TIPS FOR A CIP PROCESS

There are many types of absorption tanks such as the Dorr-type agitator, the simple propeller agitated tank, the Pachuca tank and the Draft tube-type of agit ated tank. On the selection of the most appropriate absorption tank there are several aspects to consider: the mesh of grind, the pulp solids and the apparent viscosity of the pulp as caused by clays and special chemical conditions.

The traditional Dorr tank with the slow center speed sweep and either peripheral or center column air lifts has worked quite well on the finer grinds of all minus 65 mesh and perhaps 70-80% minus 200 mesh. Pachuca tanks have also been successful and they may be capable of handling a coarser feed than the traditional Dorr tank. The draft tube in which there is a turbine which closely fits inside the draft tube. Velocity in the tube is carefully calculated to avoid under shear and abrasion of carbon. The objective in all cases is to mix the granular carbon which is typically 6 x 16 mesh, very gently but thoroughly in a pulp with minimum carbon abrasion. The major advantage of draft tube is the ease of startup and also requires one-third the horsepower input of a conventional agitator.

The type of tank most suitable for CIP process depends on the type of ore to be treated and the prevailing operating conditions. In most cases, a deep tank with turbine type mechanical agitator and low speed tip velocities would be satisfactory. Turbine type impellers give a positive agitation that assures optimum contact and reduces short circuiting. Through aeration is possible with an air sparge properly located. A mechanical agitator can easily be started up after an outage, but is usually necessary to drain and wash out a Dorr rake type or Pachuca before restarting. The turbine type impeller requires more power, but maintenance costs are negligible if rubber covered impellers are used. Carbon losses are minimal. In small plants, Pachuca tanks could provide adequate agitation and aeration.

Fig.6 Good dispersion and agitation with the right impeller speed

A tank must be designed considering the possible losses by attrition, and this is in direct relation with the speed. The impeller should be sized so that can be got a good sweeping action toward the bottom of the tank, across its bottom, up the sides and back to the center. The advantage of the draft tube is that it’s possible get improved aeration.

Some tanks can have flat bottom and conical bottom like Pachucas. Conical tanks are very used but there are some problems associated to its operation. Some plants have experienced problems with flat bottom tanks because there a tendency to sand in on the sides of the tank so only the center of the tank is active.