Environmental Engineering Reference
In-Depth Information
It is well-known that mineral separation performance is dictated by the so-called
grade-recovery curve, which establishes a compromise between the quantity (re-
covery) and the quality (grade) of the device valuable product (concentrate). Usu-
ally, it is the plant metallurgist who decides which position of the curve the plant
(or a section of it) should work at. This decision should, however, be dictated by the
profits generated by operating at a given point on the curve and should automati-
cally be taken by a RTO algorithm based on an economic criterion, such as the net
smelter return.
6.4.2 Process Control
In this section, it is assumed that the instrumentation and regulatory control are
working properly. It is also supposed that the column pulp feed has been correctly
conditioned insuring adequate pH and reagent concentration (collectors, frothers,
etc.
). The design and/or tuning of the controller, model-based or not, should rely
on adequate process models. Identification of such models is discussed in Chapter 4
and in Section 5.5.1, and also in several topics [66, 67]. As previously explained, it is
often more convenient to indirectly control the primary variables through secondary
variables which can be more easily measured at a reasonable cost. A controller that
adequately manipulates the regulatory controller set points (air flow rate, wash water
flow rate, tailings flow rate,
etc.
) must then be designed. The secondary variable set
points are the results of the RTO unit.
Undoubtedly, froth depth is the most used secondary variable, and often the only
one, for control of industrial flotation columns. The position of the pulp-froth in-
terface sets the height of the collection zone and therefore is directly related to the
time available for collection of hydrophobic particles on the air bubble surface, the
other factor being the tails flow rate. Consequently, it is partly responsible for the re-
covery of the valuable mineral. However, large froth depth set-point changes would
be necessary to obtain a significant effect on recovery, seriously limiting its use
for recovery optimization purposes. Nonetheless, pulp-froth interface position is an
important control variable since its constancy is a guarantee of stable column oper-
ation: a continuously moving interface is certainly undesirable, since it implies the
risk of feeding the column in the middle of the froth zone or deepening it so much
that a risk of froth collapse exists.
To obtain an adequate cleaning action, a minimal positive value for the bias is
required. On the other hand, if the bias is too large, the residence time of valuable
minerals in the collection zone could decrease.
Gas is at the heart of the flotation process, therefore, controlling gas rate, gas
hold-up, bubble surface area flux or BSD appears to be a way to improve metal-
lurgical performance. The control of bubble surface area flux and BSD will be dis-
cussed in Section 6.5, where future work in this area is discussed. Since measuring
gas hold-up is a more mature technology, examples of its control can be found in
the literature [34-36, 68, 69], as will be illustrated in the next section.
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