64
Ind. Eng. Chem. Prod. Res. Dev , Vol 18, No. 1, 1979
Foamed Glass and Vitreous Silica Pellets Giffin D. Jones,' W. J. McMillan, and C. N. Williams Central Research, Physical Research Laboratory, The Dow Chemical Company, Midland, Michigan 48640
Fused white pellets of glass or vitreous silica containing entrapped gas were prepared by melting and cooling the powdered glass or silica with or without small amounts of lampblack and coloring oxide in a pressure furnace at 300-500 psi. The pellets were foamed in a mold in an oven at atmospheric pressure. They had a shelf life of years.
Extensive patent literature teaches the foaming of glass by mixing ground glass with carbon, or a carbon compound which chars on heating, along with an oxidizer such as calcium sulfate and then heating in an oven (Baker, 1938; Ford, 1950, 1951; Long, 1934, 1943; Lytle, 1943). The mixture first sinters and then foams. The foam is cooled and annealed over a period of hours. The entrapped gas is mainly carbon dioxide together with some hydrogen sulfide. Calcium carbonate has been used as a blowing agent (Fox and Lytle, 1944). The use of water as a blowing agent has been taught (Fowler and Otis, 1938; Lytle, 1941; Willis, 1941). Sir William Ramsay carried out an early experiment in which silica was heated under pressure in the presence of water and a pumice-like product was obtained. It was known how to make foamed glass in a closed mold (Dewey, 1942) and foaming around a cooled metal insert was taught early (Black, 1942). It was new, however, to employ fused pellets of glass or vitreous silica with entrapped blowing agent and to foam these in a closed mold to give a white or brightly colored foam which precisely reproduces the shape of the mold, has a skin, and has fine closed cells (McMillan and Jones, 1969). Powdered silica was melted in graphite crucibles under pressure of an inert gas in a furnace having internal carbon resistors. The mass was cooled under pressure, crushed, and some of the particles reheated at atmospheric pressure in an open crucible. The results (Table I) showed that the use of finer particles of silica gave a slightly lower foam density, although still a high density in these experiments. The foams were of closed cell structure and the cell size was less than 0.1 mm in diameter. Larger cells were obtained if coarse particles of silica were used. Lower density foam was desired; therefore lampblack was mixed with the powdered silica. When 0.1% or less was used, the carbon was consumed and white foam was obtained. The use of larger amounts of carbon made the foam gray or black and gave coarse cells. (See Figure 1). The use of pressures greater than 800 psi during melting did not give much improvement in expansion during foaming a t atmospheric pressure. If the pressure in the furnace was as low as 200 psi some foaming occurred during melting. The temperature required to melt the silica was 1750 "C but foaming at atmospheric pressure began at a somewhat lower temperature (as indicated in Table I) and the foam collapsed if heated to 1725 "C. If larger amounts of carbon were used the foam collapsed at a lower temperature, for example, a t 1700 "C with 0.2% lampblack. Of course, a brief exposure did not bring the foam to full oven temperature. Ground quartz glass was substituted for crystalline powdered silica and fused with 0.0170 lampblack. In this 0019-7890/79/1218-0064$01 OO/O
case the crushed melt began to foam at 1635 "C and collapsed a t 1685 "C. Vycor (Corning Glass Co.), a glass containing 96% silica, was ground, mixed with 0.1 % lampblack, and fused in the pressure furnace. The crushed melt was foamed by heating at 1650 "C for 6 min. The foam had a density of 18.4 lb ft and was gray in color. The foam had large cells (0.2-3.4 mm) and was in large part (51%) open-celled (yet it had good abrasion resistance, 53L, of Table IV). Pyrex was ground, mixed with 0.1% lampblack, and fused in the pressure furnace. The crushed melt was foamed a t 960 " C , which was a little high. The foam was large celled and black. A borosilicate glass having a softening point of 819 "C was ground to pass a 325 mesh screen and was subsequently melted in the pressure furnace a t 1200 "C under 500 psi of carbon dioxide pressure for 1 h. The resulting unfoamed mass was heated at atmospheric pressure to 965 "C for 5 min to form a white foam of cell size 0.1-0.8 mm and a density of 8 Ib ft 3. The glass composition was 80% SO2,1.8% A1,0,, 13.0% B203,4.3% Na20,and 0.4% KzO. The preferred method of pressurizing borosilicate glass was to use ground glass containing 0.5% water pressurized under nitrogen (30Ck500 psi) at 1300 "C. The cooled mass was crushed and foamed at 830 "C. It would foam as low as 750 "C but foaming at this temperature required 1 h. The foam was annealed at 550 "C. Densities of about 5 lb ft-? were readily obtained by free foaming (Figure 2). A ring phenomenon was observed in silica foams. Three or four zones were recognizable as shown in Figure 3. The core was white and large celled. It was surrounded by a more fine-celled zone (dark) and then a fine-celled white skin. The white skin is presumably depleted in carbon and silicon monoxide by atmospheric oxidation in the foaming oven. The core may foam under reduced pressure because of the expansion of the outer portions which become hot more quickly in the foaming oven. It was possible to increase expansion by heating at below atmospheric pressure but this technique was not generally used. When a piece of the crushed pressurized fused silica was "free-foamed" it expanded as a replica of the unexpanded piece, maintaining the sharp edges. Other foaming agents were tested (cf. Figure 4 and Table 11). The substitution of graphite for lampblack gave a black foam, perhaps because the graphite was not as finely divided, Silicon monoxide was added to powdered silica and fused in the pressure furnace. The crushed melt containing 1% silicon monoxide gave a gray foam at 1745 "C. The density was 18 lb ft-3. The use of a large amount of silicon monoxide did not give a low density foam. When the amount of silicon moiloxide was reduced to 0.2%, foaniing was negligible. Likewise, the use of silicon powder c' 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
65
Table I. Silica Pressurized under Nitrogen and Foamed a t Atmospheric Pressure pressurization pressure, psi
material Supersil,= pre-dried Supersil Supersil, 0.1% lampblack Supersil ground vitreous silica, - 325 mesh 100-325 mesh 50-100 mesh 20-50 mesh
temp, "C
lOOOb
750 500
foaming
time above 1650 "C, min
temp, "C
time, min
density , Ib f t - 3
1707
70
14 5 0 -. 1600
35
30.6
>1700 1720
-120
105
1425-1710 1465-1685
35 43
34.3 33.4
60 67
1475-1700 1400-1 650
30
30.6 30.7
1760 1750
lOOOb
1000
24
32.1 37.8
a Pennsylvania Pulverizing Co., crystalline silica, 93% - 325 mesh, 0.07% A1,0,, 0.04% CaO, 0.0005% MgO, 0.03% Argon instead of nitrogen. Fe,O,, 0.013% K,O, 0.008% TiO,, 0.002% PbO,.
Table 11. Foaming Supersil with Silicon at Atmospheric Pressure Si, % 0.2 0.2 0.2
Trn,,! "C
time, min
1760 1725 1650
16 30 60
cell size, mm 0.5 0.54
density, lbift'
abrasion, s
compr. str, psi ~
16.0 13M 18.6 6M -did not foam appreciably-
.
_
_
306 165
Top cells elongated. POOOt 180 5 1600'
:
140
0
-IO
Density,
/
0
0'
i
I5
PO Density
l b s l fl'
L._-Densi+y,
1
IO
Ibr I f+'
Figure 2. Compressive strength of foamed borosilicate glass vs. density for samples foamed in a mold (left) and not in a mold (right).
O0
5
1
15
25
in
30
35
lbt / f t '
Figure 1. Compressive and abrasion strength of foamed silica samples vs. density (see Table IV for correlation of sample size with abrasion result).
gave a gray foam. In one experiment with 0.5% silicon, a large celled foam was obtained at 1630 "C. With 1.0% silicon, the foam began to shrink at 1630 "C. With 0.1% silicon, foaming began at 1645 "C. Evidently, silicon monoxide was produced and aided melting of the quartz. The use of carbon dioxide as the pressurizing gas corroded the carbon resistors in the furnace; nevertheless, silica powder was pressured with 500 psi carbon dioxide a t 1750 "C for 1 h. Upon release of the pressure a foam of 36 lb ft-3 formed. It had a broad range of cell sizes (0.1-1.5 mm) and was 12% open celled.
Water was introduced into the melting furnace as sodium tetraborate pentahydrate. Foam densities down to 5 lb ft-3 were obtained by the use of 4% water. With this coinposition the compressive strength of the foam was diminished if the temperature in the pressure furnace exceeded 1300 "C. An experiment was carried out in which pressurized fused silica containing 0.1% lampblack at 1750 "C was forced to flow through an orifice into a collector at initially the same pressure as the furnace (750 psi). The orifice frangible was opened by release of the pressure in the collector. Uniformly fine-celled foamed rod 13 in. long was obtained, 318 to 518 in. in diameter with a density of 36 lb ft-3 (Figure 5). Molding Foamable pressurized granules were molded precisely in molds made of separate sides which mated but were releasable (hlcMillan, 1966). For the high temperature
_
66
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1. 1979
Table IV. Comparison of Abrasion Results with Different Size Samples density, Ih/ft' 25 23.6 23.6
Figure 3. Alternating zones of fine cells and coarse cells in foamed samples made using small amounts of lampblack as a source of blowing agent. The foaming was done without a mold at the indicated temperatures.
4846-13 4846-21 4846-21
Novacite Novacite Quartz glass
0.01% lampblack 0.05% lampblack
0.01%lampblack
1705 "C 1720 "C 1675 "C
Fe203,and 0.015% TiOz. Table 111. Compressive Strength (psi) of Glass and Silica Foam density 1 0 Ih ft->
20 Ih ft-'
Pyrex silica
Molded Foam 45 60
200 350
Pyrex silica
Free Foamed Blocks 90 90
680 340
required with silica beads, the mold was constructed of graphite and coated with silicon carbide as a release agent. Stainless steel molds were used with pressurized borosilicate glass which was molded a t 830 "C and annealed at 550 "C (Figure 6). Foamed silica does not require annealing and had a higher strength than annealed foamed borosilicate glass of the same density (Tables 111 and VI). In order to illustrate the capability of reproducing a curved surface, the mold for the object shown in Figures 7 and 8 was employed. A pipe mold was used also and the foam tested as insulation for high-pressure steam pipe. It was satisfactory in 2-ft lengths hut handling longer lengths tended to cause breakage. Some foam was colored blue by the addition of cobalt oxide in the pressure melting furnace. Abrasion Test Friability is the main drawback of foamed glass and we developed an abrasion test in an attempt to measure this property. In this test emery paper and the specimen were rotated against each other and in the same direction with nearly the same angular velocity (ratio of 51:49, respectively), about parallel axes 1in. apart. The abrader and shaft which rested on the sample weighed 2080 g and both shafts rotated on hall bearings. The sample was held in a holder with the aid of suction which also removed dust
sample size, in. (dia.) I/*
grinder speed, rpm 50
(w.)
50
(sq.)
20
time, s
39 9 9
code for test
L M S
particles, and an air jet was used on the abrading disk to keep it clean. A fresh abrading cloth was used in each measurement. For larger samples, the abrading cloth was 80 grit Norton Durobonded Lightning Metalite cloth, and for smaller samples Norton Durobonded Fine Emery cloth was used. For the large samples (1.25 in. diameter X 1in. long) the average speed of the wheel and sample was 50 rpm and the time, in seconds, required for abrading a depth of 0.5 in. was reported. For the medium sized samples (0.5 in. X 0.5 in.) the same speed was used and the time for abrading a depth of 0.25 in. was reported. For the small samples (0.25 in. X 0.625 in.) (cut and filed), the speed was 20 rpm (cf. Tahle IV). Whether or not the test result depended upon cell size was not determined. Foamsil (foamed silica) (Corning Glass Co.) which has large cells (>1mm), had an abrasion value of 22 L and 2.5 M. The sample was gray and had a density of 12-13 lb ft-?. PC Foamglas (Pittsburgh Corning) (9-11 lb ft-?) was too brittle to test. The Foamsil sample had 18% open cells and the PC Foamglas, 9%. The Foamsil cell size was 1-2 mm; that of the PC Foamglas was 0.2-0.5 mm. A particular sample of our foamed silica was used to calibrate the abrasion test for different size samples (Table IV). It was made by melting at 1750 "C under 300 psi nitrogen pressure a mixture of Supersil with 0.1% lampblack and then cooling to a crucible temperature of 1450 "C before releasing the pressure. A fine white foam resulted but it was somewhat low in abrasion resistance, perhaps because it was not quenched fast enough (see below). The compression strength of the sample was also a little low (see Figure 1). A portion with 18.1lb/ft3 density and an average cell size of 0.1 had a compressive strength of 142 psi. Foamsil samples were used to check the effect of heat treatment on abrasion resistance (presumably by devitrification). Heating at 1500 "C caused a pronounced decrease in abrasion resistance although no crystallinity was detected by X-ray diffraction examination of the heated samples. (See Tahle V.) Discussion As is well known, the high melt viscosity of silica is dramatically decreased by the addition of inorganic oxides (Bockris and Lowe, 1954). The broad temperature range of melting of silica tends to broaden the tolerance for temperature variation in foaming. Other advantages of vitreous silica are the low coefficient of thermal expansion and high impact resistance compared to glass. (See Table
VI.) The presence of a separate phase in the pressurized glass or fused silica is deleterious to foaming. It evidently allows leakage of gas. Thus, we were unable to foam Pyroceram nor could we foam quartz glass which we devitrified by holding it under pressure just below the melting point. Impurities such as alumina (Verduch, 1958; Chaklader and Roberts, 1961) which promote devitrification are therefore undesirable.
Ind. Eng. Chem. Prod. Res. Dev., Vol. IS. No. 1, 1979 67
LAMPBLACK
GRAPHITE
16,500X
SILICON
8,OOOX
8,OOOX
NORITE
8,OOOX
Figure 4. Particle size of additives referred to in Figure 3 and Table 11.
zoo
1 5 IO
, 20
. 30
Y 45
60
90
120
7,ma. U,"
Figure 5. Foam samples made by batch extrusion of molten pressurized silica containing 0.1% lampblack.
The reaction of silica with carbon is reported to be reduced from 1400 "C to 1250 "C by vacuum whereas the reaction with carbon monoxide begins at 1300 'C (Dewey,
Figure 6. Temperature YS. time during foaming borosilicate glass in a mold to 11Ib ft-s density and cooling within the mold to a total elapsed time of 60 min followed by removal from the mold. The inset shows the location of the thermocouples in and below the mold which had relatively thin walls and was designed to mold pipe insulation.
1942). Presumably the use of super atmospheric pressure favors the reaction with carbon monoxide. The vapor
68
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
Table V. Effect of Heat Treatment o n Abrasion Resistance of FoamsiP treatment B duplicate samples boiled for 1 h in water
treatment A density, Ih/ft3
12.8 12.6 11.9 11.3
time a t
abrasion resist., s
1500 "C, h 0 0.5 1.0 2.0
22,23
L
18 L 16 L 11 L
treatment C
density, Ihift'
abrasion resist., s
density, Ibift'
13.6 12.8 13.3
19 L 19 L 21.5 L
11.7 11.5 12.2 12.5
temp, "C
time, h
abrasion, s
..
0 2.0 0.5 0.5
2.5 M 1 M 1 M 1 M
1500 1600 16351700
11.8 3.0 14 L 0.06-0.11% A1,0,; 0.015-0.03% Fe,O,. Table VI. Impact Resistance in Inch Pounds per Inch" fused Supersil containing 0.4% silicon powder vitreous silica Pyrex 7740 Measured flatwise o n cut and ground smooth.
x
6.8 5.8 3.2
in. foamed specimen
Figure 7. Sample of molded borosilicate foam (11Ib ft-? made as described in Figure 6.
Figure 8. Sample of molded borosilicate foam floating in water (photo of author, G.D.J.).
Figure 9. Drawing of furnace used to pressurize and melt glass and silica.
pressure of silicon monoxide is too low for it to be much of a flowing agent (1atm at 1880 "C) (Brewer and Edward, 1954; Gel'd and Kochnev, 1948). When water WBS heated with ground Pyrex in a stainless tube at 1050 "C, the trapped gas in the foam was 50%
water and 18% hydrogen (from corrosion of the steel). The thermal conductivity (in BTU per hour per square foot per degree F per inch of thickness) was found to be 0.43 at 9 lh ft-3 density and 0.48 at 11 (borosilicate glass). The thermal conductivity of PC Foamglas is 40. The fact
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
that the Foamglas is black does not seem to be of much value in reducing radiative heat transfer when compared to fine-celled colorless foam.
Furnace and Operation The furnace was essentially a vertical split tube graphite resistor type enclosed in a water-cooled high-pressure jacket (see Figure 9). By removing the bottom flange, samples could be placed on a graphite stand which extended up into the hot zone of the resistor. The top section was made of 310 stainless steel to prevent eddy currents where the water-cooled, schedule 80 copper pipes served as power leads to the resistor. Glass-filled Teflon chevrons around the leads provided the pressure seal. To prevent arcing and melting of the solder which bound the resistor to the copper leads and subsequent blow-out through the cooling lines, it was necessary to use silver solder. The holes in the resistor were copper-plated prior to assembly in the furnace where the soldering was done in situ. When assembled, the copper leads supported the resistor. The sample was viewed from the side or top through water-cooled sight glasses. A thermocouple port was located immediately above the side sight glass and an additional 1 in. entry was provided in the top flange. This was used as a gas exit, entry, or thermocouple well. The furnace could be operated under vacuum, atmospheric pressure, or gas pressure up to 1000 psi. The maximum temperature was approximately 2900 "C. The temperature was varied by rheostat adjustment and an indicating ammeter. The rheostat was a component part of a Weld-tronic control system using Ignitron tubes which controlled the power to the transformer. Under equilibrium conditions the furnace temperature could be controlled within 5 "C. With 1000 psi gas pressure the temperature could be raised to 1750 "C in 40 min, using 80 kVA (3400 A, 23 V). Safety features included a 1-in. blow-out frangible in the top flange (1270 psi), six thermocouples in the flanges of the main section which cut off the master switch if the temperature just inside the jacket exceeded 350 "C, automatic recording of cooling water temperature, provisions for automatic switching to an auxiliary water supply to cool the resistor in case of supply failure, a remote indicating and recording pyrometer, and visual inspection of the cooling water outlets. Low temperatures were measured by a thermocouple, but for temperatures above 1400 "C a manual optical
69
pyrometer was used. The Leeds and Northrup Rayotube remote indicating optical pyrometer required calibration for a specific operating range and was easily affected by haze or fogging of the sight glass. This was prevented by purging through a side arm. Despite the use of a continual stream of purge gas to flush the sight glass, some fogging usually occurred above 1700 "C when operating under pressure. Convection currents were set up in the sight glass tube and too large a volume of purge gas cooled the sample, making temperature readings erroneous. Several other problems were encountered in measuring the temperature of the quartz. With a heat-up period of 1 h or less, the optical pyrometer read 1 5 3 0 " higher on the foam as it emerged from the graphite crucible than it did on the crucible. Inasmuch as the top of the contents of the crucible received radiant heat, it is likely that the temperature was lower at the bottom of the crucible. The nonuniformity of temperature is shown by a sample of fused silicic acid which foamed, and the foam emerging from the crucible had a temperature of 1700 "C; yet the contents of the bottom of the crucible were only sintered. Acknowledgment We wish to thank P. R. Juckiness and A. P. Banner of the Electro and Inorganic Research Laboratory and D. S. Chisholm of the Plastics Application Laboratory for their assistance in designing and operating the furnace. Literature Cited Baker, A. H. (to Pittsburgh Corning Gorp.), U S . Patent 2 445 298 (1938). Black, H. R. (to Corning Glass Co.), U.S. Patent 2 272 930 (1942). Bockris, J. O'M., Lowe, D. C., Proc. R. Soc. London, Ser. A., 226, 423 (1954). Brewer, L., Edward, R. K., J . Phys. Chem., 5 8 , 351 (1954). Chaklader, A. C. D., Roberts, A. L., J . Am. Ceram. Soc., 44, 35 (1961). Dewey, P. H. (to Pittsburgh Plate Glass Co.), U S . Patent 2 306 330 (1942). Ford, W. D. (to Pittsburgh Corning Gorp.), U S . Patent 2514324 (1950). Ford, W. D. (to Pittsburgh Corning Gorp.), U.S. Patent 2544954 (1951). Fowler, A. A., Otis, R . M.. U S . Patent 2 117 605 (1938). Fox, J. H., Lytle, W. 0. (to Pittsburgh Plate Glass Co.). U.S. Patent 2 354 807 (1944). Gel'd, P. V., Kochnev, M. I . , J . Appl. Chem., USSR, 2 1 , 1249 (1948). Long, E. (to Saint-Gobain SA), U S . Patent 1 945 052 (1934). Long, B. (Alien Property Custodian), U.S. Patent 2337 672 (1943). Lytie, W. 0. (to Pittsburgh Plate Glass Co.), U.S. Patent 2264246 (1941). Lytle, W. 0. (to Pittsburgh Plate Glass Co.), U S . Patent 2322581 (1943). McMillan, W. J. (to the Dow Chemical Co.), U S . Patent 3231 231 (1966). McMillan, W. J., Jones, G. D. (to the Dow Chemical Co.), U S Patent 3 459 565 (1 969). Ramsay, W., Brit. Assoc. Adv. Sci. Rep. 22 (1882). Verduch, A. G., J . Am. Ceram. SOC.,41, 427 (1958). Volarovitch, M., Leontieva, J . Soc. Glass Techno/., 20, 139 (1936). Whitter, C. C. (to L. A. Whittier Glass Co.), U.S. Patent 2012798 (1935). Willis, S. L. (to Corning Glass Co.), US. Patent 2255237 8 (1941).
Received for recieu May 8,1978 Accepted August 7 , 1978
Oxidation of Sugar to Oxalic Acid and Absorption of Oxides of Nitrogen to Sodium Nitrite S. D. Deshpande and S. N. Vyas' Indian Institute of Technology, Bombay-400 076, India
During the catalytic oxidation of sugar solution, the oxides of nitrogen liberated c a n be absorbed in aqueous Na2C03 or NaOH to produce NaNO,. The problems encountered in oxidation and subsequent absorption of oxides of nitrogen are discussed. The conditions for better yields and purity of oxalic acid and NaNO, have been worked out.
Catalytic oxidation of carbohydrates and sugars by nitric acid leads to formation of oxalic acid. The choice of the 0019-7890/79/1218-0069$01.00/0
raw material and the process, however, depends on the availability of raw material and a few other considerations. Q 1979 American Chemical Society
