a Sketch of the geometry of the autoclave (after Johnston et al. 2001). A slurry is fed from the left into the first compartment, oxygen is added through the autoclave in each of the three compartments, and oxidised material is discharged from the right. Within the autoclave, the slurry (grey) is stirred by agitators (black crosses). Scales form on the agitators, walls, and floor of the autoclave. b Thin section of a spherical scale ball from the floor of the first compartment, formed concentrically around a fragment of scale (centre) that fell from an agitator. Dark bands are rich in hematite, and light bands are mixtures of rozenite and szomolnokite
Pressure-oxidation autoclaves are used to oxidise sulphide minerals
so that fine-grained gold is released for cyanidation (Fig. 2
a; LaBrooy et al.
The autoclave described in this study (Fig. 2 a; Johnstone et
The host rock and primary ore minerals are listed in Fig. 2 .
The concentrate is ground to <20 μm before being fed into
the autoclave (Fig. 2 a) to enhance reactivity.
Mineral deposits (scales) precipitate and accumulate on the inner
walls of the autoclave and on agitators, which stir the slurry as
it passes (Fig. 2 a; Johnstone et al.
In addition, fragments of scale from the walls or agitators act as
nuclei for scale growth on the autoclave floor (Fig. 2 b).
The scales are generally made up of numerous contrasting
monominerallic, biminerallic, and polyminerallic millimeter- to
centimeter-thick layers (Fig. 2 b), attesting to locally
variable chemistry in the slurry.
For example, the alternating layers of ferrous sulphates (low Fe
+++ /Fe ++ ) and hematite (high Fe
+++ /Fe ++ ) in the scale in Fig. 2 b
shows that redox states fluctuated over a wide range at that site
on a scale of hours or days.
Residual quartz undoubtedly occurs in the autoclave as well, but
this is not represented in the scales and is discharged with the
slurry (Fig. 2 a).
Intimate interlayering of Fe 2+ sulphates and hematite
in the autoclave (Fig. 2 b) shows that only minor changes in
the environment are needed to induce precipitation of minerals of
different oxidation states..
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