Cyanidation Circuit Control

 

During the last years has been a growing interest in instrumentation and automation for process control in gold cyanidation plants. The initial period in the design of detailed processes for the extraction of gold from placers and rich veins containing free gold was around a century ago, well before the development of modern commercial instrumentation. Consequently, the gold recovery processes depended almost entirely on manual control supported by simple plant tests and an analytical service provided by the assay laboratory. Initial cyanidation plants were based in this type of control. These traditional procedures have served gold operations for a long time. Sufficient proof of their adequacy is the gold recovery of 90-95% achieved from gold ores containing 10-15 g/t of gold.

It is nevertheless a matter for some surprise that there have been small changes in these early processes. Cyanidation has survived more than ninety years of technical practice without basic alterations. Characteristic of the operations involved in gold extraction are the long response times to process changes, which makes them stable and amenable to manual control. A flywheel effect is engendered throughout the process by the large and uniform tonnages handled. Once the milling rate has been selected, corresponding classifier water/solid ratios are arranged, cyanidation solution strengths determined, agitation periods and wash ratios fixed, the whole operation proceeds smoothly and steadily.

Any deviation from the normal develops slowly and can be easily noted and corrected by simple and inexpensive means well before any adverse effects are experienced. Apart from the very early days of gold mining when a reef was mined at shallow depths, expenditure on mining greatly exceeded the costs of treatment. High recovery efficiencies were early attained on reduction plants with the employment of a small percentage of the total labor complement of the mine. Attention to improving gold mining practice has therefore taken precedence over advancing the extraction procedures which were operating effectively without many instruments, these being mainly ammeters and vacuum and pressure gauges. Then, in contrast to the developments that it has pioneered in other branches of engineering, contributions by the gold mining industry to process control have hitherto been small and are virtually only beginning. Much of the instrumentation already in use in several cyanidation plants has been adopted from that developed to meet the needs of other industries such as chemical manufacturing.

Emphasis has been placed on the measurement and control of the simple process variables: level, flow, temperature and pressure. These instruments are employed in combination with recorders or computing systems. Such process controllers have helped and relieved cyanidation plant operators of many tedious, but simple control operations. Because each control loop was independent the others, instrumentation at this level made no inroads into the function of the human operator as the overall controller of the process.