Removal of cyanide from the wastewater of Zarshuran gold processing plant using coal (case study)

Contamination caused by tailing dams of mineral processing plants is one of the most important problems facing the mineral industry and that causes many environmental problems. Among these, we can point out to the contamination caused by the tailing dams of gold processing plants. This wastewater contains significant amounts of cyanide and its compounds. So far, various biological and chemical methods have been investigated for eliminating cyanide in gold processing waste. One of the methods used by researchers is the use of natural absorbents such as activated charcoal, due to the ease of use and the its reusability. In this research, the adsorption of cyanide of the tailing dam wastewater of the Zarshuran gold processing plant (35 km from Takab, West Azarbaijan Province) using Shahin Dezh coal mine (about 100 km from the plant) was investigated. Cyanide adsorption experiments were performed on a laboratory scale under two modes of using crude charcoal and processed charcoal. First, the charcoal sample was poured into the cylinder up to the height of 75 mm. In the next step, the effluent containing cyanide was added to the cylinder (up to 100 mm) and the cyanide output flow rate was measured. After the experiments, charcoal samples were dried in open air, and ash percentage analysis was performed for each of them. The resultant solution of each test was filtered with Whatman paper No. 75 and analyzed for cyanide content. According to the results of the experiments, the granulation fraction of -2±1 mm had the highest output flow rate of 8.16 mL.min-1 and the lowest flow rate was related to particles less than 1 mm in size. The highest output flow rate for processed charcoal was obtained at 10.61 mL.min-1 in the granulation fraction of -2±1 mm. Based on analysis of the amount of ash, after the cyanide adsorption operation, the amount of ash in fractions of –4.75 and +4.75 ± 2 mm decreased by 0.7 and 3.7%, but, after cyanidation, the amount of fractions + 1 mm increased by 11.8%. In all experiments, cyanide adsorption using crude charcoal has the highest absorbance value for grain size in the range of -2±1 mm. The amount of cyanide adsorbed in this fraction for crude coal, processed coal and coal mixed with cyanide using a mechanical stirrer is 42.3, 31.78 and 21.88%, respectively. In this study, isotherm adsorption models of cyanide on charcoal were also studied based on Langmuir and Freundlich. The absorption process in all granulation fractions matched most closely the Freundlich model, indicating that that adsorption of cyanide follows a multi-layer adsorption onto the heterogeneous surface of the charcoal. The adsorption phenomenon occurs in different absorbent intake regions as a result of various forces, both physical and chemical. The results of this study indicate that processing the charcoal does not have any effect on the absorption of cyanide, and physical properties such as particle size and the specific surface area of coal are the most important factors in the absorption of cyanide. The results of this study indicate that the use of coal from the Gozlu mine Shahin Dezh, located a few kilometers from the tailing dam, with a grain size of -2±1 mm, in the form of crude in the bed and bottom of the dam, can significantly reduce the cyanide contamination of underground water resources.