Owing primarily to environmental legal requirements, a copious coal resource of the United States of America is not being used to provide the share of the Nation's energy supply that it could provide. Much of the available coal contains sulfur, from 2-6% by weight, levels which have by law been declared intolerable. Many efforts have been made to find ways to remove the sulfur content, or at least to reduce it to an acceptable level but, so far, it has not been done. The problem is described in a paper by Sabri Ergun and Ernest H. Bean entitled "Magnetic Separation of Pyrite from Coals", published by the Bureau of Mines (1968), U.S. Department of the Interior, Report of Investigations 7181. The authors propose certain approaches employing dielectric heating of coals at selected frequencies to enhance the paramagnetism of pyrite by selectively heating the pyrite to transform some of it into pyrrhotite, which has nearly 1,000 times the magnetic susceptibility of pyrite. The authors state (at page 23) "In this type of heating, pyrite need not be crushed to be reactive; indeed, the opposite is true, that is, the coarser the pyrite, the more readily it will be heated. Crushing process necessary to liberate pyrite can be done after dielectric heating". However, this does not address the treatment of those coal types in which the pyrite exists in particle sizes smaller than, for example, 50 micrometers, and in some cases as small as 10 micrometers.
In a more recent paper entitled "Significance of Colloidal Pyrite Distribution for Improving Sulfur Determinations in Coal" by R. T. Greer, Department of Engineering Science and Mechanics and Engineering Research Institute, Iowa State University, Ames, Iowa 50011, published in Proceedings of the International Symposium of Analytical Chemistry in the Exploration, Mining and Processing of Materials, Johannesburg, Republic of South Africa, August 23-27, 1976, at pages 171-174, 1976, it is stated that pyrite is the major source of sulfur in coals, and that in order to free the sulfur-bearing phases from the organic matrix of the coal, it is important to require that the coal be pulverized to particles smaller than will pass through a standard 400 mesh seive. I have found that in many different types of coal, especially coals enclosing pyrite particles in sizes as small as or smaller than 50 micrometers, crushing or pulverizing the coal may not be sufficient to physically separate enough of the pyrite from the coal matrix to enable the sulfur content of the coal to be reduced to an acceptable level. I have found also that industrial processes and apparatus that are currently available for separating components of a mixture of particles have not reached the capability of handling coal that is pulverized to less than 200 mesh. Coal which is pulverized so fine resembles dust; it tends to form clumps after being pulverized and, if successfully de-agglomerated, it tends to form dust-like clouds in high tension separator apparatus which otherwise appears to be highly desirable for performing the end step of separating the pyrite from the coal.