WO2013173914A1

WO2013173914A1 - Arsenic recovery from copper-arsenic sulphides

Info

Publication number: WO2013173914A1
Application number: PCT/CA2013/000511
Authority: WO
Inventors: David G. Dixon; Laurence DYER; Luis QUIROZ CASTILLO
Original assignee: The University Of British Columbia
Priority date: 2012-05-25
Filing date: 2013-05-24
Publication date: 2013-11-28
Also published as: WO2013173914A8

Abstract

Methods are provided for precipitating arsenic from an acidic sulphate copper leach solution as a stable scorodite precipitate prior to any refining steps. The method utilizes pyrite or chalcopyrite in the leach train as the primary source of ferric ions for the precipitation of scorodite.

Description

ARSENIC RECOVERY FROM COPPER-ARSENIC SULPHIDES

CORRESPONDING APPLICATIONS

This application claims the priority benefit of United States patent application no. 61/651 ,980 filed on May 25, 2012, the contents of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to the recovery of arsenic from copper- sulphides, and more particularly the recovery of arsenic as scorodite.

BACKGROUND OF THE INVENTION

The presence of arsenic in copper ores is a common deterrent to the development of such deposits. Typically copper deposits are treated by flotation followed by toll treatment at a smelter, and, while the gold industry has largely adopted treatment routes that deal with high-arsenic concentrates, ores in which copper is the predominant element of value have been typically avoided when they contain high concentrations of arsenic. In addition, the environmental regulations relating to treatment of arsenic-containing materials and arsenic disposal have gradually become more stringent, creating additional challenges for the exploitation of such deposits.

When considering the exploitation of an arsenic-containing copper deposit, various approaches may be explored. One option is simply to avoid the arsenic-bearing zones in the ore body entirely. Alternatively, the arsenic bearing minerals may be excluded from copper concentrate during processing. A preferred alternative would be to treat the arsenic-containing concentrates in a single facility to produce copper and arsenic as separate products. Ultimately, the adopted approach depends on a multitude of factors specific to each project, which include the abundance, occurrence and distribution of arsenic minerals. Where avoidance of arsenic-rich areas is not practical, consideration may be given to physical separation of arsenic containing minerals such as tennantite and enargite from chalcopyrite, chalcocite and pyrite by selective flotation. In this case, the focus is toward producing two concentrates, a copper concentrate low in arsenic suitable for a smelter, and a concentrate high in arsenic that can be treated in a hydrometallurgical plant.

Where high arsenic grades preclude a dual concentrate approach, the entire concentrate will require hydrometallurgical treatment to produce copper cathode for sale, while fixing the arsenic in an environmentally acceptable product for disposal. Various groups have explored the idea of immobilizing the arsenic present in industrial waste as crystalline ferric arsenate (FeAs04) or scorodite. Scorodite is a stable compound which can be safely discharged to a tailings impoundment. Of the iron arsenate compounds, scorodite is one of the most stable compounds across a wide range of pH levels, and reaches its lowest solubility levels at neutral pH levels.

It is well known that scorodite is easily generated from iron/arsenic solutions in a post-leaching step by oxyhydrolysis in high-temperature autoclaves. However, autoclaves are both energy-intensive and costly to build, and thus it would be desirable to be able to precipitate scorodite in the leach solution itself under atmospheric conditions.

SUMMARY OF THE INVENTION

According to various embodiments of the invention, a method is provided for removing arsenic from an acidic sulphate copper leach solution as a stable scorodite precipitate prior to any refining steps. The method utilizes pyrite or chalcopyrite in the leach train as the primary source of ferric ions for the precipitation of scorodite.

According to various embodiments of the invention, a method is provided for producing scorodite at atmospheric pressure. The method includes the provision of a mixture comprising an acidic sulphate leach solution of dissolved copper ions and dissolved arsenic ions, wherein the leach solution is supersaturated with the dissolved arsenic ions, and an iron source, wherein the iron source is one or both of particulate pyrite and particulate chalcopyrite. The method further includes supplying an oxygen- containing gas to the mixture so as to maintain an operating potential of the leach solution sufficient to oxidize the iron source to provide at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions in the leach solution. The method further involves selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

According to various embodiments of the invention, a method is provided for producing scorodite at atmospheric pressure. The method includes the provision of a particulate mixture of a copper-arsenic sulphide concentrate and an iron source. The iron source is one or both of pyrite and chalcopyrite. The method further includes supplying an acidic sulphate solution and an oxygen-containing gas to the particulate mixture to provide leaching conditions wherein the mixture is oxidized to form a leach solution comprising dissolved copper ions, dissolved arsenic ions, and dissolved ferrous ions, wherein the leach solution is supersaturated with the dissolved arsenic ions. The method further includes maintaining the operating potential at a level sufficient to oxidize the dissolved ferrous ions to provide at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions in the leach solution. The method further involves selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

According to various embodiments of the invention, a method is provided for producing scorodite at atmospheric pressure. The method includes the provision of a particulate mixture of a copper-arsenic sulphide concentrate and an iron source, wherein the iron source is one or both of pyrite and chalcopyrite. The method further includes supplying an acidic sulphate solution and an oxygen-containing gas to the particulate mixture to provide concentrate-leaching conditions wherein the concentrate is oxidized to form a leach solution comprising dissolved copper ions and dissolved arsenic ions, wherein the leach solution is supersaturated with the dissolved arsenic ions. The method yet further involves maintaining the operating potential at a level sufficient to provide iron source-leaching conditions wherein the iron source is oxidized to form at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions. The method further includes selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

The aforementioned embodiments of the invention may further include supplying carbon with the acidic sulphate and the oxygen-containing gas to provide the concentrate-leaching conditions. The copper-arsenic sulphide concentrate may include enargite. The iron source may be predominantly pyrite. Maintaining the operating potential may involve maintaining the operating potential between 470 and 600 mV versus Ag/AgCI. The

aforementioned embodiments may include maintaining the leach solution at a pH of about 2 or less. The process may be a continuous process. A portion of the precipitated scorodite may be recirculated to the mixture

The aforementioned embodiments of the invention may be carried out at a temperature of less than 95°C. The temperature may be about 85°C.

The arsenic-depleted leach solutions in the aforementioned

embodiments may comprise less than 2 g/L of arsenic. The arsenic-depleted leach solution may comprises less than 1 g/L of arsenic.

The aforementioned embodiments of the invention may further include recirculation of a portion of the leach residues to the mixture.

According to various embodiments of the invention, a method is provided for recovering copper from a copper-arsenic sulphide concentrate at atmospheric pressure. The method includes producing scorodite according to any of the aforementioned embodiments of the invention. The method further includes separating the arsenic-depleted leach solution from the leach residues to provide a copper-enriched solution. The copper-enriched solution may include at least 80% of the copper ions from the mixture. The copper- enriched solution may include at least 90% of the copper ions from the mixture. The method may further include recirculating a portion of the leach residues to the mixture. The ratio of the recirculated portion of the leach residues to fresh particulate mixture may be 4.5:1 by weight. The initial density of solids in the slurry may be between about 30% and about 50% by weight. 1. The initial concentation of sulphuric acid in the leach solution may be about 40 g/L

Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate embodiments of the invention,

Figure 1 is a process flowsheet for atmospheric chemical leaching of copper concentrates, involving the steps of leaching (leaching of copper, arsenic, and iron, and precipitation of scorodite), liquid/solid separation of the leach residues from an arsenic- depleted leach solution with optional recirculation of leach residues to the leach reaction, and copper recovery (by SX-EW);

Figure 2 is a graph of copper extraction and arsenic concentration in the leach solution versus reaction time for Acid Series A, showing the effect of initial acid concentration on the extraction of copper and arsenic, and on the precipitation of scorodite;

Figure 3 is a plot of operating potential as a function of initial acidity of the leach solution for Acid Series A; Figure 4 is a graph of iron and arsenic concentration in a leach solution versus reaction time for Acid Series B, showing the effect of initial acid concentration on the extraction of iron and arsenic, and on scorodite precipitation;

Figure 5 is a plot of final arsenic concentration from TCLP (Toxicity

Characterization Leaching Procedure) tests on final leach residues from Acid Series A;

Figure 6 is a graph of copper and arsenic extraction and arsenic concentration in the leach solution versus reaction time for Acid Series C, showing the effect of gypsum on the extraction of copper and arsenic, and on the seeding of scorodite precipitation;

Figure 7 is a graph of copper, iron, arsenic, and acid concentration in a leach solution versus reaction time; Figure 8 is a graph of copper, iron, arsenic, and acid concentration in a leach solution versus reaction time, showing the effect leach residues on the seeding of scorodite precipitation;

Figure 9 is a graph of arsenic concentration versus reaction time for Test

1 and Test 2 described in Table 5. Diamonds denote data points corresponding to Test 1 , whereas squares denote data points corresponding to Test 2;

Figure 10 is a graph of arsenic concentration versus reaction time for Test

3, Test 4, and Test 5 described in Table 5. Diamonds denote data points corresponding to Test 3, squares denote data points corresponding to Test 4, and triangles denote data points corresponding to Test 5; and Figure 11 is a graph of arsenic concentration versus reaction time for Test 6, Test 7, Test 8, and Test 9 described in Table 5. Shaded diamonds denote data points corresponding to Test 6, triangles denote data points corresponding to Test 7, circles denote data points corresponding to Test 8, and squares denote data points corresponding to Test 9.

DETAILED DESCRIPTION

Definitions

"Atmospheric pressure", as used herein, refers to a pressure approximately equal to atmospheric pressure or a pressure greater than atmospheric pressure, and is to be understood as including those small deviations from the true atmospheric pressure that may result, for example, from depth in the mixture or local altitude and barometric conditions.

"Carbon", as used herein, may include one or more of activated carbon, coal, brown coal, coke, hard carbon derived from coconut shells or elemental carbon, and mixtures thereof.

"Copper-arsenic sulphide concentrate", as used herein, refers to a mineral concentrate that includes one or more distinct copper-arsenic sulphide minerals. Copper-arsenic sulphides include, but are not limited to enargite (Cu₃AsS₄), tennantite (Cu,Ag,Fe,Zn)i₂As₄S₁₃), and luzonite (Cu₃(AsSb)S₄.

"Leach residue", as used herein, refers to any solid, or semi-solid, residue that is not dissolved in leach solution at the end of the leaching process. Leach residues will be understood to include both refractory ore, solid leach products (e.g. elemental sulphur), and precipitates formed during the leaching process. Leach residues may also include catalysts employed in the leaching process (e.g. carbon). "Oxygen-containing gas", as used herein, includes air, oxygen-enriched air, substantially pure oxygen, or any combination thereof. "Slurry", as used herein, refers to a mixture of particulate ore and solution, and may include one or more of fresh concentrate, leach residues, and catalyst.

"P80", as used herein, refers to the particle size at which 80% of the mass of material will pass through the specified size of mesh. For use in the leaching process, the P80 particle size of the copper-arsenic sulfide concentrate can vary over a wide range. The person skilled in the art will understand that particle size will be a function of the requirements for flotation of the solids in the leach reaction. "Supersaturated", as used herein, refers to the situation where a solution contains more solute than a saturated solution and is therefore not in equilibrium. A person skilled in the art will understand that the term "supersaturated" includes both a solution that is only slightly more concentrated than a saturated solution and a solution that contains a large excess of solute.

The Leaching Reactions

The method of the invention is herein described in the context of the recovery of copper from enargite, as an example of a copper-arsenic sulfide to which the method can be applied. A person skilled in the art will understand, however, that the method is generally applicable to the precipitation of arsenic ions leached from any copper-arsenic sulphide, including tennanite and luzonite. A person skilled in the art will further recognize that, in certain embodiments, the method can be practiced in the absence of a copper-arsenic sulphide concentrate, e.g. where an acidic sulfate leach solution comprising leached copper and arsenic ions has been separated from the leached copper-arsenic sulphide ore. 1. Enargite Leaching

In one embodiment of the method, particulate concentrate comprising enargite and a particulate iron source are added to an acidic sulphate solution.

The iron source, which may be chalcopyrite, pyrite, or a mixture thereof, is included as the predominant source of ferric ions that will ultimately be used to precipitate the arsenic leached from the concentrate. The enargite and the iron source may form part of the same ore deposit, and may be inseparable from each other. However, the enargite and the iron source could be sourced from separate ore deposits. The acidic sulphate solution should have an initial iron content of at least 1 gram per liter to initiate the leaching process. Preferably the iron level is maintained above about 5 grams per liter or alternatively above about 10 grams per liter.

In the acidic sulphate solution, copper and arsenic are leached from the enargite in the presence of an oxygen-containing gas, for example air or 0₂ gas, to produce a leach solution containing copper ions and arsenic ions. In the leach reactor, copper and arsenic are leached in the presence of a catalyst, to produce copper sulphate, arsenate, and a solid sulfur residue according to the following equation:

4 CU3ASS4 + 11 0₂ + 12 H2SO4 -» 12 CuS0₄ + 4 HaAsC + 16 S + 6 H₂0 (1)

Where enargite constitutes a small proportion of the concentrate relative to the iron source, e.g. less than 10 wt%, the iron source may effectively catalyze the leaching of copper and arsenic ions from the enargite. However, where enargite forms a larger proportion of the concentrate relative to the iron source, e.g. greater than 10 wt%, particulate carbon may be added to the acidic sulphate solution as a catalyst for the leaching the copper and arsenic from the enargite. 2. Iron Source Leaching

Chalcopyrite is leached quickly in the acidic sulfate solution in the presence of either pyrite or carbon to contribute both copper and ferrous ions to the leach solution according to the equation:

CuFeS₂₍s) + 2 H₂S0₄ + 0₂→ CuS0₄ + FeS0₄ + 2 H₂0 + 2 S° (2)

Pyrite leaching follows two paths with approximately 2/3 of the sulphide sulphur converted to sulphate and 1/3 to elemental sulphur, according to the overall equation:

12 FeS₂ + 30 O₂ + H₂O - 12 FeS0₄ + 8 S + 4 H₂S0₄ (3)

3. Ferrous oxidation

As ferrous ions are leached from the iron source into the leach solution, the operating potential is maintained at a level sufficient to oxidize the ferrous ions to ferric ion according to the equation:

4 FeS0₄ + O2 + 2 H₂S0₄ - 2 Fe₂(S0₄)₃ + 2 H₂0 (4)

More particularly, the operating potential is maintained at a level sufficient to provide a ratio of ferric ions:arsenic ions in the leach solution of at least 1 :1 for subsequent precipitation of the arsenic in a controlled manner as scorodite, but not so high as to promote the precipitation of less stable arsenic- containing compounds.

4. Arsenic precipitation

The acid level in the leach solution must be maintained sufficiently high so as to permit the leach solution to become supersaturated with arsenic ions. The operating potential must also be maintained at a sufficient level so that trivalent arsenic in the leach solution is oxidized to the pentavalent state . Provided that the molar ratio of ferric ions:arsenic in the supersaturated solution is about 1 :1 or slightly higher, excess arsenic in the leach solution will precipitate with ferric ions as scorodite according to the following equation:

Fe₂(S0₄)₃ + 2 H3ASO4 - 2 FeAs0₄ + 3 H₂S0₄ (5)

The leach solution must be maintained at pH 2 or less to hold the arsenic ions in solution. Generally speaking, the lower the pH is maintained, the greater amount of arsenic that may be held in the leach solution. If the pH of the leach solution is maintained sufficiently low, e.g. such that the majority of the arsenic ions leached from the enargite is in solution simultaneously, a nucleation event can occur which results in the dramatic and rapid precipitation of the majority of the dissolved arsenic as scorodite. Alternatively, by maintaining the acidity at a lower level, at which arsenic is less soluble, precipitation of the arsenic as scorodite may be achieved gradually and steadily over the course of the leach, provided that sufficient ferric ions are made available from the leaching of the iron source. The slow release of iron and arsenic into solution may favor the formation of scorodite in preference to non-crystalline, amorphous ferric arsenate that may be generated with substantially higher levels of iron to generate.

Arsenic levels in arsenic-depleted leach solution following scorodite precipitation may be less than 2 g/L, or less than 1 g/L, less than 0.9 g/L, less than 0.8 g/L, less than 0.7 g/L, less than 0.6 g/L, less than 0.5 g/L, less than 0.4 g/L, less than 0.3 g/L, less than 0.2 g/L, less than 0.1 g/L, less than 0.05 g/L, or less than 0.01 g/L. The acid in the arsenic-depleted solution may be neutralised to pH 2.0 to 2.5 using ground limestone to facilitate removal of the last of the arsenic. The stoichiometry of the overall process with pyrite oxidation providing substantially all of the ferric ions for scorodite precipitation is: 12 Cu₃AsS₄ + 33 0₂ + 36 H₂S0₄ -» 36 CuS0₄ + 2 H₃As0₄ + 48 S + 18 H₂0

12 FeS₂ + 30 0₂ + 4 H₂0 12 FeS0₄ + 8 S + 4 H₂S0₄ 12 FeS0₄ + 3 0₂ + 6 H₂S0₄ 6 Fe₂(S0₄)₃ + 6 H₂0 6 Fe₂(S0₄)₃ + 12 H₃As0₄ - 12 FeAs0₄ + 18 H₂S0₄ 6 CusAsS + 6 FeS₂ + 33 0₂ + 10 H₂S0₄ -» 18 CuS0₄ + 6 FeAs0₄ + 28 S +

10 H₂O

Where chalcopyrite provides substantially all of the ferric ions for scorodite precipitation, the overall stoichiometry is:

Cu₃AsS₄ + 11 0₂ + 12 H₂S0₄ 12 CuS0₄ + 4 HaAsC + 16 S + 6 H₂0 4 CuFeS₂ + 4 0₂ + 8 H₂S0₄ - 4 CuS0₄ + 4 FeS0₄ + 8 S + 8 H₂0

4 FeS0₄ + 0₂ + 2 H₂S0₄ 2 Fe₂(S0₄)₃ + 2 H₂0

2 Fe₂(S0₄)₃ + 4 H₃As0₄ 4 FeAs0₄ + 6 H₂S0₄

Cu₃AsS + CuFeS₂ + 4 0₂ + 4 H₂S0 -> 4 CuS0₄ + FeAs0₄ + 6 S + 4 H₂0

The acid in the last reactor may be neutralised to pH 2.0 to 2.5 using ground limestone to facilitate removal of the last of the arsenic.

The arsenic-depleted leach solution may then be separated from the leach residues to provide a copper-enriched solution from which copper is recovered according to conventional methods.

Acid

According to the overall leach stoichiometry given above for pyrite, at least five moles of sulfuric acid should theoretically be added to the leach for every three moles of arsenic precipitated from the leach as scorodite. In practice, however, the acid requirement may fluctuate depending on the exact composition of the concentrate and the degrees of pyrite, sulfur and ferrous oxidation, and scorodite precipitation that occur during the leach.

Batch or Continuous Process

The method may be executed as a batch process or as a continuous process. In a batch process, the level of enargite in the leaching reactor (and, thus the demand for oxygen) diminishes with time. Accordingly, it may become necessary to regulate the flow of the oxygen-containing gas to the reactor, particularly when pure oxygen is used rather than air. Alternatively, in a continuous process consisting of a number of leaching tanks in series, one may simply supply the oxygen-containing gas to each tank at an appropriate flow rate. This may be facilitated in practice by supplying pure oxygen or oxygen- enriched air to the first one or two tanks and air to the remaining tanks.

Recirculation of Leach residues

A portion of the leach residues may be recirculated to the acidic sulphate solution for further participation in the method. Recycling of the enargite leach residues may serve several purposes, including:

1. increasing the effective residence time of the unleached enargite concentrate, to maximize the extraction of copper. Refractory minerals in the concentrate, such as tennantite, that are not fully leached in a first pass through the circuit are thereby given more time in the system.

2. increasing the initial acidity of the leach solution;

3. increasing the slurry density to improve carbon buoyancy, which would prevent carbon breakage in the leach process, thereby decreasing the initial carbon loading and reducing operational costs; providing seed material for scorodite precipitation, whereby the leach solution may reach arsenic saturation more rapidly. Precipitated scorodite so recirculated may act as seed for further scorodite precipitation, facilitates the steady and controlled precipitation of scorodite at a uniform rate from the beginning of the leach process, and thereby avoids the risks associated with an uncontrolled nucleation event. Steady precipitation at low supersaturation levels results in a precipitate which is more crystalline and suitable for disposal; ensuring sufficient iron from pyrite dissolution for scorodite precipitation; and returning carbon catalyst to the system for reuse.

A person skilled in the art will further understand that, in embodiments involving a continuous process with recirculation of the leach residues, ratios of the amounts of starting materials, including the relative amounts of copper- arsenic sulphide to iron source, will vary depending on the content of the leach residues being returned to the leach reaction.

Carbon Catalyst

The use of a carbon catalyst for leaching of a copper-arsenic sulphide has been disclosed previously in WO/201 /047477. The weight ratio of the carbon to the enargite present in the concentrate may be at least 1 :20. Alternatively, the carbon.enargite ratio is at least 1:9, or between about 1 :5 and

20:1 , or between about 1:2 and 4:1. Carbon is added from an external source, but may also be recirculated to make up the desired ratio in the concentrate. While carbon could be recirculated with other leach residues, carbon would typically be screened from the rest of the leach residues and returned separately. Carbon may also be retained in the leach tanks by screens, and therefore not be part of leach residues, except in the form of carbon fines which have broken off of the original coarse carbon).

Alternatively, rather than directly controlling the weight ratio of carbon to enargite being added to the acidic sulphate solution, the method may include the step of maintaining the carbon in the acidic sulphate solution at a suitable concentration, by adding coarse or granular carbon to each leaching vessel and retaining most of this carbon within the vessel, for example with the use of screens. In this way, the ground concentrate slurry passes easily from tank to tank, while the carbon is retained within the tank. In such embodiments of the method, enargite concentrate and carbon are added to an acidic sulfate leach solution. The copper and arsenic are leached from the concentrate, in the presence of an oxygen-containing gas, while maintaining the carbon at a concentration of at least 5 grams per liter of the leach solution, alternatively at least 10 grams per liter, alternatively at least 20 grams per liter, alternatively at least 20 grams per liter, alternatively at least 30 grams per liter, alternatively at least 40 grams per liter, alternatively at least 50 grams per liter, alternatively at least 60 grams per liter, alternatively at least 70 grams per liter, alternatively at least 80 grams per liter, alternatively at Ieast90 grams per liter, alternatively at least 100 grams per liter. In preferred embodiments, carbon is maintained in at a concentration between at least 40 grams per liter and about 100 grams per liter.

Carbon does not typically occur with primary copper ores and is added to the leach reaction. Hence the carbon has to be purchased and delivered to the minesite. In order for the process to be economically viable, the carbon should be efficiently recycled and reused within the system. In one embodiment this is accomplished by maintaining coarse carbon within each leaching reactor with screens. In this way, the ground concentrate slurry passes easily from tank to tank, while the carbon is retained within the tank. Coarse, hard carbon such as the coarse activated carbon derived from coconut shells can be used in this embodiment. A certain amount of attrition of the carbon occurs, particularly given the requirement for high-shear mixing to ensure adequate gas-liquid mixing. Hence, in this scenario, a certain amount of carbon passes through the screens and is lost to the tailings, and this carbon has to be made up by addition of fresh carbon. The carbon particle size may be coarse as in commercially-available activated carbon, or alternatively the carbon may be finely ground. A smaller carbon particle size may be used to obtain a larger surface area on the carbon. A larger particle size may be used to enable retention and recycling in the leaching vessels with screens and provide a more economically viable process.

Operating Potential

In order for the leaching process to proceed at an adequate rate, the operating potential of the solution (i.e. the potential at which the process is carried out) is maintained at least at about 470 mV versus Ag/AgCI (all solution potentials stated herein are expressed in relation to the standard Ag/AgCI reference electrode). Alternatively, the operating potential is maintained at least at about 480 mV, at least at about 490mV, at least at about 500 mV, at least at about 510 mV, at least at about 5 5, mV, at least at about 520 mV, at least at about 530 mV, at least at about 540 mV, at least at about 550 mV, at least at about 560 mV, at least at about 570 mV, at least at about 580 mV, at least at about 590 mV, or at least at about 600 mV. Alternatively, the operating potential is maintained in a range between 470 and 600 mV. In a preferred embodiment, the operating potential setpoint is 5 5 mV. Operating potential may be maintained or adjusted by means of controlling the flow rate of an oxygen-containing gas flow rate, or the intensity of agitation of the leaching solution, or with high-velocity gas-injection nozzles, or the slurry density level. Increasing the flow rate of the oxygen-containing gas increases the supply of oxygen, and hence increases the maximum rate at which oxygen can be utilized in the leaching reactions. Increasing the rate of agitation or injecting the oxygen through high-velocity nozzles increases the surface area of the gas-liquid interface, which also increases the utilization of oxygen. In the absence of other factors, either of these will increase the redox potential. Increasing the slurry density increases the demand for oxygen, which in turn causes the operating potential to decrease in the absence of other factors. Typically, slurry density and agitation rate are set at constant values by design of the leaching apparatus, and potential is controlled by means of the oxygen-containing gas flow rate.

Temperature

The leaching process is run at temperatures between about 50°C and the melting point of sulfur (about 110 to 120°C). Alternatively, it is run at a temperature of between about 70°C and the melting point of sulfur, or alternatively, at a temperature of between about 80°C and the melting point of sulfur. Preferably, the temperature is maintained at about 85°C. While it is preferable to perform the method under about atmospheric pressure, a person skilled in the art would realize that it can be performed under any pressure between about atmospheric pressure and those pressures attainable in an autoclave.

Grind

Ultrafine grinding of the concentrate, iron source, and the carbon is not necessary, although the process will work with ultrafine materials. The leach can be run at any slurry density that will seem reasonable to one skilled in the art. Higher slurry densities facilitate the control of solution potential by ensuring high ferric demand, and may also enhance the effectiveness of the carbon and enargite interaction.

A flowsheet for carrying out the method of scorodite precipitation and recovering the extracted copper according to one embodiment of the invention is shown in Figure 1. The method involves the three basic steps, namely, leaching 10, solid-liquid separation of leach residues from the arsenic depleted solution 12, and copper recovery by solvent extaction and

electrowinning (SX-EW) 14. In the method depicted in Figure 1 , a bulk concentrate 16 comprising particulate enargite is subjected to the leaching method. Other copper or base metal sulfides may also be present in the concentrate, including other copper-arsenic sulphides and chalcopyrite. In this embodiment, the concentrate further comprises pyrite as the predominant source of iron for subsequent scorodite precipitation, with lesser amounts of chalcopyrite. However, pyrite may be added separately, and may be from an external source.

The leaching process commences with the addition of an acidic sulphate solution 18 to the concentrate 16 comprising mixture of enargite and pyrite in a leaching tank. In this embodiment, carbon 20 is added as a catalyst for leaching of enargite

Oxygen-containing gas 22 is provided to the leaching tank, or series of leaching tanks if a series is to be used, to maintain an operating potential of the solution sufficient to leach copper and arsenic ions from the enargite. Given the relatively modest oxygen requirements of the process, this oxygen- containing gas 22 can also be supplied by a low-cost vapor pressure swing absorption (VPSA) plant, or by a more conventional cryogenic oxygen plant for larger applications.

The leaching process is further conducted under agitation whereby the solution is sufficiently agitated by impellers to suspend the solids in the leaching tanks.

Solid-Liquid Separation and Solvent Extraction

Following the leaching process, copper can be extracted from the arsenic- depleted solution. After a solid-liquid separation in which the arsenic-depleted solution is separated from the leach residues (step 12 in Figure 1) to produce a copper-enriched solution, the copper-enriched solution is subjected to conventional solvent extraction (SX) 14 and electrowinning (EW) 24 to produce pure copper cathodes according to the following overall reaction: SX-EW:

CuS0₄ (aq) + H₂0 (I) → Cu (s) + H₂S0₄ (aq) + ½ 0₂

The raffinate resulting from the solvent extraction step can optionally be recirculated to the acidic sulphate.

EXAMPLES Example 1

A series of atmospheric batch leach tests were run on samples of an enargite concentrate from a mine site to determine the conditions under which arsenic could be co-precipitated with ferric ions during atmospheric leaching of enargite, and the extent to which such precipitation could occur. The enargite concentrate of the following composition by weight:

45.3% enargite - Cu3AsS

40.8% pyrite - FeS₂

3.0% chalcopyrite - CuFeS₂

2.6% luzonite - Cu₃(As,Sb)S₄

1.8% tennantite - (Cu,Ag,Fe,Zn)i₂As₄Si₃

1.7% sphalerite - (Zn,Fe)S

0.7% galena - PbS

Balance siliceous/carbonaceous gangue

In total, the enargite concentrate, by weight, was 28.3% Cu and 9.2% As. 150g of this enargite concentrate was combined with 150g of activated carbon, 1500g of distilled water, 5.3 g of Fe₂(S0 )₃-5H₂0 to provide the initial ferric ions, and 6.0 g of FeSCy7H₂0 to provide the initial ferrous ions. In a first series of four tests, the initial acid concentration was varied between 20 and 80 g/L at a concentrate slurry density of roughly 7% (70 g/L). Table 1 summarizes the experimental conditions of this first series of tests, denoted as Acid Series A. All conditions other than initial acid concentration were the same for all four tests. No leach residues from a previous leach were added.

The mixture was agitated at 85°C in a 2.7-L jacketed glass reactor with two 2" diameter 45° 6-pitched-blade turbines at 800 rpm. The operating potential setpoint was set at 515 mV vs Ag/AgCI, and maintained by addition of pure oxygen gas. Solution and solid assays were conducted using X-ray fluorescence spectroscopy (XRF). The accuracy of selected solution assays was confirmed using atomic absorption spectroscopy (AA).

Table : Experimental conditions for Acid Series A tests.

Test MP Carbon [H₂S0₄] [Fe] T Stir E target

Con (g/L) (g/L) (g/L) (°C) Rate (mV)

(g/L) (rpm)

A20 70 70 20 1.6 80 1200 515

A40 70 70 40 1.6 80 1200 515

A60 70 70 60 1.6 80 1200 515

A80 70 70 80 1.6 80 1200 515

Results for these tests are shown in Figure 2. Several trends are apparent from Figure 2. First, the rate of copper was a weak function of acid concentration. At 40 g/L acid and above, copper extraction was more or less the same for every test. However copper extraction was depressed at 20 g/L acid. Lower oxidation potentials were observed at low acidity, as shown in

Figure 3. Evidently, enargite leaching kinetics begin to falter at solution potentials much below 470 mV vs Ag/AgCI.

Second, the arsenic behaviour was relatively complicated, showing very different trends at the different acid concentrations. At 80 and 60 g/L acid, arsenic extraction followed copper extraction, with little or no arsenic precipitation evident during the leach. At 40 g/L acid, arsenic extraction followed copper extraction initially, but then most of the arsenic precipitated after about 52 hours. The final arsenic level in solution was only about 500 mg/L. Finally, at 20 g/L acid, arsenic extraction only followed copper extraction for about 26 hours (half the time at half the acid concentration).

Precipitation was slower and more steady at the lower acidity, reaching only about 400 mg/L by the end of the test.

In a second series of four tests, the initial acid concentration was varied between 15 and 60 g/L, the concentrate slurry density was adjusted to roughly

10% (100 g/L), the leaching temperature was maintained between 80 and 85°C, and the impeller speed was set at 800 rpm. Table 2 summarizes the experimental conditions of these tests, denoted Acid Series B. Table 2: Experimental conditions for Acid Series B tests.

Test MP Carbon [H₂S0₄] [Fe] T Stir E target

Con (g/L) (g/L) (g/L) (°C) Rate (mV)

(g/L) (rpm)

B15 100 100 15 1.6 85 800 515

B30 100 100 30 1.6 85 800 515

B45 100 100 45 1.6 85 800 515

B60 100 100 60 1.6 85 800 515

Results for these tests are shown in Figure 4. The trends observed in Figure 2 for the Acid Series A tests are evident in Figure 4. The arsenic concentration reached very high levels of supersaturation before precipitating en masse, presumably by a nucleation mechanism. However, arsenic concentrations did eventually decrease to near saturation levels with time, indicating near complete precipitation of scorodite. Both the maximum arsenic concentration and the time required before the inception of rapid precipitation are roughly proportional to the initial acid concentration. This time is shown to be roughly inversely proportional to the concentrate slurry density, which fixes the overall arsenic content. In other words, the sudden arsenic precipitation occurs sooner with more arsenic in the system, as expected. Fe concentration measurements (not shown) largely stayed at around 8 g/L, suggesting that the rate of iron dissolution by pyrite oxidation was roughly matched by the rate of scorodite precipitation.

Arsenic and iron simultaneously decreased in concentration during the rapid precipitation event, indicating that an approximately 1 :1 molar ratio of ferric ions to arsenic ions is important for precipitation as scorodite.

Cu extractions from these tests (based on solid assays) were as follows:

15 g/L H₂S0₄ - 91.8%

30 g/L H₂S0₄ - 94.4%

45 g/L H₂S0₄ - 95.0%

60 g/L H₂S0₄ - 95.2%.

The dependence of the timing of arsenic precipitation on acid and arsenic concentration, and the gradual rise and sudden fall of arsenic concentration in these tests, indicates a nucleation mechanism for scorodite precipitation. The arsenic builds up in solution initially to levels far in excess of arsenic solubility in equilibrium with scorodite (which was presumed to be roughly equivalent to the final arsenic levels in solution). At a certain arsenic concentration (which depends on the initial acid concentration), a critical level of scorodite supersaturation is attained, causing spontaneous nucleation of scorodite particles. Once these scorodite nuclei are formed, precipitation occurs fairly rapidly, and scorodite saturation is approached asymptotically.

A nucleation mechanism suggests that scorodite does not form readily on activated carbon, concentrate particles, or elemental sulfur. Hence, it would appear as if seed particles, of scorodite or some other suitable seed material, are required to initiate scorodite precipitation. It is desirable for scorodite to begin precipitating as soon as the concentrate begins to leach, which may be achieved by recycling the leach residues to the acid sulphate solution. Continuous leaching will enhance the effect further, since there will always be scorodite particles in the leaching tank at steady state. Residues from these batch tests showed excellent stability in TCLP

(Toxicity Characteristic Leaching Procedure) tests. All but the most acidic residue passed were under the 5.0 mg/L Environmental Protection Agency (EPA) limit, as shown in Table 3 and Figure 5.

Table 3: TCLP As results for Acid Series B (5.0 mg/L EPA limit).

Test Final

As

(mg/L)

B15 2.3

B30 2.9

B45 4.1

B60 6.5

In order to test the effect of seed particles, two tests were run under experimental conditions identical to tests B30 and B60 as listed in Table 2, but with 20 g/L of fine gypsum powder added to the leach reactor. The experimental conditions for this test, denoted Acid Series C provided in Table 4.

Table 4: Experimental conditions for Acid Series C tests with added gypsum.

Test MP Carbon [H₂so₄] [Fe] T Stir E target

Con (g/L) (g/L) (g/L) CO Rate (mV)

(g/L) (rpm)

C30 100 100 30 1.6 85 800 515

C60 100 100 60 1.6 85 800 515

20 g/L gypsum powder added to both tests Results for Acid Series C are shown in Figure 6. The addition of gypsum powder had only a minor effect on the experiments. Precipitation still occurred via a nucleation mechanism. The level of arsenic supersaturation at nucleation decreased slightly in these tests, but the time required to reach supersaturation increased, suggesting that added gypsum impeded the leach slightly. It would appear that gypsum powder out of a bottle is not a suitable seed material for atmospheric scorodite precipitation.

Example 2

Atmospheric batch leach tests were conducted on the enargite concentrate referred to in Example 1. 28.8 kg of this enargite concentrate was combined with 30.9 kg of activated carbon, and added to 250 L of an acidic sulphate leach solution comprising 90 mg/L As, 1780 g/L Fe, 1.87 g/L Cu, and

37.0 g/L H₂S0₄.

The mixture was agitated at 85°C with two 12" diameter impellers (Lightnin A315 (bottom) and A310 (top) at 220 rpm. The operating potential setpoint was set at 515 mV vs Ag/AgCI, and maintained by addition of pure oxygen gas at a rate of 60 L/min through a slotted sparge tube. No leach residues from a previous leach were added.

Arsenic, iron, copper and free acid concentrations during the leaching process are shown in Figure 7. As in Example 1 , the arsenic concentration reached a high level of supersaturation before rapid precipitation ensued. However, final arsenic concentrations were very low (less than 300 mg/L).

Example 3

Atmospheric batch leach tests were conducted on the enargite concentrate referred to in Example 1. 13.3 kg of this enargite concentrate was combined with 7.4 kg of activated carbon, and added to 102 L of an acid sulphate leach solution comprising 455 mg/L As, 11800 g/L Fe, 6.5 g/L Cu, and 51.0 g/L H₂S0_4. In this test, a previously leached residue containing scorodite was recycled to the test.

The mixture was agitated at 85°C with one 12" diameter impeller (Lightnin A315) at 220 rpm. The operating potential setpoint was set at 515 mV vs Ag/AgCI, and maintained by addition of pure oxygen gas at a rate of 30 L/min t through a Silvent MJ4 nozzle.

Arsenic, iron, copper and free acid concentrations during the leaching process are shown in Figure 8. Scorodite precipitated from the very beginning of the test, and the peak As concentration never exceeded 1000 mg/L. The final As concentration was roughly 600 mg/L, which is higher than that of the test shown in Example 3. This reflects the higher As saturation level at higher free acid concentration.

Example 4

A series of nine large scale batch experiments were conducted for the catalyzed enargite leaching process. The conditions for these experiments are summarized in Table 5. Each test was carried out at approximately 85°C.

The results for overall copper, iron and arsenic extractions, the residual concentrations, as well as the carbon breakage and final free acid total ("Final

FAT"), are summarized in Table 6. "Cu Extraction (Test)" refers to the percentage of copper extracted from all copper initially available in a given test, including in fresh concentrate and in recycled leach residue. "Cu Extraction (Test)" was calculated using two methods, i.e. "Met-Bal" (the metallurgical balance of the metallurgical process) and "Pb-tie". For Met-Bal, extracted copper (X_Cu) was calculated based on the initial concentration of copper added to the process for a given test (including contributions from fresh concentrate and leach residues) and the final concentration of the leach residue of that test, i.e. f fin_al %Cu ₁₀₀

initial %Cu )

In the present example, it will be appreciated that the initial copper concentration varied with each test. In Test 1 and Test 2, the initial copper concentration was the concentration of the fresh concentrate, i.e. approximately 28.3%, since no leach residues were added to these tests. For subsequent tests, however, the initial copper concentration decreased as leach residue formed a greater proportion of the initial solids, and as the copper concentration of the leach solids decreased through several rounds of recirculation. For example, with a leach residue copper concentration of 0.78% in the initial solids, and a leach residue:fresh concentrate ratio of 4.66, the intial copper concentration of the initial solids would be approximately 4.59%. If the final copper concentration of the leach residue was 0.78%, c_u can be calculated as X_Cu = (1 - 0.78/4.59)- 100 = 83.2%.

Pb-tie represents a further correction for the overall loss of mass from the initial solids. Pb is insoluble in the leach solution and remains with the solids. Thus, the concentration of Pb in the leach residue may be used to calculate the final mass of the solids remaining from the solids introduced at the beginning of the test according to the following formula: final mass solid initial %Pb

^ initial mass solid final %Pb and cu can be calculated as: final %Cu Ί initial %Pb

Γ_ΐ] = [1 - x ]^■ 100

initial %Cu J final %Pb

Thus, since the lead content usually increased, the "Pb-tie" values for copper extraction are usually slightly higher than the Met-Bal values. Table 5. Summary of test conditions for catalyzed enargite leaching experiments on large scale batch reactors

PLS = Pregnant Leach Solution

ORP = Oxygen Reduction Potential

Table 6. Summary of results for catalyzed enargite leaching experiments on large scale batch reactors.

FAT = free acid total

was calculated assuming a Cu composition of the fresh concentrate of 28.3 wt% and a Cu composition of the leach residue of 0.78%.

"Cu Extraction (Cumulative)" refers to the cumulative copper extraction from total fresh enargite concentrate that would have ever been processed to make up the initial solids for the given test, and may be calculated as: final %Cu initial %Pb in concentrate^

X, Cu (cumuiative) = [1 - ] -100 initial %Cu in concentrate final %Pb

As described in Table 5, Test 1 and Test 2 differed from each other in the impellers used for slurry agitation, and the oxygen flow rate. Test 1 utilized a 12" Lightnin A310 impeller on top and a 12" Lightnin A315 impeller on the bottom. Test 1 utilized a 12" Lightnin A310 impeller on top and a 12" Lightnin R100 rushton impeller on the bottom. Test 1 and Test 2 were carried out using fresh enargite concentrate only; no leach residue from previous enargite leaches were combined with the fresh enargite concentrate. Figure 9 is a graph of arsenic concentration versus reaction time for Test 1 and Test 2 maximum arsenic concentration obtained in the pregnant leach solution of each test was between 2500 mg/L and 3000 mg/L. These maximum arsenic concentrations were followed by scorodite precipitation which decreased arsenic in solution to around 400 mg/L. This saturation-precipitation behavior was observed in each of the nine large scale batch tests.

For Tests 3 to 9, enargite leach residues were recycled and added to fresh enargite concentrate in the proportions indicated in Table 5. Leach residues for each test were sourced from the immediately preceding test, with additional leach residues being sourced from further preceding tests to achieve the desired ratios of leach residue:fresh enargite concentrate. To further reduce carbon breakage, a single impeller, i.e. a 12" Lightnin A315 impeller, was used to agitate the leach mixture. To improve gas-liquid mixing, the oxygen delivery method was changed from an open tube to one or two Silvent™ sparger micro-nozzles MJ4-100 (total length of 16.5 mm), as indicated in Table 5.

Figure 10 is a graph of arsenic concentration versus reaction time for Test 3, Test 4, and Test 5, which varied in several parameters including initial acidity and residue:fresh concentrate recycling ratio. Increased initial acidities for Test 4 and Test 5 relative to Test 3 correlated with higher arsenic concentrations at the beginning of these tests. The higher arsenic concentrations with higher initial acidities are consistent with the results for the Acid Series A and Acid Series B tests shown in Figure 2 and 4. Acid was added to Test 4 at approximately 40 hours, which correlated with a sharp increase in arsenic concentration between 40 and 48 hours.

As indicated in Table 6, carbon breakage rates of 0.17 and 0.53% per hour were observed for Test 1 and Test 2, respectively. The carbon breakage rates decreased dramatically in Test 3, Test 4, and Test 5 to less than 0.05% per hour.

Copper leaching efficiencies of greater than 90% from the fresh concentrate were observed with leach residues:fresh concentrate ratios of about 4.5:1. Accordingly, a target leach residues.fresh concentrate ratio of 4.5:1 was subsequently used for Test 6 to Test 9. A person skilled in the art will understand that the desired ratio will depend ultimately on the grade of the concentrate. The concentration of copper-enriched solutions advanced to solvent extraction will generally not exceed about 30 g/L, and the amount of fresh concentrate added to the system will be adjusted based on this parameter. If a lower grade concentrate is used, then the ratio may be lower since more fresh concentrate and less recirculated leach residues will be used to achieve a slurry density between about 30% and 50% by weight. Figure shows arsenic concentration versus reaction time for the final four large scale tests, i.e. Test 6 to Test 9. Copper-enriched solutions with high overall extractions of >98.6% and low arsenic concentrations (<300 mg/L) were consistently obtained. With the exception of Test 8, carbon breakage rates remained at almost undetectable levels. Increasing carbon concentration had no discernible effects. The residues passed TCLP environmental tests, providing final arsenic concentrations in the leach solution of less than 0.04 mg/L (<2% of the 5 mg/L regulatory limit).

While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.

Claims

What is claimed is:

1. A method of producing scorodite at atmospheric pressure, the method comprising: providing a mixture comprising: an acidic sulphate leach solution including dissolved copper ions and dissolved arsenic ions, wherein the leach solution is supersaturated with the dissolved arsenic ions, and an iron source, wherein the iron source is one or both of particulate pyrite and particulate chalcopyrite; supplying an oxygen-containing gas to the mixture to provide leaching conditions, wherein the oxygen-containing gas is supplied so as to maintain an operating potential of the leach solution sufficient to oxidize the iron source to form dissolved ferric ions in the leach solution, to provide at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions in the leach solution; and selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

2. A method of producing scorodite at atmospheric pressure, the method comprising: providing a particulate mixture of a copper-arsenic sulphide concentrate and an iron source, wherein the iron source is one or both of pyrite and chalcopyrite; supplying an acidic sulphate solution and an oxygen-containing gas to the particulate mixture to provide leaching conditions wherein the mixture is oxidized to form a leach solution comprising dissolved copper ions, dissolved arsenic ions, and dissolved ferrous ions, wherein the leach solution is supersaturated with the dissolved arsenic ions; maintaining the operating potential at a level sufficient to oxidize the dissolved ferrous ions to provide at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions in the leach solution; and selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

A method of producing scorodite at atmospheric pressure, the method comprising: providing a particulate mixture of a copper-arsenic sulphide concentrate and an iron source, wherein the iron source is one or both of pyrite and chalcopyrite, supplying an acidic sulphate solution and an oxygen-containing gas to the particulate mixture to provide concentrate-leaching conditions wherein the concentrate is oxidized to form a leach solution comprising dissolved copper ions and dissolved arsenic ions, wherein the leach solution is supersaturated with the dissolved arsenic ions; maintaining the operating potential at a level sufficient to provide iron source-leaching conditions wherein the iron source is oxidized to form at least 1 mole of dissolved ferric ions per mole of dissolved arsenic ions; and selectively precipitating the dissolved ferric ions with the dissolved arsenic ions from the leach solution as scorodite.

4. The method of claim 2 or 3, further comprising supplying carbon with the acidic sulphate and the oxygen-containing gas to provide the concentrate- leaching conditions.

5. The method of claim 2, 3, or 4, wherein the copper-arsenic sulphide

concentrate comprises enargite.

6. The method of any one of claims 1 to 5, wherein the iron source is

predominantly pyrite.

7. The method of any one of claims 1 to 6, comprising maintaining the

operating potential between 470 and 600 mV versus Ag/AgCI.

8. The method of any one of claims 1 to 7, further comprising maintaining the leach solution at a pH of about 2 or less.

9. The method of any one of claims 1 to 8, wherein the process is a

continuous process.

10. The method of any one of claims 1 to 9, further comprising recirculating a portion of the scorodite to the mixture.

11. The method of any one of claims 1 to 10, wherein the process is carried out at a temperature of less than 95°C.

12. The method of any one of claims 1 to 11 , wherein the process is carried out at a temperature of about 85°C.

13. The method of any one of claims 1 to 12, wherein the arsenic-depleted leach solution comprises less than 2 g/L of arsenic.

14. The method of any one of claims 1 to 13, wherein the arsenic-depleted leach solution comprises less than 1 g/L of arsenic.

15. A method of recovering copper from a copper-arsenic sulphide

concentrate at atmospheric pressure, the method comprising: producing scorodite according to the method of any one of claims 2 to 16; and separating the arsenic-depleted leach solution from leach residues to provide a copper-enriched solution.

16. The method of claim 15, wherein the copper-enriched solution includes at least 80% of the copper ions from the mixture.

17. The method of claim 15, wherein the copper-enriched solution includes at least 90% of the copper ions from the mixture.

18. The method of any one of claims 1 to 17, further comprising recirculating a portion of the leach residues to the mixture.