When the project metallurgist has collected all the key information, he must be focused on the prediction of gold gravity recovery. It is important to consider that in typical gold recovery circuits, material is fed to the circuit only once. Circulating loads must be minimized as the consensus is that high circulating loads are deleterious to overall circuit efficiency. Recovery downstream of the main recovery circuit is minimal or non-existent. As a result, recovery circuits are designed with appropriate retention time to maximize gold recovery at optimum concentrate grade for flotation or at appropriate cyanidation time.
Gold gravity recovery from the grinding circuit follows a different logic. Circulating loads of gravity recoverable gold are usually 2,000 to 9,000% in the absence of gravity, an indication that gravity recoverable gold, and in particular coarse particles will circulate from twenty to hundred times on average before reporting to the hydrocyclone overflow. In the presence of gravity recovery, circulating loads decrease from 500% to 1900%, the higher the recovery effort, the lower the recovery. The logic of gravity recovery is to rely on the high number of times gravity recoverable gold goes around the circulating load to achieve adequate gravity recoveries. At each pass, as little as 1% of the gravity recoverable gold may be recovered, or as much as 12-16%. The amount of gravity recoverable gold at each pass is in fact a key variable affecting how much gold will be recovered by gravity, and defines what is considered as gravity recovery effort. Basically, the gold gravity recovery effort correlates with how much gravity recoverable gold is recovered, and next to the amount and size distribution of gravity recoverable gold, is the most important factor affecting gold gravity recovery.
For example for the hydrocyclone underflow of a grinding circuit 25%, one hydrocyclone underflow out of four, is bled and screened, and the screen undersize feeds a centrifugal concentrator. Screening efficiency is 70% and the recovery unit recovers 30% of the gravity recoverable gold in its feed. The centrifuge concentrate is processed in a cleaning circuit (gold room) with an 80% of recovery. In order to determine the gravity recovery effort, it must be considered that all units, except hydrocyclones are in open circuit, the recovery effort is equal to the product of the recoveries of the various units: 25% for the bleed, 70% for the screen, 30% for the centrifuge and 80% for the gold room. This is equals to 4.2%.
Other important variable that affects gravity recovery is the number of passes, the ability of classification to keep gravity recoverable gold in the circulating load until it is recovered. This primarily dependent on the partition curve of gravity recoverable gold, which in turn is largely dictated by the target grind size of the ore, which varies between 53 µm to 230 µm. Because the link between the partition curve of gravity recoverable gold and primary grind size is sometimes not very clear, it is preferable to retrofit applications, to measure directly the gravity recoverable curve partition curve. This is done by determining the amount of gravity recoverable gold for each size class of the hydrocyclone overflow and underflow. Such curves are characterized for the purposes of simulating gravity recovery by the proportion of gravity recoverable gold in the minus 20 µm fraction reporting to the hydrocyclone underflow.