Coal assay: Encyclopedia II - Coal assay - Chemical properties of coal

Coal assay - Chemical properties of coal

Coal comes in four main types or ranks: lignite or brown coal, bituminous coal or black coal, anthracite and graphite. Each type of coal has a certain set of physical parameters which are mostly controlled by moisture, volatile content (in terms of aliphatic or aromatic hydrocarbons) and carbon content.

Moisture
Moisture is an important proerty of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analysed. Moisture may occur in four possible forms within coal:

• Surface moisture: water held on the surface of coal particles or macerals
• Hygroscopic moisture: water held by capillary action within the microfractures of the coal
• Decomposition moisture: water held within the coal's decomposed organic compounds
• Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays

Total moisture is analysed by loss of mass between an untreated sample and the sample once analysed. This is achieved by any of the following methods;

1. Heating the coal within a solution of toluene
2. Drying in a minimum free-space oven at 150 °c within a nitrogen atmosphere
3. Drying in air at 100-105 °c and relative loss of mass determined

Methods 1 and 2 are suitable with low-rank coals but method 3 is only suitable for high-rank coals as free air drying low-rank coals may promote oxidation. Inherent moisture is analysed similarly, though it may be done in a vacuum.

Volatile Matter
Volatile matter in coal is the components of coal, except for moisture, which is liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulphur. The volatile matter of coal is determined under rigidly controlled standards. In Australian and British laboratories this involves heating the coal sample to 900 +/- 5 °C for 7 minutes in a cylindrical silica crucible in a muffle furnace. American Standard procedures involve heating to 950+/-25 °c in a vertical platinum crucible. These two methods give different results and thus the method used must be stated.

Ash
Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulphur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight.

Fixed Carbon
The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.

Chemical Analysis
Coal is also assayed for oxygen conent, hydrogen content and sulphur. Sulphur is also analysed to determine whether it is a sulfide mineral or in a sulfate form. This is achieved by dissolution of the sulfates in hydrochloric acid and precipitation as barium sulphate. Sulfide content is determined by measurement of iron conent, as this will determine the amount of sulphur present as iron pyrite. Carbonate minerals are analysed similarly, by measurement of the amount of carbon dioxide emitted when the coal is treated with hydrochloric acid. Calcium is analysed. The carbonate content is necessary to determine the combustible carbon content and incombustible (carbonate carbon) content. Chlorine, phosphorus and iron are also determined to characterise the coal's suitability for steel manufacture.

Adapted from the Wikipedia article "Chemical properties of coal", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki