Gravity recovery is attractive because it can produce a high-grade product without chemical reagents. It is also frequently oversold as a complete answer for hard rock gold.
Gravity works on liberated particles
Gold must be sufficiently liberated from the surrounding mineral and fall within a size range the selected equipment can treat. Coarse native gold in quartz may respond strongly after staged crushing and grinding.
Gold locked inside sulphide grains does not become gravity-recoverable simply because the ore contains visible heavy minerals.
The recovery point matters
A centrifugal concentrator is commonly integrated into a grinding circuit so liberated gold can be removed before it circulates repeatedly. The feed stream must have controlled solids, flow and particle size.
Installing the machine on an unstable stream can create variable recovery and make the result difficult to interpret.
Grinding finer has a cost
Finer grinding may expose additional gold, but it increases power, wear and slime production. Very fine liberated particles may also become more difficult to recover by conventional gravity methods.
The target grind should be selected from size-by-size test results, not from a general belief that finer is always better.
What happens to gravity tailings?
A flowsheet is incomplete until it addresses the gold that gravity does not recover. Depending on mineralogy and test response, the next route may include flotation, leaching, sale of a concentrate or rejection of uneconomic material.
Gravity test work is therefore both a recovery test and a decision tool for the remaining circuit.