The last years the focus of many metallurgists has been to know how to evaluate the amenability of gold recovery using a gravity unit. The weight required to perform a testing program depends on the gold grade and the gold particle size of the sample or samples to be studied. It is important to mention that coarse gold and lower grade means more mass is required. For example, 5 kg is the minimum weight required to evaluate a high grade sample, whose gold content is higher than 25 g/t, and present a finer gold distribution or 60% of gold is finer than 106 micron. 150 kg is the minimum weight required to evaluate low grade samples, which may content 2 g/t of gold and 40% of free gold is coarser than 150 micron. When information is needed about gold that is closer to 20 mesh alternative procedure may be considered. In this case a pilot test may be useful.
It is important to mention the sample must be representative of the orebody. The implication of this is that although the testwork can be performed with a small mass of sample, a much large mass of sample may be necessary to obtain a good primary sample, which must be size reduced and mass reduced according to accepted practices. It is common to evaluate the response of the material for two methods. In both cases it is important to use ore from the mine stockpile or open circuit grinding product is crushed and screened at 20 mesh with oversize returned to the crusher. This mass may be 300 to 500 gr on the screen oversize and this fraction is assayed for gold. The first method comprises three stages. The undersize material from the crushing sage is processed with a gravity unit such a Knelson Concentrator, the entire concentrate assayed and tail samples taken. A sample of 15 kg of the bulk tails of this step is ground in a laboratory rod mill to 55% passing 200 meshes and it is treated in the same gravity unit. In this case the feed rate and fluidizing water pressure are lower than the first run. Then, 7 kg sample of the bulk tails from this step is ground to 80% passing 200 mesh and processed in the same recovery unit again. In this case the Knelson concentrator was used before and the flow rate and fluidization water pressure is reduced again. Essentially, by reducing the material by approximately half for each successive step provides backup material and can be used to perform particle size distribution and perform a small concentration test.
The second method is to crush the material and perform directly dry ground to approximately 75-80% passing 200 mesh in a laboratory rod mill and ground product is treated in the gold gravity unit. Knowledge of the size distribution of the gravity recoverable gold is important for gold circuit design; very coarse gold may be effectively recovered by jigs even sluices. Finer gold is collected by centrifugal units. The contribution of the finest fractions such as 20 or 25 micron is very important because very fine particles present different behavior in different gold gravity concentration units.