Product Coal Loss Due to Inappropriate Focus on Ep

The Coal Preparation fraternity is used to focussing on Ep(75/25) values when people consider separation equipment performance. However, Ep only describes a small aspect of performance, and in many cases the Ep value is completely irrelevant or even misleading. For example, when considering fine particle process efficiencies (-4 mm factions), most, if not all, raw coals are highly liberated such that most of the raw material is either Floats 1.50 relative density (RD) or Sinks 2.20 RD. Ep itself rarely has a significant influence for fines separations in the region of RD₅₀ 1.6 to 1.9 which is typical. Similarly, when considering separations for coarser particles where there is little near gravity material (NGM), e.g. for typical thermal coal separations, the Ep value may have little to no impact on the misplaced material.

In these circumstances, the amount of misplaced material is dominated by the 'tails' of the partition curve, which provide details of bypassed material. The low density tail shows the proportion of low RD material per RD fraction that is misplaced to reject, and the high density tail shows the proportion of contamination by sinks material in the product.

When partition curve data are reported, values such as t₀ and t₁ are calculated by curve fitting programs, generally using the Whiten equation. t₀ and t₁ are only curve fit parameters relating to asymptotes at artificial RD values, and so they rarely reflect the actual bypass numbers or the critical amount of bypass at RDs in the general proximity of the cutpoint. Of particular concern is the valuable material in the region of RD 1.25 to 1.50.

The impact of this issue has become more apparent in recent years where low RD separations are becoming common due to poorer washability resources being mined. Low RD separations typically sacrifice significant amounts of F1.40 material, due to the 'roll-over' of the partition curve. It is important to understand that and seek to minimise the impact.

This project has assessed the potential for alternative parameters that may be used to describe separation performance.

The project is concerned with generic parameters that can be derived from partition curves, but the focus of the data for the work relates to dense medium cyclones (DMC).

The key outcomes follow:

· One new parameter Offset t₀(0.1) and its sister parameter, Offset t₁(0.1), are recommended for inclusion as new standard partition curve parameters;

· The Offset parameters are able to demarcate, on a size-by-size basis, operating parameter influences for DMC performance;

· It has not been possible to demarcate which of medium:solids ratio, or near-gravity material, most impact upon DMC performance, but it is clear that either or both have a significant impact upon bypass values.