The experiments for determining the proper tonnage of feed to the tube-mills
"The experiments for determining the proper tonnage of feed to the tube -mills, were made on single mills running under actual working conditions. Special efforts were made to keep the character and dilution of feed constant throughout each series of experiments. Before beginning any test, the mills were run for 90 min. on the same tonnage of feed to be used in the test. Sampling of feed and discharge was then commenced, the sampling continuing 90 min. At the end of this period the volume of feed was changed and the mill again allowed to run 90 min. before sampling was commenced on the following test. Four times during each sampling period the feed was diverted into a suitable vessel and the flow of wet pulp measured. Knowing the dilution and the specific gravity of the pulp, it was then possible to calculate the tonnage per hour of dry feed. The samples of feed and discharge were retained for moisture and screen analysis, and, together with the tonnage, form the basis for the comparison of efficiency.
"Four series of tests were run. Table II gives the results from the first series, consisting of four tests made on coarse concentrator tailing, with no admixture of return feed. Table III shows the results of four tests made on dump sands, mixed with a small amount of return feed.
Tables IV and V give the results on tests run on a mixture of return feed with a relatively small amount of dump sand.
"In order to reduce the screen analyses and tonnages to terms of grinding work, a modification of the method suggested by Algernon Del Mar1 has been employed. Table I furnishes an example of this system. The work units in Tables II, III, IV and V were all calculated in the same manner as in Table I, though the calculations have been omitted on account of space; only the final results being given. The work units in feed and discharge are obtained by multiplying the reciprocal of the average size (in inches) of the sand grains corresponding to each mesh, by the proportion (not the percentage) of the sample remaining on each mesh. The sum of all these products is the work units in 1 ton of the pulp sampled. The difference between the work units in the discharge and the work units in the feed gives the work units performed upon 1 ton of the pulp in passing through the mill; while this difference multiplied by the tonnage per day gives the total work units performed by the mill in 1 day. The advantage of this system is that it furnishes a perfectly definite unit of grinding work; i.e., the work required to reduce a ton of inch-sized fragments of ore to ^-in. size. In Table I, the work units per ton in the feed are shown to be 212.8. This means that in order to reduce the feed to the size shown by the screen analysis, it has required 212.8 times the amount of work which would be required to reduce a ton of inch-sized fragments of the same ore to Jrfj-in. size. This system has the same disadvantage as all other systems for determining the amount of grinding work done from screen analyses; it disregards the fact that different sands vary in hardness, and it is, therefore, accurate for comparing grinding efficiencies only when the material ground is of the same hardness in all cases compared.
"That there are pronounced variations in the hardness of sands, even in the same plant, is shown by comparing Table II with Tables IV and V. At a given tonnage of feed to the mill, five times as much work is indicated in Table. II as in Tables IV and V. Still the horsepower required is practically the same in each case. The explanation of this apparent anomaly is that in the first case the feed consists of comparatively soft fragments of the original ore, while in the last two cases the feed is composed of the harder fragments of the original ore and of particles from the flint pebbles, which were too hard to be pulverized in the first passage through the mill.
"The principal lessons, however, to be gleaned from the tables and diagram, Fig. 17, are: first, that the power consumption is independent of the tonnage of sands fed to the mills; and second, that the proportion of pulverized sands in the discharge is highest for lower tonnages, and decreases as the tonnage is increased. At low tonnages (20 tons per day), coarse sands are slimed in a single passage through the mill the discharge containing more than 90 per cent, minus 200-mesh product. On the other hand, when tonnages in excess of 200 tons per day are fed to the mill, the discharge is only slightly finer than the feed. The third point is that when we consider tonnage, multiplying the tons passed through the mill by the grinding work done per ton, we see that without a single exception the total grinding work, or 'efficiency/ increases as the tonnage of feed is increased.
