WO2025145245A1 - Closed-loop recycling of metal sulfonate leachates - Google Patents

Abstract

It is provided a process for the close-loop recirculation of sulfonic acids in metal extraction processes relying on an alkaline earth metal base and sulfuric acid. The process comprises leaching a metal feedstock with a sulfonic acid solution metal- depleted solid residue and a concentrated metal sulfonate solution, neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution, precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution, and purifying and regenerating the crude sulfonic acid solution. The crude sulfonic acid solution can be regenerated at R > 90%.

Description

CLOSED-LOOP RECYCLING OF METAL SULFONATE LEACHATES

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 63/617,835 filed January 5, 2024, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] It is provided a method for recirculating sulfonic acid solutions in a closed loop for extracting and recovering metals.

BACKGROUND

[0003] There has been significant growth in the use of sulfonic acid solutions as leaching agents for metal extraction. Sulfonates, like nitrates, indeed form higher solubility salts with a broader range of cations than their most common inorganic peers such as sulfate, halides, and phosphate.

[0004] This broad solubilization spectrum has attracted interest for the treatment of complex feedstocks, such as electronic wastes and rare earth-rich residues, or for hard to leach metals such as lead and silver. These hydrometallurgical leachings are typically carried with concentrated sulfonic acid solutions in presence of an oxidizing agent. The metal sulfonate leachate is then processed to recover the metals of interest, yielding a spent metal-depleted sulfonate solution, that must be regenerated for the process to occur in closed loop.

[0005] Regarding the nature of the acid, the most investigated is methanesulfonic acid (MSA), as described by Binnemans and Jones (Journal of Sustainable Metallurgy, 9(1 ):26-45, 2023). MSA, the simplest of all sulfonic acids, is notably being explored for its low volatility, high redox stability, and low corrosivity, which greatly reduce operational constraints and ease material selection when compared to hydrochloric or nitric acids. This surge in interest for MSA has been buoyed by the sharp decrease in its pricing since the early 2000s, and by its label as a green acid due to its complete biodegradability.

[0006] Other sulfonic acids of potential interest are longer chain alkanesulfonic acids (such as ethanesulfonic acid - ESA) and simple arylsulfonic acids (such as benzene sulfonic acid - BSA and p-toluenesulfonic acids - PTSA). Widely available polysulfonic acids such as lignosulfonic acid, a by-product of the pulp & paper sulfite process, and sulfonated kerosene are also of industrial significance for the extraction of rare-earth metals.

[0007] Sulfonic acid recovery from the spent metal-depleted leachates remains a technical and economical challenge. Existing solutions to recover sulfonic acids involve cost-intensive strategies such as solvent extraction, anion exchange column, distillation, or melt recrystallization.

[0008] Solvent extraction approaches rely on the pH-dependant behavior of sulfonic acids. As a function of their organic tail, sulfonic acids may favor an organic phase in their acid form but will return to water if neutralized by a strong base such as sodium hydroxide. These processes yield purified and concentrated metal sulfonate solutions with recovery rates higher than 90 wt%.

[0009] Owing to the diversity of organic phases available, this approach can be adapted to a wide range of sulfonic acids and feeds: arylsufonates from wastewaters (CN101230025A, 95 wt% recovery rate); perfluorinated sulfonic acid from a viscous organic residue (US7087803, 99.8 wt% recovery rate); or linear sulfonic acids from their mixture with sulfuric acid (US3766255, 92 wt% recovery rate).

[0010] In anion exchange processes, such as described in US5028736 (naphthalene-sulfonic acids), the sulfonic acids are extracted on a basic ion exchanger. An alkaline stream is then used to regenerate the column, yielding a concentrated metal sulfonate solution that can be precipitated by lowering the temperature of the solution. Anion exchange columns can also be used to control the level of impurities in a sulfonic acid stream. US6337421 hence describes a process in which sulfate impurities are removed on an anion exchange column.

[0011] Processes relying on distillation aim at exploiting temperature gaps between the boiling point of the sulfonic acid of interest and other components. Sulfonic acids, such as MSA, are typically recovered in purified and concentrated form as a middle fraction between the higher boilers (such as sulfuric acid) and the lower boilers (water, organic species). The heat sensitivity of sulfonic acids may require the use of vacuum distillation. Examples for alkanesulfonic acid recovery involving a distillation step include US4035242, WO2018138025, EP3763701 , CA3004795, CN105237441 A. Formation of an azeotrope may favor the recovery of the sulfonic acid, such as in WO1998029385. [0012] Alternatively, melt crystallization has been proposed, e.g. US10214485 with alkanesulfonic acids. For recovery rates to be interesting, the method however needs to be combined with a distillation step.

[0013] Existing solutions hence either rely on using third parties, that are most often hazardous (solvent extraction) or expensive (ion exchange columns); or on an energy intensive step such as distillation or melt recrystallization.

[0014] Unexpectedly, chemical precipitation, the most common of all hydrometallurgical operations, has been little investigated for the recovery of sulfonic acids from concentrated spent streams. W02005073161 describes the regeneration of dilute metal sulfonate streams by a combination of solvent extraction and chemical precipitation of the metals as carbonate salts, followed by an evaporation to dryness. This approach requires the metal species to form insoluble carbonates.

[0015] Therefore, there is still a need for significant improvements in the recovery of sulfonic acids from leachates streams to enable an economically viable looping of these acids, especially if it only consumes affordable and benign chemicals owing to the high market price of sulfonic acids.

SUMMARY

[0016] It is provided a method for recirculating sulfonic acid solutions in a closed loop for extracting and recovering metals from a feedstock comprising the steps of optionally leaching the feedstock with a sulfonic acid solution metal-depleted solid residue and a concentrated metal sulfonate solution; neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution; and/or precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution; and purifying the crude sulfonic acid solution.

[0017] In an embodiment, the method comprises leaching the feedstock with a sulfonic acid solution metal-depleted solid residue and a concentrated metal sulfonate solution; neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution; precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution; and purifying the crude sulfonic acid solution.

[0018] In another embodiment, the method comprises precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution; neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution; and purifying the crude sulfonic acid solution, wherein the feedstock is a metal sulfate leachate, effluent, or solution.

[0019] In a further embodiment, the sulfuric acid is added dropwise to a stirred reactor, or crystallizer producing the regenerated crude sulfonic acid solution and precipitate from the regenerated solution.

[0020] In a further embodiment, the method described herein further comprises the step of recovering the precipitate from the regenerated solution by a solid/liquid separation to yield a crude sulfonic acid solution and a cake of alkaline earth metal sulfates.

[0021] In an embodiment, the recovered sulfonic acid has a concentration belonging to the 0.01 M to 4 M.

[0022] In another embodiment, the sulfonic acid is an alkanesulfonic acids, simple arylsulfonic acids, or a polysulfonic acid.

[0023] In an embodiment, the sulfonic acid is methanesulfonic acid, ethane sulfonic acid, p-toluenesulfonic acid, benzenesulfonic acid, lignosulfonic acid, or sulfonated kerosene.

[0024] In another embodiment, the sulfonic acid is methanesulfonic acid.

[0025] In an embodiment, the purified crude sulfonic acid solution is recycled back to the leaching step in closed-loop.

[0026] In a further embodiment, the feedstock is a metal-rich substrate or solution.

[0027] In another embodiment, the feedstock is a mineral feedstock, a waste feedstock, a mining residue feedstock, or a cake. [0028] In an embodiment, the feedstock comprises metals of the periodic table classified as transition metals, post-transition metals, metalloids, lanthanides, actinides, or a combination thereof.

[0029] In another embodiment, the feedstock comprises iron, aluminum, zinc, copper, nickel, cobalt, cadmium, chromium, lead, tin, silver, mercury, manganese, cadmium, gallium, thallium, bismuth, tungsten, alloys thereof or a combination thereof.

[0030] In another embodiment, the solid/liquid separation is performed using a gravity settler, a hydrocyclone, a classifier, a line filter, a pressure filter, a compression filter, a vacuum filter, a filtering centrifuge, or a sedimenting centrifuge.

[0031] In a further embodiment, the sulfonic acid leached metal-depleted solid residue, the precipitate of insoluble metal oxide and hydroxides, and the alkaline earth metal sulfate precipitate from the regenerated solution are further washed with rinsing water and the rising water are mixed with the solution to be recycled to the leaching step.

[0032] In an embodiment, the alkaline earth metal base comprises calcium, strontium, or barium.

[0033] In a further embodiment, the alkaline earth metal base comprises a calcium oxide, a calcium hydroxide, a calcium hydroxide hydrate, a calcium carbonate, a strontium oxide, a strontium hydroxide, a strontium hydroxide hydrate, a strontium carbonate, a barium oxide, a barium hydroxide, a barium hydroxide hydrate, a barium carbonate, or a combination thereof.

[0034] In another embodiment, the alkaline earth metal base is calcium hydroxide.

[0035] In a further embodiment, the alkaline earth metal base is introduced as a powder or an aqueous suspension to a stirred reactor operated at moderate temperatures and pressure.

[0036] In an embodiment, the metal sulfonate solution is neutralized to a pH from 2 to 12, preferably from 4 to 10, or most preferably from 9 to 10.

[0037] In another embodiment, the sulfuric acid used in the regeneration step has a concentration ranging from 30 wt% to 98 wt%, preferably from 50 wt% to 70 wt%. [0038] In a supplemental embodiment, the sulfuric acid is added dropwise at moderate temperatures and pressures.

[0039] In another embodiment, the crude sulfonic acid solution has been regenerated at a R ranging from 10% to 95%.

[0040] In a further embodiment, the method described herein further comprises recirculating residual alkaline earth metals in solution to the leaching step.

[0041] In a further embodiment, the method described herein further comprises reconcentrating the crude sulfonic acid using a water removal technology.

[0042] In an embodiment, the water removal technology is a thermal or vacuum evaporation, a distillation, an osmosis, or a nanofiltration.

[0043] In a further embodiment, the method described herein further comprises the step of purifying residual sulfates in solution by chemical precipitation using strontium oxide, strontium hydroxide, strontium hydroxide hydrates, strontium carbonate, barium oxide, barium hydroxide, barium hydroxide hydrates, barium carbonate, or a combination thereof.

[0044] In a further embodiment, the method described herein further comprises purifying residual alkali metals contaminants contained in the crude sulfonic acid solution using a strong cation exchange resin in hydrogen form.

[0045] In an embodiment, the crude sulfonic acid solution has been regenerated at R > 80%, preferably at R > 90% prior to being contacted with the strong cation exchange resin.

[0046] In another embodiment, the amount of sulfuric acid added to the solution is selected so that only a fraction of the alkaline earth metals initially in solution are precipitated.

[0047] In a further embodiment, the fraction of precipitated alkaline earth metals is regenerated at a rate R from 10% to 95%, more preferably from 20% to 90%, and most preferably from 50% to 80%.

[0048] In another embodiment, the sulfonic acid solution is reconcentrated by a factor ranging from 1 to 3 over the course of a cycle. [0049] In a further embodiment, the method described herein further comprises a hydrometallurgical step to purify the solution from a contaminant or a mix thereof, or to selectively recover an element or a mix thereof.

[0050] In an embodiment, the hydrometallurgical step is cementation, chemical precipitation, electrowinning, crystallization, stripping, solvent extraction, distillation, ion exchange columns, reverse osmosis, forward osmosis, or nanofiltration.

[0051] It is also provided a process of using the method as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Reference will now be made to the accompanying drawings.

[0053] Fig. 1 illustrates a block diagram of the steps involved in the looping process of the sulfonic acid solution in accordance to an embodiment.

[0054] Fig. 2 illustrates a block diagram of the process encompassed herein in accordance to an embodiment wherein the sulfonic acid is MSA, the alkaline earth metal base is calcium hydroxide, and the reconcentration and purification step is performed by a combination of vacuum evaporation and chemical precipitation of residual sulfates by a barium base (Ba(OH)₂ or BaCO₃) to form barium sulfate.

[0055] Fig. 3 illustrates a block diagram of the process encompassed herein in accordance to an embodiment showing a closed-loop circuit as described herein to post-treat a spent metal sulfate effluent to recover the metals as a metal hydroxides and precipitate the sulfates as a clean synthetic alkaline earth metal sulfate cake.

DETAILED DESCRIPTION

[0056] It is provided a process for the close-loop recirculation of sulfonic acids in metal extraction processes relying on an alkaline earth metal base and sulfuric acid.

[0057] It is provided a process for the recovery of sulfonic acids from metal sulfonate leachates, allowing a closed loop approach for the extraction of metals with said sulfonic acids from metal-rich feedstocks such as minerals, ores, and waste residues. The process involves the recovery of metals as hydroxides and the regeneration of the solution’s acidity by chemical precipitation with mild reagents at ambient temperatures. The reconcentration of the acid to the desired level by vacuum evaporation, the control of residual impurity levels, for instance via chemical precipitation or ion exchange column, and finally the recirculation of the sulfonic acid are also made possible. The process described herein is applicable to any aqueous leachates of metal sulfonate and mixed metal sulfonates, especially when the sulfonate is methanesulfonate.

[0058] The applicability of a method relying on chemical precipitation is directly correlated to the difference in pricing between the acid being regenerated and the chemicals used to do so. The incentive is hence quite high for regenerating sulfonic acids, who’s pricing typically range from a few to several USD per kilogram, with commodity chemicals such as lime and sulfuric acid, as is proposed herein.

[0059] This is in opposition with chemical precipitation-based approaches described in the literature for a commodity acid such as hydrochloric acid (Demopoulos et al., World of Metallurgy-ERZMETALL, 61(2):89-98, 2018). The applicability of a method applied to sulfonic acid is indeed much higher owing to the significant premium in pricing between sulfonic acids and HCI.

[0060] Another advantage of working with sulfonic acids, and methanesulfonic acid in particular, is that they are high boilers that do not form any known azeotrope with water: simple vacuum evaporation can hence constitute an easy and direct mean of reconcentration, as opposed to HCI that forms an azeotrope at 20.2 wt% with water.

[0061] The present disclosure has improved on current procedures for sulfonic acid recovery by proposing an approach relying on chemical precipitation and filtration steps to reach a purity level that allows for its closed-loop recycling. The process causes a slight dilution (by a factor usually smaller than 2), which is mostly generated by the water used to wash the various filtration cakes. Reconcentration is hence required to remove the corresponding water and adjust the acid concentration at the desired level prior to its recirculation.

[0062] The process described herein is a closed-loop circuit involving the 4 key steps described herein (see Fig. 1 ). If necessary, the circuit can be incremented to include any other complementary hydrometallurgical operations. Examples of potential additional steps include, but are not limited to cementation, chemical precipitation, electrowinning, crystallization, stripping, solvent extraction, distillation, ion exchange columns, reverse osmosis, forward osmosis, or nanofiltration. The objective of adding a step may, for instance, be of isolating specific metal species or of tuning impurity levels in the recycled stream below desired thresholds, as might be required for demanding applications.

[0063] The process provided comprises a first step 1 of acid leaching, which can involve any kind of metal-rich feedstock and a sulfonic acid solution. An oxidizing agent, such as hydrogen peroxide or oxygen might be necessary for the leaching to occur. The acid leaching step generates a metal-depleted solid residue and a concentrated metal sulfonate solution that can be recovered by solid/liquid separation (e.g. by filtration, decantation, centrifugation, etc.). More specifically, the solid/liquid separation can be performed using a gravity settler (e.g. clarifiers, lamella separators, deep thickeners, settling tanks); a hydrocyclone (e.g. conical); classifiers (e.g. screens, sieve bends, hydraulic, mechanical); line filters (e.g. cartridges, strainers); pressure filters (e.g. continuous pressure, filter press, horizontal plate, pressure Nutsche, vertical leaf, candle, tubular element); compression filters (e.g. belt press, membrane plate and frame, screw press, variable volume filter (e.g. tube); vacuum filters (e.g. rotary drum, disc, horizontal belt, tray, tilting pan, table, precoat drum); filtering centrifuges (e.g., basket, oscillating, pusher, screen scroll); or sedimenting centrifuges (e.g. tubular bowl, siphon centrifuge, baffle ring). The metal sulfonate solution is still acid at this point, although to a lesser extent than the feed solution.

[0064] Sulfonic acid forms a monophasic aqueous solution at the working concentration, both when in acid and sulfonate form, especially when the sulfonate is an alkaline earth metal sulfonate, it remains stable from a chemical, thermal, and redox point of view within the kind of operating conditions described herein, and forms fully soluble sulfonate salts with one, several, or a combination of metals of interests.

[0065] Subsequently, the metal sulfonate solution is neutralised (step 2), corresponding to a chemical precipitation that involves raising the pH of the metal sulfonate solution using an alkaline earth metal base from calcium, strontium, or barium. The neutralisation step generates a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution that can be recovered using an adequate solid/liquid separation technology. Metals that remain soluble during this step, such as alkali metals, need to be addressed through dedicated purification steps to avoid a build-up in their concentration from cycle to cycle.

[0066] The acidity is regenerated by the chemical precipitation step 3 of the alkaline earth metals using sulfuric acid. The precipitation step generates a precipitate of calcium sulfate (gypsum), strontium sulfate, or barium sulfate, and a crude sulfonic acid solution. The later, which may still contain some impurities, has been diluted by the water used to wash the various cakes produced in previous steps.

[0067] Finally, the sulfonic acid is reconcentrated and purified in step 4, which involves if need be, to adjust contaminant levels and sulfonic acid concentration to meet the specifications of the process. It can typically be achieved by combining a water-removal technology, such as evaporation or reverse osmosis, to operations that are selective of the contaminants to be removed (such as chemical precipitation, ion exchange, or stripping). The purified and regenerated sulfonic acid solution is then recycled to the leaching step, which closes the loop.

[0068] Each cycle generates some losses in sulfonic acid, for instance by chemical degradation or through an incomplete washing of the various cakes, and a make-up in sulfonic acid may be required to compensate for these losses. The make-up is ideally added prior to the leaching operation, during step 1.

[0069] In addition, during the process, solids are meticulously washed to minimize losses in sulfonic acid, and rinsing waters are mixed to the mother liquor to increase its recovery. Excluding losses occurring during the leaching step, meticulous washing of the cakes typically results in sulfonic acid recovery rates greater than 95% over a cycle, and most often greater than 98%.

[0070] As encompassed herein “metal(s)” correspond for the purpose of the present description to any species commonly classified as a transition metal, a posttransition metals, a metalloid, a lanthanide, or an actinide in the periodic table of the elements, and to alloys or combination thereof. Exceptions are for “alkali metals” and “alkaline earth metals” that are both defined separately here-below. “Metals” can hence refer to, without being limited to: iron, aluminum, zinc, copper, nickel, cobalt, chromium, lead, tin, silver, mercury, manganese, and cadmium; to combinations thereof; and to alloys thereof such as bronze, solder, steel, amalgam, alnico, electrum, white gold and pink gold alloys.

[0071] “Alkali metals” correspond herein to lithium, sodium, potassium, rubidium, and cesium. Francium is excluded for its rarity and small commercial potential.

[0072] “Alkaline earth metals” are typically defined as beryllium, magnesium, calcium, strontium, barium, and radium. Unless otherwise specified, they refer herein to calcium, strontium, and barium, the three of which are abundant, largely available commercially in base form, and form insoluble sulfate salts. The insolubility of alkaline earth metal sulfate salts is a crucial part of this invention. Magnesium, while very common, forms a soluble sulfate salt and is hence not considered herein. If need be, its level can be controlled during the neutralization step by raising the pH of the solution above ~10. Beryllium is excluded since it usually forms a water insoluble hydroxide from pH as low as 2, while radium is a radioactive element whose use is restricted and of no practical interest within the context of this invention.

[0073] “Metals”, “alkali metals”, and “alkaline earth metals” are defined independently of their oxidation state n (with n an integer > 0). It must be understood by the practitioner that they are present in the solid state (be it in “feedstocks” or following an operation such as chemical precipitation, cementation, or electrowinning) either at the oxidation state of 0; or as salts, oxides, hydroxides, or combination thereof when their oxidation state is equal to or greater than 1. In solution, they necessarily have an oxidation number n > 1 and are either present as free ion species or as complexes.

[0074] “Feedstock” refers herein to any metal-rich substrate whose metals can be fully or partially leached by an acid solution. A feedstock can be of natural (such as a mineral) or of anthropogenic origin (such as a waste, a mining residue, or a cake).

[0075] As used herein, a “leaching”, corresponds to the hydrometallurgical operation described herein, in which a solution containing at least one sulfonic acid is used to remove or extract, partially or completely, some or all metals of interest contained in the feedstock. The leaching operation may or may not require the use of third parties, such as an oxidizing agent to alter the oxidation state of the metals.

[0076] “Sulfonic acids” refer herein to any organic compound containing a sulfonic group -SO₂OH. The typical chemical formulae of a “sulfonic acid” is in the form of R- (SO₂OH)_n, where “R” is an organic group (such as an alkane or aryl group), and where “n” is an integer equal to 1 for a mono-acid and strictly greater than 1 for a polyacid. “Sulfonic acids” of practical interest within the context of this invention are those that: 1 ) form monophasic aqueous solutions within the concentration ranges covered by this invention, both when in acid and sulfonate form; 2) remain stable from a chemical, thermal, and redox point of view within the kind of operating conditions described herein; 3) form fully soluble sulfonate salts with one, several, or a combination of metals of interests. Without being limited to them, examples of sulfonic acids of practical interest include:

-Alkanesulfonic acids, especially methanesulfonic acid (MSA), but also ethanesulfonic acid, and 1-propanesulfonic acid;

-Arylsulfonic acids, especially benzene sulfonic acid and p-toluenesulfonic acid (PTSA);

-Polysulfonic acids such as lignosulfonic acid and sulfonated kerosene.

[0077] “Sulfonate” refers herein to the conjugated base of any “sulfonic acid”. Sulfonates have negatively charged groups of formulae -SO₂O’ that are associated with a cation specie, most notably a “metal”, an “alkali metal”, or a an “alkaline earth metal”.

[0078] The solution containing the leached metals is referred herein either as a “leachate”, a “metal-rich leachate”, or a “metal sulfonate solution”. It can also be referred to as a “mixed metal sulfonate solution” in occurrences where it is relevant to highlight that the solution contains a range of metals. The leachate can be of any acid pH and hence contain any combination of sulfonic acid, metal sulfonate, alkali metal sulfonate, and alkaline earth metal sulfonate.

[0079] Additional steps aimed at selectively recovering some metals are preferentially introduced just after the leaching, between step 1 and step 2. Pertinent operations include but are not limited to: chemical precipitation, ion exchange column, cementation, electrowinning, solvent exchange, stripping, and membrane filtration.

[0080] Components susceptible to be selectively removed through an additional step most notably include metals of specific interest because of their commercial value (such as Ag, Sn, or Cu) or toxicity (such as Pb, As, or Hg), but they can also be “contaminants”.

[0081] “Contaminants” refer herein to any species involuntarily introduced in solution by the leaching operation that cannot be removed by the neutralisation of the 2^nd step and whose levels need to be controlled to preserve the efficiency of the process from cycle to cycle. Contaminants, which can be organic or inorganic in nature, are to be removed by appropriate operations between steps 1 and 2, or during step 4. Within the context of this description and otherwise specified, alkali metals are considered as contaminants because they cannot be precipitated by variating the pH of the solution during step 2. Other typical contaminants may include anion species other than sulfonates and sulfates, such as halides or nitrates, and hardly extractable water- soluble organic compounds.

[0082] Sulfonic acid and/or cation sulfonate concentrations of interest herein are those that are higher than 0.01 M, preferably higher than 0.1 M, most preferably higher than 0.5 M, preferably between 0.01 M to 4M. There is indeed a higher economic incentive in recycling concentrated streams rather than dilute ones. The concentration threshold below which the invention ceases to be attractive is however dependant of the commercial value of the sulfonic acid of interest.

[0083] It is essential for the sulfonate to form a salt with at least one of the alkaline earth metals that will remain fully soluble during the neutralisation (step 2). For methanesulfonate solutions, which are used in a preferred embodiment, this would correspond as per Gernon et al. (Green chemistry, 1 (3): 127-140, 1999) to a maximum methanesulfonate concentration of 5.84 M, 5.10 M, or 3.18 M when paired with calcium, strontium, or barium, respectively. Starting from a higher concentration hence requires diluting the solution prior to the neutralisation of step 2 (for instance by mixing in the water used to wash the various cakes produced by the method) and to reconcentrate it afterwards in the fourth step.

[0084] The second step, referred herein as the “neutralisation”, is based on the identification that while most metals of common interest are insoluble at neutral pH, alkaline earth metals form fully soluble sulfonate salts independently of the pH. Hence, it is possible by raising the pH using an alkaline earth metal base to precipitate the metals of interest while maintaining quantitatively the sulfonates in solution. The target pH will be a function of the nature of the metals to be removed and may usually range from 2 to 12, more often from 4 to 10, and most often from 9 to 10. The solution fed to the second step must initially be more acid than the target pH.

[0085] The base used to increase the pH of the leachate must have an alkaline earth metal as cation, meaning herein calcium, strontium, or barium. Preferred bases do not generate any unwanted anion in solution and thus include carbonates, oxides, and most preferably hydroxides. Not being bound by theory, hydroxides generate fewer heat than oxides upon addition in an acid, and do not cause any significant emanation of gases as would be the case for carbonates. Furthermore, alkaline earth metal carbonates can only raise the pH by so much, as their solubility decreases at high pH.

[0086] The neutralisation step yields a cake of metal hydroxides, oxides, and/or mix thereof, and a sulfonate solution within which the sulfonates are predominantly paired with an alkaline earth metal. Experimental evidence from the preferred embodiment (example 1 ), using a methanesulfonic acid solution neutralized at pH 9.5 by calcium hydroxide, shows that alkaline earth metals (Ca and Ba) accounts for over 99.9 wt% of the cations in solution. Concomitantly, over 98 wt% of the metals (including Cu, Fe, Sn, Pb, Ni, Zn, etc..) were recovered in the cake. Residual levels of metals were below 100 ppm.

[0087] In an embodiment, the alkaline earth metal base is introduced as a powder or an aqueous suspension to a stirred reactor operated at moderate temperatures (10°C to 40°C) and pressures (0.5 atm to 2 atm), and most preferably at ambient temperature and atmospheric pressure.

[0088] In cases where a significant amount of magnesium is present in the leachate, it may become necessary to raise the pH of the solution above ~10 using a calcium, strontium, or barium base. This will cause the precipitation of magnesium as hydroxides and generate a calcium, strontium, or barium sulfonate solution ahead of step 3.

[0089] The third step involves the regeneration of the calcium, strontium, or barium sulfonate solution using sulfuric acid. This step exploits the large difference in solubility between the sulfonates and the sulfates of calcium, strontium, and barium. As per Gernon et al. (Green chemistry, 1 (3): 127-140, 1999), calcium, strontium, and barium methanesulfonate have solubilities of 2.92 M, 2.55 M, and 1.59 M, respectively. Meanwhile, calcium, strontium, and barium sulfate form quasi-insoluble salts whose constants at 25 °C are:

CaSO₄ Ca²⁺ + SO₄ ²', K_sp,c_aso₄ = 9.1*10-® moPL’²

SrSO₄ Sr²⁺ + SO ₄ ²', K_sp,SrSO₄ - 3.2*10^-7 mol²L-²

BaSO₄ Ba²⁺ + SO₄ ²', K_sp,_Baso₄ = 1.1*10’¹⁰ mol²l_-²

[0090] The regeneration step hence produces a cake of sulfates: calcium sulfate, most likely in dihydrate form (e.g. gypsum) at ambient conditions, strontium sulfate, or barium sulfate and a regenerated sulfonic acid solution. Not being bound by theory, the concentration of residual sulfates, calcium, strontium, and barium, can directly be estimated from the amount of added sulfuric acid and from the various solubility constants provided here-above. There are hence higher levels of residual sulfates during the regeneration of calcium sulfonate solutions than for strontium or barium sulfonate streams.

[0091] The rate of sulfonic acid regeneration, R, is defined by mass balance as the amount of alkaline earth elements in solution after the regeneration step divided by its amount before. Rates of regeneration that are targeted within the scope of this description are those comprised between 10% and 95 %, preferably between 20% and 90%, and most preferably between 50% and 80%. The fact to aim only for a partial regeneration by chemical precipitation of the alkaline earth metal sulfate is an important part of the invention as it allows to significantly reduce the amount of sulfuric acid required to reach the targeted rate R and to control the amount of excess sulfates present in solution, especially in the preferred embodiment employing a calcium base and for which the solubility of the sulfate salt is the highest. Reaching the highest regeneration rates with calcium sulfonate solutions may require combining the present invention with an ion exchange column (as described in example 2).

[0092] The regeneration step produces a sulfonic acid solution, referred herein as “crude sulfonic acid”, that still contains (100 - R) % of alkaline earth metals, in addition to any traces of metals remaining from the neutralisation step and to potential contaminants. Unless otherwise removed, these elements will be recirculated to the leaching operation of the first step. It was tested experimentally that, in most cases, the fact to recirculate a solution containing alkaline earth metals and traces of contaminants was not detrimental to the process, provided that its composition remained the same from cycle to cycle.

[0093] In some instances, it was however found that a finer control of residual element levels might be needed, as well as a reconcentration when large quantities of washing waters are required to rinse the precipitation cakes. This is the purpose of the fourth step.

[0094] “Reconcentration” is defined herein as any operation aimed at increasing the concentration of the sulfonic acid within the solution by removing water. It can be achieved through any common means, provided that its operating conditions do not lead to significant losses or degradation of the sulfonic acid. Not being limited to this technology, vacuum evaporation is particularly suitable to reconcentrate high molality solutions using forced circulation, falling film, or flash evaporators. Reverse or forward osmosis can prove more suitable for dilute solutions.

[0095] The reconcentration factor can be defined as the concentration in sulfonic acids in the concentrate divided by its concentration in the initial solution ([Sulfonic acid]out/[Sulfonic acid]_in). It is closely related to the dilution factor: the amount of water in the outlet stream divided by its amount in the inlet solution (expressed in gH2o,out/gH2o,in). The reconcentration factor varies depending on the specifics of the process, such as: the number of additional steps involved in the circuit; the technology chosen for the filtration steps (filter press, vacuum filter, centrifugation, etc.); and the targeted total recovery rate for sulfonic acid over a cycle. Not being limited to these figures, typical requirements in terms of reconcentration factors for this invention range from 1 (no reconcentration) to 3 and are positively correlated to the concentration of the stream in sulfonic acid. This is mostly because concentrated streams generate larger precipitation cakes and hence require more rinsing waters.

[0096] Experimental evidence suggests that, depending on the chosen technology, the water recovered from this reconcentration step will contain a small amount of sulfonic acids, usually less than 1 wt% of the total sulfonic acid content, preferably less than 0.5 wt%, and most preferentially less than 0.1 wt%. Recirculating this water as process water in closed loop, most notably for the washing of the cakes produced by the various filtration steps, can help mitigating the losses in sulfonic acid.

[0097] “Purification” is defined herein as any operation aimed at controlling the concentration in residual elements, including impurities. It can be performed using selective extraction methods such as, but not being limited to, chemical precipitation, stripping, distillation, and ion exchange column.

[0098] In a preferred embodiment (Example 1 ), in which the regeneration (third step) generates gypsum, the crude sulfonic acid solution fed to the fourth step contains a residual content in sulfates that can range from -100 ppm to -5,000 ppm. In this embodiment, these residual sulfates are controlled by chemical precipitation using a near stoichiometric amount of a barium base. Not being bound by theory, the lower solubility of barium sulfate reduces the level of residual sulfates within the solution. Strontium can be used instead of barium, although with a lesser effectiveness. [0099] In another embodiment (Example 2), a strong cation exchange resin, made for instance of a crosslinked polysulfonic acid, is used to control the residual level of cations within the solution. The method is especially useful to control the level of cations that cannot be precipitated by variating the pH of the solution such as alkali metals. Alternatively, an anion exchange resin may be used to control the residual level in anion impurities.

Step 1 - Leaching

[00100] The leaching step as described herein is particularly suitable for processes in which: 1 ) the feedstock, and any eventual additive (such as the oxidizing agent), does not leach any significant amounts of contaminants in the sulfonic acid solution; 2) the species to be leached are “metals”, as defined herein, that are soluble in sulfonic acid at low pH, but form insoluble oxides or hydroxides in the pH range of 2 to 12; 3) the leaching conditions do not cause any degradation of the sulfonic acid.

[00101] Leached contaminants, as defined herein (alkali metals, organic species, halide salts, etc.), are only an issue as far as they require to include specific purification steps in the circuit to prevent a build-up in their concentration from cycle to cycle.

[00102] Alkali metals, even when valuable such as lithium, are to be considered as “contaminants” within the context of the process provided herewith. This is because they do not precipitate during the neutralisation step and must hence be removed by including a dedicated step in the circuit.

[00103] Leaching may require the use of an oxidizing agent as third party. Examples of recommended oxidizing agents include e.g. hydrogen peroxide, oxygen, and ozone. On the contrary, oxidizing agents that contain halides or alkali metals are to be avoided, for instance sodium or potassium hypochlorite, chlorate, and perchlorate.

[00104] Most sulfonic acids are susceptible to deteriorate. Not being bound by theory, this may for instance come from the hydrolysis of the sulfonic group into sulfuric acid, or from the degradation of their organic tail. Leaching conditions must hence be kept mild enough to avoid, or at least hinder, the degradation of the sulfonic acid. This can for example be achieved by controlling the temperature, the rate of addition of the oxidizing agent (if any), the oxidation-reduction potential (ORP), and the duration of the leaching. [00105] It was found experimentally that a poorly controlled addition of a strong oxidizing agent, in concentrated form, could be the single most important cause of sulfonic acid degradation and that countermeasures, such as dilution, selection of a milder oxidizing agent, or modification of the feeding system, could effectively reduce sulfonic acid losses.

[00106] In typical leaching conditions, a concentrated (0.1 to 4 M) sulfonic acid aqueous solution was contacted with the feedstock at liquid/solid ratios ranging from 4 to 10 L/kg in a stirred tank reactor at near ambient temperatures (20 to 50 °C) for 30 min to 6 hours. In circumstances where an oxidizing agent is required, the reaction was performed by maintaining an ORP level in the range of 550 mV to 900 mV, either by dropwise addition of a H₂O₂ solution (10 to 50 wt%), or by maintaining a pressure of oxygen above the reactor (5 to 20 bar). The medium was then filtrated using a setting appropriate for the volume and morphology of the solid residue.

[00107] The leaching operation is usually slightly exothermic, and a cooling system might be required for the reactor to maintain its temperature. In the absence of cooling system, the temperature was typically found to stabilize 2 °C to 15 °C above room temperature depending on the operating conditions.

Step 2 - Neutralisation

[00108] The neutralisation step is performed at ambient temperature (10 to 40°C) and pressure (~1 atm) in a stirred reactor using the leachate recovered from the first step. An alkaline earth metal base, such as calcium, strontium, or barium hydroxide, or one of their hydrates, is used to raise the pH of the solution up to a target that is a function of the metals to be precipitated, usually in the 2 to 12 range. The reactor is equipped with a cooling system to quench the heat generated by the neutralisation.

[00109] At the target pH, metals of interest precipitate as oxide or hydroxides and may be separated from the alkaline earth metal sulfonate solution, most commonly by filtration or centrifugation. A flocculent might be needed to promote phase separation.

[00110] Using alkaline earth metal bases in powder or pellet form was found to lead to relatively long equilibrium times. Insufficient reaction times can cause some of the base to remain solid and to be recovered in the cake, hence reducing the purity of the metal-rich residue. For instance, experimental evidence from a preferred embodiment suggests that it is hardly possible, targeting a pH from 9 to 10, to have less than 50,000 ppm of calcium in the residue using calcium hydroxide in powder form.

[00111] On the other hand, employing suspensions of these bases (such as milk of lime), would cause significant dilution. It was found that a hybrid approach, in which 90 to 95 wt% of the base is added as powder or pellets, and the remaining as a suspension, led to the best results by limiting both the amount of alkali earth metals found in the solid residue and the dilution. The amount of base that needs to be added can be estimated prior to the neutralisation step by analyzing the leachate for its metal and sulfonate content.

[00112] In circumstances where the target pH has been overshot, most likely because of an excess of base, it is possible to compensate by adding sulfuric acid to the reaction medium. The sulfuric acid is added dropwise at moderate temperatures and pressures, yielding a crude sulfonic acid solution and an alkaline earth metal sulfate precipitate. While this corrective step generates a contamination of the filtration cake by alkaline earth metal sulfates, it was not found to otherwise reduce the efficiency of the metal precipitation.

Step 3 - Regeneration of the acidity

[00113] The regeneration of the acidity amounts to a crystallization performed at ambient temperature (10 to 40°C) and pressure (~1 atm) in a stirred reactor on the alkaline earth metal sulfonate solution recovered from the second step. A sulfuric acid aqueous solution, at concentrations ranging from 30 to 98 wt%, and more preferably from 50 to 70 wt%, is added dropwise in the reaction medium. The reactor needs to be equipped with a cooling system to quench the heat generated by the reacidification.

[00114] The addition of sulfuric acid precipitates the alkaline earth metals as insoluble sulfate salts. The volume of the precipitated cake is significant, directly correlated to the concentration in sulfonic acid and to the rate of sulfonic acid regeneration, R. It is critical to wash consciously this cake to maximize the recovery rate in sulfonic acid.

[00115] Experimental evidence suggests that a slow dropwise addition of the acid (for instance as a shower) in a slightly dilute solution (50 to 70 wt%) significantly reduced losses in sulfonic acid. Not being bound by theory, it is thought that the alkaline earth metal sulfate precipitate can entrap sulfonic acid molecules if its nucleation happens too fast.

[00116] The precipitate can for instance be separated from the crude sulfonic acid by filtration or centrifugation. Experimentally, calcium sulfate (gypsum) and strontium sulfate were found to filtrate readily, while barium sulfate was more challenging and required centrifugation.

[00117] The amount of sulfuric acid to be added can be calculated by analyzing the feed solution either for its alkaline earth metal content (e.g. by inductively coupled plasma analysis) or for its sulfonate concentration (e.g. by liquid chromatography). Target rates of regeneration, R, for this step can be comprised between 10% and 95%, preferably between 20% and 90%, and most preferably between 50% and 80%. The principal interest of leaving some of the alkaline earth metal in solution is to reduce the content of the crude sulfonic acid in residual sulfates. If needed, it is, however, possible to aim for regeneration rates greater than 95% at the cost of higher levels in residual sulfates.

Step 4 - Reconcentration and purification

[00118] The reconcentration and purification step can be divided into as many operations as required to refine the crude sulfonic acid solution and meet the process specifications. In practice, and for most applications in which the feedstock does not leach any significant amounts of contaminants, a reconcentration is sufficient.

[00119] Experimental evidence on the reconcentration, in the case of a methanesulfonic acid crude, suggests that vacuum evaporation in a forced circulation evaporator, operated under a vacuum of -0.91 bar (-27 inHg) at 50 °C yields satisfying results. The reactor was maintained at the desired temperature with a heating jacket fed with hot water (55 °C). It is thought that maintaining moderate temperatures for the heating fluid helps avoiding hot spots and reducing the rate of degradation for the sulfonic acid.

[00120] In an embodiment, the crude sulfonic acid is reconcentrated using a common water removal technology such as: thermal or vacuum evaporation (e.g. by forced circulation, falling film, thin film, wiped film, short path, or flash evaporators); distillation (e.g. simple, fractional, steam, vacuum, zone, or short path); osmosis (e.g. reverse or forward); or even nanofiltration if compatible with the sulfonic acid of interest. [00121] In a typical experiment using MSA, a solution with an initial acid concentration of 1.6 M, was for instance evaporated over the course of 3 hours by a factor of 2.5 up to a MSA concentration of 4.0 M. The water, recovered at the top of the evaporator by condensation of the vapors, had a pH of -2.2, which suggests that ~0.3 wt% of the total sulfonic acid was evaporated alongside water. The condensate was otherwise free of contaminants and could be reused in closed loop as process water, to wash the various cake for instance.

[00122] Purification steps might, however, be required for some applications. These operations might be carried every cycle, or occasionally every few cycles depending on the rate of accumulation of the contaminant.

[00123] In an embodiment, in which a calcium methanesulfonate solution is looped, it was for instance found that the level of residual sulfates had to be controlled at every cycle prior to the leaching step if lead was a metal of interest. In a typical experiment, the crude MSA typically contains in the range of -100 ppm to -5,000 ppm of residual sulfates due to the slight solubility of calcium sulfate (K_sp = 9.1*10^-6 mol²L^-2, at 25°C). This causes some of the Pb²⁺ ions to precipitate as lead sulfate during the leaching step due to its lower solubility (K_sp = 1.3*10^-8 mol²L“², at 25°C). It was found that addition of a near stoichiometric amount of a barium base (barium carbonate or barium hydroxide) could prevent the lead from precipitating in the leaching step due to the even lower solubility of barium sulfate (K_sp = 1.1*10^-10 mol²L“², at 25°C). This purification step, which takes the form of a chemical precipitation, can be performed at ambient temperature in a stirred reactor. The formed barium sulfate had to be centrifugated out of the solution due to its poor filterability.

[00124] It is worth mentioning that there is no need for sulfate control that use a strontium or a barium base for the neutralisation (step 2, see Example 3). An analysis however suggested that it is usually worth using calcium hydroxide for the neutralisation, and then a barium or strontium base for sulfate control as described in an embodiment herein (Example 1 ).

[00125] In an example, in which an alkali metal had to be removed, an ion exchange step was successfully included in the circuit described herein (Example 2). The column employed was filled with a cross-linked strong acid cation resin in hydrogen form. The resin can for instance be made of cross-linked polystyrene grafted by sulfonic acid groups. Not being bound by theory, polycations such as alkaline earth metals usually have stronger affinities for ion exchange resins than monovalent cations such as alkali metals. To avoid saturating the resin with alkaline earth metals and be selective of the alkali metals, it is hence recommended to aim for a rate of regeneration of R > 80%, and preferably of R > 90% during the third step ahead of the ion exchange column, increasing the efficiency and the running time of the cation exchange resin.

[00126] To extend the lifetime of the ion exchange resin, its regeneration should not be performed directly with sulfuric acid, which would cause an insoluble alkaline earth metal sulfate precipitate to form within the column. Instead, hydrochloric acid may be used, followed by water to rinse the chlorides. Alternatively, a NaCI solution may be employed (generating soluble alkaline earth chloride salts), followed by H₂SO₄ (generating a sodium sulfate solution).

[00127] Other purification steps can be envisioned depending on the circumstances and requirements of the process. Once satisfyingly purified, the regenerated sulfonic acid solution can be recirculated to step 1 with or without the addition of a make-up in acid.

[00128] The rate of sulfonic acid recovery for each step can be assessed by a mass balance. It is expressed in grams of sulfonic acid in the outlet stream per gram of sulfonic acid in the inlet stream (g_out/gin). Concomitantly, a dilution factor can be calculated, expressed in grams of water in the outlet stream per gram of water in the inlet stream (gH2o,out/gH2o,in). The process is efficient if it can quantitatively recover the sulfonic acid without any significant dilution of the stream.

[00129] Experimental evidence (see Table 1 ) was gathered for an embodiment, in which a calcium methanesulfonate solution is recirculated (see Fig. 2). For each step, a mass balance was established between the feed and the stream recovered after the filtration. The mother filtration liquor was first analyzed (before any rinsing of the cake), then after 1 rinsing of the cake (using ~2 L of water per kg of cake), and after two rinsing of the cake. For steps 2, and 4, the rinsing procedure was stopped before the second washing since the recovery rate was already higher than 0.995 gMSA,out/gMSA,in- As expected, rinsing increased the recovery rate in sulfonic acid, but concomitantly generated dilution.

Table 1 : Step by step rate of MSA recovery and dilution factor as a function of the number of washing operations.

[00130] The data presented in Table 1 is for an embodiment in which a calcium methanesulfonate solution is recirculated (see Fig. 2). The totals (two last entries of the table) are calculated by factoring the best recovery rates (bold figures) for each step.

[00131] In another embodiment, the closed-loop circuit described herein is employed to post-treat a spent metal sulfate effluent to recover the metals as metal hydroxides and precipitate the sulfates as a clean synthetic alkaline earth metal sulfate cake, preferentially as a synthetic gypsum that may be used in the construction industry. In this embodiment, as illustrated in Fig. 3, the first step is bypassed, and steps 2 and 3 are reverted. The repeated sequence hence corresponds to steps 3, 2, and 4, in that order. In the preferred sequence in which the alkaline earth metal is calcium, the spent acidic metal sulfate effluent is first mixed with a calcium sulfonate solution to crystallize synthetic gypsum (typical purity >95%, with restricted metal content below acceptable levels). The filtrate, a metal sulfonate solution, is then contacted with Ca(OH)₂ to raise its pH and precipitate the metal hydroxide, while simultaneously regenerating the calcium sulfonate. The crude alkaline earth metal sulfonate solution may then be reconcentrated and purified using any of the methods described in step 4.

[00132] In the latter embodiment, the alkaline earth metal sulfonate may be mixed within any proportions with other soluble alkaline earth metal salts, such as, but without being limited to halide, acetate, and nitrate salts. Suitable alkaline earth metal salts are those that do not lead to the coprecipitation of unwanted elements within the synthetic gypsum. The presence of these various anions may either be the result: 1 ) of a deliberate addition, e.g. to increase the concentration in alkaline earth metal within the solution and reduce the residual sulfate content within the filtrate of the crystallization step; 2) or of the slow accumulation of soluble contaminants within the solution owing to the closed-loop nature of the process.

EXAMPLE I Calcium methanesulfonate looping

[00133] All the processes described in the following examples were tested at the laboratory scale. The metal compositions of the various solutions were determined by Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES) analyses. Solid compositions were estimated by digesting the residue with aquae regia under the strongest oxidizing conditions and by analyzing the leachates by ICP-OES. The content in methanesulfonic acid was determined by liquid chromatography.

[00134] In the present example, which uses the process illustrated in Fig. 2, the sulfonic acid is methanesulfonic acid, the base for the neutralisation step is calcium hydroxide, the target pH for neutralisation is between 9 and 10, the regeneration is performed with sulfuric acid (50wt% solution) to precipitate a gypsum cake, and the target rate of regeneration is R ~ 50%. The reconcentration is achieved by vacuum evaporation, at which point the only significant impurities in solution are residual sulfates. Purification of these sulfates is achieved using barium carbonate.

[00135] For the sake of this example, the metal-rich feedstock was made of grounded wastes of printed circuit board and the objective was to selectively leach the lead and the tin contained in the solders.

[00136] In step 1 , -100 g of feedstock was contacted for 2 hours by 500 mL of a 3.5 M aqueous solution of methanesulfonate, 50% in acid form, and 50% as a calcium salt. The suspension was mechanically stirred at room temperature in a 2 L beaker and a solution of H₂O₂, 10 wt%, was added dropwise to act as an oxidizing agent and maintain the ORP of the solution at -600 mV. Typically, 30-35 mL of H₂O₂ solution was needed over the course of the reaction. At the end of the reaction, the suspension was filtrated over Buchner and the residue washed twice, each time with 100 mL. The rinsing waters were mixed with the filtrate to constitute the metal methanesulfonate solution. The solution initially contained -2,000 ppm of barium, introduced in step 4. Its concentration decreased gradually over the course of the reaction, forming barium sulfate, which highlighted a slight degradation of the MSA by the oxidizing agent during the leaching.

[00137] In step 2, calcium hydroxide (typically around -40 g) was slowly added to the solution to raise the pH of the solution up to a range of 9 to 10, ideally up to a pH of 9.5. The solution was continuously stirred at room temperature and the addition was made over the course of typically 1 hour to enable the dissolution of the calcium hydroxide and the stabilization of the pH. A precipitate of metal oxides and hydroxides formed readily in the beaker, and the medium was filtrated over Buchner. The residue was washed twice, each time with 30 ml_ of water. The rinsing waters were mixed with the filtrate to constitute the calcium methanesulfonate solution.

[00138] In step 3, an aqueous solution of H2SO4, 50 wt%, was added dropwise to the calcium methanesulfonate solution in a magnetically stirred beaker over the course of ~30 min. The amount of H₂SO₄ to be added was calculated from the calcium content in the feed, obtained by ICP analysis. Typically, 80 to 90 g of H₂SO₄ solution was needed to reach a regeneration rate of R ~ 50%. Gypsum formed immediately within the beaker upon addition of the sulfuric acid. The cake was filtrated over Buchner and washed twice, each with 100 ml_ of water. The rinsing waters were mixed with the filtrate to constitute the crude methane sulfonic acid solution. An ICP-OES analysis confirmed the effective rate of regeneration by determining the residual level in calcium ions within the solution.

[00139] In step 4, the solution was evaporated under a vacuum of -0.91 bar (-27 inHg) at 50 °C in a round-bottom flask. The rate of evaporation was followed both by the amount of condensed water, and by the weight variation within the round-bottom flask. Finally, 5,000 ppm of barium ions were added to the solution to precipitate residual sulfate. The barium was added in the form of barium carbonate. Barium sulfate formed readily and was removed by centrifugation to yield the regenerated sulfonic acid solution.

[00140] Losses in sulfonic acid over a cycle were estimated by liquid chromatography and a make-up of concentrated methanesulfonic acid, 98%, was added to restore the concentration in sulfonic acid. Typically, -95% of the methanesulfonic acid could be recovered over a cycle, excluding any losses caused by sampling. [00141] The regenerated sulfonic acid solution was then recirculated to step 1. Up to 5 cycles were performed to prove the method without any significant reduction of the leaching efficiency from cycle to cycle.

[00142] A typical composition of the ions in solutions at the end of steps 1 , 2, 3, and 4 is presented in Table 2. Dilution and reconcentration alter the concentration in ions: the reduction in calcium concentration between the regenerated MSA solution (step 4) and the leachate (step 1 ) is for instance fully explained by the dilution caused by the washing of the cake at the end of the leaching operation.

[00143] Hence, Al, Cu, Fe, Ni, and Zn typically accounted for -88% of the total metals present in the feedstock, while Pb and Sn only accounted for -12%. Tuning the parameters of the leaching however allowed for the selective recovery of lead and tin, which represented -85% of the leached metals. The total metal content in the leachate was of roughly 8,700 ppm. At the end of step 2, it was decreased to below 100 ppm due to the precipitation of the metals as oxides and hydroxides. Step 3 quantitatively precipitated the barium and half of the calcium (R = 50%). The solution was then reconcentrated in step 4 by a factor of -2, while some barium carbonate was added to precipitate the residual sulfates.

Table 2: Typical metal and alkaline earth metal composition of the various solutions produced over 5 cycles.

[00144] The composition of the feedstock is provided as a reference. Elements whose concentration was found to be smaller than 1 ,000 ppm in the feedstock, or smaller than 200 ppm in the solutions, are not displayed in Table 2. They include for instance: As, Cd, Co, Cr, K, Mg, Mn, Mo, Na, Sb, and Se. EXAMPLE 2

Purification of alkali metals using an ion exchange column

[00145] In one instance, the calcium methanesulfonate solution generated at the end of step 2 was found to be contaminated by sodium ions (see Fig. 2). The following example provides a course of action for purifying alkali metals using an ion exchange column.

[00146] In step 3, the sodium contaminated calcium methanesulfonate solution was first regenerated using a stoechiometric amount of H2SO4 (50 wt% aqueous solution) with respect to calcium ions. This precipitated ~90 wt% of the Ca²⁺ ions in solution (R = 90%). The resulting crude methanesulfonic acid (MSA concentration of ~1.5 M) still contained -4,500 ppm of calcium, alongside -2,500 ppm of sodium, which roughly corresponds to -0.112 mol/L of Ca²⁺ and -0.109 mol/L of Na⁺ ions.

[00147] The crude methanesulfonic acid was passed through a commercial strong cation exchange resin, made of cross-linked polystyrene grafted by sulfonic acid groups (Purolite, Puropack® PPC150H). This resin has a capacity of -1.8 eq/L, while the crude roughly contained -0.33 eq/L (-0.112 mol/L of divalent Ca²⁺ and -0.109 mol/L of monovalent Na⁺). This means that the column had a theoretical capacity of -5.4 bed volumes.

[00148] Experimental evidence suggested an initial removal efficiency up to >99 % for Ca²⁺ and 96 % for Na⁺ over the first bed volumes, after which the efficiency started to decrease. It is noteworthy that the efficiency of sodium removal decreased faster than that of calcium removal, due to the higher retention of divalent cations over monovalent ones. At some point (> 6 bed volumes), the resin starts to release the sodium to capture calcium. The resin must be regenerated before this point.

[00149] The running cycle can be extended by further precipitating calcium ions in step 3 (aim for higher R). This generates a higher level of residual sulfates in the solution, hence increasing the consumption of barium base in the subsequent step. There is hence an optimal rate of regeneration, R, to be found to minimize the costs of operating the ion exchange resin.

[00150] The filtrate recovered from the ion exchange step, having been purified from most of its sodium contaminants, can then be fed to a vacuum evaporator and to sulfate control, as described in example 1 (step 4), before being recirculated. EXAMPLE 3 Alkaline earth metal methanesulfonate looping

[00151] The present example used similar conditions to those of Example 1 to compare cycles in which the alkaline earth metal base was calcium hydroxide, strontium hydroxide, or barium hydroxide.

[00152] In step 1 , a same feedstock was representatively divided into 3 samples of -100 g each. They were contacted for 2 hours by 500 mL of a 3.5 M aqueous solution of methanesulfonate, 50% in acid form, and 50% as an alkaline earth metal salt (calcium, strontium, or barium). As for Example 1 , the ORP was maintained in the range of -600 mV through the dropwise addition of a 10 wt% H₂O₂ aqueous solution.

[00153] Comparison between the three experiments did not reveal any significant variations in leaching efficiencies between the calcium, strontium, and barium methanesulfonate solutions during step 1 , except for lead. Not being bound by theory, the lead concentration in the leachate appears to be inversely correlated to the solubility of the alkaline earth metal sulfate. It is thought that the alkaline earth metals act as sacrificial elements to capture the sulfates generated by the leaching step, for instance by the degradation of the methanesulfonic acid.

[00154] In step 2, calcium hydroxide (Ca(OH)₂, -38 g), strontium hydroxide octahydrate (Sr(OH)₂.8H₂O, -90 g), or barium hydroxide (Ba(OH)₂, -63 g) was slowly added to the solution to raise the pH of the solution up to -9.5. The solution was continuously stirred at room temperature and the addition was made over the course of typically 1 hour to enable the dissolution of the alkaline earth metal hydroxide and the stabilization of the pH. A precipitate of metal oxides and hydroxides formed readily in the beaker, and the medium was filtrated over Buchner. The residue was washed twice, each time with 30 mL of water. The rinsing waters were mixed with the filtrate to yield the three alkaline earth metal methanesulfonate solutions. The neutralisation proved equally efficient in all cases to separate the alkaline earth metal sulfonate solutions from the cakes of metal oxides and hydroxides.

[00155] In step 3, an aqueous solution of H₂SO₄, 50 wt%, was added dropwise to the alkaline earth metal methanesulfonate solutions in magnetically stirred beakers to reach regeneration rates of R ~ 50%. Precipitates, which proved to be either gypsum, strontium sulfate, or barium sulfate, formed immediately within the beaker upon addition of the sulfuric acid. The cakes were filtrated over Buchner and washed twice, each with 100 ml_ of water. The rinsing waters were mixed with the filtrate to constitute the crude methane sulfonic acid solutions.

[00156] The main difference in step 3 was observed during the filtration of the alkaline earth metal sulfate precipitates. While gypsum and strontium sulfate were readily filterable, barium sulfate formed a sticky cake that tended to plug the filter. Centrifugation is hence recommended for embodiments of this invention recirculating barium sulfonate solutions.

[00157] Experiments were interrupted after step 3 since the solutions were not recirculated for the purpose of this example. It is worth noting, however, that crude methanesulfonic acids produced from strontium and barium methanesulfonate solutions had lower residual sulfates (Table 3).

Table 3: Metal composition of the various solutions (steps 1 to 3) produced for embodiments recirculating calcium, strontium, and barium.

[00158] The composition of the feedstock is provided as a reference. Elements whose concentration was found to be smaller than 1 ,000 ppm in the feedstock, or smaller than 200 ppm in the solutions, are not displayed in the table. They include for instance: As, Cd, Co, Cr, K, Mg, Mn, Mo, Na, Sb, and Se. [00159] While the present disclosure has been described with particular reference to the illustrated embodiment, it will be understood that numerous modifications thereto will appear to those skilled in the art. Accordingly, the above description and accompanying drawings should be taken as illustrative and not in a limiting sense.

[00160] While the present disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for recirculating sulfonic acid solutions in a closed loop for extracting and recovering metals from relevant a feedstock comprising the steps of:

-optionally leaching the feedstock with a sulfonic acid solution metal-depleted solid residue and a concentrated metal sulfonate solution;

-neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution; and/or

-precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution; and

-purifying the crude sulfonic acid solution.

2. The method of claim 1 , comprising the steps of:

-leaching the feedstock with a sulfonic acid solution metal-depleted solid residue and a concentrated metal sulfonate solution;

-neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution;

-precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution; and

-purifying the crude sulfonic acid solution.

3. The method of claim 1 , comprising the steps of:

-precipitating the alkaline earth metals using sulfuric acid producing a regenerated crude sulfonic acid solution and a precipitate from the regenerated solution;

-neutralizing the metal sulfonate solution with an alkaline earth metal base producing a precipitate of insoluble metal oxide and hydroxides, and an alkaline earth metal sulfonate solution; and -purifying the crude sulfonic acid solution, wherein the feedstock is a metal sulfate leachate, effluent, or solution.

4. The method of any one of claims 1-3, wherein sulfuric acid is added dropwise to a stirred reactor, or crystallizer producing the regenerated crude sulfonic acid solution and precipitate from the regenerated solution.

5. The method of any one of claims 1-4, further comprising the step of recovering the precipitate from the regenerated solution by a solid/liquid separation to yield a crude sulfonic acid solution and a cake of alkaline earth metal sulfates.

6. The method of any one of claims 1-5, wherein the recovered sulfonic acid has a concentration belonging to the 0.01 M to 4 M.

7. The method of any one of claims 1-6, wherein the sulfonic acid is an alkanesulfonic acids, simple arylsulfonic acids, or a polysulfonic acid.

8. The method of any of claims 1-7, wherein the sulfonic acid is methanesulfonic acid, ethane sulfonic acid, p-toluenesulfonic acid, benzenesulfonic acid, lignosulfonic acid, or sulfonated kerosene.

9. The method of any of claims 1-7, wherein the sulfonic acid is methanesulfonic acid.

10. The method of claim 1 or 2, wherein the purified crude sulfonic acid solution is recycled back to the leaching step in closed-loop.

11. The method of any one of claims 1-10, wherein the feedstock is a metal-rich substrate or solution.

12. The method of claim 11 , wherein the feedstock is a mineral feedstock, a waste feedstock, a mining residue feedstock, or a cake.

13. The method of any one of claims 1-12, wherein the feedstock comprises metals of the periodic table classified as transition metals, post-transition metals, metalloids, lanthanides, actinides, or a combination thereof.

14. The method of any one of claims 1-13, wherein the feedstock comprises iron, aluminum, zinc, copper, nickel, cobalt, cadmium, chromium, lead, tin, silver, mercury, manganese, cadmium, gallium, thallium, bismuth, tungsten, alloys thereof or a combination thereof.

15. The method of claim 5, wherein the solid/liquid separation is performed using a gravity settler, a hydrocyclone, a classifier, a line filter, a pressure filter, a compression filter, a vacuum filter, a filtering centrifuge, or a sedimenting centrifuge.

16. The method of any one of claims 1-15, wherein the sulfonic acid leached metal- depleted solid residue, the precipitate of insoluble metal oxide and hydroxides, and/or the alkaline earth metal sulfate precipitate from the regenerated solution are further washed with rinsing water and the rising water are mixed with the solution to be recycled to the leaching step.

17. The method of any one of claims 1-16, wherein the alkaline earth metal base comprises calcium, strontium, or barium.

18. The method of any one of claims 1-17, wherein the alkaline earth metal base comprises a calcium oxide, a calcium hydroxide, a calcium hydroxide hydrate, a calcium carbonate, a strontium oxide, a strontium hydroxide, a strontium hydroxide hydrate, a strontium carbonate, a barium oxide, a barium hydroxide, a barium hydroxide hydrate, a barium carbonate, or a combination thereof.

19. The method of claim 18, wherein the alkaline earth metal base is calcium hydroxide.

20. The method of any one of claims 1-19, wherein the alkaline earth metal base is introduced as a powder or an aqueous suspension to a stirred reactor operated at moderate temperatures and pressure.

21. The method of any one of claims 1-20, wherein the metal sulfonate solution is neutralized to a pH from 2 to 12, preferably from 4 to 10, or most preferably from 9 to 10.

22. The method of any one of claims 1-21, wherein the sulfuric acid used in the regeneration step has a concentration ranging from 30 wt% to 98 wt%, preferably from 50 wt% to 70 wt%.

23. The method of any one of claims 1-22, wherein the sulfuric acid is added dropwise at moderate temperatures and pressures.

24. The method of any one of claims 1-23, wherein the crude sulfonic acid solution has been regenerated at a R ranging from 10% to 95%.

25. The method of claim 1 or 2, further comprising recirculating residual alkaline earth metals in solution to the leaching step.

26. The method of any one of claims 1-25, further comprising reconcentrating the crude sulfonic acid using a water removal technology.

27. The method of claim 26, wherein the water removal technology is a thermal or vacuum evaporation, a distillation, an osmosis, or a nanofiltration.

28. The method of any one of claims 1-27, further comprising the step of purifying residual sulfates in solution by chemical precipitation using strontium oxide, strontium hydroxide, strontium hydroxide hydrates, strontium carbonate, barium oxide, barium hydroxide, barium hydroxide hydrates, barium carbonate, or a combination thereof.

29. The method of any one of claims 1-28, further comprising purifying residual alkali metals contaminants contained in the crude sulfonic acid solution using a strong cation exchange resin in hydrogen form.

30. The method of claim 29, wherein the crude sulfonic acid solution has been regenerated at R > 80%, preferably at R > 90% prior to being contacted with the strong cation exchange resin.

31. The method of any one of claims 1-30, wherein the amount of sulfuric acid added to the solution is selected so that only a fraction of the alkaline earth metals initially in solution are precipitated.

32. The method of claim 31 , wherein the fraction of precipitated alkaline earth metals is regenerated at a rate R from 10% to 95%, more preferably from 20% to 90%, and most preferably from 50% to 80%.

33. The method of any one of claims 1-32, wherein the sulfonic acid solution is reconcentrated by a factor ranging from 1 to 3 over the course of a cycle.

34. The method of any one of claims 1-33, further comprising a hydrometallurgical step to purify the solution from a contaminant or a mix thereof, or to selectively recover an element or a mix thereof.

35. The method of claim 34, wherein the hydrometallurgical step is cementation, chemical precipitation, electrowinning, crystallization, stripping, solvent extraction, distillation, ion exchange columns, reverse osmosis, forward osmosis, or nanofiltration.

36. A process using the method of any one of claims 1-35.