In this process the iron is precipitated in a continuous process in a string of four tanks. Sodium is added before the first tank. The pH in the tanks is controlled by a computer program with four PID controllers with a train of on/off time cycle outputs (first on/off time cycle for tank 1, then on/off time cycle for tank 2...) The controller outputs goes to four on/off valves which adds calcine slurry or slurry from the underflow of the neutral thickeners. The pH setpoint in the last tank is fixed by the operator, the other setpoints are adjusted automatically by the computer program in such a way that the pH in the last tank is always controllable. For example, if the valve for tank 4 is open 90% of its time then the setpoints in the previous tanks are increased automatically.
Before this system were installed the operator had to go every second hour and fetch samples, take them to the process laboratory, determine the free iron content and correct the setpoints if necessary. After this system were installed they only take one sample from the process every eight hour.