Coal Preparation » Gravity Separation
The Australian Coal industry needs to find an alternative to the heavy organic liquids used to produce its coal washability analyses. Indeed, the existing Australian Standard for the Float and Sink analysis of coal is likely to be phased out over the next five years, and thus the matter needs to be urgently addressed to allow time to establish a new approach, with a new standard. Several studies have been conducted in the past, all indicating advantages and disadvantages. The purpose of this review document is to examine the options available for washability analysis within a single document.
The conclusion reached by the review document is that the main options for a new washability analysis involve water-based systems. Magnetized ferrofluids, however, can be based on kerosene or water. These approaches contrast gas pycnometry which requires no liquid media. The primary problem with water-based systems is the tendency for clays to break down, which in turn produces a degree of liberation, and hence a change in the size distribution and washability of the system. But is this really a problem, given the particles will also breakdown in a coal preparation plant, upon contacting water? By including a "wet split" step into the procedure, and recovery of the fines separately, the breakdown process could be simulated and in turn the "problem" essentially eliminated.
The fundamental result obtained from a washability analysis is the cumulative yield versus cumulative ash relationship, which provides the highest achievable yield for a given product ash. The curve provides a bench-mark for assessing process performance, and quantifying the value of a resource. The additional relationship between these results and the density of the separation is useful, but arguably less fundamental. The density can vary appreciably depending on how it is determined, which in turn can impact on the separation density and resource assessment. Thus the industry also needs to consider the basis used to specify the density in arriving at a new washability method.
It is concluded that satisfactory alternatives to the current standard on washability analysis exist, however, certain compromises will be required as all have certain disadvantages, either technical or economical. A table is presented that attempts to examine these issues in a qualitative way.