Two growing problems confront the preparation engineer—still further restrictions on stream pollution and a greater proportion of fine coal as more and more continuous miners come into use. The dewatering screens, centrifuges, and settling ponds that sufficed a few years ago must often be supplemented by more effective equipment, and in some instances the finest solids are now being recovered by vacuum filtration, once considered too costly in many coal washeries.
Slurries and Flocculants Tested: First of the slurries was the thickener feed from the washed coal section of the Michel colliery of Crow’s Nest Pass Co. Ltd., Michel, B.C.
Slurry was also obtained from the Black Diamond washery of Palmer Coking Coal Co., King County, Wash. This underflow of a ½-mm vibrating slurry screen is a waste product discharged to a settling pond. As shown by the data in Table I it contained about 20 pct material finer than 200 mesh, which analyzed 77.5 pct ash, indicating a high proportion of clay. The coarser sizes were also high in ash content because of the presence of bone and shale. Black Diamond coal is on the dividing line between bituminous and subbituminous rank.
The flocculants used were Separan 2610, Kylo 27, Jaguar MD-A, Idaho potato starch, and lime. The lime was used as a slurry, the Separan 2610 as a 0.10 pct aqueous solution, and the other flocculants as 0.50 pct aqueous solutions.
Test Procedure: Fig. 1 diagrams the filter leaf apparatus employed. This consists of a test leaf having an effective area of 0.10 sq ft., a 60°, 8½-in. diam cone to hold the slurry sample, a variable- speed electric stirrer, vacuum gage, bleed line for controlling vacuum, filtrate flask, moisture trap, and vacuum pump.
The cake rates obtained with this test procedure probably cannot be translated directly into rates for a commercial filter, but they provide useful comparative values for judging the effectiveness of flocculation.
Results of Filtration Tests: In untreated form the Crow’s Nest Pass slurry gave cake rates of 50 and 68 lb per hr per sq ft, respectively, with the fine and coarse cloths, which classes it as being filterable but providing only a moderate cake rate. Separan, Kylo, and Jaguar all behaved quite similarly with this slurry. All three increased the cake rate three to fourfold, were nearly as effective with the minimum flocculant concentration of 0.025 lb per ton as with the maximum, provided nearly as high cake rates with the fine cloth as with the coarse, and showed little reduction in cake rate between the first and second trials.
The amount of Crow’s Nest Pass slurry available did not permit testing lime and starch at the low concentrations employed for the other flocculants; they were used at concentrations from 0.10 to 4.0 lb per ton.
With this slurry the influence of floe strength is evident. At a concentration of 0.0125 lb per ton the first trial with Separan gave a rate of 116 lb, but in the second trial the rate fell to 5 lb, indicating that the floes were too weak to withstand agitation of slurry between runs. On the other hand, Kylo gave substantially the same results in the first and second trials, indicating a much stronger floe.