It will be remembered from Lesson 2 that the ash content of coal is the percentage of that coal which remains after burning. We know that the cleaner the coal the less the ash, but if we put a piece of shale on to our fire most of it remains as ash. Therefore the ash content of clean coal is much less than the ash content of shale. We learned that relative density of clean coal (say 1.3) is lower than that of shale (2.4). Consider now the relationship between the ash content and the relative density.
It is important to realize that run-of-mine coal does not only consist of clean coal (relative density 1.3) and shale (relative density 2.4) particles. It also contains particles of every relative density, from the lowest to the highest in a continuous range. In general if the coal from a certain seam is analyzed, it is found that the lowest relative density particles contain the least ash, and vice versa. As relative density increases, ash content also increases fairly uniformly. There are exceptions to this, but they will be considered in detail in Lesson 4.
Considering some general figures, suppose that the ash content of clean coal of relative density 1.3 is something of the order of 4% (ie 4 parts in 100 become ash on burning). Suppose also that the ash content of shale particles of relative density 2.4 is about 85%. The particles of relative density somewhere between 1.3 and 1.8 similarly have ash contents between 4% and 85%. We find that as the relative density increases from 1.3 to 2.4 the ash content increases from 4% to 85%. This is shown in Lesson 3 Fig. 3.
Therefore in more general terms there is a relationship between the relative density and the ash content of coal; the higher the relative density the higher the ash content. Hence, when we wash coal according to the relative density we are really washing coal according to the requirements of the markets in terms of content.
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