FI128928B

FI128928B - Hydrogen sulphide in process solutions

Info

Publication number: FI128928B
Application number: FI20176005A
Authority: FI
Inventors: Jari Kunnila; Lasse Ilves
Original assignee: Skyscape Oy
Priority date: 2017-11-10
Filing date: 2017-11-10
Publication date: 2021-03-31
Also published as: FI20176005A1

Abstract

According to an example aspect of the present invention, there is provided a method of using hydrogen sulphide dissolved in a first process solution (100) comprising metal ions for the recovery of at least two metal sulphides from said first process solution (100) comprising the steps of adding a reagent (210) to increase pH of the first process solution to provide a precipitate (260) comprising at least two metal sulphides, further adding said reagent (210) to further increase the pH of the first process solution (100) until precipitation ceases, recovering the precipitate (260) comprising at least two metal sulphides from the first process solution to provide a second process solution essentially free of hydrogen sulphide, and dissolving the precipitate comprising at least two recovered metal sulphides in an acidic solution for further processing and recovery.

Description

HYDROGEN SULPHIDE IN PROCESS SOLUTIONS FIELD

[0001] The present invention relates to a method of recycling hydrogen sulphide for the recovery of metal sulphides from a process solution.

BACKGROUND

[0002] In metal recovery processes carried out on solutions comprising sulphide ions, in particular solutions comprising hydrogen sulphide, it is typical that sulphides are precipitated in a sulphide precipitation by increasing pH to levels for specific metal sulphides to precipitate, e.g. in a solution comprising Cu” and Ni”, Cus is precipitated at a lower pH than NIS. In typical solutions, after the last metal is precipitated at its optimum pH, a relatively large amount of hydrogen sulphide is typically still dissolved in the solution. Depending on the target metal, it is necessary to stop the precipitation process at a certain pH value as beyond that value increasingly high amounts of impurities are formed in the precipitate. Precipitation is typically stopped by reducing the alkalinity of the solution, e.g. by ceasing the addition of alkaline agent to the solution.

[0003] For example US Patent 4110400 discloses a method of precipitating nickel sulphide from an acidic nickel sulphate solution having a pH in the range of about ~ 1.5 to 4 and a soluble inert sulphate salt which provides sulphate as an acid buffer N by combining with hydrogen ions formed during precipitation of nickel sulphide to = form HSO4". The method comprises, adding a slurry of finely divided nickel 2 sulphide to said solution to provide a seed concentration, and then subjecting the E 25 solution to sulphide precipitation with H,S under pressure at a temperature of about O 65 °C to less than 100 °C while vigorously agitating said solution until substantially 3 complete sulphide precipitation of nickel is obtained. N [0004] On stopping the precipitation, the hydrogen sulphide content of the solution is high. In a typical process hydrogen peroxide or other strong oxidising agents are introduced to the solution to destroy hydrogen sulphide, e.g. hydrogen sulphide is reacted with hydrogen peroxide to form water and sulphur. This is expensive and results in the formation of elemental sulphur causing further problems in the process. In addition to the costs of hydrogen peroxide or other strong oxidising agents, the cost of lost hydrogen sulphide can be substantial.

[0005] In cases where solutions comprise ferric iron, hydrogen sulphide is consumed in a reaction whereby ferric iron is reduced to ferrous iron and elemental sulphur is formed in the solution. Elemental sulphur needs to be handled or it will otherwise end up in the first target metal sulphide product as an impurity. Thus, the presence of ferric iron in the solution causes problems that elemental sulphur is formed and must be handled, and hydrogen sulphide intended for use in the precipitation of target metals is lost.

[0006] A typical process used today results in incurring costs for the destruction of hydrogen sulphide, i.e. the cost of oxidizing agents and the costs of hydrogen sulphide itself, costs arising from the presence of elemental sulphur in the products and the loss of some of the target metal and/or impurities such as further undesirable metal sulphide products in the target metals due to precipitating past the optimal pH for a given target metal.

[0007] In existing methods the pH of the solution for the last target metal sulphide is chosen to optimize both maximum possible yield and best available quality.

Maximum yield demands a higher pH and contaminates the target metal sulphide with one or more further metal sulphides. A lower pH provides a purer product but leaves valuable target metal sulphide in the solution. In other words, as a result of m optimizing pH, some additional metal sulphides precipitate in the product and some N valuable target metal does not precipitate. In some instances the remaining target - 25 metal in the process flow may cause additional problems later in the process. 2 Further, at optimum pH some hydrogen sulphide remains in solution, causing E problems for further processing. This is why it is destroyed in conventional O processes with expensive chemicals.

O R N SUMMARY OF THE INVENTION

[0008] It is an aim of the present invention to overcome at least some of the disadvantages described above and provide a method of using hydrogen sulphide dissolved in a first process solution comprising metals ions for the recovery of at least one metal sulphide from said first process solution. An embodiment comprises the steps of adding a reagent to increase pH of the first process solution to provide a precipitate comprising at least two metal sulphides, further adding said reagent to further increase the pH of the first process solution until precipitation ceases, recovering the precipitate comprising at least two metal sulphides from the first process solution to provide a second process solution essentially free of hydrogen sulphide and dissolving the precipitate comprising at least two recovered metal sulphides in an acidic solution for further processing and recovery.

[0009] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0010] According to a first aspect of the present invention, there is provided a method of using hydrogen sulphide dissolved in a first process solution comprising metal ions for the recovery of at least two metal sulphides from said first process solution, said method comprising the steps of adding a reagent to increase pH of the first process solution to provide a precipitate comprising at least two metal sulphides, further adding said reagent to further increase the pH of the first process solution until precipitation ceases, recovering the precipitate comprising at least two metal sulphides from the first process solution to provide a second process m solution essentially free of hydrogen sulphide, and dissolving the precipitate N comprising at least two recovered metal sulphides in an acidic solution for further - 25 processing and recovery.

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S R N BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGURE 1 illustrates a process according to one embodiment of the invention for the recovery of metal sulphides from a process solution comprising metal ions, whereby recovered mixed sulphides are recycled to an earlier stage of the process for redissolution, reprecipitation and recovery.

[0012] FIGURE 2 illustrates the content of metals in solution after redissolution of mixed precipitate into incoming solution, where incoming solution has a metal content of 100%

[0013] FIGURE 3 illustrates the amount of metal recovered at various pHs measured as a percentage of the original metal content of the solution

[0014] FIGURE 4 is a graph illustrating the relationship between pH, oxidation reduction potential, and concentration of ferric iron in a process solution.

EMBODIMENTS

[0015] By means of some embodiments, it has surprisingly been found that the need for strong oxidising agents such as hydrogen peroxide is eliminated, whereby no hydrogen sulphide is destroyed; dissolved hydrogen sulphide is utilized in precipitation and the precipitated metal products are more pure because otherwise lost target metals are recycled to an earlier less alkaline stage in the metal recovery process making it possible to stop the precipitation process at a lower pH, so that no ~ additional metal sulphides end up in the target product. & < [0016] Thus, by means of some embodiments the last target sulphide can be 5 precipitated at a lower pH than conventionally resulting in a purer product. After > precipitation of the last target metal sulphide, metal sulphide precipitation is E 25 continued until all of the target metal including target metal sulphide containing S impurities has been precipitated. This additional precipitation is done by increasing S the pH of the solution until the solution is essentially free from hydrogen sulphide. N No oxidizing agents are needed to destroy hydrogen sulphide. This additional precipitation purges the last target metal from the solution. The precipitate contains some impurities, such as some additional metals, which are also precipitated. This mixed precipitate is recovered and fed back to an earlier stage in the metal recovery process, for example to a PLS solution, where the mixed sulphides dissolve in a more acidic or ferric iron containing environment back into the PLS-solution. Dissolution of sulphides reduces ferric iron to ferrous iron. The amount of 5 hydrogen sulphide needed for reducing ferric iron to ferrous iron is therefore smaller than in conventional methods. Further, reintroduction of the last target metal back into PLS-solution provides for an increase in yield of that target metal. The last metal precipitated as impurity will mostly remain undissolved and will be disposed of from the process. Substantially, all of the hydrogen sulphide introduced into the process is used productively and none will be destroyed.

DEFINITIONS

[0017] In the present context, the term “pregnant leach solution” (PLS) comprises acidic metal-laden water generated from stockpile leaching or heap leaching.

[0018] Heap leaching comprises an industrial mining process to extract metals and other compounds from ore via a series of chemical reactions that absorb specific minerals and then reseparates them after their division from other earth material.

[0019] Bioleaching or bioheap leaching is a method of extracting compounds from their ores through the use of living organisms.

[0020] FIGURE 1 illustrates a process according to one embodiment of the invention in which a first process solution (100) comprising metal ions, such as Ni, 3 Co, Fe, Cu, Zn ions, and possibly sulphide ions, for example a pregnant leach - solution (PLS) is contacted with a precipitation reagent chemical (210) for 5 precipitating a product (260), which is recovered in a first product precipitation = 25 stage (200). The process solution is then directed to a product sulphide precipitation S stage (300) in which the process solution is contacted with a sulphide source (310) S and a pH adjusting chemical (320) for the precipitation of a product sulphide (370) i which is recovered. The process solution is then directed to a mixed sulphide N precipitation stage (400) in which the process solution is contacted with a pH adjusting chemical (420) for the precipitation of a mixed sulphide (480). The mixed sulphide (480) is then directed as a mixed sulphide feed (490) to an earlier stage in the process for example, being directed as a mixed sulphide feed (490) to the first process solution (100) or being directed as a mixed sulphide feed (490) to the process solution after the first product precipitation stage (200). After the mixed sulphide precipitation stage (400), the process solution may be directed to a further product precipitation stage (500), in which the process solution is contacted with a precipitation reagent chemical (510) for precipitation of a further product (590) which is recovered. The process solution is then directed to further processing (700).

[0021] FIGURE 2 illustrates the metal sulphide content of solution after mixed sulphides have been redissolved into the first process solution in accordance with an embodiment. In the first process solution the metal content is 100%, after redissolution of mixed sulphide, the amount of cobalt, nickel and zinc s increases by about 40%, i.e. approximately 140% of the original metal content is available for precipitation. The amount of cobalt in the process solution after redissolution of a mixed sulphide comprising cobalt sulphide is about 144% of the cobalt in the first process solution. The amount of nickel in the process solution after redissolution of a mixed sulphide comprising nickel sulphide is about 141% of the nickel in the first process solution. The amount of zinc in the process solution after redissolution of a mixed sulphide comprising zinc sulphide is about 138% of the zinc in the first process solution. Different process solutions having different amounts of metal provide various mixed sulphides. However, in each case, the amount of metal available after redissolution of a mixed sulphide into the first process solution increase considerably as is exemplified by Figure 2.

N > [0022] FIGURE 3 illustrates, in accordance with an embodiment, how the amount of — 25 nickel sulphide recovered from a process solution increases when the pH of the 5 process solution is increased. As the pH of the solution increases, more target = nickel sulphide is recovered but a large amount of iron is present as an impurity. + Thus, it is mandatory to stop the precipitation at a point where there is still nickel in S the process liguid. = S 30 [0023] FIGURE4is a graph showing how the concentration of ferric iron changes in relation to the pH and oxidation reduction potential (ORP) of a process solution, when mixed sulphide precipitate is redissolved into the process solution in accordance with an embodiment. At time point 0 minutes the pH, ORP and [Fe] are each measured at 100%. As time passes over a period of 60 minutes, measured at intervals of 10 minutes, the pH of the solution gradually increases while the ORP and [Fe*] gradually decrease, [Fe*] having decreased to 0% at 60 minutes, showing that practically all of the ferric iron has been reduced to ferrous iron.

DETAILED DESCRIPTION

[0024] — The present invention relates to a method of using hydrogen sulphide dissolved in a first process solution comprising metal ions for the recovery of at least two metal sulphides from said first process solution. The metal ions in the solution may include various transition metal ions, actinide ions and mixtures thereof, e.g. copper, zinc, cobalt, iron, aluminium, manganese, nickel, uranium scandium, titanium, vanadium and chromium. In an embodiment, the method comprises the steps of adding a reagent to increase pH of the first process solution to provide a precipitate comprising at least two metal sulphides. Various metals are precipitated in sulphide form from the solution at differing pH, e.g. in a solution comprising copper and nickel ions, copper sulphide precipitates out at a lower pH than nickel sulphide. A second step comprises adding further reagent to further increase the pH of the first process solution until precipitation ceases. The precipitation typically ceases due to exhaustion of the dissolved hydrogen sulphide which is the source of sulphide in the process solution. This second step provides a precipitate comprising a mixture of metal sulphides. A third step comprises recovering the precipitate comprising at least two metal sulphides from the first process solution to provide a second process solution essentially free of hydrogen 3 sulphide. A fourth step comprises dissolving the precipitate comprising at least two — 25 recovered metal sulphides in an acidic solution for further processing and recovery. 5 The acidic solution into which the precipitate is dissolved may also contain ferric = iron. In this fourth step, the recovered metal sulphides may be recycled to any so earlier stage of a metal recovery process, whereby the metal sulphides are directed S to a process solution having a lower pH than the solution from which they were = 30 precipitated. The metal sulphides dissolve in the solution and can thus be N reprecipitated separately providing metal sulphides with higher purity, thereby increasing the yield of pure metal sulphide.

[0025] In a further embodiment the first process solution is a solution from which at least one metal compound comprising at least one metal selected from the group consisting of Cu, Ni, Co, Cr, Zn, Mn and Fe has been recovered.

[0026] In one embodiment, the reagent added to increase the pH of the first process solution is an alkaline which forms soluble salts in the process solution.

[0027] Any alkaline that raises the pH of the first process solution may be used. In a further embodiment, the reagent added to increase the pH of the first process solution is non-sodium alkaline. Preferably, non-sodium alkalines that do not form precipitates such as gypsum are used. Particularly useful as a non-sodium alkaline reagent is ammonia or an aqueous solution thereof. Using ammonia or an aqueous solution thereof has the additional benefit of providing ammonium sulphate as a by- product.

[0028] In one embodiment a first metal sulphide of the precipitate comprising at least two metal sulphides comprises at least one metal selected from the group consisting of Cu, Ni, Co, Cr, Zn, Mn and Fe. A second and further metal sulphide may comprise other metals that are not target metals for example metals that are not of economic importance such as Fe.

[0029] In an embodiment one metal sulphide of the precipitate comprising at least two metal sulphides is an iron containing sulphide, such as FeS or FeS,. Such iron containing sulphides, contain ferrous sulphide, which on recycling to a previous step in a metal recovery process are dissolved to ferrous ions by the acidic solution or existing ferric iron, whereby hydrogen sulphide is formed for the recovery of = further metal sulphides. Thus, the effluent from one stage of the metal recovery 0 process is used to reduce ferric iron to ferrous iron without the need for additional 5 25 hydrogen sulphide. In a further embodiment, one metal sulphide of the precipitate = comprising at least two metal sulphides is nickel or cobalt sulphide (NiS, CoS). On > redissolution in an acidic environment NiSO, and CoSO, are formed, which provide Ni and Co ions. Ni and Co are then reprecipitated and recovered later.

™~ I [0030] In a further embodiment, the precipitate comprising at least two metal sulphides further comprises at least one impurity selected from the group consisting of sulphur, zinc, nickel and cobalt. When such a precipitate is redissolved in the acidic solution mentioned above, metal sulphides are separated from the impurity and are recovered separately by reprecipitation in the metal recovery process.

[0031] In one embodiment, the acidic solution for further processing and recovery is a pregnant leach solution, such as a pregnant leach solution e.g. a solution obtained from bioheaps in a bioheap leaching process.

[0032] Thus, in one embodiment of the reaction a first process solution, which is a solution comprising metal ions and hydrogen sulphide, for example a pregnant leach solution into which hydrogen sulphide has been added, might be contacted with a reagent for increasing the pH of the solution, such as an alkaline, particularly a non-sodium alkaline, for example ammonia or an aqueous solution thereof. At a certain pH level a first metal sulphide begins to precipitate, e.g. copper sulphide begins to precipitate for example at a very low pH. The first metal sulphide is separated from the process solution and is collected in a first receptacle. As pH of the process solution is increased above this level a second metal sulphide, e.g. zinc sulphide begins to precipitate together with the first metal sulphide, e.g. copper sulphide. At this point the first receptacle is closed so that the first receptacle contains substantially the first metal sulphide. A second receptacle is provided into which a mixture of remaining copper sulphide and zinc sulphide and possible other sulphides and precipitates are collected. In such a sulphide precipitate mixture it is typically difficult to separate metal sulphides from each other. In conventional processes, impure products are provided and/or target metals are lost to waste. The pH value of the solution is increased to a point at which substantially all of the first metal e.g. copper is precipitated out of the process solution in a precipitate mixture.

3 This precipitate mixture is collected. The process solution is directed to further = 25 processing.

2 [0033] The sulphide mixture in the second receptacle which comprises precipitates E of at least two metal sulphides are directed to an earlier stage in the process into the O process solution in which the precipitates redissolve. The pH of the process 3 solution in the earlier stage of the process is lower than the pH of the solution out > 30 of which the precipitates were precipitated. The process is then repeated with any number of receptacles as reguired, first collecting essentially pure precipitates comprising essentially one metal sulphide in one receptacle and then, as reguired,

collecting mixtures of precipitates and optionally other impurities such as elemental sulphur in a further receptacle and redirecting the precipitate mixture back to the earlier process solution for redissolution.

[0034] The following non-limiting examples illustrate the invention:

EXAMPLES Example 1

[0035] In example 1 a typical sulphate laden solution is first treated with hydrogen sulphide to precipitate ZnS. Thereafter NiS is precipitated with addition of ammonium hydroxide and hydrogen sulphide. Addition of hydrogen sulphide has been stopped at pH 3.5. Solution is practically purged from nickel at pH 3.7. At pH

3.9 solution is also purged from dissolved hydrogen sulphide. See Figures 3 and 4 Example 2

[0036] In example 2 a small amount of CoS, NiS and ZnS has been redissolved into a PLS-feed for half an hour. As a result, the target metal content of PLS-solution has increased approximately 40%. See Figure 2

[0037] tis to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not ~ intended to be limiting.

O N [0038] Reference throughout this specification to one embodiment or an 5 embodiment means that a particular feature, structure, or characteristic described in = connection with the embodiment is included in at least one embodiment of the Tr . . . . . a 25 present invention. Thus, appearances of the phrases “in one embodiment” or “in an S embodiment” in various places throughout this specification are not necessarily all R referring to the same embodiment. Where reference is made to a numerical value O . . . N using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0039] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0040] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0041] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of ™~ implementation can be made without the exercise of inventive faculty, and without N 25 departing from the principles and concepts of the invention. Accordingly, it is not 7 intended that the invention be limited, except as by the claims set forth below.

I [0042] The verbs “to comprise” and “to include” are used in this document as open so limitations that neither exclude nor require the existence of also un-recited features. S The features recited in depending claims are mutually freely combinable unless = 30 otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or N "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0043] At least some embodiments of the present invention find industrial application in mining industries, e.g. metal mining, in particular open cast mining, which provides large volumes of process solutions containing metal ion-laden solutions, for example metal sulphate-laden solutions from which metal sulphides are recovered by sulphide precipitation.

ACRONYMS LIST PLS Pregnant Leach Solution

REFERENCE SIGNS LIST ~ 400 mixed sulphide precipitation stage & o j j — 480 mixed sulphide product Tr a 490 mixed sulphide feed 3

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CITATION LIST Patent Literature US 4,110,400

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Claims

CLAIMS:

1. A method of using hydrogen sulphide dissolved in a first process solution comprising metal ions for the recovery of at least two metal sulphides from said first process solution comprising the steps of: adding a reagent to increase pH of the first process solution to provide a precipitate comprising at least two metal sulphides; further adding said reagent to further increase the pH of the first process solution until precipitation ceases; recovering the precipitate comprising at least two metal sulphides from the first process solution to provide a second process solution essentially free of hydrogen sulphide ; and dissolving the precipitate comprising at least two recovered metal sulphides in an acidic solution for further processing and recovery.

2. The method according to claim 1, wherein the first process solution is a solution from which at least one metal compound, comprising at least one metal selected from the group consisting of Cu, Ni, Co, Cr, Zn, Mn and Fe has been recovered.

3. The method according to claim 1 or 2, wherein the reagent added to increase the pH of — the first process solution is an alkaline which forms soluble salts in the process solution.

4. The method according to any of the preceding claims, wherein a first metal sulphide of the precipitate comprising at least two metal sulphides comprises at least one metal N selected from the group consisting of Cu, Ni, Co, Cr, Zn, Mn and Fe. & 25 =

5. The method according to any of the preceding claims, wherein one metal sulphide of 2 the precipitate comprising at least two metal sulphides is an iron containing sulphide. i 10

6. The method according to any of the preceding claims, wherein the precipitate 3 30 comprising at least two metal sulphides further comprises at least one impurity selected > from the group consisting of sulphur, zinc, nickel and cobalt.

7. The method according to any of the preceding claims, wherein the acidic solution for further processing and recovery is a pregnant leach solution.

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