Process Control and Mill Economics

 

Without any doubt, one of the most important advances on treatment plant economics has been the development of advanced process control. It has been note that the performance of several gold operations with flotation and/or cyanidation processes is enhanced by including control systems. For example, the earliest multi-stream X-ray diffraction units have been superceded by smaller, more versatile X-Ray fluorescence analyzers capable of detecting elements of ever-lower atomic number and bringing the benefits of on-stream analysis within reach of small operations. Combined computer-based control systems, on-stream analysis has revolutionized gold processing plants, cutting operating costs and more importantly increasing gold recoveries, grades and throughputs, mainly in flotation and cyanidation plants. In this way it is possible to minimize natural and operator induced fluctuations. No longer an operator has to work blind and pits all his skills.

Other important consideration is related to computer-based control systems, which change reagent addition and air flow rates in accordance to grades and tonnages. They are excellent to keep reagent consumption at minimum. Probably, air is the cheapest reagent and flotation control strategies based on altering flow rates are the most effective. Also, grades and flow rates of intermediate streams such rougher concentrate and cleaner tailings are often as important as those of the final products, concentrate and tailings. Typical control loops include collector addition/feed assay and air/rougher concentrate assay. On-stream analysis and computer based process management systems cannot longer be considered a luxury because they permit short and long term trends to be identified and predicted and control strategies to be developed to smooth and optimize operations.

Gold plants are used to include control systems with profile optimization that takes into account potential revenues at different grades and recoveries and operating costs. This consideration makes easy the job of the operator by providing an actualized picture of what he is controlling and may increase the operator and generate a total improvement of the productivity. Basically, the rapid response time of on-stream analysis systems facilities the task of plant metallurgist when testing new operating conditions. The grinding circuit has progressed from simple rationing of feed water to tonnage and discharge water to hydrocyclone feed slurry density to sophisticated strategies including mill weight measurement, determination of circulating load, and on-stream analysis of product. For example, on-stream size analyzers have increased milling capacity everywhere they have been installed, but more important they have contributed to maintain a steady particle size distribution to flotation or cyanidation and this have improved gold concentrate grades and cyanidation recoveries.