One of the most influential variables on the flotation process is particle size. For successful recovery by flotation, the valuable particle requires a certain degree of liberation to attach to a bubble and be recovered in the froth phase. In many ores, portions of the valuable mineral(s) in the feed are sufficiently liberated at coarser size fractions to allow early recovery by flotation in the grinding circuit. The recovery of coarse particles - particularly copper minerals and gold - in the grinding circuit is commonly known as “flash flotation” and is an area of growing interest in the minerals industry. It is an efficient method of reducing the effects of overgrinding of valuable minerals.
The objective of this project was to maximize the recovery of coarse, sufficiently liberated copper minerals and gold particles in flash flotation tests using a typical cyclone underflow size distribution.
A detailed literature review was completed in order to understand the flotation process with the primary focus on characterising coarse particle flotation behaviour. The flotation process was conceptualised as consisting of four sequential sub-processes as described in the literature:
• Particle-bubble collision
• Particle-bubble adhesion
• Elevation of stable particle-bubble aggregate
• Particle transportation through the froth phase
The probability of each sub-process occurring was used to analyse the success of coarse particle flotation. It was concluded that only the probability of particle-bubble collision was directly proportional to particle size. The other three sub-processes were found to follow an inverse relationship with particle size. The review also identified a number of key flotation conditions that are critical to the success of floating coarse particles:
• Particles require some degree of hydrophobicity for adhesion to occur.
• The agitation rate should be adjusted to maintain the slurry just above minimum suspension to limit cell turbulence and reduce particle-bubble aggregate detachment.
• Mobility induced drainage in the froth phase can be minimised by using a froth crowding device.
Laboratory flash flotation tests were conducted using ore from Newcrest’s Ridgeway copper-gold operation. A screening method was developed to replicate the provided underflow size distributions, which were the flash flotation test feed. The testwork variables were collector dosage and agitation rate. Both variables were tested independently to assess their effect on flash flotation performance. A size-by-size analysis was conducted at the optimal agitation and collector dosage rate to determine the recovery of coarse copper minerals and gold.
The results support the hypothesis that the proposed secondary cyclone underflow is more amenable to flash flotation than the existing primary cyclone underflow. Optimisation of the collector dosage and agitation rate allows this requirement to be satisfied. The collector testwork found that with increasing collector dosage the recovery and grade of the flash flotation concentrate improved. By increasing the collector dosage, coarse particles are able to adsorb sufficient collector to obtain a high degree of hydrophobicity required for successful flotation. The optimal collector dosage rates were 8 g/t and 11 g/t for the primary and secondary underflow stream respectively.
Tests at various impellor speeds support the literature that coarse particle flotation is favoured at lower cell turbulence such that the slurry is just in suspension. The size-by-size analysis proved that the recovery in each size fraction remains essentially constant. The potential increase in recovery will be caused by shifting the mass distribution of the copper minerals in the feed to size fractions with a higher recovery. The concentrate mass distribution is a function of the feed sizing of each mineral, the recovery values for each mineral, reagent addition and agitation rate.