METHOD AND APPARATUS FOR NICKEL LEACHING

Abstract

The present invention relates to a method and apparatus for optimizing the rate of nickel dissolution in an aqueous leaching solution comprising sulfuric acid and an oxidizing agent, wherein the optimization is achieved by providing a mathematical relationship that describes a starting level of oxidizing agent content in the aqueous leaching solution causing a passivation effect on the nickel surface, and then regulating the addition of oxidizing agent to the aqueous leaching solution to be close to, but somewhat less than, the starting level of oxidizing agent during the leaching process.

Description

[0001] METHOD AND APPARATUS FOR LEACHING NICKEL

[0002] The present invention relates to a method and apparatus for optimising the dissolution rate of nickel in an aqueous leach solution comprising sulphuric acid and an oxidising agent.

[0003] Background

[0004] Nickel sulphate solutions are used in electroplating / electroless plating and as raw material for production of nickel containing cathodes for high-capacity secondary lithium-ion batteries (LIBs), such as e.g. the NMC type lithium-ion battery which applies the mixed lithium oxide LiNixCoyMni-x-yO2, as the active (lithium absorbing or releasing) material. According to the S&P Global Market Intelligence, the global production of LIBs is expected to more than triple from the present (2020) annual level of 455 GWh to nearly 1500 GWh in 2025. China and Europe are expected to be the largest contributors to this growth.

[0005] Common to applying the nickel sulphate in electroplating / electroless plating and in production of active materials for the cathode of secondary LIBs, is that the nickel sulphate should be of high purity. Impurity compounds are usually electrochemically inactive and may interfere with the lithium transport in the electrode causing significant reductions in the storage capacity / volumetric energy density of the lithium-ion battery.

[0006] Nickel mainly occurs in the nature as nickel lateritic ores or as sulphide nickel ores which are extracted and chemically reduced by various hydrometallurgical and / or pyrometallurgical processes to (metallurgical grade) elementary nickel or ferronickel. However, metallurgical grade nickel has often too high impurity levels to be acceptable for use in electroplating / electroless plating or as raw material for the manufacturing of cathodes for LIBs and need to be refined.

[0007] Prior art

[0008] Nickel sulphate solutions may be produced by leaching nickel metal in an aqueous leaching agent comprising sulphuric acid and usually an oxidizing agent to speed up the dissolution rate.

[0009] Depending on the source of nickel, there may or may not be a need for postdissolution refinement of the nickel sulphate solution to satisfy the purity requirements of the battery industry. Examples of methods for forming nickel sulphate solutions for use in the battery industry which involves post-dissolution refining of the nickel are known from e.g. WO 2013 / 077296 and WO 2019 / 090389. One attractive alternative is to apply Class 1 Nickel (having a purity of at least 99.8 wt%) which may be dissolved into a nickel sulphate solution without any further need for refining to satisfy the battery industries’ purity requirements. An example of Class 1 Nickel is known from e.g. Glencore Nikkelverk AS [Ref. 1] which applies a chlorine-leaching process of nickel matte followed by removal of copper, cobalt, iron and other impurities by precipitation, filtration and solvent extraction to form a nickel electrolyte which, when applied in an electrowinning process, produces cathode plates of high-purity nickel. Examples of direct production of nickel sulphate solutions by dissolving high purity nickel are known from e.g. EP 3 967 661 Al and the applicant’s co-pending application NO 20220405.

[0010] Whatever of the two above alternatives being applied, the transformation of nickel metal to a nickel sulphate solution involves dissolving nickel metal in an aqueous sulphuric acid. However, it is well known that nickel is “reluctant” to be dissolved in sulphuric acid. The dissolution, especially when dissolving relatively large pieces / particulate / cuttings of nickel, may be painstakingly slow adding significant costs to an industrial scale process of preparing battery grade nickel sulphate solutions. Another issue is that significant amounts of hydrogen gas is produced when a metal is dissolved in an acid:

[0011] Ni (s) + H2SO4 (aq) NiSO4(aq) + H2(g) [1]

[0012] The hydrogen gas is highly explosive when mixed with air (at certain stoichiometric ratios) and may pose a safety hazard. However, combining the acid with an oxidising agent is known to both increase the dissolution rates of the metal and to produce water instead of hydrogen gas. For example, from EP 3 967 661 Al, it is known to combine the sulphuric acid with hydrogen peroxide to dissolve nickel, of which the total reaction may be given as:

[0013] Ni (s) + H2SO4 (aq) + H2O2 (1) NiSO4(aq) + 2 H2O (1) [2]

[0014] Bilczuk et al. (2016) [Ref. 2] has studied the dissolution rates of nickel metal in sulphuric acid as a function of temperature, sulphuric acid concentration, and three different oxidant types, oxygen gas, ferric sulphate and hydrogen peroxide. The study observed different kinetic regimes depending on the oxidant used for the dissolution of nickel and concluded that the order of reactivity of the different oxidants used in this work was:

[0015] H2O2 > Fe3+(pH 0) > O2> Fe3+(pH > 1) [3]

[0016] The battery industry typically requires the concentration of (unreacted) sulphuric acid in the nickel sulphate solution to be no more than 0.1 molar, preferably no more than 0.025 molar. Thus, the acid strength of the applied leaching agent may advantageously be adapted such that when an intended level of dissolved nickel (Ni2+) in the solution is obtained, for example 100 g Ni+2per litre solution, that the level of remaining sulphuric acid in the nickel sulphate solution is less than e.g. 2.5 g / litre. This requirement places a constriction on the upper allowable acid strengths of the sulphuric acid which may be applied in the dissolution of the nickel when the resulting solution is intended for use in the battery industry. The requirement of little rest acid when reaching the intended level of solute represents an upper limit of the driving force (i.e. acid strength) and thus the dissolution rates which may be obtained. This speaks for applying relatively high levels of oxidizing agent.

[0017] However, as given in e.g. Abeggs Handbuch (1939) [Ref. 3], it is observed a passivation effect on the nickel surface when exposed to hydrogen peroxide. This effect is described to completely halt the dissolution of Ni in acid free hydrogen peroxide solutions (3 % H2O2) and to significantly slow the dissolution rates in relatively dilute aqueous sulphuric acids of 0.041 to 0.057 molar H2SO4 with 0.97 to 0.366 molar H2O2. At 0.45 molar H2SO4 (and above) there is observed no surface passivation effect.

[0018] CN 110 735 041 Bl discloses an acid leaching of (insoluble or acid insoluble?) metal wastes which includes the following steps; A, the insoluble metal wastes and water are mixed and put in a dissolution kettle, a catalyst is added, and then stirring and heating are performed; the catalyst is an inorganic salt; B, oxidizing gas is fed into the dissolution kettle, and acid liquid is added, so that the pH value of the solution is adjusted to strong acidity; C, an oxidizing agent solution is added into the dissolution kettle for oxidation acid leaching of the metal wastes till the pH value of the solution is increased to weak acidity; D, the acid liquid is added into the dissolving kettle, the pH value of the solution is adjusted again to strong acidity, and then the oxidizing agent solution continues to be added till the solution in the dissolution kettle is weak acidity; and E, the steps of C and D are performed in a circulating mode till the metal wastes are totally dissolved. The acid dissolution rate of the (acid insoluble?) metal wastes is promoted through the use of catalyst, and the utilization ratio of the oxidizing agent solution is improved.

[0019] USRE 37,786 E discloses a method for selective polishing of a Cu-film on a semiconductor device which involves adding sufficient hydrogen peroxide to the polishing solution containing either aminoacetic acid or aminosulphuric acid to form a protective oxide layer functioning as an etching barrier and applying mechanical polishing removing the protective oxide film on places where a higher dissolution rate is intended. The document informs that the dissolution rate is found to be dependent on the concentration of the oxidising agent. If there is no oxidising agent, there is no etching, but that the etching rate was high upon addition of a little oxidising agent and that when increasing the amount of oxidising agent further lowered the etching rate and eventually became zero. This effect is explained to be due to formation of a protective oxide layer on the copper surface at high oxidising agent levels.

[0020] Objective of invention

[0021] The main objective of the invention is to provide a method for optimising the reaction kinetics of a leach / dissolution process of elemental nickel applying an aqueous leach agent comprising sulphuric acid and the oxidizing agent.

[0022] Another objective of the invention is to provide a leach column suitable for performing said method for optimising the reaction kinetics of a leach / dissolution process of elemental nickel.

[0023] A further objective of the invention is to provide a plant for producing nickel sulphate solutions.

[0024] Description of the invention

[0025] The invention is based on an experimental observation made by the inventors that the heat development during leach / dissolution of nickel in an aqueous leach solution comprising sulphuric acid and hydrogen peroxide is found to increase with increasing content (concentration) of hydrogen peroxide up to a certain breaking content of hydrogen peroxide at which the heat development drops suddenly and stay low for any hydrogen peroxide content above this breaking content. Since leach of nickel in sulphuric acid is an exotherm process, this observed change of heat development (heat production rate) during the leach process is directly proportional to the dissolution rate of the nickel.

[0026] Without being bound by theory, the inventors believe this observation of a sudden drop in the dissolution rate at high contents of hydrogen peroxide (at given acid strength) is the same surface passivation phenomenon as reported by Abeggs Handbuch (1939) [Ref. 3], and further that this sudden drop in dissolution rates may be explained to be due to an imbalance between the part-reactions taking place when nickel is dissolved in sulphuric acid and hydrogen peroxide. The first part reaction is hydrogen peroxide reacting with the nickel surface to form nickel oxide on its surface:

[0027] Ni + H2O2 NiO + H2O [4]

[0028] The second part reaction is that the sulphuric acid attacks the nickel oxide on the surface and forms dissolved nickel sulphate:

[0029] NiO + H2SO4 (aq) NiSO4(aq) + H2O [5]

[0030] It may be speculated that if the hydrogen peroxide content becomes higher than a certain threshold level, that reaction (4) produces more NiO on the nickel surface than reaction (5) consumes causing a build-up of a NiO-layer on the metal surface which passivates it and considerably slows down the reaction kinetics.

[0031] In any case, whatever the explanation for this observed drop in heat development may be, it is observed that when leach nickel in sulphuric acid and hydrogen peroxide, that there is an onset-level / upper limit of hydrogen peroxide content in the aqueous leach solution before at which the leach rates are observed to rapidly drop - i.e. the nickel surface becomes passivated / made more resistant towards the chemical action of the sulphuric acid and thus significantly slows the leach rate / reaction kinetics of the leach process. This onset-level of oxidising agent content causing the nickel surface passivation effect is found to be dependent on the acid strength of the aqueous leach solution. An example of this is given in the diagram shown in figure 1 which presents the onset-levels given in grams hydrogen peroxide per litre aqueous leach solution for a set of determination over a range of acid strengths, here given as pH. Each observed onset-level is marked with a black dot on the diagram. As seen from the diagram in figure 1, the onset-level of hydrogen peroxide content when the acid strength of the aqueous leach solution corresponds to pH of 0.5 is found to be approx. 10 g / 1, while at pH = 2, the onsetlevel is two orders of magnitude lower.

[0032] This relatively strong variation of the onset-level of the surface passivation effect with acid strength opens for an optimisation of the leach process by actively regulating the supply / inflow rate of hydrogen peroxide / oxi dising agent being (continuously) introduced to the aqueous leach solution. The idea is to regulate the inflow of hydrogen peroxide / oxidising agent such that the content / concentration of hydrogen peroxide / oxidising agent being present in the aqueous leach solution at any moment during the leach process is somewhat less than the above-described onset-level of hydrogen peroxide / oxidising agent, and thus obtain a maximum leach rate without risking forming the above-described surface passivation layer on the nickel metal. In practice, this may be obtained by providing a mathematical relation describing the above described onset-level of hydrogen peroxide / oxidising agent causing the surface passivation effect as a function of acid strength, more or less continuously measuring the real content / concentration of sulphuric acid and of hydrogen peroxide / oxidising agent being present in the aqueous leach agent, applying the real content / concentration of sulphuric acid to define a set value, being somewhat less than the onset-level (to avoid triggering the formation of a surface passivation of the nickel), of the content / concentration of hydrogen peroxide / - oxi dising agent to be present in the aqueous leach agent, and then applying a regulation algorithm to regulate the flow volume of hydrogen peroxide / oxidising agent being introduced to the aqueous leach solution to minimise the deviation between the real value and the set value of the hydrogen peroxide / oxidising agent content. Furthermore, even though the invention has been described in light of hydrogen peroxide as the oxidising agent, it is believed that a similar surface passivation effect is present also for other oxidising agents than hydrogen peroxide.

[0033] Thus, in a first aspect, the invention relates to a method for preparing a nickel sulphate solution, wherein the method comprises

[0034] - executing a leach process by bringing nickel metal in contact with an aqueous leach solution comprising sulphuric acid and an oxidising agent, and wherein the oxidising agent is continuously added to the aqueous leach solution, characterised in that the method further comprises:

[0035] - providing a mathematical relation describing, as a function of sulphuric acid content in the aqueous leach solution, an onset-level, Conset, of oxidising agent content in the aqueous leach solution causing a nickel surface passivation effect, and

[0036] - more or less continuously regulating the addition of oxidising agent to the aqueous leach solution by performing each of steps 1) to 4) in successive order:

[0037] 1) measuring a real content of sulphuric acid, Cacid,reai, and a real content of oxidising agent, Coxid,reai, present in the said aqueous leach solution,

[0038] 2) applying said mathematical relation to determine an actual onsetlevel, Conset, actual, at, Cacid,real,

[0039] 3) defining a set-value, Cset, actual, of the content of oxidising agent to be obtained in the aqueous leach solution from the relation:

[0040] Cset, actual K ' Conset. actual, where K is a constant chosen from the interval from 0.1 to 0.99, and

[0041] 4) applying a regulation algorithm to regulate the feeding rate of oxidising agent to the aqueous leach solution aimed at obtaining the setvalue, Cset, actual-

[0042] The wording “more or less continuously regulating the addition of oxidising agent to the aqueous leach solution” as used herein, refers to the need for regulating the feeding rate of oxidising agent to maintain an efficient leach rate. If continuous instrumental measurements of acid and oxidizing agent concentrations are provided, then the change in the feeding rate of oxidising agent could be made continuous to keep the oxidising agent level constantly at the set value. However, if concentration measurements are discrete and provided at certain time intervals, then the change in the feeding rate of oxidizing agent may be adjusted at same time intervals. If these time intervals are considerably shorter than the time it takes to complete the process, the theoretical loss in production efficiency compared to continuous measurements and adjustment of the feeding rate of oxidizing agent should be acceptably low. Thus, the term “more or less continuously regulating the addition of oxidising agent to the aqueous leach solution” as used herein encompasses either fully continuous regulation of the feeding rate of oxidising agent or intermittent regulations / corrections of the feeding rate made at regular intervals of time. In one embodiment, the regulation of the feeding rate is made at time intervals being in the range of from 5 seconds to 15 minutes, preferably from 10 seconds to 10 minutes, more preferably from 15 seconds to 5 minutes, and most preferably from 30 seconds to 2 minutes.

[0043] The wording “pre-specified rest content of sulphuric acid” as used herein refers to a parameter which may be applied to terminate the leach process. The battery industry often specifies the acceptable maximum content of unreacted sulphuric acid in the nickel sulphate solution. By setting the pre-specified rest content of sulphuric acid being equal to or below the acceptable maximum content of unreacted sulphuric acid in battery grade nickel sulphate solutions, the present method according to the invention may be made to automatically endure until the leach process has consumed sufficient amounts of the (initially supplied) sulphuric acid to obtain a remaining content of unreacted acid being on or below this maximum allowable content. Examples of suited pre-specified rest contents of sulphuric acid includes 50 g / 1 (corresponds to 0.51 molar), preferably 25 g / 1 (corresponds to 0.25 molar), more preferably 10 g / 1 (corresponds to 0.11 molar), more preferably 7.5 g / 1 (corresponds to 0.08 molar), more preferably 5 g / 1 (corresponds to 0.05 molar), and most preferably 2.5 g / 1 (corresponds to 0.03 molar) of sulphuric acid per litre aqueous leach solution.

[0044] The amount or strength of the sulphuric acid in the “virgin” leach solution depends on the battery industry’s specification of the acceptable maximum content of unreacted sulphuric acid and the requested level of dissolved nickel in the pregnant leach solution / battery grade nickel sulphate solution. A typical example of dissolved nickel contents in battery grade nickel sulphate solutions is from 10 to 200 g / 1, more preferably from 50 to 150 g / 1, and most preferably from 80 to 120 g / 1 Ni2+ions per litre pregnant leach solution / battery grade nickel sulphate solution. With the specified examples of pre-specified rest contents of sulphuric acid given above and these examples of dissolved nickel contents in battery grade nickel sulphate solutions, the initial sulphuric acid contents / acid strengths in the “virgin” leach solution will typically be in the range from about 20 g / 1 to 400 g / 1. However, the determination of the required initial sulphuric acid contents / acid strengths in the “virgin” leach solution to arrive at a pregnant leach solution containing a specific nickel ion content and maximum amount of unreacted sulphuric acid is a matter of simple stoichiometric considerations and calculations within basal common general knowledge in the field and needs no further description.

[0045] The wording “onset-level, Conset, of oxidising agent content in the aqueous leach solution causing a nickel surface passivation effect” as used herein refers to the content (concentration) of oxidising agent in the aqueous leach solution at which the leach rate of the nickel is observed to drop rapidly, (probably) due to the surface passivation effect described above caused by formation of a layer of NiO on the surface of the nickel. However, whatever the correct explanation for this may be, it is nevertheless an observation made by the inventors that there is an upper limit of how much oxidising agent which may be present in the aqueous leach solution before the leach rates suddenly slows considerably. Up to this level (content) of oxidising agent, the leach rates are observed to increase with increased content of oxidising agent, and then suddenly drop rapidly to never to recover no matter of how much additional oxidising agent is present. However, leaching may be resumed by stopping addition of oxidising agent and adding more sulphuric acid to increase sulphuric acid concentration to re-activate the metal surface by dissolving the passivating NiO-layer. The “onset-level, Conset,” corresponds to the amount / content of oxidising agent in the aqueous leach agent at which this breaking point where the leach rate is observed to drop rapidly.

[0046] The term “content of a compound” and “concentration of a compound” in the aqueous leach solution are used interchangeably herein and refer both to the same physical entity, the amount (mass) of sulphuric acid and / or oxidising agent being present in the aqueous leach solution.

[0047] Furthermore, since this onset-level is dependent on the acid strength of the aqueous leach solution, and both the acid and oxidising agent are consumed during a leach, it would be practical to have a mathematical relation describing this onset-level for all relevant acid strengths (ranges of acid content, pH-ranges) to provide the actual onset-level of oxidising agent at given (e.g. measured) acid content / strength in the aqueous leach solution in a method for regulating the content of oxidising agent and preventing adding so much oxidising agent to trigger this surface passivation effect of the nickel. Thus, the term “mathematical relation describing an onset-level, Conset,” as used herein refers to any mathematical relation which describes the acid strength relationship of the onset-level of oxidising agent. The invention is not tied to any specific method or way of obtaining a mathematical relation describing the onset-level of oxidising agent over a representative range of acid strengths but may apply any suited relation.

[0048] However, the dissolution kinetics, including the onset of such surface passivation film on nickel, is thoroughly studied and reported in the scientific literature, such as shown in e.g. Bilczuk et al. (2016) [Ref. 2], Thus in one embodiment, the person skilled in the art may derive a mathematical relation describing the onset level of oxidising agent over a representative range of acids strengths at given leach temperatures by performing a curve fitting using linear or non-linear regression analysis of literature reported data of the onset level of oxidising agent. The term “representative range of acids strengths” relates to the range of acid strengths being actual for the intended leach process from the initial “virgin” leach solution to the final pregnant leach solution, which may be determined as discussed above.

[0049] In one embodiment, the constant K is chosen from the interval from 0.2 to 0.98, and preferably from 0.3 to 0.97, more preferably from 0.4 to 0.96, more preferably from 0.5 to 0.95, more preferably from 0.6 to 0.94, more preferably from 0.7 to 0.93, more preferably from 0.8 to 0.92, and most preferably from 0.85 to 0.90.

[0050] Alternatively, in one embodiment, the mathematical relation may be derived by curve fitting using linear or non-linear regression analysis of onset data obtained by performing a set of empirical determinations of the contents / concentration levels of oxidizing agent (e.g. hydrogen peroxide) at which the passivation effect is observed over a representative range of acid strengths. Such empirical determination of the content of oxidising agent at which the surface passivation occurs is a matter of simple test runs mastered by the person skilled in the art.

[0051] In one embodiment, the nickel metal may advantageously be briquettes or cuttings of electrowon or electrorefined nickel having a characterising dimensions in the range of a thickness from 1 to 20 mm, preferably from 3 to 15 mm, more preferably from 4 to 10 mm, and most preferably from 5 to 7 mm, a length in the range of from 1 to 10 cm, and a width in the range of from 1 to 10 cm.

[0052] In one embodiment, the oxidising agent may advantageously be oxygen, ozone or hydrogen peroxide, preferably hydrogen peroxide.

[0053] The invention is not tied to any specific way of measuring the content / concentration of sulphuric acid in the aqueous leach solution, but may apply any known method conceivable to the person skilled in the art. Examples of suited methods for measuring the strength (content) of sulphuric acid in the aqueous leach solution includes refractometry measurements, sonic velocity measurements, titration with a strong base such as e.g. NaOH, etc.

[0054] Likewise, the invention is not tied to any specific way of measuring the content / - concentration of oxidising agent acid in the aqueous leach solution, but may apply any known method conceivable to the person skilled in the art. In the case of applying oxygen as the oxidising agent, examples of suited methods for measuring the content of oxidising agent in the aqueous leach solution includes iodometry titration (also known as Winkler’s method), colorimetry (using chemical reagents reacting with the dissolved oxygen forming a particular coloured compound and measuring the intensity of the colour), electroanalytical methods (using galvanic or polarographic probes measuring oxidation-reduction (redox) potentials), or luminescence (probe measuring a specific optical activity of dissolved oxygen). The invention is not tied to any specific way of regulating the feeding rate of oxidising agent to the aqueous leach solution based on measurements of the real concentration / content of the oxidising agent in the aqueous leach solution to obtain a obtain a set value of the concentration / content of the oxidising agent in the aqueous leach solution. Thus, the term “regulation algorithm” as used herein encompasses any regulation algorithm known to the person skilled in the art suitable for this purpose. Examples of regulation algorithms include PID-algorithm, feed forward algorithm, fuzzy logic control algorithm, process model-based control algorithm.

[0055] In one embodiment, the regulation algorithm may be considered a quasi on-off type algorithm which regulates the feed rate of oxidising agent by:

[0056] - start feeding oxidising agent at an initial feeding rate, and then

[0057] - more or less continuously regulating the addition of oxidising agent to the aqueous leach solution as follows: if

[0058] Coxid,real < 0.9 - Cset.actuai, increase the feeding oxidising agent by 1 0 % from the present feeding rate, or if

[0059] Coxid,real Cset. actual, reduce the feeding rate of oxidising agent by 25 % from the present feeding rate, or if

[0060] Coxid,real 1 -25 ' Cset, actual, stop feeding oxidising agent, and then start feeding Oxidising agent when Coxid,real < Cset, actual,

[0061] In the case of applying hydrogen peroxide as the oxidising agent, examples of suited methods for measuring the content of oxidising agent in the aqueous leach solution includes spectrophotometric measurements such as e.g. direct measurements of the absorbance at 240 nm of the H2O2 molecule, or monitoring the reaction of the peroxide with ferrous iron via a subsequent reaction with the dye xylenol orange and measurement of the absorbance of the solution at 550 nm, redox titration of hydrogen peroxide by potassium permanganate or by cerium sulphate.

[0062] In one particularly preferred embodiment of the method according to the first aspect of the invention applying hydrogen peroxide as the oxidising agent,

[0063] - the mathematical relation describing the onset-level, Conset, is defined to be: either:

[0064] Conset, actual 4 [g / 1] when Cacid,reai, is > 50 [g / 1], or else:

[0065] Conset, actual 0.38' Cacid,real°62[g / 1], and

[0066] - the set value of oxidising agent, Cset, is determined by the relation Cset, actual = K • Conset, actual, where K is in the range from 0.95 to 0.99.

[0067] This preferred embodiment is utilising the experimentally determined onset-levels of the hydrogen peroxide content at which the surface passivation of the nickel surface is observed for a set of test samples of the aqueous leach solution at acids strengths spanning from pH = 0.5 to pH = 2.

[0068] In a second aspect, the invention relates to a dissolution reactor 200 for manufacturing a nickel sulphate solution, comprising

[0069] - a first dissolution chamber 201-1,

[0070] - a first inlet 202-1 for an aqueous leach solution comprising sulphuric acid and optionally an oxidising agent,

[0071] - a first inlet 203-1 for a nickel metal

[0072] - a first inlet 204-1 for oxidising agent comprising a first pump 205-1, and

[0073] - a first outlet 206-1 for aqueous leach solution, characterised in that the dissolution reactor further comprises:

[0074] - a first acid strength determination module 207-1,

[0075] - a first oxidising agent content determination module 208-1,

[0076] - a first set of signal transferring lines 210, and

[0077] - a first logical control unit 211, and wherein

[0078] - the first set of signal transferring liens 210 connects the first logical control unit 211 to the first acid strength determination module 207-1, the first oxidising agent content determination module 208-1, and the first pump 205-1, and

[0079] - the first logical control unit 211 comprises a processor loaded with logic commands which when executed applies information provided by the first acid strength determination module 207-1 and the first oxidising agent content determination module 208-1 to run the first pump 205-1 to regulate via first inlet 204-1 the feed rate of oxidising agent to the aqueous leach solution as determined by the method according to the first aspect of the invention.

[0080] An example embodiment of the dissolution reactor according to the second aspect of the invention is shown schematically in figure 3. In this embodiment, the dissolution reactor is shown as a counter-current reactor by having the first inlet 202-1 for aqueous leach solution located at the bottom, the first outlet 206-1 for exiting aqueous leach solution and the inlet 203-1 for nickel located in the upper part of the reactor. However, this is not mandatory. The dissolution reactor according to the second aspect of the invention may also include co-current dissolution reactors. The term “logic control unit” as applied herein, encompasses any known and conceivable control unit able to regulate the first pump 205-1 based on the input data from the first acid strength determination module 207-1 and the first oxidising agent content determination module 208-1 to regulate via first inlet 204-1 the feed rate of oxidising agent to the aqueous leach solution as determined by the method according to the first aspect of the invention. Examples of suited logic control unit includes but is not limited to; a PID-controller, a feed-forward (open loop) controller, a fuzzy logic controller, a process-model based controller, or combinations thereof.

[0081] In one embodiment, the first logical control unit 211 may further comprise a user interface enabling an operator to feed the control unit with externally determined sulphuric acid strength and / or content of oxidising agent determined by e.g. titration with a strong base such as e.g. NaOH to determine acid strength and / or by a titration based on reaction of the peroxide with ferrous iron via a subsequent reaction with the dye xylenol orange and measurement of the absorbance of the solution at 550 nm, and / or redox titration of hydrogen peroxide by potassium permanganate or by cerium sulphate. In this case, the first acid strength determination module 207-1 and / or the first oxidising agent content determination module 208-1 may simply be a sample taking device which extracts a sample of the aqueous leach solution from the dissolution reactor.

[0082] Alternatively, in one embodiment, the first acid strength determination module 207- 1 may comprise any known sensor or probe able to monitor / measure the content / strength of the sulphuric acid, such as e.g. by utilising refractometry measurements and / or sonic velocity measurements. Likewise, in the case of applying hydrogen peroxide as oxidising agent, the first oxidising agent content determination module 208-1 may in one embodiment comprise any known sensor or probe able to monitor / measure the content of hydrogen peroxide in the aqueous leach solution, such as by e.g. spectrophotometry measuring the absorbance at 240 nm of the H2O2 molecule, and / or monitoring the reaction of the peroxide with ferrous iron via a subsequent reaction with the dye xylenol orange and measurement of the absorbance of the solution at 550 nm.

[0083] In one embodiment, the dissolution reactor 200 according to the second aspect of the invention may further comprise: a container 100 having a wall 101 and a bottom plate 102 but being open in its upper end 111, an abrasive and corrosion resilient basket 103 constituting the first dissolution chamber 201-1 located inside the container 100 such that it rests on the inner surface of the bottom plate 102 and extends a first distance upwards inside the container 100, wherein the basket 103 comprises a perforated plate 104 covering its horizontal cross-sectional area and which is located a second distance above its lover end, and where the second distance is less than the first distance, a fluid inlet 105 adapted to inject a liquid into the dissolution chamber 60 of the container 100 from below and into a space confined between the bottom plate 102, a lower part of the basket 103, and the perforated plate 104, a fluid outlet 106 adapted to extract liquid from the dissolution chamber 60 of the container 100 through the wall 101 at a height being at least the same height at which the upper end 108 of the basket 103 extends inside the container, a funnel 107 adapted to be suspended from the upper end of the container 100 and being tapered and pointing towards the bottom plate 102, and which extends a third distance downwards into the container such that the narrow lower end 109 of the funnel 107 is below the upper end 108 of the basket 103, and a removable lid 110 adapted to cover the upper end 111 of the container 100 and wherein the lid is adapted to be connected to a gas evacuation 112 for extracting eventual gases being formed inside the dissolution reactor, and wherein the first acid strength determination module 207-1 and the first oxidising agent content determination module 208-1 are located in the abrasive and corrosion resilient basket 103.

[0084] In one embodiment, the container 100 is made of a metal, preferably a stainless- steel alloy. However, any material having the mechanical strength to carry and hold the solid to be dissolved and the aqueous leach solution may be applied. In one embodiment, the inner wall of the container 100 may be lined with a corrosion resistant lining, such as e.g. a rubber, a polyethylene, a polytetrafluoroethylene, or a vinyl ester.

[0085] In one embodiment, the abrasive and corrosion resistant basket 103 and the perforated bottom plate 104 is made of a polyethylene, a polyvinyl, a vinyl ester, or a polypropylene.

[0086] In one embodiment, the lid 110 and / or the upper part 111 of the container 100 may comprise one or more openings allowing false air to enter inside the lid and dilute eventual gases developed inside the dissolution reactor.

[0087] In a third aspect, the invention relates to a plant for manufacturing a nickel sulphate solution, characterised in that the plant comprises:

[0088] - a dissolution reactor 200 according to the second aspect of the invention, wherein

[0089] - the first inlet 202-1 for an aqueous leach solution is located at the lower part of the first dissolution chamber 201-1,

[0090] - the first inlet 203-1 for a nickel metal is located at an upper part of the first dissolution chamber 201-1, and

[0091] - the first outlet 206-1 for aqueous leach solution is located below the upper inlet 3 and above the lower inlet 2 at the first dissolution chamber 201- 1, and wherein the plant comprises further:

[0092] - a first storage container 5,

[0093] - a second storage container 6,

[0094] - a first liquid conduit 10 connected in one end to the first inlet 202-1 and in a second end opposite the first end to a lower end of the first 5 and the second 6 storage containers, wherein the first liquid conduit 10 comprises a first pump 11, a first valve 12 regulating the flow of aqueous leach solution from the first storage container 5 into the first liquid conduit 1, and a second valve 13 regulating the flow of aqueous leach solution from the second storage container 6 into the first liquid conduit 10,

[0095] - a second liquid conduit 20 connected in one end to the first outlet 206-1 of the first dissolution chamber 201-1 and in a second end opposite the first end to an upper end of the first 5 and the second 6 storage containers, where the second liquid conduit 20 comprises a third valve 21 regulating the flow of aqueous leach solution from the second liquid conduit 20 into the first storage container 5 and a fourth valve 22 regulating the flow of aqueous leach solution from the second liquid conduit 20 into the second storage container 6,

[0096] - a third liquid conduit 30 connected in one end to the lower end of the first 5 and the second 6 storage containers and in a second end opposite the first end to a downstream product handling facility 9, where the third liquid conduit 30 comprises a fifth valve 31 regulating the flow of aqueous leach solution from the first storage container 5 to the downstream product handling facility 9, and a sixth valve 32 regulating the flow of aqueous leach solution from the second storage container 6 to the downstream product handling facility 9, and

[0097] - a fourth liquid conduit 40 connected in one end to the upper end of the first 5 and the second 6 storage containers and in a second end opposite the first end to an upstream supply of lean aqueous leach solution 8, where the fourth liquid conduit 40 comprises a seventh valve 41 regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution 8 to the first storage container 5, and an eight valve 42 regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution 8 to the second storage container 6.

[0098] In one embodiment, the plant according to the third aspect of the invention may advantageously further comprise a third storage container 7, wherein: the first liquid conduit 10 further connects a lower end of the third 7 storage container to the first inlet 202-1 of the first dissolution chamber 201-1 of the dissolution reactor 200 and comprises a ninth valve 14 regulating the flow of aqueous leach solution from the third storage container 7 into the first liquid conduit 10, the second liquid conduit 20 further connects the first outlet 206-1 of the first dissolution chamber 201-1 of the dissolution reactor 1 to an upper end of the third 7 storage container and comprises a tenth valve 23 regulating the flow of aqueous leach solution from the second liquid conduit 20 into the third storage container 7, the third liquid conduit 30 further connects the lower end of the third 7 storage container to the downstream product handling facility 9, where the third liquid conduit 30 comprises an eleventh valve 33 regulating the flow of aqueous leach solution from the third storage container 7 to the downstream product handling facility 9, and the fourth liquid conduit 40 further connects the upper end of the third 7 storage container to the upstream supply of lean aqueous leach solution 8, where the fourth liquid conduit 40 comprises a twelfth valve 43 regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution 8 to the third storage container 7.

[0099] The plant according to the third aspect of the invention is a further development of the plant described in the co-pending patent application PCT / EP2023 / 058389, which is included herein in its entirety. The further development consists mainly of the incorporation of the dissolution reactor according to the second aspect of the present invention which contains a logical control unit regulating the content of oxidising agent in the dissolution reactor and thus optimises the leach rates according to the method of the first aspect of the present invention.

[0100] The plant described in the co-pending patent application PCT / EP2023 / 058389 has the advantage of enabling making a batch-wise leach process (which are generally considerably faster than continuous leach processes due to allowing using higher acid strengths in the initial phases of the process due to no risk of having too strong acid exiting the leach column and pass to end-uses not tolerating high remains of free acid in the solution) becoming semicontinuous or continuous by alternately switching between at least two storage containers, one being engaged in producing a pregnant aqueous leach solution and the other being prepared for a coming dissolution cycle by being emptied of pregnant aqueous leach solution (from the previous batch) and re-filled with lean aqueous leach solution and vice-versa. This arrangement makes the dissolution process in the dissolution reactor run practically continuously and uninterrupted despite the batch-wise nature of the production of pregnant aqueous leach solution due to a fast and almost immediate switch from the enriching of one batch of aqueous leach solution with dissolved nickel sulphate to the next. Also, by using three or more storage containers, the batchwise production of pregnant aqueous leach solution may deliver a continuous and uninterrupted flow of pregnant aqueous leach solution to an end-user. For further description of the advantages, see especially from line 28 on page 2 to line 34 on page 4, further from line 33 on page 7 to line 2 on page 10 of PCT / EP2023 / 058389. The term “enriching” as used herein means an increase in the concentration of the solute / dissolved nickel sulphate in the aqueous leach solution caused by the leach / dissolution of the nickel metal. The term “pregnant or rich aqueous leach solution” as used herein means an aqueous leach solution having a high concentration of dissolved nickel sulphate, typically at a concentration level being acceptable as the intended nickel sulphate product. The term “lean aqueous leach solution” as used herein means an aqueous leach solution containing no or only a relatively small concentration of dissolved nickel sulphate.

[0101] It may be advantageous for large scale production that the dissolution reactor comprises a plurality of dissolution chambers to increase the volume rates of produced rich aqueous leach solution. This may be envisioned both for serially connected dissolution reactors or parallel connected dissolution chambers, or a combination thereof. An example embodiment comprising two serially connected dissolution chambers is schematically shown in figure 7.

[0102] Thus, in one embodiment, the process plant according to the third aspect of the invention, the dissolution reactor may further comprise:

[0103] - a second dissolution chamber 201-2 comprising a lower inlet second 202-2 for an aqueous leach solution, an upper inlet 203-2 for nickel, and an outlet 204-2, for aqueous leach solution located below the inlet 203-2 and above the second inlet 202-2, and

[0104] - a second inlet 204-2 for oxidising agent comprising a second pump 205-1, and

[0105] - a second outlet 206-2 for aqueous leach solution,

[0106] - second an acid strength determination module 207-2, and

[0107] - a second oxidising agent content determination module 208-2, wherein

[0108] - the first inlet 202-1 of the first dissolution chamber 201-1 is connected to the first liquid conduit 10,

[0109] - the outlet 204-1 of the first dissolution chamber 201-1 is connected to the second inlet 202-2 of the second dissolution chamber 201-2, and

[0110] - the outlet 204-2 of the second dissolution chamber 201-2 is connected to the second liquid conduit 20.

[0111] In one embodiment, the process plant according to the third aspect of the invention may further comprise a series of four, five, six, seven or eight dissolution chamber, each having a lower inlet for a aqueous leach solution, an upper inlet for solid compound, and an outlet for aqueous leach solution located below the inlet and above the inlet as described above, and which is serially interconnected by having the outlet of each dissolution chamber except the last of the series connected to the inlet of the next dissolution chamber, and where the inlet of the first dissolution chamber of the series is connected to the first liquid conduit and the outlet of the last dissolution chamber of the series is connected to the second liquid conduit.

[0112] In one example embodiment, the process plant according to the third aspect of the invention may further comprise a heat exchanger 50 located either in the first 10 or the second 20 liquid conduit for tempering the aqueous leach solution being enriched. Depending on the compound to be dissolved and aqueous leach solution being applied, the aqueous leach solution may advantageously be heated or cooled by the heat exchanger. For example, exothermic dissolution of metals may require cooling the aqueous leach solution passing through the heat exchanger.

[0113] In one embodiment, the process plant according to third aspect of the invention, each of the first to the twelfth valves may advantageously be actuator controlled valves regulated by a second logical controller unit 53 loaded with logic commands which, when executed, controls and regulates the actuators of the first to the twelfth valves such that the process plant is made to execute the method according to the first or the second aspect of the invention.

[0114] In one embodiment, the first 12, second 13, third 21, fourth 22, fifth 31, sixth 32, seventh 41 and the eighth 42, and if present, the ninth 14, tenth 23, eleventh 33, twelfth 43, thirteenth 15, fourteenth 16, and the fifteenth 17 valves are actuator- controlled valves, and the first 11, and if present, the second 24 aqueous leach solution pumps are actuator controlled pumps. The term “actuator-controlled valve” as used herein encompasses any known and conceivable valve comprising an actuator enabling automatically shutting-off and opening a conduit from zero to full through-flow of fluid in the conduit. The valve may advantageously e.g. be a throttle valve. The actuator may advantageously be electrically driven. The term “actuator-controlled pump” as used herein encompasses any known and conceivable pump able to pump a liquid in a liquid conduit.

[0115] List of figures

[0116] Figure 1 is a diagram presenting empirically determined onset-levels of hydrogen peroxide over a range of acid strengths given as pH causing a surface passivation of nickel in an aqueous leach solution comprising sulphuric acid and hydrogen peroxide.

[0117] Figure 2 is similar diagram as given in figure 1, but which also presents an example of the maximum level of hydrogen peroxide to be obtained by the regulation method according to the present invention.

[0118] Figure 3 is a drawing schematically illustrating an example embodiment of a dissolution reactor according to the present invention. Figure 4 is a drawing schematically illustrating another example embodiment of a dissolution reactor according to the present invention.

[0119] Figure 5 is a drawing schematically illustrating an example embodiment of a process plant according to the invention utilising one dissolution chamber and two storage containers.

[0120] Figure 6 is a drawing schematically illustrating another example embodiment of a process plant according to the third aspect of the invention utilising one dissolution chamber and two storage containers.

[0121] Figure 7 is a drawing schematically illustrating an example embodiment of a process plant according to the invention utilising two dissolution chambers connected in series and two storage containers.

[0122] Example embodiments of the invention

[0123] The invention will be described in more detail by way of an example embodiment of a process plant according to the third aspect of the invention intended for dissolving cuttings of electrolytic nickel in sulphuric acid.

[0124] The example embodiment is illustrated schematically in figure 5. As seen on the figure, the process plant applies to storage containers 5, 6 being connected to a dissolution reactor. The dissolution reactor is similar to the dissolution reactor 200 shown in figure 4. In this example embodiment, the container 101 is shaped into a vertically standing cylinder made of S325 structural steel having an inner diameter of 70 cm and a total inner height of 2 m (from the inner side of the bottom 102 to the top end 111). The inner wall of the steel container is coated with a 3-4 mm thick layer of rubber. A cylindrical basket 103 of outer diameter 65 cm and height of 160 cm is placed coaxially inside the container 101. The basket 103 is made of a 1 cm thick polyester polymer and has a perforated, with a plurality of 1 mm (|) throughgoing holes, plate 104 covering the horizontal inner cross-section of the basket at 5 cm up from its lower end, and thus forming a relatively small chamber confined between the inner surface of the bottom 102 of the container 101, the lower surface of the perforated plate 104 and the lower part of the inner wall of the basket 103. A fluid inlet 105, 202-1 penetrates the bottom plate 102 and is adapted to inject aqueous leach solution into the said chamber where it will fill the chamber and flow through the perforations and further upwards in the gasket 103. The basket 103 provides the advantage that metal cuttings will be prevented from coming in mechanical contact with the inner surface / lining of the steel container 101 and will thus represent no danger of abrasive wear on the container.

[0125] A funnel 107 having 10 mm thick wall of polyester polymer with an inner diameter of 60 cm at its upper end and an inner diameter of 40 cm at its lower end, is suspended coaxially from the top end 111 of the container 101 and protrudes downward a distance of 55 cm into the inner space of the cylindrical container 101. This makes the lower end 109 of the funnel to downwardly protrude approx. 5 cm into the upper part 108 of the basket 103. This has the advantage that metal cuttings fed through the funnel will enter and fall into the basket 103 without being in mechanical contact with the steel container.

[0126] The dissolution reactor 200 comprises further a first acid strength determination module 207-1 and a first oxidising agent content determination module 208-1 which measures the acid strength and content of oxidising agent, respectively and feed the information to a first logical controller unit 211 comprising a processor loaded with logic commands which when executed applies information provided by the first acid strength determination module 207-1 and the first oxidising agent content determination module 208-1 to run the first pump 205-1 to regulate via first inlet 204-1 the feed rate of oxidising agent to the aqueous leach solution as determined by the method according to the first aspect of the invention.

[0127] The dissolution reactor will typically be made ready for a series of dissolution cycles by filling the entire inner space of the basket and funnel with metal cuttings. The metal cuttings may e.g. be 1 about cm thick and a few cm of length and width and will fill the inner section of the cylindrical container from bottom to top. A fluid outlet 106, 206-1 is located at a height of 160 cm (from the bottom plate 102) and will make aqueous leach solution flowing up through the container 101 to exit the dissolution reactor at about the upper end 108 of the basket 103. Thus, metal cuttings being inside the funnel being above the upper end of the basket being dry. They will only be exposed to the aqueous leach solution when sinking below the fluid level of the aqueous leach solution.

[0128] The dissolution of nickel in an aqueous sulphuric acid solution produces hydrogen. This is potentially hazardous. Hydrogen gas is highly explosive at certain stoichiometric ratios with oxygen gas (in the air). The dissolution reactor is therefore equipped with a pivotally hinged lid 110 which covers the upper end of the cylindrical container 101 and is in fluid connection with a fan operated gas evacuation 112. The lid 100 may be opened to allow filling of metal cuttings.

[0129] If the plant is to produce solved nickel at a solute level of 100 g Ni per litre aqueous leach solution, the lean acid solution (no dissolved nickel) may advantageously have a sulphuric acid concentration of 170 - 175 g per litre. This corresponds to a 1.75 - 1.80 molar sulphuric acid solution. When the aqueous leach solution is enriched to its intended solute level of 100 g / 1, the rest concentration of the sulphuric acid will be approx. 0.1 molar.

[0130] The operation of the process plant may be as follows: At start up, the dissolution chamber 113 is filled with nickel cuttings and both storage containers 5, 6 and the dissolution chamber 113 may be empty of aqueous leach solution. In this case, the dissolution process may be initiated by the logical controller opening e.g. the seventh valve 41 to fill the first storage container 5 with lean aqueous leach solution (sulphuric acid) from the acid supply 8, and then closing the seventh valve 41 and opening the first 12 and the third valves 21 and engage pump 11 to circulate the aqueous leach solution through the nickel cuttings filled space of the dissolution reactor. The aqueous leach solution is circulated through the dissolution chamber 113, 201-1 until the first acid strength determination module 207-1 reports that the aqueous leach solution has reached its intended solute level of 100 g nickel per litre aqueous leach solution. Then the first dissolution cycle is terminated by the second logical controller unit 53 shutting valves 12 and 21 to disengage the first storage container.

[0131] In the meantime, the second logical controller unit 53 has prepared the second storage container by opening valve 42 to fill the storage container with lean aqueous leach solution and then close valve 42. The second storage container is therefore ready to start the second dissolution cycle the moment the first dissolution cycle terminates by simultaneously opening valves 13 and 22 when valves 12 and 21 are being closed. In this manner, the flow of aqueous leach solution through the dissolution reactor is made continuous.

[0132] While the second storage container 6 is occupied with executing the second dissolution cycle, the first storage container 5 is made ready for the third dissolution cycle by the second logical controller unit 53 opening valve 31 to empty the first storage container for rich aqueous leach solution which is passed to a downstream product handling facility 9, and then closing valve 31 and opening valve 41 to refill the first storage container with lean aqueous leach solution and then closing valve 41.

[0133] In this manner the dissolution process is made continuous by interchanging between applying the first and second storage container to enrich the acids aqueous leach solution.

[0134] List of reference numbers

[0135] 5 first storage container

[0136] 6 second storage container

[0137] 7 third storage container

[0138] 8 supply of lean aqueous leach solution

[0139] 9 product handling facility

[0140] 10 first liquid conduit

[0141] 11 first aqueous leach solution pump

[0142] 12 first valve

[0143] 13 second valve 14 ninth valve 15 thirteenth valve 16 fourteenth valve 17 fifteenth valve 20 second liquid conduit 21 third valve 22 fourth valve 23 tenth valve 24 second aqueous leach solution pump 30 third liquid conduit 31 fifth valve 32 sixth valve 33 eleventh valve 40 fourth liquid conduit 41 seventh valve 42 eighth valve 43 twelfth valve 50 heat exchanger 51 inlet for adding additives 52 aqueous leach solution strength monitoring unit 53 second logical control unit 100 container 101 container wall 102 container bottom plate 103 corrosion resilient basket 104 perforated plate 105 fluid inlet 106 fluid outlet 107 funnel 108 upper end of the basket 109 lower end of the funnel 110 lid 111 upper end of the container 112 gas evacuation 113 dissolution chamber 201-1 first dissolution chamber

[0144] 201-2 second dissolution chamber

[0145] 202-1 first inlet for an aqueous leach solution

[0146] 202-2 second inlet for an aqueous leach solution

[0147] 203-1 first inlet for nickel metal

[0148] 203-2 second inlet for a nickel metal

[0149] 204-1 first inlet for oxidising agent 204-2 second inlet for oxidising agent

[0150] 205-1 first pump

[0151] 205-2 second pump

[0152] 206-1 first outlet for aqueous leach solution

[0153] 206-2 second outlet for aqueous leach solution

[0154] 207-1 first acid strength determination module

[0155] 207-2 second acid strength determination module

[0156] 208-1 first oxidising agent content determination module

[0157] 208-2 second oxidising agent content determination module

[0158] 210 first set of signal transferring lines

[0159] 211 first logical controller unit

[0160] References

[0161] 1 Jenssen et al. (2018), “Filtration Properties of Ferric Hydroxide Precipitate in Nickel Production”, In: Davis, B., et al. Extraction 2018. The Minerals, Metals & Materials Series. Springer, Cham. https: / / doi.org / 10.1007 / 978-3-319-95022-8_112

[0162] 2 Bilczuk et al. (2016), “Kinetic study of the dissolution of metallic nickel in sulphuric acid solutions in the presence of different oxidants”, The Canadian Journal of Chemical Engineering, Vol. 94, October 2016, pp. 1872-1879, https: / / doi.org / 10.1002 / cjce.22576

[0163] 3 Abegg et al. (1937), “Handbuch der Anorganischen Chemie”, Teil 4, Nickel und seine Verbindungen, Nature 140, 1081 (1937), Seite 358, https: / / doi.org / 10.1038 / 1401081a0

Claims

CLAIMS1. A method for preparing a nickel sulphate solution, wherein the method comprises- executing a leach process by bringing nickel metal in contact with an aqueous leach solution comprising sulphuric acid and an oxidising agent, and wherein the oxidising agent is continuously added to the aqueous leach solution, characterised in that the method further comprises:- providing a mathematical relation describing, as a function of sulphuric acid content in the aqueous leach solution, an onset-level, Conset, of oxidising agent content in the aqueous leach solution causing a nickel surface passivation effect, and- more or less continuously regulating the addition of oxidising agent to the aqueous leach solution by performing each of steps 1) to 4) in successive order:1) measuring a real content of sulphuric acid, Cacid,reai, and a real content of oxidising agent, Coxid,reai, present in said aqueous leach solution,2) applying said mathematical relation to determine an actual onsetlevel, Conset, actual, at, Cacid,real,3) defining a set-value, Cset, actual, of the content of oxidising agent to be obtained in the aqueous leach solution from the relation:Cset. actual K ' Conset. actual, where K is a constant chosen from the interval from 0.1 to 0.99, and4) applying a regulation algorithm to regulate the feeding rate of oxidising agent to the aqueous leach solution aimed at obtaining the setvalue, Cset, actual.

2. The method according to claim 1, wherein the regulation of the feeding rate of oxidising agent is made at time intervals being in the range of from 5 seconds to 15 minutes, preferably from 10 seconds to 10 minutes, more preferably from 15 seconds to 5 minutes, and most preferably from 30 seconds to 2 minutes.

3. The method according to claim 1 or 2, wherein the regulation algorithm is either a PID-algorithm, a feed forward algorithm, a fuzzy logic control algorithm, or a process model-based control algorithm, or the regulation algorithm regulates the feed rate of oxidising agent by:- start feeding oxidising agent at an initial feeding rate, and then- more or less continuously regulating the addition of oxidising agentto the aqueous leach solution as follows: ifCoxid,real < 0.9 Cset, actual, increase the feeding oxidising agent by 10 % from the present feeding rate, or ifCoxid,real Cset, actual, reduce the feeding rate of oxidising agent by 25 % from the present feeding rate, or ifCoxid,real 1 -25 ' Cset, actual, stop feeding oxidising agent, and then start feeding oxidising agent when Coxid,reai < Cset, actual,4. The method according to any of the preceding claims, wherein the mathematical relation describing, as a function of sulphuric acid content in the aqueous leach solution, the onset-level, Conset, of oxidising agent content in the aqueous leach solution is obtained by curve fitting using linear or non-linear regression analysis of either:- data reported in the literature of the onset-level, Conset, of oxidising agent, or:- data of the onset-level, Conset, of oxidising agent obtained by a set of empirical determinations of the concentration levels of oxidizing agent at which nickel becomes surface passivated over a representative range of acid strengths.

5. The method according to any of the preceding claims, wherein the nickel metal is cuttings of electrowon or electrorefined nickel having dimensions selected from a thickness from 1 to 20 mm, preferably from 3 to 15 mm, more preferably from 4 to 10 mm, and most preferably from 5 to 7 mm, a length in the range of from 1 to 10 cm, and a width in the range of from 1 to 10 cm.

6. The method according to any of the preceding claims, wherein the oxidising agent is either oxygen, ozone, or hydrogen peroxide, preferably hydrogen peroxide.

7. The method according to any of the preceding claims, wherein- the measuring of the content of sulphuric acid, Cacid,reai, is obtained by either refractometry measurements, sonic velocity measurements, or titration with NaOH, and if the oxidising agent is dissolved oxygen:- the measuring of the content of oxidising agent, Coxid, is obtained by either iodometry titration, colorimetry, galvanic or polarographic probes measuring oxidation-reduction potentials, or luminescence, or if the oxidising agent is hydrogen peroxide:- the measuring of the content of oxidising agent, Coxid, is obtained by eitherspectrophotometric measurements of the absorbance at 240 nm, or redox titration using potassium permanganate or cerium sulphate.

8. The method according to any of the preceding claims, wherein the constant K is in the interval from 0.2 to 0.98, and preferably from 0.3 to 0.97, more preferably from 0.4 to 0.96, more preferably from 0.5 to 0.95, more preferably from 0.6 to 0.94, more preferably from 0.7 to 0.93, more preferably from 0.8 to 0.92, and most preferably from 0.85 to 0.90.

9. The method according to any of the preceding claims, wherein- the oxidising agent is hydrogen peroxide, and- the mathematical relation describing the onset concentration-level, Conset, is defined to be: either:Conset = 4 g / 1 when Cacid.reai is measured to be > 50 g / 1, or else:Conset = 0.38 - Cacid,reai°62g / 1, where Cacid.reai is given in units g / 1, and- the set value of oxidising agent, Cset, actual, is determined by the relation Cset, actual = K • Conset, actual where K is in the range from 0.95 to 0.99.

10. A dissolution reactor (200) for manufacturing a nickel sulphate solution, comprising- a first dissolution chamber (201-1),- a first inlet (202-1) for an aqueous leach solution comprising sulphuric acid and optionally an oxidising agent,- a second inlet (203-1) for a nickel metal- a third inlet (204-1) for oxidising agent comprising a pump (205-1), and- a first outlet (206-1) for aqueous leach solution, characterised in that the dissolution reactor further comprises:- an acid strength determination module (207-1),- an oxidising agent content determination module (208-1),- a first set of signal transferring lines (210), and- a first logical control unit (211), and wherein- the first set of signal transferring lines (210) connects the first logical control unit (211) to first the acid strength determination module (207-1), the oxidising agent content determination module (208-1), and the pump (205-1), and- the first logical control unit (211) comprises a processor loaded with logic commands which when executed applies information provided by the first acid strength determination module (207-1) and the oxidising agent contentdetermination module (208-1) to run the pump (205-1) to supply via inlet (204-1) an amount of oxidising agent as determined by the method according to the first aspect of the invention.

11. The dissolution reactor according to claim 10, wherein the dissolution reactor (200) further comprises: a container (100) having a wall (101) and a bottom plate (102) but being open in its upper end (111), an abrasive and corrosion resilient basket (103) constituting the first dissolution chamber (201-1) and being open at its bottom end and at its top end, and being located inside the container (100) such that it rests on the inner surface of the bottom plate (102) and extends a first distance upwards inside the container (100), wherein the basket (103) comprises a perforated plate (104) covering its horizontal cross-sectional area and which is located a second distance above its lover end, and where the second distance < the first distance, a fluid inlet (105) adapted to inject a liquid into the container (100) from below and into a space confined between the bottom plate (102), a lower part of the basket (103), and the perforated plate (104), a fluid outlet (106) adapted to extract liquid from the container (100) through the wall (101) at a height being at least the same height at which the upper end (108) of the basket (103) extends inside the container, a funnel (107) adapted to be suspended from the upper end of the container (100) and being tapered and pointing towards the bottom plate (102), and which extends a third distance downwards into the container such that the narrow lower end (109) of the funnel (107) is below the upper end (108) of the basket (103), and a removable lid (110) adapted to cover the upper end (111) of the container (100) and wherein the lid is adapted to be connected to a gas evacuation (112) for extracting eventual gases being formed inside the dissolution reactor, and wherein the first acid strength determination module (207-1) and the oxidising agent content determination module (208-1) are located in the abrasive and corrosion resilient basket 103.

12. The dissolution reactor according to claim 11, wherein the container (100) is made of a metal, preferably a stainless-steel alloy, and where the inner wall of the container (100) is lined with a corrosion resistant lining chosen from one of; a rubber, a polyethylene, a polytetrafluoroethylene, or a vinyl ester.

13. The dissolution reactor according to claim 11 or 12, wherein the corrosion resistant basket (103) and the perforated bottom plate (104) is made of a polyethylene, a polyvinyl, a vinyl ester, or a polypropylene.

14. The dissolution reactor according to any one of claims 11 to 13, wherein the lid (110) and / or the upper part (111) of the container (100) may comprise one or more openings allowing false air to enter inside the lid.

15. The dissolution reactor according to any one of claims 10 to 14, wherein the logic control unit is a PID-controller, a feed-forward open loop controller, a fuzzy logic controller, a process-model based controller, or combinations thereof.

16. A plant for manufacturing a nickel sulphate solution, characterised in that the plant comprises:- a dissolution reactor (200) according to anyone of claims 10 to 15, wherein- the first inlet (202-1) for an aqueous leach solution is located at the lower part of the first dissolution chamber (201-1),- the second inlet (203-1) for a nickel metal is located at an upper part of the first dissolution chamber (201-1), and- the first outlet (206-1) for aqueous leach solution is located below the upper inlet (3) and above the lower inlet (2) at the first dissolution chamber (201-1), and wherein the plant comprises further:- a first storage container (5),- a second storage container (6),- a first liquid conduit (10) connected in one end to the first inlet (202-1) and in a second end opposite the first end to a lower end of the first (5) and the second (6) storage containers, wherein the first liquid conduit (10) comprises a first pump (11), a first valve (12) regulating the flow of aqueous leach solution from the first storage container (5) into the first liquid conduit (10), and a second valve (13) regulating the flow of aqueous leach solution from the second storage container (6) into the first liquid conduit (10),- a second liquid conduit (20) connected in one end to the outlet (206-1) of the first dissolution chamber (201-1) and in a second end opposite the first end to an upper end of the first (5) and the second (6) storage containers, where the second liquid conduit (20) comprises a third valve (21) regulating the flow of aqueous leach solution from the second liquid conduit (20) into the first storage container (5) and a fourth valve (22) regulating the flow of aqueous leach solution from the second liquid conduit (20) into the second storage container (6),- a third liquid conduit (30) connected in one end to the lower end of the first (5) and the second (6) storage containers and in a second end opposite the first end to a downstream product handling facility (9), where the third liquid conduit (30) comprises a fifth valve (31) regulating the flow of aqueous leach solution from the first storage container (5) to the downstream product handling facility (9), and a sixth valve (32) regulating the flow of aqueous leach solution from the second storage container (6) to the downstream product handling facility (9), and- a fourth liquid conduit (40) connected in one end to the upper end of the first (5) and the second (6) storage containers and in a second end opposite the first end to an upstream supply of lean aqueous leach solution (8), where the fourth liquid conduit (40) comprises a seventh valve (41) regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution (8) to the first storage container (5), and an eight valve (42) regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution (8) to the second storage container (6).

17. A plant according to claim 16, wherein the plant further comprises a third storage container (7), and wherein: the first liquid conduit (10) further connects a lower end of the third (7) storage container to the inlet (2) of the first dissolution chamber (60) and comprises a ninth valve (14) regulating the flow of aqueous leach solution from the third storage container (7) into the first liquid conduit (10), the second liquid conduit (20) further connects the outlet (4) of the first dissolution chamber (60) to an upper end of the third (7) storage container and comprises a tenth valve (23) regulating the flow of aqueous leach solution from the second liquid conduit (20) into the third storage container (7), the third liquid conduit (30) further connects the lower end of the third (7) storage container to the downstream product handling facility (9), where the third liquid conduit (30) comprises an eleventh valve (33) regulating the flow of aqueous leach solution from the third storage container (7) to the downstream product handling facility (9), and the fourth liquid conduit (40) further connects the upper end of the third (7) storage container to the upstream supply of lean aqueous leach solution (8), where the fourth liquid conduit (40) comprises a twelfth valve (43) regulating the flow of aqueous leach solution from the upstream supply of lean aqueous leach solution (8) to the third storage container (7).

18. A plant according to anyone of the preceding claims, wherein the plant further comprises a second dissolution reactor comprising:- a second dissolution chamber (201-2) comprising a lower second inlet (202-2) for an aqueous leach solution, an upper inlet (203-2) for nickel, and an outlet (204-2), for aqueous leach solution located below the inlet (203-2) and above the second inlet (202-2), and- a fourth inlet (204-2) for oxidising agent comprising a pump (205-2), and- a second outlet (206-2) for aqueous leach solution,- a second acid strength determination module (207-2), and- a second oxidising agent content determination module (208-2).