The amount of ore fed to the mill influences

The amount of ore fed to the mill influences to a certain extent the best moisture content, it being necessary to retain more moisture in an overfed mill than one that is underfed because in a mill taking a heavy feed the discharge is not so free, causing the mill to be overcrowded with a tendency for masses of pulp to be thrown over the center with the pebbles unless the pulp is thin enough to overcome this tendency. G. O. Smart experimenting with a 5J^ by 22-ft. tube mill on the Rand found that 400 tons of sand containing 39 per cent, moisture by weight should be fed per 24 hr. With a small feed, 200 tons of sand per day, 27 per cent, moisture in the pulp was found to give the best grinding. The most frequent cause of inefficient operation of a tube mill due to an excess of moisture is that the launder from the head of the classifier to the tube mill feed scoop is at such a low angle of inclination that a pulp of sufficient thickness cannot be maintained; the consequence is that the oversize from the classifier increases, requiring more water to wash it down the incline, increasing the trouble by the return of more and more oversize until a point is reached when the ore feed must be turned off until this excess of oversize is ground sufficiently fine to escape from the discharge end of the classifier. With a given inclination the launder will carry a pulp of a certain consistency and to tend to thicken it will result as outlined above. It is important, when designing a mill, that this particular should be given due consideration.

We may either have the oversize from a classifier feed the tube mill at the right consistency as when a wet-crushing machine precedes the classifier, or the oversize from the classifier mixed with dry ore, as for example when the tube mill is fed with the product of dry-crushing rolls. It is evident that in the first instance the pulp must be fed at the right consistency while in the second case the oversize may contain an excess of moisture which is brought to the right consistency by the addition of dry ore. The oversize from a Dorr classifier will contain from 28 to 30 per cent, moisture and this coarse material increased to 36 per cent, moisture will not flow unless the launder is at an inclination of at least 6 in. to 1 ft. It is obvious, then, that the classifier must be placed as close to the tube mill as possible and the feed scoop of the tube mill made of such radius that a launder at this inclination can be installed. If desired the classifier may be in any position near the tube mill and a spiral screw feeder used to convey the sand to the feed scoop of the tube mill. If, as in the second instance, the oversize is fed with dry ore, the inclination of the launder should be at least 4 in. to 1 ft.

We may illustrate this falling off in capacity of a tube mill due to a dilute pulp by approximate figures. If we suppose a mill is grinding 1 ton of ore per hour to 200-mesh with 65 per cent, moisture, it may be possible to increase this capacity 50 per cent, by adding to this feed enough dry ore to bring the percentage of moisture to 40 per cent, or under. If a granular product is desired, however, we must use a greater amount of moisture as this does not promote sliming, the pulp being too dilute to hold the finer particles of ore on the pebbles, so that only the coarser particles are ground. by adding to this feed enough dry ore to bring the percentage of moisture to 40 per cent, or under. If a granular product is desired, however, we must use a greater amount of moisture as this does not promote sliming, the pulp being too dilute to hold the finer particles of ore on the pebbles, so that only the coarser particles are ground.