Abstract of Special Research Report (SRR-No.12)

National Institute of Occupational Safety and Health, Japan

Study on Prevention of Explosions and Fires caused by New Materials



: With the rapid progress of science and technology such as material science and processing technology, the research and developments on new materials have been increasingly active in recent years. In connection with this, the commodities using these functional new materials have widely been spread even in our livelihood.
    Though the research and developments for those new materials tend to further proceed, explosion or fire accidents, which have sometimes led to social issues, occurred during the manufacturing or processing of new materials.
    In this specific research report, the following research subjects were conducted to clarify the dangerous factors which may cause the explosions or fires due to new materials by considering the recent accidents.
  (1) Explosion characteristics of the principal specific gases used in the manufacturing processes of semiconductors or fine-ceramic.
  (2) Reaction hazard of rare earth metals with an organic solvent containing halogen.
  (3) Combustion characteristics of substituted solvents for Flon.
  (4) Ignition mechanism of new metal powders by shock wave.
  (5) Explosion hazard by metallic silicon and high magnetic powders.
  (6) Dust explosion hazard of fine-ceramic powders.
  (7) Evaluation of explosion hazard of organic new materials in manufacturing process and storage.
  (8) Case histories of explosion and fire caused by new materials.

Explosion Characteristics of Arsine,Silane and Phoshine in Air

Toshihiro HAYASHI

: Highly explosible or toxic gases, such as semiconductor gases, introduced into high technology industries, have inevitably brought new types of accident into working sites. Yet no sufficient data are available on hazardous properties of those gases. This paper describes experimental explosion pressures and maximum pressure rise rates of arsine, silane and phosphine mixed with air. Experiments were also carried out on hydrogen and methane as reference gases.
    Explosion tests were carried out in a cylindrical chamber of 2170 cm3 capacity. Premixture of arsine, methane or hydrogen with air was prepared by partial pressure method, and the mixture of atmospheric initial pressure was ignited at the center of the chamber by a nichrome wire heater. For silane and phosphine, no method was successful in preparing premixture with air, because of their ease of auto-ignition in air; tests were, then, tried by blowing up pressurized air from the bottom into the chamber, in which a calculated amount of test gas had been kept under decreased pressure. By controlling the air pressure in the reservoir and a duration time of the solenoid valve activation for air blow, ignition could occur just after supplying a required amount of air into the chamber, which corresponded to atmospheric initial pressure of the mixture.
    Arsine gave most severe explosion at 20 vol.% content in air, which was 1.7 times higher than stoichiometric ratio assuming ideal oxidation into As2O3. The maximum pressure (0.78 MPa) was higher than that of methane-air mixture, and the highest pressure rise rate was almost comparable to that of the optimum hydrogen-air mixture. These values suggested that, apart from its wide range of explosibility in air, arsine should be taken as one of the most dangerous flammable gases.
    For silane and phosphine, maximum explosion pressures determined in this paper, less than 0.6 MPa and less than 0.5 MPa, respectively, were lower than expected. Maximum pressure rise rate in each test was constantly found in earlier stage of an explosion, followed by relatively slow rise of pressure, and the rise rates of both gases were almost constant for wide ranges of the content in air. The highest values of rise rate recorded for both gases were much lower than even for methane. The result was attributed undoubtedly to the test method, as suggested by pressure-time records; a certain amount of test gas always ignited and burned just after blowing up air into the chamber, and then followed relatively slow combustion of residual gas. It was shown that the method of mixing gas with air should be improved to obtain reasonable explosion characteristics of easily auto-ignitable gases in air.

Explosive Reaction Rare Earth Elements with a Halogen-Containing Solvent

Takashi KOTOYORI and Takayuki ANDO

: A group of elements which are composed of 17 elements and are all belonging to Group 3A, i.e. Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, are designated as the rare earth elements.
    Nowadays rare earth elements have been extensively used as electromagnetic materials or as parts of atomic furnaces.
    Recently an explosion accident occurred, in a factory for manufacturing permanent magnets, when a sort of alloy having a composition of 20% Fe-80% Nd was being pulverized using a vibrating rod mill. The outside of the mill was water-jacketed and the air inside the mill was displaced by nitrogen gas, and further flon-113 (CCl2F-CClF2), an incombustible solvent, was used as pulverizing medium. These measures were all carried out to make sure of the safe operation, especially to prevent the occurrence of any oxidative explosions in the mill. The accident, however, arose. It appears that this explosion accident was not caused due to an explosion of any combustible-air mixture, but caused due to a process that a metal, probably Nd, reacted explosively with the halogen-containing solvent. It was presumed by the experiments performed after the accident that the latter process might be probable. Incidentally, it is well-known that explosion accidents may occur on the occasions of defatting, pulverizing or grinding of trivalent metals such as Al or alloys containing such metals in halogen-containing solvents like trichloroethylene or carbon tetrachloride. However, although there are some references in which it is mentioned that rare earth elements should be kept out of contact with chlorine-containing solvents, very few research reports are currently available on the actual state of the dangerous reaction.
    In this study, thermal analysis (DSC measurement using sealed cell) was mainly carried out on each of 6 rare earth elements, i.e. La, Nd, Sm, Gd, Ho and Er, mixed with carbon tetrachloride, which was selected as a tentative halogen-containing solvent, to clarify the reaction mechanism, and to utilize the experimental informations thus obtained for the prevention of accidents of this kind.
    Main facts clarified are as follows:
    An exothermic reaction between a rare earth element and carbon tetrachloride begins at temperatures higher than 200°C. This phenomenon may refer to the occurrence of chlorination reaction of the element. On the occasion of presence of the halide of the element concerned in the reaction system, the reaction is much enhanced, and the temperature at which the reaction begins falls to a much lower level.
    Thus, it is presumed that, when a rare earth element is defatted, pulverized or ground in the presence of a halogen-containing solvent, a halogenation reaction may arise between the element and the solvent, then the reaction gradually gets to proceed explosively in the presence of the halide of element which is the reaction product, because the halide acts as a sort of auto-catalyst on the decomposition reaction of the halogen-containing solvent.

Flammability Evaluation of Halogenatited Hydrocarbons such as Flon Substitutes

Hidenori MATSUI

: Many kinds of halogenated hydrocarbons have been used in industries. Some of Hydrochloroflnorocarbons (HCFCs) or Hydrofluorocarbons (HFCs) such as HCFC-142b and HFC-134a have been newly developed as flon substitutes whose production is predicted to in crease in future. On the other hand, many Hydrochlorocarbons (HCCs) such as trichloroethylene have been used in large quantities. The use of these substances may be inhibited in industry from the stand point of environmental pollution protection.
    Most of flon substitutes arc not easy to burn, but still more flammable than CFCs, thus it is necessary to understand combustion properties of HCFCs and HFCs to prevent explosion hazard in handling them. Combustion properties of HCCs are also important to develop the techniques for hazardous waste incinerators. Though the flammability limits and flash points of halogenated hydrocarbons have been obtained by other researchers, it is insufficient to evaluate quantitatively their flammability.
    This paper describes experimental results on oxygen indices of liquid and gaseous halogenated hydrocarbons obtained by using a diffusion combustion tester which was modified from the testing method for flammability of gases established by Ministry of Transport of Japan.
    The oxygen index directly indicated the rank of flammability of liquid or gaseous substances in a similar manner as solid. The oxygen index was strongly effected by total weight of halogen atoms in the molecule. The larger the weight of halogen atoms in the molecule, the larger oxygen index was obtained. Increase of the number of hydrogen atom in the molecule showed a tendency to decrease the oxygen index. The substances which did not cause the diffusion combustion in oxygen gas atmosphere showed no flame propagation in air even if they were premixed with air.
    It was proposed that those halogenated hydrocarbons could be categorized into three groups, namely easy combustible, hard combustible and non-combustible, corresponding to the oxygen index smaller than 30, between 30 and 60, and above 60 respectively.

Ignition Hazard Evaluation of Metal Powders

Hidenori MATSUI

: Newly developed solid materials have been used in advanced technology industries. For examples, rare earth metal such as ncodyniium-iron alloy is used for rare earth magnet, and amorphous silicon is used for solar cell. Great efforts have been put on developing new materials, but poor information about the safe handling has been presented in the production works.
    This paper describes experimental results on ignition properties of various metal powders including new metals by using a shock tube. Ignition delay time and auto-ignition temperature were determined behind the reflected shock wave. Also, activation energy of over all ignition process of the metal powders was derived from the linear relation of the ignition delay time with the reciprocal temperature. The ignition temperature was determined from observed shock wave velocity by using the shock wave equation. Ignition properties obtained from the shock tube test were compared with the data obtained by other testing methods such as ignitability test by small flame and Hartmann test on the same samples.
    The following results were obtained from the experiments.
  (1) Ignition delay time was not influenced by particle size and amount, though the longer the distance of the sample location from the end plate of the shock tube, the shorter ignition delay time was observed.
  (2) Amorphous silicon, titanium and iron-neodymium alloy powders showed quite low ignition temperatures compared with the other metal powders, and especially the lowest ignition temperature of amorphous silicon was lower than that of metal silicon by about 400 K. This result points out serious ignition hazard of these materials.
  (3) Zinc and iron powders were neither ignited by small flame nor by Hartmaim test, but they were ignited by shock tube test at around 1,100 K. Thus, the shock tube result indicated latent ignition hazard of zinc and iron powders.
  (4) Ignition delay time and activation energy derived from the shock tube test did not indicate a direct correlation with ignitability or explosibility having been obtained from other tests.

Dust Explosion Hazards of New Magnetic Materials


: A dust explosion test facility has been constructed to give comparable data for rates of pressure rise with a standard 1 m3 chamber described in ISO 6184/1. The facility uses a 30 L spherical chamber, circumferentially flanged with a pneumatically raised upper hemisphere to allow easy cleaning between tests. It ran be used to measure lean and rich limits of explosibility, explosion pressure, rate of pressure rise and limiting oxygen concentration for an explosion. The test variables influencing the explosion parameters were examined. The time delay between the closing of the air injection valve and the activation of the chemical igniter is essential in determining the turbulence intensity of the mixture at ignition, and was fixed to 130 ms in the present tests. By use of the facility, the explosion characteristics of new magnetic material dust/air mixtures have been determined. The new magnetic materials tested were neodymium-iron-boron alloy (Nd2Fe14B), samarium-cobalt alloies (SmCo5, Sm2Co17), samarium-iron-nitrogen alloy (Sm2Fe14N3) and iron carbide fine powder. The test results are summarized in Table 1, showing relatively weak explosion severity with higher explosion sensitivity. The limiting oxygen concentrations were measured by diluting the dust/air mixtures with nitrogen. Some fine dust clouds were ignited spontaneously while dispersing with high pressure air, indicating that their limiting oxygen concentrations for an explosion were -- 0%.
    Explosion reactions of metal alloys including rare earth metals were discussed, and in conclusion the test facility with a 30 L spherical chamber has been found to be very useful for evaluating dust explosion hazards of new materials.

Dust Explosibility of Fine Ceramic Powders


: The explosion characteristics of fine ceramic dust/air mixtures have been investigated experimentally. All tests were conducted at initial pressures of nominally 1.0 bar in a 30 L spherical explosion vessel. Fine ceramic powders of non-oxide compounds, which falls in four groups of carbides, nitrides, borides and silicides, were used. Average particle sizes of the dusts mostly lay between 1 and 10μm. The explosion parameters measured for each test were the lean limit of explosibility, the maximum explosion pressure and the maximum rate of pressure rise. The lean limit of explosibility was defined as the minimum concentration of dust in a cloud required to sustain the flame propagation, and obtained from the longest value of the time to a peak pressure, reflecting the slowest flame propagation from the ignition source to the vessel wall.
    The results of these tests are presented in Table 1, Among 29 kinds of dusts, 22 kinds including heavy metal compounds were ascertained to have dust explosion hazards when dispersed in the presence of an ignition source. Carbides of titanium, vanadium, zirconium and niobdenum were found to show violent explosions in the dust clouds with high values of the maximum pressure rise rate. Tungsten compounds of wilicide and boridc could not be ignited, but the tungsten carbide with mean particle size of 0.7μm exploded in the test with a 10 kJ pyrotechnical ignitor. The influence of effective ignition energy on the maximum rate of pressure rise and the lean limit of explosibility was studied, and discussion was made on the relationship between the enthalpies of complete combustion reactions and the explosion parameters obtained.

Dust Explosion Hazards of Metallic Silicon


: Silicon is an important element in view of engineering development of modern high technology, as it is extensively used hi industries of semi-conductor, organic silicon chemistry or fine ceramics. Fine metallic silicon powders, however, give rise to dust explosions.
    In this report, explosion characteristics of silicon powders with different surface states and particle sizes have been investigated experimentally, using a large-scale vertical test tube, a Hartmann-type test apparatus, a 419-L egg-shape test vessel and a 30-L spherical test vessel.
    Lower limits of the explosibility were measued hi the vertical tube for silicon dusts of A, B and C. The dust C, crashed mechanically in air, showed no explosion in the vertical tube with ignition sources of electric spark and chemical ingitor, but demonstarated violent explosions in the Hartmann apparatus. The dust A and B, prepared in argon gas atmosphere by mechanical and chemical methods, respectively formed propagating flames in the vertical tube. The results suggest that ignitability will be inhibited by the silicon oxide layer on the surface of the particles, but explosion severity will not be affected by the surface states. Values of the lower limits obtained in the 30-L vessel were smaller than those measured in the vertical tube, depending on the differences in turbulence. More turbulent mixtures would give smaller value of lower limits of explosibilities. A relation between the lower limits versus the average particle diameter is given in Fig.3. The silicon dust with mean particle size of 65μm did not explode.
    The explosion parameters defining the comparative violence of the explosion for a given system depend on dust concentration of the mixture. Fig.7 presents the variation of the explosion pressure and the maximum rate of pressure rise, i.e., K st, normalized to vessel volume on the Cubic Law, with dust concentrations. The maximum explosion pressures decrease gradually to nil as shown in Fig.3 with increasing particle sizes. The K st values were largely influenced by the mean size of the particles (Fig.9). Thus, fineness of the silicon powders increases explosion propagating rate in accordance with expectations. The maximum value of the explosion parameters were observed to attain at the higher dust concentrations with increasing particle sizes. It is general observation that the maximum explosion parameters are obtained under conditions of excess dust charge.
    Upper limit of explosibility of the dust C was not observed even at the high dust concentration of 6 kg/m3 in air, but the upper limits were apparently present for the mixtures diluted with nitrogen, that is, oxygen contents of 8 and 6 % by volume. Difmitioii of the lower and upper limits of the explosibility was given by two long delayed values of time to peak pressure form striking the ignition source, or duration of combustion. The times to peak pressures against dust concentration indicated J-letter variation and fluctuated considerably at higher concentrations in those reduced oxygen atmospheres. The reason for this scatter of data would be attributed to turbulent mixing of the mixtures. Oxygen is consumed in the presence of excess dust for full combustion under turbulence, which means that the oxygen content per unit volume of the mixture would be the factor determining the upper limits. Limiting oxygen concentrations for five silicon powders in O2 - N2 gas atmospheres were determined on the criterion of full flame propagation to the vessel wall. The limiting oxygen concentrations gradually increase from the minimum of approximately 4 vol.% when particle size increases. Thus, the most experimental data seem to show that the combustion process is heterogeneous, in which the surface reaction depends on surface area of particles.
    Poly-dimethyl-silylene powder was also tested in the 30-L spherical vessel, and the data were plotted in Fig.12. It can be seen that both maximum pressure and K st are extremely high, suggesting explosion hazard as severe as gaseous explosion. Infrared spectra of the solid residues after the explosions implied combustion of methyl radicals of the polysilane.

Evaluation of Dust Explosibility by a Testing Method

Toei MATSUDA and Hidenori MATSUI

: Testing has been performed by a method for determining dust explosibility, which has been recently adopted by the Association of Powder Process Industry and Engineering in Japan. The test is conducted by a modified Hartmann tube apparatus and provides a measure of minimum explosible concentration, to which explosion violence is suggested to be related. The test results is shown in Table 1 and 2 for fine ceramic powders and carbon related dusts, respectively. The method allows a classification of the explosion violence as severe, moderate and weak according to the minimum explosive concentrations of lower than 100, 100 to 200, and larger than 200 g/m3, respectively. When a sample dust does not establish any self-sustaining flame in the tube at a concentration lower than 500 g/m3, the dust is regarded as non-explosible by the APPIE Method. However, the data acquired in the present tests show that lean limits of flammability for sonic dusts are more higher than 500 g/m3. Therefore, it should be tested up to more higher dust concentrations, and the other testing method using a large vessel with a powerful ignition should be applied, when a combustible dust is judged as non-explosible in the APPIE method. The test results are also compared with the explosion data obtained in a 30 L spherical test vessel with a chemical ignitor.

A BAM Heat-Accumulation Storage Test on 1,2-naphthoquinone-2-diazido-5-sulfonyl chloride


: To determine the temperature value representing the thermal stability of chemical substances such as exothermic onset temperature, thermal analysis using a relatively small quantity of sample is usually first employed. However, since the degree of thermal isolation increases with increasing the quantity of substance, the temperature value of this kind has a tendency to shift gradually to lower temperature side, as a larger quantity of substance is processed. Hence, it is frequently experienced that such a temperature value of a substance processed in the industrial scale is several tens lower than that found when a very small quantity is tested, as is the case in thermal analysis. Therefore, it becomes necessary to perform a thermal stability (or isothermal storage) test using a sample of considerable quantity, to establish the upper limiting temperature which must not be exceeded in temperature control for the chemical substance.
    At present, the United States SADT test using a sample placed in the largest practical container and the BAM (BundesanstaH für Materialprüfung, Berlin) heat-accumulation storage test using a sample placed in a specially designed Dewar vessel are known as the standard thermal stability tests for chemical substances. These tests determine the so-called SADT (Self-Accelerating Decomposition Temperature) under the respectively specified conditions. The SADT is defined in the United States SADT test as the minimum constant temperature of the air environment for a thermally unstable substance at which an auto-accelerativc decomposition occurs within seven days when the substance is packaged in its largest commercial container and placed in the testing facilities. While it appears especially in Europe that researchers of chemical safety mean with the temperature value measured by the BAM heat-accumulation storage test the lowest temperature at which any subtle or very slow self-heating behaviour of a sample is observed, rather than the lowest temperature at which the thermal explosion of the sample is ultimately brought about in the Dewar vessel.
    These tests are, however, accompanied with some problems. One of them is that there are large potential hazards in the procedures of these tests, because a considerable quantity of sample is employed at each run. Accordingly, only a limited number of the SADT data of chemical substances are currently available, and it also can be said that systematic measurements have not been carried out by any organizatoin so far.
    Several years ago, an explosion accident occurred in a chemical factory, where 1,2-naphthoquinone-2-diazido-5-sulfonyl chloride (a sort of photoactive compound, hereafter referred to as NQC) burned explosively when being packaged in a container after the synthesis process. The ignition source was at that time presumed to be sparks which resulted from electrostatic charging of plastic resin materials constituting the packaging equipment concerned.
    On the other hand, it would be also necessary to grasp sufficiently in advance the thermal stability of NQC for its safe handling, because a NQC molecule contains thermally unstable diazo group and sulfonyl chloride group.
    Thus, in this document, the SADT value of NQC found by the BAM heat-accumulation storage test is reported.
    The results are as follows:
    NQC shows no self-heating behaviour up to 60 °C, but gets to show a subtle self-heating behaviour at 65 °C, and at 70 °C or over it exhibits remarkable self-heating behaviours.

Evaluation of Explosion and Fire Hazards in Manufacturing Process of Organic New Materials

Takayuki ANDO

: In manufacturing, transporting, and storage of fine-chemicals such as Pharmaceuticals, functional resins, pesticides and so on, the hazard evaluations have been extremely important. These chemicals with enhanced value are usually produced in small quantity with a variety of processes. As a result, the same reaction vessel may often be used to synthesize different kinds of chemicals with different processes. Therefore, the potential hazards of the chemicals including raw materials, intermediates, and products may be increased due to unsuitable reaction condition, mistakes in plant operation, and so on.
    To prevent explosions or fires in the fine-chemical industry, it is thus very important to evaluate the potential hazards of chemicals themselves and chemical reactions, and to investigate safety measures on manufacturing facilities and operating system on the basis of the informations on hazardous properties of the chemicals.
    In this report, several methods to investigate the thermal hazard such as runaway reaction or thermal decomposition are shown. Then, an example of application of hazard evaluation procedures is presented by considering the cause of a batch plant incident occurred recently in Japan.

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