At Mponeng the system has enabled the gold plant to achieve a steady feed from the thickeners to the leach and significant savings in reagent use as a result of improved control and field measurement failure correction. “In the past when the field instrument failed, the default condition was to add excess reagent,” Rudi Steyn, Anglogold Ashanti’s Mponeng gold plant manager, says. “However, using LeachStar the default is based on that day’s historical process requirements, which eliminates the spiking of reagents. An added bonus is that the new system has also greatly improved carbon management.”

Roger Paul, Mintek technology general manager, says the LeachStar system, one of a family of advanced process control systems developed by Mintek to enhance the operation of a number of different metallurgical operations, is taking off well and this control technology has refined the predictive nature of the cyanide system and can save significant consumption of cyanide in gold circuits.

Mintek has focused on this aspect of the gold circuit because not only is cyanide expensive, costing some US$1,200 a tonne on the open market, but increasingly there is environmental pressure to monitor and account for the use of cyanide.

One of the advantages of the LeachStar over its competitors is that it combines advanced process control with Mintek’s state-of-the-art cyanide analyser, the Cynoprobe. The Cynoprobe has excellent measurement frequency and can obtain four times as many readings when considered as readings per unit time. It also has a unit measurement accuracy of within a 5 g/t of cyanide range.

A key aspect of the LeachStar cyanide control system is the real time model predictive controller (MPC) that reduces the oscillations between spikes and deficiencies of cyanide in the circuit. “Instead of having oscillations of between 100 and 300 g/t of cyanide in the circuit the set point can be at, say, 150 g/t and the amount of cyanide can oscillate between 145 and 155 g/t,” Paul says. “Ideally the plant wants a constant cyanide level that optimises the cyanide cost versus gold recovery.”

The question that the cyanide circuit, which involves a number of tanks in circuit, typically a six-tank cyanide circuit, poses is that of where the cyanide levels are controlled? Does one control the level in the first tank where the most cyanide is, but if this is at the right volume, then the circuit can become starved of cyanide further down the line and subsequently overdosed to compensate. If the cyanide concentration is controlled at the bottom tank then there is a big feedback lag at the top, with the lag in the circuit being something like 12 to 24 hours.

Cyanide

Cyanide circuit at Mponeng.

“Because the cyanide circuit is a complex system we want to have measurements at about three or more points and this is where the software linked probes that make interactive responses become important.”

Another important aspect of a cyanide control system is the way in which the total cyanide, made up of free cyanide and weak acid associated cyanide, is measured. “There is a growing appreciation of the significance of knowing the contribution of the weak acid associated cyanide and not just the free cyanide. Mintek’s adapted cyanide probe is as a result starting to sell well.

Mintek’s LeachStar and Cynoprobe technology thus represents a real advance in the measurement and control of cyanide circuits and thus in gold processing. This takes into account that subsequent to the introduction of the carbon in pulp (CIP) technology for the processing of gold in the late 1970s most of the research in gold processing has focused on fine tuning and getting the most of existing technology.

“There also has been research looking at alternatives to cyanide due to its toxicity, but if one considers the chemistry of gold, cyanide forms such a good complex with gold that as little as 130 to 150 g/t can be used, and there is no comparable alternative. It is no surprise that cyanide has been used in gold processing for centuries,” Paul says.

Compounds such as thiourea, thiosulphate and thiocyanate that are biodegradable can all bond with gold in situ but volumes of these are required that are orders of magnitude higher in comparison with cyanide. The advantage of cyanide is that it can interact with oxygen and ultraviolet to decompose into CO2 and nitrogen.MRA