Kiyoshi Shibata and Yoshio Waseda
Institute for Advanced Materials Processing, Tohoku University, Sendai 980-8577
Recycling of materials is not always economically or environmentally beneficial, especially when the demand for its cascading use is limited by contamination of impurities. The impurity content in metal scrap will increase, when the recycling ratio becomes high. It is anticipated that the removal of such impurity requires additional load, such as energy consumption or environmental burdens, which can eliminate the importance of recycling. In this work, a system model has been developed and the requirement for the performance of the impurity removal process to meet the original purpose of the recycling is discussed. The modeled system consists of three processes; smelting of ore, simple re-melting of scrap and removal of impurities in the scrap. The relationship between the amount of recovered scrap and its impurity content was expressed as a binomial function. Contamination factor was defined as a ratio of standard deviation of the binomial equation to average content of the impurity. The amount of the production in each process was calculated, as a function of the impurity removal efficiency, yield and the contamination factor. The total load of the system and the allowable maximum load in the impurity removal process were described as a function of the recycling ratio and the performance of the impurity removal process. If the impurity removal efficiency or the yield is too low, the allowable maximum load becomes lower. The contamination factor has little effect on the maximum allowable load, but the impurity content itself affects significantly the requirement for the performance of the impurity removal process. The model, proposed in this work, is capable of giving quantitative information necessary for process development and operation control.
(Received July 17, 1998; In Final Form November 5, 1998)
recycling, scrap, impurity, energy consumption, environmental burden, mathematical model, recovery, separation, steel, non-ferrous metals
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