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The benefits of automatically controlling minerals processing plants

Controlling mineral processing plants automatically can improve productivity and help to reduce operating costs, quickly returning the cost of investment.

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 and economies of scale plus metal prices dictate a high operating efficiency is required given that they are becoming increasingly complex in nature and processing ever lower feed grades.

Processing developments

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 flowmetres and nucleonic density gauges plus the increasing availability, reliability and decreasing cost of programmable logic controllers (PLCs) and microprocessors.

These instruments have proven to be very accurate and reliable. Many observers were initially doubtful this could ever be achieved in such a hostile environment (heat, dust, abrasive slurries) typical of most mineral processing plants. The development of On Stream Analysis (OSA) by Outokumpu was a significant milestone development providing metal assays on process streams in real time.

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 valves, pumps, variable speed motors) to manipulate the process variable.

Digitising the process

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. This has allowed operator-friendly graphics, alarming functions, trending charts and the production of shift reports providing powerful tools to the production personnel.

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 have been successfully applied.

In highly variable ore feed flotation circuits, ore characterisation template guidelines are being employed which suggest to the operator what parameters should be used. The operator can gauge from the mine schedule and operating parameters and the templates what corrective action to take.

Circuit design and optimisation by computer simulation software, such as JKSimMet, has become a very powerful tool for mineral processing plants. The risk reduction when designing crushing and grinding circuits is now well-established as a key step.

Standards & guidelines

Metallurgical Accounting has industry-wide issues in mineral processing plants and AMIRA has introduced the P754 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, 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.

*Mineral Engineering Technical Services

Damian Connelly, director/principal consulting engineer

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