Acid leach process for ferronickel slag

Abstract

The present disclosure relates to a process of extracting magnesium or magnesium compounds from a ferronickel slag feed comprising magnesium and silica compounds, the process comprising: a comminution step to reduce a particle size distribution of the ferronickel slag feed; and an acid leaching step after the comminution step, comprising: leaching the ferronickel slag feed from the comminution step in a vessel with hydrochloric acid at a concentration of 5-15 wt.% HCl to form a leached slurry, controlling the hydrochloric acid concentration during leaching by adding further hydrochloric acid to the vessel to maintain the hydrochloric acid concentration at 5-15 wt.% HCl, and obtaining a leached slurry from the vessel, wherein the leached slurry comprises a liquid portion and a solids residue, and wherein the liquid portion comprises magnesium chloride.

Description

ACID LEACH PROCESS FOR FERRONICKEL SLAGThe present application claims the benefit of and priority to Australian Patent Application No. 2024903164, the entire contents of which is incorporated herein by reference.TECHNICAL FIELD

[0001] The present disclosure relates to an improved acid leach process for extracting valuable elements, metals, or metal compounds from a slag feed. More specifically, embodiments of the present disclosure relate to an improved acid leach-based process for extracting magnesium from ferrous or ferroalloy slags, such as a ferronickel slag.BACKGROUND

[0002] Slags and similar by-products from smelting processes typically include a number of potentially useful or valuable materials dispersed in the slag composition (e.g., magnesium, calcium, aluminium, rare earth metals, etc). However, in conventional industrial processes, some or all of these materials may not be extracted prior to disposal of the slag due to the technical difficulty, high reagent use, time requirements, and / or low yield of processes for recovering these materials, making the extraction non-viable from a technical and commercial perspective.

[0003] This is evident, for example, in the processing of laterite ores to make ferronickel, which produces a slag (i.e. , a 'ferronickel slag') with no or limited commercial use that is typically stockpiled as a waste product. In principle, the magnesium may be recovered from such slags by leaching with an acid and purifying the resultant solution to obtain magnesium bearing product. For example, Mubarok, M. Z., and A. Yudiarto. "Synthesis of magnesium oxide from ferronickel smelting slag through hydrochloric acid leaching-precipitation and calcination." Energy Technology 2017: Carbon Dioxide Management and Other Technologies. Springer International Publishing, 2017 describes a maximum magnesium extraction of 97% from a ferronickel slag by grinding the particles to -200 mesh (i.e., less than 74 pm particle size) and leaching with a high acid concentration (8M HCI, approx. 25 wt.% HCI) at 80°C for 24 hours and a 40:1 liquidzsolids ratio. However, each other experiment in Mubarok resulted in a substantially smaller magnesium extraction, i.e., less than 80%, which would not be commercially viable for an industrial production process.

[0004] In addition, Mubarok reported that extending the leach step to 48 hours resulted in a precipitation of gelatinous silica and a decreased magnesium extraction, and that, at lower liquicksolids ratios, the solution viscosity was higher; the Mg extraction was lower; and the likelihood of gelatinous silica formation was increased.

[0005] The formation of silica gels is well-known in the field of hydrometallurgy. For example, Queneau, P., and Berthold, C., “Silica in Hydrometallurgy: An Overview” Canadian Metallurgical Quarterly, 25:3, 201-209, 1986 describes that:“In acid solutions, silica concentrations in excess of the equilibrium solubility of monomeric silica [Si(OH)4] are encountered [...] These high levels of silica result from polymerisation [...] Aging results in the formation of amorphous silica, either in the form of a colloid, a precipitate or a gel. Colloidal particles are so small that they do not settle. Silica gel forms when these particles aggregate in a manner that incorporates most, if not all, of the water. [...] Whether a gel or precipitate forms depends on the degree of silica supersaturation, the availability of silica surface upon which fresh silica may be deposited, temperature, pH and ionic strength. ”

[0006] The formation of a gel is highly disadvantageous during acid leaching, particularly in an industrial manufacturing context.

[0007] Prasetyo, Agus Budi, et al. "Magnesium Extraction of Ferronickel Slag Waste Processed by Alkali Fusion and Hydrochloric Acid Leaching." Journal of Mining and Metallurgy, Section B: Metallurgy 57.2 (2021): 225-233 describes that a silica gel formed from a leached solution is very difficult to separate to obtain the valuable Mg compounds. To prevent formation of silica gels, Prasetyo teaches the addition of an alkali fusion pre-treatment step to reduce the amount of silica in the slag before leaching and thereby reduce the amount of silica available to form a silica gel. However, the pre-treatment step proposed by Prasetyo involved pre-treating the ferronickel slag with a 1:1 addition of sodium carbonate and reacting at an elevated temperature of 1000°C for 60 minutes to transform the silica-bonded phases within the slag (eg, magnesium silicates) to sodium silicates, which could then be washed out with water. These conditions are not suitable for an industrial manufacturing process. The alkali fusion pre-treatment step also has the disadvantage of requiring considerable additional equipment and capital expense to calcine all of the milled slag at 1000°C. Further drawbacks of this approach are the considerable increase in overall carbon dioxide emissions that are released from the sodium carbonate, as well as the need to find a suitable use, or disposal means, for the sodium silicate solution.

[0008] Importantly, the Mubarok and Prasetyo processes for magnesium extraction from ferronickel slag have not been implemented on a large or commercial scale. It is likely that this is due to the significant commercial hinderances, including one or more of: the high cost of the reagents (e.g., concentrated hydrochloric acid, magnesium oxide, ammonium hydroxide and sodium carbonate); the need to dispose of the leach residue (representing about 60 % by weight of the slag feed); low magnesium extraction yields without an extensive leach time (e.g., ~24 hours); or problems with gelatinous silica formation inhibiting operation of the leach and downstream purification steps.

[0009] There is a need for an industrial-scale manufacturing process that allows valuable components e.g., magnesium, to be extracted from low value slag, such as ferronickel slag, without prohibitive reagent consumption. There is also a need for an industrial-scale manufacturing process for extracting valuable components such as magnesium from ferronickel slag without the formation of silica gels.

[0010] Any reference to or discussion of any document, act or item of knowledge in this specification is included solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters or any combination thereof formed at the priority date part of the common general knowledge, or was known to be relevant to an attempt to solve any problem with which this specification is concerned.SUMMARY OF THE INVENTION

[0011] In a first aspect, the present disclosure provides a process of extracting magnesium or magnesium compounds from a ferronickel slag feed comprising magnesium and silica compounds, the process comprising: a comminution step to reduce a particle size distribution of the ferronickel slag feed; and an acid leaching step after the comminution step, comprising: leaching the ferronickel slag feed from the comminution step in a vessel with hydrochloric acid at a concentration of 5-15 wt.% HCI to form a leached slurry, controlling the hydrochloric acid concentration during leaching by adding further hydrochloric acid to the vessel to maintain the hydrochloric acid concentration at 5-15 wt.% HCI, and obtaining a leached slurry from the vessel, wherein the leached slurry comprises a liquid portion and a solids residue, and wherein the liquid portion comprises magnesium chloride.

[0012] In one or more embodiments, the acid leaching step occurs successively i.e. , immediately after the comminution step. In one or more embodiments, there are no intermediate treatment steps between the comminution step and the acid leaching step. In one or moreembodiments, there are no intermediate or pre-treatment steps between the comminution step and the acid leaching step to react the slag feed. In one or more embodiments, the slag feed is not treated with a fluoride compound, such as ammonium fluoride (NH4F), sodium fluoride (NaF) or hexafluorosilicic acid (also known as fluorosilicic acid, H2SiF6), before or during the acid leaching step. The absence of such a treatment step is beneficial in reducing the number of reagents required for extracting the magnesium or magnesium compounds and reducing any waste by-products. In one or more embodiments, the composition of the slag feed is maintained between the comminution step and the acid leaching step.

[0013] In one or more embodiments, the slag feed is transported directly from the comminution step to the vessel for leaching. In other embodiments, transporting the slag feed from the comminution step to the vessel includes storing the slag feed for a period of time.

[0014] In one or more embodiments, the acid leaching step consists essentially of: transporting the slag feed directly from the comminution step to a vessel comprising acid to leach the slag feed and extracting a leached slurry from the vessel.

[0015] In one or more embodiments, controlling the hydrochloric acid concentration during leaching includes adding the further hydrochloric acid in one or more batch addition(s) of HCI or a continuous feed of HCI. In one or more embodiments the further HCI is recycled from a downstream acid recovery step.

[0016] In one or more embodiments, the ferronickel slag feed further includes one or more of aluminium, calcium, chlorine, chromium, potassium, nickel, sulphur, or any combination thereof.

[0017] In one or more embodiments, the ferronickel slag feed includes one or more of ferrosilite (FeSiOs), fayalite (Fe2SiC>4), clinoenstatite (MgSiOs), and fosterite (Mg2SiO4), and other complex iron materials.

[0018] In one or more embodiments, the ferronickel slag feed further includes AI2O3, &2O3, or a combination thereof.

[0019] In an embodiment, the ferronickel slag feed is produced from smelting nickel laterite.

[0020] In an embodiment, the ferronickel slag feed comprises silica at concentrations of about 15-90 wt.%.

[0021] In an embodiment, ferronickel the slag feed comprises silica at concentrations of about 20-50 wt.%.

[0022] In an embodiment, ferronickel the slag feed comprises silica at concentrations of about 25wt.%.

[0023] In certain embodiments, the slag feed comprises silica at concentrations of about 15 wt.%, 20 wt.%, 25 wt.%, 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, 55 wt.%, 60 wt.%, 65 wt.%, 70 wt.%, 75 wt.%, 80 wt.%, 85 wt.%, or 90 wt.%, or any range therebetween.

[0024] In an embodiment, the solids residue comprises amorphous silica.

[0025] In an embodiment, the solids residue comprises amorphous silica at a concentration of about 20-90 wt.%.

[0026] In an embodiment, the solids residue comprises amorphous silica at a concentration of about 30-60 wt.%.

[0027] In certain embodiments, the solids residue comprises amorphous silica at a concentration of about 30-50 wt.%.

[0028] In certain embodiments, the solids residue comprises about 20 wt.%, 25 wt.%, 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, 55 wt.%, 60 wt.%, 65 wt.%, 70 wt.%, 75 wt.%, 80 wt.%, 85 wt.%, 90 wt.%, 95 wt.% amorphous silica, or any range therebetween.

[0029] In an embodiment, in the acid leaching step, controlling the hydrochloric acid concentration during leaching maintains the hydrochloric acid concentration at 8-12 wt.% HCI.

[0030] In an embodiment, in the acid leaching step, controlling the hydrochloric acid concentration during leaching maintains the hydrochloric acid concentration at about 10 wt.% HCI.

[0031] In one or more embodiments, the concentration of HCI in the hydrochloric acid varies during the acid leaching step.

[0032] In an embodiment, at least a portion of the further hydrochloric acid is a recycled hydrochloric acid from a downstream manufacturing step.

[0033] In an embodiment, the recycled hydrochloric acid has a higher concentration of HCI than the hydrochloric acid in the vessel.

[0034] In an embodiment, during the acid leaching step, the slag feed is passed through multiple vessels with hydrochloric acid to leach the slag feed.

[0035] In an embodiment, the concentration of HCI in each vessel is independently controlled.

[0036] In an embodiment, during the acid leaching step, the slag feed has a total residence leaching time of between 0.5 to 12 hours in the vessel(s).

[0037] In an embodiment, the total residence leaching time is between 0.5 to 8 hours or between 0.5 to 6 hours in the vessel(s).

[0038] In an embodiment, the total residence leaching time is about 8 hours in the vessel(s).

[0039] In certain embodiments, the total residence leaching time is about 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, or any range therebetween.

[0040] In an embodiment, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at a temperature between about 50-100°C.

[0041] In certain embodiments, the acid leaching step is controlled to a maximum temperature of about 80°C.

[0042] In an embodiment, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at a temperature of about 80°C.

[0043] In certain embodiments, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at a temperature of about 50°C, about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, or any range therebetween.

[0044] In an embodiment, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at an initial liquicksolid ratio of 10 mL / g to 20 mL / g.

[0045] In certain embodiments, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at an initial liquicksolid ratio of about 10 mL / g, about 15 mL / g, or about 20 mL / g.

[0046] In an embodiment, during the comminution step, the slag feed is milled to a Pgs particle size of at most 100 microns.

[0047] In an embodiment, during the comminution step, the slag feed is milled to a Pgs particle size of about 75 microns.

[0048] In certain embodiments, during the comminution step, the slag feed is milled to a Pgs particle size of about 50 microns, about 55 microns, about 60 microns, about 65 microns, about 70 microns, about 75 microns, about 80 microns, about 85 microns, about 90 microns, about 95 microns, or about 100 microns.

[0049] In an embodiment, the vessel(s) are sealed under negative pressure.

[0050] In one or more embodiments, the acid leaching step is a continuous leaching process.

[0051] In one or more embodiments, the slag feed and the hydrochloric acid are provided in a co-current flow to at least one vessel.

[0052] In one or more embodiments, the slag feed and the hydrochloric acid are provided in a counter-current flow to at least one vessel.

[0053] In one or more embodiments, the process further comprises a residue thickening step, wherein the leached slurry is thickened and separated into an overflow stream and an underflow stream, wherein the underflow stream comprises substantially all of the solids residue.

[0054] In one or more embodiments, the process further comprises an iron removal step, wherein the overflow stream is treated with oxygen and an alkaline reagent to neutralise acids within the overflow stream and to produce a mixture of an iron precipitate stream and a magnesium chloride product stream, wherein the iron precipitate is separated from the magnesium chloride product stream.

[0055] In one or more embodiments, an iron precipitate seed material is added to the overflow stream to seed a formation of the iron precipitate stream.

[0056] In one or more embodiments, the iron precipitate seed material includes a recycled material from a downstream step.

[0057] In one or more embodiments, the alkaline reagent includes a calcium-containing ore and / or a calcined product.

[0058] In one or more embodiments, the alkaline reagent includes limestone and / or quicklime.

[0059] In one or more embodiments, the alkaline reagent includes a magnesium-containing ore.

[0060] In one or more embodiments, the magnesium-containing ore includes magnesite and / or dolomite.

[0061] In one or more embodiments, the alkaline reagent includes magnesium oxide.

[0062] In one or more embodiments, the alkaline reagent includes a recycled magnesium oxide from a downstream step.

[0063] In one or more embodiments, the process further comprises a roasting step, wherein the magnesium chloride product stream is heated to produce a magnesium oxide product and an acid by-product.

[0064] In one or more embodiments, the magnesium chloride product stream has a heating residence time in the roasting step of up to 2 hours at about 700°C.

[0065] In one or more embodiments, the magnesium chloride product stream has a heating residence time in the roasting step of about 2 hours at about 700°C.

[0066] In one or more embodiments, the magnesium chloride product stream is heated in a spray roaster.

[0067] In one or more embodiments, the magnesium chloride product stream is heated in a fluidised bed roaster.

[0068] In one or more embodiments, at least a portion of the magnesium oxide product is recycled to the iron removal step.

[0069] In one or more embodiments, the process further comprises an acid regeneration step, wherein at least a portion of the acid by-product is absorbed into water to produce a recycled hydrochloric acid.

[0070] In one or more embodiments, the process further comprises a residue thickening step, wherein the leached slurry is separated into an overflow stream and an underflow stream, wherein the underflow stream comprises substantially all of the solids residue; an iron removal step, wherein the overflow stream is treated to neutralise acids within the overflow stream and to separate an iron precipitate from a magnesium chloride product stream; a roasting step, wherein the magnesium chloride product stream is heated to produce a magnesium oxide product and an acid by-product; and an acid regeneration step, wherein at least a portion of the acid by-product is absorbed into water to produce a regenerated hydrochloric acid.

[0071] In one or more embodiments, the acid by-product is gaseous hydrogen chloride.

[0072] In an embodiment, the acid regeneration step includes scrubbing of the acid by-product in a venturi scrubber with the magnesium chloride product stream before the magnesium chloride product stream is provided to the roasting step.

[0073] In an embodiment, the regenerated hydrochloric acid is recycled to the acid leaching step.

[0074] In a second aspect, the present disclosure provides a leached slurry produced by the process according to the first aspect.

[0075] In another aspect, the present disclosure provides a magnesium oxide product produced by the process according to the first aspect.

[0076] In a further aspect, the present disclosure provides a recycled acid produced by the process according to the first aspect.

[0077] In another aspect, the present disclosure provides a system for extracting one or more metals or metal compounds from a slag feed comprising: a comminution step to reduce a particle size distribution of the slag feed; and an acid leaching step after the comminution step to receive the slag feed from the comminution step and leach the slag feed with a hydrochloric acid to form a leached slurry, wherein the slag feed comprises magnesium and silica compounds, the hydrochloric acid has a concentration of between 5-20 wt.% HCI, and theleached slurry comprises a liquid portion and a solids residue, wherein the liquid portion comprises magnesium chloride.

[0078] In one or more embodiments, the system further comprises a residue thickening step, wherein the leached slurry is thickened and separated into an overflow stream and an underflow stream, wherein the underflow stream comprises substantially all of the solids residue.

[0079] In one or more embodiments, the system further comprises an iron removal step, wherein the overflow stream is treated with oxygen and an alkaline reagent to neutralise acids within the overflow stream and to produce a mixture of an iron precipitate stream and a magnesium chloride product stream, wherein the iron precipitate is separated from the magnesium chloride product stream.

[0080] In one or more embodiments, the system further comprises a roasting step, wherein the magnesium chloride product stream is heated to produce a magnesium oxide product and an acid by-product.

[0081] In one or more embodiments, the system further comprises an acid regeneration step, wherein at least a portion of the acid by-product is absorbed into water to produce a recycled hydrochloric acid.

[0082] Further features and advantages of the present disclosure will become apparent from the following detailed description.BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Various preferred embodiments of the present disclosure will now be described, by way of examples only, with reference to the accompanying figures, in which:Figure 1 illustrates a particle size distribution of sample ferronickel slag used during laboratory testing;Figure 2 illustrates magnesium extraction over time from sample ferronickel slag during laboratory-scale acid leaching in accordance with the invention;Figure 2a illustrates metal extraction (in liquor) over time from sample HY7547 during laboratory-scale acid leaching;Figure 2b illustrates metal extraction (in liquor) over time from sample HY7548 during laboratory-scale acid leaching;Figure 2c illustrates metal extraction (in liquor) over time from sample HY7549 during laboratory-scale acid leaching;Figure 2d illustrates metal extraction (in liquor) over time from sample HY8595 during laboratory-scale acid leaching;Figure 2e illustrates metal extraction (in liquor) over time from sample HY8215 during laboratory-scale acid leaching;Figure 2f illustrates metal extraction (in liquor) over time from sample HY8070 during laboratoryscale acid leaching;Figure 2g illustrates metal extraction (in liquor) over time from sample HY8071 during laboratory-scale acid leaching;Figure 2h illustrates metal extraction (in liquor) over time from sample HY8072 during laboratory-scale acid leaching; andFigure 3 illustrates a block flow diagram of an industrial beneficiation process including an extraction process in accordance with the invention.DETAILED DESCRIPTION OF EMBODIMENTS

[0084] The present invention relates to a process for extracting one or more elements, metals, or metal compounds, e.g., magnesium, from a slag feed, such as ferronickel slag. In preferred embodiments, the process can be performed on an industrial scale using recycled reagents, such as hydrochloric acid.

[0085] During investigations of magnesium extraction processes from slag, it was surprisingly found that low concentrations of hydrochloric acid (e.g., concentrations of about 20 wt.% HCI or less) could effectively leach magnesium from ferronickel slag with high yields (e.g., 90% or higher) and while also avoiding the formation of silica gels without a high temperature pretreatment step to convert silica-bonded phases within the slag to sodium silicates. This finding was contrary to the general understanding in the field that HCI concentrations above the HCIazeotropic concentration of about 20 wt.%, typically 32% HCI, or the use of a pre-treatment step, are required to leach magnesium from the slag without the formation of silica gels.

[0086] As there is presently no or limited commercial use for ferronickel slags, the potential use of relatively low hydrochloric acid concentrations is believed by the Applicant to enable the commercially viable conversion of ferronickel slag (and similar refractory slags) into useful products, without having to rely on pre-treatment reagents before leaching to alter the feed composition.ExamplesLaboratory Testing

[0087] A bulk ferronickel smelting slag was received as a crushed solid with an estimated particle size Pwo of 3.5 mm. An assay of the slag indicated a composition including:

[0088] Minerals identified in the slag included ferrierite (FeSiOs) and clinoenstatite (MgSiCh). Other minerals were not identified by XRD and the remaining elements were understood to exist in an amorphous (non-crystalline) phase in the bulk slag.

[0089] The ferronickel slag was ground to a particle size of Pgs of 75 pm for all lab experiments detailed below. A particle size distribution of the slag is illustrated in Figure 1.

[0090] The inventors believe a finer particle size may result in a faster reaction rate and hence a faster extraction of valuable elements, metals, or metal compounds from the slag during the leaching step. However, a faster reaction rate could also disperse a critical mass of silica in solution which may have potential for silica super-saturation and formation of gelatinous silica (further depending on additional factors such as the temperature, instantaneous acid strength, etc.). Accordingly, in practice consideration may be given to controlling the conditions and theamount of silica in solution to avoid the formation of gelatinous silica. In one embodiment, controlling the conditions comprises controlling the grinding conditions, for example, to obtain a desired particle size to control the amount of silica in solution and thus control or minimise formation of gelatinous silica.Bench-Scale Acid Leaching (Feed = 100 g)

[0091] Samples of between 5-32 wt.% HCI were prepared (with concentrated HCI and deionised water) and added to an agitated, baffled, sealed vessel to establish an initial HCI solution for acid leach testing. A condenser was connected to the vessel to minimise the effects of evaporation. The HCI solution was pre-heated to a target temperature (see Table 1 below) using a thermocouple controlled hot plate, after which 100 g of the ground ferronickel slag was added to the vessel to commence magnesium leaching from the slag.

[0092] Aliquots of the formed slurry were removed at set time intervals and assayed to ascertain the chemical composition over time. After a predetermined leaching duration, the final solution was also assayed for chemical composition, including Mg concentration. The final residue (solids) was washed with water, dried, and then similarly assayed. A percentage extraction of Mg was then calculated for each sample.

[0093] The invention samples included initial HCI concentrations of 5%, 10%, 15%, 20%, and 25% (w / w) HCI. As the acid is consumed during the leaching process, the acid concentration was maintained during leaching with the addition of 32% HCI as required. It will be appreciated by those skilled in the art that this acid concentration may be maintained by using any hydrochloric acid with a higher wt.% HCI than the target HCI concentration by adding a suitable amount of acid to achieve the target HCI concentration.

[0094] These invention samples were compared with three control samples with an initial concentration of 32 wt.% HCI and no HCI addition during leaching. These control sample conditions use a HCI concentration that is above the azeotropic concentration of HCI.

[0095] A summary of the results is presented in Table 1 below:Table 1 - Bench-Scale Acid Leaching Results

[0096] As evident from the differences between the initial HCI concentration and end free acid concentration of the three control samples, the HCI was consumed to drive the acid leaching reaction.

[0097] A further comparison of the magnesium extraction over time for the five acid leaches maintained at 5%, 10%, 15%, 20%, and 25% (w / w) HCI is illustrated in Figure 2, with the metal extraction (in the liquor) over time for each above sample shown in Figures 2a-2h.

[0098] These tests confirmed that a leaching HCI concentration maintained between 10-25 wt.% HCI for 8 hours could achieve a comparable Mg extraction to that achieved by the control samples that used an initial 32 wt.% HCI. Advantageously, by maintaining the HCI concentration within this range, there will always be a sufficient mass of acid to drive the leaching reactions.

[0099] The Mg extraction data also indicated little to no benefit of either leaching for more than about 4-8 hours (provided a sufficient mass of about 10% HCI or more acid to drive the leaching reactions), or by increasing the temperature from 80°C to 95°C. Advantageously, none of the invention samples formed a silica gel substance over the duration of the experiment.

[0100] Moreover, as shown in Figures 2a, 2b, 2c, 2g and 2h, a negligible concentration of silica in the liquor was found over the entire experiment for the higher concentration acid samples. It is believed that, in these samples, the leached silica immediately precipitates out of the leach liquor.

[0101] However, as shown in Figures 2d, 2e and 2f, the lower-concentration acid samples exhibit an early increase in silica concentration in the liquor which reduced over time, indicating that the silica precipitation rate was slower with the lower concentration acid samples.Purification of Magnesium

[0102] A PLS (Pregnant Leach Solution) Composite was prepared by mixing the leach solutions of the HY7547 , HY7548, and HY7549 Control samples. An assay of the PLS Composite indicated a composition including:

[0103] 5 L of the PLS Composite was added into an agitated, baffled vessel to concentrate the dissolved metals and evolve as much of the water and hydrochloric acid as possible, with results as below:

[0104] 500 mL of the resultant Evaporated PLS Composite was then subjected to a 3 hr precipitation to remove the iron and other impurities from the solution using magnesia (MgO), by pre-heating the solution to 95°C and adding magnesia in small amounts to achieve a target pH 1.5 by the end of the 3 hr period, with results shown as below:

[0105] The resultant Purified PLS Composite Liquor was then transferred to a beaker and heated to its boiling point (~100°C) and allowed to evaporate for up to 6 hrs. The solution wasthen allowed to cool to produce a crystalline solid, which was transferred to a muffle furnace purged with nitrogen gas at 700°C for 6 hrs, then allowed to cool.

[0106] The resultant calcine was calculated to achieve a Mg recovery of 95.4% from the initial Control samples (assuming all magnesium is present as MgO). Further, the test achieved a 99.1% chloride evaporation as hydrogen chloride vapour.Stability of Leachate against Gelling

[0107] Prior to conducting the experiments, the inventors believed that leaching slags with low concentrations of hydrochloric acid would increase the dissolved silica content (due to an increased solubility at lower HCI concentrations) and hence increase the risk of silica particles aggregating, over time, in a gelled form (i.e., gelatinous silica) rather than quickly precipitating out of the solution and being easier to separate.

[0108] However, as no silica gels were formed during laboratory experimentation, the inventors surprising found that relatively low concentrations of acid could be used to leach ferronickel slag without forming gelatinous silica and without compromising extraction of metals by actively controlling the HCI wt.% to a target low concentration, by adding further hydrochloric acid as HCI is consumed during the leaching step. This can be seen in Figures 2d, 2e, 2f with a controlled hydrochloric acid concentration of 5-15 wt.% HCI.

[0109] The inventors presently theorise that, by providing sufficient acid for leaching, but at a low concentration, the rates of simultaneous silica dissolving and reprecipitating can be controlled to avoid a high level of silica supersaturation, and hence reduce the risk of gelation.

[0110] Further advantages of maintaining a low concentration of hydrochloric acid during the leaching step include: (i) less downstream reagents are required to neutralise the acid for further processing, reducing manufacturing costs and environmental impact; (ii) as silica solubility does not change during the leaching stage (as the pH is being controlled through addition of further hydrochloric acid), the leaching speed and aggregation / precipitation mechanisms can be more easily predicted to control any undesirable silica gel formation (e.g., by controlling the amount of silica in solution).

[0111] The use of a relatively low concentration hydrochloric acid of about 5-15 wt.% also provides a commercial advantage in that a downstream acid regeneration step may be used to recycle hydrochloric acid to the leaching step. This is particularly useful as hydrochloric acidregeneration is typically limited to be no more than 20 wt.% HCI in industrial processes, and more commonly no more than 18 wt.% HCI. Any acid regeneration higher than this would require specialised equipment and uneconomical operating parameters, which could increase flowsheet complexity and reduce commercial viability.

[0112] It is also possible that silica gels could form after the leach step, while the leachate solution is progressively separated from the residual solids through thickening and filtration steps.

[0113] Hence, experimental tests were conducted to determine the gelling stability of a leachate obtained by leaching with a low 5% concentration of HCI (corresponding with Test ID HY8595 above). This 5% HCI concentration was selected for the gelling stability tests as this lower acid concentration was believed to have a higher likelihood of developing a gelatinous silica product.Test 1: 5% HCI, no acid addition, 6 hrs at 80°C

[0114] 100 g of the ferronickel slag was leached with 2043 g of 5 % hydrochloric acid in a 4 L beaker. The slurry was agitated by an overhead stirrer operating at 500 RPM, with the temperature was maintained at 80°C for 6 hours using a hot plate. No additional acid was added during the test.

[0115] 50 mL samples were extracted from the slurry at 1 , 2, 4 and 6 hrs and tested for gel formation by vacuum filtering using a Buchner funnel with a Whatman’s #42 ashless filter paper and washing three times with DI water. No gel formation was observed for any of these samples.

[0116] After the 6 hour leaching period, the overhead stirrer and hot plate were turned off and the resultant slurry allowed to sit in ambient conditions for 24 hours. After 24 hours, the slurry was tested for gel formation with a similar process as detailed above, with no gel formation observed.

[0117] The obtained filtrate samples were chemically analysed for their elemental composition, with the results detailed in the below Table 2.Table 2 - Chemical Analysis of Test 1 Samples

[0118] Comparing the results for 'Filtrate- 6 hour' and 'Filtrate- 30 hour', it is evident that there was only a minor change in the overall Si level in solution after standing for 24 hours after the leach. This supports the conclusion that no silica gel formation took place during this time.Test 2: 5% HCI, acid addition, 6 hrs at 80°C

[0119] 100 g of the ferronickel slag was leached with 2043 g of 5 % hydrochloric acid in a 4 L beaker. The slurry had a liquid:solid ratio of 20:1 and included 4.7% w / w solids. The slurry was agitated by an overhead stirrer operating at 500 RPM, with the temperature was maintained at 80°C for 6 hours using a hot plate.

[0120] Additional 32 % HCI was added during the leach at 5 minutes (94 g of 32% HCI), 15 minutes (94 g of 32% HCI), 60 minutes (50 g of 32% HCI), and 120 minutes (50 g of 32% HCI) to maintain the acid concentration at about 5 % HCI.

[0121] After the 6 hour leaching period, the overhead stirrer and hot plate were turned off and the resultant slurry allowed to sit and cool overnight for 12 hours at ambient conditions.

[0122] The resultant slurry was tested for gel formation by vacuum filtering using a Buchner funnel with a Whatman’s 542 hardened ashless 185 mm diameter filter paper. The slurry took approximately 3 minutes to filter. No gel formation was observed. A sample of obtained filtrate was chemically analysed for their elemental composition, with the results detailed as 'Acid leach solution' in the below Table 3.Test 3: Iron and impurity removal of Test 2

[0123] To further assess whether the silica dissolved in the filtrate would impact a downstream purification step, removing iron or further impurities, the 1650 mL of the obtained filtrate was separated to a 4 L beaker and agitated by an overhead stirrer at 250 RPM, with the temperature maintained at 90°C for 5 hours using a hot plate.

[0124] The solution was sparged with 2 L / min oxygen for the initial 1h15min, and then 1 L / min oxygen for the remaining time to achieve visible bubbles on the solution surface. 19 g of hematite was added to the solution to seed precipitation.

[0125] A sodium hydroxide solution (10% NaOH; Merck Chemicals, Analar grade) was added at intervals to the slurry to gradually raise the pH from about pH 1, at a rate of 1 pH unit per hour to approximately pH 4 at the end of three hours.

[0126] After the 5 hour period, the overhead stirrer and hot plate were turned off.

[0127] The remaining slurry (approximately 2700 mL) was filtered using a Buchner funnel with a Whatman's 541 hardened ashless 185 mm diameter filter paper. The slurry took approximately2 minutes to filter. A sample of obtained filtrate was chemically analysed for their elemental composition, with the results detailed as 'Purified acid leach solution' in the below Table 3.After the test had been conducted, the inventors have noted that it is possible that adding the hematite to the solution before at least partially neutralising the solution with sodium hydroxide solution may have resulted in some of the hematite dissolving prior to seeding precipitation. Hence, it is believed that by adding the hematite with or after the sodium hydroxide would improve the precipitation of iron and other impurities.Test 4: Synthetic, no-Si leach solution of Test 2

[0128] A synthetic leach solution was made up from analytical grade reagents with the concentrations based on the 'Acid leach solution' of the above Test 2, with the exception that no silica was added to the synthetic leach solution to help determine whether the presence of silica affected the purification step.

[0129] 1650 mL of the synthetic leach solution was added to a 4 L beaker. The solution was agitated by an overhead stirrer at 250 RPM., with the temperature maintained at 90°C for 4 hours using a hot plate.

[0130] NaOH was added to the slurry to first raise the pH to about 1, then 23 g of hematite was added to the solution to seed precipitation. After the 4 hour period the overhead stirrer and hot plate were turned off.

[0131] The remaining slurry (approximately 1700 mL) was filtered using a Buchner funnel with a Whatman's 541 hardened ashless 185 mm diameter filter paper, and the residue was washed with DI water. The slurry took approximately 3 minutes to filter. A sample of obtained filtrate was chemically analysed for their elemental composition, with the results detailed as 'Purified synthetic leach solution' in the below Table 3.

[0132] The solid-liquid separation results of both Tests 3 and 4 are provided in the below Table 4.Table 3 - Chemical Analysis of Test 2-4 SamplesTable 4 - Solid-Liquid Separation Results of Test 3-4 Samples

[0133] In both Test 3 and 4 samples, the iron removal was effectively complete. At the elevated temperature and with pH in the range 2-4, much of the silica and aluminium also precipitated. The high concentrations of sodium are from the use of a 10 % NaOH solution for pH adjustment. The filtration times were comparable.Industrial Beneficiation of Ferronickel Slag

[0134] In view of the positive Laboratory Testing results, the Applicant has designed an industrial process for beneficiating ferronickel slag to produce a commercial magnesium oxide (MgO) powder product. A block flow diagram for this process is illustrated in Figure 3 and summarised below. It is believed that the process may be similarly applied for extracting magnesium from any other suitable magnesium-rich slag feed.

[0135] Based on computer heat and mass balance modelling, it is believed that 6.51 (dry) of a typical ferronickel slag obtained from nickel laterite smelting would be required to produce 1 t of magnesium metal. However, the magnesium recovery may vary depending on, for example, the original slag composition, or the particular operating parameters used in the implementation of the process.1. Slag Storage

[0136] Ferronickel slag ("FNS"), for example from nickel laterite smelting, can be delivered to the processing site and stored until ready for beneficiation. Preferably the slag would have a composition comprising high amounts (e.g. 15 wt.% or higher) of magnesium compounds, such as magnesium silicates (e.g., MgSiOs, Mg2SiO4). Such slags would also typically have high amounts of iron silicates (e.g., FeSiCh). Smaller amounts (e.g., < 5 wt.%) of metal oxides (e.g., AI2O3, Cr20s) and other minor compounds (e.g. aluminium, calcium, chlorine, chromium, potassium, nickel, and / or sulphur compounds) may also be present.2. Slag Comminution

[0137] The particle size of the FNS may be reduced by any conventional method, for example crushing in a wet ball mill, to reduce the slag particles to a size suitable for acid leaching. The laboratory tests reduced the particle size distribution to have a Pgs of 75 pm. However, alternative (i.e. smaller or larger) particle size distributions may also be used with the downstream Acid Leach step 3 depending on the required leach kinetics, particularly foravoiding silica gelling conditions, and the equipment available. For example, alternative particle size distributions may include a Pgs of 50 pm, a Pgs of 100 pm, or a Pgs of 75 pm.

[0138] If required, the crushed slag particles may be dewatered following crushing during this step, for example using a vacuum belt or other conventional filter or dewatering equipment. The advantage of doing would be to avoid any unnecessary dilution of the acid in the following acid leach step.3. Acid Leach

[0139] Following the Slag Comminution step 2, the ground FNS is transported from the comminution step to a leaching vessel for leaching the slag. Preferably, the slag is transported directly from the comminution step to the leaching vessel. However, in other envisioned implementations, the transportation step may include a temporary storage of the slag for a period of time.

[0140] The FNS is leached with a hydrochloric acid at a target concentration between about 5- 20 wt.% HCI for about 0.5 to 12 hours to produce a leached slurry comprising a liquid portion and a solids residue. During this leaching step, the magnesium is extracted from the FNS and dissolved into the liquid portion as magnesium chloride. As shown in the laboratory testing, the Applicant found that the use of relatively low concentrations of acid did not result in formation of gelatinous silica, and hence the solids residue (now rich in precipitated silica) can still be separated from the liquid portion by conventional solid-liquid separation methods.

[0141] Preferably, the majority of HCI used in the leaching step may be recycled from a downstream step (e.g. illustrated from Acid Regeneration step 11). If required, this recycle stream can be supplemented with a make-up HCI stream to obtain the target feed HCI concentration.

[0142] In an envisioned preferred embodiment, the leaching step will be conducted with a hydrochloric acid solution maintained at a concentration in the range of 8-12 wt.%HCI, for about 6 hours. Advantageously, the HCI consumed can be supplemented with acid regenerated using conventional industrial techniques, thus avoiding the use of higher concentrations of HCI (e.g. at or above the azeotropic concentration of about 20 wt.%) which would require specialised technologies which may be difficult or costly to regenerate (e.g. requiring extractive rectification technology, or dual-pressure technology).

[0143] Due to corrosive nature of the reagents and products, the equipment and piping (including for relevant downstream steps) should be constructed of corrosion-resistant materials, such as Fibre Reinforced Plastic (FRP), or other plastics or metals coated with corrosion / abrasion resistant linings. Moreover, any produced gases should be vented for scrubbing before release to the atmosphere.

[0144] Depending on the equipment available and the volume of FNS to be leached, various leaching conditions may be selected to optimise the leaching step, including:• the selected HCI concentration is maintained during the leaching process (e.g. by adding additional HCI to the vessel as it is consumed);• the leaching may occur in one or more leaching vessels;• the HCI concentration of each leaching vessel may be independently controlled;• the leaching process, which is an exothermic reaction, may be controlled a temperature between about 50-100°C, or to a maximum temperature of about 80°C, or to an average temperature of about 80°C;• the leaching vessel(s) may be sealed under negative pressure;• the leaching step may be a batch process, or may be a continuous process; and / or• the FNS and the acid are provided to at least one leaching vessel in a co-current flow, and / or at least one leaching vessel in a counter-current flow.4. Leach Residue Thickening

[0145] After leaching, the resultant slurry is pumped from the (final) leach tank to a thickener where, optionally with the aid of flocculant, the first stage of solids / liquid separation will take place. In the modelled example, the hot acidic slurry was settled to about 35% w / w solids.

[0146] The overflow solution, including magnesium chloride, is extracted and form a feed solution for further processing. The underflow, comprising the silica, is optionally further filtered in a silica filtration step.5. Silica Filtration

[0147] The underflow slurry stream from the thickening step may be further separated to extract any residual magnesium chloride from the bulk silica.

[0148] This may be conducted by any suitable filtration device, such as a plate-and-frame filter press fitted with a membrane squeeze step for dewatering and washing of the solids, to recover a leachate including magnesium chloride and unreacted acid. This leachate may be added to the overflow for further processing. The filtered solids, comprising predominantly amorphous silica, is separated off for further treatment, sale or disposal.

[0149] In the modelled example, 6.51 of initial slag produced 41 (dry) of amorphous silica product.6. Iron Removal

[0150] The overflow solution and filtration leachate may be treated with oxygen and an alkaline reagent, such as magnesium oxide, to oxidise ferrous iron to the ferric form, neutralise any unreacted acid, and precipitate out iron in ferric compounds as the pH increases. This may be conducted in one or a plurality of tanks. The tanks may be sparged with the oxygen to distribute the oxygen evenly and minimise oxygen consumption.

[0151] As shown in Figure 3, the magnesium oxide can be recycled from a downstream step to decrease the required new reagent input to the system and the improve efficiency of the overall process. This may be a direct recycle after the Roasting step 10 (as illustrated) or the magnesium oxide may first be milled to improve its dispersibility and reactivity.

[0152] Furthermore, an iron ‘seed’ material may be provided from a downstream step (e.g. iron thickening or iron calcining) to promote the formation of large iron precipitate particles. Preferably, the iron removal step is conducted in the pH range 1-4. More preferably, the iron removal step is conducted at a temperature near 100°C. Such conditions can promote the iron precipitate to predominantly be in the form of hematite and akageneite, which may be more easily separated from the liquor (e.g. compared to ferric hydroxide).7. Iron Thickening

[0153] The resultant precipitate slurry is to be pumped from the Iron Removal final tank to a thickener where, optionally with a flocculant addition, the slurry can be settled to produce an underflow slurry. The modelled example indicates that a slurry of about 40% w / w solids would be attainable.

[0154] A portion of the thickener underflow can be recycled to the first Iron Removal tank to act as seed material for the precipitation. The remaining balance of the underflow slurry is pumped for further filtration.

[0155] The overflow from the thickener, including the magnesium, is pumped to a Roasting step 10.8. Iron Filtration

[0156] The underflow slurry can be further separated to extract any residual magnesium using suitable filter devices, such as a plate-and-frame pressure filter fitted with a membrane squeeze. The filtrate can be combined with the thickener overflow and the filter cake passed to an Iron Calcining step 9.9. iron Calcining

[0157] The discharged filter cake (with a modelled moisture content of about 25%) is fed by a conveyor to be dried and calcined. This may performed in, for example, a rotary kiln with a residence time of about 2 hours at a temperature of about 700°C, in order to complete the oxidation of the non-hematite constituents (such as akageneite and ferric hydroxide) to hematite and to remove substantially all of the remaining chloride content.

[0158] In the modelled example, 6.81 of initial slag produced 0.651 of hematite product.10. Roasting

[0159] The magnesium chloride containing streams (including the Iron Thickening step 7 overflow and the Iron Filtration step 8 filtrate) may be heated in a suitable apparatus (e.g. a spray roaster or a fluidised bed roaster) to convert the magnesium chloride to a solid magnesium oxide and a hydrogen chloride vapour.

[0160] From the modelled example, a solution strength of about 25% w / w MgCl2 can be obtained as a feed to the Roasting step, which can be roasted at 700°C to obtain MgO<S) and HCI(g). As noted above, 6.81 of initial slag was modelled to produce about 3 t of magnesium oxide, which may be purified downstream to about 1 t of magnesium metal.

[0161] A portion of the produced magnesium oxide may be recycled to the Iron Removal step 6, while the remaining magnesium oxide may be transported downstream for storage and / or sale or use as a commercial product.11. Acid Regeneration

[0162] Advantageously, the hydrogen chloride vapour produced during Roasting step 10 can be absorbed into water and the regenerated hydrochloric acid recycled to the Acid Leach step 3 to reduce the reagent costs associated with the beneficiation process.

[0163] This may be conducted in any conventional gas absorption equipment, including venturi scrubbers, packed towers, and bubble-cap columns.

[0164] In the designed model, the acid vapour is first scrubbed with the magnesium chloride stream entering the Roasting step 10, in order to improve the overall heat utilisation of the process, and the cooled gas then contacted counter-currently with water in one or more gas absorption stages.

[0165] While this is a cost-effective method of hydrochloric acid recovery, the inventors found that the maximum regenerated acid strength in this process is limited to be no more than 20 wt.% HCI, and more likely to be no more than 18 wt.% HCI. However, through laboratory experimentation, the inventors found that this HCI concentration could advantageously be used in the leaching of ferronickel slag.12. Gas Scrubbing

[0166] In light of the temperatures and use of hydrochloric acid, ideally all reaction vessels and tanks will be sealed under slight negative pressure and produced off-gases vented through additional gas scrubbing stages such as packed towers. Depending on the emission limits, the final scrubbing liquid may be either water or an alkaline solution. Sodium hydroxide and / or sodium thiosulphate may be added to the water in this step to complete HCI removal from the exit gases and to destroy any traces of chlorine gas that may have been generated in sidereactions in the Roaster.

[0167] In this specification, adjectives such as left and right, top and bottom, hot and cold, first and second, and the like may be used to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order.Where context permits, reference to a component, an integer or step (or the alike) is not to be construed as being limited to only one of that component, integer, or step, but rather could be one or more of that component, integer or step.

[0168] In this specification, the terms ‘comprises’, ‘comprising’, ‘includes’, ‘including’, or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

[0169] In this specification, the term 'consisting essentially of' is intended to be construed non- exhaustively, such that a method, system or apparatus that consists essentially of a list of elements may include one or more elements in minor proportions that are not included in the list of elements.

[0170] In this specification the term 'consisting of' means consisting only of.

[0171] In this specification, 'substantially all' means 'all or nearly all', to account for unintentional impurities (e.g. less than 5% total impurities in a process stream).

[0172] In this specification, the term 'about' indicates that a recited figure may vary according to a margin of error expected by one skilled in the art rather than requiring the precise figure. Typical margins of error are ±10% or ±5%. Accordingly, for example, the expression 'about 10' indicates that minor various above and below 10, such as 9.9 and 10.1 are included.

[0173] In this specification when a range is recited the outer limits of that range are included. For example, a range of 20-50 includes both 20 and 50 as well as values in-between.

[0174] The above description relating to embodiments of the present disclosure is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the disclosure to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present disclosure will be apparent to those skilled in the art from the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. The present disclosure is intended to embrace all modifications, alternatives, and variations that have been discussed herein, and other embodiments that fall within the spirit and scope of the above description.

Claims

CLAIMS1. A process of extracting magnesium or magnesium compounds from a ferronickel slag feed comprising magnesium and silica compounds, the process comprising: a comminution step to reduce a particle size distribution of the ferronickel slag feed; and an acid leaching step after the comminution step, comprising: leaching the ferronickel slag feed from the comminution step in a vessel with hydrochloric acid at a concentration of 5-15 wt.% HCI to form a leached slurry, controlling the hydrochloric acid concentration during leaching by adding further hydrochloric acid to the vessel to maintain the hydrochloric acid concentration at 5-15 wt.% HCI, and obtaining a leached slurry from the vessel, wherein the leached slurry comprises a liquid portion and a solids residue, and wherein the liquid portion comprises magnesium chloride.

2. The process according to claim 1 , wherein controlling the hydrochloric acid concentration during leaching includes adding the further hydrochloric acid in one or more batch addition(s) of HCI or a continuous feed of HCI.

3. The process according to claim 1 or claim 2, wherein the ferronickel slag feed further includes one or more of aluminium, calcium, chlorine, chromium, potassium, nickel, sulphur, or any combination thereof.

4. The process according to any one of claims 1-3, wherein the ferronickel slag feed includes one or more of ferrosilite (FeSiOs), fayalite (Fe2SiC>4), clinoenstatite (MgSiCh), fosterite (Mg2SiC>4), and other complex iron materials.

5. The process according to any one of claims 1-4, wherein the ferronickel slag includes AI2O3, O2O3, or a combination thereof.

6. The process according to any one of claims 1-5, wherein the ferronickel slag feed is produced from smelting nickel laterite.

7. The process according to any one of claims 1-6, wherein the ferronickel slag feed comprises silica at concentrations of about 15-90 wt.%.

8. The process according to claim 7, wherein the ferronickel slag feed comprises silica at concentrations of about 20-50 wt.%.

9. The process according to claim 8, wherein the ferronickel slag feed comprises silica at concentrations of about 25 wt.%.

10. The process according to any one of claims 1-9, wherein the solids residue comprises amorphous silica.

11. The process according to claim 10, wherein the solids residue comprises amorphous silica at a concentration of about 20-90 wt.%.

12. The process according to claim 10 or 11, wherein the solids residue comprises amorphous silica at a concentration of about 30-60 wt.%.

13. The process according to any one of claims 1-12, wherein, in the acid leaching step, controlling the hydrochloric acid concentration during leaching maintains the hydrochloric acid concentration at 8-12 wt.% HCI.

14. The process according to any one of claims 1-13, wherein, in the acid leaching step, controlling the hydrochloric acid concentration during leaching maintains the hydrochloric acid concentration between about 8 wt.% to 12 wt%, or about 10 wt.% HCI.

15. The process according to any one of claims 1-14, wherein at least a portion of the further hydrochloric acid is a recycled hydrochloric acid from a downstream manufacturing step.

16. The process according to claim 15, wherein the recycled hydrochloric acid has a higher concentration of HCI than the hydrochloric acid in the vessel.

17. The process according to any one of claims 1-16, wherein, during the acid leaching step, the slag feed is passed through multiple vessels with hydrochloric acid to leach the slag feed.

18. The process according to claim 17, wherein the concentration of HCI in each vessel is independently controlled.

19. The process according to any one of claims 1-18, wherein, during the acid leaching step, the slag feed has a total residence leaching time of between 0.5 to 12 hours in the vessel(s).

20. The process according to claim 19, wherein the total residence leaching time is between 0.5 to 8 hours in the vessel(s).

21. The process according to claim 19 or 20, wherein the total residence leaching time is about 8 hours in the vessel(s).

22. The process according to any one of claims 1-21, wherein, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at a temperature between about 50-100°C.

23. The process according to claim 22, wherein, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at a temperature of about 80°C.

24. The process according to any one of claims 1-23, wherein, in the acid leaching step, leaching the ferronickel slag feed with the hydrochloric acid is conducted at an initial liquid: solid ratio of 10 mL / g to 20 mL / g.

25. The process according to any one of claims 1-24, wherein, during the comminution step, the slag feed is milled to a Pgs particle size of at most 100 microns.

26. The process according to any one of claims 1-24, wherein, during the comminution step, the slag feed is milled to a Pgs particle size of about 75 microns.

27. The process according to any one of claims 1-26, wherein the vessel(s) are sealed under negative pressure.

28. The process according to any one of claims 1-27, further comprising: a residue thickening step, wherein the leached slurry is separated into an overflow stream and an underflow stream, wherein the underflow stream comprises substantially all of the solids residue;an iron removal step, wherein the overflow stream is treated to neutralise acids within the overflow stream and to separate an iron precipitate from a magnesium chloride product stream; a roasting step, wherein the magnesium chloride product stream is heated to produce a magnesium oxide product and an acid by-product; and an acid regeneration step, wherein at least a portion of the acid by-product is absorbed into water to produce a regenerated hydrochloric acid.

29. The process according to claim 28, wherein the acid regeneration step includes scrubbing of the acid by-product in a venturi scrubber with the magnesium chloride product stream before the magnesium chloride product stream is provided to the roasting step.

30. The process according to claim 28 or 29, wherein the regenerated hydrochloric acid is recycled as the further hydrochloric acid in the acid leaching step.

31. A leached slurry produced by the process according to any one of claims 1-27.

32. A recycled acid produced by the process according to any one of claims 28-30.

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