If we compare modern and first thickeners employed in gold ore processing plants, we will realize that there has been a steady evolution in thickener design, both of conventional tanks and high rate thickeners whose efficiency depends on the optimum flocculant addition. Basically, thickeners are almost universally employed for thickening gold concentrates ahead of filtration, final tailings for process-water recovery, and they are an integral part of the process in counter-current decantation, which are known as CCD circuits. In some occasions, thickeners are employed as a reservoir of surge capacity and for intermediate dewatering of flotation products. Most the time, thickeners are required to perform two tasks simultaneously, produce a high density underflow and a clear supernatant liquid, and usually work so smoothly that little attention need to normally be paid to them, but if they fail or are wrongly sized, the effect on throughput can be substantial.
Thickener operation is based on gravity to settle the solids. Feed enters at or near the surface and a drive mechanism moves a rotating rake to sweep the settled solids to a central underflow cone. Sizing depends on the properties of the slurry to be thickened. It has been noted that some slurries settle leaving a clear line between the supernatant liquor and the solids, and others gradually clarify without any clear demarcation. Probably, the most important factors influencing settling rate are the initial feed dilution, specific gravity and particle size of solids. The viscosity and temperature of the slurry play an important role in the settling rate.
In all thickeners there are four zones: clear overflow, zone of slurry of relatively uniform consistency where free settling conditions are important, a transition zone where settling conditions are changing from free settling into compression and the final zone is characterized by the dewatering process through solids compression forcing the liquid out. During the design, the thickener area must be sufficient to allow the slowest settling particle to reach the bottom of the tank before its associated liquor overflows. Considering the fact that the settling rate of a particle is different in each zone, sufficient space must be provided to allow the particle to pass through each zone. Also, the overflow rate must be low enough to prevent turbulence in the thickener and so that the up flow rate is not greater that the settling rate of the slowest particle.
Compression zone is a critical zone Thickener