(Fig. 4); this behaviour was due to the continuous change
in the discharge types at the underflow between spraying
and roping. The transition from a semi-rope to a rope discharge
resulted in a sharp increase in the volumetric underflow
ore concentration.
Fig. 4 shows that there was a critical underflow ore concentration
beyond which the spigot became overloaded.
Under overloaded conditions, the underflow ore concentration
remained constant at around 50% with further
increase in the feed ore concentration. Stas (1957) too
observed that when the spigot was overloaded, the underflow
ore concentration remained constant with further
increase in the feed ore concentration. There appears to
be a small disparity between Fig. 4 and Clarkson and
Wood (1993)’s proposed underflow ore concentration
upper limit of 40% (to avoid spigot overloading). However,
this disparity is probably due to the inclusion of a safety
factor in Clarkson and Wood’s underflow ore concentration
upper limit. It was not possible in this study to maintain
underflow ore concentrations at values between
approximately 31–50%; as soon as the ore concentration
exceeded about 31% the air core at the underflow collapsed