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Coal Preparation

Improved Prediction of Size Distributions and Their Effects in Materials Handling and Coal Preparation Systems

Coal Preparation » General

Published: November 93Project Number: C3004

Get ReportAuthor: Andrew Swanson, IS Fletcher | Quality Coal Consulting

 

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Summary

This project involved the holding of workshops to disseminate the results of an extensive and complex NERDDC project (No. 1290) dealing with many aspects of size distribution as it relates to coal preparation and handling.

Workshops were held at Maitland, NSW and Brisbane and Middlemount in Queensland, and were attended by over 80 people representing coal producers, industry service companies, research organisations and others. Over 60% of attendees were from coal producing companies and most of the major Australian coal companies were represented. The concept of using workshops to discuss the findings of complex research projects was very well received.

 

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Background

In the early 1970s, many new coal mines were developed in Australia in areas from which coal had not previously mined. These mines were mainly developed to meet the growing demand for export coking coal. Coal preparation was essential in most cases and common feature of plants designed during the period turned out to be a gross under capacity in their fine coal treatment circuits.

Because of limited knowledge and techniques available at the time it was not possible to accurately predict the as-mined coal size distributions, the subsequent degradation during handling, and the breakdown in preparation plants of coal and associated shales. In particular, there were no bases on which estimate the amount of fines likely to be present. Consequently, fines plant capacities were normally based on estimates from nearby plants. However, this was clearly not possible for plants in new areas or when exploiting new seams. This under capacity resulted in:

  • delays in plant operations until the extra capacity could be installed; or

  • operation at less than design throughput; or

  • in many cases, the extra fine coal being sent to tailings dams.

The other extreme, of over-designing the fines plant capacity, was normally avoided due to the fact that the capital cost (and operating costs) on a per tonne basis are many times greater for fine coal than for coarse coal. Thus it was recognised that the selection of the level of fine coal in the preparation plant feed impacted on the financial position of the project.

 

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Project outline

An extensive survey of coal producers was carried out to look for relationships between coal quality and size distribution, and information was sought by mailing a pro-forma data sheet to all NSW and Queensland black coal mines. This data sheet requested information on coal properties, mining methods, handling processes and size distributions for as mined coal and plant feed. The results from this survey together with other published data and information obtained as part of the other sampling associated with this project were assembled and conclusions drawn.

  • Stage 1 Major Plant Sampling

  • Stage 2 Minor Plant Sampling

 

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Sampling

Sampling took place at three mines and in each case a discrete parcel of coal was followed through the processing chain from coal face to plant, and n the case of two mines to the export port as well. Several samples were collected both between the face and the plant and between the plant and the port. Each sample comprised a number of increments ranging form 6 to 25, and each increment was separately sized and analysed for ash. The results obtained were statistically evaluated. Strip samples were collected from all the mines involved in the Stage 2 sampling program. The samples were subdivided and subjected to drop shatter testing. Samples of roof and floor were also collected from two of the mines and drop shatter tested.

 

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Research and development

An amount of research and development work was carried out to identify the effect on coal sizing of various wet and dry treatment and handling procedures. This was necessary to evaluate alternative techniques for pretreatment.

 

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Dry breakage experiments

Raw coal handling systems consist mainly of conveyors, chutes, bins, transfer points, stackers/reclaimers and mobile equipment (front end loaders, bulldozers, trucks). Thus there are a number of possible breakage mechanisms and dropping/impacting type breakage is expected to make a significant contribution to total size degradation.

Consideration of dropping processes led to the use of a drop shatter test, based on the standard test, being carried out with drops being from 2m and 4m, both with and without the -2m fraction. Dry tumbling of coal was examined in 2 types of devices. The normal wet tumbling drum, with and without steel cubes, was used (without water) and tumbling was carried out for varying times. JIS and Half-micum coke tumbling drums were also used with tumbling being carried out for varying numbers of revolutions. Coal was also choke fed through a jaw crusher with varying aperture sizes.

 

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Wet breakage experiments

The wet breakage experiments carried out included an examination of the wet tumbling process, as well as consideration of other means of sample agitation and degradation in water.

Samples of coal from two mines were wet tumbled using the conventional drum and the major operating parameters of tumbling speed, tumbling time and whether steel cubes were present or not, were varied.

The wet tumbling process using 200L drum with and without steel cubes provides agitation to model the processes coal experiences during preparation. Testwork was carried out to study the effect of some alternate agitation processes on coal samples which were finer than those involved in the wet tumbling tests. Coal at 16mm and 8mm top size was exposed to processes such as pumping around a closed circuit, agitating in a sump with a stirrer and rolling in a small drum with steel balls. It was hoped that this work would give further information on the time effects encountered during wet agitation.

 

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Development of new pretreatment procedures

Based on the results obtained from both Stage 1 and 2 results new procedures to pretreat strip samples and borecores were developed.

 

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Recommendations

  • Design of Raw Coal Handling Systems
    When raw coal handling systems are being designed, the results of this study should be taken into account to avoid equipment configurations that lead to excessive coal breakage, so minimising the fines content of coal. 

  • Pretreatment Procedures
    The improved method for pretreatment of strip samples and LD cores should be introduced into laboratory procedures. Briefly the technique involves:

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    1. drop shatter to reach the 'inevitable' sizing, by either stabilisation to a topsize or by  achieving the point at which the rate of breakage declines markedly, which is equivalent to the ROM size distribution;

    2. further drop shatter to model the anticipated handling system;

    3. wet tumble for five minutes to achieve in-plant size distribution.

 

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Further areas of investigation

  1. Procedures are needed to determine the minimum amount of coal breakage that should be expected for any given coal mine to provide a yardstick against which coal mining and handling practices can be assessed. Thus a project is recommended that extends the current experimental method of deriving equilibrium size and combines it with a study of geological factors that may impact on inherent coal size distribution. Geological factors which may be relevant include those of depositional origin such as coal lithotypes, those of diagnostic origin such as cleat, and those relating to subsequent tectonic history including the in-situ stress regime at the time of mining. The possibility that coal sizing can be related to an engineering materials type of property will also be considered.
     
  2. Following the planned technology transfer activities, mine sites showing significant breakage in handling systems should be investigated to determine causes and allow their elimination from future designs.
     
  3. The pretreatment procedure developed should be further refined:
  4.  
    1. to model abrasive breakage of small coal, particularly in large diameter cores

    2. to better simulate crushing steps in coal handling systems

    3. to improve procedures for the drop shatter test
       

  5. The ultimate step in such a project is the establishment of a new Australian Standard for the testing of strip and LD core samples.

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