The testing program prepared to assess the gold recovery by gravimetric concentration must study the concentrate and tails produced. Depending of the type of test, single or multiple tests, the metallurgist must be able to identify the best procedure. It is believed that a test involving several stages provides better information. For example, if the test comprised three stages, the overall recovery is based on the gold assays of the three concentrates and the tails of the third stage, whose assays are probable more reliable than those of the first two because they content some gold particles. The results are usually presented as size-by-size recoveries.
In most cases, it will possible to identify poor and very good performance. Basically, free-milling gold ores present a very variable behavior and the responses are shown as the cumulative percent of gold recoverable by gravity. Considering an intermediate response, it is possible to show cumulative recoveries for one, two and three stages. Other possible option is to present the cumulative gold recovery as a function of the K80 of the material, which is very important if the information will be used on a grind size different from that of the final product. If the respond is almost constant, the particle size distribution of concentrate and the grind size at which gold may be liberated could vary. For example, the particle size distribution could be 80% of the gold recoverable by gravity. It means that the material may require a recovery unit to recover fine gold. In the case of an intermediate response may be a common case where some fine particle and coarse particles are recovered well under similar conditions. When most of gold recovery corresponds to coarse particles, there is a special consideration in the design of the gravity recovery circuit.
There is a relationship between the particle size distribution and the percentage of gold recoverable by gravity. Usually, highly weathered ores tend to outperform the average performance of common gold ores. The total amount of gold recoverable is usually correlated to that of the first stage and this correlation can be used to predict the results of the test from those of the first stage. For example, recoveries of less than 10% for the first stage may produce overall gold recoverable near 40%. Essentially, the overall gold recovery will be higher if the gold recovery in the first stage is not low. Also, within the same orebody or close orebodies there is a correlation between gold grade and gold recoverable by gravimetric devices. It means that higher gold grades indicate a coarse size distribution of gold. A good example of this is the material of the Witwatersrand deposit.