A study of fine coal dewatering by high temperature and pressure filtration

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Virginia Polytechnic Institute

The removal of water from fine coal (minus 3/8 inch, especially minus 48 mesh Tyler) has been a major problem of the coal industry. Thermal dryers have been used exclusively as a means of reducing the fine coal moisture. Because of its high capital, operating costs, and safety hazards, an alternate method of moisture reduction is highly desirable in the coal industry.

The major effort of this thesis was directed towards the development of a method to dewater fine coal by the application of high temperature and pressure to a filter slurry. This method might have the possibility of replacing conventional thermal dryers for fine coal drying. In addition, the effect of temperature, pressure, dry cycle time, slurry density, cake thickness, and evaporation on filter cake moisture were studied simultaneously.

The fine coal sample was obtained from The Itmann Coal Mine, Consolidation Coal Company. It contained 47.3 percent minus 100 mesh Tyler and 30 percent minus 200 mesh Tyler material in size. A small eight inch in diameter laboratory pressure filter with a heater and agitating device were used for this study.

Filter cake moisture decreased with increasing temperature and pressure. The moisture reduction did not vary proportionally with increasing temperature and pressure, but diminished with increasing temperature and pressure. The effect of dry cycle time on filter cake moisture was substantially reduced as the dry cycle time was increased over 90 seconds. The filter cake moisture decreased at an average of 2.5 percent when the slurry density varied from 30 to 50 percent in 10 percent increments. The filter cake moisture decreased approximately one percent for 1/4 inch decrements from 1 1/4 to 1/2 inch cake thickness. Approximately three to five percent moisture reduction was effected by evaporation, depending upon the cake temperature and ambient conditions.

Finally, results indicated that the filter cake moisture could be substantially reduced to 9.5 percent by using a temperature of 260 degrees Fahrenheit, a pressure of 60 pounds per square inch, and a cake thickness of 1/2 inch with a dry cycle time of two minutes. Evaporation resulting from the residual heat in the cake caused a further reduction in the moisture content from 9.5 percent to 5.1 percent. The highest capacity obtained from this study was 2905.66 pounds of dry coal per hour per square foot form rate.

The total heat requirement to produce one ton of dry coal containing 5.1 percent moisture was 157,950 British Thermal Units if all the heat losses except the heat in the discharged cake could be saved. This level of heat requirement is significantly less than that of conventional thermal dryers.

The proposed method of this investigation may be capable of replacing conventional thermal dryers for fine coal drying, if measures are taken to recover as much heat as possible by counter-current heating of the intake slurry with the hot filtrate. Less cost and concurrent advantages for safety and air pollution are additional advantages over conventional thermal dryers.