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24
23
t B 0
c5
22
2’
W
5 20 e 75
6.0 a5 c10 9.5 PER CENT WATER
as
ma
85 9.0 55 PER CENT WATER
6.0
10.0
FIG. 1
It is thought that these test results are typical and that the following rules apply when sufficient pressure is used in shaping fire brick : I-Increasing the water content increases the strength of a fire brick until a limit is reached, after which further addition of water lowers the strength. 2-Increasing the water content decreases the porosity of a fire brick until a limit is reached, after which further addition of water increases the porosity. 3-An excess of water does not affect the density of a fire brick as much as an insufficient amount. Time of pugging, too, plays an important rBle in controlling the properties of fire-clay brick, and a few figures are available which show how long and short pugging affects strength. For example, one batch of brick which was made from rapidly pugged clay had an average end-crushing strength of 31,667 Ibs. Another batch made from the same clays, when pugged longer, had an end-crushing strength of 42,767 lbs. Another batch showed strengths of 36,366 and 46,000 lbs. for short and long time of pugging. From the data which have been presented as examples of the influence of water and pugging, one can appreciate how bricks made by different processes must vary. The soft-mud process involves long-time tempering with a large amount of water; the dry-press involves a shorter period of working and less water; and the stiff-mud process might. be considered as intermediate between these two. Furthermore, it must be borne in mind that no two clays are alike and that results secured by a certain process with one mix would not apply to a second. Therefore, the writer will avoid any statements concerning the methods employed in the shaping of fire brick. Each method has its advantage and produces excellent bricks for certain uses, but one cannot state that any particular process is the best for all clays. After shaping, the ware is dried on hot floors, in the open, or in tunnel driers, but this very important step concerns only the manufacturer. I n the case of large shapes, weeks are required for drying, although by controlling the humidity of the modern dryer this time may be greatly decreased. It does, however, prepare the green brick for introduction into the kiln and hence for subsequent burning. It is then that the contraction is removed from the clays and “creeping” of arches and “opening” of joints are prevented. A firm bond is developed and resistance to abrasion is increased. Sometimes a lightly burned product gives the best results, although such cases are less common. It is a t this period in manufacture that a pinkish cast is sometimes developed if damp bricks are subjected to the combined action of sulfur and water. This discoloration may therefore indicate either an excess of impurities or the full development of the coloring affect of the usual amount. However, the laboratory problem in this connection deals with the determination of the proper burning temperature for different clays. Some clays contract rapidly a t low lemperatures, and, after reaching a certain limit, remain constant in volume. Others continue to contract over a very wide range of temperature. A third class contracts but very little a t any
Vol.
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No.
12
temperature and hence is termed open-burning. Studies of such clays, when made and plotted as in the curves, give very useful information from two standpoints. They tell the manufacturer what treatment is necessary to remove shrinkage and obtain a well-burned product. They also inform the consumer how the different clays are going to behave in service, whether they will remain open or become dense a t working temperatures. In that connection, such curves will undoubtedly throw some light on the problem of spalling, which is apparently due to the vitrification of the bond clay during service and the subsequent lower resistance to heat changes. A few volumetemperature curves which indicate the variations in burning behavior of different clays are given in Fig. 2 to illustrate this point. k!
31%
L o 3 u U 4 C
E ; 30% 2 =r 75% 6
:2
10%
8:PE
/5%
:.p
10%
O w
0-
b z
a
s.d
& Z
2;
rEMPERATURC
FIG.2 A-Open-Burning Fliht Clay, B-Dense-Burning Grade Dense-Burning Plastic Clay; D-High-Grade Clay; E-Low-Grade, Dense-Burning Plastic Clay
Flint Clay: C-HighOpen-Burning Plastic
While this paper is very elementary in nature, it is hoped that it will show how every step of the many which are involved in fire-brick manufacture is important; that this industry is not so crude as is often supposed; and that most manufacturers are working to improve their products. While the naturally occurring materials are limited in application and can hardly be subjected to chemical treatment, they can be used in the most efficient manner and it is the purpose of modern manufacturers to do this. MBLLON INSTITUTE OF INDUSTRIAL RESEARCH UNIVERSITY O F PITTSBURGH, PITTSBURGH, PA.
THE SELECTION OF REFRACTORIES FOR INDUSTRIAL FURNACES By W. F. ROCHOW
Economy in the use of refractories is largely governed by the selectioq of the class of material best suited for the purpose intended, the quality of the brick, and the design of the furnace. The importance of such factors as good brick laying, care in storing and handling the brick, and proper manipulation and control of the furnace, while clearly evident, cannot be emphasized too strongly. Each class of the commercial refractories possesses some properties which make it especially adapted for certain conditions encountered in the process. Careful study of the properties and of the actual working conditions, together with practical trials, will determine the proper material to use. A superficial consideration of these factors is not sufficient, for in some cases economies have resulted from the use of refractories whose chemical composition and physical properties would indicate that they were entirely unsuited. For a long time, because of the acid character of silica brick, it was not considered good practice ever to use them where a basic material would come in contact with the brick. I n some processes, however, the temperature necessarily employed is not high enough to induce chemical reaction between the refractory and the charge. Thus silica brick have been used for
Dec., 1~919
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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
181 FIG.1-SECTION
OF
MUFFLEFURNACE
many years in kiln linings for burning such basic materials as lime, dolomite, and magnesite. Care must be taken with kilns of this kind to avoid spalling caused by rapid temperature changes and to keep the temperature within the proper limits. Moreover it is sometimes economical to use a refractory of chemical characteropposite to that of the products of the furnace, notwithstanding chemical action occurring between them, as, for example, in the glass industry silica brick are used practically t o the exclusion of every other refractory in the roofs of tanks and pot furnaces although they are subjected to the alkaline vapors from t h e glass batch. Other refractories which may here better resist the chemical action have not the same combination of other desirable properties which make silica superior. In this paper it is proposed to consider principally the silica and magnesia refractories.
1 I47
its thermal expansion. From the curve1 of Fig. 2 it is apparent that most of the expansion occurs over a narrow range and a t comparatively low temperatures. By heating and cooling slowly over this critical range up to about 500 ’ C. any difficulty due to spalling is eliminated. Notwithstanding the high thermal conductivity, the good mechanical strength of silica brick a t high temperatures makes it possible to insulate them under very difficult conditions. Unlike fire-clay and magnesia refractories there is only a narrow temperature range between the softening and fusion points of silica, and hence no deformation occurs up to the melting point. Silica brick used in arches and subjected to high heats are being insulated with such material as kieselguhr with appreciable saving. Under similar conditions the best fire-clay brick would soften gradually and eventually collapse. Where clay brick are necessarily used it is frequently impracticable to insulate them. Often i t is possible to effect an economy by cooling rather than by insulating and under some condition9 it is necessary to provide radiation or conduction of heat from the refractory. For a high-grade silica brick, Nesbitt and Bell2 secured an average crushing strength of 1831 lbs. per sq. in. a t 1350’ C. The best clay brick which they tested had a crushing strength of 1289 lbs. per sq. in. a t the same temperature. As shown by results from the load test of the American Society for Testing Material most fire-clay brick settle somewhat under a pressure of 2 5 lbs. per sq. in. a t 1350’ C., while good silica brick remain rigid under the same load a t 1500’ C. This test is of considerable value in selectiog refractories, although in most industrial applications the conditions are rarely as severe as those employed in the load test, in that only d small fraction of the brick is exposed to the intense heat of the furnace while the remainder of the brick actually supporting any load is comparatively cold. Proper consideration, moreover, should also be given other properties, for some brick having a high ultimate fusion point often show greater contraction than others of much lower refractoriness, but of higher silica content. The siliceous clays expand up to certain temperatures in accordance with the relative proportions of their free silica content. B
SILICA RSFRACTORIES
In general terms the properties of silica brick to be considered in their use are their high thermal conductivity, mechanical strength and resistance to abrasion a t furnace temperatures, thermal expansion, spalling tendency, and high softening and fusion point. The thermal conductivity of silica as compared with that of first quality fire-clay brick is illustrated by the following figures which are the results of determinations by Dud1ey.l The coefficient K expresses calories per second flowing through I sq. cm. area, I cm. thick, per I O C. difference. Mean K between O’and 100DC. 0.0016 0.0021
...............
Flint Clay Brick. Silica Brick.. ....................
Mean K between l o a n d 1000aC. 0.0025 0.0031
Along with other of its properties, the good conductivity of silica renders it the best suited material for the construction of the major portion of by-product coke ovens and other furnaces in which heat must be transferred through the refractory. Advantage also is taken of its conductivity in enameling furnace muffles. As shown in Fig. I the use of silica is not confined en. tirely to the muffle but also is used in the combustion chamber. In this type of furnace any danger from spalling is avoided by the proper control of the temperatures on heating and cooling when starting up or shutting down. As bearing on the spalling tendency of silica it is interesting a t this point to consider briefly 1
Tram. A m . Electrochem. SOC.,April 1915.
FIG.2
I n addition to chemical analysis and the load, fusion, and spalling tests used for determining the quality of silica brick, it has been pointed out that the specific gravity may be used as a criterion of the extent to which the permanent expansion should be carried in the first firing. D. W. Ross3 recently has made a 1 Montgomery and Office, J . A m . Ceram. SOC.,1918. 2 “The Necessity of Inspection and Testing of Refractory Brick,” A . S. T . M., June 1918. 3 “Silica Refractories,” U. S. Bureau of Standards, Technologic Pa)er
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
FIG.3-sKETCH
ILLUSTRATINQTEXTURE,COMPOSITION, ETC., OF
very thorough investigation of silica refractories and suggests that a specific gravity of 2.38 may be accepted as the upper limit for well-burned brick. For most American brick, that is, those made from Medina quartzite of Pennsylvania, this is undoubtedly an accurate figure, but for others a slightly higher specific gravity is sometimes found in properly burned brick. A fine grained quartzite of high purity and 2 per cent of lime are the sole constituents of unburned silica brick. Upon burning, the quartzite undergoes partial inversions to other crystalline forms, i. e., cristobalite and tridymite. These changes are accompanied by permanent volume increases and it is desirahle that this permanent expansion be attained to the maximum degree so that there may be no excessive after-expansion when the brick are used in the furnaces. The specific gravity of the crystalline forms of silica are as follows: quartz 2.65, tridymite 2.270, cristobalite 2.333, and quartz glass 2.21. Some quartzites invert to cristohalite and tridymite more slowly than others, giving a slightly higher specific gravity t o the brick made therefrom. Such brick, however, are practically identical with those having a lower specific gravity and when properly made do not show an appreciably greater expansion. The following microscopic analyses1 of two silica brick, both burned in the regular commercial manner, one made of Baraboo and one of Medina quartzite, illustrate the difference in the rate of inversion of these two quartzites. Quartz and Silicates Cristobalite BRICK Per cent Per cent Made from Raraboo quartzite. 44 52 Made from Medina quartzite.. , 13 72
.... ..
BRICKAFTER USE
IN
OPEN-HEARTH FURNACE
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No.
12
ROOFS
composition of these various zones are averaged and are plotted in Fig. 3. MAGNESIA OR MAGNESITE REFRACTORIES
Since late in the year 1914, when the supply of Austrian magnesite was cut off as a result of the war, there has been more thorough study of the properties of basic refractories than ever before. Unlike the Austrian magnesite, the domestic material does not naturally contain a sufficient amount of iron oxide. The presence of iron in any proportion lowers the fusion point, but in quantities of from 4 . 5 to 8 per cent of ferric oxide, it widens the range of vitrification and develops good bonding properties a t furnace temperatures. I n the use of the domestic product in manufacturing refractories it has become the practice to add a percentage of iron oxide and in this manner the properties are made very similar to those of the Austrian magnesite. While magnesia brick give excellent service and are almost indispensable in many metallurgical operations, unsatisfactory results may be secured unless they are used with due regard to their properties. At high temperatures magnesia brick are not mechanically strong nor resistant to abrasion. As is illustrated in the curve of Fig. 4, they expand considerably, with the maximum expansion occurring a t about 1350' C. On account of this high thermal expansion the brick have a tendency to spa11 when heated or cooled too rapidly.
Tridymite Per cent 4 15
Brick made from rock having inversion characteristics similar t o the Baraboo may be properly burned when inversion has occurred to such an extent that its specific gravity is slightly, if any, lower than 2.42. Often very much of practical value can be learned by studying the remaining portion of brick after long use. An interesting example of the manner in which silica brick deteriorate in the roofs of open-hearth furnaces may be had from the results of the investigations by E. Rengade.2 Four distinct zones have been distinguished in silica brick taken from the roof of openhearth furnaces after having been in service for the normal period of time. A number of analyses made to determine the 1
Insley and A. A. Klein, U. S. Bureau of Standards, Technical P a W
2
Academy of Science, Paris, May 1918
114.
SILICA
Vol.
FIG.4-THERJIAI.
EXPANSION OF MAGNESIA BRICK
In several applications an economy has been effected by the use of metal-encased magnesia brick in place of the regular magnesia and silica brick in parts of the furnaces. I n the manufacture of this product, rectangular or circular soft steel casings of definite gauge, open a t both ends, are rammed full with a high grade of dead burned magnesite. When properly dried, the brick are ready for use without having been burned. These brick are always laid as headers with either open end next to the heat. No cement is used in laying the rectangular brick but the spaces between those of circular cross section are filled
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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 A E M I S T R Y
with moistened furnace magnesite. At the temperature of the furnace in which they are used, the steel container melts back for a distance of one or two inches from the exposed surface and impregnates the dead burned magnesitc which it encloses. The suriace thus becomes prrretically jointless. Because of this monolithic surface and the somewhat more w e n texture of this construction. the spalling tendency is considerably less than that of magnesia or silica brick. The stiffening effect of that part of the metal case which does not luse also helps reduce spalling to a minimum. This brick is iiscd in the back walls, bulk heads, and gas ports of basic open-hearth furnaces and in the side willls ai electric steel-melting furnaces. An installatian of these brick in a &ton Heroult furnace is shown in Pig. 5 .
1149
operation, thc mccssity for greater efficiency from refractories is forced upon us. This improvement in refractories can only bc accomplished by the coiiperation oi the consumer and the producer. The consumer should be familiar with the conditions prevailing in his furnaces, such as temperature, slag, gases, dust. mechanical wear, abrasion, expansion and contraction, etc., M that he is in position to know what the brick must stand.
Fro.
I--S)dll~z. PORIIONos .DISINTSGRA-IED Reanacroav B R ~ C ASTBB Z 5 YBnns' .'HRYICB IN BLISFFUINAEB I ZNING. Nom S I M Z L A 011~ ~ Y C n l C g S IN SSN,' POPTlON 1RrJ IN UN"S*Pl nnrcL7
On the other hand, thhmanufacturer of refractories should know the limitations and possibilities of his product. He should know the effect on quality in service produced by variation in moisture, 6neiiess of griud, proportionate sizing of particles, method of molding, drying, and burning. Such information enables him to advise the consumer as to the kind of brick best suited for his particular needs
OP rCzx.iTo~ rmc FOWACR SHOWZNDPA^ BNI M B T & ~ ~ ~MACNBSITS sB Baicx SZDB WALL.LIN~NU
FEGJ-VIEIY
In addition to the physical and chemical properties of reiiactoiies the design of the shape used also is worthy of careful atteot;on. Difficult and intricate shapes should be awrded as much as possible. They cannot be repressed, are more liable to defects in workmanship, and cost more. With few exceptions they cannot be used to advantage over the standard sizes and shapes. Standard sizes are carried in stork by the refractories manuiacttmrs and ordinarily are available to the user promptly when needed. Eccause of the time required for manulacture, a number of weeks must necessady elapse before special shapes can be secured XIR~ZSON-WILBBR RBI.P.AC~ORIHS COXPANY P I T T S B U W B , PGNNSYLYINII
INTERESTING FACTS CONCERNING REFRACTORIES IN THE IRON AND STEEL INDUSTRY By C E. NBYBITT AND M I ,. BBLL
In the manuiacture oi iron and steel the part played by refractories is a most important one and up to the present time has not had proper mnsideration. Owing to inneased production, larger and more complicated furnaces. and economy of
Fzo Z-CLAY BmcR mox C 0 m . B ~ z . s ~MAIII'OIIBLASTFurnrcg SnowZAG EROSION PRoDuCBD BY DUSTC ~ a a m m o BLAST
In the iron and steel industry the temperature range is wide Temperatures in the intenor of a blast furnace vary from 260' C a t the top to 1 8 0 0 ~C. a t the tnybes. Refining temperatures are high in open-hearth, Bessemer converters, and electric furnaces, while temperatures m quenching and annealing iumaces aze moderate. Besides this wide range of temperature, refractories must meet a wide variation en physical requirements, such as wrrosivc action of acid, basic or neutral slags, sudden thermal changes, load, abrasion, impact, and expansion. Early in OUT investigation we found need forsimple tests which would give us some data on the important qualities necessary refractory brick. These tests should be easily and rapidly ex-
