WO2001085326A2

WO2001085326A2 - Method for the production of amorphous metal containing compounds

Info

Publication number: WO2001085326A2
Application number: PCT/ZA2001/000051
Authority: WO
Inventors: Roderick Ivan Edwards
Original assignee: Roderick Ivan Edwards
Priority date: 2000-05-06
Filing date: 2001-05-04
Publication date: 2001-11-15
Also published as: ZA200209872B; WO2001085326A3; AU2001261857A1

Abstract

This invention provides a method for the production of amorphous metal containing compounds including the steps of providing a soluble crystalline salt of the metal in solid form; and reacting the soluble crystalline salt directly with an alkaline solution, without first dissolving the salt. A filterable and yet relatively very reactive, granular, amorphous-, and more particularly a pseudo-crystalline hydroxide or carbonate is obtained and which has substantially the same physical form as the salt. This provides for the easy separability of the granules from the solution. In addition, the porous nature of the amorphous product allows for the virtual complete removal of soluble by-products from the interior of the particles by efficient washing procedures, as a concentrated solution, which can be utilised for other purposes in an economical fashion.

Description

METHOD FOR THE PRODUCTION OF AMORPHOUS METAL CONTAINING COMPOUNDS

INTRODUCTION This invention relates to a method for the production of amorphous metal containing compounds in a granular form. More particularly this invention relates to a method for the production of granular amorphous metal hydroxides and metal carbonates.

BACKGROUND TO THE INVENTION

Metal hydroxides and carbonates are useful intermediates in the production of other metal containing compounds, especially salts of strong inorganic acids and weak organic acids. This is particularly so for aluminium and zirconium hydroxides and carbonates. An example of the production of such a metal containing compound from a strong acid and a metal hydroxide is: AI(OH)₃ + 3HNO₃ ^■» AI(NO₃)₃ + 3H₂O ; and from a weak acid and a metal hydroxide is : Zr(OH)₄ + 2HOr -» ZrO(Or)₂ + 2H₂O where Or represents an organic anion such as octanoate.

For such reactions to be effective, the metal hydroxide or carbonate intermediate must be both pure and chemically reactive so that the metal containing product is free of contaminants and can be produced at a reasonable rate.

DESCRIPTION OF PRIOR ART

Many metals are recovered from ores, concentrates and scrap materials by leaching the raw material with a solvent such as a dilute solution of sulphuric acid to convert the metal content into a solution of the metal salt. Such solutions normally contain impurities and have to be treated to remove the unwanted constituents. A further step in the process involves the separation of the metal containing compounds from the solution to produce an intermediate or final product containing the metal in purified form.

A useful and widely practised method of such separation is crystallisation of the metal salt by evaporation or by addition of another reagent which acts to reduce the solubility of the metal containing compound in the solution, and cooling the solution to the point where crystallisation takes place. This operation is essentially a reversible process in that the crystalline compound can be redissolved simply by adding water or raising the temperature of the solution.

The above separation process has a number of advantages namely that: - the crystallisation step is itself a very effective purification process and the metal salt is usually significantly purer than the solution from which it is produced; the purification process can, if required, be repeated many times, and the salt is formed in a coarse, granular form wherein the crystal size can be controlled to be typically in the range of 0.2-2.0 mm. Separation of the solid from the solution is thus a simple matter which can efficiently be accomplished by a variety of methods, and the product can easily be washed free of contaminants present in the mother liquor.

In many cases, however, the metal salt produced by crystallisation is not directly useful and must be processed further to obtain a more suitable end product or a more versatile intermediate. It is also usually required that the product or intermediate meets stringent criteria in terms of purity, not only in terms of metal contaminants but also in terms of contamination with anions. In addition, the particle size and degree of crystallinity of the intermediate, both of which greatly affect the reactivity thereof, as well as other physical properties, are important parameters which must be controlled within tight limits during the manufacturing process.

Very often the intermediate is a metal hydroxide or a metal carbonate and these compounds can be produced by either a direct method or an indirect method.

Direct Method

The direct method involves the treatment of a solution of a metal salt such as a sulphate with a solution of an alkaline hydroxide or an alkaline carbonate under very carefully controlled reaction conditions in which the concentrations of the reactants in solution are maintained at very low levels and the solution temperature held at close to 100 °C. A typical reaction is as follows:

MSO₄ solution ⁺ 2 NH₄OH solution "^ [ M(OH)₂] _S0|ifj + (NH₄)₂SO₄ solution

The product of such reactions is usually a very fine solid which does, however, have a high degree of crystallinity. These products can be separated from solution by filtration and can be washed fairly well, but some alkaline sulphate contaminants usually remains.

In the case of aluminium, a filterable and substantially pure hydroxide 'bayerite' can be produced on a large scale by the slow crystallisation from a sodium aluminate solution. Because the hydroxide is chemically inert, a disadvantage thereof is that it can only be used to form aluminium salts using moderately strong acids. Salts of weak acids must be made in other ways, such as by using the metal itself, in the production of aluminium acetate, for example.

The method has the further disadvantage that the product has a fairly large particle size (particle size 10-20 microns) and a crystalline structure which reduces reactivity and may hence require extensive further processing by way of milling to produce a final product with the correct properties. (Often products with a mean particle size of < 1 micron are required.) In a few cases, where a very reactive product is required, reaction conditions are chosen which produce a solid of much finer sizing and lower degree of crystallinity. A disadvantage of such products is, however, that they are very difficult to separate from solution and to wash free from soluble reaction products.

For some metals the direct method always produces an unfilterable gel and for these metals an indirect method must be employed.

Indirect Method

In the indirect method, a solution of the metal salt is first reacted with a reagent, which precipitates an insoluble crystalline salt which can easily be filtered. The insoluble crystalline salt is then reacted further with an alkaline hydroxide or carbonate solution to form a hydroxide or carbonate product of much the same crystalline form as the original insoluble crystalline salt.

In the case of zirconium, a known method for the manufacture of the carbonate, includes the following steps:

A crude hydroxide is formed by hydrolysing a solution of sodium zirconate.

The hydroxide is dissolved in hydrochloric acid and a zirconium oxychloride is crystallised from this solution.

The oxychloride is redissolved in water and a basic zirconium sulphate, which is insoluble in water, is crystallised by adding a stoichiometric amount of sulphuric acid.

This insoluble salt is soaked in an ammonium carbonate solution where an ion exchange process takes place which converts the basic sulphate into a carbonate of a form similar to that of the basic sulphate crystals.

The product is thereafter filtered. The carbonate is washed to remove the alkali and sulphate.

In both the direct and the indirect methods the products obtained are thus essentially crystalline solids which are easily separable but, unfortunately, not highly reactive. On the other hand, where the crystallinity of the products is reduced towards the range of gels of random structure to increase their reactivity, substantial difficulties are encountered in separating the reaction products and such processes become unworkable on an industrial scale.

OBJECTS OF THE INVENTION

It is accordingly an object of the present invention to provide a method for the production of amorphous metal containing compounds with which the aforesaid disadvantages may be overcome or at least minimised and which is a useful alternative to the known methods described above. It is another object of the invention to provide a method whereby the hydroxide or carbonate of a metal can be prepared with an essentially amorphous fine structure but with a pseudo-crystalline macro structure similar to that of a crystalline metal salt, i.e. in a form that can easily be separated from the solution containing the soluble reaction products, while being highly reactive.

SUMMARY OF THE INVENTION

According to the invention there is provided a method for the production of an amorphous metal containing compound including the steps of: providing a soluble crystalline salt of the metal in solid form; and - reacting the soluble crystalline salt in solid form directly with an alkaline solution without first dissolving the salt, to form a granular amorphous metal containing compound.

Further according to the invention, the soluble crystalline salt comprises the crystalline sulphate of the metal.

Still further according to the invention, the alkaline solution comprises an alkaline hydroxide - or an alkaline carbonate solution.

The applicant has surprisingly found that by reacting the crystalline sulphate in solid form directly with an alkaline hydroxide - or an alkaline carbonate solution, without first dissolving the sulphate in water, a filterable and yet relatively very reactive, granular, amorphous metal hydroxide or metal carbonate is respectively obtained.

The crystalline salt is thus converted into an amorphous- and more particularly a pseudo-crystalline hydroxide or carbonate which has substantially the same physical form as the soluble crystalline salt. This provides for the easy separability of the granules from the solution.

In addition, the porous nature of the amorphous compound or end-product allows for the virtual complete removal of soluble by-products from the interior of the particles by efficient washing procedures, as a concentrated solution, which can be utilised for other purposes in an economical fashion.

The alkaline solution may be one from the group comprising sodium hydroxide, sodium carbonate, ammonium hydroxide, and ammonium carbonate, in solution.

In the case of ammonium hydroxide, the concentration of ammonia in the alkaline solution may be between 20 g/l and 200 g/l.

The step of reacting the salt with the alkaline solution may include the step of mixing the salt with the alkaline solution. Further according to the invention the step of mixing the crystalline salt with the alkaline solution includes the further step of agitating the mixture sufficiently to maintain an excess of alkaline solution at the outer surface of each reacting crystal without causing the breakdown into fragments of the partly or completely reacted crystals.

In the case of relatively coarse crystalline salts (i.e. between 0.5 mm and 2.0 mm in diameter), the alkaline solution may be introduced at one end of a fluidised bed reactor and the salt introduced at an opposite end, with the reaction taking place intermediate the ends, the arrangement being such that the agitation of the mixture results from the counter-flow of the crystals and the alkaline solution.

In the case of relatively fine crystalline metal salts (i.e. between 0.1 mm and 0.5 mm in diameter), the alkaline solution and the salt may be mixed in a low-shear mixer, the arrangement being such that the agitation of the mixture is sufficient to restrict the salt crystals from settling down without causing the breakdown into fragments of the partly or completely reacted crystals.

Preferably the soluble crystalline salt is mixed with the alkaline solution at ambient temperature and the temperature of the reaction mixture maintained below 50 °C. The method may include the further step of washing the amorphous granular compound free of soluble metal salts.

The method may include the further step of filtering the amorphous granular end - product.

The method may include the further step of drying the amorphous granular compound at a temperature below 50 °C to remove water while preserving the amorphous and reactive nature of the end-product.

Preferably the metal of the metal containing crystalline salt comprises aluminium or zirconium.

The invention will now be described below by way of non-limiting examples.

In methods according to preferred embodiments of the invention, highly reactive, filterable, granular amorphous metal containing compounds in the form of metal hydroxides or metal carbonates are produced. The methods generally include the steps of: providing or forming a soluble crystalline sulphate salt of a particular metal in solid form; mixing and reacting the sulphate salt in solid form directly with an alkaline solution, such as sodium or ammonium hydroxide or carbonate, without first dissolving the salt; agitating the mixture sufficiently to maintain an excess of alkaline solution at the outer surface of each reacting crystal without causing the breakdown into fragments of the partly or completely reacted crystals, to form a granular pseudo-crystalline metal containing compound; washing the amorphous granular compound free of soluble metal salts; filtering the amorphous granular compound; and drying the amorphous granular compound at a temperature below 50°C to remove water while preserving the amorphous and reactive nature of the end-product.

The soluble crystalline salt is mixed with the alkaline solution at ambient temperature and, if necessary the temperature of the reaction mixture is maintained below 50 °C by removing heat of reaction.

In the case of relatively coarse crystalline salts (i.e. between 0.5 mm and 2.0 mm in diameter), the alkaline solution is introduced at one end of a fluidised bed reactor and the salt introduced at an opposite end, with the reaction taking place intermediate the ends, the arrangement being such that the agitation of the mixture results from the counter-flow of the crystals and the alkaline solution. In the case of relatively fine crystalline metal salts (i.e. between 0.1 mm and 0.5 mm in diameter), the alkaline solution and the salt is mixed in a low-shear mixer, the arrangement being such that the agitation of the mixture is sufficient to restrict the salt crystals from settling down without causing the breakdown into fragments of the partly or completely reacted crystals.

Preferably, the metals are either aluminium or zirconium, so that highly pure and reactive pseudo-crystalline aluminium hydroxide, aluminium carbonate, zirconium hydroxide or zirconium carbonate are respectively formed.

EXAMPLE 1 - PREPARATION OF PSEUDO-CRYSTALLINE ALUMINIUM HYDROXIDE (PCAH) FROM AMMONIUM ALUMINIUM SULPHATE CRYSTALS (AASC)

In the case of the preparation of pseudo-crystalline aluminium hydroxide (PCAH), the soluble crystalline salt may be ammonium aluminium sulphate crystals (NH₄AI(SO₄)₂.12H₂O) (AASC) and the alkaline solution ammonium hydroxide. The crystals are prepared according to conventional methods by heating an industrial type aluminium sulphate solution with the stoichiometric amount of ammonium sulphate and stirring the solution while cooling it slowly until ammonium aluminium sulphate crystals form. The crystals are subsequently filtered, screened and dried to obtain a particular product having particle sizes of between 0.7 and 2.0 mm. Approximately 100 kg of material is prepared in this way and used as the raw material in the tests described below. Initial tests were designed to evaluate the effectiveness of the procedure under different reaction conditions and to produce material for product characterisation.

The desirable reaction occurring may be represented by:-

AASC soiid + (NH₃)_aq -> PCAH solid + ( NH₄)₂SO_{4 aq} where AASC represents ammonium aluminium sulphate in a solid crystalline form, and PCAH represents pseudo-crystalline aluminium hydroxide in a form resembling the original AASC.

The unwanted competing reaction that could occur may be represented as:-

AASC _Soiid + (NH₃)_aq -> AHG + (NH₄)₂SO_{4 aq} where AHG represents aluminium hydroxide in the usual gel state.

The relative extent to which these two reactions occurred was estimated by two easily measured parameters, the settled volume of the solid reaction product and the time taken to filter a standard batch of reacted AASC.

It was found that these two measures were closely correlated and even a small increase of settled volume of the product compared to the settled volume of the AASC resulted in a substantial increase in filtration time. The effect of various parameters on the process was investigated on a small scale in batch tests using a variable speed mechanical agitator with the following results:-

Agitation speed was found to be critical. At low speeds crystals settled out before reacting fully, creating zones depleted in ammonia where crystal dissolution took place. Consequently the subsequent reaction took place in homogeneous solution resulting in extensive gel formation.

At very high agitation speeds the PCAH particles were broken down into small fragments which also lead to an increase in settled volume and filtration time. Over a range of intermediate speeds, sufficient to suspend the crystals but insufficient to shear the PCAH, practically all the end-product was in the form of granular particles which could be filtered at a very high rate.

Ammonia concentration was also found to be important. Both very low (<20g/l) and very high (>200g/l) initial concentrations were found to produce sufficient gel to adversely affect the efficacy of the process.

Many other variables such as ammonium sulphate concentration, temperatures of reaction in the range 20-50 °C; the excess of ammonia added (above a 5% excess); and the speed of addition of the AASC to the solution were found to have no significant effect. The required reaction time was also determined. It was found that the reaction was 95% complete in 15 minutes with conversion of the remaining AASC taking a further 15 minutes.

The applicant has therefore surprisingly found that by selecting the favourable reaction conditions set out above and by reacting the AASC in solid form directly with the ammonium hydroxide solution, without first dissolving the AASC, the desirable reaction, and not the unwanted competing reaction, takes place.

CHARACTERISATION OF PCAH

Samples of the product of these tests were submitted to the following tests to illustrate the nature of the PCAH end-product:

A sample of the material was filtered to form a dry cake and then calcined at 1000 °C to determine the alumina content. This was consistently found to be in the range 17-18% AI₂O₃.

A slurry of the end-product in water containing 80 g/l AI₂O₃ was stirred for 5 minutes using a high speed, high shear mixer. At this point the pseudo-crystals had broken down completely to form a typical aluminium hydroxide gel such as is formed by the rapid mixing of an aluminium salt solution and an alkaline solution. The viscosity of the gel formed was so large that the slurry could no longer be poured from the container. This indicates that the product is at most microcrystalline. Highly crystalline products such as those disclosed by the prior art would not behaved in this fashion, although a small degree of shearing would take place.

A sample of the product was slurried with acetone, refiltered and washed with further acetone to remove as much water as possible. The material was then dried at ambient temperature under a high vacuum to remove the acetone, ground to a fine powder and subjected to X-ray diffraction (XRD) analysis. No pattern characteristic of a crystalline compound could be detected.

Samples of the product were dried under varying conditions as follows:

At low temperatures (<30°C) the product lost water under vacuum but remained amorphous.

At moderate temperatures, 50 °C, water was lost until a crystalline

tri hydrate was formed.

At 100 °C water was lost until a crystalline monohydrate was formed.

These results are consistent with those obtained by other workers who investigated the dehydration behaviour of alumina gels formed by homogeneous solution reactions - Alcoa Research Laboratories technical paper number 19, 1972, Karl Wefers and Gordon M Bell. EXAMPLE 2 - USE OF A FLUIDISED BED REACTOR

Relatively larger scale tests were further undertaken to demonstrate that the invention would be of practical application in an industrial process. In a series of tests, AASC were processed in 10 kg batches using a fluidised bed reactor to produce a washed pseudo-crystalline compound and a concentrated solution of ammonium sulphate as a by-product. The equipment used and the results obtained are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The fluidised bed reactor and the method according to the invention which takes place inside the reactor, will now be described by way of further example only with reference to the accompanying drawings wherein: figure 1 shows a schematic representation of a single fluidised bed reactor unit; and figure 2 shows a plurality of reactor units of figure 1 , arranged to form a counter-current washing circuit.

Referring particularly to figure 1, a fluidised bed reactor 12 comprises a column 14 of 150 mm internal diameter clear PVC, which enables the process to be easily observed and monitored. The column 14 defines an inlet 16 at its upper end for the ammonium aluminium sulphate crystals (AASC) and for a small stream of ammonium hydroxide for carrying the AASC downwardly along the column. The inlet 16 is also used as an inlet for flush water after the reaction has taken place.

The column 16 further defines an inlet 18 at its lower end through which a main stream of ammonium hydroxide is introduced. An outlet of an AASC hopper 20 opens into the upper inlet 16 and the lower inlet 18 is connected to an ammonium hydroxide reservoir 22 via a feedline 24. A secondary feedline 26 for a small stream of flushing ammonium hydroxide branches from the main feedline 24 and opens up in the upper inlet 16.

The overflow of the column 14 is connected to the reservoir 22 via a pipe 28.

The lower end of the column 14 is further provided with an outlet 30 for ammonium sulphate. An intermediate outlet 32 for PCAH is defined towards the lower quadrant of the column 14.

The solution is circulated through the column 14 and the feedlines 24 and 26 with a magnetic drive centrifugal pump 33.

A fine stainless steel mesh (600 micron) 34 is disposed in the lower half of the column 14. The mesh 34 acts as a flow distributor/bed support in the lower quadrant of the column 14. A 150 micron screen 35 is disposed on the overflow of the column 14 to remove fines before it reaches the circulation pump 33. Any material in the overflow is trapped on the 150 micron mesh before reaching the circulation pump 33. It will be appreciated that if material is able to pass through the 150 micron mesh, it will be fine enough not to cause blockage at the bed support mesh 34. The above arrangement proved satisfactory and robust. For example, in a 14 hour recirculation test no blockages occurred and no difficulty was encountered in any of the subsequent experimental runs.

Operating procedure.

In use, the column 14 is filled with the minimum quantity of water required to permit recirculation of liquid through the reactor 12. A 30 % excess of aqueous ammonia (25 % NH₃) is then added for a 10 kg batch of AASC. Dry graded AASC (-2 mm + 0.7 mm) were then fed into the column 14 over a 20 minute period with the recirculating rate adjusted to give a 50 % bed expansion. The system was then allowed to recirculate for about 30 minutes to ensure complete conversion of AASC to PCAH.

Recirculation was then stopped and the column drained of ammonium sulfate solution to just above the level of the settled bed 46. Referring to figure 2, the bed 46 was then washed substantially free of ammonium sulfate solution with flush water from the adjacent reactor unit 12. The concentration of ammonium sulfate in the exit stream was monitored by measuring the solution's conductivity. This allowed the washing profile to be determined and on-strength ammonium sulfate solution to be segregated and recirculated to the next run. The measure conductivity of the exit stream is depicted below in plot 1 : Plot 1 : Plot of conductivity against elution volume

volume eluted (litres)

The sudden and rapid decline of ammonium sulphate in the flush water is an indication of the high washing sufficiency achieved.

Subsequent runs followed exactly the same procedure but the initial fill from the run was made using an on-strength ammonium sulfate from the previous run to allow the concentration to build up to a maximum of 400 g/l. After the first run the quantity of ammonia added was reduced to the stoichiometric requirement for a batch, and the excess ammonia present at the end of each run was diminished to a final value of +/- 3 g/l after 4 runs.

The bed was then refluidised with water and the PCAH product drained out of the column 14, dewatered by vacuum filtration and the product used in various laboratory scale trials for conversion to final products.

Ammonium sulfate product strength. The theoretical maximum (NH₄)₂SO₄ product strength that can be achieved in a single pass operation corresponds to the case where only sufficient liquid is present to fill the intra-particle pore space and the inter-particle voidage. This can be calculated to be 582 g/l (saturated solution at 25 °C is about 1000 g/l).

In practice a considerably larger volume of liquid will have to be present to enable efficient solid/liquid contacting to be achieved. For example, if 2 I of solution are used per kg of AASC, the total volume of the system (PCAH + external liquid) will be 2.6 litres and the single-pass (NH )₂SO concentration will be reduced to 223 g/l; the required NH₃ concentration is reduced to 56 g/l. About 80 % of the ammonium sulfate could be recovered at this strength by draining off all external liquid, the remainder being recovered as a dilute solution by washing with water. However it is obvious to those skilled in the art that several reactor units 12 as illustrated in figure 1 could be linked together to form a counter-current circuit as shown schematically in figure 2. In such a circuit, the solid and solution formed in the washing cycle are made to move counter-current to one another, either by a suitable arrangement of piping and valves, or by mounting the units on a rotatable carousel so that each unit in turn acts as a reaction stage, a first wash stage, etc. During the trial ammonium sulfate product was continuously recycled from one run to the next but was diluted by addition of aqueous ammonia and with released reaction water. As a result the concentration of ammonium sulfate rose by about 60-70 g/l on each run from an initial 140 g/l to a final value of 390 g/l. Although the density of the solution thus increased substantially, no significant effect on any other operating parameter was noted. It can therefore be assumed that the system will operate up to a concentration of +/- 400 g/l (NH₄)₂SO without problem and this corresponds closely to the maximum if all the wash solutions are recycled solutions in a merry-go-round system, as shown in figure 2.

EXAMPLE 3 - USE OF MECHANICALLY AGITATED VESSELS

Fine crystals (i.e. 0.1 mm to 0.5 mm) are not suitable for processing by fluid bed technology, as the reaction product is not dense enough to have a sufficiently high settling velocity. Zirconium orthosulphate is an example of a salt, which is produced commercially in this size range, and for this material it was found that the conversion to the hydroxide took place more conveniently in mechanically agitated batch reaction tanks.

Laboratory scale tests were initially conducted on the salt in a similar fashion to those described for AASC. The results obtained were very similar to those obtained previously except that the high heat of reaction between ammonia and zirconium orthosulphate produced a marked rise in temperature.

In larger scale tests where 10 kg of crystals were processed this temperature rise was as much as 60°C above ambient and this resulted in an increase in gel formation, with the consequent increase in settled volume and filtration time. To counter this effect the reaction vessel was fitted with a cooling coil through which cooling water was passed to prevent the temperature from rising above 40°C. Subsequent tests produced settled solids, which filtered very rapidly to give a filter cake with a zirconia content of 45%.

Figure 3 illustrates how a series of batch reaction vessels 50 could be arranged to form a counter-current system through which the concentration of the soluble reaction product can be maximised. Practical systems for accomplishing this have been developed for many different systems and are well known to those skilled in the art.

The applicant has found the method according to the invention to have the following advantages: - it effectively converts a soluble crystalline metal salt into a granular and amorphous pseudo-crystalline metal containing compound which has virtually the same physical form as the original crystals; this preserves the easy separability of the compound from solution; the compound is chemically highly reactive; in addition, the porous nature of the compound allows for the complete removal of the soluble by-products from the interior of the compound granules by efficient washing procedures; and - it further allows for the recovery of the soluble by-products such as soluble salts as a concentrated solution which can be utilised further in an economical fashion.

It will be appreciated that variations in detail are possible with a method according to the invention for the production of amorphous metal containing compounds without departing from the scope of the appended claims.

Claims

1. A method for the production of an amorphous metal containing compound including the steps of providing a soluble crystalline salt of the metal in solid form; and reacting the soluble crystalline salt in solid form directly with an alkaline solution without first dissolving the salt, to form a granular amorphous metal containing compound.

2. A method according to claim 1 wherein the soluble crystalline salt comprises the crystalline sulphate of the metal.

3. A method according to claim 1 or claim 2 wherein the alkaline solution comprises an alkaline hydroxide - or an alkaline carbonate solution.

4. A method according to claim 3 wherein the alkaline solution is one of the group comprising sodium hydroxide, sodium carbonate, ammonium hydroxide, and ammonium carbonate, in solution.

5. A method according to claim 4, where, in the case of ammonium hydroxide, the concentration of ammonia in the alkaline solution is between 20 g/l and 200 g/l.

6. A method according to any one of the preceding claims wherein the step of reacting the salt with the alkaline solution includes the step of mixing the salt with the alkaline solution.

7. A method according to claim 6 wherein the step of mixing the crystalline salt with the alkaline solution includes the further step of agitating the mixture sufficiently to maintain an excess of alkaline solution at the outer surface of each reacting crystal without causing the breakdown into fragments of the partly or completely reacted crystals.

8. A method according to claim 7 wherein the alkaline solution is introduced at one end of a fluidised bed reactor and the salt introduced at an opposite end, with the reaction taking place intermediate the ends, the arrangement being such that the agitation of the mixture results from the counter-flow of the crystals and the alkaline solution.

9. A method according to claim 7 wherein the alkaline solution and the salt are mixed in a low-shear mixer, the arrangement being such that the agitation of the mixture is sufficient to restrict the salt crystals from settling down without causing the breakdown into fragments of the partly or completely reacted crystals.

10. A method according to any one of claims 6 to 9 wherein the salt is mixed with the alkaline solution at ambient temperature.

11. A method according to claim 10 wherein the temperature of the reaction mixture is maintained at below 50 °C.

12. A method according to any one of the preceding claims which includes the further step of washing the granular compound free of soluble salts.

13. A method according to any one of the preceding claims which includes the further step of filtering the granular compound.

14. A method according to any one of the preceding claims which includes the further step of drying the granular compound.

15. A method according to claim 14 wherein the granular compound is dried at a temperature of below 50 °C.

16. A method according to any one of the preceding claims wherein the metal of the metal containing crystalline salt comprises aluminium or zirconium.

17. A method for the production of an amorphous metal containing compound substantially as herein described and exemplified with reference to the accompanying drawings.