Recycling Of Valuable Metals From Waste Ni-Cd And Ni-MH Batteries

Waste batteries contain plenty of hazardous materials and valuable metals. Therefore, recycling of waste batteries is an important subject not only from the treatment for waste but also from the recovery of valuable materials. Resource utilization of waste batteries can protect environment and alleviate bottleneck for economic development constrained by resources shortage. However, a serious environmental problem is presented by waste batteries resulting from lack of relevant regulations and effective recycling technologies. Therefore, it is urgent to develop a recycling technology with high performance and without negative impact to the environment to resolve the problems resulting from waste batteries.Based on circular economy concept and the principle of“reduce, reuse and recycling”, this paper considered the recycling of waste Ni-Cd and Ni-MH batteries. In the first stage of this work, the chemical and physical characterization of waste Ni-Cd and Ni-MH batteries was carried out. According to the chemical and physical characterization results, Cd, Fe, Ni and Co were determined to the recycling target metals. A new process including crush, vacuum distillation separation and magnetic separation was proposed through the chemical and physical characterization of waste batteries. Then this paper investigated the thermodynamic and dynamics theory of the recycling of Cd from waste Ni-Cd and Ni-MH batteries by vacuum distillation separation, and use the thermodynamic and dynamics theory and the distillation fundamentals to optimize operating parameters. The residues were magnetically separated after vacuum distillation separation based on the fact that different materials are extremely different in magnetic property. In order to recover major valuable metals, this paper investigated the fundamentals of magnetic separation process to recover Fe, Ni and Co from waste Ni-Cd and Ni-MH batteries.Based on the difference in vapor pressures of various metals at certain temperature, the metal with high vapor pressure and low boiling point （such as Cd, Zn, etc.） were separated from mixed metallic materials by vacuum distillation separation. With the thermodynamic theory and dynamics analysis, it has been shown that the Cd presented in the form from hydroxide to monoxides, and then presented in the form of Cd vapor in 2/3 AA type waste Ni–Cd batteries, respectively. The plastics and fabric used as a base for the electrolyte in waste Ni–Cd batteries, which were not separated during the dismantling and crushing process, generated CO originated from the pyrolysis in vacuum distillation separation process and acted as a reduction agent to reduce oxides.The L₁₆ （4⁴） orthogonal design performed with waste Ni–Cd batteries showed that it was able to effectively recycle cadmium from waste Ni–Cd batteries of different brands and models by using vacuum distillation separation. The recycling efficiency of Cd exceeded 99.5%, with the purity of 99%, in the condition of pressure = 0.1～3.1 Pa, temperature = 1073 K, heating time = 2.5 h, and the addition of carbon powder = 2 wt %, respectively.The residues after vacuum distillation separation were magnetically separated based on the fact that different materials are extremely different in magnetic property in nonuniform magnetic field. The magnetic separation tests performed with waste Ni–Cd and Ni–MH batteries showed that the size of particles between 0.5 and 2 mm and the rotational speed of separator between 0.25 and 0.5 m/s,with the diameter of 0.16 m,were suitable for magnetic separation, with recycling efficiency exceeding 97%. Magnetic separation process enhanced the recycling of waste Ni–Cd and Ni–MH batteries. It made the subsequent metallurgical recovery process more efficient and simpler, was beneficial to the further separation and recycling, and consequently significantly enhance the resource-utilization efficiency.According to environmental impact assessment results, the process including crush, vacuum distillation separation and magneitic separation can recover major heavy metals, particularly Cd and Ni, from the waste Ni-Cd and Ni-MH batteries and convert them into harmless or reusable products. Economic assessment results demonstrated that this new process for recycling of waste Ni–Cd and Ni-MH batteries will be benefits in economical terms in the industry-scale. Therefore,the proposed process including crush, vacuum distillation separation and magneitic separation was an environmentally friendly and economically feasible method.