Settling Tests

    For many years manufacturers are employed empirical methods for determining thickener area requirements from thickening tests. These tests are performed in graduated cylinders of 1-2 L capacity. The feed sample is thoroughly mixed and left to stand, and as the solids settle, the mudline position is plotted against time. It is important to test representative samples and assure that the temperature in the lab is similar to the mine site. There three methods employed to determine the unitary area of thickeners, Coe & Clevenger, Talmadge & Fitch, and a variation of Talmadge & Fitch Method. The latter method gives accurate results and has been employed the last years to determine the unitary area of several thickeners employed to perform solid-liquid separation on gold concentrates and also to dewater final tailings.
    Basically, settling tests are used to determine settling rates, underflow density, supernatant clarity, compare flocculant performance and dosage and estimate thickener area requirements. 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.
    Settling tests and thickener 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.