Abstract: An automatic leaching system for the hydrometallurgical production of zinc comprising a pH meter and means for detecting the feed rate of calcine. The pH meter is provided with means for automatically washing the electrodes thereof so as to be capable of continuously and automatically detecting the pH value of the slurry obtained by mixing a spent electrolyte with the calcine. The system comprises a feedback control circuit and a feed-forward control circuit in which the flow rate of the spent electrolyte supplied to the system is the manipulated variable, and the pH value of the slurry is the controlled variable. These control circuits are used for the pH control so that the pH value of the slurry can be maintained constant by controlling the flow rate of the spent electrolyte.