Evaporation rates of zinc and lead in copper melt under reduced pressure

Removal of tramp elements from the remelted scrap is one of the important subjects for the recycling process of base metals such as copper. The present work was aimed to establish fundamental knowledge on the evaporation kinetics of Zn and Pb from inductively stirred liquid copper under reduced pressure. It was found that the evaporation rates of Zn and Pb from liquid copper follow the first-order rate equation with respect to Zn and Pb contents in the metal. The apparent first-order rate constant obtained is in proportional to the reciprocal of the total pressure in the reactor and is independent of the stirring condition of the melt under most of the experimental conditions. The rate of free evaporation calculated by the Hertz-Knudsen-Langmuir equation was found to be much faster than the observed rate. It was, therefore, concluded that the mass transfer in the gas phase would be the rate-limiting step in this case. However, the rate of Zn evaporation at 1693 K tended to be independent of pressure and to approach a constant value at a lower total pressure due to the effect of the mass transfer in the liquid phase. The mass transfer coefficient in the liquid phase was evaluated to be k_l=3.0 × 10^-4 m/s. The possibility of the removal of Zn and Pb by evaporation from liquid copper was discussed on the basis of the present experimental results and considerations.