FIELD

[0001] The present invention relates to a method of treating a solution comprising metal sulphates. Further, the invention relates to a method of obtaining ammonium sulphate ((NH ₄ ) ₂ S0 ₄ ). The present invention relates to a method of recovering metal compounds and (NH ₄ ) ₂ S0 ₄ from a solution comprising one or more metal sulphates.

BACKGROUND

[0002] Many processes, such as bioleaching sulphide minerals or the rayon process, produce solutions laden with sulphates, particularly metal sulphates. In order to recover metals from such solutions and purify or clean remaining effluent before releasing into the environment, the solutions must be processed. Metals can be recovered by various methods e.g. by sulphide precipitation or hydroxide precipitation. By sulphide and hydroxide precipitation, metals are selectively precipitated. Selective precipitation is conventionally accomplished by adjusting the pH value of the solution to a suitable range for each metal to be precipitated. Adjustment of the pH is usually achieved using compounds such as NaOH, CaO and CaC0 ₃ .

[0003] Adjusting the pH with NaOH introduces Na ₂ (S0 ₄ ) into the solution which is difficult to remove from the effluent due to its high solubility. CaO and CaC0 ₃ do not introduce Na ₂ (S0 ₄ ) into the solution but introduce gypsum, which buries all precipitating metals and renders them difficult to recover. The burial of precipitating metals in gypsum reduces the economic viability of the metal recovery. The gypsum is typically dumped in waste ponds or in waste rock dumps where it accrues and accumulates ad infinitum, eventually causing logistical problems.

[0004] The introduction of gypsum into the solution can be avoided by adjusting the pH of the solution with NaOH, which has its own problems as described above and a relatively high cost. SUMMARY OF THE INVENTION

It is an aim of the present invention to overcome at least some of the disadvantages described above and provide a method of recovering metal compounds and ammonium sulphate from a solution comprising one or more metal sulphates, whereby the pH of the solution is controlled one or more times by adding ammonia to the solution to precipitate one or more metal compounds. The one or more metal compounds are recovered for further processing to provide a solution comprising (NH ₄ ) ₂ S0 ₄ and water, and (NH ₄ ) ₂ S0 ₄ is recovered from the solution for further processing to provide a solution comprising essentially water.

[0005] The invention is defined by the features of the independent claim. Some specific embodiments are defined in the dependent claims. According to a first aspect of the present invention, there is provided a method for the recovery of metal compounds and ammonium sulphate from a solution comprising one or more metal sulphates, said method comprising the steps of

a) controlling the pH of the solution by adding ammonia to the solution one or more times to precipitate one or more metal compounds,

b) recovering one or more metal compounds for further processing to provide a first treated solution comprising (NH4)2S04 and water, and

c) recovering (NH4)2S04 for further processing to provide a second treated solution comprising essentially water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIGURE 1 is a flow chart illustrating a method of recovering metal compounds and (NH ₄ ) ₂ S0 ₄ from a solution comprising one or more metal sulphates in accordance with at least some embodiments of the present invention; [0007] FIGURE 2 is a flow chart illustrating a method of treating a solution comprising metal sulphates in accordance with at least some embodiments of the present invention;

[0008] FIGURE 3 is a flow chart illustrating a method of treating a solution comprising metal sulphates in accordance with at least some embodiments of the present invention.

[0009] FIGURE 4 is a flow chart illustrating a method of treating a solution comprising metal sulphates in accordance with at least some embodiments of the present invention.

[0010] FIGURE 5 is a flow chart illustrating a method of treating a solution comprising metal sulphates in accordance with at least some embodiments of the present invention.

EMBODIMENTS

[0011] DEFINITIONS

[0012] In the present context, the term "ammonia" comprises ammonia, aqueous ammonia, ammonium hydroxide and mixtures thereof.

[0013] The term "metal sulphate laden solution" includes leach solutions from mines including quarries, waste water from mines including quarries, runoff water from mines including quarries and mixtures thereof. "Metal sulphate laden solution" also includes wastewater from oil production, such as seawater containing sulphates and wastewater from pharmaceutical factories and textile factories, such as wastewaters from the rayon process, and manufacturers of fine chemicals including products for use in agriculture, as well as agricultural runoff. Also included in the term "metal sulphate laden solution" are effluents from metal finishing plants e.g. metal plating plants, such as nickel plating plants. In turn "leach solution is taken to mean acidic metal laden water generated from stockpile leaching and heap leaching, including bioheap leaching. [0014] "Controlling the pH of the solution" is intended to mean both increasing the pH of the solution until metal compounds precipitate and maintaining the pH of the solution until precipitation ceases, i.e. adjusting the pH of the solution to a suitable range for each metal or metal group to be precipitated. By means of the invention it has surprisingly been found controlling the pH of a metal- sulphate laden solution, e.g. a solution that comprises a low concentration of valuable metal sulphates, with ammonia provides a clean, economically-viable process for the recovery of metal compounds and ammonium sulphate. For example a low grade metal ore that would normally and conventionally be discarded can provide a solution comprising a low concentration of valuable metal sulphates which is then processed according to an embodiment of the invention to provide valuable ammonium sulphate in substantial amounts at little or no additional cost, and metals that otherwise would not have been recovered, e.g. metals that have been described as critical by the European Union in its report on critical raw materials, for example the revised list published in the 2014 Communication On the review of the list of critical raw materials for the EU and the implementation of the Raw Materials Initiative, may be recovered.

[0015] FIGURE 1 is a flowchart describing a method in accordance with at least some embodiments of the invention in which a metal-sulphate laden solution (100) is contacted with ammonia (102) in a first reaction chamber (200) to increase the pH of the solution until a first metal compound (401) is precipitated, which is recovered for further processing, providing a first treated solution (202). The first treated solution (202) is contacted with further ammonia (102) to increase the pH of the solution until a second metal compound (401) is precipitated and recovered for further processing. After recovery of the second metal compound (401) the pH of the first treated solution (202) is increased with ammonia (102) stepwise for the further selective precipitation of further metal compounds (402) until substantially all metal compounds (402) have been precipitated and recovered for further processing providing a second treated solution (302). The second treated solution (302), comprising ammonium sulphate and water, is directed to a third reaction chamber (400). The ammonium sulphate is recovered for further processing to provide a solution comprising essentially water.

[0016] FIGURE 2 illustrates a method in accordance with at least some embodiments of the present invention in which a metal sulphate- laden solution (100) is acidified with hydrogen sulphide (101) in a first reaction chamber (200). The acidified solution (100) is then contacted with ammonia (102) in the first reaction chamber (200) to increase the pH of the solution until a first metal sulphide (201) is precipitated. The first metal sulphide (201) is recovered for further processing, providing a first treated solution (202) which is directed to a second reaction chamber (300) into which ammonia(102) is introduced to contact the first treated solution (202), the ammonia (102) being provided to increase the pH of the first treated solution (202) which is directed to a second reaction chamber (300). In the second reaction chamber (300), the first treated solution (202) is contacted with ammonia (102), whereby the pH of the first treated solution (202) is increased. The increase in pH of the second treated solution (202) results in precipitation of metal compounds (402) which are recovered for further processing.

[0017] FIGURE 3 is a flowchart describing a method in accordance with at least some embodiments of the present invention in which a metal sulphate-laden solution (100) is contacted with ammonia (102) in a first reaction chamber (200), whereby the pH of the solution is increased and a first metal compound (401) is precipitated and recovered for further use, providing a first treated solution which is contacted with ammonia (102) in a second reaction chamber (300). Ammonia (102) is added stepwise until all available metal compounds (402) have been precipitated and recovered for further processing, providing a second treated solution (302) comprising ammonium sulphate and water. The second treated solution is directed to a third reaction chamber (400). The ammonium sulphate is recovered to provide a solution comprising water and one or more Group I sulphates and Group II sulphates.

[0018] FIGURE 4 is a flowchart describing a method in accordance with at least some embodiments of the present invention in which a metal sulphate-laden solution

(100) is treated with hydrogen sulphide (101) in a fourth reaction chamber (500). The treated solution (100) is then contacted with Al(OH) ₃ (502) in the fourth reaction chamber (500) to increase the pH of the solution until a metal sulphide (501) is precipitated. The metal sulphide (501) is recovered for further processing, providing a treated solution (602) which is directed into a fifth reaction chamber (700) into which

Al(OH) ₃ (502) is introduced in a pH controlling step to provide a solution (702). The solution (702) is then directed to a sixth reaction chamber (800) in which it is treated with hydrogen sulphide (101). This treated solution (702) is then contacted with Α1(ΟΗ) ₃ (502) in the sixth reaction chamber (800) to increase the pH of the solution until one or more further metal sulphides (801) are precipitated. The one or more further metal sulphides (801) are recovered for further processing providing a further treated solution (802). The further treated solution (802) is directed to a seventh reaction chamber (900) for precipitation of iron. The further treated solution is contacted in the seventh reaction chamber (900) with ferrous/ferric hydroxide (3002) until FeS/FeS ₂ (901) is precipitated. FeS/FeS ₂ (901) is recovered and recirculated to the metal sulphate laden solution (100) where it reduces any ferric iron in the metal sulphate laden solution (100) to ferrous iron. Of the raffinate remaining, a first portion (902) is directed to heaps (910) for heap leaching. A second portion (903) of the raffinate is directed to a first reaction chamber (200) in which the second portion of the raffinate (903) is contacted with ammonia (102), whereby the pH of the solution is increased and Al(OH) ₃ (502) is precipitated and recovered for use in the fourth (500), fifth (700) and sixth (800) reaction chambers. After recovery of Al(OH) ₃ (502), remains a first treated solution which is contacted with ammonia (102) in a second reaction chamber (300), whereby the pH of the solution is increased and Fe(OH) ₃ (3002) is precipitated. The Fe(OH) ₃ (3002) is recovered and directed for use in the seventh reaction chamber (900). Ammonia (102) is further added stepwise until all available metal compounds (402) have been precipitated and recovered for further processing providing a solution (1600) comprising water, group I and II metals and

(NH ₄ ) ₂ S0 ₄ . A third portion of the raffinate (904) is directed to an eighth reaction chamber (1000) for an aluminium precipitation step. The third portion of the raffinate (904) is contacted with CaC0 ₃ (1002) until Al(OH) ₃ (1), Fe(OH) ₃ (2), and CaS0 ₄ (3) are precipitated and recovered to provide a further solution (1001), which is subjected to a lime treatment in a ninth reaction chamber (1100), whereby the solution is contacted with CaO (1003) to precipitate Fe(OH) ₃ (4), Mn(OH) ₂ (5), Mg(OH) ₂ (6), and CaS0 ₄ (7), which are precipitated and recovered to provide clean water (1500).

[0019] FIGURE 5 is a flowchart describing a method in accordance with at least some embodiments of the present invention in which a PLS (2100) comprising at least zinc, nickel, cobalt, iron aluminium, manganese and magnesium, is treated with hydrogen sulphide (101) in a fourth reaction chamber (500). The treated solution (2100) is then contacted with Al(OH) ₃ (502) in the fourth reaction chamber (500) to increase the pH of the solution until ZnS (5100) is precipitated. The ZnS (5100) is recovered for further processing, providing a treated solution (602) which is directed into a fifth reaction chamber (700) into which Al(OH) ₃ (502) is introduced in a pH controlling step to provide a solution (702). The solution (702) is then directed to a sixth reaction chamber (800) in which it is treated with hydrogen sulphide (101) This hydrogen sulphide-treated solution (702) is then contacted with Al(OH) ₃ (502) in the sixth reaction chamber (800) to increase the pH of the solution until NiCoS (8001) is precipitated. The NiCoS (8001) is recovered for further processing providing a further treated solution (8002). The further treated solution (8002) is directed to a seventh reaction chamber (900) for precipitation of iron. The further treated solution is contacted in the seventh reaction chamber (900) with Fe(OH) ₃ (3002) until FeS (901) is precipitated essentially purging the solution (8002) of hydrogen sulphide (101). FeS (901) is recovered and recirculated to the PLS (2100) where it reduces any ferric iron in the PLS (2100) to ferrous iron. Of the raffinate remaining, a first portion (902) is directed to heaps for heap leaching. A second portion of the raffinate (903) is directed to a first reaction chamber (200) in which it is contacted with ammonia (102), whereby the pH of the solution is increased and Al(OH) ₃ (502) is precipitated and recovered for use in the fourth (500), fifth (700) and sixth (800) reaction chambers. After recovery of Al(OH) ₃ (502), remains a first treated solution which is contacted with ammonia (102) in a second reaction chamber (300), whereby the pH of the solution is increased and Fe(OH) ₃ (3002) is precipitated. The Fe(OH) ₃ (3002) is recovered and directed for use in the seventh reaction chamber (900) providing a second treated solution. The second treated solution is directed to a third reaction chamber (400) in which it is treated with H ₂ S (101)to introduce a source of sulphide ions into the solution and then is contacted with ammonia(102) whereby the pH increases and MnS (4002) is precipitated out and recovered for further use providing a third treated solution which is subjected to a treatment whereby the third treated solution is contacted with CaO (6002) in a lime boil step in which ammonia (102) is driven out of solution and recycled. CaS0 ₄ (5002) is precipitated providing a final solution comprising essentially clean water (1500). The third portion of the raffinate (904) is contacted with CaC0 ₃ until Al(OH) ₃ (1), Fe(OH) ₃ (2), and CaS0 ₄ (3) are precipitated and recovered to provide a further solution (1001), which is subjected to a lime treatment in a ninth reaction chamber (1100), whereby the solution is contacted with CaO (1003) to precipitate Fe(OH) ₃ (4), Mn(OH) ₂ (5), Mg(OH) ₂ (6), and CaS0 ₄ (7), which are precipitated and recovered to provide clean water (1500). DETAILED DESCRIPTION

[0020] The present invention relates to a method for the recovery of metal compounds and ammonium sulphate from a solution comprising one or more metal sulphates. Metal sulphates in the solution may include sulphates of various transition metals, actinides, and mixtures thereof e.g. copper, zinc, nickel, cobalt, iron, aluminium manganese, uranium, scandium, titanium, vanadium, chromium, magnesium and alloys thereof. In an embodiment the method comprises the steps of a) controlling the pH of the solution by adding ammonia to the solution one or more times to precipitate one or more metal compounds. The pH of the solution is controlled one or more times for the selective precipitation of metal compounds, e.g. metal hydroxides, from the solution. Using ammonia to control the pH of the solution provides both economical and technical advantages, e.g. compared to controlling the pH of the solution with NaOH, ammonia is very inexpensive. Further, no Na ₂ S0 ₄ , which is difficult to remove due to its high solubility, and is highly corrosive in higher concentrations, is formed. Using ammonia to control the pH of the solution provides the additional advantage that no gypsum is formed. As mentioned above gypsum buries precipitated metal compounds rendering them difficult to recover. In an embodiment, the method further comprises the steps of b) recovering one or more metal compounds for further processing to provide a first treated solution comprising (NH ₄ ) ₂ S0 ₄ and water. The first treated solution may also comprise other compounds such as magnesium sulphate. In such cases, other compounds can be removed from the first treated solution by conventional means. The metal compounds can be recovered by various means known in the art, such as filtration including membrane filtration, and centrifugation. From the first treated solution is recovered (NH ₄ ) ₂ S0 ₄ in a recovering step c) for further processing to provide a second treated solution comprising essentially water. (NH ₄ ) ₂ S0 ₄ may be recovered by means known in the art such as by evaporation, providing essentially clean water for release. Thus, by means of the invention it has been found that by combining the recovery of metals with a method of obtaining ammonium sulphate an economically viable process is provided that remains economically viable even in cases in which solutions comprising only low concentrations of valuable metal sulphates are available. A low concentration of valuable metal sulphates e.g. is a concentration of 10% by weight or less, particularly 8% or less, for example 1 to 6%, preferably 2 to 5 % suitably 4%, most suitably 3 % by weight. Naturally, embodiments of the invention are also applicable to higher sulphate concentrations. Embodiments of the invention can be applied to all metal-sulphate laden solutions from 1% by weight up to and including solutions saturated with metal sulphates. In some embodiments, the recovering step b) comprises recovering more than one metal compound, for example 2, 3, 4, 5, 6, 7, 8, 9, or 10 metal compounds for further processing to provide a first treated solution comprising (NH ₄ ) ₂ S0 ₄ and water.

[0021] In one embodiment the method comprises the steps of (a) controlling the pH of the solution by adding ammonia to the solution more than one time to precipitate more than one metal compounds, (b) recovering more than one metal compounds for further processing to provide a first treated solution comprising (NH ₄ ) ₂ S0 ₄ and water, and (c) recovering (NH ₄ ) ₂ S0 ₄ for further processing to provide a second treated solution comprising essentially water.

[0022] In some embodiments the second treated solution comprises sulphates that are not precipitated out with the addition of ammonia. In an embodiment, the second treated solution further comprises sulphates of one or more Group I or Group II metals.

These metals may include lithium, sodium, potassium, magnesium, calcium and other Group I or Group II metals, as well as mixtures thereof. In a further embodiment the second treated solution is directed to further processing, whereby non precipitated sulphates are recovered. Such sulphates may be recovered by e.g. evaporation. In an embodiment the sulphates of one or more Group I or Group II metals are recovered. In a further embodiment, the sulphates of the Group I or Group II metals are recovered before ammonium sulphate is recovered. In a further embodiment, ammonium sulphate is recovered before sulphates of Group I or Group II metals are recovered.

[0023] In a further embodiment the pH controlling step comprises (i) increasing the pH of the solution with ammonia until a first metal compound precipitates, (ii) maintaining the pH of the solution with further ammonia until the first metal compound is precipitated, and repeating steps (i) and (ii) until essentially all metal compounds are precipitated. In one embodiment the controlling step comprises (i) increasing the pH of the solution with ammonia until a first metal compound precipitates, (ii) maintaining the pH of the solution with further ammonia until the first metal compound is precipitated, optionally (iii) increasing the pH of the solution with ammonia until a further metal compound precipitates and maintaining the pH of the solution with further ammonia until the further metal compound is precipitated, and optionally (iv) repeating step iii until essentially all metal compounds are precipitated.

[0024] This method of selective precipitation ensures that as much of each metal is recovered as possible. Thus, in an embodiment the pH of the solution is maintained with the addition of further ammonia after each increase of the pH until each metal compound is precipitated and recovered. In an embodiment, 50 - 98 wt % of the metal compounds are recovered from the metal- sulphate laden solution, preferably 60 - 95 wt %, suitably 70 - 92 wt %, particularly 75 - 90 wt % of the metal compounds are recovered from the metal- sulphate laden solution. [0025] Various metal compounds are precipitated by means of embodiments of the invention. In an embodiment, metal compounds selected from the group of oxides, oxyhydroxides, silicates, hydroxides and mixtures thereof are precipitated. In a preferred embodiment, hydroxides are precipitated. As mentioned above, the metal compounds precipitated comprise metals selected from the group of transition metals, actinides, and mixtures thereof, preferably selected from the group of copper, zinc, nickel, cobalt, iron, aluminium manganese, uranium, scandium, titanium, vanadium, chromium, magnesium and alloys thereof.

[0026] Some embodiments find application in mining and in related industries. For example, in an embodiment the solution is a pregnant leach solution, optionally provided by one or more ores by a bioheap leaching process. The ores may contain metals as described above, a typical ore may comprise e.g. salts of nickel, copper, zinc, cobalt manganese, magnesium, uranium, sulphur and mixtures thereof. An ore may also comprise salts of aluminium, arsenic, calcium, cadmium, chromium, iron, sodium, silicon and mixtures thereof. [0027] In a further embodiment, the method further comprises a sulphide precipitation step. In one embodiment a pregnant leach solution (PLS) provided by such an ore is contacted with a source of sulphide ions. Contacting the PLS with a source of sulphide ions acidifies the solution, i.e. lowers the pH of the solution by the following example reaction: MeS0 ₄ (a _q) + H ₂ S( _g ) ^MeS _(aq) + H ₂ S0 _4(aq ) where Me is a metal having an oxidation state II or having two valence electrons. [0028] The source of sulphide ions is introduced to the solution for the precipitation of metal sulphides. In an embodiment the source of sulphide ions is hydrogen sulphide. Sulphates in the solution react with hydrogen sulphide to provide metal sulphides and sulphuric acid. Adding ammonia to the solution neutralises sulphuric acid to give ammonium sulphate and adjusts the pH, i.e. increases the pH, and causes metal sulphides to precipitate out of solution in turn. From a typical ore, comprising sulphates of copper, zinc, nickel, cobalt and iron, metal sulphides precipitate out in the order copper sulphide, zinc sulphide, nickel sulphide, cobalt sulphide and iron sulphide. Each metal sulphide being recovered in turn. In an embodiment the conditions of the sulphide precipitation step are selected so that metal sulphides selected from the group consisting of transition metals such as copper zinc, nickel, cobalt and iron are recovered. For example, after addition of sulphide ions to the solution and neutralisation with ammonia.

[0029] In further embodiments, the pH of the solution is increased with Al(OH)3. For example, in one embodiment the method comprises a sulphide precipitation prior to the controlling step (a), whereby the sulphide precipitation comprises the steps of increasing the pH of the solution with Al(OH) ₃ until one or more metal sulphides are precipitated. Al(OH) ₃ is ideal for precipitating sulphides from a solution having a pH of 3 or lower. In an embodiment the sulphide precipitation step further comprises recovering the precipitated one or more metal sulphides for further processing to provide a metal sulphate-laden solution, and directing the metal sulphate- laden solution to the controlling step (a) of any of the herein described embodiments.

[0030] In cases where the pH of the solution is greater than 3, the pH of the solution may be increased with Fe(OH) ₃ , which is ideal for precipitating sulphides from a solution having a pH of up to and including 5. In one embodiment, in the sulphide precipitation prior to the controlling step (a) the pH of the solution is increased with Fe(OH) ₃ .

[0031] Thus as the pH of the solution increases, the agent for increasing the pH of the solution may be switched from Al(OH) ₃ to Fe(OH) ₃ . In one embodiment one or more metal sulphides are precipitated from the solution prior to the controlling step a) using Al(OH) ₃ to increase the pH of the solution before one or more further metal sulphides are precipitated from the solution prior to the controlling step a) using Fe(OH) ₃ to increase the pH of the solution.

[0032] In various embodiments in which the pH of the solution is increased with Al(OH) ₃ or Fe(OH) ₃ , or Al(OH) ₃ and Fe(OH) ₃ , a source of sulphide ions, for example hydrogen sulphide or iron sulphide, or any readily available source of sulphide ions is provided to the solution before the solution is contacted with the pH raising agent, i.e. before the pH of the solution is raised. In a further embodiment, a source of sulphide ions is added continuously to the solution. In one embodiment, a source of sulphide ions is added to the solution until sulphide precipitation ceases. [0033] In any given metal-mining process, after metal extraction there is a raffinate stream that contains valuable chemicals that are often lost, or in the case of bioheap leaching are directed back to the heaps. In some of the embodiments described above, (NH4) ₂ S04 is driven back to the bioheaps where the ammonia is essentially lost as a pH controlling agent. In many cases this is desirable, as (NH4) ₂ S04 is very suitable as a nutrient for bacteria in many bioheaps. However, in many cases it is more desirable to harvest as much (NH4) ₂ S04 as possible. Thus, alternative pH raising agents such as Al(OH) ₃ and Fe(OH) ₃ are used. After first steps in which Al(OH) ₃ and Fe(OH) are used, the raffinate contains no valuable (NH4) ₂ S04. Ammonia is then added in subsequent hydroxide and sulphide precipitation steps before either being harvested as (NH4) ₂ S04 or being boiled off in a lime boil, thereby enabling recycling of ammonia and after the initial investment to purchase ammonia, the metal recovery process comprises a self-sufficient ammonia loop. Thus embodiments concern the recovery of (NH4) ₂ S0 ₄ andfurther embodiments concern recycling ammonia.

[0034] As mentioned above, there are some embodiments where a sulphide precipitation step is not required or not desirable. In such cases other metal compounds are precipitated in a hydroxide precipitation step as described above in which the pH of the solution is controlled stepwise for the selective precipitation of metal compounds. Such a solution might comprise metals selected from the group consisting of aluminium, arsenic, calcium, cadmium, chromium, iron, magnesium, manganese, sodium, silicon, uranium, copper, zinc, nickel, cobalt and mixtures thereof. In a further embodiment the conditions of the step for the selective precipitation of one or more metal compounds are selected so that compounds comprising metals selected from the group consisting of aluminium, arsenic, calcium, cadmium, chromium, iron, magnesium, manganese, sodium, silicon, uranium, copper, zinc, nickel, cobalt and mixtures thereof are recovered. From a typical ore, comprising sulphates of zinc, cadmium, manganese, copper, iron, magnesium, lead, iron nickel, aluminium and mixtures thereof, increasing the pH, with e.g. ammonia, causes metal compounds, such as hydroxide, to precipitate out. For example, aluminium hydroxide precipitates out at a pH of around 6, along with some iron (III) hydroxide, at a pH of around 8 to 9, copper hydroxide precipitates out along with some nickel hydroxide.

[0035] Further embodiments of the invention relate to a method of obtaining ammonium sulphate. In an embodiment the method comprises the steps of obtaining a solution comprising sulphates having a pH of less than or equal to 7, subjecting the solution to a hydroxide precipitation step, whereby the solution is treated with ammonia to precipitate out metal compounds contained therein, to provide a first treated solution comprising ammonium sulphate, and recovering the ammonium sulphate from the first treated solution. Ammonium sulphate is formed e.g. as in the following example reaction:

MeS0 ₄ (a _q) + 2NH ₄ OH _(aq) - Me(OH) _2(s) + (NH ₄ ) ₂ S0 _4(aq) where Me is a metal having an oxidation state II or having two valence electrons.

[0036] As mentioned above, the ammonium sulphate is recovered, for example, by conventional means including evaporation. The method provides an economically viable means of generating ammonium sulphate and harvesting metals from sulphate- bearing solutions from sulphidic ores, even including low-grade sulphidic ore bodies. A substantial amount of ammonium sulphate is generated as a side product in the harvesting of metals at little or no additional cost. Only a small cost for the recovery of ammonium sulphate arises. Conversely, when the aim is to produce ammonium sulphate, valuable metals are harvested as side products from sulphidic ores at no additional cost. In any particular precipitation phase with pH control ammonia can be substituted with an alternative compound provided that sufficient ammonium sulphate is generated in other precipitation phases of the process as a whole to make the entire process economically viable. [0037] The following non-limiting working examples illustrate the invention: EXAMPLES

Example 1

[0038] A pregnant leach solution (PLS) was obtained from a low grade ore. The PLS contained various salts of metals according to Table 1.

Table 1 Metal concentrations in example PLS solution

[0039] Metal extraction was carried out in three sulphide precipitation steps followed by hydroxide precipitation. In the first step, excess hydrogen sulphide was added to the PLS to provide sulphide ions. Copper sulphate reacted with hydrogen sulphide to provide copper sulphide and sulphuric acid.

CuS0 ₄ + H ₂ S -> CuS + H ₂ S0 ₄

The copper sulphide precipitated and was recovered. In the second step, to solution was added ammonia to increase the pH of the solution to a level of ca. 2.5. The pH of the solution was maintained at that level with the addition of further ammonia as zinc sulphide precipitated out. The ammonia neutralized the sulphuric acid to provide ammonium sulphate.

ZnS0 ₄ + H ₂ S -> ZnS + H ₂ S0 ₄

H ₂ S0 ₄ + 2NH ₃ -> (NH ₄ ) ₂ S0 ₄

The precipitated zinc sulphate was recovered. After recovery of zinc sulphate, to the solution was added ammonia to increase the pH of the solution to a level of ca. 3.5. The pH of the solution was maintained at that level with the addition of further ammonia as nickel sulphide precipitated out. The ammonia neutralized sulphuric acid to provide ammonium sulphate.

NiS0 ₄ + H ₂ S -> S + H ₂ S0 ₄

H ₂ S0 ₄ + 2NH ₃ -> (NH ₄ ) ₂ S0 ₄

The precipitated nickel sulphate was recovered. Further metal compounds were recovered stepwise in a hydroxide precipitation step. To the solution was added ammonia to increase the pH to precipitate hydroxides, e.g.

FeS04 + 2NH ₄ OH -> (NH ₄ ) ₂ S0 ₄ + Fe(OH) ₂

MnS04 + 2NH ₄ OH -> (NH ₄ ) ₂ S0 ₄ + Mn(OH) ₂

6NH ₄ OH + A1 ₂ (S0 ₄ ) ₃ -> 3(NH ₄ ) ₂ S0 ₄ + 2A1(0H) ₃

Each hydroxide was recovered in turn. It was found that after recovery of hydroxides the solution comprised mainly ammonium sulphate and magnesium sulphate. The magnesium sulphate was removed by conventional means, and ammonium sulphate was recovered by evaporation. It was found that the concentration of ammonium sulphate in the final solution was greater than 10 wt %, and that 1 kilogram of ammonia provides approximately 3.9 kilograms of ammonium sulphate.

[0040] It is 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. [0041] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Where reference is made to a numerical value using a term such as, for example, about or substantially, the exact numerical value is also disclosed.

[0042] 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. [0043] 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.

[0044] 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 departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0045] The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0046] At least some embodiments of the present invention find industrial application in mining industries, e.g. metal mining, in particular open cast mining, which requires the processing of large volumes of water. Other industries in which some embodiments find industrial application include the textile industry e.g. in the rayon process, the pharmaceutical industry, metal finish plants such as metal plating plants e.g. nickel plating plants as well as in the fine chemical industry, including the manufacture of chemicals for use in agricultural applications, and further in the treatment of run-off, from both agriculture and in mining..

REFERENCE SIGNS LIST

100 Metal sulphate-laden

solution

101 Hydrogen Sulphide

102 Ammonia

200 First Reaction Chamber

201 Metal Sulphide

202 First Treated Solution

300 Second Reaction Chamber

302 Second Treated solution

400 Third Reaction Chamber 401 First Metal Compound

402 Second/Further Metal

Compounds

500 Fourth reaction chamber

502 Al(OH) ₃

501 Metal sulphide

602 Treated solution

700 Fifth reaction chamber

702 solution

800 Sixth reaction chamber

801 Further metal sulphides

802 Further treated solution

900 Seventh reaction chamber

3002 Ferrous/ferric hydroxide

901 FeS/FeS ₂

902 First portion of raffinate

910 heaps

903 Second portion of raffinate

1600 Solution comprising water, group I and II metals and (NH ₄ ) ₂ S0 ₄

904 Third portion of raffinate

1000 Eighth reaction chamber 1002 with CaCOs

1 Al(OH) ₃

2 Fe(OH) ₃

3 CaS0 ₄

1001 Further solution

1100 Ninth reaction chamber

1003 CaO

4 Fe(OH) ₃

5 Mn(OH) ₂

6 Mg(OH) ₂

7 CaS0 ₄ (7)

1500 Clean water

2100 PLS

1000 Treated solution

5100 ZnS

8001 NiCoS

8002 Further treated solution

4002 MnS

6002 CaO

5002 CaS0 ₄