Process for the production of titanium dioxide, and titanium dioxide obtained thereby

The invention relates to a process for the production of titanium dioxide with a fraction of at most 10% (numerical) of crystallites with a size of less than 100 nm, as well as the titanium dioxide obtained by this process.

In the production of titanium dioxide using the sulphate process, titanium dioxide- containing raw materials (slag, ilmenite) are dried and ground and then digested with concentrated sulphuric acid. The reaction between the raw materials and the concentrated sulphuric acid is carried out in batches in lined reaction vessels. During the digestion reaction, almost all of the metal oxides present in the raw materials which react with sulphuric acid are transformed into the corresponding metal sulphates. After the reaction, a solid mass (digestion cake) remains which is dissolved with water and/or dilute sulphuric acid.

This digestion solution, which is known as black liquor, is completely freed from the undissolved components (digestion residues, gangue) by sedimentation and/or filtration processes. Further on in the process, a suspension of metatitanic acid is produced from the solids-free digestion solution by hydrolysis. The metatitanic acid can be supplied to the downstream process by washing, bleaching and, if appropriate, salt treatment, as well as filtration.

The digestion residues precipitating out during the solid/liquid separation processes as sediment or filter cake are mashed with water and/or dilute sulphuric acid and are dumped after neutralization, usually with calcium hydroxide suspension and fresh filtration.

Processes for the production of such titanium dioxides are known in the prior art. A process for the production of anatase with large crystals using added seed crystals has been described in GB 2 247 009. Alternative processes have been described in EP 0 772 905 and EP 0 782 971 . However, these processes are all complicated to control and cannot reliably produce an anatase with the desired stability of colour. - -

The object of the invention is to overcome the disadvantages of the prior art and also in particular to provide a process for the production of titanium dioxide using the sulphate process which can in a simple manner produce a titanium dioxide with a low proportion of primary crystallites with a dimension of at most 100 nm.

As an example, the process in accordance with the invention may be carried out in the following manner: the titanium dioxide-containing raw materials (slag, ilmenite) are dried and ground and then digested with concentrated sulphuric acid (for example 90% H ₂ SO ). After a maturation period of several hours, for example, the solid mass obtained from digestion of the titanium dioxide raw material is dissolved in water, for example with acidified return water. Insoluble components are separated in a clarifier. The clarified titanium-containing solution from the clarifier discharge is freed from the remaining solid in filter presses.

Dissolved trivalent iron is transformed into the divalent form by adding scrap iron, because problematically, Fe ²⁺ precipitates out as Fe(OH) ₃ together with the metatitanic acid and would then be adsorbed on it. Depending on the Fe content, FeSO ₄ . 7 H ₂ O precipitates out after cooling and is removed from the system.

In the subsequent hydrolysis, metatitanic acid, a precursor of titanium dioxide, is precipitated out as a solid from the clarified solution by heating and adding water. Advantageously, during the hydrolysis, in accordance with the invention no crystallization seeds are added and the seeds are formed in situ according to Blumenfeld. In the prior art, it is known to hydrolyze titanyl sulphate in order to precipitate out hydrated titanium oxide, either by adding seed crystals ("Mecklenburg" process) or by inducing seed crystals within the solution by adding water ("Blumenfeld" process).

The metatitanic acid precipitated out during hydrolysis is in the form of a suspension. In one embodiment, the suspension of titanium oxide particles with formula TiO(2-x)(OH) ₂ x (0 < x <1 ) may be metatitanic acid with formula TiO(OH) ₂ . Other hydrated forms such as orthotitanic acid are transformed into metatitanic - - acid by cleavage of H ₂ O.

This "titanium dioxide suspension" formed by this suspension of titanium oxide hydrate and/or hydrated titanium oxide particles with general formula TiO(2-x)(OH) ₂ x in which (0 < x <1 ), or mixtures thereof, preferably has the following properties: a Ti content, expressed as titanium dioxide, of 50 - 400 g TiO ₂ /l,

a mean particle size of 20 nm - 1000 nm, preferably 20 nm - 500 nm, particularly preferably 50 nm - 200 nm, more particularly preferably 50 - 150 nm,

a specific surface area for the particles of 200 - 500 m ² /g, preferably 200 - 400 m ² /g, particularly preferably 300 - 400 m ² /g (measured after drying at 105 °C for at least 120 min, using N ₂ porosimetry, 5-point BET),

a total pore volume for the particles of > 0.3 cm ³ /g, preferably > 0.5 cm ³ /g, particularly preferably > 0.7 cm ³ /g (measured after drying at 105 °C for at least 120 min, using N ₂ porosimetry, 5-point BET),

the crystalline phases of the particles, after drying at 105 °C for at least 120 min, are in the anatase phase. After subtracting a linear background, the ratio of the height of the most intensive reflection of the anatase structure (reflection (101 )) to the height of the most intensive reflection of the rutile structure (reflection (1 10)) is at least 20:1 , preferably at least 40:1 . Preferably, XRD analysis carried out as described in the section of the text entitled "Methods" in the description provides exclusively reflections of an anatase structure.

The metatitanic acid formed in this manner may contain free as well as bound sulphuric acid, wherein the crystalline portions of the titanium-oxygen compounds it contains, as mentioned above, are in the anatase crystal form and have a typical primary crystallite dimension of approximately 5-15 nm.

The metatitanic acid is filtered off and washed in washing tanks. The pasty filter cake is filtered and washed again after an intermediate treatment.

After cleaning up the metatitanic acid, auxiliary agents are added to the suspension - - of this intermediate product prior to calcining; these auxiliary agents control the growth of the crystals as well as counter rutile formation to some extent.

In accordance with the invention, alkali components are added as the auxiliary agents, in particular one or more potassium components, as well as phosphorus components, in particular phosphates. The potassium components may also be replaced by a sodium component or a mixture of the two.

The phosphorus component may be added in the form of phosphoric acid or in the form of a water-soluble phosphate compound, for example potassium dihydrogen phosphate. It is used in an amount of at least 0.1 % by weight, in particular at least 0.12% by weight and at most 0.3% by weight, in particular at most 0.2% by weight, more particularly preferably in the range 0.12% to 0.2% by weight of P, expressed in terms of phosphorus and expressed as the quantity of TiO ₂ in the final product in accordance with the invention. The inventors have shown that below the minimum amount of 0.1 % by weight, rutile formation is greater and above the maximum amount, crystal growth is inhibited as a function of the calcining temperature.

The potassium components are in particular added in the form of a water-soluble potassium compound, for example as potassium sulphate or as potassium hydroxide. The potassium may also be used as a counter- ion to the phosphate group, so that the pH of the final product can also be adjusted (pH 7.0 to 8.5). In this regard, in accordance with the invention, depending on the added amount of phosphorus, 1200 to 1400 ppm of potassium with respect to the quantity of T1O2 is used.

The metatitanic acid washed in the earlier stages which have been described and treated with the alkali and phosphorus components is then dewatered using filtration equipment and supplied to the calcining furnace. During calcining, water from the supplied TiO ₂ filter cake is vaporized, and the sulphuric acid still adhering from digestion is decomposed and driven off.

Calcining the starting material, which is usually at ambient temperature, is carried out - - until a constant temperature is reached, at a temperature of at least 940°C, in particular at least 960°C and less than 1020°C, in particular less than 1010°C, in particular in the range 970°C to 990°C. Calcining is continued at a constant temperature for a period of time which depends on the calcining temperature, until the primary crystallites reach the desired size with X ₅₀ (mean primary crystallite size with 50% smaller and 50% larger than the mean value X ₅₀ ) of at least 160 nm, in particular at least 200 nm, and preferably less than 300 nm, and the calcined ΤΊΟ2 contains a proportion of less than 1 .5% by weight of rutile with respect to the weight of the particles. The time period for calcining is usually 40 to 300 minutes in the constant temperature range mentioned above.

The inventors have shown that because of impurities in the raw materials, in particular niobium, which cannot be washed out, the anatase product becomes bluer and bluer with increasing calcining temperature because, in order to compensate for the pentavalent niobium, a reduction of Ti ⁴⁺ occurs to form the blue Ti ³⁺ . To compensate for the colour, in accordance with the invention, an aluminium component is added. The addition may be in the form of aluminium sulphate or aluminium hydroxide, and the amount added is at least 1 ppm, in particular at least 10 ppm and at most 1000 ppm, in particular at most 500 ppm of Al, expressed as Al and with respect to the quantity of T1O2. Quantities beyond 1000 ppm inhibit the growth of crystals and then no longer provide the desired properties for the same temperature. A weight ratio of AI ₂ O ₃ to Nb ₂ O ₅ of between 0.17 and 0.74 has been shown to be advantageous.

The calcining temperature in cooperation with the added auxiliary agents provides a product with the desired properties as regards primary crystallite size, in particular the specific surface area, proportion of crystallites with a proportion of at most 10% (numerical) with a dimension of less than 100 nm (nano-fraction) and colour.

Thus, in accordance with the invention, a particulate T1O2 can be obtained which may have the following properties: - -

• a T1O2 content of at least 99% by weight;

• an anatase content of more than 98%, in particular more than 98.5%;

• a primary crystallite size X50 of at least 200 nm, and in particular at most 300 nm (determined from the numerical distribution);

• a numerical fraction of T1O2 with a primary crystallite size of at most 100 nm of at most 10% (X10) (determined by counting these crystallites in TEM or SEM images - Feret method);

• a specific surface area of at most 8 m ² /g, in particular < 6 m ² /g (determined by BET measurements);

• 1200 to 2400 ppm of alkali, in particular potassium, with respect to the quantity of TiO ₂ ;

• an Al content of at least 1 ppm and at most 1000 ppm of Al, expressed as Al and with respect to the quantity of ΤΊΟ2;

• a weight ratio of AI ₂ O ₃ to Nb ₂ O ₅ in the range 0.17 to 0.74;

• 0.1 % by weight and at most 0.3% by weight of P, expressed as phosphorus and with respect to the quantity of ΤΊΟ2 in the final product;

• a proportion of less than 1 .5% by weight, in particular less than 1 % by weight of rutile, with respect to the weight of the particles.

Thus, the invention concerns a particulate T1O2 with the following properties:

- a TiO ₂ content of at least 99% by weight;

- an anatase content of at least 98%;

- a primary crystallite size X50 of at least 200 nm (determined from the numerical distribution);

- a numerical fraction of TiO ₂ with a primary crystallite size of at most 100 nm of at most 10% (X ₁₀ );

- a specific surface area of at most 8 m ² /g (determined by BET measurements);

- 1200 to 2400 ppm of alkali, in particular potassium, with respect to the quantity of TiO _2;

- an Al content of at least 1 ppm and at most 1000 ppm of Al, expressed as Al and with respect to the quantity of T1O2;

- a weight ratio of AI2O3 to Nb2O ₅ in the range 0.17 to 0.74; and - -

- at least 0.1 % by weight and at most 0.3% by weight of P, expressed as phosphorus and with respect to the quantity of ΤΊΟ2.

A particulate TiO ₂ as defined above, with an anatase content of at least 98.5% by weight;

A particulate ΤΊΟ2 as defined above, with a numerical fraction of ΤΊΟ2 with a primary crystallite size of at most 100 nm of at most 8%;

A particulate ΤΊΟ2 as defined above, with a specific surface area of at most 6 m ² /g (determined by BET measurements);

A particulate ΤΊΟ2 as defined above, with a primary crystallite size X50 of at most 300 nm; as well as

A process for the production of particulate TiO ₂ as defined above, in which metatitanic acid which is preferably obtainable using the sulphate process and is in the form of a suspension, is supplemented with an alkali compound in a quantity of 1200 to 2400 ppm of alkali, a phosphorus compound in a quantity of at least 0.1 % by weight and at most 0.3% by weight of P, expressed as phosphorus, and an aluminium compound in a quantity of at least 1 ppm and at most 1000 ppm of Al, expressed as Al, wherein all of the quantities are respectively with respect to the quantity of ΤΊΟ2, is preferably dewatered and dried and then undergoes calcining in a manner such that the particulate ΤΊΟ2 is held at a constant temperature of between at least 940°C and at most 1020°C for a period of time until the numerical fraction X ₅₀ of ΤΊΟ2 has a primary crystallite size of at least 200 nm, and the particulate ΤΊΟ2 obtained after cooling undergoes a grinding treatment, in particular dry grinding.

Because of its smaller quantities of particle sizes in the nanometre range, the ΤΊΟ2 material in accordance with the invention is suitable for those applications in which such a proportion of such particle sizes might be considered to be unsafe, for example in the field of foodstuffs, pharmaceuticals, cosmetics and care products for - - the human body, as well as in polymers which are used as packaging for foodstuffs or as fibres. In the latter applications, advantageously, the TiO ₂ material in accordance with the invention undergoes a subsequent treatment with an organic compound. The invention therefore also concerns:

- particulate T1O2 as described above, wherein at least a portion of the surface of the particles has been coated with an organic compound or mixtures thereof; particulate ΤΊΟ2 as described above, wherein the organic compound is selected from polyglycols including polyethylene glycols, polypropylene glycols or copolymers thereof, carboxylic acids and their alkali salts, polyvalent alcohols, including trimethylolpropane, trimethylolethane, pentaerythritol and neopentyl glycol, silanes, siloxanes and siloxane derivatives, silicone oils, alkali salts of polyphosphates, amino alcohols, salts of poly(meth)acrylic acid or poly(meth)acrylate copolymers (for example sodium, potassium or ammonium polyacrylates) or mixtures thereof, as well as:

- particulate T1O2 as described above, wherein the organic substance is used in a quantity of 0.01 % to 8% by weight, particularly preferably 0.05% to 4% by weight, more particularly preferably 0.1 % to 1 .5% by weight, with respect to the total weight of the particulate TiO ₂ .

Experimental section

Description of the measurement methods used

Determination of mean particle size

In order to determine the mean particle size of titanium oxide with general formula TiO(2-x)(OH) ₂ x in which 0 < x < 1 , the aqueous "titanium oxide suspension" was initially diluted in a solution of 1 g of Calgon/I of deionized water in order to produce a concentration of approximately 0.4 g of TiO ₂ in 60 ml of solution. The "titanium oxide" suspension diluted in this manner was initially dispersed for 5 min in an ultrasound bath (for example a Sonorex Super RK106, from Bandelin) with stirring, and then dispersed for 5 min with an ultrasound finger (Sonifier W-450 from Branson with a gold booster for amplitude reinforcement and ¾ inch horn). The particle size distribution was determined by means of a photon correlation spectrometer using Zetasizer Advanced Software, for example the Zetasizer 1000HSa, from Malvern. A measurement was carried out with multimode - - acquisition at a temperature for the measurement of 25°C. The mean particle size d ₅₀ given was the d ₅₀ value for volume distribution, which corresponds to the mass distribution with density taken into account.

Determination of phase and crystallite size in accordance with Scherrer

In order to determine the crystal phase (phase identification), an X ray diffractogram was recorded. On it, the intensities according to the Bragg relationship of the X rays deflected by the lattice planes of a crystal were measured against the angle of deflection 2 theta. A phase has a typical X ray diffraction diagram.

Measurement and analysis

The material to be investigated was painted onto the preparation support with the aid of an object carrier. The powder diffractonnetry data was analysed with the aid of the JCPDS powder diffractonnetry database. The phase was identified when the measured diffraction diagram matched the recorded pattern of spots.

Typical measurement conditions were as follows: 2 theta = 10° - 70°, measured in steps of 2 theta = 0.02°, measurement period per step: 1 .2 s.

The crystallite size was determined using the Scherrer method from the half-height widths of the anatase reflection at a 2 theta of 25.3° using the following formula:

D crystallite = K ^* I / (S ^* cos(theta)

wherein:

D crystallite: crystallite size [nm]

K: shape constant = 0.9

theta: angle of measured reflection 2 theta/2

S: half-height width of measured reflection

I: wavelength of X ray beam used.

Determination of specific surface area (multipoint method) and analysis of pore structure using the nitrogen gas adsorption method (N? porosimetry)

The specific surface area and the pore structure (pore volume and pore diameter) were determined by N ₂ porosimetry using the Autosorb 6 or 6B instrument from Quantachrome GmbH. The BET (Brunnauer, Emmet and Teller) specific surface - - area was determined in accordance with DIN ISO 9277; the pore distribution was measured in accordance with DIN 66134.

Sample preparation (N? porosimetry)

The sample weighed into the measurement cell was initially dried in the heating station for 16 h under vacuum. Next, it was heated under vacuum to 180°C over approximately 30 min. The temperature was then maintained for one hour, still under vacuum. The sample was considered to have been sufficiently degassed when in the degasser a pressure of 20 - 30 millitorr was set and, after uncoupling the vacuum pump, the needle on the vacuum gauge remained stable for approximately 2 minutes.

Measurement/analysis (N? porosimetry)

The entire N ₂ isotherm was measured using 20 adsorption points and 25 desorption points. The measurements were analysed as follows:

Specific surface area (multi-point BET):

5 measurement points in the analysis range of 0.1 to 0.3 p/pO

Total pore volume analysis:

The pore volume was determined in accordance with the Gurvich rule

(determination from the last adsorption point)

The total pore volume was determined in accordance with DIN 66134, applying the Gurvich rule. In accordance with what is known as the Gurvich rule, the total pore volume of a sample is determined from the final pressure point in the adsorption measurement:

p. pressure of adsorptive

pO. saturated vapour pressure of adsorptive

Vp. specific pore volume in accordance with Gurvich rule (total pore volume at p/Po = 0.99) reached by quasi-final adsorption pressure point obtained from the measurement. - -

Determination of mean pore diameter (hydraulic pore diameter): the relationship 4Vp/A _B ET was used for the calculation, corresponds to the "Average Pore Diameter". ABET specific surface area in accordance with ISO 9277.

Crystallite size determination of calcined TiO? samples

The method determines dimensional data of pigment crystallites, such as the number and volume distribution, the nano-fraction, shape information and typical parameters such as Χιο, X50 and Xgo- With the aid of electron microscope images, the edges of the crystals were outlined using a touch-sensitive screen. The object data obtained in this manner are brought together in an overall distribution.

The pigment was dispersed with a solvent and the suspension was dried on a support. A SEM and/or TEM image was produced at four different locations of the support,. If a nanoclassification was to be carried out in accordance with EU regulations, then the magnification has to be at least V=50000.

The SEM or TEM images were analysed using Image Pro Plus software with the aid of a macro. Here, the outer edges of the individual crystallites are outlined with the stylus, as can be seen in Figure 1 - an illustration of the measurement procedure and types of diameter.

In total, at least 500 particles have to be assessed on all the images. The dimensional data obtained in this manner was analysed using an Excel spreadsheet.

The analysis contains:

• volume and numerical distribution of the mean (primary) crystallite diameter (ECD)

• numerical distribution of the smallest (primary) crystallite diameter (Feret min)

• nano-fraction in accordance with EC regulations

• X10, X50, X90 distribution data

In order to determine the nano-fraction, the smallest diameter flank (Feret _min ) - - and the mean diameter from the crystal size data (ECD: Equivalent) were determined. The calibration was carried out from the scale bars of each individual image.

Determination of titanium, expressed as TiO?

Determination of TiO ₂ content on pressed powder pellets using X ray fluorescence analysis (XRFA)

Determination of Al

The Al content of the pressed powder pellets was determined using X ray

fluorescence analysis (XRFA).

Determination of P

The P content of the pressed powder pellets was determined using X ray

fluorescence analysis (XRFA).

Determination of Nb

The Nb content of the pressed powder pellets was determined using X ray fluorescence analysis (XRFA).

Determination of K

The K content of the pressed powder pellets was determined using X ray

fluorescence analysis (XRFA).

Determination of colorimetric measurement b ^* (in accordance with CIELAB, DIN 6174 or ISO 7724/2)

Determination of colorimetric measurements L ^* , a ^* , b ^* using the CIELAB system from the measurement of the standard colour values X, Y, Z of pressed powder pellets using the Color Eye 7000A measurement system (pellets produced in accordance with DIN 5033 T9).

Measurement protocol: illuminant C, 2° standard observer, measurement geometry: 45/0 - -

The invention will be explained in more detail by means of the following Comparative Examples and Examples.

Comparative Example A:

A titanyl sulphate solution containing 230 g/l of T1O2 was obtained using the known sulphate process for the production of titanium dioxide by digesting titanium slag in concentrated sulphuric acid, then dissolving the digestion cake obtained in water and separating the undigested residues. In a subsequent Blumenfeld hydrolysis, metatitanic acid, a precursor of titanium dioxide, was precipitated out of the titanyl sulphate solution as a solid by heating to 98°C and adding hot water. The metatitanic acid precipitated out during the hydrolysis was washed at a temperature of 80°C on suction filters with 8I of demineralized water per kg of T1O2, and then treated for 1 hour at 80°C with Ti(lll) solution in order to remove adsorbed heavy metals and then washed again on a suction filter with 8 I of demineralized water per kg of TiO ₂ . Next, the filter cake was elutriated with water to 300 g/l of T1O2 (washed metatitanic acid).

Potassium sulphate powder (1650 ppm of potassium expressed as T1O2) and 85% phosphoric acid (0.14% by weight of phosphorus expressed as TiO ₂ ) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 930°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example B:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of TiO ₂ with water.

Potassium sulphate powder (1650 ppm of potassium expressed as T1O2), 85% phosphoric acid (0.14% by weight of phosphorus expressed as T1O2) and aluminium hydroxide powder (1 10 ppm of aluminium expressed as TiO ₂ ) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 930°C in a - - laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example C:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of ΤΊΟ2 in water.

25% potassium hydroxide (1650 ppm of potassium expressed as ΤΊΟ2), 85% phosphoric acid (0.14% by weight of phosphorus expressed as ΤΊΟ2) and aluminium sulphate powder (1 10 ppm of aluminium expressed as ΤΊΟ2) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 930°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example D:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of T1O2 with water.

Potassium sulphate powder (1650 ppm of potassium expressed as T1O2), 85% phosphoric acid (0.14% by weight of phosphorus expressed as T1O2) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 980°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example E:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of T1O2 with water.

Potassium sulphate powder (1650 ppm of potassium expressed as TiO ₂ ), 85% phosphoric acid (0.14% by weight of phosphorus expressed as T1O2) and aluminium hydroxide powder (1 10 ppm of aluminium expressed as T1O2) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, - - stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 1040°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example F:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of ΤΊΟ2, with water.

Potassium sulphate powder (1650 ppm of potassium expressed as ΤΊΟ2), 85% phosphoric acid (0.14% by weight of phosphorus expressed as TiO ₂ ) and aluminium hydroxide powder (1000 ppm of aluminium expressed as ΤΊΟ2) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 980°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Comparative Example G:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of T1O2 with water.

Potassium sulphate powder (1 100 ppm of potassium expressed as T1O2), 85% phosphoric acid (0.09% by weight of phosphorus expressed as TiO ₂ ) and aluminium hydroxide powder (1 10 ppm of aluminium expressed as T1O2) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 930°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Example 1 :

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of T1O2, with water.

Potassium sulphate powder (1650 ppm of potassium expressed as T1O2), 85% - - phosphoric acid (0.14% by weight of phosphorus expressed as TiO ₂ ) and aluminium hydroxide powder (1 10 ppm of aluminium expressed as TiO ₂ ) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 980°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Example 2:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of TiO ₂ , with water.

25% potassium hydroxide (1650 ppm of potassium expressed as ΤΊΟ2), 85% phosphoric acid (0.14% by weight of phosphorus expressed as ΤΊΟ2) and aluminium hydroxide powder (1 10 ppm of aluminium expressed as TiO ₂ ) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 980°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill.

Example 3:

A washed metatitanic acid was produced as described in Comparative Example A and adjusted to 300 g/l of TiO ₂ , with water.

Potassium sulphate powder (1650 ppm of potassium expressed as TiO ₂ ), 85% phosphoric acid (0.14% by weight of phosphorus expressed as TiO ₂ ) and aluminium hydroxide powder (60 ppm of aluminium expressed as TiO ₂ ) were added with stirring to 6 kg of this washed metatitanic acid in a mixing tank with a propeller stirrer, stirring was continued for a further 1 hour, drying was carried out in a drying cabinet at 105°C and then calcining was carried out in batches for 90 min at 980°C in a laboratory muffle furnace. The calcined material was ground in a spiral jet mill. - -

Table 1 - Summary of experimental conditions

Visual perceived colour by observer of TiO ₂ powder: CIE b ^* > 0.5 yellowish white

CIE b ^* 0 to 0.5 neutral white

CIE b ^* < 0 bluish white - -

The product in accordance with the invention is distinguished by a crystallite size with a small numerical fraction of crystallites of at most 100 nm of at most 10%, with a warm shade, which is of advantage, in particular with cosmetic products.