||Lead-acid batteries as the main product for the world’s lead production, is also a major source of secondary lead. With the increasing depletion of lead mine and the rapid increase of waste lead production, recycling the waste lead-acid batteries has become an inevitable trend of modern society. Waste lead-acid batteries are generally treated by breaking and separation, producing grids, lead pastes, and plastics. Lead paste (including lead mud and lead sand) is the most complicate portion in the waste lead-acid batteries. If discarded directly, it worsed cause serious environmental problems. During the waste lead-acid batteries recycling process, lead paste is abundant of lead sulfate whose recovery ratio worsed highly influence battery recycling profit and market competition. Thus, this study aimed to improve and upgrade the existing battery recycling technology for lead paste.|
In this study, ammonium bicarbonate as the desulfurization reagent was used to convert lead sulfate (PbSO4) to lead carbonate (PbCO3). The advantages of this innovative technology included high purity and price of lead products, high recycling efficiency of waste lead-acid battery, low emission of acidic air pollutants, less corrosion problems of pipelines, low energy consumption, and low odorous pollution problems. An elemental analyzer (EA) was applied to analyze the sulfur content of the lead paste before and after the reaction. According to the desulfurization efficiency of lead paste, we explored the influences of ammonium bicarbonate addition, stirring time, and stirring speed on the desulfurization efficiency. In particular, ammonium bicarbonate can be reproduced by absorbing carbon dioxide in the flue gas with ammonia solution. This innovative technology could not only produce ammonium bicarbonate, but also reduced the emission of greenhouse gases from stacks, and further investigated the absorption efficiency of CO2 and the desulfurization efficiency of lead paste, and thus established an innovative and comprehensive resource recycling system. Finally, the lead paste was then conducted in a pyrometallurgical reduction process to produce lead ingots. By adding calcium hydroxide, this study also investigated the stirring time and stirring speed on the removal efficiency of sulfate ion to the ammonium sulfate filtrate.
Sodium carbonate and ammonium bicarbonate as desulfurization reagents, both accomplished complete reactions, and had considerable desulfurization effect, ammonium bicarbonate demonstrated better desulfurization efficiency than that of sodium carbonate. Experimental results confirmed that ammonium bicarbonate can be applied in the lead paste’s desulfurization technology. The optimum operating parameters for lead paste and ammonium bicarbonate reaction were as follows: the mass ratio of 0.5 for ammonium bicarbonate and lead paste, the reaction time of 60 min, and the stirring speed of 150 rpm. In this case, lead mud and lead sand almost achieved their highest desulfurization efficiency (99.31% for lead mud and 91.52% for lead sand).
This study concluded that ammonium bicarbonate can be successfully applied for lead paste desulfurization, which can be produced by absorbing carbon dioxide with ammonia solution. The concentration of carbon dioxide simulated in the flue gas was 15%. With different concentration of ammonia solution (0.5~18%) to absorb CO2 for 1 hr, the pH values of the solution ranged between 6.3 and 10.3, indicating that the major reaction product was ammonium bicarbonate (NH4HCO3). The optimal conditions for the absorption of CO2 were as follows: the concentration of ammonia solution of 10%, CO2 absorption time of 1 hr, and the intake air flow of 1 L/min. The maximum absorption efficiency of 15% CO2 was 93.52%. The optimal conditions for the desulfurization of lead paste were as follows: the concentration of ammonia solution of 2%, CO2 absorption time of 1 hr, the intake air flow of 3 L/min, the reaction time of 60 min, and the stirring speed of 150 rpm. Under this circumstance, the desulfurization efficiency of lead mud was 94.65%, the desulfurization efficiency of lead sand was 87.78%, which was slightly lower than the chemical reagent ammonium bicarbonate.
For conducting the experiments of ammonium sulfate filtrate reacting with calcium hydroxide, the stirring speed can be negligible for the removal efficiency of sulfate ion, the optimal amount of calcium hydroxide added was 2.5 g and the reaction time was 70 min, which could achieve the removal efficiency of sulfate ion higher than 70%. By purging CO2 into the ammonia solution filtrate, ammonium bicarbonate could be regenerated. The main component of the desulfurized lead paste was lead sulfate and converted to lead carbonate which can be reduced to form lead ingots at 850℃. Compared to traditional reflection smelting furnace operating at 1200℃, this innovative technology could not only maintain low energy consumption, but could also reduce the emission of SO2, and thus improve the corrosion problem.