In the case of all these the usual method is to introduce the stream to a tank through a downcast launder or pipe to a point near the bottom and allow it to make its exit on a level with the surface of the pulp.

For Pachucas an alternative system has been devised by Huntington Adams1 at the Natividad Mine in Oaxaca. In each case the pulp stream enters at the surface but the outflow is obtained by cutting a section out of the stream issuing from the top of the air lift by means of a receiving box fitted with an adjustable sliding lid. By opening or closing this aperture the level of the pulp in each tank is regulated. Arrangements are also made for bypassing the pulp around any individual tank to allow of emptying for cleaning or repairs. The advantage of this system over the usual one seems to lie in ensuring a complete mixture of the inflowing untreated ore with that already in the tank and preventing the possibility of more than the normal proportion of ore passing into and out of any given tank untreated. At one mill known to the writer, however, where the system was tried the proper regulation of the stream cut from each tank was found so difficult that it had to be abandoned.

The advantages claimed for the continuous system are (1) an increase in the time of agitation for any given unit of capacity; (2) less labor and attention. These apply to all methods of agitation, but there is an additional advantage in the case of Pachuca tanks in that it obviates the loss of head due to their excessive height because the pulp makes its exit from the series only a few feet lower than the point at which it enters.

The only drawback to the continuous system seems to be the inevitable short circuiting of a portion of the pulp. The amount thus passing untreated through any given tank in the series may be considerable and will be impossible to estimate when the system of passes does not ensure a complete mixture of the incoming stream with the pulp contents of the tank before it can reach the exit.

Even when such perfect mixture takes place there will still be 1 Engineering and Mining Journal, Oct. 7th, 1911,

SLIME TREATMENT.

101

a definite proportion going through untreated. Assuming a series of 100-ton tanks and a flow of 10 tons per hour, then approximately 1 ton out of the 10 tons flowing into the first tank will pass out immediately untreated into the second tank of the series: onetenth of a ton per hour of this original pulp will pass directly from the second to the third tank and so on in diminishing fractions to the end of the series.

As the action is continuous, however, the above does not represent the whole of the results of short circuiting. In order to demonstrate the modus operandi of this principle L. P. Hills in the Mining and Scientific Press of Feb. 8, 1913 makes the following calculations:

"Assume a series of 100-ton tanks with a flow of 10 tons per hour. Consider the efflux from the first tank for any given hour. That 10ton portion is composed of approximately:

0.909 tons of the influx of the given hour
0.825 tons of the first preceding hour
0.751 tons of the second preceding hour
0.683 tons of the third preceding hour
0.621 tons of the fourth preceding hour
0.565 tons of the fifth preceding hour
0.513 tons of the sixth preceding hour
0.466 tons of the seventh preceding hour
0.424 tons of the eighth preceding hour
0.386 tons of the ninth preceding hour
0.351 tons of the tenth preceding hour

The remaining 31⁄2 tons in diminished portions back to the initial inflowing 10 tons. The great bulk of the pulp receives exceedingly long treatment, the time of treatment of different portions varying between wide limits."

Another drawback to the continuous method exists in the case of Pachuca tanks. These tanks when kept full of pulp rapidly build up a deposit of hard slime around the sides thus seriously reducing the capacity available for pulp in motion, and proportionately reducing the time of treatment and increasing the fraction of the flow which has to pass untreated to the next tank in the

series.

It is therefore necessary to cut out each tank every two or three weeks for cleaning, and even so the available space for agitation is never at any time up to the maximum cubic capacity. Besides this permanent reduction in space there is the time lost in emptying and cleaning.

Loss in Extraction Due to Continuous System.-At one important mill in Mexico it was shown that the continuous system with Pachuca tanks as at that time being operated was responsible for a loss in extraction of 10 to 15 grams of silver per metric ton and the tails were reduced by that amount when the charge system was substituted for the continuous. The probability is, however, that the greatest contributary cause of this difference was due to the system of passes between tanks which allowed a straight flow from one tank to the next only a little below the pulp level. This while not attended with serious results when the tonnage and rate of flow were small yet was the cause of considerable short circuiting as the mill capacity, and therefore the volume of pulp, was increased, the velocity of flow carrying some of the pulp right across the tank to the exit before it had time to sink and become mixed with the rest of the charge. When instead of running the whole series of tanks in tandem they were connected into three or four separate series and the stream of pulp split up into an equal number of parallel streams the bad effect was reduced to a minimum, and in a case of this kind assuming that each tank is cleaned out frequently the added convenience of the continuous system would probably outweigh the remaining drawbacks.

Slime Treatment by Leaching in Filter Presses.-In New Zealand in the early days of slime treatment one of the first methods to be applied commercially was the use of filter presses and at first the dissolution of the gold was effected in the press as well as the subsequent washing out of the metal-bearing solution. This worked well with surface ores on which the action of the cyanide was rapid, but later it was found necessary to conduct the treatment by agitation, reserving the use of the filter for dewatering and washing only.

06

THE MERRILL AUTOMATIC SLUICING PRESSURE SLIME FILTER
METHOD OF SLUICING

FIG. 19.-Merrill Slime Press. Method of Sluicing.

A. Standard container or frame.

B. Feed channel through which the slime pulp enters each frame.

C. Channels from which water or solution is drawn off during process of filtration.

D. Partially sluiced slime cake.

E. Filter cloth with portion removed showing corrugations of filter plate.

F. Filter plate.

G. Horseshoe clamp for holding filter cloth against filter plate.

H. Sluicing pipe, containing water under 60 to 90 lbs. pressure admitted at either or both ends.

I. Sluicing nozzles located at center of each container or frame.

NOTE.-The sluicing pipe is automatically revolved forward and back through an arc of about 180 degrees, by means of the mechanism shown at the head of the press. As the cake is washed away by the action of the jet of water playing upon it. the mixture of slimeresidue and water flows into the channel underneath the sluicing pipe and through the outlet cocks to the waste conduit below.