CHEMICAL DATA NOTEBOOK SERIES #111: SILICON TETRACHLORIDE
BY FRANK L. FIRE
Silicon tetrachloride is a water-reactive, toxic, corrosive, irritating, mobile, volatile, colorless to light yellow-colored fuming liquid with a sharp, pungent, suffocating odor. It is commonly used in the production of compounds such as ethyl silicate, high purity silica, various silicone compounds, fused silica glass, and other chemicals. It reacts vigorously with water, liberating heat, hydrogen chloride (which dissolves in the water to form hydrochloric acid), and siloxane.
Known principally as a corrosive liquid, silicon tetrachloride will not burn. Its boiling point is 135°F; its melting point ranges from 90°F to 94°F; and it decomposes at temperatures produced by fires of ordinary combustibles, producing chlorine, hydrogen chloride, and silicon dioxide. Its specific gravity is 1.48; molecular weight, 170; and vapor density, 5.86. It reacts vigorously with water. Its chemical formula is SiCl4.
Silicon tetrachloride`s main hazard is corrosivity. The liquid and fumes will attack most metals if any moisture is present and will attack human tissue–causing severe burns–even if moisture is not present. Contact with the eyes will cause tearing, inflammation, and even permanent damage.
Water reactivity presents a significant problem. As silicon tetrachloride is exposed to the atmosphere, it begins to react with the moisture present, producing hydrogen chloride and generating heat. Eventually, the heat buildup will be great enough to accelerate the generation of hydrogen chloride, which in turn will generate more heat. If a significant quantity of silicon tetrachloride is exposed, the hydrogen chloride generated will cause breathing problems; the hydrochloric acid generated will be corrosive to human tissue.
Inhaling silicon tetrachloride vapors or hydrogen chloride will result in respiratory problems, causing the following symptoms: breathing difficulties, burning sensations, coughing, headaches, nausea, and vomiting. Exposure to high concentrations of silicon tetrachloride can cause inflammation of the bronchial tubes, chemical pneumonitis, and pulmonary edema–any of which could cause death. These serious symptoms may be delayed for from 12 to 48 hours.
Silicon tetrachoride is very mobile (the liquid has a very low surface tension and therefore will flow very easily and rapidly) and volatile (the liquid has a high vapor pressure and will evaporate rapidly and produce high amounts of vapor in a short time). Once accidentally released, therefore, silicon tetrachloride will flow more rapidly and evaporate considerably faster than water.
Since the effects of inhaling silicon tetrachloride vapors are the same as those caused by inhaling any corrosive liquid`s vapors, silicon tetrachloride`s toxicity may be in question. If the material is ingested, all tissues in contact with the liquid will be irritated or possibly severely burned. The mouth, lips, esophagus, and stomach could be severely burned; nausea and diarrhea can result; and, in severe cases, collapse, unconsciousness, and death can occur. There is no listed TLV-TWA (threshold limit value-time weighted average) for silicon tetrachloride. However, many references indicate that its toxicity is the same as that for hydrogen chloride. It, therefore, makes sense to use a ceiling value of five ppm (parts per million of air).
Products of the decomposition of silicon tetrachloride are more dangerous than the products produced by reaction with water. Although hydrogen chloride is produced as a result of silicon tetrachloride`s reaction with water and its decomposition, the additional product (chlorine) formed during the product`s decomposition is a highly dangerous, toxic, corrosive, oxidizing material.
Silicon tetrachloride is considered stable during normal transportation. Shippers of the product protect it from contact with water or moisture in the air. It reacts vigorously and dangerously with sodium, potassium, alcohols, alkalies, and acetone.
The release of any appreciable quantity of silicon tetrachloride is dangerous because of the material`s hazards and the different manners in which the hazards themselves produce hazards. A release should trigger the community`s emergency response plan, providing the incident commander with all the resources needed to mitigate the emergency.
If a container of silicon tetrachloride is leaking, contain and recover, or cover the product as soon as possible. Because of its high mobility, the liquid will move rapidly and rapidly percolate into the soil. Its high volatility will cause it to generate vapors and react with the moisture in the air. Large amounts of hydrogen chloride gas may also be generated. Therefore, at the start of the incident, consider evacuating areas downwind of the immediate spill area.
Approach the release of the volatile silicon tetrachloride from upwind and uphill because the product may be reacting with the moisture in the air and releasing dangerous amounts of hydrogen chloride. Also, the heat generated by the material`s reaction with water could be significant and dangerous.
For releases of most hazardous liquids, it is recommended that a containment pond to hold the product be constructed by pushing up earth or sand around the released material. If the proper equipment is available, a containment pit may be dug to hold the product. Removing contaminated soil may be more difficult from a pit than a pond, however.
The silicon tetrachloride should be covered with a sheet of plastic or other impervious material to protect it from moisture and contain the vapors and any hydrogen chloride being formed. Professionals from a salvage firm should siphon the product from the containment pond or pit into secure containers, using tools and equipment compatible with silicon tetrachloride.
If the product cannot be covered and the location of the spill or evacuation is presenting difficulties, a water spray may be used to sweep hydrogen chloride gas or silicon tetrachloride dust from the air. Be careful. Do not allow the water to contact the released liquid. It may seem strange to suggest that water be used to mitigate an incident that involves a water-reactive material; but, in some cases, it may not be possible to cover the liquid to slow the generation of hydrogen chloride–it may be raining, for example, in which case the spilled product will heat up and produce acid gas more rapidly. The water fog or spray could control the movement of the gas. It will be difficult to control the runoff in the rain.
Prevent silicon tetrachloride from entering the sewer system or waterways. Recover accumulated liquid pools with appropriate compatible hoses, pumps, and storage containers or vacuum trucks. Absorb any remaining spilled liquid with cement powder, clay, earth, fly ash, sand, commercial sorbents, or other compatible substances. Once used, sorbent materials pose the same hazards as the spilled product and must be handled in the same safe manner as the pure product. Their use adds to the overall volume of contaminated material. All contaminated solids must be removed and disposed of in accordance with all federal, state, and local regulations.
Silicon tetrachloride`s entering a sewer system could cause problems throughout the municipality served by that system. As soon as the product contacts water in the sewer system, hydrogen chloride gas will be formed, and it will begin to dissolve in the water. As more and more hydrogen chloride and acid are formed, dangerous reactions might take place within the sewer.
Silicon tetrachloride entering a pond or lake could be fatal to aquatic life near the point of entry. The product will dissolve in the water and slowly spread away from the point of entry. If the entry is into a river or stream, the contamination will be carried away and mixed throughout the water at a rate proportional to the speed of the moving water. If the volume of moving water is great, the material will dissolve and be diluted faster. Immediately notify possible downstream users of the water. The environmental authorities must determine the extent of contamination and when the water will be safe for use again.
The deadly chlorine gas that forms as a decomposition product when silicon tetrachloride is exposed to a fire`s heat is not only toxic and corrosive but also a powerful oxidizing agent. Its presence (in addition to the oxygen in the air) will worsen the fire.
The pressure relief device on a silicon tetrachloride container will be operating from the first time the container is heated. Vapors will be rushing into the atmosphere. Water from firefighters` hoses will react with that vapor, producing hydrogen chloride, which is also soluble in that water. Another strange comment made in connection with such a situation is that it is better that hydrogen chloride be generated instead of chlorine.
Protect all containers of silicon tetrachloride from the heat of a fire by applying water to them with unmanned monitors from as far away as possible. Never approach a container of liquid being heated by a fire. The container`s internal pressure may rise faster than its pressure relief device can relieve it, possibly causing the container to fail catastrophically.
PROTECTIVE CLOTHING AND EQUIPMENT
Choose protective clothing and equipment that will prevent the possibility of skin or eye contact, including rubber boots, gloves, face shields, splashproof safety goggles, and other impervious and chemical-resistant clothing. Total encapsulating suits with self-contained breathing apparatus must be worn to prevent contact with silicon tetrachloride`s vapors. Suits made of Teflon(TM) may offer protection. Ask the manufacturers of the suits and the silicon tetrachloride for their recommendations. Just remember that protection is a relative term. A material may be used because it resists a particular chemical in the laboratory, but the same material used in a total encapsulating suit may fail because it is thinner than that used in the lab tests. Other factors affecting the suit`s protection level include the integrity of the seams and the coverings on the zippers, the concentration of the chemical in question, and the duration of the exposure.
An SCBA with full face piece (or the equivalent) must be worn when participating in an incident involving the release of silicon tetrachloride.
Inhalation. Move a victim who has inhaled the vapors of silicon tetrachloride and hydrogen chloride to fresh air and keep him warm and at rest. If breathing stops or becomes difficult, administer artificial respiration. (Avoid mouth-to-mouth contact, since it may expose you to the chemicals in the victim`s lungs or vomit.) Get medical attention immediately.
Eye contact. Immediately flush the contaminated eye(s) with lukewarm, gently flowing water for 20 minutes, holding the eyelid(s) open. Do not rinse contaminated water into the nonaffected eye. If irritation persists, repeat flushing. Obtain medical attention immediately.
Skin contact. Remove all contaminated clothing at once. Wash all affected body areas with large amounts of water. Get medical attention immediately.
Ingestion. If the victim is conscious, administer large quantities of milk of magnesia or water immediately. Do not attempt to make the victim vomit. Get immediate medical attention. n
IDENTIFICATION NUMBERS AND RATINGS
(Chemical Abstract Services)
(Standard Transportation Commodity Code)
(Registry of Toxic Effects of Chemical Substances)
(United Nations/North America)
(Chemical Hazard Response Information System)
(U.S. Department of Transportation)
(International Maritime Organization)
NFPA 704 Rating
(National Fire Protection Association)
FRANK L. FIRE is the vice president of marketing for Americhem Inc. in Cuyahoga Falls, Ohio. He`s an instructor of hazardous-materials chemistry at the University of Akron as well as an adjunct instructor of haz mats at the National Fire Academy. Fire is the author of The Common Sense Approach to Hazardous Materials and an accompanying study guide, Combustibility of Plastics, and Chemical Data Notebook: A User`s Manual, published by Fire Engineering Books. He is an editorial advisory board member of Fire Engineering.