NO317932B1 - Quality improvement of titanium-containing materials - Google Patents

Description

QUALITY IMPROVEMENT OF TITANIUM CONTAINING MATERIALS.

The present invention relates to the removal of impurities from naturally occurring and synthetic titanium-containing materials. The invention is particularly suitable for the improvement of titanium-containing materials used in the production of titanium metal and titanium dioxide pigments by means of industrial chlorination systems.

Embodiments of the present invention have the common feature that caustic leaching and sulfuric acid leaching under pressure are used to improve the quality of titanium-containing materials, e.g. titanium-containing slag, obtained from hardstone ilmenites. Additional steps may be used as described below.

In industrial chlorination processes, raw materials containing titanium dioxide are fed together with coke to chlorination apparatuses of different constructions (fluidized bed, shaft, molten salt), which work at a maximum temperature in the range of 700-1200°C. The most common type of industrial chlorination equipment is of fluidized-bed construction. Gaseous chlorine is fed through the titanium oxide and carbonaceous charge and converts titanium dioxide to titanium tetrachloride gas which is then removed in the outlet gas stream and condensed to liquid titanium tetrachloride for further purification and processing.

The chlorination process as carried out in industrial chlorination equipment is suitable for converting pure titanium dioxide raw material into titanium tetrachloride. However, most introduced materials (eg impurities in the raw materials) cause difficulties that greatly complicate either the chlorination process itself or the subsequent steps of condensation purification and waste disposal. The following table provides an indication of the types of problems to expect. In addition, each unit of introduced material that does not enter the products will significantly contribute to the formation of waste for treatment and disposal. Some introduced material (eg special metals or radioactive substances) results in waste classifications that may require special disposal in supervised repositories.

Preferred added materials for chlorination are therefore high-quality materials where the mineral rutile with 95-96% Ti02 is the most suitable of today's raw materials. Shortage of rutile has, however, led to the development of other raw materials obtained by improving the quality of naturally occurring ilmenite with 40-60% TiC>2, e.g. titanium-containing slag with approx. 86% TiC>2 and synthetic rutile with a TiC>2 content varying from 92-95%. These quality improvement processes have had iron removal as their main focus, but have been extended to remove magnesium, manganese and alkaline earth impurities as well as some aluminium.

In the known technique, synthetic rutile is formed from titanium-containing materials, e.g. ilmenite, via different techniques. According to the commonly used technique as it is often used in Western Australia, the titanium-containing material is reduced by coal or coke in a rotary kiln at a temperature above 1100°C. In this process, the iron content of the mineral is essentially metallized. Sulfur additions are also made to partially convert the manganese impurities into sulphides. After reduction, the metallized product is cooled, separated from adhering coke, and then subjected to aqueous aeration to remove virtually all contained metallic iron as a separable, fine iron oxide. The titanium-containing separation product is treated with 2-5% aqueous sulfuric acid to dissolve manganese and any remaining iron. There is no significant chemical removal of alkali metals or alkaline earth metals, aluminium, silicon, vanadium or radionuclides, in this process as described or operated. Furthermore, the removal of iron and manganese is incomplete.

Later discoveries have provided a method which carries out reduction at lower temperatures and provides hydrochloric acid leaching after the aqueous aeration and iron oxide separation. According to these descriptions, the process is effective for the removal of iron and manganese, alkali and alkaline earth metal impurities, a significant portion of the introduced aluminum and some vanadium as well as thorium. The process can be carried out as a retrofit on existing oven-based installations. However, the process is inefficient for full vanadium removal and has little chemical impact on silicon.

In another known invention, a relatively high degree of removal of magnesium, manganese, iron and aluminum is achieved. In such a process, ilmenite is first thermally reduced to substantially completely reduce the iron(III) oxide content (ie, without significant metallization), usually in a rotary kiln. The cooled, reduced product is then leached under a pressure of 241.3 kPa at 140-150°C with excess 20% hydrochloric acid to remove iron, magnesium, aluminum and manganese. The leaching liquids are spray roasted to regenerate hydrogen chloride, which is recycled to the leaching step.

In other processes during the ilmenite grain refining by thermal oxidation followed by thermal reduction (either in a fluidized bed or a rotary kiln). The cooled, reduced product is then subjected to atmospheric leaching with excess 20% hydrochloric acid to remove the harmful impurities. Acid generation is also carried out by spray roasting in this process.

In all of the above-mentioned hydrochloric acid-based leaching processes, the impurity removal is the same. The removal of vanadium, aluminum and silicon is not fully effective.

In yet another process, ilmenite is thermally reduced (without metallization) with carbon in a rotary kiln, followed by cooling in a non-oxidizing atmosphere. The cooled, reduced product is leached under a gauge pressure of 137.9-206.8 kPa at 130°C with 10-60% (typically 18-25%) sulfuric acid, in the presence of a seed material which supports hydrolysis of dissolved titanium dioxide and which therefore supports the leaching of impurities. This process requires the use of hydrochloric acid instead of sulfuric acid. Under such circumstances, similar impurity removal as that achieved with other hydrochloric acid-based systems must be expected. Where sulfuric acid is used, radioactivity removal will not be complete.

A commonly used method for improving the quality of ilmenite into higher quality products is to melt ilmenite at temperatures above 1500°C under coke addition in an electric furnace, forming a molten, titanium-containing slag (for casting and crushing) and a pig iron product. Of the problem impurities, only iron is removed in this way and then only incompletely as a result of compositional limitations of the process.

In another process, titanium-bearing ore is roasted with alkali metal compounds, followed by leaching with a strong acid other than sulfuric acid (see AU-B-70976/87). According to this description, substantial removal of various impurities is achieved where "substantial" is defined to mean more than 10%. In the context of the present invention, such a poor removal of impurities and in particular of thorium and uranium, would not be described as an efficient process. No specific phase structure is indicated after voting for this process, but it is clear from the provided analytical results (where product analyses, unlike raw material analyses, do not add up to 100% and where analyzes for added alkali metal are not given) that it can be a significant retention of the additive in the final product. Under the conditions given, it is described here that it is to be expected that alkali iron(III) titanate compounds which cannot be subjected to subsequent acid leaching will be formed. The resulting retention of alkali will make the final product unsuitable as a raw material for the chloride pigment process.

In yet another process, titanium-bearing ore is treated by alternating leaching with an aqueous solution of alkali metal compound and an aqueous solution of non-sulfur mineral acid (US 5,085,837-A). This process is specifically limited to ores and concentrates and does not address prior processing that aims to artificially alter the phase structures. As a consequence, the process requires the use of extensive reagent and harsh process conditions to be at least partially efficient and is unlikely to be economically implemented to provide a feedstock for the chloride pigment process.

A wide range of potential feedstocks are available for upgrading to high titanium dioxide content materials suitable for chlorination. Examples of primary titanium dioxide sources which cannot be satisfactorily upgraded by the previously known processes for the purpose of producing a material suitable for inclusion include hardstone (non-weathered) ilmenites, siliceous leucoxenes, many primary (non-climate -exposed) ilmenites and large anatase springs. Many such secondary sources (eg titanium dioxide-containing slag) also exist.

In particular, it is for titanium-containing materials containing high levels of silicon dioxide, aluminum oxide and magnesium oxide, e.g. the titanium-containing slags originating from hardstone-ilmenite sources, no previously described quality improvement method is available which is effective for producing a raw material for the commercial chloride pigment manufacture. The combination of silicon dioxide, which cannot be economically removed by the previously identified techniques, and alumina and magnesium oxide, which together during the thermal processing support the formation of pseudobrookite - anoaso-white type phases which cannot be subjected to leaching with hydrochloric acid under commercially realistic conditions, limits the use of such substances to sulfate pigment process raw materials. Because the pigment process is expected to supply all of the increase in pigment demand, such a limitation is a serious impediment to the chloride process.

A large proportion of the world's identified titanium dioxide reserves are in the form of hardstone ilmenites.

There is thus a clear incentive to arrive at an improvement in the quality of such titanium-containing materials which economically enables the manufacture of high-quality products which are suitable as raw materials for the chloride pigment process.

The present invention provides a combination of process steps that can be incorporated into more general processes for improving the quality of titanium-containing materials and to make such processes applicable to the treatment of a wider range of raw materials and to produce products of higher quality than could otherwise be achieved.

The purpose is achieved according to the invention by the features indicated in the description below and in the subsequent patent claims.

Accordingly, the present invention provides a method for improving the quality of a titanium-containing material by removing impurities and this process comprises alternating leaching of the material in a caustic leach and a sulfuric acid leach under pressure.

In a particular embodiment of the invention, the caustic leaching is carried out economically and efficiently despite the need for the use of excess caustic during the leaching by means of the circulation of caustic leaching liquids after solid/liquid separation via a caustic regeneration step that uses lime addition to precipitate complex aluminosilicates and to regenerate caustic solution. The complex aluminosilicates are then separated from the regenerated caustic solution which is recycled to the leachate.

The treatment of titanium-containing materials containing both aluminum oxide and silicon dioxide in such a way has not previously been described and it is shown here that it is only possible under specific operating conditions to carry out such a process without precipitation of complex aluminosilicates in the caustic leach.

It has surprisingly been discovered that by limiting the concentration of silicon dioxide, aluminum oxide and titanium oxide and other impurities in the caustic cutting liquor, e.g. by leaching at low slurry densities and recycling leaching liquids through caustic regeneration, the complex aluminosilicates that are otherwise formed during caustic leaching can often be avoided.

It has also surprisingly been found that complex aluminosilicates formed by the caustic leaching can indeed be removed by the subsequent acid leaching together with other impurities. This is particularly surprising as silicon dioxide in titanium-containing materials cannot under most circumstances be removed by acid leaching.

As a result and in a further embodiment, it is possible to carry out a simple process involving a two-stage treatment in which complex aluminosilicates are formed in a first stage and consumed by acid leaching in a second stage, and in which the silicon dioxide removal is achieved in the acid leaching stage together with the other benefits of acid leaching in the more general quality improvement.

In particular, it is described that the easy formation by caustic leaching and removal by acid leaching of complex aluminosilicates depends on the caustic:silica ratio in the leaching liquid (which determines whether the aluminosilicates are of the sodalite type or exist in another form), where high caustic:silica ratios allow easier removal. Thus, the circulation of caustic leaching liquids through a caustic leaching and caustic regeneration with lime (which keeps the caustic:silica ratio high), followed by sulfuric acid leaching under pressure, is in many cases a highly effective means of improving the quality of titanium-containing materials, especially those that stem from hardstone ilmenite.

It has been found that the method of the invention can remove iron, magnesium, aluminum, silicon, calcium, magnesium, manganese, phosphorus, chromium and vanadium, which impurities form a so-called complete list of impurities in hardstone-ilmenite sources of titanium dioxide.

Additional steps may be incorporated into the process as desired, eg: (1) The titanium-containing material may be roasted in any suitable apparatus and at any temperature under reducing or oxidizing conditions prior to leaching. Such roasting can be carried out to increase the response of the material to the leaching steps or to reduce the formation of sulfur dioxide during the leaching by oxidation of trivalent titanium dioxide in the titanium-containing material. (2) Additives may be added to the titanium-containing material prior to such roasting step to increase the response of the material to the leaching steps, or for any other purpose. (3) The titanium-containing material may be ground prior to roasting or leaching to increase reaction rates or preparation for agglomeration steps which are enhanced by the formation of a broad particle size distribution in the material to be agglomerated. (4) An agglomeration step can be carried out via which the additives are incorporated into the titanium-containing material before roasting. (5) Physical separation of the material (eg, magnetic separation of the final product to selectively remove and recirculate iron-rich material) for further quality improvement. (6) The final titanium-containing product may be agglomerated by any suitable technique to provide a size suitable in the synthetic rutile market. After agglomeration, the product can be burned at temperatures sufficient to produce sintered bonds and thereby avoid dust loss in fluidized bed chlorination equipment. (7) Regardless of final product agglomeration, the final product may be calcined to remove volatiles {e.g. water, sulfur dioxide and sulfur trioxide). (8) A caustic solution draining or caustic solution evaporation (for washing water removal) can be carried out. (9) The effluent from the sulfuric acid leach can be neutralized to form solid sulphates and hydroxides for disposal. (10) The effluent from the sulfuric acid leaching can be treated for the regeneration of sulfuric acid from aqueous sulphate solutions which are formed during the process. (11) Other leaching steps, filtration steps and washing steps can be incorporated into the process as desired. E.g. a hydrochloric acid leach can be carried out to support the removal of trace levels of radioactivity. Pressure filtration of the complex aluminosilicate that precipitates during the caustic recovery can be carried out to support solid/liquid separation. (12) Flocculating agents and other auxiliary substances can be used to support solid/liquid separation.

The invention will be explained in more detail with reference to a number of laboratory experiments which serve to illustrate the techniques described here.

Example 1

This example is intended to demonstrate the ineffectiveness of treatments found to be effective for quality improvement of other titanium-containing substances on materials such as titanium-containing slags produced from hardstone ilmenites.

Commercial titanium-containing slags of the composition indicated in Table 1 were subjected to oxidation roasting in air at 750°C for 30 minutes, followed by reduction roasting in a 1:3 hydrogen:carbon dioxide (volumetric basis) gas mixture at 680°C for 1 hour. The cooled product from this thermal treatment contained no iron (III) and no trivalent titanium oxide. The phase composition for the material was suggested by X-ray diffraction as pseudobrookite.

The thermally treated material was leached by refluxing 10% caustic soda solution at 10% slurry density. After filtration and washing, the solid residue had a composition as indicated in Table 2.

It is clear that the caustic leaching has no significant effect on the content of silicon dioxide or aluminum oxide in the material.

The residues from the caustic leaching were subjected to a leaching with refluxing 20% hydrochloric acid at 30% slurry density for 6 hours. After filtration and washing, the solid residue had a composition which is also indicated in Table 2.

It is clear that a roasting/leaching process using 10% caustic soda at 10% slurry density and 20% hydrochloric acid at 30% slurry density as leaching agents is, so to speak, totally ineffective in terms of quality improvement of slag.

Example 2

The treatment indicated in Example 1 was repeated except that the caustic leaching was carried out under pressure at 165°C.

The mixtures of the caustic and acid-leached products are given in Table 3. It is clear that the caustic leaching had no significant effect on the content of silicon dioxide or aluminum oxide in the material. The acid leach, despite being virtually ineffective in providing any quality improvement suitable for the chloride pigment process, had a substantial effect on the silica and alumina content.

There was no such effect on a slag sample directly subjected to a hydrochloric acid leach.

It is clear that caustic pressure leaching has altered the state of the silica to allow its subsequent removal by hydrochloric acid leaching, but did not result in any direct removal. Investigations showed the formation of a complex aluminosilicate precipitate in the caustic leach. The caustic leaching was carried out under conditions where silicon dioxide could be leached, but was not soluble.

The results of this experiment, combined with the results of subsequent examples where effective caustic leaching is demonstrated, show the dependence of the removal of silicon dioxide and aluminum oxide by caustic leaching on the leaching conditions.

Example 3

A sample of the slag whose composition is indicated in Table 1 was mixed with 2% borax, converted into pellets and subjected to reduction roasting at a 19:1 ratio of hydrogen:carbon dioxide, calculated on a volumetric basis, at 1000°C for 2 hours . The phase composition of the cooled product after this thermal treatment was suggested by X-ray diffraction to be

be pseudobrookitis.

A sample of the thermally treated material was leached by refluxing 10% caustic soda solution at 5% slurry density. After filtration and washing, the solid residue had a composition as indicated in Table 4. It is clear that the caustic leach is very effective for the purpose of removing silica despite the much poorer performance of a leach carried out at a 10% slurry density in Example 1 , where the complex aluminosilicates are formed.

The remainder of the caustic leach was subjected to a pressure leach at 150°C with 20% sulfuric acid at 5% slurry density for 6 hours. After filtration and washing, the solid residue had the composition shown in table 4.

It is clear that the combined effects of a low slurry density caustic leach and a subsequent sulfuric acid pressure leach (which is able to decompose pseudobrookite) could significantly improve the quality of the slag into a very high quality product that had a suitable composition as a chloride pigment process raw material.

The leach liquid from this caustic leach was preserved and, after analysis, was treated with micronized lime at a weight ratio of 1.3 units of lime per unit dissolved silicon dioxide. The resulting complex aluminosilicate precipitate and any excess lime was removed by filtration and the "regenerated" caustic solution was preserved for reuse by leaching.

A further sample of the thermally treated material was leached with the regenerated caustic solution under the same conditions as indicated above. There was no significant difference between the results of the leaching with fresh caustic soda and the results of the leaching with regenerated soda.

Example 4

This example is intended to show the ineffectiveness of acid leaching alone in the removal of silica from titanium-containing materials such as titanium-containing slags, prepared from hardstone ilmenites.

Commercial titanium-containing slag of the composition shown in Table 1 was subjected to roasting for 2 hours in an atmosphere of 1:19 hydrogen:carbon dioxide gas mixture, calculated on a volume basis, at 1000°C. After cooling in the roasting atmosphere, the roasted slag was pressure leached at 135°C in a 20% sulfuric acid at 25% w/w slurry density for 6 hours.

The composition of the leach residue is given in Table 5. Such direct acid leach treatment of roasted, titanium-containing material can be predicted to result in little improvement in product quality by leaching, and no removal of SiC>2.

Example 5

A sample of slag without addition of additive was formed without any thermal treatment and was then treated by the same leaching steps as indicated in Example 3.

The composition of the final product is given in Table 6. Substantial removal of the impurities was achieved without thermal treatment.

Claims (4)

1. Method for upgrading titanium-containing material through the removal of impurities from the titanium-containing material, characterized in that the process includes: (a) leaching of the material in a caustic leaching solution to avoid precipitation of complex aluminosilicates, where the sludge density in the caustic leaching washing solution is 5 weight percent or less of the combined weight of a slurry of the material and the caustic washing solution; {b) separating a residue from the caustic leaching and the caustic leaching solution; and (c) leaching with sulfuric acid under pressure said residue to remove impurities. 2. Method as stated in claim 1, characterized in that it further comprises regeneration of the caustic leaching solution from step (b) by precipitation of complex aluminosilicates. 3. Method as stated in claim 2, characterized in that it further comprises separation of the regenerated caustic leaching solution and the precipitated complex aluminosilicates and recycling of the regenerated leaching solution to step (a). 4. Method as stated in any of the preceding claims, characterized in that the titanium-containing material comprises a titanium-containing slag extracted from hard ilmenite.