March, 1926
I,VDCSTRIIL 9,VD ENGI,VEERING CHEMISTRY
Possibilities of Application This recording device, although a t present very crude, may become a t some future time an important apparatus, not only for research work in filtration, but also in the field of chemical engineering. For instance, it is possible to use it in connection with large commercial filter presses by dividing the flow from the entire apparatus into a part small enough to be proper for the recorder capacity or by operating the float in a weir tank through which the total flow passes. Every filter press run would then make its own record of timt:, total discharge, and pressure. These records would then be examined and filed away the same as other recording device charts. Excessive pressure periods could be detected. The economical time to change filter cloths could be determined. Moreover, the ratio between total solids and filtrate could be checked up and many other things could be found to keep the management well in touch with its factory operation. The device could also be used with any other types of filter, such as the continous filters
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in which the discharge line should be a straight line. Discrepancies in suction, per cent of solids, and fouling of filter base would be indicated by deviation of the curve from the straight line or by change of angle. I n the field of research there are many interesting applications of this recorder. For instance, in studying the effect of pressure upon the rate of flow through a cake of constant quantity of solids, a number of curves can be made for different pressures and the angularity and curvature noted. For rigid cakes the curves ought to be straight lines, whose angularity is in direct proportion to the pressure, but in nonrigid cakes the angularity would vary in some other way with the pressure. The effect, caused by varying the per cent of solids, all other factors remaining constant, can be very easily studied by comparison of the curves. Research work in filter cloths and other filter bases can be simplified by use of the recorder. A later article will probably appear giving other filtration curves and describing the method used in analyzing them.
Determination of Oxygen and Hydrogen in Metals by Fusion in Vacuum' By Louis Jordan and James R. Eckman BUREAUOF
STANDARDS;
An absorption train suitable for the absorption and gravimetric determination of water vapor, carbon dioxide, carbon monoxide, and hydrogen passing through the train a t low pressures has been developed. By means of this train one or all of these gases present in the mixture of gases evolved from a metal sample fused in vacuum may be accurately determined. The high-frequency induction furnace is used for the fusion of the metal samples. This type of furnace permits the fusion of the samples in any desired type of crucible within a small silica tube. Moreover, the metal may be held molten a t temperatures of 1500" C. or more while the walls of the tube remain comparatively cool. Thus, no difficulty is encountered from the failure of the tube to hold a vacuum. Three methods of fusion previously employed in the determination of gases in metals fused in vacuum were
WASHINGTON, D. C.
applied to pure iron, a low-carbon steel, and a high-carbon steel. These methods are (a) direct fusion of the metal in a refractory oxide crucible, (b) fusion in a refractory oxide crucible with the addition of antimony and tin, and (c) fusion in a gas-free graphite crucible. Neither (a)nor (b) is satisfactory for determining oxygen in ferrous alloys. Method (c) gives the most dependable values for total oxygen. The values obtained for hydrogen offer no choice between the three methods for the determination of this element. Fusion of ferrous alloys in graphite determines, besides any oxygen present as such, the oxygen t h a t may be present in the metal as oxides of carbon, iron, silicon, manganese, aluminium, zirconium, and titanium. The fusion in graphite method is also applicable to the determination of oxygen and hydrogen in many nonferrous metals and alloys.
.. .. .. EARLY all metals contain small amounts of oxygen, hydrogen, and nitrogen, frequently spoken of as "gases in metals," whether they exist as oxides, hydrides, and nitrides, or in some other form. Many differences in quality of metals not readily attributed to differences in composition, as determined by the usual chemical analyses, or to different physical treatments, are supposed to be due to the presence of "gases" in the metals. Apparently, the two most frequently used methods for determining a gas in a metal have been modifications of the Ledebur method for oxygen2 and of the Allen method for n i t r ~ g e n . ~The original Ledebur method determines only those oxides reducible by hydrogen a t 900" c r 1000" C., and is not a t all applicable to ferrous metals containing
N
1 Received October 15, 1925. Published b y permission of the Director, the National Bureau of Standards. 2 Cain and Pettijohn, Bur. Standards, Te'h. P a p e r 118 (1919). 3 Jordan and Swindells, I b z d . , Sci. Paper 467 (1922).
carbon (steels, cast irons, and pig irons), since part of the reduction of oxides in such materials is brought about by the carbon or carbides in the metal. It is open to the further objection that the metal sample must be in the form of finely cut or crushed chips in order that the hydrogen may penetrate the metal. The preparation of such chips is very liable to result in surface oxidation of the metal and introduce large errors in the results for oxygen. The Allen method for nitrogen determines only that present as nitrides which are either soluble in acids or are decomposed by the hot alkaline solution in the distillation step of the procedure. The method does not determine any uncombined nitrogen, if such be present. Vacuum Fusion Methods of Analysis Methods for extracting the gases from metals by heating and melting under reduced pressures have been developed
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I N D U S T R I A L A N D ENGA'NEERIXG C H E M I S T R Y
and used by many investigators. A rather complete review of such methods as applied to ferrous alloys has been published by Alleman and D a r l i n g t ~ n . ~ Three types of vacuum fusion methods have been used by various investigators-namely, fusion of the metal ( a ) in a refractory oxide crucible,4 (b) in an oxide or silicate crucible with an antimony-tin alloy,5 and ( c ) in a gas-free graphite crucible.6 A study of these three methods has been made a t the Bureau of Standards. From this study there has been developed a method of the third type, which is more generally applicable and more accurate than previous methods for the determination of total oxygen in iron and steel. The method also determines hydrogen. A similar method for the determination of nitrogen by vacuum fusion is now in course of development and will be reported later. In order to extract gases completely from a metal in vacuum it is necessary to keep the sample molten for some time. In the case of the higher melting metals this requirement has introduced difficulty in securing a vacuum chamber which remains impervious to gases a t elevated temperature. Heating the sample, in the form of a bar or wire, by its electrical resistance' has been employed. The walls of the vacuum chamber may thus be kept considerably cooler than the sample, but the heating necessarily ceases with the fusion of any portion of the sample. Goerens and Paquet, by the addition of an antimony-tin alloy to iron or steel, were able to melt their samples at temperatures which did not cause fused silica vacuum tubes to become porous.
Vol. 18, No. 3
melting and temperature readings with an optical pyrometer are made through this glass cap. G is a crucible of sintered magnesia, which serves as the crucible for holding the molten sample in direct melting or in melting with antimony and tin, and as a heat insulator surrounding the Acheson graphite crucible, F , when melting the sample in graphite. Gravimetric Determination of Gases
In many previous methods for the analysis of gases from metals fused in vacuum the gases evolved have been collected and their composition determined by volumetric methods. The amounts of oxygen and hydrogen present in most cases, however, are not large. It seemed desirable to attempt to avoid volumetric methods and to absorb the gases in solid reagents and determine the gases gravimetrically. The train used for the absorption of the gases in the present work is shown diagrammatically in Figure 2. The details of the special form of absorption tubes are given in Figure 3. Tubes A and C (Figure 2 ) are filled with phosphorus pentoxide with plugs of glass wool a t each end. These tubes absorb, respectively, the water vapor originally present in the gases coming from the silica tube and the water formed by the oxidation of free hydrogen by copper oxide in the furnace E. This furnace operates a t approximately 300" C. Tubes B and D contain ascarite, protected from loss of moisture by phosphorus pentoxide at each end of the tube. Tubes B and D absorb, respectively, the carbon dioxide originally present in the gases pumped from the vacuum furnace and that formed by oxidation of carbon monoxide in the copper oxide furnace, E. Fusion of Samples by High-Frequency Induction As based on the amounts of water and carbon dioxide collected by tubes A , B, C, and D in the fusion of metals I n the present investigation the samples have been heated containing carbon or of samples in graphite crucibles, conby means of high-frequency induction. In this manner it clusions regarding the gases originally present in the metal is possible to heat a are limited to the calculation of the equivalent oxygen metal sample contained and hydrogen. No distinction can be made between the in a refractory oxide or oxygen of metallic oxides and of oxides of carbon, or between a g r a p h i t e c r u c i b l e carbon monoxide and carbon dioxide. Some distinction A within a fused-s i 1i c a between oxygen present as metallic oxides and that present v a c u u m c h a m b e r to as carbon monoxide or carbon dioxide may be possible in temperatures of 2000 O the case of carbon-free metals. I n such a case the excess C. and higher, while the of oxygen obtained by melting in graphite over that obtained walls of the silica tube by melting in a refractory oxide should represent oxides remain cool enough in the metal reduced by carbon. t o m a i n t a i n a good The by-pass, N , provides for the evacuation of the silica vacuum. tube a t the beginning of an analysis without pumping the The inductor coil (D, air it contains through the absorption tubes. Tube I , filled Figure 1) of the high- with phosphorus pentoxide, is a guard tube to prevent diffusion frequency furnace used of moisture from the vacuum pump when little or no gas is about 6 inches long is flowing through the last tube of the train, D. The by-pass by 2 inches inside diam- M is used whenever there is any flow of gases through stopeter. This coil s u r - cocks G and H towards the vacuum pump, since there is rounds the closed end then no danger of backward diffusion of moisture. of a fused-silica tube, The efficiency of this train in absorbing water and carbon C, 24 inches long by dioxide, and in oxidizing and absorbing hydrogen and carbon 1.5 inches inside diam- monoxide, a t low pressures (below 15 mm. of mercury) D eter. The open top of was tested by passing known volumes of hydrogen, carbon Figure 1-HiBh-Frequency V a c u u m Fur- this tube is sealed and nace for Melting M e t a l S a m p l e s monoxide, and mixtures of the two through the train a t connected to the gas- low pressures and determining the recovery by the increase absorption train by means of a tubulated Pyrex glass cap and in weight of the absorption tubes. de Khotinsky cement.8 Observation of the specimen during Five tests each were made with hydrogen and with carbon 4 J. Franklin I n s f . , 185, 161, 337, 461 (1918). monoxide. The average recovery of the hydrogen was Goerens and Paquet, Ferrum, 12, 57, 73 (1918). 99.5 per cent and of the carbon monoxide, 100.0 per cent. 6 Walker and Patrick, Original Corn., 8th Inlern. Cong. A p p l . Chem., The composition of a mixture of hydrogen and carbon mon21, 139 (1912). oxide, shown by standard gas analysis methods to contain 1 Austin, J . Iron Steel Inst., 86, 11, 236 (1912); Ryder, Trons. Am. Electrochem. Soc., 33, 197 (1918). 61.7 per cent H Pand 37.0 per cent GO, was determined by 8 The water-cooled, screw-top, metal cap shown in Figure 1 was used this absorption train as 61.8 per cent Hz and 37.4 per cent with larger diameter or shorter silica tubes. A more complete description CO. The train thus was proved quite suitable for the use of the development and tests of the vacuum furnace and absorption train desired. is given in Bur. Standards, Sci. PaPer 614.
March, 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-+cz===-c a r
Figure 2-Train
-
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.N-
1
A
for Absorption o f Oxygen a n d Hydrogen Evolved f r o m M e t a l s Fused i n V a c u u m
Three Methods of Fusing Ferrous Alloys
Comparative tests of the three types of vacuum fusion methods were made, using the vacuum furnace and the absorption train described. IN REFR.4CTORY OXIDE CRUCIBLES-Difficulty iS FUSIOK t o be anticipated in the fusion of steels and cast irons in refractory oxide crucibles on account of the possible reaction between the carbon of the ferrous alloys and the refractory oxides of the crucibles yielding oxygen (as carbon monoxide) which was not present in the metal sample. Tests were made of the suitability of a variety of refractory oxide crucibles-namely. magnesia-zircon, cbmmercial electrically sintered magnesia, silictl-free magnesia, pure aluminium oxide, alundum, zircon, and zirconia-alumina crucibles. Only the zirconia-10 per cent alumina crucibles proved at all suitable for the fusion of a 0.25 per cent carbon steel in vacuum without excessive evolution of carbon monoxide from reduction of the crucible oxides. When used with a higher (0.7 per cent) carbon steel, the reduction of the oxides of this type of crucible also became excessive. Determinations of oxygen and hydrogen were made on samples of approximately 100 grams each of electrolytic iron (previously fused), a 0.25 per cent carbon steel, and a 0.72 per cent carbon steel. These values are given in Table I. The value for electrolytic iron is the average of two, and for the lowcarbon steel the average of three determinations. These results, and all others here reported, have been corrected by carefully determined blanks in the furnace and absorption train.
All analyses reported in Table I were made on samples of metal as one or two solid pieces with carefully ground and cleaned surfaces. Samples as millings or drillings, though very carefully cut in air or even under oil, are liable to give results for oxygen that are greatly in error, probably on account of oxidation during cutting, or adsorption of air after cleaning. FUSION WITH ASTIMOSY-TIS-4LLoY-Analyses of samples of the same three metals were next made by the antimonytin fusion method. Samples of approximately 30 grams each were fused with twice their weight of a gas-free alloy of equal weights of antimony and tin. The values in Table I for electrolytic iron by this method are the average of two determinations, and those for the lowcarbon steel the average of four results. A single determination was made on the high-carbon steel. FUSIOX IS GRAPHITECRucrBms-Samples of the same metals were finally fused in gas-free acheson graphite crucibles. The samples weighed about 30 grams each. The values in Table I are again the average of several determinations except for the high-carbon steel. Oxides Determined by Fusion of Ferrous Alloys in Graphite. The results for oxygen in the electrolytic iron and the low-carbon steel are considerably higher by fusion in graphite than by either of the other two methods. This indicates a more complete reduction of oxides in the samples. A--
i 4.7cm-
m.
Table I-ComDarison
of Three M e t h o d s of V a c u u m Furion Analysis Electrolytic Low-Carbon High-Carbon Iron Steel, 0.25'3 C Steel. 0.72% C % Oz %Hz %Oz '3 Hz '70 Oz '3 Hz
METHOD Fusion in zirconiaalumina 0.0092 0.0013 0.0093 0.0015 0 021' 0.0009" Fusion with,Sb-Sn in magnesia 0 . 0 0 ~ o.0017 ~ 0.0105 O.OOIO o 042 o.oois Fusioningraphite 0.0163 0,0011 0.0208 0 . 0 0 1 5 0 0027 0.0003 a The extraction of gas was not carried to completion in this test as the very high pressure developed in the vacuum furnace indicated rapid reaction between the molten steel and the oxides of the crucible
Figure 3-Design
of Absorption T u b e s
Walker and Patrick stated that their method of fusion with graphite determined nearly all the oxygen present as Fe203, &03, and Si02. Tests of the reduction of these oxides
IYDUSTRIAL AND ENGINEERING CHEMISTRY
282
were repeated in the present investigation, and in addition tests were made with manganous carbonate (used as a convenient substitute for manganous oxide), titanium oxide, and zirconium oxide. It was found possible to obtain practically complete reduction of all these oxides by fusion a t 1450’ to 1500’ C. with gas-free, carbon-saturated iron. Oxygen present in iron or steel as any of these oxides will, therefore, apparently be obtained by this third method of fusion. Agreement of Analyses. Table I1 shows the agreement obtained in analyses by fusion in graphite of thirteen irons and steels. Check analyses do not always agree. Such lack of agreement seems to be characteristic of certain irons and steels rather than to be due to any unreliability in the method of analysis. This is not surprising, inasmuch as the samples used for analysis are single solid pieces of metal which may be expected to show any nonuniform distribution of oxygen existing in the metal. The lack of agreement in check analyses of certain steels and irons is plainly shown in Samples 2, 12, and 13. The excellent agreement of check analyses is shown in Samples 1, 5, 6,9, 10, and 11. In this table results for oxygen and hydrogen are given to the fourth decimal in order to show more clearly the magnitude of the usual variations in analyses of the same sample. The analytical results by this method, however, are more properly reported to not more than the third decimal or more than two significant figures for oxygen and to not more than the fourth decimal or more than one significant figure for hydrogen; values under 0.001 per cent oxygen or 0.0001 per cent hydrogen are best reported as “less than” the values mentioned, of Analyses f o r Different I r o n s and Steels Oxygen Hydrogen Per cent Per cent MATERIAL 0.0096 0.0002 Steel 0.0096 0.0003 0.0105 0.0031 Steel 0.0160 0.0015 0.0277 0.0016 0.0178 0.0015 0.0087 0.0003 Welding steel 0.0081 0.0013 0.0330 0.0003 Carburizing steel 0.0310 0.0001 0.0162 9.0010 Fused electrolytic iron 0.0163 0.0011 0.0185 0.0015 Fused electrolytic iron 0.0183 0.0018 0.1410 0.0018 Oxidized iron 0.1400 0.0021 0.0015 0.1270 0.0742 0.0005 Oxidized iron 0.0812 0.0013 0.0695 0.0001 0.0003 0.0700 0.0844 0.0002 0.0812 0.0002 0.0029 0.0004 Cast iron 0.0027 0.0005 0.009s 0.0008 Cast iron 0.0009 0.0098 0.0135 0.0001 Cast iron 0.0135 0.0001 0.0103 0.0007 Cast iron 0.0043 0.0009 0.0090 0.0012 0.0010 0.0011 Cast iron 0.0079 0.0004 0.0034 0.0001 0.0012 0.0129
T a b l e 11-Agreement Sample
1 2
3 4
5 6 7 8
9 10
11 12 13
COMPARISON OF RESULTS--A comparison of the analytical results summarized in Table I shows that in the case of the electrolytic iron and low-carbon steel, graphite fusion determines the total oxygen more completely than the other two methods. In the direct fusion of low-carbon metal there is not present sufficient carbon to bring about the reduction of oxides of manganese, silicon, aluminium, etc., which are reduced by fusion in graphite in the presence of iron. On the other hand, even with low-carbon steels and
Vol. 18, No. 3
electrolytic iron containing a few hundredths of one per cent carbon, an appreciable decarburization is caused by fusing the metal in refractory oxides. This decarburization is accompanied by the production of carbon monoxide representing oxygen from the crucible refractory rather than from the metal. This same reaction with the crucible refractory is especially marked in the direct fusion of high-carbon steel. Even an incomplete analysis by direct fusion indicated an oxygen content of a high-carbon steel as 0.021 per cent, while fusion of the same steel in graphite showed only 0.0027 per cent oxygen as actually present in the steel. Direct fusion i n refractory oxide containers is therefore not satisfactory for the determination of oxygen in either low-carbon or high-carbon ferrous alloys. Fusion of electrolytic iron, low-carbon and high-carbon steels with antimony-tin gives results practically the same as those given by direct fusion, and for the same reasons. For pure iron and low-carbon steel the results are lower than the true values given by fusion in graphite because of the lack of sufficient carbon to reduce the oxides of the metal, whereas for high-carbon steel the oxygen is higher than the correct value owing to reactions with the crucible refractory. Practically the same value for hydrogen is given by all three methods with pure iron and low-carbon steel. Fusion of the high-carbon steel with antimony-tin did not give quite the same value for hydrogen as fusion in graphite. There is, however, no obvious choice of methods based on the values obtained for hydrogen. Fusion in graphite is the most convenient of the three methods of melting the samples in vacuum by high-frequency induction, and allows the widest range in the amount and the form of the sample. The heating takes place in the graphite crucible and is uniform and easily controlled regardless of the size, shape, or closeness of packing of the metal sample-three factors which necessarily often vary and introduce difficulties in the uniform heating and complete fusion of metal samples by direct induction in the metal, as is required in direct fusion or fusion with antimony-tin. Acknowledgment
The authors wish to acknowledge the assistance of several of their associates in the Division of Metallurgy of the Bureau of Standards. They are especially indebted to J. R. Cain for aid in the initiation and earlier portions of this work, to W. P. Barrows for many test analyses of the method a s finally developed, and to R. J. Kranauer for faithful and painstaking aid in the preparation of samples, crucibles, and insulators, and in the care of absorption tubes.
Flue Gas Stops Dust Explosions Engineers of the Department of Agriculture have erected a feed-grinding mill a t t h e experimental farm, Arlington, Va., to demonstrate the practicability of using inert gas from boiler flues for preventing dust explosions. A regular feed-grinding unit for grinding oat hulls has been installed, and a pipe line from the power house brings in the flue gas. Although this mill is equipped for grinding oat hulls for stock feed, the principles involved are applicable t o mills t h a t grind many other materials. The department is anxious t h a t mill owners, operators, and others interested in dust-explosion prevention visit t h e experimental mill and observe some of the tests, both with and without the inert gas present in the system. I n this way operators will have a n opportunity t o observe the efficiency of inert gas as a means of preventing explosions in grinding equipment and t o study the possibility of using inert gas in their own plants. Arrangements may be made t o observe the tests, or information on any particular phase of the work may be obtained by writing t o the Bureau of Chemistry, Washington, D. C.
