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An early application of microscopy to coal science made headlines in the United Kingdom in the mid-19th Century, when the then still infant microscope was called upon to assist in solving the so-called "torbanite case". It concerned a legal battle that was fought over the question, whether or not a certain deposit should be classified as torbanite, i.e. oil (kerosene) shale, or as coal. The answer had commercial ramifications, since the two came under different legislation and were taxed differently (Stach et al., 1982). While none of the then conventional testing methods could provide a conclusive answer, the microscope did so by facilitating the identification of remnants of mainly land plants in coal, whereas torbanite consisted predominantly of aqueous, planktonic algae, such as the Botryococcus braunii, and the related, but more ancient, saltwater-tolerant Gloeocapsomorpha prisca (Derenne et al, 1992).
The optical microscope has played a key role in the development of coal science and technology, including the study of coke and other coal-derived industrial products. Today, optical methods supplement and extend the conventional chemical and technological coal testing procedures, such as proximate and ultimate analysis, as well as the various tests that target the thermoplastic and other technologically important coal properties. It is the purpose of this survey to familiarise the reader with some of the applications of light-microscopic techniques to coal research and such investigations that are of particular interest to the coal industry.
This review has been divided into eight parts. The first two parts provide some background to the origin of coal, its classification and geological distribution. This is followed by a discussion of various aspects of coal microscopy, after which parts four and five concentrate on metallurgical coke, as one of the main products of the beneficiation of bituminous coal. Microscopic methods of coke quality assessment are discussed in part six, while part seven deals with special purpose investigations, for example, of air-borne dust and other contaminants related to the coal industry. The survey is concluded by an alphabetic list of the references cited in the text.
Coal consists of a mixture of plant-derived, carbon-enriched, mainly solid components whose chemical and physical properties relate to its atomic order and molecular arrangement. This, in turn, determines the coal's optical appearance, such as colour, reflectance, bireflectance and polishing relief, all of which are properties that are accessible to the optical microscope. The attraction of using microscopic techniques in the quality assessments of coal and its solid downstream products is the capability of the polarising, incident-light microscope to differentiate both visually and quantitatively between coal components not only of different origin and composition, but also to make predictions about their likely response to different technological processes.
Coal utilisation has benefitted greatly from the application of microscopic techniques. Particularly in carbonisation research, they have been employed as quick and comparatively inexpensive methods of supplementing, or even replacing other kinds of coke quality assessment. The main advantage of the incident-light microscope is that is allows the trained operator to assess not only how but also why a coke will performs in a certain manner under a given set of operational conditions.