THE ESSENTIAL FACTORS GOVERNING CAPACITY
The amount of ore ground to a particular mesh by a tube mill will depend upon the character of the ore, which is not pertinent to the machine itself and, to the following factors which may be varied according to the work to be done and the cost of material.
I. Diameter and length.
II. Speed of rotation.
III. Size of feed and discharge.
IV. Volume of feed.
V. Amount of moisture in pulp.
VI. Load of pebbles.
VII. Size and character of pebbles.
VIII. Character of the lining.
IX. Character of the discharge opening.
The following table by M. K. Rodgers 1 will illustrate the fact that the amount of ore ground in a tube mill differs with the character of the ore. The re<ive crushing -duty of Hardinge mills on three different ores, other conditions being the same, shows a remarkable difference in tonnage.
I. DIAMETER AND LENGTH
The most usual diameter of tube mills now in use is 5 ft., few being of greater diameter, but by the nature of the problem of ore grinding we must recognize that the diameter of the mill, which regulates the height or amplitude of fall of the cascading pebbles, should be proportioned to suit the conditions of the feed. If the mill is fed coarse particles of ore the pebbles should fall a greater distance than when grinding fine particles; therefore, the diameter of the mill should be such that the largest-sized pieces of ore fed to the mill are effectively broken by the pebbles. We find this observed to a certain extent in practice for it will be observed that the tube mills taking the product of light stamps and consequently medium-size ore particles are from 4 to 4^ ft. in diameter while those taking the feed of heavy stamps are from 5 to 6 ft. in diameter.
The greatest portion of the ore fed to a tube mill is ground by the impact of pebble against pebble and the lesser portion by attrition when pebble slides on pebble or on the lining. Some operators have contended that the fine grinding is done more particularly by the sliding of the pebbles, that is by attrition, but we know that to attain capacity of 200-mesh material we must cascade the pebbles and not let them slide any more than possible. We must have the mill of such length that most of ore is reduced to the desired fineness by the time it reaches the discharge end of the mill. Should we use a short mill the proportion of oversize would be so great that it would require a complication of tube mills and classifiers to accomplish the work that can be done by one cylindrical mill of suitable length. The finer the ore is crushed the less chance there is of further reduction; hence the length of the mill is proportioned to give the least amount of oversize with the greatest capacity. If all we require is a medium-sized product suitable for concentrating, we use a short tube of large diameter, but for sliming ores the long cylindrical tube is preferred.
At Tonopah, Nev., and South Africa it was found that the usual 22-ft. tube mill may be shortened 4 to 6 ft. without any decreased capacity, which is accounted for by the fact that in this type of mill an excess of length causes overgrinding, useless work being done on ore already ground to the desired fineness. At the El Oro, Mexico, a portion of the end of the mill has been used as a classifying chamber by the use of a perforated diaphragm plate 6 in. from the discharge head of the mill.
Until lately the 8-ft. conical mill has been the standard diameter for this type of mill but lately Hardinge mills with 10-ft. major diameter have been made. No doubt this will be followed by the 10-ft. diameter cylindrical tube mill of short length. The success of the Hardinge mill may be attributed in great part to the great diameter of the cylindrical portion of the mill, this enterprising metallurgist leading the way for the cylindrical-mill manufacturers.
Cylindrical mills are now made 8 ft. in diameter. That illustrated in Fig. 11, made by the Power
In a previous paragraph I have stated that a number of short tube mills with classifiers would unnecessarily complicate a reduction plant and had in mind for comparison a mill 5 or 6 ft. in length. That the 22- or 24-ft. tube mill has been shortened I have also indicated but this contraction is apt to be more radical than would appear at first sight, for from the nature of the problem of ore reduction we can predict that in future a tube mill 8 or 10 ft. in diameter and 6 to 8 ft. long or a ball mill of smaller dimensions will take the place of the intermediate crusher, to be followed by a tube mill 5 to 6 ft. in diameter and 10 to 12 ft. long, both discharging into the same classifier with the oversize going to the longer mill for final reduction.
