Coal beneficiation processes

Many of the elements that cause concern are trapped within the fly ash after coal combustion and sometimes in the bottom ash by-products. Coal beneficiation processes prior to utilization may serve as a means of reducing the levels of at least some trace elements. Elements of concern that occur at significant levels in the processing residues may give rise to waste disposal or control problems such as leaching into the natural environment via ground or surface water infiltration. 

Ash from pulverized coal combustion is a strategic material that has many critical applications from a source of aggregate to the most important source of pozzolan for addition to Portland cement concrete. Environmental control measures on the emissions of coal combustion have resulted in a loss of quality for these materials. In response we have seen the advent of beneficiation processes applying both proven and new technologies to produce high-quality consistent products from these materials. Currently we estimate that about one-fifth of all ash products marketed are processed through some form of beneficiation method. We expect that the demand for quality and consistency will continue and the relative amount of process ash products will increase in the future. 

The calorific value of coal is an important property. For example, the gross calorific value can be used to compute the total calorific content of the quantity of coal or coke represented by the sample for payment purposes. It can also be used to compute the calorific value versus sulfur content to determine whether the coal meets regulatory requirements for industrial fuels. The gross calorific value can be used to evaluate the effectiveness of beneficiation processes. Finally, the gross calorific value can be required to classify coal (ASTM D-388). 

Coal washability determination of the theoretical limits for the removal of mineral impurities from coal by beneficiation processes that rely on specific gravity separation (ASTM D-4371). 

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. 

Trace Element Removal During Physical Cleaning. Comnercial coal cleaning processes employ physical means for beneficiation and are aimed at removing ash forming minerals and sulfur, although removal of the mineral matter also results in reduced levels of some trace elements. Trace element extraction efficiencies for various physical cleaning processes have been reported. 

A combination of coal beneficiation and relatively high-temperature roasting of the char is required for production of low-sulfur char from high-sulfur coal. When an equilibrium recycle gas composition (at about 70 psi H2) is used, char must be roasted at about 1400°F for periods of about 1 hr, as in the U.S. Steel Clean Coke process. Alternatively, the use of low-sulfur coal permits production of low-sulfur char under a wider range of hydrocarbonization conditions so that higher liquid yields, for example, may be obtained. 

Coal beneficiation involves a series of steps to separate the mineral matter from the combustible portion of the coal. Current coal characterization for beneficiation is usually limited to measurements of the particle specific gravity distribution (washability). It is further assumed that the properties of the coal feed stream and related mineral matter remain constant during the separation or cleaning process, but the compositions of the streams do change. These changes are important in understanding the lack of expected separations. The effects of specific mineral constituents on different unit operations are described. Better measurement and analytical systems will permit improved control of the processes and better separations. 

The composition of any individual coal beneficiation feed particle ranges from a nearly uniform metamorphized plant component through an almost infinite mixture series with macerals-minerals to an opposite end member as a nearly uniform mineral component. The behavior of a beneficiation feed during processing is determined by... 

Mineral matter analysis can provide a more accurate and complete representation of the minerals present in bituminous coal and the effects of beneficiation processes upon them. Comparisons with conventional analytical methods are made which demonstrate some of the potential advantages of this technique in the monitoring or design of coal cleaning operations. 

Keller, Jr., D. V., "Otisca T-Process, A New Coal Beneficiation Approach for the Preparation of Coal Slurries", Coal Gasification, Liquifaction, and Conversion to Electricity Conference, University of Pittsburgh, August 1982. 

It is known that low rarrk and/or oxidized coals are not a srritable feedstock for beneficiation by the oil agglomeration method. The research carried out at the Alberta Research Council has shown, however, that bridging liqtrids, comprising mairtly bitumen and heavy refinery residues are very efficient in agglomeration of thermal bitirminorrs coals. Similar results had earher been reported in the flotation of low rank coals the process was much improved when 20% of no. 6 heavy oil was added to 2 fuel oil. 

Loosely linked with dispersion is the process of removing small particulate material from the surface of larger pieces. This has been employed in the process of coal beneficiation, where 20 tons per hour of a fine waste coal mixed with an equal amount of water can be treated. The equipment is relatively simple, involving an ultrasonically vibrating tray over which the mixture is passed, and it requires only 2 kW of ultrasonic energy. ... 

However, in certain cases, gasification can be the only way to use coal especially, if high ash contents are present. This is true for the residues of coal washing or other beneficiation processes. Then the price level is very low because there is no competing use. There can be even costs for disposal. 

Manufacture. Titanium chloride is manufactured by the chlorination of titanium compounds (1,134—138). The feedstocks usually used are mineral or synthetic mtile, beneficiated ilmenite, and leucoxenes. Because these are all oxygen-containing, it is necessary to add carbon as well as coke from either coal or fuel oil during chlorination to act as a reducing agent. The reaction is normally carried out as a continuous process in a fluid-bed reactor (139). The bed consists of a mixture of the feedstock and coke. These are fluidized by a stream of chlorine iatroduced at the base (see Fluidization). The amount of heat generated in the chlorination process depends on the relative proportions of CO2 or CO that are formed (eqs. 1 and 2), and the mechanism that... 

Flotation. The appfication of flotation (qv) to coal cleaning is a relatively new development iu the United States. In 1960, only 0.6% of the clean coal produced came from flotation. However, by 1983 flotation accounted for about 5% of the clean coal production (Table 2). Utilization of the flotation process is expected to grow rapidly because more fine size coal is produced as a result of beneficiation schemes that require significant size reduction of the taw coal prior to cleaning to enhance the fiberation of pyrite and ash minerals. 

General 1. Use cokeless iron- and steel-making processes, such as the direct reduction process, to eliminate the need to manufacture coke. 2. Use beneficiation (preferably at the coal mine) and blending processes that improve the quality of coal feed to produce coke of desired quality and reduce emissions of sulfur oxides and other pollutants. 

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