BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION This invention relates to the leaching of metal values from metal chalcogenic minerals.

2. DESCRIPTION OF RELATED ART

U.S. Pat. No. 1 ,672,924 discloses the recovery of sulfur from ferrous sulphide and pyrhotite by treatment with sulphurous acid, giving an oxidized form of iron and elemental sulfur as products.

U.S. Pat. No. 3,896,208 discloses recovery of metals from copper and iron sulfide minerals by treatment with hydrochloric acid and oxygen or air at atmospheric pressure and temperatures above 105°C. Free sulfur is a by-product.

U.S. Pat. 4,980,134 discloses the leaching of gold and other noble metals using an oxidizing agent and a lixiviant such as sodium chloride, ammonium chloride, etc. which acts to form a soluble complex with the noble metal.

U.S. Pat. 5,147,617 discloses recovering gold from ores by treating the ore with an oxidizing agent and an anion exchange resin for absorption of the gold in the presence of dissolved oxygen U.S. Pat. 5,260,040 discloses the extraction of gold from gold-bearing material by treating with an acidic lixiviant solution containing thiourea and ferric ions and also a complexing agent such as EDTA.

The prior art disclosures do not provide a leaching solution with the characteristics of the present invention, that of a solution which acts at an approximately neutral pH, at ambient temperatures and with little or no environmental toxicity, to leach metals from chalcogenide minerals.

SUMMARY OF THE INVENTION In this disclosure the term "chalcogenide" refers to metalliferous minerals which contain one or more of the chalcogen elements. The chalcogen elements are sulfur (S), arsenic (As), antimony (Sb) selenium (Se) and tellurium (Te). The most common chalcogenide minerals are metal sulfides, containing a sulfide form of sulfur (S). The term sulfide-type mineral is used for minerals with one or more metal cation combined

with one or more of As, Sb, Se, or Te, whether separate from, or in association with, other sulfide minerals.

In this disclosure the term "rock matrix" refers to any ore, geologic host-rock, processed mineral concentrates, or any mine waste material such as fine tailings and* coarse waste rock, in which the chalcogenide minerals are found.

This invention is a near neutral pH, two-component, aqueous chemical leaching solution, comprised of any suitable oxidizing agent and any suitable chelating agent, both in sufficient concentration to effect a useful rate of metal ion extraction, and maintain the solubility of said metal ions, from chalcogenide minerals in target ores, concentrates, or mine waste material.

The methods used in the prior art do not provide the advantages of the processes of this disclosure. These advantages include the use of ambient temperature and pressure, reaction within minutes, the use of environmentally-benign chemicals, and reaction at neutral to slightly alkaline pH. Because of these advantages, the processes of this disclosure may be used in heap leach mining and in in situ leach mining of sulfide ores containing large amounts of acid-reactive carbonate rocks, such as limestone or dolomite. These processes avoid the use of toxic chemicals such as the use of cyanide solution to extract precious metals. Other bulk extraction methodologies this invention can be utilized with include solution mining in general, heap leaching, stope leaching, batch and continuous leaching methods such as agitation and percolation, and the methods of treating mine wastes such as dump leaching, and the leaching of fine tailings and agglomerated tailings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The aqueous leaching solutions of this invention are comprised of an oxidizing agent and a chelating agent. These solutions may be used to leach the following metals: copper, Cu; zinc, Zn; lead, Pb; iron, Fe; bismuth, Bi; cadmium, Cd; cobalt, Co; molybdenum, Mo; nickel, Ni; gallium, Ga; germanium, Ge; gold, Au; silver, Ag; indium, In; mercury, Hg; platinum, Pt; palladium, Pd; rhodium, Rh; ruthenium, Ru; iridium, Ir; osmium, Os; rhenium, Re; thallium, Tl; tin, Sn; and vanadium, V, from metal chalcogenide minerals. The leached metals will exist as chelated metal ions in the post-leach solution.

The chalcogenide elements S, As, Sb, Se, and Te also can be recovered by this invention. These elements will exist in post-leach solution in either fine, solid phase, native element form or as dissolved oxyanions.

The metal chalcogenide minerals which are leached in this invention are usually referred to as sulfides, arsenides, antimonides, selenides, and tellurides. These minerals include, but are not limited to, the following: argentite (or acanthite), pyrargyrite, proustite, polybasite, stephanite, hessite, calaverite, petzite, sylvanite, bismuthinite, covellite, chalcocite, bornite, chalcopyrite, cubanite, enargite, stannite, tennantite, tetrahedrite, sperrylite, cooperite, stibiopalladinite, braggite, vysotskite, laurite, molybdenite, millerite, niccolite, siegenite, cobaltite, skutterudite, smaltite, pentlandite, gersdorffite, arsenopyrite, pyrrhotite, pyrite/marcasite, molybdenite, galena, bournonite, jamesonite, sphalerite/wurzite, greenockite, cinnabar, realgar, orpiment, and stibnite. These minerals can be leached from sedimentary, igneous, metamorphic or mixed host rock types, from processed mineral concentrates of said minerals, or from such waste rock.

Without wishing to be held to this explanation, it is believed that the leaching reaction in general occurs rapidly in aqueous solution according to reaction (1). (1) METAL CHALCOGENIDE + OXIDANT + FREE CHELATE -> METAL-BOUND-CHELATE + SPENT OXIDANT + OXIDIZED CHALCOGEN SPECIES

This general reaction form is virtually identical for metal sulfides, metal arsenides, metal antimonides, metal selenides, metal tellurides and other metal chalcogenides (mixtures of S, As, Sb, Se and Te as anion).

In metal chalcogenide minerals, before reaction with the leach solution, most often the chalcogenide element is of a (2-) or (3-) or (1-) valence. Upon oxidation, the chalcogenides become either the elemental (native) form, e.g., S°, Se°, Te°, As", or Sb°, which are typically fine amorphous particles, as observed in lab tests, or undergo further oxidation to soluble oxyanions. In laboratory tests with metal sulfides such as galena and sphalerite, it was observed that finely-sized native sulfur particles formed; chemical analysis of several post-leach solutions have shown that about one-sixth of the original sulfide present in the minerals was converted to sulfate anion, after the reaction has come to completion. From leaching cobaltite it was observed that light-yellow arsenic,

or whitish arsenic oxide particles were formed, with a similar level of arsenic oxyanion formed in solution.

Other oxyanions not shown in these examples, such as bisulfite, bisulfate, thiosulfate, dithionite, and dithionate, and analogous species for the other chalcogens, are expected to be formed and present in the post leach solutions.

To the best of current understanding, in most cases, the metal cation remained at the same valence in the chelated metal form as it was in the original mineral form. One common exception is iron cations: for minerals with ferrous iron, Fe ²⁺ , in their structure, the oxidant in the leach solution is expected to oxidize ferrous iron to ferric iron, Fe ³⁺ in a soluble chelated form. Another exception is for copper in the mineral chalcocite,

Cu ₂ S; the leach solution will oxidize structural cuprous copper, Cu'\ in the mineral to cupric copper, Cu ²⁺ in a soluble chelated form.

The chemical formulation of this invention, a near-neutral oxidant + chelate aqueous solution for mineral leaching, is simple and is also intended to be flexible. The variability in the type of ingredients, and in the concentration of the ingredients is dependent upon the expected metal chalcogenide content in the rock matrix of ore or waste to be leached, and the desired concentration of dissolved metals in the reacted leach solution, also known as the "post-leach solution" or the "pregnant solution". For every molar equivalent of metal to be leached, one molar equivalent of oxidant is required to break the chalcogenide bond, and from one-half to five molar equivalents of chelate are required to effect the high solubilization or leach efficiency level desired. A preferred range is 1 to 2 molar equivalents of chelate for every molar equivalent of metal. Additional oxidant may be consumed due to losses by extraneous side reactions. In this invention, the leach solution is prepared in advance of use. This solution is injected into rock matrix, such as an ore deposit for solution mining or applied onto broken or processed ore rock or waste rock or tailings.

Any oxidizing agent having the following characteristics may be used with this invention: water soluble in oxidized and reduced form, of sufficient oxidative power to oxidize chalcogenide elements, and not of sufficient oxidative power to substantially oxidize the chelating agent.

Suitable oxidizing agents include, but are not limited to, hydrogen peroxide, dissolved air, dissolved oxygen, chelated ferric iron, sodium perborate, potassium iodate, sodium percarbonate, sodium hypochlorite (bleach), potassium monopersulfate, sodium persulfate, chlorine, bromine, and iodine. The following oxidizing agents have been tested and found suitable: potassium monopersulfate, sodium hypochlorite solution at 0.01%, 0.1%, and 1 % by weight, water soluble polymer containing molecular bromine at 54% by weight, sodium persulfate, molecular iodine in excess potassium iodide, hydrogen peroxide, sodium perborate tetrahydrate in sodium carbonate matrix, potassium iodate, and sodium percarbonate in sodium carbonate. The oxidant concentration is typically between 0.01 to 0.5 moles/liter in water, depending on the concentration of metal chalcogenide in the ore or waste matrix, and the anticipated level of adverse reactions that consume additional oxidant. The quantity used is calculated on the basis of the active oxidizing species or compound, for example, when formulating with bleach, the basis is on the active ingredient HOC1 (molarity and/or weight) instead of NaClO. It is important to note that any suitable oxidant can be used.

Any chelating agent having the following characteristics may be used with this invention: is soluble in water, forms a metal chelate complex which is water-soluble, has affinity for at least one metal in said metal chalcogenide mineral, and is resistant to oxidation by said oxidizing agent.

Suitable chelating agents include, but are not limited to diethylenetriaminepentaacetic acid (DTP A), ethylenediaminetetraacetic acid (EDTA), ethyleneglycol bis(aminoethyl)-tetraacetic acid (EGTA), hydroxyethylethylenediaminetriacetic acid (HEDTA or HEETA), nitrilotriacetic acid (NT A), triethylenetetraminehexaacetic acid (TTHA), tartaric acid, citric acid (cit), gluconic acid, 5-sulfosalicylic acid (5-SSA), ethylenediamine (en), triethylenetetramine (trien), triaminotriethylamine (tren), triethanolamine (TEA), N-hydroxyethylethylenediamine (hen), o-phenanthroline (phen), l,2-dihydroxybenzene-3,5-disulfonic acid, disodium salt (also known as disulfopyrocatechol, or Tiron), chromotropic acid (DNS), thioglycolic acid, thiourea, cryptands, polyethylenimine (PEI), poly (p-vinylbenzyliminodiacetic acid), NAH-LP142, SMC-10A, SMC-49, SMCAV74C, nitrilomethylenephosphonic acid

(NTPO, ATMP), ethylenediaminetetra(methylenephosphonic acid) (EDTPO), hydroxyethylidenediphosphonic acid (EHDP, HEDPA), sodium tripolyphosphate (STPP), and acetylacetone (acac).

The chelate concentration is typically between 0.05 to 1.0 moles/liter in water and is coupled to the oxidant concentration used, which is in turn dependent on the grade of metal chalcogenide to be leached. By weight, a solution concentration as high as 25 weight % is possible, however above this level chelate solutions tend to become significantly more viscous than water and become less effective. The quantity of chelate used is calculated on the basis of the free chelate species or compound. It is important to note that any suitable chelant can be used.

The pH of the pre-leach oxidant + chelate solution must be between pH 2.5 and 12, with a preferred range being between pH 5 and 9. When leaching minerals from carbonate host rocks, such as limestone or dolomite, or in host rocks or mineral concentrates that contain as low as 1 weight % carbonate or other acid-consuming mineral, the pH of the pre-leach solution should not be lower than pH 5.5 so as to prevent acidic dissolution of the host rock

Any appropriate chemical compound(s) that provides acidity or alkalinity may be used to adjust the pH of the pre-leach solution. Examples of such useful compounds include hydrochloric acid, sodium hydroxide, and potassium hydroxide. Example 1 : Leaching Rates Over Time:

Leaching kinetic experiments were conducted to examine the speed of reactivity when a mineral sulfide is added to a stirred solution containing an oxidizing and chelating agent combination. Three different oxidants were tested, each on a separate day, using sodium perborate, sodium hypochlorite, and hydrogen peroxide, respectively. Each experiment consisted of a 100 ± 0.5 g aqueous slurry consisting of the following initial reactant amounts and conditions: mineral: 0.548 millimoles of galena (PbS lead sulfide),

-270+400mesh, gangue: 14.0 millimoles of dolomite = (Ca, Mg)CO ₃ , -100+400 mesh, oxidant: 8.38 millimoles, either NaBO ₃ , or NaClO, or H ₂ 0 ₂ . chelant: 4.19 millimoles EDTA, prepared from ethylenediaminetetraacetic acid, disodium salt (Na ₂ EDTA) reagent,

fluid: distilled, deionized water (DDIW), initial pH: 8 ± 0.1 , adjusted with dilute HC1 and NaOH, temperature: 25 ± 0.2 °C (initial; and maintained).

The temperature, pH, and Eh (oxidation/reduction potential, or ORP) parameters were monitored using probes and computerized data acquisition.

"Gangue" is a term used in geology and mineral extraction fields that is defined as rock matrix or solid material which hosts (contains) the minerals of value, but which has no value in and of itself; it is the bulk or host or waste material not being sought for its value. The dolomite was added to simulate conditions present if the galena were present in a dolomite ore body or similar carbonate host rock or mine waste matrix.

These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) excess oxidant, (b) excess metal chelator, and (c) excess dolomite gangue, relative to PbS; metal : oxidant, 1 : 10; metal : chelate, 1 : 5; PbS : gangue, 1 : 25.5. These kinetic experiments were each conducted in a sealed 180 mL teflon batch reactor vessel with teflon-coated magnetic stirring bars for constant stirring of the solution. All ingredients except the PbS were combined and brought to constant temperature and pH. Then the vessel was purged with nitrogen gas, N ₂ , for at least 5 minutes to remove dissolved oxygen. Then the PbS was added, and the computer data collection and fluid sampling were both initiated.

Sampling of the leachate for soluble Pb was conducted primarily within the first hour. One mL samples were periodically taken and immediately filtered through 0.22 μm syringe filters to remove mineral particles, which stopped the dissolution reactions in the collected samples. The results indicated that the combination of hydrogen peroxide oxidant and

EDTA chelant resulted in a greater than 70% recovery of soluble Pb occurring within 10 minutes after the start of leaching. With the mixture of sodium perborate and EDTA, greater than 70% Pb recovery occurred within 40 minutes. With sodium hypochlorite bleach and EDTA (active oxidant was HOC1), the reaction was slower as indicated by the very low Pb extraction after 60 minutes, which was under 5% recovery.

These experiments were left to continue with stirring and temperature control overnight. The final samples were taken and the recoveries calculated: 72.9% Pb

recovery for the H ₂ 0 ₂ /EDTA leach, 69.2% Pb recovery for the NaBO ₃ /EDTA leach, and 57.9% Pb recovery for the NaClO/EDTA leach.

These results confirmed that the reactions with hydrogen peroxide and sodium perborate had essentially reached completion and remained stable well within the first hour of leaching.

Example 2: Effect of Using Different Oxidants

Batch leaching tests were conducted to examine how metal extraction from a mineral sulfide matrix might differ depending on which oxidizing agent was used in the oxidant + chelant leaching fluid. Four different oxidants were tested: sodium hypochlorite (bleach); a dry source of bromine, Br ₂ (a commercial product,

"Bio-D-Leachant"); iodine, I ₂ ; and sodium perborate tetrahydrate, NaBO ₃ »4H ₂ O, which is a dry source of hydrogen peroxide.

Each experiment consisted of a water slurry comprised of the following initial reactant amounts and conditions: mineral: 0.15 grams of galena (PbS = lead sulfide), -270+400 mesh, gangue: 2.70 grams of dolomite - (Ca,Mg)C0 ₃₎ -100+400 mesh, oxidant: 0.63 millimoles, either NaClO, Br ₂ , 1 ₂ , or NaBO ₃ chelant : 1.16 grams (3.12 mmol) of Na ₂ EDT A, fluid: 22.60 grams DDIW, initial pH: 8.5 ± 0.1 adjusted with dilute HC1 and NaOH.

Temperature, pH, and Eh were maintained and monitored as in Example 1.

These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) equimolar oxidant, (b) excess metal chelator, and (c) excess dolomite gangue, relative to PbS; metal : oxidant, 1 : 1 ; metal : chelate, 1 : 5; PbS : gangue, 1 : 17.8.

In general these batch tests were performed by combining ingredients into small 35 mL LDPE plastic bottles, which were capped and mixed for 24 hours using a vertical, 1 RPM revolving mixing device. These bottles were left to continue mixing overnight. After 24 hours had elapsed the leachate was sampled for soluble metals. That is, from each bottle, a one mL sample was taken and filtered through a 0.22 μm syringe filter to remove mineral particles and stop the leach reaction. The results of leaching galena with various oxidants and EDTA for 1 day are tabulated in Table 1.

Table 1 : Extraction Results using various Oxidants with EDTA Chelate

Oxidant %Extr. Pb %Extr. Zn %Extr. S

Bleach (HOC1) 16.1% 4.3% 3.2%

Bromine (Br ₂ ) 31.1% 8.1% 3.7%

Iodine (I ₂ ) 38.5% 7.8% 3.6%

Perborate (H ₂ O ₂ ) 47.5% 4.7% 9.9%

A second fluid sample of the perborate leach solution was taken after 6 days elapsed test time, yielding 48.5% Pb, 4.9% Zn, and 7.1% S extractions; which confirmed that the leaching reactions had essentially come to completion before 1 day's time, and had remained stable several days longer. Example 3. Effect of Varying Concentration of the Chelating Agent.

Batch leaching tests were conducted to examine how the concentration of the chelant in the leach solution might affect the degree of metal extraction from a mineral sulfide matrix. In batch tests conducted along with those mentioned in Example 2, bleach, EDTA, dolomite and galena were combined at nominal Na ₂ EDTA concentrations of 1 wt%, 2 wt% -and 5 wt%, respectively.

Each experiment consisted of a water slurry comprised of the following initial reactant amounts and conditions: mineral: 0.15 grams of galena (PbS = lead sulfide), -270+400 mesh, gangue: 2.70 grams of dolomite = (Ca,Mg)CO ₃ , -100+400 mesh, oxidant: 0.63 millimoles, NaClO bleach, chelant: 0.232 grams (0.63 mmol) of Na ₂ EDTA for 1 wt% chelate, or,

0.464 grams (1.24 mmol) of Na ₂ EDTA for 2 wt% chelate, or, 1.162 grams (3.12 mmol) of Na ₂ EDTA for 5 wt% chelate, fluid: 22.60 grams DDIW, initial pH: 8.5 ± 0.1 , adjusted with dilute HC 1 and NaOH. These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) equimolar oxidant, (b) equivalent or excess metal chelator, and (c) excess dolomite gangue, relative to PbS; metal : oxidant, 1 : 1; metal : chelate, 1 : 1 (for the 1 wt% chelate batch); metal : chelate, 1 : 2 (for the 2 wt% chelate batch); metal : chelate, 1 : 5 (for the 5 wt% chelate batch) and PbS : gangue, 1 : 17.8.

The experiment was performed as in Example 2. Table 2 shows the extraction of metals after 24 hours. Table 2: Extraction Results Using Varied Amounts of EDTA.

Ratio of Reactants %Extr. Pb %Extr. Zn %Extr. S

(1) NaClO : (l) PbS : (1) EDTA 2.1% 4.6% 2.1%

(1) NaClO : (1) PbS : (2) EDTA 10.9% 4.3% 2.1%

(1) NaClO : (1) PbS : (5) EDTA 16.1% 4.3% 3.2%

These results indicated that some excess chelate had to be present in the leach solution to effect a reasonable level of metal extraction from metal chalcogenide minerals. One to two molar equivalents of chelate were shown to be minimally effective, and five molar equivalents were only slightly more effective than two molar equivalents of chelate. Example 4: Leaching a Multiple of Metal Sulfide Minerals In the previous examples, quantities of only one metal sulfide mineral type (PbS, galena, in these cases) were added for determination of leaching effect. In this example, three metal sulfide mineral types were added purposely for determining the leaching effect for each respective metal. The three mineral types added were galena, PbS; chalcopyrite, CuFeS ₂ , and sphalerite, ZnS. One small batch was mixed in the same manner and with identical chemical formulation as the 5 wt% Na ₂ EDTA solution described in example 3, except that in addition to the 0.15 grams (0.63 mmol) of PbS added, exactly 0.0290 grams (0.30 mmol) of ZnS and 0.0088 grams (0.037 mmol) of CuFeS ₂ ere added as well. A slight compensative addition of bleach oxidant was also made to keep the oxidant-to-metal ratio equimolar.

These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) equimolar oxidant, (b) equivalent or excess metal chelator, and (c) excess dolomite gangue, relative to all sulfide minerals; metal ; oxidant, 1 : 1; metal : chelate, 1 : 5 metal sulfides: gangue, 1 : 15.1. The ingredients were combined in 35 mL plastic bottles; capped and mixed with rotation. The results of leaching for 1 day is recorded below in Table 3. Table 3. Extraction Results For 3 Metal Sulfides.

Metal Sulfides Added %Extr. Pb %Extr. Cu %Extr. Zn %Extr. S

PbS 16.1% n.s. 4.3% 3.2%

PbS + CuFeS ₂ 17.7% 28.1% 1.1% 2.5% n.s. not significant

Trace levels of Pb, Cu, Zn, and S in the dolomite gangue was included in calculations, and the sulfur extraction was calculated from soluble sulfate. Extractions of Ca and Mg were both 5% ± 0.4%. The final Eh and pH conditions were practically the same for both post-leach solutions at +0.18v Eh and pH 8.8 respectively. In summary it is clear from these results that with a sufficient amount of oxidant along with an excess amount of chelate present in a two-part oxidant + chelate leach solution, several metals were leached simultaneously from rock or waste material containing a multiple number of metal chalcogenide minerals. Example 5: Effect of Various Types of Chelating Agents Used Batch leaching tests were conducted to examine how each of several chelating agents used in separate leach tests on the same mineral sulfide matrix might affect the degree of metal extraction.

Each experiment consisted of a water slurry comprised of the following initial reactant amounts and conditions: minerals: 2.50 grams of metal sulfide ore, -100+400 mesh, containing

15.7% Pb, (as the lead sulfide galena, PbS), 1.5% Zn, (as the zinc sulfide sphalerite, ZnS), 0.6% Cu, (as the copper-iron sulfide chalcopyrite, CuFeS ₂ ), 2.2% Fe, (from pyrite, FeS ₂ , and chalcopyrite, CuFeS ₂ ), and the remainder, about 2.00 grams, as dolomite, (Ca,Mg)CO ₃ , oxidant: 2.56 millimoles H ₂ O ₂ , hydrogen peroxide from 30 wt% reagent, chelant: 9.75 ± 0.25 millimoles (see list below for chelate types), fluid: 22.1 + 0.1 grams, mixture of DDIW + dissolved chelant, initial pH: 8.0 ± 0.1 , adjusted with dilute HC1 and NaOH.

These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) an excess of oxidant above equimolar, (b) excess metal chelator, and (c)

excess dolomite gangue, relative to the metal sulfides, for all batches; metals : oxidant, 1 : 2.3; metals chelate, 1 : 2.6 to 1 : 2.9; and metal sulfides: gangue, 1 : 2.9.

The ingredients were combined in small 35 mL plastic bottles; capped and mixed with rotation for just 1 hour, after which the leachate was sampled for soluble metals. The results of leaching the pulverized metal sulfide ore with hydrogen peroxide and various chelating agents for 1 hour are presented in Table 4. Table 4: Extraction Results Using Different Chelating Agents.

Chelant %Extr. Pb %Extr. Zn %Extr. Cu %Extr. Fe

TEA: 3.1% 1.6% 0% 0%

Tiron: 72.6% 0.6% 0.9% 0.6%

NTA: 35.2% 1.1% 9.3% 5.8%

HEETA: 77.1% 0.8% 1.9% 2.2%

EDTA: 76.5% 0.8% 1.8% 2.7%

MgEDTA: 11.4% 0.3% 0% n.d.

EGTA: 66.5% 0.5% 1.1% 0.8%

DTPA: 94.8% 0.6% 1.2% 2.3% n.d. not determined.

Clearly, DTPA was the most effective of all chelates in this experiment, and achieved 95% recovery of the lead in the ore. A closer examination of the data showed that some chelates had a higher affinity for some metal ions than for others.

Example 6: Effect of Leach Solution Formula Appl ied To Different Metal

Chalcogenide Minerals.

This example demonstrated that metals were leached from several different chalcogenide minerals contained in pulverized ore material. In this example three metal chalcogenides were tested with oxidant + chelant leach solutions. These were natural cobaltite ((Co,Fe)AsS) ore, natural tetrahedrite (Cu ₁₂ Sb ₄ S ₁₃ ) ore, and natural tennantite

(Cu, ₂ As ₄ S,_) ore. The cobaltite was in a dolomite matrix, the tetrahedrite was in a siderite (FeCO ₃ ) matrix, and the tennantite was in a dolomite matrix.

Batch leaching tests were begun using hydrogen peroxide as the oxidant and MgEDTA as the chelant. The batches were sampled first one hour after initiation of leaching, and then again 12 days later. Leaching of these same samples were repeated

an additional 16 days later, at day 28, with a fresh supply of leach solution, using more of the same oxidant but then using DTPA as the chelant instead of the MgEDTA. These three batch experiments consisted of a water slurry comprised of the following initial reactant amounts and conditions: minerals: 2.500 + 0.004 grams of metal chalcogenide, - 100+400 mesh, containing either 0.50 g. cobaltite ore + 2.00 g. dolomite (low-metals) gangue, or 2.50 g. tetrahedrite ore (in siderite), or 0.50 g. tennantite ore + 2.00 g. dolomite (low-metals) gangue, oxidant: 2.56 millimoles H ₂ O ₂ , from 30 wt% reagent (for initial tests), and 1.98 millimoles H ₂ O ₂ , from 30 wt% reagent (for after 28 day tests), chelant: 9.5 + 0.1 millimoles MgEDTA (for initial tests), prepared from a ethylenediaminetetraacetic acid, magnesium disodium salt solution, and 9.5 + 0.1 millimoles DTPA, from diethylenetriaminepentaacetic acid, pentasodium salt solution (for after 28 day tests), fluid: 22.1 + 0.2 grams, mixture of DDIW + dissolved chelant, initial pH: 8.0 + 0.1 , adjusted with dilute HC1 and NaOH. These ingredient mixtures afforded the following initial molar ratios, which assured that there was (a) an excess of oxidant above equimolar, (b) excess metal chelator, and (c) excess gangue, relative to the metal chalcogenides, for all batches; metals : oxidant, 1 : 1.7 to 1 : 3.6; metals : chelate, 1 : 2.0 to 1 : 4.4; metal chalcogenides: gangue, 1 : 4.9 to 1:13.2.

The ingredients were combined in 35 mL plastic bottles; capped and mixed with rotation for only 1 hour, then sampled for soluble metals by withdrawing 1 mL of postleach solution and filtering. The samples were immediately resealed and allowed to continue mixing for 12 days longer, and sampled again for soluble metals. Bottles were re-sealed, mixing was stopped and they were left undisturbed for an additional 16 days. Each sample was then drained of the former MgEDTA leach solution, and fresh peroxide + DTPA solution was added. Bottles were re-capped and allowed to mix with rotation for only 2 hours; then sampled again for soluble metals on the same day.

The results of leaching these three metal chalcogenides with hydrogen peroxide and both chelating agents are presented in Table 5. Table 5: Extraction Results Of Leaching Different Chalcogenides COBALTITE %Extr. %Extr. %Extr. %Extr. %Extr. %Extr. %Extr. Time Chelant Co Ni Cu Fe S As Sb

1 hour MgEDTA <1 68 0 3 0 0 12 days later 1 3 12 1 0 3 <1

2 hours DTPA 5 12 3 5 9 12 2 TOTAL leached 7% 15% 83% 6% 12% 15% 2%

TETRAHEDRITE %Extr. %Extr. %Extr. %Extr. %Extr. %Extr. %Extr.

Time CJislani Cu Fe Zn Pb S As Sb

1 hour MgEDTA <1 0 0 0 1 <1 <1

12 days later 3 2 1 43 0 15 <1

2 hours DTPA 1 1 0 47 1 3 <1

TOTAL leached 4% 3% 1% 90% 2% 18% 1%

TENNANTITE %Extr. %Extr. %Extr. %Extr. %Extr. %Extr.

Time Chelant Cu Fe Zn S As Sb 1 hour MgEDTA 40 0 69 4 0 0

12 days later 38 0 20 0 0 <1

2 hours DTPA 6 4 5 9 6 4

TOTAL leached 84% 4% 94% 13% 6% 5%

Initial composition of solids used in Table 5 were as follows: Cobaltite + dolomite mix (partial analysis): 2.08% Co, 3.37% Ni, 0.50% Cu,

2.26% Fe, 3.10% S. 8.76% As, 0.20% Sb, 17.9% Ca, 9.4% Mg, Tetrahedrite ore (partial analysis): 3.60% Cu, 3.40% Fe, 0.28% Zn, 0.06% Pb, 9.10% S, 0.27% As, 2.70% Sb, 0.23% Ca, 1.0% Mg.

Tennantite + dolomite mix (partial analysis): 2.25% Cu, 2.76% Fe, 0.09% Zn, 2.08% S, 0.17% As, 0.20% Sb, 17.4% Ca, 9.4% Mg.

These results confirmed that metal chalcogenide minerals in addition to those that are strictly "metals sulfides" were successfully leached from host rock or matrix.

They also confirmed that more metal is solubilized with each successive volume of fresh oxidant + chelate lixiviant. These findings furthermore indicated that continuous pumping, injection, or applica~ion of fresh leach solution would lead to near complete remove of all metals of interest in any of the solution mining or mine waste remediation scenarios envisioned in this disclosure.

Extraction of all metals increased on account of adding fresh amounts of lixiviant (oxidant + DTPA chelate) to the pulverized ores, but also because of time. While some metal was extracted in the first hour of the peroxide + MgEDTA application, in half of the cases additional metals were solubilized as the peroxide and MgEDTA remained in solution for the full 12 days. Finally, it is surmised that optimal extraction can be accomplished by using a different chelating agent that demonstrates more effective extraction properties with the metal ion of interest than either of the two chelants used in this example.

It will be apparent to those skilled in the art that the examples and embodiments described herein are by way of illustration and not of limitation and that other examples may be used without departing from the spirit and scope of the present invention, as set forth in the appended claims.