WO2013061092A1

WO2013061092A1 - Potash product and method

Info

Publication number: WO2013061092A1
Application number: PCT/GB2012/052687
Authority: WO
Inventors: Pedro Lucas Gervásio LADEIRA
Original assignee: Verde Potash Plc
Priority date: 2011-10-27
Filing date: 2012-10-29
Publication date: 2013-05-02
Also published as: BR112013004384A2; BR112013004384B1

Abstract

To extract water-soluble potassium salts from a mineral feedstock, the mineral feedstock is reacted with sodium chloride in the presence of calcium carbonate at a temperature of between 825C and 950C. An apparatus for carrying out the reaction comprises a first reactor for reacting the feedstock and a second reactor for leaching the water-soluble potassium salts from a reaction product or calcine, and for producing the desired soluble potassium product, for example for use in fertiliser.

Description

Potash Product and Method

The invention relates to a potash product and method, and in particular to a potash product suitable for use in fertilisers and a method and apparatus 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, including potassium-containing silicates such as feldspars and micas.

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%.

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 such as KCI can be mined and a product with an increased potassium content, for use in fertilisers, can then be produced by

P/66807.WO01 - 29.10.12 dissolving the potassium salts in water followed by evaporation of the water. However, many ores contain potassium compounds which are not water soluble, such as the silicates described above. 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 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 mineral feedstock with a metal chloride, preferably sodium chloride, in the presence of a metal carbonate, preferably calcium carbonate. When the metal chloride comprises sodium chloride, a small proportion of the sodium chloride may optionally be replaced by, or substituted by, calcium chloride. The reaction forms a product containing soluble potassium (in the form of a water-soluble potassium compound, or salt). Under preferred reaction conditions, the reaction may be carried out at temperatures of above 825C or 835C, and below 860C, 880C, 905C, 915C, 925C or 935C. Optionally, the reaction may be carried out at a temperature of between 850C and 1000C, and preferably at a temperature of between 850C and 950C.

Potassium-containing minerals have previously been reacted with metal chlorides in attempts to produce soluble potassium salts, although no such reaction has yet been found to be sufficiently effective to form the basis for a commercial process for making a soluble potash product. But the inventor has found that the proportion of the potassium in the mineral feedstock which is converted to soluble potassium (the reaction yield), as well as the energy efficiency of the process, may be dramatically improved by adding appropriate quantities of CaC0₃ and NaCI to the mineral feedstock to form the reaction feedstock, which may additionally enable the reaction to be carried out in an advantageously elevated temperature range.

P/66807.WO01 - 29.10.12 Advantageously, a portion of the NaCI in the reaction feedstock may be replaced by, or substituted by, a quantity of CaCI₂ as described in more detail herein.

After the reaction, the soluble potassium product may advantageously be lixiviated or leached by water or by a solution of a low concentration of hydrochloric acid. To form the reaction feedstock, a potassium-containing ore (such as a potassium-containing feldspar or mica) may be blended with pre-determined quantities of the metal chloride and the metal carbonate, and the blend or mixture ground or comminuted to an advantageously small particle size, such as less than 500 micrometres or less than 250 or 150 or 100 or 50 micrometres particle size, before reaction with the chloride and the carbonate. The mineral, reagent (chloride) and/or carbonate may alternatively be ground separately (preferably to the same ranges of particle sizes as mentioned above) and then blended or mixed with the other components of the reaction feedstock. The particle sizes of the mineral feedstock, the chloride reagent and the carbonate may be assessed by sieving the ground materials, and a particular particle size may be achieved when, for example, a predetermined fraction such as 90% or 100% of a ground material passes through a sieve of dimensions corresponding to a particle size identified above.

The reaction feedstock is preferably retained in powder form for the reaction but can optionally be pelletised before the reaction.

In the reaction feedstock, the chloride-containing reagent preferably comprises NaCI, or a mixture of NaCI and CaCI₂. The carbonate preferably comprises CaC0₃. The mineral, reagent and carbonate are preferably blended, or mixed, in proportions (measured by weight of undried materials) of between 70:10:20 and 60:20:20 for typical potassium-containing mineral feedstocks containing between about 5 to 8% K₂0 equivalent and about 9.5 to 1 1 % K₂0 equivalent (of K in a form which is available for reaction), or of about 6% to 12% K₂0

P/66807.WO01 - 29.10.12 equivalent. Advantageously, larger quantities of NaCI, or of the mixture or combination of NaCI and CaCI₂, may be used, for example to give proportions between about 70:10:25 and 60:10:25 as described below. The required quantities of the reagent and the carbonate in the reaction feedstock may depend on the quantity, or concentration, of available potassium in the mineral feedstock. This may be assessed as follows. The molar quantity of NaCI, or the combined molar quantity of NaCI and CaCI₂, in a reaction feedstock is preferably between 3 and 9, or between 5 and 7, times the molar quantity of K₂0 equivalent of available potassium in the mineral feedstock. In other words the molar ratio of metal chloride (generally NaCI or NaCI and CaCI₂ combined) to K₂0 equivalent in a reaction feedstock should be between 3:1 and 9:1 , or between 5:1 and 7:1. The molar quantity of metal carbonate, preferably CaC0₃, in a reaction feedstock is preferably between 1 and 2.2, or between 1.4 and 1 .8, times the molar quantity of K₂0 equivalent of available potassium in the mineral feedstock. In other words the molar ratio of metal carbonate (generally CaC0₃) to K₂0 equivalent in a reaction feedstock should be between 1 :1 and 2.2:1 , or between 1 .4:1 and 1 .8:1 .

In terms of percentages (throughout this document, such values are set out in weight % unless indicated otherwise), the reaction feedstock preferably contains between 15% and 30% or between 20% and 28%, and preferably about 25%, of the reagent (preferably NaCI or a mixture of NaCI and CaCI₂) by weight. The reaction feedstock preferably contains between 5% and 25%, or 7% and 15%, or 8% and 12%, and preferably about 8% or 9% or 10% of the carbonate (preferably CaC0₃) by weight. These proportions of the materials in the reaction feedstock are for typical mineral feedstocks containing about 8 to 12% K₂0 equivalent, such as about 10% K₂0 equivalent, of K in a form which is available for reaction. The percentages of the NaCI, or NaCI and CaCI₂ mixture, and CaC0₃ in the reaction feedstock may be proportionally increased or reduced in accordance with the quantity of available K in the mineral feedstock if that quantity is more than or less than about 8% to 12% K₂0 equivalent.

P/66807.WO01 - 29.10.12 The inventors have found that where the reagent consists primarily or exclusively of NaCI, a minimum of 8% NaCI is stoichiometrically required to react with the mineral feedstock (for a typical potassium-containing silicate mineral feedstock as described above), but that a proportion of the NaCI (about 40%) volatilises during the reaction. Therefore, at least about 40% more NaCI than the stoichiometrically-required amount may advantageously be included in the reaction feedstock. The inventors have also found that the yield of soluble potassium obtained from a reaction feedstock may advantageously be increased if the metal chloride primarily comprises NaCI, but also comprises a quantity of CaCI₂. In particular, a quantity of about 5% to 12%, or about 6% to 10%, of the amount of the metal chloride in a reaction feedstock may be CaCI₂ and the balance may be NaCI. For example if a reaction feedstock contains 25% metal chloride (by weight), it may contain about 23% NaCI and 2% CaCI₂. In other words, in this case, by comparison with a feedstock containing only NaCI, a proportion of about 8% of the NaCI has been replaced with CaCI₂. The inventors have found that this may not only increase the yield of soluble potassium, but also enable a small reduction in the reaction temperature, above about 825C or 830C or 835C and below 860C or 880C or 900C.

The inventors have found that in order to implement a commercial process for extracting a large proportion of the available potassium from mineral feedstocks containing potassium feldspars and micas, a number of factors need to be optimised in combination. This combination is not taught by the known prior art, and it is the inventors' understanding that this lack of prior art teaching has to date prevented the effective extraction of soluble K salts from K-silicate mineral feedstocks. The present invention addresses this problem in the prior art.

First, the reaction temperature should be as high as possible while avoiding excessive sintering or melting of the feedstock, where K and Na can become disadvantageously incorporated into the silicate solid structure. If excessive sintering or melting occurs, then the reaction product may be in the form of a

P/66807.WO01 - 29.10.12 fused mass, or clinker, which is not suitable for a subsequent leaching or lixiviation step to extract the soluble potassium product. In the prior art, temperatures up to about 820C have been proposed but as found by, for example, Tschirner in UK Patent GB 1 17,870, when attempts have been made to process K silicate minerals with reagents at temperatures higher than 800- 820C, the feedstock has been found to fuse and a clinker or fused mass is produced which is not suitable for leaching. The prior art therefore advocates that the reaction temperature should not exceed 820C. However, the inventors have appreciated that a higher proportion of the available potassium can be extracted from a mineral feedstock, and the reaction time can be reduced, if a higher temperature is used. Further, in a preferred aspect the invention provides a specific reaction feedstock composition that enables higher- temperature processing without forming a fused mass. Regarding the Ca in the reaction feedstock, the reaction mechanism is not fully understood, but the inventor's understanding is that the presence of some Ca is required in order to disrupt, or break up, the structure of the

potassium-containing silicates in a mineral feedstock, and in particular to disrupt the structure of potassium feldspars in the feedstock. A smaller proportion of Ca is believed to be required to make potassium available from any K-containing micas in the feedstock.

However, in order to optimise the performance of a commercial process, the inventor has found that the quantity of metal carbonate, or CaC0₃, in the reaction feedstock should be minimised. Consequently it is particularly preferred that the ratio (by weight) of metal carbonate, or CaC0₃, or limestone, in a reaction feedstock should be between 1 :10 and 1 :5, or between 1 :9 and 1 :7, or particularly preferably about 1 :8, for a typical mineral feedstock containing between 8 and 12%, or about 10% K₂0 equivalent. These figures are for a mineral feedstock primarily containing K-bearing feldspar. For mineral feedstocks primarily containing K-bearing mica, lower quantities of metal carbonate may be possible.

In less-preferred embodiments, the quantity of CaC0₃ (limestone) required may be as follows. The reaction feedstock may comprise a ratio (by weight) of

P/66807.WO01 - 29.10.12 CaC0₃, or limestone, to potassium-bearing feldspar of between 1 :6 and 4:6, preferably between 1.5:6 and 3:6, and particularly preferably about 1 :3.

(A feedstock may comprise other minerals as well as the potassium-bearing feldspar and these ratios are therefore based only on the K-bearing feldspar content of a feedstock.) The reaction feedstock may advantageously comprise less limestone for each part of potassium-bearing mica in the mineral feedstock, such as between 0% and 50%, or between 10% and 50%, or between 15% and 30%, as much limestone for each part of potassium-bearing mica in the mineral feedstock.

The reaction feedstock also contains NaCI, providing chloride to react with the available K within the feedstock, to produce the soluble KCI product. In the reaction, some of the CI reacts to form other products, such as HCI by reaction with any hydroxyls or water present in the feedstock, and in the combustion flue gas. Consequently, the stoichiometrically-required quantity of NaCI may be between 10% and 25% or 30% more than the amount required to react only with the K available for reaction in the mineral feedstock to form KCI.

However, at the desired reaction temperature, some volatilisation of NaCI will occur, and so the reaction feedstock should preferably contain at least 40% more than the stoichiometrically-required amount of NaCI, up to about twice or 2.5 times the stoichiometrically-required amount.

In a preferred embodiment, volatilized NaCI is recovered and recycled for use as a reagent in a further batch of reaction feedstock.

When a reaction feedstock containing CaC0₃ is heated as described above to a temperature above 825C, or to between 850C and 950C, the limestone decomposes to CaO and C0₂. The inventors have found that the evolution of C0₂ is very beneficial in the reaction, and aids the extraction of K from the mineral feedstock. It is believed that the C0₂ contributes, with the Ca, to breaking up the mineral structure of the feedstock, in particular the

K-containing feldspars. However, the decomposition or decarbonation of limestone requires a significant amount of energy and this greatly increases the amount of heat energy required to heat a reaction feedstock to the desired reaction temperature. A problem then arises as decarbonation progresses and

P/66807.WO01 - 29.10.12 the amount of limestone in the feedstock which has not decomposed reduces, and the amount of heat energy absorbed by the decomposition reaction correspondingly reduces. The problem is that the temperature of the reaction feedstock tends to increase very rapidly when all of the CaC0₃ has been decarbonated to CaO and before the energy supply can be reduced and/or the heat energy within the feedstock and the reactor can dissipate. When the reaction feedstock is heated, there is therefore a strong tendency for a violent temperature overshoot to undesirably elevated temperatures as soon as the decarbonation process is complete, or almost complete. This temperature overshoot may disadvantageously cause excessive volatilisation of NaCI and/or excessive sintering or melting of the reaction feedstock and/or product.

One solution to this problem is to operate the reaction at a lower temperature, as in the prior art, but this increases the reaction time and decreases the yield. In a second solution, a lower heat energy input could be used in order to reduce the size of any temperature overshoot, but the reaction feedstock would then be heated more slowly to the desired processing temperature of more than 825C. This would again disadvantageously give more time for NaCI volatilisation.

The inventor has appreciated that the problem can be solved by a different approach, by minimising the amount of CaC0₃ in the feedstock, to the levels described above. This limits the mass of CaC0₃ which needs to be heated and decarbonated. This in turn reduces the amount of energy required to heat the feedstock to the desired reaction temperature, and reduces the magnitude of any temperature overshoot when the decarbonation reaction terminates.

At the same time, the elevated reaction temperature of perhaps 825C to 850C or more tends to increase the volatilisation of NaCI, which the skilled person would perceive as a significant disadvantage. However, by enabling accurate temperature control and reducing any tendency to a temperature overshoot (by minimising the mass of CaC0₃) and by controlling or minimising the reaction time, NaCI volatilisation is kept to an acceptable level. In addition volatilized NaCI may be captured and recycled.

P/66807.WO01 - 29.10.12 As noted above, the reaction may be carried out at a temperature between 825C and 1000C, but preferably between 825C and 950C or less. This is because NaCI volatilization becomes very significant above 950C. Any temperature overshoot beyond 950C, or even beyond 900C or 930C, should therefore preferably be avoided.

In a particularly preferred embodiment, the reaction may be performed in a reactor which enables minimisation of the temperature overshoot after CaC0₃ decarbonation. The inventor favours a rotary kiln, in which the feedstock can readily be agitated or mixed during and at the end of the decarbonisation process, in order to dissipate heat rapidly from the feedstock.

In the prior art, in GB 1 17,870 of Tschirner for example, the ratios of mineral feedstock:limestone:NaCI in reaction feedstocks range between 54:27:19 and 47:37:16. The presence of between 27% and 37% limestone makes it extremely difficult to avoid large temperature overshoots if elevated processing temperatures, above 820C, are used. The inventor has found that significantly reducing the CaC0₃ content of the feedstock means that the temperature of the reaction can be controlled effectively, preferably enabling reaction above 825C in the temperature ranges described above, for example between 850C and 950C, with reduced reaction times and higher yields.

In a commercial process, the reaction feedstock may be fed to a kiln or reactor, preferably a rotary kiln, where the main reactions will take place.

It may be preferable not to dry the reaction feedstock, or the components of the reaction feedstock, before the reaction. The presence of water or hydroxyls in the feedstock, or in the combustion flue gas, may assist in the formation of HCI, and accelerate the extraction of K from the mineral feedstock. Formation of HCI may also enable useful recycling of the HCI from the reaction exhaust gases as described herein.

The reaction should preferably be carried out at a temperature sufficient to melt the chloride-containing reagent, at least partially, so if the reagent consists of or comprises NaCI the reaction temperature should be greater (and preferably

P/66807.WO01 - 29.10.12 substantially greater) than the melting point of NaCI, which is about 804C. The reaction temperature is therefore preferably greater than 825C, or 835C, or 850C, or less preferably above 900C, or 920C. The reaction temperature should not exceed 930C or 950C or 1000C however, to avoid excessive melting and the formation of a glassy reaction product from which it is difficult to leach the desired potash product, even when the amount of CaC0₃ in the feedstock is minimised as described above. Consequently, the reaction temperature should preferably be less than 980C or 970C or 950C or 930C. The preferred temperature range if the metal chloride contains only NaCI may be 850-950C, or 850-930C. Under preferred conditions, such as when a portion of the NaCI is replaced with CaCI₂ as described above, the preferred temperature range may be reduced, to about 825C to 900C, or 830C to 850C. When the reaction is carried out within these preferred temperature ranges, the rate of reaction is advantageously rapid, and excessive agglomeration or sintering of the reaction product is avoided. This means that the reaction product may be in the form of powder or may be lightly sintered so that it can, if required, be easily crushed to powder and effectively and rapidly leached.

Although higher temperatures might contribute to the reaction kinetics and the diffusion of potassium through mica and feldspar layers, liquid phase formation tends to produce a vitreous product, which reduces leaching yield. Therefore, excessive liquid phase formation must be avoided.

In the reaction, it is believed that a sodium-containing chloride may act as a reagent for the extraction of potassium from the feedstock. Alkalis

(carbonates) may act to replace potassium in the ore (mineral) structure.

The inventors have observed that the presence of carbonates (e.g. CaC0₃) in the reaction contribute more to the potassium extraction yield than their equivalent oxide (e.g. CaO). C0₂ released from the carbonate during heating of the material fed to the kiln - decarbonation - seems to play an important role in the reaction mechanism.

The inventors also believe that a swap between CaO and K₂0 occurs in the silicate structure. Calcium silicates are more stable than potassium silicates,

P/66807.WO01 - 29.10.12 and by providing sufficient energy and residence time the reaction favours the formation of calcium silicate, thus releasing potassium to become bioavailable in the potash product. Heating of the reagents may be provided by the traditional fossil fuels, but alternative fuels or sources of heat may also be applied. High sulphur petroleum coke may be a particularly suitable fuel as it supplies sulphate to the reaction product, thus adding potassium sulphate, which may be desirable in fertiliser products.

The effectiveness of this process may advantageously be improved by, for example, increasing residence time in the reactor, or by increasing

temperature in the reactor (within the limits described above). A reaction time of 1 hour or less, or between 45 or 60 minutes and 90 minutes, in a rotary kiln has been found to be sufficient at the preferred ranges of the temperatures and particle sizes described herein. The reaction rate may also be increased by increasing the fineness of the blend of the feedstock and other reagents.

Depending on the type of reactor used for the reaction, after the reaction, the reaction product is optionally quenched, for example to reach less than 100C in less than 10 minutes, or less than 5 or 3 minutes (quenching may not be required after reaction in a rotary kiln, for example), and is treated to remove the desired soluble potassium products. This may involve leaching, or lixiviation, in water, for example to extract soluble potassium chloride, potassium hydroxide and potassium carbonate. Leaching may be carried out for a period of 5 minutes or 15 minutes to 1 hour or 3 hours. The product may then be concentrated from the leachate, for example by evaporation and crystallisation. The leaching process should selectively extract only desired products from the reaction product, such as KCI (and any other soluble K salts such as sulphates, as mentioned above if sulphur-containing petroleum coke is used), CaCI₂ and NaCI. The inventors have found that this can be achieved by leaching with water, optionally at a low temperature such as 25-30°C, but preferably at an elevated temperature such as more than 75°C, or 90°C, or

P/66807.WO01 - 29.10.12 95°C or even 98°C. The preferred temperature is 95C. This may extract KCI, CaCI₂ and NaCI from the reaction product, and not undesired substances such as Fe or Al chlorides. The same outcome may be achieved using a HCI leach, as long as a low HCI concentration such as less than 0.1 % HCI is used.

Preferably, the HCI leach solution should have a pH value of more than 1 .7, or more than 2. An HCI leach may be carried out at a lower temperature than leaching in water, such as between 55°C and 65°C, and preferably about 65°C. An HCI leach may be completed more quickly than a leach in water. An HCI leach may take between 7 and 15 minutes, preferably about 10 minutes, and a leach in water may take between 45 and 75 minutes, preferably about 60 minutes. The leached compounds may then be extracted and separated. For example, KCI forms the desired potash product and NaCI and CaCI₂ may be recycled into a further reaction feedstock. Potassium may be recovered not only from the treated ore (i.e. the solid product of the reaction), but some portion of the potassium extracted from the mineral feedstock may also be found vaporized, as part of the process exhaust gases. This portion of volatile potassium salt may be captured by condensation or scrubbing of the exhaust gases.

Other substances may also be volatised, or generated as gases, during the reaction. In particular NaCI or other useful reagents may be volatilized. These can be recovered from the process exhaust gases and recycled into further reaction feedstock. Also, HCI gas may be generated in the reaction. This can be recovered, for example by water scrubbing, and used in either of two ways. It may be used to generate HCI leach solution as described above, or an HCI solution may be reacted with limestone to produce CaCI₂, which can be added to a further reaction feedstock. A calcium- or magnesium-containing ore or substance may be added to the reaction feedstock, or blend, in order to decrease the melting point of the blend or of a component of the blend during thermal treatment. In this case the reaction temperature may be high enough to melt the blend, or the component of the blend. Adding a calcium-containing ore or substance may also supply

P/66807.WO01 - 29.10.12 calcium as a reactant, and may improve the agricultural properties of the product.

In further aspects, the invention may advantageously provide a soluble potassium-containing product, which, after leaching, can be used as a fertiliser or as a component of a fertiliser, and the invention may advantageously provide a fertiliser containing the soluble potassium or soluble potassium product. As noted above, embodiments of the invention may produce a potassium- chloride product. This can also be used as a fertiliser or a component of a fertiliser. Thus, in a further aspect, the invention may provide a fertiliser containing a potassium-chloride product. The mineral feedstock may comprise biotite, chlorite, glauconitic argilite, potash-feldspars (microcline, sanidine and orthoclase), muscovite, sericite or any potassium-containing minerals 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% Al₂0₃, 3.37 FeO/24.31 % Fe₂0₃, 53.48% Si0₂, 4.22% H₂0. The composition of glauconite 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.

Although attempts were made in the prior art to produce soluble potassium from potassium silicates, insufficient observations on mechanism, as CaO-K₂0 and Na-K swaps in the silicate structure, importance of avoiding sintering and

P/66807.WO01 - 29.10.12 carbonate utilization, both as source of alkali and C0₂, have been provided in the past.

Other Features

Optionally, a reaction feedstock as set out in the claims may comprise between 1 and 4 parts limestone, by weight, to 6 parts potassium-bearing feldspar, preferably between 1 .5 and 3 parts limestone to 6 parts

potassium-bearing feldspar, and particularly preferably about 1 part limestone to 3 parts potassium-bearing feldspar.

Optionally, the molar quantity of sodium in a reaction feedstock as set out in the claims may be between 1.2 and 2, or 2.5, times the molar quantity of potassium in the mineral feedstock, or of potassium contained in

potassium-bearing feldspar and mica in the mineral feedstock.

Optionally, a reaction feedstock as set out in the claims may contain between 10% and 20%, or between 10% and 15%. Of CaC0₃ by weight, and/or between 12% and 20%, or between 15% and 20% NaCI by weight. Optionally, a reaction product as set out in the claims may be leached with water at a temperature about 75C to 90C or 95C or 98C for between 45 and 75 minutes, or about 60 minutes, or at a temperature above 95C or 98C or 99C for between 20 and 60 minutes, or between 20 and 30 minutes. In a method as set out in the claims, gaseous substances may be generated or volatilized during the reaction, including one or more of HCI, NaCI, CaCI₂ and KCI, and one or more of the gaseous substances may be recovered, preferably by dissolution, or scrubbing, optionally in water. Optionally, volatilized KCI recovered from the reaction may be added to the soluble potassium salt product.

Optionally, volatilized NaCI and/or CaCI₂ recovered from the reaction may be recycled by being added to a reaction feedstock for a further iteration of the method.

P/66807.WO01 - 29.10.12 In terms of percentages, in alternative embodiments, the reaction feedstock may optionally contain between 8% and 30%, or 12% and 20%, or 15% and 20% of the reagent (preferably NaCI or a mixture of NaCI and CaCI₂) by weight. The reaction feedstock may contain between 5% and 25%, or 10% and 20%, or 10% and 15%, of the carbonate (preferably CaC0₃) by weight. These proportions of the materials in the reaction feedstock are for typical mineral feedstocks containing about 5 to 8% K₂0 equivalent, or about 9.5 to 1 1 % K₂0 equivalent, of K in a form which is available for reaction. The percentages of the NaCI, or NaCI and CaCI₂ mixture, and CaC0₃ in the reaction feedstock may be proportionally increased or reduced in accordance with the quantity of available K in the mineral feedstock if that quantity is more than or less than about 5 to 8% K₂0 equivalent, or about 9.5 to 1 1 % K₂0 equivalent.

Optionally, HCI recovered from a reaction may be used to form a solution of HCI in water for leaching further reaction product.

Specific Embodiments

Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a schematic diagram of an apparatus embodying the invention; Figure 2 is a graph showing potassium recovery as a function of x % CaC0₃ in the feed mixture (reaction feedstock); and

Figure 3 is a graph showing the variation of K recovery as a function of varying proportions of NaCI and CaCI₂ in the reaction feedstock.

In an example of this process, a mass of 279.6g of crushed verdete of composition 10.85% K₂0, 16.06% Al₂0₃, 3.3% MgO, 60.00% Si0₂,

6.43% FeO, 3.24% H₂0, w/w) was blended with 39.9g of calcitic limestone and 78.9g of commercial grade sodium chloride and ground in a ball mill to a fineness of 100% -100 mesh (about 150 micrometres). The ground feed was

P/66807.WO01 - 29.10.12 burned in an indirectly heated quartz rotary batch kiln for 1.5 hour at a temperature of 950C. Off-gases were scrubbed in a water scrubber during the experiment. The calcine obtained (358.3g) and the scrubber solution were analysed for most relevant elements (K, Fe, Ca, Al, Na, Mg, Si, CI, Mn). After roasting, the kiln was cooled down and rinsed with water to collect any residual elements stuck to its walls. The kiln rinse solution was also analysed.

Calcine (roasted material) loss on ignition (burning) was 10.3% w/w (as a percentage of the feed), mainly due to NaCI and KCI volatilization and decarbonation of CaC0₃. Volatilized NaCI and KCI were absorbed mainly by the scrubber solution and partially stuck to the kiln walls. The scrubber solution presented a pH of 0. This suggests that HCI vapour is generated and released in the pyro (burning) process, and can optionally be used later for the calcine leaching step.

The calcine (roasted material) was quenched and leached in water at 99C for 1 hour (leachate:calcine ratio 5:1 ). Chemical analysis of the leaching solution showed the presence of soluble KCI as product in the liquid fractions.

Diffraction analysis of the heat treated blend of verdete:CaC0₃:NaCI after leaching showed only a minor quantity of KCI in the insoluble residue (filter cake).

The overall potassium recovery (including K recovered from scrubber solution, leaching solution, kiln walls) was 65.0% w/w. Na recovery was 73%, making this reagent recyclable. Less than 8% of Ca was recovered. Through XRD analysis it was possible to verify the presence of Ca-silicates, which are more stable than K-silicates.

Figure 1 is a schematic diagram of a commercial production process implementing an embodiment of the invention. The mineral feedstock

(Verdete) 2 is ground and blended in a mill 4 with crushed limestone 6

(containing CaC0₃) and NaCI 8. In addition, recycled CaCI₂ and NaCI 10 are ground and added into the mill.

P/66807.WO01 - 29.10.12 This mixed, or blended, reaction feedstock, consisting of a powder of particle size less than 100 μηι (such that 90% of the powder passes through a 100 micrometre mesh) is fed to a rotary kiln 12. The reaction feedstock is heated in the kiln to 950C for 1 hour to produce a reaction product 14. The reaction product is still in powder form, or can easily be crushed to powder.

During the reaction, gases produced in the kiln are passed through a scrubber 16. HCI and other soluble compounds in the exhaust gas are dissolved in water and placed in a tank 18. Limestone 20 is added to the tank to neutralise the HCI solution and produce CaCI₂. The CaCI₂, and NaCI and KCI volatilized from the kiln, are separated from the water by evaporation and collected 22 for recycling.

The reaction product 14 is quenched in water, which also forms a leaching solution. Leaching is carried out for 1 hour at 99°C in tanks 24, followed by a crystallisation process 25 to extract KCI, NaCI and CaCI₂ from the leachate.

The chlorides recovered from the leachate and the recycled chlorides 22 are combined 26. The KCI 28 forms the desired potash product. The NaCI and CaCI₂ 30 are recycled to form further reaction feedstock.

Unwanted materials recovered from the leachate are dried and sent to landfill 32. A second example of the invention illustrates the influence of the percentage of CaC0₃ (limestone, calcium carbonate) in the recovery of potassium.

Ten tests were carried out in triplicate at a temperature of 900°C, in a muffle furnace with samples of representative K-silicate ore containing approximately 10.0% K₂0.

Each sample was prepared to make up around 120 g of mixture (reaction feedstock). A base mixture was prepared containing ore and NaCI in the ratio 70:20, and varying amounts of limestone were added to make up mixtures for the different samples of reaction feedstock. Limestone (Calcium Carbonate)

P/66807.WO01 - 29.10.12 purity was better than 52% CaO.

These mixtures were ground to 100% - 90 micrometers (100% of the mixture passing a 90 micrometre sieve) and then homogenized before being placed into the muffle furnace already heated up to the test temperature, which was 900°C. Residence time in the muffle furnace for each sample was fixed at 90 minutes.

After calcining the material was submitted to a hot-water leaching under reflux to determine the amount of K amenable to leaching i.e. the amount of K rendered water soluble by the high-temperature reaction. For leaching, the ratio between hot water and calcine was 3:1 . Leaching was carried out at 90°C for 30 minutes. The amount of K recovered in the leach liquor was calculated as % extraction in relation to the amount of K contained in the feed mixture and plotted for each concentration of limestone in the mixture. The results are shown in Figure 2.

Under the conditions evaluated of temperature and salt (NaCI) concentration without any CaC0₃ K recovery is only around 24%.

Potassium recovery increases with the increase in percent addition of CaC0₃ and surprisingly good results are obtained at about 6 to 8 % CaC0₃ beyond which additions become less significant.

A third example of the invention illustrates the improvement in K extraction by the use of CaCI₂ in addition or as a substitution in part of the NaCI as a chloridizing agent in the high-temperature reaction. In this example the process was carried out in a pilot plant with capacity for 150 kg/h of feed mixture (reaction feedstock) for 45 minutes of retention (reaction) time.

This feed mixture contained: 20% ore, 10% CaC0₃, 18% NaCI, 2% CaCI₂. The flow diagram (process route) comprised:

P/66807.WO01 - 29.10.12 • Pre-heating system with two-stage cyclones;

• Rotary kiln (0,9 m diameter x 9,8 m length);

• Calcine cooling system with one cyclone and bag house;

· Petroleum coke dosing system;

• Exhaust-gas handling and dust collecting;

• Exhaust-gas scrubbing and emission control

This operation, with this type of mixture, ran continuously for a period of 40h. The example referred herein is related to a sample period of 18 h during which an average K recovery value of 61 .3% was achieved.

This same operation in this same pilot plant under the same conditions with the same mixture without the 2% of CaCI₂ (i.e. with 20% NaCI) gave a K recovery of 56%. So, a yield increase of 5.3 percentage points was obtained by substituting 10% of the NaCI with CaCI₂.

Recovery measurements were obtained by taking samples at predetermined intervals from product fines. Product samples (calcine) were leached in water to evaluate the amount of K rendered soluble by the high temperature reaction. K was then determined in the leach liquor and calculated as % K extracted in relation to the feed mixture that fed the kiln, which was also sampled. Laboratory tests were carried out to show in more detail the effect of the inventor's proposal to substitute a portion of the NaCI in the reaction feedstock with CaCI2. The results are shown in Figure 3. Reaction feedstock samples containing ore/CaC0₃/metal chloride in the ratio 70/10/20 were prepared, with the metal chloride portion of the feedstock containing different proportions of NaCI and CaCI₂, ranging from 20% NaCI and 0% CaCI₂ to 0% NaCI and 20% CaCI₂. The samples were otherwise prepared as described in the second Example above, before being heated in a muffle furnace followed by water leaching and analysis of the leachate.

P/66807.WO01 - 29.10.12 The results show that small amounts of CaCI₂ substituted for NaCI, from about 1 % up to 3%, and preferably about 3%, of the reaction feedstock, provide very surprisingly beneficial results. 2% substitution can mean an absolute increase in K recovery of some 5 to 7%, which is very significant.

A fourth example of the invention concerns the presence of Sulphur in the calcine, due to the use of petroleum coke as source of energy in the pilot plant test work as described in Example 3 above.

Feed mixture (reaction feedstock) used for this run was: 65% ore/ 10% CaC0₃ / 25% NaCI. Petroleum coke containing 4.5% S was fed to the feed mixture, in the ratio (as described below) expected for an industrial implementation of the invention. In view of the reduced scale of the kiln in the pilot plant, by comparison to a commercial-scale plant, additional heat was required to bring the pilot kiln to the desired reaction temperature. This heat was provided to the pilot kiln by diesel oil. The feed rate to the kiln was 1 14 kg/h and the petroleum coke consumption was 4.5 kg/h.

The leaching process was carried out using hot water in a ratio of 1.7 parts of calcine to 1 .0 part of water and the liquor obtained presented the following composition:

TDS-total dissolved solids

P/66807.WO01 - 29.10.12 Part of the S0₄ ⁼ ions will react with the calcium present in the solution (formed from the reaction of CaC0₃ and HCI generated in the reaction), producing CaS0₄, which will precipitate. The excess of sulphate ions will remain in the brine.

This brine will be evaporated to recover NaCI.

Once it reaches KCI saturation, the main product will be recovered by crystallization, by cooling down brine temperature.

According to the ionic concentrations of the brine and the temperature which KCI will be crystallized, together with it will also crystallize in minor amount, the double salt: Na₂S0₄.3K₂S0₄. The concentration of this double salt on the final product will depend on the amount of S0₄ ⁼ available, which will depend on the S content of the petroleum coke.

A bulk sample of the resulting calcine after reaction was leached with water at 70°C in order to evaluate the characteristics of the brine obtained, aiming at the hydro-chemical plant, namely KCI recovery, NaCI recovery, solid-liquid separation and so forth.

The inventor found that sulphate ions were advantageously found in the brine, derived from the sulphur in the petroleum coke. In a preferred embodiment of the invention, a high sulphur petroleum coke may therefore be used if sulphate is desired in the industrial potassium product.

P/66807.WO01 - 29.10.12

Claims

1 . A method for forming soluble potassium, comprising the step of heating a reaction feedstock in order to react a potassium-containing mineral feedstock with sodium chloride in the presence of calcium carbonate at a temperature between 825C and 950C.

2. A method according to claim 1 , in which the potassium-containing mineral feedstock comprises a potassium-bearing feldspar and/or a

potassium-bearing mica and/or a potassium-bearing silicate.

3. A method according to any preceding claim, in which the

potassium-containing mineral feedstock is blended with NaCI and CaC0₃ to form the reaction feedstock.

4. A method according to any preceding claim, in which a reaction feedstock contains more than 6%, or 7%, or 8%, and less than 25%, or 15% or 12%, CaCOs by weight.

5. A method according to any preceding claim, in which the reaction feedstock contains between 8% and 30%, and preferably between 15% and 30%, NaCI by weight.

6. A method according to any preceding claim, in which a reaction feedstock contains 60% to 70% mineral feedstock, 7% to 15%, or 10% to 20%, CaCOs and 15% to 30% NaCI.

7. A method according to any preceding claim, in which the reaction feedstock further comprises CaCI₂.

8. A method according to claim 7, in which a quantity of CaCI₂ in the reaction feedstock is between 5% and 12%, or between 6% and 10%, of a total, or combined, quantity of NaCI and CaCI₂ in the reaction feedstock.

P/66807.WO01 - 29.10.12

9. A method according to any of claims 3 to 8, in which a ratio of the molar quantity of K₂0-equivalent in the mineral feedstock to the molar quantity of NaCI, or of NaCI and CaCI₂ in combination, in the reaction feedstock is between 1 :3 and 1 :9.

10. A method according to any of claims 3 to 9, in which a ratio of the molar quantity of K₂0-equivalent in the mineral feedstock to the molar quantity of CaC0₃ in the reaction feedstock is between 1 :1 and 1 :2.2.

1 1 . A method according to any preceding claim, in which the reaction is at a temperature above 825C, or 830C, or 835C, or 840C, or 845C, or 850C.

12. A method according to any preceding claim, in which the reaction is at a temperature below 950C, 930C, 910C, 900C, 890C, 880C, 870C, 860C or 850C.

13. A method according to any preceding claim, in which the reaction feedstock comprises NaCI and not CaCI₂, and the reaction temperature is between 825C and 930C, and preferably between 830C and 900C or between 835C and 880C.

14. A method according to any of claims 1 to 12, in which the reaction feedstock comprises NaCI and CaCI₂, and the reaction temperature is between 825C and 900C, and preferably between 825C and 880C or 830C and 850C.

15. A method according to any preceding claim, in which the mineral feedstock contains available potassium of between 6% and 15% K₂0, or between 8% and 12% K₂0, or of about 10% K₂0.

16. A method according to any preceding claim, in which the reaction feedstock comprises the mineral feedstock, calcium carbonate or limestone, and CaCI₂ in a ratio of 60 to 70 parts : 7 to 13 parts : 22 to 28 parts (by weight), or preferably 63 to 67 parts : 8 to 12 parts : 23 to 27 parts.

P/66807.WO01 - 29.10.12

17. A method according to any preceding claim, in which the reaction feedstock comprises the mineral feedstock, calcium carbonate or limestone, NaCI and CaCI₂ in a ratio of 60 to 70 parts : 7 to 13 parts : 20 to 25 parts : 1 to 5 parts (by weight), or preferably 63 to 67 parts : 8 to 12 parts: 21 to 24 parts : 2 to 4 parts.

18. A method according to any preceding claim, in which the reaction is carried out in a rotary kiln, preferably coupled to a suspension preheater and/or a scrubber.

19. A method according to any preceding claim, in which the reaction forms a product containing water soluble potassium salts, and the method comprises the step of leaching the reaction product to obtain a solution from which a potassium salt can be concentrated, for example by evaporation,

crystallization, solvent extraction, precipitation and/or sublimation.

20. A method according to claim 19, in which the reaction product is leached with water, preferably at a temperature above 75C or 90C or 95C or 98C or 99C, preferably for between 5 and 60 minutes, or between 5 and 30 minutes, or between 5 and 15 minutes, or for about 30 minutes.

21 . A method according to claim 19, in which the reaction product is leached with HCI solution of less than 0.15% HCI, or less than 0.1 % HCI, or pH greater than 1.6, preferably at a temperature between 55C and 65C, or of about 65C, preferably for between 7 and 15 minutes, or about 10 minutes.

22. A method according to any preceding claim, in which the mineral feedstock has a particle size of less than 150 micrometres, or less than 100 micrometres.

23. A method according to any preceding claim, in which the sodium chloride and/or the calcium chloride, if present, has a particle size of less than 150 micrometres, or less than 100 micrometres.

P/66807.WO01 - 29.10.12

24. A method according to any preceding claim, in which the calcium carbonate has a particle size of less than 150 micrometres, or less than 100 micrometres.

25. A method according to any preceding claim, in which the mineral feedstock, the sodium chloride, the calcium chloride if present, and the calcium carbonate are blended together and ground such that at least 90% passes a 90 micrometre sieve (170 mesh), to form the reaction feedstock.

26. A method according to any preceding claim, in which the mineral feedstock, sodium chloride, calcium chloride if present, and calcium carbonate are not dried before the reaction.

27. A method according to any preceding claim, in which HCI recovered from the reaction is reacted with CaC0₃ or limestone or Ca(OH)₂ or hydrated lime, to form CaCI₂ which is added to a reaction feedstock for a further iteration of the method, preferably in addition to or as a partial substitution of the NaCI.

28. A method according to any preceding claim in which a reaction product, or calcine, is quenched after the reaction, preferably by immersion in water or in HCI solution.

29. A method according to claim 28, in which the water or HCI solution is used for leaching the reaction product.

30. A method according to claim 28, in which the reaction product is quenched from a reaction temperature to less than 100C in less than 25s, or less than 15s, and preferably less than 10s.

31 . A method according to any preceding claim, in which the mineral feedstock comprises biotite, glaucomitic argillite, chlorite, greensand, potassium feldspar, biotite or kamafugite or comprises a mineral containing potassium silicate.

P/66807.WO01 - 29.10.12

32. A method according to any preceding claim, in which the reaction is at a temperature between 850C and 950C.

33. A product comprising soluble potassium chloride, hydroxide and/or carbonate, produced using the method of any of claims 1 to 32.

34. An apparatus for producing a soluble potassium salt product, comprising a first reactor for reacting a potassium-containing mineral feedstock with sodium chloride, and optionally calcium chloride, in the presence of calcium carbonate, coupled to a second reactor for carrying out a leaching and evaporation/crystallization step to concentrate a soluble potassium species generated in the first reactor.

35. An apparatus according to claim 34, comprising a third reactor coupled to the first and second reactors for recovering HCI evolved from the first reactor and recycling the HCI as a leaching solution to the second reactor.

P/66807.WO01 - 29.10.12