RU2102507C1 - Aqueous solution for leaching precious metals (versions) - Google Patents

Abstract

FIELD: precious metal recovery. SUBSTANCE: solution contains bromide and perbromide ions, molecular bromine, and at least 2 wt % bromate ion and alkali or alkali-earth metal ion. Equivalent content of molecular bromine ranges from about 10 to some 40 wt %, and molar ratio of bromate ion to sum of molecular bromine and perbromide ion lies in the range of about 0.05-0.8. precious metals, including gold and silver, are leached from their sources. EFFECT: improved leaching efficacy. 2 cl

Description

 The invention relates to hydrometallurgical extraction of precious metals and, in particular, to an improved hydrometallurgical process for the extraction of gold and silver from ores and other sources through the use of bromate-perbromide leaching solution.

В связи с высокой стабильностью аниона комплекса цианида золота для окисления золота в присутствии иона цианида достаточно даже кислорода воздуха. Извлечение золота из раствора осаждением цианида можно проиллюстрировать следующим уравнением реакции:

По другому варианту золото можно извлечь из раствора цианида адсорбцией аниона комплекса цианида золота на активированном углероде, десорбцией горячим щелочным раствором и извлечением электролизом или посредством повышения pH. Типичная схема извлечения посредством активированного углерода иллюстрируется приведенными ниже уравнениями реакции:

Этот метод широко применяется в промышленном масштабе, однако цианидное выщелачивание имеет и недостатки. Так, скорости выщелачивания при использовании щелочных цианидных растворов низки, в случае золотых руд время контактирования обычно составляет от 24 до 72 ч. Из-за токсичности цианида следует принимать меры предосторожности для поддержания растворов цианида на щелочной стороне во избежание выделения газообразного цианида водорода. Следует соблюдать жесткие ограничения, связанные с охраной окружающей среды, требующие тщательного контроля и регулирования всех очистных потоков процесса. Отработанные выщелачивающие цианидные растворы следует подвергать очистке до выброса в окружающую среду.Noble metals such as gold and silver are recovered from ores by leaching with an alkaline cyanide solution. During the reaction with the cyanide ion and oxygen, the noble metal is converted into a cyanide complex (gold cyanide anion), which is absorbed in the leach solution. The dissolution of gold, for example, is illustrated by the following reaction:

Due to the high stability of the anion of the gold cyanide complex, even atmospheric oxygen is sufficient to oxidize gold in the presence of a cyanide ion. The recovery of gold from the solution by precipitation of cyanide can be illustrated by the following reaction equation:

Alternatively, gold can be recovered from a cyanide solution by adsorption of an anion of a gold cyanide complex on activated carbon, desorption by a hot alkaline solution and extraction by electrolysis, or by increasing the pH. A typical activated carbon recovery scheme is illustrated by the following reaction equations:

This method is widely used on an industrial scale, but cyanide leaching also has drawbacks. Thus, the leaching rate when using alkaline cyanide solutions is low, in the case of gold ores, the contact time is usually from 24 to 72 hours. Due to the toxicity of cyanide, precautions should be taken to maintain cyanide solutions on the alkaline side to avoid the release of gaseous hydrogen cyanide. Strict environmental protection restrictions must be observed, requiring careful monitoring and regulation of all treatment effluents. Spent leach cyanide solutions should be cleaned prior to release to the environment.

Gold on an industrial scale is also leached using aqua regia, a mixture of concentrated hydrochloric and concentrated nitric acids in accordance with the following reaction equation:
Au + 4H ⁺ + 4Cl ^- + NO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ __ → AuCl $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ + NO + 2H ₂ O (4)
Gold can then be recovered by reduction with zinc metal or by increasing the pH of the leach solution. However, aqua regia is an expensive compound and highly corrosive and emits toxic gases. In addition, it easily dissolves base metals and relatively slowly dissolves gold in an aqueous solution.

Thiourea was also used as a leaching agent for dissolving gold from ores in accordance with the following reaction equation:
Au + Fe ³⁺ + 2CS (NH ₂ ) ₂ ⇄ Au [CS (NH ₂ ) ₂ ] ⁺ + Fe ²⁺ (5)
Thiourea is more effective, however, it is susceptible to oxidative degradation and therefore its consumption when extracting gold from gold ores is high.

 Other processes have been developed to use halogens, halides or other halide-containing compounds to extract precious metals from ores. For example, Shaeffer in US Pat. No. 2,677,223 describes a process in which ore is calcined in a tank, water is added to the calcined ore, and liquid bromine is added to form a mixture which is stirred, whereby gold is dissolved in water as bromide. After filtering to separate solids, gold is precipitated from the solution with oxalic acid or sulfate ion.

Fink and others in US patent 2283198 note that chloride and bromide ions accelerate the dissolution of gold in an aqueous solution of bromine. This invention establishes that chlorine or hypochlorite can be used as intermediate oxidizing agents, as indicated by the reactions:
2Br ^- + Cl ₂ _ → 2Cl ^- + Br (6)
or
2Br ^- + ClO ^- + 2H ⁺ _ → Cl ^- + Br ₂ + H ₂ O (7)
Therefore, they proposed a process for extracting gold from gold ores by leaching using a solvent prepared by adding free chlorine to a solution containing bromide and a large excess of chloride salt. In another embodiment, the leaching agent is prepared by adding hypochlorite and mineral acid to a solution containing bromide and a large excess of chloride salt. Fink et al. Describe the precipitation of gold from a solution by any of the well-known methods, such as adding zinc or iron sulfate.

 In US patent 2304823, Harrison describes the extraction of gold from ores by dissolution in a treatment solution containing iodine, potassium iodide, water and nitric acid. You can then add mercury or zinc to extract the noble metal from the solution. It is claimed that this method is applicable to the processing of refractory materials such as refractory sulfide, telluride and the like.

 In US Pat. No. 3,625,674, Jacobs describes the oxidation of gold by an alcoholic solution of molecular iodine, resulting in AlI, which can be decomposed by heating to form metallic gold and iodine vapor, the iodine being recovered, regenerated and reused to process a new portion of the ore. The AgI obtained in this process can also be used to produce other gold compounds, such as gold sodium thiomalate, used in industrial processes.

 In U.S. Pat. No. 3,709,681, Wilson describes a process in which a finely ground source of a noble metal is treated with a solution containing a ketone solvent, dissolved iodine, bromine or chlorine, a halide salt, and predominantly glacial acetic acid. The noble metal contained in the treatment solution is recovered by transferring the base metal, such as aluminum foil, onto the surface.

 In US Pat. No. 3,957,505, Homick et al. Describe a process in which a gold-containing material is processed in an aqueous solution consisting of iodine and a water-soluble salt of hydroiodic acid to obtain a solution containing dissolved gold iodide salts. Gold metal is precipitated from the solution by mixing the solution with a reducing agent such as hydroxylamine, hydrazine, sodium thiosulfate and the like. Iodine from the spent aqueous solution is regenerated by acidifying the solution and adding an oxidizing agent, such as hydrogen peroxide, potassium permanganate, sodium chromate, chlorine or bromine to precipitate elemental iodine.

 US Pat. No. 4,557,759 to McGrew et al. Describes a process for the hydrometallurgical recovery of gold from gold-containing materials by leaching materials with a leaching agent containing iodine. The leaching agent is prepared by saturating an aqueous solution of a salt of hydroiodic acid with iodine. When sulfide is added to this reagent, iodine reacts with sulfide and turns into iodide. Then, an additional amount of elemental iodine is added to this iodide-containing solution until the desired total iodine concentration and the ratio of iodine to iodide are achieved, at which the maximum leaching efficiency is ensured. Then the leaching agent circulates through the ore zone until all the gold contained is dissolved. Next, gold is extracted on activated charcoal. Excess iodide formed during the process is re-oxidized in under the electrochemical method in a special diaphragm element to regenerate the leaching agent. The desired concentration of iodine in the leaching agent is from 1 to about 20 g / l.

 In addition to ores, there are a large number of other sources of gold and silver, which provide the possibility of economical extraction of precious metals from them. In fact, in terms of the content of the metal to be extracted, many of these secondary sources are significantly richer than ores. Gold can be obtained from a variety of metal wastes, including industrial wastes, gold-plated electronic circuit boards, as well as alloys with copper, zinc, silver or tin in ornamental gold used in jewelry. Silver is contained in emulsions of photographic and x-ray films, silver scrap and in numerous industrial wastes.

 In US Pat. No. 3,495,976, Bazilevsky describes a process for recovering gold from a coated substrate by dissolving gold in an aqueous solution of potassium iodide and free iodine. Gold is extracted from the solution by adding concentrated sulfuric acid and heating the resulting mixture to a temperature near the boiling point, as a result of which molecular iodine is distilled off and favorable conditions are created for the deposition of gold.

 US Pat. No. 4,190,489 to Bahl et al. Describes a composition and method for etching gold, in particular gold layers on ceramic substrates. The composition is prepared, for example, by mixing potassium bromide (75 g), elemental bromine (25 g) and water (100 ml). This solution is effective for extracting gold from ceramic substrates at almost room temperature.

 Patent 4,375,984 to Bahl et al. Describes a process in which an alkali metal bromide-bromine solution is used to etch gold from a sub-belt. In this invention, an etching solution can be prepared, for example, by mixing potassium bromide (2 g), bromine (1 g) and water (25 ml). Gold dissolved in the etching solution is recovered as metallic gold either by precipitation using alkali metal hydroxide, or by decomposition to remove the etching solution. The etching bromide-bromine solution of an alkali metal can be regenerated by adding acid to it.

In PCT Publication W085 / 00384, Kalocsai and Zoltan describe the dissolution of metallic gold in a reagent containing a protium solvent, such as water or alcohol, a non-reducing source of a cation, such as sodium, potassium, containing 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 and a reagent can be released. Alternatively, the solution further contains a strong oxidizing agent such as hydrogen peroxide, sodium peroxide, potassium peroxide, sodium permanganate, potassium permanganate, potassium chromate or iron (III) sulfate. Among the described reagents Kolacsai and other example reagents there is a composition (example 26) containing Br ₂ in a volume fraction of 1% 1% NaBr in terms of mass to volume and 0.6% NaOH in terms of mass to volume and having a pH of 7.35.

 US Pat. No. 4,637,865 to Sergent et al. Describes a process for extracting a noble metal from a source material by contacting the source material with an aqueous leach solution containing a leach agent consisting of an N-haloantoin compound. Leaching solutions containing 1,3-dibromo-5,5-dimethylhydantoin, 1-bromo-3-chloro-5,5-dimethylhydantoin and 1,3-dichloro-5,5-dimethylhydantoin are described.

 The noble metal can be recovered from the leach solution by precipitation of a less noble metal, an ion exchange process, treatment with activated carbon, solvent extraction, or electrodeposition.

 In U.S. Patent 4,439,235, Simpson describes a process for recovering precious metals from carbonaceous ground ores, in which the ground ore is contacted with an aqueous solution of hypochlorite, an iron ion, and an acid.

 US Pat. No. 4,391,923 to Falanga et al. Discloses a process in which gold palladium is etched with an etching solution of potassium iodide and iodine, and the metal is removed from the solution by adding an alkaline component, mainly KOH, to raise the pH to at least 12.5 and precipitate from a solution of metallic gold. Borohydride is used to precipitate palladium. A similar process is described in US Pat. No. 4,391,922 to McDonald.

 In Jalles' book "Bromine and its Compounds" by Academic Press, New York, 1966, p. 173, it is claimed that a mixture of sodium broccate and sodium bromide called "mountain salts" was used in the extraction of gold ores. It is claimed that the ratio of the respective components varies, but usually amounts to 43% sodium bromate and 57% sodium bromide, i.e. 2 moles of sodium bromide per 1 mole of sodium bromate.

 US Pat. No. 3,222,276 to Belohlav et al. Describes a process for preparing an aqueous bromine solution from an aqueous solution of bromide-bromate salts and mineral acid. According to the process, a concentrated aqueous solution of bromide and bromate salts in a molar ratio of 5: 1 is pumped into the mixing zone, where it is mixed with mineral acid to convert the bromide-bromate solution into a concentrated aqueous bromine solution. This concentrated aqueous bromine mixture is pumped into the second mixing zone, where it is diluted with a large amount of water to create a diluted stream of aqueous bromine, and this last stream is further diluted with a large volume of water to obtain a highly diluted aqueous bromine solution, providing a very large yield of these amounts of mineral acid and bromide-bromate salts. The described process is advisable to use when brominating water in swimming pools. This reference material does not mention the use of bromine or bromides to extract precious metals from their sources.

 Although known processes that use molecular bromine have been effective in extracting gold from source materials, pure bromine is a toxic, easily vaporizing liquid that forms a choking gas, and requires special handling. Bromine may be somewhat soluble in water or incorporated into a perbromide solution of an alkali metal, but these aqueous materials exhibit significant bromine vapor pressure, and as a result of this, special rules are usually followed when using them. Alternatively, molecular bromine can be obtained by acidification of alkali metal bromates, but bromates themselves are only a limited source of molecular bromine. In addition, bromate salts have a high crystallization temperature, which makes them inconvenient to use as leaching agents for noble metals. Mixtures of bromides and bromates, such as the “mountain salts” described by Janes, have found application in the technology for the recovery of precious metals, but they do not represent a sufficient source of molecular bromine and are not as effective as such known solutions in which the bromine vapor pressure is relatively high.

 The technical result achieved by carrying out the invention is the creation of an improved process for the hydrometallurgical extraction of gold and silver from ores or their other sources, in particular, the creation of such a process that provides a significant source of bromine for dissolving the metal, which does not require the use of liquid bromine or solutions having significant bromine vapor pressure; the creation of such a process in which the use of cyanide is excluded; creating a process that can be used to extract gold from various types of ores, including refractory ores; and creating a process in which a leach product is obtained from which metallic gold or silver can be easily extracted.

 In addition, the creation of compositions that are applicable and effective for leaching gold and silver from source materials; the creation of such compositions that contain a significant source of molecular bromine; and the creation of such compounds that do not exhibit high bromine vapor pressure.

 This is achieved by creating an aqueous composition containing a bromide ion, a perbromide ion, molecular bromine and at least 2 by weight bromate ion and a metal ion selected from alkali and alkaline earth metals. The equivalent molecular bromine content of the composition is between about 10% and about 40% by weight. The ratio of the molar concentration of bromate ion to the sum of the molar concentrations of bromine and perbromide ion in the composition is between about 0.05 and about 0.8.

 The invention is directed to the creation of a leach solution designed to leach a noble metal containing gold or silver from its source. The solution has a pH between about 2 and about 10 and contains from about 0.01 to about 1% by weight equivalent of molecular bromine, from about 0.01% to about 1% by weight of bromide ion, and from about 0.005 to about 15% by weight of total halide ion. The equivalent concentration of molecular bromine, expressed in moles per liter, is the sum of the actual molar concentration of molecular bromine, the molar concentration of the perbromide ion, the triple molar concentration of the bromate ion and the molar concentration of the hypobromite ion and hypobromic acid in the solution.

 The present invention has developed a method for leaching a noble metal, including gold or silver, from ore containing such a metal. The method comprises contacting the ore with a bromine aqueous leach solution, wherein the leach solution has a pH between about 2 and between about 10 and contains from about 0.01 to about 20% by weight of the equivalent of molecular bromine, from about 0.005 to about 20% by weight of the bromide ion and from about 0.005 to about 30% by weight of the total halide ion. The equivalent concentration of molecular bromine, expressed in moles per liter, is the sum of the actual molar concentration of molecular bromine, the molar concentration of the perbromide ion, the triple molar concentration of the bromate ion and the molar concentration of the hypobromite ion and hypobromic acid in the solution. In this way, an aqueous leach product containing a noble metal is obtained.

 It was found that gold and silver can be effectively extracted from source materials by means of a leaching solution obtained from a unique initial composition containing highly concentrated bromine in the form of a combination of alkali or alkaline earth metal perbromide and alkali or alkaline earth metal bromate. In particular, it was found that the initial composition, which is prepared by mixing the constituent solution of perbromide salt with the constituent solution of bromate salt, may contain a significant concentration of equivalent molecular bromine, although it has a very low bromine vapor pressure, which simplifies its use and minimizes the risk associated with using molecular bromine to leach gold and other noble metals. Moreover, the leach solution of the present invention can be easily prepared by diluting the starting composition and used to leach noble metals without any serious problems associated with the presence of bromine vapor.

 The use of the initial composition is not complicated by the crystallization of bromate salts or any other precipitation of them from solution. The leach solution prepared from this composition is highly effective for leaching gold from refractory ores, and no pretreatment is required other than conventional calcination. However, if desired, a pure ore concentrate can be prepared by conventional processing, which may include pressure oxidation.

In accordance with an example of the invention, an initial leach solution concentrate containing perbromide and bromate salts is first prepared. In the preparation of the leach solution, this concentrate is diluted to obtain a leach solution. The pH can be adjusted either before or after dilution by adding an acid or its source, for example, HBr, HCI, H ₂ SO ₄ or Cl ₂ , or a base, for example, NaOH, KOH or Ca (OH) ₂ .

При приготовлении с использованием концентрата раствор бромида металла первоначально имеет концентрацию по меньшей мере около 25% по массе и преимущественно насыщается до его предела растворимости, т.е. до 45 50% по массе в случае NaBr или 55 60% по массе в случае CaBr₂. Какова бы ни была первоначальная концентрация раствора бромида металла к раствору добавляют жидкий или парообразный Br₂ до насыщения его ионом бромида, т.е. в полном стехиометрическом эквиваленте с содержанием Br^-. Когда Br₂ добавляют к раствору NaBr, который первоначально находится у предела растворимости, количество введенного брома, которое может быть определено иодометрическим титрованием, эквивалентно массовой концентрации в результирующем пербромидном растворе, составляющей около 40 50% Br₂. Так как реакции по уравнениям (8) и (9) обратимы, часть брома присутствует в виде Br₂, но в основном он присутствует в виде Br $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ В растворе, насыщенном относительно как первоначальной растворимости NaBr, так и бромирования иона Br^-, равновесие таково, что раствор содержит около 63 64% по массе NaBr₃ от 4 до 4 1/2% Br₂ и от 2 1/2 до 3% NaBr.In the preparation of the starting composition in accordance with the invention, the constituent solution of an alkali or alkaline earth metal perbromide is mixed with the constituent solution of an alkali or alkaline earth metal bromate. A perbromide solution is prepared by adding bromine to an aqueous solution of a bromide ion. For example, sodium perbromide and calcium perbromide are prepared by saturating a triple amount of an appropriate aqueous solution of NaBr or CaBr _{2 with} molecular bromine:

When prepared using a concentrate, the metal bromide solution initially has a concentration of at least about 25% by weight and is predominantly saturated to its solubility limit, i.e. up to 45 50% by weight in the case of NaBr or 55 to 60% by weight in the case of CaBr ₂ . Whatever the initial concentration of the metal bromide solution, liquid or vaporous Br _{2 is} added to the solution until it is saturated with a bromide ion, i.e. in full stoichiometric equivalent containing Br ^- . When Br _{2 is} added to a NaBr solution, which is initially at the solubility limit, the amount of bromine introduced, which can be determined by iodometric titration, is equivalent to the mass concentration in the resulting perbromide solution of about 40 to 50% Br ₂ . Since the reactions according to equations (8) and (9) are reversible, part of the bromine is present in the form of Br ₂ , but basically it is present in the form of Br $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ In a solution saturated with respect to both the initial solubility of NaBr and the bromination of the Br ^- ion, the equilibrium is such that the solution contains about 63 64% by weight of NaBr ₃ from 4 to 4 1/2% Br ₂ and from 2 1/2 to 3% NaBr.

An alkali or alkaline earth metal bromate constituent solution is prepared by adding liquid bromine or bromine vapor to an aqueous solution of a metal hydroxide, most preferably an alkali metal hydroxide. Hydroxyl ions and molecular bromine reacts in accordance with the following reaction equation, forming both bromate and bromide ions:
3Br ₂ + 6OH ^- _ → 5Br ^- + BrO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ + 3H ₂ O (10)
Under alkaline conditions, this reaction predominantly shifts to the right. Advantageously, the concentration of the initial caustic (or other alkaline) solution and the amount of molecular bromine added to it are controlled in such a way that when the bromate solution is mixed with an alkali or alkaline earth metal perbromide solution in predetermined relative proportions, the resulting mixture has a pH from about 6.5 to about 7.5. When using a bromate solution to prepare a concentrate, the concentration of the initial caustic solution and the degree of bromination are selected so that the bromate solution contains at least about 15% by weight of equivalent molecular bromine, i.e. at least about 4% by weight of bromate ion. Advantageously, the bromate component of the acid concentrate comprises from about 5% to about 8% by weight of the bromate ion, which is approximately equivalent to from about 20 to about 30% by weight of molecular bromine. To obtain a constituent bromate solution having such a concentration of equivalent molecular bromine and satisfying the stoichiometric requirements defined by equation (10), the initial concentration of the caustic solution is preferably in the range of 10 to 20% by weight in the case of sodium hydroxide. Equivalent molar ratios can be calculated for other alkali metals
Alternatively, the bromate constituent solution can be prepared by dissolving an alkali metal bromate or an alkaline earth metal bromate salt in water. In fact, this is an advantageous method of preparing a constituent solution containing alkaline earth metal bromate, since difficulties may arise in preparing such a solution by adding molecular bromine to a solution or suspension of lime or magnesium. In this alternative method of preparing the constituent solution, alkali or alkaline earth metal bromide is also introduced in such a way as to obtain a general composition substantially equivalent to that obtained by dissolving Br ₂ in a caustic solution.

 In the preparation of the starting composition in accordance with the invention, the perbromide solution and the bromate solution are mixed in ratios from about 4 parts by weight of the perbromide solution to 1 part by weight of the bromate solution to about 4 parts by weight of the bromate solution per 1 part by weight of the perbromide solution. Mostly the two components of the solution are mixed in approximately equal proportions. Whatever ratios are used, the pH of the resulting composition should be from about 6.5 to 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 approximately 0.05 to 0.8. If the bromide ion was completely saturated with bromine during the preparation of the perbromide constituent solution, then the molar concentration of the bromide ion in the initial composition in accordance with the invention is equal to the sum of the molar concentration of molecular bromine and the five-fold molar concentration of bromate ion.

Эквивалентное содержание молекулярного брома в концентрате находится между примерно 10% и примерно 40% преимущественно между примерно 20% и примерно 40% по массе. Более преимущественное эквивалентное содержание молекулярного брома составляет по меньшей мере около 25% по массе. При использовании высококонцентрированных описанных выше составляющих растворов можно приготовить концентрат, содержащий 34% по массе или более эквивалентного молекулярного брома.In the concentrate in accordance with the invention, the concentration of bromate ion is at least about 2%, usually ranging from about 2% by weight 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 the bromide ion (calculated based on the absence of dissociation of the perbromide ion) is usually in the range of from about 3 to about 19%, and the preferred compositions usually contain high concentrations of bromide ion in the range from about 6 to about 17%
As for the leaching potential of the compositions that can be obtained from the concentrate, the most important parameter of the concentrate is its equivalent molecular bromine content. This is the fraction of molecular bromine that can actually be obtained by acidifying the starting concentrate in the preparation of the leaching agent. The equivalent concentration of molecular bromine in the concentrate is defined in molar units as the sum of the actual molar concentration of molecular bromine, the molar concentration of the perbromide ion and the triple molar concentration of the bromate ion. The equivalent bromine concentration further includes the molar concentration of the hypobromite ion and hypobromic acid, which is expressed by the equilibrium:

The equivalent molecular bromine content of the concentrate is between about 10% and about 40%, preferably between about 20% and about 40% by weight. A more advantageous equivalent molecular bromine content is at least about 25% by weight. Using the highly concentrated constituent solutions described above, a concentrate containing 34% by mass or more of equivalent molecular bromine can be prepared.

In the desired pH range from about 6.5 to about 7.5, the contents of molecular bromine in the concentrate usually do not turn into bromate and bromide, i.e., equation (10) does not shift to the left. As a result, there is a stable equilibrium between the perbromide ion and Br ₂ , and the composition of the concentrate is stable within the limits discussed above.

It was found that despite the high content of equivalent molecular bromine, including significant proportions of Br ₂ and Br $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ the vapor pressure of the concentrate in accordance with the present invention is extremely low. For example, a concentrate containing about 34% by weight of equivalent bromine exhibits a total vapor pressure of only 23 mmHg. at 0 ^o C and a total vapor pressure of only 112.5 mm Hg at 35 ^o C. For comparison, the vapor pressure of liquid bromine is 75 mm RT.article. at 0 ^o C and 357.5 mm Hg at 35 ^o C, and for sodium perbromide 44 mm Hg at 0 ^o C and 214 mm Hg at 35 ^o C. Thus, the concentrate corresponding to the present invention can be used in the preparation of a leach solution for noble metals with greater convenience, reliability and safety than liquid bromine or sodium perbromide.

Effective aqueous bromine leaching solutions for the recovery of precious metals can be prepared by diluting the concentrate of the present invention. Before or after dilution, the pH can be adjusted by adding either a mineral acid or a source thereof, for example HBr, HCl or Cl ₂ , or a base such as NaOH or KOH. If the concentrate is acidified, it is preferable to add HBr than HCl. The leach solution is effective over a wide pH range, but the process is preferably carried out in the range of from about 2 to about 10, even more preferably in the range of from about 5 to about 7.5. Typically, acidic pH values are preferred to convert the bromate ion to molecular bromine. The acidification is carried out before dilution, thus obtaining a more acidic concentrate having a pH in the range of from about 0.25 to about 2.5 and an equivalent molecular bromine concentration in the range of from about 28 to about 40% by weight.

In connection with the dilution, it should be mentioned that it is advisable to introduce part of NaBr or another halide salt into the solution. The Eh / pH diagrams constructed on the basis of thermodynamic data show a progressive larger solubility field at lower values of Eh during the formation of the AuBr vector $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ (see equations 13 and 14), as the concentration of Br ^- ions increases from 10 ^-5 to 1.0 M. Water and, if desired, the halide salt are added in such relative amounts that the equivalent molecular bromine content in the leach the solution ranged from about 0.01 to about 20% by weight, the concentration of bromide ion from about 0.005% to about 20% by weight and the total concentration of halide ion from about 0.005 to about 30% by weight. In practice, in most cases, in particular when extracting precious metals from ores, the leaching solution should contain from about 0.01 to about 1% by weight and, preferably, from about 0.02 to about 0.5% by weight, equivalent molecular bromine, from about 0.005 to about 10%, preferably from about 0.01 to about 1%, by weight of bromide ion, and from about 0.005 to about 15%, mainly from about 0.01 to about 1.5% by weight of total halide and she. However, for some purposes, for example, to extract metallic gold from electronic printed circuit boards or from jewelry scrap, a more concentrated leach solution can be used. It can be prepared from the concentrates described above by moderate dilution with water. In some cases, you can even directly use the concentrate to dissolve metallic gold or silver.

 The rate of dissolution of the noble metal in the leach solution in some cases increases if the concentration of halide ions in the solution is higher than that provided by the bromine-rich concentrate. For this, the process of preparing the leach solution includes the introduction of a chloride salt or bromide salt from an ion source than a concentrate. The actual content of molecular bromine and the final content of bromide ion are affected by equilibrium shifts accompanying the process of acidification and dilution. Therefore, the above reactions according to equations (8) and (9) are shifted to the left, causing the transformation of the perbromide ion into bromide and into molecular bromine; the reaction according to equation (10) also shifts to the left, causing the transformation of the bromate ion and bromide ion into molecular bromine. The dilution tends to shift the reaction according to equation (11) to the right, which leads to the conversion of molecular bromine into bromide ion and hypobromic acid. The final result is that a significant component of the equivalent molecular bromine content in the leach solution is the concentration of hypochlorous acid.

When the initial concentrate or leach solution is acidified by the addition of Cl ₂ , then not only the bromate content, but also the bromide ion is converted into molecular bromine. This can further increase the oxidizing and complexing power of the leach solution to leach gold or silver from the source material.

В зависимости от природы руды относительные соотношения руды (или другого материала-источника) и выщелачивающего агента может быть таким, чтобы выщелачивающая суспензия содержала от примерно 0,454 до примерно 45,36 кг активного агента на тонну источника. Активный агент в этом случае определяется как сумма количеств бромида металла, бромата металла, пербромида металла, гипобромитной соли, бромноватистой кислоты металла и молекулярного брома в выщелачивающем растворе.Gold and silver are recovered from a source, such as ground gold ore, by contacting the source material with an aqueous bromine leach solution. In the case of gold, its oxidation and complexation proceed in accordance with the equations
2Au + 3Br ₂ + 2Br ^- _ → 2AuBr $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ (13)
or

Depending on the nature of the ore, the relative ratios of ore (or other source material) and leach agent may be such that the leach suspension contains from about 0.454 to about 45.36 kg of active agent per ton of source. The active agent in this case is defined as the sum of the amounts of metal bromide, metal bromate, metal perbromide, hypobromite salt, hypobromic acid metal and molecular bromine in a leach solution.

If the source material is refractory ore, it may be necessary to pretreat it to remove sulfur and carbon materials. This can be achieved by known methods, such as calcination or oxidation under pressure. Calcination may be a sufficient pretreatment if it is carried out at a temperature of at least about 500 ^° C. The leaching composition and method of the present invention can also be successfully used to extract gold from non-refractory ores of high fractions, low-fraction refractory and pure ores, scrap electronic components jewelry scrap and similar low-grade and pure ores. The composition and method can be used to extract silver from various sources, including film.

 The ore suspension in the leach solution is predominantly mixed in order to facilitate the transition of the noble metal from the ore particles to the aqueous phase. Thus, a leach product is obtained containing gold or silver in complex with bromide ions. Leaching can be carried out at room temperature. Advantageously, the contact is continued for 2 to 6 hours to achieve the maximum transition of gold or silver from the ore.

 After processing the source of the precious metal with a leach solution, the leach product is separated from the leached ore, for example, by filtration. The filtrate (leachate) is washed with an aqueous washing medium, the used washing solution is combined with the filtrate, and the combined filtrate and washing solution are treated to recover the noble metal. It is advisable, especially in the case of silver, to wash the filter cake with a concentrated acid solution, such as HBr, but preferably HCl. With this washing of the precipitate on the filter, further amounts of metal from the precipitate can be leached effectively. An HCl solution is particularly effective for this purpose.

 Gold can be recovered from the combined filtrate and wash solution by conventional methods, such as zinc or aluminum precipitation, ion-exchange method or electrodeposition. Similarly, the leach solution in accordance with the present invention can be used for silver recovery.

 The following examples illustrate the present invention.

Example 1
A solution was prepared by dissolving sodium bromide (27.7 g) in water (29 g). A sodium perbromide solution was prepared by adding liquid bromine in an amount (43.0 g) sufficient to saturate the bromide ion in the solution, i.e. stoichiometrically equivalent to the initial bromide ion content. The resulting sodium perbromide constituent solution contained 43% equivalent molecular bromine.

 A sodium hydroxide solution was prepared containing 16.7% by weight of sodium hydroxide. To this solution (75.0 g) was added liquid bromine (25.0 g) to obtain a composition containing 6.7% by weight of bromate ion (7.9% by weight as sodium bromate; 25% by weight of equivalent molecular bromine ) An acid concentrate was prepared by mixing in equal proportions by weight the perbromide and bromate constituent solutions. The concentrate thus prepared contained (in mass fractions) 31.82% sodium perbromide, 2.14% bromine, 14.80% sodium bromide, 3.94% sodium bromate and 47.30% water. It had an equivalent molecular bromine concentration of 34% by weight.

The bromine concentrations in both the initial concentrate and the constituent solution of sodium perbromide were confirmed by adding excess potassium iodide to the corresponding solutions and then titrating the iodine released with sodium thiosulfate using starch as an indicator. Titration of the total equivalent molecular bromine content in the concentrate was performed by adding concentrated mineral acid to convert the bromate content to Br ₂ . The concentrate was also titrated without adding acid to determine the actual concentration of bromine, expressed as molecular bromine and perbromide ion. This titration showed the presence of 21.5% bromine in the concentrate.

Using the isoteniscopic method, the total vapor pressure was measured as a function of temperature for liquid Br ₂ , for the constituent solution of sodium perbromide in the present example, and for the starting concentrate of the present example. Based on the data obtained, the corresponding values of the enthalpy of evaporation were calculated. The results of these measurements and calculations are given in table. 1.

Example 2
The constituent solutions of sodium perbromil and sodium bromate were prepared as described in Example 1. A number of concentrates were prepared using various ratios of constituent solutions. The composition of the obtained concentrates is shown in table. 2.

Example 3
In order to compare the vapor pressure of the solutions prepared in accordance with the present invention with the already known aqueous solutions based on bromine, a solution was prepared in accordance with a method comparable to the method described in US patent 4190489, Bahl and others and the composition of the present according to the invention, the equivalent bromine content in each of them was 34%. To prepare the composition in accordance with the aforementioned US patent, 26 g of KBr was dissolved in 40 g of water, and then 34 g of Br _{2 was} added to the resulting solution. To prepare the composition of the present invention, 14.26 g of NaBr, 45.49 g of H ₂ O, 6.25 g of NaOH and 34 g of Br _{2 were} mixed. The bromate content and vapor pressure were determined as follows: titration with K1 thiosulfate using a weak acid, shows the actual content of Br ₂ (Br ₂ + Br $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ ), while titration with K1 thiosulfate using a strong acid results in the conversion of bromate to bromine and shows the total concentration of bromate and bromine. Therefore, the bromate content in two solutions was determined by titration with K1 thiosulfate, first using acetic acid, in order to determine the actual concentration of bromine, and then by titration using H ₂ SO ₄ , in order to determine the total content of equivalent molecular bromine ( Br ₂ + Br $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ + BrO $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ ) and subtracting the difference. The vapor pressure of the solution at 25 ^o C was determined using the isoteniscopic method.

Example 4
From the concentrate described in Example 1, a leach solution was prepared and used in leaching tests to extract gold and silver from a concentrate of refractory smelting ore, which initially contained 12.5% by weight of carbon and 15.5% by weight of sulfur. An analysis of this ore by calcination showed the presence of 219 902 g of gold per 1 ton of ore and 198.751 g of silver per 1 ton of ore. A similar calcination analysis showed the presence of 205.594 g of gold per ton of ore and 181.333 g of silver per ton of ore. Due to the high concentration of carbon and sulfur in this ore, it became necessary to pre-treat the ore before leaching. Among the commonly used methods for the oxidation of carbon and sulfur in carbonaceous and refractory ores before the extraction of precious metals from them by leaching, oxidation under pressure and firing. In this case, the ore was fired.

During the firing process, ore concentrate (451 g) was loaded into a batch quartz furnace with a diameter of 100 mm. This furnace was placed in an electric clamshell furnace, locked at the ends by a rotary device, and rotated at a frequency of about 5 rpm. Oxygen was passed through the kiln as its contents were heated to a temperature in the range of 600-707 ^° C, an average of about 650 ^° C. The temperature was controlled by supplying electricity to heat and opening the furnace to the environment for cooling. After heating the ore in the presence of an oxygen stream for 120 min, the kiln was cooled and samples of calcined products were taken for analysis. During firing, a 23% weight reduction occurred. In the fired product, the total sulfur content was 3% sulfate 8.5% indicating that the residual sulfide level was 0.17%
A series of leaching tests were carried out, during which gold and silver were recovered from the calcined product using an aqueous bromine leaching agent. To prepare the leaching agent, part of the concentrate from Example 1 (1.4 g) and 48% hydrobromic acid (0.8 g) were introduced into a small bottle with a cap. The contents were mixed well to ensure conversion of sodium bromate to bromine. The mixture was then poured into a 100 ml flask containing sodium bromide (1 g) and water was added to the mark, i.e. The total solution volume was adjusted to 100 ml.

 A sample of the calcined product (22.75 g, one analysis per tonne equivalent of dried unfired concentrate) and an aqueous bromine leach solution from a volumetric flask were placed in an Erlenmeyer flask with a capacity of 250 ml, equipped with a cap. The resulting suspension was stirred with an automatic stirrer for a predetermined period of time at room temperature. Separate experiments were performed in which mixing was completed after 4, 8, 12 and 24 hours, respectively. After the completion of the mixing cycle, the suspension was filtered, and the residue was washed with hydrochloric acid with a molar concentration of 4 mol / L. The first filtrate and wash solution were combined and analyzed for gold and silver. The results are presented in table. 3. The percent extraction data in this table are based on the concentrations of gold and silver in the solution and the average sample values according to Haren and Chemex.

Example 5
To determine the effect of pH on the leaching of gold from a refractory concentrate, a series of leaching experiments were carried out, such as those described in Example 3, but at different pH values. After mixing the calcined product with a bromine aqueous leach reagent, the pH of the suspension was adjusted to a predetermined value in the range from 2 to 10 by the addition of hydrobromic acid or sodium hydroxide. After adjusting the pH, the suspension was stirred for 4 hours and filtered. The filter cake was washed and the gold content of the combined filtrate and wash solution was determined. The experimental results for this example are shown in table. 5. These data show that the starting concentrate according to the present invention can be diluted to produce leach solutions that are effective leach agents for leaching gold at various pH levels.

Example 6
In order to optimize the concentration of active agents necessary for leaching from a refractory concentrate, a series of leaching experiments were carried out under conditions comparable to the conditions of example 4, but with different dilutions of the concentrate. In the experiments of this example, pH adjustment was not carried out. A suspension of the calcined product and the aqueous bromine reagent was stirred for 4 hours and filtered. The filter cake was washed and the gold content of the combined filtrate and wash solution was determined. The experimental results for this example are shown in table. 6, and each result given in this table is the average of three experiments. It should be noted that to leach approximately 217.7 g (7 ounces) of gold from 1 ton of refractory ore concentrate, 19.96 kg of acid concentrate (44 pounds) or 6.8 kg of equivalent bromine was found to be necessary.

Example 7
To study the effect of NaBr concentration on the leaching of gold and silver from a refractory concentrate, a series of leaching experiments were carried out. The experiments were similar to those carried out in Example 4, but the NaBr concentration was changed. An aqueous bromine leach solution contained 1% by weight of the concentrate from Example 1 and various amounts of NaBr. The residue was washed with water rather than hydrochloric acid with a molar concentration of 4 mol / L. The results are presented in table. 7. Considering the sample by calcining pure ore, the gold recovery ranged from 96 to 100% and the silver recovery was 2–11%. It is interesting to note that the addition of NaBr does not affect the gold recovery, while silver recovery depends on the concentration of NaBr. Comparing silver recovery in these leaching experiments with the data from Example 4, it can be concluded that washing the residue with hydrochloric acid at a molar concentration of 4 mol / L definitely improves silver recovery without any effect on gold recovery. Residues in the leaching experiments (Table 7) were analyzed by calcination. In the table. Figure 8 shows the metallurgical balance and the estimated percentage of solubilized gold, based on the calculated net ore.

Example 8
A series of leaching experiments were conducted in which gold and silver were recovered from low fraction pure ore using an aqueous bromine leaching agent. The process was carried out in accordance with Example 4. The leach solution contained 0.5% by weight of the concentrate from Example 1 and 1% by weight of NaBr. The residue was washed with hydrochloric acid with a molar concentration of 4 mol / L. The first filtrate and wash solution were analyzed for gold and silver to obtain data on solubilized metals. The results are presented in table. 9. If we consider the sample calcination of pure ore (Hazen) as the basis, the gold recovery was 100%. The silver recovery ranged from 50 to 100%.
Example 9
In a subsequent series of leaching experiments using the concentrate of Example 1, the concentration of the concentrate in the leach solution varied from 2.0 to 6.0 g / L. These experiments show that the degree of gold recovery is maximum at a concentration of 4.0 g / l.

 In further experiments, the leaching time was 2, 4, 6, 12, 18 and 24 hours. As a result of these experiments, it was found that more than 98% of all leached gold was dissolved within 2 hours. Based on the results of recent experiments, the leaching time in subsequent experiments was 6 hours

Confirming experiments were carried out using two different firing products, which were obtained by calcining samples of Canadian flotation concentrate at 650,750 ^o C. These experiments were carried out under the following conditions, which were found optimal: at a concentration of 4 g / l, pH 5.0 6.0 and leaching for 6 hours. In experiments using the first calcination product, the gold content in the residue was from 0.592 to 0.650 ounces per ton, the extracted gold content was from 94.2 to 94.5% and the rich Au concentrate was from 9.51 up to 9.96 ounces per ton. In experiments using the second calcining product, the corresponding values were from 0.714 to 0.768 ounces per ton of Au in the residue, the content of recovered Au ranged from 96.0 to 96.3% and the rich gold concentrate ranged from 17.29 to 17.73 ounces per ton.

Claims (2)

 1. An aqueous solution for leaching precious metals containing bromide ion, molecular bromine and an alkali or alkaline earth metal ion, characterized in that the solution additionally contains perbromide ion and bromate ion, the equivalent molecular content of bromine in the solution being 10 to 40 wt. equivalent molecular content of bromate ion 2 6 wt. equivalent molecular content of perbromide ion 10 55 wt. the ratio of molar concentrations of molecular bromine to perbromide ion is 0.05 0.8, the molar concentration of bromide ion is equal to the molar concentration of molecular bromine and five-fold molar concentration of bromate ion.  2. An aqueous solution for leaching precious metals, containing molecular bromine, bromide ion and a halide ion, characterized in that the solution contains, by weight. Equivalent molecular bromine 0.01 1.0
Bromide ion 0.01 1.0
Total halide ion 0.005 15.0 s

Images