LABOR PROTECTION AND SAFETY ENGINEERING IN POWDER METALLURGY

ON T H E P Y R O P H O R I C P R O P E R T I E S , E X P L O S I O N H A Z A R D ,

AND T O X I C I T Y OF P O W D E R S AND D U S T S OF I R O N AND

I T S C O M P O U N D S

Y u . M. G o r o k h o v

(Review)

Insti tute of the Problems of Materials Science,Ukraine, Academy of Sciences Translated from Poroshkovaya Metal lurgiya, No. 1 (19), pp. 105-110, January-February, 1964 Original ar t ic le submitted January 22, 1962

Iron powders are presently being evermore widely used in various branches of industry. A number of shops and

departments have already been constructed and are being constructed for the production of iron powder. Therefore the problems of the pyrophoric properties, explosion hazard, and toxic i ty of these powders have acquired a specia ! import ance.

It is known from the data in the l i terature [1-9] that fine powders and dust of iron, iron sulfide, ferrovanadium, ferrosilicon, and ferrocerium, and also of iron pentacarbonyl, burn spontaneously in air (spontaneous combustion) or when a mixture of powder is heated in air to cer tain temperatures.

P y r o p h o r i c P r o p e r t i e s

Pyrophoric powders of iron are obtained as the result of:

1. the reduction of H z [1-4] or CO [4] for rolling scale, oxides, hydroxides, and ore concentrates at t empera - tures below 600-400 ~

2. thermal decomposit ion of iron carbonyl and also of salts of organic acids (oxalate, tartrate, and formate) [1, 3-6].

3. the decomposit ion of organometa l l ic compounds [4]. For example , in the at tempt to obtain CzHsFeI by the action of C~HsMgI on FeI, the formation of strongly pyrophoric iron was observed.

4. the dis t i l la t ion of amalgam (dist i l lat ion of mercury) [4, 5].

5. the action of a lkal i meta l on the salts of heavy metals [4]. Pyrophoric iron wasobtained from nitrates by reduction with me ta l l i c sodium.

6. fine mechanica l pulver izat ion in a cyclone mil l [2]. Spontaneous combustion of powder can occur when grinding sponge iron in a bal l mi l l when the rules of operation are not followed.

7. mechanica l cold processing of iron. The iron dust is capable of spontaneous combustion [7].

8. the e lec t r i ca l method of production [1, 6].

It is necessary, however, to note that meta l powders demonstrated their pyrophoric properties only after washing off the sur face-ac t ive substances and removal from their surface of the organic solvent in a vacuum at low temperature.

From the point of view of fire hazard, the ignit ion point of powder or dust is of great impor tance: the lower it is, the more dangerous is the powder. The sources or impulses of powder and dust ignit ion are numerous and diverse. This can be an impac t or friction, e lec t r ica l discharge or static e lec t r ic i ty , an open fire, e levated temperature from some source, a chemica l reaction, spontaneous combustion from physical and chemica l processes, etc.

Various methods are described in the l i terature for determining the ignit ion point of powders and dusts [8, 9,

etc.] . The most widespread of these are as follows:

1. powders or dust are p laced in an open furnace. The lowest temperature at which ignit ion or burningoccurs

for 1-2 sec is considered the ignition point.

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TABLE 1. Ignit ion Point of Powders and Dust of Iron and Its Compounds Based on Var i -

ous Literature Sources

Metal or alloy Part icle size, mm Ignition point, ~ Source

Dusts of iron powder (e lect rolyt ic and sponge freed from iron carbonyl), of ferrovanadium, and ferrositicon

Iron Alloy F e - S b Misch metat with 30% Fe Misch meta l with 40% Fe Misch meta l with 60% Fe Ferrocerium with 30% Fe F errocerium Ferroeerium F errocerium

Iron powder Iron in a layer of powder Iron in a cloud of dust

0.05

0.15

0.2-0.3

0.3-0.5

315-850

216

230-300

187

194

210

135-140

160 -175

170-190

190-200

>500

280-460

315 -780

[1] [9] [9] [9] [9] [9] [9] [91 [9] [9]

[lO] [11] [11]

TABLE 2. Part icle Size of Iron Powders Obtained by Different Methods [2]

Method of production Average grain size, mm

Reduction of pure iron oxide Reduction of ore (sponge iron) Reduction of ro l l i ng -mi l l scale Electrolysis

C arbonyl Cyclone Pulverization of mel t of iron and its alloys Compressed-ai r a tomizat ion in molten state

0.05-0.35

0.05-0.25

0.I0-0.25

0.09

0.05-0.10

0.12-0.25

0.05-0.18

0.12 -0.35

2. same as (1), but in a stream of air, and with the powder or dust in a precipi ta ted condit ion (aerogel) .

3. a current of air is created so that the dust or powder is in a suspended state (aerosol).

4. ignit ion of powders or dust as an aerogel or aerosol from other sources or impulses.

First of al l we need note that the ignit ion point (burning) is considerably affected by the atmosphere and method

of obtaining the specimen. If a powder is tested several seconds after it is obtained, the ignit ion point in the case of argon (and only of argon) is very low (considerably lower than room temperature) [9]. For example , pyrophoric iron retains thsi character is t ic at -78"C [4]. The ignit ion point increases with holding t ime in air, at first rapidly and then slowly [9].

The data on determining the ignit ion point of various iron powders are shown in T a b l e l . We see from the tab le that the ignit ion point can serve as a qual i ta t ive character is t ic of the degree of the pyrophoric properties of dust under specific conditions.

The pyrophoric character of powders is determined by their structure and depends most of al l on par t i c le d i - mension. In some works [1-3, 5,9] iron powders reduced from oxides are pyrophoric at temperatures below 600-

400"C and igni te in the air. This powder when reduced at higher temperatures (in various works above 600-900"C) will not igni te at all in the air.

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It was demonstrated in work [1] that powder reduced at elevated temperatures has larger grains and this weakens its pyrophoric properties. It was indicated in work [4] that at a certain temperature the pyrophoric properties disappear. This is accompanied by an increase in the volume of the metal which apparently is a consequence of recrystalliza- tion. It was also noted in work [9] that with an enlargement of particle size, the pyrophoric properties of iron powder disappear. It was shown in work [3] that if pyrophoric powder reduced at a low temperature (540~ ignites at 820~ sintering of its particles will occur and the pyrophoric properties disappear.

Table 2 shows data on particle size of powders obtained by different methods.

Thus if we consider that powders having a particle size less than 0.05 mm are pyrophoric, then iron powders obtained by the usual methods (Table 2) do not pertain to such. But it is necessary to bear in mind that this table shows the average grain sizes of powders and that this does not at all preclude the presence in the powder of apprec- ciably smaller particles, especially since dusts of these powders occur during production and use.

As was already stated, iron powders produced in cyclone mills are pyrophoric [2], the ignition point of iron with particles of 0.05 mm is 261~ (Table, 1), etc. Thus we can apparently consider these powders to be to some degree potentially pyrophoric regardless of their par t icle size.

The pyrophoric properties of powders are greatly affected by the condition of the surface of the powders (de- gree of oxidation of the surface) and moisture in the air, which can enter into reaction with the powder, accelerat- ing oxidation and decomposition. However, this problem is still too inadequately studied to make any definite con- clusion.

The pyrophoric properties of powders can be somewhat reduced by their careful oxidation within narrow limits. For example, to reduce the pyrophoric properties of iron powders when grinding in a cyclone mill, the working cham- ber is filled with inert gas to which is added 3-5% oxygen for the formation of protective oxide films on the powder particles [2]. When grinding sponge iron in mills it is necessary to provide access for air, depending on the oxidation conditions, so that the active surface that is forming is sufficiently oxidized.

The pyrophoric properties of powders, as was indicated in work [4], can be affected by distortions in the crystal lattice of the powders and by structural defects. For example, when producing iron carbonyl powders, imperfections in the crystal structure are caused by the low formation temperature of powders and by carbon and oxygen impurities

[33.

It was revealed in works [3, 9] that the enthalpy of pyrophoric iron powders is increased owing to the developed surface and distortions in the crystal lattice. It was possible to demonstrate that pyrophoric iron powders have a high energy reserve and this is expressed in a 0.1 A distortion of the lattice. X- ray diffraction studies of the crystal structure of deformed metal filings and electrolytic and carbonyl powders, and also powders produced by grinding in cyclone and ball mills, revealed distorted lattice parameters and two and three kinds of stresses.

As we have already seen, certain authors relate the absence of pyrophoric properties with recrystallization [3, 4]. Substances in an active state can be obtained if the process occurs at a lower temperature than the temperature at which rectification of the structural defects and collective recrystallization occur. Recrystallization can also be caused by intermixing during the reduction process at temperatures appreciably below the upper temperature limit for retaining pyrophoric properties (500-600"C). For example, a temperature of aT0*C is sufficient for iron.

The upper temperature limit for retaining pyrophoric properties can be elevated by a mixture of Also s : FezO z. If powder from precipitated hydroxide upon reduction with hydrogen has this limit at 5a0-540~ then an admixture of 2% A120 s increases it to 650~ and an admixture of 20% AlzO 3 increases it to 700"C [4].

The effect of the state of the starting substance is evidenced in that grinding of iron oxalate in an agate mortar increases the upper temperature level for retaining pyrophoric properties from 530 to 590~ [4].

The effect of structure on the pyrophoric properties in friction is confirmed by such data as: in the F e - C e system hardened alloys become less pyrophoric than those slowly cooled; hardened alloys of the M n - S b and F e - S b systems do not have noticeable pyrophoric p roper t i e s - they are manifest, however, after annealing at 200"C. Most frictional pyrophoric alloys have a distorted lattice. True, these distortions are not sufficient for the powders of these alloys to

ignite at room temperature [9].

In conclusion it is necessary to point out that in some cases, even in active metal powders there may not be latt ice defects [5]. In work [4], for example, there are indications of the absence of differences in the x-ray diffrac- tion pictures of pyrophoric and nonpyrophoric iron specimens.

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E x p l o s i o n H a z a r d Unfortunately, there are no numerical data in Soviet or foreign l i terature concerning dangerously explosive

concentrations of dust and powders of iron and its compounds. But it is known that aerosol systems in which sub- stances being energet ica l ly oxidized with the l iberat ion of heat (reduced iron, iron sulfide) are the dispersed solid

phase, where dangerously explosive concentrations can form [8]. Based on the degree of explosion hazard, powders

can be divided into three categories [1]: the first category includes powders of a luminum and magnesium (the dust is ex t remely explosive), the second (dust of average explosiveness) includes iron powders (e lec t ro ly t ic and sponge free from iron carbonyt) and powders of ferrovanadium and ferrosilicon, and the third (slightly explosive and non- explosive) includes contaminated iron.

It is known that charging into an empty mi l l of powder or siftings of iron below a certain quantity can lead to explosion. This quantity, evidently, for a given mi l l volume determines the upper concentration l imi t of ignit ion (explosion). Ignition and explosion of ground mater ia l is possible when grinding ferroalloys [12], so that to prevent such explosions in the charge it is necessary to introduce an inert addit ive ( local izer) in a quantity of 5-8 mass % of the mass of mate r ia l being mi l led .

One of the best local izers is finely pulverized feldspar. However, i t is necessary to note that this method is app l icab le only for mills with periodic loading, since in mills with a continuous supply of mater ia l it is p rac t ica l ly impossible to mainta in a constant ratio of the charge and local izer . Another method of preventing explosions is grinding in an inert gas medium (nitrogen, carbon dioxide). In this case it is necessary to use a closed crushing sys- tem, i .e . , one in which the process occurs continuously and the mater ia l being ground is co l lec ted in separators.

Thus, powder and dust of iron and its compounds can be dangerously explosive under cer tain conditions. The causes for the occurrence of an explosion are the same as those previously examined for the occurrence of the pyro- phoric properties. In general , combustion of dust and powders of iron and its compounds is, so to speak, the first stage of the process which can grow into an explosion under certain conditions.

Powders and dust of iron in a suspended s tate are the most dangerously explosive. If they are in an aerosol state, their burning veloci ty and, consequently, l iberat ion of heat per unit t ime will be so great that they can lead toan ava lanche - l ike ]propagation of burning, increase of pressure, and to explosion. According to the data ci ted in a review [11], the max imal pressure during burning of iron powder can reach 245 kN/m z.

T o x i c i t y

Powders and dust of pure iron are nontoxic or l i t t l e toxic [13]. Sometimes, owing to minute iron part icles entering the skin and their oxidation there, yellow spots will appear on the hands or face which gradually disappear when the person no longer works with iron [13].

One of the sources of industrial intoxicat ion and disease is Fe~3 a. Under prolonged exposure to Fe20 3 in the form of dust or haze, i t can be deposited in the lungs, causing the development of a unique disease, siderosis. An increased inc idence of nasa1 and nasopharyngeal disorders is noted in this disease. Recently obtained data indica te that under the effect of dustof iron oxides, in part icular, the dust of iron scale, fibrogenic changes can develop in the lungs [13, 14].

Of the other iron compounds whose harmful effect is known, iron pentacarbonyl and iron salts are of interest for powder metal lurgy. Pentacarbonyl is ex t remely toxic when inhaled or taken internally, or even when absorbed through undamaged skin, it caused acute pulmonary edema. Iron pentacarbonyl can also cause burns.

Iron salts usually do not cause industrial intoxicat ion, although bivalent (lower oxide) compounds have certain general toxic effects. Chlorides act more strongly than sulfides. Tr ivalent (higher oxides) compounds are less toxic, but can burn the mucous membrane of the throat [13].

In prac t ice , different mixtures of iron dust with other substances are found in many cases. In powder metal lurgy mixtures of iron with graphite, copper, brass, sulfur, nickel, etc. , are used for manufacturing articles. At present there are p rac t i ca l ly no data on the harmful effect of various dust mixtures of iron.

Dusts of iron alloys with vanadium, nickel , molybdenum, and beryl l ium are less toxic than dusts of vanadium,

nickel , molybdenum,, and berylI ium oxide taken separately [14], but are evident ly more toxic than the dust of pure iron. I t was established in experiments on animals that inhalat ion of the aerosol from e lec t r ic welding caused more vigorous changes than inhalat ion of FezO s dust. Maximum permissible concentrations of iron oxides with admixtures of fluorine or manganese compounds in the atmosphere of work rooms have been es tabl i shed: 4 m g / m s.

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Thus, the action of Fe20 s dust, which under prolonged exposure can cause chronic pulomonary diseases, has been studied the most. It is also evident that different dust mixtures of iron and its alloys with other more toxic elements can also harmfully affect man.

Methods of obtaining powders, the admixtures accompanying them, and the physical properties of powders, dusts, etc., should have an effect on the toxicity of powders and dusts of iron and its compounds, just as on the pyro- phoric properties. Therefore, to determine the toxicity of powders and dust of iron, its compounds, and mixtures, we need carry out a number of experimental studies using appropriate scientific procedures.

LITERATURE C I T E D

1. A. Buoik, Safety Engineering in Powder Metallurgy [in Russian from the Polish], Hutnik (Stalingrod), 8, 21 (1954). 2. G.V. Samsonov and S, Ya. Plotkin, Production Iron Powder [in Russian], MetaUurgizdat, Moscow (1957). 3. I .M. Fedorchenko and R. A. Andreivskii, Principles of Powder Metallurgy [in Russian], Izd. AN UkSSR, Kiev

(1961). 4. D.A. Pospekhov, ZhPKh, 2...22, 1, 5 (1949). 5. F. Aizenkol'b, Powder Metallurgy [in Russian], Metallurgizdat, Moscow (1959). 6. t~. M. Natanson, Colloidal Metals [in Russian], Izd. AN UkSSR, Kiev (1959). 7. Referativnyi zhurnal "Khimiya," 10, 28348 (1954). 8. M.G. Godzhello, Explosions of Industrial Dusts and Their Prevention [in Russian], MKKh RSFSR, Moscow (1952). 9. Kh. Novomyi, Uspekhi khimii, 2_~7, 3, 353 (1958).

10. A.A. Shillovskii, Principles of Pyrotechnics [in Russian], Oborongiz, Moscow (1954). 11. A.A. Rodd, Collection: Protection of Chemical Enterprises from Fire and Explosion" [in Russian], NIIT~Kb_IM,

Moscow (1961), p. 3291 12. N.R. Anders and V. S. Rakovskii, Principles of Hard Alloys Production [in Russian], Metallurgizdat, Moscow

(1961). 13. N.V. Lazarev, Toxic Substances in Industry (Handbook for Chemists) [in Russian], part 2, Goskhimizdat,

Leningrad (1954). 14. B.A. Krivoglaz, V. G. Boiko, V. P. Neis, A. A. Model', L. A. Zaritskaya, and E. P. Krasnyuk, Poroshkovaya

metallurgiya, 5, 109 (1962).

All abbreviations of periodicals in the above bibliography are letter-by-letter transliter-

ations of the abbreviations as given in the original Russian journal. Some or a11 of this per/-

odical l i terature may well be avai lable in English translation. A complete list of the cover-to-

cover English translations appears at the back of this issue.

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