WO2014087118A1

WO2014087118A1 - Potash product, method and apparatus

Info

Publication number: WO2014087118A1
Application number: PCT/GB2012/053043
Authority: WO
Inventors: Pedro Lucas Gervasio Ladeira
Original assignee: Verde Potash Plc
Priority date: 2012-12-06
Filing date: 2012-12-06
Publication date: 2014-06-12
Also published as: BR112015013081A2

Abstract

A product containing potassium in soluble form for use as a fertiliser is extracted from a potassium-containing mineral feedstock by heating with a reagent comprising one or more of a calcium oxide, a calcium carbonate, a magnesium oxide or a magnesium carbonate. During the extraction the basicity of the blended feedstock and reagent is greater than or equal to 0.65. The calcine produced by the reaction is ground and pelletised or compacted to form a fertiliser product.

Description

Potash Product, Method and Apparatus

The invention relates to a potash product and method, and in particular to a potash product suitable for use in fertilisers and a method for making the product.

Background of the Invention

There is a pressing demand for fertilisers to help grow crops to feed the world's ever-growing population. Moreover, many of the food-growing regions of the world have to import most of their fertilisers, in many cases from sources thousands of miles away, undesirably creating carbon dioxide through fossil- fuel consumption as the fertilisers are transported. In many countries, soils are regarded as being deficient in potassium and, therefore, fertilisers containing potassium, or potash, are highly valued. Many countries have low-grade deposits of potassium-containing minerals. Attempts have been made to upgrade such minerals to make viable fertilisers, but with limited success.

Potash fertilisers usually contain potassium in the form of potassium chloride, sulphate or carbonate. Fertilisers do not contain potassium oxide, K₂0, which is a highly reactive, caustic compound, but it is important to be able to quantify the potassium content of fertilisers and this is conventionally expressed as a K₂0-equivalent figure. For example, potassium oxide is about 83% potassium by weight but potassium chloride is only 52% potassium by weight. Thus, if a fertiliser is 30% potassium chloride by weight, its standard potassium rating, or K₂0 equivalent, would be only 19%.

KCI is often used as a fertiliser, but has certain disadvantages. Chloride is a micronutrient for plant growth and is supplied to the soil from a variety of sources. Generally, there is not a deficiency of chloride ions in the soil but there is a concern regarding excess of chloride ions which can be brought about when KCI is used as a fertiliser. For many vegetables the yield can fall dramatically if the chloride content is above 2000ppm. Potash, or potassium, is referred to as the quality nutrient for its ability to improve the taste, texture and nutritional value of many crops, but it has been observed that the quality of coffee produced from chloride-free fertilisers was better than from fertilisers containing chloride (such as KCI).

A potash fertiliser preferably has an advantageously high potassium content, with the potassium in a form which is soluble in water. Ores containing water- soluble potassium salts can be mined and a product with an increased potassium content, for use in fertilisers, can then be produced by dissolving the potassium salts in water. However many ores contain potassium compounds which are not water soluble. Various attempts have been made in the prior art to process such potassium-containing ores to produce a product with an increased potassium content, suitable for use as a fertiliser, but none of these processes has proved economically or technically attractive.

Statement of Invention

The invention provides a method, an apparatus and a potash product, or fertiliser, as defined in the appended claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent subclaims.

In a first aspect, the invention may therefore provide a method of reacting a potassium-containing ore, or feedstock, with a reagent comprising limestone or other source of one or more of a calcium oxide, a calcium carbonate, a magnesium oxide and/or a magnesium carbonate to form a product containing potassium in a form which is soluble in water, or citric acid solution, or which can react with soil substances to release potassium, and optionally also other substances which beneficially become bioavailable for crop growing.

The feedstock is preferably derived from an ore, which is mined or is otherwise naturally occurring. This may be termed a mineral feedstock. The ore, or feedstock, advantageously has a potassium content greater than 5%, or 8%, or 10% K₂0 equivalent.

The mineral feedstock may comprise biotite, potash-feldspars (microcline, sanidine and orthoclase), muscovite, sericite or any potassium-containing mineral such as K-feldspar, illite, alunite, lepidolite, leucite, phlogopite, zinnwaldite, glauconite, silvinite and carnallite.

Verdete is polymineralic rock composed mainly of K-feldspar, quartz and a phyllosilicate (mica group), of average composition 5.49% K, 0.27% Na, 2.28% Mg, 1.9% Al, 19.62% Fe, 25.00 % Si, 0.47% H and 44.97% O. Expressed in terms of oxides, this equates to 6.62% K₂0, 0.36% Na₂0, 3.785% MgO, 3.58% AI₂0₃, 3.37 FeO, 24.31 % Fe₂0₃, 53.48% Si0₂, 4.22% H₂0. The composition of Verdete varies according to local geology which may change the K content by ±5% depending on the base composition of the mineral: for example whether it is illite rich, or biotite rich etc.

However, the true stoichiometry of the K in Verdete is complex due to the presence of illite, smectite and microcline intimately dispersed within the ore. This impacts on the chemical representation of the K.

The reagent may comprise calcium oxide, calcium carbonate, magnesium oxide and/or magnesium carbonate, but these may not be pure oxides or carbonates. Reagents containing only a proportion of calcium oxide, calcium carbonate, magnesium oxide and/or magnesium carbonate may also be used.

The feedstock may be ground or comminuted to an advantageously small particle size, and may then be blended with the reagent, which is preferably also ground or comminuted an advantageously small particle size. Preferably, however, the feedstock and the reagent are ground or comminuted together. The inventors' experiments have surprisingly found that grinding the components separately then blending the ground ore and ground reagent together provided potassium solubility results in the calcine, after reaction, approximately 3 times worse when compared to grinding the feedstock and reagent together. The blended material is then preferably fed directly as powder to a reactor, where the main reactions will take place. Alternatively, the feedstock and the reagent may be blended together, or ground together, and pelletized before processing in the reactor. Pelletization may be carried out using a pelletizing drum or a pelletizing pan. This may have the advantage of producing a pelletized reaction product but the inventors have found that feeding the reagent and feedstock into the reactor in powder form produces a reaction product containing a higher proportion of soluble potassium.

The material fed into the reactor, whether in powder form or pelletized form, may be termed the reactor feed, or kiln feed.

The average particle size, or the maximum particle size, of the feedstock and/or the reagent after grinding or comminuting may be preferably less than 150 micrometres or 88 micrometers. In a specific embodiment, the feedstock and/or the reagent are ground such that 90% of the materials passes through a 170 mesh sieve, or to produce a maximum particle size of 75 micrometers. The reaction is carried out with the feedstock and the reagent in the solid state, and using a small particle size may advantageously increase the rate of the reaction, decrease required residence time in the reactor and improve reaction yield.

The reaction is carried out at high temperatures, preferably above 1000C, to dry the feedstock and the reagent, and to allow the thermal treatment, or calcination, to occur. During the high temperature exposure, the feedstock and the reagent react to convert the potassium in the feedstock into a soluble form (which may be termed soluble potassium).

The reaction products may then advantageously be rapidly cooled, or quenched, in order to maintain the newly-obtained soluble form of the potassium and avoid a reverse reaction back to the initial compounds. Fast cooling may be carried out using air and/or water as quenching media. The reaction product may typically leave the reactor at a temperature of more than 1 100C and rapid cooling may advantageously prevent changes in the crystalline structure of the reaction product and prevent the formation of secondary structures which can develop during slower cooling. Preferably, the reaction product may be cooled to 700C or less, in less than 25 minutes, and preferably less than 7 minutes.

The inventors' understanding based on their experiments during thermopotash development is that a swap between CaO and K₂0 occurs in the silicate structure. Calcium silicates are more stable than potassium silicates, and by providing sufficient energy and residence time the reaction favours the formation of calcium silicate, thus releasing potassium to become bioavailable in the thermopotash product.

A calcined product (or calcine) containing potassium in a soluble form (acid- soluble or water-soluble potassium) may thus be obtained. In the case of feeding powder mixture (or blend) directly to the reactor, a liquid phase is formed at a burning zone in an extent of 10% to 20% of the powder mixture and small stones of sintering material (calcine) with less than 30 mm in diameter are produced and leave the reactor to enter the quenching system. A drying operation may be conducted to remove superficial moisture from the product if a water quenching process is used. Water spraying the material leaving the kiln system may advantageously quench it to a temperature below 700C (and to a temperature at which water rapidly evaporates) without leaving residual moisture, thus avoiding any need for drying.

The inventors have found that the calcined product leaving the reactor has a larger particle size than the feedstock, and may be referred to as granulated. The inventors' experiments have shown that this as-produced granulated thermopotash leaving a thermal treatment will react with citric acid aqueous solution, but to a more limited extent than may be desirable for commercial usage. An additional grinding step should therefore preferably be carried out in order to significantly increase thermopotash solubility in citric acid. After grinding, according to customers' needs, the ground product may advantageously be pelletized or compacted, making it easier to be transported and applied on the field in a granular form. The inventors have found in agronomic tests that parameters including fineness and pellet hardness control potassium release from such pelletized products.

In preferred embodiments of the invention, sources of potassium, chlorine and/or other agents may be added before the heat treatment, in order to reduce the melting point of the feedstock during thermal treatment. Also, elementary sulphur may be fed to the final grinding mix (after heat treatment) in order to provide the fertilizer with sulphur, which is an important micronutrient; this may also improve the agricultural properties of the end product. In a further preferred embodiment of the invention a suspension pre-heater and/or pre-calciner system may be employed before the reaction is performed. The reaction may be performed, for example, in a rotary kiln reactor. This may advantageously reduce the thermal consumption of the process. The inventors' research results have shown that a product temperature up to 950°C can be reached at the outlet of the pre-calciner. In this case, the dimensions of the rotary kiln reactor may then advantageously be reduced.

The reaction between the feedstock and the reagent may be carried out at an elevated temperature, such as above 1000C, up to 1600C. The temperature may be above 1 100C, 1200C, 1300C, 1400C or even 1500C. An optimum temperature range may be about 1250C, optionally plus or minus 50C or 100C.

Vitreous product is poorly dissolved or digested when distributed on the soil. Therefore, excessive liquid phase formation (which occurs at over 1300C) should be avoided during the reaction.

Heating to elevated temperature may be provided by traditional hydrocarbon fuels, but alternative fuels or heat sources may also be used. The efficiency of this process may advantageously be increased by, for example, increasing residence time in the reactor, or by increasing temperature in the reactor, or by adding salts to act as flux. In further aspects, the invention may advantageously provide a product (termed thermopotash) containing soluble potassium, which can be used as a fertiliser or as a component of a fertiliser, and may also advantageously provide a fertiliser containing the soluble potassium or the soluble-potassium product.

Reactions of various potassium-containing silicates, aluminosilicates and ferrites are set out below. The empirical formula of verdete or similar potassium bearing ore is K₀.₆Nao_.5Fe₍3 ₊ i .3₎Mgo_. Fe₍2 ₊ o.2₎Alo.₃Si₃.₈0io(OH)2. The following reactions refer to K₂0 which, as noted above, is highly reactive and caustic, and which in the presence of water may produce KOH. Depending on the details of processes embodying the invention, K₂0 may be formed and trapped within small stones produced by partial sintering in the reactor (when the reactor feed is in powder form). Alternatively, the K₂0 may react with C0₂ or S0₃ in the reactor, for example to form K₂C0₃ or K₂S0₄. In the following equations, reference is nevertheless made to K₂0 as this is the appropriate commercial reference material.

K₂0*Si0₂ + CaO = K₂0 + CaO*Si0₂

K₂0*2Si0₂ + CaO = K₂0 + CaO*2Si0₂

K2O S1O2 + CaO = K₂0 + CaO Si0₂

2KAISi₃0₆ + CaO = K₂0 + CaO*AI₂0₃*6Si0₂

2KAISi₂0₆ + CaO = K₂0 + CaO*AI₂0₃*4Si0₂

2KAISi0₄+ CaO = K₂0 + CaO*AI₂0₃*2Si0₂

2KFe0₂+ CaO = K₂0 + CaO*AI₂03*Fe₂03

K₂0*Si0₂ + MgO = K₂0 + MgO*Si0₂ K₂0*2Si0₂ + MgO = K₂0 + MgO*2Si0₂ K₂0*4Si0₂ + MgO = K₂0 + MgO Si0₂ 2KAISi₃0₆ + MgO = K₂0 + MgO*AI₂0₃*6Si0₂

2KAISi₂0₆ + MgO = K₂0 + MgO*AI₂0₃*4Si0₂

2KAISi0₄+ MgO = K₂0 + MgO*AI₂0₃*2Si0₂ 2KFe0₂+ MgO = K₂0 + MgO*AI₂0₃*Fe₂0₃

After reaction and quenching takes place, less complex silicates and K₂0 are formed, and the product becomes soluble in citric acid solution. This product may be termed Thermopotash. In order to increase to a very high level the solubility of potassium in the product, any small stones in the calcine, or the pellets of the calcine if the reactor feed was pelletized, may advantageously be ground.

The inventors' experiments in agronomic tests have shown that a grain size (particle size) distribution for the ground product with a maximum of 25% passing a 0,5mm sieve and a minimum of 95% passing a 3,36mm sieve presented very good results.

In laboratory tests, comminution to 50 mesh sieve showed that this fineness is sufficient to release potassium in citric acid solution in a very efficient way.

In order to obtain an alternative product, in response to customer demand, another embodiment may be produced. Farmers traditionally use fertilisers in a granulated shape. Granular product is easier both to transport and to apply on the field. To meet these requirements, the inventors have developed two processes of thermopotash granulation (or peptization). One is granulation in a pelletizing pan using binders like cornflour or maniocflour with caustic soda solution. Pre-gelatinized starch can also be used. Fineness of the ground thermopotash suitable for this process should be in the order of 44 micrometers. The other process developed to granulate thermopotash is compaction. Laboratory experiments have shown that one advantage of this process is a lower binder consumption, compared to granulation in a pelletizing pan as described above. Grain size distribution of the ground thermopotash suitable for this process can be 100% passing a 100 mesh sieve, with a maximum particle size of 149 micrometers.

The inventors' understanding is that the main quality parameters for granulated thermopotash are the grain hardness, abrasion resistance and grain fragmentation when submerged in water. The latter is found to be surprisingly important, probably because of the limited solubility of thermopotash in water. In this way, to increase the biorelease velocity or rate for the soil, the fragmentation should advantageously occur as soon as possible after the grains are applied to the soil. Specific Embodiments

Specific embodiments of the invention will now be described by way of example.

In a first embodiment, crushed potassium-bearing verdete ore of composition 10.97% K₂0, 16.71 % Al₂0₃, 1.65% MgO, 60.51 % Si0₂, 8.43% Fe₂0₃. was mixed with crushed limestone of composition 94.57% CaC0₃, 2.49% Si0_2, and 1.41 % AI2O₃. The blend was then ground down to 90% passing a 170 mesh sieve in a ball mill in order to achieve a better homogenization of the components.

This blend called kiln feed was then fed to a 100 kg/h, diesel-fired rotary kiln and heated up to above 1 150C whilst combustion gas (flue gas or exhaust gas from the diesel combustion, rich in C0₂ with residual 0_2, high in inert N₂ and with traces of CO) is flowed over it. The kiln feed was held in the reactor for 1.5 hours, including an initial period at a lower temperature to dry the material and then approximately 30 minutes at between 1200C and 1300C (from burning zone to exit of the rotary kiln) to ensure reaction between the potassium-bearing ore and the reagent (high grade calcific limestone) takes place. The product - or calcine - was then quenched with room temperature spray water and blowing air, in order to decrease product temperature down to 200C in less than 25 minutes.

The inventors have found that a sufficiently fast quenching yields better results than slow cooling. Fast quenching prevents reactions from occurring in the reverse sense. Tests were carried out using only water or blowing air as quenching fluids. Experimental results have shown both fluids can be applied, providing similar quenching efficiency.

This product contains soluble potassium and can be used directly as a fertiliser or as a component of a fertiliser, providing K element to a NPK (nitrogen phosphorus potassium) formulation in a fertiliser product.

The product, after further grinding procedure to become 100% passing a 100 mesh sieve, was analysed by leaching with citric acid, to identify the proportion of the potassium in the verdete which had been converted to a soluble form. Up to 80% of the potassium oxide was found to be soluble in the citric acid.

In a further embodiment, the ground mixture of ore and reagent was pelletized before reaction in the rotary kiln. This method advantageously produced a pelletized thermopotash product, but it was found by the inventors in agronomic tests that the pelletized thermopotash produced from the pelletized kiln feed has a disadvantageously low release of potassium to the soil, to an extent that it may be of limited use for an economical application. Further grinding should be applied to the product in order to improve release. In preferred embodiments, the product thermopotash may yield more rapid potassium release by grinding it to a suitable fineness of below 100 mesh after the quenching process. Potassium release from the final product may advantageously be controlled by grinding intensity or fineness. The end product may be produced as fine powder or be granulated, using water and agglomeration additives (pre jellified flour, for example) as granulation agents. Release of potassium may then be controlled by pellet hardness and use of fragmentation additives.

Agronomical tests with powder thermopotash were carried out by the inventors in several experiences and very good results were achieved. Thermopotash got even better results than KCI in some tests, especially in middle and long term tests.

It was proven that for both additives and thermopotash granulating, special agglomerators such as pre jellified flour (or manioc flour with caustic soda) may advantageously be used to obtain the adhesive properties required for the granulating operation.

Granulation processing was tested using two kinds of technologies: granulation pan and compaction. Both presented successful results after a great number of tests varying the kind of additive, their quantities in the formulations, drying temperature and drying time. The inventors have found that the most important parameter of granulation is the grain hardness. The table below is a comparison between the two technologies tested:

PARAMETER GRANULATION COMPACTION

FEEDING FINENESS 325 MESH 100 MESH

BINDERS CONSUMPTION HIGH MODERATE

MOISTURE BEFORE DRYING 12% 3%

HEAT CONSUMPTION AT DRYING HIGH LOW

GRANULATION EQUIPMENT GRANULATION PAN COMPACTOR

COATING NECESSITY? YES YES

GRANULE HARDNESS > 2KGF > 2KGF

FRAGMENTATION WHEN SUBMERGED INTO WATER VERY GOOD GOOD

PRESENCE OF CORNERS IN

GRANULE SHAPE ROUND THE GRAIN

SULFUR ADDITION FEASIBLE FEASIBLE

In the compaction process, KCI may advantageously be used to replace part of the additives required.

In further embodiments, elemental sulphur was added to the thermopotash before the granulation process, with good results in both granulation pan and compaction processes.

During the burning (reaction) process, it was found that sintering is beneficial to maximize reaction yield. Formation of a liquid phase advantageously allows for improved kinetics (though excess melting should be avoided, to limit the formation of vitreous phases on cooling). Sintering is better accomplished by increasing feed fineness, and/or adding flow, and/or increasing reaction temperature. In the following table, in tests 1 and 6, it is shown that by increasing reaction temperature by 125K, thermopotash solubility was increased by 45%. Also comparing test 4 with test 6, by operating at 50K higher temperature, the percentage of thermopotash soluble in citric acid solution increased from 0.76% to 6.29%. Test Mixture composition Tempe rature (°C) Solubility (%) % soluble K20

1 55% \«rdete/45% limestone 1 100 35,23 0,08

2 55% wrdete/45% limestone 1 125 37,88 0,08

3 55% wrdete/45% limestone 1 150 36,82 0, 14

4 55% wrdete/45% limestone 1 175 40,60 0,76

5a 55% wrdete/45% limestone 1200 64,59 4,01

5b 55% wrdete/45% limestone 1200 68, 19 4,44

6 55% wrdete/45% limestone 1225 80,00 6,29

Experiments using a flash calciner test have shown that it is possible to achieve up to 1000C in a pre-heating and pre-calciner system, but this pre- calcined product presents only 3% of K₂0 solubility (but 85% of the loss of ignition required for the limestone decomposition reaction). Therefore, a processing configuration embodying the invention should advantageously achieve a reactor temperature from 1200C to 1250C in order to allow the main reactions to form the thermopotash to take place, for example by using a rotary kiln. Kinetics are further significantly improved by partially melting the kiln feed in the burning zone, at a final step of the reaction, which takes place for example in the last zone of the rotary kiln.

This configuration using pre-heating, pre-calciner and rotary kiln should preferably be applied to reduce the thermal consumption (energy consumption) of the thermal potash production.

A TGA/DSC (thermogravimetric analyser/differential scanning calorimeter) was run up to 1300C with the purpose of thermally characterizing a blend of 55% verdete ore and 45% limestone. A graph showing the results is presented below.

From room temperature up to 500C we can observe free H₂0 type losses and other H₂0 losses (points 1 , 2 and 3). From 500C to 900C (point 4), the blend has a significant decarboxylation loss (%C0₂) at 20,75%. The C0₂ loss calculated as contributing to the formation of CaC0₃ is approximately 56%. This endothermic reaction has an entalphy of -690 to -870 kJ/kg.

From 1070C to 1125C (point 6), an exothermic reaction takes place: the crystal formation from reaction of CaO (from limestone) and silicates (+15 to 20 kJ/kg). From 1150C to 1300C three other peaks indicating endothermic reactions (points 7, 8 and 9) can be observed, each peak probably indicating partial melting of the material. The loss of ignition of the entire path (room temperature up to 1300C) was calculated at 21 ,35%.

The total reaction's Entalphy was calculated at -720 to -910 kJ/kg. The inventors have found that a very relevant parameter for reaction yield may be basicity, expressed as the coefficient (CaO+MgO)/Si0₂, calculated using the weight percentage of each of the oxides in the feed material. It was found that basicity has to be kept above 0.65, or even above 0.70. The higher the CaO content, the more the K₂0 is released from silicates. In the table below, comparing basicity 0,62 and basicity 0.79, it is clear that solubility increases 27% at the same processing conditions. In this way, the optimized mixture for TK (thermopotash) process may advantageously be 57% verdete ore and 43% limestone (TK 57/43).

Magnesium may replace calcium as reagent, although practical results have shown that its contribution to basicity is less expressive than calcium. As an example, burning a 54% verdete/46% limestone pelletized mixture, ground to 100% passing 100 mesh sieve, at 1 150°C for 1 hour yielded 75% K₂0 solubility, whilst burning a 54% verdete/46% dolomite pelletized mixture under the same conditions yielded only 35% citric-acid-soluble K₂0.

Test % Verdete % Limestone % Dolomite Solubility (%) K₂0 Solubility(%)

11 54 46 - 78 74.4

12 54 - 46 64 35.4

13 70 30 - 24 16.8

14 70 - 30 13 14.3

15 80 20 - 6 6.9

16 80 - 20 3 3.7 Thermopotash may be leached with acid solution to advantageously release soluble potassium. TK 54/46 (the product of burning the 54% verdete/46% limestone kiln feed described above) was ground (100% < 100 Mesh) and leached with solid:solution 1 :100 ratio (i.e. the ratio of solid thermopotash to liquid leaching solution), stirring being applied for 30 minutes at room temperature. Up to 82% K₂0 could be extracted from the thermopotash to the solution, as shown in the following table.

Further concentration of the leachate, for example by evaporation and/or crystallisation, may provide a more concentrated potassium product.

Although attempts have been made in the prior art to produce soluble potassium from potassium silicates, the inventors' new contributions relating to composition (explained through the basicity concept), kinetics (CaO-K₂0 swap in the silicate structure, importance of sintering and flux utilization, desirability of additional grinding procedure to release potassium, pellet hardness), agglomeration technique, and Mg and S reagent utilization, provide significant advantages over prior art methods and may enable a commercially-effective process for producing soluble potassium through a thermal technique for the first time.

Claims

1. A method for forming a product containing potassium in a soluble form, comprising the step of reacting a potassium-containing mineral feedstock with a reagent comprising one or more of a calcium oxide, a calcium carbonate, a magnesium oxide or a magnesium carbonate.

2. A method according to any claim 1 , in which the step of reacting the feedstock with the reagent is carried out at an elevated temperature, preferably above 1000C, and particularly preferably at about 1250C, optionally plus or minus 50C or 100C.

3. A method according to claim 1 or 2, further comprising the step of quenching the reaction product.

4. A method according to claim 1 , 2 or 3, in which the reagent is a mineral reagent.

5. A method according to any preceding claim, in which the feedstock and the reagent are ground together to form a kiln feed for reaction.

6. A method according to any of claims 1 to 4, in which the feedstock and the reagent are pelletized before the reaction, and the product is preferably in the form of pellets.

7. A method according to any preceding claim, in which the feedstock comprises verdete, potassium feldspar, biotite or kamafugite, or comprises a mineral containing potassium silicate.

8. A method according to any preceding claim, in which the mineral feedstock has a particle size of less than 500 micrometres and preferably less than 250 micrometres or 150 micrometres. A method according to any preceding claim, comprising the step of adding one or more of potassium, chlorine and sulphur to the reaction between the feedstock and the reagent.

A method according to any preceding claim, comprising the step of grinding the reaction product to a predetermined particle size.

A method according to claim 10, comprising the step of pelletizing or granulating the ground reaction product to form a granulated fertilizer product or a granulated component of a fertilizer product.

A method according to any preceding claim in which a mixed or blended kiln feed for the reaction has a basicity greater than or equal to 0.65, or preferably greater than or equal to 0.7.

A method according to any preceding claim, comprising the step of adding elemental sulphur to the reaction product.

A fertiliser comprising a product produced using the method of any of claims 1 to 1 1.

An apparatus for producing a product containing potassium in a soluble form, comprising a first reactor for reacting a potassium-containing feedstock with a reagent comprising one or more of a calcium oxide, a calcium carbonate, a magnesium oxide or a magnesium carbonate, coupled to a second reactor for carrying out a quenching process.