An investigation of the effects of water injection on froth flotation.
The majority of platinum production in South Africa is from the UG-2 chromitite seam. Fine grinding of this material followed by flotation has resulted in a chromite entrainment problem which affects the efficiency of the downstream smelting furnace. Froth washing offers the potential for selective removal of chromite and possible improvements in platinum grade and recovery from the flotation concentrate. This project was aimed at improving flotation efficiency using froth washing on a laboratory scale and evaluating its application to various stages on a platinum flotation plant. A synthetic mixture of minerals was chosen for the laboratory batch flotation test work to enable monitoring of the flotation and entrainment of fully liberated particles. Smith and Warren (1989) described entrainment as a function of water recovery. This simple model was used to compare the entrainment factor for froth washing tests. Subsequent work focused on mixtures of two components, i.e. limestone (gangue) and Alsil P (floatable mineral) and the use of froth washing to reduce gangue entrainment. Several methods of froth washing were investigated and the use of a submerged wash water bar was chosen. Variables included the position for water injection, the rate of water addition, concentration of reagents and the stage of flotation (rougher, cleaner and scavenger). Results indicated that the effectiveness of froth washing was mostly dependent on the stage of introduction and the positioning of the wash water. Laboratory tests found that the best positioning for washing was 3cm from the lip of the cell. Although tests on the scavenger stage alone were disappointing, froth washing from the start of flotation resulted in an increase in recovery of floatable material of approximately 10 percent when comparing the same mass recovery. The tests conducted at Lonmin Platinum showed that the concentrate grade was increased by washing, but the flow of platinum (recovery) was reduced. Observations and results showed that the positioning of the wash bar at the upper surface of the froth inhibited its flow. A few tests, with the wash bar at the froth/pulp interface showed promise, although the mass flow decreased by approximately 15 percent the PGM content increased by 19 percent.