Calcium carbide (CaC2) is manufactured by heating a lime and carbon mixture to 2000 to
2100癈 (3632 to 3812癋) in an electric arc furnace. At those temperatures, the lime is reduced by
carbon to calcium carbide and carbon monoxide (CO), according to the following reaction:
CaO + 3C ?CaC2 + CO
Lime for the reaction is usually made by calcining limestone in a kiln at the plant site. The sources of
carbon for the reaction are petroleum coke, metallurgical coke, and anthracite coal. Because impurities
in the furnace charge remain in the calcium carbide product, the lime should contain no more than
0.5 percent each of magnesium oxide, aluminum oxide, and iron oxide, and 0.004 percent phosphorus.
Also, the coke charge should be low in ash and sulfur. Analyses indicate that 0.2 to 1.0 percent ash
and 5 to 6 percent sulfur are typical in petroleum coke. About 991 kilograms (kg) (2,185 pounds [lb])
of lime, 683 kg (1,506 lb) of coke, and 17 to 20 kg (37 to 44 lb) of electrode paste are required to
produce 1 megagram (Mg) (2,205 lb) of calcium carbide.
The process for manufacturing calcium carbide is illustrated in Figure 11.4-1. Moisture is
removed from coke in a coke dryer, while limestone is converted to lime in a lime kiln. Fines from
coke drying and lime operations are removed and may be recycled. The two charge materials are then
conveyed to an electric arc furnace, the primary piece of equipment used to produce calcium carbide.
There are three basic types of electric arc furnaces: the open furnace, in which the CO burns to
carbon dioxide (CO2) when it contacts the air above the charge; the closed furnace, in which the gas is
collected from the furnace and is either used as fuel for other processes or flared; and the
semi-covered furnace, in which mix is fed around the electrode openings in the primary furnace cover
resulting in mix seals. Electrode paste composed of coal tar pitch binder and anthracite coal is fed
into a steel casing where it is baked by heat from the electric arc furnace before being introduced into
the furnace. The baked electrode exits the steel casing just inside the furnace cover and is consumed
in the calcium carbide production process. Molten calcium carbide is tapped continuously from the
furnace into chills and is allowed to cool and solidify. Then, the solidified calcium carbide goes
through primary crushing by jaw crushers, followed by secondary crushing and screening for size. To
prevent explosion hazards from acetylene generated by the reaction of calcium carbide with ambient
moisture, crushing and screening operations may be performed in either an air-swept environment
before the calcium carbide has completely cooled, or in an inert atmosphere. The calcium carbide
product is used primarily in generating acetylene and in desulfurizing iron.
11.4.2 Emissions And Controls
Emissions from calcium carbide manufacturing include particulate matter (PM), sulfur oxides