The study of gold gravity circuits is a difficult task that many metallurgists have to face. One of the problems is related mass needed to obtain good and reliable information. The presence of flakes with variable weight, the free content and sampling error have an enormous impact on the results. Obviously, when the procedures followed are reasonable, the results will be good. If we consider 10 mg flakes at 10 g/t of gold, 8 to 12 kg do not produce a big error at higher. When the material has free gold in the 48-300 mesh fraction, the results are good and a minimum of 5 to 6 kg is needed for a 8-11% error. This is particularly noted in the circulating load stream.
It is important to indicate the importance of the information that can be obtained when the metallurgical lab has to process samples of reasonable weight. In this case, good and reliable information is obtained when most gold particles are under 48 mesh and more reliability will be obtained when gold is present at finer sizes. Essentially, good size-by-size information is obtained at finer sizes, but no at coarser sizes. This is an important consideration when is indispensable study the gold recovery size-by-size, the presence of coarse tends o make more difficult the sampling procedure and the responsible for the work must keep in mind this idea.
The other common problem in many gold operations is related to total assaying of free gold in large gold mill samples and the main antecedent is found in the old gold operations. One solution is to amalgamate the material, but try to do this on fine particles such those find in flotation or small scale tests is sensitive due to the process has limitations. Centrifugal concentrators are the source of many gold concentrates in full scale plants or metallurgical labs. Some of them show a peculiar behavior at particles finer than 38 micron and the separation is insensitive to slurry density, size distribution, fluidization water. For example the Knelson concentrator yields gold recoveries similar to amalgamation. It is believed that twice as much mass as is recovered by a Knelson laboratory unit can be fully assayed with approximately one assay tone or less per size fraction, and a larger concentrate mass could complicate the determination of free gold because the equipment collected free gold and gold bearing minerals.
According to some metallurgist the optimum centrifugal concentrator must yield a concentrate whose mass is approximately 1 – 2% of total mass fed to the unit. Also, this would indicate the scarce presence of gold bearing minerals because the recovery of sulphide minerals is variable. An indirect proof of this is that when feeds known to contain little gravity recoverable gold are fed, gold recovery tends to be low, even if gold grade is high. Another fairly direct proof that most of gold recovered in a centrifugal unit such as Knelson is free is related to the performance of a cleaning stage. The examination of cleaning tails using a microscopic device indicate the presence of free gold particles such flakes and most of free gold was collected in the cone ring. Obviously the experience of the metallurgist is important to determine the possible presence of free gold and use the appropriate technique or method to get a reasonable result.