AU2013100642A4 - Method of pretreating gold ore - Google Patents

Abstract

The present invention provides a method of pretreating gold ore for hydrometallurgically recovering gold from gold ore containing pyrite that enables to enhance the gold-leaching speed while the generation of sulfur dioxide is suppressed. The method of pretreating gold ore comprises a step of converting 60% or more of pyrite contained in the gold ore to pyrrhotite represented by a following formula: Fei-xS (where x=0-0.2). [1/4] [Fig. 1] 5.89.7it hokit F ES:.p c.E (D __A ____ 5crpnr 1.6 -A-1. 0 1 2 3 4 50 A60 A60 9 Lea chi ng ti me / h

Description

I SPECIFICATION [Title of the invention] Method of Pretreating Gold Ore [Technical field] [0001] The present invention relates to a method of pretreating gold ore for hydrometallurgically recovering gold from gold ore which contains pyrite. [0002] As a method for recovering gold from sulfide ore containing gold, a technique relying on the hydrometallurgical process is known. Traditionally, the leaching of gold from the sulfide ore into a solution has been conducted by using reagents such as cyanide, thiourea, thiosulfate, halogen gas or the like. Recently, a gold-leaching solution containing chloride ions, iron ions, copper ions and bromide ions is proposed as a less toxic leaching solution as described in Japanese Patent Application Publication No. 2008-106347 (Patent document 1) and Japanese Patent Application Publication No. 2009-235525 (Patent document 2). [0003] Further, as a pretreatment for facilitating the leaching of gold from sulfide ore, a method of subjecting sulfide ore to oxidizing roasting is known and, recently, a pretreatment method comprising the oxidizing roasting process combined with one or other processes has been proposed. For example, Japanese Patent Application Publication No. 2010-235999 (Patent Document 3) proposed a method of subjecting copper sulfide ore to leaching treatment at a temperature below the melting point of the sulfur, allowing the resulting fine sulfur particles and remaining non-leached sulfide particles to float up from the leached residue through utilization of the difference in their hydrophobicity from other iron oxide and gangue components, and separating iron oxide or gangue by precipitation or as ore tailings, whereby the gold contained in the residual solution is 2 concentrated. Thereafter, the condensed components containing gold is subjected to sulfur removal and then to the oxidizing roasting so as to transform the iron component into iron oxide (hematite) which, in turn, is dissolved in sulfuric acid, whereby the residue containing the concentrated gold is recovered. [0004] With respect to pyrite only, it has been known that pyrite decomposes into pyrrhotite, which is readily-soluble to acid, and sulfur. Japanese Patent Application Publication No. 2005-042155 (Patent document 4) suggests utilizing the reaction to remove pyrite from the residue obtained after leaching copper sulfide ore containing pyrite and to enrich the noble metal. [Prior art literatures] [0005] Patent document 1: Japanese Patent Application Publication No. 2008-106347 Patent document 2: Japanese Patent Application Publication No. 2009-235525 Patent document 3: Japanese Patent Application Publication No. 2010-235999 Patent document 4: Japanese Patent Application Publication No. 2005-042155 [Summary of the invention] [Problem to be solved by the invention] [0006] The method disclosed in Japanese Patent Application Publication No. 2009-235525 (Patent document 2), which does not use highly toxic cyanide, thiourea, thiosulfate, halogen gas, or the like and facilitates leaching of the gold contained in copper sulfide ore, is highly practical for leaching the gold included in the copper sulfide ore. However, when this method is applied to the pyrite ore, the gold-leaching speed is not sufficient. [0007] As such, a pretreatment which uses the oxidizing roasting by supplying oxygen as disclosed in Japanese Patent Application Publication No. 2010-235999 (Patent document 3) is considered to remove sulfur in advance and to facilitate iron leaching. [0008] However, if the method of the oxidizing roasting of sulfide ore, including the method disclosed in the Patent document 3, is adopted, the following chemical reactions, 2CuS+302--2CuO+2SO2, 4CuFeS2+1302--4CuO+8SO2+2Fe2O3 and 4FeS2+1102--2Fe2O3+8SO2 will occur predominantly, and accordingly the problem of generation of sulfur dioxide (SO 2 ), which is known as environmental contaminant, cannot be avoided. In particular, as the content of pyrite in the gold ore is higher, the amount of generation of sulfur dioxide becomes larger. Accordingly there is a problem to be solved in terms of practical use. [0009] As for pretreatment for enhancing the gold-leaching speed, it is desirable, from the aspects of the safety and the protection of the environment, to decrease the sulfur dioxide which is generated during a treatment method of ores for gold-leaching, thereby enhancing the safety and reducing the influence on the environment. It is assumed that a pretreatment applicable to gold ore containing a lot of pyrite, which has been considered difficult to put to practical use, would greatly contribute to the progress of gold mining development. [0010] The Patent document 4 is related to a process predicated on recovering noble metal with pyrometallurgy in view of the problem residing in recovering noble metal with hydrometallurgy. As such, there is no supposition that noble metal should be leached with a hydrometallurgical process (see paragraphs 0007-0008, 0078 of the patent document 4). Further, it does not suggest the effect achieved by utilizing a hydrometallurgical process at all. [0011] 4 Therefore, the present invention has been invented under the above mentioned situations, and has an object of providing a method for pretreating gold ore for hydrometallurgically recovering gold from gold ore containing pyrite, and has an object of enhancing the gold-recovering speed while the generation of sulfur dioxide is suppressed. [Means for solving the problem] [0012] The present invention, in one aspect, provides a method of pretreating gold ore for hydrometallurgically recovering gold from gold ore which contains pyrite (FeS2), the method comprising a step of converting 60% or more of pyrite contained in the gold ore to pyrrhotite represented by a following formula: Fei-xS (where x=0-0.2). [0013] In one embodiment of the method of pretreating gold ore according to the present invention, the content of the pyrite in the gold ore prior to pretreatment is 5 to 80 mass%. [0014] In another embodiment of the method of pretreating gold ore according to the present invention, S (mass %)/Au (mass ppm) in the gold ore prior to pretreatment is 1 to 20. [0015] In a further embodiment of the method of pretreating gold ore according to the present invention, the pretreatment involves heat treatment. [0016] In a further embodiment of the method of pretreating gold ore according to the present invention, the heat treatment comprises heating the gold ore to 450 0 C or more under a non-oxidative atmosphere. [0017] In a further embodiment of the method of pretreating gold ore according to the present invention, the heat treatment is carried out under the conditions of retaining the gold ore at 600-750 0 C for 5-60 minutes.

[0018] In a further embodiment of the method of pretreating gold ore according to the present invention, gaseous elemental sulfur is generated by the heat treatment and is separated from the gold ore by solid-gas separation. [Effect of the invention] [0019] By conducting the pretreatment according to the present invention on the gold ore containing the pyrite ore and then conducting a hydrometallurgical process, improved gold-recovering speed can be attained, while the generation of noxious sulfur oxide is suppressed. Especially, the improved gold leaching speed is remarkable when using a particular gold-leaching solution. In other words, the present invention provides a highly practical gold-leaching method which excels in the safety and preservation of the environment. [Brief explanation of the drawings] [0020] Fig. 1 is a graph showing the relation between the leaching time and the Au grade in the residues with respect to Example and Comparative Example. Fig. 2 is a TG/DTA curve obtained during thermal analysis under a nitrogen atmosphere for ground pyrite concentrate used in Example 1. Fig.3 is an XRD chart prior to heat treatment with respect to the pyrite ore concentrate used in Example 1. Fig.4 is an XRD chart after heat treatment with respect to the pyrite ore concentrate used in Example 1. [Mode of practicing the invention] [0021] The present invention will be explained in details in the following. [0022] 1. Pretreatment One embodiment of pretreatment of gold ore for hydrometallurgically recovering gold contained in the gold ore according to the present invention includes a step of converting 60% or more of pyrite (FeS2) contained in the gold ore to pyrrhotite represented by the following formula: Fei-.S (where x=0-0.2). [0023] (1) Gold ore The object of the present invention is gold ore containing pyrite. This is because the present invention aims at the enhancement of the leaching ratio of gold in the pyrite, which is difficult to dissolve and has a low gold leaching ratio. However, the other conditions, such as the concentration of gold in the ore, for example, are not questioned. The gold ore, which is the object of treatment, may be those having been subjected to conventional beneficiation such as floatation or gravity separation. It is also possible to grind the ore to smaller particle sizes so that the contact of gold-leaching solution with gold within the ore is facilitated. The gold concentration of the gold ore is typically in the order of 0.1-100 ppm by mass, and more typically in the order of 1-20 ppm by mass. [0024] In addition to pyrite, the gold ore may contain chalcopyrite, galena, sphalerite, arsenopyrite, antimonite, and pyrrhotite. In a typical example of the present invention, gold ore containing at least 5 mass% of pyrite, more typically at least 10 mass %, and yet more typically at least 30 mass% of pyrite, is used. In this type of gold ore, as the ratio of sulfur content to gold content (S/Au) in the ore becomes higher, it is generally difficult to efficiently recover gold. Therefore, by using such gold ore having a high concentration of pyrite, the effect of the pretreatment of the present invention is remarkably achieved. Specifically, S (mass%)/Au (mass ppm) is 1 to 20, preferably 1.5 to 20, more preferably 1.5 to 10. There is no particular upper limit to the content of the pyrite in the gold ore and 100 mass% is allowable but typically the content is at most 80 mass%. [0025] (2) Conversion to pyrrhotite In the conventional technique, the ores were subjected to oxidizing roasting under the presence of the oxygen or air, and hence the sulfur contained in the sulfide ores was combined with the oxygen, resulting in generation of sulfur oxide. In the present invention, such oxidizing roasting is not carried out substantially. Instead, according to the present invention, 60% or more of pyrite (FeS2) contained in the gold ore is converted to pyrrhotite represented by the following formula: Fei-xS (where x=0-0.2). This reaction is typically expressed by the formula FeS2->FeS+S. Pyrrhotite is a kind of iron sulfides having Fe:S=0.8 to 1:1 in the stoichiometric ratio. In the present invention, the conversion of Fe in the pyrite to pyrrhotite is judged by XRD analysis. Namely, when a peak derived from Fei-xS is recognized through XRD analysis, it is judged that the pretreated ore includes pyrrhotite and that Fe in the pyrite is converted to pyrrhotite. The degree of conversion can be assessed to some extent by whether the peak derived from FeS2 exists certainly or at the minimum. The condition for XRD analysis is set at starting (20) angle of 30, finishing (20) angle of 900, sampling width of 0.020, scanning speed of 4 0 /min, divergence slit of 10, scattering slit of 10, receiving slit of 0.3mm, divergence longitudinal limitation slit of 10mm, voltage of 40kV, and electric current of 20mA. In Examples, Rint Ultima2200 available from Rigaku Corporation (formerly Rigaku Denki) was used. The XRD result for the ore prior to conversion is compared with that after conversion. If the intensified peak corresponds to that for pyrrhotite, it is considered that pyrrhotite is observed. [0026] The conversion rate is acceptable if it is 60% or more, preferably 80% or more, yet preferably 90% or more, and yet further preferably 95% or more. In the present invention, the conversion rate is calculated according to the following formula. Conversion rate = Fe amount derived from pyrrhotite in the gold ore after pretreatment/Fe amount derived from pyrite in the gold ore prior to 5 pretreatment. The Fe amount derived from pyrrhotite in the gold ore after pretreatment is calculated according to the following procedure. 50g of the pretreated gold ore after pretreatment is subjected to leaching at 85 0 C for 180min with agitation in IL of hydrochloric acid (1.Omol/L) containing lmol/L of Fe 3 +, which is then filtered. The Fe concentration in the filtrate is determined by ICP-AES (in Example, Model:SPS4000 available from Hitachi High-Technologies Corporation (formerly S11) was used.) (Fe initially contained in the hydrochloric acid should be deducted.). The Fe concentration after deduction is assumed as all derived from pyrrhotite. Since pyrite is insoluble to hydrochloric acid, such assumption is permitted. Based on the determined Fe concentration, the amount of liquid and the amount of ore, Fe amount is calculated. Specifically, it is represented as follows: Fe amount derived from pyrrhotite = (determined Fe concentration - initial Fe concentration) (g/L) x liquid amount (IL) + ore amount (50g) The Fe amount derived from pyrite in the gold ore prior to pretreatment is calculated according to the following procedure. 0.2g of gold ore prior to pretreatment, 4g of sodium peroxide, ig of sodium carbonate are charged in a crucible made of zirconium and heated with a gas burner for alkali fusion. After the crucible is cooled with water, 30mL of 35 mass% hydrochloric acid is charged for leaching of the melt. The post-leaching solution is subjected to determination by ICP-AES (in Example, Model:SPS4000 available from Hitachi High-Technologies Corporation (formerly SII) was used.). Based on the determined Fe concentration, the amount of liquid and the amount of sample, Fe amount is calculated. Specifically, it is represented as follows: Fe amount derived from pyrite = determined Fe concentration (g/L) x liquid amount (30mL) + sample amount (0.2g). [0027] The step of conversion can be carried out by heat treatment. From the standpoint of suppressing the generation of sulfur oxides, it is preferable to conduct the conversion step in a condition that oxygen feed is suppressed (in a non-oxidative atmosphere). The condition of such suppressed oxygen feed means in the present invention that the molar ratio of oxygen/pyrite ore = 1/ 2 or less. Also, the non-oxidative atmosphere means that the molar ratio of oxygen /pyrite = 1/5 or less, preferably 1/10 or less. [0028] If the mixing of oxygen is suppressed, the amount of sulfur oxide generation is low and accordingly there is no necessity of installing a separate sulfuric acid production facility. A shower tower will be enough to remove it. If the non-oxidative atmosphere is used, even the shower tower may be dispensed with. [0029)] The gold ore after the conversion step exhibits remarkably enhanced solubility into the gold leaching solution as will be explained hereafter and the leaching speed of gold may increase by approximately ten times more than the case without the conversion step. In the pyrolysis according to the present invention, since most pyrite (FeS2) is converted to pyrrhotite (iron (II) sulfate) and not converted to hematite (Fe203), it was anticipated that the ratio of gold-leaching would not be sufficient. Therefore, it was quite amazing that such remarkable result has been attained. [0030] As the non-oxidative atmosphere for conducting the conversion step, reductive atmosphere such as ammonia, carbon monoxide and hydrogen sulfide, and inert atmosphere such as rare gas (e.g. argon or helium), nitrogen and carbon dioxide may be cited. Among them, inert atmosphere is preferable in terms of preventing unexpected reaction to occur. Alternatively, the exhausted gas used in the pyrolysis may be reused by recycling. [0031] During the conversion step, it is necessary to maintain the temperature of the gold ore at least 450 0 C, preferably at least 550 0 C and more preferably at least 650 0 C to enhance the pyrolysis of pyrotite.. Also, it is preferable to keep the retention temperature for at least 5 minutes, IU preferably for at least 15 minutes. This is to sufficiently progress the pyrolysis reaction. However, if the temperature of the gold ore is excessively high, the energy for heating the ore and the processing time become too excessive, and accordingly the retention temperature is preferably 800 0 C or less, and more preferably 750 0 C or less. Similarly, the time for maintaining the retention temperature is preferably 120 minutes or less, more preferably 60 minutes or less. [0032)] Although there is no particular restriction to the type of the heating furnace for the conversion step, a tubular furnace or a rotary kiln, for example, may be used. [0033] The elemental sulfur generated by pyrolysis of pyrite has been gasified in the high temperature furnace and accordingly the elemental sulfur can be subjected to solid/gas separation and then can be delivered together with the atmospheric gas to a venting system. However, if the elemental sulfur is sent to the venting system, the sulfur will deposit with the decreasing temperature and may cause trouble such as clogging of the gas flue. Therefore, it is desired to recover the sulfur with a wet scrubber. Alternatively, the gaseous elemental sulfur may be cooled together with the pyrrhotite generated in the conversion step. In this case, they are recovered as solids, which in turn are sent together to the gold-leaching step. The elemental sulfur is separated in the leaching step as leaching residue without interfering with the leaching of gold. In this case, this method is economical because the wet scrubber becomes unnecessary. [0034] Depending on the operational limitation, there may be a case where pyrolyzed gold ore and unpyrolyzed gold ore are comingled and subjected to the iron-leaching step and subsequent steps. However, even in such case, the gold ore which has been subjected to the pyrolysis step is contained and accordingly such embodiment, too, belongs to the technical scope of the present invention.

11 [0035] 2. Hydrometallurgical process The effect of the present invention can be exerted by recovering gold from the pretreated gold ore through hydrometallurgical process. Hydrometallurgical process includes, but not limited to, gold leaching in a cyanide bath combined with autoclave treatment and gold leaching in an acidic bath. [0036] Gold leaching using a cyanide bath generally includes reacting gold ore containing pyrite with water and oxygen in a pressure-resistant container at a high temperature under a high pressure (e.g. 200'C, 30atm) to convert iron sulfide into iron oxide, then leaching gold. The process is called "autoclave treatment" as an autoclave is used as the pressure resistant container. In case where the pretreatment is not conducted, the oxidation reaction of iron sulfide is represented by the following formula. 4FeS2+1502+8H20 -- 2Fe203 +8H2 S04 - (1) On the contrary, in case where the pretreatment is conducted, the oxidation reaction of iron sulfide is represented by the following formula. 4FeS+902 +4H 2 0 -- 2Fe2 03 +4H 2 S04 - (2) Though not intending to limit the invention by any theory, if the reaction occurs in a container, the reaction (2) is considered to more easily proceed than the reaction (1) since the reaction (2) produces less product than the reaction (1) for one equivalent of iron sulfide. [0037] Also, in the gold leaching using an acidic bath, it is generally important to bring gold locked in the iron sulfide ore into contact with a leaching solution. The pretreatment according to the present invention can bring gold in the iron sulfide ore into contact with the leaching solution in a shorter period since it can convert pyrite in the gold ore to iron sulfide soluble to acid. [0038] 12 Though the time required for the hydrometallurgical process following the pretreatment can be reduced in either hydrometallurgical process, the gold leaching using an acidic leaching solution is advantageous since it can be conducted under a mild operational condition (under atmospheric pressure, less than 100 0 C) and without toxic cyanide. The gold leaching using an acidic bath will be hereafter explained in detail. [0039] Types and steps for performing gold leaching using an acidic bath for the pretreated gold ore are not restrictive but the following gold leaching step, which includes contacting with a gold-leaching solution containing halide ions, copper ions and iron ions while supplying an oxidant, thereby to leach gold component in the gold ore, can be mentioned as a gold leaching step exhibiting a great effect. [0040] The leaching of gold proceeds as follows. The dissolved gold reacts with halide ions, particularly chloride ions or bromide ions, to form a gold halide complex, particularly chloride complex or bromide complex of gold. Though chloride ions may be singly used as the halide ions in the gold leaching solution, the combined use of chloride and bromide ions allows formation of a complex at a lower oxidation-reduction potential, thereby enhancing the leaching efficiency of gold. Further, iron ions in the form of ferric ions formed under supply of oxidant, or ferric ions from the beginning, function to oxidize the gold. The gold-leaching solution preferably contains copper ions. Although the copper ions do not directly participate in the reaction, the oxidation of the iron ions is accelerated in the presence of the copper ions. [0041] As the source of chloride ions, though there is no particular restriction, hydrogen chloride, hydrochloric acid, metal chloride and chorine gas, etc. may be cited for instance. From the aspects of economy and safety, it is preferable to feed the ions as metal chloride salt. Cited as metal chloride salts are copper chloride (cuprous chloride, cupric chloride), iron I J chloride (ferrous chloride, ferric chloride), chloride of alkaline metal (lithium, sodium. potassium, rubidium, cesium, francium), alkaline earth metal (beryllium, magnesium, calcium, strontium, barium, radium) can be cited. Sodium chloride is preferred from the standpoints of cost and easy availability. It is also preferable to use copper chloride and iron chloride because they are utilized also as sources of copper ions and iron ions. [0042] As the source of the bromide ions, although there is no particular restriction, hydrogen bromide, hydrobromic acid, metal bromide and bromine gas can be cited. As metal bromide, copper bromide (cuprous bromide and cupric bromide), iron bromide (ferrous bromide, ferric bromide), bromide of alkaline metal (lithium, sodium. potassium, rubidium, cesium and francium), bromide of alkaline earth metal (beryllium, magnesium, calcium, strontium, barium, radium), and from the economical standpoint and easy availability, sodium bromide is preferred. Also, copper bromide and iron bromide are preferred because they can be also used as sources of copper ions and iron ions. [0043] Copper ions and iron ions are usually supplied in the form of their salts, for example, halide salts. The copper ions are preferably supplied in the form of copper chloride and/or copper bromide, and the iron ions are preferably supplied in the form of iron chloride and/or iron bromide, from the standpoint that they can be also used as sources of chloride ions and/or bromide ions. As the copper chloride and iron bromide, it is preferable to use cupric chloride (CuCl2) and ferric chloride (FeCl3), respectively, but cuprous chloride (CuCl) and ferrous chloride (FeCl2) may also be used because they are respectively oxidized into cupric chloride (CuCl2) and ferric chloride (FeCl3) by supplying oxidant to the leaching solution. [0044] The concentration of the chloride ions in the gold-leaching solution used in the gold leaching step is preferably 30g/L-180g/L. The concentration of the bromide ions in the gold- leaching solution is preferably lg/L-100g/L 14 from the standpoints of the reaction rate and the solubility, and more preferably 10g/L-40g/L from the economical standpoint. And, the total concentration of the chloride ions and bromide ions is preferably 120g/L 200g/L. Also, the weight ratio of bromide ions to chloride ions in the gold leaching solution is preferably at least 1. [0045] The oxidation-reduction potential (reference electrode is Ag/AgCl electrode) of the leaching solution at the beginning of the gold leaching step (right before contacting of the ores with leaching-solution) is preferably at least 550mV, more preferably at least 600mV, from the standpoint of acceleration of the gold-leaching. Also, during gold-leaching process, it is preferred to maintain the potential at 550mV or more and more preferably at least 600 mV. Also, to promote the gold-leaching, the pH of the leaching solution is preferably maintained at 2.0 or less and preferably 1.8 or less. The temperature of the gold-leaching solution is preferably at least 45'C, and more preferably at least 60'C from the standpoint of acceleration of the gold-leaching. However, excessively high temperature will cause evaporation of the leaching solution or increase the costs for heating, and accordingly 95'C or less is preferable and 85'C or less is more preferable. [0046] Accordingly, in a preferred embodiment of the present invention, a mixed solution containing at least one of hydrochloric acid and hydrobromic acid, at least one of the cupric chloride and cupric bromide, and at least one of ferric chloride and ferric bromide may be used as the gold-leaching solution in the gold leaching step on the condition that both of chloride ions and bromide ions are contained in the leaching solution. [0047] The oxidation-reduction potential is controlled by supplying the oxidant while conducting the gold-leaching step. If the oxidant is not supplied, the oxidation-reduction potential will be decreased and thus the leaching reaction will not proceed. Though there is no particular restriction to the oxidant, oxygen, air, chlorine, bromine and hydrogen peroxide or the ID like may be cited. An oxidant having excessively high oxidation-reduction potential is not necessary and the air is sufficient. The air is preferred from the standpoint of the cost and safety. [0048] After pretreatment but before the gold-leaching step, various treatments for removing impurities in the gold ore may be performed. For example, elemental sulfur can be removed by heating the pretreated gold ore to a temperature at which the elemental sulfur is molten and then separating the elemental sulfur and gold by filtration. [0049] After the leaching of gold and the subsequent solid/liquid separation, gold can be recovered from the resulting gold solution. Although there is no particular restriction to the method for recovering the gold, adsorption on activated carbon, electrowinning, solvent extraction, reduction, cementation and ion exchange or the like may be utilized. Sulfur component may remain as sulfate, sulfide and elemental sulfur in the post gold-leaching solution but the gold leached in the solution can be separated from them by solvent extraction. [0050] Further, it is also effective to recover gold during the leaching reaction, whereby the concentration of gold in the leaching solution is lowered, and as a result the leaching ratio of gold is increased. This can be performed, for example, by introducing activated carbon with or without lead nitrate into the gold-leaching solution during the leaching reaction. [Examples] [0051] In the following, the present invention will further be specifically explained by way of working examples. It should be noted that the present invention is not restricted to the examples. The analysis of the metals used in the working examples was performed according to ICP-AES. However, the analysis of the gold used in the examples was conducted according to ICP-AES for quantitative analysis after causing deposition of gold in the 10 specimens by cupellation process (JIS M8111). [0052] [Comparative Example 1] Pyrite ore concentrate (produced in Papua New Guinea) was prepared as gold ore. The content of pyrite in this pyrite ore concentrate was determined by XRD and chemical analysis, and 17 mass% of pyrite was confirmed. In addition, the ratio of S (mass%)/Au(mass ppm) in the concentrate was 1.4. The pyrite ore concentrate was milled and ground in a ball mill to adjust the particle size to 50pm at the particle size d80, namely, the particle size at which the cumulative weight becomes 80% in the distribution curve of cumulative weight particle sizes. The d80 was the average of three measurements which were conducted using the laser diffraction particle size distribution analyzer (Shimadzu Corporation Model No. SALD2100). Subsequently, leaching operation was conducted on the ground pyrite ore concentrate (200g), using a hydrochloric acidic gold leaching solution having the composition as listed in Table 1, with pulp concentration of 100 g/L at a temperature of 85 0 C for 90 hrs. Air was blown in (0.1 L/min per IL of the concentrate) during the leaching operation with continuous agitation and the oxidation-reduction potential (ORP: vs. Ag/AgCl) was maintained at 530mV or higher. Also, during the leaching, the pH of the gold leaching solution was maintained at 1.0-1.1 by appropriately adding hydrochloric acid. [0053] 1/ [TABLE 1] Gold leaching solution FeCl 6H2O(g/L) 10 CuCl2 2H20(g/L) 48 NaCl(g/L) 25 NaBr(g/L) 103 All chloride ions(g/L) 40 All bromide ions(g/L) 80 ORP(mV) 717 (vs.Ag/AgCl) pH 1.52 [0054] During the leaching test, samples of the leaching residue were periodically taken and the Au grade was determined. Fig. 1 shows the relation between the leaching time versus Au grade in the residue obtained from the test. Refer to the plotting of FeS2 (refer to the plot of "without FeS2 pyrolysis" in Fig. 1). From this result, it is ascertained that it took 90 hours for the Au grade in the residue, which was approximately 6g/t at the start, to decrease to 0.9g/t. [0055] <Example 1> The ground pyrite ore concentrate (1.5kg) which was identical with that of Comparative Example 1 was charged in a tubular furnace and one hour was spent to raise the temperature to 700'C (the rate of temperature increase=10 0 C /min) under the nitrogen atmosphere. It was thereafter heated for one hour. After allowing it to cool to a room temperature, it was confirmed that the peak of FeS2 contained in the original ore had disappeared and a peak of FeS had been found by the XRD analysis before and after the heat treatment, the elemental sulfur resulting from the heat treatment was naturally removed from pyrite ore by solid-gas separation.

16 [0056] For the pyrite concentrates prior to and after the heat treatment, the presence of Fei-xS was investigated by XRD analysis under the condition explained earlier. For the pyrite concentrate prior to the heat treatment (see Fig. 3), there was no peak for Fei-xS but there was a peak for FeS2. In contrast, for the pyrite concentrate after the heat treatment (see Fig. 4), the peak for Fei-xS was confirmed but the peak for FeS2 was not confirmed. Accordingly, it was determined that the pyrite was converted to pyrrhotite. The conversion rate was calculated as 98% or more according to the method explained earlier. [0057] Subsequently, using a hydrochloric acidic solution of the gold leaching solution having the same composition as in Comparative Example 1, leaching was conducted on the heat-treated pyrite ore concentrate with pulp concentration of 100g/L at the solution temperature of 85 0 C for 18 hours. Air was blown during the leaching (at the rate of 0.1L/min per IL of the concentrate) while agitation was kept and the oxidation-reduction potential (ORP:vs Ag/AgCl) was maintained at least 400mV. During the leaching, hydrochloric acid was appropriately added to keep the pH of the gold-leaching solution at 1.0-1.1. [0058] During leaching test, samples of the leaching residue were periodically taken and the Au grade was determined. Fig. 1 shows the relation between the leaching time versus Au grade in the residue obtained from the test (refer to the plot of "with FeS2 pyrolysis" in Fig. 1). From this result, it is ascertained that it took only 12 hours for the Au grade in the residue, which was approximately 6g/t at the start, to decrease to 0.6g/t. Incidentally, when using a gold leaching solution with no bromide ions, roughly similar results were obtained though the Au leaching speed was lower than the case with bromide ions. [0059] <The change in Fei-xS and conversion rate caused by pyrolysis condition> 19 Using 1.5 kg of the ground pyrite ore concentrate used in Example 1, the presence of Fei-xS and conversion rate were investigated when the retention temperature and the retention time was changed as shown in Table 2. The presence of Fei-xS and the conversion rate was determined in the same procedure as in Example 1. The test was conducted using a tubular furnace under the nitrogen atmosphere. The elemental sulfur generated by pyrolysis was evaporated and purged by a nitrogen stream. The temperature was increased at a rate of 10 0 C/min for all tests. Cooling was conducted by allowing it cool to a room temperature. The results are shown in Table2. [0060] [Table 2] Heating Condition Presence of Conversion Retention Retention Fei-xS rate temp.( 0 C) time(min) (%) Before heat treatment 0 550 60 X 48 550 120 A 54 600 5 A 49 600 30 A 61 600 60 o 76 650 60 o 95 700 60 o 98 [0061] The standards for "Presence of Fei-xS" are as follows. o: Fei-xS peak is confirmed but FeS2 peak is not confirmed or is at minimum. A: Fei-xS peak and FeS2 peak are both confirmed. X: Fei-xS peak is not confirmed. [0062] From the result shown in Table 2, the presence of Fei-xS, which was ZU confirmed when heated to 550'C or more, shows pyrolysis of crystalline pyrite. It is understood that the retention temperature of 650'C or more and the retention time of 60 minutes or more is most preferable as the conversion rate exceeds 80%. [0063] <Example 2: The temperature at which pyrolysis occurs.> On the ground pyrite ore concentrate used in Example 1, the weight change and the endothermic/exothermic heat at respective temperatures were monitored, using the thermal analysis device (Model TG/DTA6300 manufactured by Seiko). The results are shown in Fig. 2. From the fact that the mass decrease begins at 450'C and simultaneously the change of calorific value is observed. It is confirmed that the pyrolysis of the pyrite begins. Under the nitrogen atmosphere, pyrolysis does not occur until the temperature reaches 450'C. It should be noted that, from the results of the XRD analysis, a long period of time is necessary for pyrolisys and accordingly heat treatment at 600'C or higher is desirable.

Claims (7)

1. A method of pretreating gold ore for hydrometallurgically recovering gold from gold ore which contains pyrite (FeS2), the method comprising a step of converting 60% or more of pyrite contained in the gold ore to pyrrhotite represented by a following formula: Fei-xS (where x=0-0.2).

2. The method of pretreating gold ore according to claim 1, wherein the content of the pyrite in the gold ore prior to pretreatment is 5 to 80 mass%.

3. The method of pretreating gold ore according to claim 1 or 2, wherein S (mass%)/Au (mass ppm) in the gold ore prior to pretreatment is 1 to 20.

4. The method of pretreating gold ore according to any of one of claims 1 to 3, wherein the pretreatment involves heat treatment.

5. The method of pretreating gold ore according to claim 4, wherein the heat treatment comprises heating the gold ore to 450'C or more under a non-oxidative atmosphere.

6. The method of pretreating gold ore according to claim 4 or 5, wherein the heat treatment is carried out under the conditions of retaining the gold ore at 600-750'C for 5-60 minutes.

7. The method of pretreating gold ore according to any one of claims 4 to 6, wherein gaseous elemental sulfur is generated by the heat treatment and is separated from the gold ore by solid-gas separation.