WO2025228545A1

WO2025228545A1 - Extraction method

Info

Publication number: WO2025228545A1
Application number: PCT/EP2024/062358
Authority: WO
Inventors: Nicholas John
Original assignee: Bluecap Resources
Current assignee: Bluecap Resources
Priority date: 2023-05-05
Filing date: 2024-05-03
Publication date: 2025-11-06
Anticipated expiration: 2026-11-03
Also published as: GB202306674D0

Abstract

A method for the extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said method comprising: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 1 and 120°C for a period of between 1 second and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; and d) recovering at least a first metal from the first liquid fraction; and an apparatus for the same.

Description

Extraction Method

Field of the Invention

The present invention relates to methods for extracting valuable and/or desirable materials from waste. In particular, the invention relates to methods for recovering metals from the waste material generated by industrial processes. Such waste material is generally slag, and in particular slag which is rich in silica/silicates.

Background to the Invention

Smelting and/or refining of metals typically generates large quantities of glassy or stony waste, dross or scoria, often and herein referred to as “slag”. This solid waste, dross, or scoria, usually contains metals, which may have been the target metal of the original process, or may have been present as impurities in the metal or ore. Such metals may be of significant value and/or may represent an environmental hazard if left unprocessed. For example, slag may contain significant quantities of lead or arsenic, which may be an environmental risk if left untreated. Being unsuitable for agricultural or construction use, such slags tend to be left in visible heaps near to the site of the original smelting operation, with the documented potential for harmful leaching of (for example) lead into lower watercourses. Lead is, however, a valuable material if extracted and purified so may also represent a valuable resource. Other metals such as zinc, copper, gold or silver may be valuable but present less environmental hazard. As a result, there is both an economic and environmental need to process slag material to extract desirable metals.

Some types of slag can be processed effectively for residual metals by use of sulphuric acid (H₂SO₄). However, materials containing a high silica/silicate content (e.g. greater than 10% by weight) tend to form a gel when treated with sulphuric acid. This makes the resulting mixture difficult to handle and makes separation of the solid and liquid components impractical. As a result, it has previously been difficult to treat silicate-rich slag materials for recovery of desirable metals in an economically viable way. Slag heaps have therefore been accumulating in the global environment for many hundreds of years, and in some cases, represent an environmental hazard. Processing such materials into commercially valuable products may therefore achieve both commercial and environmental benefits. In view of the above, it would be an evident benefit to provide a new method for the extraction of metals and/or minerals from silicate-rich slag. It would be a particular advantage if the method allowed for efficient processing, recycling of chemical processing materials and/or generation of valuable industrial material products. It would be a further advantage if the process could utilise mild conditions (e.g. without requiring high pressures and/or temperatures) and/or readily available, biodegradable and/or easily handled reagents in order to minimise the risk, cost and/or environmental impact of the process. Brief Description of the Invention

The present inventors have now established that the extraction of desirable metals from slag materials with silicate-rich content may be significantly improved by the use of methanesulphonic acid (also known as methanesulfonic acid or MSA: with formula CH₃SO₃H and structure H₃C-S(=O)₂-OH), in a process which addresses at least some of the above advantages and/or additional advantages which are set out in the present description. In a first aspect, the invention therefore provides a method for the extraction of at least one metal from a silicate-rich solid waste material (e.g. slag material) comprising said at least one metal said method comprising: a) contacting said silicate-rich solid waste material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 1 to 120°C, preferably 20 and 120°C for a period of between 1 second to 24 hours, preferably 10 minutes and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; and d) recovering at least a first metal from the first liquid fraction.

In one particular embodiment, the method comprises regenerating and/or recycling at least a part of the MSA. Such regeneration may be by any suitable means but will preferably be by means of sulphuric acid (H₂SO₄) and in particular may be by use of excess sulphuric acid.

In a further embodiment, the extraction of at least one metal from a silicate-rich slag material may comprise: a) contacting said silicate-rich slag material with an acidic solvent comprising a mixture of methanesulphonic acid (MSA) and sulphuric acid to form a first extraction mixture; In a further aspect, the invention also provides an apparatus for extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said apparatus being configured: i) to accept a solid silicate-rich slag material into a first vessel; ii) to accept an acidic solvent comprising methanesulphonic acid into said first vessel to generate a first extraction mixture; iii) to maintain said first extraction mixture in said first vessel at a temperature of between 1 to 120°C, preferably 20 and 120°C for a period of between 1 second to 24 hours, preferably 10 minutes and 24 hours; iv) to separate said first extraction mixture into a first solid fraction and a first liquid fraction; and v) to recover at least a first metal from said first liquid fraction.

Summary of the Figures

Figure 1 shows a block diagram representing the general process of extracting at least a first metal from silicate-rich slag using a methanesulphonic acid (MSA) solution (e.g. comprising methanesulphonic acid (MSA) and sulphuric acid).

Figure 2 shows a block diagram representing the extraction process including regeneration and recycling of at least a part of the MSA. Similar to Figure 1 the MSA solution may comprise MSA alone or MSA with another acid such as H2SO4.

Figure 3 shows a block diagram representing the cyclic process using MSA mixed with sulphuric acid and with lead (Pb) extracted from the first solid fraction.

Figure 4 shows a block diagram of one example of the implementation of the process of the present invention. The MSA leach solution presented in Figure 4 may comprise MSA and H2SO4. The “Leach Filtrate” in this figure represents the first liquid fraction while the “Leach Residue” represents the first solid fraction.

Detailed Description of the Invention

The present invention relates to methods for extracting at least one metal from a solid industrial waste material. In particular from silicate-rich slag. Such silicate-rich slag is typically difficult to process because common extraction methods with, for example, sulphuric acid tend to generate a silica gel which prevents the extraction mixture from being readily processed.

The method of the present invention provides for extraction of at least one metal from a silicate- rich slag material comprising said at least one metal. Metals which are relevant for extraction in the present context may be any metal which is of commercial use/value and/or presents an environmental hazard. Typical metals which may be extracted by the method of the present invention in various embodiments include lead (Pb), zinc (Zn), aluminium (Al), gallium (Ga), iron (Fe), tin (Sn), antimony (Sb), copper (Cu), silver (Ag), gold (Au), arsenic (As), tantalum (Ta), titanium (Ti), niobium (Nb) rare earth metals and any combination thereof. Each metal may be extracted separately or several metals may be extracted together either for combined use or for further separation. For example, Cu, Ag and Au may be extracted together for further processing and/or separation.

Generally, the “at least one metal” referred to herein will be a desired metal or metal-containing product. Preferred examples of the “at least one metal” for extraction include Pb, and Zn. In one preferred embodiment, the method of the invention will comprise extraction of at least Pb and Zn. It is preferred that such metals are separated individually and not mixed with a significant amount of other metals. The extracted metals, salts, oxides etc will be of sufficient purity to be of commercial value, preferably of the purity necessary for optimal marketability. Suitable purity will depend upon the metal extracted and the target market and may, for example, be at least 40%. This is described in greater detail herein below.

Certain metals and/or non-metals may be extracted as waste or by-product material and thus need not be separated with any specific level of purity. Silica and/or silicates will be a waste product of the present methods, as may be aluminium salts and/or iron salts.

Alternatively, the silica and/or silicates may be a useful by-product of the present methods. For example, silica and/or silicates are particularly useful as an additive in the cement industry.

Typically, silica and/or silicates which are used as an additive in the cement industry are used in the form of a gel. The term “unwanted components” as referred to herein, may therefore refer to by-products of the reaction (especially non-metal by-products), although these may be useful for certain applications.

Iron may be produced as a desired product (e.g. at concentrations acceptable for steelmaking, especially for primary metal smelters). Alternatively, or in addition, iron may be present in such levels and oxidation states that some iron is lost with the waste materials without significant detriment to the process. Iron may thus be a desired product, a waste product or both.

The methods of the present invention include step a) of contacting a silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture. Such contacting will typically take place in a first vessel.

Prior to contact with the acidic solvent, the silicate-rich slag material may be in any form but will preferably be in the form of solid particles. Such particles may be or any appropriate size that allows the metals to be taken up into solution. Generally, this will be by exposure of the metals at the surface of the particles and thus the desired size may depend on the nature of the slag being treated, the metals to be extracted and the size of the grains of metal or metal compounds in the slag material.

Suitable particles may be formed by physical treatment of the source material, which itself may be a solid of any physical size and shape. The solid particles for use in any of the method of the invention may be, for example, a weight average of 10|j.m to 1cm in largest dimension, such as 50|j.m to 5mm. Any suitable method, such as grinding, crushing, milling, etc., may be used to reduce larger pieces of slag material to an appropriate size. Separation method such as screening and/or cyclonic separation may be used to separate material of a desired (small) size from larger remaining particles, which may then be re-sized (e.g. re-ground and re-screened). In one embodiment, the material is “screened” such that a certain proportion of particles pass a mesh “screen” with a certain aperture size. In one embodiment, at least 50%, such as 50 to 90%, e.g. 70 or 80% (by number) of particles pass through a screen of aperture size 500|j.m, preferably 200|j.m, more preferably 150|j.m. The optimum screen size may depend upon the metals which are to be extracted, the size distribution of the desired contained metal or mineral and/or the desired balance between speed of extraction and energy/time spent in grinding operations.

The acidic solution is contacted with the silicate-rich slag in any suitable amount. This amount will preferably allow for the metal(s) to be taken up into solution without causing gelling (or substantially without causing gelling). Generally, around 10 to 600g of solid slag material will be used per litre of acidic solution. This may be 10 to 200 g/L or 15 to 150 g/L. In one embodiment, this will be 20 to 10Og/L or 30 to 75 g/L (e.g. 40 to 60 g/L). In alternative embodiments, such as at larger scales, the preferred range may be 200 to 600 g/L, such as 300 to 550 or 350 to 500 g/L. The first extraction mixture may be maintained at an elevated temperature, optionally with agitation, in order to promote extraction of certain metals (e.g. metals other than lead) into the solution phase. In step b) the extraction material may be maintained at a temperature of 1 to 160°C, such as 1 to 120°C, preferably 20 to 160°C, preferably 20 to 120°C. This will preferably be above around 50°C, preferably above around 80°C (e.g. 80°C to 100°C) more preferably above around 90°C (e.g. 90 to 100°C). Where appropriate, pressurised reaction vessels may be used to allow temperatures to be elevated above the temperature at which the reaction mixture would boil at atmospheric pressure. In an alternative embodiment, all steps described herein may be conducted at or around atmospheric pressure (e.g. 1 ±0.1 Atm).

The contact time in step b) will be sufficient for a desired proportion of the at least one metal to be extracted from the slag material and may depend upon the temperature and the material particle size. Suitable contact times may be 1 second to 24 hours, preferably 10 minutes to 24 hours, preferably 1 to 12 hours, such as 2 to 6 or 3 to 5 hours. The residence/contact times indicated herein for any step or embodiment may be the average residence times for material undergoing that step. This applies particularly for continuous process implementations where actual residence times for any particular particle or sample may be variable and/or difficult to determine.

Separation of the first extraction mixture into a first solid fraction and a first liquid fraction at step c) may be by any suitable solid/liquid separation method. Filtration through suitable screens and/or filters is one suitable embodiment, as are methods such as thickeners or cyclonic separation. Similar methods may be used for any solid/liquid separation step described herein.

Step d), relating to the extraction of at least a first metal from the first liquid fraction may take the form of many individual steps, some of which are described herein, with many also being known to those of skill in the art.

The first liquid fraction, as referred to herein, may alternatively be referred to as the leach filtrate (or first leach filtrate). Typically, the first liquid fraction may comprise metals such as zinc and iron.

Typically, step d) comprises adjusting the pH of the solution through the addition of one or more salts (e.g. soluble salts) or reagents. It is generally preferred if the pH of the solution is increased. Typically, the addition of acidic salts and acidic reagents is avoided. Generally accepted salts and reagents may be selected from sulphide reagents (e.g. Na2S, K2S, H2S), wherein Na₂S is especially preferred. For the extraction of some metals, the addition of metal salts such as alkali metal or alkaline earth metals salts is preferred. Examples of suitable alkali or alkaline earth metal salts are calcium carbonate, sodium hydroxide, calcium hydroxide, potassium hydroxide, and especially calcium carbonate and/or calcium hydroxide. Calcium salts are desirable, as described herein, because this can result in generation of valuable calcium sulphate (gypsum) during the regeneration step (see below).

It is preferred that steps a) to d) do not involve electrolysis (e.g. do not include electrolysis for the extraction of at least a first metal from a silicate-rich slag material). The steps a) to d) generally involve solubilisation and precipitation techniques. In a preferred embodiment, the recovery of the first metal does not include any electrolysis step.

In one embodiment, no electrolysis is utilised in any step of the present method.

The nature of silicate-rich slag is described herein below but will typically contain at least 10% by weight of silicate, measured as silica (SiO₂). Such silicate-rich slag materials have previously been difficult to process because they tend to form gels with sulphuric acid. The silicate-rich slag may also contain iron. In one embodiment, the silicate-rich slag material may also be rich in iron. Iron-rich slag may contain 10 to 50% or 20 to 50% by weight of iron, such as 30 to 40% or 32 to 38% by weight of iron (measured as elemental iron).

The acidic solution used in all of the various embodiments of the present invention comprises methanesulphonic acid (MSA). Suitable levels of MSA in the acidic solution will generally be around 50 to 500 g/L, preferably around 65 to 200 g/L, such as 80 to 150 g/L. A level of around 90 to 120g/L has been found to work effectively.

The total amount of acid present in the acidic solution may be made up from a mixture of MSA and sulphuric acid. In one embodiment, the total weight of acid in the acidic solution may thus be around 60 to 500 g/L, preferably around 80 to 200 g/L, such as 100 to 150 g/L. A level of around 100 to 140g/L total acidity has been found to work effectively.

It is generally preferred that the acidic solution comprises both methanesulphonic acid (MSA) and sulphuric acid (H₂SO₄). This may be at any suitable ratio by weight (including those described herein), such as between 10:1 and 1 :10 MSA to sulphuric acid (e.g. 10:1 to 1 :1 or 5:1 to 2:1 MSA to sulphuric acid). The methanesulphonic acid used in the acidic solution (e.g. in step a)) of the present invention may be gradually neutralised during the separation steps leading to the recovery of at least one metal from the first liquid fraction. In one embodiment, the MSA may be regenerated and thus re-used following such extraction steps. The methods of the present invention may thus include: z) regenerating the methanesulphonic acid following extraction of at least a first metal.

Such regeneration will typically be by means of adding a strong acid component such as a mineral acid. In one embodiment, the MSA is regenerated by addition of sulphuric acid, such as at least a 1 :1 mole ratio of sulphuric acid. The regenerated MSA may then form at least one component of the acidic solution to be used in future iterations of step a). By such a method, the MSA is recycled, reducing waste and reducing the cost of the process and of the resulting waste-handling. It is preferable that at least 50% (e.g. 50 to 99%) by weight of the MSA may be regenerated and recycled to step a). This will preferably be at least 60% or at least 70%, more preferably at least 75% (e.g. at least 80%).

In certain cases when the acidic solution comprises both MSA and H₂SO₄, MSA may be regenerated by the further addition of sulphuric acid. When MSA is regenerated from a solution comprising both MSA and H₂SO₄, a further addition of H₂SO₄ is added so the ratio of MSA/ H₂SO₄, in the acid solution is an appropriate ratio (e.g. as described herein such as 1 :1).

Regeneration of the MSA is typically carried out using sulphuric acid. This is advantageous for several reasons. Firstly, it is a stronger acid than MSA and thus allows regeneration of the MSA. It is also readily available with established handling methods etc. Additionally, the use of sulphuric acid for acid regeneration has a considerable advantage where at least one calcium salt is used in one or more pH control steps during the process (see also below). This is because the use of acidification with sulphuric acid results in precipitation of at least some of the calcium out of the regeneration solution in the form of insoluble calcium sulphate, which can be removed by any appropriate solid/liquid separation method (e.g. filtering, cyclonic separation etc). Calcium sulphate (also known as gypsum) is a valuable product for use in plaster and plaster products such as plasterboard (drywall). In one embodiment, hydrated calcium sulphate of at least 90% purity (e.g. 90 to 99.9% purity) is generated at the regeneration step and separated from the regenerated methanesulphonic acid solution.

In one embodiment, the silicate-rich slag material comprises lead. Correspondingly, the at least one metal may comprise lead (Pb). Lead may be present in the slag material at an amount of 1 to 30% by weight, such as 1 to 20% or 1 to 10% by weight (measured as elemental Pb). An example lead content may be 2 to 8% by weight.

In one embodiment, the acidic solution may contain a mixture of MSA and sulphuric acid. This has a particular advantage because it allows the lead from the slag material to precipitate from the extraction material. Without being bound by theory, it is believed that the lead will precipitate in the form of insoluble lead sulphate. This allows the lead to be separated from other metals by removal with the first solid fraction rather than in the first liquid fraction.

Alternatively or in addition, the process of lead extraction may be enhanced by the addition of sulphates and/or sulphides in order to increase lead precipitation. This may take place in the vessel where the slag and acidic solution are contacted or may take place in a secondary vessel (e.g. by transfer of the first liquid fraction to a secondary vessel and addition of at least one sulphate or sulphide).

In one preferred embodiment, at least 50% (e.g. 50 to 100%) of the recovered lead is present in the first solid fraction. This will preferably be at least 80% or at least 90%.

The amount of sulphuric acid in the acidic solution may be any suitable amount which avoids the formation of an unmanageable gel when contacted with the high-silicate raw material. Generally, this will be less than 10Og/L (e.g. 0 to 100 g/L). In one embodiment, the sulphuric acid is present in an amount sufficient to precipitate at least 80% of the lead from the high- silicate slag as lead sulphate in the first solid fraction. To achieve these two goals, it is preferable that the sulphuric acid is in a suitable “sweet spot”, allowing easy processing but good recovery of lead. Typically, the sulphuric acid content will typically be around 15 to 80g/L. This will preferably be around 20 to 60 g/L or 25 to 50 g/L.

The ratio of MSA to sulphuric acid in the acidic solution may be important to ensuring that the silicates do not significantly form a gel during extraction while allowing the lead to precipitate out as lead sulphate. Again, there is a “sweet spot” where too much sulphuric acid causes gelling while too little causes lead to be lost or less easily separated. In one embodiment, acidic solvent thus comprises methanesulphonic acid and sulphuric acid at a ratio of 10:1 to 1 :1 by weight. This will preferably be 5:1 to 2:1 and more preferably between 4:1 and 3:1 MSA:H₂SO₄ (all ratios by weight).

A suitable amount of sulphuric acid may be present in the acidic solution by using excess sulphuric acid when regenerating the MSA. Thus, the regeneration step z) may utilise a 10 to 100% excess of sulphuric acid, and typically 20 to 50% excess. In one embodiment, an amount of 10 to 80g/L of sulphuric acid is added at the regeneration step, preferably 30 to 50g/L. In one embodiment, around 10 to 50% of the total weight of acid in the acidic solution is added as sulphuric acid in the regeneration step. This may be, for example 25 to 40% or 30 to 38% of the total acidity. Thus, for example, where the desired total weight of acid is 120g/L, around 30 to 48g/L or sulphuric acid may be added at the regeneration step, amounting to 25 to 40% of the total.

Where lead is one of the at least one metals to be extracted, this can be removed from the first solid fraction after precipitation by sulphuric acid in the extraction mixture. In order to recover lead from the first solid fraction, this may be treated by any appropriate method. Suitable methods include the steps of: q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

The first solid fraction, as referred to herein, may alternatively be referred to as the leach residue (or first leach residue). Essentially the first solid fraction or leach residue is the undissolved material from the first extraction mixture. Typically, the first solid fraction may comprise metals including lead, copper, silver, gold and/or tin, but is not limited thereto.

The aqueous solution used in step q) may be any effective solution but will preferably be a salt solution, such as a solution of NaCI (brine). Suitable concentrations of brine may be around 75 to 300 g/L, preferably around 100 to 200g/L such as 130 to 170 g/L. Step q) may be carried out at any effective temperature but generally a temperature of 20 to 40’C may be used. The amount of aqueous solution may be any effective amount but will typically be around 1 to 500 g of solid per litre of aqueous solution. This may vary depending upon the scale and the equipment used and so may be, for example, 1 to 20 g/L for lower concentrations or 100 to 500 g/L for large scale production.

In one embodiment, particularly on moderate scales, the amount of solid may be 1 to 20g of solid per litre of solution. This will preferably be around 2 to 12 g/L or 4 to 10 g/L. The extraction in the second extraction mixture may be for any effective time, such as 1 minute to 3 hours, 10 minutes to 1 hour or 20 to 40 minutes. Where appropriate, the (e.g. brine) solution may be separated from at least a portion of the solubilised lead (e.g. by precipitation) and may then be recirculated for further use. Thus, steps q) and r) or steps q), r) and s) may be repeated several times (e.g. 1 to 4 times).

The separation of the second extraction mixture (step r)) may be by any appropriate method, such as those described herein or known in the art.

The precipitation of lead sulphide in step s) may be under any appropriate conditions. A typical method may include addition of a gaseous sulphide (e.g. H₂S) or a soluble sulphide such as Na₂S or K₂S. Addition of sodium sulphide is preferred. The sulphide may be added in any effective amount but will generally be added until the pH of the second liquid fraction reaches pH 6 to 8, preferably pH 6.5 to 7.5 (e.g. 6.9 to 7.1).

In one embodiment, at least 70% (e.g. 70 to 99%) of the lead present in the high-silicate slag material is recovered in the lead extraction steps (e.g. steps q) r) and s)). This will preferably be at least 75% or at least 80% by weight.

In one embodiment, the amount of lead in the lead salts (e.g. PbS) recovered in the lead extraction steps (e.g. steps q), r) and s)) will be at least 40% or at least 50% (e.g. 40 to 90% or 50 to 90%) based on elemental lead content. This will preferably be at least 60% or at least 65%, more preferably at least 70%.

Other materials may be present in the lead salts recovered but will typically be at limited levels in order to achieve lead material of the optimum technical and commercial value. Possible ranges of other materials that may be present include (independently) any one or more of the following:

Ag content: 0 to 2000g/t, preferably 100 - 1500 g/t

Cu content: 0 to 5%, preferably 0.5% - 3%

Zn content: 0 to 5%, preferably 0.5% - 3% Fe content: 0 to 25%, preferably 5% - 20% S content: 0 to 40% (e.g. 20% - 40%, especially for PbS) As content: 0 to 8%, preferably 0.5% - 5% PbS content: >90% Moisture content: <10% Any of these limits may be applied to other products described herein, where appropriate and technically viable.

The lead is preferably extracted from the second liquid fraction by precipitation with a sulphide. However, other metals may be present in the second solid fraction and so this may be processed further. In one embodiment, Cu, Ag and/or Au may be recovered from the second solid fraction. Such extraction methods are known in the art and any suitable method may be used. Suitable methods include acid, Fe2(SC>4)3 or H2O2 leaching methods for copper. Cyanide leaching or thiosulphate leaching methods are suitable for Au/Ag.

In one embodiment, the at least one metal may comprise zinc (Zn). Zinc will typically be recovered from the first liquid fraction, optionally following a clean-up step (see below). Zinc will generally be recovered by precipitation from the first liquid fraction (or from the cleaned-up liquid fraction) by: h) precipitation of at least one zinc salt from the first liquid fraction; and i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction.

Since precipitation and separation may not recover all of the zinc from the liquid fraction, the zinc-depleted liquid fraction may be re-extracted for zinc and steps h) and i) repeated as necessary. Thus the process may include: k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution.

Typically 1-4 repetitions (to give 2-5 total occurrences) of steps h) and i) may be employed. A total of 3 zinc precipitation and separation steps (h) and i)) are preferred.

Zinc precipitation may be brought about by any appropriate means. In particular, addition of a reagent or soluble salt which generates an insoluble (or sparingly soluble) zinc salt may be used. Typical reagents include sulphides such as H2S, Na2S or K2S. Na2S is preferred.

The purity of the zinc salt(s) recovered may be in the range 25 to 67% or 25 to 70% by weight of elemental zinc (ZnS will be up to 67% zinc while ZnO and mixtures may be higher). This will preferably be at least 30% by weight zinc or at least 40% (e.g. around 40 to 60%) by weight elemental zinc content. More preferably at least 45%.

It will be preferable that the zinc extract does not contain large amounts of contaminant metals. In one example embodiment, the zinc salt(s) recovered contain no more than 7% by weight of lead (e.g. 0 to 5% or 0 to 3% or 2 to 5%). In a further embodiment, the zinc salt(s) recovered contain no more than 18% by weight of iron (e.g. 0 to 15% or 5 to 15%). In a further embodiment, the zinc salt(s) recovered contain no more than 3% by weight of copper (e.g. 0 to 3% or 0.5 to 2% or 0.2 to 2%). In a further embodiment, the zinc salt(s) recovered contain no more than 2500ppm by weight of cadmium (e.g. 0 to 2000ppm or 0 to 10OOppm or 100 to 2000ppm). Other materials may be present in small quantities, such as silver (e.g. 20-50 g/t) or gold (e.g.2-5g/t). These levels are disclosed herein with respect to the zinc product but all such levels may be appropriate with other products indicated herein, where technically viable.

In one embodiment, at least 50% (e.g. 50 to 95%) of the zinc present in the high-silicate slag is recovered at the zinc extraction stage (e.g. steps h), i) and optionally k)). This will preferably be at least 60% or at least 65%.

At any stage in the process, particularly after separation of the first solid fraction from the first liquid fraction, liquid components may be subject to a “clean-up” step. This will preferably be before recovery of any of the “at least one metal” which is desired to be extracted from the liquid component. For example, this may take place after the separation step c) but before metal recovery step d) (which may itself be recovery of zinc by steps h) and i)).

A clean-up step may be carried out by: f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; and g) separation of components precipitated in step f) from said first liquid fraction;

Unwanted components in this context will typically include silicon compounds such as silica/silicates and may include aluminium salts. Although iron salts may also be precipitated to a certain extent in step f), these are typically not unwanted materials. However, it may not be desirable and/or practical to avoid precipitation of a certain amount of iron during the clean-up step. ln a preferred embodiment, 50 to 99.99% of the silicon and/or aluminium is removed by cleanup steps f) and g). This will preferably be at least 95%, at least 98% or at least 99%. Such steps may evidently be repeated if necessary or desirable.

In a further embodiment of the present invention, iron may be precipitated from the liquid fraction, preferably following extraction of zinc. In one embodiment, therefore, the method of the present invention may include: l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z).

Without being bound by theory, it is believed that different oxidation states of iron may precipitate under different conditions. Oxidation or reduction steps may therefore be used to control the oxidation state of the iron. This may allow greater purity of the precipitated iron product.

In the various methods of the present invention, the pH, especially of the various liquid components, is adjusted at various stages. In one preferred embodiment, the pH is adjusted during at least one step by means of a calcium salt. Suitable calcium salts include quick lime (calcium oxide), slaked lime (calcium hydroxide) and/or calcium carbonate.

In one embodiment, pH is adjusted at steps f) and/or I). The pH is preferably adjusted by means of at least one calcium salt. Preferably, pH will be adjusted at step f) by means of addition of calcium carbonate. Preferably pH will be adjusted at step I) by means of addition of calcium hydroxide.

One embodiment of the present invention may include recovery of at least Pb and Zn (and preferably Fe) from a silicate-rich slag material by a cyclic method comprising use of an acidic solvent comprising MSA (and preferably also comprising H₂SO₄). Such a method may comprise: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 1 to 120°C, preferably 20 and 120°C for a period of between 1 second to 24 hours, preferably 10 minutes and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; d) recovering at least a first metal from the first liquid fraction; e) recovering lead from the first solid fraction; and z) regenerating the methanesulphonic acid following extraction of at least a first metal.

In this and in all methods of the present invention step d) may optionally comprise: f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; g) separation of components precipitated in step f) from said first liquid fraction;h) precipitation of at least one zinc salt from the first liquid fraction; i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction; k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution; l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z).

In this and in all methods of the present invention, step e) may optionally comprise: q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

Step e) may additionally comprise recovery of additional metals such as Cu, Ag and/or Au as described herein.

Viewed from a further aspect, the present invention refers to an apparatus for the extraction of at least one metal, said apparatus being configured to carry out the method of any preceding claim. The apparatus of the present invention includes various features which embody the key contributions of the present invention. In general, the apparatus has features configured: i) to accept a solid silicate-rich slag material into a first vessel; ii) to accept an acidic solvent comprising methanesulphonic acid into said first vessel to generate a first extraction mixture; iii) to maintain said first extraction mixture in said first vessel at a temperature of between 1 to 120°C, preferably 20 and 120°C for a period of between 1 second to 24 hours, preferably 10 minutes and 24 hours; iv) to separate said first extraction mixture into a first solid fraction and a first liquid fraction; and v) to recover at least a first metal from said first liquid fraction.

Part i) may be achieved any appropriate feed mechanism such as a conveyor, pump, screwfeeder or batch feeding process. The solid silicate-rich slag material may be in any suitable form, such as a dry powdered solid or a slurry.

Part ii) may be achieved by any appropriate liquid-feeding process adapted to handle strong acids, such as a suitably refractory pipeline (e.g. of glass or stainless steel).

Part ii) may be achieved by use of any suitable refractory first vessel, such as a glass, ceramic or stainless-steel lined vessel equipped with heating and optionally (and preferably) agitation means.

Part iv) may be achieved with any appropriate separation means such as filtration, settling and/or cyclonic separation equipment adapted to accept the strong acidic solution. Such means may be formed of a refractory material such as glass or stainless steel.

Part v) may be achieved, for example, by configuring the apparatus to pass the first liquid fraction from part iv) on to a second vessel configured to extract zinc from said first liquid fraction by means of precipitation as an insoluble zinc salt. Such apparatus may be configured for pH adjustment and separation steps as described herein including optional re-extraction by repeating appropriate steps by recycling of materials. The second vessel may, in some configurations, be the same physical vessel as the first vessel.

The apparatus may additionally be configured to pass the first solid fraction from part iv) on to a third vessel configured to extract lead from said first solid fraction by means of extracting with an aqueous solution. This third vessel may be configured for extraction and/or precipitation as described in any appropriate embodiment herein.

The apparatus may be additionally configured to: vi) regenerate at least a part of the MSA by addition of a mineral acid and recycle the thus- regenerated MSA to part ii)

Such regeneration may take place in a fourth vessel configured to accept addition of a mineral acid such as sulphuric acid. The vessel may be configured for regeneration of MSA as described in any embodiment herein. The vessel may be configured to separate a solid such as gypsum generated during the recycling step (as described herein). The fourth vessel may, in some configurations, be the same physical vessel as the first and/or second vessels.

In use, the apparatus of the invention will, in some embodiments, be configured such that the first vessel contains an acidic solvent such as those described herein (e.g. a mixture of MSA and sulphuric acid) and contains particles of a silicate-rich slag material (as described herein in any embodiment). The particles of slag material may be such that least 50% of particles pass through a screen of aperture size 500|j.m or any appropriate size, such as those sizes described herein.

Definitions

As used herein, the term “slag” or “slag material” is used to indicate any solid (e.g. stony and/or glassy) waste, dross, or scoria, generated from the smelting or refining of at least one metal.

The term “silicate-rich slag” is used to indicate that at least 10% by weight of the slag is silica/silicate (measured as silica (SiO₂) content). Typical amounts of silica/silicate in silicate- rich slag will be 10 to 60% by weight, preferably 12 to 50% or 15 to 40% by weight. Highly suitable silicate-rich slag material will contain 15 to 40% or 20 to 30% by weight silica.

The “metals” extracted by the methods of the present invention may be any metal, particularly those which are valuable and/or present an environmental hazard. For the avoidance of doubt, silicon is not a metal and therefore extraction of silicon and/or silica/silicate (SiOx) is not extraction of a metal for the purposes of the present invention. The term “metal” or “metals” includes both elemental metal(s) in addition to metal ions (e.g. in the form of metal salt(s) and/or solution(s)). Although aluminium is a metal, in one embodiment, aluminium is not a metal which is desirable for extraction. Thus, in one embodiment, the “at least one metal” to be extracted by the methods of the present invention may be at least one metal excluding aluminium. Thus, although aluminium may in fact be recovered, this may form part of the waste stream from the process and/or may not be in a commercially useful form.

As used herein, the term “substantially”, as well as terms “about”, “approximately”, “around” and similar terms indicate that the value specified is close to the value indicated and/or is appropriate to achieve the described result. Typically, such values may vary by ±20% of their stated value, preferably ±10% or ±5%, such as ±1%. Evidently, the stated value will be a preferred embodiment. Where it is specified that conditions do not cause substantial gelling, this may be that such conditions do not increase the viscosity of a mixture by more than 50% (e.g. may reduce viscosity by dilution, such as by -20% or increase by up to 50%), preferably not by more than 20% or not by more than 10%.

Times, temperatures and other conditions specified herein for various steps and embodiments may be absolute measured conditions, or may be average conditions to which a particular mixture is subjected. In particular, where continuous processes are used, contact or residency times may be averages. Similarly, in both batch and continuous processes, contact periods at specified temperatures may be those periods spent at the target temperature or may be periods during which the average temperature is in the target range, allowing for time heating and/or cooling the mixture and/or the physical and/or chemical effects on temperature caused by adding reagents (e.g. exothermic reactions).

Percentages are given by weight unless indicated otherwise. Where the percentage content of a metal is given, this is the content calculated as the elemental metal, unless otherwise stated. Content of silicate(s) will typically be calculated as SiO₂ unless otherwise stated.

Examples

Example 1 - cyclic extraction method - Pb, Zn, Fe extraction

A cyclic metal extraction method was tested using MSA as leaching acid and regenerating/recycling the acid. The overall method is illustrated schematically in Figure 4.

1A - MSA leaching MSA Leaching of Slag was conducted on silicate-rich Turkish slag having the following analysis: 5.03 % Pb, 2.55 % Zn, 26.0 % SiO₂,35.6 % Fe, 0.82 g/t Au, 61 g/t Ag. 80% of particles passed through a screen of 150 pm aperture.

MSA (concentration 120 g/L) was added to solid slag material in an amount of 50 g slag per Litre leach solution. The solution was maintained for 4 hours at a temperature of 95°C.

The solid and liquid components were separated by filtration.

1B - Clean-up removal of Si/AI

Si/AI was Precipitated from the liquid phase in 2 stages:

Stage 1 - CaCO₃ was added to adjust pH to 3.7. The solution was maintained with aeration for 1 hour at 23°C.

Stage 2 - CaCO₃ was added to adjust and maintain pH at 4.5. The solution was maintained with aeration for 2 hours at 23°C.

Some Fe is precipitated in this step but was found to be insufficient to merit recovery. The bulk of the Fe is precipitated after Zn recovery.

1C - Zinc recovery

Zinc was recovered by ZnS Precipitation from the cleaned-up liquid phase in 3 stages.

Stage 1 - MSA was added to acidify the pH to 2 over a period of 15 minutes. Subsequently, Na₂S was added to adjust pH to 4 over 30 minutes. The temperature was maintained at around 23°C. Precipitated ZnS was separated by filtration.

Stage 2 & 3 - repeat of stage 1 using the liquid fraction from the ZnS filtration step.

1D - Total Fe Precipitation

The remaining iron in solution was precipitated by increasing the pH of the liquid phase. Slaked lime (Ca(OH)₂) was added to the liquid fraction from the final ZnS filtration step to adjust pH to 7.6. The solution was maintained at ~25°C with aeration for approximately 2 hours. The precipitated iron salts were separated from the liquid fraction by filtration.

1E - MSA regeneration MSA Regeneration was carried out by addition of 40 g of H₂SO₄ per L of solution generated from the Fe precipitation step. The solution was maintained at approx. 25°C for around 30 minutes. The calcium ions from the earlier neutralisation steps were precipitated as gypsum (calcium sulphate) at good commercial purity and recovered by filtration.

1F - MSA Makeup and cyclic processing

MSA was added to the regenerated acid solution at ~25°C to give a total acid titration of 120 g/L MSA. This regenerated and made-up solution was used for MSA leaching step in the next cycle. Steps 1 A to 1 F were thus repeated on a cyclic basis.

1G - Pb Recovery

The Pb recovery steps were carried out on the solid component separated at step 1 A.

1G.1 - Brine Leach

The MSA Leach Residue from step 1 A was treated with aqueous NaCI (brine) at a concentration of 150 g/L. The brine solution was added at a level of 6.67 g solid residue per litre of brine solution. The mixture was maintained at around 29°C for 30 minutes. The solid and liquid fractions were separated by filtration.

1G.2 - Lead precipitation

PbS Precipitation was carried out on the liquid fraction from step 1G.1 (Brine Leachate solution). Na₂S was added to the liquid brine leachate to adjust the pH to 7.0 over around 30 minutes. The temperature was around 25°C.

The brine was recovered and recycled back to step 1 G.1

Example 2 - recoveries from cyclic testing

The recovery of various metals in the cyclic method of Example 1 was analysed, along with the purity of the recovered salts.

Total Pb, Zn, and Fe recoveries obtained in the locked cycle tests were as follows: 82.0 % to 84.4 % for Pb, 67.4 % to 75.2 % for Zn and 57.7 % to 72.9 % for Fe. Purities were found to be:

PbS product grades ranged from 69 % to 74.8 % Pb.

ZnS product grades ranged from 33.3% to 45.0%.

Fe-oxide precipitates contained between 36.0 % and 45.6 % Fe. High purity CaSO₄ products were produced in the MSA regeneration step.

The cyclic method was run for 6 cycles and the recoveries of the various components measured for each cycle. The results are shown in the table below: tThe concentrations were calculated based on the Ca contents. The >100 percent products may contain stucco or anhydrite which could have been formed during sample drying process. The following embodiments represent preferable implementations of the invention which may be used individually or in any technically viable combination of two or more.

In a preferable embodiment, the invention refers to a method for the extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said method comprising: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 20 and 120°C for a period of between 10 minutes and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; and d) recovering at least a first metal from the first liquid fraction.

In a preferable embodiment, the invention refers to a method as described herein wherein the silicate-rich slag comprises at least 10% by weight silicate, measured as silica (SiO₂).

In a preferable embodiment, the invention refers to a method as described herein wherein the silicate-rich slag comprises at least 10% by weight iron (Fe).

In a preferable embodiment, the invention refers to a method as described herein comprising the step: z) regenerating the methanesulphonic acid following extraction of at least a first metal.

In a preferable embodiment, the invention refers to a method as described herein wherein at least a part of the methanesulphonic acid is regenerated by means of addition of sulphuric acid (H₂SO₄).

In a preferable embodiment, the invention refers to a method as described herein, being a cyclic method in which at least a part of the methanesulphonic acid is regenerated and the thus- formed regenerated methanesulphonic acid is returned to step a) and contacted with a further portion of silicate-rich slag material.

In a preferable embodiment, the invention refers to a method as described herein wherein regeneration of at least a part of the MSA generates solid calcium sulphate, which is separated from the regenerated methanesulphonic acid.

In a preferable embodiment, the invention refers to a method as described herein wherein the acidic solvent comprises both methanesulphonic acid and sulphuric acid.

In a preferable embodiment, the invention refers to a method as described herein wherein the acidic solvent comprises methanesulphonic acid and sulphuric acid at a ratio of 1 :1 to 5:1 by weight.

In a preferable embodiment, the invention refers to a method as described herein wherein the at least one metal comprises lead (Pb). In a preferable embodiment, the invention refers to a method as described herein comprising: e) recovering lead from the first solid fraction.

In a preferable embodiment, the invention refers to a method as described herein wherein step e) comprises: q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

In a preferable embodiment, the invention refers to a method as described herein wherein copper, gold and/or silver are extracted from said second solid fraction.

In a preferable embodiment, the invention refers to a method as described herein wherein the at least one metal comprises Zinc (Zn).

In a preferable embodiment, the invention refers to a method as described herein comprising: h) precipitation of at least one zinc salt from the first liquid fraction; and i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction.

In a preferable embodiment, the invention refers to a method as described herein comprising: k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution.

In a preferable embodiment, the invention refers to a method as described herein wherein the zinc is separated from the first liquid fraction by means of precipitation of zinc sulphide.

In a preferable embodiment, the invention refers to a method as described herein comprising: f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; and g) separation of components precipitated in step f) from said first liquid fraction; wherein steps f) and g) preferably take place before steps h) and i) (where present).

In a preferable embodiment, the invention refers to a method as described herein wherein the unwanted components comprise silicon and/or aluminium.

In a preferable embodiment, the invention refers to a method as described herein wherein increasing pH is carried out by addition of a calcium salt such as calcium carbonate.

In a preferable embodiment, the invention refers to a method as described herein wherein increasing pH is carried out to a pH of around 3 to 6, preferably 4 to 5.

In a preferable embodiment, the invention refers to a method as described herein wherein: l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z).

In a preferable embodiment, the invention refers to a method for extraction of at least Pb and Zn (and preferably Fe) from a silicate-rich slag material by a cyclic method comprising use of an acidic solvent comprising MSA (and preferably also comprising H2SO4). Such a method comprising: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 20 and 120°C for a period of between 10 minutes and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; d) recovering at least a first metal (e.g. zinc) from the first liquid fraction; e) recovering lead from the first solid fraction; and z) regenerating the methanesulphonic acid following extraction of at least a first metal; wherein step d) optionally comprises: f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; g) separation of components precipitated in step f) from said first liquid fraction; h) precipitation of at least one zinc salt from the first liquid fraction; i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction; k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution; l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z). and wherein step e) optionally comprises: q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

In a preferable embodiment, the invention refers to a method as described herein wherein the silicate-rich slag material has a particle size such that at least 80% of particles pass through a sieve with aperture size 500 .m.

In a preferable embodiment, the invention refers to an apparatus for extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said apparatus being configured: i) to accept a solid silicate-rich slag material into a first vessel; ii) to accept an acidic solvent comprising methanesulphonic acid into said first vessel to generate a first extraction mixture; iii) to maintain said first extraction mixture in said first vessel at a temperature of between 20 and 120°C for a period of between 10 minutes and 24 hours; iv) to separate said first extraction mixture into a first solid fraction and a first liquid fraction; and v) to recover at least a first metal from said first liquid fraction.

In a preferable embodiment, the invention refers to an apparatus as described herein wherein the first vessel contains a mixture of MSA and sulphuric acid and particles of said silicate-rich slag material of which at least 50%, of particles pass through a screen of aperture size 500 .m.

In a preferable embodiment, the invention refers to an apparatus as described herein configured to pass the first liquid fraction from part iv) on to a second vessel configured to extract zinc from said first liquid fraction by means of precipitation as an insoluble zinc salt.

In a preferable embodiment, the invention refers to an apparatus as described herein configured to pass the first solid fraction from part iv) on to a third vessel configured to extract lead from said first solid fraction by means of extracting with an aqueous solution.

In a preferable embodiment, the invention refers to an apparatus as described herein being additionally configured to: vi) regenerate at least a part of the MSA by addition of a mineral acid and recycle the thus- regenerated MSA to part ii)

Claims

1 ) A method for the extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said method comprising: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 1 and 120°C for a period of between 1 second and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; and d) recovering at least a first metal from the first liquid fraction.

2) The method of claim 1 wherein step d) comprises adjusting the pH of the first liquid fraction, preferably increasing the pH, through the addition of one or more salts or reagents, preferably one or more basic salts or reagents.

3) The method of claim 2 wherein the one or more salts or reagents is selected from a list comprising: sulphide reagents (e.g. H2S, K2S, Na2S, preferably Na2S) or metal salts, such as alkali or alkaline earth metal salts (e.g. CaCO₃ and/or Ca(OH)₂).

4) The method of any preceding claim wherein in step b) the temperature is in the range 20 to 120°C and/or the period is in the range of 10 minutes and 24 hours.

5) The method of any preceding claim wherein the silicate-rich slag comprises at least 10% by weight silicate, measured as silica (SiCh).

6) The method of any preceding claim wherein the silicate-rich slag comprises at least 10% by weight iron (Fe).

7) The method of any preceding claim comprising the step: z) regenerating the methanesulphonic acid following extraction of at least a first metal.

8) The method of any preceding claim wherein at least a part of the methanesulphonic acid is regenerated by means of addition of sulphuric acid (H₂SO₄). 9) The method of any preceding claim, being a cyclic method in which at least a part of the methanesulphonic acid is regenerated and the thus-formed regenerated methanesulphonic acid is returned to step a) and contacted with a further portion of silicate-rich slag material.

10) The method of claim 8 or claim 9 wherein regeneration of at least a part of the MSA generates solid calcium sulphate, which is separated from the regenerated methanesulphonic acid.

11) The method of any preceding claim wherein the acidic solvent comprises both methanesulphonic acid and sulphuric acid.

12) The method of claim 11 wherein the acidic solvent comprises methanesulphonic acid and sulphuric acid at a ratio of 1 :1 to 5:1 by weight.

13) The method of any preceding claim wherein the at least one metal comprises lead (Pb).

14) The method of any preceding claim comprising: e) recovering lead from the first solid fraction.

15) The method of claim 14 wherein step e) comprises: q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

16) The method of claim 15 wherein copper, gold and/or silver are extracted from said second solid fraction.

17) The method of any preceding claim wherein the at least one metal comprises Zinc (Zn).

18) The method of claim 17 comprising: h) precipitation of at least one zinc salt from the first liquid fraction; and i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction.

19) The method of claim 18 comprising: k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution.

20) The method of claim 19 wherein the zinc is separated from the first liquid fraction by means of precipitation of zinc sulphide.

21) The method of any preceding claim comprising: f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; and g) separation of components precipitated in step f) from said first liquid fraction; wherein steps f) and g) preferably take place before steps h) and i) (where present).

22) The method of claim 21 wherein the unwanted components comprise silicon and/or aluminium.

23) The method of claim 21 or claim 22 wherein increasing pH is carried out by addition of a calcium salt such as calcium carbonate.

24) The method of any of claims 21 to 23 wherein increasing pH is carried out to a pH of around 3 to 6, preferably 4 to 5.

25) The method of any preceding claim wherein: l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z).

26) A method for extraction of at least Pb and Zn (and preferably Fe) from a silicate-rich slag material by a cyclic method comprising use of an acidic solvent comprising MSA (and preferably also comprising H₂SO₄). Such a method comprising: a) contacting said silicate-rich slag material with an acidic solvent comprising methanesulphonic acid (MSA) to form a first extraction mixture; b) maintaining the first extraction mixture at a temperature of between 1 and 120°C for a period of between 1 second and 24 hours; c) separating the first extraction mixture into a first solid fraction and a first liquid fraction; d) recovering at least a first metal (e.g. zinc) from the first liquid fraction; e) recovering lead from the first solid fraction; and z) regenerating the methanesulphonic acid following extraction of at least a first metal; wherein step d) optionally comprises one or more of steps f) to m): f) precipitation of unwanted components from said first liquid fraction by means of increasing pH; g) separation of components precipitated in step f) from said first liquid fraction; h) precipitation of at least one zinc salt from the first liquid fraction; i) separation of said precipitated zinc salt from the first liquid fraction to give a zinc- containing solid fraction and a zinc-depleted liquid fraction; k) returning said zinc-depleted liquid fraction to steps h) and i) at least one additional time (e.g. 1 to 4 times) to increase the amount of precipitated zinc salt and to generate a zinc-barren solution; l) the zinc-barren solution is neutralised to a pH of around 6 to 8 whereby to precipitate any remaining iron; and m) separation of the precipitated iron before regenerating said MSA at step z). and wherein step e) optionally comprises one or more of steps q) to s): q) contacting said first solid fraction with an aqueous solution whereby to give a second extraction mixture comprising dissolved lead; r) separation of said second extraction mixture into a second solid fraction and a second liquid fraction; and s) precipitation of lead sulphide from said second liquid fraction.

27) The method of any preceding claim wherein the silicate-rich slag material has a particle size such that at least 80% of particles pass through a sieve with aperture size 500|j.m.

28) An apparatus for extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said apparatus being configured: i) to accept a solid silicate-rich slag material into a first vessel; ii) to accept an acidic solvent comprising methanesulphonic acid into said first vessel to generate a first extraction mixture; iii) to maintain said first extraction mixture in said first vessel at a temperature of between 1 and 120°C for a period of between 1 second and 24 hours; iv) to separate said first extraction mixture into a first solid fraction and a first liquid fraction; and v) to recover at least a first metal from said first liquid fraction.

29) The apparatus of claim 28 configured to pass the first liquid fraction from part iv) on to a second vessel configured to extract zinc from said first liquid fraction by means of precipitation as an insoluble zinc salt.

30) The apparatus of any of claims 28 or claim 29 configured to pass the first solid fraction from part iv) on to a third vessel configured to extract lead from said first solid fraction by means of extracting with an aqueous solution.

31 ) The apparatus of any of claims 28 to 30 being additionally configured to: vi) regenerate at least a part of the MSA by addition of a mineral acid and recycle the thus- regenerated MSA to part ii)

32) An apparatus for extraction of at least one metal from a silicate-rich slag material comprising said at least one metal, said apparatus comprising a first vessel and being configured to carry out the method of any preceding claim.

33) The apparatus of any of claims 25 to 32 wherein the first vessel contains a mixture of MSA and sulphuric acid and particles of said silicate-rich slag material of which at least 50%, of particles pass through a screen of aperture size 500|j.m.