SAFETY PRACTICES
Safe Handling of Alkali Metals The alkali metals are increasing in industrial importanc-sodium for the manufacture of titanium, sodium-potassium alloy for heat transfer, and lithium for high energy fuels. Proper handling begins with the provision of specific storage facilities for alkali metals and with equipment design involving anticipation of the particular operating problems which may be encountered when handling these materials.
MARSHALL SITTIG Ethyl Corp., 100 Park Ave., New York 17, N . Y .
A
S THE alkali metals assume increasing prominence in
today’s industrial world ( I S ) , the problem of their safe handling becomes increasingly more important. Sodium is produced in substantial quantities and has essentially attained the status of a heavy chemical. Potassium is exhibiting steady growth, and lithium, after dropping from a war time peak associated with lithium hydride production for military hydrogen manufacture, is now growing rapidly sparked t o a degree by requirements for hydrogen bomb manufacture. I n addition to these three metals, it is proper to mention sodium-potassium alloy (NaK) as a growing relative of the alkali metal family. Rubidium and cesium are produced only in gram quantities and are not discussed further here. The principal hazards u hich may be encountered in the handling of the alkali metals are: Explosions or fire resulting from contact with water, chlorinated hydrocarbons, or other reactive agents Fires resulting from their exposure to air Personnel injury from direct contact with hot alkali metals Caustic soda burns from the residue of a sodium-water reaction All these hazards may be effectively combated by use of known handling techniques (3,4,10,11,14). Particular attention is called t o the recently issued Sodium-NaK Supplement (6) t o the “Liquid Metals Handbook” which contains extensive material on handling both nonradioactive and radioactive sodium and sodium-potassium alloy. STORAGE PROBLEMS
The alkali metals are available from the manufacturers in a variety of solid forms-generally in brick form or cast solid in containers ranging from drums t o tank cars. Sodium is available in two grades. The grade of lesser purity is actually a very pure material running about 99.8% sodium with potassium and calcium as the major impurities. A second high purity grade now is available for titanium manufacture and other special applications; it is filtered through a sintered metallic plate and is loaded into the shipping container under argon rather than nitrogen. Sodium is available in bricks ranging from 1 pound t o 24 pounds in weight, cast solid in steel drums and in 80,000-pound tank cars. Potassium is available in 99+% purity with sodium as the major impurity. It may be obtained in 0.5-ounce sticks and in I-, 2.25-, and 4.25-pound bricks as well as in cast solid form in steel drums. Lithium is available in two grades-a regular grade averaging 99 +% lithium and a low sodium grade averaging 0.005% sodium or less. It is available commercially in small shot, wire, and in cylindrical bricks ranging from 0.2 to 2 pounds each. February 1956
The precautions t o be followed in storing any of the valuable alkali metals are similar. The important thing is that the area must be dry, because hydrogen gas explosions may result from the contact of the metals with water. No automatic sprinkler system or water or steam piping should be used in the room. Sufficient heat should be provided, without using open flames in heaters, t o prevent condensation of moisture in the room if the weather changes. In any event, a designated alkali metals storage area should be provided. This area may be a separate room, an area in a large room, or simply a steel box such as used for the storage of paints and flammable solvents. Yellow lines on floors may be used t o mark off an alkali metals storage area within a room. Fire extinguishers for use on alkali metal fires must be provided in the storage area, but only extinguishers containing the agents recommended in the following sections. On the laboratory bench the alkali metals may be stored dry in rust-free friction top metal cans or in cans under a layer of dry hydrocarbon. Glass containers should be avoided where possible, and if used, they should be placed inside metal pails t o confine alkali metal spillage if the glass container should break. I n storing intermediate amounts of alkali metals in the plant, only that amount immediately needed should be removed from the storage area, and the alkali metals should not be withdrawn for intermediate storage in reaction areas. If the alkali metals are handled on a very large scale as is the case with sodium, t h e storage problem actually becomes easier because the molten metal is stored in a tank farm and pumped, or otherwise conveyed, completely within closed systems,
PROTECTIVE EQUIPMENT FOR HANDLING
Clothing. The degree of protective clothing required in handling the alkali metals depends entirely on the quantity of metal to which a person is exposed and on the temperature of that metal. In manufacturing operations or in large volume systems of high temperature heat transfer, a maximum of protection is required. Aprons, leggings, and face covering with masks may be employed. The Knolls Atomic Powcr Laboratory specifies that a plastic face shield and helmet, with duck apron, should be worn in addition t o goggles where alkali metals are being handled or circulated a t temperatures above 425“ F. (16). Loose-fitting coveralls are specified also. A somewhat different and appreciably simpler problem is encountered when handling a few hundred pounds of the alkali metals for introduction to chemical process equipment a t temperatures just above the melting points of the metals. Conventional factory protective equipment of goggles and gloves generally is
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adequate. All clothing, including gloves, should be loose fitting and easily removable. In handling the alkali metals in the labor:it,ory, goggles and gloves are the only essential additions t o conventional laboratory clothing. When dispersions of the alkali metals in hydrocarbons are handled, rubber or plastic gloves and aprons are desirable in addition to goggles or a face shield. As the Knolls Laboratory “Guide to A411raliMetals Handling” ( 1 5 ) states: “The absolute minimum equipment which must he worn whenever the smallest amount of liquid metal is handled in the open is complete protection for the eyes, hands, and arms.” Dyne1 work clothing has been found desirable for working with the alkali metals, as it has considerable resistance in case of splashes or spills. In any event, clothing worn by operators in alkali metal handling areas should be fire resistant. Blanketing Gases. Kitrogen, which is used for blanketing molten sodium, should have a moisture content less than 1 mg. per cubic foot, should have an oxygen content less than 0.5YG by volume, and should have a hydrogen content less than 1.5y0 by volume. Ansmering these specifications, nitrogen is equally suitable for use in blanlret,ing potassium metal. Lithium, hov-ever, reacts with nitrogen, forming the characteristic black nitride coating; t,hus, argon or helium must be used as blanketing gases. The black nitride, Li3N, increases the viscosity of molten lithium appreciably, so that particular care must he taken to keep niirogen out of liquid lithium-circulating systems. Potassium readily forms a superoxide, KO2)tvliicli may undergo vigorous reactions-with oil, for example. Therefore effective blanketing is particularly important in the case of potassium (7‘). Dry Boxes. It may be particularly convenient to handle sodium or liquid sodium-potassium alloys in the laboratory in nitrogen-filled dry boxes, descriptions of Tvliich are availahle ( I , 9). Equipment Design for Safe Handling. An integral factor in the safe handling of the alkali metals is t,he proper design of equipment. This covers a number of items and is apart, from the provision of first-aid in fire-fighting equipment. Rather it concerns the design of an alkali nietal handling system from the ground up with all necessaiy precautions built into the equipment ( 1 7 ) . This involves the maintenance of closed systems, the proper provision of blanketing gases, and the provision of spray shields-a t flanges and pump cells, for example-to prevent the spraying of hot metal under pressure in case of a lea!stem cleaning has been described in some detail (6, 1 7 ) . FIRE EXTINGUISHING
Choice of Extiiiguishing Agents. Fighting alkali metal fires simply consists of forming a protective blanket over the metal so that the metal cools below its burning temperature in air. I t should be emphasized that prompt action is vital and that the action taken during the first minute is the most important one as far as controlling the fire is concerned. In general, the preferred agents for extinguishing sodium and potassium fires are dry sodium carbonat,e (aoda ash), dry sodium chloride, and dry powdered graphite. Although these extinguishing materials are satisfactory for sodium and potassium, sodium carbonate and sodium chloride are not recommended for lithium fires, because lithium liberates free sodium and the residue is more reactive than the original lithium. Thus, for lithium fires, powdered graphite and zirconium silicate are the only recomniended agents. These various agent,s may be dispensed under inert gas prepsure from extinguishers or through permanent piped systems. The agents may also be applied, to localized fires, with long handled shovels in the plant or with scoops in the laboratory. Extinguishing agents which should not, be used for alkali metal fires include water, aqueous foams, carbon dioxide, or chlorinated hydrocarbons. A preferred practice designates yellow as the standard color for approved extinguishers, as distinguished from red for the unapproved (for alkali metals) types listed above. Pails of powdered agents may be painted yellow and stenciled in red “For Alkali Metal Fires Only” or “For Lithium Fires Only.” I n general, from 2 to 8 pounds of fire extinguishing agents are required per pound of hot metal ( 6 )although a ratio of 1 t o 1 has been recently indicated as adeqnate ( 1 6 ) . However, graphite is particularly effective on lithium firrs and a light dusting with less than 1 pound of graphite per pound of lithium may form a lithium carbide crust thick enough to smother the fire. February 1956
Fires of dispersions of alkali metals in hydrocarbons present a special problem. Such dispersions can be ignited by contact either with water or fire, or by simply spilling them on porous materials whereupon the hydrocarbon is absorbed and the finely divided metal exposed t o atmospheric moisture. Such a fire is first a hydrocarbon fire and can be brought under control with conventional dry chemical (bicarbonate) extinguishers. Blanketing with inert gases or simply closing all the openings in a vessel are also effective means of fighting fires within R piere of apparatus. Use of Perforated Trays. I n the laboratory as well as in the plant, it is a recognized practice to place metal pans or trays beneath individual pieces of equipment, which are capable of containing all the alkali metal within that equipment. A more recent innovation which has been developed by the workers a t the Knolls Atomic Power Laboratory involves the principle of oxygen starvation as a means of fire control through the provision of perforated covers on such trays. Experiments have indicated that holes should be drilled in the steel covers of such trays giving a masimum open area of about 35y0of the total area. Using this design condition, any burning sodium which falls onto the tray and through the holes extinguishes itself within the tray. In practice at the Knoll? Laboratory, pans made of ‘/*-inch coldrolled steel about 16 inches square and 3 inches deep were fabricated and a//r-inch holes were drilled in the cover, spaced such that 3OrO open area was provided. These holes were punched and dished in order to minimize liquid metal holdup on the covers. CONCLUSION
A large amount of work is currently being carried out in both government installations and industrial plants involving the handling of the alkali metals. This work has already resulted in a veiy well-developed technology of alkali metals handling such that these metals, which but a few years ago were only laboratory curiosities, can now be handled with safety and assurance. It is hoped that workers in this field will continue, within the limits of security, t o publish their results on new and more effective techniques for the safe handling of the alkali metals. LITERATURE CITED
Brieeleb, G., Chem. Ing. Tech. 21, 234 (1949). Cottrell, W. B., and RIann, L. A , , Sodium Plumbing, ORNL1688,Aug. 14, 1953. Ethyl Corp., New York, K. Y., “Handling Sodium-In the Laboratorv. In the Plant.” 1953. Hawkes, A. k., Hill, E. F., and Sittig, N., J . Chem. Ed. 30, No. 9, 467-70 (1953). Hill, P. L., “illkali Metals Area Safety Guide, Y-12 Alkali and Liquid Metals,” Carbide & Carbon Chem. Corp., Oak Ridge, Tenn., Aug. 13, 1951. Jackson, C. B., in “Liquid Metals Handbook,” Sodium-SaK Suppl., U. S. Govt. Printing Office, Washington, D. C., July 1955. Jackson, C. B., and Adams, R. RI., in “Liquid Metals Handbook,” 2nd ed. (revised), Navexos P-733 (Rev.), U. S. Govt. Printing Office, Washington, D. C., Jan. 1954. Kelman, L. R., Wilkinson, W. D., and Yaggee, F. L., Argonne Natl. Lab. Rept. ANL-4417, July, 1950. Ketchen, E. E., Trumbore, F. A , , Wallace, W. E., and Craig, R. S., Rev.Sei. Instr. 20, 524 (1949). Manufacturing Chemists Assoc., Washington, D. C., Chemical Safety Data SD-47, 1952. Natl. Safety Council, Chicago, Ill., Data Sheet D-Chem-37. Nelson, D. B., Knolls Atomic Power Lab., Rept. AECU-1273, March 1, 1951. Sittig, M., Chem. W e e k 74, N o . 26,47-54 (1954). Sittig. Rl.. Natl. Safety A-ews 68, 26, 27, 111-14 (July, 1953). Steiner, F. C., “Guide t o Alkali Metals Handling,” Knolls Atomic Power Lab., Manual LMSC-1, Schenectady, N. Y . July 1, 1954. Steiner, F. C., private communication, Aug. 2, 1955. Woolen, W. B., in Report No. X/R 1381, iltomic Energy Research Establishment, Harwell, England, 1954. RECEIVED for review April 23, 1956. ACCEPTEDNovember 2 3 , 1955. Division of Industrial and Engineeting Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, March-April 1956.
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