Method of leaching copper sulfide ores containing chalcopyrite

Abstract

An object of the present invention is to provide a method of efficiently leaching copper from copper sulfide ores containing chalcopyrite at room temperature. A method of recovering copper from copper sulfide ores, characterized by comprising: using a sulfuric acid solution having a chloride ion concentration adjusted to 6 g/L or more but less than 18 g/L and a pH adjusted to 1.6 or more but less than 2.5 as a leaching solution; and carrying out copper leaching with the addition of a chloride ion-resistant sulfur-oxidizing bacterium to the leaching solution, when recovering copper from copper sulfide ores containing chalcopyrite, is provided.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of efficiently recovering copper from copper sulfide ores and particularly from primary copper sulfide ores such as chalcopyrite.

BACKGROUND ART

[0002] Hitherto, copper metallurgy has been carried out mainly by pyrometallurgy whereby copper metal is obtained by electrolyzing crude copper that is obtained via concentrate smelting. However, in view of energy saving and influences upon the environment, hydrometallurgy (SX-EW: solvent extraction-electrowinning) not involving smelting has been rapidly adopted in recent years. Hydrometallurgy is regarded as a technology to replace pyrometallurgy. In the SX-EW method, copper is dissolved from copper ores with the use of sulfuric acid, copper is removed from the leach solution with the use of an organic solvent, and electrolytic copper is obtained via electrolysis. A typical solvent used for hydrometallurgy of copper ores is sulfuric acid. Thus, target ores used for hydrometallurgy have been limited to copper oxide ores that are readily dissolved in sulfuric acid. However, in general, there are fewer copper oxide ore reserves that can be used are than copper sulfide ore reserves. Thus, the use of copper sulfide ores with large ore reserves as target ores for hydrometallurgy has been examined.

[0003] Examples of known leaching operations for copper sulfide ores via hydrometallurgy include a leaching operation performing an agitated batch reaction with the use of sulfuric acid or hydrochloric acid and a leaching operation (heap leaching method): forming ore heaps, supplying sulfuric acid or hydrochloric acid to the tops of the ore heaps, and recovering liquid dripping therefrom due to the force of gravity. However, with such heap leaching method, leaching takes several years. In addition, leaching rate of copper is extremely low, resulting in poor efficiency. Further, a method of efficiently leaching copper with the use of microorganisms such as an iron-oxidizing bacterium and recovering copper (bacterial leaching method) has also been used. According to such bacterial leaching method, iron (II) ions in a leaching solution are oxidized into iron (III) ions that functions as powerful oxidants with the use of an iron-oxidizing bacterium. At such time, sulfur contained in ores is oxidized by iron (III) ions so that sulfuric acid is produced. Thereafter, copper in ores is eluted in the form of copper sulfate due to the presence of the produced sulfuric acid.

[0004] The aforementioned bacterial leaching method has been applied in practice with the use of secondary copper sulfide ores containing chalcocite (Cu.sub.2S), covellite (CuS), or the like, which have been found in secondary enriched zones of porphyry copper deposits. However, such technology has been developed mainly for the purpose of using primary copper sulfide ores containing copper chalcopyrite (CuFeS.sub.2), which most abundantly exist as copper resources.

[0005] However, chalcopyrite is substantially insoluble in sulfuric acid. In addition, the copper leaching rate is extremely slow. Therefore, a variety of techniques in addition to the addition of an oxidant in order to improve the leaching rate have been suggested.

[0006] For instance, high temperature-pressure treatments (JP Patent No. 3046986, JP Patent Publication (Kokai) No. 2001-515145 A, and JP Patent Publication (Kokai) No. 2003-328050 A), the maintenance of a certain redox potential (of a Ag/AgCl reference electrode) by adjusting iron content and the ratio of trivalent irons to divalent irons (JP Patent Publication (Kokai) No. 10-265864 A (1998)), the maintenance of a certain redox potential by adding activated carbon and iron to a leaching solution (JP Patent Publication (Kokai) No. 2005-15864 A), and other techniques have been reported. Although all of the aforementioned methods are effective for improving leaching rate to some extent, such methods are problematic because of the high costs of energy and reagents used. Further, at an advanced stage of a leaching reaction, the leaching rate is significantly lowered due to a leaching inhibition phenomenon caused by residual sulfur on the surface of a sulfur-containing concentrate, which is also problematic. Therefore, in practice, there is no practicable technology involving hydrometallurgy with the use of primary copper sulfide ores containing chalcopyrite.

DISCLOSURE OF THE INVENTION

[0007] In view of the aforementioned reasons, it is an objective of the present invention to provide a method of recovering copper from primary copper sulfide ores containing chalcopyrite as a main constituent under versatile conditions for real operation in an efficient and cost-effective manner.

[0008] As a result of intensive studies to achieve above objectives, the inventors of the present invention have found that the copper leaching rate can be promoted at room temperature when recovering copper from primary copper sulfide ores containing chalcopyrite via hydrometallurgy in a manner such that the chloride ion concentration of a leaching solution is adjusted and a chloride ion-resistant sulfur-oxidizing bacterium is added to the leaching solution. Also, they have found that such leaching-promoting effect is enhanced by adjusting the initial copper (II) ion concentration and the initial iron (II) ion concentration in the leaching solution. These have led to the completion of the present invention.

[0009] Specifically, the present invention encompasses the following inventions.

(1) A method of recovering copper from copper sulfide ores, characterized by comprising: using a sulfuric acid solution having a chloride ion concentration adjusted to 6 g/L or more but less than 18 g/L and a pH adjusted to 1.6 or more but less than 2.5 as a leaching solution; and carrying out copper leaching with the addition of a chloride ion-resistant sulfur-oxidizing bacterium to the leaching solution, when recovering copper from copper sulfide ores containing chalcopyrite.

(2) The method according to (1), wherein the initial copper (II) ion concentration and the initial iron (II) ion concentration are adjusted to 0.5 g/L or more but less than 5 g/L.

[0010] (3) The method according to (1) or (2), wherein the chloride ion-resistant sulfur-oxidizing bacterium is of an Acidithiobacillus sp. TTH-19A strain (NITE P-164).

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a graph showing time course changes in the bacterial density of a leaching solution (pH 1.8) upon copper leaching with the use of leaching solutions of Examples 1 and 2 and Comparative Examples 1 to 4 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: chloride ion concentration=6 g/L; Example 2: chloride ion concentration=12 g/L; Comparative Example 1: chloride ion concentration=0 g/L; Comparative Example 2: chloride ion concentration=3 g/L; Comparative Example 3: chloride ion concentration=18 g/L; and Comparative Example 4: chloride ion concentration=30 g/L).

[0012] FIG. 2 is a graph showing time course changes in the pH of a leaching solution (pH 1.8) upon copper leaching with the use of leaching solutions of Examples 1 and 2 and Comparative Examples 1 to 4 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: chloride ion concentration=6 g/L; Example 2: chloride ion concentration=12 g/L; Comparative Example 1: chloride ion concentration=0 g/L; Comparative Example 2: chloride ion concentration=3 g/L; Comparative Example 3: chloride ion concentration=18 g/L; and Comparative Example 4: chloride ion concentration=30 g/L).

[0013] FIG. 3 is a graph showing time course changes in the copper concentration of a leaching solution (pH 1.8) upon copper leaching with the use of leaching solutions of Examples 1 and 2 and Comparative Examples 1 to 4 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: chloride ion concentration=6 g/L; Example 2: chloride ion concentration=12 g/L; Comparative Example 1: chloride ion concentration=0 g/L; Comparative Example 2: chloride ion concentration=3 g/L; Comparative Example 3: chloride ion concentration=18 g/L; and Comparative Example 4: chloride ion concentration=30 g/L).

[0014] FIG. 4 is a graph showing time course changes in the bacterial density of a leaching solution upon copper leaching with the use of leaching solutions of Examples 1, 3, and 4 and Comparative Examples 5 and 6 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: pH=1.8; Example 3: pH=1.6; Example 4: pH=2.0; Comparative Example 5: pH=1.4; and Comparative Example 6: pH=2.5).

[0015] FIG. 5 is a graph showing time course changes in the copper concentration of a leaching solution upon copper leaching with the use of leaching solutions of Examples 1, 3, and 4 and Comparative Examples 5 and 6 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: pH=1.8; Example 3: pH=1.6; Example 4: pH=2.0; Comparative Example 5: pH=1.4; and Comparative Example 6: pH=2.5).

[0016] FIG. 6 is a graph showing time course changes in increase in copper concentration of a leaching solution upon copper leaching with the use of leaching solutions of Examples 1, 5, and 6 and Comparative Examples 7 and 8 with the addition of a chloride ion-resistant sulfur-oxidizing bacterium (1.times.10.sup.7 cells/mL) (Example 1: initial copper (II) ion concentration=0 g/L, initial iron (II) ion concentration=0 g/L; Example 5: initial copper (II) ion concentration=0.5 g/L, initial iron (II) ion concentration=0.5 g/L; Example 6: initial copper (II) ion concentration=1.0 g/L, initial iron (II) ion concentration=1.0 g/L; Comparative Example 7: initial copper (II) ion concentration=0.1 g/L, initial iron (II) ion concentration=0.1 g/L; and Comparative Example 8: initial copper (II) ion concentration=5.0 g/L, initial iron (II) ion concentration=5.0 g/L).

[0017] Hereinafter the present invention will be described in detail. The present application claims the priority of Japanese Patent Application No. 2006-204454 filed on Jul. 27, 2006 and encompasses contents described in the specification and/or drawings of the patent application.

[0018] The method of recovering copper from copper sulfide ores of the present invention characterized by comprising: using a sulfuric acid solution having a chloride ion concentration adjusted to 6 g/L or more but less than 18 g/L and a pH adjusted to 1.6 or more but less than 2.5 as a leaching solution; and carrying out copper leaching with the addition of a chloride ion-resistant sulfur-oxidizing bacterium to the leaching solution, when recovering copper from copper sulfide ores containing chalcopyrite.

[0019] Copper sulfide ores containing chalcopyrite as target ores of the present invention may be copper sulfide ores containing chalcopyrite as a main constituent or copper sulfide ores that partially contain chalcopyrite, for example. The chalcopyrite content is not particularly limited.

[0020] Further, when it comes to hydrometallurgy of copper using a sulfuric acid solution as a leaching solution, the method of the present invention can be used in any types of leaching operations. For example, not only agitated batch leaching but also heap leaching and dump leaching where copper is leached into sulfuric acid by sprinkling sulfuric acid over ore heaps may be optionally adopted.

[0021] Dissolution and leaching of copper sulfide ores proceed through a series of reactions shown in Equation 1 to Equation 3 below.

Cu.sup.2++Fe.sup.2+Cu.sup.++Fe.sup.3+ (Equation 1)

CuFeS.sub.2+Cu.sup.++Fe.sup.3+.fwdarw.Cu.sub.2S+2Fe.sup.2++S (Equation 2)

Cu.sub.2S+4Fe.sup.3+.fwdarw.2Cu.sup.2++4Fe.sup.2++S (Equation 3)

[0022] According to the method of the present invention, the equilibrium of the above Equation 1 is shifted toward the right by increasing chloride ion concentration in the leaching solution (D. M. Muir, M. D. Benari, B. W. Clare et al, Hydrometallurgy, 9, 257, 1981). As a result, the reaction in Equation 2 can be accelerated.

[0023] According to the method of the present invention, the chloride ion concentration of a leaching solution is adjusted to 6 g/L or more but less than 18 g/L. The chloride ion concentration of a leaching solution may be optionally adjusted with the addition of, for example, sodium chloride. When the chloride ion concentration of a leaching solution is less than 6 g/L, a small leaching-promoting effect is obtained. Meanwhile, when the concentration exceeds 18 g/L, growth inhibition is induced even with the use of a chloride ion-resistant sulfur-oxidizing bacterium, which is undesirable.

[0024] Elemental sulfur that is produced in the above reactions (Equations 2 and 3) and causes a coating phenomenon is removed in the reaction described below (Equation 4) with the addition of a chloride ion-resistant sulfur-oxidizing bacterium. As a result, deceleration of the leaching rate is prevented so that efficient leaching can be realized.

S+1.5O.sub.2+H.sub.2O sulfur-oxidizing bacterium.fwdarw.H.sub.2SO.sub.4 (Equation 4)

[0025] Herein, the term "chloride ion-resistant sulfur-oxidizing bacterium" indicates any sulfur-oxidizing bacterium without particular limitation as long as such sulfur-oxidizing bacterium is not inhibited in terms of growth or ability to oxidize sulfur at a high chloride ion concentration of a leaching solution (6 g/L or more but less than 18 g/L) as described above.

[0026] A preferred example of such sulfur-oxidizing bacterium that can be used is the Acidithiobacillus sp. TTH-19A strain that was deposited under accession number NITE P-164 at the Patent Microorganisms Depositary, National Institute of Technology and Evaluation (NPMD) (2-5-8 Kazusakamatari, Kisarazu-shi, Chiba), an Independent Administrative Institution, on Jan. 13, 2006.

[0027] Meanwhile, Cu.sub.2S produced above (Equation 2) is dissolved by the reaction described below (Equation 5). Thus, it is advantageous to set the pH at a low level for leaching. Although sulfuric acid produced above (Equation 4) accelerates leaching, the growth of a sulfur-oxidizing bacterium is inhibited when the pH is less than 1.6.

[0028] Further, in a case of a pH of more than 2.5, the leaching reaction is inhibited due to production of jarosite and the like. Thus, the pH of a leaching solution is preferably 1.6 or more but less than 2.5.

Cu.sub.2S+4H.sup.++O.sub.2.fwdarw.2Cu.sup.2++2H.sub.2O+S (Equation 5)

[0029] The amount of the aforementioned sulfur-oxidizing bacterium added to a leaching solution is not particularly limited. However, in general, such bacterium is added so as to result in a bacterial density of 1.times.10.sup.6 to 1.times.10.sup.7 cells/mL. It is not necessary to adjust a bacterial density that varies in a time-dependent manner to a specific level.

[0030] Furthermore, in another preferred embodiment of the method of the present invention, in addition to the adjustment of the chloride ion concentration of a leaching solution described above, the initial copper (II) ion concentration and the initial iron (II) ion concentration are adjusted. As a result, the above reaction (Equation 1) is further promoted so that it becomes possible to accelerate the leaching rate.

[0031] Preferably, the initial copper (II) ion concentration of a leaching solution is adjusted to 0.5 g/L or more but less than 5 g/L. The initial copper (II) ion concentration of a leaching solution is adjusted with the addition of, for example, copper (II) sulfate. Alternatively, when pregnant leach solution (PLS) is subjected to solvent extraction, such as via the SX-EW method for recovering copper, a raffinate obtained after solvent extraction is repeatedly used as a lixivant. Accordingly, the copper (II) ion concentration of such raffinate may be adjusted to a certain concentration. When the initial concentration is less than 0.5 g/L, a small leaching-promoting effect is obtained. On the other hand, when the initial concentration exceeds 5 g/L, the amount of reagent added is increased and the solvent extraction yield declines, resulting in undesirable poor cost-effectiveness.

[0032] In addition, preferably, the initial iron (II) ion concentration of a leaching solution is adjusted to 0.5 g/L or more but less than 5 g/L. The iron (II) ion concentration of a leaching solution may be adjusted with the addition of, for example, iron (II) sulfate. When the iron (II) ion concentration of a leaching solution is less than 0.5 g/L, a small leaching-promoting effect is obtained. On the other hand, when the concentration exceeds 5 g/L, the amount of reagent added is increased, resulting in undesirable poor cost-effectiveness.

BEST MODES FOR CARRYING OUT THE INVENTION

[0033] Hereinafter, the present invention is more specifically described by way of examples and comparative examples. However, the present invention is not limited thereto.

EXAMPLES 1 AND 2

[0034] Concentrate (mined in Candelaria) containing chalcopyrite as a main constituent was used as a target ore. The quality of the concentrate was as follows: Cu=28% by mass; Fe=28% by mass; and S=32% by mass.

[0035] Three grams of the above concentrate was mixed with 300 mL of a leaching solution (containing: ammonium sulfate=3 g/L; potassium hydrogen phosphate=0.5 g/L; magnesium sulfate heptahydrate=0.5 g/L; and potassium chloride=0.1 g/L) that had been adjusted to a pH of 1.8 with sulfuric acid and poured into a 500 mL Shaking flask. To the leaching solution in the flask, sodium chloride was added so that the chloride ion concentration became 6 g/L (Example 1) or 12 g/L (Example 2), and a chloride ion-resistant sulfur-oxidizing bacterium (Acidithiobacillus sp. TTH-19A strain: accession no. NITE P-164) was further added at a density of 1.times.10.sup.7 cells/mL. Then the flask was shaken at room temperature for a certain period of time. The bacterial density, pH, and copper concentration of the supernatant of the resulting leaching solution were measured. Then, time course changes in the measurement results were examined.

COMPARATIVE EXAMPLES 1 TO 4

[0036] Shaking leaching (Comparative Example 1) was performed at room temperature in the same manner as in Example 1, except that sodium chloride was not added to the leaching solution described in Example 1. In addition, shaking leaching was performed at room temperature in the same manner as in Example 1 except that sodium chloride was added to the leaching solution described in Example 1 so that the chloride ion concentration became 3 g/L (Comparative Example 2), 18 g/L (Comparative Example 3), or 30 g/L (Comparative Example 4).

[0037] The experimental results of Examples 1 and 2 and Comparative Examples 1 to 4 are shown in FIG. 1 (changes in bacterial density), FIG. 2 (changes in pH), and FIG. 3 (changes in copper concentration).

[0038] As shown in FIG. 1, in the cases of Examples 1 and 2 and Comparative Examples 1 and 2, bacterial growth was observed. However, in the cases of Comparative Examples 3 and 4, bacterial growth was not observed. This is because the levels of chloride ion concentrate in Comparative Examples 3 and 4 exceeded the upper level of the growth conditions for the chloride ion-resistant sulfur-oxidizing bacterium (Acidithiobacillus sp. TTH-19A strain: accession no. NITE P-164).

[0039] As shown in FIG. 2, in the cases of Comparative Examples 3 and 4 in which bacterial growth was not observed, production of sulfuric acid due to oxidation of sulfur represented by Equation 4 did not take place, resulting in increases in pH. Simultaneously, concentrate particles were floating on the surface of the leaching solution due to a coating phenomenon caused by elemental sulfur.

[0040] As shown in FIG. 3, the copper concentration of Example 1 (Day 34: 1.0 g/L) was higher than that of Comparative Example 1 (Day 30: 0.74 g/L). Thus, it was confirmed that the copper leaching rate was fast in the case of Example 1. In the case of Example 2, the copper leaching rate became faster (Day 34: 1.4 g/L).

[0041] Meanwhile, in the case of Comparative Example 2, the increase in copper leaching rate was small (Day 34: 0.84 g/L). Thus, the effect of the addition of sodium chloride was not observed.

[0042] Based on the above results, it has been found that efficient copper leaching can be carried out by adding chloride ions (6 g/L or more but less than 18 g/L) and chloride ion-resistant sulfur-oxidizing bacteria to a leaching solution at a pH of 1.8.

EXAMPLES 3 AND 4

[0043] Shaking leaching was performed at room temperature in the same manner as in Example 1 except that sulfuric acid was added to the leaching solution described in Example 1 so that the leaching solution was adjusted to a pH of 1.6 (Example 3) or a pH of 2.0 (Example 4). The bacterial density and copper concentration of the supernatant of the leaching solution were measured. Then, time course changes in measurement results were examined.

COMPARATIVE EXAMPLES 5 AND 6

[0044] Shaking leaching was performed at room temperature in the same manner as in Example 1 except that sulfuric acid was added to the leaching solution described in Example 1 so that the leaching solution was adjusted to a pH of 1.4 (Comparative Example 5) or a pH of 2.5 (Comparative Example 6). The bacterial density and copper concentration of the supernatant of the leaching solution were measured. Then, time course changes in the measurement results were examined.

[0045] The experimental results of Examples 1, 3, and 4 and Comparative Examples 5 and 6 are shown in FIG. 4 (changes in bacterial density) and FIG. 5 (changes in copper concentration).

[0046] As shown in FIG. 4, in the cases of Examples 1, 3, and 4 and Comparative Example 6, bacterial growth was observed. However, in the case of Comparative Example 5, bacterial growth was not observed. This is because the growth of the chloride ion-resistant sulfur-oxidizing bacterium (Acidithiobacillus sp. TTH-19A strain: accession no. NITE P-164) was inhibited at a low pH.

[0047] As shown in FIG. 5, the copper concentration in the case of Example 3 (Day 30: 1.2 g/L) and the copper concentration in the case of Example 4 (Day 30: 0.96 g/L) were almost equivalent to or exceeded the copper concentration in the case of Example 1 (Day 34: 1.0 g/L). However, in the case of Comparative Example 6 (Day 30: 0.82 g/L), a leaching reaction was inhibited due to production of jarosite and the like so that it was confirmed that the copper leaching rate was slow.

EXAMPLES 5 AND 6

[0048] Shaking leaching was performed at room temperature in the same manner as in Example 1 except that the initial copper (II) ion concentration and the initial iron (II) ion concentration in the leaching solution were adjusted to 0.5 g/L (Example 5) or 1.0 g/L (Example 6). An increase in the copper concentration in the supernatant of the leaching solution was measured. Then, time course changes in the measurement results were examined.

COMPARATIVE EXAMPLES 7 AND 8

[0049] Shaking leaching was performed at room temperature in the same manner as in Example 1 except that the initial copper (II) ion concentration and the initial iron (II) ion concentration in the leaching solution were adjusted to 0.1 g/L (Comparative Example 7) or 5.0 g/L (Comparative Example 8). An increase in the copper concentration in the supernatant of the leaching solution was measured. Then, time course changes in the measurement results were examined.

[0050] FIG. 6 shows changes in increases in copper concentrations in the cases of Examples 1, 5, and 6 and Comparative Examples 7 and 8. Increases in the copper concentrations in the cases of Example 5 (Day 30: 1.1 g/L) and Example 6 (Day 30: 1.2 g/L) exceeded the increase in the copper concentration in the case of Example 1 (Day 34: 1.0 g/L). However, it was impossible to confirm the effect of adjusting the initial copper (II) ion concentration and the initial iron (II) ion concentration in the leaching solution in the case of Comparative Example 7 (Day 30: 0.98 g/L). In the case of Comparative Example 8, a large increase in the copper concentration was obtained. However, when the initial copper (II) ion concentration and the initial iron (II) ion concentration in the leaching solution exceeded 5 g/L, the amount of reagent added was increased and the yield of solvent extraction declined, resulting in undesirable poor cost-effectiveness.

[0051] All publications, patents, and patent applications cited herein are incorporated herein by reference in their entirety.

INDUSTRIAL APPLICABILITY

[0052] According to the present invention, efficient leaching of copper from copper sulfide ores containing chalcopyrite can be achieved at room temperature. A high temperature-pressure treatment is not necessary for the method of the present invention. Thus, it is possible to promote a copper leaching rate by merely adjusting the chloride ion concentration of a leaching solution with sodium chloride, for example. Therefore, the method of the present invention is simple and highly cost-effective. In addition, with the addition of a chloride ion-resistant sulfur-oxidizing bacterium to a leaching solution, sulfur can be changed into sulfuric acid, such sulfur being produced as a by-product during a leaching reaction for copper sulfide ores so as to adhere to ore surfaces, causing deterioration in leaching performance. Accordingly, it becomes possible to prevent a coating phenomenon caused by sulfur by-products on ore surfaces and to perform efficient leaching of copper as a result of the consumption of the sulfuric acid produced during copper leaching.

Claims

1. A method of recovering copper from copper sulfide ores, characterized by comprising: using a sulfuric acid solution having a chloride ion concentration adjusted to 6 g/L or more but less than 18 g/L and a pH adjusted to 1.6 or more but less than 2.5 as a leaching solution; and carrying out copper leaching with the addition of a chloride ion-resistant sulfur-oxidizing bacterium to the leaching solution, when recovering copper from copper sulfide ores containing chalcopyrite.

2. The method according to claim 1, wherein the initial copper (II) ion concentration and the initial iron (II) ion concentration are adjusted to 0.5 g/L or more but less than 5 g/L.

3. The method according to claim 1 or 2, wherein the chloride ion-resistant sulfur-oxidizing bacterium is of an Acidithiobacillus sp. TTH-19A strain (NITE P-164).