Processing of evaporite minerals

Abstract

A method for the processing a mixed particulate evaporite minerals with particles of size at least 0.25 mm where the minerals comprise a first evaporite with a first metal contaminant and a second eva

Description

FIELD OF THE INVENTION The invention relates to the processing of evaporite minerals to form useful products. The invention relates in particular to the processing of potassium containing evaporite minerals in the production of potash (KCI), which is widely used as an agricultural fertiliser. The invention provides improved handling of secondary metals that are found in potassium-containing evaporite ore deposits, thereby allowing products of economic value to be obtained from these secondary metals, which would otherwise be lost to waste streams. BACKGROUND The term potash refers to various soluble salts of potassium that are widely used as agricultural fertilizers. Potassium reserves are largely in the form of evaporite mineral ores, so-called because the minerals were formed via the evaporation of salt water. The resulting minerals body comprises a variety of minerals comprising Na+, K+ and Cl’ as the primary components of economic value, alongside a variety of secondary metal components, such as Mg2+, Ca2+ and Fe2+ / Fe3+. The primary mineral sources of potassium chloride used in the production of potash are carnallite (KCI.MgCI2-6(H2O)) and sylvite (mineral form of KCI), although these are typically found alongside halite (mineral form of NaCI) and other minerals, such as kainite (KMg(SO4)CI-3H2O), langbeinite (K2Mg2(SO4)3), kieserite (MgSO4'H2O), bischofite (MgSO4'6H2O), polyhalite (K2Ca2Mg(SO4)eH2O), schoenite (K2Mg(SO4)2'6(H2O), rinneite (K3Na[FeCl6]), erythrosiderite (K2FeCI5 (H2O)) and douglasite (K2[FeCI4(OH2)2]). In the case that a mineral source of potassium chloride comprises evaporites containing secondary metals (such as Mg, Ca or Fe), separation processes are required to isolate the target KCI product from these secondary metals. In general, these processes rely on the relatively low solubility of KCI and NaCI to allow solid KCI and NaCI products to be separated from a brine comprising salts of other metals, such as magnesium, iron and calcium. For example, in a typical process for recovering potassium chloride from carnallite, a brine is formed by dissolution of the carnallite in water until a saturation point is reached. Due to the higher solubility of magnesium, the brine will continue to absorb magnesium ions from the carnallite with the simultaneous displacement of potassium ions from the saturated solution, resulting in the precipitation potassium chloride which can be recovered as a solid. Once the brine reaches saturation in magnesium, it is discarded as an industrial waste stream. A process in which the dissolution of an evaporite comprising a high solubility component occurs simultaneously with the precipitation of a lower solubility component is referred to herein as “decomposition”. In practice, carnallite is mined alongside other evaporite minerals and the process described above generally involves dissolving carnallite and one or more other evaporites (in particular, sylvite and halite). Potassium and sodium are less soluble than magnesium and therefore the decomposition of carnallite occurs with the precipitation of both KCI (from carnallite and sylvite) and NaCI (from halite). KCI and NaCI can be separated by a hot leach process that relies on the fact that the solubility of KCI is relatively temperature sensitive, whereas the solubility of NaCI is relatively temperature insensitive. The NaCI can be recovered as a byproduct and finds applications in road de-icing or, after additional purification, as an industrial grade commodity. In the case that the ore body comprises evaporite minerals containing other metals, such as the iron found in rinneite, the decomposition process results in a waste brine that contains a mixture of secondary metals, in particular iron and magnesium. It is extremely difficult to treat a brine with this mixed composition with a view to recovering any components of commercial value from the secondary metals, since each effectively contaminates the other. Accordingly, the mixed brine is a complex waste material that requires either direct disposal or long-term storage in a suitably designed and engineered facility (such as a tailings dam), where loss of containment poses an environmental risk. This waste brine also represents a loss of water from the processing plant inventory which requires replacing, and this is important from an environmental sustainability consideration. There is therefore a need in the art for methods for the processing of evaporite minerals with improved handling of waste streams. In particular, there is a need for methods of processing evaporite minerals that provide for the recovery of products of economic value from the metals that would otherwise be discarded in waste brines under conventional processing methods. Such methods would desirably provide for the selective handling of different evaporite mineral types such that the secondary metals can be isolated from one another without forming complex mixed brines from which economical recovery of products of value is impossible. Still further, such methods would desirably provide for the reuse of the process water. There is also a need in the art for methods of converting the secondary metal content of waste brines to commercial products of economic value, particularly for sale as fertilizers. The applicant has now identified that improved handling of evaporite ores may be achieved by a process which exploits differences in the rates of dissolution / decomposition of different evaporites to allow different secondary metals to obtained in different brines such that products of value may be recovered. The applicant has also identified novel methods for the recovery of products of value from these dissolved metals. SUMMARY OF THE INVENTION In a first aspect, the invention provides a method for the processing of mixed evaporite minerals, wherein the mixed evaporite minerals comprise a first evaporite mineral comprising a first metal contaminant and a second evaporite mineral comprising a second metal contaminant, wherein at least one of the mixed evaporite minerals comprises potassium, wherein the method comprises the steps of: (a) providing particles of the mixed evaporite minerals, wherein the particles have a particle size of at least 0.25 mm; (b) adding the particles from step (a) to an aqueous liquid and selectively dissolving and / or decomposing the first evaporite mineral in the aqueous liquid; (c) separating particles of the second evaporite mineral from the aqueous liquid to provide a first brine comprising the first metal contaminant and solid particles of the second evaporite mineral; wherein the ratio of the first metal contaminant to the second metal contaminant in the first brine obtained in step (c) is higher than in the particles of the mixed evaporite minerals provided in step (a). In a second aspect, the invention provide a method for the processing of an evaporite mineral body comprising one or more evaporite minerals, wherein the one or more evaporite minerals comprise potassium, optionally sodium and at least one other metal, wherein the method comprises the steps of: (a) decomposing the one or more evaporite minerals in an aqueous liquid to form a precipitated potassium salt, optionally a precipitated sodium salt, and a first brine comprising the at least one other metal; and (b) adding at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the first brine from step (a) to recover a salt of the at least one other metal and a second brine. DETAILED DESCRIPTION The first aspect of the invention provides a method for the processing of mixed evaporite minerals, wherein the mixed evaporite minerals comprise a first evaporite mineral comprising a first metal contaminant and a second evaporite mineral comprising a second metal contaminant. At least one of the mixed evaporite minerals comprises potassium. The method comprises the steps of: (a) providing particles of the mixed evaporite minerals, wherein the particles have a particle size of at least 0.25 mm; (b) adding the particles from step (a) to an aqueous liquid and selectively dissolving and / or decomposing the first evaporite mineral in the aqueous liquid; and (c) separating particles of the second evaporite mineral from the aqueous liquid to provide a first brine comprising the first metal contaminant and solid particles of the second evaporite mineral. The ratio of the first metal contaminant to the second metal contaminant in the first brine obtained in step (c) is higher than in the particles of the mixed evaporite minerals provided in step (a) as a result of selective dissolution / decomposition of the first evaporite mineral and separation of the second evaporite mineral as a solid. The method of the first aspect provides a solution to the problem that mixed evaporite ores result in the formation of waste brines comprising intractable mixtures of secondary metals, from which it is difficult or impossible to recover components of economic value. The applicant has recognised that solubility differences between different evaporite ores can be exploited to achieve the selective dissolution / decomposition of a first evaporite mineral containing a first metal contaminant while a second evaporite mineral of lower solubility and containing a second metal contaminant can be separated as a solid without dissolution. Key to the invention is the recognition that the exploitation of differences in solubility can be leveraged by controlling the particle size of the evaporite minerals. As used herein, the term “evaporite” relates to a natural mineral salt deposit which is formed by the evaporation of a body of water. The mixed evaporite minerals which are processed by the method according to the invention may be obtained by mining an ore body comprising mixed evaporite minerals. The mixed evaporite minerals comprise a first evaporite mineral comprising a first metal contaminant and a second evaporite mineral comprising a second metal contaminant. In the context of the invention, the term “metal” encompasses metal ions which are present in the evaporite minerals. In a preferred embodiment, the first metal contaminant is an alkaline earth metal. More preferably, the first metal contaminant is magnesium. As at least one of the evaporite minerals comprises potassium. In some embodiments, at least one of the first evaporite mineral and second evaporite mineral comprises potassium. Optionally, both the first evaporite mineral and the second evaporite mineral may comprise potassium. Preferably, at least one of the mixed evaporite minerals comprises chloride (i.e. a chlorine ion). More preferably, at least one of the first evaporite mineral and second evaporite mineral comprises chloride. Still more preferably both the first evaporite mineral and the second evaporite mineral may comprise chloride. Accordingly, the evaporite minerals provide the building blocks for extracting a potassium salt from the mixed evaporite minerals, wherein the potassium salt is preferably potassium chloride. The particles provided in step (a) of the method have a particle size of at least 0.25 mm. Particle sizes above this threshold allow a solubility differential between the first evaporite mineral and the second evaporite mineral to be exploited such that the first evaporite mineral is selectively dissolved and / or decomposed in the aqueous liquid, and the second evaporite mineral remains substantially in solid particle form. By contrast, particles of lower particle size will dissolve more rapidly in aqueous solution thereby reducing the degree of solubility-based selectivity that is achievable. Optionally, the particles of the mixed evaporite minerals in step (a) may have a particle size of at least 0.5 mm, or at least 1 mm, or at least 1.5 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm. The minimum particle size in step (a) may be selected to optimise the solubility differential between the first and second evaporite minerals. Step (a) may therefore comprise classifying the mixed evaporite minerals by particle size to provide a coarse fraction and a fine fraction, wherein the coarse fraction has a particle size of at least 0.25 mm, or at least 0.5 mm, or at least 1 mm, or at least 1.5 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm. The coarse fraction is then used in step (b), whereas the fine fraction may be processed separately, for example in accordance with the further steps disclosed below. The classifying may be carried out using a mesh screen. Preferably a vibrating mesh screen is used to classify the mixed evaporite minerals, as the mechanical vibrations improve the efficiency of the separation step. The method of classifying particles is not particularly limited, and any known method may be used. For the avoidance of doubt, references herein to minimum or maximum particle sizes refer to the ability of the particles to pass through a screen of corresponding mesh size. Accordingly, where the particles provided in step (a) are required to have a particle size of at least 0.25 mm, this shall be taken to refer to the particle size fraction that does not pass through a sieve with 0.25 mm openings (60 Mesh). Prior to particle size classification, the method may further comprise the step of providing an ore body comprising mixed evaporite minerals comprising the first evaporite mineral and the second evaporite mineral and comminuting the ore body to provide mixed evaporite particles. This may be achieved by any known method, including crushing. The upper limit of the size of the mixed evaporite particles provided in step (a) is not particularly limited. However, to optimise the rate of dissolution and increase the speed of the process, preferably the mixed evaporite particles provided in step (a) have a particle size of 80 mm or less, preferably 60 mm or less, preferably 50 mm or less, preferably 40 mm or less, preferably 30 mm or less. Providing this upper limit optimises the rate of dissolution and / or decomposition in step (b), which improves efficiency of the process. The term “selectively dissolve and / or decompose” in the context of the invention means that a higher proportion of the first evaporite mineral dissolves and / or decomposes than of the second evaporite mineral. Therefore, the remaining brine may comprise dissolved and / or decomposed firstand second metal contaminants, however, relative to the mixed evaporite mineral particles provided in step (a), the ratio of the first metal contaminant to the second metal contaminant in the first brine obtained in step (c) is higher. Preferably, the molar ratio of first metal contaminant to second metal contaminant in the first brine obtained in step (c) is at least 90:10, more preferably at least 95:5, more preferably at least 98:2, more preferably at least 99:1. Examples of evaporites that are suitable as the first evaporite mineral include those that comprise an alkaline earth metal. In particular, the first evaporite mineral may be a magnesium-containing evaporite mineral. Still more preferably, the first evaporite mineral may comprise both potassium and magnesium. Examples of suitable magnesium-containing evaporite minerals include carnallite, langbeinite, schoenite, kainite, polyhalite, kieserite or bischofite. Preferably, the first evaporite mineral is carnallite. The structures of these evaporite minerals are summarised in Table 1. It is also preferred that the first evaporite mineral is non-magnetic, such that it is not attracted or repelled by a magnetic field. In addition to the first and second evaporite minerals, the mixed evaporite minerals used in the method according to the invention may further comprise sylvite, which is potassium chloride (KCI) in its natural mineral form. In the case that the mixed evaporite minerals comprise sylvite, this will undergo decomposition / dissolution alongside the first evaporite mineral and the potassium chloride content thereof will simply add to the overall potassium inventory in the process without affecting the separation of the first and second metal contaminants. Potassium derived from sylvite will then be recovered along with the potassium content of the first evaporite mineral. The mixed evaporite minerals may optionally further comprise halite, which is sodium chloride (NaCI) in its natural mineral form. Similarly to sylvite, any halite in the mixed evaporite minerals will add to the overall sodium inventory in the process without affecting the separation of the first and second metal contaminants. It will be recovered alongside potassium prior to separation in subsequent steps as discussed below. Step (b) preferably comprises decomposing the first evaporite mineral. As set out above, a decomposition process is one in which the dissolution of the first metal contaminant displaces other metal ions from solution such that they precipitate as solid salts. In particular, a decomposition process would include processes in which the dissolution of magnesium from the first evaporite mineral occurs simultaneously with the precipitation of potassium salts, and optionally sodium salts (in particular potassium chloride and sodium chloride). These precipitated salts may be separated from the first brine alongside the second evaporite mineral by solid-liquid separation to leave a first brine containing the dissolved first metal contaminant. The precipitated salts obtained in step (b) will typically have a particle size that is much smaller than the particles of the second evaporite mineral. Accordingly, the precipitated salts may be separated from the second evaporite mineral by particle size classification. Alternatively, the precipitated salts may be passed with the second evaporite mineral to further processing steps. The dissolution or decomposition of step (b) is preferably carried out at ambient temperature and pressure. Preferably, the temperature of the aqueous liquid at step (b) is from 5-50 °C, or from 10-45 °C, or from 10-40 °C, or from 15-35 °C. The method of the invention preferably comprises recovering a potassium salt from the one or more precipitated salts. The precipitated salts may be a mixture of KCI and NaCI. The recovery step may comprise a hot leach step and / or a crystallisation step. However, any suitable method for separating a salt product from a mixture of precipitated salts may be employed. The second evaporite mineral may in principle be any evaporite mineral with a lower rate of dissolution / decomposition that the first evaporite mineral. However, it is particularly preferred that the second evaporite mineral is magnetic since this provides access to a second separation process which allows the first and second evaporite minerals in the fine fraction to be separated. Preferably, the second evaporite mineral comprises iron, more preferably, the second evaporite mineral comprises a mixture of potassium and iron. Examples of magnetic evaporite minerals comprising iron and potassium include rinneite, erythrosiderite and douglasite. Preferably, the second evaporite mineral is rinneite. The invention is particularly applicable to the processing of mixed evaporites in which the first evaporite mineral is carnallite and the second evaporite is rinneite. However, the principles set out herein may be applied on an equivalent basis to other combinations of evaporites, in particular other combinations of magnesium-containing and iron-containing evaporites, including those set out above. A particularly preferred mixed evaporite comprises carnallite, rinneite and optionally one or both of sylvite and halite. The mass ratio of the first evaporite mineral to the second evaporite mineral in the mixed evaporite minerals may suitably be in the range from 1:99 to 99:1, or from 5:95 to 95:5, or from 10:90 to 90:10, or from 20:80 to 80:20. In a preferred embodiment, the first evaporite mineral is carnallite and the second evaporite mineral is rinneite, and the mass ratio of carnallite to rinneite is in the range from 1:99 to 99:1, or from 5:95 to 95:5, or from 10:90 to 90:10, or from 20:80 to 80:20. The mixed evaporite minerals may optionally comprise from 10 to 80 wt% halite, for example from 20 to 85 wt.% halite, or from 40 to 80% halite, or from 50 to 75% halite, with the remainder constituted by potassium bearing evaporites (i.e. the first and second evaporite minerals and optionally any further potassium bearing evaporites, such as sylvite). As an example composition, the mixed evaporite minerals may comprise from 10 to 80% halite, from 1 to 40% carnallite, from 1 to 40% rinneite and from 0 to 40% sylvite. As a further example composition, the mixed evaporite minerals may comprise from 25 to 75% halite, from 5 to 30% carnallite, from 2 to 20% rinneite and from 0 to 20% sylvite. As a further example composition, the mixed evaporite minerals may comprise from 50 to 75% halite, from 8 to 20% carnallite, from 2 to 20% rinneite and from 1 to 15% sylvite. As a further example composition, the mixed evaporite minerals may comprise from 50 to 75% halite, from 8 to 20% carnallite, from 5 to 15% rinneite and from 1 to 10% sylvite. Although example compositions have been disclosed for illustrative purposes, the process of the invention can be applied to various ratios of mixed evaporite minerals. When the second evaporite mineral is magnetic, the method may further comprise the steps of: (d) providing a fine fraction of mixed evaporite minerals, wherein the fine fraction has a particle size below 6mm; (e) separating the first evaporite mineral and second evaporite mineral in the fine fraction using a magnetic separator to form a magnetic fine fraction and a non-magnetic fine fraction. Optionally, the particles of the fine fraction in step (d) may have a particle size of below 5 mm, or below 4 mm, or below 3 mm, or below 2 mm, or below 1.5 mm, o below 1 mm, or below 0.5 mm, or below 0.25 mm. The maximum particle size in step (a) may be selected to optimise the efficiency of magnetic separation. Preferably, the particle size of the fine fraction of step (d) has a particle size below that of the particles in step (a); meaning that the fine and coarse fractions are capable of being separated in a single step by a sieve of the specified mesh size. Particles that do not pass through the openings of the sieve form a coarse fraction and may be used in steps (a) to (c). Particles that do pass through the openings form the fine fraction that may be used in steps (d) and (e). The coarse and fine fractions may optionally be obtained by comminution of an ore body as described above, followed by classification of the comminuted particles by size. Optionally, the fine fraction may be further comminuted prior to step (e). Step (e) separates magnetic particles of the second evaporite mineral from nonmagnetic particles of the first evaporite mineral. Whereas the separation in steps (a) to (c) is optimised for larger particles, the magnetic separation in steps (d) and (e) is optimised for the separation of fine particles. Accordingly, the method of the invention advantageously provides for the separation of the first and second metal contaminants in both a coarse fraction of the evaporite minerals and in a fine fraction of the evaporite minerals. The method of the invention may further comprise the step of: (f) dissolving and / or decomposing the particles of the magnetic fraction in an aqueous liquid to form a second brine comprising the second metal contaminant, wherein the ratio of the first metal contaminant to the second metal contaminant in the second brine is lower than in the fine fraction of mixed evaporite particles. Accordingly, the method of the invention is operable to form a first brine in step (c), wherein the first brine is enriched in the first metal contaminant and to form a second brine in step (f), wherein the second brine is enriched in the second metal contaminant. Preferably, the molar ratio of second metal contaminant to first metal contaminant in the second brine obtained in step (f) is at least 90:10, more preferably at least 95:5, more preferably at least 98:2, more preferably at least 99:1. The method of the invention suitably further comprises separating one or more precipitated salts from the second brine. The one or more precipitated salts preferably comprise a potassium salt and optionally a sodium salt, preferably wherein the potassium salt is potassium chloride, preferably wherein the sodium salt is sodium chloride. The method of the invention preferably comprises recovering a potassium salt from the one or more precipitated salts. The precipitated salts may be a mixture of KCI and NaCI. The recovery step may comprise a hot leach step and / or a crystallisation step. However, any suitable method for separating a salt product from a mixture of precipitated salts may be employed. The recovery of a potassium salt obtained from step (f) may be combined with the recovery of potassium salt obtained from step (c). The method of the invention therefore involves a separation process for larger particles that is based on selective dissolution / decomposition and optionally further comprises a further separation process for smaller particles that is based on magnetic separation. In a preferred embodiment of the invention, particles of the second evaporite mineral that are separated in step (c) may be comminuted and added to the fine fraction prior to step (d) or may be added to the magnetic fraction from step (d). Alternatively, the particles of the second evaporite mineral that are separated in steps (a)-(c) may be added to the starting ore body prior to the comminution and classification steps described above. In this way, the particles that are separated in step (c) may be recycled into steps (d)-(f) to recover the mineral content thereof. Similarly, the non-magnetic fine fraction (comprising particles of the first evaporite mineral) that is separated in step (e) may be added to the aqueous liquid in step (b) alongside the coarse mixed evaporite minerals. In this way, the particles that are separated in step (e) may be recycled into steps (a)-(c) to recover the mineral content thereof. The method according to the invention may further comprise the step of: (g) recovering a salt of the first metal contaminant from the first brine to provide a third brine. The reagents used in step (g) to recover a salt are not particularly limited, and are dependent on the desired salt product of the first metal contaminant. Step (g) preferably comprises the addition of at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the first brine. Preferably, the ammonium salt is ammonium hydroxide and / or the phosphate salt is selected from monoammonium phosphate, diammonium phosphate, and triammonium phosphate. The salt recovered in step (g) is selected from a magnesium phosphate and a magnesium ammonium phosphate. It is particularly preferred that the salt recovered in step (g) is selected from struvite, K-struvite dittmarite, newberyite or bobierrite. These products may be in hydrated or anhydrous form. In a preferred embodiment, the salt recovered in step (f) may be selected from hydrated struvite (MgNH4PO4-6H2O), hydrated K-struvite (MgKPO4'6H2O), anhydrous struvite (MgNH4PO4) or anhydrous K-struvite (MgKPO4). Most preferably, the salt product is recovered as hydrated struvite (MgNH4PO4'6H2O) or hydrated K-struvite (MgKPO4'6H2O). These products may advantageously be used as fertilizers. Preferably, step (g) further comprises recovering residual ammonium salts (in particular ammonium chloride) from the first brine. The third brine is preferably recycled and used as the aqueous liquid in step (b). In this way, any residual mineral content of the third brine is recycled into the process and maybe recovered in a subsequent cycle. The method may further comprise the step of: (h) recovering a salt of the second metal contaminant from the second brine to provide a fourth brine. The reagents used in step (h) to recover a salt are not particularly limited, and are dependent on the desired salt product of the second metal contaminant. However, in a preferred embodiment, step (h) comprises the addition of at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the second brine. Preferably, the ammonium salt is ammonium hydroxide and / or the phosphate salt is selected from monoammonium phosphate, diammonium phosphate, and triammonium phosphate. The salt recovered in step (g) is preferably selected from an iron phosphate, an iron ammonium phosphate and an iron magnesium phosphate. It is particularly preferred that the salt recovered in step (g) is vivianite, in hydrated form (Fe2+Fe22+(PO4)2'8H2O) or in anhydrous form (Fe2+Fe22+(PO4)2. Preferably, step (g) further comprises recovering residual ammonium salts (in particular ammonium chloride) from the first brine. The fourth brine is preferably recycled and used as the aqueous liquid in step (e), thereby minimising waste streams. In this way, any residual mineral content of the fourth brine is recycled into the process and maybe recovered in a subsequent cycle. The method of the invention therefore achieve a number of advantages over conventional processes for the processing of mixed evaporite ores. By providing a way to separate the secondary metal components of evaporite ores into separate brines, the method of the invention enables a greater proportion of the mineral content of the evaporite ore to be converted into products of economic value. In a conventional process, any magnesium that enters the brine is not only contaminated with other metals, such as iron, but is lost when disposed. In this regard, the magnesium content of carnallite is 8.75 wt%, which is comparable to the potassium content (14.07 wt%) for which the carnallite was originally mined. The process therefore allows ca. 620 kg of useful magnesium to be recovered for every tonne of potassium recovered from carnallite. Therefore the elemental mass recovery (in useable form) from the ore is greatly increased. A similar benefit is obtained in the recovery of useful iron products from the rinneite, or other iron-bearing component of the mixed evaporite ore starting material. The invention is therefore particularly applicable to the processing of mixed evaporites in which the first evaporite mineral comprises magnesium and in which the second evaporite mineral comprises iron. The invention is particularly applicable to the processing of mixed evaporites in which the first evaporite mineral is carnallite and the second evaporite is rinneite. However, the principles set out herein may be applied on an equivalent basis to other combinations of evaporites, in particular other combinations of magnesium-containing and iron-containing evaporites, including those set out above. A particularly preferred mixed evaporite comprises carnallite, rinneite and optionally one or both of sylvite and halite. A further benefit is gained whilst recovering ammonium chloride from the spent brines, the reaction removes a chloride ion from solution for each ammonium ion present. The source of the chloride ions is the evaporate minerals themselves. Hence a further portion of the ore is converted (in useable form) to an elemental mass recovery. Still a further benefit of the invention is that the recovery of magnesium and iron products from the produced brines results in the formation of a process water stream that is substantially depleted in mineral content. This process water can be recycled into the process, significantly improving the water economy of the process, eliminating the need to dispose of significant quantities of waste brine and allowing any residual mineral content of the depleted brine (in particular any dissolved potassium) to be recovered in a subsequent processing cycle. Yet another benefit of the invention is that the separation of the secondary salt contaminants in the first and second evaporite minerals avoids the problem of a mixed magnesium / iron brine exceeding the saturation limit of FeCh In the case that the saturation limit of FeCh is exceeded, then FeCh would precipitate alongside the alkali metal salts (NaCI and KCI) and then reports to the downstream steps for the recovery of KCI. This results in operational difficulties associated with elevated corrosion levels caused by the FeCh undergoing reaction with water to produce ferrous hydroxide and HCI. Yet a further benefit of the invention is that methodology is provided to convert the mineral content of waste brines into useful ammonium and phosphate salts, such as struvite, K-struvite, dittmarite, newberyite, bobierrite and vivianite, which find applications in the fertilizer industry. In a second aspect, the invention provides a method for the processing of an evaporite mineral body comprising one or more evaporite minerals, wherein the one or more evaporite minerals comprise potassium, optionally sodium and at least one other metal, wherein the method comprises the steps of: (a) decomposing the one or more evaporite minerals in an aqueous liquid to form a precipitated potassium salt, optionally a precipitated sodium salt, and a first brine comprising the at least one other metal; and (b) adding at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the first brine from step (a) to recover a salt of the at least one other metal and a second brine. The at least one other metal may be an alkaline earth metal, preferably magnesium. As set out above, magnesium is a component of a number of different evaporite minerals, such as carnallite, langbeinite, schoenite, kainite, polyhalite, kieserite or bischofite. Accordingly, the one or more evaporite minerals may comprise any of these magnesium-containing evaporites. In particular, the one or more evaporite minerals may comprise carnallite. The one or more evaporite minerals may further comprise sylvite and / or halite. As set out above, in conventional processes for the processing of evaporites ores, in particular to recover potassium chloride, a saturated magnesium-containing brine is obtained as a waste stream, which must be stored or discarded. This is disadvantageous for a number of reasons. The need to dispose of large quantities of a magnesium-containing brine is problematic for environmental reasons and represents the loss of water from the process which might otherwise be recovered and reused. More significantly, the magnesium containing brine contains material of economic value in the magnesium itself and in minor amounts of unrecovered potassium salts. The method of the second aspect provides a solution to this problem by providing a method of recovering salts of economic value from the first brine. In addition, the second brine obtained after recovery of the salts may be recycled to step (a), thereby conserving water and enabling residual amounts of potassium to be recovered. Step (b) comprises the addition of at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the first brine. Preferably, the ammonium salt is ammonium hydroxide and / or the phosphate salt is selected from monoammonium phosphate, diammonium phosphate, and triammonium phosphate. The salt recovered in step (b) is preferably a magnesium phosphate or a magnesium ammonium phosphate. It is particularly preferred that the salt recovered in step (b) is selected from struvite, K-struvite dittmarite, newberyite or bobierrite. These products may be in hydrated or anhydrous form. In a preferred embodiment, the salt recovered in step (b) may be selected from hydrated struvite (MgNH4PO4-6H2O), hydrated K-struvite (MgKPO4-6H2O), anhydrous struvite (MgNH4PO4) or anhydrous K-struvite (MgKPO4). Most preferably, the salt product is recovered as hydrated struvite (MgNH4PO4'6H2O) or hydrated K-struvite (MgKPO46H2O). As discussed above, magnesium salts recovered from a waste brine have less economic value if they are contaminated by other metals, in particular the iron found in evaporites such as rinneite. Accordingly, the first brine is preferably substantially free of iron, and more preferably is substantially free of transition metals. In this context, the term “substantially free” means that the total molar ratio of iron and / or other transition metals to magnesium is no more than 5:100, preferably no more than 2:100, more preferably no more than 1:100 and most preferably no more than 0.5:100. In the case that the evaporite mineral body comprises both iron and magnesium, separation of these metals into separate brines may be achieved in accordance with the methods of the first aspect of the invention. In an alternative method, the at least one other metal may be a transition metal, preferably iron. Iron is also a component of a number of different evaporite minerals, such as rinneite, erythrosiderite and douglasite. Accordingly, the one or more evaporite minerals may comprise any of these iron-containing evaporites. In particular, the one or more evaporite minerals may comprise rinneite. The one or more evaporite minerals may further comprise sylvite and / or halite. In an analogous way to that described for magnesium, the disposal of iron-containing waste brines presents an environmental concern and also involves the loss of the economic value of the iron itself and any minor amounts of unrecovered potassium salts in the brine. The invention therefore provides for the recovery of iron salts of economic value from the iron-containing brine. In addition, the second brine obtained after recovery of the iron salts may be recycled to step (a), whereby residual amounts of potassium may be recovered. In this case, the salt recovered in step (b) is preferably an iron phosphate or an iron ammonium phosphate, more preferably vivianite. As above, the iron salts would have reduced economic value if contaminated by other salts. Accordingly, the first brine is preferably substantially free of magnesium, and more preferably is substantially free of alkaline earth metals. In this context, the term “substantially free” means that the total molar ratio of magnesium and / or other alkaline earth metals to iron is no more than 5:100, preferably no more than 2:100, more preferably no more than 1:100 and most preferably no more than 0.5:100. The precipitated potassium salt is in step (a) is preferably potassium chloride, and wherein the optional precipitated sodium salt in step (a) is preferably sodium chloride. Example 1 A mixed evaporite ore (8x1 o5 kg / hour) comprising carnallite (13.5 wt%), sylvite (7.2 wt%), rinneite (6.6 wt%) and halite (72.7 wt%) is crushed and screened through a 10 mesh (2 mm) screen to form a coarse fraction (6.27x1 o5 kg / hour) of particle size 2 to ca. 14 mm and a fine fraction (1.73x105 kg.hour) of particle size <2mm. The fine fraction is subjected to primary magnetic separation to separate rinneite from non-magnetic components. The magnetic fraction (rinneite) is then ground in a ball mill circuit to below 0.4 mm before being subjected to a secondary magnetic separation. The magnetic fraction is fed to a rinneite composition circuit, in which it is decomposed in water (1.45x105 kg / hour) to form KCI and NaCI as solid precipitates and an iron-containing brine according to the following reaction scheme: K3Na [FeCle] (s) 3KCI (s) + NaCI (aq) + FeCI2 (aq) The non-magnetic fractions (e.g., carnallite, sylvite or halite) from the primary and secondary magnetic separations are combined with the coarse fraction from screening and then fed to a carnallite decomposition circuit treating the carnallite alongside sylvite and halite. The carnallite decomposition circuit exploits the size-based selective decomposition whereby coarse rinneite exhibits only very slow decomposition, whereas the carnallite, sylvite and halite of comparable size undergoes extremely rapid full decomposition or dissolution to form KCI and NaCI as solid precipitates and a magnesium-containing brine according to the following reaction scheme: KCI (s) KCI (aq) NaCI (s) NaCI (aq) KMgCI2(H2O)6 (s) 6H2O (I) + KCI (s) + MgCI2 (aq) The non-decomposed coarse residue after carnallite decomposition (i.e. the rinneite) is separated from the Mg brine and recirculated to the ball mill circuit where it is ground to -0.4mm and added to the rinneite decomposition circuit to recover the KCI content associated with the rinneite phase. Solid KCI and NaCI is recovered by solid-liquid separation from the two decomposition circuits and is then sent to a hot leach and crystallization circuits to ultimately recover final KCI and NaCI products (1.06x105 kg / hour 1.06x105 kg / hour of KCI and 1.28x105 kg / hour 1.28x105 kg / hour of NaCI). The two-step process generates a magnesium-rich brine and a separate iron-rich brine from which products of value can be obtained. The magnesium-rich brine is processed to produce struvite (a magnesium ammonium phosphate fertilizer) (9.1 x104 kg / hour) according to the following reaction scheme: MgCI2 + NH3 + (NH4)2HPO4 Mg(NH4)PO4 (s) + 2NH4CI 5 The iron-rich brine is processed to produce vivianite (a ferrous phosphate fertilizer) (2.0x1 o4 kg / hour) according to the following reaction scheme: 3FeCI2 (aq) + 2NH3 (aq) + 2(NH4)2HPO4 (aq) Fe3(PO4)2 (s) + 6NH4CI (aq) The struvite (Mg(NH4)PO4.6H2O) and vivianite (Fe3(PO4)2.8H2O) products are recovered in their hydrated forms 10 Finally, the ammonium chloride is removed from the two brine solutions as a further saleable by-product. The brines, now stripped of their contained Mg and Fe but retaining any K, are recycled back into the processing circuit, and re-used.

Claims

1. A method for the processing of mixed evaporite minerals, wherein the mixed evaporite minerals comprise a first evaporite mineral comprising an alkaline earth metal and a second evaporite mineral comprising iron, wherein at least one of the mixed evaporite minerals comprises potassium, wherein the method comprises the steps of:(a) providing particles of the mixed evaporite minerals, wherein the particles have a particle size of at least 0.25 mm;(b) adding the particles from step (a) to an aqueous liquid and selectively dissolving and / or decomposing the first evaporite mineral in the aqueous liquid;(c) separating particles of the second evaporite mineral from the aqueous liquid to provide a first brine comprising the alkaline earth metal and solid particles of the second evaporite mineral;wherein the ratio of the alkaline earth metal to the iron in the first brine obtained in step (c) is higher than in the particles of the mixed evaporite minerals provided in step (a).

2. A method according to claim 1, , preferably wherein the alkaline earth metal is magnesium.

3. A method according to claim 1 or claim 2, wherein the first evaporite mineral is selected from carnallite, langbeinite, schoenite, kainite, polyhalite, kieserite or bischofite, preferably wherein the first evaporite mineral is carnallite4. A method according to any preceding claim, wherein the first evaporite mineral comprises potassium and magnesium.

5. A method according to any preceding claim, wherein the alkaline earth metal is non-magnetic.

6. A method according to any preceding claim, wherein the mixed evaporite minerals further comprise sylvite.

7. A method according to claim 1, wherein one or more salts precipitate in step b), and step (c) further comprises separating the one or more precipitated salts from the first brine.

8. A method according to claim 7, wherein the one or more precipitated salts comprise a potassium salt and optionally a sodium salt, preferably wherein the potassium salt is potassium chloride, preferably wherein the sodium salt is sodium chloride.

9. A method according to claim 7 or claim 8, further comprising separating the one or more precipitated salts from the second evaporite mineral by particle size classification.

10. A method according to claim 8 or claim 9, further comprising recovering a potassium salt from the one or more precipitated salts, optionally wherein the step of recovering a potassium salt comprises a hot leach step and / or a crystallisation step.

11. A method according to any preceding claim, wherein step (a) comprises classifying mixed evaporite minerals by particle size to provide said particles having a particle size of at least 0.25 mm as a coarse fraction and a fine fraction having particle size less than the coarse fraction.

12. A method according to any preceding claim, wherein the particles of the mixed evaporite minerals in step (a) have a particle size of at least 0.5 mm, or at least 1 mm, or at least 1.5 mm, or at least 2 mm, or at least 3 mm, or at least 4 mm, or at least 5 mm, or at least 6 mm.

13. A method according to any preceding claim, wherein the second evaporite mineral is magnetic.

14. The method according to claim 13, wherein the second evaporite mineral comprises potassium.

15. A method according to claim 14, wherein the second evaporite mineral is selected from rinneite, erythrosiderite and douglasite, preferably wherein the second evaporite mineral is rinneite.

16. A method according to any of claims 13 to 15, further comprising the steps of:(d) providing a fine fraction of mixed evaporite minerals, wherein the fine fraction has a particle size below 6 mm;(e) separating the first evaporite mineral and second evaporite mineral in the fine fraction using a magnetic separator to form a magnetic fine fraction and a non-magnetic fine fraction.

17. A method according to claim 16, wherein the fine fraction has a particle size below 5 mm, or below 4 mm, or below 3 mm, or below 2 mm, or below 1.5 mm, or below 1 mm, or below 0.5 mm, or below 0.25 mm.

18. A method according to claim 16 or claim 17, wherein the fine fraction has a particle size below that of the particles in step (a).

19. A method according to claim 16, wherein step (d) further comprises comminuting the particles in the fine fraction prior to magnetic separation.

20. A method according to any of claims 16 to 19, further comprising the step of:(f) dissolving and / or decomposing the particles of the magnetic fraction in an aqueous liquid to form a second brine comprising iron, wherein the ratio of the alkaline earth metal to the iron in the second brine is lower than in the fine fraction of mixed evaporite particles.

21. A method according to claim 20, wherein step (f) further comprises separating one or more precipitated salts from the second brine.

22. A method according to claim 21, wherein the one or more precipitated salts comprise a potassium salt and optionally a sodium salt, preferably wherein the potassium salt is potassium chloride, preferably wherein the sodium salt is sodium chloride.

23. A method according to claim 22, further comprising recovering a potassium salt from the one or more precipitated salts, optionally wherein the step of recovering a potassium salt comprises a hot leach step and / or a crystallisation step.

24. A method according to any of claims 19 to 23, further comprising comminuting the particles of the second evaporite mineral and optionally the precipitated salt obtained from step (c) and adding the comminuted particles to the fine fraction prior to step (d) or to the magnetic fine fraction obtained from step (d).

25. A method according to any of claims 19 to 24, further comprising adding the non-magnetic fine fraction to the aqueous liquid in step (b).

26. A method according to any preceding claim, further comprising the step of(g) recovering a salt of the alkaline earth metal from the first brine to provide a third brine.

27. A method according to claim 26, wherein step (g) comprises the addition of at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the first brine.

28. A method according to claim 27, wherein the ammonium salt is ammonium hydroxide and / or wherein the phosphate salt is selected from monoammonium phosphate, diammonium phosphate, and triammonium phosphate.

29. A method according to any of claims 26 to 28, wherein the salt recovered in step (f) is selected from a magnesium phosphate and a magnesium ammonium phosphate.

30. A method according to claim 29, wherein the salt recovered in step (g) is selected from struvite, K-struvite, dittmarite, newberyite or bobierrite.

31. A method according to any of claims 26 to 30, wherein the third brine is recycled and used as the aqueous liquid in step (b).

32. A method according to any preceding claim, further comprising the step of(h) recovering a salt of the iron from the second brine to provide a fourth brine.

33. A method according to claim 32, wherein step (h) comprises the addition of at least one of ammonia, aqueous ammonia solution, an ammonium salt, phosphoric acid and a phosphate salt to the second brine.

34. A method according to claim 33, wherein the ammonium salt is ammonium hydroxide and / or wherein the phosphate salt is selected from monoammonium phosphate, diammonium phosphate, and triammonium phosphate.

35. A method according to any of claims 32 to 34, wherein the salt recovered in step (h) is selected from an iron phosphate, an iron ammonium phosphate and an iron magnesium phosphate.

36. A method according to claim 35, wherein the salt recovered in step (h) is vivianite.

37. A method according to any of claims 32 to 36, wherein the fourth brine is recycled and used as the aqueous liquid in step (e).