1. Background

[GLOSS]Coal[/GLOSS] is introduced as an industrial fuel in Combustion File (CF) 62 and described in detail in CF177.

This CF describes how to make an analysis of the distribution of [GLOSS]mineral matter[/GLOSS] in a sample of coal, and the mineral matter associations. This information may be used for determining the possibility of reducing the mineral matter content of a coal by further crushing or washing.

As a general rule, the washability of a run-of-mine coal or a specific coal type is determined by a float-sink test. This allows for the determination of the proportion of the various density fractions and the respective ash (mineral matter) yield in each fraction. On the basis of this data, washability curves can be constructed that establish the densities of separation between “clean coal”, “middlings” and residue.

If information is required on the potential of reducing the concentration, or even concentrating a particular mineral, in a specific density fraction, or to increase the yield of a lower ash coal by further crushing, microscopic analysis using essentially the same method used for maceral analysis is required.

Methods of coal sampling and sample preparation are similar to those involved for analysis of coals for [GLOSS]macerals[/GLOSS] as detailed in CF181.

 

2. Mineral Matter Distribution Analysis

The mineral matter distribution analysis must be carried out on coal samples in their original size distribution, as further crushing alters the viewed and measured associations.

Samples can be derived from run of mine coal or core samples.

Apart from this, a polished section must be prepared. In general, the sample must be dried and mixed with a synthetic resin (such as [GLOSS]Araldite®[/GLOSS]). Enough synthetic resin should be added to give sufficient strength to the sample for polishing.

The prepared sample is then subjected to several stages of polishing on high speed polishing machines using chromium oxide or high-grade alumina.

 

3. Instruments of Analysis

A twin eyepiece reflected light microscope is required. One of the oculars must have an adjustable eyepiece in which a micrometer [GLOSS]graticule[/GLOSS] is placed.

The polished surface is placed in the slide of an automatic point counter. The point-to-point distance (s, [mm]) used is chosen according to the mean size of the largest grain (dmax, [mm]) according to:

s = 1/2dmax

Point-to-point measurements are made with a point counter, which consists of a traversing carriage attached to the microscope stage and a counting unit.

The carriage displaces the sample in a series of equally spaced steps and after each step the particle in the centre of view is identified and recorded by pressing the corresponding button on the counting unit. The counting unit and carriage are connected so that each time a count is made the carriage advances one step.

Table 1 gives the size and minimum number of polished surfaces required for different size fractions for mineral distribution analysis.

The objectives on the microscope used are dependent on size fraction and are given in Table 1.

 

 

Table 1:     Size and minimum number of samples required for mineral matter distribution analysis

 

To analyse coarser size fractions would call for such a large number of measurements and such a large size of polished block, the time required for analysis would be out of proportion to the information generated. Furthermore, it has been shown that mineral distribution analysis should be carried out on closely graded size fractions.

Mineral matter distribution analysis can generally be approached from three viewpoints:

·          Distribution of the mineral matter as a whole

·          Distribution of individual minerals

·          Distribution of different types of mineral associations

As for maceral analysis, the results are expressed in terms of % by volume, and the accuracy is between 2-3%.

 

4.  Distribution of total mineral content, individual minerals and types of associations

The analysis for the distribution of total mineral matter content reveals the proportion of minerals in the individual coal grains. Different categories of associations of coal and minerals can be established.

The distribution of individual minerals is only concerned with one specific mineral, i.e. pyrite, quartz, calcium carbonate (limestone). Table 2 gives a full description of the various minerals in coal after Stach et al.

The distribution of the different types of associations can give an idea as to whether the content of low mineral content or mineral free coal can be reduced by further crushing and subsequent washing and separation.

In this type of analysis, only two fractions are determined:

·          Fractions where the mineral matter is distributed regularly;

·          Fractions where the mineral matter is distributed irregularly.

If the mineral matter associated with the coal is regularly distributed in all grains analysed, it is impossible to reduce the mineral matter content by further crushing and washing. However, a reduction can be obtained if the distribution is irregular.

Mineral Group

First stage of coalification: syngenetic formation, synsedimentary, early diagenetic (intimately intergrown)

Second stage of coalification: epigenetic formation

 

 

Transported by water or wind

Newly formed

Deposited in fissures, cleats and cavities (coarsely intergrown)

Transformation of syngenetic minerals (intimately intergrown)

Clay minerals

Kaolinite, illite, sericite, clay minerals with mixed layer structures montmorillonite, tonstein (smectite)

–

Illite, chlorite

Carbonates

–

Siderite – ankerite concretions; dolomite, calcite, ankerite, siderite

Ankerite. calcite, dolomite; ankerite in fusite

 

Sulphides

–

Pyrite concretions. Melnikovite-pyrite, coarse pyrite, (marcasite) concretions of FeS₂-CuFeS₂ -ZnS, pyrite in fusite.

Pyrite, marcasite, zinc sulphide (sphalerite) lead sulphide (galena) copper sulphide (chalcopyrite) pyrite in fusite

Pyrite from transformation of syngenetic concretions of FeCO₃.

Oxides

–

Haemetite

Goethite lepidocrocite (needle iron ore)

Goethite lepidocrocite (needle iron ore)

Quartz

Quartz grains

Chalcedony and quartz from the weathering of felspars and mica

Quartz

 

Phosphates

Apatite

Phosphorite. apatite

–

 

Heavy minerals

Zircon. Rutile, tourmaline, orthoclase, biotite.

–

Chlorides, sulphates, nitrates.

Chlorides, sulphates, nitrates.

 

Table 2: Minerals in coal (after Stach et al, 1975)

Sources

[1] E. Stach et al., Stach’s Textbook of coal petrology, ISBN 3443390684, Borntraeger, Berlin (1975).

[2] D. W. Van Krevelen, Coal – Typology, Chemistry, Physics and Constitution. ISBN 044440600, Elsevier, Amsterdam (1981).

[3] J. F. Unsworth, D. J. Barratt and P. T. Roberts. Coal Quality and Combustion Performance – An International Perspective. ISBN 0444887032, Elsevier, Amsterdam (1991).

[4] L. Thomas, Handbook of Practical Coal Geology, ISBN 0 471 93557 3, John Wiley and Sons, Chichester (1992).