Hydrocyclones demonstrate a trait that is useful for gold mills – minerals with higher specific gravities having a progressively finer cut-size (whether this is desired or not). In plants with free-milling gold, this leads to gold concentrating in the milling circuits (see attached). On a size-by-size basis, concentrations of up to 100 times can occur in cyclone underflow. In this respect, hydrocyclones are outstanding rougher concentrators. Many operations take advantage of this effect by installing gravity-concentration units in the grinding circuit. Treating a split of the cyclone underflow is perhaps the most common, but ball-mill discharge and cyclone feed streams have also been treated.
There are a number of considerations for gravity circuits fed by bleed streams. First is the effect on the overall circuit water balance. As the gravity tailing (which represents nearly 100% of the feed mass) is typically returned to the ball-mill discharge sump, any water addition in the gravity circuit will affect the grinding circuit’s overall water balance. In most cases, the water balance can be adjusted with other streams in the ball-mill circuit, but the overall water balance should be carefully considered.
Most circuits treat cyclone underflow, but ball-mill discharge is also sometimes used as gravity-circuit feed. In either case, feed to the gravity circuit can be pre-concentrated to a certain extent. For cyclone underflow streams, weirs can be constructed in the underflow tubs. In this case, cyclone underflow is fed to a weired area in the underflow tub, and gravity-circuit feed taken from this area. Slurry excess to the capacity of the gravity circuit overflows back to ball-mill feed. For those circuits treating ball-mill discharge, a sluice can
often be arranged to provide some pre-concentration for the gravity circuit.
Flash flotation is also used in comminution circuits for gold recovery. The application on cyclone underflow is generally similar to integration of a gravity circuit. Milling-in-cyanide is also conducted by many operations. Cyanide and lime are added to the grinding circuit, often with trimming of both pH and cyanide levels during the subsequent leach process. There is relatively little information published concerning leach kinetics, but the addition of cyanide to the milling circuit clearly initiates the leach process earlier. This practice is not recommended for those ores that contain preg-robbers (which necessitates the use of carbon-in-leach (CIL)), or when pre-aeration is required to passivate cyanicides. In the case of cyanicides requiring pre-aeration, adding cyanide to the milling circuit can dramatically increase the total amount of cyanide required.