Recovery of Vanadium and Nickel from a High CaCO3 Containing Petroleum Coke Ash by Roasting and Acidic Leaching

In this study, it was aimed to extract vanadium (V) and nickel (Ni) from a petroleum coke ash (PCA) using a roasting process without additives, followed by leaching with sulfuric acid (H2SO4). The experiments were designed based on the Taguchi approach, taking into account the parameters of temperature, acid concentration, time, and solid ratio. Additional leaching tests were conducted on the non-roasted PCA for comparison, to assess the effect of roasting on the extractions of V and Ni. The results showed that no extra reducing agent was needed as the PCA contained high levels of CaCO3, which could be used as a reducing agent during roasting. It was found that roasting was essential for high Ni extractions, but had no strong effects on V extractions. The Ni extraction was found to be between 13.3 and 80.8% for the non-roasted PCA and between 43.6 and 99.3% for the roasted PCA. The V extraction was between 36 and 97.9% for the non-roasted PCA and between 45.4 and 99.9% for the roasted PCA. The optimal leaching conditions were determined to be a sulfuric acid of 4.5 M, a solid ratio of 10%, a temperature of 75 degrees C, and a time of 75 min. In addition, it was determined that the leaching conditions had a great effect on the oxidation state of vanadium ions, and an increase in the acid concentration led to the formation of V3+ ions (green color) instead of VO2+ ions (blue color) in the pregnant leach solution. The final pregnant leach solution containing 1056.50 mg/L V, and 251.85 mg/L Ni was achieved with an extraction yields of > 98%. The experimental results were greatly fitted by the shrinking core model and the activation energy (E-a) for V and Ni was calculated as 3.60 and 4.01 kJ/mol, indicating that the leaching mechanism can be explained by the diffusion control model.