Leaching experiments to study the release of trace elements from mineral separates from Chinese coals

Leaching experiments were carried out for two samples of mineral separates from Chinese coals for the purpose to simulate the release of trace elements in acid mine drainage. The leaching solution has a ferric ion concentration of 10 ^{- 3}M, pH ≈ 2. The experiments were carried out at room temperature. The grain size of the mineral separates is 75-130μm. The behavior of the release of eight trace elements (Co, Ni, Zn, Cd, Al, Cr, As, and Pb) were measured during the period of 312h. The results from four leaching tests using two samples with two solid-to-liquid ratios show that the concentrations of Co, Ni, Zn, and Cd continuously increased during the leaching period and they were correlated with the sulfate content in the same leachate. This indicates that the elements were released during the oxidation and dissolution of pyrite. The elements were easily dissolved in the leaching solution. On the other hand, the concentrations of Al, Cr, As, and Pb in the leachates varied considerably under same leaching conditions and were sensitive to the condition of the leachate during the leaching process. The release of these four elements was affected by the pH of the leachate and by absorption and co-precipitation. They are less likely to migrate. Therefore, the release and migration of the eight elements in acid coal-mine drainage are controlled mainly by three factors, the oxidation of pyrite, the pH of the leachate, and mineralogy of country rocks.