A case study of optimising UG2 flotation performance part 2: Modelling improved PGM recovery and Cr2O3 rejection at Northam's UG2 concentrator

Abstract In Part 1 of this 2 part series of papers the relationships between Platinum Group Metals (PGM), mass and water recovery and %Cr 2 O 3 (as chromite) in concentrate were defined for laboratory, pilot and plant scales. The entrainment of chromite in final concentrate was shown to be related to its slow floating kinetics. In 2004, Northam Platinum mine embarked upon an upgrading program on its UG2 plant which included the installation of two column cells with external spargers as final cleaners. Optimisation resulted in a PGM recovery increase of 6% whilst %Cr 2 O 3 in final concentrate was reduced from 4.0% to 2.2%. In this second paper, the Northam circuit is modelled before and after modification and installation of the final cleaner column cells. Simulation shows that the external sparger, driven by a dedicated recirculation pump, imparts energy into the system and provides the column with a PGM recovery capability equal to that of a mechanically driven cell. The deep froth bed of the columns enhances chromite rejection. As a result of this, greater operating flexibility has allowed PGM recovery at Northam to be increased by increasing mass pull whilst at the same time reducing %Cr 2 O 3 in concentrate. Modelling shows that Northam’s reduction in %Cr 2 O 3 can be fully described by reducing only the laboratory to plant scale-up factor for slow floating rate of chromite by 25%, whilst leaving the normal scale-up factors for fast floating fraction and rate unchanged. The improvement in PGM recovery was simulated using the same set of rougher and cleaner feed PGM kinetics and scale-up factors. This suggests that chromite entrainment is modelled via its slow floating rate and PGM recovery by true flotation is primarily modelled via its fast floating fraction and rate. Floatable gangue is modelled via its full set of fast and slow kinetics as its passage into final concentrate (and cleaner tailings circulating load) is a mix of true flotation and entrainment. This case study shows that a standard flotation circuit and one incorporating Northam’s column cell may be predicted from laboratory scale rate tests and flotation kinetics. The impact of Northam’s technology on other UG2 ores may be predicted by applying the change in chromite scale-up factors determined by simulation. The degree of improvement is dependant on the ore’s mineralogy and specifically its selectivity between PGM minerals, chromite and floatable gangue. This is illustrated by simulating the impact of the Northam column cell on Barrick’s Sedibelo UG2 ore.