The Marathon mill
In a class by itself is the Marathon mill, which is a cylindrical tube mill using steel rods for grinding instead of balls or pebbles.
"The mill contains a charge of these rods sufficient to do grinding work, and the tube is revolved at a speed comparable with that of the tube mill. Grinding is performed by the falling of the pieces of cylindrical steel rods one upon the other, or by their attrition against each other. It is claimed that by the use of rods instead of pebbles or balls, the pulp is subjected to more grinding action, as-in contradistinction to the mill charged with pebbles, the contact is over a long line instead of being limited to a point. A test of the Marathon mill has been carried out at the concentrator of the Detroit Copper Mining Co., of Arizona, at Morenci, in which the advantages of the mill have been pointed out. The diameter of the rods used varies from 2 in. to % in., these various diameters causing them to fit compactly and thus requiring a minimum falling distance to deliver a crushing blow.
"Two tests were made of the Marathon mill, and in the first test the most favorable results were obtained with a pulp which contained 36.7 per cent, solids. In test No. 2 the amount of water and slime in the feed was reduced to a minimum by shovel-wheels to secure a less volume of pulp and a consequent slower velocity of material through the mill. The tonnage in test No. 2 was greatly increased over that in test No. 1. The feed contained an average of 63.5 per cent, solid and only 2.92 per cent. 200-mesh slime. The speed in all of the tests of the Marathon mill was 30 r.p.m. In a diameter of 3 ft. at 30 r.p.m., there is no dead zone in the periphery of the mill, while excessive slipping of rods is prevented by having the liner plates thicker at each edge than in the middle. This results in a corrugated interior surface, and the lining is said to retain these corrugations until it is worn out. The rods are said to wear evenly from end to end, do not get crosswise in the mill, and are reduced to a diameter of % in. before they begin to crush, flatten out or break into pieces. When worn to this thinness, some rods roll up and are discharged; the short-length pieces retain their positions in the charge of rods without any serious result, but may reduce crushing efficiency to some extent. It is said to be good practice to take out the entire charge of rods every 10 days, take out the small and disabled rods, and replace the loss in weight with 1^-in. rods. As soon as the weight of rods consumed per day was determined, their loss in weight was replaced daily with fresh rods. The mill must be closed down to add the daily charge of rods. During the test the Marathon mill was tilted y in. per foot, but later tests showed that it would produce equally as good results when run horizontally.
"In No. 1 test the mill crushed 236.5 tons of dry feed in 24 hr. and in test No. 2 the mill crushed 440 tons of dry feed in 24 hr. The horsepower consumed per hour in test No. 1 was 18.6 and in test No. 2, 22.5. This makes 0.5316 tons of dry feed per horsepower-hour in test No. 1 and 0.8148 tons of dry feed per horsepower-hour in test No. 2." 1
It has always been the opinion of the careful operator that mills of this character lose efficiency by uneven wearing of rods and the consumption of steel is liable to be excessive for many rods must be discarded before completely worn out.
The ball mill is by no means a new invention and some of the ideas incorporated in the latest machines were tried years ago. Both 6 by 6-ft. and 6 by 8-ft. mills were in use in the '60s, but like other inventions the time had not arrived when the object for which the machine was specially adopted was a factor in metallurgical processes and the use of alloy- steels was unknown.
It will perhaps be noted that most of the improvements in facilitating discharge and obtaining capacity pertain particularly to ball -mills partly because there is more competition in this class of machinery and partly because a ball-mill has a different function than a tube mill in the metallurgy of ores; the ball-mill is simply used for grinding while the tube-mill may be used not only for grinding but also for obtaining a partial solution of the metals contained in the ore. Where an ore is ground with cyanide solution in a tube mill, the heat generated in the mill, the thorough mixing as the pulp is tumbled over in the mill and its intimate contact with the solvent, give ideal conditions for getting the precious metals in solution and we generally expect about 75 per cent, extraction in the tube mill alone and never under 50 per cent.
