And
this process is now universally giving every satisfaction, the old
reverberatory furnaces having likewise been discarded in favor of the
Ca-lifornian drop furnaces.
(b.) CHLORINATION.
This
mode of treatment is an adaptation of the German process (Plattner's),
and was first introduced by Mr. G. F. Deetken, U. S., Mineral Surveyor
and Assayer, Grass Valley, California, to whom many improvements are
due in overcoming the difficulties surrounding the utilization of
different kinds of sulphurets; and it may be stated that, although it
is very effective with gold, it is not at all successful with silver,
as the latter metal, on treatment with chlorine gas, passes away as a
chloride in the waste residues.
The
chlorination process includes calcination in the already mentioned drop
furnaces; the previously concentrated sulphurets are roasted "dead" in
these furnaces, which have for a charge of one ton of sulphurets at a
time, an area of 130 square feet, the dome, or cover, rising but 24
inches in the centre above the floor of the hearth. The sulphurets are
delivered through a cast-iron funnel at the top of the first hearth,
from trucks, cars, &c.; and when they are very fine a "
dust-chamber " will save as high as 5 per cent. of the ore calcined.
With these drop furnaces from 5 to 6 tons of sulphurets can be calcined
at the expense of but 2 cords of soft firewood, in three shifts, or 24
hours—a considerable improvement on our reverbatory furnaces'
capabilities, assuming that in both cases the pyrites are delivered in
as dry a state as possible. These drop furnaces may be worked in
distinctly different ways, viz., for producing sulphuric acids as a
by-product, and for oxydation and chloridizing calcination. When for
the latter, as interesting ourselves chiefly, the furnaces used,
consist of two hearths, constructed at different levels, or one about
12 feet above the other, and placed " end for end;" they are connected
with each other by means of a vertical flue, of the same width as that
of the hearths ; and this flue, 12 inches deep, is constructed so as to
lead to the lower hearth, zig-zag fashion, or over a series of terraces
built in the flue right down to the bottom hearth. The fireplace,
common to both hearths, having been built in front of the lower hearth,
the flames therefrom primarily affect the pyrites, as separated only
from the lower hearth by a low bridge in that hearth; then these
flames, &c, ascend through the flue or drop to the upper
hearth, subjecting the pyrites there in a like manner; finally passing
out through the damper into the stack and open air. As soon as a charge
has been fed into the upper hearth the hopper is closed, the usual
burning and raking takes place for a specific period, and then the
partly calcined pyrites are raked into the drop-flue, where they
descend like a thin sheet into the ascending flames, which latter
subject every particle of these sulphurets to severe calcination. At
the bottom of the drop-flue is what is termed the back-hearth, whence
the roasted ore can be withdrawn for outside treatment by means of a
trap-door. If, however, the ore is to be calcined farther in the lower
hearth, it is raked into the latter, and the final roasting takes
place, including the addition of coarse salt after three hours' work.
By means of this salt the gold is freed from its oxydes, and is besides
rendered easier for chlorination by removing lead or other sulphates
obnoxious to the final process ; consequently the quantity of salt to
be added depends on the greater or lesser percentage of lead in the
ore. To follow the actual working of the furnaces in this paper would
take
