On the mechanism of flame inhibition by alkali metal salts

Combustion and Flame

85

On the Mechanism of Flame Inhibition 'by Alkali Metal Salts J.D. BirchaH Imperial Chemicat Industrie~ Ltd.. Mond Division. Runcorn, Cheshire. Engkmd Alkali metal salts, in particular sodium and potassium bicarbonate, are increasingly used as fire-extinguishing powders. Studies have been made in the past of the relative efficiency of alkali-metal salts as flame inhibitors, and the oxahltes have been shown to be particularly effective. In the present work it has been found that the high efficiency of the alkali oxalates is due to their ready decomposition in a flame, with the generation of sufimicron particles of alkali carbonate. Other salts such as alkali ferrocyanides have been found to behave in a similar way, With such salts the surface area required for the extinction of a diffusion flame varies with particle size. there being an optimum size below whleh decomposition occurs before the flame front is reached and above which little decomposition occurs within the residence time of the particle in the flame. Inhibition is considered to occur via a homogeneous mechanism, involving the volatilization and/or reaction of the initially formed submicron particles to provide gaseous hydroxide as the inhibiting species. In order that the generation of alkali oxide or hydroxide may be facilitated, the anion associated with the alkali metal should be of low acid strength.

Introduction It is well established that the efficiency o f finely ,divided solids in extinguishing flame is a function o f the surface area o f the solid presented to the flame [ I ] . In practical fire fighting, however, there is a lower limit to the particle size o f p o w d e r that can be used from pressurized containers and thus to the efficiency that can be realized with, say, N a H C O 3 [2]. T h e caking o f the p o w d e r increases with size reduction, and it becomes increasingly difficult for the particles to penetrate the flames [3"1. Laboratory tests carried out by various workers show large differences in the flame-extinguishing efficiency o f substances. T h e alkali-metal salts are a m o n g the most effective substances, potassium salts being generally better than sodium salts, and the anion a s s o c i a t e d with the metal has an important bearing on efficiency. Consistently in zhe litera-

ture, the alkali oxalates are reported as being particularly effective: some l a b o r a t o r y investgations, for example, have shown potassium oxalate to be better than sodium becarbonate by a factor o f 30, while tests on hydrocarbon liquid fires suggest a factor o f o v e r 4 [4. 5, 6, 7]. T h e mechanism o f inhibition by alkali-metal salts remains obscure, and the role o f the ~ssociated a n i o n - - i n particular, the high activity o f the o x a l a t c s - - r e m a i n s unexplained. T h e present investigation was carried out to study the effect o f the acid radical on the flameextinguishing efficiency o f alkali-metal salts. Experimental

Data

The Effect of Injecting Various Powders into Town's Gas-Air F l a m e A n a p p a r a t u s (Fig. 1) was constructed by which known weights o f powdered material cotfld be Combusthm & Fhmw, 14, 96 t t970) Copyright C 1970by The Combustion IltsUtute Published by AmericanElsevierPublishingCompany, Inc,

J, D. Birehall .......

.____

,,•

6OC

ta.I 5OC iJ_

~'SCMS

40C Na HC03 3OC

2OC HCOa IOC

II "OCMS

0

IL O %

AIR INLET

Figure l, App~tratus for iniecting powdered material ~nto town's gas-airIlame.

injected into a town's gas-air flame confined in a silica tube of 2.3-cm diameter. A turbulent di ffusion flame filled the cross section of the tube so that all the powder injected passed into the flame. The efflux from the flame could be collected and examined chemically, by X-ray and by electron and optical microscopy. The total gas flow rate was held constant at 10 liters min-~: the fuel-air ratio, however, could be varied, and rates giving overall compositions of 10°h~ v/v. 2 0 % v/v, and 3 0 % v/v fuel in air were studied. All the powders examined were sized between 200 and 240 B.S.S., corresponding to a mean particle diameter of 70 it, and the weight injected was increased progressively until 7 out of 10 tests resulted in the extinction of the flame.

v/v F U E L

2f0 30 IN AIR

Figure 2. Efficiencyof NaHCO.~and KHCOa(70-/t powders) in extinguishingtown's gus-air flame,

The results* for sodium and potussium bicarboTaate are given in F;g. 2. It will be seen that in bo~Lh eases the weight of powder required for extinction passes through a minimum in the mi,Jdle range of fuel-air compositions. At the minima, 200 mg N a H C O 3 was required for extinction, while only 100 m g of KHCO 3 was sufficient. This order of difference is in good agreement with results obtained by other workers. For both substances little change in particle size was observed alter passage through the flame, although the normally translucent crystals became opaque and their surfaces were converted to normal carbonate. The alkali oxalates, potassium ferrocyanide, and sodium nitroprusside gave results of particular interest, shown in Fig. 3. Again the potassium salts were found to be generally more e f * The extinguishing surface area is proporliomd Io the extinguishingweight['orall powders.

Flame Inhibition by kikali ~¢(~1S~lts

87

22C

s.s.s.s.s.s.s.s.s.~c <

z~14C

~2o

~ec --~ Na2C20a

Be

No~ [F,(C.),NO] ~H~O

_~ .,o

R,I\ ~ /

:)0

-~

i,. ~ = ~ K ,

is in conflict with results obtained by Friedrich [6], who allowed a cloud of powder to fall through a diffusion flame. Friedrlch invariably rated hydrates as being more effective than anhydrous salts, It is possible that the differenl results obtained in the Friedrieh test and in the study reported here may be due to differences in heat transfer to the particles under the two conditions. In the Friedrieh test the particles pass through the flame, whereas in the present method the particles travel with the flame gases so that diffusion ofstcam away from the particle may be slow and heat transfer reduced by ~.he steam envelope, An examination of the material issuing from the flame in the present test showed that hydrated materials wcre less reduced in size than the anhydrous salts--indeed, a proportion passed through unchanged.

F¢ (CN), . 3H:O

R, Fe(CN)o o,

.... i8

~

so

°/b "Iv FU~'L IN AIR

Figure 3. EI1L'ctof a~kalioxalates, llzrrocy~nides,etc.. o~ t~wn'.~gas Ilame(~$Ipowders70-t0.

fective than their sodium analogs, and anhydrous salts were more effective than the corresponding hydrates. With the most effective of these salts, the weight required for extinction was relatively insensitive to fuel/air ratio. In the case of the oxalates, ferrocyanides, and sodium nltroprusside, the efflux from the Hame was a firtely particulate smoke, many particles being less than I # in diameter (Fig. 4), Since it is well established that lhe efficiency of a powder in extinguishing flame is dependent on the surface area of solid presented to the flame, the above results suggest that the high efficiency ofcerta}n alkali salts (e.g., oxatates and ferrocyaaides) is due to their decomposition in the flame to give a solid reaction product having a large specific surface. The finding that hydrated salts were less effective !hart the corresponding anhydroos salts

Fi~ore 4,. Pohlss~umoxa(at¢snloke ( × 40,O(NII, [Reduced 25%'~,

88

7~=.~~:

J. D, Birchall

':

~=

-?'9

Figure 5. 70-/1 NaHCOa before passage through hot tube ( x 100). [Reduq:ed 26~/~].

Figure 6. 70-t4 NaHCO~ alter hot tube ( x [00). [ Reduced 261'/n],

Figur¢: 7, 70-/1 KHCO a before passage through hot tube ( x 100), [Reduced 26o/o].

Figure g, 70-p KHCO,~ aller passage through bot tube ( × ib0), [Reduced 26%~],

Figure 9. K =CO~ smoke resulting. I'rom passage of 70-/~ K ~ C : O 4 ' H 2 0 throtLgh hot tube ( × i~)(g;), [Reduced 4tJrl/o ],

89

Flame Inhibitionby AlkaliIVletalSalts The Increase in Surface Area on Passing Various Salts through a Hot Zone The observation that certain substances decomposed in flarrtes to give solid products having a large surface area was confirmed by passing a suspension of 70-,u powder in air through a silica tube 60 cm long and 5 cm in diameter heated to 1000°C. The eftlux from the tube was collected and photomicrographed. Typical results are illustrated in Figs. 5-9. Little change was evident in the case of sodium and potassium bicarbonate, although the heated crystals were opaque because of the formation of normal carbonate at the crystal surface and the magnitude of this change was in the order of decreasing thermal stability, that is, KHCO 3 was more stable than NaHCO 3. With KzCzO, ~. H,O and K,~Fe(CN)c,-3HzO. however, the average particle size was ca, I ,u. This represents at least a sixtyfold increase in specific surface. In the case of the oxalates the smoke consisted primarily of the alkali carbonate, while with ferrocyanide the smoke contained mainly alkali cyanate and carbonate and traces of cyanide together with iron compounds. It appears that substances which when heated undergo reactions of the type Solid I --->solid II + gas in which solid II is finely particulate, may potentially be effective flame-inhibiting agents.

that contained suspended submicron particles, identified by X-ray as mainly potassium carbonate. The reactions involved in the decomposition of oxalate are: K2C20 a • H20 ~ K2C20,~ + H,O K2C20 a-'> K2CO a + CO A proportion of this carbonate could, of course, have been formed in the cold probe and receiver by the carbonation of some precursor such as alkali oxide or hydroxide. Platinum probes were inserted in the flame above the point of contact with the solid, and a potential of 6 V was applied across the probes; the current through the ~ystem could be measured by de microammeter. The decrepitation of potassium oxalate crystals and cavity formation were associated with a large increase in flame current. These effects were investigated using a range of substances in the form of cylindrical pellets 0.6 cm in diameter, the end of the pellet being presented to Ific edge of a laminar town gas diffusion flame. Cavity size and flame current were noted (Table I ). Table I. Electrical Effects in Flames and Combustion Inhibition Substance Sodiumchloride Sodium bromide Potassium chloride

Effect of Large Bodies of Various Salts in Contact with a Laminar Diffusion Flame Friedrich [6] noted that, if a large crystal (or agg, cgation of crystals) of potassium oxahtte was presented to the edge era diffusion frame, a cavity--or zone in which combustion was inh i b i t e d - w a s formed above the crystal. This observation was confirmed by presenting crystals on the end of platinum wire to the edge of a laminar town gas diffusion flame. The crystal decrepitated, and by inserting a probe imo the flame cavity it was possible to withdraw gases

Flame Current. ttA 2 20

Potassium bromide 40 Potassium iodide S0 Potassium iodule " [54) Sodiumcarbonate anhyd, 2 Sodiumcarbomltc10H20 20 Sodinmbicarbonate 2 Potassiumcarbonate 50 Potassium bicarbonate 50 Potassiunloxatateanhyd. 50 100 Potassiumoxillale}1,O > 300

Potassium formate Potas,,ium propionatc

>300 >300

Cavity

None

Smarl Smallto moderal¢ Moderate Moderateto large None Small None Snlallto moderate Sr~allto mOdel'S/to Moderate Currentand c~lvit) hlrge~lssoon ;is pelIt!lis introduced but decrease rapidly Largeand consistent Largeand consistent

J. D. Birehall

9O The results in Table 1 suggest that: 1. Potassium salts have a more profound effect on both flame conductivity and combustion inhibition than sodium salts. 2. Substances decompo:sing at low temperature are more effective than thermally stable substances. This is well illustrated by the difference between anhydrous sodium carbonate and the decahydrate. In the case of potassium oxalate, a high flame current and a large cavity resulted z~ssoon as the pellet touched the flame: the currcat and cavity then declined rapidly as the pellet surface became coated with the relatively inactive potassium carbonate. Potassium formate and propionate pellets gave consistently high flame currents and large cavities because the melting of the pellets continuously exposed a fresh surface to the flame. Massive potassium bicarbonate has little effect on a diffusion flame. In at1 experiment to demonstrate ',he effect of small particles, potassium bicarbonate was finely ground and particles less than 5/z in diameter were clutriated from the powder by a stream of air. This stream of particle-laden air, if directed at the edge of a diffusion flame, caused cavity tbrmation and a flame current of 250 IrA. The above test, in which pellets of various substances are held in contact with a flame, thus indicates the ability of the substance to feed small particles of decomposition product into the flame. Inhibition might then result from reactions at the particle surface or from gas-phase reactions that would require the volatilization of the particles. The observed effect on the conductivity of the flame requires the volatilization of the solid, and the fact that the flame current and the size of cavity were related strongly indicates a homogeneous mechanism of inhibition. Extinction o f a Diffusion F l a m e il)y Particulate

Solids The extinction of free-burning fires essentially involves the projection of the inhibiting sub-

NICKEL CRUCIBL~

SEE INSET

AIR..-----.~ t~

Y-I

N SIEVE- -

MS

II ,.otis NIC :nu

~:~

O,V.RG!.,AS$ SECTIONS2" 1.0.

~1

i!i~

N

SIEVE-31'0

COOUNG~ ASBF.STO! WASHER~

I

-5CMS Figure 10. Modified Friedrich apparatus.

stance into a diffusion flame. The effect of the particle size and the composition of particulate solids on a diffusion flame was studied using a modification of the apparatus designed by Friedrich [6] and illustrated in Fig. 10. A weighed quantity o f powder in a nickel crucible was tipped into the mouth of a tube 5 cm in diameter and 93 cm long. A stream of air was passed down the tube, and the dispersion of the powder in this stream was assisted by sieves in tbe tube. The powder appeared at the base of the tube as a cloud. The diffusion flame (fuel at 1.2 liters min-~ through a I-era-diameter jet) was arranged to be in the path of the powder cloud. The stream of air passing down the tube was so adjusted as to neutralize the upward convection of the flame so that the diffusion flame was confined to a narrow, flat zone, The weight of powder introduced into the dispersion tube was increased progressively until a minimum weight giving reprodueible extinction was found.

Flalne Inhibition by Alkali Metal Salts

91

~ '~OOC 5

deerepitating solids (represented by sodium bicarbonate) requireJ is sensibly constant with particle size in the range studied, that ofdecrepitating solids (such as potassium oxalate hydra!e) passes through a minimum with decreasing particle size. This is illustrated in Table 2 and Fig. 11. The efficiency with which the alkali-metal content o f the various salts is utilized in inhibition varies greatly. This is clear from a comparison o f the amounts o f alkali metal required for extinction with the salts at a particle size of 70 ,u (Table 3),

C03 •

~

300¢

2

P.,

4•y o

o O

~2ooc ni

Q

o

/ IOCX:

'

o/ ej

4

50 IOO ISO 200 PARTICLE SIZE (micron)

250

Figure I I, Variation with particle size in the surlhce area of powder required to extinguish town's gas diffusion ltamc.

The substances investigated were sized by sieving and air elutriation. I f the results are expressed in terms o f the s u r f a c e a r e a o f powder required for extinction, it is found that, whereas the surface area o f non-

Observation o r particles o f potassium ferrocyanide trihydrate o f various sizes in the region o f the flame indicated the reason for the rise in the surface area required tbr extinction with very small particles. Potassium ferrocyanide decomposes in a flame with the formation o f glowing particles (iron oxides) which serve to indicate the particle track and the extent o f decomposition, Very large particles ( > 2 0 0 /0 were observed to pass through the flame with little reaction, whereas smaller particles (200-70 p) decomposed within the flame itself. Very small particles ( < 30 #) decomposed before reaching the edge o f the luminous flame, and the minute, particulate decomposition products were then carried away from the flame and failed to penetrate to the flame front,

Table 2. Variation with P~rtiele Size in tile SurfiiceArea o1"Powders Required to Extinguisha TowuGas DiffusiuuFlame I)~rlicle

Size. It 231 181 128 90 70 31 23 19 12 9

Surlhce Area of Original Solid Required for Exlinction, crn: .....................................................................................................................................

NulICO.~

K2C204'H~O

. . . . . . .,. 1102 ,.. 621 ~769 48 2367 41 2762 '" . . . . . . ,,, 90() 2310 1430 . . . . . .

K~Fe(CN)(,'3II:O 1260 447 554 125 46 626 563 . . . . . . . 1078

Nu ,Fc(CN)~NO' 21120 Nu=Fc(CN)~,' HiH:O . . 1925

.

.

.

. .., >300(I 495 35(I 272

IgS8

. . . .

851 596 ,,, . . . . . . . . . .

.

.

.

.

. .

92

J. D. Birchall

Table 3. Utilization of Alkali Metal in Various Salts Having a Particle Size of 70 p

Salt

Alkali Metal to Extinguish. mg

NaHCO~ K,CzO~.HzO K~Fe(CN)6' 3HzO Na,Fe(CN)5(NO)-2H~O Na~Fc(CN)," 10 HzO

71 I I 8 4

The results of the above experiments with a diffusion flame confirm that the high extinguishing efficiency of certain alkali-metal salts is due to the decomposition in the flame of relatively large particles, with the formation of submicron particles of a solid decomposition product. The results obtained by Freidrieh may be explained in these terms. For example, Friedrich found thai: 1.2 g of 44 It K,,CO3 was required to extinguish a CO diffusiml~ flame, corresponding to st powder surface area (calculated assuming spherical particles) of about 700 em". The weight o f 44-,u K 2CzO 4 • H zO required was 0. I g, giving 0.075 g of K2CO 3 on decomposition. If the new surface of decomposition product K2CO 3 is assumed to'have the same activity as preprepared KzCO 3, its area must be 700 cm-" and the specific surface must be of the order of 10,000 em-" g - i corresponding to a particle size of about 2 it.

Discussion The M e c h a n i s m o f Inhibition

Two mechanisms of inhibition may be postulated as resulting fl'om the presence of small particles in a flame: " I. A heterogeneous mechanism such as radical recombination on a particle surface, that is, R+M~RM.

RM+R'-~-RR'+M

[12]

2. A homogeneous mechanism involving at least partial volatilization of the small par-

titles, producing some gaseous inhibiting species such as K, K ÷, K O H [10]. Dolan and Dempster [8] note that in the ease of the alkali halides the inhibition of methaneair combustion cannot be explained in purely thermal terms, since the efficleney of the solids continues to be dependent on particle size at diameters below that at which thermal equilibrium between particle and flame would be reached. These authors suggest that at least partial vaporization occurs and that the gaseous species interfere in the methane-air reaction. Rosser et al. [9J similarly calculate that the volatilization of small ( < 5-/0 particles is pos-sible in a flame. Friedman and Levy It0] have calculated the equilibrium concentration of potassium species in a stoichiometric methaneair flame containing potassium and conclude that K O H and K would be the only important species, the ratio KOH/K being at least 2.6. These authors suggest further that the reaction a f chain-propagating species (H, O, OH) with elemental potassium would proceed only by three body processes, such as K + O H + M-->KOH + M

(I)

so that reaction with K O H would be more favorable, that is. KOH + H - + H 2 0 + K

AH = - 36 kcal (2)

KOH + OH---~-H,O + KO

A H = - 13 kcal (3)

The suggestion of K O H as an inhibiting species explains the observation of numerous workers that oxygenated salts are more effective flame inhibitors than nonoxygenated salts such as KCI [13]. Although gaseous KCI could possibly inhibit via reactions such as KCI + H->HCI + K

AH = --4.0 kcal (4)

this reaction would be slower than reactions 2 and 3. The' production of K O H would be slow (reaction I). Oxygenated salts probably pyrolyze stepwise in a flame, ultimately producing

Flame Inhibition by Alkali Metal Salts

93

molten KaO. This may yield gaseous KOH by reactions such as K20(I) + H20(g)-~ 2KOH(g)

AH=+21kcal (5)

Some evidence in support of inhibition by gaseous K O H has been found in the work reported here.

Formation of KOH in Flames Containing Potassium Salts Dunderdale and Darie [11] investigal:ed the behavior of sodium salts in flames and concluded that sodium atoms are produced by reactions such as NaCI + H + HCI + Na

(6)

and that the reaction Na + H~Om NaOH + H

(7)

is then responsible for the Ibrmation of NaOH. The N aOH then reacts with other anion-lbrming entities such as COn, SO,, and SO a. In the present investigation, a town's gas diffusion flame was led with finely particulate potassium halides. The smoke from the, flame was "'condensed" on a cold surface, and the solid analyzed with the following msuhs: Halide

Alkalinity of Collected Solid. % K,O

KCI KBr KI

Nolle detected 0.007 0.12

The concentration of "'K20" found in the product from the flame increases in the order of increasing flame-~nhihiting efficiency. ]For example, Fricdrich's i esultsshow that theminimum quantities of the tqree halides required to extinguish a natural gas diffusion flame are as follows: KCI. 28.6 raM; KBr. 19.6 raM; KI, 15.3 mM [6].

Effect of Anion-Forming Entities on the Efficiency of Salts Dunderdale and D urle [ I 1] suggested that anionforming entities react with the alkali hydroxides formed in l~ames preferentially in the order so~- > 0:

> S O l - > CO~-

In consequence, little or no hydroxide should result from the presence of. say, KaSO.~ in a flame, and the introduction of, say. chlorine into a flame should reduce the efficiency of KOH-forming additives by reaction with the K O H to form the less active halide. Significantly, no extinction could beobtained with K2SOa, using the apparatus shown in Fig. 10. Simple experiments showed that, when a crystal of KaC,O.~-H20 was introduced into a laminar diffosion flame of town's gas containing 2 % v/v chlorine, no flame cativy was produced, although the crystul dcerepitated and fed small particles into the flame. In more complex experiments, the effects o f chlorine and of the ammonium halides, which dissociate readily to give the hydrogen halide, on the efficiency of K2C20~ and K,Fe(CN) 6 were studied. The efficiency of anhydrous potassium oxalate and potassium ferrocyanide in extinguishing a 20 per cent town's gas-air flame was determined with and without the addition of chlorine to the fuel-air mixture. Powders of size 70 ,u were used. The results are reported in "fable4, Effectof Cldorine on Ihe Ef0eiencyof K2CzO~ and K~Fe(CN)~ in E~tingulsbinga 200/0Town'sGas-Air Flame Chlorine in 20% Fuel-Air Mixture, % viv

Mini,11umWeiglltof 70-11 Powder to Exlinguish Flame, n~g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

K:C..Oa 0 1 2 3 4

28 48 80 94 99

K~F'c(CN),, II

82 89 I00

94

J. D. Birchall

Table 4. A reasonable explanation o f the reduced efficiency of the salts when the chlorine was added is that K O H (or intermediates in the formation o f K O H ) was converted to relatively inactive KCI. In other experiment~, equimolar concentrations of the ammonium halides were mixed with anhydrous potassium ferrocyanide--all the solids being in size 70/~--and the weight of mixtures required to extinguish a town's gas-air flame was determined, The results are presented in Table 5. The deleterious effect of the ammonium halide (or rather the halogen acid) increases in the order CI > Br > I and thus in the order of increasing acid strength, The present work shows that two types of solid may be distinguished: thosewhieh, when injected as relatively large (i.e., 70-#) particles into a flame. show little or no change in particle size, and tho~e which, like the alkali oxalates, decompose to generate submicron particles of a solid product. These secondary particles are sufficiently small to volatilize and react in flames to form some gaseousinhibiting species. Theavailableevidence indicates that the inhibiting species is gaseous alkali hydroxide, as suggested by Friedman an0 Levy [ 10], At some limiting small particle size such that the complete volatilization of any alkali compound is assured, differences in efficiency between salts of the same alkali metal would be related only to the faeilily with which pyrolysis would give alkali hydroxide in the flame and Talde 5. Effectof 23 Mole °/o Additionof AmmoniumHalide to 70 p K4Fe(CN)~,on the Extinction of a 20% v/v Town's Gas-Allr Flame

..................................................... Weightof K.,Fe(CN)~,Required to Ammonium ExtinguishFlame ~nPresenceof Halide 23 Mole °/o Halide,. mg None NHaCI NH~Br NHal

d 34 14 9

thus tO the nature o f the anion. The order of efficiency of such ultimate particles would be, for example, Oxide > cyanate > carbonate > iodide > bromide > chloride > sulfate > phosphate Thus the efficiency o f an alkali salt in extinguishing flame will depend o n the following: 1. The change in surface area accompanying the reaction, solid (I)---> solid ( l I ) + gas. 2. The chemical nature of the secondary particles. 3. The particle size and ballistic parameters which determine a residence time in the flame sufficient to allow the decomposition of the original solid within the flame front. The practical significance of the findings reported here will be obvious from the following comment. The average specific surface of commercially available sodium bicarbonate fireextinguishing powders is 3000 cm z g - t so that a mass of the order of I g is required for extinction in the apparatus shown in Fig. 10. With the most effective decrepitating solids examined, extinction was possible with 0.1 g of material. This reveals the possibility of producing fireextinguishing powders up to 5-10 times more effective on a weight basis than the agents ( K H C O 3 and NaHCO:,) at present in use. Compounds other than those investigated, however, would be required to avoid toxicological problems.

The author wishes to thank the Directors of hnperial Chemical Industries LM., Moral Division. for permission to publish this paper.

References i. i-ltl~o. D., and GgI~GSrEN, M. J,. "The extlnction of flammable liquid fires by dry chemical extinguishing agents." Fire Res. Note 239. Fire Research Station. Ministry of Technology (1956).

Flame Inhibition I~y Alkali Metal Salts 2. DESSART,H, and MALAME,L.. Fire lntern., No. 7. p. 38 (1965). 3. P:IETERS,H. A. J.. Chem. Weekblad. 54, 429~37 (1958), 4. DUFRAISSE,C,. LE BRAS, and GERMAN, M.. Colnlll, Rend.. 21)7. 1221 (1938). 5. DUFRAISSE,C., and GERMAN.M.. Colnpl. •end.. 236, 164-167(1953), 6. FRItIDRICrl,M .. V.F.D.B, Z,. 9 (1960). Speci:tl Issue No, 2. *'The aetlon of dry extinguishing media." 7. LEE, T. O,. and ROIII~RTSON,A, F., "International symposium on the use of models in fire research.'" Nat. Acad. SeL PubL 786,p. 93 (1961). Also Fire Res. Abstr, Rev., No, I, p. 13 (1960). 8. DOLAN,J. E.. and DEMPSTER,P, B,, ,Z Appl. Chem., 5, 510-517(1955).

95 9. RossER, W. A., INAMLS. H., and Wlsl~, H., Combustion & Flame. 7, 107-119 (;963). I0. FKIEDMAN,R., and Lzw,, J. B., Combustion & Flame, 7. 195-201 (1963). I I. DUNDEaDALt~,J.. and DURIE,R. A., J. ]nst. Fuel, 37, 493 (1964), 12. DEWITTI~, M., VREBOSCH,J., and VAN TIGOELEN,A,, Combustion & Flame, 8, 257-266 (1964). 13. THOMAS,C. A., and HOCHWAL'F,C. A., [nd, Eng. Cheot,. 20, 675 (1928).

( R e c e i v e d April 1969; ret,ised S e p t e m b e r 1969)