FIELD

The present application relates to methods for the simultaneous leaching and extraction of precious metals. For example, the present application relates to methods of leaching and extracting gold and/or palladium from a substance comprising gold and/or palladium such as a gold- and/or palladium-containing ore which is optionally done in one step using a compound of the present application.

BACKGROUND

Gold is an element in the periodic table which belongs to the same group as silver and copper. It is usually found in combination with these metals in ores. The average concentration of copper and silver in Earth's crust is 50 and 0.07 ppm (parts per million) respectively while for gold it is just 0.005 ppm1.

Ore deposits with a concentration of 0.5 ppm or higher are considered to be economically recoverable. Due to its limited sources, gold recovery not only from ores, but also from secondary sources has become more and more important during the last decades. The annual production of gold from the gold mining industry is more than 2500 tonnes worldwide2. In addition, about 900 tonnes of secondary gold is recovered from different sources such as but not limited to anode slime and jewelry, dentistry and electronic scraps3.

The most commonly used process for gold recovery from ore includes the use of highly toxic inorganic cyanides (e.g., NaCN, KCN) to convert gold(0) into a water-soluble Au(CN)2−coordination complex by a process known as leaching. An example of a known process10for gold recovery using cyanide leaching is shown inFIG. 1. In process10, low grade ore12is crushed and ground14then leached16with a basic solution of NaCN for 16 to 48 hours depending on ore type. Because of some environmental accidents in various gold mines around the world, gold leaching by cyanidation has been prohibited in many countries4. Therefore, considerable efforts have been made to find an alternative to cyanide and a variety of leaching reagents have been studied and proposed5,6.

Generally, following gold dissolution in the cyanide solution, gold is recovered by activated carbon adsorption (e.g. step18in process10ofFIG. 1wherein, for example 0.1 to 1 kg activated carbon per ton ore is used), or by the zinc cementation process. The activated carbon adsorption process is considerably more common7,8. For example, 4 to 8 kg gold can be adsorbed by 1 ton activated carbon in 4 to 8 steps over a time period of 4 to 8 hours.

As shown inFIG. 1, following the carbon adsorption step18, the loaded activated carbon is washed20with low concentrated HCl to remove impurities such as adsorbed Zn, Ca, Fe, Cu and Ag then gold desorption (elution)22is done by using, for example 1% NaOH and 0.1 to 0.2% NaCN solution at a high temperature (e.g. 110° C.) for 36 to 72 hours. Pure gold24can be obtained, for example by electrowinning or reduction. The whole process time for gold recovery using a process like the process10shown inFIG. 1is 46-110 hours.

Processes for gold recovery which use activated carbon may suffer from several drawbacks such as but not limited to low selectivity, very long procedures, loss of gold product, high temperature requirements, and further consumption of cyanide for desorption of gold from activated carbon, all of which may bring additional costs during the gold recovery process9.

Although considerable effort has been undertaken to replace cyanide, none of the reported leaching reagents has been used in the industrialization of gold production due, for example to drawbacks such as (i) high reagent consumption, (ii) complex chemistry, (iii) lack of industrial techniques for the recovery of gold from their resulting solutions, and (iv) low rate of gold recovery compared to cyanide. Drawbacks such as toxicity, cost, long reaction times and poor selectivity are also associated with known systems. Thus, it may be desirable to develop more effective leachants with, for example, higher efficiency and/or lower toxicity from both an environmental and an economical viewpoint.

Cyanide Leaching

For more than a century, cyanidation has remained the dominant process for extraction and recovery of gold from ore. Metallic gold can be dissolved in an alkaline solution of potassium or sodium cyanide in the presence of dissolved molecular oxygen (reaction 1):



In neutral or acidic conditions, over 99% of the cyanide will exist as highly poisonous HCN gas. By increasing pH, it is converted to free cyanide ion so that at a pH of 9.3, CN−and HCN are in equilibrium, with 50% of each present. At a pH of 11, over 99% of the cyanide remains in solution as CN−.10The free cyanide ion is a very strong ligand which can form a highly stable complex with gold, Au(CN)2−, in aqueous solution. With stoichiometric ratios, gold dissolution in alkaline cyanide solution is slow, but by increasing the cyanide concentration, the leaching rate will increase until a maximum is reached (0.075 w/w % KCN or 0.06% NaCN) and after that the rate of dissolution remains constant11.

Before cyanide treatment, the gold ore is typically crushed and ground to decrease the size of the ore particles to 75 microns or less to provide a larger contact surface area between the gold and the leaching solution. Depending on the ore type, the cyanide consumption varies from about 0.25 to 2 kg of cyanide per tonne of ore and the rate of gold dissolution in cyanide takes 16 to 48 hours11. The cyanide consumption increases when the refractoriness of the gold ore is increased. A refractory gold ore is a gold-containing ore that is resistant to recovery by direct cyanidation. Other minerals and metals are also dissolved in the alkaline cyanide solution and they usually consume cyanide and oxygen and thus reduce the overall efficiency of gold leaching.

For example, copper minerals such as chalcocite (Cu2S) and cuprite (Cu2O) can form a variety of cyanide complexes such as CuCN, Cu(CN)2−, Cu(CN)32−and Cu(CN)43−and iron sulfides like pyrrhotite (Fe7S8), pyrite (FeS2) and arsenopyrite (FeAsS) form highly stable Fe(CN)64−and Fe(CN)63−complexes12. In addition, most sulfide minerals have a detrimental effect on gold leaching since they may passivate the surface of gold and consume cyanide and oxygen. However, some other minerals such as galena (PbS) can improve gold leaching kinetics by preventing formation of a passivation layer on the gold surface13.

Although cyanide is still the main leaching reagent for gold recovery in the mining industry, it suffers from several drawbacks such as but not limited to high toxicity, slow leaching kinetics and low gold extraction for refractory ores. Considerable efforts have thus been made to find an alternative to cyanide.

Gold Recovery from Cyanide Solution

There are several techniques for gold recovery from cyanide leach liquors like carbon adsorption, zinc cementation and solvent extraction with carbon adsorption being by far the more common technique14,15. In the carbon adsorption technique, after gold is leached into cyanide solution, activated carbon is applied for selective gold adsorption to separate AuCN2−from other metals and impurities. 0.1 to 1 kg activated carbon per tonne of ore is usually applied in 4 to 8 steps for complete adsorption of Au(CN)2−complex from cyanide solution which takes 4 to 8 hours. The loaded activated carbon is usually washed with a low concentration HCl solution to remove other impurities such as Fe, Cu, Zn, Ca, and Ag. The dicyanoaurate(I) complex is then removed from the activated carbon in an elution step by washing the loaded activated carbon with a fresh basic sodium cyanide solution at 110° C. for 36 to 72 hours10,16. The desorbed Au(CN)2−complex is finally reduced to elemental gold by electrowinning or reduction.

The activated carbon method suffers from several drawbacks such as but not limited to low selectivity, very long procedures, loss of some gold product, and high temperature requirements17.

Alternatives to Cyanide

Due to the high toxicity and environmental problems of cyanide, there has been a quest to find useful alternatives. In recent years, some alternatives to cyanide have been reported to leach gold ore efficiently. Some of the useful reported leaching reagents are thiosulfate, thiocyanite, thiourea, and chloride in combination with an oxidizing agent like HNO3, H2O2and hypochlorite.

Thiosulfate Leaching

Thiosulfate is the most studied alternative to cyanide. Gold can be leached in alkaline aqueous solutions (pH=9.5-10.5) of thiosulfate in the presence of oxidizing agents like O2and copper(II) ions. The rate of gold dissolution becomes slower in the absence of copper (II) ions18. Ammonia is usually used to accelerate the rate of gold leaching in this media. It has an efficient role to stabilize the intermediate oxidation products of gold, decreasing the rate of thiosulfate oxidation by Cu2+, preventing the formation of insoluble components like sulfides on the gold surface and keeping a high concentration of Cu2+by forming Cu(NH3)42+during the leaching process19,20. Oxygen has a dual role by oxidation of Cu(NH3)2+to Cu(NH3)42+or direct oxidation of the gold surface. The overall balanced equation of gold dissolution in thiosulfate media is shown in the following reaction21(2):
Au+Cu(NH3)42++2S2O32−→Au(S2O3)23−+Cu(NH3)2++2NH3(2)

Compared to the cyanidation process, thiosulfate leaching has some advantages such as but not limited to fast leaching kinetics, lower toxicity and higher gold recovery in the case of some refractory gold ores22,23. However, it suffers from some major drawbacks such as but not limited to complex chemistry, toxicity of ammonia, ineffectiveness of activated carbon for desorption of leached gold, and high consumption of thiosulfate.

For example, the copper(II) itself consumes thiosulfate resulting in high consumption of both thiosulfate and copper and the resulting tetrathionate (S4O62−) decomposes to elemental sulfur and forms sulfides such as CuS which increases the gold passivation during the leaching process (reaction 3)24,25.
2Cu(NH3)42++8S2O22−→2Cu(S2O3)35−+8NH3+S4O62−(3)
Thiourea

Thiourea is another well-studied leaching reagent which can dissolve gold in acidic media based on the following reaction (4)26:
Au+2CS(NH2)2→Au(SC(NH2)2)2+(4)

Different oxidizing reagents such as but not limited to hydrogen peroxide, sodium peroxide, oxygen, ozone and ferric ion can be used in combination with thiourea to dissolve gold. Among these oxidizing reagents, ferric ion in sulfuric acid solution is a useful one (reaction 5)27.
Au+2CS(NH2)2+Fe3+→Au(SC(NH2)2)2++Fe2+(5)

However, thiourea is not stable in acidic media in the presence of ferric ion and is decomposed to sulfur and cyanamide28. Addition of a reducing agent such as SO2decreases the thiourea consumption by preventing its oxidation29. The kinetics of gold leaching in thiourea solution are much faster than the cyanidation process because of nongaseous oxidants such as but not limited to hydrogen peroxide and ferric sulfate which are used instead of oxygen which is used in the cyanidation process30. However, gold recovery and reagent consumption with cyanide is more economical than thiourea31.

Complexation with base metals such as copper accelerates thiourea consumption and decreases gold leaching kinetics. Thermal degradation, oxidation by the ferric sulfate and air are the other reasons for high consumption of thiourea32. Thiourea's commercial application has been hindered due to its high consumption and no existence of applicable industrial techniques for the recovery of gold from its solution. Although thiourea has a lower toxicity compared to cyanide, it is suspected to be a carcinogen agent and is treated with caution33.

Chloride Solution Containing an Oxidizing Agent

Concentrated hydrochloric acid in combination with powerful oxidizing agents is known as a strong leaching reagent for leaching precious metals, for example from scraps and secondary sources34. A hot solution of concentrated HCl mixed with concentrated HNO3(known as aqua regia) or hydrogen peroxide can dissolve gold according to the following chemical reactions (see reactions 6 and 7) resulting in the formation of a stable AuCl4−complex35.
Au+4HCl+HNO3→HAuCl4+2H2O+NO  (6)
2Au+3H2O2+8HCl→2HAuCl4+6H2O  (7)

Apart from these oxidants, chlorine gas can also be used which forms the same gold species36. Chlorine had been used to dissolve gold from ores and concentrates during the second half of the 19thcentury until it was gradually replaced by the more economical alkaline cyanide leaching. In all cases, the dissolution rate is faster compared to cyanide, however, due to high concentration of HCl, all of these solutions are highly corrosive and toxic and in the case of gold ore treatment, their consumption is not economical37.

Chloride/Hypochlorite

Chloride/hypochlorite solutions have been recognized as another alternative leaching reagent to cyanide which can dissolve gold in a wide range of pH values38. Depending on the solution's pH, three different oxidizing species can be formed in hypochlorite solutions. At pH>7.5, hypochlorite ion (OCl−) is the dominant species while for pH values between 3.5 and 7.5, hypochlorous acid (HOCl) acts as oxidizing agent and for pH less than 3.5, nascent chlorine gas (Cl2) is formed. Among these three species, HOCl is the most effective oxidizing agent to leach gold as the [AuCl4]−(reaction 8)39.
2Au+3HOCl+3H++5Cl−→2[AuCl4]−+3H2O  (8)

In a solution containing 100 g/L NaCl, the [AuCl4]−is stable in the pH range of 0-8 and potentials greater than 0.9 V40. The chloride-hypochlorite solution is a useful leaching reagent, for example for refractory gold ores. Because of low acidity, it does not produce a corrosion media; however the reagents consumption is still high41,42. The main drawback of this leaching reagent is that the percentage of leached gold is usually less than 85%43.

SUMMARY

The methods of the present application are directed to eliminating the use of both cyanide (which is highly toxic) and activated carbon (which is one of the most expensive steps in known processes) for the selective recovery of precious metals such as gold, for example, in the mining industry. For example, the sulfur-based ligand extractant(s) of the present application can be used in a simultaneous leaching and solvent extraction system to accelerate the leaching process over known processes. Extractants with higher selectivities are useful in light of environmental and/or economic concerns. No solvent extraction system has ever been implemented in the industrialization of gold production. The methods of the present application can, for example, shorten the entire gold processing time to hours at room temperature with high selectivity and minimal amounts of acid and oxidants, while the current gold recovery process takes 2-3 days, employs highly toxic cyanide and uses temperatures of up to 110 degrees Celsius in some steps of the process.

The methods of the present application can be implemented into current industrial processes such as gold extraction processes with minimal financial costs and effort. The entire cost of gold recovery by the cyanidation process is at least $10,000 per kg of gold of which the cost of gold recovery by the activated carbon step represents circa 25% of the overall cost (i.e. $2500 per kg gold). However, the cost of activated carbon is not the only issue; the majority of the expense is in the time required in the extraction step as well as the use of elevated temperatures. The activated carbon extraction step is a lengthy process using 36-72 hours at 110° C. The present methods eliminate the use of cyanide and activated carbon using highly selective ligands that presently cost, for example, $7-12/kg and in doing so, decreases overall gold recovery times while maintaining high extraction efficiencies. An environmental benefit arises from the elimination of sodium cyanide in the gold mining process. Other advantages may include, for example, greater simplicity, lower costs, considerably shorter extraction times at room temperature and the provision of a cleaner, safer and more environmentally friendly alternative to the existing cyanide process.

Accordingly, the present application includes a method of leaching and extracting gold and/or palladium from a substance comprising gold and/or palladium, the method comprising:

treating a mixture comprising an aqueous phase comprising an acid, an oxidizing agent and the substance, and an organic phase comprising a water-immiscible organic solvent and a compound of Formula I:


wherein

R1is —NR4R5or aryl;

R2and R3are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl; or

R2and R3together with the nitrogen atom to which they are attached, form a heterocycloalkyl or a heteroaryl, or a heterocycloalkyl or a heteroaryl substituted at one or more carbon atoms with C1-4alkyl;

R4and R5are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl; or

R4and R5together with the nitrogen atom to which they are attached, form a heterocycloalkyl or a heteroaryl, or a heterocycloalkyl or a heteroaryl substituted at one or more carbon atoms with C1-4alkyl;

X is O or S;

Y is S, NR6or CR6R7; and

R6and R7are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl,

under conditions to leach the gold and/or palladium from the substance and extract the gold and/or palladium by forming a complex between the leached gold and/or palladium and the compound of Formula I, in one step.

In an embodiment, the compound of Formula I is a compound of Formula I(a):


wherein R2, R3, R4, R5and Y are as defined for the compound of Formula I.

In an embodiment, for example, in the compound of Formula I(a), only one of R2, R3, R4and R5is H.

In an embodiment, for example, in the compound of Formula I(a), R2and R3together with the nitrogen atom to which they are attached form a heterocycloalkyl or a substituted heterocycloalkyl, wherein the heterocycloalkyl is selected from aziridinyl, azetidinyl, pyrrolidinyl, piperidinyl, azepanyl, azocanyl, imidazolidinyl, oxazolidinyl, thiazolidinyl, piperazinyl, hexahydropyrimidinyl, morpholinyl, 1,3-oxazinanyl, thiomorpholinyl, 1,3-thiazinanyl, 1,3-diazepanyl, 1,3-oxazepanyl, 1,3-thiazepanyl, 1,4-diazepanyl, 1,4-oxazepanyl, 1,4-thiazepanyl, 1,3-diazocanyl, 1,3-oxazocanyl, 1,3-thiazocanyl, 1,4-diazocanyl, 1,4-oxazocanyl, 1,4-thiazocanyl, 1,5-diazocanyl, 1,5-oxazocanyl and 1,5-thiazocanyl.

In another embodiment, for example, in the compound of Formula I(a), R2and R3together with the nitrogen atom to which they are attached form morpholinyl, pyrrolidinyl or 4-methylpiperidinyl.

In an embodiment, for example, in the compound of Formula I(a), R4is H and R5is C1-6alkyl or C3-8cycloalkyl.

In an embodiment, Y is NR6.

In an embodiment, R6is H, C1-6alkyl or C3-8cycloalkyl.

In an embodiment, the compound of Formula I is a compound of Formula I(a)(i), I(a)(ii), I(a)(iii) or I(a)(iv):



In another embodiment of the present application, the compound of Formula I is the compound of Formula I(a)(i):



In another embodiment of the present application, the compound of Formula I is the compound of Formula I(b)(i):



In an embodiment, the molar ratio of the compound of Formula I to the gold and/or palladium is about 3:1 to about 4:1.

In another embodiment, the acid is HCl having a concentration in the aqueous solution of about 1 M to about 2 M. In a further embodiment, the oxidizing agent is HNO3having a concentration in the aqueous solution of about 0.1 M to 1.0 M. It is an embodiment that the water-immiscible organic solvent is selected from dichloromethane, chloroform and chlorobenzene.

In an embodiment, the conditions to leach the gold and/or palladium from the substance and extract the gold and/or palladium by forming a complex between the leached gold and/or palladium and the compound of Formula I in one step comprise stirring the mixture for a time of about 2 hours to about 10 hours at a temperature of about 10° C. to about 40° C.

In an embodiment, the method further comprises separating the mixture into an aqueous phase and an organic phase comprising the complex between the leached gold and/or palladium and the compound of Formula I.

In an embodiment, the method further comprises stripping the gold and/or palladium from the complex between the compound of Formula I and the gold and/or palladium by a method comprising contacting the organic phase with an aqueous solution comprising an acid and thiourea under conditions to obtain a gold and/or palladium-containing strip solution and a gold and/or palladium-reduced organic phase comprising the compound of Formula I.

In another embodiment, the method further comprises separating the gold and/or palladium-containing strip solution from the gold and/or palladium-reduced organic phase comprising the compound of Formula I and recovering gold and/or palladium from the gold and/or palladium-containing strip solution by electrowinning or reduction.

In a further embodiment of the present application, the method further comprises recycling the compound of Formula I from the gold and/or palladium-reduced organic phase, for example, for use in the step of contacting a gold and/or palladium-containing substance with the mixture.

In an embodiment, the method further comprises recovering gold and/or palladium from the organic phase by direct reduction.

In an embodiment, the substance comprising gold and/or palladium is a gold-containing substance. In another embodiment of the present application, the gold-containing substance is a gold-containing ore.

The present application also includes a use of a compound of Formula I:


wherein

R1is —NR4R5or aryl;

R2and R3are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl; or

R2and R3together with the nitrogen atom to which they are attached, form a heterocycloalkyl or a heteroaryl, or a heterocycloalkyl or a heteroaryl substituted at one or more carbon atoms with C1-4alkyl;

R4and R5are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl; or

R4and R5together with the nitrogen atom to which they are attached, form a heterocycloalkyl or a heteroaryl, or a heterocycloalkyl or a heteroaryl substituted at one or more carbon atoms with C1-4alkyl;

X is O or S;

Y is S, NR6or CR6R7; and

R6and R7are each independently selected from H, C1-10alkyl, C3-10cycloalkyl, C1-6alkyleneC3-10cycloalkyl, heterocycloalkyl and aryl,

for leaching and extracting gold and/or palladium in one step from a substance comprising gold and/or palladium.

Other features and advantages of the present application will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the application, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.

