GB2037725A - Pre-calcination additives for hydrous TiO2 - Google Patents

Abstract

A process for the production of calcined pigmentary titanium dioxide wherein a solid particulate source of at least two of the elements sodium, potassium, zinc, antimony and aluminium are mixed with a quantity of water and the resulting mixture is formed into a free-flowing particulate solid for addition to the hydrous TiO2 before the latter is passed to calcination.

Description

SPECIFICATION Pre-calciner additives The present invention relates to the production of pigmentarytitanium dioxide by the calcination of hydrous titanium dioxide in the presence of additives.

It is known, prior to the calcination of hydrous titanium dioxide, to addition the titanium dioxide with additives to improve the properties of the calcined product, for example when intended for subsequent use as a white pigment. Such additives are, inter alia, sources of potassium, sodium, phosphate, zinc, antimony and/or aluminium.

Hitherto, such additives have been added separately to the hydrous titanium dioxide in the dry finely-divided form in which they are supplied, e.g.

as potassium sulphate, sodium carbonate, zinc oxide, antimony oxide and/or aluminium sulphate. The separate additions of such finely divided dry materials have a number of disadvantages. For example, there is the likelihood of loss of material due to spillage etc. (which also makes accurate additioning more difficult) and there is the loss of time and increased labour required in making such separate additions. Finally, there is a problem in ensuring uniform distribution of such separate additions in the hydrous titanium dioxide, both prior to, and during calcination.

It is an object of the present invention to provide a process whereby loss of additives may be reduced and/or whereby the accuracy of addition and/or the distribution of the additives in the hydrous titanium dioxide, prior to and/or during calcination, may be improved. There may also be a saving in time and labour compared with previously used processes.

Accordingly, the present invention is a process for the production of calcined pigmentarytitanium dioxide comprising mixing a solid particulate source of at least two of the following elements, sodium, potassium, zinc, antimony and aluminium with a quantity of water, forming the resulting mixture into a free-flowing, particulate solid and thereafter adding a predetermined amount of said free-flowing particulate solid to hydrous titanium dioxide prior to its calcination.

The invention also comprises a free-flowing parti culate solid suitable for addition to hydrous titanium dioxide and containing at least two solid particulate sources of the following elements, sodium, potas sium, zinc, aluminium and antimony together with added water.

The hydrous titanium dioxide to be treated bythe process of the present invention is normally obtained from the thermal hydrolysis of titanyl sulphate solutions which solutions have, in turn, been omitted by the digestion of an iron-containing titaniferous material such as ilmenite or a titanifer ous slag in concentrated sulphuric acid followed by the subsequent dissolution of the resulting solid mass with water or with dilute sulphuric acid, normally in the presence of steam. The hydrolysis of the titanyl sulphate solution is usually carried out by heating the solution in the presence of nuclei, which may be anatase - or rutile - inducing, as desired.

Prior to hydrolysis the iron sulphate content of the titanyl sulphate solution may have been reduced, for example by crystallisation, and/or the acid content of the solution may have been adjusted.

After hydrolysis, the hydrous titanium dioxide pulp is recovered, for example by settling and/or by filtration and by washing. At least one stage during the washing sequence is commonly a leaching process carried out with an aqueous solution containing trivalent titanium in order to improve the removal of impurities such as iron.

The filtration may be carried out using Moore filters and/or rotary filters and is continued until the hydrous titanium dioxide has an acceptably low level of undesirable impurities, for example, of iron.

After the washing process the hydrous titanium dioxide is commonly resuspended (or "repulped") in water between filtrations and at least some of the addition may be made to the resuspended hydrous pulp before the final filtration stage and prior to passing the filter cake to the calciner or all, or some, of the addition may be made to the filter cake during its passage from the final filter to the calciner.

It is preferred to add the free-flowing particulate solid of the present invention during the passage of the hydrous titanium dioxide from the final filter to the calciner.

The hydrous titanium dioxide is commonly conveyed from the final filter to the calciner by means of a screw conveyor the movement of which assists in the distribution of the particulate solid through the hydrous titanium dioxide.

The proportions of the separate additives in the free-flowing particulate solid to be added to the hydrous titanium dioxide will normally vary with the requirements of the process. Generally the freeflowing particulate solid will contain a source of potassium, expressed as the sulphate, in the range 10 to 50, and preferably in the range 20 to 45, parts by weight and in the case of sodium, expressed as the carbonate, the amounts are conveniently in the range 0 to 23 and are preferably in the range 0 to 18, parts by weight.

The source of zinc, expressed as zinc oxide, where included, is normally in the range 20 to 180 and preferably in the range 70 to 110, parts by weight.

As to the source of aluminium, this is normally present in the free-flowing solid of the present invention in an amount in the range 70 to 300 parts by weight and preferably in an amount in the range 75 to 250, these amounts being expressed as A12(SO4)3 16H2O (which is the form in which the source of aluminium is usually added).

Antimony, when added, is normally present in the additive in an amount in the range 10 to 35 parts by weight and preferably is present in an amount in the range 15 to 30, these amounts being expressed as antimony oxide, Sb203.

Using the above proportions of additives it has been found that the amount of water to be added is conveniently in the range 10 to 30 and is preferably in the range 10 to 25, parts by weight.

After mixing the ingredients, including the water, the mixture may either be ground to provide a free-flowing particulate solid or it may, if desired, be pelletised to provide a similarly free-flowing particulate solid, but of larger average particle. The pelletisation is conveniently achieved by trundling and this may be continued until pellets having an average size of up to about 1/4" in diameter are formed.

It is preferred to utilise, as the source of potassium, potassium sulphate, and as the source of sodium, anhydrous sodium carbonate, for example as soda ash.

Zinc oxide is the preferred source of zinc, aluminium sulphate of aluminium and antimony oxide as the source of antimony where these additives are present in the free-flowing solid of the invention.

The sources of potassium, sodium, zinc, antimony and aluminium may, of course, be compounds other than those mentioned above and alternative sources are well known in the art. It is however essential that such sources when used are capable of forming, when mixed with the appropriate amount of water, the free-flowing particulate solid of the present invention. It is preferred not to add as the source of potassium, potassium carbonate, since the presence of this compound in the additive may necessitate drying the product at an elevated temperature, after pelletisation, in order to produce a free-flowing powder.

The amount of the free-flowing particulate solid added to the hydrous titanium dioxide will vary depending upon the type of pigment to be produced and the desired amount of each additive will be known to the manufacturerfor his pigments. Generally, however an amount of 0.5 to 7 parts by weight per 100 parts by weight of TiO2 is added, the TiO2 being expressed in the an hydrous form.

Generally, the additive which varies most in quantity between different pigments is the source of zinc, where this is to be added to the pigment. The free-flowing particulate solid of the present invention may conveniently be made up in two forms, one containing an amount of zinc in the lower half of the broader range of previously suggested amounts of this additive and one containing an amount of zinc in the upper half of this range.

The following Examples show methods of carrying out the present invention.

Example 1 A mixture was produced in a paddle mixer containing 100 parts by weight of zinc oxide, 25.5 parts by weight of potassium sulphate and 13.6 parts by weight of anhydrous sodium carbonate (as soda ash) and to this mixture was added, during mixing, 21 parts by weight of water.

After the addition of the water the mixture broke down into fine free-flowing pellets and these were discharged through a vibrating screen which retained particles greater than 6 m.m. in diameter.

The product was added to hydrous titanium dioxide as it passed from the final filter to the calciner in a screw conveyor in an amount of 1.6 parts by weight of product to each 100 parts by weight of titanium dioxide.

The additioned titanium dioxide was then calcined and the product compared with (a) material to which similar separate additions had been made to resuspended hydrous titanium dioxide prior to final filtration of the material before calcining and (b) material to which similar separate additions had been made to a similar hydrous titanium dioxide during its passage from the final filter to the calciner.

In Cases (a) and (b) the material was calcined under the same conditions as for the additioned pigment of the present invention.

It was found that it was much quicker and easierto add the free-flowing particulate product of the present invention to the hydrous titanium dioxide and that no losses occured during the addition whereas there was a significant loss of additives where these were made separately, particularly on the walls of the container from which the additions were made.

The additioned and calcined products from the process of the present invention and from (a) and (b) above, when compared, were found to be identical and could not be differentiated.

Example 2 The process of Example 1 was repeated with the exception that in place of the water there was added a hot solution (50%) of aluminium sulphate (as A12(SO4)3 16.H2O) in sufficient quantity to give 30 parts of aluminium, expressed as Al2O3, per 100 parts of zinc oxide present in the mixture.

The product was a free-flowing particulate solid which was easily handled and did not adhere to the walls of its container.

Example 3 A mixture was formed in a paddle mixer as described in Example 1 but containing only 25.5 parts by weight of potassium sulphate, 13.6 parts by weight of sodium carbonate and 28 parts by weight of solid aluminium sulphate.

Water, 21 parts by weight, was added during mixing and as mixing progressed free-flowing pellets were formed which, when sieved as described in Example 1, were easily handled and showed no stickiness or tendency to adhere to the walls of their container.

Example 4 The process of Example 1 was repeated except that the sodium carbonate was replaced by an equimolar amount of potassium sulphate.

The product was slightly more crisp than that from Example 1 and gave a product with excellentflow characteristics which showed no tendency to adhere to its container.

Example 5 The process of Example 1 was repeated except that the sodium carbonate and potassium sulphate were replaced by an equimolar quantity of potassium carbonate.

The product was noticeably softer and stickier and required heating for 10 minutes at 140"C to give an acceptably free-flowing powder.

Claims (10)

1. A processforthe production of calcined pigmentarytitanium dioxide comprising mixing a solid particulate source of at least two of the following elements, sodium, potassium, zinc, antimony and aluminium with a quantity of water; forming the resulting mixture into a free-flowing particulate solid and thereafter adding a predetermined amount of said free-flowing particulate solid to hydrous titanium dioxide prior to its calcination.

2. A process as claimed in claim 1 wherein the free-flowing particulate solid is added to the hydrous titanium dioxide during the passage of the latter from the final filter to the calciner.

3. A process as claimed in claim 2 wherein the hydrous titanium dioxide is conveyed to the calciner by means of a screw conveyor.

4. A process as claimed in any of the preceding claims wherein the amount of additives in the free-flowing particulate solid to be added to the hydrous titanium dioxide is for potassium, in the range 10 to 50 parts by weight (when expressed as the sulphate); for sodium, 0 to 23 parts by weight (expressed as the anhydrous carbonate); for zinc, 20 to 180 parts by weight (expressed as the oxide); for aluminium, 70 to 300 parts by weight (expressed as A12(504)3 6H2O); for antimony, 10 to 35 parts by weight expressed as Sb203) and for water, 10 to 30 parts by weight.

5. A process as claimed in claim 4 wherein the amount of the source of potassium is in the range 20 to 45 parts by weight; of sodium, 0 to 18 parts by weight; of zinc, 70 to 110 parts by weight; of aluminium, 70ato 250 parts by weight; of antimony, 15 to 30 parts by weight and of water, 10 to 25 parts by weight.

6. A process as claimed in any of the preceding claims wherein the amount of particulate freeflowing solid added to the hydrous titanium dioxide is in the range 0.5 to 7 parts by weight per 100 parts by weight of TiO2 (expressed as anhydrous TiC2).

7. A process as claimed in any of the preceding claims wherein the free-flowing particulate solid comprises pellets having an average size of up to 1/4" diameter.

8. A free-flowing particulate solid suitable for addition to hydrous titanium dioxide and containing at least two solid particulate sources of the following elements, sodium, potassium, zinc, aluminium and antimony, together with added water.

9. A process as claimed in any one of claims 1 to 7 and substantially as described in any one of Examples 1 to 5.

10. Calcined titanium dioxide when prepared by a process as claimed in any one of claims 1 to 7.

New claims or amendments to claims filed on 31 March 1980 Superseded claims 8 New or amended claims: Original claims 9 and 10 renumbered as 8 and 9 respectively.