ect
follows the mere contact of air with sulphur particles at atmospheric
temperature; but if the particles be raised to a temperature of 500
degrees Fahr., the sulphur is oxidised to the gaseous sulphur dioxide.
The same action effects the elimination of the arsenic and antimony
associated with gold and silver ores, as when heated to a certain
constant temperature these metals readily oxidise.

The science of calcination consists of the method by which the sulphide
ores, having been crushed to a proper degree of fineness, are raised
to a sufficient temperature and brought into intimate contact with
atmospheric air.

It will be obvious then that the most effective method of roasting
will be one that enables the particles to be thoroughly oxidised at the
lowest cost in fuel and in the most rapid manner.

The roasting processes in practical use may be divided into three
categories:

_First or A Process._--Roasting on a horizontal and stationary hearth,
the flame passing over a mass of ore resting on such hearth. In order to
expose the upper surface of the ore to contact with air the material is
turned over by manual labour. This furnace of the reverberatory type is
provided with side openings by which the turning over of the ore can
be manually effected, and the new ore can be charged and afterwards
withdrawn.

_Second or B Process._--Roasting in a revolving hearth placed at a
slight incline angle from the horizontal. The furnace is of cylindrical
form and is internally lined with refractory material. It has
projections that cause the powdered ore to be lifted above the flame,
and, at a certain height, to fall through the flame and so be rapidly
raised to the temperature required to effect the oxidation of the
oxidisable minerals which it is desired to extract.

The rate, or speed, of revolution of this revolving furnace obviously
depends upon the character of the ore under treatment; it may vary from
two revolutions per minute down to one revolution in thirty minutes. Any
kind of fuel is available, but that of a gaseous character is stated to
be by far the most efficient.

Any ordinary cylinder of a length of 25 ft., and a diameter of 4 ft. 6
in., inclined 1 ft. 6 in. in its length, will calcine from 24 to 48 tons
per diem.

Another form of rotating furnace is one in which the axis is horizontal.
It is much shorter than the inclined type, and the feeding and removal
of the ore is effected by the opening of a r