AU2019262079B2 - Processing of silicate minerals - Google Patents

Processing of silicate minerals Download PDF

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AU2019262079B2
AU2019262079B2 AU2019262079A AU2019262079A AU2019262079B2 AU 2019262079 B2 AU2019262079 B2 AU 2019262079B2 AU 2019262079 A AU2019262079 A AU 2019262079A AU 2019262079 A AU2019262079 A AU 2019262079A AU 2019262079 B2 AU2019262079 B2 AU 2019262079B2
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leach
acid
fraction
coarse
fine
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Gary Donald Johnson
Mark Daniel Urbani
Nicholas John VINES
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Silica Technology Pty Ltd
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Silica Tech Pty Ltd
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/20—Silicates
    • C01B33/36—Silicates having base-exchange properties but not having molecular sieve properties
    • C01B33/38—Layered base-exchange silicates, e.g. clays, micas or alkali metal silicates of kenyaite or magadiite type
    • C01B33/42—Micas ; Interstratified clay-mica products
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B3/00—Extraction of metal compounds from ores or concentrates by wet processes
    • C22B3/04—Extraction of metal compounds from ores or concentrates by wet processes by leaching
    • C22B3/06—Extraction of metal compounds from ores or concentrates by wet processes by leaching in inorganic acid solutions, e.g. with acids generated in situ; in inorganic salt solutions other than ammonium salt solutions
    • C22B3/08—Sulfuric acid, other sulfurated acids or salts thereof
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C01B33/187—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by acidic treatment of silicates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/113—Silicon oxides; Hydrates thereof
    • C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
    • C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
    • C01B33/187—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by acidic treatment of silicates
    • C01B33/193—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof by acidic treatment of silicates of aqueous solutions of silicates
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B33/00—Silicon; Compounds thereof
    • C01B33/20—Silicates
    • C01B33/36—Silicates having base-exchange properties but not having molecular sieve properties
    • C01B33/46—Amorphous silicates, e.g. so-called "amorphous zeolites"
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B1/00—Preliminary treatment of ores or scrap
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B26/00—Obtaining alkali, alkaline earth metals or magnesium
    • C22B26/10—Obtaining alkali metals
    • C22B26/12—Obtaining lithium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B9/00—General methods of preparing halides
    • C01B9/08—Fluorides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B21/00—Obtaining aluminium
    • C22B21/0015—Obtaining aluminium by wet processes
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B21/00—Obtaining aluminium
    • C22B21/04—Obtaining aluminium with alkali metals earth alkali metals included
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B26/00—Obtaining alkali, alkaline earth metals or magnesium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B26/00—Obtaining alkali, alkaline earth metals or magnesium
    • C22B26/10—Obtaining alkali metals

Abstract

A process for the processing of silicate minerals, the process comprising passing an ore or concentrate (1) containing a silicate mineral to a pre-treatment phase, the pre-treatment phase including a size separation step (20) to produce a coarse fraction (4) and a fine fraction (5), whereafter the coarse fraction (4) is passed to an acid leach step (50, 60, 70, 80) in which the coarse fraction is contacted with at least sufficient acid, on a stoichiometric basis, to react with the mica rich mineral contained in both the coarse (4) and fine fractions (5), the fine fraction (5) subsequently being added to the acid leach step (70) whereby the mica in the fine fraction (5) is also leached, the leach step (50, 60, 70, 80) producing a leach slurry (15) that is passed to a separation step (90) forming a pregnant leach solution (17) and a leach residue (18), the pregnant leach solution (17) in turn being passed to one or more recovery steps. The leach residue (18) contains amorphous silica (23), and is passed to a treatment step (110) in which gangue minerals (24) are separated from the amorphous silica (23).

Description

“Processing of Silicate Minerals”
Field of the Invention
[0001 ] The present invention relates to a process for the processing of silicate minerals. More particularly, the process of the present invention is intended for application in the processing of micas.
[0002] In one form the process of the present invention relates to the extraction of lithium, and optionally potassium, rubidium and caesium, from lithium rich mica minerals, including but not limited to lepidolite and zinnwaldite.
[0003] In a further form, the process of the present invention further relates to treatment of a leach residue from a leach of a mica rich ore or concentrate to produce amorphous silica.
Background Art
[0004] The major sources of commercially mined U2CO3 have historically come from brine solution and spodumene containing ores. Lepidolite, a lithium containing mica, is present in many pegmatite deposits, and co-exists with spodumene in some pegmatites. The presence of lepidolite is problematic for refineries that produce Li2C03 from spodumene concentrate. As such, the lithium content of lepidolite holds no value and is rejected at the spodumene concentrator.
[0005] Lepidolite can contain up to 7.7% L12O. The lepidolite in pegmatite bodies can be separated from the gangue minerals by flotation, or classification. However, this separation is not 100% efficient and results in gangue minerals present in the concentrate.
[0006] A process to extract lithium from lepidolite, a phyllosilicate, and produce U2CO3 is described in International Patent Application PCT/AU2015/000608 (WO 2016/054683). In the process of International Patent Application PCT/AU2015/000608, lepidolite is preferably milled to produce a product having a particle size of Pso <75 micron. This process requires fine grinding of the lepidolite to achieve acceptable results. The milled lepidolite is leached in sulfuric acid under atmospheric pressure and temperatures up to boiling. The leach stage produces a pregnant leach solution, which is subjected to a series of process steps in which one or more impurity metals are removed, prior to recovering lithium as a lithium containing salt product.
[0007] Precipitated silica (amorphous silica) is produced commercially by neutralising a solution of sodium silicate with a mineral acid, usually sulphuric acid, with the by-product sodium sulphate being removed by filtration and subsequent washing. After drying, the precipitated silica consists of 86-88% S1O2 and 10-12% water, which is present both in the molecular structure and physically bound on its surface.
[0008] During filtration, the filter cake may contain considerable quantities of water so that drying may need to evaporate up to six times the amount by weight of water. For this reason, drying accounts for a considerable proportion of the production costs and the drying method employed varies depending on the target properties of the final products.
[0009] The two primary methods of sodium silicate production are the furnace process and the hydrothermal process. The furnace process involves the fusion of silica sand and soda ash, yielding a vitreous sodium silicate and liberating carbon dioxide gas. The vitreous sodium silicate can be dissolved in water. This mostly concerns sodium silicates with a mole ratio higher than 2.5 (typically ± 3.4). The bulk of commercial sodium silicate is manufactured by the furnace process.
[0010] Sodium silicate is one of the key reagents in producing amorphous silica. It is produced by the direct fusion of precisely measured portions of pure silica sand (S1O2) and soda ash (Na2C03) in a fuel or electrically fired furnace at temperatures above 1000 °C. The proportion of sand and soda ash determines the molar ratio S1O2: Na20 in the silicate product.
[001 1 ] In the hydrothermal process, solutions of soluble silicates may be produced either by dissolving the soluble silicate lumps in water at elevated temperatures (and partly at elevated pressure) or for certain qualities also by hydrothermally dissolving a reactive silica source (mainly silica sand) in the respective alkali hydroxide solution. In general, solutions are subsequently filtered to remove any residual turbidity and adjusted to yield products to a particular specification. [0012] The process of the present invention has as one object thereof to substantially overcome the problems associated with the prior art or to at least provide a useful alternative thereto.
[0013] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia or any other country or region as at the priority date of the application.
[0014] 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.
[0015] Throughout the specification and claims, unless the context requires otherwise, the word “mica”, “micas” or obvious variations thereof will be understood to refer to the group of complex hydrous aluminosilicate minerals that crystallise with a sheet or plate-like structure. Specifically, the mica referred to herein is to be understood to refer to lithium containing mica.
[0016] Throughout the specification and claims, unless the context requires otherwise, amorphous silica refers to a silica (S1O2) produced by precipitation, and consequently the terms amorphous silica and precipitated silica are to be understood to refer to the same product.
Disclosure of the Invention
[0017] In accordance with the present invention there is provided a process for the processing of silicate minerals, the process comprising passing an ore or concentrate containing a silicate mineral to a pre-treatment phase, the pre treatment phase including a size separation step to produce a coarse fraction and a fine fraction, whereafter the coarse fraction is passed to an acid leach step in which the coarse fraction is contacted with at least sufficient acid, on a stoichiometric basis, to react with the silicate mineral contained in both the coarse and fine fractions, the fine fraction subsequently being added to the acid leach step whereby the silicate mineral in the fine fraction is also leached, the leach step producing a leach slurry that is passed to a separation step forming a pregnant leach solution and a leach residue, the pregnant leach solution in turn being passed to one or more recovery steps.
[0018] In one form of the present invention, the silicate mineral is a mica rich mineral. Preferably, the mica rich mineral is a lithium bearing mica rich mineral.
[0019] In a further form of the present invention, the leach residue contains amorphous silica.
[0020] Preferably, the leach residue is passed to a treatment step in which gangue minerals are separated from the amorphous silica.
[0021 ] Still preferably, the treatment step comprises a flotation step, wherein gangue minerals are separated from amorphous silica and report to a flotation concentrate whilst purified amorphous silica reports to the flotation step sinks.
[0022] The gangue minerals of the leach residue may comprise one or more minerals that are refractory to acid leaching, including feldspar and quartz.
[0023] Preferably, the particle size distribution of the leach residue is similar to that of the ore or concentrate prior to the size separation step and, as such, is amenable to separation by a flotation step.
[0024] Preferably, the mica rich minerals include lepidolite and/or zinnwaldite.
[0025] Preferably, the pre-treatment phase further comprises one or both of a concentration step and a milling step. The concentration step may be a flotation step. The milling step may preferably be a fine milling step.
[0026] Still preferably, the fine milling step produces a product having a particle size of: a. Pso <250 micron; b. Pso <150 micron; or c. P80 <75 micron.
[0027] Preferably, the size separation step comprises the separation of milled material by particle size, thereby producing the coarse fraction and the fine fraction. [0028] Preferably, the size separation step produces a coarse fraction having a particle size of predominantly +25 micron and a fine fraction having a particle size of predominantly -25 micron.
[0029] Still preferably, the size separation step produces a coarse fraction having a particle size of predominantly +15 micron and a fine fraction having a particle size of predominantly -15 micron.
[0030] In one form of the present invention, acid may be added to the leach step at or about the time the fine fraction is added thereto, in an amount such that the stoichiometric amount of acid to leach both the coarse and fine fractions is not exceeded.
[0031 ] The acid leach step is preferably carried out with an excess of H2SO4.
[0032] The amount of acid added to the coarse fraction in the leach step may preferably be about 1000 g/L.
[0033] Still preferably, the total sulfate concentration is close to the saturation limit of the solution at the leaching temperature. For example, this may be 6.0M S at >90°C.
[0034] The coarse fraction is leached in the leach step for a period of time, to produce a pregnant liquor containing a high concentration of sulfuric acid, prior to addition of the fine fraction to the leach step. Preferably, the concentration of sulfuric acid in this pregnant leach liquor is about 500 g/L.
[0035] The fine fraction is preferably added to the leach step and the contained mica is leached with the acid in the coarse pregnant liquor.
[0036] Preferably, the leach slurry from the leach step contains a free acid concentration of greater than about 150 g/L FI2SO4. Still preferably, the free acid concentration of the leach slurry is about 200 g/L.
[0037] The leach step preferably results in at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and caesium being extracted into solution, thereby forming the pregnant leach solution (“PLS”). A significant proportion of contained silicon preferably reports to the leach residue as amorphous silica, for example greater than 95%, preferably greater than 99% of contained silicon. [0038] Preferably, the leaching step is conducted under atmospheric conditions.
[0039] The leaching step is preferably conducted at a temperature close to boiling. Still preferably, the leaching step is conducted at up to 120°C, for example at or about 105°C.
[0040] In one form of the present invention, the leach step comprises four leach stages, with leach slurry being passed progressively from a first of the four leach stages through to the fourth thereof. Still preferably, the fine fraction is added to the third of the four leach stages.
[0041 ] The retention time in the four leach stages is preferably: a. between about 12 to 24 hours; or b. about 18 hours.
[0042] Still further preferably, in the leach step greater than about 90% lithium extraction is achieved.
[0043] The treatment step to which the leach residue is passed preferably includes a beneficiation step. The beneficiation step preferably comprises one or more of sizing, gravity separation and froth flotation.
[0044] Preferably, the beneficiation step comprises froth flotation.
[0045] Still preferably, the beneficiation step comprises a reverse flotation process.
[0046] The leach residue is preferably passed to a conditioning step prior to passage to the beneficiation step. The conditioning step adjusts the leach residue to a pH in the range of 2-7.
[0047] In addition, the conditioning step further preferably adds a cationic-anionic collector. The cationic-anionic collector may be provided in the form of N-tallow- 1 ,3-diaminopropane dioleate (Duomeen® TDO).
[0048] In one form of the present invention, the flotation concentrate contains crystalline gangue minerals, including for example feldspar, unreacted mica, and quartz, whereas the purified amorphous silica reports to the flotation sinks. [0049] The amorphous silica product of the treatment step, a purified amorphous silica, consists of similar morphology to the mica concentrate. That is, the purified amorphous silica is of a coarse size and subsequently dewaters readily by decantation and filtration. The filtered amorphous silica wet cake contains less water than commercial precipitated silica wet cake produced via prior art processes.
Brief Description of the Drawings
[0050] The process of the present invention will now be described, by way of example only, with reference to one embodiment thereof and the accompanying drawings, in which:-
Figure 1 is a flow sheet of the process of the present invention, including the treatment of a leach residue to produce amorphous silica; and
Figure 2 is a graph showing two plots of lithium extraction versus time, wherein one plot represents a process in which a finely milled product is simply leached in acid for 24 hours, whereas the second plot represents a process conducted in accordance with the present invention, wherein a lepidolite concentrate was pre-milled to Pso 75 micron then separated in +25 micron and -25 micron fractions, and then the coarse fraction is leached for 8 hours, after which the fine fraction was added to the reactor, without additional acid, and the leach run out to 24 hours.
Best Mode(s) for Carrying Out the Invention
[0051 ] The present invention relates to a process for the processing of silicate minerals. More particularly, the process of the present invention is intended for application in the processing of micas.
[0052] In one form the process of the present invention relates to the extraction of lithium, and optionally potassium, rubidium and caesium, from lithium rich mica minerals. The lithium rich mica minerals include, but not limited to, lepidolite and zinnwaldite.
[0053] In one form, the process of the present invention further relates to treatment of a leach residue from a leach of a mica rich mineral to produce amorphous silica. The process of the present invention provides, in a preferred form, for the production and purification of amorphous silica from mica by acid leaching and subsequent reverse flotation of the leach residue. The process is conducted at a particle size distribution of the mica that enables both efficient mica leaching and effective purification by reverse flotation.
[0054] In very general terms, in one embodiment of the present invention, a lithium containing mica rich mineral, lepidolite, is pre-concentrated, if required, by a mineral separation process, for example flotation. The lepidolite ore or concentrate is then subjected to a pre-treatment phase comprising, for example, milling and size separation. The size separation step utilises screens and/or hydrocyclones to produce a slurry containing the coarse particles and a slurry containing the fine particles.
[0055] The lithium, potassium, rubidium, caesium, fluorine and aluminium present in the coarse lepidolite are extracted by strong sulfuric acid leaching in an acid leach step, producing leach liquor containing lithium, potassium, rubidium, caesium, fluorine, aluminium, a high concentration of sulphuric acid and a leach residue containing amorphous silica and unreacted gangue minerals. The fine lepidolite is introduced into the coarse leach slurry after a sufficient period to enable the coarse lepidolite to leach. The fine lepidolite is leached due to the high free acid concentration present in the coarse leach slurry.
[0056] Lepidolite is a lilac-grey or rose coloured lithium phyllosilicate (mica group) mineral and a member of the polylithionite-trilithionite series. The standard chemical formula for Lepidolite is, but is not limited to, K(Li,AI)3(AI,Si)4010(F,OH)2. It occurs in granite pegmatites, high temperature quartz veins, greisens and granites. Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl. Lepidolite can contain up to 7.7% U2O. The lepidolite in pegmatite bodies can be separated from the gangue minerals by flotation, or classification. However, this separation is not 100% efficient and results in gangue minerals present in the concentrate.
[0057] It is envisaged that the processes of the present invention are applicable to any lithium bearing mica ores, such as lepidolite, but also including zinnwaldite, biotite, glauconite and muscovite. Zinnwaldite is a lithium containing silicate mineral in the mica group, generally light brown, grey or white in colour, and having the chemical formula KLiFeAI(AISi3)Oio(OH,F)2. [0058] In one form of the present invention the process comprises the method steps of:
(i) Separation of a mica rich mineral from gangue minerals, such as quartz and feldspar, by froth flotation, if required, to produce a mica concentrate;
(ii) Milling the mica concentrate;
(iii) Separating the coarse particles from the fine particles in the milled product;
(iv) Leaching the coarse mica in sufficient sulfuric acid solution required to leach the mica contained in the coarse and fine fractions under atmospheric conditions to enable lithium, potassium, rubidium, caesium and aluminium to be converted to soluble sulfates and to also extract any fluorine present in the coarse fraction;
(v) Leaching the fine mica in the coarse leach slurry, which contains sufficient sulfuric acid solution to leach the mica contained in the fine fraction;
(vi) Separating the leach liquor and leach residue contained in the leach slurry by decantation and/or filtration;
(vii) Subjecting the leach residue to reverse flotation to concentrate the crystalline gangue minerals, such as unreacted mica, feldspar and/or quartz from amorphous silica, in which a purified amorphous silica reports to the sinks; and
(viii) Dewatering the purified amorphous silica by decantation, filtration and/or drying to produce a filter cake or dry powder product.
[0059] In one embodiment of the present invention, a lepidolite ore or concentrate is treated in accordance with the present invention as shown in Figure 1 . The relative grades of the metals in lepidolite are described only by way of example, and the process of the present invention is expected to be able treat any lepidolite bearing material and any lithium bearing mica, independent of grade. [0060] In Figure 1 there is shown a flow sheet in accordance with the present invention and in which the embodiment depicted is particularly intended for the processing of lepidolite containing ore or concentrate 1 to extract lithium therefrom, with the addional recovery of amorphous silica.
[0061 ] The lepidolite containing ore or concentrate 1 is passed to a pre-treatment phase comprising a milling step 10, with water 2, in which the ore or concentrate 1 is milled to reduce the particle size, for example to Pso <250 micron. More particularly, the particle size is reduced to < Pso 150 micron or < Pso 75 micron.
[0062] Milled lepidolite 3 is directed to a further feature of the pre-treatment phase, a size separation step, for example a size separation device 20, in a conventional process such as hydrocyclones, in which a coarse fraction, the coarse mica 4, is separated from a fine fraction, the fine mica 5. The size separation step produces a coarse fraction having a particle size of predominantly +25 micron and a fine fraction having a particle size of predominantly -25 micron, with a 50/50 mass split between the coarse and fine fractions. Optionally, the size separation step produces a coarse fraction having a particle size of predominantly +15 micron and a fine fraction having a particle size of predominantly -15 micron. With such a size separation, it is envisaged that the coarse fraction will constitute a higher proportion of the mass split than the fine fraction.
[0063] The use of hydrocyclones requires water to fluidise the slurry, part of which requires removal prior to leaching. The coarse mica 4 is dewatered in a conventional process, such as thickening 30. The water 7 can be returned to the size separation process step 20. The fine mica 5 is also de-watered in step 40 and the water 8 can be returned to the size separation process step 20.
[0064] The dewatered coarse mica 6 is repulped in leach liquor providing greater than 40% solids w/w, the leach liquor containing in the order of 200 g/L free acid, before being directed to an atmospheric acid leach step, comprising a‘coarse leach’ in which it is directed to a first leach tank 50 (TK1 ) in which it is contacted with concentrated sulphuric acid, whereby a free acid level of about 1000 g/L is reached. The slurry from leach tank 50 is passed to a second leach tank 60 (TK2) in which steam 1 1 is added to maintain the reaction temperature, at or about boiling, for example 105°C. In these leach tanks or reactors at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and caesium are extracted from the coarse mica into solution forming a pregnant leach solution (“PLS”).
[0065] The coarse leach slurry 12 is passed to a third leach tank 70 (TK3) where it is reacted with the de-watered fine mica 8, after same is first repulped with leach liquor, which again contains in the order of 200 g/L free acid. Steam 13 is added to leach tank 70 to maintain the reaction temperature, again at or about boiling, for example 105°C. The slurry from leach tank 70 is passed to a fourth leach tank 80 (TK4) in which steam 14 is added to maintain the reaction temperature. Again, in these reactors at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and caesium are extracted, this time from the fine mica, in addition to the further leaching of the coarse fraction, into solution forming a pregnant leach solution (“PLS”).
[0066] A coarse and fine leach slurry 15 is passed from the leach tank 80 to a solid liquid separation step, for example a filter 90, which enables the PLS to be recovered from the leach residue at or near the leaching temperature. The solid liquid separation stage produces a PLS 17 containing the bulk of the extracted lithium, potassium, aluminium, rubidium, fluorine and caesium and a leach residue 18 with high silica content, which is washed with water 16. The wash water 16 can be combined with the PLS 17.
[0067] The PLS 17 may be passed on to one or more recovery steps in which any one or more of the contents, such as lithium and the like, may be recovered.
[0068] The silica containing leach residue 18 is passed to a treatment step. For example, the leach residue 18 is re-pulped in water 19 and conditioned with sulphuric acid 20 and collector 21 , for example a cationic-anionic collector, in the conditioning tank 100. The cationic-anionic collector may be provided in the form of N-tallow-1 ,3-diaminopropane dioleate (Duomeen® TDO).
[0069] The conditioned flotation feed 22 is directed to a reverse flotation process 1 10 in which crystalline gangue minerals concentrate to the flotation concentrate 24 and the purified amorphous silica 23 reports to the flotation sinks.
[0070] The amorphous silica product of the treatment step, the purified amorphous silica 23, consists of similar morphology to the original mica concentrate. That is, the purified amorphous silica 23 is of a coarse size and subsequently dewaters readily by decantation and filtration. The filtered amorphous silica wet cake contains less water than commercial precipitated silica wet cake produced via prior art processes.
[0071 ] The present invention may be better understood, in part, with reference to the following non-limiting example.
EXAMPLE
[0072] In Figure 2 there is shown a graph of two plots of lithium extraction versus time.
[0073] The first plot describes the leaching of lepidolite concentrate, pre-milled to P80 25 micron and leached with 1 100 kg/t sulphuric acid at 105°C over 24 hours. The second plot describes the leaching of lepidolite concentrate, pre-milled to a coarser Pso 75 micron, then separated in +25 micron and -25 micron fractions. After 8 hours of leaching of the coarse fraction the fine fraction was added to the reactor, without additional acid at that point, and the leach run to 24 hours. The total acid addition was 1 100 kg/t and the leach temperature was 105°C. As can be seen with reference to Figure 2, the‘split leach’ resulted in a higher lithium extraction despite the coarser particle size.
[0074] The total sulfate concentration in the leach steps of tanks 50 and 60 is such that it is close to the saturation limit of the solution at the leaching temperature. For example, it is understood by the Applicants that this could be 6.0M S at >90°C. With these conditions for the leaching of the coarse fraction, and the subsequent addition and leaching of the fine fraction, the Applicants have noted >90% metal, for example lithium, extraction is achieved within 18 hours and is significantly higher than leaching the mica concentrate without undergoing the size separation prior to leaching.
[0075] It is envisaged that the use of the leach step that comprises a number of stages, with the coarse fraction being first leached with the full stoichiometric amount of acid, or close thereto, allows more complete leaching of the coarse fraction that might otherwise not leach if the fine fraction were present immediately. The process of the invention is, amongst other things, intended to provide as little dilution of the leach liquor as the leach step progresses through the several stages thereof. This is in part facilitated by repulping of the fine and coarse fractions with leach liquor.
[0076] It can be seen from the above description that the process of the present invention is conducted at a particle size distribution of the mica that enables both efficient mica leaching and effective purification by reverse flotation.
[0077] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims (36)

Claims:
1 . A process for the processing of silicate minerals, the process comprising passing an ore or concentrate containing a silicate mineral to a pre-treatment phase, the pre-treatment phase including a size separation step to produce a coarse fraction and a fine fraction, whereafter the coarse fraction is passed to an acid leach step in which the coarse fraction is contacted with at least sufficient acid, on a stoichiometric basis, to react with the silicate mineral contained in both the coarse and fine fractions, the fine fraction subsequently being added to the acid leach step whereby the silicate mineral in the fine fraction is also leached, the leach step producing a leach slurry that is passed to a separation step forming a pregnant leach solution and a leach residue, the pregnant leach solution in turn being passed to one or more recovery steps.
2. The process of claim 1 , wherein the silicate mineral is:
a. a mica rich mineral; or
b. a lithium bearing mica rich mineral.
3. The process of claim 1 or 2, wherein the leach residue contains amorphous silica.
4. The process of claim 3, wherein the leach residue is passed to a treatment step in which gangue minerals are separated from the amorphous silica.
5. The process of claim 4, wherein the treatment step comprises a flotation step, whereby gangue minerals are separated from amorphous silica and report to a flotation concentrate whilst purified amorphous silica reports to the flotation step sinks.
6. The process of claim 5, wherein the gangue minerals of the leach residue comprise one or more minerals that are refractory to acid leaching, optionally including feldspar and quartz.
7. The process of any one of the preceding claims, wherein the particle size distribution of the leach residue is equivalent to that of the ore or concentrate prior to the size separation step and is amenable to separation by a flotation step.
8. The process of any one of the preceding claims, wherein the mica rich minerals include lepidolite and/or zinnwaldite.
9. The process of any one of the preceding claims, wherein the pre-treatment phase further comprises one or both of a concentration step and a milling step.
10. The process of claim 9, wherein the concentration step is a flotation step.
1 1 . The process of claim 9 or 10, wherein the milling step is a fine milling step.
12. The process of claim 1 1 , wherein the fine milling step produces a product having a particle size of:
a. Pso <250 micron;
b. P80 <150 micron; or
c. P8o <75 micron.
13. The process of any one of the preceding claims, wherein the size separation step comprises the separation of milled material by particle size, thereby producing the coarse fraction and the fine fraction.
14. The process of claim 13, wherein the size separation step produces:
a. a coarse fraction having a particle size of predominantly +25 micron and a fine fraction having a particle size of predominantly -25 micron; or
b. a coarse fraction having a particle size of predominantly +15 micron and a fine fraction having a particle size of predominantly -15 micron.
15. The process of any one of the preceding claims, wherein acid is added to the leach step at or about the time the fine fraction is added thereto, in an amount such that the stoichiometric amount of acid to leach both the coarse and fine fractions is not exceeded.
16. The process of any one of the preceding claims, wherein the acid leach step is carried out with an excess of H2SO4.
17. The process of claim 16, wherein an amount of acid added to the coarse fraction in the leach step is about 1000 g/L.
18. The process of claim 16 or 17, wherein the total sulfate concentration is:
a. close to the saturation limit of the solution at the leaching temperature; or
b. about 6.0M S at >90°C.
19. The process of any one of the preceding claims, wherein the coarse fraction is leached in the leach step for a period of time, to produce a pregnant liquor containing a high concentration of sulfuric acid, prior to addition of the fine fraction to the leach step.
20. The process of claim 19, wherein the concentration of sulfuric acid in this pregnant leach liquor is about 500 g/L.
21 . The process of claim 19 or 20, wherein the fine fraction is added to the leach step and the contained mica is leached with the acid in the coarse pregnant liquor.
22. The process of claim 21 , wherein the leach slurry from the leach step contains a free acid concentration of:
a. greater than about 150 g/L H2SO4; or
b. about 200 g/L.
23. The process of any one of the preceding claims, wherein the leach step results in at least a proportion of the contained lithium, potassium, aluminium, rubidium, fluorine and caesium being extracted into solution, thereby forming the pregnant leach solution.
24. The process of claim 23, wherein contained silicon reports to the leach residue as amorphous silica, at a rate of:
a. greater than 95% of contained silicon; or
b. greater than 99% of contained silicon.
25. The process of any one of the preceding claims, wherein the leaching step is conducted under atmospheric conditions.
26. The process of any one of the preceding claims, wherein the leaching step is conducted at a temperature:
a. close to boiling;
b. up to 120°C; or
c. at or about 105°C.
27. The process of any one of the preceding claims, wherein the leach step comprises four leach stages, with leach slurry being passed progressively from a first of the four leach stages through to the fourth thereof.
28. The process of claim 27, wherein the fine fraction is added to the third leach stage.
29. The process of any one of the preceding claims, wherein the retention time in the four leach stages is:
a. between about 12 to 24 hours; or
b. about 18 hours.
30. The process of any one of the preceding claims, wherein greater than about 90% lithium extraction is achieved in the leach step.
31 . The process of any one of claims 4 to 30, wherein the treatment step to which the leach residue is passed includes a beneficiation step.
32. The process of claim 31 , wherein the beneficiation step comprises:
a. one or more of sizing, gravity separation and froth flotation; b. froth flotation; or
c. a reverse flotation process.
33. The process of claim 31 or 32, wherein the leach residue is passed to a conditioning step prior to passage to the beneficiation step.
34. The process of claim 33, wherein the conditioning step:
a. adjusts the leach residue to a pH in the range of 2-7;
b. adds a cationic-anionic collector; or
c. adds a cationic-anionic collector in the form of N-tallow-1 ,3- diaminopropane dioleate (Duomeen® TDO).
35. The process of any one of claims 5 to 34, wherein the flotation concentrate contains crystalline gangue minerals and the purified amorphous silica reports to the flotation sinks.
36. The process of claim 35, wherein the crystalline gangue minerals include feldspar, unreacted mica, and quartz.
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