A mixture of steam, flammable gas and high temperatures erupted with explosion and fire with two fatalities.
As industrial chemical processes go, manufacturing calcium carbide is one of the simpler ones. Still, it requires enormous amounts of heat and can produce a dangerous mix of flammable products and toxic gasses. On March 21, 2011, there was an explosion in an electric-arc furnace (EAF) at Carbide Industries, LLC, in Louisville, Kentucky. Two workers were killed and two others injured.
The headline for this post came from a statement made by Rafael Moure-Eraso, chairperson of the Chemical Safety Board as he summarized the investigation of this incident. He said, “This accident is literally a case study into the tragic, predictable consequences of running equipment to failure even when repeated safety incidents over many years warn of impending failure.”
Manufacturing calcium carbide is very basic. Coke, which is essentially pure carbon, and lime are fed into an EAF and heated to about 3800 ºF. At this point it all melts and a reaction causes the new compound to be formed. It also releases carbon monoxide, which is both toxic and flammable. Residual hydrogen in the coke is also liberated as flammable gas. The final product is typically around 80% pure and is mixed with other impurities brought in with the feedstocks. When mixed with water, in liquid form or humidity, calcium carbide releases acetylene. This commercial production approach began in the late 19th century when electric power was available in amounts sufficient to run an EAF.
Carbide Industries’ Louisville facility operated a single 50 MW EAF and could produce about 120,000 tons of calcium carbide annually. Most of the product was sold to steel mills or industrial gas producers.
Making Calcium Carbide
Once the furnace is charged with raw materials, the arc furnace lights up and melts the material. After the initial heating, it becomes a continuous process adding more feedstock as the mixture melts and drains out of the furnace into cars. Once cooled, the solid chunks are ground to the desired particle size.
Given the high temperatures produced by an EAF, the enclosure itself is typically a combination of steel and refractory material. Most furnaces have a lid with holes for the three enormous carbon electrodes to pass through. To keep the weight of the lid manageable, it is usually made of refractory material in a steel shell. To preserve the steel, it is constructed with a maze of pipes and internal passages carrying cooling water to remove the heat. Lid sections are bolted together and water flows from section to section. Given the heating and cooling cycles, along with general abuse equipment takes in such a difficult environment, keeping the lid in working order is a challenge but minimizing leakage is critical.
At the Louisville plant, the control room was 12 ft. from the furnace cover. Aware of the potential for catastrophic problems, the control room windows were dual-pane, half-inch thick glass with wire reinforcing. Unfortunately, that was not enough.
When the furnace is working, the interior is kept at positive pressure to prevent air being drawn in and creating an explosive mixture. Creating that pressure is no problem given the volume of gasses produced by the process provided the containment is tight enough to keep it from dissipating too quickly through leaks in the shell. Those gasses are supposed to go to a wet scrubber to be cleaned and then flared.
Operators watch the top of the furnace from the control room where they judge what’s happening inside by watching the flames from gasses escaping around the electrodes. In some cases, a few times per year according to reports from plant employees, the furnace would have a “blow” where it would over-pressurize with an eruption of feed material and hot gasses out onto the floor outside the control room. Although these happened with some regularity, there was never an attempt to determine the cause.
On March 21, 2011, at about 5:40 pm, the furnace was operating normally but experienced a series of major blows in rapid succession, then a large explosion followed by two or three additional smaller events. Debris from the large explosion crashed through the windows into the control room, fatally burning the operators inside. It also flew out of the tap hole at the bottom of the furnace where the cars capture molten carbide as it comes out of the furnace.
The smoke from the resulting fires engulfed the five-story building until fire fighters got the scene under control. The operators in the control room died within 24 hours.