FIGURE 1. Cumulative size distribution by weight of pyrite in experimental coals prior to pulverization tests. TABLE 1.-Source and analytical data on experimental coals TABLE 2.-Particle size and pyritic sulfur data on pulverized products Pittsburgh No. 8 (West Vir- In the pulverization experiments with the two-gun pulverizer, the coal was fed to the classifier where it flowed downward in normal fashion to the two opposing nozzles or guns and was pulverized by impaction against itself in the two opposing air streams. The riser pipes, discharging from the impact zone and normally delivering air and coal after impact back to the classifier, were for these tests disconnected from the classifier and arranged to discharge into dust-collector bags. In this manner, samples produced in these tests represented the product after a single pass of the coal through the impact zone of a two-gun mill. The amount of compressed air utilized by the jet pulverizer impact zone was approximately 200 standard cfm at approximately 100 psig and 50°F. and the coal feed rate was approximately 750 pounds per hour. In the pulverization experiments with the one-gun unit coal was accelerated in a single nozzle or gun and impacted at high velocity against and through a labyrinth target composed of alumina balls contained in a perforated basket. The material and air discharged from this special impact zone were delivered directly to dust collector bags for collection as samples representative of product from a single pass through the impact zone of a one-gun mill. The amount of compressed air utilized by the one-gun unit impact zone was approximately 56 standard cfm at 100 psig and 60°F. and the coal feed rate was 1,150 to 1,500 pounds per hour. Characterization of pulverized coal products Each pulverized coal product was sampled and screened on 16-, 30-, 60-, 100-, and 200-mesh sieves. In addition, the minus 200-mesh sieve fraction with one exception was further classified by air elutriation in a roller particle size analyzer into five additional fractions; air flow rates were chosen to yield the following sizes: 0 to 10 microns, 10 to 20 microns, 20 to 40 microns, 40 to 80 microns, and a plus 80-micron residue. The minus 200-mesh fractions from the air jet mill products have not yet been subjected to further classification in the roller apparatus. Each of the sieve fractions and the roller fractions was analyzed for pyritic sulfur and examined microscopically. The analytical data obtained on the products from pulverizations using the hammer mill and the impact mill are listed in table II. The yield and pyritic sulfur contents of the various sieve fractions of the experimental coals after pulverization in a one-gun and a two-gun air Jet mill are shown in table III. DISCUSSION Significance of analytical data on experimental coals Beneficiation of coal as mined is dependent in a large measure upon the size and distribution of the particles of mineral matter. Washability tests using float-sink techniques are widely used to determine the degree of beneficiation or cleaning that is attainable by conventional coal cleaning methods. In these tests data are obtained on the amount of material which reports to the float and to the sink fractions in liquids of various specific gravities. Thus, the average specific gravity of each free-settling particle is the determining factor in these tests. The sink fraction may contain coal pieces which have one or more grains of pyrite embedded in them. If these pyrite grains are large, the material may be returned to the crusher, pulverized further, and separated in a second operation. If these pyrite grains are small, simple crushing does not release them and high yields of sink material are obtained. In commercial practice the efficiency of cleaning is never as good as that obtainable in the laboratory and even higher yields of sink material are obtained. Pyritic sulfur in general represents some 40 to 80 percent of the total sulfur in most coals. The nature and mode of occurrence of sulfur in coal are important factors in the development of improved methods for its removal. In the present study estimates of the relative sizes of the pyrite grains present in the experimental coals have been made using micrometric methods (see fig. 1). From 70 to 98 percent of the pyrite grains are smaller than 20 microns in diameter and from 0 to 2.4 percent are 75 to 250 microns in size. On a weight basis, the pyrite grains with a diameter less than 20 microns represent only 2.5 percent or less of the total pyrite present in three of the test coals, namely, West Virginia Pittsburgh No. 8, Illinois No. 6, and Missouri Tebo. In the Ohio Meigs Creek No. 9 seam coal, this size pyrite represents 13.4 percent of the total pyrite; in the Kentucky No. 11 seam coal, it represents 42.7 percent. Thus, except for the Kentucky No. 11 seam coal, the pyrite present in particles less than 20 microns in diameter is negligible. For three of the test coals, most of the pyrite is in grains larger than 75 microns (200 mesh) in size. In the West Virginia Pittsburg No. 8 seam coal, 88.9 percent by weight of the pyrite is estimated to be present in particles 75 to 250 microns in diameter; for the Illinois No. 6 seam coal, this figure is 88.2 percent and for the Missouri Tebo seam coal, 90.8 percent. TABLE 3.-Particle size and pyritic sulfur analysis of sieve fractions produced by air jet pulverization |