Summary of the invention
The foregoing objects and advantages of the invention are achieved in accordance with the present invention in which a pelletized product is produced from a finely divided feed material, or a mixture of finely divided feed materials and water, by pressing the material through die openings in a commercial pelletizer such as a Kahl, California, or a Bliss pellet mill and permitting excess water to be expelled from the die during pelletizing. While the invention may be utilized in the processing of various raw materials, it has been found especially useful in producing a pelletized fuel from a finely divided material such as coal or a mixture of coal with a combustible biomass and/or waste material, and the invention will be described with specific reference to such materials, it being understood that the invention is not so limited.
The die of the improved pelletizer includes one or more water outlet openings preferably at a location, or locations, spaced upstream from the pellet discharge outlet of the die. As the materials being processed is pressed through the die, pressure builds in the material progressively from the entrance to a point adjacent the outlet. This build-up of pressure causes excess water in the material to flow through the openings in the die wall. Continued movement of the material expels the dewatered and pelletized material from the die openings in the conventional manner. In one embodiment, a pressure enhancing means is provided for increasing the pressure in the material as it passes through the dies to enhance dewatering and produce a more compact pellet.
The dimensions of the water outlet openings in the die walls are determined by the nature of the material being processed as well as the material particle size. The water outlets generally may be substantially greater than the minimum particle size of the material being processed, and where the material contains an element of fibrous material such as paper pulp sludge or the like, the openings may be greater than for material consisting essentially of solid particulates such as coal fines. The pressurization/dewatering process which enable the use of die wall opening greater than the particle size without expelling excessive particles from the openings cannot be determined for certain, but it is believed the particles tend to bridge the openings in a manner to permit the water under high pressure to be expelled without the particle entering and plugging the openings. The expelled water is collected and led from the pellet mill for discharge as waste or for further treatment.
In accordance with one embodiment of the invention, the die of a pellet mill such as a Kahl pelletizer may be made as a plurality of plates assembled together as by bolting, with the water outlets being provided by space between adjacent plates, which space can be controlled as by the surface finish of mating plate surfaces, or the use of shims, spacers, or the like between the plates.
In accordance with another embodiment, a back pressure plate is supported in closely spaced relation to the outlet of the pellet mill, with the back pressure plate being driven for rotation about the pellet mill axis at a rate different from the rate of the pressure rolls of the mill. The back pressure plate has a substantially flat surface with spaced openings extending therethrough so that, in operation, the flat surface will underlie a portion only of the die openings while the plate openings will underlie the remaining die openings. By driving the pressure rolls and back pressure plate at different speeds, a portion of the die outlets will be blocked as the pressure rolls pass thereover on each revolution, thereby increasing the pressure in the blocked dies. Continued operation will result in these die openings being unobstructed on a subsequent pass, of the pressure roll to expel the dense pellets.