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Coal Preparation

Online Particle Size Monitoring in Coal Preparation

Coal Preparation » Fine Coal

Published: April 17Project Number: C24046

Get ReportAuthor: Brad Garraway | QCC Resources

The principle of using laser diffraction as a means of measuring a volume size distribution in slurry is well established. Laboratory devices have been in use since the 1960's for monitoring ultrafine solids in cosmetics, pharmaceuticals, paper manufacture and minerals processing.

Outotec have produced an industrial version of the well-known Malvern laboratory laser sizing device that can be installed as a real time online instrument for use in monitoring, process control and quality assurance of fine particle streams. This industrialized version is known as the PSI500i  particle size analyser.

The PSI500i installation base has a global reach of over 100 units in mineral processing. These analysers are mainly employed in monitoring and control of grinding circuits. In Australia, the analyser has found application at 15 sites, again principally in grinding circuits for base metals and gold concentrators.

At the time of initiating this project, an analyser had been purchased but not installed for a Queensland coal preparation plant.

The PSI500i consists of an inline pipe sampler where a cutter extracts a continuous sample for analysis. The sample is diluted and analysed for particle size distribution (all within the PSI500i analyser unit) and accurately produces sizing results for streams with particle sizes in the 0 to 0.5 mm particle size range. The cycle time for the device from sample extraction to reporting is approximately 3 minutes.

For this project trial, the PSI500i was installed at a Coal Preparation Plant in the Hunter Valley region on the overflow stream from a classifying cyclone cluster. For the purpose of this study the device was a standalone instrument; it was not linked to the existing plant's PLC and was not used to control any device in the existing plant.

This project was concerned with three key investigations; determining the robustness of the PSI500i unit in an operational coal plant, comparison of the PSI500i reported sizing distribution with industry standard sieve analysis, and identification of potential applications of the technology within the coal industry.

During the trial, the PSI500i unit was found to be prone to computer memory failures leading to low continuous availability of reported data. However, the poor machine availability is not considered to be representative of the application to coal but very likely related to transport damage and limitations of the test plant installation. If an undamaged machine was installed as a permanent system, connected to the site control system, with a potable water supply and uninterruptable power supply, the reliability would be expected to be similar to that proven at other sites.

Comparison between the laboratory sieve analysis and the laser diffraction results found an offset between the two distributions. This was not unexpected, as the sieve analysis will tend to report the second smallest dimension of a particle and the laser diffraction analysis reports a spherical volume equivalent diameter. This highlights the importance of aligning the sizing and analysis method with the intended purpose of measuring the size distribution in the first instance.

Whilst neither of these numbers is incorrect, the spherical volume equivalent diameter was converted to a cubical volume equivalent dimension, considered to be more representative of coal particles, using a shape correction factor. This results in a better agreement between the laboratory analysis and the measured results at the larger particle sizes but tends to diverge as the particle size approaches 0.038 mm. At this size range as the particle size decreases, the mineral content of the particles tends to increase making the cubical shape factor less applicable. The laser diffraction solids distribution is a volumetric calculation and the sieve analysis is a mass calculation which may also affect the result as the volumetric calculation will under represent the finer particles with higher mineral content and thus denser particles when compared to mass bases analysis.

Further work is required to define the optimum shape factor for the cyclone overflow and investigate whether different shape factors can be applied by particle size to improve the correlation between the two results. However, while the sieve analysis is very important to the performance of screening operations, it is not necessarily important for cyclone performance or ultrafine coal operations where other measurements may drive performance.

A number of applications in which a rapid particle size measurement would be advantageous in a coal preparation plant were identified.

The PSI500i can accept feed streams from up to 3 different sample points; therefore it would be possible that one device could service a number of applications.

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