This general average of 12.28 per cent. of mercury is pronounced
higher than the usual run of the ore, which, it is stated, does not go
above 7 to 8.50 per cent.
The furnace in which the ore is treated is cylindrical, 2 meters in
diameter, and 3.70 meters high from a brick grate, supported by three
arches to the arched roof. At the level of the grate is a charging
orifice, and near the roof are openings into two chambers, from the
bottom of which extend 12 lines of aludels, clay vessels, open at both
ends, the middle being expanded. The mouth of one fits into the back
end of the one following, a channel being thus formed through which
the fumes to be condensed are passed. The lines of aludels which are
laid on the ground terminate in a chamber, and for half the distance
between the furnaces and these chambers the ground slopes downward,
while for the other it slopes upward. Two furnaces are always placed
side by side, and the pair have from 1,100 to 1,150 aludels.
The operation is as follows: A layer of poor quartz is spread over the
brick grate; this is followed by a layer of smalls, and then by a
layer of still finer stuff, all of it being low grade ore. On top of
this are piled two-thirds of the _china_ of the charge on which the
_metal_ is put. Then follows a layer of _requiebro_, another lot of
_china_, and finally the _vaciscos_, shaped into balls, the whole
charge amounting to about 11½ tons, which is put in from an hour and a
half to two hours by three men. The charging orifice is then closed,
the aludels are luted, and everything made tight. The fires under the
brick grate are lighted and kept going for twelve hours, during which
time furnaces, charge, and condensing apparatus are heated up. During
this period, the temperature in the condensing-chamber at the end of
the line of aludels runs up 40 or 50 degrees Celsius, and some
mercury, evidently part of the native quicksilver, is noticed in it.
The temperature of the aludels in the immediate vicinity of the
furnaces is about 140 degrees C. During this period, the consumption
of fuel is four parts to every part of quicksilver produced. At its
close, the fire is drawn, and the second period begins. The air
entering through the brick arch is heated to from 200 to 300 degrees
by contact with the layer of poor stuff, the cinnabar is ignited, and
its sulphur oxidized, and the quicksilver vaporized and, condensing in
the aludels, flows toward the depression in the central portion of the
line. The temperature goes on increasing, until, twelve hours after
the beginning of this period, the thermometer shows 212 degrees C. at
the first aludels. This lasts for 18 hours, and then the third or
"cooling period" begins, which takes from 24 to 26 hours, and during
the beginning of which the temperature in the furnaces still rises. It
is then opened and cooled down. A very elaborate series of
observations made on the temperatures of various parts of the
condensing apparatus of the Almaden furnaces has shown that at the
aludels nearest to them the heat increases steadily until it reaches
249 degrees C., 44 hours after the beginning of the operation; that in
the middle of the line, at the depression, the maximum is 50 degrees
50 hours after starting the fires; and that at the end it does not
surpass 39 degrees. In the final condensing chamber, the temperature
varied, running downward from 40 degrees during the heating period to
14 degrees, rising again to 29 degrees toward the close.
The loss of the quicksilver during the operation has been vary
variously estimated, some stating that it is 50 per cent. and more,
while others place it at 30 per cent. Escosura, in his work, gives the
details of an operation checked by a royal commission in 1872,
according to which the loss in working ore running 9.55 per cent. was
only 4.41 per cent.--a loss which he considered inevitable. In 1806,
two Idria furnaces were put up at Almaden, but the engineers are not
favorably impressed with them. The first cost is stated to be more
than ten times greater than that of an aludel furnace, while the
capacity is only 50 per cent. greater. One pair of Idria furnaces in
five years produced 120,000 kilogrammes of quicksilver, against
843,000 kilogrammes made by eight sets of the Bustamente furnaces, the
cost per kilogramme of quicksilver being respectively 0.121 and 0.056
peseta.
* * * * *