（Yunnan Yuntong Zinc Co.,Ltd. Kunming China 650102 ）
Abstract: The leaching slag through leaching indium from Waelz oxidizing contained a large amount of jarosite. The reasons of the alum are analyzed theoretically. The influence on indium leaching rate about control of the acidity and the amount of reagent was considered. The control of creation and destruction of alum was proposed to increase the leaching rate of indium. The production practice shows that the leaching rate of indium increased from 60% to about 96%, and the indium content in the leach residue containing also decreased from 900g/t to 100 g/t ~ 200 g/t.