Orway Mineral Consultants (OMC) has undertaken hundreds of grinding circuit optimisation studies in mineral processing plants globally. In undertaking these, several common problems are encountered repeatedly, irrespective of other variables. One re-occurring problem is cyclone control. The cyclone in grinding circuits plays a critical role in classifying the particles, separating the product from material requiring further grinding. They are selected and configured to achieve a target grind size under optimal conditions for the project: pressure, water split, feed and overflow density. However, the performance of the cyclone within the circuit is more influenced by ore properties, mill power draw and feed tonnes, ie specific energy input and energy requirement of the ore. This can be highly variable in some operations, resulting in a product which does not meet specifications. The question of best practice cyclone control is frequently debated but the pros and cons are not often well-understood holistically within the context of the overall comminution circuit performance. When the product size of a circuit goes out of specification, many control systems adjust the water balance in the circuit to constrain the product size, or metallurgical teams look to optimise cyclone pressure, number of cyclones operating, water addition or components. When considered from first principles, this doesn’t make sense as the cyclones do not undertake any grinding and classification efficiency is a minor contributor. The misunderstanding appears to stem from attempts to optimise the unit process (classification) in isolation from the system (grinding). Actions taken to control the grind size in a grinding circuit are different to what is done to theoretically adjust the cut size of a cyclone.
This paper discusses common problems seen with cyclone operation and control and discusses the OMC recommended cyclone control philosophy to maintain equilibrium and stability in grinding circuits. Several case studies are presented demonstrating the benefits.