T H E J O U R i V A L O F I Y D C ‘ S T R I A L A N D ENGINEERING C H E M I S T R Y
Apr.9 = 9 = 7
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TABLEV-REMELTINGFERROSFOR REFINING Tilting Furnace, Water-cooled Hearth CHARGE PROPER IN POUNDS Disregarding Slag in Furnace Furnace ANALYSIS FERRO Mill Time at Expt. FBRRO U C Si Slag UOz Scale CaFz Hr. Min. Start NO. 7 1 .................. 6 . 0 (No.7 0 ) ( a ) 2 5 . 5 4 . 2 2 . 5 None 5.0 1 0 Cold 83 9 . 0 (No. 82)(a) 4 5 . 0 3 . 6 1 . 2 9 . 0 (No. 82) 5 . 0 0:;s 1 20 Cold 84 7 . 0 (No. 65)(a) 3 8 . 0 5 . 8 1 . 0 7 . 0 (hTo. 82) ... 1 . 7 5 0 . 7 5 50 Hot 90 7 . 0 (No. 89)(a) 5 6 . 0 1 . 2 0 . 6 5 . . . 3.0 .... 1.2 1 .. Cold 91 .................. 9 . 0 (No.86)(a) 6 8 . 5 5 . 0 0 . 2 5 . . . 6.0 . . . . 2.5 . . 50 Hot
.... ....
.................. .................. ..................
Expt.
No.
71..
.........
83... 83
RATIOSPER 10% U Original Rifined Ferro CFe’roSi C Si 1.64 0 . 9 8 0 . 2 5 1.28
........ 0.80
........... 1 . 5 3 90 ........... 0 . 2 2 91 ........... 0 . 7 3
0.27 0.26 0.12 0.04
0.30 0.52 0.22 0.27
0.29 0.30
0.12 0.11
--RECOVERY I n Ferro Charged U C Si 1.50 0.25 0.15 4.05 2.67 3.56 6.17
0.25 0.41 0.07
0.45
0.11
0.07 0.04 0.02
(POUNDS)In Ferro Recovered U C Si 1.25 0 . 0 3 0.16 2.70 0.07 1.90 0 . 1 0 0 . 9 7 0.024 0.126 4 3 ,. 36 02 o , 0 9
0.06 0.06
..
Lbs. Percentage Analysis Ferro of Product Kw.-H. Poured U C Si 49 3.5 36.5 0 . 9 4.7 55 4.0 56 1.7 1.6 31.25 4.0 47.5 2.45 1.4 49 2.25 43.0 0.95 1.S5 36 7.0 66.0 1 . 8 0.75 5.0(b)
REMARKS S i c hearth &walls, walls crumbled and contaminated Ferro
...---.
mith .Si
Magnesite hearth Magnesite hearth 0.035 Magnesite hearth‘ Increase in O.O53(c) 0.038(d) siliceous impurities in CaFz
si
due to
( a ) Crushed to pea size and smaller.
Plus any ferro left in furnace from preceding run. ( b ) Taken from furnace in regulus, when cold. (6) Poured. ( d ) Regulus.
I n all of these runs t h e recovery of t h e ferro charged was poor, much less being poured t h a n was charged, There was always some spatter, small globules of ferro being shot u p into t h e air above t h e hearth, oxidizing, a n d dropping back into t h e hearth, t h e Fez03t h u s produced tending t o decrease t h e percentage of U in t h e recovered ferro, as metallic U in t h e ferro will be oxidized b y Fez03. The experiments indicate t h a t i t is probably desirable t o produce a low carbon ferro in one operation, rather t h a n first t o make a high carbon ferro a n d t h e n t o refine it. Vanadium has not been determined in most of t h e ferros. It cannot average as high as 0 . 2 5 per cent, though individual ferros, made with a large amount of fresh UOz, will run a trifle higher t h a n t h e average because V is more readily reduced t h a n U, while one made from a charge consisting largely of old slag from which t h e V is already largely extracted will run lower t h a n t h e average. A few analyses of t h e ferros indicated t h a t prabably 0.4 per cent a n d certainly not over 0 . j per cent is t h e maximum. Traces of V were present in all. 9 s V is used in almost all tool steels, t h e very small amount of V t h a t would be introduced into steel by t h e ferro-uranium certainly would not be classed as a harmful impurity, though for experimental work on t h e value of U in steel it is desirable to have a ferrouranium as low as possible in V in order not t o have another variable t o contend with in t h e V. It appears t h a t b y using a pure UOz, a low-ash coke, and a pure iron as raw materials, with C a F z as slag former, a n d using a tilting direct arc t y p e furnace with water-cooled magnesite hearth and sides, i t should be possible t o produce commercially, without a second refining operation, ferro-uranium of any desired U content, say 40-70 per cent, with carbon averaging below 2.0 pei cent, silicon below 0.7 j per cent, vanadium below 0 . j per cent, a n d with aluminum, sulfur, phosphorus and manganese all so low as t o be negligible. If experiments with such a ferro show t h a t uranium steels high in uranium are not valuable, b u t t h a t a little uranium is useful, a n d if t h e amount required is so low t h a t t h e carbon introduced b y a high carbon ferro is harmless, then t h e furnace might have a n uncooled carbon hearth, a n d t h e ferros would contain 4 - j per cent carbon, If uranium is found useful only as a deoxidizer or
scavenger of oxygen and nitrogen, aluminum would not be harmful a n d might be advantageous, a n d t h e slag former might be wholly or in p a r t A1203. Grateful acknowledgment is made t o t h e Department of Chemistry a t Cornel1 University for t h e use, under a cooperative agreement, of its laboratory facilities, which are particularly well adapted t o a problem of this nature. ITHACA, NEWYORK
INFLAMMABILITY OF CARBONACEOUS DUSTS IN ATMOSPHERES OF LOW OXYGEN CONTENT By H. H. BROWNAND J. K. CLEMENT Received November 20, 1916 INTRODUCTION
As was stated in a previous paper,’ if dust could be entirely confined within t h e machinery of a mill in which combustible dust is produced, a n d a method could be found for preventing explosions in these machines, a long step would be taken in t h e prevention of dust explosions in mills. T o keep a dust cloud from forming in t h e machines appears t o be almost, if not utterly, impossible. It is possible, however, by proper cleaning t o remove foreign material from t h e grain and t h u s lessen t h e possibility of a spark being formed in t h e machine which might ignite t h e dust. But cleaners a n d separators do not always take out all t h e foreign material, so t h a t even under t h e best conditions foreign materials may get into t h e machines, or other conditions develop which might cause sparks, or other sources of heat, t o be formed within t h e machine, thus creating a very dangerous condition. However, if there were present within t h e machine a n a t mosphere which would not support combustion, t h e dust could not ignite and a n explosion could not take place. For some time t h e possibility of preventing explosions within machines b y t h e use of inert gases has been under consideration. I n a n article on “Coal Dust Explosions and Their Prevention,” J. Harger2 recommends a low oxygen content in t h e atmosphere of t h e mines as a preventive of explosions. The writer states in part as follows: “ T h e only way t o absolutely prevent dust explosions is t o reduce t h e oxygen percentage below t h e lower limit, which varies with t h e different coal dusts. 1 “Inflammability
of Carbonaceous Dusts,” THIS JOURNAL, 9 (1917).
269. 2
J . SOL.Chem. Ind.. 31 (1912). 413.
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T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
A reduction of 2 per cent in practice will do this for all mines, b u t t o get a guarantee of absolute immunity 1 7 l / 2 per cent oxygen is t h e figure. A 1 7 l / 2 per cent oxygen atmosphere is ideal for a coal mine, if i t were not too expensive t o obtain, which should not be t h e case.” He recommends not over I per cent carbon dioxide in this atmosphere, as a higher percentage would be dangerous from t h e point of view of respiration. Referring t o decreasing t h e oxygen content of t h e atmosphere he states: “This method is, in my opinion, the only one which can be thoroughly applied t o mines or machines making inflammable dust.” The most likely source of such inert gases around a mill are the flue gases. These contain a large amount of nitrogen, 79 per cent, t h e other 2 1 per cent containing varying amounts of oyxgen and carbon dioxide. Under normally good working conditions the oxygen content is about 1 1 per cent. Small amounts of carbon monoxide are often present, especially when t h e oxygen content is low, but this is usually under one per cent. For use in enclosed machines t h e carbon dioxide content would not necessarily have t o be as low as in a mine or in an atmosphere where men were working so this would not have t o be removed from t h e flue gases before they could be used in t h e grinding systems of mills. The purpose of this investigation was t o determine t o what amount t h e oxygen content of t h e air must be reduced in order t o prevent t h e propagation of explosions of various carbonaceous dusts. EXPERIMENTAL
APPARATUS-The Bureau of Mines has made a number of tests upon t h e inflammability of coal dusts in mixtures of air and natural gas. For this work t h e apparatus used in t h e usual testing of coal dusts was adopted with but one change. This was an auxiliary attachment t o t h e base plate for putting the gas into t h e globe.’ It consists of a brass tube lying just above t h e glass injection funnel a n d passing upward through t h e base plate a t t h e right of t h e funnel. After passing through t h e base plate it extends upward for a distance of I in. and then encircles t h e t o p of t h e funnel. The portion of the brass tube forming this circle has eight small holes, spaced a t equal intervals from each other, drilled through its upper wall. The outward end of t h e brass tube is connected t o t h e gas holder. GAS MIXTURES-The gas mixtures used contained approximately 79 per cent of nitrogen, t h e remaining 2 1 per cent being made up of carbon dioxide and oxygen. T o obtain t h e gas mixtures, commercial carbon dioxide, oxygen and nitrogen were used. T h e gas mixtures were made in a large gas holder, t h e amount of each gas used being measured roughly by t h e height of gas in t h e holder. The mixtures were then analyzed before a n d after each series of tests. METHOD OB OPERATION-TO test t h e inflammability of t h e dusts in these gas mixtures in t h e apparatus 1
“Inflammability of Carbonaceous Dusts.”
Vol. 9, No. 4
referred t o above, it was necessary t o displace all t h e air in t h e globe with t h e gas mixture. Evacuating and refilling were considered, as were other methods, but t h e only method which proved t o be practical was t o allow a sufficient amount of t h e gas t o enter t o displace all the air. To determine t h e amount of gas necessary t o displace t h e air in t h e globe (capacity of 1390 cc.), measured amounts of a gas containing 9.5 per cent carbon dioxide and I I . 2 per cent oxygen were allowed t o flow into t h e globe through t h e gas injecting tube around t h e t o p of t h e funnel, and t o force t h e air out of a valve in t h e tube t o which the pressure gauge is attached. A sample taken after 4 . 5 liters had been allowed t o flow into the globe showed 8.2 per cent COz a n d 1 3 per cent oxygen. Four and a half liters more were allowed t o flow into t h e globe. A sample taken showed 9.6 per cent COS and 11.4 per cent oxygen. Nine liters of the gas were allowed t o flow through another globe. A sample of t h e gas in the globe showed 9.5 per cent C 0 2 and 11.4 per cent oxygen. These results indicate t h a t 9 liters of gas will displace all t h e air in t h e globe. Therefore, t o be certain t h a t all t h e air is displaced, I O liters of gas were allowed t o flow into t h e globe before each test. The gas flowed a t a rate of 0.9 t o 1.0liter per minute. To blow t h e dust into this gas mixture b y a puff of air would change t h e oxygen content and introduce an appreciable error. Therefore, a second connection was made t o t h e gas holder so t h a t gas could be drawn from it and compressed b y a vacuum a n d compression pump, into t h e bulb attached t o t h e funnel which contained t h e dust t o be tested. The method of testing was, therefore, not unlike t h a t used for testing t h e explosibility of carbonaceous dusts in air, except t h a t the explosion flask was filled with a known gas mixture and t h e dust was injected by the same gas mixture under a pressure of 2 0 cm. of mercury. The heating of t h e igniting coil was so regulated t h a t t h e dust might be injected as soon as possible after all t h e air had been displaced. This was done t o obviate a n y possible change in t h e gas mixture by leakage or any other way. SERIES O F TESTS-A few preliminary tests were made with dusts which gave a n indication of t h e lower limit t o be expected. Five typical dusts were then tested, using t h e following gas mixtures: Per cent Oz
Per cent COz
Per cent Nn
Pittsburgh Standard Coal Dust was tested with the following gas mixtures: Per cent Oz 19.8 19.2
Per cent COz
18.1 16.9 16.0
1.2 1.9 2.8 4.6 8.8
0.8
0.0
Per cent Nz 79.0 78.9 79.1 78.5 75.2 99.2
It will be observed t h a t t h e sum of t h e oxygen a n d carbon dioxide content does not always add t o 2 1
A P ~ . ,1917
T H E J O U R N A L O F , I N D U S T R I A L AiVD E N G I N E E R I N G C H E M I S T R Y
per cent. This is due t o t h e inaccurate method of measuring t h e gas as i t was mixed. These figures are t h e mean of two analyses made before a n d after each series of tests. I n no case was there a variation of over 0.2 per cent in t h e t w o analyses. As in all cases where n o inflammation was noticeable, a pressure was obtained above t h a t given by t h e blank-that is a test made without a n y dust-it was thought t h a t these might be due t o a decomposition caused by t h e dust striking t h e hot coil a n d not due t o a slight ignition. T o determine this, a series of tests was made using 99.2 per cent pure nitrogen in t h e globe. With all grain dusts a pressure of 0 . 4 t o 0.5 lb. more t h a n t h e blank was obtained, t h a t is. a pressure of 0.6 t o 0 . 7 lb. The average of 0.6 lb. was therefore taken as t h e zero correction, in all tests where gas mixtures are used. Coal dust gave a pressure of 0 . 2 lb. more t h a n t h e blank, or a pressure of 0.4lb. This pressure was taken as t h e zero correction for coal dust. REsuLTs-The results obtained with t h e five typical dusts and coal dust are given in Fig. I, t h e average lbs. pressure being plotted against t h e percentage of oxygen in t h e gas mixture. This method was adopted 16
14
12
B
I
w
43 10
(R Q
e: w a
a"* z 3
E
w-
s6
@
E 4
2
10
12
14
16
18
20
22
OXYGEN, PER CEXT FIG I
instead of plotting pressures against percentage of COe since i t is t h e amount of oxygen in t h e air rather t h a n t h e amount of carbon dioxide t h a t determines inflammable limits If t h e carbon dioxide replaced t h e oxygen in air, then plotting against carbon dioxide would gir-e t h e same curves because t h e balance of t h e 2 1 per cent would be oxygen. But in t h e gas mixtures used, t h e carbon dioxide was sometimes one per cent lower t h a n would be required t o make t h e s u m of it and t h e oxygen u p t o 2 1 per cent. This one
349
per cent or less is nitrogen, a n inert gas, and, therefore, would have a similar effect t o t h a t of t h e carbon dioxide in preventing combustion. The values as given in t h e curves are t h e average of all results obtained in each series of tests. For t h e pressures above 2 pounds, t h e results given in each series were quite uniform a n d concordant. This is also t r u e for pressures averaging under 0 . 5 lb. But those results between 0.5 lb. a n d 2 lbs. are less uniform. I t will be observed t h a t these curves are quite similar in their direction, falling very rapidly as t h e percentage oxygen is decreased until a point is reached where t h e curves flatten. This point is in nearly every case a t pressures near or less t h a n I lb. I t is considered t h a t rapid oxidation will t a k e place if t h e oxygen cont e n t of t h e atmosphere is above t h a t indicated b y t h e break in t h e curve, b u t cannot t a k e place if t h e oxygen content is less t h a n this amount. coxcLusIoNs-Although n o large scale tests have been made with which t o compare these results, from t h e general action of t h e dusts in t h e tests, i t may be concluded t h a t a n explosion of a grain dust cannot be initiated in a gas mixture containing 1 2 or less per cent of oxygen, t h e remainder being inert gases. And this limit could be extended t o 1 4 or 14.3 per cent of oxygen if elevator dusts alone are considered. The results would suggest t h a t t h e maintaining of a n atmosphere of inert gases in all systems grinding or handling carbonaceous materials which form dangerous dusts would be a n effective means of preventing many dust explosions; for even though a n ordinarily dangerous amount of dust may be present a n d a spark or other source of heat may be formed, t h e dust would not be ignited or an explosion be propagated because t h e oxygen content of t h e atmosphere would be too low t o support combustion. The results of these tests show t h a t a lower oxygen content in t h e inert gas mixture is necessary t o prevent an explosion of grain dust t h a n would be required t o prevent a coal dust explosion. And, t h e results obtained with coal dust would indicate t h a t a lower oxygen content in t h e mine atmosphere would be necessary t o prevent a coal dust explosion t h a n was recommended b y Harger. However, his recommendation of 1 7 . 5 per cent oxygen was very close t o t h a t determined by t h e authors. I t may be noted in this connection t h a t t h e results published in a previous paper show t h a t with one exception coal dust is less inflammable in air t h a n t h e other dusts used in t h e tests with inert gases. This one exception is wheat flour, a comparatively coarse dust. Large scale tests are being planned t o demonstrate further t h e effectiveness of this preventive. The results which may be obtained then may alter t h e above results slightly. However. i t is considered, as a result of t h e present tests, t h a t a n inert gas mixture cont'aining 1 2 per cent or lower of oxygen will prevent a dust explosion from starting or propagating. BUREAUO F
CHEMISTRY
WASHINGTON, 1
Lor. c i t .
D. C .
