GB2035978A

GB2035978A - A Process for the Treatment of Halide Titanium Dioxide Pigments to Improve Durability

Info

Publication number: GB2035978A
Application number: GB7930026A
Authority: GB
Original assignee: Tioxide Group Ltd
Current assignee: Tioxide Group Ltd
Priority date: 1978-11-09
Filing date: 1979-08-30
Publication date: 1980-06-25
Also published as: FR2440976A1; FR2440976B1; CA1140097A; GB2035978B; AU519446B2; ZA794869B; JPS5757409B2; AU5082979A; DE2944791A1; JPS5571630A

Abstract

A process for improving the durability of titanium dioxide pigment produced by the oxidation of a titanium tetrahalide when incorporated into a binding medium which is exposed to light comprising fluid energy milling particulate reactor discharge in the presence of steam and thereafter subjecting the milled discharge to dry heat at a temperature of at least 400 DEG C.

Description

SPECIFICATION A Process for the Treatment of Halide Titanium Dioxide Pigments to Improve Durability The present invention relates to a process for improving the durability of titanium dioxide pigments made by the halide process, particularly when incorporated into binding media which are then exposed to light.

It is known that titanium dioxide pigments, both when made by the so-called sulphate or halide processes, ultimately cause breakdown of the binding medium into which they are incorporated, for example to form pigmented surface coatings or in polymers etc. and it is desirable to produce titanium dioxide pigments which cause as little breakdown of binding medium as possible, i.e. pigments which are of improved durability. One method of testing such pigments is to incorporate the pigment in the desired binding medium and to expose the product (when applied to a suitable substrate in the case of surface coatings) either to natural sunlight or to artificially-produced radiation for a prolonged period of time before examining the product for evidence of breakdown of the binding medium.Where the product is exposed to artifically-produced radiation, this is commonly carried out in an apparatus known as a Marr Weather-O-Meter (which name is a Registered Trade Mark).

It is an object of this invention to provide a process whereby the durability of titanium dioxide pigment made by the halide process, and particularly that of pigment made by the so-called chloride process, may be improved.

Accordingly, the present invention is a process for the treatment of reactor discharge from the oxidation of a titanium tetrahalide comprising subjecting particulate reactor discharge to fluid energy milling in the presence of steam and thereafter to dry heat at a temperature of at least 4000 C.

The particulate reactor discharge is normally produced by the oxidation of titanium tetrachloride in the vapour phase by means of oxygen or an oxygen-containing gas,7or example air or oxygenenriched air.

The reactor discharge is then separated from its reaction environment, for example by cooling of the reaction products and by filtration or by cyclone separation, and is then subjected to fluid energy milling in the presence of steam.

Such fluid energy milling may be carried out in any suitable apparatus, for example in a "Microniser" (which name is a Registered Trade Mark) or in a tube or pipe mill, for example of the type shown and described in U.K. Patent Specifications 1,019,833 and 1,019,834. The steam is usually used as the milling medium and is injected in to the mill at super-atmospheric pressure and temperature.

It has been found that excellent results are obtained using steam at a temperature in the range 1 500C to 3500C and particularly at temperatures in the range 2000C to 3000C and, of course, at corresponding pressures, at least in mills of the type previously referred to. Optimum steam/pigment ratios can readily be determined by experiment and will depend to some extent on the type of fluid energy mill used. Generally, however, such ratios in the range 1.0 to 6.0 and particularly in the range 1.5 to 3.5, have been found to be very effective.

After subjecting the reactor to fluid energy milling in the presence of steam, the thus-treated pigment is subjected to dry heat at a temperature of at least 4000C. It is preferred to subject the pigment to dry heat at a temperature in the range 4000C to 8000C and preferably to one in the range 5000Cto6000C.

By the term "dry heat" is meant subjecting the pigment to the required temperature without the introduction into the treatment vessel of added steam. For example, the pigment may be treated by exposure to a heated gas stream (not containing added steam) either by direct addition of the pigment to the gas stream or in a fluidised bed of particulate solid heated and fluidised by a suitable hot gas such as air or it may be heated (and preferably agitated, e.g. by stirring or tumbling) in a heated oven or furnace.

The optimum duration of the exposure of the pigment to dry heat may readily be determined by trial and experiment and it is clearly commerically advantageous to carry out the process for the shortest time which is consistent with obtaining the desired improved durability of the pigment in order to reduce unit costs and to obtain the maximum throughput. Additionally the optimum duration of the process may vary with the nature of the particulate titanium dioxide to be treated.It is believed, however, that exposure to dry heat as an agitated mass for a period of 1 to 60 minutes and particularly for a period of 5 to 30 minutes or exposure in a fluidised state for 0.2 to 60 seconds and particularly for 1 to 10 seconds, will normally give a pigment of high durability when compared with similar pigment which has not been treated by the process of the present invention or which has been subjected to either fluid energy milling in the presence of steam, or to dry heat, alone.

It is believed that the process of the present invention gives titanium dioxide pigment of high durability because the initial fluid energy milling in the presence of steam hydrolyses or otherwise decomposes impurities on the surface of the reactor discharge and that the subsequent exposure to dry heat under the conditions described modifies the hydrolysed surface and/or removes those products of hydrolysis or other form of decomposition from the surface of the titanium dioxide particles.

The following Examples show the improvement in durability (expressed as the durability ratio) obtained in reactor discharge treated by the process of the present invention when compared with that of a similar starting material which has not been treated by the process of the present invention and also show the effect of varying the steam temperature during fluid energy milling.

Example 1 Reactor discharge recovered from the vapour phase oxidation of titanium tetrachloride was divided into three separate portions. The separate portions were treated as follows, (a) Portion untreated (b) Portion subjected unstirred to dry heat in an oven at 5500C for 1 5 minutes (c) Portion subjected to treatment in the fluid energy pipemill shown in U.K. Specification 1,019,833 using steam at 2500C (but not further treated).

Part of the product from (c) was then heated to a temperature of 5500C for a period of 1 5 minutes as described for sample (b) (i.e. this product-designated(d)-was made according to the process of the present invention. The durability ratio of each of the samples (a) to (d) was determined as set out below and the following results were obtained Sample Durability ratio (a) 0.62 (b) 0.45 (c) 0.47 (d) 0.38 Example 2 The process described in Example 1 was repeated with a sample of reactor discharge having a different durability ratio and the following results were obtained: Sample Durability ratio (a) 0.57 (b) 0.55 (c) 0.46 (d) 0.39 Example 3 Reactor discharge obtained as described in Example 1 was taken and divided into three portions.

The durability ratio of the untreated reactor discharge was determined and each of the three portions was subjected to fluid energy milling in a pipemill of the type described in U.K. Patent Specification 1,019,833 in the presence of steam. The temperature of the steam varied between 2500C and 3500C for the three samples, as shown.

The durability ratios of the samples which were fluid energy milled in the presence of steam at the various steam temperatures were determined and the products of this milling were subjected to dry heat (at 5500 C) for 1 5 minutes and the durability ratios were again determined.

The results were as shown below: Pipemllled and Untreated discharge Pipemllled discharge heated discharge (DR) (DR) (55000) (DR) 2500C 0.49 0.37 0.59 3200C 0.59 0.40 3500C 0.64 0.55 Example 4 The process described in Example 3 was repeated with a different sample of reactor discharge and this was fluid energy milled in the presence of steam at differing temperatures in the range 2000C to 3300C, as shown, prior to dry heat treatment (unstirred) at 5500C for 1 5 minutes.

The results were as shown below: Pipemllled and Untreated discharge Pipemllled discharge heated 1550 OCJ (DR) (DR) discharge (DR) 2000C 0.44 0.35 0.66 2500C 0.50 0.32 3000C 0.53 0.38 3300C 0.60 0.37 Example 5 Four different samples of reactor discharge from the vapour phase oxidation of titanium tetrachloride were taken and treated as described in Example 1. The durability ratios were determined as set out below and the following results were obtained.

Sample Durability Ratio (a) 0.64 0.62 0.49 0.47 (b) 0.51 0.50 0.39 0.44 (c) 0.61 0.57 0.53 0.49 (d) 0.44 0.45 0.34 0.38 The measurement of the Durability Ratio is defined and carried out as follows.

The property measured is the mass loss (as defined in Experimental and Theoretical Studies of the Durability of Paint System pigmented with Titanium Dioxide-Evans and Murley, VI FATIPEC CONGRESS,1962) from a paint film containing the experimental pigment(s) when compared with a similar film containing a standard pigment(s); the mass loss being induced by accelerated weathering, using a weathering cycle generally accepted by the paint trade and as defined in British Standard 3900, Section F3.

Films of a paint(s) containing the test pigment(s) and other paint films containing one or more standard pigments are prepared under standard conditions by incorporating the pigments by bead milling into an air-drying alkyd resin solution, normally Uralac (a Registered Trade Mark) P470 which is a long oil linseed pentaerythritol alkyd resin, to produce paint(s) which are spun onto a stainless steel substrate(s) to a dry film thickness of approximately 35,us.

These films, after drying, are exposed alongside each other, i.e. under identical conditions, in the twin-arc Marr Weather-O-Meter.

The mass loss of the panel coated with the paint(s) containing the test pigment(s) is compared with the mass loss of the panel(s) containing the standard pigment(s), after a series of exposure times.

A durability ratio (D.R.) expressed as experimental film loss/standard film loss of 1.00 indicates equivalent performance of the panel coated with a paint containing one of the standard pigment(s). A D.R. of less than 1.00 indicates performance superior to that of the standard(s).

Reactor discharge when treated by the process of the present invention will usually be treated further, for example in the manner in which halide pigments are normally treated to improve their dispersion in organic media, their gloss and to further improve their durability.

The gloss of such pigments may be improved by additional milling, for example by additional fluid energy milling and/or mechanical milling and/or sand milling as an aqueous slurry. This property may also be improved by one or more classification steps whereby particles of less than optimum size are removed or at least reduced in proportion.

The properties of dispersion and of further improvements in durability may for example be achieved by coating the pigment particles with one or more hydrous oxides of elements such as silicon, aluminium, cerium, zirconium or titanium or with a phosphate of one or more of these elements, as is well known in the art.

Additionally or alternatively, the pigment particles may be coated with one or more organic compounds, for example with a polyhydroxy compound such as pentaerythritol, trimethylol propane or an ethanolamine. Such organic coatings are, again, well known in the art and are commonly applied to assist in improving further the dispersion of the pigmentary particles in organic media.

After coating titanium dioxide particles prepared by the process of the present invention it is normally advisable to subject the coated particles to more milling to break up aggregates which tend to form during the coating processes.

The final pigment may then be sold, for example either in the form of an aqueous slurry or as a dried particulate solid. It is of advantage, if the pigment is to be sold in the form of an aqueous slurry, to avoid the necessity for drying the pigment prior to redispersing it in water to form the desired slurry and any milling which may be required in this case may be provided by sand milling or by mechanically milling an aqueous slurry.

The oxidation of the titanium tetrahalide, for example titanium tetrachloride, to produce the reactor discharge which is to be treated by the process of the present invention will normally be carried out under conditions which are well known for such an oxidation to produce pigmentary titanium dioxide. For example, the oxidation is carried out at an elevated temperature, for example at least 8000 C, and the titanium tetrahalide is maintained in the vapour phase. The oxidation commonly takes place in the presence of additives, for example in the presence of a minor proportion of an aluminium halide, normally the chloride, and, if desired, a minor proportion of a silicon halide, normally the chloride. Minor proportions of water vapour and/or a source of an alkali metal ion, particularly potassium, are also commonly added to the oxidation.Such additives, which as noted above are well known in the prior art, improve the quality of the titanium dioxide produced for pigmentary purposes.

The temperature in the oxidation reactor is achieved by the introduction of heat into the reaction zone. This may be provided directly by the combustion products of a suitable gas, for example of carbon monoxide, or by the provision of a gas which has been heated by its passage through an electric arc. The gas in the latter process may be an inert gas or it may be one of the reactants, for example oxygen. Alternatively, one or both of the reactants may be preheated indirectly, for example by means of a heat exchanger prior to its introduction into the reaction zone. Where the reactants are preheated to reaction temperature they should, of course, be separately introduced into the reaction zone to prevent premature reaction. Normally, in such a process, one reactant, usually the oxygen, is heated to a temperature in excess of the desired reaction temperature and the other reactant, the titanium tetrahalide, is heated to a temperature sufficient to maintain it in the vapour phase; the temperature of the mixed gases being sufficient to produce the required temperature in the reaction zone.

Claims

1. A process for the treatment of reactor discharge from the oxidation of a titanium tetrahalide comprising subjecting particulate reactor discharge to fluid energy milling in the presence of steam and thereafter to dry heat at a temperature of at least 4000 C.

2. A process as claimed in claim 1 wherein the titanium tetrahalide is titanium tetrachloride.

3. A process as claimed in claim 1 or 2 wherein the fluid energy milling is carried out using steam at a temperature in the range 1 500C to 3500C.

4. A process as claimed in claim 3 wherein the steam is at a temperature in the range 2000C to 3000C.

5. A process as claimed in any of the preceding claims wherein the steam/pigment ratio during fluid energy milling is in the range 1 to 6.

6. A process as claimed in claim 5 wherein the steam/pigment ratio is in the range 1.5 to 3.5.

7. A process as claimed in any of the preceding claims wherein the pigment is subjected to dry heat at a temperature in the range 4000C to 8000 C.

8. A process as claimed in claim 7 wherein the pigment is subjected to dry heat at a temperature in the range 5000C to 6000 C.

9. A process as claimed in any of the preceding claims wherein the pigment is subjected to dry heat for a period of 1 to 60 minutes.

10. A process as claimed in claim 9 wherein the pigment is subjected to dry heat for a period of 5 to 30 minutes.

11. A process as claimed in any of the preceding claims wherein the pigment is subjected to dry heat, when in the fluidised state, for a periodof 0.2 to 60 seconds.

12. A process as claimed in claim 11 wherein the pigment is subjected to dry heat for a period of 1 to 10 seconds.

13. Particulate titanium dioxide when prepared by a process as claimed in any one of claims 1 to 12.

1 4. A process as claimed in any of the preceding claims 1 to 12 and substantially as shown in Examples 1 to 4.