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The reliable pH analyser system

Halland Instruments AS

Applications overview:

APPLICATION: Automatic pH control of the Jarosite process.

Process description:
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 cycles 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 where 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 where installed they only take one sample from the process every eight hour.

pH range: 1 to 2 pH
Solids content: 50 to 70 g/l
Temperature: 85 C - 95 C
Flow: 250 m/h
Tanks: 4 tanks each with a capacity of 180 m
Number of pH analyser: 4


The Jarosite process.
Four tanks of 180 m in a square.
The pH analysers are mounted in the black hut with white door.


Jarosite process, the distributor tank delivers calcine slurry to the three tanks in operation. The green valve is open.

Jarosite process, trend last 24 hours, scale 0-3 pH, three tanks in use. The white lines are the measurement and the setpoint in the first tank, the red lines are the second tank and the green lines are the third tank. The system has excellent response to the disturbance at 0400 o’clock.

Trend Jarositt