W. G. SYKES, R. H. JOHNSON,and R. M. HAINERArthur D. Little, Inc.Cambridge, Mass.

Ammoniumnitrateexplosion hazardsA number of experiments have indicatedthat confined AN when heated to thepoint where a decomposition reactionbegins will detonate in a short time.

r OR MORE THAN FIFTY years am-

monium nitrate (AN) has been an im-portant industrial chemical in ex-plosives and agricultural fertilizers.Markets for ammonium nitrate havecontinued to expand and currentlythere is widespread acceptance of ANmixed with various fuels as a low-cost replacement for dynamite andblasting agents.

As an explosive ingredient or as anexplosive material, AN is exceedinglystable. It is classified as an oxidizingagent for transport and storage andin the pure form can be stored atambient conditions in large amountswithout fear of self-ignition or spon-taneous explosion. In many ways itis an ideal chemical material.

Ammonium nitrate decomposes withthe liberation of heat and gases andhas excess oxidizing capability overthat of the oxidation of ammonia; itcan therefore oxidize as much as 6%by weight of hydrocarbon or otherfuel. As a result, fires in AN cannotbe smothered. In the past, many firesinvolving moderate quantities of ANhave been fought without hazard. Inother circumstances, AN has been in-volved in incidents with extensive anddevastating damages and many of

these incidents were not expected onthe basis of prior experience. It isbelieved that the extensive use of oiland AN presents a hazard which maylead to the development of explosionfrom fire in frequent and unexpectedways. Certain features of the acceler-ated decomposition of masses offueled, liquid ammonium nitrate maycause detonation in circumstanceswhich, until recently, would not havebeen expected.

History of ammonium nitrateThe safety of ammonium nitrate,

in spite of its potential for rapid oxi-dation and explosion, has been thebasis of many of its uses. Early ap-plications of AN to military purposeswere based upon the fact that ANcould not be exploded with rifle fire.In many ways, the expansion of theuses of AN have been based on itsrelative safety in comparison to othercompounds of equal effect; in fact,hazard from explosion had been soremote that a major use is as a chem-ical fertilizer which for the most part,is handled without concern for hazard.

The history of accidents involvingAN (1) can be looked at as a seriesof surprising and devastating events

The S. S. Grandcamp just a few min-utes prior to the explosion on board.

in what is otherwise an exceedinglyuseful and steadily expanding market.The development of uses for AN andin part the expansion of its produc-tion rate has led to periods of in-creased and unexpected hazard. Thecareful study of the conditions andcauses for an unexpected consequencehas led to its removal as a conse-quence only to be followed later bya new and as yet unexpected con-sequence.

Boostering to detonation by anotherexplosive is one major recurring causefor hazard from AN which, oneslearned, in newer circumstances hashad to be releamed. Incidents of thistype include Morgan (1918), Kriewald(1921), Oppau (1921), Tessenderloo(1942), and Roseburg (1959). Of theseOppau deserves special mention sincebefore the accident many thousandsof explosive charges had been usedto break up the heavy cake of thedouble salt 2 nitrate-1 sulfate withoutincident. Also at Roseburg, the am-monium nitrate was mixed with astoichometric amount of fuel oil, afact that undoubtedly increased thescale of the damage.

Heating with confinementA number of experiments have in-

dicated that confined AN (as in aclosed pipe) when heated to the pointwhere a decomposition reaction be-gins will detonate in a short time.Fuel, such as oil or vapor, in contactwith the confined AN reduces ehe timeinterval for heating to explosion byproviding additional highly exothermicreactions that can be initiated fromrelatively low temperatures. The sen-sitivity of AN to initiation is, in gen-eral, considerably increased at ele-

66 January 1963 CHEMICAL ENGINEERING PROGRESS, (Vol. 59, No. I)

Texas city shortly after the explosion of the S. S. High Flyer. The S. S. Grand-camp was moored beyond the left edge of the picture.

vated temperatures so that a smallaccidental explosion in the presenceof a larger heated sample may pro-duce initiation that otherwise is un-expected. Incidents where fire wasnot observed prior to explosion includeKensington (1896), Gibbstown (1916).Nixon (1924), Merano (1936), Milan(1944), and Benson (1944).

It was learned that an air lancein a high pan could become con-taminated with oil or could becomeplugged and act as a bomb. This dis-covery has led to corrective measureswhich can eliminate this hazard.

Precautions for designing drain-offtubes for high pans and for otherprocessing equipment can be takenthat reduce the amount of AN presentand that do not allow the develop-ment of pressure. Further precautionswith regard to potential zones of con-finement in storage can also be takento reduce the hazard of decompositionunder confinement. Experimentationboth at the Bureau of Mines and Pic-catiny Arsenal have elaborated specificconditions and, along with other workon the importance of pressure, haveincreased considerably our understand-ing of this form of hazard.

FiresThe most perplexing aspect of am-

monium nitrate is its behavior in afire. Large amounts of AN have beeninvolved in a number of fires whichhave burned or been extinguishedwithout incident. These fires are com-mon enough so that some have noteven been reported in the literature.Accidents involving fire in AN havebeen divided into incidents of twokinds: first, where the origin of firewas unknown and spontaneous heat-

ing could be involved; second, wherethe origin of fire was from some ex-ternal source. In either event, the ad-dition of organic matter, as in thecase of FGAN (fertilizer grade am-monium nitrate coated with 0.5% to1.0% wax, etc.), seems to have con-tributed to the energy release in thedecomposition reaction and, therefore,to the hazard.

The practice at government ord-nance works, prior to 1947, of bag-ging and shipping FGAN at approxi-mately 90°C was apparently responsi-ble for a number of fires caused byspontaneous heating. Between 1946and 1949, thirteen freight cars loadedwith FGAN burned without explosion.During the same period, in other carscharred materials were found eventhough fire did not develop. The firesin both the S.S. Grandcamp (1947)and the S.S. Ocean Liberty (1947)have been attributed to spontaneousheating. In the absence of free acid,it has been demonstrated that AN inpaper bags is not subject to spon-taneous heating if the material is as-sembled at ambient temperature. In-cidents involving proven spontaneousheating have not been reported formore than a decade.

Explosions

There were a number of cases inprocessing plants where fires in ANhave transformed to explosion. Theseinclude Oakdale (1916), Emporium(1925), Gibbstown (1932), and Her-cules (1953). In each of these fires,organic material was present in someform.

There have been four ship ex-plosions from fire. Three ships carriedFGAN: S.S. Grandcamp, S.S. High

Fiver (1947), and S.S. Ocean Liberty;1947), while the S.S. Tirrenia (1953)carried nitrate of unspecified composi-tion. A fire in a ship carrying pureAN (CPAN) in casks was extinguishedwithout incident: S. S. Hallfried(1920).

There were several attempts to re-produce the conditions that led to ex-plosion in ships carrying FGAN, usingscaled down models. In no case was3. detonation obtained.

As a general summary, fires in pureAN have been less intense and moremanageable while fires in AN withintermixed organic matter have beenmore intense, extensive and havesometimes involved conversion to ex-plosion. In several cases whistlingnoises increasing in pitch were heard,indicating localized pressure condi-tions with the presence of acceleratingchemical reactions.

After the use of FGAN was discon-tinued, incidents involving AN wererare in spite of the fact that increas-ing amounts of AN were being pro-duced. However, burning to detona-tion has taken place in two incidents,Traskwood (1960) and Norton (1961)(2), each of which had certain condi-tions which require further study.Both could have involved a mixtureof AN and organic material (accident-ally in Traskwood) and in each casea fire burned for some thirty minutesbefore explosion took place.

Mechanisms of burning

There are two mechanisms of burn-ing for AN and its organic mixtureswhich involve quite different proc-esses (3), although both processescan occur in the same incident. Onemechanism is that of propellant burn-ing where, if the AN composition isinitiated from an external source,burning can take place on the sur-face of the mass only with the con-sequent free and unconfined ventingof the gases. Where this processdominates, apparently large amountsof AN can be consumed without anexplosion; e.g., Barksdale (1920),Cleveland (1922), Muscle Shoals(1925), Gibbstown (1940), St. Steph-ens (1947), Presque Isle (1947), andIndependence (1949). The evidenceindicates that in these cases wherefire alone occurred, the process of de-composition and oxidation of fuel isrestricted to a limited zone on thesurface of relatively cold material.

The second method of AN decom-position involves the heating, melting,and collecting of sizeable quantitiesof liquid which finally will fume-off.This process will be accelerated in

CHEMICAL ENGINEERING PROGRESS, (Vol. 59, No. I)January 1963 67

the presence of catalysts or organicmaterial.

Decomposition reactionsAN is capable of a number of de-

composition reactions which can begenerally classified as involving theoxidation of ammonia to nitrogen andwater and which provide oxidizingagents such as N2O, other oxides ofnitrogen, or free oxygen as by-prod-ucts. These processes of decomposi-tion are exothermal and for the mostpart can be described satisfactorilyby first order rate constants.. Thefeature of importance is that an ar-bitrary quantity of liquid AN, be-cause of its liquid phase exothermaldecomposition reactions which arethemselves an increasing function ofrising temperature, can have an ac-celerating decomposition in which theheating is equally distributed through-out the entire volume of the liquidphase instead of being in thin zonesover the surface as in the case ofpropellant burning. Without interrup-tion, this self-accelerating, run-awayreaction would inevitably lead to ex-plosion if a critical mass or criticalventing cross section were attained.All of the behavior of AN in partiallyvented enclosures verifies this view.

Temperature limiting mechanismA unique feature of AN is its dis-

sociative volatilization to ammoniaand nitric acid—a process that absorbslarge amounts of energy, transfersammonium nitrate away from the re-action zone, and, if adequate ventingis present, limits the rise of tempera-ture to a function of the applied pres-sure. This volatilization interrupts andlimits the self-accelerating and run-away reaction (4). The temperaturelimit at 1 atm. is approximately 292° Cfor pure AN decomposing to N2O andwater. Other forms of decompositionwhich involve oxidation of organic

O

matter or catalysis to other oxidativeproducts with greater release of ex-othermic energy can effectively raisethe energy dissipated within the re-active mass, but will not raise thetemperature of AN higher than avalue determined by its own dissocia-tive vapor pressure.

The dissociative vapor pressure hasbeen measured by Reik (5), by Feick(6), and by Brandner (7). These dataare summarized in Figure 1.

For a given pressure, the dissocia-tive vapor pressure curve gives thehighest attainable temperature in thecondensed phase. This temperatureapplies even though high temperatureflames or oxidative reactions proceedadjacent to the liquid surface. Al-

though these added sources of heatwill increase the rate of gas formation,the endothermy of evaporation limitsthe temperature of the liquid, justas for water, the maximum tempera-ture at 1 atm. pressure is 100°C. Thistemperature-limiting mechanism is themajor means for controlling the de-composition of AN and its mixturewith reducing agents. A mechanismwhich overcomes this temperaturelimiting feature can cause extremehazard.

Rate of decompositionThe rate of decomposition (3) has

been measured by a number of in-vestigators on pure AN and in com-bination with other materials. In gen-eral, acids, chlorides (allowed to goacid), chromâtes, and a few othermaterials markedly affect the rate ofdecomposition at low temperatures.For example, dichromate is an activecatalyst that raises the rate of decom-position in pure AN, presumably bychanging the decomposition productsand increasing the exothermy overthat obtained in uncatalyzed reactions.

It is clear that the rate of decom-position is a function of the tempera-ture. The unusual feature of AN isthat, at the temperature-limiting levelfor 1 atm. pressure, pure AN or anyof its catalyzed products do not pro-duce rates of decomposition that con-vert to shock-wave detonation proc-esses in 'laboratory-size" samples. Thisfull statement with its limitations ofpressure rise and sample size is neces-sary since any mechanism whichcould develop rapidly acceleratingrates of reaction and shock processescould convert a controlled deflagra-tion of AN to an uncontrolled detona-tion.

Effect of pressureThe higher the applied pressure,

the higher the self-limiting tempera-ture, and therefore the greater the re-action rate. It is clear that at somevalue of the applied pressure, theprocess of liquid fume-off in largevolume can produce shock processeswhich detonate the remainder of theliquid. Pressure can be generated fromseveral sources: 1. externally appliedpressure, 2. pressure produced bygaseous reaction products ventedthrough a limited aperture, 3. pres-sure produced from the momentumchange of gases evolved at the sur-face of the liquid, and 4. pressureproduced by inertia of liquid sur-rounding an expanding bubble with-in the liquid.

Measurements with partially ventedbombs and with indirect evidence

from other kinetic rates of reactionnear explosion suggest that fueled ANwill detonate in relatively small sam-ples with applied pressures of theorder of 300 lb./sq. in. (20 atm.).

Lacking experimental evidence inthe transition to detonation region, itis difficult to state whether one or acombination of these mechanisms ismost significant. Certainly, the ap-parent pressures required to reachthe point for deflagration to detona-tion based on vented bomb tests (300lb./sq. in. for FGAN) preclude thepossibility of detonation based on con-finement by the static head of mate-rial present.

To develop an understanding of thetransition process, simple mathemati-cal analyses of some of the gas flowprocesses that would occur in themechanisms, previously described,have been attempted. Based on theseanalyses it seems that resistance toflow through a small vent appropriateto the sample size would producesuch pressures that the initial pres-sures would be within the static headrange and a final rapid pressure risewould be possible with confinementprovided by the inertia of the ma-terial confining the liquid.

Another effect would be the pres-sure generated by the momentum re-quired to accelerate the reacting gasesaway from the liquid. Such a processwould be an important factor in thefinal pressure rise.

Surface shock processesPressure can raise the rate of burn-

ing for propellant type surface burn-ing as well as for the fume-off ofliquid. In the propellant case thegas-phase flame is driven closer tothe surface with an increase in heattransfer but with only moderate in-creases in burning rate. Eventually,surface shock processes may be de-veloped which could initiate thinheated layers of more sensitive mate-rial at the surface.

For liquid fume-off, the pressuremechanism is more direct. In Figure1, the limiting temperature for anyzone in the liquid phase is given; ifthe pressure rise is a step function,the increase in temperature of theliquid is directly related to its rate ofreaction and this rate in itself is anexponential function of temperature.

For pure AN at the self-limitingpoint the rate of temperature rise fora step increase in pressure is of theorder of l°C/sec., while at the ANboiling point for one atmosphere (thatis with high energy influx) the rateof rise may be as much as 5° to10°C/sec. Because heat flow into the

68 January 1963 CHEMICAL ENGINEERING PROGRESS, {Vol. 59, No. I)

500 450T(°C)

400

E«3

aj"3

"5.o.

1.4 1.61000/T(°K)

Figure 1. Fume-off of ammonium nitrate at limiting temper-ature as a function of pressure.

3 4Pressure, atm.

Figure 2. Depth of fuming-off liquid required to satisfy con-ditions of continuity and acceleration.

molten liquid is not necessary in theself-heating process that generates atemperature rise, a step function in-crease in pressure can lead to a veryrapid temperature rise of the fumingliquid and a very rapid increase inthe rate of decomposition for theliquid that remains. Thus, the pres-sure dependence of the rate of gasevolution is far greater in the case ofliquid fume-off than in surface burn-ing.

Critical depths of liquidAn order of magnitude estimate of

the depth of liquid required to givea rate of gas evolution adequate todevelop a high pressure can be madeby assuming the following: 1. theflame results solely from the burningof gaseous fume-off products; 2. theflow is one dimensional near the flame.

The following criteria are imposed:I. the continuity in the gas flow ismaintained; 2. the overpressure on theliquid (resulting from the reaction inthe gases) is equated to the rate atwhich momentum is added to the gasstream in the vicinity of the flame.(This implies that the pressure be-comes atmospheric reasonably close tothe flame area.)

The velocity of sound in the burn-ing gas is approximately 1.8 X 105

cm./sec. and the density of the burn-ing gas is 2.2 X 10~* P g./cu. cm.where P is the pressure in atmos-pheres. (These correspond to a gas ofy = 1,4, molecular weight 27, and

temperature 1500°K.) The gaseousevolution from self-heating of pureammonium nitrate is approximatelygiven byC>(P) = 2.5 X IQ"2 P3'2 g./sec.-cu. cm.

If M denotes the ratio of the veloc-ity of gas flow to sonic velocity, thenthe depth of liquid D required tomaintain continuity of the gas flow is

D = 1.6 X 103 M P-2-2

Equating the overpressure to therate of momentum increase (recallingthat 1 atm. is approximately 106

dynes/sq. cm.), one obtainsD = 2.5 X 102 (P - 1)/M P3-2

Equating these two expressions forD, one finds

M = 0.4 (P - 1)/Pwhich leads to

D = 640P-2-r(P-l)°-5cm.This expression is plotted in FigureA.

On the basis of these simplifyingassumptions it can be concluded thatfor CPAN a depth of liquid accumu-lation of approximately 175 cm. wouldbe adequate to support an unlimitedpressure rise; further, at the vaporpressure limit the critical depth wouldbe reduced from about 175 cm. toabout 6 cm. The presence of fuel oilprobably has a significant effect onthe rate of fume-off and introducescorrespondingly greater hazard infires.

Notice that M is a monotonie in-creasing function of P. In one incidentof a fire burning to detonation it was

reported that whistling noises of in-creasing pitch were heard before thedetonation; this would appear con-sistent with an increasing velocity ofgas flow.

The attainment of these criticaldepths of liquid clearly requires aninitial depth in excess of those for agiven pure liquid ammonium nitrateand for a given catalyzed liquid am-monium nitrate since some of theliquid would be consumed in attain-ing the temperature and associatedreaction rate in equilibrium with therising pressure. The initial depth mustbe such that all points on the criticaldepth versus pressure curve are at-tainable from self-heating of the liquidoriginally present.

Effect of foamingA further effect in the fume-off of

accumulated, self-heating liquid is theviolent splashing and foaming that oc-curs. In a fire, one can conceive thata cavity in a mass of unheated ma-terial might be partially filled withliquid AN below the self-limiting tem-perature, this liquid being collectedfrom the liquid film adjacent to theburning and decomposing surface. Theaverage temperature of this film wouldbe well below the surface tempera-ture which in turn can be no higherthan the limited temperature. Self-heating in this liquid becomes rapidonce some critical volume is reached.

In the final stages of liquid fume-off the very rapid gas evolution ex-

CHEMICAL ENGINEERING PROSRESS, (Vol. 59, No. I) January 1963 69

poses thin films or sprays of liquidAN to surface fiâmes with the at-tendant accelerated volatilization, in-creasing gas volume and energy re-lease. These accelerating processes canproduce surging in the burning proc-ess and contribute to the dynamiceffects of pressure. An increase inpressure will, in turn, raise the limit-ing temperature, accelerate furtherthe rate of decomposition of theliquid, increase the volume of gas tobe vented and enhance the probabil-ity of converting burning to detona-tion.

Confinement mechanismsThe detailed descriptions of the

fume-off mechanisms for liquid am-monium nitrate provide a means forunderstanding the most puzzling as-pect of the burning-to-detonationtransfer in AN fires, i.e., how a proc-ess that apparently requires pressuresgreatly in excess of the hydrostatichead of solid or liquid material pres-ent can be operative in the absence ofa mechanism for maintaining suchpressures under static conditions. Atleast two separate mechanisms cancause a rapid increase in reaction ratewith confinement. The combination ofthese mechanisms can give a basis fordeveloping a pressure-accelerated re-action in a sufficient amount of liquidAN which has sufficient rapidity sothat inertial confinement alone mavbe adequate for transition to detona-tion once a critical condition is passed.It is evident that additional confine-ment makes these processes morelikely to occur.

New experiments requiredThe possibility in a fire of a runa-

way reaction within large quantitiesof decomposing liquid AN in thepresence of fuels may well be an im-portant consideration in the wide-spread use of oil and AN. An ade-quate understanding of this phenom-enon is required to protect propertyand human life, and new experimenta-tion of a type that has heretofore notbeen attempted is needed.

This experimentation is complicatedand demanding, since the specificationof the composition of the amount ofmaterial burned and of its arrange-ment alone are not sufficient condi-tions for the accumulation of sizablequantities of liquid which come tofume-off in pressure-generated surges.The unique conditions which need tobe controlled are the accumulation ofliquid nitrate, the presence of fuel orcatalysts, fire within the pile andthe capacity of the venting. No knownexperiments have been made with

significant quantities of liquid nitratewithin a fire. In the past, the possi-bility of liquid decomposition in theconfinement of air-lances, floor drainsor other piping, has been pointed outas a source of hazard, but the possibili-ties of these mechanisms leading todetonation in a pile alone has not beeninvestigated directly. The storage con-figuration of bulk or bagged AN isimportant, but it cannot be stated atthis time how to provide storage thatwould eliminate the hazard of an ex-tensive fire and the thermal decom-position of accumulated liquid.

As is shown by Traskwood (1960),the possibility of an incident that isinitiated by the accidental associationof fuel and ammonium nitrate can-not be dismissed even with AN thatmay be produced for agricultural pur-poses. The further understanding ofthe role of the liquid in the burning-to-detonation transfer is essential toprovide the key for preventing furtherdisasters in ammonium nitrate, a verycommon and useful material. #

LITERATURE CITED1. Arthur D. Little, Inc., Report on Hazards

in the Handling, Storing, and Transport-ing of Ammonium Nitrate, Contract Teg38325 (April 1953).

2. Van Dolah and Malesky, "Fire and Ex-plosion in a Blasting Agent Mix Build-ing," U.S. Bureau of Mines, RI 6015(1962).

3. Arthur D. Little. Inc., Report on anInvestigation of the Properties and Haz-ards of Ammonium Nitrate. Commandant,U. S. Coast Guard Headquarters (Novem-ber 1952).

4. Feick and Hainer, On the Thermal De-composition of Ammonium Nitrate, J.Am. Chem. Sac., 76, p. 5860 (19M).

5. Reik, Monatsh, es, p. 1055 (1902).

Sykes Johnston Hainer

Wendell G. Sykes received his B. S. inChemical Engineering Practice atM. 1. T. He has been associated withArthur D. Little, Inc., since 1950. Hisinterests have been in the general fieldof instrumentation with applications inthe field of explosives, electronics, in-dustrial hazards, meteorology andphysics.Reed H. Johnston received his B. Sc.in Physics and Mathematics from Mc-Gill University and his Ph. D. in Physi-cal Chemistry from Stanford University.He is a physicist on the staff of ArthurD. Little, Inc., where he is engaged inthe various applications of physics toindustrial problems including industrialhazard evaluation.Raymond M. Hainer received his B. A.from the College of Wooster and hisPh. D. in physical chemistry fromBrown University. At Arthur D. Littlesince 1945, he has worked in areasof molecular spectroscopy, industrialproblem solving, and new productdevelopment.

6. Feick, J. Am. Chem, Soc. 76, p. 5858(1954).

7. Brandner, et al., J. Chem. Eng. Data,J, No. 2, p. 227 (1962).

Questions and answersTranscript of the comments whichfollowed the presentation of thisarticle at the A.I.Ch.E. Denvermeeting, August, 1962.

S. B. Johnson, Monsanto Chemical Co.—I would like to make three commentsregarding incidents mentioned in yourpaper. First, at Roseburg, Oregon, aspointed out in the ICC hearing, theexplosion involved dynamite and nitro-carbo-nitrate, not ammonium nitrate.

Second, I investigated the Trask-wood accident within a few hours afterthe explosion. I would like to describewhat happened as we know it. Thewreck included a box car of baggedammonium nitrate, a hopper car ofbulk ammonium nitrate, a tank car of99% nitric acid, 3 tank cars of ammoni-um nitrate in urea solution, about 3 or4 tank cars of petroleum products (re-fined crude oil and fuel oil), 3 cars ofgasoline, 3 cars of drummed lubricat-ing oil, and 2 cars of roll paper. Thereasons for the detonation have beenextensively investigated since it oc-curred, and tests by Spencer, Phillips,Monsanto, and others have determinedthat burning petroleum products suchas refined crude, when mixed withconcentrated nitric acid, will immedi-ately detonate. It has also been de-termined that a stoichiometric mixtureof urea and ammonium nitrate is veryshock sensitive and could have con-tributed to the initial detonation.

There have been a number of in-vestigations and considerable studyinto what happened at Traskwood. Ithink that even though a year and ahalf has passed since Traskwood,it is still premature to say that am-monium nitrate exploded from the fire,or that it exploded because it wasmixed with oil in the fire. I mentionedthat the Bureau of Mines has under-taken a study of ammonium nitrateproperties for the ManufacturingChemists Association. One of themajor reasons for the Bureau of Mines'study is to determine the cause of theTraskwood explosion.

We are sincerely interested in find-ing out what happened at Traskwood,so that it will not happen again. Whenthis study is completed, I think wecan tell you more about it. In sometests that have been run by Spencer,Monsanto, and others—it has been de-termined conclusively that burningpetroleum products such as the partial-ly refined crude oil. when mixed with

70 January 1 9 6 3 CHEMICAL ENSINEERINS PR06RE55, (Vol. 57, No. I)

concentrated nitric acid will immedi-ately detonate. There is no questionabout this. This has been repeatedtime and time again. It has also beendetermined that certain urea and am-monium nitrate mixtures, are verysensitive—that they are very shocksensitive, and that this could havecaused the initial detonation at Trask-wood, Now we are not trying to saythat ammonium nitrate did not deto-nate at Traskwood. We only want tofind out what did happen so that itcan't happen again.

Let's assume that there was an ex-ternal source of energy, either fromthe concentrated nitric acid and pe-troleum products or from the urea-nitrate, and that this in turn propa-gated to the ammonium nitrate. Wedo believe that the boxcar of ammoni-um nitrate was detonated. We wereunable to find any parts of that car.We were also unable to find any partsof one of the fuel oil cars, and we wereunable to find any parts of one of theammonium nitrate-urea cars.

All of the cars involved were a totalloss, but of these three cars, there wereno identifiable parts in the area. Ithink that until we find out from theBureau of Mines study, and there mayeven then be some question, it is pre-mature to say that ammonium nitratewas the initial cause of this explosion,or that the fuel and ammonium ni-trate that might have combined inthe fire was the cause of it.

We do know that the ammoniumnitrate in the hopper car did not ex-plode. We found two major pieces ofthis hopper car—it looked as if it hadbeen hit by the force of the explosion.One part—was on one side of thetracks, 200-300 ft. away; the other partwas on the other side of the tracks,200-300 ft. away. Prilled ammoniumnitrate was scattered over the entirearea. The crater was probably 20-ft.deep and 60-80-ft. across. We foundprilled ammonium nitrate in thecrater. Prills were every place, indicat-ing that they fell back into the areaafter the explosion. There was fuel oiland there was concentrated nitric acidscattered over the area. The nitricacid car ruptured from internal pres-sure as did many of the other cars.

I do think that we must take theseother factors into consideration andshould not prematurely judge Trask-wood. This is true in many cases whenthere is an explosion. We've got toget all of the facts before we canjudge it.

Third, we had a man at Nortonshortly after the explosion. It was re-ported to our man at the site thatthere was a case of dynamite in the

building. It was not reported, in theBureau of Mines report on the Nortonincident, that dynamite existed. Thisis very hard to substantiate, and wewere unable to get a documented state-ment that dynamite was present.

I think that all of these facts mustbe recognized before we can say thatammonium nitrate by itself, from firealone, fueled or unfueled will explode.

W. G. Sykes, A. D. Little, Inc.-I willanswer your comments in order. Firstyou are quite right that in the Rose-burg incident the ammonium nitratewas mixed with fuel oil (presumably6% fuel oil). Based on my own experi-ence, I am sure that pure ammoniumnitrate would also have been detonatedby the dynamite under the conditionsat Roseburg, although the total yieldof the explosion would have been lessfor unfueled nitrate.

Second, I would like to commentthat it has been difficult for those whowere not directly involved to obtaininformation about Traskwood. Wehave had word-of-mouth information,but it seems unfortunate that there hasbeen no report issued about this po-tentially disturbing accident.

Perhaps Traskwood does not indi-cate the existence of a new effect, butuntil the questions raised by this ac-cident have been answered—and cer-tainly they have not yet been answered—those who are shipping ammoniumnitrate should consider the possibilityof similar accidents.

Third, in regard to your commentabout dynamite at Norton. This isnot the only case where such a ques-tion has been raised. Certainly it wouldbe in the interest of the owner to claimthat dynamite was not present. Sowhom do you believe—the evidence isgone.

Ralph Miller, Spencer Chemical Co.—While investigating the cause ofTraskwood, we discovered the reac-tion between nitric acid and burningpetroleum. Pouring nitric acid intoburning petroleum gives an instan-taneous detonation with velocitiesnearly equivalent to nitroglycerine inthe 20,000 ft./sec. range. It is a verysubstantial booster.

W. H. Doyle, Factory InsuranceAssoc.—Has any consideration beengiven to the solubility of the decompo-sition products of ammonium nitratein molten nitrate as a possible meansof reducing the heat carried away bythe endothermic reaction?

Sykes—We wrote our paper becausewe do not feel that enough attentionhas been given in the past to phenom-

ena that may occur in the liquid. I donot know the answer to your question.

P. Dyer, Phillips Petroleum—One fur-ther comment on the Norton, Virginia,incident: I believe that in your discus-sion of it you mentioned that this wasan incident where there was probablyno confinement. On the contrary, ifyou look at the type of mixing equip-ment which probably was there—al-though this may be information thatis not completely reported—you willfind there is a very distinct possibilitythat there might have been severeconfinement of a fine oiled materialsomewhere in the mixing equipment.Therefore, it is premature to conclude,from the incident, that even oiled am-monium nitrate will detonate in a firewithout confinement.

We can overlook the possibility ofhollow shafting for instance, in themixing equipment, which could haveled to the incident—again all this pointsup that it is certainly premature to saythat we have conclusive evidence-that even the oiled material will goto detonation in fire alone. These ques-tions have been raised and, as indi-cated, the Bureau of Mines is search-ing for answers to them. Whether it'sgoing to find a conclusive answer,certainly, we can't say at this time, butthe project is thorough and is beinghandled on a completely objectivebasis.

It is very significant to record that12 ammonium nitrate producing com-panies are underwriting this independ-ent objective research project to try tofind clear-cut answers.

John Lawrence, Armour AgriculturalChemical Co.—I would like to askyour opinion about the relative safetyof a properly vented tank containinga hot 96Î? to 97% solution of ammoni-um nitrate and water. Would such atank be almost foolproof if there wereno fuel present and no pressure build-up?

Sykes—There are several factors toconsider in areas where data are lack-ing. First would be the critical vol-ume-temperature relationship to per-mit self-heating for these solutions.Presumably the hazard would be lessthan for pure ammonium nitrate, butI doubt that reaction rate vs. tempera-ture data is known. Second, the liquiddepths for a runaway reaction givenin our paper are only approximationsat best, and it would be impossible tostate without further experimentationwhat depths of liquid would bedangerous if a self-heating processstarted and went to a fume-off in alarge tank. #

CHEMICAL ENGINEERING PROGRESS, (Vol. 59, No. I) January 1963 71

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