Pyrite physical removal from coal

The goal of beneficiation is to remove as much sulfur from a fuel as possible before it is ever burned. When burned, fuel with lower sulfur content will produce less sulfur dioxide. Beneficiation is usually accomplished by a physical process that separates one form of sulfur, pyritic sulfur, from coal. Pyritic sulfur consists of sulfur minerals (primarily sulfides) that are not chemically bonded to coal in any way. The name is taken from the most common form of mineral sulfur usually found in coal, pyrite, or iron sulfide (FeS2). 

Growing concern over environmental effects of acid rain has resulted in increased Interest in development of pre-combustion removal of sulfur from coal. Physical coal cleaning processes are effective for pyritic sulfur removal but do little to reduce the organic sulfur content of coal This paper reports the removal of organic sulfur from coal, employing ethyl or methyl alcohols as the solvent/ reactant. The process is based on the observation that, under supercritical conditions, reactions occur that selectively remove organic sulfur from the coal matrix. 

This classification is useful in that the removal of minerals from coal in preparation plants is strongly influenced by the mineral s physical mode of occurrence. Fine-grained quartz, clay, and pyrite disseminated within macerals are least susceptible to removal by physical cleaning methods whereas rock fragments and minerals in layered, nodular, and fissure modes break free and are more easily removed. 

Precombustion cleaning involves the removal of any, or at least of a part, of pollution-generating impnrities from coal by physical, chemical, or biological means. A substantial amount of the coal nsed in ntility boilers does receive some form of cleaning before it is burned. The major objective of many of the precombustion cleaning processes is the reduction of the sulfur content (usually pyritic, FeS2, sulfur). The wider use of conventional coal-cleaning processes will allow the sulfur dioxide emissions to be reduced markedly. 

Physical cleaning can remove only matter that is physically distinct from the coal, such as small dirt particles, rocks, and pyrite. Physical cleaning methods cannot remove sulfur that is chemically combined with the coal (organic sulfur), nor can they remove nitrogen from the coal. Currently, physical cleaning can remove 30%-50% of the pyritic sulfur and about 60% of the ash-forming minerals in coal. 

A number of processes are being used to remove sulfur and sulfur oxides from fuel before combustion and from stack gas after combustion. Most of these efforts concentrate on coal, since it is the major source of sulfur oxides pollution. Physical separation techniques can be used to remove discrete particles of pyritic sulfur from coal. Chemical methods can also be employed for removal of sulfur from coal. 

Nitrogen, unlike pyritic sulfur, is mosdy chemically bound in organic molecules in the coal and therefore not removable by physical cleaning methods. The nitrogen content in most U.S. coals ranges from 0.5—2.0 wt %. 

The full potential for removing pyritic sulfur from various coals by physical coal cleaning is significant but difficult to achieve. However, SO2 control by precombustion removal of pyrite could be an important S02-emissions reduction strategy. The cleaned coal produced could be used in coal-fired utilities, constructed both pre-and post-NSPS, as well as in industrial boilers. To realize the potential for coal cleaning in actual practice, however, new techniques must be demonstrated in the laboratory and then at the "proof-of-concept" scale (approximately one ton of coal per hour). These new coal beneficiation techniques could be advanced physical-coal-cleaning (PCC) processes, or they could employ microbial desulfurization or chemical desulfurization to remove organic sulfur. These latter processes could be used by themselves or in concert with PCC processes. 

Advanced physical cleaning techniques are expected to be more effective than older techniques (Feibus et al., 1986). And increased efficiency can be achieved by grinding the coal to a much smaller size consistency whereupon the coal will release more of the pyrite and other mineral matter. Thermal treatment can be used to reduce moisture and modify surface characteristics to prevent reabsorption. New coal-cleaning processes can remove more than 90% of the pyritic sulfur and undesirable minerals from the coal. 

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