The determination of the variables that impact the gravity recoverable gold is one of the first analysis required to design an optimum gravity circuit. It is important to note the difference between the potential of a gold ore for gravity recovery and a prediction of how much gold will be recovered by gravity. Basically, gold recovery is a function of three very different sets of variables: ore properties, grinding circuit and gravity circuit. When characterizing the ore, only the first should be measured as the final flowsheet is normally not selected yet. It is therefore inappropriate at this point to try mimicking the gravity circuit for the purpose of predicting gravity recovery.
The characterization process involves a laboratory testing program on a centrifugal concentrator. Basically, a representative sample of 50 to 80 kg is crushed to 100 passing 850 micron for a first recovery attempt. A first concentrate of about 90-100 g is produced and screened from 212 micron to 20 micron. The five coarser sizes are further upgraded with a hydrosizer and examined with a microscope. The tailing is split and a 20-25 kg sample is ground to 50-55% passing 75 micron for secondary recovery with the same centrifuge, but lower fluidization water and feed rate to recover finer particle more effectively. The tailing is treated at a final grind of 75-80% passing 75 micron. The concentrates of the last two stages are screened from 20 micron to top size and the fractions are assayed to extinction. A 500 g sample of each of the three tailings is screened from 20 micron to top size for fire assaying, up to 30 g or one assay ton per fraction.
The results of several samples are plotted in the same chart. In this way, there will be obtained three curves: low, average and high results. The curve called “low”, shows an ore for which gravity recovery would be inappropriate because the gravity recoverable gold content is low and fine. This type of material is usually found in the American cordillera, both in North or South America, and especially in Peru. Other factors that would contribute to a low gravity recovery are the specific gravity of the gangue, gold particles with a high silver content and a coarse grind size. the curve called “average” is for a typical response of free milling ore, which should yield a gravity recovery of 28-35% for many applications. The curve called “high” is characteristic of an ore very amenable to gravity recovery, which could easily yield gravity recoveries in excess of 50% at full scale. Gold ores that are extremely amenable to gravity recovery have yielded gravity recoveries of 90-92%.
Other alternative is to use a lab centrifugal concentrator to produce a gold rougher concentrate that is upgraded to a concentrate of few grams by hand panning or a shaking table. This procedure is appropriate to produce a gravity tailing that can be used to evaluate downstream recovery using flotation or cyanidation. In this case the amount of gravity recoverable gold is not an absolute predictor of plant recovery. Most the time, it will be necessary to ensure a reasonable weight of sample when the material is difficult. This is typically the case when flotation is the major recovery method with either low grade or complex gold ores. Metallurgist must keep in mind that piloting does not guarantee an accurate estimate of gold gravity recovery, as the piloting recovery effort may be higher, typically, or lower than what will be used at full scale.
Gravity recoverable gold