Environmental Engineering Reference
In-Depth Information
the operating costs and the market value of the sellable product ( e.g. the Net Smelter
Return).
It is evident that to develop some form of real-time optimization (RTO) for a
given process, some previous steps must first be completed, namely to dispose of:
(a) physical or virtual sensors for measuring the critical variables of the process; (b)
efficient control strategies able to adequately face the inherent complexity of multi-
variable processes with interactions; (c) mathematical tools to continuously modify
the secondary variable set points to maximize the economic profit of the considered
process. Many industrial and/or academic research groups have been working on
these subjects. The LOOP group (Universite Laval) has been especially interested
in the application of all these aspects to the optimization of flotation column oper-
ation. Their experience and that of other research groups (particularly the McGill
Mineral processing group on sensors) will be described in this chapter. The chap-
ter is organized as follows. First, Section 6.2, Process Description, presents a brief
description of the column flotation process and its main operating variables. Then,
Section 6.3, Sensor Development and Applications discusses the available methods
for measuring the main variables specifictoflotation columns. Section 6.4, Auto-
matic Control discusses current approaches for automatic control. Finally, Section
6.5, Future Trends and Industrial Applications, reviews possible research orienta-
tions of flotation column sensors and control research and opportunities for indus-
trial applications.
6.2 Process Description
The flotation column being considered in this chapter was conceived by P. Boutin
and R. Tremblay in the early 1960s [1, 2], and was first commercialized by Column
Flotation of Canada (CFC) in the 1970s [3]. The first successful industrial appli-
cation was at Les Mines Gaspe (North-East Quebec, Canada) in 1981 [4]. Since
then, this unit has acquired great popularity in the mineral separation business ow-
ing to its capability for producing clean concentrates at good recoveries. Although
the original CFC column had a squared section (36 in or 72 in), present columns are
of a circular section, typically 3 m in diameter. Large capacity plants though, have
chosen rectangular shapes, internally baffled to approach plug flow conditions ( e.g. ,
20 m by 4 m baffled at 1 m by 1 m). A schematic representation of a flotation col-
umn is given in Figure 6.1. A conditioned pulp stream is fed at 2-3 m from the top
of the column, whereas a flow of gas (usually air) is injected at the bottom through
a bubbling system or sparger. A unique feature of the flotation column is the addi-
tion of a fine spray of water above the overflowing concentrate stream; its objective
will be explained later. Under normal operating conditions, the column exhibits two
distinct regions, each having a different objective and air content (volume fraction).
The lower zone, between the feed port (roughly) and the air bubbling system at the
bottom, typically contains less than 20% of air. The zone between the feed port and
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