MADE THERE FROM

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a PCT International Application of U.S. Patent Application No. 13/444,730 filed on April 1 1 , 2012. The entire disclosure of the above referenced application is incorporated herein by reference.

FIELD

[0002] This invention relates to the recovery of lead and other metals from polymetallic lead-bearing mineral resources, such as ores and tailings, and a composite polymetallic concentrate made there from.

BACKGROUND

[0003] This section provides background information related to the present disclosure which is not necessarily prior art.

[0004] After mining, polymetallic lead-bearing ores are typically crushed, ground, and separated into individual mineral concentrates (such as a lead concentrate, a zinc concentrate, a copper concentrate, and other mineral

concentrates) through one or more sequential flotation processes. These individual mineral concentrates are then further processed using other metallurgical techniques that often use high temperatures to extract the metals from the minerals. [0005] A disadvantage of the current methods for the sequential flotation of polymetallic mineral resources into two or more individual mineral concentrates is that there is cross-contamination of the minerals contained in each of the individual concentrates produced {e.g., a portion of the lead minerals in the mineral resource report to the other individual concentrates), thereby limiting the overall recovery of the metals contained in the complex or polymetallic ores, because these contaminant metals are typically lost in downstream processing.

Current processing technology cannot efficiently recover these contaminant metals, so the seller is usually not compensated for these contaminant metals, and may even be penalized for contaminant metals in the concentrate.

[0006] Another disadvantage of the current methods for the sequential flotation of polymetallic mineral resources into two or more individual mineral concentrates is that there is additional loss of metal to the final waste product or tailing from the milling process. Because of these residual metal values, tailings are often the subject of environmental regulation, as they are in the United States. In the United States most tailings are deemed hazardous waste as defined RCRA (40 CFR Part 261 ), but benefit from the Bevill exclusion, an amendment to the Resource Conservation and Recovery Act (RCRA) which allows tailings to be stored in at certain permitted locations. However there are continuing obligations of monitoring and management of these permitted locations where tailings are stored.

[0007] Table 1 shows an example of the distribution of the metals when separate lead, zinc, and copper concentrates are prepared by a sequential flotation process. As indicated in Table 1 , there is cross contamination of metals in the various concentrate, and a significant amount of metal remains in the tailing, and thus is not recovered.

[0008] Between the metals that are not recovered at all and is lost to the tailings, and the metals that report as contaminants in another metal

concentrates and are lost in subsequent processing, a significant amount of the metal present in a polymetallic mineral resource can be lost. Moreover the lost metals can create environmental issues in the tailings, residues, and slags where they end up.

[0009] In some instances, for example where the ore body is primarily zinc and lead, and other associated metal values are low, it may be more expedient to produce a bulk concentrate by floating substantially all of the lead and the zinc. The resulting bulk concentrate can be processed, for example according to the Imperial Smelting Process. This is only appropriate for ores of certain compositions, for example where there is more zinc than lead. There are significant financial penalties to the seller of bulk type concentrates, discouraging their production whenever individual concentrates can be produced instead. Moreover there can still be significant losses of the individual metal constituents, and the resulting metal products often contain significant impurities.

SUMMARY

[0010] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0011] Embodiments of the present invention provide for the improved recovery of lead, zinc, and other metals from polymetallic mineral resources containing lead, zinc, and other metals, for example from ores containing galena (PbS), sphalerite/blende (ZnS), chalcopyrite (CuFeS ₂ ), and additional minerals of value. These additional minerals may include metal compounds that contain lead, zinc, copper, nickel, cobalt, silver, cadmium, and others. Not only do embodiments of the present invention provide increased metal recovery, these methods can also reduce the complexity and ultimately the cost of the milling operation, can reduce the amount of metals that are contained in the final waste product or tailing from the milling process, and can reduce the metal lost in recovery from the resulting concentrates.

[0012] Generally, in a preferred embodiment of this invention, a method for improved recovery of metals contained in polymetallic mineral resources comprises the production of a composite concentrate consisting of a lead mineral and at least two other metallic mineral compounds. In contrast to current methods of producing individual metal concentrates, the composite concentrate is prepared to maximize the content of the lead in the concentrate, rather than to minimize the content of contaminating metals. This reduces the inhibiting effect that suppressing contaminating metals has on the recovery of the lead, resulting in more recovery of the lead, and less lead reporting to other concentrates or tailings or other waste.

[0013] In some embodiments, chemical additives or reagents can be added to the flotation process in such a manner as to increase the flotation of other metallic mineral compounds (such as Pb, Zn, Cu, Ni, Co, Ag, Cd, and other metallic mineral compounds) which are further processed by collection, dewatering, and filtering into a final composite concentrate. Additional chemical additives can be added to the flotation process in such a manner to depress a portion of the zinc and/or other compounds contained in ore, for example where it is desired to prepare a separate concentrate of zinc and/or such other metals. However, these additives are preferably selected, and their quantities controlled, so as not to unduly suppress the flotation the metals that are desired in the composite concentrate, chiefly lead but also including other metals that either cannot be efficiently recovered from a separate concentrate or whose value would otherwise go uncompensated (or even penalized) in a separate concentrate. This suppression of undesirable metals, e.g., zinc which can be expensive to recover from a composite concentrate, is balanced against the resulting suppression of other metals, e.g., silver, which would also be suppressed and whose value would be lost in a zinc concentrate or a tailing.

Additional zinc can be accepted in the composite concentrate if the additional cost of processing the zinc is less that the value of the additional other metals recovered.

[0014] However, preferably not all of the zinc is captured in the composite concentrate, and a secondary zinc concentrate is prepared. This is because zinc can be efficiently and economically recovered separately, after the flotation of the composite concentrate. A significant portion of the zinc is collected in the composite concentrate because its suppression might unduly suppress the collection and reduce the overall recovery of lead, copper, and silver and other metals.

[0015] At least one additional flotation process may be used to recover additional zinc compounds into a zinc concentrate. Chemical additives can be added to these flotation processes in such a manner as to increase the flotation of the zinc compounds or other metallic mineral compounds which are further processed by collection, dewatering, and filtering into final concentrate(s).

[0016] Table 2 shows the metal distribution of a composite concentrate (produced for downstream hydrometallurgical metal recovery), zinc concentrate, and tailing prepared from the same ore as the separate concentrates (typically produced for downstream pyrometallurgical metal recovery) shown in Table 1 .

Table 2 shows that a substantial portion of the metals report to the composite concentrate, and that a much lower portion of the metals report to the tailings. Table 3 shows the improved recovery of metals and the reduction of metals reporting to the tailings between the conventional preparation of individual concentrates versus the preparation of a composite concentrate according to the principles of this invention: Table 3

Mill Recovery Comparison Metal

Pb Zn Cu Ag

Cross Contamination

Improvement 1.38% 5.31% 12.85% 0.00%

Tailings Improvement 3.89% 15.83% 32.83% 1.07%

Net Improvement 5.27% 21.14% 45.68% 1.07%

[0017] The lead from this composite concentrate is then recovered with subsequent downstream processing. Conventional pyrometallurgical processes would experience complications in recovering the lead from the composite

concentrate, and would be unable to recover the other metals (except perhaps zinc) which would report to a slag or a dross. However, the lead can be recovered from the composite concentrate using a Flubor process, developed by Engitec SA, and disclosed in U.S. Patent No. 5,039,337, issued August 12, 1991 , entitled Process for Producing Electrolytic Lead and Elemental Sulfur from Galena, the entire disclosure of which is incorporated herein by reference. In accordance with the Flubor process, lead is leached from the lead concentrate with an acidic aqueous solution of ferric fluoroborate to form ferrous fluoroborate, lead fluoroborate, and elemental sulfur according to the reaction:

2 Fe(BF ₄ ) ₃ + PbS -> 2 Fe(BF ₄ ) ₂ + Pb(BF ₄ ) ₂ + S the remaining solid residue, composed of elemental sulfur and metal-bearing gangue is removed. The solution of ferrous fluoroborate and lead fluoroborate circulates to a diaphragm electrolytic cell, where pure lead is deposited at the cathode while at the anode ferrous ion is oxidized to ferric ion. The solution of ferric fluoroborate regenerated at the anode is reused in the leaching step. By operating under suitable conditions, lead can be selectively dissolved and separated from the other metals, small amounts of which are contained in galena together with said lead. Sulfur produced by the reaction can be separated from the metal-bearing gangue by extraction with a solvent, or by flotation. The advantages of the Flubor Process include the reduced energy consumption and reduction of slag and S0 ₂ emissions that are typical by-products of pyrometallurgical recovery processes.

[0018] Because of the variety of metals, and the relatively low levels of concentration, conventional pyrometallurgical processes could not economically recover the metals remaining in the metal-bearing gangue. However, the inventors have discovered that the metals can be recovered from the metal-bearing gangue from a polymetallic concentrate using a hydrometallurgical process, such as a recovery process based upon the Ezinex™ Process, developed by Engitec

Technologies, S.p.A. This process, originally adapted for recovery of zinc metal from EAF dust, involves leaching of metals primarily using ammonium chloride NH ₄ CI, and successively recovering metals from the solution in order of the their

electronegativity by additions of zinc and subsequently electrowinning. The zinc recovered in the process can be recycled for the purification of the solution by precipitation of Pb, Cu, Ni, Co, Ag, and other metals in the form of a cement.

[0019] Various embodiments of the present invention provide flexibility in handling the co-products from the production of the original composite

concentrate. In some embodiments some or all these co-products can be processed into separate concentrates that can be added to the initial composite concentrate for processing with the composite concentrate. In other embodiments, some or all of these co-products can be processed into separate concentrates and added to the metal -bearing gangue resulting from the initial lead recovery process, and

processed. In still other embodiments, some or all of these co-products can be processed into commercially acceptable concentrates and simply sold for processing by others. This flexibility in handling polymetallic mineral resources allows for the most economical exploitation, even as the composition of the polymetallic mineral resource, and the various metal prices change.

[0020] The solids remaining after the lead leaching and recovery can be processed themselves, or concentrates from the further processing of material remaining after the production of the lead concentrate, can be added before processing. The metals can then be leached from this material with an aqueous- based leaching solution containing chloride ions and ammonium ions, prepared, for example, by dissolving, in water, chlorides of alkaline and/or alkaline-earth metals together with ammonium chloride.

[0021] The concentration of chloride ions varies within the range of 50- 250 g/l; the concentration of ammonium ions varies within the range of 20-150 g/l. The pH of the solution is neutral, i.e. within the range of 6.5 - 8.5. The leaching is effected under heat, at a temperature varying within the range of 100°C - 160°C, and a pressure varying within the range of 150 kPa-1000 kPa. The duration of the leaching phase varies according to the nature of the solid matrix and the content of metals to be recovered. The leaching typically lasts from one to ten hours.

[0022] The pH is adjusted to precipitate iron. The pH is further adjusted and the solution is subjected to a sequential cementation recovery in which the addition of less noble metals are used to precipitate more noble metals (Ag, Cu, Pb, Co, Ni, Cd and other metals can be recovered in this manner). After the sequential recovery of these metals either separately or in groups, zinc can be recovered from the solution by electrowinning.

[0023] The metals recovered during the sequential cementation are valuable and readily marketable. The determination of whether to cement the metals separately or in groups is partially a factor of the marketability of the metals in groups versus the economics of separate cementation. It is also a factor of the quantity of the individual metals. The material remaining after the leaching has had most of the metal removed, so that it possibly is no longer considered a hazardous waste in most jurisdictions, and can be used in road and building construction. Having gone through such a severe leaching the comparatively mild leaching used in the TCLP (Toxicity Characteristic Leaching Procedure) testing potentially allows the test to be passed and the waste material to be not hazardous.

[0024] Table 4 compares the payable metal recovery from a

conventional process of preparing multiple individual concentrates with the payable metal recovery from the creation of a composite concentrate in accordance with the principles of this invention. Payable metal recovery refers to the metal values for which the seller is actually compensated, which is less than the actual metal value present, because of various discounts for metal contaminants in the product.

Because of the improved recoveries from the starting material, and the improved recoveries from the concentrates, for the feed material represented in Tables 1 and 2, the producer would be paid for an additional 9.8% lead, an additional 20.5% zinc, and additional 36.5% copper, and an additional 34.8% silver. The additional metal product not only represents more value recovered, but it also means less metal is present as a contaminant in the resulting tailings, residues and slags.

[0025] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF DRAWINGS

[0026] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0027] Fig. 1 is a flow chart of a process for the improved recovery of lead and other metals from poly-metallic lead-bearing mineral resources in accordance with the principles of this invention.

[0028] Fig. 2 is a flow chart showing the preparation of the composite concentrate and secondary zinc concentrate in accordance with the principles of this invention. [0029] Fig. 3 is a flow chart of the process for recovering non-ferrous metals from the materials remaining after the recovery of lead according to the process of Fig. 1 .

[0030] Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

[0031] Example embodiments will now be described more fully with reference to the accompanying drawings.

Preparation of a Composite Concentrate

[0032] A preferred embodiment of a process in accordance with the principles of this invention is shown in Fig. 1 . As shown in Figure 1 , at 22 a polymetallic mineral resource containing lead, zinc and other metals (e.g., copper, nickel, cobalt, and/or silver) is crushed or ground in a mill as is known. Typically, these ores contain metals in an oxidized form, more commonly as metal sulfides, such as galena (PbS) and sphalerite (ZnS). At 24 a composite concentrate is prepared from the crushed/ground ore, by flotation. In contrast to conventional methods of producing lead concentrates, which are treated with various

suppressants to suppress the flotation of other metals, such as sphalerite, even at the cost of recovering lead, according to a preferred embodiment of this invention, suppressants are only used to the extent that they do not substantially affect the recovery of metals, chiefly lead, desired in the composite concentrate. The result is a greater recovery of lead, which is affected to some extent by the suppressants. However, it also results in other metals, such as some of the sphalerite, and various copper, nickel, cobalt, silver metals reporting to the composite concentrate. A lead concentrate is not conventionally formed in this manner because the additional metal values associated with the lead concentrate will not be fully recovered in

conventional lead recovery processes, and in some cases may even result in an economic penalty by the purchaser, who may have trouble recovering the additional metals in the lead concentrate, even though they can be valuable. The resulting concentrate is a composite concentrate, because in addition to increased lead, it has increased amounts of other metals that are usually excluded from a conventional lead concentrate, which, depending upon the source mineral resource, may include zinc, copper, nickel, cobalt, silver, and cadmium.

[0033] A zinc concentrate may be produced, which in some cases can be efficiently processed using conventional metal recovery techniques, such as a roasting/leaching/electrowinning process.

[0034] The step 24 for the preparation of a composite concentrate is shown in more detail in Fig. 2. After the polymetallic ore is ground and a

suppressant such as ZnS0 ₄ is added to suppress zinc, a collector such as xanthate is added to collect sulfides and an iron suppressant, such as NaCN is added to suppress iron (the NaCN addition also has the additional benefit of neutralizing any copper sulfate present in the ore) . At 102 the ground ore is subjected to a froth flotation separation. A frother, such as an aliphatic alcohol is added. Dilution water is added at 104 and the froth containing the lead and other metal values is cleaned, and the composite concentrate is collected. At 106, the material that does not float is subjected to a second flotation. A frother, such as an aliphatic alcohol is added. An activator, such as CuS0 ₄ is added to promote the flotation of zinc. A collector such as xanthate is added to make the minerals hydrophobic. Dilution water is added and at 108 the froth bearing the zinc is cleaned, and at 1 10 filtered, resulting in a zinc concentrate. This zinc concentrate can be added to the composite concentrate (for example if the other metals in zinc concentrate were more valuable that the additional cost of handling the extra zinc) or, as potentially a better economic alternative, combine it with the residue from the composite concentrate after going through the Flubor process and then taken to the co-product process. In some cases it can be more economically processed using conventional zinc concentrate recovery techniques, such as a roasting/leaching/electrowinning process.

[0035] The tailing from the Zinc Cleaner Flotation at 108 can have sufficient non-zinc metal values that in addition to a zinc concentrate, a non-zinc metal concentrate may be produced as well. Depending upon the composition of this non-zinc metal concentrate, it can be subjected to the lead recovery process with the composite concentrate. If these non-zinc metal values do not include lead, then some or all of these non-zinc metal values can be can be subjected to a regrinding and flotation, refloated as part of the process of making the composite concentrate, or added to the composite concentrate, for example after the cleaner step and before filtration. If the tailing from the zinc cleaner flotation at 108 contains zinc, it can be added back to the zinc rough flotation at 106 to recover additional zinc in the zinc concentrate. Alternatively, these non-zinc metal values can be sold for processing by third parties. By recirculating zinc cleaner tailing or capturing the zinc cleaner tailing and adding directly to downstream processes, there may be additional overall achieved metal recovery from the polymetallic resource. Recovery of Lead from the Composite Concentrate

[0036] Referring again to Fig. 1 , at 26, the composite concentrate is subjected to a leaching step with an aqueous solution of ferric fluoroborate in fluoroboric acid leaches lead from the composite concentrate. At 28, the leachate is separated from the solids which include gangue as well as the other metals, which are not soluble in fluoroboric acid. At 30, lead metal is recovered by electrowinning, and the solution is recirculated for leaching of new concentrate.

[0037] At 32, the material remaining after the production of the composite concentrate is subjected to additional flotation and filtering operations to produce one or more concentrates. This will typically include at least a zinc concentrate. What concentrates will be produced will depend upon the content of the starting mineral resource, and the relative prices of the metal constituents, and can include a secondary lead concentrate, a zinc concentrate, and a copper concentrate. Preferably enough of the metal is recovered from the material at step 32, that the resulting tailings can be disposed of without the need for special handling. Typically at least a zinc concentrate is produced which, particularly if it is lead-bearing, can be subjected to a lead recovery process with the composite concentrate in step 26. As indicated by path 34, some or all of the zinc concentrate can be included in the leaching step 26, but if it does not contain lead, it will simply interfere with the operation of the lead recovery steps 26-30, and require a larger system to handle the volume of material. As indicated by path 36, some or all of the zinc concentrate can be included in the further processing of the residuals from the filtering step 28. Finally, as indicated by path 38 some or all of the concentrates can be sold for processing by third parties. [0038] It may be that there are sufficient non-zinc metal values in the tailing from the production of zinc concentrate at 32 that in addition to a zinc concentrate, a non-zinc metal concentrate may be produced as well. Depending upon the composition of this non-zinc metal concentrate, it can be subjected to the lead recovery process with the composite concentrate at 26, as indicated by path 42. If these non-zinc metal values do not include lead, then some or all of these non-zinc metal values can be can be included in the further processing of the residuals from the filtering step 28, as indicated by path 44. Finally, as indicated by path 46 some or all of these non-zinc metal values can be sold for processing by third parties.

Recovery of Other Metals From the Composite Concentrate

[0039] At 40, the solids from the filtering step 28, and any co-product concentrates from the production of the lead concentrate, are leached primarily using ammonium chloride NH ₄ CI and successively recovering metals from the solution in order of the their electronegativity by additions of zinc, and subsequently

electrowinning. The zinc recovered in the process can be recycled for the recovery of other metals, including Pb, Zn, Cu, Ni, Co, Ag, and other metals.

Leaching of Metals

[0040] The leaching of Additional Metals can be effected with an aqueous-based leaching solution containing chloride ions and ammonium ions, prepared, for example, by dissolving, in water, chlorides of alkaline and/or alkaline- earth metals together with ammonium chloride. [0041] The concentration of chloride ions preferably varies within the range of 50-250 g/l; and the concentration of ammonium ions varies within the range of 20-150 g/l.

[0042] The pH of the leaching solution is neutral, i.e., within the range of 6.5 - 8.5.

[0043] The leaching is preferably effected under heat, at a temperature varying within the range of about 100°C to about 160°C, and a pressure varying within the range of between about 150 kPa and about 1000 kPa.

[0044] The duration of the leaching phase varies according to the nature of the solid matrix and the content of metals to be recovered. The leaching typically lasts from one to ten hours.

[0045] Leaching under the above operative conditions results in the dissolution of the non-ferrous metals present in the solid matrix and at the same time, the oxidation of the metallic sulfides possibly present. The final pH of the solution can decrease to values lower than 1 in relation to the composition of the feeding to the reactor and operative conditions.

[0046] The leaching solution comprising chloride ions and ammonium ions, is capable of effectively dissolving the non-ferrous metals of interest, reducing the addition of sulfuric acid and/or sulfates in the leaching solution. The addition of sulfuric acid and sulfate ions can be fact undesired, as, at the end of the extraction process, they generally should be eliminated from the leaching solution (for example, by precipitation in the form of calcium sulfate) with a consequent increase in energy costs, consumption of chemical reagents and production of waste-products to be disposed of.

[0047] The dissolution reaction of the metallic sulfides is believed to be the following:

Me ₂ S _n + 2n NH ₄ CI + n/2 0 ₂ → 2 Me(NH ₃ )mCl _n + n H ₂ 0 + n S (1 ) in which, when Me = Ag, Cu, Pb, Ni, Co and Zn, then n = 1 or 2, m = 0 or 2.

[0048] The dissolution reaction of metallic oxides is the following:

Me ₂ O _m + m NH ₄ CI → 2 Me(NH ₃ ) _m Cl _m + m H ₂ 0 (2) in which, when Me = Ag, Cu, Pb, Ni, Co and Zn, then m = 1 or 2.

[0049] The leaching solution can advantageously contain Cu ²⁺ ions, introduced, for example, by adding a copper salt such as CuCI ₂ . It is believed that the copper ions substantially act as catalyst, favoring the dissolution reaction of the metallic oxides. These ions, in fact, oxidize the sulfides present, reducing in turn the Cu ⁺ ions; the Cu ⁺ ions are then oxidized again to Cu ²⁺ by the oxygen present in the reaction environment.

[0050] It is believed that the following reactions are at the basis of this catalytic effect of the Cu ²⁺ ions:

Me ₂ S _n + 2n NH ₄ CI + 2n CuCI ₂ → 2 Me(NH ₃ ) _m Cl _n + 2n CuCI + 2n HCI + n S (3)

2n CuCI + 2n HCI + n/2 0 ₂ → 2n CuCI ₂ + n H ₂ 0 (4)

The sum of reactions (3) and (4) leads to the overall reaction (1 ). [0051] At the end of the leaching phase a solution is obtained, containing ions of the non-ferrous metals leached from the solid matrix (extraction solution) and a solid leaching residue consisting of the part of the solid matrix which has not dissolved.

[0052] After separating the solid leaching residue, the leached metals present in the extraction solution are separated from this by means of precipitation (cementation). The metals are thus recovered in the elemental state.

Cementation

[0053] As described above, the precipitation of the metals is preferably effected by means of cementation (also known as "chemical displacement precipitation"). Cementation is the reaction through which metals are precipitated in the elemental state, from a solution containing it in dissolved form, by the addition to the solution of another metal in the elemental state (precipitating metal) having a lower (or more negative) reduction potential with respect to the reduction potential of the other metals.

[0054] Cementation allows the leached metals present in the extraction solution to selectively precipitate separately or in groups, by suitably selecting the precipitating metal on the basis of its reduction potential.

[0055] Although cementation is selective with respect to the

precipitated metals, the cement obtained generally also contains impurities of one or more of the other leached metals. The concentration of these impurities mainly depends on the difference between the reduction potential of the metals which form the impurities and that of the precipitating metal, in addition to the concentration of the respective ions in the solution subjected to cementation.

[0056] The cement typically contains the main metal precipitated in a highly pure form (higher than 95% by weight with respect to the weight of the cement; the remaining part consists of impurities of other metals in the elemental state).

[0057] The cements obtained can be re-used as is, or they can be subjected to simple known purification processes, to obtain metals having an even higher purity.

[0058] Because the concentrates typically contain more than one non- ferrous metal to be recovered, the cementation is preferably effected in a plurality of steps in series (multi-step cementation), in each of which one or more of the leached metals precipitates.

[0059] In each step, the precipitating metal is added to the solution subjected to cementation in powder form, thus favoring the chemical displacement reaction which leads to the precipitation of the metallic cement. The precipitating metal is generally added in an excess quantity with respect to that of the metal to be precipitated. Although in each step a different precipitating metal can be added, in a preferred embodiment, the metal added in each of the cementation steps is always the same.

[0060] In this preferred embodiment, the cementation is effected as follows: [0061] Referring to Fig. 3, after the leaching step at A, in a first cementation step at B1 , a first quantity of precipitating metal (for example zinc) is added at 305 to the extraction solution, obtaining the precipitation at 306 of the non- ferrous metal having the highest reduction potential among the metals present in solution (for example silver).

[0062] The precipitating metal is added to the solution in an excess quantity with respect to the metal or metals to be precipitated, so as to cause substantially complete precipitation of the metals to be recovered. The excess precipitating metal (or metals) is calculated in relation to the specific chemical displacement reaction which takes place in the cementation step. The precipitating metal is typically added in an excess of 1 to 50% with respect to the stoichiometric quantity with respect to the metal to be precipitated.

[0063] The extraction solution is left to decant and the precipitated metal, in the elemental state, is subsequently separated from the supernatant solution, for example by filtration.

[0064] The supernatant solution 307 containing the remaining leached metals, (and possibly a residual quantity of ions of the first precipitated metal), is subjected to a second cementation step at B2, wherein, by the addition of a second quantity of precipitating metal at 308, obtaining the precipitation at 309 of the non- ferrous metal having the highest reduction potential among the remaining metals present in the solution (for example copper). [0065] Due to the favorable reduction potential of the first metal (e.g., silver), the precipitation of the second metal (e.g., copper) may be accompanied by the possible precipitation of a further quantity of the first metal.

[0066] After separating the cement of the second metal, the

supernatant solution 310 can be subjected to a third cementation step at B3, in which a further non-ferrous metal (for example lead) is precipitated at 312 (the one having the highest reduction potential among those still in solution) by the addition of a third quantity of the precipitating metal at 31 1 . The precipitation of the cement of the third metal is accompanied by the possible precipitation of increasingly less significant quantities of the previous metals precipitated (silver and copper).

[0067] After the precipitation of the third metallic cement (for example lead), the supernatant solution 313 is subjected to possible further cementation steps, such as at B4, similar to the previous steps, until all the non-ferrous metals of interest present in the extraction solution have precipitated and been recovered. At 315 at least one metal (for example nickel and cobalt) are recovered by the addition of a fourth quantity of the precipitating metal at 314.

[0068] As described above, in each cementation step, the metal used as the precipitating metal can be any metal having a reduction potential lower than the reduction potential of at least one of the leached metals present in solution. In all the cementation steps, the same precipitating metal is preferably used. In this case, the precipitating metal must have a lower reduction potential with respect to the reduction potential of each of the leached metals present in solution. A metal particularly suitable for the purpose is zinc, due to its low cost and greater tendency to oxidize with respect to the non-ferrous metals typically to be recovered. The standard reduction potential of zinc for the pair Zn ² 7Zn is in fact equal to -0.76 V.

[0069] This cementation process can be used to recover silver, copper, lead, cobalt, nickel, cadmium and other metals more noble than zinc. Typically copper and the silver are recovered together, and the cobalt and the nickel are recovered together.

[0070] At the end of the last cementation step B4, after recovering all the leached non-ferrous metals, the supernatant solution 313 substantially only contains the ions of the metal used as precipitant in the various cementation steps (in addition to possible residues of ions of non-precipitated leached metals). The supernatant solution can be advantageously subjected to electrolysis to recover the precipitating metal in elemental form, so that it can be re-used in subsequent recovery process cycles or purified and used. In a preferred embodiment, the electrolysis of the final extraction solution is preferably effected in an open cell, with a titanium cathode and graphite anode, according to the process described in U.S. Patent No. 5,468,354, Process for Heavy Metal Electrowinning, and U.S. Patent No. 5,534,131 , for Process for Heavy Metal Electrowinning, the entire disclosures of which are incorporated herein by reference.

[0071] The particular composition of the electrolytic solution, which contains CI ^" and NH ₄ ⁺ ions, allows the electro-deposition of metallic zinc to be obtained at the cathode and the evolution of gaseous chlorine at the anode. As it is formed, the gaseous chlorine reacts rapidly with the ammonium ions present in solution around the anode forming ammonium chloride with evolution of gaseous nitrogen.

[0072] The possible electrolysis of the final extraction solution not only allows the recovery of the metallic zinc, but also the regeneration of the leaching solution, which can be re-used in the process.

[0073] The reactions involved in the electrolysis process are the following: at the cathode:

Zn(NH ₃ ) ₂ CI ₂ + 2 e ^" → Zn + 2 NH ₃ + 2 CI ^" at the anode:

2 CI ^" → Cl ₂ + 2 e ^" close to the anode:

Cl ₂ + 2/3 NH ₃ → 1/3 N ₂ + 2 HCI The overall chemical reaction of the electrolytic cell is:

Zn(NH ₃ ) ₂ CI ₂ + 2/3 NH ₃ → Zn + 1 /3 N ₂ + 2 NH ₄ CI

[0074] The electrolytic process described above is particularly advantageous as it avoids the evolution of gaseous chlorine, which is a toxic gas, in favor of the evolution of gaseous nitrogen.

[0075] The zinc electro-deposited on the titanium cathode is finally recovered, for example, in the form of a metallic sheet which can be then melted into ingots. Pure zinc powder can be produced from the molten mass. The zinc powder thus recovered can be re-used in new recovery process cycles of non-ferrous metals according to the present invention. Various embodiments of the present invention provide flexibility in handling co-products from the production of the original composite concentrate. In some embodiments, some or all of these co-products can be processed into concentrates that can be added to the composite concentrate for further processing. In other embodiments, some or all of these by-products can be processed with the metal-bearing gangue resulting from the initial lead recovery process. In still other embodiments some or all of these by-products can be turned into commercially acceptable concentrates and simply sold for processing by others. This flexibility in handling polymetallic mineral resources allows for the most economical exploitation, even as the composition of the ore changes, and the various metal prices change.

EXAMPLE

[0076] A 1 ,000 gram ore sample of the composition shown in Table 5 and 500cc of water (approximately 67% solids) was ground for eight minutes in a Denver Equipment Co. laboratory rod/ball mill charged with rods. This resulted in a screen distribution of 85 to 90% minus 200 mesh. The following reagents were added to the grind: 8cc of sodium isobutyl xanthate (1 % solution), 8cc of ZnS0 ₄ (12.5% solution), and 1 cc of NaCN (1 .25% solution).

[0077] After grinding, the conditioned slurry (30 - 40% solids) from the mill was placed in a Denver Equipment Co. 500 gram stainless steel flotation cell and operated at 1350 rpm. 5 drops (approximately 0.075 grams/ton) of an aliphatic alcohol frother was added to the flotation cell. The cell was operated for 5 minutes to produce a rougher composite concentrate that was further cleaned in one stage with no additional reagent additions using water addition to dilute the solids. The froth (composite concentrate) was collected, filtered, dried, and processed for analytical assays.

[0078] Subsequently, in the same Denver Equipment Co. 500 gram stainless steel flotation cell, the tailing from the composite rougher stage were conditioned for 2 minutes with 5cc of CuS0 ₄ (1 .25% solution), 1 cc of sodium isobutyl xanthate (1 % solution), and 1 drop (approximately 0.015 grams/ton) of aliphatic alcohol frother. Then, the cell was operated for 3 minutes (at 1350 rpm) to produce a zinc rougher concentrate that was further cleaned in one stage with no additional reagent additions using water addition to further dilute the solids. The froth (zinc concentrate) was collected, filtered, dried, and processed for analytical assays.

[0079] The remaining slurry (tailing) was collected, filtered, dried, and processed for analytical assays.

[0080] Based on the above test procedure, collection of products, and analytical assays, the results were as follows:

Table 6 Metal Distribution (Recovery) for Example

Pb Zn Cu Ni Co Ag

Feed 100.00% 100.00% 100.00% 100.00% 100.00% 100.00%

Composite 97.12% 28.81 % 91 .55% 41 .87% 35.57% 44.48% Concentrate

Zinc 0.34% 66.60% 0.94% 10.41 % 1 1 .68% 16.85% Concentrate

Final Tailing 2.54% 4.59% 7.51 % 47.72% 52.75% 38.67%

For comparison purposes, Metal Distribution and Product Assay data was collected from a commercial operating facility producing lead, zinc, and copper concentrates for sale to pyrometallurgical smelters from an ore feed similar to that in Table 5:

Concentrate

Zinc 0.60% 77.73% 0.76% N/A N/A 42.31 % Concentrate

Copper 0.35% 0.45% 58.43% N/A N/A 1 .98% Concentrate

Final Tailing 6.22% 15.07% 27.32% N/A N/A 37.54%

Assays

[0081] In the preferred embodiment a composite concentrate is prepared by flotation separation that contains at least about 85% of the lead, at least about 80% of the copper from the original polymetallic resource and at least 15% but no more than about 50% of the zinc into the composite concentrate. This includes regrinding and refloating of the composite concentrate to maximize the recovery of Pb and Cu to the composite concentrate(s). This is in contrast to the prior art where the steps to produce high purity individual concentrates (such as deliberately suppressing recoveries of certain metals) would depress the total recoveries. It is also in contrast to the prior production of bulk concentrates, where zinc would be collected in the bulk concentrate, and not be left in the tail. To achieve this excess xanthates can be used to increase the flotation of lead and copper. Recoveries in the composite concentrate can also be improved by one or more regrinding and flotation steps of the material that does not float to further separate the lead and copper metal from the rock matrix. Suppressants can be used during the flotation separation to suppress the zinc, but preferably not so much that the recoveries of lead and copper are adversely affected. In a more preferred embodiment, at least about 95 percent, and more preferably 98 percent of the lead is recovered in the composite concentrate, and at least about 90 percent, and more preferably 95% of the copper is recovered in the composite concentrate.

[0082] Preferably at least some of the zinc is present in the composite concentrate, because its suppression can suppress the lead and the copper.

Depending upon the starting polymetallic mineral resource, the composite

concentrate may have between about 20% and about 70% lead, between about 2% and about 30% zinc, and between about 2% and about 10% copper. The lead content is preferably greater than the zinc content in the composite concentrate. The composite concentrate contains between about 85% and 98% of the lead, between about 20% and about 50% of the zinc, and between about 75% and about 98% of the copper of the polymetallic mineral resource. More preferably the zinc is between 20% and 40% of the composite as a natural (i.e., without additions to specifically enhance or suppress zinc flotation) float. [0083] The zinc tailing is preferably processed into a separate zinc concentrate. Sufficient zinc is left in the tail to make this process economic, and to reduce the bulk of the composite concentrate so that the lead can be efficiently and economically recovered. The zinc concentrate can, in some embodiments, be added to the composite concentrate before further processing, but this can unnecessarily increase the size of the system needed for recovering lead to handle the additional volume of material. The zinc concentrate can, in other embodiments be added to the remains of the composite concentrate after the lead has been removed, so that the copper and other metals in the composite concentrate and the zinc (and any other metals in the zinc concentrate) can be recovered in a single process. Lastly, the zinc concentrate can be processed separately, or sold for separate processing.

[0084] The lead is conveniently recovered by leaching it with fluoroboric acid according to a Flubor or similar process, and then the leached lead is recovered by electrowinning. The remains of the composite concentrate after the leaching of the lead is subjected to a second leaching step to recover the other metals. The leached metals can be recovered individually, or in groups, including copper, nickel, cobalt, and silver, successively by the introduction of appropriate amounts of less electronegative metals. Different metals can be used for each step, but preferably only zinc is used at least in part because the zinc can be recovered from the leaching solution by electrowinning.

[0085] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.