Recovery of lead from lead paste in spent lead acid battery by hydrometallurgical desulfurization and vacuum thermal reduction

- Vacuum thermal reduction avoided the harm of lead particulates in the reduction process.
- Hydrometallurgical desulfurization solved the problem of the emission of sulfur dioxide to the atmosphere.
- The vacuum thermal recovery process of lead has the potential of economic and ecological benefits.

Lead sulfate, lead oxides and lead metal are the main component of lead paste in spent lead acid battery. When lead sulfate was desulfurized and transformed into lead carbonate by sodium carbonate, lead metal and lead oxides remained unchanged. Lead carbonate is easily decomposed to lead oxide and carbon dioxide under high temperature. Namely, vacuum thermal process is the reduction reaction of lead oxides. A compatible environmental process consisted of hydrometallurgical desulfurization and vacuum thermal reduction to recycle lead was investigated in this research. Lead paste was firstly desulfurized with sodium carbonate, by which, the content of sulfur declined from 7.87% to 0.26%. Then, the desulfurized lead paste was reduced by charcoal under vacuum. Under the optimized reaction conditions, i.e., vacuum thermal reduction at temperature 850 °C under 20 Pa for 45 min, a 22.11 × 10−2 g cm−2 min−1 reduction rate, and a 98.13% direct recovery ratio of fine lead (99.77%) had been achieved, respectively.