BLACK SANDS OF PARACALE. QAO 
As summarized in the above table, lot 1, uncrushed, will give 
a very small extraction, but after tube-milling a high extrac- 
tion is obtained in forty-eight hours where previous amal- 
gamation is used. A high extraction can be obtained without 
previous amalgamation only when the time is greatly increased. 
lot 2 will give a much higher extraction at 100-mesh than at 
60-mesh where both products have been previously amalgamated. 
The tube-milled product without previous amalgamation gave 
only a moderate extraction even with long time. This moderate 
extraction could probably be increased by more efficient agitation. 
The general conclusion is that cyanidation will give a high ex- 
traction when the following process is used: (1) Tube-milling, 
(2) amalgamation, (3) air agitation, (4) filter pressing or de- 
cantation, and (5) zine precipitation. Insufficient samples were 
on hand to make confirmatory tests on a large scale, and exact 
determination of the best strength of solution, thickness, etc., was 
not possible. 
Many mills which formerly shipped their concentrates to smel- 
ters are now using cyanidation for the recovery of the values. 
A few notes on this practise which is very similar in the essen- 
tials to the required practise on the black sand concentrates are 
herein given. The agitation of the pulp may be done in either a 
suitably designed Pachuca or in a tank using mechanical stirrers. 
Hamilton® recently advocated the latter type. As stated by him: 
For agitation, an ordinary tank with mechanical stirrer and a small air- 
lift at the periphery for aerating, will usually be found much more 
convenient than a Pachuca, especially when the plan is to settle and decant. 
Not the least trouble is experienced concerning the sticking of the paddles 
if they are set sufficiently high above the bottom for the granular part of 
the charge to settle beneath them, the only other precaution necessary 
being to give a sufficient speed to the paddles so that they will pick up the 
charge again from the bottom. If these two points are observed devices 
for raising and lowering the paddles are quite unnecessary. 
It is generally known that decantation has drawbacks among 
which the incomplete washing out of the dissolved values is a 
serious consideration. The trend of recent practise is toward 
filter-pressing which permits nearly perfect washing combined 
with the use of small amounts of solutions. As stated by 
McFarren :° 
At first the rich cyanide solution was removed by decantation as none 
of the filtering devices were suitable. However the Kelley filter press 
was found adaptable and is now being used for filtering concentrates. 
*Eng. & Min. Journ. (1912), 93, 841. 
* Salt Lake Review, April 30 (1912), 19. 
