Coal assay
Encyclopedia
Coal Analysis 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
.
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.
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;
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.
matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. 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 (1650 ±10 °F) for 10 min.6000calfc
mineral or in a sulfate
form. Sulfide content is determined by measurement of iron
content, as this will determine the amount of sulfur present as iron pyrite
or dissolution of the sulfates in hydrochloric acid with precipitation as barium sulfate.
Carbonate minerals are analysed similarly, by measurement of the amount of carbon dioxide emitted when the coal is treated with hydrochloric acid. Calcium is ana
he 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.
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. To determine in-place coal tonnages however, it is important to preserve the void space when measuring the specific gravity.
content, rank, ash and mineral content and porosity. Coal is usually washed by passing it over a bath of liquid of known density. This removes high-ash content particles and increases the saleability 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 the minimum work.
Floatsink testing is achieved on crushed and pulverised coal in a process similar to metallurgical testing
on metallic ore
.
of a coal during complete combustion. This is required particularly for coals used in steam-raising.
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 1000 °C to as high a temperature as possible, preferably 1600 °C (2,912 °F). The following temperatures are recorded;
is the free swelling index test. This involves heating a small sample of coal in a standardised crucible to around 800 degrees Celsius (1500 °F).
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.
Coke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...
, power generation or for iron ore smelting
Smelting
Smelting is a form of extractive metallurgy; its main use is to produce a metal from its ore. This includes iron extraction from iron ore, and copper extraction and other base metals from their ores...
in the manufacture of steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
.
Chemical properties of coal
Coal comes in four main types or ranks: ligniteLignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...
or brown coal, bituminous coal
Bituminous coal
Bituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...
or black coal, anthracite and graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
. 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 is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analysed quantitatively. Moisture may occur in four possible forms within coal:- Surface moisture: water held on the surface of coal particles or maceralMaceralA maceral is a component of coal or oil shale. The term 'maceral' in reference to coal is analogous to the use of the term 'mineral' in reference to igneous or metamorphic rocks. Examples of macerals are inertinite, vitrinite and liptinite.- Inertinite :...
s - Hydroscopic moisture: water held by capillaryCapillaryCapillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
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 clayClayClay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
s
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 with tolueneTolueneToluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
- Drying in a minimum free-space oven at 150 °C (302 °F) within a nitrogen atmosphere
- Drying in air at 100 to 105 °C (212 to 221 F) 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
VolatileVolatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
matter in coal refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur. The volatile matter of coal is determined under rigidly controlled standards. In Australian and British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
laboratories this involves heating the coal sample to 900 ± 5 °C (1650 ±10 °F) for 10 min.6000calfc
Ash
Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur 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 content, hydrogen content and sulfur. Sulfur is also analysed to determine whether it is a sulfideSulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
mineral or in a sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
form. Sulfide content is determined by measurement of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
content, as this will determine the amount of sulfur present as iron pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
or dissolution of the sulfates in hydrochloric acid with precipitation as barium sulfate.
Carbonate minerals are analysed similarly, by measurement of the amount of carbon dioxide emitted when the coal is treated with hydrochloric acid. Calcium is ana
he 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.
Relative density
Relative densityRelative density
Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water...
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. To determine in-place coal tonnages however, it is important to preserve the void space when measuring the specific gravity.
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 and 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 and its behaviour quantified. It is necessary to know these data before coal is mined, so that suitable crushing machinery can be designed to optimise the particle size for transport and use.Float-sink test
Coal plies and particles have different relative densities, determined by vitriniteVitrinite
Vitrinite is one of the primary components of coals and most sedimentary kerogens. Vitrinite is a type of maceral, where "macerals" are organic components of coal analogous to the "minerals" of rocks. Vitrinite has a shiny appearance resembling glass . It is derived from the cell-wall material or...
content, rank, ash and mineral content and porosity. Coal is usually washed by passing it over a bath of liquid of known density. This removes high-ash content particles and increases the saleability 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 the minimum work.
Floatsink testing is achieved on crushed and pulverised coal in a process similar to metallurgical testing
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
on metallic ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
.
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.(I.E 1500 revolution for 8 min.) The abrasion index is determined by measuring the loss of mass of the four metal blades.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 outputEnergy value of coal
The energy value of coal, or the fuel content, raj is the amount of potential energy in coal that can be converted into actual heating ability. The value can be calculated and compared with different grades of coal or even other materials...
of a coal during complete combustion. This is required particularly for coals used in steam-raising.
Ash fusion test
The behaviour of the 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 unsatisfactory 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 1000 °C to as high a temperature as possible, preferably 1600 °C (2,912 °F). The following temperatures are recorded;
- Deformation 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 cokeCoke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...
is the free swelling index test. This involves heating a small sample of coal in a standardised crucible to around 800 degrees Celsius (1500 °F).
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.
See also
- Coking coalCoke (fuel)Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...
- Bituminous coalBituminous coalBituminous coal or black coal is a relatively soft coal containing a tarlike substance called bitumen. It is of higher quality than lignite coal but of poorer quality than Anthracite...
- CoalCoalCoal is a combustible black or brownish-black sedimentary rock usually occurring in rock strata in layers or veins called coal beds or coal seams. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure...
geology - [Coal is energy source]
External links
- Coal Preparation Journal