Figure 9.2a shows the Dorr classifier as it was in 1940, and it has changed little since then. Mineral companies quickly accepted the rake classifier and thickener, and closed — circuit wet grinding became part of mineral processing technology. With its reciprocating rakes, the Dorr classifier soon had competition from the Akins classifier (Figure 9.2b), which used a screw conveyor.
HYDROCYCLONES
For the 50 years leading up to 1955, the only serious competitor for the Dorr rake classifier was the Akins spiral classifier, which operated in a similar manner using gravity as the separating force but with coarse particles being removed by a rotating spiral rather than a rake. The settling pool was quieter in the spiral classifier, and the classification would have been marginally sharper, but the difference was not large and the choice of a
classifier would have been made according to capital and maintenance costs rather than classification efficiency. By 1950, it had become evident that rake and spiral classifiers would be an impediment to the design of the high-throughput grinding circuits then required because of their high capital cost and limited capacity to transport coarse particles back to the mill. The demand for iron and copper minerals in particular was growing, and the new concentrators required high-capacity circuits. The idea of high-energy classification in cyclones using centrifugal force was well known, and reliable centrifugal slurry pumps were available that were capable of pumping high volumes of slurries at high pressures. The time for hydrocyclones had come, and they were about to change wet-grinding circuits from relatively low — to very high-capacity processes.
The inner and outer spiraling flows in hydrocyclones, which carry the fine and coarse particles into the product streams, are shown in Figure 9.3. The hydrocyclone may seem to be a very simple processing unit because it consists only of a few pipes and a cone, but its performance as a size separator is not easy to predict accurately. Two reasons are
1. It is influenced by both the size and specific gravity of the particles in the feed so that particles of equal size from different minerals will behave differently with a greater proportion of particles of the heavier mineral than of the lighter mineral entering the “coarse” product.
2. The fraction of ore in the feed that short circuits directly to the coarse product is difficult to predict.
Despite these problems, hydrocyclones are the classifiers most commonly used in wet mineral processing plants because they are inexpensive to install and operate, reasonably efficient as separators, and nothing goes wrong with them. How they have come to occupy their present dominant position in wet-grinding circuits will be discussed in this chapter. An offshoot of the hydrocyclone is the dense-medium cyclone in which finely divided material with a high specific gravity, such as magnetite or ferrosilicon, is added to the slurry. This increases the specific gravity of the liquid, and the dense — medium cyclone can be controlled so that lower specific gravity minerals float and higher specific gravity minerals sink. Dense-medium cyclones are used to separate coal from silica and silica from heavier metallic minerals such as hematite, galena, and sphalerite. In the rest of this section, for simplicity we refer to hydrocyclones as cyclones.