The need for automatic control of mineral processing plants requires continuous accurate measurement of process streams and can result in increased productivity, improved metal recovery, improved concentrate grades, reduced operating costs and lower product variability.
Modern mineral processing plants are very capital intensive. Economies of scale, teamed with high metal prices, dictate the need for even higher operating efficiency.
This is especially important given that minerals processing plants these days are becoming increasingly complex in nature and processing ever lower feed grades. Process control in mineral processing plants has made rapid developments since the early 1970s due to the introduction of real time sensors such as magnetic flowmeters and nucleonic density gauges, plus the increasing availability and reliability of PLCs and microprocessors.
The development of On Stream Analysis (OSA) by Outokumpu was a significant milestone providing real-time metal assays on process streams. Further developments have been made where XRF analysis of iron ores on moving conveyors have been installed (Pilbara). On stream monitoring of ash in coal is becoming increasingly used in coal preparation plants. Weighing of the ore is now more accurate with modern electronic weightometers and they offer less drift and greater reliability. On line moisture measurement is also becoming a reality.
On line particle size analysers have been successfully utilised in large grinding circuits where energy efficiency is a major cost issue. Energy is always the single largest cost in mineral processing plants. Cameras and image analysis software are also used to generate size distributions of crushed ore on moving conveyors.
Other important sensors are pH meters and level and pressure transducers, all of which provide a signal relating to the measurement of a process variable. This allows the final control element (servo valve, pump or variable speed motor) to manipulate the process variable.
The availability of continuing lower cost and higher powered digital computers over the last thirty years has revolutionised process control in mineral processing plants. At the same time the development of high level languages allowed more user-friendly software and greater flexibility. A better understanding of the process has also lead to the development of mathematical models of unit processes and algorithms that can be applied to the process. The use of the ‘Froth Cam’ to look at the froth and compare with a database and then report the condition of the froth and suggest improvements is another example.
For large grinding circuits ‘artificial intelligence’ systems are still in their infancy and reasonably crude but they will suggest solutions to the operator when faced with challenges in maximising productivity. Expert systems are still some way off becoming a reality although ‘rules of thumb’ and fuzzy logic apply.
Metallurgical Accounting has industry wide issues in mineral processing plants and AMIRA has introduced P754 a ‘Code of Practice and Guidelines’. The biggest problem is accurate measurement and mass balancing around mineral processing circuits. There are sampling issues, limitations of the two product formulae, and errors in measurement of moisture in mill feed samples which result in poor reconciliations from mine to mill to shipment.
A number of times the capital investment in process control has been queried by boards with respect to the cost benefits. A financial analysis reveals the payback is usually hours or days and the benefits of installation of automatic control systems result in energy savings, increased metallurgical efficiency, increased throughput, decreased reagent consumption and process stability.
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