Desulfurization is an important process to reduce the sulphur content in liquid steel. This helps to meet low sulfur specifications for finished steel products.
A mixture of calcium carbide and precipitated, carbon-containing calcium carbonate is most effective in desulfurizing ferrous melts. Its effectiveness is due to intimate mixing and better reactivity than powder injection reagents.
Calcium carbide is a grayish-black solid substance that can be crushed to produce powder or granules. It is a key ingredient in acetylene production, as it reacts with water to release acetylene gas (C2H2), which can be used for cutting and welding.
It is also a key component of the reducing process during steelmaking, which helps to remove sulphur impurities from the melt and improves the quality of the final product. The demand for low-sulfur steels in specialty applications and modern continuous casting operations is driving growth for this product.
When used as a slag conditioner in electric arc furnace or BOF processes, it reacts with the oxidising slag to form a reducing slag that has a lower eutectic point. Carbidoelement additions may be made in discrete additions or continuously provided to the melt. When added continuously, the rate at which oxygen is supplied to the melt to oxidize the oxidizable components should be controlled.
Calcium carbonate is used in a wide range of industrial applications, including as a raw material for the production of builders’ lime in its limestone form, or as a cement additive to increase the strength of concrete. It is also used to improve the appearance of concrete and to purify metals by increasing their refractoriness.
In foundries and steelworks, it is often used to line furnaces and kilns. This is because it is highly heat-resistant and has good chemical stability, ensuring that the linings will remain leakage-free and intact for a long time.
Various types of ground and precipitated calcium carbonates are used for PVC, depending on the specific application requirements. These can be classified into two broad groups: GCC and PCC. Both are surface treated with stearic acid, which controls moisture absorption and improves processing and dispersion. This treatment is important to prevent reactivity and to protect against mechanical damage during melt processing. The coating also provides resistance to corrosion.
Calcium Hydroxide, Ca(OH)2, is a white odorless powder. It is produced by reacting quicklime (calcium oxide) with water. It is also known as slaked lime, caustic lime, builders’ lime, and pickling lime.
In steelmaking, it acts as a desulfurizer. It reduces the amount of sulfur in the molten metal to avoid cracking during forming. It is a necessary component in electric arc furnaces, AOD converters, and refining ladles.
It is also used as a desulfurizing agent in the production of alumina and as a clinker additive in iron and aluminum production. Its low toxicity makes it useful as a flocculant in waste and sewage treatment.
When dissolved in water, it forms a solution called limewater or milk of lime. This is an effective water treatment for sludge, and it can be used as a corrosion inhibitor in cooling towers. It is one of the chemicals added to industrial scrubbers that remove nitrogen and sulfur oxides from exhaust gases produced by fossil fuel power plants.
Calcium sulfate, CaSO4 x 2H2O, occurs naturally in the form of gypsum (calcium sulfate dihydrate). It can also be produced from the hydrates of the mineral plaster of Paris or synthetically by mixing sulfuric acid H2SO4 with calcium nitrate Ca(NO3)2. It has the property of dehydrating rapidly to insoluble anhydrite (or dead-burned gypsum) at high temperature.
In addition to lowering the S content in liquid steel, Ca treatment also modifies the shape of inclusions. It can transform inter-dendritic Al2O3 galaxies into fine Type III inclusions, which are less detrimental to the machinability and ductility of the finished product.
However, the use of CaO in the GBFS process must be balanced against the risk of excessive reoxidation. Without careful control, reoxidation can result in formation of high melting point calcium aluminates, which are more harmful, especially to continuous caster nozzles than the original inclusions. This can be prevented by using protective slag blankets and/or inert gas or by installing refractory shrouded nozzles.
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