XCT Prediction of Breakage and Washability of Borehole Cores

The most critical parameters for the efficient design and planning of coal mining and processing operations depend on the accurate prediction of coal breakage and washability characteristics of the coal resources. The aim is to establish the most “effective” liberation of the coal constituents for separation. In this regard, borehole core samples are an integral part of the evaluation process, providing important information of coal reserve size, the mining conditions, the coal quality and its washability characteristics.

The aim of this project was to apply and extend the use of X-ray computed tomography (XCT) technology to predict breakage and washability of coal borehole samples with different coal constituents (bright and dull coals). Borehole and channel coal samples were obtained from operating mines in the Bowen Basin. The channel samples consisted of ten bright and ten dull coal chunks, sized from 0.2 to 0.4 meter with a weight between 5 to 18 kg per lump coal. The borehole samples included seven cylinders with a length between 20 and 35 cm, diameter of 10 cm, and weight between 1.5 to 3 kg. These samples were subject to traditional analysis and characterisation methods to determine baseline washability information.

This project was based on the two successful ACARP projects: C21050 “Application of X-ray Computed Tomography for Coal Washability Analysis” and C23024 “Non-destructive Characterisation of Borehole Coal Samples Using X CT Technology”. In C21050, a high-resolution X-ray micro CT (HRXMT) scanner was used to scan packed particle beds of coal samples taken from the DMC circuits to provide the details for coal washability analysis using XCT. In C23024, a milli XCT scanner (millimetre resolution) for large channel samples (taken from coal seams, representing the borehole samples) and the HRXMT scanner for small samples of broken coal particles (produced by a modified drop shatter test applied to the channel samples) were used to obtain and predict the coal washability of the large unbroken channel samples.

In this project, only a powerful milli (medical) XCT scanner was used to scan the large coal samples to obtain and predict the coal breakage and washability, without using the HRXMT scanner which is only suitable for the small samples. The coal breakage was produced by applying the modified drop shatter testing developed in C23024 which involved wrapping each piece of the channel and borehole samples in a few layers of a tough plastic film (known as pallet wrap) to both confine the particles and maintain their relative shape during fragmentation by dropping from 2m heights. Each piece was first scanned by the milli CT. It was then broken by the drop shatter testing according to a predetermined schedule using the modified drop-shatter method and then scanned again in the confined state in order to examine the fracture pattern. These breaking and scanning steps were repeated as per the pre-determined number of drops (to achieve the pre-determined energy inputs). Finally, the final broken sample was released from confinement and screened into the desired size fractions for the float and sink analysis using heavy liquids and the procedure described in the Australian Standard AS 4156.1-1994. The coal breakage was analysed and predicted using the population balance method and software developed in-house at the University of Utah.

Based on the outcomes generated by the research activities, the objectives of the project were successfully achieved, namely:

• A non-destructive methodology using XCT for characterising coal breakage and washability of borehole and channel samples was developed;
• XCT instrumentation capability of analysing breakage and washability of borehole and channel samples was demonstrated.

The outcomes of this project demonstrate that the application of XCT technology can provide benefits to the coal industry, including a non-destructive, efficient technique to characterise borehole coal samples for mine planning and processing strategy, as well as for exploration and development of new coal mines. The technique is environmentally safe, fast and cost-effective. However, many challenges remain if the XCT technique can be routinely utilised to save time, human resources and money for the coal industry. The major challenges being faced by the XCT technology include the following:

• The voxel resolution of medical XCT scanners is rather limited for studying washability of borehole samples:

• coarse size fractions produced by breaking a borehole sample can be quickly imaged by XCT but they do not have a sufficient number of particles as required by washability analysis to obtain reasonable statistics.

• Powerful micro XCT scanners should be used for studying washability of borehole samples:

• They would be capable of scanning big samples with fine voxel resolution and provide a sufficient number of particles (2000 as per AS 4156.1-1994) for washability analysis.

Future research and development of XCT for the coal industry must consider highspeed X-ray tomography, now available, for coarse particle analysis, as large as 200 mm, with scan rates of approximately 1-2 kg/min. at a voxel resolution of about 150 microns.