Metal Value Recovery From Carbonaceous Ores

This invention relates to a process for recovering at least one first metal, e.g., gold, from an ore containing the first metal and carbonaceous material. In particular, the invention relates to a process for recovering the first metal which involves processing the first metal, carbonaceous material-containing ore so as to facilitate the recovering of the first metal from the ore.

Carbonaceous ores, i.e., ores which contain elemental carbon (e.g., graphite) and/or organic compounds, often contain valuable metals, such as gold, silver, the platinum group metals and the like. Once characteristic of such ores which has made them difficult and expensive to process is that the presence of carbon and organic compounds inhibits metal recovery using conventional, e.g., cyanide, processing. In other words, the presence of organic material in such carbonaceous ores tends to interfere with metal extraction, e.g., by cyanidation. For example, a substantial amount of carbonaceous ore is not amenable to conventional cyanidation techniques because of the presence of carbon (which often acts like activated carbon), and relatively long chained organic hydrocarbon-type compounds containing sulfur, nitrogen, carboxylic acid groups and the like.

Various procedures have been investigated in an attempt to enhance metal recovery from these difficult-to-process ores, including roasting, kerosene pretreatment, flotation and aqueous chlorination. These measures are either substantially ineffective to increase metal recovery from carbonaceous ores or are relatively expensive and involve processing with chlorine and chlorinated components which are often corrosive or otherwise difficult to handle. See: B. J. Schermer, et al. "Processing Refractory Carbonaceous Ores for Gold Recovery," Journal of Metals, March, 1971, pp. 37-40; D. Raicevic and R. W. Bruce, "Gold Recovery from a Refractory Carbonaceous Gold Ore," Canadian Mining Journal, March, 1976, pp. 40-45; W. J. Guay, "How Carlin Treats Gold Ores by Double Oxidation," World Mining, March, 1980, pp. 47-49; and J. A. Eisele, et al., "Recovery of Gold and Silver from Ores by Hydrometallurgical Processing," Separation Science and Technology, 18 (12 and 13), pp. 1081-1094, 1983.

There is a growing world-wide interest in metal recovery from carbonaceous ores. Thus, in spite of the substantial work which has been done to provide for such metal recovery, a need currently exists to provide for a process for metal recovery from carbonaceous ores.

Therefore, one object of the invention is to provide a process for recovery of at least one first metal from ores containing the first metal and carbonaceous material, i.e., carbonaceous ores.

Another object of this invention is to provide a process to improve the effectiveness of conventional metal recovery procedures, in particular cyanidation, using carbonaceous ores. Other objects and advantages of the present invention will become apparent hereinafter.

A process for recovering at least one first metal, in particular gold, from at least one ore containing the first metal and carbonaceous material has been discovered. In a broad aspect, the process comprises contacting the carbonaceous ore with at least one added metal component other than alkali and alkaline earth metal components in an amount effective to at least promote the oxidation of the carbonaceous material. The contacting occurs at conditions effective to (1) chemically oxidize at least a portion of the carbonaceous material, and (2) at least partially liberate the first metal from the ore. The first metal is then recovered from the ore. In one embodiment, at least one additional oxidant is present during the contacting. This additional oxidant is present in an amount effective to provide at least one of the following: maintain at least partially the promoting activity of the added metal component; produce at least a portion of the added metal component; and/or oxidize at least a portion of the carbonaceous material. The preferred added metal components (promoters) have been found to be soluble redox catalysts which have sufficient oxidizing potential to either oxidize carbon and/or to activate the additional oxidant to oxidize carbon in the carbonaceous ore. The additional oxidant can provide a reservoir of oxidizing capacity which enhances the overall rate of oxidation and ultimate recovery of the first metal. The added metal component is preferably selected from the group consisting of iron components, soluble manganese (predominating in plus three (3+) components and mixtures thereof. The various embodiments of this invention can be practiced singly or in any combination of embodiments, with selection and optimization generally being a function of the ore type and desired metal value recovered.

The present invention provides substantial benefits. For example, improved yields of first metal are often achieved under less severe conditions by practicing the present process, especially when compared to recovering first metal from the carbonaceous ore without utilizing the present process of this invention. The present process is relatively easy to operate and control. Relatively low concentrations of added promoters are used and relatively mild operating conditions may be employed. Operating and capital costs are often reduced relative to previous chlorination/oxidation procedures which require substantial amounts of chemicals and/or expensive metallurgy to combat corrosion problems. Thus, the present invention can provide a cost effective approach to recovery of first metal from carbonaceous ores.

The process of the present invention is useful for metal recovery from carbonaceous ores, as defined above. Recovery of preferred first metals such as gold, silver, the platinum group metals and mixtures thereof, in particular gold, can be achieved. A large number of ore bodies and large amounts of carbonaceous ores are susceptible to be treated in accordance with the present process. Examples of such ores include: oxidized and carbonaceous ores from various locations in north central and northeastern Nevada, such as the Carlin ore, Jerritt Canyon ore, the Cortez ore and the Witwatersrand ore; ores from the Prestea and Ashanti gold fields in Ghana; the Natalkinsk and Bakyrichik ores from the Soviet Union; various Canadian ores such as the gold ore from the McIntyre Mine, located near Schamacher, Ontario; and the like ones. The carbonaceous ores may include oxidized ore material, possibly even a major amount of oxidized ore material. Also, the carbonaceous ores may contain metal pyrites. However, in another embodiment, the carbonaceous ore which contains metal pyrites can be processed for pyrite removal by physical and/or chemical means to reduce the pyrite content of the ore prior to the contacting step of the present invention. For example, subjecting the ore to various procedures such as grinding, particle size fractionation, flotation and the like can reduce the amount of metal pyrites in the core.

The present process employs at least one added metal component other than alkali and alkaline earth metal components. Such metal components may include alkali and/or alkaline earth metals provided that they also contain one or more additional metals which are effective in the present invention. Such added metal components are present during the contacting step in an amount effective to at least promote the oxidation of the carbonaceous material in the ore. Thus, such added metal components are present in an amount effective to promote the oxidation of the carbonaceous material and/or to oxidize the carbonaceous material.

Without wishing to limit the invention to any specific theory of operation, it is believed that the added metal promoters, preferably soluble and in combination with an added oxidant, oxidizes the carbon surface and/or oxidatively decarboxylates the long chain hydrocarbon components which have gold cyanide absorbing and/or complexing properties, to allow for example, cyanide to complex with the gold and be elected to improve ultimate metal recovery; the process effectively reduces the tendency of the carbonaceous material to absorb and/or complex with the gold electing complex.

The added metal component is preferably selected from the group consisting of iron components, copper components, cobalt components, vanadium components, manganese plus three components and mixtures thereof. More preferably, the added metal component enhances the oxidizing potential of the metal component and is selected from the group consisting of iron components in which iron is present in the 3+ oxidation states in an amount effective to at least promote the oxidation of the ore's carbonaceous material, copper components in which copper is present in an amount in the 2+ oxidation state effective to at least promote the oxidation of the ore's carbonaceous material, cobalt components in which cobalt is present in an amount in the 2+ oxidation state effective to at least promote the oxidation of the ore's carbonaceous material, vanadium components in which vanadium is present in the 3+ or 5+ oxidation states in an amount effective to at least promote the oxidation of the ore's carbonaceous material, manganese components in which manganese is present in the 3+ oxidation state in an amount effective to at least promote the oxidation of the ore's carbonaceous material, and mixtures thereof.

In one embodiment, the iron, copper and cobalt, vanadium and manganese components are soluble and preferably selected from iron complexes with ligands, copper complexes with ligands, and cobalt complexes with ligands, vanadium components with ligands, manganese components with ligands, and mixtures thereof. Such complexes preferably include at least a portion, more preferably a major portion and still more preferably substantially all, of the metal in the preferred oxidation state noted above.

Examples of iron complexes useful in the present invention include iron complexes with polyfunctional amines, for example, ethylenediamine, propylene diamine, ethanol amine, glycine and asparagine and salts thereof; phosphonic acids and phosphonic acid salts, for example, ethane-1-hydroxy-1,1-diphosphonic acid; pyridine and substituted, chelating pyridine derivatives, for example, 1,10-phenanthroline, 2,2'-bipyridyl, glyoxime and salicylaldehyde derivatives; aquo; and CN--. Particularly preferred iron complexing agents for use in the present invention are those selected from the group consisting of substituted chelating derivatives of pyridine, aquo, CN- and mixtures thereof.

Examples of copper complexes useful in the present invention are copper, in particular copper 2+, complexes with pyridine, 1,10-phenanthroline, imidazole, substituted, non-chelating derivatives thereof and mixtures thereof. These derivatives include substituents such as hydroxy, carboxy, amino, alkyl and argyl groups.

Cobalt, in particular cobalt 2+, complexes of chelating Schiff's bases are preferred. These ligands include, for example, ligands uitlizing 1,2 diamines, 1,3-diamines, substituted 1,2-dionemonoximes, substituted 1,3-dionemonoximes, substituted salicylaldehydes and mixtures thereof, such as bis-(salicylaldehyde)ethylenediimine and bis(2,3-butandionemonoxime) ethylenediimine. Examples of vanadium and manganese complexes involving oxyanions are sulfate, nitrate and carboxylates, e.g., acetates.

Especially suitable salt forms of complexing agents are the potassium, sodium and ammonium salts. Mixtures of complexing compounds can be very desirably employed.

As will be recognized by those skilled in the art, the stability of the complexes formed will often be affected by the pH of the aqueous composition employed in the present contacting step. Some stability of the complex or complexes may have to be sacrificed because of the pH of the aqueous composition during the contacting which pH may be preferred for various processing reasons. This reduced complex stability has surprisingly been found not to have an undue adverse effect on oxidation. The particular pH employed can also affect the salt form of the complexing agent employed, and such complexing salts are complexing agents within the scope of this invention.

The present contacting occurs at conditions effective to (1) chemically oxidize at least a portion of the carbonaceous material in the ore and (2) at least partially liberate the first metal from the ore. By "liberated from the ore" is meant that the desired first metal in the ore after the present contacting can be more effectively recovered using conventional (e.g., cyanide extraction) processing relative to the uncontacted ore. In certain instances, at least a portion of the carbonaceous material in the ore normally acts in a manner akin to activated carbon to "pick-up" the first metal after it has been extracted by cyanidation, thus impeding or reducing the overall recovery or yield of the first metal. The term "liberated from the ore" is meant to include reducing this "activated carbon" and/or the complexing effect to provide improved yields of first metal in the metal recovery step, i.e., the first metal becomes more amenable to recovery. This contacting preferably leads to a metal recovery step which involves reduced operating and capital costs and/or provides increased yields of first metal relative to recovering first metal from an uncontacted ore. The present contacting preferably acts to oxidize carbonaceous material in the ore, render an increased amount (relative to uncontacted ore) of the first metal in the ore amenable to conventional (cyanide extraction) metal recovery, and provide for a more effective and/or effective first metal recovery step.

The present contacting preferably takes place in the presence of an aqueous medium or composition. The added metal component or components, which are preferably soluble in the aqueous medium, may be added to the aqueous medium prior to the contacting. Any suitable, aqueous medium can be employed in the present process. The pH of the aqueous medium may be acidic, neutral or basic depending, for example, on the composition of the ore or ores being treated, the specific added metal component or components being employed, and the presence or absence of other components or entities during the contacting. Preferably, the pH of the aqueous composition is in the range of about 1 to about 10, more preferably about 2 to about 8. The pH of the aqueous medium may be adjusted or maintained, e.g., during the contacting step, for example, by adding acid and/or base.

The aqueous medium comprises water, preferably a major amount of water. The medium is preferably substantially free of ions and other entities which have a substantial detrimental effect on the present process. Any suitable acid and/or base or combination of acids and/or bases may be included in, or added to, the medium to provide the desired pH. For example, hydrogen halides, preferably hydrogen chloride, sulfurous acid, sulfuric acid, metal salts which decompose (in the aqueous medium) to form such acids, alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxide, metal salts which decompose (in the aqueous medium) to form such bases, mixtures thereof and the like may be employed. The quantity and composition of the aqueous medium may be selected in accordance with the requirements of any given ore to be treated and as may be found advantageous for any given mode applying the present process in practice.

The amount of added metal component or components employed may vary widely provided that such amount is effective to function as described herein. Such added metal component or components are preferably present during said contacting in an amount less than about 2%, more preferably in the range of about 10 ppm. to about 1% by weight, calculated as elemental metal, based on the amount of ore present. One of the substantial advantages of the present process is that large amounts of added metal components are not required. Thus, in order to reduce costs still further while achieving benefits of the present invention, low concentrations of added metal components are preferably selected. Preferably, the mole ratio of complexing agent to metal ion that is used to form the promoter component is in the range of about 0.01 to 5, more preferably about 0.5 to about 2.0. Preferred concentrations of added metal are in the range of about 20 to 10,000 ppm, more preferably about 50 to about 1,000 ppm., by weight based upon the aqueous composition, calculated as elemental metal. It is generally convenient to provide the metal complex in combination with, preferably in solution in, the aqueous compositions used in the contacting step of this invention.

In one embodiment, the present invention involves the use of at least one additional oxidant, i.e., an oxygen reservoir, in an amount effective to provide at least one of the following: maintain at least partially the promoting activity of the added metal component, produce at least a portion of the added metal component, and/or oxidize at least a portion of the carbonaceous material in the ore. The additional oxidant or oxidants may be present during the contacting step and/or during a separate step to form and/or regenerate the added metal component or components. Any suitable oxidant capable of performing one or more of the above-noted functions may be employed. The additional oxidant is preferably selected from the group consisting of molecular oxygen (e.g. in the form of air dilute or enriched air, or other mixtures with nitrogen or carbon dioxide), singlet oxygen, ozone, inorganic oxidant components containing oxygen and at least one second metal and mixtures thereof. More preferably, the additional oxidant is selected from the group consisting of molecular oxygen, oxidant components containing oxygen and at least one second metal and mixtures thereof. Still more preferably the additional oxidant is selected from the group consisting of oxidant components containing oxygen and at least one second metal and mixtures thereof. One particularly preferred system involves an oxidant component containing oxygen and at least one second metal, and molecular oxygen in an amount effective to maintain the oxidant component in the desired oxidized state and/or to produce the desired oxidant component and/or to oxidize at least a portion of the carbonaceous material. Care should be exercised to avoid large excesses of the additional oxidant so as to minimize reactions that could solubilize deleterious elements, i.e., arsenic, etc. The amount of additional oxidant employed is preferably in the range of about 10 to about 200%, preferably about 80% to about 140% of that needed to oxidize the carbonaceous ore to allow for improved liberation of the first metal in the present process.

The oxidant component, i.e., reducible second metal component, or oxidant components useful in the present invention may be chosen from a wide variety of materials. The second metal or metals are preferably not the same as the first metal sought to be recovered from the ore. Preferably, the second metal is a metal which forms reducible metal oxides which are reduced during the conduct of the process of this invention. Many of the transition metals have this property. Typical examples of metals which have this property include minerals and other compounds which are generally solids under the condition of this process, such as, manganese, tin, lead, bismuth, germanium, antimony, indium and certain of the rare earth metals and minerals, e.g., cerium, praseodymium and terbium and mixtures of rare earth minerals which typically have varying ratios of lanthanum, cerium, etc. Such reducible second metal components are preferably capable of becoming at least partially reduced at the present contacting conditions to form reduced second metal component.

The present contacting results in at least a portion of the reducible second metal component being chemically reduced to form a reduced component. This reducible/reduced component can exit the contacting zone and be separated from the ore, in particular the contacted ore, i.e., partial to substantial separation. This component can be used on a once-through basis, or may be regenerated to reducible second metal component and recycled to the contacting zone. In the case of a once-through basis, it is preferred to minimize the amount of reduced metal component exiting with the ore. Such regeneration can be done by electrochemically oxidizing the manganese component or oxidizing manganese with molecular oxygen, preferably promoted for purposes of enhanced yield and rate, at elevated temperatures to convert the reduced component to a reducible second metal component.

Manganese is a more preferred second metal. In one embodiment, the reducible manganese component includes manganese in the 4+ oxidation state. One particularly useful reducible manganese component is manganese (manganic) dioxide and its minerals. Typical examples of such ores are psilomelane pyrolusite, manganite, birnessite and manganese-bearing minerals from the spinel group. Particularly, useful ores are silver, manganese-containing ores in which at least a portion of the silver is locked by the manganese-bearing minerals.

The amount of additional oxidant employed in the present invention is chosen to facilitate the desired functioning of the present contacting step. Without limiting the invention to any specific theory or mechanism of operation, it may be postulated that when additional oxidant is employed such additional oxidant acts in conjunction with the added metal component to oxidize the carbonaceous material in the ore and "liberate" the first metal from the ore. Although the metal component may taken an active part in the oxidation and liberation functioning, when additional oxidant is employed, such metal component preferably acts as a catalyst and may, and preferably is, used more than once in the present contacting step, e.g., is recycled to the present contacting step or is employed to contact more than one increment of ore.

The amount of additional oxidant employed preferably acts to facilitate the desired oxidation of carbonaceous material and liberation of first metal from the ore. The specific amount of additional oxidant employed varies depending on many factors, for example, the specific ore or ores being treated, the specific metal component and additional oxidant being employed, and the specific degree of carbonaceous material oxidation and first metal liberation desired. If a reducible second metal component is used, it preferably is used in an amount in the range of about 0.1% or less to about 10% or more by weight of the carbonaceous ore. Preferably, the amount of second metal component employed in the present contacting step should be sufficient to provide the oxidation/metal liberation to the desired degree. More preferably, the amount of second metal component employed should be about 40% to about 250%, more preferably about 80% to 120%, of that required to achieve the desired degree of carbon oxidation. Substantial excess of second metal component should be avoided since such excesses may result in materials separation and handling problems, and may result in reduced recovery of the desired metal or metals.

Although one or more of the additional oxidants may be utilized in a separate oxidation or regeneration step, it is preferred that such additional oxidants, and in particular reducible second metal components, be present and effective during the contacting step of the present invention.

The contacting of the present invention takes place at a temperature and pressure and for a time sufficient to obtain the desired results. A combination of temperature and pressure effective to maintain water (the aqueous medium) in the liquid state is preferred. In one embodiment, temperatures of about 20% to about 140.degree. C. are preferred with temperatures in the range of about 20.degree. C. to about 110.degree. C. and in particular between about 25.degree. C. to about 80.degree. C. being especially useful. Contacting pressure may be in the range of about atmospheric to about 500 psia or more. Pressures in the range of atmospheric to about 100 psia have been found to provide satisfactory results.

Contacting times vary widely depending, for example, on the mode in which the contacting is performed. Such contacting time may range from minutes to weeks or even months. For example, if the contacting occurs in a stirred tank with the carbonaceous ore present in a slurry with the aqueous medium and the added metal component, the contacting time preferably is in the range of about 0.1 hours to about 60 hours, more preferably about 1 hour to about 24 hours. On the other hand, if the contacting takes place with the carbonaceous ore placed in a heap with the aqueous medium and added metal component being made to flow through the heap, the contacting time is preferably in the range of about 1 day to about 3 months, more preferably about 7 days to about 60 days.

The present process may be conducted on a batch or continuous basis. The present contacting step may be conducted on a pad, with the carbonaceous ore to be treated situated in a heap; or in a vat, tank or other suitable vessel or arrangement, e.g., with the ore to be treated present in a slurry with the aqueous medium and added metal component. The primary criterion for the contacting step is that the desired carbonaceous material oxidation and first metal liberation take place. Preferably, the first metal-containing carbonaceous ore and the added metal component and the additional oxidant, if any, are brought together in intimate contacting generally in contact with an aqueous medium. The carbonaceous ore is preferably subjected to particle size reduction, e.g., by crushing, grinding, milling and the like, prior to contacting to render the ore more easily and/or effectively processed in the present contacting step. Air or other gaseous additional oxidant may be dispersed through, or otherwise contacted with, this admixture during the contacting step to achieve the desired result. Amounts of acid and/or base can be added to the initial admixture and/or may be added during the contacting to provide the desired pH.

The solid ore remaining after the contacting step may be subjected to any suitable metal recovery processing step or steps for the recovery of the first metal. For example, this solid ore may be neutralized with any suitable acidic or basic material, such as sulfuric acid, carbonates, white lime, milk of lime and the like, and then subjected to a conventional sodium cyanide extraction, followed by activated carbon treatment and zinc dust precipitation. Alternately, the solid ore after contacting can be neutralized and subjected to an ammonium thiosulfate or an acid thiourea extraction followed by zinc dust precipitation. Still further, the solid ore after contacting can be subjected to a brine extraction followed by ion exchange to recover the desired first metal or metals. The conditions at which these various recovery processing steps take place are conventional and well known in the art, and therefore are not described in detail here. However, it is important to note that conducting the metal recovery processing on the ore after the contacting of the present invention preferably provides improved metal recovery performance relative to conducting the same metal recovery processing without this contacting.

One processing arrangement which provides outstanding results involves the agglomeration of the first metal-containing carbonaceous ore and reducible second metal component. The ore and reducible second metal component are preferably subjected to crushing, grinding, or the like processing to reduce particle size to that desired for improved carbonaceous material oxidation and first metal liberation, generally a maximum particle diameter of about 1/2 inch or less. The ore and component particles are mixed with sufficient aqueous medium and if desired added metal promoter. This intimate admixture is formed into agglomerates by conventional processing, such as extruding, pilling, tableting and the like.

The agglomerates are placed on a pad, to form a heap which is built up by addition of agglomerates, preferably over a period of time in the range of about 15 days to about 60 days. During the time the heap is being built up, and preferably for a period of time ranging up to about 3 months, more preferably about 1 month to about 3 months after the last agglomerates are added to the heap, an aqueous medium containing the added metal component is made to flow through the heap, e.g., from the top to the bottom of the heap. If desired, air or other gaseous additional oxidant can be contacted with the heap during the contacting. After contacting the heap, the aqueous medium is collected and processed for disposal, processed for second metal and/or added metal recovery, and/or added metal component regeneration, and/or recycled to the heap. This contacting provides another important benefit in that at least a portion of the "cyanacides," such as copper, which may be present in the ore and/or metal sulfide-containing material is removed and/or deactivated. Such "cyanacides" cause substantial increases in cyanide consumption if present in cyanide extraction processing. Therefore, removing and/or deactivating cyanacides in the present contacting step provides for more effective metals recovery by cyanide extraction.

After the heap-aqueous composition contacting has proceeded to the desired extent, an aqueous basic (e.g., white lime, milk of lime or the like basic components) composition is contacted with the heap to neutralize the heap if a pH below 7 was used. After this neutralization, the agglomerates may be placed on a second heap, which is preferably larger than the heap previously described.

In addition, the neutralized agglomerates may be broken apart and reagglomerated prior to being placed on the second heap to provide for any incidental acid neutralization (if required) and/or to expose the treated ore for subsequent cyanidation. This can be done using conventional means, such as subjecting the agglomerates to grinding, milling or the like processing, and then forming the second agglomerates by extruding, tableting, pilling, pelletizing or the like processing.

In any event, if a second, preferably larger, heap is formed on a pad, then a dilute aqueous cyanide, preferably sodium cyanide, solution is made to contact the second heap. Typically, this cyanide contacting is performed in the presence of air. Preferably, the cyanide solution is percolated through the second heap. The cyanide solution, after being contacted with the second heap, contains the first metal. This solution is collected and sent to conventional further processing for recovery of the first metal.

Both heaps are preferably maintained at ambient conditions, e.g., of temperature and pressure. Also, both heaps may be built up and worked (contacted) with the aqueous contacting solutions and the cyanide solution for as long as the economics of the particular application involved remain favorable.

When an agitated leach in vessels is used for the process, contact times may vary depending, for example, on the specific ore being contacted, the other components present during the contacting and the degree of metal recovery desired. Contact times in the range of about 5 minutes or less to about 48 hours or more may be used. Preferably, the contact time is in the range of about 4 hours to about 36 hours, more preferably about 8 hours to about 24 hours. During this time, agitation can be advantageously employed to enhance contacting. Known mechanical mixers can be employed.

While the present invention has been described with respect to various specific examples and embodiments, it is to be understood that the present invention is not limited thereto and that it can be variously practiced within the scope of the following claims.