This invention relates to a method of digesting metal containing material to form metal containing sulphates. The invention also relates to a method of separating certain metal values from other material in metal containing material.

Background Art Many different processes are known for extracting metal values from metal containing material such as ore or slag. One such process is the sulphate process which is widely used to beneficiate titaniferous material. The sulphate process is a batch process and the first step is to digest the TiO2 bearing ore in excess hot H2SO4. The excess H2SO4 is calculated not only to ensure that sufficient H2SO4 is present to sulphate all sulphatable species in the titaniferous material but also to ensure that sufficient H2SO4 is present in order that for each mole of Ti02 present in the titaniferous material, at least two moles of H2SO4 is present.

This excess ensures that during the H2SO4 digestion process, TiO2 in the titaniferous material reacts with the excess H2SO4 to form H2TiO(SO4)2 also known as acid titanium sulphate which is a water soluble compound.

H2TiO (SO4) 2 can also be represented as TiOSO4. H, S04.

The H2TiO is extracted with water from the reaction product (in the form of a filter cake) and is then precipitated as TiO (OH) 2 whereafter it is separated, washed and calcined to form TiO,.

The H2SO4 used to digest part of the titaniferous material may be concentrated or may be diluted, and the H2SO4 digestion step takes place at a furnace (kiln) temperature of about 200°C. In the case of the digestion of ilminite slag the temperature is typically about 220°C. The reason for this temperature is that H2SO4 has a high vapour pressure and a boiling point of 290°C and furthermore, H2SO4 decomposes into S03 and H2O at 340°C and 1 atmosphere pressure. The furnace temperature is accordingly retained below said temperatures and H2SO4 digestion is not used at furnace temperatures above 340°C thereby to minimise decomposition of the H2SO4.

Furthermore due to the high vapour pressure of H2SO4 most H2SO4 digestion

processes operating at above 160°C require the use of pressure vessels, such as lead vessels. This causes the digestion process to be carried out at pressures substantially above atmospheric pressure. This equipment is expensive and cumbersome.

Disclosure of the Invention The inventor has now found that if HIS04 digestion of metal containing material is carried out without the utilisation of pressure vessels and at a furnace temperature above 340°C (and typically at 600°C), that is where H2SO4 decomposes to form S03 and HZO, very good digestion of metal values take place to form metal sulphates. It is totally unexpected that H2SO4 can be used at furnace temperatures so far above its boiling point without the use of expensive autoclave high-pressure technology. It would be expected that at such temperatures (typically at 600°C), all the acid would rapidly evaporate and/or decompose and would therefore not remain in contact with the material for long enough to effectively sulphate sulphatable species.

It is accordingly an object of the present invention to provide an alternative method of digesting metal containing material to form metal containing sulphates. It is also an object of the invention to provide a method of treating

metal containing material to separate at least certain metal values from other material in the metal containing material.

According to a first aspect of the present invention there is provided a method of digesting metal containing material comprising the steps of -providing the metal containing material ; and reacting the metal containing material with a source of at least one sulphur oxide compound at a furnace temperature above 340°C to form at least one metal containing sulphate.

The at least one metal containing sulphate which forms may be soluble in a suitable solvent, preferably water.

The metal containing material may contain a number of different metals. At least some of the metals may be in the form of metal oxides, and/or metal sulphides and/or metal phosphates. The metal containing material may comprise metal containing ore. Alternatively it may comprise a metal containing slag. In one embodiment of the invention it may comprise titaniferous material such as for example ilmenite ore, for example ilmenite, rutile, anatase, leucoxene and titaniferous magnetite. Alternatively the titaniferous material may comprise titaniferous slag which may include pseudo

brookite and/or perovskite. It may also comprise plasma dissociated zircon (PDZ).

Due to Zircon sand's (ZrSiO4) unreactivity, it must be activated before it can react with the source of sulphur oxide compound in the form of H2SO4. Activation may be either by thermal dissociation e. g. plasma flame to produce ZrO2. SiO2 or by chemical reaction of metal (I and/or II) oxides or precursor salts thereof to produce MI2ZrSiOs or MIIZrSiO5. During this chemical activation reaction the aid of a flux like B203 and/or CaF2 can be used. The dissociated zircon or chemical activated zircon can react with H2SO4. In the case where chemical activated zircon is used, the metal (I and/or II) sulphate would form during the H2 SO4 digestion reaction.

The material may comprise at least one selected from the group comprising ocean manganese nodules ; transition metal sulphide ores e. g. manganese iron, copper, nickel cobalt, sulphide ; other sulphide ores e. g. indium sulphide ores, zinc sulphide ores, sulphided auriferous ores ; (nickel-iron-magnesium) lateritic ores ; platinum group metal ores ; copper converter slags ; complex pyretic ores ; refractory carbonate ores containing the precious metals ; alumina containing clays ; tungsten and molybdenum alloys and oxides ; rare earth ores e. g. monazite ; magnesium micra e. g. phlogopite ; pyrophyllite ; titanium containing

ores and materials e. g. rutile ; synthetic rutile and ilmenite ; and vanadium ores.

One or more compounds selected from the following group may be digested by means of the digestion process (especially at a temperature of 600°C) : TiO2 ; Al203 ; Al203. 2SiO2. 2H20 ; ZrO2 ; ZrO2. Si02 ; Fe2TiO5 ; CaTi03 ; FeTiO3 and Fe304. Moderate to high yields (50 to 95%) are obtained for TiO2 and Al203 whereas low yields (less than 20%) are obtained for these compounds with conventional processes where the furnace temperature is at about 200°C.

Higher yields are also obtained for most of the rest of the compounds listed above compared to the conventional method where the furnace temperature is at about 200°C and where the method is carried out substantially above atmospheric pressure. According to the present invention, if the furnace temperature is at 600°C, no digestion of the following compounds are observed namely, SiO2, ZrSiO4 and A16S'2013- The metal containing material may be sized to have a particle size of 85oµm and smaller. Preferably the metal containing material is sized to have a particle size of 75 um and smaller. In some applications the particle size may be smaller than 15pm. The finer the particle size of the metal containing material the faster the reaction rate and higher the yield.

The metal containing material may be sized by means of grinding, milling or the like. Preferably it is by milling. The milling may comprise dry milling or wet milling. Wet milling may take place in the presence of sulphuric acid (preferably concentrated sulphuric acid) thereby forming a wet mixture of the titanium containing material and the H2SO4.

The source of a sulphur oxide compound may comprise H2SO4 or a source of H2SO4. In this specification the term"source of H2SO4"comprises any compound other than H2SO4 which forms H2SO4 or a decomposition product thereof during the digestion process, the decomposition product including but not being limited to SO3. The source of H2SO4 may comprise a sulphate salt such as (NH4) 2 SO4 or NH4 HSO4.

Preferably however, the source of sulphur oxide compound comprises H2SO4.

The H2SO4 may be concentrated or diluted. Preferably it is provided in a concentrated form from 1 mol. d to 40 mol. d M-3 ; preferably from 12 mol. dm~3 to 20 mol. dm~3 ; and preferably at a concentration of about 18 mol. dry 3.

With furnace temperatures above 340°C which is above the decomposition temperature (340°C), of H2SO4 it is believed that the H2SO4 decomposes to

form S03 and H2O. It is believed that these decomposition products may react with the metal in the metal containing material to form metal containing sulphate. Accordingly it is believed that the source of sulphur oxide may comprise SO3 or a source of SO3. In this specification the term"source of SO3" comprises any compound other than SO3 which forms S03 during the digestion process. A source of S03 may comprise H2SO4. Alternatively or additionally it may comprise (NH4) SO4 and/or NH4HSO4. Preferably water is also present to allow a mixture of SO3 and H2O to be present for reaction with the metal in the metal containing material.

The source of sulphur oxide compound is preferably provided in a stoichiometric excess, where 100% stoichiometric equivalent is calculated by determining the molar amount of source of sulphur oxide to fully sulphate all the sulphatable species of the metal containing compound. Preferably a 4/3 stoichiometric excess is provided. In certain cases a 3/2 stoichiometric excess may be provided.

In determining the stoichiometric equivalent where the metal containing material is titanium containing material and where H2SO4 is used as the source of the sulphur oxide compound, the molar ratio of H2SO4 : Ti values may be calculated as 2 : 1, that is to form H, TlO (SO4),. Alternatively and preferably, in

determining the stoichiometric equivalent in such a case, the molar ratio of H2SO4 : Ti values is calculated as 1 : 1, that is to form TiOSO4. It will be appreciated that the molar ratio of the H2SO4 to titanium containing material will depend on the composition of the titanium containing material and more particularly it will depend on the nature and ratio of sulphatable species in the titanium containing material. For example, titaniferous material containing (a) TiO2, (b) Fe203, (c) Al203, (d) MgO, (e) CaO and (f) SiO (a to f indicating the molar amounts present the composition) the following molar amount of H2SO4 will be the stoichiometric equivalent, namely (la + 3b+3c+ld + le+ 0f). Usually the stoichiometric excess will not be more than 0, 6 times the stoichiometric molar amount to be added where the stoichiometric molar amount is calculated as above. It also has to be taken into account that when FeO is present (such as in ilmenite) the H, S04 will oxidise the FeO to Fe203 which will then be sulphated. In calculating the H, S04 to be added it has to be taken into account that some HS04will be used to oxidise the FeO to Fez03.

Preferably the furnace temperature at which the source of sulphur oxide compound is reacted with the metal containing material (hereinafter referred to as the digestion stage) is above 400°C, preferably above 450°C, more preferably from 450°C to 750°C and most preferably from 550°C to 650°C.

The use of higher temperatures is also foreseen, especially up to 1000°C.

Where H2SO4 is used as the source of the sulphur oxide compound it has been found that the product temperature of the H2SO4 during this stage is preferably above 240°C, preferably from 270°C to 340°C. Most preferably from 270°C to 320°C. This temperature is preferably maintained until the temperature of the reaction product starts to rise. At this stage most of the H, S04 has been consumed, either in the oxidation reaction, sulphating reaction, or as lost to the system due to evaporation and/or dissociation. In use, the particle size of the metal containing material and the amount of H2SO4 may be adjusted to ensure that at the end of the digestion stage, substantially all the sulphatable species in the metal containing material are sulphated.

The reaction product of the digestion stage may be subjected to heat treatment (hereinafter referred to as the dead burn stage) to ensure that substantially all unreacted sulphate is lost from the product. Preferably, where the metal containing material comprises titanium containing material and titanium containing sulphate forms, only TiOSO4 is present as the titanium containing sulphate. This step is considered to be most preferred, and is preferably carried out subsequent to the digestion stage. Where H2SO4 is used as the source of sulphur oxide compound the product temperature during the dead burn stage is usually higher than 320°C to cause the sulphate in the form of H2SO4 to

dissociate into H2O and S03 (S03 can also further dissociation into SO2 and °2) which can then be recycled with the vapour products produced during digestion. The product temperature may be from 400 to 550°C. The temperature during this stage is determined by the type of metal containing material which is used and will be less than the decomposition temperature of commercially important sulphates. However, for certain materials it may be beneficial or desirable to thermally decompose specific metal containing sulphates, typically to form metal oxides. In such cases the temperature will be chosen to be above the decomposition temperature of the metal containing sulphate in question but below the temperature of the commercially important metal containing sulphate (s) or vice versa.

The digestion stage and dead burn stage are preferably carried out as two separate stages, but it is foreseen that they can be combined into a single stage.

Prior to the digestion stage the product may go through a warming stage wherein it is warmed to the temperature required for the digestion stage.

Preferably the warming occurs as fast as possible.

Subsequent to the digestion stage, but preferably only subsequent to the dead burn stage the product is allowed to cool (hereinafter referred to as the cooling

stage). The cooling may be natural, but it may also be forced to speed up this stage.

Preferably the metal containing material is reacted with H2SO4 by mixing them together to form a wet mixture and then subjecting it to at least the digestion and dead burn stages. Preferably the mixture of the metal containing material and H2SO4 goes through all four the above stages. Different furnaces may be used during the different stages. For example, a first furnace such as tunnel kiln may be used for the warming and digestion stages and a second furnace such as a rotary kiln may be used for the other stages.

The reaction product of the digestion stage may also be milled prior to subjecting it to the dead burn stage.

The digestion process is preferably carried out at about atmospheric pressure, preferably in a range of 5% above or below atmospheric pressure, preferably in a range of 2% above or below atmospheric pressure.

The reaction conditions are preferably such to prevent SO3 from decomposing.

At temperatures above about 1000°C, S03 decomposes to sulphur dioxide and

oxygen which is not desirable.

Where HIS04 is used as the source of the sulphur oxide compound the process may also include adding an additional source of sulphate or SO3 to the metal containing material to be digested with the H2SO4, which additional source is not H2SO4. The additional source may comprise a source of sulphate. The additional source may comprise ammonium sulphate ((NH4) 2SO4), or ammonium bisulphate (NH4HSO4). Preferably it comprises ammonium sulphate. Ammonium sulphate converts to ammonium bisulphate in the presence of sulphuric acid, or decomposes to ammonium bisulphate with the release of ammonia when heated to above 100°C, typically between 160 and 240°C.

If the additional source comprises ammonium sulphate, an amount thereof equal or less than the H2SO4 used in the process is added, which amounts are calculated on a molar basis. During the reaction (especially the dead burn stage, if applicable) ammonium bisulphate decomposes to produce off gases including NH3, SO3 (or SO2 and02) and H20. These gases may be recycled.

If such an additional source of sulphate or SO3 is used it may be taken into account when calculating the amount of H2SO4 (as the source of the sulphur

oxide compound) to be used. That is, the sulphate and/or Su, of the additional source can be subtracted from the H2SO4 required.

The process may also include the step of removing sulphate (if present), especially H, SO, (where H2SO4 is used) from the reaction product of the metal containing material with the source of sulphur oxide compound. In cases where titanium containing sulphate forms, sulphate is removed in order that substantially only TiOSO4 is present as the titanium containing sulphate. The sulphate, especially HS04may be removed by means of heat treatment, preferably in the form of the dead burn stage as set out above. It will be appreciated that when an additional source of sulphate orS03 is present, the treatment should preferably also remove the sulphate and SO3 originating from such a source.

The reaction time is typically between half an hour and six hours depending on the thickness of the cake formed during the reaction, operating temperatures and material finess of the metal containing material. The digestion reaction is completed when all sulphatable components in the material have been converted to sulphates.

The digestion method may also include the step of separating the metal

containing sulphate from other material. Where water soluble and water insoluble species are present in the reaction product, the reaction product may be treated with water to allow water-soluble species to dissolve in the water to form a leachate, which leachate may then be separated from undissolved solids.

Where titanium containing sulphate forms, it may be in the form water soluble of H2TiO which may soluble in water. Alternatively it may be in the form of TiOSO4 which is only soluble in water in the presence of relatively high concentrations of SO42-.

Where the metal containing sulphate is soluble in water the leachate may be treated to separate various sulphates if more than one water-soluble metal containing sulphate is contained in the leachate.

Standard processes may be used for this purpose and may include pH control, neutralisation, hydrolysis, electrolysis etc.

The invention also relates to a product formed by the digestion process including water-soluble metal containing sulphate. The water-soluble metal containing sulphate which forms is effectively anhydrous. This is often not the case for many conventional low temperature sulphate leach processes where the sulphates are hydrated or dissolved in water.

According to another aspect of the present invention a method of separating at least certain metal values from other material in a metal containing material comprises the steps of -providing the metal containing material ; -reacting the metal containing material with a source of at least one sulphur oxide compound at a furnace temperature above 340° to form a solid reaction product including at least one metal containing sulphate ; and -leaching the solid reaction product including the metal containing sulphate with water to form a leachate and separating the leachate from water insoluble solids.

According to yet another aspect of the present invention a method of separating at least certain metal values from other material in a metal containing material comprises the steps of -providing the metal containing material ; -reacting the metal containing material with a source of at least one sulphur oxide compound at a furnace temperature above 340°C to form a solid reaction product including at least one metal containing sulphate which is soluble in a suitable solvent ; and -dissolving the at least one metal containing sulphate in the said suitable

solvent to form a leachate and separating the leachate from the insolubles.

The method may also include the step of treating the leachate to separate different dissolved sulphates in cases where more than one such dissolved sulphate is formed.

The invention also relates to a product formed by the process.

The invention will now be further described by means of the following non- limiting examples.

Example 1 Sources of natural baddeleyite (ZrO) is substantially depleted. Zircon sand (ZrSiO4) is another source of zirconia but is very unreactive and before the ZrO2 can be retrieved, the lattice must be dissociated to form ZrO,. Si02. One way to achieve this dissociation is by means of a plasma flame where zircon is exposed to high temperatures for a short retention period in order that dissociation occurs to form plasma dissociated zircon (PDZ). Plasma dissociation of zircon is about 85% efficient, that is 15% (mass/mass) remains as ZrSiO4 and 85% (mass/mass) is converted to ZrO2. SiO2.

In this example 100kg of PDZ was provided and it contained 85kg ZrO2. SiO4 and 15kg ZrSiO4. The PDZ was dry milled to have a particle size smaller then 15um (° = 5um). The milled PDZ was then mixed with 142kg (77. 2fol) H2SO4 (98% purity) to provide a 50% stoichiometric excess of H2SO4. It is believed that metal oxides containing a single metal (and not a mixture of metals) such as Al203, TiO2 and ZrO2 need an excess HISO, to react significantly. It is believed that the excess H2SO4 has to compensate (react) with the HO that forms during the neutralisation reaction in order that no free H20 is present e. g. ZrO2+3H2SO4#H2ZrO(SO4)2+HSO4+H3O+ The resultant slurry mixture of the PDZ and H, S04 was poured into aluminium containers. These containers were introduced into a top-hat furnace and then ramped up to 600°C and soaked until the off-gasses started to turn blue-grey. Zr (SO4) 2 is stable at 600°C with the result that little decomposition occurs at this temperature.

A digested filter cake formed which was crushed. The crushed cake was leached with 180l of water to dissolve the Zr (SO4) 2. The acid zirconium sulphate leach can be used as a feedstock to manufacture zirconium downstream salts. The insoluble products of the cake comprised 15kg ZrSiO4, 8. 5kg ZrO2. SiO4 (which did not react under the experimental conditions) and

25kg Si02.

The furnace included a ceramic lined gas outlet which lead into a scrubber for the S03 and H2O resulting for the decomposition of the H2SO4. The scrubber comprised a stirred vessel containing water and slaked lime. The scrubber was driven by a reticulation pump and included a ceramic venturi. During scrubbing gypsum formed which was filtered out after the scrubbing was completed. The water used during the scrubbing was recycled.

The pressure in the furnace was slightly below atmospheric pressure. This was due to the S03 and H2O being sucked out of the furnace.

The atmosphere in the furnace included S03 and H20 due to the complete decomposition of H2SO4. Furthermore, °2 and N2 was also present due to air being sucked into the furnace as a result of the reduced internal pressure.

It is foreseen that if the furnace is a hermetically sealed unit the gas pressure could rise to just above atmospheric pressure in order that gas would be forced into the scrubber/recycle unit.

Example 2 One current process for treating ilmenite involves smelting iron ore in high temperature furnaces to produce pig iron plus a high-grade slag (i. e slag with a high titanium value, typically 85%). Such slags can be used in either the conventional sulphate or conventional chlorination process to produce TiO2 pigments. These slags have a very high melting point and are quite corrosive to refractories. The furnaces used in such reactors are expensive to build and maintain and they are also troublesome.

The present invention allows ilmenite or similar materials i. e. titaniferous magnetite to be handled in a less expensive and more benign manner. Such feedstocks can be combined with fluxes like Six,,, CaO, dolomite (CaO : MgO) etc. These fluxes will lower the slag melting point and hence enable cheaper and more conventional furnaces to be used to smelt and remove the molten iron. The resulting slag would not normally be treatable by conventional processes to recover the titanium values because of the presence of too high levels of impurities from the flux and hence the presence of CaTi03 if CaO is used CaTi03 is extremely stable and is not digested by the conventional sulphate processes. However these slags would be treatable by the present

invention.

In the present example titanium slag from Highveld Steel and Vanadium at Witbank was used. This slag has similarities to ilmenite slags that were combined with fluxes. The major morphologies (phases) of this slag are as follows : Augite, aluminium-Ca (Mg, Fe + 3, Al) (Si, Al) 206 Perovskite, syn-CaTiO3 Armalcolite, ferrian, syn- (MgFe) (Ti3Fe) Olo Pseudo brookite, syn-Fe2TiO5 A chemical analysis was conducted on the slag and the composition is indicated in Table 1.

Table 1 : Chemical composition of Highveld Steel and Vanadium slag and H2SO4 required to digest the slag. Molar mass of Metal Mass % of metal Kmol of metal Kmol SO42- metal oxides in oxides in oxides in slag oxide per required to slag (g) slag 100kg slag sulphate metal oxides 79. 90 TiO2 28 0.350 0. 350 40. 31 MgO 13 0.323 0.323 101. 96 Al203 11. 5 0. 113 0. 339 159. 69 Fe2O3 7 0.044 0. 132 56. 08 CaO 15 0.267 0. 267 60. 08 Si02 21 0. 350---- Totals 1. 411 41. 7% excess 0. 589 2. 000

2 Kmol SO42- amounts to 200kg H2SO4 at 98% purity.

The slag in an amount of 100kg was dry milled to a particle size of smaller than 25, um and was then mixed with 200kg H2SO4 (98% purity). The amount of H2SO4 to be added was calculated on the basis as set out in Table 1. The

H2SO4provided a 41, 7% excess H2SO4 calculated on the basis of a Ti02 : SO42- molar ratio of 1 : 1. An excess of H2SO4 is used to ensure complete oxidation of all reduced species in the slag and complete sulphation thereof.

The resultant wet slurry mixture of 200kg H2SO4 and 100kg slag was poured into open aluminium trays in order that the slurry did not exceed a depth of 200mm. The containers were introduced into a top hat furnace, and the furnace was ramped up to 600°C to provide a product temperature between 240°C and 320°C. This period is known as the digestion stage during which oxidation and sulphation of the slag occurs.

After about six hours at this temperature, the product temperature started to rise above 320°C. At this stage most of the oxidation and sulphation had occurred. This treatment was continued until the crust temperature of the cake (which formed due to the reaction) reached 450°C at which time the furnace was switched off in order that decomposition of the formed metal sulphates did not occur. This period of increased product temperature is known as the dead burn stage and during this period all remaining H2SO4 is lost from the product in order that only TiOSO4 is present as the sulphated titanium value and that no excess H2SO4 is present in the cake to form H2TiO (SO4) 2. This stage took about 4 hours.

The S 03, S °2 and °2 which form during the digestion and dead burn stages may be recycled. The said gasses may be fed through a ceramic lined tube from the furnace to a mild steel scrubber containing oleum to recycle it as H2SO4.

The product was then allowed to cool for another 4 hours and dead burning also occurred during the first period of cooling.

The cakes were then tipped out of the trays and crushed. The reaction yielded approximately 200kg cake and had a composition as set out in Table 2. The results indicate that the sulphation efficiency was not less the 85%. The crushed cake was then leached with 375t water at 60°C, thereby leaching the soluble sulphates from the cake. After 2 hours of leaching the leachate was filtered from the solids. The concentration of the metal sulphates in the leachate are also indicated in Table 2. The insolubles are safe to be dumped. Table 2 : Cake composition, solubility of sulphated species and concentration of sulphated species present in leachate. Molar mass of Sulphate Kmol of metal Mass of Solubility of Concentration sulphated species in sulphates per Sulphate sulphated of sulphated species in cake Cake 100kg cake species in species in species in (g) Cake water leachate 159. 96 TiOSO4 0.300 48. 0 kg soluble in 0. 750 mol/ presence of SO42- 120. 37 MgS04 0. 275 33. 1 kg very soluble 0. 688 molle 342. 15 Al2 (SO4) 3 0. 096 32. 9 kg very soluble 0. 240 mol/f 399. 87 Fe2(SO4)3 0.037 15.0 kg soluble 0. 093 mol/ 136. 14 CaS04 0. 227 30. 9 kg insoluble-- 60. 08 SiO2 0. 300 18. 0 kg insoluble Undigested 15. 0 kg insoluble slag Other 7.5 kg very soluble negligible 491.5kg(#400l) Totals 200. 4 kg

The leachate was then treated to reduce Fe3+ to Fe2+. This was done by adding iron filings to the leachate and reduction was enhanced by heating the solution from 40 to 60°C. During reduction the solution changed colour from pale green to light purple. The remaining iron metal was then removed from the solution by filtration, but magnetic separation is also possible. The solution

was then heated to 90°C to hydrolyse the solution, thereby converting TiOSO4 to TiO (OH) 2. The hydrolyses was completed after about 2 hours.

The hydrolysed product, TiO (OH) 2, is also known as crude anatase pulp (CAP) and is insoluble in water. The CAP was filtered from the leachate. The CAP had a high purity and also has the advantage that it is in activated form.

Accordingly it is a lucrative feedstock for the sulphate process to produce titanium pigments.

Example 3 In this example ilmenite slag was treated. The slag had the chemical composition as set out in Table 3.

Table 3 : Chemical composition of slag and H2SO4 required to digest the slag. Molar mass Petal Mass % of Kmol of metal Kmol SO42- of oxides in metal oxides oxide per required to metal oxides slag in slag 100kg slag sulphate metal in slag oxides 79. 90 TiO2 85 1. 064 1. 064 159. 69 Fe203 10 0. 063 0. 189 Totals 1. 253 59. 6% excess 0. 747 2000

2 Kmol SO42-amounts to 200kg H2SO4 at 98% purity.

The slag in an amount of 100kg was milled with 200kg H2SO4 (98% purity) to a particle size of smaller than 25um. An alumina lined pin mill, using 600kg, 6mm alumina beads was used for this purpose at 500rpm for 1 hour. The amount of H2SO4 to be added was calculated on the basis as set out in Table 3.

The H2SO4 provided a 59. 6% excess H2SO4 calculated on the basis of a TiO2 : 2-molar ratio of 1 : 1. An excess of H2SO4 is used to ensure complete oxidation of all reduced species in the slag and also complete sulphation thereof.

The resultant wet mixture of 200kg H2SO4 and 100kg slag was in the form of a slurry at 60 to 80°C The slurry was pumped into aluminium trays in order that the slurry did not exceed a depth of 200mm. After 1 to 2 hours the slurry solidified and it is believed that this hard polymerised cake is the first step of digestion involving co-ordination of the H2SO4 with slag via hydrogen-oxygen- change bonding. The containers were introduced into a top hat furnace, and the furnace was ramped up to 600°C to provide a product temperature between 240°C and 320°C. This period is known as the digestion stage during which oxidation and sulphation of the slag occurs.

After about six hours at this temperature, the product temperature started to rise above 320°C. At this stage most of the oxidation and sulphation had occurred. This treatment was continued until the crust temperature of the cake (which formed due to the reaction) reached 450°C at which time the furnace was switched off in order that decomposition of the formed metal sulphates did not occur. This period of increased product temperature is known as the dead burn stage and during this period all remaining H2SO4 is lost from the product in order that only TiOSO4 is present as the sulphated titanium value and no excess H2SO4 is present in the cake to form H2TiO (SO4) 2. This stage took about 4 hours.

TheS03, SO2 and02which form during the digestion and dead burn stages may be recycled as HIS04 as described in example 2.

The product was then allowed to cool for another 4 hours and dead burning also occurred during the first period of cooling.

The cakes were then tipped out of the trays and crushed. The reaction yielded approximately 188 kg cake and had a composition as set out in Table 4.

Table 4 : Cake composition and solubility of sulphated species. Molar mass of Sulphate Kmol of metal Weight of Solubility of sulphated species in sulphates per sulphated sulphated species in cake cake 100kg cake species in species in cake (g) cake 159. 96 TiOSO4 0. 904 144. 6 kg insoluble (low SO42-) 399. 87 Fe2 (SO4) 3 0. 054 21. 6 kg soluble Undigested--15. 0 kg insoluble slag other 7. 0 kg insoluble/soluble Totals 188. 2 kg

The results indicate that the sulphation efficiency was not less than 85%. Insolubles and solubles may be separated by leaching the reaction product with water. Insoluble TiOSO4 is a lucrative feedstock to produce synthetic rutile for the chloride process.

It will be appreciated that the process according to the present invention uses H2SO4 at furnace temperatures significantly above the decomposition temperature of H2SO4. In the examples no pressure vessels (autoclaves) were used and it would be expected that the HIS04 would gassify before it had time to react with the metal containing material. Unexpectedly and surprisingly this did not happen. The H2SO4 and S03+ H20 remained in contact with the

metal containing material for long enough to react completely with the sulphatable portion thereof.

It is known that some metal sulphates are not stable at 600°C, e. g. ferric sulphate decomposes at 480°C. However these sulphates do not decompose during the digestion step at 600°C as long as H2O and SO3 are still present.

It will also be appreciated that many variations in detail are possible without thereby departing from the scope and spirit of the invention. One such variation involves the digestion of manganese ores. In the conventional sulphate route extraction processes the ore has to be pre-reduced so that MnO2 phases are reduced to MnO. This is typically done in a high temperature rotary furnace using carbon as a reductant. The use of the present invention removes the necessity for the MnO2 to be reduced to MnO, and the MnO, containing ore feedstock can be treated with H2SO4 to produce soluble manganese sulphate in a single reaction stage.

Another variation involves the digestion of rare earth containing ores e. g. monazite according to the present invention. In this variation the rare earth sulphates formed are soluble in acid leachates. In yet another variation of the'process of the present invention, the process may be used to digest precious metal mixtures and ores. e. g. Pt, Au, Pd and Ag.