CA1340169C - Hydrometallurgical process for extracting gold and silver ores with bromate/perbromide solutions and compositions therefor - Google Patents

Abstract

An aqueous composition adapted for dilution to provide a leaching solution effective for leaching of a precious metal comprising gold or silver from a source thereof. The composition comprises bromide ion, perbromide ion, molecular bromine, at least about 2% by weight bromate ion, and a metal ion selected from among alkali metal and alkaline earth metal. The equivalent molecular bromine content of the composition is between about 10% and about 40%
by weight and the ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in the composition is between about 0.05 and about 0.8. A leaching solution produced by dilution of the aqueous composition, and a method for recovery of precious metals from sources thereof by use of the leaching solution, are also disclosed.

Description

~ 13~0169 COMPOSITION AND METHOD FOR RECOVERY
OF GOLD AND SILVER FROM SOURCES THEREOF
Backqround of the Invention This invention relates to the hydrometallurgical recovery of precious metals, and more particluarly to an improved hydrometallurgical process for extracting gold and silver from ores and other sources by use of a bromate/perbromide leaching solution.
Conventionally, precious metals such as gold and silver have been recovered from ores by leaching with alkaline cyanide solution. By reaction with cyanide ion and oxygen the precious metal is converted to a cyanide complex (gold cyanide anion) which is taken up in the leaching solution. The dissolution of gold, for example, is illustrated by the following reaction:

4Au+8CN- +02+2H20 ~4Au(CN)2 +40H- (1) Because of the high stability of the gold cyanide complex anion, even oxygen of the air is sufficent to oxidize gold in the presence of cynanide ion. Recovery of gold from the cyanide solution by precipitation may be illustrated by the following reaction equation:

Au(CN)2+2NaCN+Zn+2H20 Zn(CN)4 2 ( ) Alternatively, gold may be recovered from cyanide solution by adsorption of the gold cyanide complex anion onto activated carbon, desorption with a hot alkaline solution, and recovery by electrowinning or by raising the pH. A

*
., '"X

.
typical scheme for recovery via activated carbon is illustrated by the reaction equations set forth below:

AU(cN)- activated > Au(CN) adsorbed (3) Hot alkaline OH
5Au AU(cN)2 desorbed While widely practiced on a commercial scale, cyanide leaching suffers from well known disadvantages.
Thus, leaching rates with alkaline cyanide solutions are slow, contact times in the range of 24-72 hours being common in the case of gold ores. Because of the toxicity of cyanide, care must be exercised to maintain cyanide solutions on the alkaline side in order to prevent the release of hydrogen cyanide gas. Severe environmental restrictions must be observed, requiring careful monitoring and control of all process purge streams. Spent cyanide leaching solutions must be sub~ected to waste treatment operations before discharge to the environment.
Gold has also been leached commercially by use of aqua regia, a mixture of concentrated hydrochloric and concentrated nitric acid, according to the following reaction equation:

Au+4H +4Cl +NO3 > AuC14+NO+2H2O (4) Gold may then be recovered by reduction with zinc metal or raising of the leaching solution pH. However, this method is relatively unattractive because aqua regia is expensive, and highly corrosive and emits toxic fumes. Moreover, it 1~40169 ~ .
readily dissolves base metals and dissolves gold only relatively slowly in aqueous solution.
Thiourea has also been used as a lixiviant for the dissolution of gold from ores according to the following reaction equation:

Au+Fe +2CS(NH2)2 > Au[CS(NH2)2] +Fe 2+ (5) Although thiourea is effective, it is subject to oxidative degradation and is, thus, prone to high consumption levels in extracting gold from its ore.
Other processes have been developed for the use of halogens, halides or other halide-bearing compounds for the recovery of precious metals from ores. For example, Shaeffer U.S. patent 267,723 describes a process in which ore is roasted in a vat, water added to the roasted ore, and liquid bromine added to produce a mixture which is agitated, thereby dissolving the gold in the water in the form of a bromide. After filtration to separate the solids, gold is precipitated from the solution by oxalic acid or sulfate ion.
Fink et al. U.S. Patent 2,283,198 note that chloride and bromide ions accelerate the dissolution of gold in aqueous bromine solution. This disclosure states that chlorine or hypochlorite may be used as intermediate oxidizing agents, as indicated by the reactions:

2Br +C12 2 Cl- (6) or 2Br +ClO +2H Cl +Br2+H2O (7) They, therefore, proposed a process of extracting gold from its ores by leaching with a solvent prepared by adding free '' 13~016~
chlorine to a solution containing a bromide and a large excess of chloride salt. Alternatively, a leaching agent is prepared by adding hypochlorite and a mineral acid to a solution containing a bromide and a large excess of chloride salt. Fink et al. describe precipitation of gold from the solution by any of the well known methods, such as addition of zinc or ferrous sulfate.
Harrison U.S. patent 2,304,823 describes the recovery of gold from ores by dissolution in a treatment solution containing iodine, potassium iodide, water and nitric acid. Mercury or zinc may then be added to recover the precious metal from solution. The method is said to be applicable to treatment of refractory material such as refractory sulfide, telluride or the like.
Jacobs U.S. patent 3,625,674 describes oxidizing gold with an alcoholic solution of molecular iodine, pro-ducing AuI which may be decomposed by heating to produce metallic gold and iodine vapors, the iodine being recovered and recycled for treatment of additional ore. AuI obtained from the process is also suitable for forming other gold compounds for use in the industrial arts, such as gold sodium thiomalate.
Wilson U.S. patent 3,709,681 describes a process in which a finely divided source of noble metal is treated with a solution containing a ketone solvent, dissolved iodine, bromine or chlorine, a halide salt, and preferably glacial acetic acid. The noble metal content of the treatment solution is recovered by displacement onto a non-noble metal surface such as aluminum foil.
Homick et al. U.S. patent 3,957,505 describe a process in which gold bearing material is treated in an aqueous solution consisting of iodine and a water soluble iodide salt to produce a solution containing dissolved gold iodide salts. Gold metal is precipitated from the solution by mixing of the solution with a reducing agent such as 13~016~

hydroxylamine, hydrazine, sodium thiosulfate and the like.
Iodine from the spent aqueous solution is recovered by acidification of the solution and addition of an oxidizing agent such as hydrogen peroxide, potassium permanganate, sodium chromate, chlorine or bromine to precipitate elemental iodine.
McGrew et al. U.S. patent 4,557,759 describe a process for the hydrometallurgical recovery of gold from materials containing gold by leaching the materials with a lixiviant containing iodine. The lixiviant is prepared by saturating an aqueous solution of iodide with iodine. When a sulfide is added to this reagent, iodine reacts with the sulfide and is converted to iodide. Additional elemental iodine is then added to this iodide bearing solution until the desired concentration of total iodine and ratio of iodine to iodide are achieved for maximum leaching efficiency. The lixiviant is then circulated through the ore zone until all the gold is dissolved. Gold is subsequently recovered on activated charcoal. The excess iodide formed during the process is reoxidized to iodine electrochemically in a special diaphragm cell to regenerate the lixiviant. The desired concentration of iodine in the lixiviant is between 1 and about 20 grams per liter.
In addition to ores, there is a substantial number of additional sources of gold and silver which offer the opportunity for economical recovery. In fact, many of these secondary sources are substantially richer than ores with respect of the content of the metal to be recovered.
Gold is available from numerous scrap sources, including industrial wastes, gold plated electronic circuit boards, and as an alloy with copper, zinc, silver or tin in the karat gold used in jewelry. Silver is available from photographic and x-ray film emulsions, from scrap sterling, and from numerous industrial sources.

13~101fi9 -Bazilevsky U.S. patent 3,495,976 describes a process for recovering gold from a plated substrate by dissolving the gold in an aqueous solution of potassium iodide and free iodine. Gold is recovered from the solution by addition of concentrated sulfuric acid and heating of the resultant mixture near the boiling point, which distills off molecular iodine and effects precipitation of the gold.
Bahl et al. U.S. patent 4,190,489 describe a composition and method for etching gold, particularly gold layers on ceramic substrates. The composition is prepared, for example, by mixing potassium bromide (75g), elemental bromine (25g), and water (100 ml.) This solution is effective for recovery of gold from ceramic substrates at essentially room temperature.
Bahl et al. 4,375,984 describe a process in which an alkaline metal bromide/bromine solution is used to etch gold from a substrate. In this disclosure, the etching solution may be prepared, for example, by mixing potassium bromide (2g), bromine (one gram), and water (25 ml.) Gold dissolved in the etching solution is recovered as metallic gold, either by precipitation using an alkali metal hydroxide or by decomposition in which the etching solution is driven off. The alkali metal bromide/bromine etchant solution may be regenerated by the addition of an acid thereto.
Kalocsai Bl 4,684,404, based on re-examination of U.S. patent 4,684,404, describes dissolution of metallic gold in a reagent comprising a protic solvent such as water or alcohol, a nonreducing cation source such as sodium, potassium, ferrous or ammonium, and a source of free bromine such as molecular bromine, bromine water, or an inorganic or organic bromine containing compound from which bromine can be liberated in the reagent. Optionally, the solution further contains a strong oxidizing agent such as ~ 13 10169 hydrogen peroxide, sodium peroxide, potassium peroxide, sodium permanganate, potassium permanganate, potassium chromate or ferric sulfate. Among the exemplary reagents disclosed by Kalocsai is a composition (Example 28) containing 1.0% v/v Br2, 1% w/v NaBr, and 0.6% w/v NaOH, and having a pH of 7.35.
Sergent et al. U.S. patent 4,637,865 describe a process for extracting a precious metal from a source material by contacting the source material with an aqueous leaching solution containing a leaching agent comprised of an N-halohydantoin compound. Leaching solutions are described containing 1,3-dibromo-5,5-dimethylhydantoin, l-bromo-3-chloro-5,5 dimethylhydantoin and 1,3-dichloro-5,5-dimethylhydantoin. Precious metal may be removed from the leaching solution by precipitation of the less noble metal, ion exchange, treatment with activated carbon, solvent extraction or electrowinning.
Simpson U.S. patent 4,439,235 describes a process for removing precious metal values from comminuted carbonaceous ores in which the comminuted ore is contacted with an aqueous solution of hypochlorite, iron ion, and acid.
Falanga et al. U.S. patent 4,319,923 teaches a process in which gold and palladium are etched with a potassium iodide/iodine etching solution and the metal is recovered from the solution by addition of alkaline compound, preferably KOH, to increase the pH to at least 12.5 and precipitate metallic gold from the solution. A
borohydride is used to precipitate palladium. A similar process is described in MacDonald U.S. patent 4,319,922.
Jolles, "Bromine and its Compounds", Academic Press, New York, 1966, page 173, states that a mixture of sodium bromate and sodium bromide has been used under the name of "mining salts" in the extraction of gold ore.
Jolles states that the proportion of the respective ~ 13401~
components vary but are usually 43% sodium bromate and 57%
sodium bromide, i.e., two moles sodium bromide to one mole sodium bromate.
Belohlav et al. U.S. patent 3,222,276 describe a process for producing an aqueous bromine solution from an aqueous solution of bromide/bromate salts and mineral acid. In accordance with the process, a concentrated aqueous solution of bromide and bromate salts at a 5:1 molar ratio is pumped to a mixing zone where it is mixed with a mineral acid to convert the bromide/bromate solution into a concentrated aqueous bromine solution. This concentrated aqueous bromine mixture is pumped to a second mixing zone where it is diluted with a larger volume of water to produce a diluted stream of aqueous bromine, and the latter stream is further diluted with a large body of water to produce a highly dilute aqueous bromine solution in substantially quantitative yield from the mineral acid and the bromide/bromate salt solution. The process as described is useful in the bromination of swimming pool water. The reference contains no mention of the use of bromine or bromides in the recovery of precious metals from sources thereof.
Although prior art processes which use molecular bromine have been effective for recovery of gold from source materials, pure bromine is a toxic, fuming liquid which generates a suffocating vapor and must be subjected to special handling. Bromine may be dissolved to a certain extent in water, or incorporated in an alkali metal perbromide solution, but these aqueous materials exhibit a substantial bromine vapor pressure, so that their use also commonly entails special handling. Alternatively, molecular bromine can be generated from the acidification of alkali metal bromates, but by themselves bromates provide only a limited source of molecular bromine.
Additionally, bromate salts have a high crystallization ,~ 1310169 temperature which makes them inconvenient to use as leaching agents for precious metals. Mlxtures of bromides and bromates, such as the "mining salts" described by Jolles, have found their place ln the technology of precious metal recovery, but have not provlded a sufficient source of molecular bromine to be as effective as those known solutions whose bromine vapor pressure is relatively high.
SUMMARY OF THE INVENTION
Briefly, therefore, the present invention is directed to an aqueous composition havlng a pH of between about 6.5 and about 7.5 comprising bromide ion, perbromide ion, molecular bromlne, at least about 2% by welght bromate ion and a metal ion selected from among alkali metal and alkaline earth metals. The equivalent molecular bromine content of the composition is between about 10% and about 40%
by weight and the ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in the composition is between about 0.05 and about 0.8.
The invention is further directed to a leachlng solution having a pH of between about 6.5 and about 7.5 adapted for leachlng of a preclous metal comprising gold or silver from a source thereof. The solution contains between about 0.01% and about 1% by weight equivalent molecular bromine, between about 0.01% and about 1% by weight bromide ion, and between about 0.005% and about 15% by weight total halide lon. The equlvalent molecular bromine concentration in moles per liter is equal to the sum of the actual molar g B

~ 1340169 concentration of molecular bromlne, the molar concentratlon of perbromlde lon, three tlmes the molar concentration of bromate ion, and the molar concentratlon of hypobromlte ion and hypobromous acld ln the solutlon.
The inventlon is further directed to a method for leaching of a precious metal comprlsing gold or silver from an ore contalnlng such metal. The process comprlses contacting the ore wlth an aqueous bromlne leachlng solutlon, the leaching solutlon having a pH of between about 6.5 and about 7.5 and containing between about 0.01~ and about 20% by weight equivalent molecular bromine, between about 0.005% and about 20% by weight bromide ion, and between about 0.005% and about 30% by weight total hallde lon. The equivalent molecular bromine concentration in moles per liter ls equal to the sum of the actual molar concentratlon of molecular bromlne, the molar concentratlon of perbromlde lon, three tlmes the molar concentration of bromate lon, and the molar concentratlon of hypobromite lon and hypobromous acld ln the solutlon. An aqueous leachate contalnlng the preclous metal ls thereby produced.
Other obiects and features wlll be ln part apparent and ln part polnted out herelnafter.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance wlth the present invention, lt has been dlscovered that gold and sllver may be efflclently recovered from source materials by use of a leaching solution derived from a unlque precursor composltlon that contalns a high concentration of avallable bromine in the form of a combinatlon of an alkali metal or alkaline earth metal perbromide and an alkali metal or alkallne earth bromate. In partlcular, lt has been found that the - lOa -n 13901~9 precursor composition, which is prepared by mixing a perbromide salt component solution and a bromate salt component solution, may contain a substantial concentration of equivalent molecular bromine, yet have a very low bromine vapor pressure which facilitates handling and minimizes the hazards of using molecular bromine for leaching of gold and other precious metals. Moreover, the leaching solution of the invention may be readily prepared by dilution of the precursor composition and used for the leaching of precious metals without any serious problems of containment of bromine vapor.
Use and handling of the precursor composition is not hampered by bromate salts crystallizing or otherwise precipitating from the solution. The leaching solution prepared from this composition has been demonstrated to be highly effective for the leaching of gold from refractory ores, without the need for any preparatory processing other than conventional roasting. If preferred, however, a clean ore concentrate can be prepared by conventional processing, which may include pressure oxidation.
In accordance with a particularly preferred embodiment of the invention, a leaching solution precursor concentrate containing perbromide and bromate salts is initially produced. In the preparation of the leaching solution of the invention, this concentrate is diluted to provide the leaching solution. If desired, the pH may be adjusted either before or after dilution by addition of an acid such as HBr, HCl, H2SO4, or C12, or a base, such as NaOH, KOH or Ca(OH)2.
In the preparation of the precursor composition of the invention, a component solution of an alkali metal or alkaline earth metal perbromide is mixed with a ~ .
component solution of alkali metal or alkaline earth metal bromate. The perbromide solution is prepared by addition of bromine to an aqueous solution of a bromide ion. For example, sodium perbromide and calcium perbromide are prepared by saturating the Br- content of the respective aqueous NaBr or CaBr2 solution with molecular bromine:

NaBr + Br2 > NaBr3 (8) CaBr2 + 2Br2 ~ ~ Ca(Br3)2 (9) When prepared in the course of providing a concentrate, the metal bromide solution initially has a concentration of at least about 25% by weight, preferably essentially saturated to its solubility limit, i.e., 45-50% by weight in the case of NaBr, or 55-60% by weight in the case of CaBr2.
Whatever the initial concentration of the metal bromide solution, liquid or vapor Br2 is added to the solution to the extent of saturating the bromide ion therein, i.e. in full stoichiometric equivalence with the Br- content.
Where the Br2 is added to a NaBr solution that is initially at its solubility limit, the amount of bromine introduced, as may be determined by iodometric titration, is equivalent to a weight concentration in the resulting perbromide solution of about 40-50% Br2. Because of the reversibility of the reactions of equations (8) and (9), a portion of the bromine is present as Br2, but most is present as Br3-. In a solution saturated with respect to both initial NaBr solubility and bromination of Br- ion, the equilibrium is such that the solution contains about 63-64% by weight NaBr3, 4 to 4 1/2% Br2 and 2 1/2 to 3% NaBr.
The alkali metal or alkaline earth metal bromate component solution is prepared by addition of liquid 134016~
bromine or bromine vapor to an aqueous solution of metal hydroxide, most preferably an alkali metal hydroxide.
Hydroxyl ions and molecular bromine react in accordance with the following reaction equation to produce both S bromate and bromide ions:

3Br2 + 60H- - > 5Br- + BrO3- + 3H2O (10) Under alkaline conditions, this reaction proceeds essentially quantitatively to the right. Preferably, the strength of the initial caustic (or other alkaline) solution and the amount of molecular bromine added thereto are controlled so that, when the bromate solution is mixed with the solution of alkali metal or alkaline earth metal perbromide in predetermined relative proportions, the resulting mixture has a pH of between about 6.5 and about 7.5. Where the bromate solution is used in the preparation of a concentrate, the strength of the initial caustic solution and the degree of bromination are selected so that the bromate solution contains at least about 15% by weight equivalent molecular bromine, i.e., at least about 4% by weight bromate ion. Preferably, the bromate solution component of the concentrate contains between about 5% and about 8% by weight bromate ion, roughly equivalent to between about 20% and about 30% by weight molecular bromine. To provide a bromate component solution having such concentration of equivalent molecular bromine and satisfying the stoichiometric requirement set forth by equation (10) the initial concentration of the caustic solution is preferably in the range of 10-20% by weight in the case of sodium hydroxide. Equivalent molar proportions may be computed for other alkalis.

-~ 1340169 Alternatively, the bromate component solution may be prepared by dissolving an alkali metal bromate or alkaline earth metal bromate salt in water. This in fact is the preferred method for preparing a component solution comprising an alkaline earth metal bromate, since difficulty may be encountered in the preparation of such solution by addition of molecular bromine to a lime or magnesia solution or slurry. In this alternative method of preparing the component solution, an alkali metal or alkaline earth metal bromide is also incorporated so as to produce an overall composition essentially equivalent to that obtained by dissolving Br2 in a caustic solution.
In the preparation of the precursor composition of the invention, the perbromide solution and bromate solution are mixed in proportions of between about 4 parts by weight perbromide solution per part by weight bromate solution and about 4 parts by weight bromate solution per part by weight perbromide solution. Preferably, approximately equal portions of the two component solutions are mixed. Whatever relative proportions are used, the pH
of the resultant composition should be between about 6.5 and about 7.5, and the ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in the composition is between about 0.05 and about 0.8. Where the bromide ion has been fully saturated with bromine in the preparation of the perbromide component solution, the molar concentration of bromide ion in the precursor composition of the invention is equal to the sum of the molar concentration of molecular bromine and five times the molar concentration of bromate ion.

In the concentrate of the invention, the bromate ion concentration is at least about 2%, typically ranging from about 2% to about 6% by weight, the equivalent perbromide content is preferably at least about 10%, ranging from about 55% to about 10% by weight, and the concentration of bromide ion (as computed on the basis of no dissociation of perbromide ion) generally ranges from about 3% to about 19%, the preferred compositions typically containing bromide ion weight concentrations in the range of about 6% to about 17%.
With respect to the potency of leaching compositions that may be prepared from it, the most significant parameter of the concentrate is its equivalent molecular bromine content. This is the proportion of molecular bromine that is made actually available upon the acidification of the precursor concentrate in the preparation of the leaching agent. The equivalent molecular bromine concentration of the concentrate is defined in molar terms as the sum of the actual molar concentration of molecular bromine, the molar concentration of perbromide ion, and three times the molar concentration of bromate ion. The equivalent bromine concentration further includes the molar concentration of hypobromite ion and hypobromous acid, as reflected by the equilibrium:

Br2 + H2O > H+ + HOBr + Br- (11) HOBr ~ ~ H + OBr (12) The equivalent molecular bromine content of the concentrate is between about 10% and about 40%, preferably between about 20% and about 40%, by weight. More preferably the ~ 1340169 equivalent Br2 content is at least about 25% by weight. By using the highly concentrated component solutions as described above, a concentrate can be prepared containing 34% by weight or more equivalent molecular bromine.
At the desired pH of between about 6.5 and about 7.5, the molecular bromine content of the concentrate is generally not converted to bromate and bromide, i.e., equation (10) does not proceed to the left. As a consequence, there is a stable equilibrium between perbromide ion and Br2, and the composition of the concentrate is stable within the ranges discussed above.
Despite the very high proportions of equivalent molecular bromine, including significant fractions of Br2 and Br-3, it has been discovered that the vapor pressure of the concentrate of the invention is quite low. For example, a concentrate containing about 34% by weight equivalent bromine exhibits a total vapor pressure of only 23mm Hg at 0~C, and a total vapor pressure of only 112.5mm Hg at 35~C. By comparison, the vapor pressures of liquid bromine are 75mm Hg at 0~C and 357.5mm Hg at 35~C and sodium perbromide are 44mm Hg at 0~C and 214mm Hg at 35~C.
Thus, the concentrate of the invention can be used in the preparation of a precious metal leaching solution with greater convenience, assurance, and safety than can either liquid bromine or sodium perbromide.
Effective aqueous bromine leaching solutions for recovery of precious metals may be prepared by dilution of the concentrate of the invention. Prior to or after dilution, the pH may be adjusted by addition of either a mineral acid such as HBr, HCl, or C12, or a base, such as NaOH or KOH. Where the concentrate is acidified, it is preferred that HBr be added rather than HCl. The leaching .
solution is effective over a wide range of pH, but operation is preferably carried out within a range of about 2 to about 10, more preferably in a range of between about 5 and about 7.5. A somewhat acid pH is generally preferred to promote the conversion of bromate ion to molecular bromine. Acidification is preferably carried out prior to dilution, thus producing a more acidic concentrate having a pH of between about 0.25 and about 2.5 and an equivalent molecular bromine concentration in the range of between about 28% and about 40% by weight.
In conjunction with dilution, a portion of NaBr or other halide salt is advantageously incorporated into the solution. Eh/pH diagrams constructed from thermodynamic data show progressively larger solubility field at lower Eh values for the formation of the AuBr4~
complexion (see equations 12 and 13 infra) as Br~ ion concentration increases from 10-5 to l.OM. Water, and optionally the halide salt, are added in such relative amounts that the equivalent molecular bromine content of the leaching solution is between about 0.01% and about 20%
by weight, the bromide ion concentration is between about 0.005% and about 20%, and the total halide ion concentration is between about 0.005% and about 30% by weight. In most applications, particularly in the recovery of precious metals from ores, the leaching solution should contain between about 0.01% and about 1% by weight, preferably about 0.02% to about 0.5% by weight, equivalent molecular bromine, between about 0.005% and about 10%, preferably about 0.01% to about 1%, by weight bromide ion, and between about 0.005% and about 15%, preferably about 0.01% to about 1.5%, by weight total halide ion. However, in certain applications such as, for example, recovery of metallic gold from an electronic circuit board or jewelry scrap, a more concentrated leaching solution may be used.
Such may be prepared from the above described concentrates by modest dilution with water. In some instances, the concentrate may even be used directly for dissolution of metallic gold or silver.
The rate of dissolution of precious metal in the leaching solution is in some instances accelerated if it contains a concentration of halide ions that is higher than that provided by a bromine saturated concentrate. For this reason, preparation of the leaching solution preferably involves incorporation of chloride salt or bromide salt from a source other than the concentrate. It may be noted that both the actual molecular bromine and the ultimate bromide ion content are also affected by the shifts in equilibria which accompany the acidification and dilution process. Thus equations (8) and (9), supra, are driven to the left, converting perbromide ion to bromide and molecular bromine; and equation (10) is also driven to the left, converting bromate ion and bromide ion to molecular bromine. Dilution tends to drive equation (11) to the right, resulting in conversion of molecular bromine to bromide ion and hypobromous acid. As a net result, the hypobromous acid concentration is a significant component of the equivalent molecular bromine content of the leaching solution.
Where the precursor concentrate or leaching solution is acidified by addition of C12, not only the bromate but the bromide ion content thereof are converted to molecular bromine. This may further enhance the oxidizing and complexing power of the leaching solution for leaching of gold or silver from a source material.

Gold and silver are recovered from a source thereof, such as comminuted gold ore, by contacting the source material with the aqueous bromine leaching solution. In the case of gold, oxidation and complexing of the gold is believed to proceed in accordance with the equations:

2Au +3Br2 + 2Br- > 2AuBr4 (12a) or H+
2Au + 3HOBr + 5Br- > 2AuBr4 + 3H2O (13) Depending on the nature of the ore, the relative proportions of ore (or other source material) and leaching agent may be such that the leaching slurry contains between about 1 and about 100 lbs. active agent per ton of source.
Active agent in this instance is defined as the sum of the amounts of metal bromide, metal bromate, metal perbromide, metal hypobromite, hypobromous acid, and molecular bromille in the leaching solution.
If the source material is a refractory ore, it may be necessary to pretreat it for removal of sulfur and carbonaceous material. Such may be accomplished by methods known to the art such as roasting or pressure oxidation.
Roasting may be sufficient pretreatment if carried out at a temperature of at least about 500~C. The leaching composition and method of the invention also may be used advantageously for recovery of gold from high grade non-refractory ores, low grade refractory and clean ores, electronic component scraps, jewelry scrap and similar low grade refractory and clean ores. The composition and method may be used for recovery of silver from various sources, including photographic film.

,~
.
~~e The slurry of ore in leaching solution is preferably agitated to promote transfer of precious metal from the ore particles to the aqueous phase. A leachate is thus produced containing gold or silver complexed with bromide ions. Leaching may be carried out at ambient temperature. Preferably, contact is maintained for a period of 2 to 6 hours to achieve the maximum transfer of gold or silver from the ore.
After treatment of the precious metal source with the leaching solution is completed, the leachate is separated from the leached ore, as by filtration. The filtrate (leachate) is washed with an aqueous washing medium, the spent wash solution is combined with the filtrate, and the combined filtrate and wash solution is treated for recovery of the precious metal therefrom.
Advantageously, particularly in the case of silver, the filter cake is washed with a strong acid solution, such as HBr but preferably HCl. Washing the filter cake in such fashion may be effective to leach further quantities of metal from the cake. A solution of 6N HCl is especially Gold may be recovered from the combined filtrate and wash solution by conventional means such as zinc or aluminum precipitation, ion exchange, carbon adsorption, or electrowinning. In similar fashion, the leaching solution of the invention may be used for recovery of silver.
The following examples illustrate the invention.

Example 1 A solution was prepared by dissolving sodium bromide (27.7 grams) in water (29.3 grams). A sodium perbromide solution was prepared by adding liquid bromine .~
in an amount (43.0 grams) sufficient to saturate the bromide ion, i.e., stoichiometrically equivalent to the initial bromide ion content, in the solution. The resulting sodium perbromide component solution contained 5 43% equivalent molecular bromine.
A sodium hydroxide solution was prepared containing 16.7% by weight sodium hydroxide. Liquid bromine (25.0 grams) was added to this solution (75.0 grams) producing a composition which contained 6.7% by 10 weight bromate ion (7.9% by weight as sodium bromate; 25%
by weight equivalent molecular bromine). A concentrate was prepared by mixing equal parts by weight of the perbromide and bromate component solutions. The concentrate so prepared contained 31.82% by weight sodium perbromide, 15 2.14% by weight bromine, 14.80% by weight sodium bromide, 3.94% by weight sodium bromate and 47.30% by weight water.
It had an equivalent molecular bromine concentration of 34%
by weight.
The bromine concentrations of both the precursor 20 concentrate and the sodium perbromide component solution were confirmed by adding to the respective solutions an excess of potassium iodide and then titrating the iodine released with sodium thiosulfate using starch as an indicator. Titration of the total equivalent molecular 25 bromine content of the concentrate was effected by the addition of a strong mineral acid to convert the bromate content to Br2. The concentrate was also titrated without addition of acid in order to determine the actual bromine concentration in terms of molecular bromine and perbromide 30 ion. This titration showed a 21.5% bromine in the concentrate.

134~169 Using the Isoteniscope method, the total vapor pressure was measured as a function of temperature for liquid Br2, the sodium perbromide component solution of this example, and the precursor concentrate of this example. From the data obtained, the corresponding enthalpies of vaporization were calculated. The results of these measurements and calculations are set forth in Table 1.

Table 1 Vapor Pressure Data Vapor Pressure/mm Hg Temperature/~C Br2 a NaBr3 b ConcentrateC
0 75.0 44.0 23.0 95.5 56.0 30.5 15 10 120.5 68.0 38.0 151.0 86.0 48.0 189.0 108.5 60.0 234.0 138.0 69.0 289.0 173.0 86.0 20 35 357.5 214.0 112.5 a. ~ Hv = 7.29 Kcal mole -1 b. ~ Hv = 7.65 Kcal mole -1 c. ~ Hv = 7.36 Kcal mole -1 Example 2 Sodium perbromide and sodium bromate component solutions were prepared in the manner described in Example 1. A series of concentrates was prepared using varying proportions of the two component solutions. The composition of the concentrates obtained are set forth in Table 2.

Example 3 In order to compare the vapor pressure of solutions prepared according to the invention with previously known aqueous bromine-based solutions, a solution was prepared by a formulation method comparable to Bahl, et al. U.S. patent No. 4,190,489, and a composition was prepared according to the invention, each containing 34% by weight equivalent bromine. For the Bahl et al.
formulation, 26 g KBr was dissolved in 40 g water and then 34 g Br2 was added to the resulting solution. For the composition prepared according to the invention, 14.26 g NaBr, 45.49 g H2O, 6.25 g NaOH and 34 g Br2 were mixed.
Bromate content and vapor pressure were calculated as follows: Titration with Thiosulfate-KI using a weak acid determines actual Br2 content (Br2 + Br3~) while titration with Thiosulfate-KI using a strong acid converts bromate to bromine and determines the sum of bromate and bromine concentration. Therefore, the bromate content of the two solutions was determined by Thiosulfate-KI titration first with acetic acid to determine the actual bromine concentration, and then by Thiosulfate-KI titration with H2SO4 to determine the total equivalent molecular bromine (Br2 + Br3- + BrO3-) and subtracting the difference.
Solution vapor pressure at 25~C was obtained by using the Isoteniscope method.

1~40169 O ~n o o o O ~ ~ ~
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O C cr r.
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o co ~n ~ o ~--~ o ~ -,,.". :--_ w ~ ~ Z
. . . . .

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r~
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o ~ Vl o a~ a~ o o ~ ~1 ~

e -~4 Table 3 pH, Br2 Concentration, and Vapor Pressure Measurements:
Composition of Invention * vs. Bahl Formulation **
~c~ o ~~
~r~ ~mcndcd 5 Parameter Claim 1Bahl Formulation pH 6.6 3.0 Wt. % Br2 (with HAC) 24.2 33.1 Wt. % Br2 (with H2SO4)32.5 33.4 Wt. % Br2 (present as BrO3-) 8.3 0.3 10 Vapor Pressure at 25~C 70.5 106 mm-Hg * 14.26 g NaBr, 45.49 g H20, 6.25 g NaoH, 34 g Br2.
** 26 g KBr, 40 g H2O, 34 g Br2.

Example 4 A leaching solution was prepared from the concentrate of Example 1 and used in leaching tests for recovery of gold and silver from a refractory ore concentrate initially containing 12.5% by weight carbon and 15.5% by weight sulfur. A fire assay of this ore performed by Chemex of Canada showed 7.07 oz. gold per ton and 6.39 oz. silver per ton. A similar fire assay provided by Hazen of the U.S. showed 6.61 oz. gold per ton and 5.83 oz.
silver per ton. Because of the high concentration of carbon and sulfur in this ore, it was necessary to pretreat the ore prior to leaching. Pressure oxidation and roasting are among the commonly used methods for oxidizing carbon and sulfur in carbonaceous and refractory ores before the recovery of precious metals therefrom by leaching. In this instance, the ore was pretreated by roasting.
In the roasting operation, an ore concentrate (451 g) was charged into a 100 mm diameter quartz batch kiln. The kiln was placed in an electrically heated 13~0169 clamshell furnace sealed with rotary fittings at the ends, and rotated at about 5 rpm. Oxygen was passed through the kiln while the contents thereof were heated to a temperature ranging from 600-707~C, averaging approximately 650~C. Temperature was controlled by application of electric power to heat and opening of the furnace to the surroundings for cooling. After the ore was heated in the presence of a stream of oxygen for 120 min., the kiln was cooled and the calcine products sampled and analyzed. A
22% weight loss occurred during roasting. The calcine contained 3% total sulfur and 8.5% sulfate, indicating that the residual sulfide level was 0.17%.
A series of leaching tests was carried out in which gold and silver were recovered from the calcine using an aqueous bromine leaching agent. To prepare the leaching agent, a portion of the concentrate of Example 1 (1.4 grams) and 48% hydrobromic acid (0.8 grams) were introduced into a small capped bottle. The contents were mixed well to assure conversion of sodium bromate to bromine. The mixture was then transferred into a 100 ml. flask containing sodium bromide (1 g), and water was added up to the mark, i.e., to produce a total solution volume of 100 ml.
A specimen of calcine (22.75 grams; one assay ton equivalent of dried unroasted concentrate) and the aqueous bromine leaching solution from the volumetric flask (100 ml.) were placed in a capped 250 ml. Erlenmeyer flask. The resultant slurry was mixed using an automatic mixer for a predetermined period of time at room temperature.
Individual runs were made in which mixing was terminated after 4, 8, 12 and 24 hours, respectively. After termination of the mixing cycle, the slurry was filtered 13~0169 and the residue washed with 4M hydrochloric acid. Head filtrate and wash solution were combined and analyzed for gold and silver. The results are presented in Table 4.
The data for percent extraction in this table are based on the concentrations of silver and gold in the solution and the average assay values of Hazen and Chemex.

Table 4 LEACHING OF REFRACTORY CONCENTRATE

Sample Size: 22.75 g Calcine or 29.16 g unroasted 10 Fire Assay: 6.84 oz/t Au; 6.11 oz/t Ag (unroasted ore) Feed Preparation: 325 mesh, roasted at 650~C

B Conditions: 22~ C, pH = 5.0 - 5.5, 18.5~ ~olids~ O 40\;~

Solution Leach Time Au Ag % Extraction Test No. Hour oz/t oz/t Au Ag 1 4 7.10 3.98 100 65 2 8 7.13 4.13 100 68 3 12 6.86 4.54 100 74 4 24 7.00 4.33 100 70 Example 5 To determine the effect of pH on leaching gold from refractory concentrate, a series of leaching tests was conducted in the manner described in Example 4 but at varying pH. After the calcine was mixed with the aqueous bromine leaching reagent, the pH of the slurry was adjusted to a specified value in the range of pH 2 to pH
by the addition of hydrobromic acid or sodium hydroxide. After pH adjustment, the slurry was mixed for four hours and filtered. The filter cake was washed and the gold value in the combined filtrate and wash solution was determined. The test results of this example are set forth in Table 5. These data demonstrate that the precursor concentrate of the invention may be diluted to produce leaching solutions which are effective lixiviants at varying pH levels for leaching gold.

Table 5 LEACHING OF REFRACTORY CONCENTRATE

Sample size: 22.75 g Calcine or 29.16 g unroasted 20 Fire Assay: 6.84 oz/t Au; 6.11 oz/t Ag (unroasted ore) Feed Preparation: 325 mesh, roasted at 650~C
B Conditions: 22~C; pH = 2 - 10; 18.5~ aolid~A ~ ~ID 5O\;~
Leach Time pH pH Au Extraction 25 Test No. Hour Initial Final oz/t %
1 4 2.1 3.2 6.58 96 2 4 5.6 6.3 6.86 100 3 4 8.3 7.5 6.83 100 4 4 9.8 9.1 6.90 100 Example 6 In order to optimize the concentration of active agents needed to leach gold from a refractory concentrate, a series of leaching tests was carried out under conditions comparable to those of Example 4 but at varying dilutions of the concentrate. No pH adjustment was made in the tests of this example. The slurry of calcine and aqueous bromine reagent was mixed for 4 hours and filtered. The filter cake was washed and the gold value of the combined filtrate and wash solution was measured. The results of the experimental runs of this example are set forth in Table 6, each test result reported in this table being based on the average of 3 runs. Note that 44 pounds of the concentrate or 15 pounds of bromine equivalent was found necessary to leach about 7 oz. of gold from 1 ton of refractory concentrate ore.

Table 6 1340169 LEACHING OF REFRACTORY CONCENTRATE

Sample Size: 22.75 g calcine or 29.16 g unroasted Fire assay: 6.84 oz/t Au; 6.11 oz/t Ag (unroasted ore) Feed Preparation: 325 mesh; roasted at 650~C
~a Conditions: 22~C; pH = 5.0 - 6.5; 18.5~ Eolids ~ O
4 hrs. mixing Usage of Conc.* Au Extraction 10 Test No. lb/t ore oz/t %

1 123 6.86 100 2 88 6.94 100 3 44 6.85 100 *Concentration of reagent of Example 1 per ton of ore in leaching slurry.
Example 7 To study the effect of NaBr concentration on the leaching of Au and Ag from a refractory concentrate, a series of leaching tests was carried out. The tests were similar to those of Example 4, but the concentration of NaBr was varied. The aqueous bromine leaching solution contained 1 wt.% concentrate of Example 1 and varying amounts of NaBr. The residue was washed with water instead of 4M HCl. The results are presented in Table 7.
Considering the fire assay of head ore (Hazen) as the .
basis, the gold recovery ranged from 96% to 100% and silver recovery range was 2-11%. It is interesting to note that the addition of NaBr does not have any effect on the Au recovery, whereas the recovery of Ag is affected by the concentration of NaBr. Comparing the Ag recovery in these leaching tests with those of Example 4, it may be concluded that washing the residue with 4M HCl definitely improves the Ag recovery without having any effect on the Au recovery. The residues of the leaching tests (Table 7) were fire assayed by Hazen. Set forth in Table 8 are the metallurgical balance and the calculated percentage of gold solubilized on the basis of the calculated head.

Table 7 LEACHING OF REFRACTORY CONCENTRATE
15 Sample Size: 50.00 g Calcine Fire assay: 8.52 oz/t Au; 6.60 oz/t Ag (Calcine) Feed Preparation: 325 mesh; roasted at 650~C
B Conditions: 22~C; pH = 5.0 - 6.5; 20.0~ Eolids~ 50\'~
4 hrs. mixing Solution NaBr Au Ag % Extraction Test No.Wt. % oz/t oz/tAu Ag 1 0 8.77 0.12100 1.8 2 2.5 8.44 0.1799 2.6 3 5.0 8.27 0.6097 9.1 4 10.0 8.18 0.8596 12.9 5 20.0 8.97 0.73100 11.1 Table 8 LEACHING OF REFRACTORY CONCENTRATE
METALLURGICAL BALANCE

Sample Size: 50.00 g Calcine Fire assay: 8.52 oz/t Au (14.62 mg Au) Feed Preparation: 325 mesh; roasted at 650~C
Conditions: 22~C, pH - 5.0 - 6.5; 20.0~ solidEA ~ .~ 5 ~LJ 4 hrs. mixing Run No. 1 2 3 4 5 Filtrate Volume, ml 370 360 294 302 486 Au Conc., mg/L 40.9 40.8 47.0 46.3 31.8 Au Conc., mg 15.13 14.69 13.82 13.98 15.46 Residue 15 Au Conc., oz/t 0.372 0.322 0.186 0.276 0.342 Au Conc., mg 0.64 0.55 0.32 0.47 0.58 Au Solubilized, % 96.0 96.4 97.8 96.7 96.4 Calculated Head 9.19 8.88 8.24 8.42 9.35 oz/t 20 Overall Balance, % 108 104 97 99 109 Example 8 A series of leaching tests was carried out in which gold and silver were recovered from a low grade clean ore using an aqueous bromine leaching agent. The procedure of Example 4 was followed. The leaching solution contained 0.5 wt.% of the concentrate of Example 1 and 1 wt.% NaBr.
The residue was washed with 4 M HCl. The head filtrate and wash solution were analyzed for Au and Ag to obtain the solubilized metals. The results are presented in Table 9.
Considering the fire assay of head ore (Hazen) as the basis, the gold recovery was 100%. The silver recovery ranged between 50-100%.

Table 9 LEACHING OF LOW GRADE CLEAN ORE

15 Sample Size: 29.16 g ore Fire Assay: 0.148 oz/t Au; 1.99 oz/t Ag Feed Preparation: 200 mesh ~'~
Conditions: 22~C; pH = 5.0 - 6.5; 23 Eoli~S ~t.~o ~0\~5 Solution Leach Time Au Ag % Extraction Test No.Hour oz/t oz/t Au Ag 1 4 0.148 2.35 100 100 2 4 0.150 2.30 100 100 3 4 0.166 1.00 100 50 4 4 0.146 1.10 99 55 4 0.167 1.02 100 51 6 4 0.177 2.30 100 100 7 4 0.195 2.30 100 100 Example 9 In a further series of leaching tests using the concentrate of Example 1, the concentration of concentrate in the leaching solution was varied from 2.0 to 6.0 g/L.
These tests indicated that gold recovery was maximized at about 4.0 g/L concentrate.
Further tests were conducted at leaching times of 2, 4.6, 12, 18 and 24 hours. The results of these tests indicated that over 98% of all leachable gold was solubilized after 2 hours. Based on the results of the latter tests a leaching time of 6 hours was chosen for further tests.
Triplicate confirmatory tests were conducted on two separate ore calcines that had been obtained by roasting samples of Canadian flotation concentrate at 650~C
- 750~C. The confirmatory tests were conducted using what were considered generally optimum conditions: 4g/L
concentrate, pH 5.0-6.0, and leaching time 6 hours. In the tests on the first calcine, gold in the residue ranged from 0.592 to 0.650 oz/t, Au recovery ranged from 94.2% to 94.5%
and Au Head was calculated as ranging from 9.51 to 9.96 oz/t. In the tests on the second calcine, the ,, corresponding figures were 0.714 to 0.768 oz/t Au in residue, 96.0 to 96.3% Au extraction, and 17.29 to 17.73 oz/t Au Head.
In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.
As various changes could be made in the above methods and products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Claims (27)

1. An aqueous composition having a pH between about 6.5 and about 7.5 comprising bromide ion, perbromide ion, molecular bromine, at least about 2% by weight bromate ion, and a metal ion selected from the group consisting of alkali metal and alkaline earth metal, the equivalent molecular bromine content of the composition being between about 10%
and about 40% by weight and the ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in the composition being between about 0.05 and about 0.8.

2. A composition as set forth in claim 1 wherein the equivalent molecular bromine content is between about 20% and about 40%.

3. An aqueous composition as set forth in claim 1 wherein the molar concentration of bromide ion is equal to the sum of the molar concentration of molecular bromine and five times the molar concentration of bromate ion.

4. An aqueous composition as set forth in claim 1 containing between about 2% and about 6% by weight bromate ion and between about 55% and about 10% by weight equivalent perbromide ion.

5. An aqueous composition as set forth in claim 4 containing at least about 25% by weight equivalent molecular bromine.

6. A leaching solution adapted for leaching of a precious metal containing gold or silver from an ore containing said precious metal, said solution having a pH of between about 6.5 to about 7.5 and containing between about 0.01% and about 1% by weight equivalent molecular bromine, between about 0.01% and about 1% by weight bromide ion, and between about 0.005% and about 15% by weight total halide ion.

7. A method for leaching of a precious metal containing gold or silver from an ore containing said precious metal, the process comprising contacting said ore with an aqueous bromine leaching solution, said leaching solution having a pH of between about 6.5 and about 7.5 and containing between about 0.01% and about 20% by weight equivalent molecular bromine, between about 0.005% and about 20% by weight bromide ion, and between about 0.005% and about 30% by weight total halide ion, thereby producing an aqueous leachate containing the precious metal.

8. A method as set forth in claim 7 wherein said leaching solution contains between about 0.01% and about 1% by weight equivalent molecular bromine, between about 0.01% and about 1% by weight bromide ion, and between about 0.005% and about 15% by weight total halide ion.

9. A method for leaching of a precious metal containing gold or silver from a source material containing such metal, the method comprising contacting said source material with an aqueous bromine leaching solution prepared by acidification of a precursor composition initially having a pH of between about 6.5 and about 7.5 and comprising bromide ion, perbromide ion molecular bromine, at least about 2% by weight bromate ion, and metal ion selected from the group consisting of alkali metal and alkaline earth metal, said precursor composition having an equivalent molecular bromine content of between about 10% and about 40% by weight and the ratio of the molar concentration of bromate ion to the sum of the molar concentrations of molecular bromine and perbromide ion in said precursor composition being between about 0.05 and about 0.8, said leaching solution having a pH of between about 2 and about 10 and containing between about 0.01% and about 20% by weight equivalent molecular bromine, between about 0.005% and about 20% by weight bromide ion, between about 0.005% and about 30% by weight total halide ion, thereby producing an aqueous leachate containing the precious metal.

10. A method as set forth in claim 9 wherein said precursor composition is acidified by mixing of hydrobromic acid therewith.

11. A method as set forth in claim 10 wherein the preparation of said leaching solution further comprises dilution of said precursor solution with water so that the equivalent molecular bromine concentration is reduced to a concentration in the range of between about 0.01% and about 1% by weight.

12. A method as set forth in claim 11 further comprising incorporation in the solution of a bromide or chloride salt from a source other than said precursor composition.

13. A method as set forth in claim 12 wherein gold ore is contacted with said leaching solution in a proportion of between about 1 and about 100 lbs. of active agent per ton of ore, said active agent being defined as the sum of the concentrations of metal bromide, metal bromate, metal perbromide, metal hypobromite, hypobromous acid, and molecular bromine in said solution.

14. A method as set forth in claim 13 wherein the ore comprises a refractory ore containing sulfur, and the ore is roasted for oxidation of sulfide contained therein prior to contact with said leaching solution.

15. A method as set forth in claim 14 wherein the ore is roasted by contact with an oxygen-containing gas at a temperature of at least about 500°C.

16. A method as set forth in claim 14 wherein:
a comminuted ore comprising said precious metal is mixed with said leaching solution to produce a leaching slurry;
the slurry is agitated and precious metal is transferred from said ore to the aqueous phase of the slurry, thereby producing an aqueous leachate containing said precious metal complexed with bromide ions;
said aqueous leachate is separated from said slurry; and said leachate is treated for separation of said precious metal therefrom.

17. A method as set forth in claim 16 wherein, after separation of said leachate therefrom, the leached ore is washed with an aqueous washing medium, the spent wash solution is combined with the separated leachate, and the resultant combined leachate and spent wash solution is treated for separation of precious metal therefrom.

18. A method as set forth in claim 17 wherein said precious metal comprises silver, and said aqueous washing medium comprises a solution of hydrobromic or hydrochloric acid having a strength of about 6N.

19. A method as set forth in claim 17 wherein said washing medium comprises hydrochloric acid having a strength of about 6N.

20. A method as set forth in claim 9 wherein the molar concentration of bromide ion in said precursor composition is equal to the sum of the molar concentration of molecular bromine and five times the molar concentration of bromate ion, the acidification of said precursor composition producing molecular bromine by dissociation of perbromide ion and decomposition of bromate ion.

21. A method as set forth in claim 9 wherein said precursor composition initially contains between about 2% and about 6% by weight bromate ion and between about 55% and about 10% by weight equivalent perbromide ion.

22. A method as set forth in claim 21 wherein said precursor composition initially contains at least about 25%
by weight equivalent molecular bromine.

23. A method as set forth in claim 8 wherein said ore is selected from the group consisting of nonrefractory ores and clean ore concentrates.

24. A method as set forth in claim 8 wherein:
a comminuted ore comprising said precious metal is mixed with said leaching solution to produce a leaching slurry;
the slurry is agitated and precious metal is transferred from said ore to the aqueous phase of the slurry, thereby producing an aqueous leachate containing said precious metal complexed with bromide ions;
said aqueous leachate is separated from said slurry; and said leachate is treated for separation of said precious metal therefrom.

25. A method as set forth in claim 24 wherein, after separation of said leachate therefrom, the leached ore is washed with an aqueous washing medium, the spent wash solution is combined with the separated leachate, and the resultant combined leachate and spent wash solution is treated for separation of precious metal therefrom.

26. A method as set forth in claim 25 wherein said precious metal comprises silver, and said aqueous washing medium comprises a solution of hydrobromic or hydrochloric acid having a strength of about 6N.

27. A method as set forth in claim 26 wherein said washing medium comprises hydrochloric acid having a strength of about 6N.