The charge must be occasionally stirred with
the stirrer (fig. 10) so as to expose fresh surfaces to the action of
the air, and to prevent adhesion to the sides of the crucible. The
stirrer should not be removed till the calcination is finished. The
temperature should be raised at the end to a good red heat; and (to
ensure the decomposition of any sulphate that may be formed) the roasted
ore should be rubbed up in a mortar with a pinch of anthracite, and
again calcined. It is then mixed with fluxes as described, and fused in
the same crucible.
The calcination of an ore is a work occupying a good deal of time, and,
in most cases, it is better to take advantage of the desulphurizing
power of red lead or nitre. Red lead by itself will do, but a large
quantity of it will be required; 1 part of a metallic sulphide needs
from 20 to 50 parts of red lead to yield a button free from sulphur;
whereas at most from 2 to 2-1/2 parts of nitre are sufficient. There is
sometimes an advantage in having a considerable excess of oxide of lead
in the slag, but where there is no such reason, 2 parts of red lead to 1
of ore is enough. A charge which will do for most sulphides is the
following: 20 grams of ore, 40 to 100 grams of red lead, 20 grams of
"soda," 5 of borax, and sufficient nitre (or perhaps flour) to give a
button of about 25 grams of lead.
Pages:
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181