Iron and Steel Production
Coal Dust As Fuel
Iron and steel have become an essential part of our lives. From ships to cars and numerous household items, there isn’t a doubt that iron and steel are vital in our everyday lives. Approximately 64% of steel manufactured on a global basis was derived from iron produced in blast furnaces which use coal as their primary fuel. In 2003, the quantity of crude steel produced on a worldwide scale was put at 965 million tons with about 543 Mt used in the manufacturing process.
The raw materials used in iron production from a blast furnace include iron ore, coke (made from coking coals) and a small amount of limestone. However, some blast furnaces use pulverised coal injection (PCI) methods such that cost is saved and better performance is derived. The PCI method was developed initially in the 19th century, but it was not until the 1970s that iron and steel manufacturers widely adopted this technique. It was an upsurge in the cost of coke, due to the rise in global demand in addition to increased competition for this resource, that led manufacturers to turn their attention towards this method.
The idea behind the PCI method is quite simple. It involves the primary air, also referred to as the conveying gas, carrying coal dust (pulverised coal) which is introduced through a lance into the tuyere (the mid-bottom inlet of the blast furnace). Subsequently, a blowpipe in the tuyere delivers secondary hot air (also termed the blast) and then mixes with the primary air which as mentioned earlier, conveys foundry coal dust. This mixture is channeled to the furnace, creating a balloon-like cavity, otherwise known as a raceway. This raceway propagates the combustion of coke and coal, liquefying the solid iron ore and releasing molten iron in the process.
Green Sand Molding Process
Greensand describes molding sand that is neither baked nor dried but possesses an inherent moistness. The raw sand in its ore form is processed such that the grain sizing is evenly distributed. Organic clays act as binders for these grains during the course of processing into molding sand.
The addition of foundry coal dust helps to ensure that the casting quality is excellent as sand expands when hot molten metal is emptied into the mold. The usage of other additives including pitch, cellulose, and silica are also allowed. The sand, along with additives and water are blended in a mullor, otherwise referred to as a mixer. The sand is deemed ready to make a mold when it has mixed with other substances in the mullor.
The pouring of molten iron into a green sand mold containing coal dust will cause the release of reducing gases and volatile organic compounds following the application of heat and consequently prevents iron oxide formation during the intermediate phase of burn-on production. Burn-on is deposited iron oxide, and is prevented by the pyrolysis of the coal dust.
In the final phase of the molding process, coking of coal dust starts at the mold surface, leading to its softening and expansion. The critical quartz sand expansion in the base silica sand occurs alongside the softening and coking of coal dust. Consequently, the sand grains are readjusted and the occurrence of expansion-type defects is regulated.
The coal dust used in foundries for iron casting requires low ash coal dust which must possess a minimal sulfur and chloride content, inherent moisture of about 2-4%, and volatile content of 30% or thereabout. Summarily, foundry coal dust decreases defects associated with expansion and hydrogen pin-holing. An improvement in the dimensional stability of molds is also due to the inclusion of this substance in greensand.
Utilisation in green sand castings of fly ash - which is chemically and physically similar to coal dust - has been shown to be effective especially for non ferrous metal casting(4). In addition to offering similar performance to green sand alone, the authors note that there are economic benefits to using less sand and more ash, as it is cheaper; physical and mechanical properties are similar also. Furthermore, environmental performance is increased. Such lessons can be applied to coal dust.
Overall, it can be said that the use of coal dust use decreases burned-on defects, improves surface finish and prevents metal penetration.
As A Replacement Or Supplement In Silica Mould Casting
Whilst greensand is a popular choice, and the inclusion of coal dust and similar compounds is an important step in reducing cost and modulating defects and burn on, green sand is far from the most common casting method. Silica casting is fundamentally the same, but utilises only high purity silica sand as the mould building material.
An important consideration when replacing silica sand with any other material, for example coal dust, is that of moisture. Residual moisture present in the mould will lead to gas defects and mistakes in the casting process(5). With this and any other modification on the usual casting protocol, modelling can lead to better outcomes(6).
Preventing Wetting In Castings
The prevention of surface defects is a major challenge in the molding space, where materials prone to wetting are often used. Wetting directly causes surface defects, and the use of sufficient amounts of coal dust in the sand can alleviate these phenomena.
Under high temperature conditions, coal dust/powdered anthracite will pyrolyse and deposit a thin film of solid carbon at the liquid metal-sand interface. This deposited layer will prevent metal penetration into the sand, and vice-versa. It is, however, two layers. One on each of the sand and the metal that ensures non-wetting behaviour and the prevention of the formation of burrs which would later need to be machined off of the casted product. Coal dust that is predominantly anthracite, has a good coking capacity, has no more than 30% volatiles by weight, with lower than 0.8% by weight sulfur and has low ash contents are preferred(7).
Naturally, a pressure increase is to be expected due to the evolution of gases from the pyrolysis of the coal dust, causing a moderate pressure increase, in addition to the evaporation of water from the sand. Such pressure increases are well within the tolerance of any sand casting mold. The only potential mild issue is that hydrogen could penetrate the metal, at high enough temperatures, if enough of it is present, owing to its small atomic radius(8).
Interestingly, wetting is an important consideration in a study that looked at coal dust storage. As coal dust is highly flammable, storage of it needs to be taken seriously. A spray based on sodium dodecyl sulfate has been developed which prevents degradation and fire in stored coal dust(9).
Refractory Applications
As mentioned earlier, anthracite has a degree of refractoriness, particularly when it has been calcined.
Refractory Bricks
Refractory bricks are capable of enduring high temperature and are characterized by a low thermal conductivity which allows for greater efficiency. Applications that require high thermal, chemical or mechanical stress calls for the use of dense refractory bricks. However, kiln brick – a more porous refractory brick – is more suitable for less harsh situations. Kiln bricks are weaker than the dense ones, but they are advantageous in the sense that they are lightweight and better insulators.Coal dust as an additive in the production of refractory bricks has been used for almost one hundred years(10).
One of the major substances involved in the production of refractory brick otherwise referred to as firebrick is coal dust. Open hearth furnaces, electric arc furnaces, metallurgy furnaces, cement rotary kilns, and glass kilns are constructed with firebricks made from refractory coal dust. As an additive, it is necessary that coal dust is mixed with clay and water. Subsequently, the mixture undergoes a firing process which involves air drying for 120 minutes at a temperature of 30oC and passed through a temperature of 110oC. In the last phase of firing, the mixture sample is fed into a furnace and heated to a temperature of 1050oC in 6 hours.
For refractory bricks containing coal dust, it has been found that with a finer grind (i.e. smaller particles), compressive strength and levels of porosity are increased, whilst thermal conductivity decreases. Naturally, the balance of thermal conductivity and compressive strength are key to a successful firebrick. Researchers have concluded that thermal conductivity decreased whilst compressive strength and porosity levels increased with an overall increase in the percentage of coal dust used(11).
Specifically, coal dust provides the thermal insulation required by refractory bricks to perform when needed. Refractory coal dust has the following effect on the firebrick:
- Reduces thermal conductivity
- Increase the crushing strength and porosity of the refractory brick
- Enhance the firebrick’s ability to withstand thermal and corrosive factors
- Promotes the ability to withstand thermal shock.
Percentage composition in standard coal dust refractory bricks attributed to coal dust ranges from 38 to 68%, with grind sizes ranging from 20 to 500 μm. It has been stated that with increasing coal content, the mechanical strength increases and thermal conductivity decreases(12).
As with all refractories, the concept of porosity is crucial. High levels of porosity corroborate with higher levels of thermal insulation due to a greater capacity of air, as air is a poor temperature conductor(13), but it should be noted that too much porosity can be a bad thing. Excess porosity is often associated with a lessening of mechanical strength.
Permeability is one of the major deciding factors in the longevity of refractory materials(14), and is highly influenced by porosity. Pores are created after a combustible material within the overall refractory mix burns off during firing. The shape of the pores is related to the material(15) and the overall process is known as ‘burnout’. When coal dust burns, it typically leaves a spherical hole(16). As a general rule, the larger particle size of the additive, the larger the pore size will be realised.
Coal dust is also used as an additive in the manufacture of highly thermally stable bricks from high iron content red clay mud in developing countries(17). These bricks are valued for their low water retention and longevity.
Refractory Linings
As mentioned earlier with respect to blast furnace linings, coal dust as anthracite can be used in refractory linings, oftentimes as a part of a monolithic structure. Aluminium smelting makes extensive use of carbonaceous linings, utilising anthracite alongside coke and other carbon-based materials. Anthracite derived monolithic linings are highly valued due to their low ash content and long term stability. Cathodes used in the electrolysis stage of aluminium smelting can contain up to 45% of calcined anthracite; whilst maintaining stability at high temperature and no loss of electrical conductivity this is in addition to more pot-lining or tundish-type scenarios where the addition of coal dust aids to ensure the molten metal is held at constant temperature(18).
Other Refractory Uses
For joining other refractories in smelter linings, a highly effective refractory paste composed of calcined anthracite and a resinous pitch can be used(19). When these resins are anthracite-rich, their performance is regarded as superior in terms of compressive strength at the higher temperature ranges, when compared to asphalt-type resins(20). Coal dust has been used in refractory cements and concrete as early as 1910. Whilst the addition of coal dust does not contribute to the pozzolanic (curing) activity of the concrete, the inclusion does increase the overall thermal performance, making it a good choice for high temperature environments.
You must be logged in to post a comment.