During the testing program is important to evaluate several concentration processes in order to get the highest gold recovery. Gravity concentration tests give a partial recovery and the sample must be tested again by other method in order to study the global performance. Considering the fact that gravity equipment is able to recover free gold particles, fine particles need to be recovered by other techniques. Froth flotation is an old process that allows recovering fine particle of free gold or associated with sulphides.
Laboratory Flotation Machine
Gold concentration by flotation is opened up due to there are several factors that can modify the surface mineral with the assistance of reagents which make the valuable particles floatable. Collectors and frothers are necessary to make possible the concentration. The adsorption of collectors may be influenced by adding other reagents which can be either supportive or inhibitive in effect. In these cases is common to make reference to activators and depressants. Air is added to transport the particles to the surface. Other reagents can be added when the non-valuable minerals interfere with the process.
The first part of the program includes rougher kinetics tests conducted on the gravity separation tailings or the whole sample. By these tests is possible to evaluate the effect of primary grind product, pH, collectors, modifiers and frothers on gold recovery. Sometimes is critical to select the collector, but as base line is common to select Potassium Amyl Xanthate (PAX) as the main collector and a Dithiophosphate (e.g. A-208, A-404) as a secondary collector. The frother can be MIBC and the first tests can be done at natural pH. PAX is selected because is a strong collecting agent whose action is sometimes enhanced by a secondary collector. If there aren’t frothing problems, MIBC is a good option, otherwise a strong and less selective frother such Pine Oil or Dowfroth D-250 can be tested. Since lime tends to affect gold flotation, to perform flotation tests at natural pH is the first option. About the particle size to be evaluated, it is common to evaluate flotation at 150, 125, 100, 90, 80 and 75 µm K80. Results indicate the most appropriate particle size.
Flotation tests by using a mechanical skimmer
Based on rougher flotation results, the next stage is to evaluate the fineness of regrind in the cleaning circuit. Since the rougher concentrate contains non-liberated particles is necessary to evaluate the effect of regrind on gold concentrate quality. Also, sometimes, the concentrate is reprocessed by cyanidation and this process is performed at 85-95% passing 75µm. Then, this evaluation has two important effects for the process. In this way, is common to evaluate the cleaning circuit at 70, 50, 45, 40, 35, 30, and 25 µm. Then, the program must try to find the optimum particle size. If the design consider gravimetry and flotation, the particle is not necessary to be very fine, but is must be enough to assure the separation among valuable minerals and gangue. Some programs are based on experimental design techniques that try to evaluate several conditions and specific targets. In this way, the program can be focused in getting recoveries as high as 90% and concentrates with high content of precious metals. Tests are selected under specific conditions and results are evaluated mathematically. Obviously, the metallurgist must be able to judge these results against the reality due to the numbers are an indication and need an interpretation.