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LABOR PROTECTION AND SAFETY ENGINEERINGIN POWDER METALLURGY

S E L F - I G N I T I O N O F M A G N E S I U M P O W D E R SE. I . Po po v UDC 621.762.3

In view of the wide use of magnesium powders in industry, it is important to know their critical valuesof ignitabili ty under various conditions. This will make i t possible to asses s the degree of hazard encoun-tered in any situation and take precautio ns enabling such powders to be handled safely in any proc ess . Un-fortunately, the published information on the spontaneous combustion of magnesium powders is lacking inagr eem ent , and it is thus impos sible to obtain a reliable pictur e of the problem involved (Table 1).

Lack of information on particle shapes and somet imes even on particle sizes hinders compa rison anduse of existing data. Apart from this, contamination by detr i tus from, for example, mill ing cutters and millballs and lining can affect the ignition behavior of powders . The iron content of some of the powders was ashigh as 4 %. Compa rison of the self- ignition temp erat ures of powders of different particle sizes and shapesis made even more diff icult by the fact that different authors employed different experimental procedures.

In the work described below, the ignition behavior of magne sium powders of different particle shapesand sizes was studied under identical experimental conditions so as to obtain comparable results and makeit possible to examine the effects of individual factors . Ignition tempera ture s were determ ined by the dif-ferential therma l method. Before deciding on the precise test conditions, a study was made ofthe dependenceof the ignition temp era tur e upon two pa ra me te rs , namel y, the rate of flow of air and the rate of heating ofthe specimen. The values of these two pa ram et er s were chos en (1 lit er/ rain , 25 deg C/min) giving the low-est ignit ion temper ature s. Air of the required moisture content was prepa red by bubbling dried air throughwater in a con stan t-te mper atur e vessel. The actual ignit ion temperatu re was taken to be the lowest tem-perat ure reco rded in f ive parallel experim ents, provided that i t differed by not more than 10 deg C from themean tempe rature.

The effect of particle size on ignition temperatur e was investigated using powders produced by ma-chining (in a milling machine). To obtain differ ent fract ions , the powders were pa ssed throu gh variou ssieves. The particle size distr ibution of the f inest powders (-50 I~) was determined by sedimentation anal-ysis (Fig. 1) {performed in ethyl alcohol as dispersing medium, using a Shimadzu automatic apparatus).

The variation of the ignition tempe rat ure of the powders with their partic le size is depicted in Fig. 2.It will be seen that the ignition tem per atu re sha rply ris es with inc rea se in parti cle size up to ~ 100 p. Forco ar se r powders, the dependence of their ignition tempe ratu re on particle size is less pronounced.

The ignition temperature of a powder depends not only on its total specific surface but also, and verymarkedly, on i ts particle size distr ibution. This is clea rly i l lus trated by the result s of experimen ts oncoar se powders with additions of various amounts of f ines. Thus, a powder of 4- to 6-m m particle sizeignited at 597~ but the addition to it of 25 and 50% of a powder of -1 5- p parti cle size lo wered its ignitiontemp erat ure to 575 and 550~ respectivel y. Fine particles in a mixture heat up more rapidly and then actas ignit ion init iators, thereby determin ing the ignition tempe ratur e of a multisized mixture.

The same conclusion follows also from a comp ariso n of the ignition tempe ratu res of the two -5 0- pfractio n spec ime ns whose parti cle size distri butio ns ar e shown in Fig. 1 (curves 1 and 2). Both spe cime nshad the same limiting particle diame ter, but specimen No. 1 contained a higher proportion of the f inestfrac tion s (0-10 and 10-20 g), as a resu lt of which it ignited at a temp era tur e of 517~ while specime n No. 2ignited at 535~ It is interest ing to note that three f ract ions having differe nt upper particl e size limitsbut almost the same surface (thanks to the presence of very fine particles) were found to have almost iden-tical ignition te mpe rat ure s (535, 537, and 540~ resp ecti vely ).

Moscow. Tran slat ed fr om Poro shk ova ya Metallurgiya, No. 7 (139), pp, 93-96, July, 1974. Originalarticle submitted January 28, 1972.

9 1974 Cons u l t an t s Bureau , a d i v i s i on o f P l enum Pub l i sh i ng Corpora t ion , 227 Wes t 17 t h S t ree t , New Y ork , N . Y . I0011 .No par t o f t h i s pub l i ca t i on may be reproduced , s t o red i n a re t r i eva l sy s t em , o r t ransmi t t ed , i n any f orm or by any means ,e l ec t ron i c , mec han i ca l , pho t ocopy i ng , m i cro f i l m i ng , record i ng o r o t herw i se , w i t hou t wr i t t en perm i ss i on o f t he pub l i sher . Acopy o f t h i s a r t i c l e i s ava i l ab l e f rom t he pub l i sh er f o r $15 .00 .

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50

~0

30

20

0

" < 0

!- . . , ,

\\ \- - < . .

1 0 2 0 , YO ~ 0 5 0 5 0particle diameter,Fig. 1. Parti cle size dis tr i -bution of magnesium powdersof-50-# par t ic le s ize .

o 600g , , o /K 5oo

4o 0o o ,0 t , a 1 ,0 2 ,oparticle size, mm

Fig. 2. Variatio n of ignitiontemperature of magnesiumpowders with particle size.

TABLE 1. Self-Ignition Tem pera tur es of Mag-nesium Powders

Type of powder Self-ig-nitiontemp., ~ Source

Dust 4 o o [ ~Dry dust 420--440 [2]Moist dust 360--370 [2]powder (-270 ~) 570 [3]Fine powder 480 [41Coarse powder 560 [41Powder 520--550 [5]powder (- 40 /l) 532 [6]powder ( 4 0 - 66 p) 504--548 [6]Powder 520 [7]Dry dust 450 [8]Powder 490 [9]

TABLE 2. Ignition Temp erat ures of MagnesiumPowders of Various Particle Shapes

Ignition tempera-of preparation shape [d ryair ~ttmid air

Comminution in mortar Petal-shaped 540 510Machining in mi ll in g Irregular,agged 560 530machineComminntien in impact- Irregular, ounded 575 545eddy millComminution in ball Sphero idal 580 550millGranulation Spherical 585 552

To examine the effects of particle shape, experime nts were c arr ied out on powders of the same par ti-cle size (-71 p) produ ced by diffe rent method s (Table 2). As can be seen from the table, the ignition tem -pera ture is strongly affected by the particle shape of the powder. The easies t to ignite are powders com -posed of f lattened partic les, which have a well-developed surface. As particle shape approache s sphe rical ,the ignition tempe rature of the powder r ise s.

All magnesiu m powders, irr espe ctiv e of their particle size or shape, burn spontaneously at tempe r-atures below 600~ i .e. , before the metal begins to melt . This compar ative ease of self- ignition is attr i-butable to the poor protective qualit ies of magnesi um oxide f i lms and to the high vapor pres sur e of mag-nesium at test tempera tures . The heat being evolved during oxidation reactions on the particle surfa cesrais es the temp erat ure of magne sium and brings about i ts partial evaporation. The resultant magn esiumva po r- ai r mixture is then ignited by the hot particles. The f lame produced during the combustion isblindingly bright and white. The pr oce ss is accompa nied by the evolution of mag nes ium oxide in the for mof white smoke. In the cas e of par tic les with smoot h surfa ces it was found that, during the oxidation re-actions preceding ignition, f lashes appeared at some points, after which the f lame spread over the wholesurface. With irregu lar -sh ape d part icles , on the other hand, their sharp edges ignited f irst , which intensi-fied the ignition process.

Humidifying the air was found to lower the ignition tem pe rat ure s of all the powders investigat ed; ata relative humidity of 1.8%, the fall in ignition tem per atu re was as much as 30-40 deg C (Table 2). Athumidities of 1.8-15%, the self- ignition temp era ture s remain ed vir tually unchanged. At st i l l higher moi s-ture contents, the ignition temperatures began to r ise, returning to their original values at a humidity of50%. Fur the r increas e in humidity to 100% raised the ignition tem pera ture s by another 10-15 des C.

The results obtained clearly show that the self- ignition behavior of magnesium powders is stronglyaffected by their particle size and shape and by the humidity of the air . They also explain the discrepa nciesin the data obtained by different authors. Determinatio ns made with allowance for these fact ors yield more

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reliable starting data, with the aid of which it should be possible to minimize hazards in the handling ofmagnesium powders.L I T E R A T U R E C I T E D

1. L. N. Bidulya, Casting Production Technology [in Russian], Metallurgizd at, Moscow (1956).2. V, V. Krymov, Safety Engineer ing in the Handling of Magnesi um Alloys [in Russian] , Oborongiz (1955).3. F. Clark, New Powder Metallurgy Techniques [Russian transla tion], Metall urgizdat (1965).4. Yu. Kh. Shaulov, Liquid and Solid Rocket Fuels [in Russia n], Moscow (1955).5. H. Nowotny, Usp. Khim. , 27...., No. 3 (1958).6. G. Const antinides , Ann. Chim. , 49 (1952).7. Har tmann , Mining Met ., 2-9, 331 (1945).8. S. Wernke, Fertigung stee hnik , 4, 133 (1954).9. A. Jacobs on, U. S. Bur . Mines Rept. Inv. , 65, 176 (1964).

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