Conventional crushing, screening, grinding and classification circuits are renowned for extreme inefficiency, wear and maintenance. This has not changed significantly since their evolution over a century ago. The equipment has however become significantly larger so as to increase capacity for production.
Crushing and screening circuits of various configurations, before or after tumbling mills, are mandatory for overcoming limitations of tumbling mills. Tumbling mills are conventionally used to do the final size reduction for process feed. Tumbling mills have energy efficiencies in the order of 2% (based on theoretical particle surface area calculations). Recirculation inside tumbling mills is not readily quantifiable; comminution relies on sporadic, uncertain attrition and impact events. Oversize particles in their discharge stream are separated and re-circulated to their feed stream with a re-circulating mass flow that can be substantially greater than the primary feed stream. This separation is done with other process equipment that consumes high quantities of energy and maintenance resources.
Tumbling mill reline-work-environments have a confined-space, and the work-floor inside the mill is an uneven bed of process particles and grinding media. Mill liners, which are consumable, need to be handled in and out of the mill via a horizontal inlet with a specially designed machine. This is a known hazardous-work-environment.
Tumbling mills trap some heavy liberated minerals due to gravity and geometry, preventing them from flowing further in the process stream. This is undesirable for various process control and security reasons.
Long retention times, typical for tumbling mills, tend to smooth and round process particles. This can be undesirable for some processes such as some flotation circuits.
In recent times high pressure grinding rolls in conjunction with tumbling mill attrition, or high intensity attrition in stirred mills, have proven to increase overall efficiency of comminution circuits, reportedly up to 40%. However, grinding rolls are limited in their functionality by virtue of the geometry of their rolling surfaces; they fall short of superseding all of the conventional equipment in comminution circuits.
There is a need to combine and synergise the comminution types of high pressure and high intensity attrition into one machine that will receive run-of-mine feed and have no need of classification circuits. The resulting efficiencies, reduction in wear and maintenance will be revolutionary.