PbS quantum dot (QD) image sensors have emerged as promising chips for a wide range of infrared (IR) imaging applications due to their monolithic integration with silicon-based readout integrated circuits. However, avoiding primary toxic Pb usage and reducing the cost of PbS QDs remains crucial for widespread application. We present a novel cost-effective and environmentally friendly hydrometallurgical process for recovering PbCl₂ from spent lead-acid battery paste to synthesize high-quality PbS QDs. The method recovers PbCl₂ with a production ratio of 97% and a purity of 99.99%. PbS QDs and photodetectors synthesized from recycled PbCl₂ (R-PbCl₂) have comparable performance and quality to those made using commercial PbCl₂. R-PbCl₂-derived photodetectors exhibit a high external quantum efficiency of 49.6% and a high specific detectivity of 6.95 × 10¹² Jones compared to published studies. Additionally, based on R-PbCl₂, a PbS QD image sensor with 640 × 512 resolution successfully discriminated common solvents. Moreover, through life-cycle assessment and economic cost analysis, this novel synthesis route offers great advantages in the environmentally friendly use of chemical reagents and reduces the production cost of PbS QDs by 23.2% compared to commercial PbCl₂. Thus, this work not only contributes to the green recycling of spent lead paste but also provides a low-cost strategy for synthesizing PbS QDs and their optoelectronic application.