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

近几十年来，人们提出了许多回收废钢的方法，但是，由于其处理速度，去除杂质元素的湿法工艺非常罕见。我们再次提出采用基于电化学方法的湿法工艺去除回收钢中的主要有害元素锌和铜。在浓硝酸溶液中，锌和铜容易溶解，而钢的溶解通过钢的完全钝化可以忽略不计。此外，在硝酸中的钝化膜的稳定性取决于硝酸根离子的浓度。最后提出了用氨水溶液电解法从废钢中去除和回收铜的方法。在氨水溶液中，铜和锌以氨络合物的形式溶于中性至碱性溶液中，而铁则保持稳定的钝化状态。为了加速铜的溶解速率，将含有足够量的铜的废钢阳极极化，并且溶解的铜在相同电流下沉积到对电极。阳极溶解电流效率接近100%，阴极沉积电流效率达到70%.沉积铜的纯度超过99.9%。在本研究中，我们设计了一个具有50 A和50 V输出能力的双电位器。由此，对电极的电极电位也被控制以维持铜的适当沉积电位。为了进一步研究铜的阳极溶解加速机理，采用旋转圆盘电极对铜的阳极溶解机理进行了研究。溶解是由氨在停滞状态下向电极的扩散控制的。在消除扩散效应的条件下，速率控制步骤是继电子转移反应之后的化学反应。

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.