Cyanide Control at a Gold Plant, Gauteng, South Africa

The following are examples of actual data from a Gold plat to illustrate the comparative control between Stream A and B.

To evaluate the control capabilities on the TAC 2000 it was decided that Stream A would be controlled manually by the operators and Stream B is controlled automatically by the TAC 2000 analyser.

This evaluation of the cyanide control at this specific Gold Plant gives us an ideal platform to perform a comparison between Automatic TAC 2000 control versus manual operator control as the pulp being treated by Stream A and B is exactly the same pulp at the same density as it is only split for Stream A and B after the thickener, the flow rates are also similar.

In the three periods shown on the graphs below the difference in control is clearly visible, the manual being a classic saw tooth, the operators will add cyanide until they measure a high level of cyanide and will then cut back or close the cyanide dosing valve. The valve is than opened again when the cyanide level is measured to be low. Due to the large inertia of the cyanide in the Leach tank, the measured cyanide will have large variations. This type of control is very typical for manual control on gold plants worldwide.

The variation in cyanide level is also attributable to the fluctuation in incoming feed of pulp, again the problem here is that the operator does not have and prior warning of these fluctuations and can thus not take immediate action to counter them unlike the TAC 2000 analyser which gets the flow input and can thus make rapid changes in cyanide flow to compensate for the change in pulp flow.

One can also see the clear difference in the manually controlled stream on 4 May during the early hours of the morning and during the day shift when more emphasis is put on the cyanide control.

If one looks at the level of cyanide as being the major leaching agent then there are a number of times when the leaching of gold was compromised on Stream A as the cyanide value went below 100 ppm. Stream B never went down to this critical minimum levels. One could possible lower the setpoint of Stream B significantly and still have a large enough margin to ensure that all leachable gold is leached, this should translate into cyanide savings. During the times where the cyanide was higher than the setpoint, one could argue that the cyanide was wasted.

The TAC 2000 is fitted with our new ‘Dynamic’ control algorithm that is specifically designed to accommodate gold plants that have ore that has an unusually large variation in cyanide consumption, variations in pulp flow and even cyanide delivery anomalies. This kind of automatic control within 2 – 5% of the setpoint is no longer just the domain of large stable plants with constant feed of high quality consistent ore.


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