_Calcination or Roasting and Calcining Furnaces._--The roasting should
be conducted so as to eliminate as much of the arsenic and antimony as
possible, and to leave just enough sulphur as is necessary to combine
with all the copper present when the calcined ore is smelted. The
process is effected either in heaps, stalls, shaft furnaces,
reverberatory furnaces or muffle furnaces. Stall and heap roasting
require considerable time, and can only be economically employed when
the loss of the sulphur is of no consequence; they also occupy much
space, but they have the advantage of requiring little fuel and
handling. Shaft furnaces are in use for ores rich in sulphur, and where
it is desirable to convert the waste gases into sulphuric acid.
Reverberatory roasting does not admit of the utilization of the waste
gases, and requires fine ores and much labour and fuel; it has, however,
the advantage of being rapid. Muffle furnaces are suitable for fine ores
which are liable to decrepitate or sinter. They involve high cost in
fuel and labour, but permit the utilization of the waste gases.

Reverberatory furnaces of three types are employed in calcining copper
ores: (1) fixed furnaces, with either hand or mechanical rabbling; (2)
furnaces with movable beds; (3) furnaces with rotating working chambers.
Hand rabbling in fixed furnaces has been largely superseded by
mechanical rabbling. Of mechanically rabbling furnaces we may mention
the O'Harra modified by Allen-Brown, the Hixon, the Keller-Gaylord-Cole,
the Ropp, the Spence, the Wethey, the Parkes, Pearce's "Turret" and
Brown's "Horseshoe" furnaces. Blake's and Brunton's furnaces are
reverberatory furnaces with a movable bed. Furnaces with rotating
working chambers admit of continuous working; the fuel and labour costs
are both low.

In the White-Howell revolving furnace with lifters--a modification of
the Oxland--the ore is fed and discharged in a continuous stream. The
Bruckner cylinder resembles the Elliot and Russell black ash furnace;
its cylinder tapers slightly towards each end, and is generally 18 ft.
long by 8 ft. 6 in. in its greatest diameter. Its charge of from 8 to 12
tons of ore or concentrates is slowly agitated at a rate of three
revolutions a minute, and in from 24 to 36 hours it is reduced from say
40 or 35% to 7% of sulphur. The ore is under better control than is
possible with the continuous feed and discharge, and when sufficiently
roasted can be passed red-hot