Graphite material purification process

Abstract

The present invention relates to an improved graphite material purification process, the process comprising the steps of: (i) Passing a concentrate of a graphite material to be purified to an oxidative leach stage; (ii) Passing a product of the oxidative leach stage of step (i) to a solid liquid separation stage producing a liquid product and a solid product; (iii) Passing the solid product of the solid liquid separation stage of step (ii) to a caustic leach stage; (iv) Passing a product of the caustic leach stage of step (iii) to a solid liquid separation stage producing a liquid product and a solid product; (v) Passing the solid product of the solid liquid separation stage of step (iv) to an acid leach stage; and (vi) Passing a product of the acid leach stage of step (v) to a solid liquid separation stage producing a liquid product and a solid product, wherein the solid product comprises a purified graphite material.

Description

GRAPHITE MATERIAL PURIFICATION PROCESSField of the Invention

[0001] The present invention relates to an improved graphite material purification process.

[0002] More particularly, the graphite material purification process of the present invention is intended to be applicable to the purification of low graphite content materials.

[0003] In particular, the graphite material purification process of the present invention is intended for the production of graphite material that is of a purity suitable for use in the production of lithium-ion batteries (LiBs).Background Art

[0004] Graphite that is to be utilised in the production of lithium-ion batteries (LiBs) is required to be highly pure, for example greater than or equal to about 99.95% Loss on Ignition (LOI). This requirement presents challenges to producing suitable graphite product from material with relatively low graphite content, for example 60 to 70% Cg, and in which there are relatively high levels of silicate, sulphide, titanium and base metal minerals. These challenges are heightened if the contaminant minerals are highly disseminated throughout the graphite ore and it is desired to reduce or minimise carbon losses.

[0005] Presently employed methods for the purification of graphite for use in the production of LiBs employ particularly aggressive acid leaching steps, typically employing a leach step requiring high concentrations and volumes of hydrofluoric acid. This brings significant environmental and occupational health and safety concerns. These methods also have low levels of carbon recovery, for example below about 85%. Further, these methods generally also start with graphite materials with greater or equal to 90% carbon content.

[0006] There would be significant advantage and benefit to providing an improved graphite material purification process that reduced carbon losses and / or reduced reliance on large volumes of highly concentrated acids and / or did not require the use of hydrofluoric acid.

[0007] The graphite material purification process and product of the present invention have as one object thereof to overcome substantially one or more of the abovementioned problems associated with prior art processes, or to at least provide a useful alternative thereto.

[0008] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. This discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0009] Throughout the specification and claims, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

[0010] It is to be understood that the ranges provided herein include the stated range and any value or sub-range within the stated range. For example, a range from about 1 micrometer (pm) to about 2 pm should be interpreted to include not only the explicitly recited limits of from about 1 pm to about 2 pm, but also to include individual values, such as about 1 .2 pm, about 1 .5 pm, about 1 .8 pm, etc., and sub-ranges, such as from about 1.1 pm to about 1 .9 pm, from about 1 .25 pm to about 1.75 pm, etc. Furthermore, when “about” and / or “substantially” are / is utilised to describe a value, they are meant to encompass minor variations (up to + / - 10%) from the stated value.

[0011] It is further to be understood that references to the % recovery or % removal of an element or mineral, or similar, are, unless the context demands otherwise, a reference to the % of that component recovered or removed relative to the original content of the feed to the described process. Other references to %are, again as long as the context does not require otherwise, to be taken as references to weight or wt % (rather than volume or v %).Disclosure of the Invention

[0012] In accordance with the present invention there is provided an improved graphite material purification process, the process comprising the steps of:(i) Passing a concentrate of a graphite material to be purified to an oxidative leach stage;(ii) Passing a product of the oxidative leach stage of step (i) to a solid liquid separation stage producing a liquid product and a solid product;(iii) Passing the solid product of the solid liquid separation stage of step (ii) to a caustic leach stage;(iv) Passing a product of the caustic leach stage of step (iii) to a solid liquid separation stage producing a liquid product and a solid product;(v) Passing the solid product of the solid liquid separation stage of step (iv) to an acid leach stage; and(vi) Passing a product of the acid leach stage of step (v) to a solid liquid separation stage producing a liquid product and a solid product, wherein the solid product comprises a purified graphite material.

[0013] Preferably, sulphide minerals and at least a portion of acid soluble minerals are solubilised in the oxidative leach stage (i).

[0014] Silica minerals are preferably solubilised in the caustic leach step (iii). Additional alkaline soluble impurities are also preferably solubilised in the caustic leach step (iii).

[0015] Preferably, impurity minerals partially leached in the caustic leach stage (iii) are solubilised in the acid leach stage (v).

[0016] In one form of the present invention, the lixiviant of acid leach stage (v) is selected from hydrochloric acid or sulphuric acid. Preferably, the lixiviant of acid leach stage (v) is hydrochloric acid.

[0017] Preferably, the solid product of the solid liquid separation stage of step (vi) is further passed to one or a number of re-pulp filtration stages (vii) whereby a proportion of remaining impurities are removed from the solid product.

[0018] Still preferably, the repulped and filtered solid product of step (vii) is passed to a drying stage (viii) to provide the purified graphite material.

[0019] Still further preferably, the purified graphite material in step (vi) is classified in or after the drying step. The purified material of step (vii) is preferably classified in the drying stage (viii) into multiple products having different particle size and surface area properties, for example into at least three fractions. In one form the classification utilises dry powder methods, for example cyclone classification.

[0020] In one form, the oxidative leach stage of step (i) comprises a first oxidative leach portion and a second oxidative leach portion. Preferably, the first oxidative leach portion is operated under conditions in which sulphide minerals present are oxidised to sulphates. Preferably, the second oxidative leach portion is operated under conditions in which sulphates are leached into solution. More preferably, the second oxidative leach portion is operated under conditions in which, if present, iron, magnesium and potassium minerals are leached.

[0021] Preferably, the first oxidative leach portion operates:(i) with a target redox potential of > about 425 mV (versus Ag / AgCI);(ii) with a pH of > 10;(iii) with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w;(iv) at a temperature of between about 30 to 60oC; and / or(v) with a residence time of between about 30 to 60 minutes.

[0022] Still preferably, the target redox potential is:(i) in the range of about 800 to 1200 mV (versus Ag / AgCI); or(ii) in the range of about 950 to 1000 mV (versus Ag / AgCI).

[0023] Preferably, at least one of sodium hypochlorite or sodium chlorate is utilised as the oxidant in the first oxidative leach portion of the oxidative leach stage of step (i).

[0024] Preferably, sodium hypochlorite is added based on:(i) about 125 to 200% of the stoichiometric amount based on sulphide sulphur content;(ii) a mole ratio of 7:2; and / or(iii) 50 to 150 kg / t feed material based on sulphide sulphur content.

[0025] In one form of the present invention, the first oxidative leach portion of the oxidative leach stage of step (i) comprises an initial pH adjustment.Preferably, NaOH is added to adjust the pH. Still preferably, NaOH is added in the range of about 10 to 50 kg / t feed, dependent upon the amount of sulphide sulphur in the feed. Yet still preferably, NaOH is added in the range of about 20 to 25 kg / t feed.

[0026] The oxidative leach stage of step (i) further comprises a second oxidative leach portion in which, if present, iron, magnesium and potassium minerals are leached. Preferably, hydrochloric acid is utilised as the lixiviant inthe second oxidative leach portion. Preferably, at least a portion of the hydrochloric acid utilised as the lixiviant in the second oxidative leach portion is recycled from elsewhere in the process.

[0027] The second oxidative leach portion preferably operates:(i) at a temperature of about 40 to 10OoC;(ii) over multiple leach stages;(iii) with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w;(iv) with a residence time in the range of about 2 to 6 hours;(v) with an HCI addition rate of between about 150 to 350 kg / t feed; and / or(vi) with a background HCI acidity of between about 30 to 60 g / L.

[0028] Still preferably, the second oxidative leach portion operates:(i) with a residence time of between about 2 to 4 hours; and / or(ii) with an HCI addition rate of about 320 kg / t feed.

[0029] Preferably, aluminium and / or calcium minerals are at least partially leached in the second oxidative leach portion of the oxidative leach stage of step (')■

[0030] In one form, the caustic leach stage of step (iii) comprises a two-stage leach.

[0031] Preferably, the caustic leach stage of step (iii) operates:(i) in a counter-current manner;(ii) under pressure;(iii) at a temperature of between about 100 to 220oC;(iv) with a starting slurry density of between about 150 to 250 g / L solids;(v) with a total residence time of between about 2 to 16 hours; and / or(vi) with a background NaOH concentration of between about 100 to 500 g / L.

[0032] Still preferably, the caustic leach stage of step (iii) further operates:(i) at a pressure of about 2 bar; and / or(ii) with a background NaOH concentration of about 350 g / L.

[0033] Preferably, caustic containing leachate from the caustic leach stage of step (iii) is recycled to a first caustic contact of the caustic leach stage.

[0034] In one form, the caustic containing leachate from the caustic leach stage of step (iii) is preferably recycled by way of a caustic regeneration step to which lime and / or hydrated / slaked lime is fed, and from which regenerated caustic is recovered and a residue of silicates and aluminium produced.

[0035] In embodiments where the lixiviant of acid leach stage (v) is hydrochloric acid, the hydrochloric acid leach stage of step (v) is conducted:(i) in multiple stages;(ii) at a temperature of between about 60 to 10OoC;(iii) with a slurry density of between about 15 to 35% w / w, for example about 20% w / w;(iv) with a total residence time of between about 2 to 6 hours; and / or(v) with a background HCI acidity of between about 30 to 60 g / L.

[0036] Preferably, the hydrochloric acid leach stage of step (v) is conducted:(i) at a temperature of about 80oC; and / or(ii) with a total residence time of between about 2 to 4 hours.

[0037] The hydrochloric acid is preferably added to the hydrochloric acid leach stage of step (v) at a rate of between about 150 to 350 kg / t feed to acidify residual alkalinity. The rate of addition of hydrochloric acid to the hydrochloric acid leach stage of step (v) is preferably undertaken with consideration of background acid requirements.

[0038] In embodiments where the lixiviant of acid leach stage (v) is sulphuric acid, the sulphuric acid leach stage of step (v) is conducted:(i) in multiple stages;(ii) at ambient temperature;(iii) with a slurry density of between about 15 to 35% w / w, for example about 20% w / w;(iv) with a total residence time of between about 2 to 6 hours; and / or(v) with a background free acid concentration of between about 25 to 50 g / L H2SO4.

[0039] The process of the present invention further comprises a first effluent treatment, or brine treatment, stage in which liquid products from the oxidative leach stage of step (i), the caustic leach stage of step (iii), and the acid leach stage of step (v) are combined. Preferably, the acidity of the liquid products of steps (i) and (v) are neutralised at least in part by the caustic leach stage products.

[0040] Preferably, base metals present in the first effluent treatment stage are hydrolysed therein.

[0041] In one form, the first effluent treatment stage comprises three reactors operating at a temperature of about 60 to 80oC and at about pH 6.5, pH 10.5 and pH 12, respectively, with the addition of an alkali. The added alkali is preferably lime or hydrated lime.

[0042] Preferably, sodium sulphate is added to the first effluent treatment stage, whereby calcium is precipitated as insoluble gypsum and soluble sodium chloride formed.

[0043] Preferably, the total residence time in the three reactors of the first effluent treatment stage is about 2 to 4 hours. Still preferably, from the reactors a slurry is passed to a solid liquid separation step and a dewatering step.

[0044] A reagent recovery step, including a crystallisation stage, is preferably provided and to which a liquid product of the slurry solid liquid separation step is passed. Alkali and alkaline earth components are preferably removed in the crystallisation stage through the addition of sodium carbonate, with or without additional reagents.

[0045] The crystallisation stage preferably evaporates water and increases the sodium chloride concentration to between about 250 to 350 g / L. Preferably, a mixed salt product is crystallised in the crystallisation stage. Still preferably, the crystallisation stage is operated with one or more evaporative multi-effect evaporators.

[0046] Concentrated sodium chloride solution containing crystallised solids from the crystallisation stage is passed to a solid liquid separation step to clarify and concentrate the solution. The solid liquid separation step may preferably be conducted using a centrifuge or a press filter. Preferably, the sodium chloride solution is about 27.5 wt% NaCL

[0047] Preferably, the clarified and concentrated sodium chloride solution is passed to a chlor-alkali electrolysis step, a further component of the reagentrecovery step. Sodium hydroxide, hydrochloric acid and sodium hypochlorite reagents are preferably amongst the reagents recovered.

[0048] A depleted anolyte from the chlor-alkali electrolysis step and reagent recovery step is preferably recycled to the crystallisation step whereby the sodium chloride content is replenished. A caustic catholyte is preferably recycled to the caustic leach stage of step (iii). Preferably, the caustic catholyte is concentrated in an evaporation step before being recycled to the caustic leach stage of step (iii). A chlorine gas generated at anodes of the chlor-alkali electrolysis step and reagent recovery step is captured and used to regenerate to hydrochloric acid and sodium hypochlorite for reuse in oxidative leach stage of step (i) and the caustic leach stage of step (iii).

[0049] In one form of the present invention, the graphite material to be purified is first passed to a size reduction step, for example grinding or milling, to provide a fine ground or milled material to be purified.

[0050] In accordance with the present invention there is further provided a purified graphite material product produced in accordance with the improved graphite material purification process described hereinabove.Brief Description of the Drawings

[0051] The present invention will now be described, by way of example only, with reference to two embodiments thereof and the accompanying drawings, in which:-Figure 1 is a schematic representation of a flow chart depicting an improved graphitic material purification process in accordance with a first embodiment of the present invention; andFigure 2 is a schematic representation of a flow chart depicting an improved graphitic material purification process in accordance with a second embodiment of the present invention.Best Mode(s) for Carrying Out the Invention

[0052] The present invention provides an improved graphite material purification process, the process comprising the steps of:(i) Passing a concentrate of a graphite material to be purified to an oxidative leach stage;(ii) Passing a product of the oxidative leach stage of step (i) to a solid liquid separation stage producing a liquid product and a solid product;(iii) Passing the solid product of the solid liquid separation stage of step (ii) to a caustic leach stage;(iv) Passing a product of the caustic leach stage of step (iii) to a solid liquid separation stage producing a liquid product and a solid product;(v) Passing the solid product of the solid liquid separation stage of step (iv) to an acid leach stage; and(vi) Passing a product of the acid leach stage of step (v) to a solid liquid separation stage producing a liquid product and a solid product, wherein the solid product comprises a purified graphite material.

[0053] The sulphide minerals and at least a portion of acid soluble minerals are solubilised in the oxidative leach stage (i), whilst silica minerals are solubilised in the caustic leach step (iii). Additional alkaline soluble impurities are also solubilised in the caustic leach step (iii). Impurity minerals partially leached in the caustic leach step (iii) are solubilised in the acid leach stage (v).

[0054] The acid leach stage (v) is a leach in hydrochloric acid or sulphuric acid. In accordance with a first embodiment of the present invention the acid leach stage (v) is a leach in hydrochloric acid, whilst in accordance with a secondembodiment of the present invention the acid leach stage (v) is a sulphuric acid leach.

[0055] The solid product of the solid liquid separation stage of step (vi) is further passed to one or a number of re-pulp filtration stages (vii) whereby a proportion of remaining impurities are removed from the solid product. The repulped and filtered solid product of step (vii) is passed to a drying stage (viii) to provide the purified graphite material.

[0056] The purified graphite material in step (vi) is classified in or after the drying step. The purified material of stage (vii) is classified in the drying stage (viii) into multiple products having different particle size and surface area properties, for example into at least three fractions. In one form the classification utilises dry powder methods, for example cyclone classification.

[0057] In one form, the oxidative leach stage of step (i) comprises a first oxidative leach portion in which sulphide minerals present are oxidised to sulphate. The first oxidative leach portion operates:(i) with a target redox potential of > about 425 mV (versus Ag / AgCI);(ii) with a pH of > 10;(iii) with a slurry density of between about 15 to 35% w / w, for example about 20% w / w;(iv) at a temperature of between about 30 to 60oC; and / or(v) with a residence time of between about 30 to 60 minutes.

[0058] The target redox potential is:(i) in the range of about 800 to 1200 mV (versus Ag / AgCI); or(ii) in the range of about 950 to 1000 mV (versus Ag / AgCI).

[0059] At least one of sodium hypochlorite or sodium chlorate is utilised as the oxidant in the first oxidative leach portion of the oxidative leach stage of step (i). Added sodium hypochlorite is based on:(i) about 125 to 200% of the stoichiometric amount based on sulphide sulphur content;(ii) a mole ratio of 7:2; and / or(iii) 50 to 150 kg / t feed material based on sulphide sulphur content.

[0060] Adjustment of the pH occurs first in the first oxidative leach portion of the oxidative leach stage of step (i). NaOH is added to adjust the pH, in the range of about 10 to 50 kg / t feed, for example in the range of about 20 to 25 kg / t feed, dependent upon the amount of sulphide sulphur in the feed.

[0061] The oxidative leach stage of step (i) further comprises a second oxidative leach portion in which, if present, iron, magnesium and potassium minerals are leached. Hydrochloric acid is utilised as the lixiviant in the second oxidative leach portion. At least a portion of the hydrochloric acid utilised as the lixiviant in the second oxidative leach portion is recycled from elsewhere in the process.

[0062] The second oxidative leach portion operates:(i) at a temperature of about 40 to 10OoC;(ii) over multiple leach stages;(iii) with a slurry density of between about 15 to 35% w / w, for example about 20% w / w;(iv) with a residence time in the range of about 2 to 6 hours;(v) with an HCI addition rate of between about 150 to 350 kg / t feed; and / or(vi) with a background HCI acidity of between about 30 to 60 g / L.

[0063] The second oxidative leach portion operates:(i) with a residence time of between about 2 to 4 hours; and / or(ii) with an HCI addition rate of about 320 kg / t feed.

[0064] Aluminium and / or calcium minerals are at least partially leached in the second oxidative leach portion of the oxidative leach stage of step (i).

[0065] In one form, the caustic leach stage of step (iii) comprises a two-stage leach. The caustic leach stage of step (iii) operates:(i) in a counter-current manner;(ii) under pressure;(iii) at a temperature of between about 100 to 220oC;(iv) with a starting slurry density of between about 150 to 250 g / L solids;(v) with a total residence time of between about 2 to 16 hours; and / or(vi) with a background NaOH concentration of between about 100 to 500 g / L.

[0066] For example, the caustic leach stage of step (iii) further operates:(i) at a pressure of about 2 bar; and / or(ii) with a background NaOH concentration of about 350 g / L.

[0067] Caustic containing leachate from the caustic leach stage of step (iii) is recycled to a first caustic contact of the caustic leach stage. For example, the caustic containing leachate from the caustic leach stage of step (iii) is recycled byway of a caustic regeneration step to which lime and / or hydrated / slaked lime is fed, and from which regenerated caustic is recovered and a residue of silicates and aluminium produced.

[0068] The hydrochloric acid leach stage of step (v) is conducted:(i) in multiple stages;(ii) at a temperature of between about 60 to 10OoC;(iii) with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w;(iv) with a total residence time of between about 2 to 6 hours; and / or(v) with a background HCI acidity of between about 30 to 60 g / L.

[0069] For example, the hydrochloric acid leach stage of step (v) is conducted:(i) at a temperature of about 80oC; and / or(ii) with a total residence time of between about 2 to 4 hours.

[0070] The hydrochloric acid is added to the hydrochloric acid leach stage of step (v) at a rate of between about 150 to 350 kg / t feed to acidify residual alkalinity. The rate of addition of hydrochloric acid to the hydrochloric acid leach stage of step (v) is undertaken with consideration of background acid requirements.

[0071] The process of the present invention further comprises a first effluent treatment, or brine treatment, stage in which liquid products from the oxidative leach stage of step (i), the caustic leach stage of step (iii), and the acid leach stage of step (v) are combined. The acidity of the liquid products of steps (i) and (v) are neutralised at least in part by the caustic leach stage products. Base metals present in the first effluent treatment stage are hydrolysed therein.

[0072] The first effluent treatment stage may comprise three reactors operating at a temperature of about 60 to 8O0C and at about pH 6.5, pH 10.5 and pH 12, respectively, with the addition of an alkali. The added alkali is, for example, lime or hydrated lime. Sodium sulphate is added to the first effluent treatment stage, whereby calcium is precipitated as insoluble gypsum and soluble sodium chloride formed. The total residence time in the three reactors of the first effluent treatment stage is about 2 to 4 hours. A slurry is passed from the reactors to a solid liquid separation step and a dewatering step.

[0073] A reagent recovery step, including a crystallisation stage, is provided and to which a liquid product of the slurry solid liquid separation step is passed. Alkali and alkaline earth components are removed in the crystallisation stage through the addition of sodium carbonate, with or without additional reagents.

[0074] The crystallisation stage evaporates water and increases the sodium chloride concentration to between about 250 to 350 g / L. A mixed salt product is crystallised in the crystallisation stage. The crystallisation stage is operated, for example, with one or more evaporative multi-effect evaporators.

[0075] Concentrated sodium chloride solution containing crystallised solids from the crystallisation stage is passed to a solid liquid separation step to clarify and concentrate the solution. The solid liquid separation step may for example be conducted using a centrifuge or a press filter. The sodium chloride solution is about 27.5 wt% NaCL

[0076] The clarified and concentrated sodium chloride solution is passed to a chlor-alkali electrolysis step, a further component of the reagent recovery step. Sodium hydroxide, hydrochloric acid and sodium hypochlorite reagents are amongst the reagents recovered.

[0077] A depleted anolyte from the chlor-alkali electrolysis step and reagent recovery step is recycled to the crystallisation step whereby the sodium chloride content is replenished. A caustic catholyte is recycled to the caustic leach stage of step (iii). The caustic catholyte is concentrated in an evaporation step before being recycled to the caustic leach stage of step (iii). A chlorine gas generated atanodes of the chlor-alkali electrolysis step and reagent recovery step is captured and used to regenerate to hydrochloric acid and sodium hypochlorite for reuse in oxidative leach stage of step (i) and the caustic leach stage of step (iii).

[0078] The present invention further provides a purified graphite material product produced in accordance with the improved graphite material purification process described hereinabove.

[0079] In Figure 1 there is shown an improved graphite material purification process 10 in accordance with a first and preferred embodiment of the present invention.

[0080] The improved graphite material purification process 10 comprises the steps of:(i) Passing a concentrate 12 of a graphite material to be purified to an oxidative leach stage 14, for example an oxidative muriatic leach;(ii) Passing a product of the oxidative leach stage 14 of step (i) to a solid liquid separation stage (not shown) producing a liquid product 16 and a solid product 18;(iii) Passing the solid product 18 of the solid liquid separation stage of step (ii) to a caustic leach stage 20;(iv) Passing a product of the caustic leach stage 20 of step (iii) to a solid liquid separation stage (not shown) producing a liquid product 22 and a solid product 24;(v) Passing the solid product 24 of the solid liquid separation stage of step (iv) to an acid leach stage 26, for example a leach in hydrochloric acid; and(vi) Passing a product of the acid leach stage 26 of step (v) to a solid liquid separation stage (not shown) producing a liquid product 28and a solid product 30, wherein the solid product 30 comprises a purified graphite material.

[0081] The solid product 30 of the solid liquid separation stage of step (vi) is further passed to a re-pulp filtration stage (vii) 32, whereby a proportion of remaining impurities are removed from the solid product 30, providing a repulped and filtered solid product 34 and a liquid product 36. The repulped and filtered solid product 34 is passed to a drying stage (viii) 38 to provide a further purified graphite material 40.

[0082] The further purified graphite material 40 is classified in or after the drying stage (viii) 38. The solid product 34 from the re-pulp filtration stage (vii) 32 is classified in the drying stage 38 into multiple products having different particle size and surface area properties, for example into at least three fractions. In one form the classification utilises dry powder methods, for example cyclone classification.

[0083] In one embodiment, the graphite material concentrate is a natural graphite. In an alternative embodiment, the graphite material concentrate is a synthetic graphite.

[0084] In one embodiment, the natural graphite concentrate has a carbon content of at least 70 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of at least 75 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of at least 80 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of at least 85 %w / w.

[0085] In one embodiment, the natural graphite concentrate has a carbon content of between 70 %w / w and 95 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 75 %w / w and 95 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 80 %w / w and 95 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 85 %w / w and 95 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 70 %w / w and 90 %w / w. In one embodiment, the natural graphite concentrate has acarbon content of between 75 %w / w and 90 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 80 %w / w and 90 %w / w. In one embodiment, the natural graphite concentrate has a carbon content of between 85 %w / w and 90 %w / w.

[0086] In one embodiment, the method further comprises the beneficiation of a graphite material (not shown) to prepare the graphite material concentrate 12. Beneficiation can include any methods known in the art to remove gangue materials and increase the graphite concentration. Suitable beneficiation steps can include one or more of ore sorting, coarse and fine grinding for gangue liberation, froth flotation, screening and classification, and magnetic separation (LIMS / WHIMS).

[0087] The graphite material concentrate 12 is repulped, for example using recycled process water, and adjusted to a pH in the range of 10-11 with caustic soda ahead of a first oxidative leach portion of the oxidative leach stage 14 of step (i). The oxidative leach stage 14 is operated under conditions in which sulphide minerals present are oxidised to sulphate and solubilised. In a preferred embodiment, oxidative leach stage 14 comprises an oxidative acid leach.Oxidative leach stage 14 will further leach at least a portion of other acid soluble minerals present in the graphite concentrate. Sodium hypochlorite is utilised as the oxidant in the first oxidative leach portion. In one embodiment, the oxidative leach stage 14 does not comprise the contact of the graphite material concentrate with hydrofluoric acid.

[0088] In embodiment, the graphite material concentrate is not subjected to a caustic baking step prior to the oxidative leach stage 14. The inventors have found that the oxidative leach stage 14 can replace energy intensive caustic bake steps used in the prior art to assist with the solubilisation of silicates.

[0089] In the embodiment shown in Figure 1 , the oxidative leach stage 14 of step (i) comprises a first oxidative leach portion and a second oxidative leach portion. The first oxidative leach portion is operated under conditions in which sulphide minerals present are oxidised to sulphate. In a preferred embodiment, the first oxidative leach portion comprises contact of the graphite concentrate withan oxidant. The first oxidative leach portion operates with a target redox potential of > about 425 mV (versus Ag / AgCI), with a pH of > 10, with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w, at a temperature of between about 30 to 60°C, and with a residence time of between about 30 to 60 minutes. The target redox potential is in the range of about 800 to 1200 mV (versus Ag / AgCI), for example in the range of about 950 to 1000 mV (versus Ag / AgCI).

[0090] In one embodiment, at least one of sodium hypochlorite or sodium chlorate, or a mixture thereof, is utilised as the oxidant in the first oxidative leach portion of the oxidative leach stage of step (i). Added sodium hypochlorite is based on about 125 to 200% of the stoichiometric amount based on sulphide sulphur content, a mole ratio of 7:2, and / or 50 to 150 kg / t feed material based on sulphide sulphur content.

[0091] In a preferred embodiment, the first oxidative leach portion of the oxidative leach stage of step (i) comprises an initial pH adjustment. The initial pH adjustment targets a pH of 10-11. NaOH is added to adjust the pH, in the range of about 10 to 50 kg / t feed, for example in the range of about 20 to 25 kg / t feed, dependent upon the amount of sulphide sulphur in the feed. At least a portion of the NaOH utilised may be recycled from elsewhere in the process 10.

[0092] The oxidative leach stage 14 of step (i) further comprises a second oxidative leach portion. In a preferred embodiment, the second oxidative leach portion comprises contacting the graphite concentrate with an acidic lixiviant. The second oxidative leach portion is used to solubilise at least a portion of the sulphates resulting from the first oxidative leach portion. The second oxidative leach portion will further leach at least a portion of the iron, magnesium and potassium minerals present in the concentrated graphite. In one embodiment, hydrochloric acid is utilised as the lixiviant in the second oxidative leach portion. At least a portion of the hydrochloric acid utilised as the lixiviant in the second oxidative leach portion is recycled from elsewhere in the process 10, including brine treatment and reagent recovery steps to be described hereinafter.

[0093] The second oxidative leach portion operates at a temperature of about 40 to 100°C, over multiple leach stages, with a slurry density of between about 15 to 35% w / w, for example about 20% w / w, with a residence time in the range of about 2 to 6 hours, with an HCI addition rate of between about 150 to 350 kg / t feed, and with a background HCI acidity of between about 30 to 60 g / L. For example, the second oxidative leach portion operates with a residence time of between about 2 to 4 hours, and with an HCI addition rate of about 320 kg / t feed. In one embodiment, the second oxidative leach portion does not comprise the contact of the graphite material concentrate with hydrofluoric acid.

[0094] Aluminium and / or calcium minerals are at least partially leached in the second oxidative leach portion of the oxidative leach stage 14 of step (i).

[0095] The caustic leach stage 20 of step (iii) comprises contact of the solid product 18 with a caustic solution. In one embodiment, caustic leach stage 20 is a two-stage leach operated in a counter current manner. The caustic leach stage of step (iii) operates at a temperature of between about 100 to 220°C. In one embodiment, caustic leach stage 20 is operated at low pressure. In this embodiment, the caustic leach stage 20 is operated a pressure of up to 5 bar, for example 2 bar. In an alternative embodiment of the present invention, the caustic leach step 20 is operated under high pressure. In this embodiment, the caustic leach stage 20 is operated a pressure of 20-25 bar. High pressure, high temperature leaching lower residence times and / or required background NaOH concentration. However, lower pressure and temperature leaching is generally less expensive to operate.

[0096] The caustic leach stage 20 of step (iii) operates at a starting slurry density of between about 150 to 250 g / L solids, with a total residence time of between about 2 to 16 hours, and with a background NaOH concentration of between about 100 to 500 g / L. For example, the caustic leach stage of step (iii) further operates at a pressure of about 2 bar or 200 kPa, with a background NaOH concentration of about 350 g / L.

[0097] Caustic containing leachate from the caustic leach stage 20 of step (iii) is recycled to a first caustic contact of the caustic leach stage 20. For example, thecaustic containing leachate from the caustic leach stage of step (iii) is recycled by way of a caustic regeneration step 42 to which lime and / or hydrated / slaked lime is fed, and from which regenerated caustic is recovered and a residue 44 of silicates and aluminium produced.

[0098] Silica minerals are solubilised in the caustic leach stage 20 of step (iii). Additional alkaline soluble impurities are also solubilised in the caustic leach stage 20. Impurity minerals partially leached in the caustic leach stage 20 are solubilised in the acid leach stage 26, to be described hereinafter.

[0099] The hydrochloric acid leach stage 26 of step (v) comprises contact of solid product 24 with a hydrochloric acid solution. In one embodiment, the hydrochloric acid leach stage 26 of step (v) is conducted in multiple stages, at a temperature of between about 60 to 100°C, with a slurry density of between about 15 to 35% w / w, for example about 20% w / w, with a total residence time of between about 2 to 6 hours, with a background HCI acidity of between about 30 to 60 g / L. For example, the hydrochloric acid leach stage 26 of step (v) is conducted at a temperature of between about 60 to 80°C, with a total residence time of between about 2 to 4 hours. In one embodiment, the acid leach stage 26 does not comprise the contact of the graphite material concentrate with hydrofluoric acid.

[0100] The hydrochloric acid is added to the hydrochloric acid leach stage 26 of step (v) at a rate of between about 150 to 500 kg / t feed, for example about 320 kg / t feed. The rate of addition of hydrochloric acid to the hydrochloric acid leach stage of step (v) is undertaken with consideration of background acid requirements.

[0101] The product of the hydrochloric acid leach stage 26 is passed to a solid liquid separation step to separate a solid product 30 that comprises a purified graphite material. The solid product 30 is further passed to a re-pulp filtration stage 32, whereby a proportion of remaining impurities are removed from the solid product 30, providing a repulped and filtered solid product 34 and a liquid product 36. The repulped and filtered solid product 34 is passed to a drying stage (viii) 38 to provide a further purified graphite material 40.

[0102] The purified graphite material 40 preferably has a carbon content of at least 99.9 %w / w. In one embodiment, the purified graphite material 40 preferably has a carbon content of at least 99.92 %w / w. In one embodiment, the purified graphite material 40 preferably has a carbon content of at least 99.93 %w / w. In one embodiment, the purified graphite material 40 preferably has a carbon content of at least 99.95 %w / w. In one embodiment, the purified graphite material 40 preferably has a carbon content of at least 99.95 ± 0.02 %w / w.

[0103] The process 10 of the present invention further comprises a first effluent treatment stage, for example a brine treatment stage 46, in which liquid products from the oxidative leach stage 14 of step (i) the caustic leach stage 20 of step (iii), and the acid leach stage 26 of step (v) are combined. The acidity of the liquid products of steps (i) and (v) are neutralised at least in part by the caustic leach stage products. Base metals present in the first effluent treatment stage are hydrolysed therein.

[0104] The brine treatment stage 46 may comprise three reactors operating at a temperature of about 60 to 80°C and about pH 6.5, pH 10.5 and pH 12, respectively, with the addition of an alkali. The added alkali is, for example, lime or hydrated lime. Sodium sulphate is added to the first effluent treatment stage, whereby calcium is precipitated as insoluble gypsum and soluble sodium chloride formed. Iron is also precipitated at this time. The total residence time in the three reactors of the first effluent treatment stage is about 2 to 4 hours. A slurry is passed from the reactors to a solid liquid separation step and a dewatering step. The precipitates of gypsum and iron are removed as a residue 48 in the solid liquid separation step.

[0105] A reagent recovery step 50, comprising in part a crystallisation stage, is provided and to which a liquid product 52 of the slurry solid liquid separation step is passed. Alkali and alkaline earth components, including calcium carbonate, are removed in the crystallisation stage through the addition of sodium carbonate, with or without additional reagents. The calcium carbonate and any other precipitated components / compounds are ultimately removed as a residue 54.

[0106] The crystallisation stage evaporates water and increases the sodium chloride concentration to between about 250 to 350 g / L. A mixed salt product is crystallised in the crystallisation stage. The crystallisation stage is operated, for example, with one or more evaporative multi-effect evaporators.

[0107] A concentrated sodium chloride solution containing crystallised solids from the crystallisation stage is passed to a solid liquid separation step to clarify and concentrate the solution. The solid liquid separation step may for example be conducted using a centrifuge or a press filter. The concentrated sodium chloride solution is about 27.5 wt% NaCL

[0108] The clarified and concentrated sodium chloride solution is passed to a chlor-alkali electrolysis step, a further component of the reagent recovery step 50. Sodium hydroxide 56, hydrochloric acid 58 and sodium hypochlorite 60 reagents are amongst the reagents recovered and that are then able to be recycled or reused within the process 10.

[0109] A depleted anolyte from the chlor-alkali electrolysis step and reagent recovery step 50 is recycled to the crystallisation step whereby the sodium chloride content is replenished. A caustic catholyte is recycled to the caustic leach stage of step (iii). The caustic catholyte is concentrated in an evaporation step before being recycled to the caustic leach stage 20 of step (iii). A chlorine gas generated at anodes of the chlor-alkali electrolysis step and reagent recovery step is captured and used to regenerate hydrochloric acid and sodium hypochlorite for reuse in oxidative leach stage 14 of step (i) and the caustic leach stage 20 of step (iii).

[0110] The present invention further provides a purified graphite material product produced in accordance with the improved graphite material purification process described hereinabove.

[0111] In Figure 2 there is shown an improved graphite material purification process 100 in accordance with a second embodiment of the present invention. The improved graphite material purification process 100 comprises the steps of:(i) Passing a concentrate 102 of a graphite material to be purified, for example in the form of a moist filter cake, to an oxidative leach stage 104, for example an oxidative muriatic leach;(ii) Passing a product of the oxidative leach stage of step (i) to a solid liquid separation stage (not shown) producing a liquid product 106 and a solid product 108;(iii) Passing the solid product 108 of the solid liquid separation stage of step (ii) to a caustic leach stage 110;(iv) Passing a product of the caustic leach stage 110 of step (iii) to a solid liquid separation stage (not shown) producing a liquid product 112 and a solid product 114;(v) Passing the solid product 114 of the solid liquid separation stage of step (iv) to a sulphuric acid leach stage 116; and(vi) Passing a product of the sulphuric acid leach stage 116 of step (v) to a solid liquid separation stage (not shown) producing a liquid product 118 and a solid product 120, wherein the solid product 120 comprises a purified graphite material.

[0112] The solid product 120 of the solid liquid separation stage of step (vi) is further passed to one or more re-pulp filtration stages (vii) 122 whereby a proportion of any remaining impurities are removed from the solid product 120. A repulped and filtered solid product 124 of step (vii) 122 is passed to a drying stage (viii) 126 to provide a further purified graphite material 128.

[0113] The graphite material concentrate 102 is repulped using recycled process water and adjusted to a pH in the range of 10-11 with caustic soda ahead of a first oxidative leach portion of the oxidative leach stage 104 of step (i), in which sulphide minerals present are oxidised to sulphate. Sodium hypochlorite is utilised as the oxidant in the first oxidative leach portion.

[0114] The first oxidative leach portion operates with a target redox potential of > about 525 mV (versus Ag / AgCI) over multiple stages, for example over two stages. This redox potential is understood to be necessary to oxidise the sulphide minerals present.

[0115] The oxidative leach stage of step (i) further comprises a second oxidative leach portion in which, if present, iron, magnesium and potassium minerals are leached. Hydrochloric acid is utilised as the lixiviant in the second oxidative leach portion. At least a portion of the hydrochloric acid utilised as the lixiviant in the second oxidative leach portion is recycled from elsewhere in the process.

[0116] The second oxidative leach portion preferably operates at a temperature of about 80°C over multiple leach stages, for example over three leach stages, with a background HCI acidity of between about 50 to 100 g / L. This level of acidity is achieved with the addition of fresh HCI. Aluminium and / or calcium minerals are at least partially leached in the second oxidative leach portion of the oxidative leach stage of step (i). The slurry of 20 to 30% w / w solids is then dewatered, for example using plate and frame pressure filter equipment. A HCI rich filtrate liquor is recycled to the oxidative leach stage 104 or is bled from the circuit depending upon the water balance. A filter cake is repulped in water and filtered, for example in a two-stage counter current re-pulp filtration circuit, to remove soluble chloride salts and acid from the solids to protect the caustic leaching circuit. Fresh HCI addition at 33% w / w is added to the leach circuit to account for gangue leaching and losses from the water balance.

[0117] The caustic leach stage of step (iii) comprises a pre-heating portion in which the solid product of step (ii) has sodium hydroxide added thereto, and a leach portion in which gangue silicate minerals and soluble silica and aluminium are leached.

[0118] A moist filter cake from the oxidative leach stage 104 is repulped with recycled caustic soda process liquor to nominally 25 to 35% w / w pulp density. Fresh caustic soda is added targeting 150 to 250 g / L NaOH ahead of advancing the slurry to the pre-heating portion of the caustic leach stage 110, for examplemultiple pre-heating stages that utilise steam vented from the autoclave discharge pressure let down vessels to heat up the feed slurry to the autoclave. Off-gases vented from the slurry pre-heating stages are collected and scrubbed with the scrubber slurry advancing to effluent treatment, to be described hereinafter.

[0119] Pre-heated slurry then advances to the leach portion of the caustic leach stage 110, comprising a multistage autoclave operating at between about 180 to 220oC to leach gangue silicate minerals and soluble silica and aluminium as sodium silicate and sodium aluminate, respectively, over a residence time of about 2 to 4 hours. An autoclave slurry then discharges through, for example, multiple let-down vessels ahead of filtration to separate the solid and liquid phases using, again for example, plate and frame pressure filters. A caustic rich filtrate 130 advances to a caustic regeneration stage (not shown). Solids are repulped in caustic soda to solubilise any precipitated salts that may have formed during autoclave discharge or filtration steps, and are filtered again. These solids are further washed through another water repulp and filtration step before advancing to the sulphuric acid leach stage 116. Caustic repulp filtrate liquors will be concentrated as required based on the water balance, through evaporation, prior to cycle to the autoclave feed repulp stage.

[0120] Dewatered caustic rich leach liquors are then treated with lime slurry carrying 20 to 25% w / w solids at about 80 to 120oC over 2 to 4 hours to precipitate calcium silicate solids and regenerate caustic soda. A resulting slurry is thickened with the overflow solution recycled to the autoclave feed re-pulping stage. Thickened slurry in the underflow stream is filtered, for example on a horizontal belt filter operating with washing to wash and recover the solids.Filtrate and wash liquors are bled from the circuit to manage build-up of soluble silica and other impurities.

[0121] It is envisaged that the caustic leach stage of step (iii) may, in one form of the present invention, operate at a lower pressure and with higher background caustic levels, as is seen described hereinabove with reference to the first embodiment of the present invention.

[0122] Moist filter cake from the caustic leach stage 110 is repulped with recycled sulphuric acid leach filtrate liquor to nominally 15 to 25% w / w pulp density ahead of advancing to the sulphuric acid leach stage 116 of step (v). The sulphuric acid leach stage 116 is conducted at ambient temperature in a number of stages, for example over three stages, with a total residence time of between about 2 to 4 hours. Fresh sulphuric is added to the leach to maintain background free acid in the range of between about 25 to 50 g / L H2SO4.

[0123] A leach discharge is dewatered, for example using conventional plate and frame pressure filter equipment, as the solid liquid separation stage of step (vi). An acid rich filtrate liquor will be recycled to the leach stage 116 or bled from the circuit depending on the water balance.

[0124] The solid product 120 of the dewatered leach discharge, for example as a moist filter cake, is passed to one or more re-pulp filtration stages (vii) 122. The re-pulp occurs in water and the re-pulped solid product is filtered in a filter circuit, for example a two-stage counter current re-pulp filtration circuit, to remove soluble sulphate salts and acid from the solids to meet any specific product requirements and to minimise crystallisation of impurity salts during product drying. The repulp solids density in each stage employed is about 5 to 10% w / w solids to ensure product quality criteria are met.

[0125] The repulped and filtered solid product 124 of step (vii) 122, as a moist filter cake, is passed to the drying stage (viii) 126. The drying stage (viii) 126 employs, for example a flash dryer in which water is removed yielding the further purified graphite material 128 as the final product, the further purified graphite material 128 having for example <0.5% w / w moisture. The dried further purified graphite material 128 is collected, for example using cyclones, and a bag house 132 with a portion recycled to a feed hopper and a cake chopper to facilitate material handling.

[0126] The process of the present invention further comprises a first effluent treatment stage 134 in which liquid products from the oxidative leach stage 104 of step (i) and the caustic leach stage 110 of step (iii) are combined. The acidity of the oxidative leach stage products is neutralised at least in part by the causticleach stage products. Base metals present in the first effluent treatment stage 134 are hydrolysed therein.

[0127] The first effluent treatment stage 134 comprises, for example, three reactors operating at a temperature of about 60 to 80oC and at about pH 6.5, pH 10.5 and pH 12, respectively. The total residence time in the three reactors is about 2 to 4 hours. The raising of the pH acts to hydrolyse base metals.Ferrous or ferric chloride is added to facilitate removal of base metals and improve the dewatering properties of the slurry and requires oxidation with air sparged into the reactors.

[0128] From the reactors a slurry is passed to a solid liquid separation step, for example a thickener, and a dewatering step, for example a filter, operating in series to dewater precipitated solids. Filtrate from the filter is recycled to the thickener and the thickener overflow solution then advances to crystallisation, described hereafter.

[0129] A crystallisation stage 136 is provided in which a neutralised liquid product of the slurry solid liquid separation step is passed thereto. The crystallisation stage 136 is, for example, operated with one or more evaporative multi-effect evaporators and a mixed chloride salt product is crystallised therein. Evaporated water is vented to atmosphere whilst the mixed chloride crystal is dewatered, for example using a centrifuge. The mixed chloride salt comprises >70% NaCI on a dry weight basis. Steam required to evaporate water is, in one form, provided by an electric steam boiler rather than a diesel fired boiler in an effort to avoid use of fossil fuels.

[0130] The process of the present invention further comprises a second effluent treatment stage 138 in which liquid products from the sulphuric acid leach stage 116 of step (v) and the repulp filtration stage 122 are combined.

[0131] In the second effluent treatment stage 138 the combined liquid product carrying predominantly sulphate salts is neutralised, for example with a 25% lime slurry, to a pH of about 12 in multiple reactors, for example in three reactors, at atemperature of between about 30 to 50oC, with a total residence time of between about 2 to 4 hours.

[0132] Ferrous sulphate and sparged air, or in one form ferric sulphate, are added to the reactors of the second effluent treatment stage 138 to facilitate precipitation of base metals and improve dewatering properties of the combined product. A slurry of combined product is then passed to a solid liquid separation stage, with the solid recycled and the liquid forwarded to a sulphate removal stage. In the sulphate removal stage, sodium sulphate and calcium sulphate salts are precipitated as gypsum and barium sulphate, through the sequential addition of calcium chloride and barium chloride. Each stage operates at ambient temperature having a residence time of about 2 hours in each of the two reactors, giving a total circuit residence time of about 4 hours. Slurry carrying low levels of solids are then filtered, for example using a press filter, to recover these solids. Filtrate from the filter is mixed with process water to meet discharge targets of <360 mg / L sulphate.

[0133] The improved graphite material purification process of the present invention is intended to contemplate combinations of the first and second embodiments thereof described hereinabove. For example, it is envisaged that components or features of one embodiment may be utilised in the method of the other embodiment, including in particular it is envisaged that the chlor-alkali electrolysis step and associated integers / features of the first embodiment may be utilised in the process of the second embodiment.

[0134] The present invention further provides a purified graphite material product produced in accordance with either of the embodiments of the improved graphite material purification processes described hereinabove, or in accordance with any combination thereof. For example, as noted immediately above, it is envisaged that components or features of one embodiment may be utilised in the method of the other embodiment, including in particular it is envisaged that the chlor-alkali electrolysis step and associated integers / features of the first embodiment may be utilised in the process of the second embodiment.

[0135] The process of the first embodiment of the present invention may be better understood with reference to the following non-limiting examples.Example - First Embodiment of the Present Invention

[0136] Table 1 provided below illustrates and summarises test work conditions applied to three (3) graphite concentrate samples, together with their starting compositions and the final purified graphite material product quality.Table 1 (continued over the following two pages)

[0137] An investigation of the regeneration of caustic, in accordance with the first embodiment of the process of the present invention described hereinabove, was conducted. This investigation utilised steps (i), (ii), (iii), (iv) and the caustic regeneration step of the process 10 described above and shown in Figure 1 , other than as varied as herein noted. Specifically, a Locked Cycle Trial wasundertaken, in which graphite material prepared by way of steps (i), (ii), (iii) and (iv), with the caustic leach of step (iii) being a single leach step. The caustic regeneration step specifically utilised the addition of hydrated or slaked lime. Filtered regenerated liquor was then recycled to the next caustic leaching cycle and repeated over four additional cycles. The starting graphite material and process liquors employed in the first caustic leach cycle are set out in Table 2 below.[Remainder of page left blank intentionally]Table 2 - Locked Cycle Test Feed Material and Starting Solutions

[0138] The Locked Cycle Test Work results are shown below in Table 3. These results confirm that regeneration of caustic liquors was able to be achieved through addition of lime and the reuse of these liquors in the caustic leaching step (iii) did not inhibit the dissolution of silica and aluminium.[Remainder of page left blank intentionally]Table 3 - Locked Cycle Test Work ResultsExample - Second Embodiment of the Present Invention

[0139] Table 4 provided below illustrates and summarises test work conditions applied to samples of a single graphite concentrate, together with starting compositions, general operating conditions and the final purified graphite material product quality.Table 4 (continued over the page)

[0140] It can be seen from the above description that the improved graphite material purification process of the present invention provides an improved graphite material purification process that overcomes reliance on hydrofluoric acid and consequently has less environmental impact and is generally safer than prior art processes. Additional benefits may be realised in terms of decreased reagent consumption and, broadly, increased sustainability relative to prior art processes. In one embodiment of the present invention, the process does not include the contact of the graphite material concentrate with hydrofluoric acid. In one embodiment of the present invention, the process does not include a roast step or a calcination step.

[0141] It is envisaged, where the graphite material to be purified is first passed to a size reduction step, for example grinding or milling, to provide a fine ground or milled material to be purified, that impurity removal efficiency may be improved and / or that a high specific surface area product may be achieved. In a preferred embodiment, the size reduction step reduces the particle size below 20 microns. Such products may be suitable for specific LiB product applications.

[0142] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

Claims1 . An improved graphite material purification process, the process comprising the steps of:(i) Passing a concentrate of a graphite material to be purified to an oxidative leach stage;(ii) Passing a product of the oxidative leach stage of step (i) to a solid liquid separation stage producing a liquid product and a solid product;(iii) Passing the solid product of the solid liquid separation stage of step (ii) to a caustic leach stage;(iv) Passing a product of the caustic leach stage of step (iii) to a solid liquid separation stage producing a liquid product and a solid product;(v) Passing the solid product of the solid liquid separation stage of step (iv) to an acid leach stage; and(vi) Passing a product of the acid leach stage of step (v) to a solid liquid separation stage producing a liquid product and a solid product, wherein the solid product comprises a purified graphite material.

2. The process of claim 1 , wherein the oxidative leach stage of step (i) comprises a first oxidative leach portion and a second oxidative leach portion.

3. The process of claim 2, wherein the first oxidative leach portion is operated under conditions in which sulphide minerals present are oxidised to sulphates.

4. The process of claim 2 or 3, wherein the first oxidative leach portion operates:(i) with a target redox potential of > about 425 mV (versus Ag / AgCI);(ii) with a pH of > 10;(iii) with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w;(iv) at a temperature of between about 30 to 60oC; and / or(v) with a residence time of between about 30 to 60 minutes.

5. The process of any of claims 2 to 4, wherein at least one of sodium hypochlorite or sodium chlorate is utilised as the oxidant in the first oxidative leach portion of the oxidative leach stage of step (i).

6. The process of any of claims 2 to 5, wherein the second oxidative leach portion is operated under conditions in which, if present, iron, magnesium and potassium minerals are leached.

7. The process of any of claims 2 to 6, wherein the second oxidative leach portion operates:(i) at a temperature of about 40 to 10OoC;(ii) over multiple leach stages;(iii) with a slurry density of between about 15 to 35% w / w solids, for example about 20% w / w;(iv) with a residence time in the range of about 2 to 6 hours;(v) with an HCI addition rate of between about 150 to 350 kg / t feed; and / or(vi) with a background HCI acidity of between about 30 to 60 g / L.

8. The process according to any of the preceding claims, wherein the caustic leach stage of step (iii) operates:(i) in a counter-current manner;(ii) under pressure;(iii) at a temperature of between about 100 to 220oC;(iv) with a starting slurry density of between about 150 to 250 g / L solids;(v) with a total residence time of between about 2 to 16 hours; and / or(vi) with a background NaOH concentration of between about 100 to 500 g / L.

9. The process according to any one of the preceding claims, wherein the caustic leach stage is operated at a pressure below 5 bar.

10. The process according to any one of claim 1 to 8, wherein the caustic leach stage is operated at a pressure between 20 and 25 bar.11 . The process according to any of the preceding claims, wherein the lixiviant of acid leach stage (v) is selected from hydrochloric acid or sulphuric acid.

12. The process according to claim 11 , wherein the lixiviant of acid leach stage (v) is hydrochloric acid, and the acid leach stage (v) is conducted(i) in multiple stages;(ii) at a temperature of between about 60 to 100°C;(iii) with a slurry density of between about 15 to 35% w / w, for example about 20% w / w;(iv)with a total residence time of between about 2 to 6 hours; and / or(v) with a background HCI acidity of between about 30 to 60 g / L.

13. The process according to any of the preceding claims, wherein a caustic containing leachate from the caustic leach stage of step (iii) is recycled by way of a caustic regeneration step to which lime and / or hydrated / slaked lime is fed, and from which regenerated caustic is recovered and a residue of silicates and aluminium produced.

14. The process according to any of the preceding claims, wherein a reagent recovery step, including crystallisation and chlor-alkali electrolysis stages, is provided and to which a liquid product of the slurry solid liquid separation step (iv) is passed.

15. The process according to any of the preceding claims, wherein the process further comprises a first effluent treatment, or brine treatment, stage in which liquid products from the oxidative leach stage of step (i), the caustic leach stage of step (iii), and the acid leach stage of step (v) are combined.

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