The feed launder is of insufficient inclination

If, when grinding to a slime, the pulp becomes too dilute and the feed launder is of insufficient inclination to carry the right consistency of pulp, the ore feed should be shut off until the quantity of oversize has been reduced to the point where it will flow to the feed scoop at the right consistency. As a last resort the mill may be stopped, the excess moisture allowed to run out and the mill started again with the pulp at the right degree of dilution. During the time the source of ore supply is cut out the tube mill is not doing its full share of work for there will be no coarse pieces to be ground with the medium-grade oversize, so that it is important at all times to maintain the right moisture content. Constructing engineers often overlook the necessity of placing this feed launder at the best inclination; in fact a great percentage of mills erected have this defect.

If we stand at the discharge end of a tube mill when it is doing effective work, we observe first that the pulp is at a higher temperature than the atmosphere, that the issuing pulp contains but little oversize and no particles have gone through the mill without some reduction and if we stick our fingers in the pulp it will form a smooth coating and will be of such consistency that a drop will hang at the end of the finger but will not be liquid enough to fall off. With a little practice dipping the finger in the pulp the right consistency can be maintained without the necessity of taking the specific gravity.

The right amount of moisture is not the least amount that can be used in the mill but the amount that is found by careful testing to be right, but practically it comes very close to being the least amount of moisture that will form a flowing pulp.

W. J. Pentland says in respect to the amount of moisture in tube mill grinding:

"We think that the pulp should be thick enough to permit the sand particles to cling to the pebbles as they arise with the mill. When the pulp is so thin that no particles cling to the pebbles, poor work is being done, and only the coarse sand is being satisfactorily ground."

It would appear, then, that we must proportion the amount of moisture so that the greatest number of particles of a size larger than that desired in the final product, will cling to the pebbles the pulp occupying the voids in the pebbles being composed of those particles already ground sufficiently to be discharged.

To compare the tonnages at different degrees of dilution we may vary the amount of solution added to the tube mill and take the time required to fill a tub of known capacity. Knowing the discharge may be calculated with the help of a curve such as is shown in Fig. 19. To be accurate a sizing test of the dry pulp must be made, for the object is to compare the amount of 200 product with each variation of moisture. An example will suffice to show the method of calculating tonnages with the use of the specific-gravity curve. The specific gravity of the ore is 2.6. Compare the tonnages of 200-mesh product from a mill where it takes 75 sec. to fill a 20-gal. tub with pulp of 1.445 specific gravity and where it takes 100 sec. to fill the same tub with pulp of 1.585 specific gravity. The sizing tests on the two samples showed 75 per cent. 200-mesh for the first and 84 per cent. 200-mesh for the second.

A 20-gal. tub will hold 166 Ib. water. If the pulp is 1.445 specific gravity the tub will hold 166 X 1.445 or 240 Ib. pulp. Referring to the curve, we find that pulp of 1.445 specific gravity contains 50 per cent, dry pulp, so that in the tub there is 120 Ib. dry pulp; 120 Ib. in 75 sec. is 69 tons a day and of this 75 per cent, is 200 mesh, or 51 tons. Computing the second filling of the tub in the same way, we find it to be 56 tons a day, so that by decreasing the moisture from 50 to 40 per cent, with the same amount of dry pulp we gain 5 tons 200-mesh material per day.

SPECIFIOGKAVITY FORMULAS

Let
a = specific gravity of wet pulp.
S specific gravity of dry slime.
V = total volume of wet pulp.
m = total weight of dry slime in wet pulp.
c = volume of solution in wet pulp.
d = specific gravity of solution.
P = percentage of dry slime in wet pulp.
m = (SV(a - d)) / (s - d)
S = m / (V - c)
a = (m + cd) / V
P = 100S(a - d) / a(S - d)

An approximate value for P where the specific gravity of the solution is taken as 1 is:

P = 100S(a - 1) / a(S - 1)

From the formula S = m / (V - c) may obtain the specific gravity of the dry pulp. Take a beaker and balance it, then add a known weight of dry pulp to which add a known weight of water and weigh the wet pulp and note the resultant volume of wet pulp ; then by this formula the specific gravity is found. This is a good method of checking results found in the usual way.