INTEGRATION OF CARBON SEQUESTRATION WITH SELECTIVE HYDROMETALLURGICAL RECOVERY OF METAL VALUES

Abstract

Processes are provided in which successive steps of hydrometallurgical value extraction may be carried out using the products of carbon capture and an electrolytic reagent-generating process. The electrolytic process provides an acid leachant and an alkali hydroxide, with the alkali hydroxide then available for use either directly as a precipitant in the hydrometallurgical steps, or available for conversion by carbon capture to an alkali metal carbonate that can in turn be used as the precipitant in the selective hydrometallurgical steps.

Claims

1. A process for processing a comminuted mineral feedstock, comprising: a) leaching metal values from the comminuted mineral feedstock with an acid leachant, to produce a solid siliceous residue and a loaded leach solution; b) precipitating iron and/or aluminum from the loaded leach solution with addition of: a first alkali metal carbonate precipitant, to produce a carbon dioxide off gas, or, a first alkali hydroxide precipitant, to produce an Fe/Al depleted solution and an iron and/or aluminum hydroxide or oxide precipitate product; c) precipitating nickel and/or cobalt from the Fe/Al depleted solution or from a Ni/Co ion exchange eluant obtained from the Fe/Al depleted solution by selective extraction of nickel and/or cobalt on an ion exchange medium, wherein the precipitating is with addition of: a second alkali metal carbonate or bicarbonate precipitant, or, a second alkali hydroxide precipitant, to produce a Ni/Co depleted solution and a nickel and/or cobalt carbonate or hydroxide precipitate product; d) before or after step (c), precipitating iron and/or aluminum and/or manganese from the Ni/Co depleted solution with addition of an oxidant and with addition of: a third alkali metal carbonate or bicarbonate precipitant, or, a third alkali hydroxide precipitant, to produce an Fe/Al/Mn depleted solution and an iron and/or aluminum and/or manganese hydroxide precipitate product; e) precipitating magnesium from the Fe/Al/Mn depleted solution with addition of: a fourth alkali hydroxide precipitant, or a fourth alkali metal carbonate or bicarbonate precipitant, to produce a Mg-depleted solution and a magnesium hydroxide or carbonate precipitate product; f) subjecting the Mg-depleted solution to an electrolysis process to produce the acid leachant and: one or more of the alkali hydroxide precipitants, or an alkali hydroxide product.

2. The process of claim 1, further comprising reacting the alkali hydroxide product of the electrolysis process directly or indirectly with a carbon source to produce one or more of the alkali metal carbonate or bicarbonate precipitants.

3. The process of claim 2, wherein reacting the alkali hydroxide product with a carbon source comprises scrubbing carbon dioxide from a CO.sub.2 containing gas by treating the CO.sub.2 containing gas with a scrubbing solution comprising the alkali hydroxide product, to produce one or more of the alkali metal carbonate or bicarbonate precipitants.

4. The process of claim 3, wherein the alkali hydroxide product comprises NaOH, wherein scrubbing carbon dioxide from the CO.sub.2 containing gas comprises precipitating Na.sub.2CO.sub.3 hydrates from the scrubbing solution in a crystallisation process to produce a solid Na.sub.2CO.sub.3 crystallizer product.

5. The process of any one of claims 1-4, further comprising precipitating calcium from the Mg-depleted solution with a fifth alkali hydroxide precipitant, to produce a calcium hydroxide product, and generating one or more of the alkali metal carbonate or bicarbonate precipitants by treating the calcium hydroxide product with a carbon source.

6. The process of claim 5, wherein the carbon source is a CO.sub.2 containing gas or a metal carbonate.

7. The process of claim 3, 4 or 6, wherein the CO.sub.2 containing gas comprises air.

8. The process of claim 4, wherein one or more of the alkali metal carbonate or bicarbonate precipitants comprises the solid Na.sub.2CO.sub.3 crystallizer product.

9. The process of any of claims 1-8, wherein the alkali metal carbonate or bicarbonate precipitant comprises NaHCO.sub.3, Na.sub.2CO.sub.3 or K.sub.2CO.sub.3.

10. The process of any one of claims 1-9, wherein the alkali hydroxide precipitant comprises NaOH or KOH.

11. The process of any one of claims 1-10, wherein the acid leachant comprises a mineral acid, HCl or H.sub.2SO.sub.4.

12. The process of any one of claims 1-11, wherein the electrolysis process comprises a chloralkali process producing the alkali hydroxide precipitant and/or the alkali hydroxide product, a Cl.sub.2(g) product and a H.sub.2(g) product, further comprising reacting the Cl.sub.2(g) product and the H.sub.2(g) product to produce HCl as the acid leachant.

13. The process of any one of claims 1-11, wherein the Mg-depleted solution comprises Na.sub.2SO.sub.4, wherein the electrolysis process comprises a salt splitting process comprising electrolytic generation of: the alkali hydroxide product and/or the alkali hydroxide precipitant; and, H.sub.2SO.sub.4 as the acid leachant.

14. The process of any one of claims 1-13, wherein precipitating magnesium from the Fe/Al/Mn depleted solution with the alkali hydroxide precipitant, further comprises addition of a CO.sub.2(g) precipitant to produce the Mg-depleted solution and the magnesium carbonate precipitate product.

15. The process of claim 14, wherein the CO.sub.2(g) precipitant comprises the carbon dioxide off gas from the step of precipitating iron and/or aluminum from the loaded leach solution.

16. The process of any one of claims 1-15, wherein the oxidant comprises chlorine gas (Cl.sub.2(g)) or sodium hypochlorite (NaOCl).

17. The process of any one of claims 1-16, wherein the nickel and/or cobalt hydroxide precipitate is a mixed Ni/Co hydroxide product.

18. The process of any one of claims 1-17, further comprising magnetically separating material from the comminuted mineral feedstock.

19. The process of any one of claims 1-18, further comprising subjecting the loaded leach solution to a resin in leach process so as to selectively remove nickel values from the loaded leach solution, to obtain a purified nickel product.

20. The process of any one of claims 1-19, further comprising washing and/or alkalization of the solid siliceous residue.

21. The process of any one of claims 1-20, further comprising washing and/or alkalization of the iron and/or aluminum hydroxide or oxide precipitate product.

22. The process of any one of claims 1-21, further comprising adding a hematite seed material to the step of precipitating iron and/or aluminum so as to seed the precipitation of a hematite product.

23. The process of any one of claims 1-21, wherein the iron and/or aluminum hydroxide or oxide precipitate product comprises a hematite seed material, and the hematite seed material is recirculated to the step of precipitating iron and/or aluminum so as to seed the precipitation of a hematite product.

24. The process of any one of claims 1-23, further comprising recycling a brine comprising the Fe/Al/Mn depleted solution to a comminuting step to provide the comminuted mineral feedstock.

25. The process of any one of claims 1-24, wherein the mineral feedstock comprises a nickel saprolite ore or tailing, an olivine ore or tailing, an asbestos ore or tailing, a mafic mineral, a saprolite material, an ultramafic rock, olivine or wollastonite.

25. A process for processing a comminuted mineral feedstock, comprising: optionally magnetically separating material from the comminuted mineral feedstock; a) leaching metal values from the comminuted mineral feedstock with an acid leachant, to produce a solid siliceous residue and a loaded leach solution; optionally subjecting the loaded leach solution to a resin in leach process so as to selectively remove nickel values from the loaded leach solution, to obtain a purified nickel product, optionally, washing and/or alkalization of the solid siliceous residue; b) precipitating iron and/or aluminum from the loaded leach solution with addition of: a first alkali metal carbonate or bicarbonate precipitant, to produce a carbon dioxide off gas, or, a first alkali hydroxide precipitant, to produce an Fe/Al depleted solution and an iron and/or aluminum hydroxide or oxide precipitate product, optionally a hematite product; optionally, washing and/or alkalization of the iron and/or aluminum hydroxide precipitate product; optionally, adding a hematite seed material to the step of precipitating iron and/or aluminum, and further optionally wherein the iron and/or aluminum hydroxide or oxide precipitate product comprises the hematite seed material; c) precipitating nickel and/or cobalt from the Fe/Al depleted solution or from a Ni/Co ion exchange eluant obtained from the Fe/Al depleted solution by selective extraction of Ni and/or cobalt on an ion exchange medium, wherein the precipitating is with addition of: a second alkali metal carbonate or bicarbonate precipitant, or, a second alkali hydroxide precipitant, to produce a Ni/Co depleted solution and a nickel and/or cobalt carbonate or hydroxide precipitate product; d) before or after step (c), precipitating iron and/or aluminum and/or manganese from the Ni/Co depleted solution with addition of an oxidant and with addition of: a third alkali metal carbonate or bicarbonate precipitant, or, a third alkali hydroxide precipitant, to produce an Fe/Al/Mn depleted solution and an iron and/or aluminum and/or manganese hydroxide precipitate product; optionally recycling a brine comprising the Fe/Al/Mn depleted solution to a comminuting step to provide the comminuted mineral feedstock; e) precipitating magnesium from the Fe/Al/Mn depleted solution with addition of: a fourth alkali hydroxide precipitant, or a fourth alkali metal carbonate or bicarbonate precipitant, to produce a Mg-depleted solution and a magnesium hydroxide or carbonate precipitate product; f) subjecting the Mg-depleted solution to an electrolysis process to produce the acid leachant and: one or more of the alkali hydroxide precipitants, or an alkali hydroxide product; and, g) sequestering carbon dioxide from a CO.sub.2 containing gas, by reacting the CO.sub.2 containing gas directly or indirectly with the alkali hydroxide product, in one or more of: the nickel and/or cobalt carbonate precipitate product; or, the magnesium carbonate precipitate product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0071] FIG. 1 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process.

[0072] FIG. 2 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process.

[0073] FIG. 3 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process.

[0074] FIG. 4 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process.

[0075] FIG. 5 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process, showing the use of Na.sub.2CO.sub.3 to precipitate Mg.

[0076] FIG. 6 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide and a chloralkali electrochemical process, showing the use of NaOH in combination with CO.sub.2(g) to precipitate Mg.

[0077] FIG. 7 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by an electrolytic salt splitting anion exchange process.

[0078] FIG. 8 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide (DAC) and an electrolytic salt splitting anion exchange process.

[0079] FIG. 9 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide (DAC) and an electrolytic salt splitting anion exchange process.

[0080] FIG. 10 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, with reactants for the hydrometallurgical process provided by capture of carbon dioxide (DAC) and an electrolytic salt splitting anion exchange process.

[0081] FIG. 11 is a schematic illustration of an integrated process for hydrometallurgical value extraction from a mineral feedstock, which includes an initial step of magnetic beneficiation to adjust the metal content of the treated material.

DETAILED DESCRIPTION

[0082] Processes are provided in which successive steps of hydrometallurgical value extraction are carried out using the products of carbon capture and an electrolytic reactant regeneration process, such as a chloralkali process or an electrolytic salt splitting anion exchange process. The electrolytic reactant regeneration process provides an acid leachant and an alkali hydroxide, with the alkali hydroxide (e.g. NaOH) then available for use either directly as a precipitant in the hydrometallurgical steps, or available for conversion to an alkali metal carbonate (e.g. Na.sub.2CO.sub.3) or bicarbonate (e.g. NaHCO.sub.3) that can in turn be used as the precipitant in the hydrometallurgical steps.

[0083] In an alternative embodiment, the alkali hydroxide from the chloralkali process may be used to precipitate a calcium hydroxide product, with the calcium hydroxide product then available for use directly in carbon dioxide gas scrubbing, or for use to accept a carbonate that is provided by a CO.sub.2 scrubbing process.

[0084] In some embodiments, a crystalliser step may be introduced to precipitate Na.sub.2CO.sub.3 or Na.sub.2CO.sub.3 hydrates from a CO.sub.2 enriched solution that is being treated with the alkali hydroxide (NaOH) product of the chloralkali process. In such processes, a crystalliser may be used to reduce water content in the hydrates by modulating temperature, pressure and NaOH concentration. The solid Na.sub.2CO.sub.3 product may then be used as a carbonate precipitant.

[0085] By using a carbonate precipitant to precipitate iron and aluminum from the leach solution, at a suitably low pH, the carbonate will decompose to release a concentrated stream of CO.sub.2, and the concentrated CO.sub.2 stream may in turn be sequestered or fixed.

[0086] FIG. 1 illustrates a process in which metal values are leached from a comminuted (crushing and grinding) mineral feedstock with an acid leachant (HCl leaching), to produce a solid siliceous residue (Amorphous Silica Residue for Cement Manufacture) and a loaded leach solution. As illustrated, the residue may be washed. Crushing and grinding in a recycled brine solution containing a variety of chloride or sulfate salts, such as magnesium and sodium salts, may be carried out so as to avoid or minimize the need for the addition of non-brine water. HCl acid leaching may be carried out at relatively high acid concentrations, such as 30-36% HCl by weight in watera typical product from an HCl production facility attached to a chlor-alkali plant.

[0087] As illustrated in FIG. 11, in an embodiment of the invention, the ferromagnetic content of the crushed ore may be modulated using a magnetic separator, for example so as to increase or decrease the iron and nickel hydroxide products of the process. For example, with an (ultra)mafic sand input comprising olivine or wollastonite, the ratio of MgSiO.sub.4 and CaSiO.sub.4 content to nickel and iron may be optimised via magnetic separation. In a further alternative, a resin in leach process may be used to selectively remove nickel content in the acidic leach prior to selective precipitation steps, to obtain a purified nickel product.

[0088] Conditions for leaching may include a leaching temperature of from 80 C. to boiling point, to 115 C. or higher. Acid addition during HCl leaching may for example range from 500 to 1000 kg HCl per dry tonne of solid feed, varying with the chemical composition of the feed. Leaching times may for example be for effective residence times of from 1 hour to 8 hours. Leaching may for example be carried out in a single stage or two or more countercurrent stages. In a single stage process, the acid and ore are added together and allowed to react at a leaching temperature to completion. In a multistage leach, fresh ore is contacted with partly reacted solution so as to maximize the use of the acid (low terminal acidity) and in the second or subsequent stage, the partly leached ore (from the first stage) is contacted with high acid to maximize extraction of Mg/Ni/Co/Fe, etc. The multistage process may involve additional solid/liquid separation steps to ensure countercurrent movement of solids and liquids.

[0089] The raw materials for the present processes may contain a variety of silicate minerals including magnesium, iron, nickel and cobalt and minor impurity elements. The chemistry of acid leaching, with HCl, may therefore be represented the following reactions:

Mg.sub.2SiO.sub.4+4HCl=2MgCl.sub.2+SiO.sub.2+2H.sub.2O

Ni.sub.2SiO.sub.4+4HCl=2NiCl.sub.2+SiO.sub.2+2H.sub.2O

Fe.sub.2SiO.sub.4+4HCl=2FeCl.sub.2+SiO.sub.2+2H.sub.2O

[0090] Other minerals present in source materials such as iron oxides or aluminum oxides may also react with HCl to form additional salts in solution:

FeO(OH)+3HCl=FeCl.sub.3+2H.sub.2O

AlO(OH)+3HCl=AlCl.sub.3+2H.sub.2O

[0091] Natural mineral source materials are of course not pure compounds, so that the source minerals my contain a variety of elements (eg. Mg, Ni, Co, Fe in one silicate mineral) and may be hydrated or weathered. Geological descriptions of suitable feed materials include: nickel saprolite ores, olivine ores, and asbestos ores and tailings.

[0092] The product of HCl leaching is a weakly acidic solution containing various chloride salts. A silica rich residue is recovered as a solid product. This residue may for example be washed to remove salts and excess acid with fresh water, and/or alkalized (alkali conditioning) with a base to adjust pH, and then directed to cement manufacture where the silica may be used as a replacement for other materials (thus lowering the carbon intensity of cement manufacture) and as a strengthener to improve the yield strength of concrete, with the silica acting as a supplementary cementitious material (SCM) in a high performance concrete.

[0093] Iron and/or aluminum are precipitated (Iron and Aluminum Precipitation) from the loaded leach solution with an alkali hydroxide (NaOH) or alkali metal carbonate or bicarbonate precipitant (Na.sub.2CO.sub.3 as illustrated in FIG. 1). When Na.sub.2CO.sub.3 is used as a precipitant, this produces a carbon dioxide off gas (CO.sub.2 Off Gas), an Fe/Al depleted solution and an iron and/or aluminum hydroxide or oxide precipitate product (Fe/Al Hydroxide Precipitate as illustrated, comprising magnetite in select embodiments). As illustrated, the residue is washed to provide the precipitate. When an alkali hydroxide (e.g. KOH or NaOH) is used as the precipitant, the iron and aluminum content in the solution is generally precipitated as a mix of oxide and hydroxide solids by raising the pH with an alkali hydroxide (KOH or NaOH) solution. The NaOH solution may for example be added as a 50% solution, and may be diluted with recycled brine solution for process convenience and enhanced pH control (it may be hard to control pH when adding a very strong base). The added NaOH neutralizes excess acid and precipitates Fe/Al and other trivalent cations if present:

HCl+NaOH=NaCl+H.sub.2O

FeCl.sub.3+3NaOHFeO(OH)+3NaCl+H.sub.2O

2FeCl.sub.3+6NaOHFe.sub.2O.sub.3(hematite)+6NaCl+3H.sub.2O

AlCl.sub.3+3NaOHAlO(OH)+3NaCl+H.sub.2O

2AlCl.sub.3+6NaOH=Al.sub.2O.sub.3+6NaCl+3H.sub.2O

CrCl.sub.3+3NaOHCrO(OH)+3NaCl+H.sub.2O

2CrCl.sub.3+6NaOH=Cr.sub.2O.sub.3+6NaCl+3H.sub.2O

[0094] The pH adjustment may for example be conducted with stoichiometric amounts of alkali hydroxide. Over-addition of NaOH may result in precipitation of Ni/Co (undesirable) so control of base addition must be maintained. The Fe/Al precipitation temperature may for example be 75 C. to boiling point. Seed (precipitate) may be recycled, for example in the form of hematite, to ensure growth of suitably sized particles, and materials, for enhanced solid/liquid separation. An initial mineral seed, such as hematite, may be used to initiate the process of precipitating a select material, such as hematite. Fe/Al precipitation time may for example be 1 to 8 hours. NaOH may for example be added progressively through precipitation tanks (continuous) so as to enhance precipitation of coarser/separable precipitates. The Fe/Al precipitation product may be separated by S/L separation and washed.

[0095] The Fe/Al precipitation residue may for example be treated to form commercial products, such as hematite. For example, drying and partial reduction may be used to form magnetite and a mixed Al/Cr oxide. The magnetite can be separated using magnetic separation and the Al/Cr oxide can be sold as a product for the refractory market.

[0096] Nickel and cobalt may be selectively recovered in a variety of ways. In an HCl based leaching process, Ni and Co will be present in solution as NiCl.sub.2 and CoCl.sub.2 salts, and these salts can be recovered by ion exchange, for example using a Dow M4195 resin to extract Ni and Co in a Na-form resin. The resin can then be stripped with HCl solution to form a strong, purified solution of Ni/Co chloride salts. The resin may then be treated with NaOH solution after acid stripping to return to the resin loading step.

[0097] In select embodiments, the recovery of Ni/Co is by way of a mixed hydroxide precipitate (MHP). This can be done directly from the solution coming from the iron precipitation step, or can be effected starting with the ion exchange eluant containing nickel and cobalt chloride. In these processes, a solution of sodium hydroxide is added to from the precipitates:

NiCl.sub.2+2NaOHNi(OH).sub.2+2NaCl

CoCl.sub.2+2NaOHCo(OH).sub.2+2NaCl

[0098] Other metals may also precipitate with the Ni/Co in minor amounts. For example Mn, Fe (remaining iron in solution).

[0099] The selectivity of Ni/Co MHP precipitation can be enhanced by using two stage MHP precipitation, in which a second stage precipitate is recovered and recycled to the first stage or to the discharge from the main leaching step (where acid is present to redissolve the Ni/Co and other metals from the second stage leach).

[0100] The mixed hydroxide precipitate may be recovered by S/L separation and washing. A pressure filter may be used with a squeeze cycle to minimize the entrained moisture in the washed Ni/Co MHP cake prior to shipping.

[0101] The Ni/Co MHP precipitation may be carried out between 25-90 C. with a terminal pH in the range of 5-8. The addition of base can also be controlled by stoichiometry rather than, or in addition to, pH. The Ni/Co MHP precipitation time may for example be 1-8 hours. Seed recycling may be used to maximize particle size and minimize contamination. The Ni/Co MHP process (as in all steps) may be conducted continuously.

[0102] As illustrated in FIG. 1, in an alternative embodiment nickel and/or cobalt may be precipitated from the Fe/Al depleted solution with a second alkali metal carbonate or bicarbonate precipitant (Na.sub.2CO.sub.3 as illustrated), to produce a Ni/Co depleted solution and a nickel and/or cobalt carbonate precipitate product (Ni/Co Carbonate (to battery manufacture)).

[0103] Most of the iron and aluminum are removed from solution in the first iron removal step. Manganese is generally not removed from solution in either the initial iron control or the Ni/Co MHP precipitation steps. Accordingly, a second stage of iron precipitation may be implemented with increased pH so as to maximize the removal of iron with an oxidant added to oxidize Mn and Fe to facilitate more complete removal and purification of all species. Suitable oxidants include gaseous chlorine or sodium hypochlorite (NaOCl). Example reactions include:

2FeCl.sub.2+NaOCl+4NaOH2FeO(OH)+5NaCl+H.sub.2O

MnCl.sub.2+NaOCl+2NaOH=MnO.sub.2+3NaCl+H.sub.2O

AlCl.sub.3+3NaOHAlO(OH)+3NaCl+H.sub.2O

[0104] Conditions for iron and/or aluminum and/or manganese scrubbing may be designed to maximize precipitation of the impurity elements while minimizing formation of magnesium hydroxide. The oxidant (eg. NaOCl) may be added so as to achieve a suitably high oxidation/reduction potential (ORP) to maximize the oxidative removal of Fe/Mn. Scrubbing temperature may for example be 25 C. to the boiling point. As in other precipitation steps, seed recycle can be used to improve performance. Scrubbing time may for example be 1 to 8 hours.

[0105] Alternatively, as illustrated in FIG. 1, iron and/or aluminum and/or manganese may be scrubbed from the Ni/Co depleted solution with a third alkali metal carbonate or bicarbonate precipitant (also Na.sub.2CO.sub.3 as illustrated) and an oxidant, such as the illustrated sodium hypochlorite, to produce an Fe/Al/Mn depleted solution and an iron and/or aluminum and/or manganese hydroxide precipitate product (Fe/Al/Mn Hydroxide Precipitate). As illustrated, brine comprising the Fe/Al/Mn depleted solution may be recycled to the comminuting step to provide the comminuted mineral feedstock.

[0106] Magnesium remaining in solution may be precipitated from the Fe/Al/Mn depleted solution with an alkali hydroxide precipitant (NaOH as illustrated), to produce a Mg-depleted solution and a magnesium hydroxide precipitate product (Mg Hydroxide Precipitate):

MgCl.sub.2+2NaOHMg(OH).sub.2+2NaCl

[0107] This may for example be carried out by adding NaOH to MgCl.sub.2 solution, or by reversing the order of addition. In either case, the process may be carried out so as to provide a near complete removal of Mg as Mg(OH)2 from solution. This generally requires a near stoichiometric addition of NaOH.

[0108] The Mg-depleted solution may then be subjected to further purification, for example in an ion exchange resin separation step, or sent directly to an electrolysis to produce the alkali hydroxide precipitant and the acid leachant (in FIG. 1, Chlor-Alkali Plant to make HCl and NaOH for Recycle, in FIG. 7 Salt Splitting Plant to make H.sub.2SO.sub.4 and NaOH for Recycle). Standard chloralkali brine pretreatments may be carried out on the Mg-depleted solution to provide a higher purity Mg-depleted brine, for example essentially free of undesirable solids and ions, for example involving brine saturation/evaporation and softening, for example by primary and polish filtration steps and high-performance ion exchange softening. In an HCl based extraction process, the final Mg-depleted solution is NaCl.sub.(aq) with some minor contaminants in solution. This NaCl.sub.(aq) solution is directed to a chlor-alkali plant for manufacture of NaOH, Cl.sub.2 and H.sub.2, involving conventional steps, with the Cl.sub.2 and H.sub.2 available to be burned and water-scrubbed to form a strong HCl solution for recycle to leaching. Excess heat from Cl.sub.2 and H.sub.2 combustion may for example be recovered as steam and used to evaporate excess water from solution.

[0109] As illustrated in FIG. 1, carbon dioxide may be scrubbed from a CO.sub.2 containing gas (Air as illustrated) by treating the CO.sub.2 containing gas with a scrubbing solution comprising the alkali hydroxide precipitant (NaOH as illustrated), to produce one or more of the alkali metal carbonate or bicarbonate precipitants (Na.sub.2CO.sub.3 as illustrated).

[0110] In the foregoing process, the step of scrubbing carbon dioxide from the CO.sub.2 containing gas may include a crystallisation step to precipitate Na.sub.2CO.sub.3 hydrates from the scrubbing solution, the alkali hydroxide precipitant being NaOH. The solid Na.sub.2CO.sub.3 crystallizer product may then be directed to provide one or more of the alkali metal carbonate or bicarbonate precipitants.

[0111] FIG. 2 illustrates a process analogous to the process illustrated in FIG. 1, with potassium compounds in place of the sodium compounds of FIG. 1.

[0112] FIG. 3 and FIG. 4 illustrate alternative embodiments which involve precipitating calcium from the Mg-depleted solution with a fourth alkali metal hydroxide precipitant (NaOH as illustrated), to produce a Ca-depleted solution and a calcium hydroxide product. The calcium hydroxide product is then available for carbon sequestration reactions, for example by generating the metal carbonate precipitant for the iron and/or aluminum precipitation step by treating the calcium hydroxide product with a carbon source, such as air (FIG. 3) or a metal carbonate that is in turn derived from KOH-mediated carbon capture (FIG. 4). In these processes, the Ca-depleted solution is subjected to electrolysis to produce one or more of the first, second, third or fourth alkali metal hydroxide precipitants and the acid leachant.

[0113] The alkali hydroxide precipitant may accordingly be NaOH (FIGS. 1, 3 and 4) or KOH (FIG. 2). The process acid leachant as illustrated is HCl. These products may be produced in a chloralkali process.

[0114] FIG. 5 and FIG. 6 illustrate alternative embodiments, in which alternative pathways are used to form MgCO.sub.3 rather than Mg(OH).sub.2 in the magnesium precipitation step. These embodiments reflect adaptations related to the use of Mg(OH).sub.2 from the present processes for: (1) direct air capture (DAC) of CO.sub.2 to form MgCO.sub.3; or, (2) ocean alkalinity enhancement (OAE) to form Mg(HCO.sub.3).sub.2 by direct addition of Mg(OH).sub.2 to the ocean environment. The use of Mg(OH).sub.2 to form MgCO.sub.3 by contact with air containing CO.sub.2 can in some circumstances suffer from unfavourable kinetics. The embodiments illustrated in FIG. 5 and FIG. 6 accordingly provide alternative routes to forming MgCO.sub.3 in approaches that may be adapted to optimize carbon sequestration.

[0115] FIG. 5 illustrates a process in which MgCO.sub.3 is formed by direct neutralization of the Fe/Al/Mn depleted solution, so that Na.sub.2CO.sub.3, for example produced in and recovered from a direct air capture (DAC) process, reacts with MgCl.sub.2 (aq) in the Fe/Al/Mn depleted solution to form MgCO.sub.3(s):

MgCl.sub.2+Na.sub.2CO.sub.3=MgCO.sub.3+2NaCl

[0116] In select embodiments, essentially the full amount of NaOH produced by the chloralkali process is directed to the DAC system to produce Na.sub.2CO.sub.3 from CO.sub.2 captured directly from the atmosphere. In such a process, sufficient Na.sub.2CO.sub.3 is produced to provide the alkali metal precipitant for all aspects of the process, including recovery of MgCO.sub.3. In this way, sorbent regeneration for DAC, i.e. NaOH, is combined with long term mineralisation of the CO.sub.2. MgCO.sub.3 mineralisation thereby creates carbon negative products in the form of carbonates, that may for example be used as filler or construction aggregate.

[0117] FIG. 6 illustrates an alternative process involving the formation of MgCO.sub.3 by direct addition of CO.sub.2 gas, with addition of NaOH, to the Fe/Al/Mn depleted solution, to react with MgCl.sub.2 (aq) in solution to form MgCO.sub.3(s):

MgCl.sub.2+2NaOH+CO.sub.2(g)=MgCO.sub.3+2NaCl+H.sub.2O

[0118] As illustrated in FIG. 6, a portion of NaOH from the chloralkali process may be directed to the Mg precipitation stage, together with CO.sub.2(g) (for example recovered as a CO.sub.2 off gas from iron and aluminum precipitation with Na.sub.2CO.sub.3), forming MgCO.sub.3 in-situ. Alternatively, CO.sub.2(g) for Mg carbonate precipitation may come from sources external to the present process.

[0119] Reactions in various stages of the present process may be represented as follows:

Neutralization

[0120] Alkali hydroxide: 2HCl+2NaOH=2NaCl+2H.sub.2O [0121] Alkali metal carbonate: 2HCl+Na.sub.2CO.sub.3=2NaCl+H.sub.2O+CO.sub.2(g)

Iron Precipitation

[0122] Alkali hydroxide: 2FeCl.sub.3+6NaOH2FeO(OH)+2H.sub.2O+6NaCl [0123] 2FeCl.sub.3+6NaOHFe.sub.2O.sub.3 (hematite)+6NaCl+3H.sub.2O [0124] Alkali metal carbonate: 2FeCl.sub.3+3Na.sub.2CO.sub.3+H.sub.2O 2FeO(OH)+6NaCl+3CO.sub.2(g)

Nickel Recovery

[0125] Alkali hydroxide: NiCl.sub.2+2NaOHNi(OH).sub.2+2NaCl [0126] Alkali metal carbonate: NiCl.sub.2+Na.sub.2CO.sub.3=NiCO.sub.3+2NaCl

Magnesium Recovery

[0127] Alkali hydroxide: MgCl.sub.2+2NaOHMg(OH).sub.2+2NaCl [0128] Alkali metal carbonate: MgCl.sub.2+Na.sub.2CO.sub.3=MgCO.sub.3+2NaCl [0129] Direct CO.sub.2: MgCl.sub.2+2NaOH+CO.sub.2(g)=MgCO.sub.3+2NaCl+H.sub.2O

[0130] In alternative embodiments, NaHCO.sub.3 may take the place of Na.sub.2CO.sub.3 in reactions in various stages of the present process.

[0131] FIGS. 7-10 illustrate processes in which metal values are leached from a comminuted (crushing and grinding) mineral feedstock with a sulfuric acid leachant (H.sub.2SO.sub.4 leaching), to produce a solid siliceous residue (Amorphous Silica Residue for Cement Manufacture) and a loaded leach solution. As illustrated, the residue may be washed.

[0132] Iron and/or aluminum are precipitated (Iron and Aluminum Precipitation) from the loaded leach solution with either an alkali hydroxide precipitant (FIG. 7) or an alkali metal carbonate or bicarbonate precipitant (Na.sub.2CO.sub.3 FIGS. 8-10). Use of the alkali metal carbonate or bicarbonate precipitant produces a carbon dioxide off gas (CO.sub.2 Off Gas), an Fe/Al depleted solution and an iron and/or aluminum hydroxide or oxide precipitate product (Fe/Al Hydroxide Precipitate, which may be an oxide, such as hematite). The concentrated CO.sub.2 Off Gas may be sequestered using a variety of approaches. As illustrated, the residue may be washed to provide the precipitate, and the precipitate may be used in magnetite manufacture.

[0133] Nickel and/or cobalt are precipitated from the Fe/Al depleted solution with the alkali hydroxide precipitant (e.g. NaOH, FIG. 7) or the alkali metal carbonate or bicarbonate precipitant (e.g. Na.sub.2CO.sub.3, FIGS. 8-10), to produce a Ni/Co depleted solution and a nickel and/or cobalt hydroxide (FIG. 1, MHP) or carbonate precipitate product (FIGS. 8-10, Ni/Co Carbonate (to battery manufacture)).

[0134] Iron and/or aluminum and/or manganese may be scrubbed from the Ni/Co depleted solution with the alkali hydroxide precipitant (FIG. 7) or with the alkali metal carbonate or bicarbonate precipitant (FIGS. 8-10, Na.sub.2CO.sub.3) and an oxidant, such as the illustrated sodium persulfate (Na.sub.2S.sub.2O.sub.8), to produce an Fe/Al/Mn depleted solution and an iron and/or aluminum and/or manganese hydroxide precipitate product (Fe/Al/Mn Hydroxide Precipitate).

[0135] As illustrated, brine comprising the Fe/Al/Mn depleted solution may be recycled to the comminuting step to provide the comminuted mineral feedstock.

[0136] Magnesium may be precipitated from the Fe/Al/Mn depleted solution with the alkali hydroxide precipitant (NaOH as illustrated in FIGS. 7 and 8), or with the alkali metal carbonate or bicarbonate precipitant (FIG. 9) or with a combined feed of the alkali hydroxide precipitant and CO.sub.2 (in a carbon dioxide capture step,

[0137] FIG. 10) to produce a Mg-depleted solution and a magnesium hydroxide (FIGS. 7 and 8) or carbonate (FIGS. 9 and 10) precipitate product, The Mg-depleted solution may then be subjected to an electrolysis to produce the alkali hydroxide precipitant and the acid leachant (Salt Splitting Plant to make H.sub.2SO.sub.4 and NaOH for Recycle).

[0138] Carbon dioxide may be scrubbed from a CO.sub.2 containing gas (Air as illustrated) by treating the CO.sub.2 containing gas with a scrubbing solution comprising the alkali hydroxide precipitant (NaOH as illustrated), to produce one or more of the first, second, third and fourth alkali metal carbonate or bicarbonate precipitants (Na.sub.2CO.sub.3 as illustrated), for use respectively in i) iron and aluminum precipitation, ii) Ni/Co precipitation, iii) iron and aluminum precipitation with manganese removal, and iv) Mg precipitation.

[0139] In the foregoing process, the step of scrubbing carbon dioxide from the CO.sub.2 containing gas may include a crystallisation step to precipitate Na.sub.2CO.sub.3 hydrates from the scrubbing solution, the alkali hydroxide precipitant being NaOH. The solid Na.sub.2CO.sub.3 crystalizer product may then be directed to provide one or more of the alkali metal carbonate or bicarbonate precipitants.

[0140] The process acid leachant as illustrated is H.sub.2SO.sub.4. As such, processes are provided that use of a sulfate based system for treatment of magnesium silicates. In select embodiments, (FIG. 7) H.sub.2SO.sub.4/NaOH/Na.sub.2SO.sub.4 salt splitting is used to produce amorphous silica for cementing, iron residue, mixed nickel and cobalt hydroxide and magnesium hydroxidewhich is then available for carbon sequestration. In alternative embodiments, various direct air carbon capture (DAC) steps are integrated into the sulfate system (FIGS. 8-10). In particular, FIG. 8 illustrates a process wherein a portion of the alkali hydroxide precipitant NaOH is used to remove CO.sub.2 from air. The resulting sodium carbonate is then used in the iron removal and the nickel/cobalt precipitation stages. FIG. 9 illustrates a process in which there is complete use of NaOH for DAC to form Na.sub.2CO.sub.3. The addition of Na.sub.2CO.sub.3 to the Mg precipitation stage results in MgCO.sub.3 precipitation directly for carbon sequestration. FIG. 10 illustrates an alternative embodiment in which the alkali hydroxide precipitant NaOH is combined with CO.sub.2 added directly to the Mg precipitation stage, to form MgCO.sub.3.

[0141] Steps in the sulfate process may be characterized by reactions therein, as follows:

Acid leaching (simplified); [0142] Mg.sub.2SiO.sub.4+2H.sub.2SO.sub.4=2MgSO.sub.4+SiO.sub.2+2H.sub.2O [0143] Ni.sub.2SiO.sub.4+2H.sub.2SO.sub.4=2NiSO.sub.4+SiO.sub.2+2H.sub.2O [0144] Co.sub.2SiO.sub.4+2H.sub.2SO.sub.4=2CoSO.sub.4+SiO.sub.2+2H.sub.2O [0145] Fe.sub.2SiO.sub.4+2H.sub.2SO.sub.4=2FeSO.sub.4+SiO.sub.2+2H.sub.2O [0146] MnO.sub.2+2FeSO.sub.4+2H.sub.2SO.sub.4=MnSO.sub.4+Fe.sub.2(SO.sub.4).sub.3+2H.sub.2O [0147] 2FeO(OH)+3H.sub.2SO.sub.4Fe.sub.2(SO.sub.4).sub.3+4H.sub.2O [0148] 2Al(OH)+3H.sub.2SO.sub.4Al.sub.2(SO.sub.4).sub.3+4H.sub.2O
Iron/aluminum removal (with product); [0149] H.sub.2SO.sub.4+2NaOH=Na.sub.2SO.sub.4+2H.sub.2O [0150] Al.sub.2(SO.sub.4).sub.3+6NaOH2Al(OH).sub.3+3Na.sub.2SO.sub.4 (Aluminum hydroxide) [0151] Fe.sub.2(SO.sub.4).sub.3+6NaOH2Fe(OH).sub.3+3Na.sub.2SO.sub.4 (Iron hydroxide) [0152] Al.sub.2(SO.sub.4).sub.3+6NaOH2Al(OH)+3Na.sub.2SO.sub.4+2H.sub.2O (Aluminum oxyhydroxide) [0153] Fe.sub.2(SO.sub.4).sub.3+6NaOH2FeO(OH)+3Na.sub.2SO.sub.4+2H.sub.2O (Iron oxyhydroxide) [0154] Fe.sub.2(SO.sub.4).sub.3+6NaOH=Fe.sub.2O.sub.3+3Na.sub.2SO.sub.4+3H.sub.2O (hematite) [0155] 3Al.sub.2(SO.sub.4).sub.3+12NaOH2NaAl.sub.3(SO.sub.4).sub.2(OH).sub.6+5Na.sub.2SO.sub.4 (Alunite) [0156] 3Fe.sub.2(SO.sub.4).sub.3+12NaOH2NaFe.sub.3(SO.sub.4).sub.2(OH).sub.6+5Na.sub.2SO.sub.4 (Jarosite)

Nickel and Cobalt Precipitation

[0157] NiSO.sub.4+2NaOHNi(OH).sub.2+Na.sub.2SO.sub.4 [0158] CoSO.sub.4+2NaOHCo(OH).sub.2+Na.sub.2SO.sub.4

Iron/Aluminum/Manganese Removal Stage 2

[0159] Al.sub.2(SO.sub.4).sub.3+6NaOH2Al(OH).sub.3+3Na.sub.2SO.sub.4 (Aluminum hydroxide) [0160] Fe.sub.2(SO.sub.4).sub.3+6NaOH2Fe(OH).sub.3+3Na.sub.2SO.sub.4 (Iron hydroxide) [0161] Al.sub.2(SO.sub.4).sub.3+6NaOH2Al(OH)+3Na.sub.2SO.sub.4+2H.sub.2O (Aluminum oxyhydroxide) [0162] Fe.sub.2(SO.sub.4).sub.3+6NaOH2FeO(OH)+3Na.sub.2SO.sub.4+2H.sub.2O (Iron oxyhydroxide) [0163] 3Al.sub.2(SO.sub.4).sub.3+12NaOH2NaAl.sub.3(SO.sub.4).sub.2(OH).sub.6+5Na.sub.2SO.sub.4 (Alunite) [0164] 3Fe.sub.2(SO.sub.4).sub.3+12NaOH2NaFe.sub.3(SO.sub.4).sub.2(OH).sub.6+5Na.sub.2SO.sub.4 (Jarosite) [0165] MnSO.sub.4+Na.sub.2S.sub.2O.sub.8+4NaOH=MnO.sub.2+3Na.sub.2SO.sub.4+2H.sub.2O

Magnesium Hydroxide Precipitation

[0166] MgSO.sub.4+2NaOHMg(OH).sub.2+Na.sub.2SO.sub.4

Salt Splitting (Anion Exchange Membrane)

[0167] 2Na.sub.2SO.sub.4+4H.sub.2O=4NaOH+2H.sub.2SO.sub.4+2H.sub.2+O.sub.2

[0168] In alternative embodiments, processes make use of NaOH, NaHCO.sub.2 or Na.sub.2CO.sub.3 precipitants, with some alternative chemistries shown below:

Neutralization

[0169] Alkali hydroxide: H.sub.2SO.sub.4+2NaOH=Na.sub.2SO.sub.4+2H.sub.2O [0170] Alkali metal carbonate: H.sub.2SO.sub.4+Na.sub.2CO.sub.3=Na.sub.2SO.sub.4+H.sub.2O+CO.sub.2(g)

Iron Precipitation

[0171] Alkali hydroxide: Fe.sub.2(SO.sub.4).sub.3+6NaOH2Fe(OH).sub.3+3Na.sub.2SO.sub.4 [0172] or Fe.sub.2(SO.sub.4).sub.3+6NaOH=Fe.sub.2O.sub.3+3Na.sub.2SO.sub.4+3H.sub.2O [0173] Alkali metal carbonate: Fe.sub.2(SO.sub.4).sub.3+3Na.sub.2CO.sub.3+H.sub.2O 2FeO(OH)+ [0174] 3Na.sub.2SO.sub.4+3CO.sub.2(g)

Nickel Recovery

[0175] Alkali hydroxide: NiSO.sub.4+2NaOHNi(OH).sub.2+Na.sub.2SO.sub.4 [0176] Alkali metal carbonate: NiSO.sub.4+Na.sub.2CO.sub.3=NiCO.sub.3+Na.sub.2SO.sub.4

Magnesium Recovery

[0177] Alkali hydroxide: MgSO.sub.4+2NaOHMg(OH).sub.2+Na.sub.2SO.sub.4 [0178] Alkali metal carbonate (with Na.sub.2CO.sub.3): MgSO.sub.4+Na.sub.2CO.sub.3=MgCO.sub.3+Na.sub.2SO.sub.4 [0179] Alkali metal carbonate with NaOH/CO.sub.2(g): MgSO.sub.4+2NaOH+CO.sub.2=MgCO.sub.3+Na.sub.2SO.sub.4+H.sub.2O

[0180] The present processes may be integrated with other carbon sequestration processes, such as ocean alkalinity enhancement. This present processes for the production of synthetic brucite and calcium hydroxide accordingly address environmental risks of direct ocean alkalinity enhancement with untreated mafic rocks. The present processes also create a less carbon intensive source of magnesium and calcium hydroxides to be used as feedstock in carbon capture and storage, including direct air capture technologies. The use of the brucite or calcium hydroxide products of the present processes in a direct air capture (DAC) process may be carried out so as to eliminate calcining and slacking steps that are otherwise required in these processes. The present processes provide for the use of basaltic sands in less carbon intensive industrial purposes, by producing low carbon sources of nickel and iron hydroxides as well as amorphous silicate (SiO.sub.2).

[0181] Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Terms such as exemplary or exemplified are used herein to mean serving as an example, instance, or illustration. Any implementation described herein as exemplary or exemplified is accordingly not to be construed as necessarily preferred or advantageous over other implementations, all such implementations being independent embodiments. Unless otherwise stated, numeric ranges are inclusive of the numbers defining the range, and numbers are necessarily approximations to the given decimal. The word comprising is used herein as an open-ended term, substantially equivalent to the phrase including, but not limited to, and the word comprises has a corresponding meaning. As used herein, the singular forms a, an and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a thing includes more than one such thing. Citation of references herein is not an admission that such references are prior art to the present invention. Any priority document(s) and all publications, including but not limited to patents and patent applications, cited in this specification, and all documents cited in such documents and publications, are hereby incorporated herein by reference as if each individual publication were specifically and individually indicated to be incorporated by reference herein and as though fully set forth herein. The invention includes all embodiments and variations substantially as hereinbefore described and with reference to the examples and drawings.