Leaching of Various Metals from Coals and Coal Ashes

Abstract

Leaching of various metals from stockpile of coal or coal ash might be a potential problem for Japan which is now attempting to increase the consumption of coal as a substitutional energy source for petroleum.Leaching of some coals and ashes was carried out by using four buffer solutions with different pHs. Among various types of ashes from Taiheiyo coal, a slag which was once melted at 1620K was found to be most stable for leaching with a solution of pH5.6, followed by a raw coal and a residual char after steam gasification. From both a high temperature ash and a low temperature ash, a considerable amount of Ca and Mg was leached out at pH5.6. The extent of leachability depended on the presence of surrounding matrix and also on the porosity of the solid. Toxic elements like Be, Cd and Hg were almost absent in the leached solution. With decreasing pH, a great deal of Al and Fe appeared, whereas they were not found in a solution of pH5.6. Even from a slag, many metal ions were leached out to a solution of pH2.2. The composition of solution depended upon the type of coal used, and it could be anticipated from the elemental composition of mineral matters originally present in coal.