THE GROWING POPULARITY OF THE TUBE MILL
In crushing and grinding machinery a certain type gains or loses popularity, maybe by reason of good advertising, by successful installations, or by some big company adopting that type of machine. One cannot look about at the present time without noticing the popularity of the tube mill, either in the form of a ball mill, a long cylindrical mill, a short tube of big diameter and short length, the marathon mill, or of the conical mill, with its big diameter and sizing cone. The need of finer grinding, whether for concentration, amalgamation, cyaniding or flotation, has made it necessary that we have a machine that covers a wider range of reduction than that formerly possible, so the tube mill has been lengthened, and shortened, and expanded, and contracted until we have a choice of many sizes to suit the particular work necessary. Ball mills are made with screens on the surface of the cylinder, and without screens, operating like the usual tube mill, by displacement. The 5-ft. cylindrical mill was thought to be the ultimate diameter a few years ago, now we have them 8 ft. in diameter while the conical mill still leads the way in increasing diameter, by giving us a 10-ft. mill. With the increase in diameter, the tube mill has been shortened, for a forced feed with a return of oversize i$ economically right, and a short mill is particularly adapted for this class of work. Costs have been lowered by studying the various factors that promote economy, so that we have grinding in these large-diameter tube mills at as low as 7 cts. per ton of ore ground.
I have made a chart showing the usefulness of the tube mill in stage grinding, whereat advocates of rolls and Chilian mills will, of course, be disappointed because, in the first place, I have considered rolls useful only as secondary crushers, and the Chilian not at all.
Our primary crusher is a Blake or a gyratory, both equally effective. Our secondary crushers may be stamps, disc crushers, rolls or ball mills. It our object is amalgamating with or without concentrating, and we desire nothing finer than 40-mesh, the stamps alone are sufficient. If we desire a 90-mesh product for amalgamating or concentration, we may regrind with a conical mill, or a cylindrical tube mill of short length and big diameter, in a closed circuit with a classifier. If we desire a 200-mesh reduction for cyanide treatment, we must use a cylindrical tube mill, in circuit, with a classifier.
When we use a disc crusher for secondary reduction for concentrating ore, it must be followed by a conical or a cylindrical tube mill of big diameter and short length, and when for the production of a slime for cyanide treatment, by the cylindrical tube mill.
If we have rolls as secondary crushers, and the object is concentrating, they may be followed by the conical mill, a cylindrical tube mill of medium diameter and short length, and if for 200 reduction, by the cylindrical tube mill.
If ball mills are used as secondary crushers, they may be able to deliver a product sufficiently fine for concentration, but if not, they must be followed, as in the last instance, by a conical or cylindrical mill, and if for an all-sliming process, by the cylindrical tube mill.
From a summing up of my ideas on the subject certain facts may be gathered: First, the stamp mill is still with us for amalgamating and a medium-size product, and in combination with the cylindrical tube mill, for cyanide treatment of a slime. Second, the popularity of the conical mill for a product up to 90- mesh, in competition with a cylindrical tube mill of big diameter and short length in circuit with a classifier. Third, the deserved popularity of the cylindrical tube mill with varying diameter and length according to the size of the feed.
