Pyritic sulfur from coal, chemical removal

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. 

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). 

Chemical Removal of Pyritic Sulfur from Coal... 

A new approach for the chemical removal of pyritic sulfur from coal is described. The process is based on the discovery that aqueous ferric salts selectively oxidize the pyritic sulfur in coal to chemical forms which can be removed by vaporiza-tion, steam, or solvent extraction. Data for removal of the pyritic sulfur from four major coals (Lower Kittanning, Illu nois No. 5, Herrin No. 6 and Pittsburgh) are presented together with a discussion of the process chemistry. The effect of variables, such as coal particle size, acid and iron concern tration, reaction time, and temperature are discussed. The results show that near complete removal of pyritic sulfur can be obtained under mild conditions, resulting in a reduction of the total sulfur content of the coals from 40 to 80%, depending on the original pyritic sulfur content. 

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. 

A fascinating little company in Canada was producing pyrites to recover sulfur from coal gas. In 1882, the Vesey Chemical Company of Montreal commenced burning spent oxide from the city s gasworks into sulfur dioxide. The spent oxide was iron ore that had been used to remove hydrogen sulfide from coal gas. The plant was purchased by Nichols Chemical in 1902 and shut down. 

Analytical Procedures. The samples of bituminous coals were analyzed for pyritic sulfur, sulfate sulfur and total sulfur using conventional wet chemical procedures adapted from the American Society of Testing Materials (23, 24). The ASTM D 3177-75, The Eschka method was used for the total sulfur, and ASTM D-2492-79was used for sulfate and pyritic sulfur determinations. The treated samples were filtered through a Whatman 2 filter paper to separate the coal particles from the liquid medium. The filtered sample was washed with 0.1 N HCl followed by distilled water to remove traces of absorbed sulfate and iron. 

The concept of chemically removing pyrites from coal has not previously been advanced as a solution to the sulfur oxide air pollution problem because iron pyrites are not soluble in any known liquids. For example, the acids hydrochloric, hydrofluoric, sulfuric, or combinations of... 

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. 

Inexpensive coal or petroleum coke fuel will contain sulfur. The hydraulic cleaning processes involving pulverization to 0.1 mm particle size and flotation separation of pyrite (FeSj) particles from the coal, will remove roughly half of the initial sulfur. Sulfur chemically bound to the coal molecule will be entrained with the char to some extent. 

Biological processes are also being studied to investigate abiHty to remove sulfur species in order to remove potential contributors to acid rain (see Air pollution). These species include benzothiophene-type materials, which are the most difficult to remove chemically, as weU as pyritic material. The pyrite may be treated to enhance the abiHty of flotation processes to separate the mineral from the combustible parts of the coal. Genetic engineering (qv) techniques are being appHed to develop more effective species. 

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