Method for the preparation of titanium dioxide.

0001The invention relates to a new method of preparing microcrystalline titanium dioxide, having the average crystal size &lt;100nm, in which <ul id="ul0001" list-style="none"><li>a) the solid titanium dioxide hydrate is treated with a base,</li><li>b) the precipitate treated with the base is treated with hydrochloric acid, and</li><li>c) the precipitate treated with the base and the hydrochloric acid is neutralized.</li></ul>

0002The properties of the microcrystalline titanium dioxide differ from the properties of the usual titanium dioxide known as white pigment. The differences are due to the dissimilarities in the crystal size, for the crystal size of the microcrystalline titanium dioxide (10-100nm) is about 5 to 10 times smaller than the crystal size of the normal titanium dioxide (160 to 250nm). The crystal size becoming smaller the coverage of titanium dioxide in the range of visible light disappears and the pigment becomes transparent. On the other hand, the permeability of UV-light decreases. Accordingly, the microcrystalline titanium dioxide is suitable for a UV-protective agent. Thanks to the small crystal size and the great specific surface area titanium dioxide can be used as an effect pigment among other things in catalysts, ceramics and paints.

0003The preparation of microcrystalline titanium dioxide needs a process of its own in which the difficulty lies in the creation, controlling and retaining of the small particle size throughout the process. The control of the purity and the crystal size distribution are important factors when assessing the various methods of preparation. Moreover, the preparation processes must be economically profitable and ecologically beneficial. Different crystal forms can be achieved by different methods. Rutile is known as a durable crystal form and its permeability to UV-light is less than that of anatase.

0004It is possible to prepare microcrystalline titanium dioxide by many different methods either by gas phase or by precipitating technique.

0005The titanium source or the titanium containing material can be e.g. titanium tetrachloride, titanium alcoxide or titanium hydrate prepared from ilmenite. For the preparation of rutile precipitation methods have been patented. In the JP patent 86/049250 a method of preparing microcrystalline titanium dioxide, after-treated with aluminum and/or silicon dioxide, from titanium hydrate prepared from ilmenite and ammonia has been briefly presented. By using ammonia the bringing of foreign metal cations into the pigment is avoided but, on the other hand, you are compelled to handle troublesome ammonia reagent. The crystal form of the product is not mentioned in the patent specification, but it is not necessarily rutile. In a later application JP 57/67681 of the same Japanese applicant a similar method of preparation is presented by which finely-divided anatase is created. Rutile is prepared in the same patent with a recipe starting from titanium tetrachloride. The neutralization is carried out with NaOH and water additions. The precipitation concentration is 30g/liter TiO₂.

0006In DE 3817909 preparation of microcrystalline titanium dioxide both from ilmenite and titanium tetrachloride is presented. The titanium hydrate mass prepared from ilmenite is treated with sodium hydroxide to a titanium hydrate cake containing sodium. The pH is first set to the value 2.8 to 3.0 with a hydrochloric acid addition and at a later stage of the cooking the ratio between the acid and the titanium dioxide is adjusted to the value 0.26. At the end of the cooking the slurry is neutralized to the pH-value 7.5, after which the titanium dioxide is filtered and washed. Then the pigment is sand ground and after-treated with aluminum and/or silicon dioxide. The crystals of the microcrystalline titanium prepared in this way become acicular. A drawback of the method is hereby for many purposes e.g. that the pigment is not calcinated, whereby the crystals would become rounder and the crystal size could be regulated. A further drawback of this known method is that the hydrochloric acid amount which is added to create the rutile crystals is dependent on the titanium dioxide concentration.

0007The wash of the titanium dioxide so as to become free from the precipitation salts is rather laborious, as the final neutralization is carried out to the pH-value 7.5.

0008The same drawbacks also apply for the preparation of microcrystalline titanium dioxide from titanium tetrachloride presented in the same DE application.

0009In the CA patent specification 962,142 a very similar method of preparation is disclosed as in the above-mentioned DE publication in which the starting material is ilmenite but after the neutralization of the precipitation (with ammonia to the pH-value 6.5) the composition is cooked before the filtration and the wash and the so-called calcination chemicals (K₂O, P₂O₅) are added and the titanium dioxide is calcinated. This method does not any longer produce titanium dioxide which is sufficiently microcrystalline, but the crystal size achieved is 50 to 150nm.

0010In the method presented in the JP patent application 59223231, in which the starting material is titanium hydrate mass, acicular rutile crystals are produced, which are coated with organic aluminum compounds. The titanium dioxide is not calcinated, for which reason the regulating of the crystal size is troublesome. The filtration difficulties are overcome by means of an organic after-treatment. Hereby, however, no conventional inorganic treatment recipes of titanium dioxide can be applied.

0011In the corresponding method of preparation starting from the titanium hydrate mass the JP publication 62/235215 suggests solving the filtration and drying problem by means of a filtration film of 100Å and freezing drying. The method is troublesome, apt to disturbancies and unnecessarily expensive.

0012An aim of the present invention is to prepare microcrystalline titanium dioxide particles in rutile crystal form, form a crystal size and crystal size distribution of the product as advantageous as possible, use as cheap and easily handled chemicals and devices as possible and to save as much process chemicals as possible.

0013The above-mentioned aims have been reached in the present invention by means of the three-stage method mentioned, which is mainly characterized in that the base treatment of the stage a) is carried out to an alcalic pH-value and to the final hydrochloric acid content is adjusted 8 to 25g/liter for the precipitation of the rutile-formed titanium dioxide.

0014Accordingly, the invention differs at least in two points from the prior art.

0015Firstly, it has been noted that rutile crystals are produced in a solution in which the measured HCl-concentration is about 8 to 25 g/liter independent of the concentration of the titanium dioxide. Accordingly, microcrystalline titanium dioxide having a more preferable crystal form is produced when a lower hydrochloric acid concentration than usual is used. Neither is a precise knowledge of the titanium dioxide concentration necessary any more, nor must the acid addition be increased as a function of the titanium dioxide concentration. At the same time chemicals are saved because greater consistencies thus also become possible.

0016Secondly, it has been noted that the pH-value of the final neutralization taking place in stage c) has a decisive effect on the filtrability of the precipitation mass and to its washing off of salts. The optimal pH-values of the final neutralization varying between about 4.0 to 6.0 were found and are thus lower than previously. As the hydrochloric acid used in the precipitation is neutralized in the very final neutralization stage mentioned it can be seen that also synergy occurs between the two steps mentioned as far as the quality of the product and the saving of chemicals are concerned. Accordingly, the invention is decisively helpful in the technical realization of the microcrystalline titanium dioxide and is very inexpensive.

0017In the first stage of the process according to the present invention the titanium-containing substance is in aqueous medium brought into contact with a base.

0018The titanium-containing substance can be any titanium compound of a commercial process which is capable of precipitation and recrystallization. According to a preferred embodiment it is a washed titanium dioxide hydrate precipitate obtained from a sulfate process, whereby it has been prepared by <ul id="ul0002" list-style="none"><li>i) bringing ilmenite, its concentrate, or any other unpurified titanium dioxide raw-material to react with sulphuric acid,</li><li>ii) dissolving the solid reaction product produced by means of water and e.g. waste acids of the process,</li><li>iii) reducing and clarifying the dissolved reaction composition,</li><li>iv) concentrating the solution obtained by dissolution from the reaction composition,</li><li>v) precipitating the titanium containing substance from the solution by means of hydrolysis, and</li><li>vi) washing the precipitation mass obtained in stage v) for the use as a titanium containing substance.</li></ul>

0019When the titanium containing substance is this kind of an intermediate product of a sulfate process the method is preferably carried out in the following manner:

0020The solid titanium dioxide hydrate is treated with the base, preferably with an aqueous solution of sodium hydroxide at an elevated temperature, after which the obtained titanium mass treated in base is washed, preferably hot, filtered and elutriated again. Then the temperature of the slurry is raised to about 60°C. Hereby, it is preferable that before the raise of temperature the pH is adjusted by hydrochloric acid to the value of about 1.5 to 2.0, and preferably to the value about 1.8. Finally, the concentration of the hydrochloric acid is adjusted to the mentioned final hydrochloric acid content of the stage b) for the precipitation of the microcrystalline titanium dioxide.

0021Hereby, it is advantageous that the base treatment i.e. the stage a) is carried out at the temperature of about 95°C. The base treatment of the stage a) is preferably carried out so that the base concentration corresponds to about 300 to 350g NaOH/liter of H₂O.

0022The concentration of the stage b) is in the beginning preferably about 95 to 180g/liter. As has been stated above about 8 to 25g/liter has been adjusted as the final hydrochloric acid content of the stage b). It is more preferable to adjust the content to the value 8 to 15g/liter and most preferable to adjust it to the value 8 to 12g/liter i.e. about 10g/liter.

0023After the final hydrochloric acid content of the stage b) mentioned has been adjusted, the composition is preferably still heated before the neutralization of the stage c). When washed titanium dioxide hydrate precipitate obtained from the sulfate process is used the heating preferably comprises a slow heating to the boiling point and cooking for about two hours. The precipitate to be treated is at the stage b) in the form of slurry, the content of which is at least 70g/liter, preferably 70 to 180g/liter.

0024The neutralization of the composition obtained from the stage b) is carried out in stage c). The neutralization is carried out by raising the pH to the value above about 4.0, but below about 6.0 and preferably to the value about 4.4 to 5.0. The neutralization is preferably carried out with sodium hydroxide or sodium carbonate.

0025After the neutralization of the stage c) the reaction composition is optionally treated further in stage d). After the stage c) a filtration and wash of the microcrystalline titanium dioxide precipitate is almost always carried out. When, according to the present invention the neutralization is carried out cautiously to the pH-value below 6 and still more preferably below 5, the filtration and wash succeeds particularly well and almost all sodium and chloride ions affecting detrimentally the further treatment of the product can be removed from the precipitate.

0026When the filtration and wash stage is performed, the further treatment stage d) preferably comprises at least the calcination of the purified neutralized composition, preferably at the temperature 350 to 800°C. Hereby it is particularly preferable to adjust the crystal size of the microcrystalline titanium dioxide by means of the calcination temperature. The crystal size and the crystal size distribution can be readily and precisely regulated by the calcination parameters of salt-free, rutile-formed titanium dioxide mass. The crystals of the calcinated pigment are ellipsoidal in form (oval).

0027The further treatment d) of the method according to the invention can also comprise stages which are similar to the aftertreatment stages of the usual titanium dioxide pigment. Hereby, similar dispersion and weather-resistance properties are achieved as with titanium dioxide pigment. The product obtained from the calcination can be rough-ground e.g. with a hammer grinder and finely ground e.g. by sand grinder.

0028After the grinding the microcrystalline titanium dioxide is treated with treatment recipes of titanium dioxide pigment in use so that the surface of titanium dioxide is coated with aluminum, silicon and/or zirconium oxide hydrate or oxide. On the surface of the pigment there might be from 0 to 10% of aluminum oxide, from 0 to 10% of silicon dioxide and zirconium oxide calculated as zirconium from 0 to 5% (all percentages in the patent are by weight). The treated pigment is dried, before which it is treated with an organic auxiliary agent, such as e.g. trimethylolethane or silicon (see FI patent specification 57124).

0029The microcrystalline TiO₂-pigment can finally be ground with an efficient mill. The finely-ground microcrystalline TiO₂-pigment treated with various coatings is suitable for use as effect pigments e.g. in car paints, as UVA- and UVB-protective agent in cosmetic, as UV-protective agent in timber protection or in packing plastics for foodstuffs to protect a UV-light sensitive foodstuff.

Example 1

0030The preparation of titanium dioxide by sulfate method is began by the ilmenite concentration reaction together with sulphuric acid. The created solid reaction cake is dissolved by means of water and waste acids. The impurities are removed and the iron sulfate is crystallized away. The titanium containing solution is concentrated and the titanium dioxide hydrate is precipitated by hydrolyzation. This precipitation mass is washed in several stages so as to become free of salts. 1400g of this washed filtration cake is elutriated with 1200ml of distilled water. The slurry is made strongly basic by adding at 60°C 1070ml of NaOH-solution (700g/liter). The temperature of the slurry is raised to 95°C and the slurry is agitated for two hours at this temperature. After this the sulfate ions are removed from the titanium hydroxide mass by washing the slurry for so long with hot distilled water that no sulphates are found in the filtrates measured by the barium chloride precipitation test.

0031The filtrate cake which is free from sulfates is elutriated into distilled water so that the concentration of the dispersion expressed as titanium dioxide is about 180g/liter. The pH of the slurry is set to 1.8 by adding 37% hydrochloric acid to the slurry. Continuously agitating the temperature of the slurry is raised to 60°C. In this temperature the slurry is agitated for 30 minutes, after which acid content of the slurry is set to 10g HCl/liter by an addition of 37% hydrochloric acid for the formation of rutile formed crystals. The temperature of this dispersion is raised slowly under constant agitation to 90°C. The slurry is cooked under agitation at this temperature for 120 minutes.

0032Finally, the slurry is neutralized with sodium carbonate or sodium hydroxide so that the pH settles at 4.7 to 4.8. If the neutralization result is more basic the washing off of the sodium ions becomes more difficult and if the neutralization result is more acidic the washing off of the chloride ions becomes more difficult. The neutralized slurry is filtered and washed with (4 liters of) distilled water. The dry content of the filtrate cake is about 30%. An X-ray diffraction diagram was taken of the dried filtrate cake and the microcrystalline titanium dioxide is found to be of rutile crystal form.

0033The titanium dioxide filtrate cake is calcinated at a temperature of 500°C for one hour. As the average crystal size is measured about 25nm from the electron microscope picture of the calcinated rutile-formed product the distribution being from 10 to 50 nanometres. The sodium content of the calcinated product is below 0.1% and the chloride content below 0.05%. The electron microscope picture of the created product is presented in the attachment 1 together with a crystal size distribution diagram.

0034The titanium dioxide free of salts is elutriated by means of dispersing agent into distilled water as a thick slurry. The grinding is carried out in a sand mill. The crystals ground apart from each other are after-treated according to the planned use with an aluminum, silicon and/or zirconium compound. As recipe is used known recipes for the treatment of titanium dioxide pigments, such as e.g. FI patent specification 62130.

0035The treated microcrystalline TiO₂ is dried. Before drying an organic agent, such as trimethylolethane (TME) or silicone (see FI patent specification 57124) is added. The dried microcrystalline TiO₂ is spray-ground into finely-divided powder having the particle size below 200nm.

Example 2

0036Microcrystalline titanium dioxide was precipitated and calcinated as in example 1, and the crystal size obtained was 25nm. The water dispersion of the calcinated microcrystalline titanium dioxide was ground in a sand mill as in example 1. The pigment slurry was diluted to a TiO₂-content of 225g/liter and heated to the temperature 40°C. An amount corresponding to 0.5% of Al₂O₃ calculated on the TiO₂ of the slurry of acidic aluminum sulfate solution was added to the slurry. During this addition the pH of the slurry sank to the value 2.5. The slurry was neutralized with sodium carbonate solution to the value 6.4. After the neutralization the treated titanium dioxide pigment was recovered by filtering. The filtrate cake was washed with water from which the salts had been removed by means of an ion exchanger. 4% of dimethylpolysiloxane calculated on the TiO₂ was added to the washed filtrate cake. The pigment was washed and ground in a spray mill. The particle size of the dried and spray-ground pigment was below 200nm. LDPEplastic films were prepared, to which different amounts of microcrystalline titanium dioxide was added. The thickness of the films was 25»m and the contents of microcrystalline titanium dioxide were 0, 0.5, 1.0, and 2.0%. The light and UV-radiation permeabilities of these films were measured. Appendix 2.

Example 3

0037A titanium hydrate mass according to example 1 treated with base, which had been washed and filtered, was elutriated in water so that the concentration of the dispersion expressed as titanium dioxide is about 180g/liter. The temperature of the slurry was raised to 60°C under constant agitation. The acid content of the slurry was set by an addition of 37% hydrochloric acid to 10g HCl/liter. The temperature of this dispersion is raised slowly under constant agitation to 90°C. The slurry is cooked at this temperature for 120 minutes.

0038The slurry is neutralized, filtered and washed as in example 1. The titanium dioxide filtrate cake is calcinated at the temperature of 500°C for one hour. The product produced is on the basis of an X-ray diffraction diagram rutile-formed microcrystalline titanium dioxide. Of the chemicals used in the preparation below 0.05% of sodium has remained in the titanium dioxide on the basis of atom absorption spectrophotometric determinations and below 0.15% of chloride on the basis of X-ray fluorescense determinations.

Example 4

0039The filtrate cake free from sulfates prepared according to example 1 is elutriated into distilled water so that the concentration of the slurry expressed in titanium dioxide is about 95 to 100g/liter. The pH of the slurry is set to 1.8 by adding 37% hydrochloric acid to the slurry. Under constant agitation the temperature of the dispersion is raised to 60°C. At this temperature the solution is agitated for 30 minutes, after which the acid content of the slurry is set by a 37% hydrochloric acid addition to 9g HCl/liter so as to produce rutile-formed crystals. The temperature of this slurry is raised slowly under constant agitation to 90°C. The slurry is cooked under agitation at this temperature for 120 minutes.

0040Finally, the slurry is neutralized as in example 1 to the pH-value 4.7 to 4.8 and washed as in example 1. The crystal form of the precipitated and dried product was checked by an X-ray diffraction method. It was found to be of rutile form. The crystal size is regulated by calcination parameters.

Example 5

0041The filtrate cake free from sulfates prepared according to example 1 is elutriated into distilled water so that the concentration of the slurry expressed in titanium dioxide is about 140g/liter. The pH of the slurry is set to 1.8 by adding 37% hydrochloric acid to the slurry. Under constant agitation the temperature of the slurry is raised to 60°C. At this temperature the slurry is agitated for 30 minutes, after which the acid content of the slurry is set by a 37% hydrochloric acid addition to 10g HCl/liter so as to produce rutile-formed crystals. The temperature of this slurry is raised slowly under constant agitation to 90°C. The slurry is cooked under agitation at this temperature for 120 minutes.

0042Finally, the slurry is neutralized as in example 1 to the pH-value 4.7 to 4.8 and washed as in example 1. The crystal form of the precipitated and dried product was checked by an X-ray diffraction method. It was found to be of rutile form.

Comparative Example 1

0043The filtrate cake free from sulfates prepared according to example 1 is elutriated into distilled water so that the concentration of the dispersion expressed in titanium dioxide is about 120g/liter. The pH of the slurry is set to 1.8 by adding 37% hydrochloric acid to the slurry. Under constant agitation the temperature of the slurry is raised to 60°C. At this temperature the slurry is agitated for 30 minutes, after which the acid content of the slurry is set by a 37% hydrochloric acid addition to 6g HCl/liter. The temperature of this slurry is raised slowly under constant agitation to 90°C. The slurry is cooked under agitation at this temperature for 120 minutes.

0044Finally, the slurry is neutralized as in example 1 to the pH-value 4.7 to 4.8 and washed as in example 1. The crystal form of the precipitated and dried product was checked by an X-ray diffraction method. It was found to be of anatase form. The titanium dioxide filtrate cake is calcinated at the temperature of 500°C for one hour. In the calcination the product does not turn to rutile.

Example 6

0045Precipitated and washed rutile-formed titanium dioxide is calcinated according to example 1 in various temperatures for one hour. The crystal size can be handily regulated by changing the calcination conditions, as appears from the electron microscope pictures. <tables id="tabl0001" num="0001"><table frame="all"><title>Table</title><tgroup cols="4" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center">Temperature</entry><entry namest="col2" nameend="col2" align="center">Crystal size</entry><entry namest="col3" nameend="col3" align="center">Top</entry><entry namest="col4" nameend="col4" align="center">Size distribution</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="right">400°C</entry><entry namest="col2" nameend="col2" align="right">24nm</entry><entry namest="col3" nameend="col3" align="right">25nm (39%)</entry><entry namest="col4" nameend="col4" align="right">10-50nm</entry></row><row><entry namest="col1" nameend="col1" align="right">500°C</entry><entry namest="col2" nameend="col2" align="right">26nm</entry><entry namest="col3" nameend="col3" align="right">25nm (37%)</entry><entry namest="col4" nameend="col4" align="right">10-50nm</entry></row><row><entry namest="col1" nameend="col1" align="right">550°C</entry><entry namest="col2" nameend="col2" align="right">30nm</entry><entry namest="col3" nameend="col3" align="right">27nm (33%)</entry><entry namest="col4" nameend="col4" align="right">10-50nm</entry></row><row><entry namest="col1" nameend="col1" align="right">600°C</entry><entry namest="col2" nameend="col2" align="right">35nm</entry><entry namest="col3" nameend="col3" align="right">31nm (36%)</entry><entry namest="col4" nameend="col4" align="right">10-60nm</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="right">650°C</entry><entry namest="col2" nameend="col2" align="right">45nm</entry><entry namest="col3" nameend="col3" align="right">40nm (25%)</entry><entry namest="col4" nameend="col4" align="right">10-70nm</entry></row></tbody></tgroup></table></tables>