AU2004203325A1 - Process for precipitation of syngenite type minerals - Google Patents

Description

AUSTRALIA

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RELATED ART: NAME OF APPLICANT: ACTUAL INVENTOR(S): ADDRESS FOR SERVICE: INVENTION TITLE: Tennant Limited Frank Trask LORD AND COMPANY, Patent and Trade Mark Attorneys, of 4 Douro Place, West Perth, Western Australia, 6005, AUSTRALIA.

PROCESS FOR PRECIPITATION OF SYNGENITE TYPE MINERALS DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO'S: AUSTRALIAN PROVISIONAL PATENT APPLICATION NUMBER 2003903862 FILED ON 25 JULY 2003 The following Statement is a full description of this invention including the best method of performing it known to me/us:

TITLE

PROCESS FOR PRECIPITATION OF SYNGENITE TYPE MINERALS FIELD OF THE INVENTION The present invention relates to a process for precipitation of syngenite type minerals, in particular to a process for precipitation of potassium syngenite and ammonium syngenite for use as slow release fertilizers or as cements in the production of agglomerated granulated fertilizers.

BACKGROUND OF THE INVENTION It is well known that potassium is one of the basic nutrients essential for plant growth, and water-soluble potassium minerals in the form potassium chloride are not in short supply. However, the application of potassium chloride to various agricultural and horticultural crops necessarily introduces large quantities of undesirable chloride ions into the soil and consequently, significant attention has been directed towards developing processes that convert readily available potassium chloride to agronomically beneficial potassium salts, such as potassium sulphate.

Many of these processes produce potassium syngenite (K 2 S0 4 .CaSO 4

.H

2 0) as an intermediate that is then converted to potassium sulphate, or a reaction by-product whose formation is to be avoided.

Fernando Lozano and Wint have investigated the production of potassium sulphate from the reaction of gypsum and potassium chloride (Fernandez Lozano, JA, and Wint, A, "Double Decomposition of Gypsum and Potassium Chloride Catalyzed by Aqueous Ammonia", The Chemical Engineering Journal, 23(1982) 53-61; Fernandez Lozano, JA, and Wint, A, "Production of Glaserite and potassium sulphate from gypsum and sylvinite catalyzed by ammonia", Chemical Engineering Journal 67(1997) Abu-Eishah et al investigated the same reactions using phosphogypsum as the source of calcium sulphate (Abu-Eishah,SI, Bani-Kananeh, AA, and Allawazi, MA, "K 2 S0 4 production via the double decomposition reaction of KCl and phosphogypsum." Chemical Engineering Journal, 76(2000) 197-207). All of these studies were concerned with preventing or minimising the formation of potassium syngenite, as it was considered to be a low value mineral.

US Patent 4554139 describes a process for producing potassium sulphate from potassium chloride salts, gypsum and another sulfate source, wherein syngenite is formed and then decomposed with mineral acids to produce crystalline potassium sulphate. The resultant gypsum is recycled. Notably, sodium sulphate is added to the reaction mixture to increase the concentration of sulphate and promote the formation of syngenite.

US Patent 4554151 describes a process that produces potassium bisulphate from the decomposition of potassium syngenite. The syngenite is formed in the same manner as US Patent 4554139, and is by the same authors.

US Patent 6334990 relates to a process for the preparation of potassium sulphate, sodium carbonate and sodium bicarbonate from a potash brine. Syngenite is precipitated by reaction of sodium chloride and potassium chloride with calcium sulphate and sodium sulphate. The syngenite is then treated to produce potassium sulphate via a complex route.

However, potassium syngenite and its ammonium analogue, ammonium syngenite, have been shown to be potentially valuable slow-release fertilizers.

Hill, Anderson and Nestell investigated conditions under which potassium syngenite would form in 1934 and 1920, respectively, (see Hill, A.E. "Ternary Systems. XIX.

Calcium Sulphate, Potassium Sulphate and Water", Journal of The American Chemical Society, 26(1934), 1071-1078; Anderson, E and Nestell, RJ, "The Formation of the Double Salts of Calcium and Potassium sulphates at 100 0 The Journal of Industrial and Engineering Chemistry 12(1920) 243-246).

US Patent 4883530 relates to the use of ammonium syngenite as a slow release fertilizer. It alludes to ammonium syngenite((NH4)2SO4.CaSO4.H20) as a well known product that is produced by the reaction of ammonium sulphate and calcium sulphate (anhydrite) in an aqueous solution.

There has been minor production of potassium syngenite (Garrett, DE, "Potash", Chapman and Hall, 1996, 690) at the Pasquasia Operations of Italkali of Sicily, where low-grade potassium brine from the precipitation of Schoenite is reacted with gypsum to precipitate syngenite, whereupon it is recycled within the plant.

Massive solar salt fields in Australia and other countries precipitate halite (NaC1) from seawater. The process is conducted in large solar ponds that depend on hot and dry winds to remove moisture to allow for the precipitation of NaCI. After the optimum amount of NaCl has been precipitated, the remaining solutions, which are called bitterns, are released to the open sea. These bitterns contain abundant magnesium and potassium salts. There is almost nowhere in the world where the air has a low enough humidity to precipitate either the potassium or magnesium salts on a continuous basis.

There have been numerous attempts to recover potassium from bitterns on a world basis. The Lake McLoed Project of Texada Salt was a venture in Australia that failed to achieve potassium production because of poor understanding of the humidity conditions needed to get these minerals to precipitate. Apart from a few specialized solar projects (Great Salt Lake Chemical and Dead Sea Chemical), there is no production of potash from any source other than primary sedimentary deposits of chlorides or mixed potassium magnesium sulphate. Most recovery plants have centred around evaporation, but others have attempted to salt out the material using ammonia and methanol, to name a few projects. All of these processes require very high energy requirements, complex flow sheets with many steps, and very high capital input to handle the large volumes.

The present invention attempts to overcome at least in part some of the aforementioned disadvantages.

SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention there is provided a process for precipitation of potassium syngenite or ammonium syngenite comprising reacting calcium sulphate hemihydrate with aqueous solutions of potassium or ammonium salts, respectively.

In accordance with a second aspect of the present invention there is provided a process for recovering potassium syngenite from concentrated potassium salt solutions, such as from salt bitterns, natural salt brines, and waste solutions from potash processing plants, the process comprising: a) reacting calcium sulphate hemihydrate with the concentrated potassium salt solution to precipitate potassium syngenite; and b) separating the resulting precipitated potassium syngenite from the mother liquor.

In accordance with a third aspect of the present invention there is provided a process for agglomerating water-soluble potassium salts comprising mixing calcium sulphate hemihydrate with water-soluble potassium salts and moisturizing the mixture with water or an aqueous solution of potassium sulphate to promote precipitation of potassium syngenite, whereupon the precipitated potassium syngenite adheres to and thereby binds the water-soluble potassium salts in an agglomerated form.

In accordance with a fourth aspect of the present invention there is provided a process for agglomerating water-soluble ammonium salts comprising mixing calcium sulphate hemihydrate with water-soluble ammonium salts and moisturizing the mixture with water or an aqueous solution of ammonium sulphate to promote precipitation of ammonium syngenite, whereupon the precipitated ammonium syngenite adheres to and thereby binds the water-soluble ammonium salts in an agglomerated form.

In accordance with a fifth aspect of the present invention there is provided a process for agglomerating fertilizer particles comprising mixing calcium sulphate hemihydrate with the fertilizer particles and moisturizing the mixture with an aqueous solution of potassium sulphate, or ammonium sulphate, to promote precipitation of potassium syngenite, or ammonium syngenite, respectively, whereupon the precipitated potassium syngenite, or precipitated ammonium syngenite, respectively, adheres to and thereby binds the fertilizer particles in an agglomerated form.

In accordance with a sixth aspect of the invention there is provided an agglomerated fertilizer composition comprising particles of at least one fertilizing agent and a binding agent to promote agglomeration of the particles of the or each fertilizing agent, wherein the binding agent is potassium syngenite or ammonium syngenite.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION Conventional chemistry has viewed the reaction between bassanite (calcium sulphate hemihydrate) and water to form gypsum as a simple hydration reaction. There is now common agreement among researchers that when bassanite is reacted with water to form gypsum, it occurs in accordance with the following reaction equilibria: i. CaSO 4 .0.5H 2 0 H20 Ca2+(q)+ S0 4 2 AH -18 kJ/mol ii. Ca2+(a+ SO 4 2 +2H 2 0 CaSO 4 .2H 2 0 AH° +lkJ/mol Total free energy -17kJ/mol (see Solberg,C., and Hansen,S., "Dissolution of CaSO 4 .0.5H20 and precipitation of CaSO 4 .2H20 A kinetic study of synchrotron X-ray powder diffraction." Cement and Concrete Research 31(2001) 641-646) As a result of their high solubility, the Ca 2 and S0 4 2 ions remain in solution for anywhere up to 30 minutes. The residence time of the ions in solution is dependent on surface area of the bassanite particles, ionic strength of other salts in the solutions, and several other factors. Many different additives and temperatures can vary the time for conversion to gypsum, and is a well known art. For example see Brant and Bosbach, and Badens et.al (Brandt, and Bosbach,D., "Bassanite (CaSO 4 .0.5H20) dissolution and Gypsum (CaSO 4 .2H 2 0) precipitation in the presence of cellulose ethers." Journal of Crystal growth 233(2001) 837-845; Badens,E., Vessler,V., and Boistelle,R., "Crystallization of gypsum from hemihydrate in presence of additives", Journal of Crystal growth 198/199(1999) 704-709). There are many well known arts of controlling the set time and the strength of the gypsum that is formed. The formation of gypsum in this manner from bassanite is referred to as a throughsolution route.

Whilst the prior art has relied on gypsum, phosphogypsum, and calcium sulphate anhydrite as the source of solvated calcium and sulphate ions in solution, it has now been found that the ready dissolution of bassanite (calcium sulphate hemihydrate) to afford excess solvated calcium and sulphate ions in solution can be advantageously employed to precipitate microcrystals of gypsum which, in turn, act as seed crystals for precipitation of potassium syngenite or ammonium syngenite from aqueous solutions of potassium or ammonium salts, respectively, according to the following reactions: i. CaS04.0.5H 2 0 H 2 0 Ca 2 S042(a) ii. Ca 2 2SO 4 2 2K(aq) K 2 S0 4 .CaSO 4

.H

2 04, ii. Ca 2 2SO 4 2 -(aq 2NH 4

(NH

4 )2S0 4 .CaSO 4

.H

2 04 In relation to the first aspect of the present invention, the process for precipitation of potassium syngenite or ammonium syngenite is performed by reacting calcium sulphate hemihydrate with aqueous solutions of potassium or ammonium salts, respectively.

Typically, in excess of a stoichiometric amount of calcium sulphate hemihydrate as an aqueous solution is reacted with an aqueous solution of highly soluble potassium salts including, but not limited to, potassium halides, potassium sulphate, potassium nitrate, and mixtures thereof, or highly soluble ammonium salts including, but not limited to, ammonium halides, ammonium sulphate, ammonium nitrate, and mixtures thereof.

Preferably, up to 10% in excess of calcium sulphate hemihydrate is reacted in accordance with the present invention. Further, it will be understood that other cationic species, such as sodium, magnesium, and other alkali metals, alkaline earths, and transition elements, may also be present at relatively high concentrations in the aqueous solutions.

Interestingly, it has been found that potassium syngenite will precipitate from aqueous solutions with potassium concentrations as low as 2% w/w equivalent of potassium sulphate. The precipitation reaction for potassium syngenite or ammonium syngenite in accordance with the present invention may thus be conducted on aqueous solutions whose potassium or ammonium concentrations range from relatively low to saturation point.

In order to precipitate potassium syngenite or ammonium syngenite, it is preferable to keep the reaction temperature as low as practically possible, as lower temperatures promote stable formation of potassium syngenite or ammonium syngenite.

In relation to the second aspect of the present invention, the process for recovering potassium syngenite from concentrated potassium salt solutions, such as from salt bitterns, natural salt brines, and waste solutions from potash processing plant comprises the following steps: a) reacting calcium sulphate hemihydrate with the concentrated potassium salt solution to precipitate potassium syngenite; and b) separating the resulting precipitated potassium syngenite from the mother liquor.

It is proposed to take concentrated potassium salt solutions, such as from salt bitterns, natural salt brines, and waste solutions from potash processing plant that have been concentrated to a density around 1.31 to 1.32 g.cm- 3 At this density Epsomite is typically ready to precipitate, and the ratio of concentration is in the order of 55 times.

Potassium concentration is in the order of 33 g.L- 1 of K20. Conventional addition of gypsum would reduce the K 2 0 concentration to about 21 grams per litre of K20. By contrast, addition of Bassanite reduces the K20 concentration to between 10 and g.L 1 K20 by promoting precipitation of potassium syngenite. The reaction with gypsum takes 12 hours to go to completion, whilst the reaction with Bassanite is completed within 15 minutes. Thus, the process of the present invention allows for a reasonable recovery of K20 as potassium syngenite with minimal investment and materials handling.

It is envisaged that a process plant would extract gypsum from a pre-concentration pond of a solar salt works or other source, and transform it into Bassanite by any of a number of known methods that are commercially available. Bassanite would then be mixed in up to a 10% excess ratio to the potassium ions to be removed from the bitterns, and given a one minute residence time in a high intensity mixer. The resulting mixture would then be sent to a holding tank for 15 minutes reaction time, after which it would be centrifuged to remove the fine acicular crystals of potassium syngenite. The separated potassium syngenite crystals would be washed with fresh water to remove NaC1 and other soluble salts that are not desired in the fertilizer.

Typically, the potassium syngenite precipitate resulting from the reaction mixture of step a) is separated from the mother liquor by filtration, centrifugal means, or clarification by counter current decantation and washing.

The process of the present invention can be used to convert potassium chloride, which is not wholly desirable in agriculture, to potassium syngenite, which itself is regarded as a slow-release fertilizer. The by-product of this process is calcium chloride.

In this way it is possible to produce an agronomically acceptable form of potassium from concentrated potassium halide solutions which previously would have been discarded.

It is common practice to use gypsum as a binding agent in the production of granular fertilisers, wherein the fertilising agents are mixed with gypsum and water, and heated

I

and cured over a long period of time to form resultant agglomerates, wherein the adhesive binding the fertilising agents is gypsum.

US Patent 4019889 relates the use of calcium sulphate hemihydrate to produce a whole range of slow release fertilizers by mixing from between 10 and 50% bassanite (CaS0 4 .0.5H 2 0) and then reacting the mechanical mixtures with heated water in an amount sufficient for crystallization of calcium sulfate (gypsum), thereby forming a granuable mixture. The slow release fertilizer that was produced depended on the low solubility of the calcium sulphate (gypsum) that covered the soluble components to protect them from dissolution.

It has now been demonstrated that Bassanite (CaS04.0.5H 2 0) does not transform directly to gypsum by simple hydration when reacted with water at low temperatures, but it is actually a very soluble mineral that produces a supersaturated solution of calcium and sulphate ions in respect to gypsum. Various reactions take place and several compounds can be formed by taking advantage of the supersaturated condition of the solutions in respect to gypsum. Accordingly, as described above, it has been found that both potassium and ammonium syngenite can be precipitated quickly and efficiently. In contrast, the diffusion of ions from the partial dissolution of gypsum is slow.

The reactions which afford rapid precipitation of potassium syngenite or ammonium syngenite can be advantageously used to form binders for the granulation of fertilizers, the production of slow release fertilizer themselves, and any other chemical reaction where an elevated amount of calcium or sulphate is needed to enhance the equilibrium by the addition of excess calcium or sulphate ions when compared to the use of gypsum.

In relation to the third aspect of the present invention, the process for agglomerating water-soluble potassium salts comprises mixing calcium sulphate hemihydrate with water-soluble potassium salts and moisturizing the mixture with water or an aqueous solution of potassium sulphate to promote precipitation of potassium syngenite, whereupon the precipitated potassium syngenite adheres to and thereby binds the water-soluble potassium salts in an agglomerated form.

The water-soluble potassium salts include, but are not limited to, potassium chloride, potassium sulphate, potassium nitrate, potassium syngenite, and mixtures thereof.

Typically, the water-soluble potassium salts are mechanically and integrally mixed with up to 10 w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w water or up to w/w of a dilute solution of potassium sulphate to moisturize the resulting mixture.

Residual moisture from the introduced spray promotes dissolution of powdered calcium sulphate hemihydrate and a small amount of the water-soluble potassium salts on the exterior surface of particles in the mixture. Solvated calcium, potassium, and sulphate ions react rapidly to precipitate crystalline and micro-crystalline potassium syngenite on the exterior surface of the particles, thereby binding said particles together in an agglomerated form.

The above process is conducted at ambient temperature, and curing times are in the order of fifteen to sixty minutes, depending on various factors including, but not limited to, the volume of material in the tumbler, mixing speed, and humidity of material in the tumbler.

In relation to the fourth aspect of the present invention, the process for agglomerating water-soluble ammonium salts comprises mixing calcium sulphate hemihydrate with water-soluble ammonium salts and moisturizing the mixture with water or an aqueous solution of ammonium sulphate to promote precipitation of ammonium syngenite, whereupon the precipitated ammonium syngenite adheres to and thereby binds the water-soluble ammonium salts in an agglomerated form.

The water-soluble ammonium salts include, but are not limited to, ammonium chloride, ammonium sulphate, ammonium nitrate, ammonium syngenite, and mixtures thereof.

Typically, the water-soluble ammonium salts are mechanically and integrally mixed with up to 10 w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w water or up to w/w of a dilute solution of ammonium sulphate to moisturize the resulting mixture.

Residual moisture from the introduced spray promotes dissolution of powdered calcium sulphate hemihydrate and a small amount of the water-soluble ammonium salts on the exterior surface of particles in the mixture. Solvated calcium, ammonium, and sulphate ions react rapidly to precipitate crystalline and micro-crystalline ammonium syngenite on the exterior surface of the particles, thereby binding said particles together in an agglomerated form.

The above process is conducted at ambient temperature, and curing times are in the order of fifteen to sixty minutes, depending on various factors including, but not limited to, the volume of material in the tumbler, mixing speed, and humidity of material in the tumbler.

In relation to the fifth aspect of the present invention, the process for agglomerating fertilizer particles comprises mixing calcium sulphate hemihydrate with the fertilizer particles and moisturizing the mixture with an aqueous solution of potassium sulphate, or ammonium sulphate, to promote precipitation of potassium syngenite, or ammonium syngenite, respectively, whereupon the precipitated potassium syngenite, or precipitated ammonium syngenite, respectively, adheres to and thereby binds the fertilizer particles in an agglomerated form.

The fertilizer particles include any fertilizing agent and mixtures thereof, that can be applied to soils, crops, or hydroponic systems, in an agglomerated form.

Typically, the fertilizer particles are mechanically and integrally mixed with up to w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w of a dilute solution of potassium sulphate or ammonium sulphate to moisturize the resulting mixture, such that the exterior surface of particles in the mixture are at least partially coated with a dilute solution of potassium sulphate or ammonium sulphate. Residual moisture from the introduced spray also promotes dissolution of powdered calcium sulphate hemihydrate on the exterior surface of particles in the mixture.

Solvated calcium, potassium or ammonium, and sulphate ions react rapidly to precipitate crystalline and micro-crystalline potassium syngenite or ammonium syngenite on the exterior surface of the particles, thereby binding said particles together in an agglomerated form.

The above process is conducted at ambient temperature, and curing times are in the order of fifteen to sixty minutes, depending on various factors including, but not limited to, the volume of material in the tumbler, mixing speed, and humidity of material in the tumbler.

In this way, Bassanite can be used as a precursor binding agent to make agricultural agglomerates of water-soluble potassium salts, including potassium sulphate, watersoluble ammonium salts, including ammonium sulphate, and other fertilizer agents, provided the fertilizer agent is mixed with an aqueous solution of potassium sulphate, or ammonium sulphate.

When Bassanite is substituted for gypsum, the amount of bassanite required to generate a binding material, comprising potassium syngenite or ammonium syngenite, can be decreased in comparison to the amount of gypsum that is generally required, and the time to form the agglomerate is reduced substantially. There is no need for long and extended curing periods or high processing temperatures, and the agglomerates formed have a superior strength compared to the prior art gypsum method. Advantageously, potassium syngenite and ammonium syngenite are also slow release fertilizers in themselves.

In relation to the sixth aspect of the invention, the agglomerated fertilizer composition comprises particles of at least one fertilizing agent and a binding agent to promote agglomeration of the particles of the or each fertilizing agent, wherein the binding agent is potassium syngenite or ammonium syngenite.

The agglomerated fertilizer composition is typically produced according to the process of the third, fourth, or fifth aspect of the invention as outlined above. The agglomerated fertilizer composition comprises at least one fertilizing agent, preferably a mixture of a plurality of fertilizing agents, that can be applied to soils, crops, or hydroponic systems, in an agglomerated form.

Particles of the fertilizing agent, or mixture of fertilizing agents, are bound together in an agglomerated form by crystalline or microcrystalline potassium syngenite or ammonium syngenite that has been precipitated on the exterior surface of the particles of the fertilizing agent(s), typically in accordance with the processes outlined above.

Several embodiments of the present invention are now further described by reference to the following Examples.

Example 1.

A solution containing 60 g.L of Mg as MgSO 4 and 20 g.L'1 of K as KC1 was made up to represent seawater bitterns that had been concentrated 53 times to a terminal density of 1.33 g.L To one litre of this mixture, 3.5 grams of equivalent pure Bassanite was added. The reaction mixture was stirred for 1 minute, and left to react for another 14 minutes. The resulting precipitate was filtered and washed with acetone, resulting in an optically pure sample of potassium Syngenite (7.8 g), corresponding to a recovery in terms of the Bassanite of 98%. This experiment recovered 40% of the potash in the artificial bittern, and was run at 25 0 C. When an excess of Bassanite was added, a mixture of gypsum and potassium Syngenite precipitated, and the concentration of K reduced to 10.9 g.L" 1 This indicates that, at this temperature, the ultimate recovery is in the order of Example 2.

A sample of fine crystals of ammonium sulfate was dried and mixed with 3.5% by weight of very fine powdered Bassanite (96% pure). The mixture was tumbled in a rotating bottle, and sprayed with 1.0%.by weight water. The resultant mixture was allowed to agglomerate and cure for 15 minutes to afford a well bound granular product. Examination under a polarizing microscope revealed that individual crystals of ammonium sulphate were bonded together by acicular needles of ammonium Syngenite running normal to the grains.

Example 3.

A sample of fine sand was dried and mixed with 3.5% by weight of very fine powdered Bassanite (96% pure). The mixture was tumbled in a rotating bottle, and sprayed with 2.5 by weight saturated ammonium sulphate solution. The resulting mixture was allowed to agglomerate and cure for 15 minutes, affording a sample of well bound granules. Examination under a polarizing microscope revealed that the individual sand grains were bonded together by acicular needles of ammonium Syngenite running normal to the grains.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims (18)

1. A process for precipitation of potassium syngenite or ammonium syngenite comprising reacting calcium sulphate hemihydrate with aqueous solutions of potassium or ammonium salts, respectively.

2. A process according to claim 1, wherein the potassium salts are selected from a group of highly soluble potassium salts including, but not limited to, potassium halides, potassium sulphate, potassium nitrate, and mixtures thereof.

3. A process according to claim 1, wherein the ammonium salts are selected from a group of highly soluble ammonium salts including, but not limited to, ammonium halides, ammonium sulphate, ammonium nitrate, and mixtures thereof.

4. A process according to any one of the preceding claims, wherein an aqueous solution of calcium sulphate hemihydrate is reacted with aqueous solutions of potassium or ammonium salts, respectively. A process according to any one of the preceding claims, wherein calcium sulphate hemihydrate is reacted up to 10 excess of the stoichiometric ratio of potassium or ammonium salts.

6. A process for recovering potassium syngenite from concentrated potassium salt solutions, such as from salt bitterns, natural salt brines, and waste solutions from potash processing plants, the process comprising: c) reacting calcium sulphate hemihydrate with the concentrated potassium salt solution to precipitate potassium syngenite; and d) separating the resulting precipitated potassium syngenite from the mother liquor.

7. A process according to claim 6, wherein the precipitated potassium syngenite I separated by filtration, centrifugal means, or clarification by counter current decantation and washing.

8. A process according to claim 6 or claim 7, wherein an aqueous solution of calcium sulphate hemihydrate is reacted with the concentrated potassium salt solutions of step a).

9. A process according to any one of claims 6 8, wherein the calcium sulphate hemihydrate is reacted in up to a 10 excess ratio to the potassium salt. A process for agglomerating water-soluble potassium salts comprising mixing calcium sulphate hemihydrate with water-soluble potassium salts and moisturizing the mixture with water or an aqueous solution of potassium sulphate to promote precipitation of potassium syngenite, whereupon the precipitated potassium syngenite adheres to and thereby binds the water-soluble potassium salts in an agglomerated form.

11. A process for agglomerating water-soluble potassium salts according to claim wherein the water-soluble potassium salts include potassium chloride, potassium sulphate, potassium nitrate, potassium syngenite, and mixtures thereof.

12. A process for agglomerating water-soluble potassium salts according to claim or claim 11, wherein the water-soluble potassium salts are mechanically and integrally mixed with up to 10 w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w water or up to 5 w/w of a dilute solution of potassium sulphate to moisturize the resulting mixture.

13. A process for agglomerating water-soluble ammonium salts comprising mixing calcium sulphate hemihydrate with water-soluble ammonium salts and moisturizing the mixture with water or an aqueous solution of ammonium sulphate to promote precipitation of ammonium syngenite, whereupon the precipitated ammonium syngenite adheres to and thereby binds the water-soluble ammonium salts in an agglomerated form.

14. A process for agglomerating water-soluble ammonium salts according to claim 13, wherein the water-soluble ammonium salts include ammonium chloride, ammonium sulphate, ammonium nitrate, ammonium syngenite, and mixtures thereof. A process for agglomerating water-soluble ammonium salts according to claim 13 or claim 14, wherein the water-soluble ammonium salts are mechanically and integrally mixed with up to 10 w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w water or up to 5 w/w of a dilute solution of ammonium sulphate to moisturize the resulting mixture.

16. A process for agglomerating fertilizer particles comprising mixing calcium sulphate hemihydrate with the fertilizer particles and moisturizing the mixture with an aqueous solution of potassium sulphate, or ammonium sulphate, to promote precipitation of potassium syngenite, or ammonium syngenite, respectively, whereupon the precipitated potassium syngenite, or precipitated ammonium syngenite, respectively, adheres to and thereby binds the fertilizer particles in an agglomerated form.

17. A process for agglomerating fertilizer particles according to claim 16, wherein the fertilizer particles are mechanically and integrally mixed with up to 10 w/w powdered calcium sulphate hemihydrate in a tumbler, rotating bottle, or other suitable mixing means, and sprayed with up to 5 w/w of a dilute solution of potassium sulphate or ammonium sulphate to moisturize the resulting mixture.

18. An agglomerated fertilizer composition comprising particles of at least one fertilizing agent and a binding agent to promote agglomeration of the particles of the or each fertilizing agent, wherein the binding agent is potassium syngenite or ammonium syngenite.

19. An agglomerated fertilizer composition according to claim 18, produced according to the processes of any one of claims 10 to 17. A process for precipitation of potassium syngenite substantially as hereinbefore described with reference to Example 1.

21. A process for recovering potassium syngenite from concentrated potassium salt solutions, such as from salt bitterns, natural salt brines, and waste solutions from potash processing plants substantially as hereinbefore described with reference to Example 1.

22. A process for agglomerating water-soluble ammonium salts substantially as hereinbefore described with reference to Example 2. DATED THIS 23RD DAY OF JULY 2004. Tennant Limited By their Patent Attorneys LORD AND COMPANY PERTH, WESTERN AUSTRALIA.