High-Temperature Resistance-Type Electric Laboratory Furnaces WALLACE hl. HAZEL
AND Whf.
J . O'LEARY, Norton Company, Chippawa, Ontario, Canada
V
in laboratories d i e r e speed and volume of work are prime factors, to require further mention. An important reason for using elect'ric furnaces in these laboratories is t o minimize deterioration of platinumware at elevated temperatures. The authors are constantly fusing refractory materials t h a t cont'ain small amounts of iron, ferrosilicon, carbides, free carbon, and iron oxides; in their hands, when such fusions are made with alkaline fluxes over gas-fired burners, the atmosphere surrounding the crucibles is often sufficiently reducing t o cause alloying of part of the iron with the platinum (6), with consequent erratic results in the iron determination and damage t o the crucibles. This situation can be improved by prelining the crucibles with flux, but it is complet'ely eliminated when fusions are conducted in a furnace whose atmosphere is subject t o accurate control. A loosely covered furnace like the types described is fully oxidizing. The conventional externally ivound furnace for carbon combustions has also been subjected t o criticism since the advent of the more refractory high-carbon alloys and cemented carbides. Some analysts (9,12) dispense entirely with the core, and wind their combustion tube directly with resistance wire in order t o obtain sufficient temperature to burn such alloys; t o hold the element firmly in place, this practice requires t h a t the combustion tube and winding be coated with a refractory cement. This procedure is satisfactory a t intermediate temperatures, provided the resistance wire does not' melt any constituent in the cement and cause fluxing; the best grades of cements, however, contain very little low-melting constituents. When silica, glazed porcelain, or siliceous clay tubes are wound direct'ly with Kanthal wire, failure is likely to occur above 1100" C.; the manufacturers of this resistance wire advise against having it in cont'act with free silica, low-melting silicates, phosphates, or ferric oxide, because these react with and flux the wire (IO). The analysis of refractory materials in these laboratories requires high-temperature fusions, ignitions, and combustions; in order to make more efficient use of t'he various resistance wires available on the market the authors have adopted the furnace construct'ion described below as applied to a crucible furnace.
ISITORH t o tlie Reseal ch and Chemical Laboratories of this company have displayed so much interest in the practical operation of some small electric furnaces t h a t it seeins worth while t o describe these in some detail; their simplicity and efficieiicy will undoubtedly appeal to chemists and engineers who are not fully satisfied with the performance of the conventional types of laboratory furnaces. Nearly everybody who has used a n electric furnace is familiar v i t h the usual method of external winding. A refractory core is wound on t h e outside with a length of resistance wire calculated t o raise the furnace t o a predetermined temperature; this winding is held in place by a refractory cement which i n turn is surrounded by insulation. However, because of t h e thermal insulating properties of such refractory cores there is a n appreciable drop in temperature between the resistance wire and t h e center of the furnace formed by the core; i t is not uncommon with this type of winding t o find the center of the furnace 200" t o 300" C. cooler t h a n the resistance wire, across an air gap of only 3 inches. Shortening the winding in order to raise the furnace temperature may result in burning out the wire; if the wire does not melt it is liable t o fuse some ingredient i n t h e retaining cement, in the insulation, or i n the core, which ultimately fluxes the wire and causes a burnout. Consequently, unless operated a t moderate temperatures (700" t o 900" C.) , externally wound furnaces may have poor service records. On the other hand there is a definite need in some laboratories for furnaces t h a t will operate serviceably a t more elevated temperatures, from 1100" t o 1500" C. and higher. For instance, the ignition of precipitated alumina or silica to constant weight cannot ordinarily be accomplished under 1250" C. (1,6,11); i n the better texts on analytical chemistry a blast lamp is usually recommended for this purpose instead of standard types of electric furnaces. The inconvenience and disadvantages of blast lamps are too well known, particularly
Core The use of cement to hold elements in place is usually disadvantageous; the cores, shown in Figure 1, were therefore made to support the element a t all points to prevent sagging. The resistance wire is wound on the inside of the core and is left exposed, so as to obtain maximum heat transfer to the material that is being heated. Laboratory muffle and tube furnaces of various manufacturers embodying this construction have been on the market for some time, but in the authors' hands they have needed frequent repair because of the necessarily high operating temperatures. The core is made of Alundum; this contains no bond constituents that might melt, react with the heating element, or distill out and contaminate crucibles or samples. This core is also made in two pieces; both pieces can be wound continuously with a single length of wire, the two halves can be wound separately and then connected in series, or with suitable current adjustments the two halves can be connected in parallel. Separate winding of both halves has the doubtful advantage that a single defective unit can be removed for repair; the authors have, however, preferred to wind both halves together with a single length of wire, because failure of one unit in a series after normal service is usually followed rapidly by failure of the others due to progressive oxidation, recrystallization, and the like.
FIGURE 1. IKTERNALLY ~TOUN ALUSDUN D CORE
109
IhDUSTRl21, 2 h D ESGINEEKIAG CHEhlISTRf
108
TABLE
A.
Iv.
EXPERIVEKT.tL
RESULTS
Degree of Vulcanization of Rubber Sulfur k
h
kh 1 24 1.14
1.00 0.i9
B
I l i r t u r e s of Rubber a n d Ileoprene NeoRubber prene k
+
Rubber Xeoprene 100 Sulfur 3 Zinc oxide 3 Ljght rnagncsiuni carbonate 5 gtearic acid 2 Nonox 1 Mercaptobenzothiazole 1 C
+
33
33 0
67 100
'
j
100 , 90 ; 75 60 , 25 ~
5s
0.40 0 26
no
1 .OO 0.52
0.86
0.34 0 23
1
n o
n
10 ''5 30 i.5
1.00 0 83 o 70 0.54 0.32
89 89
1, 1
oo
0 Q5 0.91 0.80 0.69
Ruhhei C o m p o u n d e d n i t h 3Iineral H a r r t e s k h Barytes
I00 Rubber Sulfur Zinc o u d ? Stearic acid 2 Nonox 1 Xlercapt obenzuttiiazole 1
:
E Kubbei Sulfur Zinc oside Stearic acid
I 00
o
Mixtures of Rubber a n d Perbuiiaii PerbuRubber nan h
Rubber Perbunan 100 Sulfur a Zinc oxide 3 Stearic acid 2 Nonox I .\lercaptobenzothiazolr 1 D
o
1 10067
kh
h
0 ,
+!I 150
250
1 00 0.91 0.81 0.80
Rubber ('urnpounded with Diatomite Diatomite k
100 3
(I .i ) 2 0 2
F.
hh
Summary and Conclusions
h
/.I,
1.0u
1 00
0.83 0.81
0 8.5 0 74 0 63
1.0u 0 71 0 60 0 4; 0 34 0 2ii
I.
I>
vti
0.il
Iiorosealb
1 .O(I
tioroseal a b
..
0.16
0.42
If .4 = 100 sq, rni,, L = 0.1 I'III., ( p . - 711) = T6 rni. of mercury, and t = 1 hour, then Q = -0.29 r r , a t normal temperature and pressure. The negative ate< that the gas passe from the high- to the Ion--pre
0.79 0.63 0.43 0 22
/: h
i.no
0.iO
j
The product k h , 1.46 X loF6 X 7.15 X = 1.03 X is directly proportional to the permeability of a standard sheet under standard conditions. For a comparison of the permeability of the material of this test piece to nitrogen xith that of a second material, this value is referred directly to the corresponding value for the second material. If it is desired to interpret the result directly in terms of permeahilit!-, the values of kh and of the chosen conditions may h r inswted in F:quation 3 :
Tahle IT aiitl Figiii,es 18 tcl I T give t u i ~ h e rexperimental result? ol~tainedwith the iamc appii'atii\ iititlei, similar condi tion::. I n these experiments, tlie ga. usetl was commercial nitrogen, tlie temperature was 30" C., antl the trqt pieces n-ere unco.i-ered square roils 0.4 x 0.4 x 15.0 cin. In the mixes t'he proportions are given as parts hy weight. Yulcanization was 1 hoiir a t 150" c'. The results are given as relative valiiea.
1 00 0.86 0.66 0 31i
1 00 0.94 0.82
VOI,. 12. NO. 2
0 0;
Coinmercial grade G. Goodrich plasticized polyvinyl chloride.
The rate at which a gas passe. throupli a sheet of ruhherlike niat,erial and the rate a t n-hich a gas is ahmrhed 11y a Iilock of tlie material are l)ot,li rlepetitlrnt upon the same fa(*tors, solubility and tliffusioii constant. The niagnit,ude of these factors can I)e measiiretl 1)y aiworption esperiments carried out under specifietl contiitions and the results can ess the perniealility of tlie material in sheet form. Experimental metliod. antl sui1 ahle apparatus are r1escril)eil for following absorption atid t he maniier of interpreting tlie results iii terms of pernieatdity is given. Resides such advantages as accurate tmiperatiirct cont,rol. convenient size arid ease of handling of the apparatits, t,he m a l l size of the test piece. anti t'lie elimination of the difficulty of producing iiiiiforni thin sheets free froin ~~inlioles, the inetliotl lias the advantage t h a t I)ot,Ii factors. wlitl~ility and dilfusion constant, caii lie awessecl ititlepeiitlently; this is not normally possible lvit,li cliiwt periiieatioii mensrirement~. Some experimental results are talxilateti.
.4cknowledgmenI. The other experimental details were
Po p
2 . 2 cm. ot Iiiercury 205.7 cm. of mercury (barometric pressui'e, it;,?:e's( pressure, 129.5) r0 = 2.40 em. (scale divisions \yere centimeters) r , = 9.14 cm. (scale division- n-ere centimeters) rr = 3.00 c m (scale divisions were centimeters) T' = 2.80 cc. b = 6.23 X I O F 3 < < I , ~ I I I . I', = 2.34 cv. o = 0.40 ~ 1 1 1 . ( = 30.0 * 0.1 C ' . = =
These values, inserted i n Equation 15. give h = 7.15 X IO-', where concentration is measured in cubic centimeters of gas at normal temperature and pressure per rubic centimeter, :ind pas pressure i.5 measured in centimeters of mercury. The averagr value of u is used to calculate the diffusion constant from the relationship k = sq. cni. per second, where 4.63 x 10-3 ?)O( p = 0,514, Substituting these values, k = 0.514 sq. cm. per second = 1.46 X eq. cm. per second. Correction may be made for small variations in the dimensions of the test pieces by substituting for the value of u2 the true cross-sectional area. For example, if the cross-sectional dimensions of the test piece had been 0.395 and 0.403 cm., the value 0.39.5 X 0.403 n-ould have been naed in place of (0.4)*.
I: (Y
Grateful acknon-ledgment is made to the l)unlop Rublwr Co.. Ltd., for periiiissiori t o pulhlish this inrre.;tigation.
Literature Cited (1) Dayilea, H. .I., Proc. Roy. .Yoc., A97, 286 1:1920). Kaq-ser, Ann. P h y s i k , 43, 544 (1891). Morgan, L. B., and Naiintrin, \Y.J . 5 . . P J V CRubber . Tech. Conf., p. 599, London, 1938. (4) Morris and Street, IKD. Eso. ( ' H E M . , 21, 1215 (1929). (5) Reychler, J . chim. p h y s . , 8, 617 (1910). CHEM.,30,409 (1938). (6) Taylor, R. L., and Kemp, A . R (7) Yenable and Fuwa. [ h i d . , 14, (2) (3)
.
PREaENrED before t h e Division o f Rubber Ctieiiii.tri. a t the 98th Meeting of t h e American Chemical Society, Boston, \Iaqa.
