Dynamic modeling of cadmium substance flow with zinc and steel demand in Japan

Abstract The demand for cadmium has gradually decreased lately, because of its toxicity and consequent regulations. However, cadmium is coproduced in zinc smelters, and therefore the production of cadmium ingots and cadmium emissions are inevitably affected by zinc demand. Most of the zinc is used for galvanizing steel, and therefore it is of interest to investigate the linkage between cadmium flow and zinc and steel demand. In addition, cadmium is used for secondary batteries, Ni–Cd batteries, alloys, etc. Therefore, a substantial amount of cadmium is still being stocked and discarded in the form of products, and management of the cadmium contained in these end-of-life products is further required in the future. To describe the flow and stock of cadmium in Japan, a dynamic substance flow analysis was conducted in this study. The linkage between cadmium, zinc and steel was quantitatively expressed: 1 t of galvanized steel sheet requires approximately 20 kg zinc, which induces 90 g cadmium in production. The emission intensity of cadmium into air was in the range 0.189–0.296 g Cd/t Zn, whereas the emission intensity into water has decreased by more than 50% during 2003–2008. It is likely that the in-use stock of cadmium and the amount of cadmium contained in discarded products have already passed their peak, and have decreased recently. The recovery rate of cadmium from Ni–Cd batteries has increased. However, it was estimated that 80%, or 700 t, of the cadmium in discarded Ni–Cd batteries, a significant proportion of the total of 840 t of discarded cadmium, had still not been recovered in 2008. The discarded Ni–Cd batteries might have gone into landfill or incineration plants, or been stored away in households. To reduce cadmium emissions, recovery of used Ni–Cd batteries needs to be enhanced. The recovery rate of zinc from end-of-life products to reduce the consumption of zinc ore, which results in the decrease in primary cadmium production, also needs to be enhanced. It is also suggested that the sustainable use of cadmium, e.g., the CdTe photovoltaic systems that use cadmium as a raw material, should be investigated.