A large feed has the same advantage over a small feed
"A large feed has the same advantage over a small feed that a short tube mill has over a long one; i.e., the pulp passes oftener through the classifiers, the pulverized sands being promptly removed from the circuit, and the work of the tube mill being expended more upon grinding coarse sands than upon grinding sands already sufficiently fine.
11 Extremely large tonnages, however, have two important drawbacks: first, the gain in efficiency is small; second, the proportion of coarse sands in the discharge is greatly increased. The small gain in efficiency for large tonnages is best shown by reference to the diagram : above a feed of 200 tons of dry sands per day, the curves flatten, indicating practically no gain in efficiency. The increase of coarse sands in the discharge is shown in Tables II, III, IV and V, and is also indicated in Table VI. This last table shows the increase of return feed when the number of tube mills in operation is reduced. As the tonnage is crowded into fewer mills, the amount of return feed increases out of all proportion to the tonnage fed to each mill. This excess of return feed must be elevated, concentrated, classified and returned to the tube mills. The extra cost of performing this work, when the tonnage of feed per mill exceeds 200 tons per day, is easily in excess of the slight reduction in cost due to increased grinding efficiency. Hence, an average daily feed of 200 tons of dry sands per mill has been selected as the tonnage giving best results. This 200-ton feed includes 50 tons per day of concentrator tailings ('initial feed') and 150 tons of sands returned from the classifiers after having passed one or more times through the tube mills.
"When operations were started at the Lucky Tiger plant, all five mills were run as much of the time as possible. It was soon noticed, however, that the mills grinding the return sands were receiving very little feed and one of these mills was stopped. Later, three mills were forced to do all the grinding when the feed was light, and a fourth mill run during heavy feeds. Recently (after completing the above tests), a greatly increased initial feed was crowded into three mills, the fourth mill being run only during exceptionally heavy feeds. By this means the total daily feed (initial and return feed) has been increased to 200 tons per mill, which is the tonnage shown by the tests to give maximum efficiency.
This crowding of the feed through the tube mills has resulted in practically doubling the capacity per mill. When five mills were being run, the average daily tonnage of initial sand from the concentrating plant was barely 26 tons per tube mill. Now, with three mills in operation, the average tonnage of initial sands per mill is 50 tons per day. This increase in capacity has occasioned no decrease in the grinding work done, the screen analyses of the final overflow from the classifiers still averaging 80 per cent, through 200-mesh, as it did during the time five mills were used.
"The power required to grind a ton of sands has decreased in inverse ratio as the capacity of the mills has increased. It now requires 22 hp.- hr. to grind a ton of concentrator tailings to such a fineness that 80 per cent, passes a 200-mesh screen. This is half what was required when the mills were run on a light feed.
"The decrease in pebble consumption has been even more noticeable. With a feed of 26 tons of initial sands per day, it was necessary to charge 2,860 Ib. of quarry rock and 467 Ib. of imported pebbles per day into each mill. This is at the rate of 110 Ib. of quarry rock and 17.9 Ib. of imported pebbles per ton of sands ground. When the feed per mill was increased to 41 tons of initial sands per day (80 tons per day of combined initial and return feed), the imported pebbles charged to each mill amounted to 640 Ib. per day, or 15.5 Ib. per ton of initial coarse sands. No quarry rock was used. When the tonnage was further increased to 50 tons of initial sands, or 200 tons of combined initial and return sands per day, the daily consumption of imported pebbles dropped to 410 Ib. for each mill, or 8.2 Ib. per ton of initial sands fed."
