FIELD OF THE INVENTION

The present invention relates to hydrometallurgical methods for recovering metals from sulfide minerals. The present invention also relates to a use of a sulfide mineral as an iron reductant in hydrometallurgical methods.

BACKGROUND OF THE INVENTION

The ever-increasing need of batteries for consumer electronics and e.g. electric vehicles increases the need for valuable metals recovered from the Earth’s crust through mining. Metals such as nickel and cobalt are essential for several battery chemistries.

With the increasing demand for raw materials for batteries, mining industry will have to look for alternatives for high grade and good quality ores. Recovering valuable metals from low grade, polymetallic and refractory ores increases the energy, chemicals, and water consumption of the process. Sulfide ores are often complex in structure, and may contain various valuable metals, such as copper, zinc, nickel, and cobalt, with a large fraction of iron that is currently considered as waste.

The major conventional technology for recovering sulfide ores is froth flotation followed by treating the obtained metal concentrates in smelters. The main drawback in froth flotation is the requirement of the relatively high grade and simple ores. If the ores are complex or mineral liberation requires too fine milling, flotation is ineffective and plenty of valuables end to the tailings. The use of smelters for producing metals from concentrates may be environmentally problematic due to emissions, moreover, these plants require massive amounts of feed material and are not flexible.

Another conventional technology is hydrometallurgical treatment. Hydrometallurgical treatment is more flexible than froth flotation, but requires high acid input and possibly strong and expensive oxidation system. Typically, a hydrometallurgical process consists of three main steps: leaching, solution concentration and purification, and metal or metal compound recovery. The leaching step b) utilizes leaching chemicals that may work by altering the redox state of the ore, or by altering the pH.

One hydrometallurgical option is bioleaching (e.g. BIOX ^® type of process). This process utilizes certain microorganisms, such as bacteria, that can produce the leaching chemicals (e.g., sulfuric acid and oxidant) from the sulfide mineral input itself, when supplemented with nutrients and air. Thus, chemical costs can be greatly decreased. However, the drawback is a slower processing capacity compared to conventional chemical leaching. Moreover, the system is somewhat sensitive to changes in operative parameters.

One of the main problems in hydrometallurgical processes is high iron dissolution. Selective leaching of valuable metals without major iron dissolution is challenging. Sometimes valuable metals exist in the same mineral as iron (e.g. (Fe _x Ni _y )å9S8, CuFeS2, (Zn,Fe)S) and thus destruction of iron containing mineral is required for liberation of the valuable metal. Oxidative leaching of sulfide ores is one of the major process options. In this case, the sulfide minerals in the ore will dissolve in the order of nobility. High oxidation power is often required for dissolution of nickel and cobalt sulfide minerals. Less noble minerals like pyrrhotite (FeS) will dissolve before the dissolution of these more noble minerals take place. Especially when low grade ores are dissolved, the iron concentration in pregnant leach solution (PLS) may be very high compared to concentration of valuable metals.

Hydrometallurgical processes require rather complex down-stream processing to produce metal products. In majority of these, ferric iron present in the PLS is a major problem and needs to be removed by chemical precipitation before recovery of valuable metals can take place. A fraction of the valuable metals co-precipitate with the ferric iron. The co-precipitated valuable metals are lost in the process, with no possibility to recover in a profitable manner. The amount of co-precipitating Cu, Zn, Ni, and Co depends on iron concentration and precipitation conditions and it is typically approx. 10 wt-% of the total amount of these valuable metals in the ore, but in some cases it can be significantly higher. The co-precipitation undoubtedly decreases the efficiency of the metal recovering process.

Thus, improved processes for metal recovery from sulfide ores are needed. BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to eliminate the drawbacks present in the prior art.

An object of the present invention is to provide a method for metal recovery from sulfide ores with an enhanced valuable metal efficiency and economic feasibility.

A further object of the present invention is to provide a method for metal recovery from sulfide ores with a reduced consumption of process chemicals.

A yet further object of the present invention is to provide a method for recovering valuable metals, as well as iron from sulfide ores with a decreased waste production and a decreased need of external acid.

These objects are attained with the invention having the characteristics presented below in the independent claims. Some preferable embodiments are disclosed in the dependent claims.

The features recited in the dependent claims and the embodiments in the description are mutually freely combinable unless otherwise explicitly stated.

The exemplary embodiments presented in this text and their advantages relate by applicable parts to all aspects of the invention, both the method and the use aspects, even though this is not always explicitly mentioned.

A method for recovering metals from a sulfide-containing feed material according to the present invention comprises the steps of a) mixing the feed material with an aqueous solution, forming a working suspension comprising sulfide minerals, the sulfide minerals originating from the feed material, b) dissolving valuable metals from the feed material in the working suspension by a leaching process with leaching chemicals, forming a pregnant leach solution (PLS) comprising ferric iron and valuable metal ions, c) circulating the PLS or at least part of the PLS back to step a), wherein the PLS constitutes the aqueous solution in step a), d) reducing the ferric iron contained in the PLS by the sulfide minerals originating from the feed material, forming a reduced PLS comprising ferrous iron and valuable metal ions, and e) optionally recovering the valuable metal ions from the reduced PLS, thus forming a ferrous sulfate solution.

The present invention provides a use of a pregnant leach solution in a hydrometallurgical process for oxidizing and/or dissolving sulfide minerals contained in a feed material. In one embodiment, the pregnant leach solution is derived from leaching of sulfide minerals.

The present invention further provides a use of sulfide minerals or a sulfide- containing waste material in a hydrometallurgical process for reducing ferric iron contained in a pregnant leach solution produced in the hydrometallurgical process. In one embodiment, the pregnant leach solution is produced in leaching of sulfide minerals in said hydrometallurgical process.

The inventors have surprisingly found that the ferric iron produced in a leaching process may be reduced to ferrous iron using the sulfides originating from the feed material by recirculating the leachate solution (pregnant leachate solution PLS) back to step a) and mixing the PLS with the sulfide minerals originating from the feed material. In an embodiment, only a feed material containing sulfide minerals is needed in reducing the ferric iron to ferrous iron by utilizing recycling and double leaching. In the method of the present invention, ferric iron oxidizes the feed material and reduces at the same time to ferrous iron leading thus to an efficient recovery of the valuable metals.

It is an advantage of the present invention that the valuable metals can be recovered before iron removal. Valency of iron has a significant effect on valuable metals recovery. Selective recovery of valuable metals is possible using either sulfide precipitation, solvent extraction or ion-exchange technologies if the iron is in its ferrous form instead of ferric form.

It is an advantage of the present invention that the need of external acid may be reduced, or completely eliminated. Using the method according to the present invention, the chemicals needed for reducing ferric iron into ferrous iron originate directly from the process itself. In conventional hydrometallurgical treatment, dissolution of sulfide minerals, such as pyrrhotite, consumes large amounts of acid. This acid consumption can be completely avoided or partly reduced by using the method according to the present invention. The ferric iron contained in the PLS can oxidize and/or dissolve the sulfide minerals contained in the feed material. Thus, the process is compatible with the concept of circular economy.

It is an advantage of the present invention that iron may be reduced into ferrous iron, whereby the iron recovery may take place after recovering the valuable metals. When recovering iron as ferrous iron after the valuable metals recovery, loss of the valuable metals induced by co-precipitation is eliminated.

It is a further advantage of the present invention that the ferrous iron may also be recovered from the ore as a by-product and not as a waste material, increasing the economic feasibility of the method. A further advantage of the present invention is a reduced total process time.

It is an advantage of the present invention that valuable metals may be recovered from sulfide ores that are poor in valuable minerals. Using the method according to the present invention, the recovery of valuable metals may be economically feasible from even these ores with low target metal content. A further advantage of the present invention is that it may utilize sulfide minerals previously considered as waste, making the invention compatible with the concept of circular economy.

It is also an advantage of the present invention that valuable metals may be recovered from fine sulfide minerals containing feed materials, such as flotation concentrates, for example. Using the method according to the present invention, the recovery of valuable metals may be economically feasible from even such fine feed materials.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 presents a simplified process chart for a metal recovery method according to the present invention,

Figure 2 presents a simplified process chart for a metal recovery method according to an embodiment of the present invention, comprising iron recovery and filtration steps. DETAILED DESCRIPTION OF THE INVENTION

In a first aspect of the invention, a method for recovering metals from a sulfide- containing feed material is provided. In the method, feed material is processed in an aqueous solution with leaching chemicals in a leaching process, whereby the metal ions of the feed material dissolve.

Figure 1 provides a simplified process scheme for the present invention. In the process, feed material is directed to a mixing step a). Thereafter, a working suspension produced in the mixing step a) is directed to a leaching step b), wherein the sulfide minerals contained in the feed material are dissolved, producing a pregnant leach solution (PLS). After the leaching step, the PLS or at least part of the PLS is circulated back to step a), now also termed a reduction step d), wherein sulfide minerals originating from the feed material reduce ferric iron contained in the PLS, with a simultaneous partial oxidation and/or dissolution of the feed material. The reduced PLS or at least part of the reduced PLS is then directed back to the leaching step b) for a next round of leaching. The reduced PLS or at least part of the reduced PLS may be directed to the valuable metal recovery process e).

In a second aspect of the present invention, a use of a pregnant leach solution in a hydrometallurgical process for oxidizing and/or dissolving sulfide minerals contained in a feed material is provided. In one embodiment, the pregnant leach solution is derived from leaching of sulfide minerals.

In a third aspect of the present invention, a use of sulfide minerals or a sulfide- containing waste material in a hydrometallurgical process for reducing ferric iron contained in a pregnant leach solution produced in the hydrometallurgical process. In one embodiment, the pregnant leach solution is produced in leaching of sulfide minerals in said hydrometallurgical process.

According to an embodiment of the present invention, the feed material originates from a sulfide mineral. Sulfide minerals are a class of minerals containing a sulfide (S ² ) or persulfide (S2 ² ) as the main anion. In one embodiment, the feed material originates solely from sulfide mineral(s).

In an embodiment of the present invention, the sulfide mineral may comprise iron sulfide minerals comprising nickel, cobalt, zinc, copper, manganese, or any combination thereof. Usable minerals may be selected, for example, from pyrite, pyrrhotite, troilite, pentlandite, talnakhite, millerite, heazlewoodite, nickeline, maucherite, gersdorffite, mackinawite, linnaeite, polydymite, cobaltite, sphalerite, chalcopyrite, chalcocite, covellite, bornite, cubanite, or any combination thereof. Iron sulfide minerals also comprise a small fractions of other metals. These fractions of other metals may be considered as valuable metals due to their commercial value. In an embodiment, the valuable metals may be selected from a group comprising Cu, Zn, Ni, and Co. In an embodiment, the valuable metals are selected from Cu, Zn, Ni, and Co.

In an embodiment, the feed material originates from a waste material, such as tailings or gangue. For example, old nickel flotation tailings containing nickel, cobalt, zinc and/or copper sulfides and possibly pyrite and/or pyrrhotite may be used as a feed material for the present invention. Certain low-grade ore bodies may even be considered as a waste material in the mining industry. These low-grade ore bodies are not mined or are separated from the high grade ore as a gangue. In an embodiment, the feed material originates from low-grade ore bodies or gangue. The method of the present invention is suitable for recovering valuable metals in an economically feasible way from waste materials and from ores that have previously been considered as inextractible.

In an embodiment of the present invention, the feed material is directed into a reactor and mixed with an aqueous solution. In an embodiment, the feed material may be pre-treated prior to directing into the reactor. Pre-treatment of the feed material may comprise e.g. grinding, crushing, sieving, magnetic separation, flotation, density separation, screening, or any combination thereof.

In an embodiment of the present invention, a working suspension is formed upon mixing the feed material with the aqueous solution. In an embodiment, the working suspension comprises sulfide minerals originating from the feed material.

The method according to an embodiment of the present invention comprises a step wherein the valuable metals are dissolved from the feed material in a leaching process by bringing the working suspension comprising the feed material into contact with leaching chemicals. The leaching step b) may be carried out using methods and leaching chemicals known in the art. In an embodiment, the leaching chemicals comprise acid, preferably sulfuric acid, and oxidant. In the leaching process, the leaching chemicals dissolve the mineral in the aqueous solution. As a result, a pregnant leach solution (PLS) forms. The PLS comprises the metal ions originally contained in the feed material.

The leaching process of sulfide minerals is a series of chemical redox reactions. The leaching process typically comprises the steps of i) spontaneous oxidation of sulfides in the ore to thiosulfate, sulfate or elemental sulfur by a ferric ion, ii) oxidation of ferrous ion into ferric ion, iii) oxidation of thiosulfate and possibly elemental sulfur from step i) to sulfate.

The ferric iron produced in step ii) is used in step i) to oxidize the sulfide. Large amounts of ferric iron are present in the PLS due to the intermediate reaction (step ii)). In most minerals, iron is present in its ferrous form. In some minerals, such as goethite, however, iron may exist in its ferric form.

In an embodiment of the present invention, the leaching process may be a bioleaching process, wherein the leaching chemicals are produced by microorganisms, such as bacteria. The microorganisms catalyse the dissolution reaction by oxidizing ferrous iron into ferric ion and thiosulfate into sulfate using oxygen. There are numerous suitable microorganisms known in the art. A skilled person is able to select the best one for the process in question depending on the conditions of the process, for example. In an embodiment, bacteria such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans , Leptospirillum ferrooxidans, or any combination thereof, may be used in the bioleaching process.

Along with the dissolution of iron in the leaching process, the valuable metals are dissolved in a similar manner. Thus, the PLS comprises the metal ions originating from the feed material, alongside with sulfate anions. In an embodiment of the present invention, the PLS is circulated from the leaching step b) back to step a) of the process, i.e. back to the step where the feed material is mixed with an aqueous solution. According to the present invention, the PLS constitutes the aqueous solution in step a). This circulation enables to reduce the ferric iron contained in the PLS by directly utilizing the feed material itself.

In an embodiment of the present invention, the PLS is mixed with the feed material in step a), forming a working suspension comprising both sulfides originating from the feed material and ferric iron dissolved in the leaching step. The ferric iron contained in the PLS is reduced by the metal sulfides originating from the feed material, yielding ferrous iron, elemental sulfur, thiosulfates and sulfates. By utilizing the feed material as a source of iron reductant, the need of external chemical reductants may be decreased or even completely eliminated.

In an embodiment, step d) may be termed a reduction step or a pre-leaching step. The ferric iron contained in the PLS oxidizes sulfide ions originating from the feed material, the sulfide ions being simultaneously leached from the feed material into the working suspension. Reduction of the ferric iron may be depicted with the reaction:

Ferrous sulfide FeS in the reaction comprises the sulfide minerals originating from the feed material. Using the feed material directly as an iron reductant has the advantage of reducing or completely eliminating the need of external acid. In conventional hydrometallurgical methods, large amounts of acid are required to dissolve sulfide minerals in the leaching step. This acid consumption may be remarkably reduced or completely avoided by using the leached ferric iron to oxidize and/or dissolve the sulfide minerals contained in the feed material.

The ferric iron ions Fe ³⁺ originate from the leached feed material and are carried to the reduction step along with the circulated PLS. As a result of the reduction reaction, iron is in its ferrous form Fe ²⁺ . An advantage of the present invention is that the iron contained in the PLS after the reduction step is in ferrous form, enabling valuable metals recovery before iron removal. In one embodiment, the ferric iron concentration in the PLS is at most 1 g/L. In one embodiment, the ferric iron concentration in the PLS is < 1 g/L. In one embodiment, the ferric iron concentration in the PLS is in the range of 0.1 g/L - 1 g/L. In one embodiment, the ferric iron concentration in the PLS is 0.2 g/L, 0.3 g/L, 0.4 g/L, 0.5 g/L, 0.6 g/L, 0.7 g/L, 0.8 g/L or 0.9 g/L.

Circulation of the PLS back to step a) of the process serves two purposes: i) the ferric iron contained in the PLS reduces to ferrous iron, therefore removing the need for a separate iron removal step and enabling valuable metals recovery prior to iron removal, and ii) the ferric iron contained in the PLS serves as a pre-leaching chemical for the sulfide minerals contained in the feed material, thus reducing or eliminating the need for external acids as a leaching chemical. Thus, there is no need for the removal of ferric iron before the recovery of the valuable metals.

In an embodiment of the present invention, a reduced PLS is formed in the reduction step d). In other words, after the reduction step, the reduced PLS comprises the ferrous ions obtained in the reduction step, as well as the valuable metals dissolved in the leaching step. In addition, the reduced PLS also comprises non-leached feed material. The reduced PLS or at least part of the reduced PLS is directed back to the leaching step b) for the next round of leaching. In one embodiment, there is no need for milling/grinding or floatation between the leaching processes in the method of the present invention. In one embodiment, there is no need for feeding additional oxygen between the leaching processes in the method of the present invention

The valuable metals may be recovered from the PLS in step e) of the method, as also depicted in Figure 1 .

In an embodiment of the present invention, the valuable metals may be recovered as metal sulfides, metal sulfates, or metal hydroxides. The present invention is suitable for producing metal sulfides, common metal compounds in metal recovery industry. The valuable metals may also be recovered as sulfates. An advantage of recovering the valuable metals as sulfates is that the sulfates may be directly suitable for the production of rechargeable batteries. The method according to the present invention is also suitable for producing metal hydroxides, especially in small plants.

According to an embodiment of the present invention, recovering the valuable metal ions may be carried out by ion-exchange, solvent extraction or chemical precipitation methods, or any combination thereof.

In an embodiment, recovering the valuable metals may be carried out by ion- exchange methods. Ion-exchange involves the use of ion-exchange resins. Ion-exchange resins are insoluble matrices typically in the form of small microbeads, with a radius of 0.1-0.8 mm. The microbeads are typically fabricated from an organic polymer substrate. The microbeads are typically porous, providing a large surface area. Ion-exchange resins function by binding or trapping ions from e.g. a solution, accompanied by releasing other ions into the solution.

In an embodiment, the valuable metals are recovered by using chelating ion- exchange resins. Chelating ion-exchange resins may comprise reactive functional groups covalently attached to a polymer backbone. The reactive functional groups, such as iminodiacetate or b/s-picolylamine, chelate to the valuable metal ions to trap them.

In an embodiment, recovering the valuable metals may be carried out using solvent extraction methods. In solvent extraction, a diluent, preferably an organic solvent, such as oil, is mixed with the PLS to form and emulsion and allowed to separate. The metal will be extracted from the PLS to the diluent phase, resulting in a diluent phase bearing the metal, and a raffinate containing the remains of the PLS.

In an embodiment, recovering the valuable metals may be carried out using chemical precipitation methods. The valuable metals are precipitated out of the PLS into a solid form as an insoluble compound suitable for further purification. In an embodiment, the valuable metals may be precipitated as sulfides, sulfates or hydroxides.

In an embodiment of the present invention, the method further comprises a step of removing or recovering ferrous iron from the tailings. Figure 2 presents a simplified process scheme for such a method. Ferrous iron may be removed from the tailings as ferrous sulfate using crystallization. The invention provides a method for acquiring ferrous sulfate as a profitable byproduct alongside with the valuable metals, whereas in a conventional process iron will be lost to iron- gypsum-precipitate that is considered as waste.

In addition to the embodiments described herein, the method for recovering metals from sulfide-containing feed material may further comprise steps and processes needed for an optimal metal recovery. These steps, such as filtration and washing steps, may be carried out using conventional process equipment and process chemicals known to a person skilled in the art. These further steps may be incorporated or carried out in any appropriate phase of the processing cycle.

In an embodiment, the method may further comprise a filtration step or several filtration steps. The filtration step or filtration steps may be carried out for example after step a), after step b), and/or after step d).

Embodiments of the present invention comprising filtration steps are depicted in Figure 2. A filtration step may optionally be performed after steps a) and/or d), i.e., after mixing the feed material with the aqueous solution and/or after reducing the ferric iron contained in the PLS. Liquid portion of the working suspension is directed to the valuable metals recovery step e), and solid portion of the working suspension is directed to the leaching step b) for a next round of leaching. Another filtration step may optionally be performed after the leaching step b). Liquid portion of the PLS is circulated (step c)) back to step a) and/or d) of the process for reduction. Solid leach residue is discarded and/or directed to a tailings pond.

The method according to the present invention may also be used in a ferrous iron production process. In this embodiment, iron is considered the valuable metal. The concentration of metals such as copper, zinc, nickel, and cobalt is likely low in the feed material, such that these metals may be considered as impurities. These impurities need to be removed from the working suspension. With the presented method, the impurity removal is enhanced with the help of the iron reduction process. The metals considered as impurities may be removed from the process prior to iron removal, rendering the iron recovery process more efficient and yielding a more pure iron product. Iron may be recovered from the process as ferrous sulfate by crystallization.

EXAMPLE By using the method according to the present invention, the ferric ion concentration of pregnant leach solutions may be greatly reduced. In laboratory experiments, the ferric ion concentration in a PLS has been reduced from 30 g/L to 1 g/L. In the experiment, a pregnant leach solution containing cobalt (Co) in a concentration of 690 mg/L, nickel (Ni) in a concentration of 290 mg/L, and iron as ferric iron (Fe ³⁺ ) in a concentration of 35 g/L. 100 mL of the PLS was mixed in a vessel at 60 °C with 38 g of fresh feed material, containing 230 g/kg of pyrrhotite. During agitation, concentration of Fe ³⁺ decreased to 0.9 g/L.