pH control model

Abstract

A method of controlling either the flow rate Ḃ of a pH modifier or the concentration [B] of a pH modifier during solvent extraction of a metal from a pregnant leach solution using a pH modifier by:

Description

Field of the Invention The present invention relates to a method and use of such a method for controlling the extraction of one or more target metals, such as one of cobalt, lithium, nickel, or manganese, from a leachate obtainable from a source material. The present invention has a particular, but not exclusive, application to the sustainable recovery of battery materials, with the source material obtainable from black mass, lithium-containing electrochemical devices, or virgin feedstock from mines, black mass, battery factory waste or scrap, precursor battery materials, and combinations thereof. The present invention relates to a model to control the pH during the execution of the method to avoid saponification, loss of valuable solutions, and waste disposal problems. Recovered materials may be recycled, re-engineered and upcycled with a reduced carbon footprint and reduced cost compared with virgin materials. The present invention relates a solvent extraction system configured to extract one or more of the target metals from the source material by way of improved pH control. The present invention also relates to a computer-implemented execution of the control model. Background to the Invention The need to recycle materials is becoming more important every year with a commitment to develop sustainable green processing technologies for recycling at scale. As the world seeks to defossilize the global energy supply chain in order to meet net-zero goals, the materials that are used to create batteries to power the defossilization of the world’s economy are in ever increasing demand as sourcing of critical minerals and metals becomes ever more problematic. Although it will continue to be necessary to obtain battery materials from primary sources, it is useful to also obtain battery materials by way of recycling or further processing secondary sources, such as used, defective, out of warranty, or end of life batteries, especially EV batteries. Batteries have a limited lifespan and eventually need to be recycled. The complete recovery of critical metals, including lithium, cobalt, nickel, and manganese, during the recycling process of lithium-based batteries is desirable to avoid the loss of valuable product and to reduce heavy metal waste disposal. The separation of critical metals from other components found within lithium-based batteries can be performed using a counter-current solvent extraction process, involving an organic phase, comprising a cationic extractant, and an aqueous phase, comprising the critical metals. The critical metals are extracted from the aqueous phase into the organic phase according to the following equation, wherein RH is the cationic extractant in x stoichiometric quantity and Mn+ is the critical metal ion of n valency: xRH^org) + ^(aq) ^x^(org) + n^(aq) U) The transfer of metals from the aqueous phase into the organic phase is highly dependent on the pH of the aqueous phase, and each metal has its own limited pH range in which its extraction is selective over other metals and / or efficient. However, since the release of hydrogen ions during the extraction process results in a reduction in the pH, it is important to retain the pH within range so as to maintain effective extraction. There are a number of existing ways in which the pH of the aqueous phase may be maintained within a desired range to extract critical metals using counter-current solvent extraction and to mitigate crud formation. CN117701883A describes an extractant, comprising an organic acid, a diluent, and a surfactant and the use of the extractant to improve manganese-magnesium separation and reduce the number of counter-current solvent extraction stages during manganese extraction. This document describes the use of sodium hydroxide to saponify the extractant prior to extracting manganese and the use of the diluent reduces the viscosity of the extractant to reduce the rate of saponification, thereby avoiding excess emulsification of the phases. US3666446 describes a solvent-ion exchange reaction in which the pH of the metalbearing liquor is regulated by an ion exchange reaction, wherein hydrogen ions in the aqueous liquor are exchanged for metal cations in an ion extractant. This document describes sequentially raising the pH of the metal-bearing liquor by contacting the liquor with an ion extractant configured to extract hydrogen ions, thereby increasing the pH of the liquor and eliminating the requisite to add a base. US4011297 describes a method of adjusting pH in-situ during the metal extraction process through a feedback mechanism whereby pH electrodes measure the pH of a mixed aqueous and organic phase as basis for adapting the rate of addition of base to match the rate at which hydrogen ions are being released. This document also describes a process in which pH measurements are taken at multiple stages of the extraction process and are each independently controlled at each stage. Whilst existing methods do allow for the recovery of critical metals from an aqueous solution, they either rely on controlling the pH through saponification or by responding to real-time pH changes in-situ using multiple pH measurements. In processes that rely on saponification, pH is managed through the addition of a base or a pH modifier to the organic extractant prior to the extraction process. The saponified extractant molecules then extract the target metal ions by exchanging the cation of the pH modifier, thereby avoiding the participation of protons. This limits the extent of pH variation since the pH is a measure of protons in solution. The saponification and subsequent metal extraction occurs according to the following equations: RH(prg) +Y0H(aq) # RY + H2O xRY + ^(aq) Rx^^org) + XY(aq) (2) (3) The production of metal salt RY ultimately controls the pH during extraction due to mitigating the production of hydrogen ions, yet is soluble in both the organic and aqueous phases. In circumstances wherein the target metal is highly concentrated within the organic phase, a solid phase may form (crud) which results in the loss of valuable materials and it is necessary to implement further removal and / or filtration stages, requiring further expenditure. In addition, saponification can lead to a decrease in extraction efficiency since it takes a long time for the system to achieve the desired level of saponification and is also slow to react to change in the feedstock. As such, materials which are provided when the saponification has not reached the desired level are either not extracted or are inefficiently extracted, resulting in a significant amount of waste. Processes that rely on controlling pH by responding in real-time to fluctuations in measured pH are often inefficient during the start-up of the process or when there is a change of composition in the metal-containing aqueous phase, causing delays in attaining and maintaining the desired pH. Often, pH sensors are provided downstream to where separation takes place due to the ineffectiveness of measuring the pH of an aqueous-organic mixture, and so changes in the composition of the metal-containing aqueous phase at an upstream position are not accounted for until the aqueous phase has already been processed and reached the downstream pH sensor. In addition, counter-current solvent extraction apparatus commonly comprise of multiple organic-aqueous mixer tanks. Thus, in-situ processes necessitate multiple pH electrodes, base feeds, and corresponding feedback systems, which adjust the base volume addition according to the real-time pH measurements, yet are costly to procure and maintain. The present invention seeks to at least partially address and overcome the limitations of existing systems and methods. It is therefore an object of the present invention to mitigate at least some of the problems stated above and other known problems with the existing methods. Summary of Invention According to the first aspect of the present disclosure, there is provided a method of controlling the extraction of one or more target metals from a pregnant leach solution using a solvent extraction system, the method including the steps of: a. determining the concentration of a target metal in the pregnant leach solution ([Mn+]), the flow rate of the pregnant leach solution (P), and the pH of the pregnant leach solution (pHj); b. (i) calculating a desired pH modifier flow rate (B) using the following equation: • mx P(10-PHi —10“PHE+([Mn+]jXnxE)) B-----------M----------’ wherein pHe is a target pH for a raffinate, m is the hydroxyl coefficient of a pH modifier in its aqueous-dissociated form, n is the stable oxidation state of the target metal ion, E is a target extraction efficiency value expressed as a value between 0 and 1, and [B] is the concentration of the base, or (ii) calculating a desired pH modifier concentration ([B]) using the following equation: rnl m x P(10-PHi —10-PHE + ([Mn+]jXnxE)) [ ] — g wherein pHe is a target pH for a raffinate, m is the hydroxyl coefficient of a pH modifier in its aqueous-dissociated form, n is the stable oxidation state of the target metal ion, E is a target extraction efficiency value expressed as a value between 0 and 1, and B is the pH-modifier flow rate; and c. controlling a pH modifier flow rate (B) to achieve the calculated desired pH modifier flow rate, or controlling the pH modifier concentration ([B]) to achieve the calculated desired pH modifier concentration. The one or more target metals from the pregnant leach solution preferably include at least one of cobalt, lithium, nickel, or manganese, which may be sourced from one or more of black mass, lithium-containing electrochemical devices, battery factory scrap, consumer batteries, mixed hydroxide precipitate (MHP), blends of virgin extract, or mixtures thereof. These sources typically contain other materials or metals such as aluminium, iron, copper, graphite, and possibly even cadmium. Therefore, it is of high interest to separate these different materials from one another so that the valuable materials may be reused to form useful materials. Useful materials may include precursors to active electrode materials, cathode or anode active materials for use in batteries, as well as materials for any other use, such as, for example, speciality and technical material manufacture such as alloys, coatings, composites, alloys, performance additives agrichemical, construction materials, fertilizers, electrical components, and pharmaceutical derivatives. The present method provides for improved control of the extraction of one or more target metals from a pregnant leach solution by utilising a model to control a pH modifier flow rate or concentration. This method does not rely on downstream monitoring of the pH of an output stream, such as a raffinate, which is slow to react to change in the composition of a pregnant leach solution input stream. The present method allows a target pH to be reached more quickly during start-up, meaning that there is a significantly shorter period during which metals within the pregnant leach solution are co-extracted due to improper control of the pH. Co-extraction of metals is undesirable as it produces a waste stream which needs to be processed further. In addition, this method allows for target pH values to be achieved and maintained throughout multiple stages of separation, culminating in a complete extraction protocol being achieved in a shorter time frame time. The precise control of pH also allows a lower level of extractant to be used, which reduces costs and also reduces the risk of the extractant becoming too viscous, which occurs when the pH is not adequately controlled. If the extractant becomes viscous, this adversely affects the extraction process. The pregnant leach solution enters the solvent extraction system at an established pH and flow rate, and is of a pre-determined concentration of one or more of the target metals, which may be determined using inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP-OES) or atomic absorption spectroscopy (AAS). The undesirable materials and / or metals that comprise the pregnant leach solution leave the system as the raffinate. To perform the present method, the pregnant leach solution containing the one or more target metals may be brought into contact with an organic extractant and a pH modifier, such as a base. The use of organic extractants for removing target metals from solution is known in the art and are typically anionic, wherein target metal ions replace their released hydrogen cations. Despite being anionic in nature, such organic extractants are known in the art as ‘organic cationic extractants’ due to their ability to extract cations. Useful organic cationic extractants may include those comprising of organophosphorus acids, such as a phosphinic acid (hypophosphorous acid), a phosphoric acid, a phosphonic acid, or an alkyl-phosphate. Certain organophosphorus acids are preferred over others for metal extraction because of their high selectivity for specific metals. For example, Cyanex™ 272 (bis(2,4,4-trimethylpentyl)phosphinic acid) may be used for extracting cobalt, DEHPA™ (di(2-ethylhexyl)phosphoric acid) may be used for extracting manganese, and Verstatic™ Acid 10 (neodecanoic acid) may be used for extracting nickel and lithium. Other organic cationic extractants may include those comprising of phenols (e.g. catechol for extracting iron), oximes (e.g. hydroxyamines for extracting copper), beta-diketones (for extracting copper), and sulphonic acids (e.g. p-toluenesulphonic acid (p-TsOH) for extracting copper), as well sulphur-containing diamides, and acidic ionic liquids. The organic cationic extractant may also comprise of a mixture of any of these examples. The organic extractant may be mixed with a diluent, which may include those comprising of a hydrocarbon solvent, such as a kerosene, for example Exxsol™ D80 and Escaid™ 110. The concentration of organic extractant that is required to sufficiently extract the target metal is dependent on the concentration of the target metal ion and the complex the target metal makes with the extractant. For example, four equivalents of the organic extractant may be required to complex with a divalent metal cation. The pH modifier may be used to adjust the pH of the pregnant leach solution so that the raffinate reaches its desired pH range. Suitable pH modifiers may include a base, an amphoteric agent, or a mixture thereof. Another pH modifier may be an acid. The base may be a metal hydroxide. Examples of bases may include sodium hydroxide, ammonia, ammonium hydroxide, potassium hydroxide, calcium hydroxide, nickel hydroxide, manganese hydroxide, cobalt hydroxide, lithium hydroxide, magnesium hydroxide, or a mixture of any thereof. The extraction efficiency of different metals depends on the pH of the pregnant leach solution, and by adjusting the pH of the solution it is possible to control which metal is preferably extracted. By entering pre-determined values into the equation in step b.(i) to execute the present method, the flow rate of the pH modifier at a pre-determined concentration is established, wherein the volume of the pH modifier entering the system supplies a sufficient amount of anions to neutralise the free hydrogen ions released when the target metal cations react with the organic cationic extractant. This ultimately avoids saponification by preventing an excess of base entering the system. Furthermore, since the equation predicts the dosage volume of base from start-up or commissioning, instead of adjusting the base dosage in response to deviations in pH, there is a smooth and stable variation in pH from the initial pH to the target pH which may be reached rapidly in comparison to prior art systems. The target extraction efficiency value E defines the extent of which the target metal is extracted from the pregnant leach solution on a scale of 0 to 1, wherein an E value equal to 0 refers to no extraction of the target metal from the pregnant leach solution and an E value equal to 1 refers to complete extraction of the target metal from the pregnant leach solution. As such, E is a number up to 1. The value of E may be determined from previous, well-controlled experiments, wherein the conditions and desired target raffinate pH are entirely equal to those to be employed when implementing the method of the current disclosure. Conditions that may affect the value E include: a differing amount of organic extractant, an inefficiency in the extractant regeneration, a differing ratio between the volume of organic phase to aqueous phase, mixing or agitation efficiency, and temperature. As such E may be determined based on experimentation or during operation. The hydroxyl coefficient of a pH modifier in its aqueous-dissociated form m accounts for the stoichiometric value of hydroxide anions released upon dissociation of the pH modifier in aqueous solution. In some cases, this value is equal to the oxidation state of the cation; for example: NaOH + H2O -+ Na+ + 1OH" + H2O Ca(OH)2 + H2O -+ Ca2+ + 2OH“ + H2O m = 1; m = 2. However, in some cases, the stoichiometric value of hydroxide anions released upon dissociation of the pH modifier in aqueous solution is different to the oxidation state of the cation; for example: CaCO3+H2O^Ca2++HCO3’ +1OH" m = 1. It will be appreciated that when the pH modifier is an acid, the coefficient m will represent the stoichiometric value hydrogen ions released upon dissociation of the pH modifier in aqueous solution. The temperature of the solvent extraction system during the execution of the method may be within the range of 10 °C and 70 °C, preferably between 18 °C and 60 °C, more preferably between 20 °C and 60 °C. The selected temperature should ideally match the temperature at which the target extraction efficiency value E was determined. The method may be most efficiently executed when the flow rate of the pregnant leach solution is at least five times higher than the flow rate of the pH modifier to avoid high raffinate dilution. The flow rate of the pH modifier may be i) at least 2 times lower than the flow rate of the pregnant leach solution, preferably at least 3 times lower, 4 times lower, or 5 times lower than the flow rate of the pregnant leach solution. The flow rate of the pH modifier may be at most 30 times lower than the flow rate of the pregnant leach solution, preferably at most 30 times lower, 25 times lower, 20 times lower, or 10 times lower than the flow rate of the pregnant leach solution. The method may also be most efficiently executed when the flow rate of the pregnant leach solution is at most 30 times higher than the flow rate of the pH modifier to avoid excessively high concentrations of the pH modifier entering the system, which may result in local precipitation of metals and a reduction in solution homogeneity. The ratio between the pregnant leach solution flow rate and the pH modifier flow rate may be referred as the ‘raffinate dilution factor’. The method may also be executed by determining the pH modifier concentration by entering pre-determined values into the equation in step b.(ii), including a pre-defined pH modifier flow rate. The equation in step b.(ii) is an equal alternative to equation b.(i), but may be preferable over the equation in step b.(i) in some instances. For example, when the equation in step b.(i) determines that the pH modifier flow rate is at least five times lower than the flow rate of the pregnant leach solution, it is preferable instead to alter the pH modifier concentration determined by the equation in step b.(ii) to maintain the raffinate dilution factor and the reaction efficiency. According to a second aspect of the present disclosure, there is provided a solvent extraction system configured to extract one or more target metals from a pregnant leach solution including means for: a. determining the concentration of a target metal in the pregnant leach solution ([Mn+]) the flow rate of the pregnant leach solution (P), and the pH of the pregnant leach solution (pHj); b. a processor configured to calculate the pH modifier flow rate according to the first aspect; c. a controller configured to operate a pump to provide the calculated desired pH modifier flow rate or pH modifier concentration. The system may comprise of a plurality of solvent extraction tanks and a pH modifier tank in fluid communication with one another. A solvent extraction tank may include a mixing tank and a settling tank in fluid connection with one another. The mixing tank may include an agitator configured to bring organic solutions and aqueous solutions into contact with one another. The mixing tank may also include a first inlet for the organic solution, a second inlet for the aqueous solution, and / or a third inlet for the pH modifier. The mixing tank may optionally include one or more further inlets configured as recycle inputs or drains. The mixing tank include at least one outlet for the organic-aqueous mixture. The mixing tank may be configured to mix the pregnant leach solution and the organic extractant into an organic-aqueous mixture. The organic-aqueous mixture may then pass into the settling tank via fluid connection in which the two phases separate over time to form a lower aqueous layer and an upper organic layer. The settling tank may include a first outlet for the removal of the lower aqueous layer and a second outlet for the removal of the upper organic layer. The solvent extraction tank may include one or more recycle lines to recycle one or both of the organic phase and the aqueous phase from the settling tank back into the mixing tank. This is because solvent extraction generally requires multiple passes to extract a target metal. According to a third aspect of the present invention, there is provided a computer readable storage medium containing instructions configured to cause one or more computers to control a system to perform the method of the first aspect. The method and system according to the present disclosure allow for the recovery of target metals from a pregnant leach solution. The flow rate or the concentration of the incoming pH modifier may be calculated to obtain a target pH of the raffinate so as to ensure that a pre-determined extraction efficiency of the desired target metal is met. It will be appreciated that features described in respect of one aspect may be combined with any features described in respect of another aspect and all such combinations are expressly considered and disclosed herein. Brief Description of the Drawings Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing in which corresponding reference symbols indicate corresponding parts, and in which: Figure 1 is a schematic flowsheet depicting various stages of the system in the present disclosure; Figure 2 is a graph illustrating the relationship between the pH of an aqueous solution comprising metal ions (x axis) and the percentage of the metal ions (lithium (Li2+), nickel (Ni2+), manganese (Mn2+), cobalt (Co2+), copper (Cu2+), calcium (Ca2+), and aluminium (Al2+)) extracted into Cyanex™ 272, an organic cationic extractant; Figure 3 is an extraction profile graph presenting the percentage of metal ions extracted from an aqueous phase into an organic phase at each solvent extraction tank stage utilising the method described in the present invention and a solvent extraction system comprising three solvent extraction tanks to extract cobalt from a pregnant leach solution; Figure 4a is a graph presenting the measured pH of a raffinate over 200 minutes in a solvent extraction system employing the method in the present invention to extract cobalt from a pregnant leach solution; Figure 4b is a graph presenting the measured percentage of cobalt extraction over 200 minutes in a solvent extraction system employing the method in the present invention to extract cobalt from a pregnant leach solution; Figure 5 is a graph illustrating the relationship between the pH of an aqueous solution comprising metal ions (x axis) and the percentage of the metal ions (lithium (Li2+), nickel (Ni2+), manganese (Mn2+), cobalt (Co2+), copper (Cu2+), and calcium (Ca2+)) extracted into DEHPA™, an organic cationic extractant; Figure 6 is an extraction profile graph presenting the percentage of metal ions extracted from an aqueous phase into an organic phase at each solvent extraction tank stage utilising the method described in the present invention and a solvent extraction system comprising four solvent extraction tanks to extract cobalt from a pregnant leach solution; Figure 7a is a graph presenting the measured pH of a raffinate over 300 minutes in a solvent extraction system employing the method in the present invention to extract manganese from a pregnant leach solution; Figure 7b is a graph presenting the measured percentage of manganese extraction over 5 hours in a solvent extraction system employing the method in the present invention to extract manganese from a pregnant leach solution; and Figure 8 is a schematic of an example computer system. The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. It will be appreciated that features depicted and described in respect of one aspect of the present disclosure may be combined with any feature described in respect of any other aspect of the present disclosure. Detailed Description Figure 1 is a flowsheet depicting various elements comprising the solvent extraction system according to some embodiments the present invention. Such elements include a series of three mixer tanks 3, 8, 12 and three settling tanks 4, 9, 20 in fluid communication with one another, an inlet 2 for a pregnant leach solution from a first tank 1 into mixing tank 3, an inlet 17 into mixing tank 12 for a pH modifier from a second tank 13, an inlet 19 for an organic extractant from a third tank 18, an inlet 5 from settling tank 4 for the target metal, an outlet 22 from settling tank 20 for the raffinate 23 (or depleted pregnant solution), and a pH sensor 24 associated to settling tank 20. The first tank 1 may be a holding tank. The holding tank 1 may be configured to receive a pregnant leach solution, such as a pregnant leach solution from a hydrometallurgical recovery system, such as that described in GB patent application no. 2401211.4, the contents of which is incorporated by reference in its entirety. From the first tank 1, a pregnant leach solution of a pre-determined pH comprising the target metal enters mixing tank 3 via an inlet 2. A first organic phase transported from settling tank 9 enters mixing tank 3 via an outlet 10 and is mixed with the pregnant leach solution to form a mixed phase. The mixed phase is transported to settling tank 4 where it is allowed to separate into a second organic phase atop of a first aqueous phase. The second organic phase comprises the target metal, whilst the first aqueous phase comprises the undesired metals and target metal that was not extracted. The second organic phase comprising the target metal is removed from the system as a target metal-loaded solution 6 via an outlet 5. The first aqueous phase is transported to mixing tank 8 via an outlet 7 from settling tank 4 and is mixed with a third organic phase comprising the target metal transported from settling tank 20 via an outlet 21. The mixed phases are transported to settling tank 9 and allowed to separate into the upper first organic phase, comprising the target metal, and a lower second aqueous phase, comprising the undesired metals and target metal that was not extracted. The second aqueous phase is transported into mixing tank 12 via an outlet 11, wherein a pH modifier from a second tank 13 and an organic extractant from a third tank 18 also enters mixing tank 12 via inlets 17, 19 and are subsequently mixed together with the second aqueous phase. The flow rate of the pH modifier is adjustable using a pump 16 that is operated by a controller 15 following an output from a processor 14 that calculates the required base flow rate using Equation 1. The mixed phases are transported into settling tank 20 and allowed to separate into the third organic phase, comprising the target metal, and a lower third aqueous phase. The pH of the third aqueous phase is measured by a pH sensor 24 associated with settling tank 20 for the purposes of monitoring. The third aqueous phase, also referred to as a raffinate 23, is removed from settling tank 20 via an outlet 22. The non-limiting examples below can be used to assist in describing the solvent extraction system depicted in Figure 1 and illustrating the embodiments of this invention. Example 1: Extracting cobalt from a pregnant leach solution. The method described in the current invention was utilised to efficiently extract cobalt from a pregnant leach solution comprising cobalt, nickel, lithium and manganese employing a solvent extraction system comprising three solvent extraction tanks, a pregnant leach solution inlet, an Cyanex™ 272 inlet, a sodium hydroxide inlet associated to a controllable pump, a raffinate outlet, a target metal outlet, and a pH sensor associated to one of the solvent extraction tanks. It should be noted that the example is not limited to the use of sodium hydroxide as the pH modifier and that other suitable bases may be used to perform the example method. Table 1 states the initial conditions of the pregnant leach solution entering the solvent extraction system. Table 1: Initial conditions of a first example pregnant leach solution. Pregnant Leach Solution Components Co Ni Li Mn Composition (g / L) (Mn+) 4.0 4.6 1.5 0.02 Flowrate (L / min) (P) 0.05 PH (pHi) 3.7 Based on the prior art for extracting cobalt using Cyanex™ 272, it is known that a pH of 6.1 (pHe) of the raffinate is sufficient to extract a high percentage of cobalt over other metals, including nickel. This is demonstrated in Figure 2, which depicts an S-curve graph for describing the relationship between pH of the raffinate (x axis) and the percentage of the metal ions (manganese (Mn2+), cobalt (Co2+) and nickel (Ni2+)) in sulphate solution extracted into Cyanex™ 272. Whilst Figure 2 also demonstrates that manganese is also efficiently extracted at pH 6.1, it was only present at a low concentration in the initial pregnant leach solution. Sodium hydroxide was selected as the pH modifier with a pre-determined concentration ([B]) of 1 mol / L. A target cobalt extraction (E) of 100% was established. Equation 4 was used to determine the desired flow rate of the pH modifier (B) to achieve a 100% cobalt extraction efficiency: • mx P(10-PHi-10_PHE+([Mn+]jXnxE)) B ----------------M---------------’         (4) wherein m (the hydroxyl coefficient of sodium hydroxide in its aqueous-dissociated form) is 1 and n (the stable oxidation state of the cobalt ion) is 2. By inputting the variables into Equation 4, the desired pH modifier flow rate was calculated to be 0.0067 L / min. A solvent extraction system was configured as described in Figure 1 to extract cobalt from the pregnant leach solution using Cyanex™ 272 as the organic extractant. The pump 16 associated to the sodium hydroxide tank 13 was configured to run at a flow rate of 0.0067 L / min (6.7 mL / min) into mixing tank 12. Cyanex™ 272 entered mixing tank 12 and was brought into contact with the sodium hydroxide and the aqueous phase from mixing tank 9. Whilst agitating, Cyanex™ 272 extracted cobalt and small amounts of the other metal ions from the aqueous phase. The Cyanex™ 272-aqueous mixture flowed into settling tank 20 and separated into an upper cobalt-loaded organic phase and a cobalt-depleted raffinate rich in nickel and lithium. A pH sensor 24 associated to settling tank 20 was utilised to monitor the pH to ensure the method was proceeding as anticipated. The cobalt-loaded organic phase flowed into mixing tank 8 and was mixed with the aqueous phase (partially cobalt-depleted pregnant leach solution) from settling tank 4. Mixing tank 8 was of a slightly lower pH to that of mixing tank 12 (ca. pH 5.3) to encourage co-extracted nickel and lithium to become less stable within the organic phase and to return to the aqueous phase. After mixing the organic and aqueous phases within mixing tank 8, the organic-aqueous mixture flowed into settling tank 9 to settle into an upper cobalt-loaded organic phase and a partially cobalt-depleted aqueous phase. The cobalt-loaded organic phase flowed into mixing tank 3 to be mixed with the pregnant leach solution from a first tank 1. The pH within mixing tank 3 was lower (ca. pH 4.5) to favour cobalt extraction whilst limiting coextraction of nickel. The mixture of cobalt-loaded organic and partially cobalt-depleted pregnant leach solution flowed into settling tank 4 in which the fully cobalt-loaded organic phase settled atop of the partially cobalt-depleted pregnant leach solution. The percentage of metal ions present in the organic phase after extraction from the aqueous relevant to the amount present in the initial pregnant leach solution is presented in Figure 3. Stage 1 represents the organic phase in settling tank 20, Stage 2 represents the organic phase in settling tank 9, and Stage 1 represents the organic phase in settling tank 4. Whilst cobalt, nickel and lithium are partially extracted from the aqueous phase into the organic phase at Stage 1, the reduction in pH at Stage 2 results in more nickel returning to the aqueous phase and encourages cobalt to be extracted into the organic phase. By Stage 3, only a small percentage of manganese is extracted into the cobalt-loaded organic phase. Using the solvent extraction system and method described in Example 1 and a sodium hydroxide flow rate defined by Equation 4, the raffinate pH reached that of the target raffinate pH within 20 minutes and remained stable for over 3 hours (Figure 4a). The rapid attainment of stable pH resulted in an equally rapid increase in the cobalt extraction yield compared to lithium, nickel and manganese also present in the system (Figure 4b - manganese excluded from graph due to low initial concentration), ensuring a high product yield. The method also enables daily process start-ups where overnight and / or 24-hour operations are not possible. In comparison, the automated pH control systems defined in the prior art typically experience unavoidable fluctuations, resulting in inconsistent results and low metal extraction yields. Example 2: Extracting manganese from a pregnant leach solution The method described in the current invention was utilised to efficiently extract cobalt from a pregnant leach solution comprising cobalt, nickel, lithium and manganese employing a solvent extraction system comprising three solvent extraction tanks, a pregnant leach solution inlet, a DEHPA™ inlet, a sodium hydroxide inlet associated to a controllable pump, a raffinate outlet, a target metal outlet, and a pH sensor associated to one of the solvent extraction tanks. It should be noted that the example is not limited to the use of sodium hydroxide as the pH modifier and that other suitable bases may be used to perform the example method. Table 2 states the initial conditions of the pregnant leach solution entering the solvent extraction system. Table 2: Initial conditions of a second example pregnant leach solution. Pregnant Leach Solution Components Co Ni Li Mn Composition (g / L) (Mn+) 20.2 20.3 6.5 17.7 Flowrate (L / min) (P) 0.02 PH (pHi) 3.2 Based on the prior art for extracting manganese using DEHPA™, it is known that a pH of ca. 3.5 (pHe) of the raffinate is sufficient to extract over other present metals (such as cobalt and nickel). This is demonstrated in Figure 5, which depicts an S-curve graph for describing the relationship between pH of the raffinate (x axis) and the percentage of the metal ions (manganese (Mn2+), cobalt (Co2+) and nickel (Ni2+)) in sulphate solutions extracted into DEHPA™. Sodium hydroxide was selected as the pH modifier with a pre-determined concentration ([B]) of 5 mol / L. A target manganese extraction (E) of 100% was established. Equation 4 was used to determine the desired flow rate of the pH modifier (B) to achieve a 100% manganese extraction efficiency: • mx P(10-PHi-10_PHE+([Mn+]jXnxE)) B -              M             ’         (1) wherein m (the hydroxyl coefficient of sodium hydroxide in its aqueous-dissociated form) is 1 and n (the stable oxidation state of the cobalt ion) is 2. By inputting the variables into Equation 4, the desired pH modifier flow rate was calculated to be 0.0026 L / min. A solvent extraction system configured to extract manganese from a pregnant leach solution comprised a series of four solvent extraction each comprising a mixer tank and a settling tank, an input for a pregnant leach solution from a first tank into the mixer tank of the first solvent extraction tank, inputs for sodium hydroxide from a second tank and DEPHA™ from a third tank into the settling tank of the fourth solvent extraction tank, an output for the target metal from the settler tank of the first solvent extraction tank, an output for the raffinate from the settling tank of the fourth solvent extraction tank, and a pH sensor associated to the settling tank of the fourth solvent extraction tank. A pump associated to the sodium hydroxide tank was configured to run at a flow rate of 0.0026 L / min (2.6 mL / min) into its associated mixer tank. In Stage 1, DEHPA™ entered the fourth mixing tank and was brought into contact with the sodium hydroxide and the aqueous phase from the third mixing tank. Whilst agitating, DEHPA™ extracted manganese and small amounts of the other metal ions from the aqueous phase. The DEHPA™-aqueous mixture flowed into the fourth settling tank and separated into an upper manganese-loaded organic phase and a manganese-depleted raffinate. A pH sensor associated to the fourth settling tank was utilised to monitor the pH to ensure the method was proceeding as anticipated. In Stage 2, the manganese-loaded organic phase flowed into the third mixing tank and was mixed with the aqueous phase (partially manganese-depleted pregnant leach solution) from the second settling tank. The third settling tank was of a slightly lower pH to that of the fourth settling tank (ca. pH 3.2) to encourage co-extracted nickel, lithium and cobalt to become less stable within the organic phase and to return to the aqueous phase. After mixing the organic and aqueous phases within the third mixing tank, the organic-aqueous mixture flowed into the third settling tank to settle into an upper cobalt-loaded organic phase and a partially manganese-depleted aqueous phase. In Stage 3, the manganese-loaded organic phase flowed into the second mixing tank to be mixed with the aqueous phase from the first settling tank. The pH within the second mixing tank was lower (ca. pH 3.1) to favour manganese extraction whilst limiting co-extraction of lithium, nickel and cobalt. In Stage 4, the manganese-loaded organic phased flowed into the first mixing tank to be mixed with the pregnant leach solution. The mixture of manganese-loaded organic and partially manganese-depleted pregnant leach solution flowed into the first settling tank in which the fully manganese-loaded organic phase settled atop of the partially manganese-depleted pregnant leach solution. The percentage of metal ions present in the organic phase after extraction from the aqueous relevant to the amount present in the initial pregnant leach solution is presented in Figure 6. Stage 1 represents the organic phase in the fourth settling tank, Stage 2 represents the organic phase in the third settling tank, Stage 3 represents the organic phase in the second settling tank, and Stage 4 represents the organic phase in the first settling tank. Whilst cobalt, nickel and lithium were partially extracted from the aqueous phase into the organic phase at Stage 1, the reduction in pH at Stage 2 resulted in nickel, lithium and cobalt returning to the aqueous phase and encouraged manganese to be extracted into the organic phase. At Stage 3, a further reduction in pH encouraged manganese extraction into the organic phase and for nickel, lithium and cobalt to return to the aqueous phase. By Stage 4, much of the manganese has been extracted to form a manganese-loaded organic phase. Using the solvent extraction system and method described in Example 2 and a sodium hydroxide flow rate defined by Equation 4, the raffinate pH reached that of the target raffinate pH within 5 minutes and remained stable for over 5 hours (Figure 7a). The rapid attainment of stable pH resulted in an equally rapid increase in the cobalt extraction yield compared to lithium, nickel and manganese also present in the system (Figure 7b), ensuring a high product yield. The method also enables daily process start-ups where overnight and / or 24-hour operations are not possible. In comparison, the automated pH control systems defined in the prior art typically experience unavoidable fluctuations, resulting in inconsistent results and low metal extraction yields. Example 3: Comparison between methods described in the prior art and the current method Table 2 provides data obtained from a model solvent extraction system, wherein the pH control relied on an automated feedback system as described in the prior art. Table 3 provides data recorded using a solvent extraction system utilising the pH control method described in the current invention. Both systems were carried out on a pilot scale, wherein 100 L of pregnant leach solution was processed per hour, and on a larger scale, wherein 1000 L of pregnant leach solution was processed per hour. In comparison to the system described in the prior art, the current invention allowed for the pH to stabilise around 10 times quicker, requiring around 10 times less pregnant leach solution before the pH became stable and utilising around 10 times less organic extractant. This reduction in time and solution volumes is particularly significant in terms of improving the efficiency of the system, as well as reducing excess chemical and energy wastage and their associated costs. Table 2: Data from a solvent extraction system utilising an automated feedback pH control method, demonstrating the association between time taken to reach pH stability and the volumes of pregnant leach solution (PLS) and organic extractant. Process Plant Type Volume per hour of PLS Processed / L / h Time Taken to Reach pH Stability Zh PLS Volume Used to Reach pH Stability ZL Organic Extractant Volume of Pure Cyanex™ 272 or DEHPA™ ZL Volume of Organic Diluent (e.g. Exxsol™ D80) ZL Pilot 100 3 300 180 420 Large Scale 1000 5 5000 3000 7000 Table 3: Data from a solvent extraction system utilising the pH control method described in the current invention, demonstrating the association between time taken to reach pH stability and the volumes of pregnant leach solution (PLS) and organic extractant. Process Plant Type Volume per hour of PLS Processed / L / h Time Taken to Reach Stability Zh PLS Volume Used to Reach Stability ZL Organic Extractant Volume of Pure Cyanex™ 272 or DEHPA™ ZL Volume of Organic Diluent (e.g. Exxsol™ D80) ZL Pilot 100 0.3 30 18 42 Large Scale 1000 0.6 600 360 840 Example 4: A computer system configured to perform the method of the present invention Figure 8 shows an example computer system 1500. The herein described method may be a computer-implemented method implemented on a computer system, such as the computer system 1500. The computer system 1500 may comprise a central processing unit 1510, memory 1520, one or more storage devices 1530, an input I output processor 1540, circuitry to connect the components 1550 and one or more input I output devices 1560. The computer-implemented model may utilise artificial intelligence or machine learning. The method according to any aspect of the present disclosure may rely on collective inputs rather than instantaneous inputs. Instantaneous inputs may drive control of the active process, whereas historic or collective inputs may be used to refine future control parameters. In this example, working memory 1520 may be used for holding (which may be transient), processing, and manipulating flow rate, pH, and reaction data. Permanent memory 1530 may be used to store operating system code comprising codes configured to perform the methods as herein described, or configured to control a system to perform the methods as herein described. These codes may be loaded and implemented by processor 1510. The processor 1510 may be further configured as controller. Alternatively, the computer system may comprise a controller (not shown) configured to control a pH modifier flow rate. The computer may be comprised in the computer system or may be in communication with the computer system. The controller may comprise an input / output device 1560 of the computing system. This specification uses the term “configured” in connection with systems and computer program components. For a system of one or more computers to be configured to perform particular operations or actions means that the system has installed on it software, firmware, hardware, or a combination of them that in operation cause the system to perform the operations or actions. For one or more computer programs to be configured to perform particular operations or actions means that the one or more programs include instructions that, when executed by data processing cause the apparatus to perform the operations or actions. Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, in tangibly-embodied computer software or firmware, in computer hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a tangible non-transitory storage medium for execution by, or to control the operation of, data processing apparatus. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of them. Alternatively, or in addition, the program instructions can be encoded on an artificially generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. The term “data processing apparatus” refers to data processing hardware and encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can also be, or further include, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). The apparatus can optionally include, in addition to hardware, code that creates an execution environment for computer programs, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, or a combination of one or more of them. A computer program, which may also be referred to or described as a program, software, a software application, an app, a module, a software module, a script, or code, can be written in any form of programming language, including compiled or interpreted languages, or declarative or procedural languages; and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data, e.g., one or more scripts stored in a mark-up language document, in a single file dedicated to the program in question, or in multiple coordinated files, e.g., files that store one or more modules, sub programs, or portions of code. A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a data communication network. The processes and logic flows described in this specification can be performed by one or more programmable computers executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by special purpose logic circuitry, e.g., an FPGA or an ASIC, or by a combination of special purpose logic circuitry and one or more programmed computers. Computers suitable for the execution of a computer program can be based on general or special purpose microprocessors or both, or any other kind of central processing unit. Generally, a central processing unit will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a central processing unit for performing or executing instructions and one or more memory devices for storing instructions and data. The central processing unit and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device, e.g., a universal serial bus (USB) flash drive, to name just a few. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user’s device in response to requests received from the web browser. Also, a computer can interact with a user by sending text messages or other forms of message to a personal device, e.g., a smartphone that is running a messaging application, and receiving responsive messages from the user in return. Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front end component, e.g., a client computer having a graphical user interface, a web browser, or an app through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back end, middleware, or front end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (LAN) and a wide area network (WAN), e.g., the Internet. The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data, e.g., an HTML page, to a user device, e.g., for purposes of displaying data to and receiving user input from a user interacting with the device, which acts as a client. Data generated at the user device, e.g., a result of the user interaction, can be received at the server from the device. While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any invention or on the scope of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially be claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. Similarly, while operations are depicted in the drawings and recited in the claims in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system modules and components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. For example, the actions recited in the claims can be performed in a different order and still achieve desirable results. As one example, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In some cases, multitasking and parallel processing may be advantageous. In summary, the present invention provides for improved control of a solvent extraction system that reduces the time taken to achieve a desired pH and thereby reduces the amount of materials which are co-extracted. The present invention also avoids saponification and makes better use of chemicals by more appropriately controlling the pH of the system.

Claims

1. A method of controlling the extraction of one or more target metals from a pregnant leach solution using a solvent extraction system, the method including the steps of:a. determining the concentration of a target metal in the pregnant leach solution ([Mn+]), the flow rate of the pregnant leach solution (P), and the pH of the pregnant leach solution (pHj);b. i) calculating a desired pH modifier flow rate (B) using the following equation:6 m X P(10“PHi- 10“PHE+([Mn+]jXnxE))B----------------isi---------------'wherein pHe is a target pH for a raffinate, m is the hydroxyl coefficient of a pH modifier in its aqueous-dissociated form, n is the stable oxidation state of the target metal ion, E is a target extraction efficiency value expressed as a value between 0 and 1, and [B] is the concentration of the pH modifier, orii) calculating a desired pH modifier concentration ([B]) using the following equation:r , m x P(10-PHi —10-PHE+([Mn+]jXnxE))[ ] — gwherein pHe is a target pH for a raffinate, n is the stable oxidation state of the target metal ion, E is a target extraction efficiency value expressed as a value between 0 and 1, and B is the pH modifier flow rate; andc. controlling a pH modifier flow rate (B) to achieve the calculated desired pH modifier flow rate or controlling the pH modifier concentration [B] to achieve the calculated desired pH modifier concentration.

2. The method according to claim 1, wherein the method includes contacting an organic extractant, a pH modifier and a pregnant leach solution containing the one or more target metals, optionally wherein the organic extractant comprises a cationic organic extractant.

3. The method according to claim 1 or 2, wherein the one or more target metals include at least one of cobalt, lithium, nickel, or manganese.

4. The method according to any preceding claim, wherein the pregnant leach solution is obtained from one or more of: black mass, lithium-containing electrochemical devices, battery factory scrap, consumer batteries, mixed hydroxide precipitate (MHP), blends of virgin extract, or a mixture thereof.

5. The method according to any preceding claim, wherein the pH modifier is selected from one or more of a metal hydroxide, sodium hydroxide, ammonia, ammonium hydroxide, potassium hydroxide, calcium hydroxide, nickel hydroxide, manganese hydroxide, cobalt hydroxide, lithium hydroxide, magnesium hydroxide, or a mixture of any thereof.

6. The method according to any preceding claim when dependent on claim 2, wherein the cationic organic extractant comprises a phosphinic acid, a phosphoric acid, a phosphonic acid, an alkyl phosphate, a carboxylic acid, an oxime, a phenol, a beta-diketone, a sulfonic acid, a sulfur-containing diamide, an acidic ionic liquid, or a mixture thereof.

7. The method according to any preceding claim when dependent on claim 2, wherein the organic extractant comprises bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex™ 272), di(2-ethylhexyl)phosphoric acid (DEHPA™), or neodecanoic acid (Versatic™ Acid 10), chatecol, p-toluenesulfonic acid (tosylic acid, pTsOH), or mixtures thereof.

8. The method according to any preceding claims, wherein the method is carried out between 10 °C and 70 °C, preferably between 18 °C and 60 °C, more preferably between 20 °C and 60 °C.

9. The method according to any proceeding claim, wherein the flow rate of the pH modifier is:i) at least 2 times lower than the flow rate of the pregnant leach solution, preferably at least 3 times lower, 4 times lower, or 5 times lower than the flow rate of the pregnant leach solution; and / orii) at most 30 times lower than the flow rate of the pregnant leach solution, preferably at most 30 times lower, 25 times lower, 20 times lower, or 10 times lower than the flow rate of the pregnant leach solution.

10. A solvent extraction system configured to extract one or more target metals from a pregnant leach solution including means for:a. determining the concentration of a target metal in the pregnant leach solution ([Mn+]), the flow rate of the pregnant leach solution (P), and the pH of the pregnant leach solution (pHj);b. a processor configured to calculate the pH modifier flow rate or pH modifier concentration according to claim 1;c. a controller configured to operate a pump to provide the calculated desired pH modifier flow rate or pH modifier concentration.

11. The solvent extraction system according to claim 10, wherein the system comprises a plurality of solvent extraction tanks and a pH modifier tank in fluid communication with one another, including one or more pH sensors configured to measure the pH within the solvent extraction tanks.

12. The solvent extraction system according to claim 10 or 11, wherein the solvent extraction tank includes a mixing tank and a settling tank which are in fluid connection with one another.

13. The solvent extraction system according to any of claims 10 to 12, wherein the mixing tank includes an agitator, a first inlet for the organic solution, a second inlet for the aqueous solution, and / or a third inlet for pH modifier addition.

14. The solvent extraction system according to any of claims 10 to 13, wherein the mixing tank is configured to mix the pregnant leach solution and the organic extractant into an organic-aqueous mixture.

15. The solvent extraction system according to any of claims 10 to 14, wherein the settling tank includes a first outlet for the removal of an aqueous layer, and a second outlet for the removal of an organic layer.

16. A computer readable storage medium containing instructions configured to cause one or more computers to control a system to perform the method of any of claims 1 to 9.5 17. The computer readable storage medium according to claim 16, wherein thesystem is the system according to any of claims 10 to 15.s