In this study, the influence of clay particles on bioleaching process of Sarcheshmeh Copper Mine in the south of Iran was investigated. Shake flasks experiment was carried out in a completely randomized design with six treatments including washed (T1), sieved (T2), and untreated (T3) of low-grade ore particles b2 mm, all inoculated with a mixture of sulfur and iron oxidizing bacteria and non-inoculated treatments (T4, T5 and T6) with the same ore particle sizes and three replications. The results showed that bacteria number in T3 treatment decreased significantly compared to T1 and T2 treatments. oxidation–reduction potential (ORP) decreased in T3 and T6 treatments due to the presence of clay particles. The highest concentration of extracted copper (50 mg/L) was obtained in the inoculated washed and sieved treatments (T1 and T2) and the least copper concentrations was measured in those treatments with high level of clay particles (T3 and T6). It was concluded that copper extractionwas increased significantly by removing the clay particles fromlow-grade ore and thiswas pronounced when clay particles were washed by water.

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