Selective mechanochemical alkaline leaching of zinc from zinc plant residue

• Zinc was selectively leached from PHFC in alkaline solution using MCAL1 method.
• Maximum zinc recovery by MCAL method was reached to 99.99%.
• In comparison with CAL2 (maximum zinc recovery of 82.4%), MCAL was more efficient.
• During CAL method, an insoluble layer of Ca3[Mn(OH)6]2was formed on ZnO particles.
• This insoluble layer was continuously removed during attrition through MCAL.

A large amount of plant residue containing heavy and hazardous metals is generated in Iranian lead–zinc plant annually. The plant residue is known as hot filtercake (HFC) which contains 8–20% zinc, 6–16% manganese, 5–25% calcium and 0.5–3% cobalt. This paper is devoted to the mechanochemical alkaline leaching (MCAL) of zinc from HFC. The possibility of complete selective zinc leaching from HFC by the MCAL method was investigated through a 2 × 4 × 5 full factor design wherein effects of NaOH concentration, leaching time and milling speed were evaluated. In addition, the results were compared with the results of conventional leaching method.The results from the conventional alkaline leaching (CAL) indicated that just after a while from the beginning of the reaction, as confirmed by XRD investigations, an insoluble calcium manganese hydroxide layer forms on active surface of ZnO particles which prevents diffusion of leaching solution. Consequently, leaching process was halted and maximum recovery did not exceed 82.4%. Mechanochemical leaching results revealed that intensive milling in an attritor enhanced leaching performance, selectively. This is attributed to a wide structural change and abrasion of insoluble layer during MCAL. Subsequently, zinc recovery of 99.9% was obtained. The statistical analysis of the results showed that NaOH concentration, milling time and milling speed were statistically significant factors affecting the leaching process at 95% confidence level. In conclusion, approximately complete zinc recovery from HFC using the MCAL method can assist cobalt and manganese extraction process from the plant residue in the subsequent process.