A leach circuit and a process for the recovery of nickel and cobalt from laterite ores

Abstract

There is disclosed a leaching circuit for treating a mineral ore to produce at least one target metal in solution comprising that includes at least two ore leaching steps, at least a first of which comprises a pressure leach step configured to operate at above ambient pressure and temperature, and at least one counter current solid-liquid flow, not including any apparatus or process that is not directly involved in the leaching process, between the two ore leaching steps. The invention also extends to a method for treating a mineral ore in such a leaching circuit.

Description

[0001] A LEACH CIRCUIT AND A PROCESS FOR THE RECOVERY OF NICKEL AND COBALT FROM LATERITE ORES

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the acid leaching of metals from ores. More particularly, the present invention relates to the recovery of the metals nickel and cobalt from lateritic ores.

[0004] BACKGROUND TO THE INVENTION

[0005] The application of various leaching techniques to the recovery of base metals from laterite ores is a well-developed field of mineral processing. Base metals in the present context include nickel, cobalt, iron, zinc, copper, aluminium, magnesium, manganese, and chromium.

[0006] Laterite (from the Latin word "later"' meaning brick or tile) is a surface geological formation often found in hot and wet tropical areas, which is enriched in iron and aluminium which is developed by intensive and long lasting weathering of the underlying parent rock. Nickel laterites are commonly formed as a result of the laterisation of nickel bearing silicate rocks such as olivine. Laterite ores can be broadly classified by their iron and silica or magnesium contents into two types, namely saprolite and limonite.

[0007] There are various methods in use today for treating nickel laterite process ores and the method to produce Nickel Pig Iron (NPI) is perhaps the simplest and most recent development. Pioneered in China, it essentially involves treating high grade limonite-type or lower grade saprolite-type laterite ores in a blast furnace or submerged arc furnace to produce a Ni-bearing pig iron that can be used in the manufacture of certain lower grade stainless steels. Sulphur can also be added to the process to produce a nickel sulphide matte that can be further processed to produce nickel metal or other products such as nickel sulphate. The NPI process is probably the most polluting of all the Nickel process routes. The more conventional ferro-nickel process also known as the RKEF (Rotary Kiln Electric Furnace) process is another pyro-metallurgical process that is generally used in the treatment of higher grades of saprolite. It produces a superior grade nickel-iron product to nickel pig iron that is used in the manufacture of higher grade of stainless steels.

[0008] Another example is the Caron process, which is a combination of pyrometallurgical and hydrometallurgical technologies to produce nickel metal. This process is now largely considered to be inferior in relation to other available processes.

[0009] There are also more exotic processes under development that are based on leaching with hydrochloric or nitric acid.

[0010] The remaining current technologies are hydrometallurgical processes that use sulphuric acid and are based on autoclave leaching, tank leaching or heap leaching or combinations thereof. It is in relation to these hydrometallurgical processes that the current invention is most relevant. All of these processes consume large amounts of sulphuric acid. A typical flow sheet for a sulphuric acid-based leach plant is given in Figure 1 .

[0011] With reference to Figure 1 , mined laterite ore is first crushed and ground to an optimised size (comminution) as determined by metallurgical testing. Beneficiation of the ore may also take place within or ahead of this comminution circuit, represented by the first block in Figure 1 , to improve the nickel grade of the feed to the leach circuit, which is represented by the second block in Figure 1.

[0012] The solution produced by leaching is commonly referred to as the Pregnant Leach Solution, or “PLS”. In addition to the nickel and cobalt values present in the PLS, there are also other unwanted elements such as iron and aluminium that have leached into the PLS. This is also represented by the second block in Figure 1 . There is also a significant quantity of unreacted sulphuric acid (H2SO4), which is added to the leaching step, remaining in the solution after leaching. With reference to Figure 1 , limestone is first added to the PLS to neutralise the unreacted sulphuric acid that remains in the PLS. This is represented by the third block in Figure 1 .

[0013] Further limestone is then added to precipitate especially iron and aluminium amongst other unwanted elements from the PLS. This is represented by the fourth block in Figure 1.

[0014] The solid and solution products resulting from the acid neutralisation (blocks 3 and 4 in Figure 1 ) and the iron and aluminium precipitation (block 4 in Figure 1 ) are then separated from each other by a process of counter current decantation. The solids comprise a mixture of calcium sulphate, iron hydroxide, aluminium hydroxide, and other precipitates, and these solids are discarded as waste. The PLS then contains the leached nickel and cobalt values as well as remnant iron and aluminium and other species, notably magnesium, manganese and calcium, lead and zinc.

[0015] The next step in the process, which is represented by the fifth block in Figure 1 , is to precipitate out of the PLS the nickel and cobalt and leave most of the other elements behind in the PLS. The nickel and cobalt are commonly precipitated from the solution using Hydrogen Sulphide gas to produce a nickel cobalt mixed sulphide product, or alternatively lime, magnesia, soda ash or caustic soda is used to produce a mixed hydroxide product. These products can be either shipped elsewhere for treatment or further refined on site.

[0016] The wastewater from this nickel cobalt precipitation step, represented by the fifth block in Figure 1 , can be partially recycled into the upstream circuit. However, because of the buildup of magnesium and other metals in the circuit it is necessary to incorporate a large bleed from this stream to be dealt with outside of the main flowsheet. Where the net evaporation rate will allow, this waste solution can be pumped into evaporation ponds to dry, but this method is becoming less and less popular as it can adversely affect the environment. Where it is not possible or desirable to use this approach the more common method for treating this solution is to precipitate the magnesium from solution with lime after which the water can be re-used, albeit saturated with calcium. This still occurs in the fifth block in Figure 1 .

[0017] The further steps in the process, represented by the sixth and final blocks in Figure 1 , include downstream processing of the precipitated nickel and cobalt, to provide nickel and cobalt products as offtake.

[0018] Hydrometallurgical options for the leaching of nickel laterite ores come down to the following sulphate-based processes.

[0019] High Pressure Acid Leaching: HPAL is the most common hydrometallurgical process for low-grade laterite ores. It utilises expensive high tech autoclave technology presently used to process limonite and mixed limonite / saprolite laterite ores with a magnesium content of up to about 5% Mg. Nickel extraction is high, typically >90%. Cobalt recovery is also high and is a valuable byproduct, while acid consumption is typically in the range of 300-400kg / t.

[0020] Enhanced Pressure Acid Leaching: EPAL is a combination of HPAL autoclaves and Atmospheric Tank Leach (AL) technologies and was commercialised by BHP Biliton at the Ravensthorpe operation in Western Australia. It is a hybrid process aimed at processing both limonite and saprolite ore types. Nickel and cobalt extractions are typically >90%, and acid consumption is typically in the range of 400-600 kg / t ore.

[0021] Heap Leaching (HL): Significant effort has been invested by numerous organizations into the development of sulphuric acid heap leaching as a potentially lower capital cost alternative to HPAL. Following pilot testing at Qaldag, in Turkey by European Nickel, Heap Leaching is presently being developed by Brazilian Nickel in Northern Brazil. Compared with HPAL, EPAL and AL, Heap Leaching yields lower nickel and cobalt extractions, typically 70- 80%, and higher acid consumptions in the range of 500-700 kg / t ore, but with lower capital costs and less engineering complexity. Atmospheric Tank Leaching (ATL): Despite the higher capital cost and higher acid consumption, ATL offers certain advantages over HL because of higher extraction, smaller footprint, and a shorter ramp up time. Nickel and cobalt extractions are typically around 90% and acid consumption is typically in the range of 700-900 kg / t ore, depending on mineralogy, ore grade and the particular process concept adopted. Only one single stage ATL facility treating saprolite has been commercialized, that being a satellite to the Ravensthorpe EPAL operation.

[0022] All of the above laterite leach technologies suffer from the same problems. These include that the leach solutions that they produce contain iron, aluminium, and sulphuric acid, and these must be removed from the solution during the downstream refining process, which is achieved by the addition of limestone. The limestone reacts with the iron to produce a gelatinous iron hydroxide, gypsum, and carbon dioxide. The limestone also reacts with the sulphuric acid to produce more gypsum and more carbon dioxide. The combined waste products from these operations in terms of either tonnes or volume are greater than the original ore mined, and due to the gelatinous nature of the iron precipitate it is difficult to dewater and safely dispose of. In addition, the production of so much carbon dioxide means that the carbon emission resulting from the limestone addition is also a significant negative issue.

[0023] There is a need for a laterite leaching process that will produce a leach solution (a Pregnant Leach Solution or PLS) that is comparably low in both iron, aluminium, and sulphuric acid content, while maintaining a high recovery of the metal values.

[0024] It is further required that such a process should have the following characteristics:

[0025] • Lower operating costs

[0026] • Reduced CO2 emissions,

[0027] • Less waste product

[0028] • An easier to manage waste product Achieving these outcomes using industry standard equipment and operating procedures.

[0029] In this specification the following terms have the following meanings:

[0030] • “Counter-current” flow means the movement of solid and solution between two leaching steps being caused to flow in opposite directions relative to each other. For example, in a system with two leaching steps, the product of leach step 1 is dewatered to produce a solid and solution, and the solution leaves the leach circuit, while the solid is transported to leach step 2. The product of leach step 2 is then dewatered and also produces a solid and a solution, and the solid then leaves the leach circuit, while the solution is transported back to leach step 1. The movement of the solid and solution between leach steps 1 and 2 is in opposite directions which is referred to as counter-current. The same is also true if the movement of the solid and the solution were to be reversed. This movement may be achieved by any of the various means of dewatering and liquid, slurry or solids transport available such as CCDs, thickeners, filters etc, and pumps and launders etc.;

[0031] • “Solid” refers to fresh, leached or partially leached ore, usually in combination with solution in the form of a slurry, pulp, filter cake, or similar;

[0032] • ’’Solution” refers to fresh and or recycled water, which may or may not contain dissolved solids, industrial chemicals and reagents such as acids and flocculants, as well as colloidal or suspended solids as might be expected in an industrial operation;

[0033] • “LPAL” means a low-pressure acid leach process where “low” refers to pressures above the ambient (atmospheric) and below those typically associated with HPAL (40bar and above); and “HPAL” means a high-pressure acid leach process that typically involves utilising pressures of 40bar and above.

[0034] OBJECTIVE OF THE INVENTION

[0035] It is an objective of the invention to provide a process for the recovery of nickel and cobalt from laterite ores which at least partly overcomes the abovementioned problems.

[0036] SUMMARY OF THE INVENTION

[0037] In accordance with this invention there is provided a leaching circuit for treating a mineral ore to produce at least one target metal in solution comprising that includes at least two ore leaching steps, at least a first of which comprises a pressure leach step configured to operate at above ambient pressure and temperature, and at least one counter current solid-liquid flow, not including any apparatus or process that is not directly involved in the leaching process, between the two ore leaching steps; with the first ore leaching step - being configured to receive an intermediate PLS which includes unconsumed acid from the second ore leaching step to leach the fresh ore, and producing after at least partial dewatering by means of solid / liquid separation a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the second ore leaching step; with the second ore leaching step - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step, and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the second ore leaching step, and producing after dewatering by means of solid / liquid separation waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the first ore leaching step.

[0038] There is further provided for the pressure leach step to comprise an LPAL or an HPAL process.

[0039] There is still further provided for the leaching circuit to include at least two counter current solid-liquid flows between the ore leaching steps, not including counter current flows from any apparatus that is not directly involved in a leaching step.

[0040] There is still further provided for the leaching circuit to include one or more alternative leaching steps selected, without limitation, from the group comprising heap leaching, vat leaching or any form of elevated pressure or elevated temperature leaching, and any other suitable form of leaching for the ore.

[0041] There is still further provided for the ore to include any one or more of a laterite, oxide or sulphide ore.

[0042] According to a further aspect of the invention there is provided for the leaching circuit to include three ore leaching steps, a first of which comprises the pressure leach step, a second of which comprises a primary tank leach and a third of which comprises a secondary tank leach; with the first leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the primary tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the primary tank leach; with the primary tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS which includes unconsumed acid from the secondary leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary tank leach; with the secondary tank leach - receiving partially leached ore and at least partly dewatered solids from the primary tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary tank leach tank, and producing after dewatering waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the primary tank leach.

[0043] According to a yet further aspect of the invention there is provided for the leaching circuit to include three ore leaching steps, a first of which comprises a primary pressure leach step, a second of which comprises a tank leach and a third of which comprises a secondary pressure leach step; with the first ore leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the tank leach; with the tank leachreceiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS which includes unconsumed acid from the secondary pressure leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary pressure leach step; with the secondary pressure leach step - receiving partially leached ore and at least partly dewatered solids from the tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary pressure leach step, and producing after dewatering waste solids as an offtake and an intermediate PLS which includes unconsumed acid for the tank leach.

[0044] According to a further aspect of the invention there is provided a method for treating a mineral ore in a leaching circuit as defined above comprising the steps of: feeding at least one fresh mineral ore that includes at least one target metal to the first ore leaching step of the circuit to be leached in the first ore leaching step and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the second ore leaching step, feeding partially leached ore and at least partly dewatered solids from the first ore leaching step and leach solution comprising dilutant, preferably water, and excess acid to the second ore leaching step to be leached in the second ore leaching step, feeding an intermediate PLS which includes unconsumed acid from the second ore leaching step to join mineral ore being fed into the first ore leaching step of the circuit to be leached in the first ore leaching step, and producing after dewatering waste solids as a circuit offtake from the second ore leaching step and final PLS containing target metal values from the pressure leach step. There is further provided for the method to include providing at least two counter current solid-liquid flows between the ore leaching steps, not including counter current flows from any apparatus that is not directly involved in a leaching step.

[0045] There is further provided a method for treating a mineral ore in a leaching circuit as defined above that includes three ore leaching steps, a first of which comprises a primary pressure leach step, a second of which comprises a tank leach and a third of which comprises a secondary pressure leach step, comprising the steps of: in the first leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the primary tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the primary tank leach; in the primary tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS which includes unconsumed acid from the secondary leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary tank leach; and in the secondary tank leach - receiving partially leached ore and at least partly dewatered solids from the primary tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary tank leach, and producing after dewatering waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the primary tank leach. There is further provided a method for treating a mineral ore in a leaching circuit as defined above that includes three ore leaching steps, a first of which comprises a primary pressure leach step, a second of which comprises a tank leach and a third of which comprises a secondary pressure leach step; in the first ore leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the tank leach; in the tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS which includes unconsumed acid from the secondary pressure leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary pressure leach step; and in the secondary pressure leach step - receiving partially leached ore and at least partly dewatered solids from the tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary pressure leach step, and producing after dewatering waste solids as an offtake and an intermediate PLS which includes unconsumed acid for the tank leach. There is further provided for the method to include splitting the ore into at least two streams before directing it through the leach circuit, preferably as shown in Figure 4.

[0046] There is still further provided for the method to include splitting the ore by one or more means including but not limited to, a simple mass split, sizing, gravity separation, magnetic separation, froth flotation, or other appropriate means.

[0047] There is also provided for the PLS from the leach circuit to be processed in a circuit including sulphuric acid and magnesium recycling, preferably as shown in Figure 5.

[0048] There is still further provided for the method to preferably include one or more alternative leaching processing steps, including but not limited to Heap Leaching, Vat Leaching or any form of elevated pressure or elevated temperature leaching, and any other applicable form of leaching for the ore.

[0049] There is still further provided for the mineral ore to be any one or more of a laterite, oxide or sulphide ore.

[0050] There is also provided for the ore to be split to contain different grades of magnesium.

[0051] There is also provided for the ore to be split according to characteristics of differing consumption rates of sulphuric acid per unit mass.

[0052] There is also provided for the method to include utilising fractions of the products of splitting elsewhere in the process to optimise the process efficiency.

[0053] There is also provided for the method to include recovering magnesium leached during the process as magnesium sulphate, and preferably to be crystallised from the solution to produce magnesium sulphate crystals. There is also provided for the method to include the step of roasting the magnesium sulphate crystals recovered from the process to produce a gas comprising sulphur dioxide and or sulphur trioxide, and magnesium oxide in solid form.

[0054] There is still further provided for the method to include the step of using the gas comprising sulphur dioxide and or sulphur trioxide to manufacture sulphuric acid.

[0055] There is also provided for the method to include the step of using the magnesium oxide as a precipitant in the post leach processing of the PLS.

[0056] There is still further provided for the method to include the step of adding sulphur to the roasting process to produce a gas comprising sulphur dioxide and or sulphur trioxide, and magnesium oxide in solid form.

[0057] There is still further provided for the method to include the step of adding a carbon-based reducing agent, including any one or more of the group containing carbon, carbon monoxide, natural gas or other gas liquid or solid containing carbon, hydrogen or any other suitable reducing agent to the roasting process.

[0058] BRIEF DESCRIPTION OF THE DRAWINGS

[0059] A preferred embodiment of the invention is described by way of example only and with reference to the accompanying drawings in which:

[0060] Figure 1 is a flow diagram showing a typical flow sheet for a prior art sulphuric acid-based leach plant;

[0061] Figure 2 is a flow diagram showing a flow sheet of a first embodiment of leach plant according to the invention;

[0062] Figure 3A is a flow diagram showing a flow sheet of a second embodiment of leach plant according to the invention; Figure 3B is a flow diagram showing a flow sheet of a third embodiment of leach plant according to the invention, similar to the second embodiment but including a second pressure leach step to remove iron and aluminium from the circuit as a separate product;

[0063] Figure 4 is a flow diagram showing a flow sheet of a fourth embodiment of leach plant according to the invention; and

[0064] Figure 5 is a flow diagram showing a flow sheet of a fifth embodiment of leach plant according to the invention.

[0065] DETAILED DESCRIPTION OF THE INVENTION

[0066] Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings.

[0067] The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

[0068] The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

[0069] Unless specified or limited otherwise, the terms "mounted", "connected", "engaged" and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings and are thus intended to include direct connections between two members without any other members interposed therebetween and indirect connections between members in which one or more other members are interposed therebetween. Further, "connected" and "engaged" are not restricted to physical or mechanical connections or couplings. Additionally, the words "lower", "upper", "upward", "down" and "downward" designate directions in the drawings to which reference is made.

[0070] The terminology includes the words specifically mentioned above, derivatives thereof, and words or similar import. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent.

[0071] As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items.

[0072] These objectives of the invention are to be realised by the adoption of a novel and inventive leach circuit.

[0073] In order to achieve the stated intentions in accordance with the present invention, a leaching circuit is provided within which at least one counter current solid-liquid flows can be identified, not including any apparatus that is not directly involved in the leaching process such as counter current decantation or other means of thickening or dewatering, pumping or means of transporting or processing solids and / or solutions.

[0074] A first embodiment of the invention is shown in the flowsheet in Figure 2. In this first example of the invention, the whole (fresh) ore is combined with the dewatered Intermediate PLS (Pregnant Leach Solution) from a Tank Leach step, before entering a LPAL (Low Pressure Acid Leach) step. The LPAL in this embodiment of the invention operates at a temperature of approximately 160 degrees Celsius and is a low-cost unit in terms of both capital and operation in comparison to a HPAL (High Pressure Acid Leach), which commonly operates in a range of 240 to 270 degrees Celsius. After solid / liquid separation the intermediate PLS from the Tank Leach step contains ferric iron and aluminium plus nickel and cobalt and other elements and compounds including sulphuric acid. The ferric iron and aluminium in the intermediate PLS will be precipitated from the leach solution mainly as hematite and alunite, and in so doing more sulphuric acid will be released into the solution.

[0075] The more easily leached components of the whole (fresh) ore, such as lizardite and other serpentines, will react and consume remnant acid in the intermediate PLS. The sulphuric acid produced by the precipitation of the iron and aluminium from solution will also be consumed by these components of the ore until such time as the iron and aluminium levels in the ore, and the sulphuric acid levels in the PLS no longer react together at an effective rate.

[0076] The final PLS thus produced is therefore low in iron, aluminium, and sulphuric acid, a result which has not been achieved elsewhere in the leaching of nickel laterite ores with sulphuric acid.

[0077] It is intended that the autoclave be operated with the emphasis on minimising the levels of iron, aluminium, and sulphuric acid in the PLS at the expense of nickel and cobalt extraction from the ore to the PLS. Efficient extraction of these values is achieved in the Tank Leach section of the circuit.

[0078] The part leached ore from the LPAL, having been separated from the PLS in the LPAL solid / liquid separation section of the flowsheet, is combined with fresh solution (water and acid) before entering the Tank Leach step. Sulphuric acid, forming part of the fresh solution, is added either before or during the Tank Leach. In this embodiment of the invention, the temperature in the tank leach is maintained in the region of 95 to 100 degrees Celsius. It is comprised of agitated tanks in series, in keeping with common tank leach design. The Tank Leach step is operated with the emphasis on nickel and cobalt recovery. Within reason, excess acid can be added to this part of the circuit in the knowledge that it will be consumed in the LPAL section of the invention. Ore that is leached to the maximum extent possible in the circuit is expelled as solid waste from the circuit following dewatering after the Tank Leach step.

[0079] It will be appreciated that the solids that are being leached, and the solution that the metals are being leached into, are caused to travel counter current between the means of leaching. In Figure 2 the two means of leaching are a Tank Leach and a LPAL. In other embodiments of the invention, the tank leach might be wholly or partly replaced or extended by the inclusion of alternative means of leaching, for example, an autoclave leaching step.

[0080] There may be other counter current flows within the leach circuit such as a counter current decantation facility that is used for separating the solids and the solution, or other means of thickening or dewatering, pumping or means of transporting or processing solids and / or solutions and these may facilitate the counter current flow between the means of leaching (the tank leach and LPAL).

[0081] Excluding those ancillary apparatus, there is at least the two means of ore leaching between which at least one counter current solid-liquid flow is included At least one of the means of ore leaching operates at above ambient pressure, and the final PLS leaving the leach circuit is derived from the discharge of this above ambient pressure means of leaching by a process of solid / liquid separation.

[0082] A second embodiment of the invention is given in Figure 3A, where the main difference in comparison to Figure 2 is the addition of a second counter current circuit. This second circuit is nested within the first counter current circuit in the Tank Leach. The counter current flow is achieved by splitting the single Tank Leach into two stages that are referred to in Figure 3A as the Primary and Secondary Tank Leaches.

[0083] The flowsheet in Figure 3A is considered to be more efficient than that in Figure 2. The ore in combination with the intermediate PLS from the Primary Tank Leach enters the LPAL autoclave where, as explained for the single counter current circuit in Figure 2, the acid in the intermediate PLS and the acid formed by the precipitation of the iron and the aluminium, is consumed by the more easily leached components of the ore.

[0084] The part leached ore from the LPAL is combined with the Intermediate PLS from the Secondary Tank Leach before it enters the Primary Tank Leach where it continues to leach by virtue of the excess acid available in the solution and so lowers sulphuric acid content of the intermediate PLS for the LPAL.

[0085] In effect, the neutralisation of the excess acid that was being performed in the LPAL in Figure 2, has been shifted in part or wholly to the Primary Tank Leach.

[0086] The now twice leached ore, after dewatering, is combined with fresh solution for leaching in the Secondary Tank Leach. At this stage of the process, it is only the harder to leach components of the ore that remain. Therefore most, or all of the sulphuric acid used in the overall leaching circuit is being added to the point where it is most needed and where its strength is not diluted through leaching the easier to leach components. In this way a high overall nickel and cobalt extraction can be expected while still achieving an overall low acid consumption and a high purity PLS.

[0087] In other embodiments of the invention, the primary or secondary tank leach might be wholly or partly replaced or extended by the inclusion of alternative means of leaching, for example, an autoclave leaching step.

[0088] A third embodiment of the invention is given in Figure 3B, where the main difference in comparison to Figure 3A is the addition of a second pressure leach step (Secondary LPAL) instead of the Secondary Tank Leach. Such a configuration might be considered beneficial, for example, where the ore is high in iron, or where there is a build-up of iron in the circuit. In Figure 3B the Secondary LPAL is the final stage of ore leaching in this circuit and plays a dual role of both leaching the harder to leach iron minerals, mainly goethite, and precipitating the leached iron as hematite. The hematite is then discarded from the circuit as waste in the ensuing dewatering step.

[0089] By precipitating and ejecting much of the leached iron from the circuit at this point, rather than leaving it in the solution to precipitated later in the Primary LPAL, this duty is therefore largely removed from the Primary LPAL, and its focus is shifted to leaching the easier to leach ores using the remnant acid in the PLS and any further acid it produces through the precipitation of the now much lower amounts of iron and aluminium.

[0090] A fourth embodiment of the invention is given in Figure 4. In this instance, the ore is split into two parts, preferably to produce easier and harder to leach fractions of the ore. Such a split can often be achieved with laterite ores during mining, where for example, limonite and saprolite can be mined separately. Alternatively, screening of the ore into two fractions, with or without the need for crushing, will often produce the desired result. Other ore sorting treatments are also available for this purpose and would be known to one conversant with the art. These might include separations by gravity or conductivity or even colour or other physical or chemical signature.

[0091] This embodiment of the invention is considered to be the most efficient of the three presented thus far. In the flowsheet shown in Figure 4 the mined ore is split into two fractions discussed earlier. Stream 1 has a low magnesium content consistent with a harder to leach limonite ore, and stream 2 has a higher magnesium content consistent with an easier to leach saprolite or mixed saprolite / limonite ore or transitional ore. Splitting the ore in this way allows each ore type to be directed through the circuit in the manner best suited to its leach characteristics. The leach circuit itself however remains in essence the same as that presented in Figure 3. Both streams, while taking separate paths through the leach circuit, are still subject to two stages of counter current leaching.

[0092] Stream 1 , the harder to leach ore fraction, is first leached in the Primary Tank Leach where it neutralises much of the excess sulphuric acid that was added to the Secondary Tank Leach. After which it reports to the Secondary Tank Leach where it is subject to much stronger leaching conditions to ensure efficient recovery of the nickel and cobalt values.

[0093] Stream 2, the easier to leach ore fraction, is first leached in the LPAL where it neutralises any remnant acid left over from the Primary Tank Leaching process, plus the acid that is produced during the precipitation of the iron and aluminium.

[0094] Stream 2 then joins stream 1 in the Secondary Tank Leach.

[0095] It is envisaged that in other embodiments of the invention, acid may be added in lesser amounts elsewhere in order to trim the acid levels and optimise the process. Similarly, it is envisaged that portions or fractions of ore may be diverted to trim the acid levels elsewhere in the circuit. In fact, there are several embodiments of the invention that might be considered by one conversant with the art having first been introduced to the invention.

[0096] In other embodiments of the invention, the tank leach might be wholly or partly replaced or extended by the inclusion of alternative means of leaching, for example, an autoclave leaching step.

[0097] Note that in this embodiment of the invention only a fraction of the mass of the whole ore, here referred to as the “Coarse” fraction, passes through the LPAL autoclave, thus keeping the operating and capital costs low.

[0098] It is believed that this invention provides a low cost means of increasing the capacity of an existing autoclave-based operation such as a HPAL or an EPAL type operation.

[0099] A fifth embodiment of the invention is given in Figure 5, in which acid that would normally be lost to the circuit due to the addition of limestone and lime, as shown in Figure 1 , is recycled.

[0100] In this fifth embodiment of the invention, the mined ore is screened into coarse and fine fractions before being ground to an appropriate size for leaching, as determined by metallurgical testing. The leach circuit is the same as that presented in Figure 4, with counter current decantation (CCD) used to dewater the leached ore from the Tank Leach, and a single stage of thickening used to produce the final PLS. As per Figure 1 , the PLS reports to the iron and aluminium precipitation section of the circuit. Instead of using limestone for this purpose however, magnesium oxide (MgO) that has been prepared on site is used. Nickel and cobalt are then precipitated from the PLS by a further addition of MgO to produce a mixed hydroxide product (MHP) that will either sent off site or processed further.

[0101] The main elements left in the barren PLS solution will now be magnesium and lesser amounts of manganese. The manganese can be removed from the solution through the addition of further MgO and or by other means and can be further processed into to a saleable product. The relatively pure magnesium sulphate solution that remains is then concentrated and crystallised from the solution before drying and roasting to produce sulphur dioxide for acid production, and the MgO that will be recycled back into the circuit as a precipitant.

[0102] The first to fourth embodiments shown in Figures 2 to 5 include steps relating to ‘dewatering’ and ‘thickening’ of the leach residue. It will be appreciated that this dewatering constitutes solid / liquid separation, which may be achieved by any suitable manner of solid / liquid separation.

[0103] It is important that the final leach process prior to the PLS leaving the leach circuit is performed at a pressure above ambient in order to achieve a required purity. The exact pressure will be determined empirically for each application, but it is expected to be above 5 bar(g).

[0104] In respect of the process of the invention, in all of the embodiments, it is important to note that the ore leach step which receives acid in excess of the stoichiometric requirement for its leaching is intended to result in an intermediate PLS that includes unconsumed acid, which is directed counter current to a previous ore leaching step. In the other leaching steps, which do not receive acid in excess of their stoichiometric requirement, there may also be some unconsumed acid, but this is coincidental despite the aim of producing a PLS (final or intermediate) that has zero acid from such steps. The acid in such steps may vary according to prevailing conditions, and the process will invariably be controlled in a manner designed to minimize that. That is the opposite of the ore leach step which receives acid in excess of the stoichiometric requirement, where the intention is to manage the process to create an intermediate PLS that includes unconsumed acid.

[0105] The above-described processes according to the invention are capable of being operated with lower operating costs, reduced CO2 emissions, produces less waste product and easier to manage waste product, and all this using industry standard equipment and operating procedures.

[0106] By way of example, the process of the invention is expected to consume between 50 and 100 kg / t less acid than prior art processes, which is a vast improvement over prior art processes.

[0107] It will be appreciated that the embodiments described above are given by way of example only and are not intended to limit the scope of the invention. It is possible to alter aspects of the embodiments without departing from the essence of the invention.

Claims

CLAIMS1 . A leaching circuit for treating a mineral ore to produce at least one target metal in solution comprising that includes at least two ore leaching steps, at least a first of which comprises a pressure leach step configured to operate at above ambient pressure and temperature, and at least one counter current solid-liquid flow, not including any apparatus or process that is not directly involved in the leaching process, between the two ore leaching steps; with the first ore leaching step - being configured to receive an intermediate PLS from the second ore leaching step to leach the fresh ore, and producing after at least partial dewatering by means of solid / liquid separation a final PLS as a circuit offtake, and producing partly leached ore and at least partly dewatered solids for the second ore leaching step; with the second ore leaching step - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step, and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the second ore leaching step, and leaching the partially leached ore and at least partly dewatered solids and producing after dewatering by means of solid / liquid separation waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the first ore leaching step.

2. A leaching circuit as claimed in claim 1 in which the pressure leach step comprises a low-pressure autoclave or an HPAL.

3. A leaching circuit as claimed in claim 1 which includes at least two counter current solid-liquid flows between the ore leaching steps, not including counter current flows from any apparatus that is not directly involved in a leaching step.

4. A leaching circuit as claimed in claim 1 that includes three ore leaching steps, a first of which comprises the pressure leach step, a second of which comprises a primary tank leach and a third of which comprises a secondary tank leach; with the first leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the primary tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the primary tank leach; with the primary tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS from the secondary leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary tank leach; and with the secondary tank leach - receiving partially leached ore and at least partly dewatered solids from the primary tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary tank leach, and producing after dewatering waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the primary tank leach.

5. A leaching circuit as claimed in claim 1 that includes three ore leaching steps, a first of which comprises a primary pressure leach step, a second of which comprises a tank leach and a third of which comprises a secondary pressure leach step;with the first ore leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the tank leach; with the tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS from the secondary pressure leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary pressure leach step; and with the secondary pressure leach step - receiving partially leached ore and at least partly dewatered solids from the tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary pressure leach step, and producing after dewatering waste solids as an offtake and an intermediate PLS which includes unconsumed acid for the tank leach.

6. A leaching circuit as claimed in any of the preceding claims that include one or more alternative leaching steps selected, without limitation, from the group comprising heap leaching, vat leaching or any form of elevated pressure or elevated temperature leaching, and any other suitable form of leaching for the ore.

7. A leaching circuit as claimed in any of the preceding claims in which the ore include any one or more of a laterite, oxide or sulphide ore.

8. A leaching circuit as claimed in claim 1 and illustrated in Figure 4 that is configured for the ore to be split into at least two streams and directing it through the leach circuit.

9. A leaching circuit as claimed in claim 8 in which the ore is split to contain different grades of magnesium.

10. A leaching circuit as claimed in in claim 8 in which the ore is split according to characteristics of differing consumption rates of sulphuric acid per unit mass.11 . A leaching circuit as claimed in in claim 8 in which the ore is split by one or more means including but not limited to, a simple mass split, sizing, gravity separation, magnetic separation, froth flotation, or other appropriate means.

12. A leaching circuit as claimed in any one of claims 1 to 11 which is configured to recycle Fe and / or Al precipitate back into the leach circuit operatively to at least partly neutralise solid waste and reduce nickel and cobalt losses to the waste solids.

13. A leaching circuit as claimed in any one of claims 1 to 12 which is configured for the PLS from the leach circuit to be processed in a circuit including sulphuric acid and magnesium recycling.

14. A method for treating a mineral ore in a leaching circuit as claimed in claim 1 comprising the steps of: feeding at least one fresh mineral ore that includes at least one target metal to the first ore leaching step of the circuit to be leached in the first ore leaching step, and producing after at least partial dewatering by means of solid / liquid separation a final PLS as a circuit offtake, and producing partly leached ore and at least partly dewatered solids for the second ore leaching step, feeding partially leached ore and at least partly dewatered solids from the first ore leaching step and leaching solution that includes acid in excess of the stoichiometric requirement for leaching in the secondore leaching step, leaching the partially leached ore and at least partly dewatered solids and in the second ore leaching step, producing and feeding an intermediate PLS which includes unconsumed acid from the second ore leaching step to join mineral ore being fed into the first ore leaching step of the circuit to be leached in the first ore leaching step, producing after dewatering by means of solid / liquid separation waste solids as a circuit offtake from the second ore leaching step, and final PLS containing target metal values from the first ore leaching step.

15. A method for treating a mineral ore as claimed in claim 14 which includes directing at least two counter current solid-liquid flows between the ore leaching steps, not including counter current flows from any apparatus that is not directly involved in a leaching step.

16. A method for treating a mineral ore in a leaching circuit as claimed in claim 4 which includes the steps of: in the first leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the primary tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the primary tank leach in the primary tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS from the secondary leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary tank leach; and in the secondary tank leach -receiving partially leached ore and at least partly dewatered solids from the primary tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary tank leach, and producing after dewatering waste solids as a circuit offtake and an intermediate PLS which includes unconsumed acid for the primary tank leach.

17. A method for treating a mineral ore in a leaching circuit as claimed in claim 5 which includes the steps of: in the first ore leaching step - receiving and leaching fresh ore and receiving an intermediate PLS from the tank leach, and producing after at least partial dewatering a final PLS as a circuit offtake and partly leached ore and at least partly dewatered solids for the tank leach; in the tank leach - receiving partially leached ore and at least partly dewatered solids from the first ore leaching step and an intermediate PLS from the secondary pressure leach step following dewatering of its offtake, and producing after dewatering an intermediate PLS for the first ore leaching step and partly leached ore and at least partly dewatered solids for the secondary pressure leach step; and in the secondary pressure leach step - receiving partially leached ore and at least partly dewatered solids from the tank leach following dewatering of its offtake and leaching solution that includes acid in excess of the stoichiometric requirement for the leaching in the secondary pressure leach step, andproducing after dewatering waste solids as an offtake and an intermediate PLS which includes unconsumed acid for the tank leach.

18. A method for treating a mineral ore as claimed in any one of claims 14 to 17 which includes the step of splitting the ore into at least two streams before directing it through the leach circuit.

19. A method for treating a mineral ore as claimed in claim 18 which includes splitting the ore by one or more means including but not limited to, a simple mass split, sizing, gravity separation, magnetic separation, froth flotation, or other appropriate means.

20. A method for treating a mineral ore as claimed in any one of claims 18 and 19 which includes the step of utilising fractions of products of ore splitting elsewhere in the process to optimise the process efficiency.

21. A method for treating a mineral ore as claimed in any one of claims 14 to 20 which includes recovering magnesium leached during the process as magnesium sulphate, and preferably to be crystallised from the solution to produce magnesium sulphate crystals.

22. A method for treating a mineral ore as claimed in any one of claims 14 to 21 which includes the step of roasting the magnesium sulphate crystals recovered from the process to produce a gas comprising sulphur dioxide and or sulphur trioxide, and magnesium oxide in solid form.

23. A method for treating a mineral ore as claimed in any one of claims 14 to 22 which includes the step of using the gas comprising sulphur dioxide and or sulphur trioxide to manufacture sulphuric acid.

24. A method for treating a mineral ore as claimed in any one of claims 14 to 23 which includes the step of using the magnesium oxide as a precipitant in the post leach processing of the PLS.

25. A method for treating a mineral ore as claimed in any one of claims 22 to 24 which includes the step of adding sulphur to the roasting process to produce a gas comprising sulphur dioxide and or sulphur trioxide, and magnesium oxide in solid form.

26. A method for treating a mineral ore as claimed in any one of claims 22 to 25 which includes the step of adding a carbon-based reducing agent, including any one or more of the group containing carbon, carbon monoxide, natural gas or other gas liquid or solid containing carbon, hydrogen or any other suitable reducing agent to the roasting process.

27. A method for treating a mineral ore as claimed in any one of claims 14 to 26 which includes the step of recycling Fe and / or Al precipitate back into the leach circuit operatively at least partly neutralising solid waste and reducing nickel and cobalt losses to the waste solids.

28. A leaching circuit for treating a mineral ore to produce at least one target metal in solution substantially as herein described and with reference to Fig 2, 3A, 3B, 4 or 5.

29. A method for treating a mineral ore in a leaching circuit to produce at least one target metal in solution substantially as herein described and with reference to Fig 2, 3A, 3B, 4 or 5.

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