Laws of Gravity Gold

Ten Laws of Gravity Gold Recovery

DON’T

Try to Evaluate the Economic Impact of Gravity Recovery
Get a Statistically Significant Sample of the Ore to Evaluate Gravity Response on a Size-by-Size basis
Design the Gold Recovery Circuit with a Good Understanding of Gold’s Behaviour in Grinding Circuits
When GRG Recovery Is Not Indicated, Look at Other Options
Design the Circuit for Ease of Operation and Mechanical Reliability Rather than Absolute Performance
Improve Gold Room Performance
Be Aware of Scale-up Problems
Pay Attention to Screening
“Overfeed” the Primary Unit
Recycle

Go for the BMW of Gravity Circuits: Nothing Can Be Too Good!
Extract a Large Number of Samples, Look for a Few Gold Grains, Preferably Very Large Ones, and Use them to Justify Circuit Design
Gold Gravity Is Just Like Any Other Gravity Recovery
Try to Produce Bullion By Gravity at All Cost (literally…)
Design the Circuit to Maximize the Recovery Effort… Most of the Time
Focus Only on Primary Recovery
Assume that Laboratory Recoveries Are Easily Reproduced at Plant Scale
Don’t Pay Attention to Screening
Maximize Primary Stage (Unit) Recovery
Keep Operation “Simple” by Avoiding Recycling

The following statements are generally applicable:

Gold grinds between 5 and 20 times slower than its gangue.
About 99% of all GRG fed to a cyclone reports to its underflow, unless it is a primary cyclone in a two-stage classification circuit.
GRG below 25 pm still reports to cyclone underflows in a proportion ranging between 75 and 95%.
Gold particles above 75 pm circulate between 50 and 100 times in a grinding circuit, unless they are recovered by gravity.
Once a particle of GRG breaks, more than 90% of its fragments are gravity recoverable.

The recommendations can be summarized as follows:

Proper design of the concentrate receiving tank, to minimize entrainment of fine gold with the return water.
Proper design of the bottom of the tank for easy concentrate removal: taper, water addition, and correct choice of valve.
Screening and magnetic removal of tramp iron from concentrate prior to tabling.
Centrifuge-based scavenging of fine table tails.
Recovery of spillage on the floor of the gold room (to be pumped to the primary concentrate tank).
Recycling of the tails from the gold room to the primary gravity unit.
Nitric acid treatment of the final concentrate when copper blasting wire is used.
Design of the gold room for headroom as much as floor space, to take advantage of gravity flow.
Recycling of the cleaner table tailing and rougher table middling to the primary concentrate hopper
Intensive cyanidation of primary concentrate or table tails if appropriate.

Original text by A.R. Laplante