Elk City mining news. (Elk City, Idaho) 1903-1913, 1905-07-01
CONTINUED FROM LAST WEEK. |

NUM- IAOl w If n. /
SEPARATION OF THE SOLUTION FROM
THE SLIME. This is the greatest prob
lem of slime treatment. There are two
standard methods in general use, known
respectively as the decantation and the
filter press methods. There are two
certain patented processes which have
much to recommend them, nevertheless
have not been widely adopted, probably
because of the royalty question.
In the decantation method, after the
gold has been dissolved the slime is al
lowed to settle for a certain length of
time and the clear solution on top is
drawn off by means of a decantation
pipe, siphon or pipes arranged around
the sides of the tank at different heights.
The sludge remaining behind usually
contains from so percent. to 80 percent
of moisture. To recover the values
retained in it, wash water or a weak
cyanide solution containing low gold
values is added and the sludge agitated
to insure a thorough mixture, after
which it is again allowed to settle and
the clear solution again removed.
In general, two or three determinations
will reduce the value of the sludge to a
point where it does not pay to bother
with it any further. The clear solution
withdrawn is sent through the zinc box
es and the gold and silver recovered in
the usual manner. The greatest OBJ
jections to the decantation process are:
First, it requires very large amount
of tank room, and, second, it causes the
accumulation of large quantities of So
luton so low in gold value and so weak
in cyanide that it becomes a serious
problem to percipitate the gold from it.
Some varieties of slime will settle
very quickly, while others require so
much time that decantation is imprac-
ticable. Stacked lime has the peculiar
property of coagulating slime and
causing it to settle very rapidly. For
this reason it is extensively used in
slime treatment. The amount used
varies from one to ten pounds per ton
of dry slime. The proper quantity to
add can be determined only by actual
experiment.
Another method of separating slime
and solution is that which is known as
filter pressing. This was developed on
the gold fields of Western Australia,
where a process was required that need
ed minimum amount of water. After
the gold is dissolved the sludge is forced
into filter press under a heavy pres
sure. Clear solution flows out through
the filter cloths and is conveyed to the
precipitation department, while the
sludge after washing is discharged and
the press made ready for new charge.
The filter press consists of a series OF
hollow frames from one to three inches
thick, according to the nature of the
slime, and from two to four feet square.
Between each frame is filtering sur.
face covered with a cloth. When the
press is together it consists of a series
of from thirty to fifty Independent
chambers, each with separate filterine
surface. Arrangements are made foi
delivering the pulp to the center of each
chamber and for washing the cakes
after they have been formed. The
greatest objections to filter pressing
are, first, the high cost of the press,
and, second, the cost of operation
Every time the press is filled it is nee.
essary to open it for the purpose of dis
charging it. Nevertheless, filter press.
ing is the neatest and best way Of
treating shines, provided, of course,
that the shines can be pressed. Some
slime is of such a nature that it will
pack in the press so that only very thir
cakes can be made and the cost of treat
ment becomes too great.
Special presses have been designed
for slime treatment. The largest ones
have a capacity of several tons Of dry
slime per single charge. The time re-
quired to put charge through a press
varies from two to three hours, depend
ing upon the porosity of the cakes. The
greatest difficulty in filter pressing is
to get cake of uniform density. In
charging a press the heavier particles
tend to settle in the lower part of the
chamber, and this makes that part
of the cake the most porous. In wash
ing cake the wash water will follow
along the lines of the least resistance,
and if the cake is not uniform the wash
ing will be imperfect. For this reason