 9 g of HCl / l by the addition of 37% hydrochloric acid solution to give rutile crystals. The temperature of this slurry is slowly raised to 90 ° C with constant stirring. The slurry is boiled with stirring at this temperature for 120 min.

Finally, the slurry is neutralized as in Example 1 to pH

4.7-4.8 and washed as in Example 1. The crystalline form of the precipitated and dried product was checked by X-ray diffraction method. It was found to be in rutile form. The crystal size is controlled by calcination parameters.

Example 7

The sulphate-free filter cake prepared according to Example 1 is slurried in distilled water so that the concentration of the slurry, expressed as titanium dioxide, is about 140 g / l. The pH of the slurry is adjusted to 1.8 by adding 37% hydrochloric acid to the slurry. With constant stirring, the temperature of the slurry is raised to 60 ° C. At this temperature, the slurry is stirred for 30 minutes, after which the acid content of the slurry is adjusted to 10 g of HCl / l by the addition of 37% hydrochloric acid to give rutile crystals. The temperature of this slurry is slowly raised to 90 ° C with constant stirring. The slurry is boiled with stirring at this temperature for 120 min.

Finally, the slurry is neutralized as in Example 1 to pH

4.7-4.8 and washed as in Example 1. The crystal form of the precipitated and dry1289900 product was checked by X-ray diffraction method. It was found to be in rutile form.

Example 8

The sulphate-free filter cake prepared according to Example 1 is slurried in distilled water so that the concentration of the dispersion, expressed as titanium dioxide, is about 120 g / l. The pH of the slurry is adjusted to 1.8 by adding 37% hydrochloric acid to the slurry. With constant stirring, the temperature of the slurry is raised to 60 ° C. At this temperature, the slurry is stirred for 30 minutes, after which the acid content of the slurry is adjusted to 6 g HCl / l by the addition of 37% hydrochloric acid solution. The temperature of this slurry is slowly raised to 90 ° C with constant stirring. The slurry is boiled with stirring at this temperature for 120 min.

Finally, the slurry is neutralized as in Example 1 to pH

4.7-4.8 and washed as in Example 1. The crystalline form of the precipitated and dried product was checked by X-ray diffraction method. It was found to be mainly in the anatase form. The titanium dioxide filter cake is calcined at 500 ° C for one hour. The rutile content of the product remained below 91% even after calcination.

Example 9

The rutile titanium dioxide precipitated and washed according to Example 1 is calcined at different temperatures for one hour. The crystal size can be conveniently adjusted by changing the calcination conditions, as shown by electron micrographs.

Table

<td>Temperature</td><td>The crystal size</td><td>Top</td><td>The size distribution</td>

<td>400 ° C</td><td>24 nm</td><td>25 nm (39%)</td><td>10-50 nm</td>

<td>500 ° C</td><td>26 nm</td><td>25 nm (37%)</td><td>10-50 nm</td>

<td>550 ° C</td><td>30 nm</td><td>27 nm (33%)</td><td>10-50 nm</td>

<td>600 ° C</td><td>35 nm</td><td>31 nm (30%)</td><td>10-60 nm</td>

<td>650 ° C</td><td>45 nm</td><td>40 nm (25%)</td><td>10-70 nm</td>

Example 10

The hydrochloric acid solution of titanium tetrachloride (200 g / l TiO2 and 345 g / l HCl) and sodium carbonate solution (250 g / l Na2CO3) are used as starting materials for microcrystalline titanium dioxide. Pour 638 ml of sodium carbonate solution and 12 ml of distilled water into the bottom of a 2-liter three-necked flask. Place a stirrer in the middle opening of the three-necked flask and a thermometer and titanium tetrachloride measuring container in the side openings. The stirrer is started and the flask is heated by the surrounding heating mantle n. To a temperature of 40 ° C. Slowly add titanium tetrachloride from the 350 ml graduated container to the flask. The solution is stirred all the time but not heated. After all the titanium tetrachloride solution has been added, the titanium dioxide content in the slurry is 90 g / l and the hydrochloric acid content is 10 g / l. Slowly raise the temperature of the dispersion to 80 ° C. At this temperature, the slurry is stirred for 30 minutes.

Finally, the slurry is neutralized with sodium hydroxide or sodium carbonate to pH 4.5. At this pH, filtration and washing are most successful. The precipitated rutile titanium dioxide is filtered off and washed with two liters of distilled water. The washed and filtered cake has a dry matter content of about 30% and a measured Na content of less than 0.1% and a Cl content of less than 0.05%.

The washed and filtered cake is calcined at 500 ° C for one hour. In this case, the average crystal size becomes about 25 nanometers and the distribution is 20-50 nm. The agglomerates formed in the calcination are ground in a sand mill. A thick aqueous dispersion is prepared from titanium dioxide with the aid of an organic dispersion aid. Grinding is successful because titanium dioxide is as free of salts as possible.

The crystals ground apart are post-treated with aluminum, silicon and / or zirconium compounds, depending on the application. Known recipes for the treatment of titanium dioxide pigments, such as FI patent publication 62130 (1980), are used as recipes.

The treated microcrystalline TiO 2 is dried. Before drying, an organic substance such as trimethylolethane (TME) or silicone is added, cf. F I Patent Publication 57124 (1978). The dried microcrystalline T1O2 is ground to a fine powder with a particle size of less than 200 nm.

Example 11

The aqueous dispersion of calcined microcrystalline titanium dioxide was ground in a sand mill as in Example 10. The pigment slurry was diluted to a TiO 2 content of 225 g / l. The pigment slurry was heated to 40 ° C. To the slurry was added an aluminum-containing solution of 4.5% Al 2 O 3 corresponding to the TiO 2 of the slurry and the zirconium-containing solution as 0.7% Zr based on TiO 2. A solution of silicon containing 2.2% SiO 2 based on TiCl 4 was then added to the solution and stirred. After neutralization, the treated titanium dioxide pigment was recovered by filtration. The filter cake was washed with desalted water using an ion exchanger. Trimethylolethane was added to the washed filter cake and it was finally dried and ground in a jet mill. The dried and spray-milled pigment has a particle size of less than 200 nm. The coating concentrations of TiO2 analyzed from the final product were 4.0% Al2O3, 2.2% S1O2, 0.4% Zr, 0.7% C.

Example 12

The mixed mass precipitated from the titanium tetrachloride solution and the sodium carbonate solution according to Example 10 is finally neutralized to pH 6.0. The slurry is washed and filtered as in Example 10 and calcined at 500 ° C for one hour. The Na content of the pigment is 0.9% when analyzed. When this pigment is dispersed in water as in Example 10, the slurry pellets.

I;

9 900

Example 13

The mixed mass precipitated from the titanium tetrachloride solution and the sodium carbonate solution according to Example 10 is finally neutralized to pH 3.0. The slurry is washed and filtered as in Example 8, but the filtration is not successful and the slurry passes through the filter cloth.

Example 14

The rutile titanium dioxide precipitated and washed according to Example 10 is calcined at different temperatures for one hour. The crystal size can be conveniently adjusted by changing the calcination conditions.

Table

Temperature

The size distribution

The crystal size

Top

<td>400 ° C</td><td> 22</td><td>nm</td><td> 22</td><td>nm</td><td> (42</td><td> %)</td><td> 10-40</td><td>nm</td>

<td>500 ° C</td><td> 25</td><td>nm</td><td> 23</td><td>nm</td><td> (40</td><td> %)</td><td> 10-45</td><td>nm</td>

<td>550 ° C</td><td> 26</td><td>nm</td><td> 26</td><td>nm</td><td> (35</td><td> %)</td><td> 10-50</td><td>nm</td>

<td>600 ° C</td><td> 30</td><td>nm</td><td> 31</td><td>nm</td><td> (30</td><td> %)</td><td> 10-50</td><td>nm</td>

<td>700 ° C</td><td> 42</td><td>nm</td><td> 32</td><td>nm</td><td> (24</td><td> %)</td><td> 10-60</td><td>nm</td>