(_a_) It is very slow, since a long period of time is
required for the oxidising effect to penetrate through
massive lumps of ore.
(_b_) A large amount of capital is tied up in the material
at the roast-yards.
(_c_) The losses occasioned by wind and rain are very
considerable.
(_d_) It is difficult to use up a large quantity of fines in
the roast-heaps.
(_e_) Difficulties arise owing to damage by the fume, and
from interference by litigation.
There is one special instance of a modern smelter making a great
success of heap-roasting—namely, at Rio Tinto—but the circumstances are
peculiar, as the roasting is followed by leaching operations of the
immense ore heaps _in situ_.
This branch of roasting need not be considered at length, and the older
standard text-books give full descriptions of the various methods
employed. The following particulars are important, however, when under
exceptional circumstances such work has to be undertaken:—
The maximum and best average size under ordinary conditions is 40
feet by 24 feet, by 7 feet high above the bed of fuel. The height is
important, and varies with the quantity of sulphur in the ore. The
lower the sulphur content, the higher the pile; with about 40 per cent.
sulphur, the best height is 6 to 7 feet; with 15 per cent. of sulphur,
up to 9 feet; and if still less sulphur be present, the height may
even be a little greater. Such a heap holds about 240 tons, and if the
quantity of ore to be dealt with exceeds this, a number of such piles
should be constructed. The time occupied in roasting is about 70 days,
with 10 days more for removing and rebuilding.
The selection of a proper site is important.
(_a_) The prevailing direction of the wind must be considered,
so as to keep the fumes away from the works and offices.
(_b_) The yards must be protected from winds, so as to prevent
losses of dust, as well as uneven burning.
(_c_) The ground must be perfectly dry or drained.
Along the upper edges of the roast-yard a deep trench should be cut,
so as to catch rain-water, and prevent it from washing soluble copper
salts out of the pile; drainage trenches must also be provided to
carry any copper-bearing liquors to some point where the copper can
conveniently be precipitated on scrap iron. Enormous losses of copper
may occur if these precautions are not observed; thus, at one period in
the old roasting process in Tennessee, as much as 34 per cent. of the
copper in the heaps was lost in 186 days.
_Preparing of the Floor._—Remove roots and subsoil, fill space with
broken stone or rough tailings, cover with 4 to 6 inches of clayey
loam, and beat down well. The floor is then fairly impervious, and
does not crack on drying. The ground should be given a gentle slope so
as to facilitate draining. A layer (about 6 inches thick) of fine ore
is next put down, then 9 inches of fuel; channels are now mapped out
by means of logs set in both directions, leading to rough chimneys.
The pile is then constructed, with the lower parts of the very coarse
materials, smaller stuff being put towards the top and sides. On the
very top and at the outside of the pile are placed the fines, but this
top cover is only put on when the burning is well started. This process
is still worked at Tyee, B.C., and at some other localities, but is
most probably only a temporary plan, to be replaced by a more efficient
method as development progresses.
B. _Kilns for Lump Ores._—Kilns possess the advantage that they
permit of arrangements being made for the recovery of SO_{2} for acid
manufacture, and the subject belongs more properly to that branch of
technology. Few large smelting works employ kilns for roasting lump
ores, though there are important exceptions at works both in Britain
and on the Continent of Europe. Kilns are used at the Cape Copper
Company’s smelter at Britton Ferry, for this purpose.
(=b=) =The Roasting of Fines.=—Fines (and particularly fine
concentrates) are the usual materials subjected to roasting. The finer
the particles, the more rapid and complete is the oxidation, but the
losses by dust are heavier. The size limit is thus liable to some
variation, but often the material roasted is that under ⅜-inch in size.
_Roasting Furnaces—Requirements._—For the roasting of fines there is
simply required a place where the material can be gently heated in the
presence of a constantly renewed air supply. The fuel has itself a
reducing action, it must therefore be separated from the charge, and
hence the furnace employed is of the reverberatory type. Muffles are
never used for the oxidising roasting of copper ores. Since only a
moderate temperature is necessary for the operation, the furnace needs
but a small fireplace, and it is provided with a large hearth area. The
fuel used is one yielding the fairly long oxidising flame required.
_Developments of Roasting Practice._—The main objects sought in
roasting practice have been—
(1) To have as large a surface of material exposed to heat and
air as possible.
(_a_) By elongating and multiplying the beds of
the furnace.
(_b_) By furrowing and rabbling the charge.
(2) Continually to expose fresh surfaces of ore to oxidation.
(_a_) First by hand-rabbling.
(_b_) Later by movable furnace hearths.
(_c_) By mechanical rabbling.
(3) To obtain a continuous output—
(_a_) By mechanical charging, rabbling,
and discharging.
The Development of the Roasting Furnace.
A. FIXED HEARTH.—In Great Britain from 1583 onward, roasting in _small
reverberatory furnaces_ seems to have been the usual method, and up
to 1850 the furnaces appear to have been only of moderate dimensions,
with a single hearth, 16 feet × 13 feet 6 inches, constructed of
firebricks set on end, and with a fire-box 7 feet × 2 feet 3 inches × 18
inches. Rabbling was done by a long rake, the material being charged
and worked through one door. This method of working wasted time,
made the process intermittent, and caused continual cooling down of
the furnace, involving large fuel costs and much labour. The first
improvements were to lengthen the hearth, to add more working doors,
and to put the charge into the furnace by a hopper passing through the
roof. It was next found best to elongate the hearth still further, and
to drop the level of the bed in stages by about 2 inches at a time,
thus ensuring better control of working. By this means the best type of
_hand-calciner_ was arrived at, consisting of four beds, each 16 feet ×
16 feet, the whole charge being moved forward from one bed to the next
at each stage of the process.
In roasting, the ore is first placed in the coolest part of the
furnace, and is worked towards the fire, so that the charge travels
in one direction, and the flame and furnace gases in the opposite
direction to meet it.