dumped until arsenic compounds became popular as insecticides later in the century. In the 21st century, the use of gangue has been considered from an environmental point of view. For example, in 2002, about 130 million tons of gangue were produced per year from coal mining in China. This, mixed with the 60 million tons of coal mud also produced, could be used for power generation; and the coal mining gangue could be combined with coal fly ash to produce building materials. The estimated cost of such projects would be up to 4 billion yuan, but would be expected to save just over 4 million tons of standard coal per year. See also References An Elementary Outline Of Mechanical Processes, by G. W. Danforth. 1912; 49. Elimination Of Gangue History of mining Ore deposits