Development of an innovative process for the selective recovery of metals from spent refinery catalysts

In the present work was studied and developed the treatment of spent refinery catalysts to recovery valuable metals contained in them. It was hypothesized that, using the sulfide precipitation could be possible obtain a selective precipitation of molybdenum, nickel, vanadium and aluminium after the early solubilisation of them by leaching or bioleaching processes. The refinery catalysts are used extensively in the hydro-treating processes for the production of clean fuels from fractions distilled from crude oil. The hydro-treating processes include the elimination of nitrogen compounds, sulphur and metals from charged of the catalytic cracking. The metals present in the charged cause the poisoning of the catalysts reducing their activity; the catalysts deactivated are classified as solid wastes by the United States Environmental Protection Agency (USEPA). Environmental regulations and interesting amounts of strategic metals guide towards the development of new feasible and sustainable process not only to treat this waste but also to recover valuable metals such as V, Al, Ni and Mo. The work was divided in two phases: the first phase consisted in the solubilization of spent refinery catalysts by bioleaching processes with sulphur-oxidising bacteria or leaching process by sulphuric acid. Once solubilised the spent catalysts, the metal-containing wastewater was treated by adding of sulfide source and sodium hydroxide to precipitate and recovering the four metals at different pH values (second phase). The sulfide sources used were Na2S chemical compound and H2S gas produced by sulfate-reducing bacteria in a lactate feed anaerobic baffled reactor. Tests were conducted to determine whether could be possible obtain the selective precipitation of all metals presents at different pH values as sulfide (molybdenum and nickel) and as hydroxide (aluminum and vanadium); the molybdenum precipitation occurred at low pH value 0.5, nickel at pH value 3.5 aluminum at pH value 4 and vanadium at pH value 6. The pH values were settled by preliminary solubility tests and preliminary chemical speciation studies conducted with Medusa software. The selective precipitation tests were done using two synthetic base metals solution called leach liquor and bioleach liquor (solution 1 and solution 2 respectively) simulating the typical metals concentration of liquor after an operation of leaching and bioleaching processes respectively. About the experiments conducted using as sulfide source Na2S, the percentages of precipitation were higher than 60% for all metals investigated (Al= 65 %; Mo= 87 %; Ni= 52 %; V= 64 %;) in the synthetic bioleaching solution (solution 2) and in the synthetic leaching solution (Al= 55 %; Mo= 75 %; Ni= 54 %; V= 67 %;) (solution1). The indices of purity of precipitates were calculated considering the total weight of metal target precipitated (for example the molybdenum at pH 0.5) on the total weight of precipitates (Mo, Ni, V, Al) at pH 0.5; in both cases of bioleaching synthetic solution and leaching synthetic solution the purity indices of molybdenum were 0.86 and 0.70 at pH= 0.5 for leaching and bioleaching synthetic solutions respectively; while about the nickel the purity indices were 0.76 and 0.92 at pH=3 for leaching and bioleaching synthetic solutions. Regarding the experiments conducted using as sulfide source the H2S gas produced by anaerobic baffled reactor containing sulfate-reducing bacteria the average sulfate reduction rate in the ABR was 130 mg L-1 d-1 and the average dissolved sulfide concentration 190 mg L-1 mg L-1. The precipitations of Mo at target pH of 0.5 was 36-72% and V precipitation was 64-70% at pH 6 depending of the initial concentrations of them. Percent Ni precipitation was up to 40 % at target pH of 3.5. The purity indices of Mo and V precipitates were 0.97 and 0.90 at pH 0.5 and pH 6, respectively. Were also conducted the technical and economic analysis processes by Super Pro simulating chemical process software on four processes (C-C: leaching solution-Na2S; C-B: leaching solution- H2S; B-C: bioleaching solution- Na2S; B-B: bioleaching solution-H2S) were determinate the payback time of the processes (PBT= time of return of capital invested), the production of liquid waste, the percentages of base metals recovered and the total recovery of matter. The results obtained shown that the process C-C was the best among the all factors investigated (PBT, production of liquid waste, percentages of base metals recovered and total recovery of matter). The PBT of C-C process was less than 2 years, the recovery of base metals were all higher than 80%, the total recovery of matter was around the 90% and the production of liquid waste was around 3 tons/ tons of catalysts treated.