Small-scale pyrometallurgical plant for recycling spent lead acid batteries: CFD analysis of a submerged lance furnace, environmental and economic assessment, and multiobjective optimization

Abstract

A thorough analysis of a small-scale Spent Lead Acid Batteries recycling plant was carried out. The plant included a Top Submerged Lance furnace as a smelter for which a transient model gave insight into the plant operation times and energy requirements. A Computational Fluid Dynamics simulation of the furnace was used to determine the hydrodynamics, thermal efficiency, and to establish a correlation of fuel flow and the smelter temperature. An environmental and economic assessment was conducted based on two indicators: the Environmental Impact Load (EIL) and the Net Present Value (NPV) as computed from the simulation of the pyrometallurgical recycling process in Aspen Plus®. The EIL considered the equivalent CO₂, SO₂; and 1,4-dichlorobenzene emissions, however the CO₂ equivalent emissions were two orders of magnitude higher than those of SO₂ and 1,4-dichlorobenzene and dominated the EIL. Six input variables: smelter and refining temperatures, the flow of fluxes (C, Fe₂O₃, Na₂CO₃), and water flow were systematically varied in more than 2500 Aspen® simulations to conduct a sensitivity analysis of the effect of the input variables on EIL and NPV. The results indicated that the smelter temperature was, by two orders of magnitude, the most relevant input variable for SLAB recycling in a pyrometallurgical process. The fluxes of carbon and iron (III) oxide had an impact on the process, but on a much lower scale. An optimization of the plant performance based on a function ($f={\alpha }_{\text{EIL}}\text{EIL}+{\alpha }_{\text{NPV}}\text{NPV})$ that considered weighting factors for EIL and NPV indicated that the lowest temperature selected for the smelter (1173 K) yielded the lowest values of EIL and the highest NPV. Smelter temperatures below 1173 K were not considered as they would reduce the flowability of the slag. The flow of Fe₂O₃ had some, but relatively minor impact on NPV. This analysis that applies for the first time a set of simulation tools to address both the environmental and economic impact of a small-scale recycling plant indicated that any efforts devoted to optimize the plant performance in this areas should focus on the reduction of the temperature of the smelter.