Page 48 - Introduction to Mineral Exploration

2: MINERALOGY OF ECONOMIC DEPOSITS 31

Sieve size % Sn erals mention can be made of the presence
of limonite coatings on quartz grains in sand
F 60 required for glass making. Extra cost in ex-
–y 1 + y 2 fraction ploitation will arise from the need for an
S 40 acid leach, or other method, to remove the
coatings. Coal fragments in gravel will render
it valueless as gravel processing plants do not
F 40
–y 2 + y 3 fraction include equipment for eliminating the coal.
S 60 Lastly it has become apparent recently that
the alkaline-silica reaction, which produces
“concrete cancer,” is due in some cases to the
F 15
–y 3 + y 4 fraction presence of certain types of opaline silica in
S 85 the aggregate.

F 9 2.2.6 Miscellaneous examples of the use of
–y 4 + y 5 fraction microscopy in ore evaluation and
S 91 mineral processing

F 8 Chromite ores
–y + y fraction
5 6 Chromite is never pure FeCr 2 O 4 and iron(III)
S 92 may substitute for chromium, particularly
along grain boundaries and fractures, producing
F 7.5 “off color grains” which can be detected in
–y 6 + y 7 fraction polished sections by the experienced observer.
S 92.5 The magnetite rims will introduce mineral
processing difficulties, as will badly fractured
FIG. 2.3 Mineral liberation size investigation of a chromite grains present in some podiform
possible tin ore. F, float; S, sink; y 1 to y 7 , grain (sieve) deposits; these may disintegrate rapidly into a
sizes. For discussion see text.
very fine-grained powder on grinding.

Nickel sulfide ores
Mineral liberation size
The normal opaque mineralogy is magnetite–
This is usually investigated as follows. Suppose pyrrhotite–pentlandite–chalcopyrite. The
we have a simple example such as a tin ore magnetite and pyrrhotite are separated mag-
consisting of cassiterite (ρ= 6.99) and quartz netically and normally become waste. Nickel
(ρ< of the minerals are separated, using heavy present in flame-like exsolution bodies (Fig.
liquids such as (ρ= 3.2) sodium-polytunstate 2.2e) and in solid solution in the pyrrhotite will
and lithium-tungstate. The float and sink frac- be lost. In an ore investigated by Stephens
tions are analyzed for tin grade. Figure 2.3 (1972) use of the electron probe microanalyzer
shows a possible result, where little improve- revealed that 20% of the nickel would be lost
ment in separation would be obtained by grind- in this way. Minute grains of PGM minerals
ing beyond the y 5 –y 6 fraction. are often included in the pyrrhotite, again lead-
ing to losses if this has not been noted. Also in
such ores the chalcopyrite may contain large
2.2.5 Deleterious substances
exsolved bodies of cubanite, a magnetic min-
These have been discussed in under “Undesir- eral, which could mean a substantial copper
able substances” in section 2.2 with reference loss in the pyrrhotite concentrate. In such a
to arsenic, mercury, phosphorus, calcite, and case the ore must be roasted (at extra expense)
topaz in metallic ores. Among industrial min- to destroy the magnetism of the cubanite.