Lime is the purifying element in iron and steel production, where it is needed for slag formation and desulphurization. The process of iron making is the reduction of iron oxide to iron. Iron oxides normally occur as iron ores, which also contain quantities of impurities particularly silica. Thus the commercial iron making process also involves the removal of these impurities, usually by forming a slag with lime or dolime.
Lime is normally added as limestone. The heat in the blast furnace decomposes the limestone to lime, which then reacts with the impurities. Quick Lime is needed for further processing in the steelworks, where such harmful constituents as phosphorus and sulphur are captured by the lime.
Specially customized mixtures are provided for particularly high-quality steel grades with extremely low sulphur content.
Lime products containing magnesium help to regenerate the refractory materials in the furnaces.
Flue Gas Desulphurization
Lime and limestone play a significant role in the removal of pollutants from flue gas streams of coal-fired power plants, incinerators and industrial facilities. Flue gas desulfurization (FGD) primarily refers to the removal of sulfur dioxide (SO2). However, lime and limestone are also used in the removal of other pollutants such as hydrogen chloride (HCl), sulfur trioxide (SO3), fine particulates and mercury. In the US, air pollution control applications were the second largest use of lime in 2003, consuming over 3.4 million tons of lime. Lime and limestone products are used in both wet and dry FGD processes.
In the wet FGD processes these products are slurried with water and sprayed into a flue gas scrubber vessel. The acidic gases, normally SO2 and HCl, are absorbed into the water where they chemically react with the lime and limestone. The reaction products, primarily calcium sulfite, can then be oxidized to produce calcium sulfate, a salable gypsum byproduct.
There are three basic types of dry FGD processes:
- Dry injection processes inject dry hydrated lime directly into the flue gas stream
- Spray dryer processes inject finely atomized lime slurry into a separate vessel. Water from the slurry is evaporated before the solids contact the vessel walls
- Circulating fluidized bed processes inject dry hydrated lime in a separate fluidizing vessel
With all three processes, the acidic gases combine with lime to form a dry product which is removed from the flue gas stream in particulate control devices such as bag houses or electrostatic precipitators (ESP's).
Lime is an important commodity for the pulp and paper industry. Most pulp and paper facilities operate their own kilns, and regenerate lime after use. Thus, this industry is primarily a market for “make-up” lime from commercial manufacturers.
Sulfate Process: By far the largest application of lime in pulp manufacture is as a causticizing agent in sulfate (Kraft) plants. Here, the waste sodium carbonate solution is recovered and reacted with high calcium lime to generate caustic soda for reuse in the process. Most large sulfate plants recover about 90-98 percent of the lime by dewatering the waste calcium carbonate mud, then calcining it in rotary kilns
Sulfite Process: Sulfite plants use quicklime to prepare calcium bisulfite, an acid cooking liquor used to digest or dissolve the non-cellulosic constituents of wood chips (paper and pulp stock). This process is on the wane due to waste disposal problems
Bleaching: In pulp manufacturing processes, the interaction of lime and chlorine is used in the preparation of calcium hypochlorite bleach liquor. Calcium hypochlorite, the oldest known and lowest cost bleach, is used extensively to bleach the pulp to the desired degree of whiteness
Miscellaneous: Lime is used in the treatment of pulp and paper mill liquid wastes as a coagulant in color removal, a filtration conditioner and to a lesser extent as a neutralizing agent. It is also used in the recovery of valuable by-products from pulp and paper mill wastes. It is used commercially to recover alcohol, calcium lignosulfonate, and yeast
Most of the process water in the chemical industry requires treatment with lime to correct its pH & mineral content. Acidic waste waters can also be treated with lime before being recycled or released into the sewage system.
Lime is a basic industrial chemical that is used as a component in many other chemical process:
- Alkali plants with access to natural soda ash use the lime-soda process to manufacture caustic soda.
- Calcium carbide ,the oldest source of acetylene is formed by mixing quicklime to coke and heating to a temperature of 2000 degrees c.Calcium cyanide,a nitrogen fertilizer is made by heating calcium carbide in the presence of nitrogen.
- Lime is used in the purification of citric acid.Hydrated lime is added to a solution of citric acid to remove suspended solids.
- Lime is used in the process to manufacture magnesia
- Lime is also employed in the manufacturer of many other inorganic and organic chemicals and pharmaceuticals
- Calcium hypochlorite bleaches are produced by reacting lime with chlorine
- Dolomite lime is reacted with acetic acid to produce magnesium acetate
Building and Construction Industry
The dominant construction-related use of lime is soil stabilization for roads, building foundations, and earthen dams. Lime is added to low quality soils to produce a usable base and sub base.
Hydrated Lime has long been acknowledged to be a superior anti-stripping additive for asphalt pavements. It also helps resist rutting and fracture growth at low temperatures, reduce age hardening, and improve the moisture resistance and durability. A recent study shows lime holds significant advantages over other additives for hot mix asphalt roads, including long term cost savings and increased durability.
Perhaps the oldest and most traditional use of lime has been in mortar and plaster, because of its superior plasticity and workability. This area provides additional information about the construction applications of lime.
Road Construction Industry
Water & Waste Treatment:
Lime (calcium oxide), also known as quicklime, is a chemical compound derived from limestone. According to the National Lime Association, lime is the most used chemical by weight in potable and industrial water treatment.
Water Softening: Treating water with hydrated lime (calcium hydroxide) removes hardness caused by carbonate minerals. Treating with lime remedies non-carbonate hardness that is, hardness resulting from non-carbonate calcium minerals and magnesium salts
PH Adjustment: Water utilities use hydrated lime to prepare water for treatment processes by adjusting the water's pH. To minimize corrosion of pipes and equipment, utilities add lime to neutralize acidic water
Coagulation and Flocculation: Water treatment facilities add lime to optimize conditions for coagulation and flocculation, the processes that make suspended particles clump together and settle out of suspension
Disinfection: Raising water alkalinity to a pH of 10.5 to 11 by adding lime inhibits the growth of bacteria and some viruses
Purification: Lime removes impurities such as fluoride, iron, manganese and organic tannins from water. The magnesium in dolomitic lime is especially effective in removing silica from water. A side effect of the lime-enhanced softening process is arsenic removal, while disinfection with lime also serves to remove most heavy metals