Connected
with the mill there ought to be one or more machines for breaking the
blocks of quartz into small pieces. When the quartz is brought to the
mill, the smaller pieces are picked out and sent direct to the
stampers, and the blocks to the stone-breakers. A stone-breaking
machine will break about eight tons of quartz per diem. As the price of
labour is very low in India, it might be practicable to have all the
stone broken by hand, but the saving effected (lif any) by employing
manual labour would be very small.
A self-feeding aparatus is almost indispensable. It not only saves labour, but also ensures regularity in feeding.
The water trough is usually placed under the self-feeding hopper.
In
front of the coffers are three or more troughs containing quicksilver,
and below these are the tables. The tables should be evenly and
securely fixed and in such a manner as to admit of the inclination
being altered if necessary. They should be at least twenty feet in
length. The strakes (subdivisions of the table formed by fastening
narrow strips of wood to the floor) should be about fourteen inches in
breadth. In all well-planned mills there are breaks in the tables,
generally at intervals of three feet, the upper edge of the lower
strake being about two inches below the slightly overlapping edge of
the one above. The tables should be made quite smooth, and the utmost
nicety is required in setting them, so that the inclination may be the
same throughout, and any line at right angles to the strakes absolutely
horizontal. The same care should be employed in putting on the
blankets; they should lie flat and cling to the boards.
Closely-woven green baize is perhaps the best material for blanketing.
At
the extreme end of the tables there is another trough containing
quicksilver, and finally a waste trough through which the sand,
pyrites, lime, &c, pass to settling boxes.
The boxes are cleaned out from time to time during the working day, often at intervals of a few hours.
The
separation of the pyrites from the sand &c, constituting the
tailings i? now generally effected by some form of buddle.
Borlase's
buddle with Munday's patent scrapers is believed by many to be the
best. It consists of a circular wooden trough or basin from eighteen to
twenty-four feet in diameter. It is about one foot six inches in depth.
The tailings consisting of crushed quartz pyrites, &c., are made to
pass along a sluice and fall into a box whence they are conveyed by
pipes to the sides of the trough. The greater specific gravity of the
pyrites causes the mineral to separate from the quartz sand and to
remain on the floor of the buddle, while the latter being carried down
the table passes away through a discharge pipe. In order to prevent the
loss of the valuable material, two or three rims or stops are fixed on
the floor of the basin. The pyrites are raked by knives, which, as well
as the pipes conveying the material to the trough, are made to revolve
round the central shaft. A machine making seven or eight revolutions
per minute is recommended. The knives are raised or lowered by means of
screws*.
The
concave buddle has had the attention of mechanical engineers for some
years, and several improvements have been adopted from time to time.
Other ore-dressers have also undergone modifications with a view to fit
them for the use of the gold miner, and some forms of percussion tables
have given good results.
The
pyrites, however excellent the system of concentration may be, are
never entirely free from a certain proportion of quarlz sand; but this
is not to any serious extent objectionable when the material comes to
be roasted in the furnace. Numerous forms of furnace have been devised
for roasting pyrites; one kind
