Coal assay techniques are specific analytical methods designed to measure the particular physical and chemical properties of coals.
These methods are used primarily to determine the suitability of coal for coking, power generation or for iron ore smelting in the manufacture of steel.
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 property of coal, as all coals are mined wet.
Groundwater and other extraneous moisture are known as adventitious moisture and are 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.
- Hydroscopic 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:
- Heating the coal within a solution of toluene.
- Drying in a minimum free-space oven at 150°C within a nitrogen atmosphere.
- 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 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.
它相当于燃烧过程中碳、氧、硫和水（包括粘土中的水）排出后的大块矿物质。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.
Coal is also assayed for oxygen, hydrogen and sulphur content.
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 content, 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 also 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.
An analysis of coal ash may also be carried out to determine not only the composition of coal ash, but also to determine the levels at which trace elements occur in ash. This data is useful for environmental impact modelling and may be obtained by spectroscopic methods.
Physical and Mechanical Properties
Relative density: Relative density or specific gravity of the coal depends on the rank of the coal and degree of mineral impurity. Knowledge of the density of each coal ply is necessary to determine the properties of composites and blends. The density of the coal seam is necessary for conversion of resources into reserves.
Relative density is normally determined by the loss of a sample’s weight in water. This is best achieved using finely ground coal, as bulk samples are quite porous.
Particle size distribution: The particle size distribution of milled coal depends partly on the rank of the coal, which determines its brittleness, and on the handling, ·crushing, milling it has undergone. Generally, coal is utilised in furnaces and coking ovens at a certain size, so the crushability of the coal must be determined. It is_ necessary to know this data before coal is mined, so that suitable crushing machinery can be designed to optimise the partide size for transport and use.
Float-sink Test, Coal 1s usually washed by passing it over a bath of liquid of known density. This removes high-ash content particles and increases the salability of the coal as well as its energy content per unit volume. Thus, coals must be subjected to a float-sink test in the laboratory, which will determine the optimum particle size for washing, the density of the wash liquid required to remove the maximum ash content with minimum work. Float-sink testing is achieved on crushed and pulverised coal in a process similar to metallurgical testing on metallic ore.
Abrasion Testing: Abrasion is the property of the coal which describes its propensity ‘and ability to wear away machinery and undergo autonomous grinding. While carbonaceous matter in coal is relatively soft, quartz and other mineral constituents in coal are quite abrasive. This is tested in a calibrated mill, containing four blades of known mass. The coal is agitated in the mill for 12,000 revolutions at a rate of 1,500 revolutions per minute. The abrasion index is determined by measuring the loss o°f mass of the four metal blades.
Special Combustion Tests
Specific Energy: Aside from physical or chemical analyses to determine the handling and pollutant profile of a coal, the energy output of a coal is determined using a bomb calorimeter which measures the specific energy output of a coal during complete combustion. This is required particularly for coals used in steam-raising.
Ash Fusion Test: The behaviour of a coal’s ash residue at high temperature is a critical factor in selecting coals for steam power generation. Most furnaces are designed to remove ash as a powdery residue. Coal which has ash that fuses into a hard-glassy slag known as clinker is usually unsati factory in furnaces as it requires cleaning. However, furnaces can be designed to handle the clinker, generally by removing it as a molten liquid.
Ash fusion temperatures are determined by viewing a moulded specimen of the coal ash through an observation window in a high temperature furnace. The ash, in the form of a cone, pyramid or cube, is heated steadily past l ,000°C to as high a temperature as possible, preferably 1, 600″C. The following temperatures are recorded:
Def ormatio11 temperature, this is reached when the corners of the mould first become rounded.
Softening (sphere) temperature, this is reached when the top of the mould takes on a spherical shape.
Hemisphere temperature: This is reached when the entire mould takes on a hemisphere shape.
Flow (fluid) temperature: This is reached when the molten ash collapses to a flattened button. on the furnace floor.
Crucible swelling index (Free Swelling Index): The simplest test to evaluate whether a coal is suitable for production of coke is the Free Swelling Index test. This involves heating a small sample of coal in a standardised crucible to around 800°C. After heating for a specified time, or until all volatiles are driven off, a small coke button remains in the crucible. The cross-sectional profile of this coke button compared to a set of standardised profiles determines the Free Swelling Index,