Chemical Data Notebook Series: Calcium Carbide
HAZARDOUS MATERIALS
By itself, calcium carbide has only one hazard: It’s corrosive. However, when calcium carbide contacts water, a very rapid chemical reaction occurs—acetylene gas is liberated. This is the real hazard associated with calcium carbide because, when under pressure, acetylene will explode.
There are many other names for calcium carbide, including acetylenogen, calcium acetylide, calcium dicarbide, carbide, and ethyne (calcium derivative). Calcium carbide’s chemical formula is CaC2. Its molecular weight is 64.
Its UN/NA designation is 1402, its STCC (Standard Transportation Commodity Code) is 4916408, its CAS (Chemical Abstract Services) number is 75-20-7, and its RTECS (Registry of Toxic Effects of Chemical Substances) or NIOSH (National Institute for Occupational Safety and Health) designation is EV9400000. NFPA’s 704 system rates calcium carbide as 1-4-2-W, The U.S. Department of Transportation classifies it as a “Flammable Solid, Dangerous When Wet,” and the International Maritime Organization classifies it as 4.3, a flammable solid. It usually exists as approximately 75% pure. The remainder is made up of calcium oxide (lime).
Properties
Calcium carbide exists in a powder or lump form and is slightly corrosive. It has a faint odor, similar to that of garlic.
With a specific gravity of 2.22, calcium carbide will sink in water. It will begin reacting at first contact with water, but not fast enough to keep from sinking.
Although it’s a stable chemical, calcium carbide reacts violently with a few other chemicals, including acids, alkalies, lead fluoride, magnesium, selenium, silver nitrate, sulfur, sodium peroxide, and tin chloride. There are reports that explosive compounds may be formed when calcium carbide contacts copper or brass. Remember this when you select the tools to handle a calcium carbide spill.
It’s unlikely that you’ll ever see calcium carbide melt, even in a fire, because the temperature required is 4,172° F. Calcium carbide boils at 4,055° F, which is just as unlikely to be reached. The substance has no vapor pressure, no flash point, no flammable range, and no ignition temperature, but it’s still listed as a flammable solid!
Calcium carbide doesn’t burn, but the acetylene gas it produces in reaction to water is flammable to the point of being explosive. Generating this gas is the major use of calcium carbide.
Any business that bottles acetylene will generate the gas itself from calcium carbide, which is shipped to these companies in siftproof, airand watertight containers. Acetylene isn’t shipped over long distances because it has such a wide flammable range (2.5% to 80%), burns at such a high temperature (11,526 calories per gram, which produces a flame temperature of 5,710° F when burned in oxygen), and is very unstable. If it’s shocked when it’s under even slight pressure, acetylene will detonate!
The chemical reaction that produces acetylene from calcium carbide is:
CaC2 + 2HOH—CH + Ca(OH)
The acetylene will vigorously bubble out of the solution, which is made up of calcium hydroxide (slaked lime), a caustic material, dissolved in water. This reaction can be catastrophic when it occurs out in the open. When acetylene is being generated for commercial use, the reaction occurs in a closed container. Water is added at an intake valve, and acetylene is removed at another port.
Hazards
A large amount of dust will be created during the shipping and handling of calcium carbide. It’s dangerous to inhale this dust. Asphyxiating acetylene would be generated by any calcium carbide dust reaching the lungs, and the calcium oxide in the dust would irritate the lungs.
Calcium oxide can also irritate the skin and eyes. Ingesting the substance is hazardous to the mouth, lips, and throat.
Since the generation of acetylene is the major hazard involved with calcium carbide, you must take precautions to prevent an explosion. Acetylene has a vapor density of 0.897, which means it’s slightly lighter than air. As it’s generated in a calcium carbide release, the gas will rise as it bubbles out of the solution. It’s at this point that the danger is greatest, because the concentration of explosive gas will be at its highest just above the reacting calcium carbide. From this point, the gas will begin to disperse in the air. The stronger the wind, the faster the dispersion will occur.
Containment efforts
It’s not always apparent when calcium carbide is mixing with water; it may even react with moisture in the air. So containment efforts have to take into consideration all the forms in which calcium carbide can be released and all the ways in which it can come in contact with water.
No matter where the release occurs, you must follow certain procedures to prevent catastrophic results from the spill.
- Approach the spill from upwind;
- Remove all ignition sources, including the apparatus;
- Secure the area around the spill as quickly as possible;
- Avoid any action that might agitate the material or cause it to come into contact with water.
Emergency responders are often so concerned that water will reach a water-reactive material that they may forget to try to prevent the water-reactive material from reaching water. Don’t allow the material to spill into sewers or waterways. And remember that moisture in the soil that contains the calcium carbide may be another source of water that begins the reaction.
Keep calcium carbide dust from swirling around—and from getting wret—by covering it with a tarp made of plastic, rubber, or some other material that won’t allow water to pass through.
This is an important procedure. Some emergency responders may think if they don’t apply water to the spill, the material will remain dry. However, they often forget about the possibility of rain. Even if it doesn’t rain, strong winds may blow the material around, and if the ground is wet, reactions will begin immediately.
If water has contacted the calcium carbide or if rain seems likely and the material can’t be covered, one solution is to create containment dikes, ponds, and pits. You can dig trenches or pits to contain the solids or solution. Use sand, clay, soil, or another substance as diking material—just as you would for any liquid spill. These materials may also be used to block entrances to sewers and waterways. Just remember that any moisture in these materials will begin the acetylene-generating reaction.
If the calcium carbide does enter a waterway, you must notify all appropriate agencies and downstream users of the water. There will be an immediate reaction when the material contacts the water. However, some of the larger lumps will sink and take longer to react. Remove them by dredging the bottom of the waterway. But be careful, because as long as the dredged-up lumps are wet, they will generate acetylene.
Glossary
Asphyxiation—Unconsciousness or death caused by the lack of oxygen.
Dispersion—The act of widely scattering a material or dispelling it from a general area.
Narcosis—Deep unconsciousness caused by a drug or chemical.
Reactor—A vessel, usually closed, in which a chemical reaction is carried out.
Vapor density—The density of a vapor or gas as compared to air.
Water-reactive—Having the property of entering into a chemical or physical reaction when contacting water.
In a fast-moving stream, the rock-like material may be carried downstream, releasing acetylene as it moves. In this case, the generation of the gas will be spread over a larger area, and this may lessen the danger, unless, of course, the acetylene released downstream is ignited and the area affected is more highly populated or has more exposures.
The most dangerous situation would be if calcium carbide entered an enclosed area containing water, such as a sewer. Acetylene’s very wide flammable range will allow tremendous amounts of the gas to build up and move throughout a sewer system. The gas buildup and the explosion that results when an ignition source is reached could be catastrophic and cause extensive damage. This could be much more devastating than the results of an isolated spill that occurs out in the open. Remember, it’s imperative that you try to prevent calcium carbide from entering a sewer.
Handling fires
If water does reach a calcium carbide spill in the open and the acetylene is ignited, the procedures to follow will depend on the circumstances, location, exposure, and danger to human life.
In some cases, you can handle burning acetylene as you would a fire caused by a container leaking pressurized gas. The rule here is: Don’t extinguish the fire unless the flow of gas can be immediately stopped. The rationale, of course, is that as long as the gas is burning, it won’t accumulate and cause a devastating explosion. If there’s no danger to human life or exposures, it may be better to allow the acetylene to continue to burn.
You should even encourage the generation of more gas by adding water to any unreacted carbide until all the carbide has reacted and the fire goes out due to lack of fuel. Just remember that a deliberate burn that starts out with no danger to human life and exposures can get out of hand and cause severe damage. You must know all the dangers and consequences of deliberate burns and accept the responsibility for them.
Emergency responders may start the deliberate burn if they feel they can’t prevent the accumulation of acetylene gas and that this accumulation will present a greater danger of explosion if it’s ignited at some site downwind. Deliberately igniting the gas is much more dangerous than allowing a burning gas to continue burning, because the act of ignition may cause an explosion if there’s enough gas accumulated. Again, the incident commander must be aware of all dangers and be prepared for the consequences of his decisions.
Because of acetylene’s vapor density, it’s unlikely that vapors will build up. However, if they do, they can be dispersed by using a water spray or fog applied to the suspected area. But be sure not to allow the water to contact the exposed calcium carbide.
The incident commander can use any noncombustible, nonhazardous dry powder to extinguish the fire: dry sand, clay, graphite, soda ash (sodium carbonate), dry Portland cement, common table salt (sodium chloride), or lime (calcium oxide). Lime is the same chemical as the calcium oxide that might be present in the calcium carbide, so the commander should already be aware of its corrosiveness and water-reactivity.
These materials may smother the fire or dry up the water to slow down or stop the generation of gas. Carbon dioxide may or may not work, and, of course, foams contain water that might intensify the reaction. Try to avoid spreading the material or generating dust during extinguishment.
It’s not necessary to wear fully encapsulated chemical suits when dealing with calcium carbide. But do wear protective clothing that will shield your skin and eyes. This includes rubber gloves and boots, face shields, safety goggles, and any other clothing that will resist the material. Normal turnout gear will protect the part of the body that it covers. Sleeves and pant cuffs should be tied down. You must wear positive-pressure self-contained breathing apparatus in any atmosphere where dust has been generated. However, no type of clothing will protect you from the explosion of acetylene gas or from the tremendous heat generated by burning acetylene.
First aid
Because acetylene is such a reactive gas, inhaling it can cause dizziness, apparent intoxication, headaches, increased respiration, and narcosis. Give the victim fresh air, and administer artificial respiration if the person’s breathing becomes impaired or stops.
Use large amounts of water to wash the parts of the body that have contacted the material. Flush the eyes immediately for at least 15 minutes, occasionally lifting the eyelids. In all cases of skin or eye contact, medical attention is necessary. If the acetylene has been ignited, any burns will be severe. They should be treated medically as soon as possible.
Emergency responders may try to contain the material by shoveling it into water-tight containers. They may even use front-end loaders and backhoes to pick up the calcium carbide and load it into covered trucks or large, secure containers.
If you are involved in cleanup and removal, remember that you must follow all federal, state, and local regulations. Also remember that the contaminated soil collected now contains a water-reactive substance, and any moisture present or subsequently in contact with the soil will cause a release and buildup of acetylene. And any sparks from cleanup tools could be the ignition source for an explosion.
But, if there’s no impending windor rainstorm, it’s safer to cover the material, monitor the situation, and secure the area until the professional cleanup crews arrive. Emergency responders should handle the removal themselves only when it’s absolutely necessary. An experienced, educated, well-trained incident commander should be the one to decide whether it is.
