Gravity separation, one of the oldest separation techniques, has become increasingly popular in modern plants, with new equipment enhancing the range of separations possible. When coupled with generally low capital and operating costs and lack of chemicals to cause environmental concerns, this often provides an attractive process for the recovery of gold.
Gravity separation relies upon the differences in density of minerals to provide efficient separation. The ease and efficiency of separation is dependent on a number of factors, including relative density, particle size and shape, liberation – all of which affect the selection of equipment type.
In the case of gold, gravity tools can be useful in solving a number of problems. These can include what is termed spotty or coarse gold, which makes mass balancing and gold accounting extremely difficult. By utilizing gravity ahead of the leach train, early recovery of gold in the process can also have financial benefits and avoid potential losses. Gravity recovery is also a useful diagnostic tool and can, and has been, used to check for the potential salting of samples. Removal of coarse gravity-recoverable gold can also enhance leach kinetics in plant practice. Use of gravity recovery as a safety net on tailings has also been exploited at several operations, where unleached gold, either as coarse particles or sulfide locked, are recovered from the tailings by gravity means and re-treated, usually with a re-grind prior to a re-leach. The range of equipment available for gravity separation includes standard mineral jigs, Kelsey jigs, In-Line Pressure jigs, spirals, tables, Mozeley sizer, Knelson, Falcon Superbowl and others.
Conventional jigs are often used to recover heavy minerals that are liberated
at a coarse particle size from crushing/grinding circuits, thus avoiding subsequent over-grinding and loss.
Centrifugal jigs use enhanced forces generated by their spinning motion to enable finer particle sizes and closer specific gravity (SG) minerals to be separated. The Kelsey jig is the most common example of this type of separator.
Spirals are one of the oldest gravity separators. There is a wide range of profiles available including low-grade, medium-grade, high-grade and fine mineral models, plus ones incorporating different wash water techniques. Careful monitoring and control of size distribution is important in achieving optimum results with spirals.
Mozley gravity separator
The MGS is a low-capacity high-performance gravity separator suitable for treating difficult fine particle feeds below 75 μm.
Falcon and Knelson concentrators
These are centrifugal type gravity separators also suited to fine particle-size
feeds. These units come in batch and continuous configuration for both laboratory testing and operational application.
Tables are often used in the laboratory as a preliminary test to ascertain an ore’s amenability to gravity separation or upgrade, or as a tool in their own right. Size of tables used in the laboratory environment vary but usually range from third or quarter production size up to half and on occasion full size.
A laboratory diagnostic tool used to produce the highest concentration of material, super-panners are often used in conjunction with other gravity devices as a final cleaning step, and in the preparation of samples for mineralogical work.