WO1994005600A1

WO1994005600A1 - The production of fluorotitanates

Info

Publication number: WO1994005600A1
Application number: PCT/GB1993/001654
Authority: WO
Inventors: Philip John Brockerton; Sean Paul Lynch; Iain Stewart Mackirdy; David Barry Mobbs
Original assignee: Laporte Industries Ltd
Current assignee: Evonik LIL Ltd
Priority date: 1992-08-26
Filing date: 1993-08-05
Publication date: 1994-03-17
Anticipated expiration: 1995-02-26
Also published as: CN1083795A; GB9218170D0

Abstract

A process for the production of a substantially pure salt of fluorotitanic acid from an impure titanium source comprising dissolving the titanium source in hydrofluoric acid, treating the thus formed solution with an ammonium or alkali metal halide or nitrate in sufficient quantity to precipitate ammonium or alkali metal fluorotitanate and recovering the said fluorotitanate, wherein a mineral or synthetic rutile is used as the titanium source and the said source is dissolved directly in concentrated hydrofluoric acid.

Description

The production of fluorotitanates

This invention relates to the production of fluorotitanates.

Fluorotitanic acid is understood to be the compound H2 iF₆ and the products envisaged in this invention are the corresponding alkali metal salts and the ammonium salts.

Fluorotitanates find use in the metallurgical industries, for example as a constituent of aluminium master alloys or as a source of highly pure titanium metal. Potassium fluorotitanate is in common use for such purposes and is currently subject to stringent industry requirements in respect of purity, for example l-^TiFg at least 97.5%, elemental composition vanadium <0.25%, silicon <0.2%, zirconium, calcium or magnesium <0.05% any other metal compound <0.1% by weight. Fluorotitanates have been produced by processes involving the dissolution of the titanium content of titanium bearing minerals in hydrofluoric or hydrofluorosilicic acid followed by precipitation of the required fluorotitanate. European Patent Specification No. 209760 describes the production of alkali metal fluorotitanates by treating rutile ore with a 36% hydrofluorosilicate solution, evaporating to dryness over 6 hours, extracting the residue in demineralised water and precipitating potassium fluorotitanate by neutralising to a pH of 3 to 4 with potassium carbonate or potassium hydroxide. If this process is implemented with other than a highly pure ore the product may be contaminated with metallic hydrous oxides or carbonates. To counter this the use of a sodium compound as the base is taught where impure ores are treated so that the relatively soluble sodium fluorotitanate may be separated from precipitated impurities.

United States Patent Specification No. 2568341 discloses the production of potassium fluorotitanate by adding pulverised ilmenite ore containing 38% of iron calculated as FeO to a quantity of 28.6% hydrofluoric acid the addition being conducted at a rate sufficiently low to avoid, aided by the addition of increments of water, more than a gentle boiling of the reacting mass. The quantity of hydrofluoric acid is taught to be 10% to 20% in excess of that required to convert the titanium and iron components of the titaniferrous material to titanium tetrafluoride and iron fluoride. The iron content of the reacted mass is then reduced to the ferrous state by the addition of iron powder and, preferably, sulphuric acid, the insolubles are separated and potassium fluorotitanate precipitated at a temperature of about 70°C by the addition of potassium chloride. The use of potassium chloride, or as an alternative, potassium nitrate, is intended to form soluble compounds of iron and thereby reduce the contamination of the fluoro itanate product but the large quantity of iron present would make it difficult to attain the degree of purity now required. This process is adapted for the treatment of any titaniferous raw material containing significant quantities of elements other than titanium.

US Patent Specification No. 4497779 also discloses the use of potassium chloride to precipitate potassium fluorotitanate from a solution of fluorotitanic acid. In this case the solution of fluorotitanic acid is obtained by the dissolution of the highly pure titanium mineral anatase using 6.3% hydrofluoric acid solution over a contact period of 6 hours at a temperature of 190°F. The potassium fluorotitanate so produced is acceptably pure. If the process is to be applied to a titanium source which contains impurities such as iron or silicon, such as ilmenite, the recovery of titanium is considerably reduced due to incomplete dissolution and if any ferric iron is present in the liquor it must be reduced to the ferrous state by the addition of iron or other reductant.

It would be of advantage to be able to produce pure potassium fluorotitanate or other salts of fluorotitanic acid from an iron, or other impurity-containing titanium source without the use of a separate ferric iron reduction step.

The present invention provides a process for the production of salts of fluorotitanic acid by the dissolution of a source of titanium in hydrofluoric acid to form a solution containing fluorotitanic acid, or precursor thereof such as titanium tetrafluoride, treating the solution with an ammonium or alkali metal halide or nitrate to precipitate the ammonium or alkali metal fluorotitanate and recovering the said fluorotitanate, the process being characterised by the use of mineral rutile or synthetic rutile as a source of titanium and the dissolution of the source of titanium directly in concentrated hydrofluoric acid.

Despite the significant amount of impurities in both mineral rutile, which tends to average about 95% Ti(>2 wt %, and synthetic rutile, which tends to average about 90% to

95% Ti(>2 wt %, the process of this invention can be made to produce salts of fluorotitanic acid meeting the industry requirement without an intervening reduction stage and/or using a relatively short dissolution time and/or a relatively small excess of fluorine.

The use of mineral or native rutile as a source of titanium is not excluded from the present invention although it is relatively unreactive with hydrofluoric acid and may require a long period of digestion. It is found that the product of a titaniferous ore, including particularly suitably weathered il enites such as beach sands, alluvial deposits or leucoxeneε, which have been subjected to beneficiation to remove a proportion of their iron content and to convert a majority or substantially all of their titanium dioxide content into the rutile form, a product hereafter referred to as synthetic rutile, is particularly suitable for use in the practice of this invention.

An example of a beneficiation process which may be used to produce a synthetic rutile may involve treating a suitable titaniferous ore such as an ilmenite to form a pseudobrokite phase therein, for example by means of an oxidising roast. Thereafter the excess iron is reduced, for example by heating in the presence of a carbonaceous reductant, to metallic iron or to reduced iron oxide which is selectively leached from the ore to leave a residual rutile titanium dioxide product. The synthetic rutile so produced may typically contain over 90% of titanium dioxide together with a significant quantity of iron oxides, for example ferric oxide, and other impurities.

Synthetic rutiles such as have been described above are particularly reactive with hydrofluoric acid in comparison with native rutile.

The synthetic rutile utilised according to the invention may suitably contain about 0.5% to about 15% of iron calculated as Fβ2θ₃ although it is preferred that it contain not more than about 10% and particularly preferred that it contain not more than about 6% of iron on the same basis. All percentages are by weight.

Despite the use of concentrated hydrofluoric acid it is found that the process is quite manageable with regard to temperature control. The synthetic rutile, in finely divided form is suitably added to the hydrofluoric acid at a rate such that boiling does not occur, preferably so that 100°C is not exceeded, the. reaction mixture being agitated to suspend the synthetic rutile. Preferably the temperature is maintained at at least 80°C by the input of heat if necessary, for example, very suitably at about 90°C + 5°C until at least 80% very suitably at least 90% of the synthetic rutile has been dissolved. Despite this control, reaction times of the order of only about one hour may be attained. The concentrated hydrofluoric acid preferably has a concentration of at least 40% by weight. It is found that excellent dissolution can be obtained by using 60% by weight hydrofluoric acid and it should not be necessary to exceed this markedly e.g. by the use of more than 75% HF wt %. A very suitable range of concentration is 50% to 65% wt. The hydrofluoric acid should preferably be used in at least the stoichiometric quantity to react with the titanium present to form H₂TiFg, e.g. a 6:1 molar ratio but particularly preferably an excess is used. Since the use of an undue excess is wasteful we prefer not to use more than 7.5 moles of HF per atom of Ti and a very suitable quantity is 6.2 to 7.2 moles. The temperature at which the dissolution is conducted is preferably at least 60°C and may be up to the boiling point although it has been found that excellent dissolution can be obtained using a temperature of 90°C.

The time taken to dissolve the ore given suitable selection of the other parameters may preferably be from 0.5 to 2 hours, a very suitable direction being from 0.5 to 1 hour assuming that the ore is in a particulate form with not more than 1.0% wt being above 20 mesh USS (841 microns).

Preferably at least 90% wt of the ore is in t e size range of 200 to 20 mesh USS (74 to 841 microns) .

It is found that the efficiency of dissolution falls off markedly using an acid concentration below 40% wt or a temperature below 60°C or a duration below 0.5 hours and that the use of parameters having values above the preferred maxima quoted above do not give increased efficiency to any worthwhile degree.

The solution produced by the dissolution of the source of titanium may suitably be separated from the solid residue by any suitable means such as decantation, filtration or centrifugation. The solid residue generally contains a substantial proportion of the silicon, aluminium, zirconium and chromium values present in the original source of titanium as well as proportion of the iron values. It is a feature o_ the present invention that the use of a combination of a limited excess of concentrated hydrofluoric acid in the dissolution stage and the use of a halide or nitrate in the fluorotitanate precipitation stage results in a lower precipitation pH than that applying in the prior art where precipitation is conducted by means of strong bases. Preferably the precipitation pH is below 2 for example from 0 to 1.

The precipitation of fluorotitanate is preferably conducted by forming an aqueous solution of the halide or nitrate, preferably the chloride, suitably at a concentration below 50% wt and above 10% wt, for example from 10% wt to 30% wt and adding the solution to that obtained by dissolution of the source of titanium. The quantity of the precipitating salt is preferably in stoichiometric excess. This excess is preferably at least 5% and for example up to 50% on a molar basis. The precipitation is conducted at a preferred temperature of below 40°C and preferably at a temperature of from 10°C to 35°C. The slurry of fluorotitanate salt so produced is preferably washed, separated and dried by normal means. The product so obtained may have a purity considerably exceeding the industry requirements stated above.

The invention will now be illustrated by reference to the experimentation described below. in the experiments a synthetic rutile having the following composition was used as a source of titanium:

the % figures being by weight.

The synthetic rutile had a particle size of 98.8% wt greater than 200 mesh USS (74 microns) and 0.0% greater than 20 mesh USS (841 microns) . Samples of the synthetic rutile were dissolved in hydrofluoric acid under varying conditions of acid concentration and excess, and treatment time, temperature and the degree of dissolution of the ore was noted. The treatment of the synthetic rutile with the acid was conducted as follows. The synthetic rutile was added to the hydrofluoric acid with stirring so as to attain full addition in less than 1 minute. Once the addition had been completed the reaction vessel was heated to maintain a temperature of 90°C + 5°C and maintained under agitation for the specified reaction duration.

The parameters used and the results are summarised in Table I.

TABLE I

In an example of a preferred process according to the invention a sample of the same synthetic rutile as used in the experiments described above was treated with hydrogen fluoride under the following conditions:

Hydrofluoric acid Concentration - 60% wt.

HF/Ti0₂ molar - 7/1

Dissolution 90°C 1 hour

The residue was found to amount to 10% of the weight of the ore. The supernatant solution was separated by filtration and treated with a 50% molar excess of potassium chloride added as a 20% aqueous solution. The temperature was maintained at 25°C and the pH of the solution during crystallisation was 0.5. The crystals of potassium fluorotitanate so formed were separated by filtration, washed and dried, and were then analysed to give the following elemental composition.

This shows that a highly pure material may be produced using short disssolution times according to the invention.

Claims

1. A process for the production of a substantially pure salt of fluorotitanic acid from an impure titanium source comprising dissolving the titanium source in hydrofluoric acid, treating the thus formed solution with an ammonium or alkali metal halide or nitrate in sufficient quantity to precipitate ammonium or alkali metal fluorotitanate and recovering the said fluorotitanate, characterised in that a mineral or synthetic rutile is used as the titanium source and the said source is dissolved directly in concentrated hydrofluoric acid.

2. A process as claimed in Claim 1 wherein a synthetic rutile, as herein defined, is used as the titanium source.

3. A process as claimed in Claim 2 wherein the synthetic rutile comprises not more than 6% by weight iron calculated as Fe₂0₃.

4. A process as claimed in any one of Claims 1 to 3 wherein dissolution occurs at from 60°C to 95°C.

5. A process as claimed in any one of Claims 1 to 4 wherein the concentration of hydrofluoric acid is 40 to 75 wt%.

6. A process as claimed in any one of Claims 1 to 5 wherein the molar ratio of HF to Ti in the dissolution system is from 6.2 to 7.2.

7. A process as claimed in any one of Claims 1 to 6 wherein the particulate size of the titanium source is such that 90% wt of the ore is in the range 74 to 841 μm ( 200 to 20 USS Mesh) .

8. A process as claimed in any one of the preceding Claims wherein precipitation of fluorotitanate is conducted by forming an aqueous solution of the halide or nitrate and adding the solution to that obtained by dissolution of the titanium source.

9. A process as claimed in Claim 8 wherein the aqueous solution of the halide or nitrate has a concentration of from 10 to 30 wt%.