Development of Wet Recycling Process to Remove Tramp Elements from Scraped Steels

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Many recycling processes for scraped steels have been proposed in these decades, however, wet processes to remove tramp elements are very rare because of their processing speed. We proposed again to use wet process based on electrochemical method to remove zinc and copper which are major harmful elements for using recycled steels. In a concentrated nitric acid solution, zinc and copper easily dissolved contrasting steel dissolution which are negligible by complete passivation of steels. Moreover, stability the passive films on steels in nitric acid depends upon the concentration of nitrate ions. We finally proposed a method to remove and recover coppers from scraped steels by using aqueous ammonia solution and electrolysis. In aqueous ammonia solution, copper and zinc dissolved as ammonia complexes in neutral to alkaline solution, however, iron remains stable passive state. To accelerate dissolution rate of copper, the scraped steels containing sufficient amount of copper was anodically polarized and the dissolved copper was deposited to the counter electrode by the same current. Current efficiency of the anodic dissolution was almost 100 % and that of cathodic deposition reached to 70 %. The purity of the deposited copper exceeds 99.9 %. In this research, we designed a bi-potentiostate having a capability of 50 A and 50 V of output. Using this, the electrode potential of the counter electrode also controlled to maintain a suitable deposition potentials for copper. To extend the acceleration of copper dissolution, anodic dissolution mechanism of copper was analyzed using a rotating disk electrode. The dissolution is controlled by diffusion of ammonia to the electrode in a stagnant condition. In a condition of eliminating diffusion effect, the rate determining step is found a chemical reaction following the electron transfer reaction.