Roasted zinc concentrate (calcine) is leached continuously in a string of five tanks.
The liquid flow into the first tank is a mixture of spent acid and overflow from the jarosite thickener. Oxygen is used for the oxidation of iron. This process is also a purification process. Accurate pH control is essential for a good result.
The first three tanks have screw feeders for adding calcine and the pH values are automatically controlled by three independent control loops. The controller outputs goes to the variable frequency drives for the electric motors on the screw feeders.