FIELD OF THE INVENTION

The invention is related to a process for recovering precious metals from ore. The invention in more particularly relates to a method of extracting gold from gold-bearing ore.

BACKGROUND OF THE INVENTION

Various leaching processes are known for extracting precious metals (e.g., gold, platinum, silver) from minerals. In known processes, a leaching liquor dissolves gold from an ore into a stable solution phase containing gold ligand (i.e., lixiviants). The dissolved gold is collected by a recovery process, to produce solid gold metal. The most commonly used gold lixiviant is cyanide salts. Cyanidation, however, has a number of problems. For example, cyanide is an extremely toxic chemical which, if released into the environment, may cause serious environmental damage. Thiosulfate has been proposed as the most promising alternative for leaching of gold from ores. Thus, it is not surprising that numerous thiosulfate leaching studies have been published in the last decades.

It is well known that atmospheric oxygen as oxidising agent alone is not efficient enough to leach gold in the thiosulphate system because of the slow kinetics of both the gold oxidation half reaction (reaction 1 ) and the oxygen reduction on the surface of gold (reaction 2).

Au + 2S ₂ O ₃ ^2" — [Au(S ₂ O ₃ ) ₂ ] ^3" + e- E ₀ = 0.150 V (1 ) O ₂ + 2H ₂ O + 4e-— 4OH ^" E ₀ = 0.401 V (2)

Therefore, certain additives have been proposed to accelerate the overall dissolution rate of gold in thiosulfate solution. Such proposed additives are ammonia (NH ₃ ) combined with cupric ions (Cu ²⁺ ), NH ₃ combined with nickel salts, oxidised with 0 ₂ giving nickelous oxide (Ni ₃ 0 ₄ ) , ferric sulfate + Na- EDTA (sodium salt of ethylenediaminetetraacetic acid) combined with thiourea, and EDA/cupric - ethylenediamine combined with cupric ions.

One of the major drawbacks of the thiosulphate leaching is the high consumption of the thiosulphate reagent. Patent publication WO 2007/098603 discloses that the addition of EDA/cupric to the thiosulfate system can reduce the consumption of thiosulfate. The same effect can be achieved by using ferric-EDTA with thiourea in the thiosulphate system. Moreover, applying of nickel with ammonia can reduce the thiosulfate consumption also. However, there are still some crucial technical challenges to overcome, such as high concentration of toxic EDA, ammonia or thiourea in solution.

OBJECT OF THE INVENTION

The object of the present invention is to overcome disadvantages of the prior art processes and to provide a new and advantageous hydrometallurgical method for recovering gold. The method of the invention is based on a thiosulfate leaching media.

The invention provides a method where the advantages of a less toxic thiosufate method is utilised with combination of lowered thiosulfate consumption. In this invention the overall reagent consumption is lower compared to prior art methods and there is no need for high concentrations of toxic ammonia, EDA, thiourea and heavy metals.

A special advantage of the present invention is that it provides a process where gold is easy to recover from the leach solution by ion exchange. The chemistry of the process is very simple as only oxygen and/or other oxidants are used for oxidation. In addition no toxic substances report to tailings pond. SUMMARY OF THE INVENTION

The invention provides a process for extracting gold from a gold-bearing raw material, such as gold-bearing ore. The process includes, first, grinding the gold bearing raw material, second, providing a leach liquor comprising thiosulfate anions and trace amounts of thallium ions. Next, the leaching liquor is brought into contact with the gold-bearing ore in the presence of oxidant such as dispersed oxygen to form a pregnant leach solution including gold dissolved from the gold-bearing ore. Finally, at least part of the gold is recovered from the pregnant leach solution.

In another aspect of the present invention, a method of removing thallium containing reagent from the leaching liquor and recycling thallium back to the leaching step is provided. In accordance with the present invention, a thallium precursor dissolving in an aqueous media is used. Preferably organic thallium salts or thallium salts of mineral acids are used the form Tl ions or Tl complexes in the leach liquor. The thallium salt can be selected from a group of thallium salt of sulphuric acid, hydrochloride acid, nitric acid and ethylenediaminetetraacetic acid. Further, the thallium precursor added to the leaching step of the process may be formed of a mixture of two or more salts of thallium.

In the method of this invention it is essential to control and adjust the concentration of dissolved thallium ions in the leaching liquor, thus a method of recovering and recycling thallium in the leaching process is provided. Dissolved thallium may be recovered from the leaching liquor and at least partly recycled and in a controlled amount back to the leaching step. Excess of the thallium ions and compounds may be removed from the process.

LIST OF DRAWINGS

Figure 1 represents a flow diagram of the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Next the invention is described in more detail with reference to the accompanied drawing.

In the beginning of the process of the present invention, ground gold bearing raw material is added to a leaching liquor comprising thallium ions and thiosulphate salt to perform leaching in a leaching step 10. The equipment of the leaching step 10 is designed to provide the leaching liquor with an appropriate amount of oxidant. The oxidant may be dispersed air, oxygen enriched air or oxygen. The appropriate amount of the oxidant ensures proper reaction rate of the second half reaction (2). As a result of the thallium catalyzed reactions (1 ,2) gold is dissolved in the lixiviant.

The reactor in the leaching step 10 may operate in atmospheric pressure or in an elevated pressure meaning that an autoclave reactor may be applied.

The amount of the thallium compound in the leaching liquor is controlled and adjusted so as to form a desired thallium ion concentration. The dissolved Tl -ion concentration in the leaching liquor is preferably between 1 and 50 mg/l.

According to the first preferred embodiment of the invention as shown in Fig. 1 , ion exchange resin is added to the leaching step 10 to form a loaded resin which contains gold from the leach liquor. The ion exchange resin is selected from commercially available resins capable of removing gold. For example, suitable resins are strong base anion exchangers. After leaching, the leaching pulp is subjected to screening so as to separate loaded resin from the non-dissolved solids and the leach liquor.

Gold bearing resin is conducted to process step 14 to elute gold from the resin to form gold containing liquid. The dissolved liquid from the gold bearing eluant may subjected to a gold electrowinning process 13 to recover metallic gold from the liquid. The regenerated gold resin is fed back from the gold elution step 14 to the leaching stage 10. Dissolved thallium ions are recovered from the leaching pulp with another ion exchange resin in a resin-in-pulp step 1 1 after the leaching step 10. As the thallium is removed from the liquor with aid of the thallium resin, the lodaded thallium resin is separated with screening from the tailings and barren solution. The separated thallium loaded resin is fed to thallium elution step 12 for eluting thallium from the resin and to recycle recovered thallium containing solution back to the thiosulphate leaching step 10. The regenerated thallium free resin is recycled back to the resin-in-pulp step 1 1 for reuse. The concentration of thallium ions in the leaching step 10 is controlled.

The thiosulphate containing barren solution (reclaimed solution) is separated (step 15) from the tailings and recycled back to the thiosulphate leaching step 10. EXAMPLE 1

A 24-hour batch leaching test was carried out in a 3 liter agitated reactor using 0.1 mol/l Na ₂ S ₂ 0 ₃ solution with 10 mg/l Τ (as sulfate) and only atmospheric oxygen as the oxidant. The slurry density of the pulp was set to 33% in the beginning of the test. Slurry temperature was not controlled during the experiment and remained at ambient temperature. The ore sample contained 77% Si0 ₂ , 16.8% Al ₂ 0 ₃ , 2.1 % K ₂ 0, 1 .1 % FeO, and 5.9 ppm Au, determined by fire assay. Gold occured mainly as fine-grained native grains containing an average of 5% Ag. Gold was present also in tellurides mainly as calaverite AuTe ₂ , sylvanite AuAgTe ₄ and kostovite CuAuTe ₄ . The ore was mildly refractory because of the Au-tellurides. The ore sample was ground to 75% -80 μΐη. Gold was determined in the leach residue by graphite furnace atomic absorption spectrometry (GFAAS). Thiosulfate was analyzed by high performance liquid chromatography (HPLC).

The said lixiviant extracted 70% of the gold after 24 hours of leaching. The degradation rate of thiosulfate was very low, not detectable by HPLC. The typical recovery of gold using standard cyanide leaching for the same ore was slightly higher, 80%. The respective Cu ² 7EDA/thiosulphate experiment gave 60 % gold extraction, lower than with the method presented in this work.