Evaporate in a
weighed dish, ignite, and weigh.
The following analysis will illustrate the composition of a sample of
Cornish wolfram as brought into the market:--
Tungstic acid 50.1%
Cassiterite 10.9
Ferrous oxide 24.6
Manganous oxide 5.4
Niobic oxide, alumina, &c. 3.5
Silica 1.2
Copper oxide 2.7
Zinc oxide 0.22
Arsenic 0.51
Sulphur 0.20
-----
99.33
NIOBIC AND TANTALIC OXIDES.
These oxides are commonly met with in samples of wolfram and tinstone,
especially niobic. They are probably present in the form of columbite, a
niobate of iron and manganese; and tantalite, a tantalate of the same
metals.
On boiling with hydrochloric acid they are both liberated, and remain
for the greater part (all the niobic) in the insoluble residue with the
tungstic acid. On removing the latter with dilute ammonia they remain as
a white insoluble precipitate, very prone to run through the filter on
washing. They may be dissolved in hydrofluoric acid either at once or
after fusion with bisulphate of potash, and extraction with cold water.
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