Platinum and rhenium extraction from a spent refinery catalyst using Bacillus megaterium as a cyanogenic bacterium: Statistical modeling and process optimization

• Extraction of Pt and Re from a spent catalyst was studied using Bacillus megaterium.
• Response surface methodology was applied to evaluate and optimize two main factors.
• Optimum values for pulp density and glycine conc. were obtained 4% and 12.8 g/l.
• Maximum recovery of Pt and Re at optimum condition was 15.7% and 98%, respectively.
• Kinetics of metal extraction was investigated using modified shrinking core model.

The present study evaluated the potential of Bacillus megaterium as a cyanogenic bacterium to produce cyanide for solubilization of platinum and rhenium from a spent refinery catalyst. Response surface methodology was applied to study the effects and interaction between two main effective parameters including initial glycine concentration and pulp density. Maximum Pt and Re recovery was obtained 15.7% and 98%, respectively, under optimum conditions of 12.8 g/l initial glycine concentration and 4% (w/v) pulp density after 7 days. Increasing the free cyanide concentration to 3.6 mg/l, varying the pH from 6.7 to 9, and increasing the dissolved oxygen from 2 to 5 mg/l demonstrated the growth characteristics of B. megaterium during bioleaching process. The modified shrinking core model was used to determine the rate limiting step of the process. It was found that diffusion through the product layer is the rate controlling step.