When the ores contain a good deal of quartz or stony matter,
the fluxes just given (for average ores) will do; but the proportion of
soda should be diminished, and that of the borax, oxide of lead, and
flour increased as the quantity of metallic oxides become greater. If
the ore contains practically no quartz, the soda may be altogether
omitted, and some glass or powdered quartz added. The following charge
may be taken as an example: weigh up 20 grams of the powdered ore, 15
grams each of "soda" and borax, 60 grams of oxide of lead, and 5 grams
of flour. Mix and place them in an E crucible, and cover with a layer of
from a quarter to half an inch of common salt. Place in the furnace as
before. The salt will give off a considerable amount of fume, which
will, to a certain extent, conceal the state of the charge: when the
crucible has been in the furnace for about 25 minutes remove it and pour
out the contents immediately. With ores that produce a thick slag the
addition of 5 grams of fluor spar will be an advantage. It may happen
that with an unknown ore the first assay will be more or less
unsatisfactory: but from it the necessity for adding more or less flour
will be learnt, and a second assay, with the necessary modification of
the charge, should give a good result.
~Ores containing much Sulphides.
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