Layer modeling of zinc removal from metallic mixture of waste printed circuit boards by vacuum distillation

• Low-level chamber pressure did not exert significant effect on zinc removal.
• Central composite design selected the process variables with significant effects.
• The zinc in metallic mixture could be efficiently removed by vacuum distillation.
• The Blake-Kozeny–Burke-Plummer equation was introduced into mass transfer equation.
• A layer model was established to elucidate the mechanism of zinc removal.

A layer model was established to elucidate the mechanism of zinc removal from the metallic mixture of waste printed circuit boards by vacuum distillation. The removal process was optimized by response surface methodology, and the optimum operating conditions were the chamber pressure of 0.1 Pa, heating temperature of 923 K, heating time of 60.0 min, particle size of 70 mesh (0.212 mm) and initial mass of 5.25 g. Evaporation efficiency of zinc, the response variable, was 99.79%, which indicates that the zinc can be efficiently removed. Based on the experimental results, a mathematical model, which bears on layer structure, evaporation, mass transfer and condensation, interprets the mechanism of the variable effects. Especially, in order to reveal blocking effect on the zinc removal, the Blake-Kozeny–Burke-Plummer equation was introduced into the mass transfer process. The layer model can be applied to a wider range of metal removal by vacuum distillation.