Dec., 19x9
T E E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C E E M I S T R Y
1rgr
into a fire-clay bridi heated to 1350' C. by applying a load of 16wlbs. Results are depth of penetration expressed in inches. -Maehine-Med0.27 0.46
-Hand-Mad0.72
0.29
0.55
0.41
The degree of fineness to which the raw material is ground in d a y brick produces results similar to those of silica brick, namely, the finer the grind, the greater the spailing loss. The results given here are the averages obtained from two brands of clay brick, onc of coarse grind and the otlier of finc grind. Loss by Smiling Coarse Grind.. .......................... Fine Grind ..............................
Per cent 8.47 49.95
Rcduction in strength by heating is shown by the impact test, A steel ball approximately zi/slbs. in weight was dropped by a suitably designed machine from successive increasing heights of 2 in. upoii the heated brick. The brick tested was a wellknown ladle hrick ma& largely of plastic clay. Hsifrrr OP F ~ L OP L BGL IO Teniperntttrc............... 30" c. Average height. inches.. 51 . 2
....
Fit;. 7 - W i a ~ nIN Am Fuxlrnc~PRODUCBLI I.AY'IUI.Y
BY
Si.** A-ON
AND
TBMPFRATrrRB C B A N O B S
finer the grind the higher the loss, which in turn means a decided shortening of the life of the structure when subjected to severe thermal changes as illustrated in this table. FINSNBSS OF GRLNDZNE Mesh 4 8 12
SP*'rnO
LOSS
Per Cent 30.4 47.6 63.2
The interesting facts stated above in regard to silica brick are generally true with clay brick, except that they are not so marked, owing to the fact that the raw materials have entirely different properties. However, the method of manufacture has D pronounced effect on the qualitics of clay brick. This is illustrated by the average spalling valuc of sin different brands of brick,
BBEAKBBIC= 340' c. 4800 c. 40.3 33.6
Thc effect of extreme variation in degree of burn is well illustrated in a recent shipment of clay checker brick. The average spalling loss was r q per rent for the avuage burn and 60 per cenr for the hard-burned bricks. From all the comparative data it,is evident that refractories require. more than general consideration. They demand a most thorough study by both producer and consumer. Investigators should try to adopt simple practical tests whicir can be mn ih quantity and which give data showing variations in quality, which reflect in the life of the structure. The consumer judges his refractories by the life obtained and demands a product giving a more uniform life. Variations in uniformity are largely due to methods of manufacture. It is believed that a much more uniform product can be secured if a careful study is made of the variationsin manufacture whichaffect the important qualities. CdnNBGIL? sr€%x.CCmP*NY P I r n m J R C A , P*N)ISY'"ANI*
SUPERIOR REFRACTORIES By ROSSC . PDRDY
There is a demand for better quality in refractories, the real urgency and volume of which are growing apace with the rapid development in more eficicnt manufacturing equipment. The most vital limiting factor in the development and adaptatik of electric furnaces is adequate refractories, thc present furnaces having many compromises in design and operation, and serious limitation in use because of failure of the refractories to withstand the service conditions that otherwise would be placed upon them. New inventions of processes to lessen cost, improve quality, recover by-products, eliminate material losses, etc., etc., are not war babies, but the natural result of actual necessity for new and greater refinement in materials and economics in processes. The increase in mechanical and dielectric strains on high tension insulators resulting from extensive development of electric power, the increased strain on spark plugs and the increase in hazards due to failure of spark PLO.8 B r o c ~ s C u r ~ TII-OSIMILARSILIC~ ~oa S S A P ~ SSBOWIYE V A X I A T ~ N plugs in air craft are but two of many examples of the inm Fsr~irmss01,G m m creasing urgencies that can be met only by materials that necessitate more intense heat treatment. Examples of inthree of which are machine-made, but by differentprocesses, creases in demand for improved quality and refinement are while three are hand-made brick. more nufae~ousand perhaps mare urgent for metals than for AYBKACE PBRCENTACB Loss BY SPALLLINO AT 1350' C. ceramic ware. YMecbiaeMade-Hand-MadNew alloys that require higher temperatures for tempering. 26.9 11.7 9.4 7.3 3.4 16.7 new glasses that necessitate mare intense heat treatment to Again this variation in quality of refractories is shown by the melt and refine, new labor-saving devices that entail more results obtained from the load test in which a steel ball is pressed severe temperature changes, new by-prcducts that must be
1152
T H E Y O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
reclaimed t o make the essential process commercially feasible ; all of these and many more create an actual, not fanciful, demand for a refractory that will be superior to that which hitherto has been considered adequate. These new requirements and these changes in requirements call for a distinct applied chemical engineering, a specialty in ceramic engineering, which must be employed jointly by the producer and users of refractories. What is meant by a superior refractory? Simply one that will better meet the requirements in each case. No doubt the superior refractory in most cases will be found by proper selection of the refractories already available, but there are cases, and they are multiplying rapidly, where new refractory products will have to be developed. There are known requirements for refractories made of pure silicon carbide, fused alumina, sintered magnesia, fused spinels, crystallized sillimanite, calcined zirconia, etc., refractories that are not now much beyond the laboratory stage in development. It will be only such as these last-named refractories that will meet the excessively high temperature requirements and have at the same time the many other desired heat, strength, and di-electric properties. There is no denying of the demand for such special refractory materials, but the plain truth is that most of the new requirements will be met either by intelligent adaptation of the materials in general use to-day or by readily made modifications of them. T o make proper selection of a suitable refractory requires an understanding of the several properties that are requisite in each case and a knowledge of their relative importance. This may sound pedantic and unnecessary repetition of that which has often been said and written, but when one hears chemists, metallurgists, and engineers of recognized ability discussing refractories altogether in terms of the temperature of fusion and of chemical analysis, i t is made plain that all of our present refractory difficulties are not chargeable to the producer. How do refractories fail? T o answer this question intelligently and in language t h a t will need no interpretation, it is necessary to deal with the simple fundamentals, and, to prevent confusion of ideas, let us take the better known fire-clay refractories for illustration of these fundamentals. The most refractory fire clays may be either nearly IOO per cent pure kaolinite or a relatively complex mixture of refractory minerals, such as kaolinite, quartz, diaspore, bauxite, etc. Such a variety of mineral mixtures, so far as they concern us a t this time, may be equally as refractory although representing comparatively wide variation in chemical composition and mineral constitution. The fundamental concepts of what is essential in refractories would, however, apply alike to each. FUSION
At no time in the history of the clay refractory, either during fabrication or use, is the process of fusion of the several mineral components carried t o completion. A well-made clay refractory article will have had the fusion of the minerals carried only to that point a t which maximum strength and greatest constancy in volume are attained without distortion of the fabricated refractory article. Retaining of shape of the fabricated article is the result only of stopping the progress of fusion very much short of attainment of ultimate possible strength and constancy in volume of the refractory constituents. Chemists would speak of this partial fusion as arrested reaction. It is a very important consideration how far these reactions should progress in the manufacturing of the refractories and to what further extent they will be carried in use. Now the interesting fact in this partial fusion or arrested reaction consideration is that the degree of fusion is affected by such factors as compactness of the mass, homogeneity of the mixture, size of the mineral particles, and other physical factors. These physical factors result from plasticity and fineness of the clays, the degree to which the clays are pulverized, mixed, and
Vol.
11,
No.
12
tempered; and also t o the method of manufacture of the refractory article. These physical factors have a very decided and strong influence on the rate of fusion and it is this rate of fusion that is of more importance than the possible ultimate fusibility. SOFTBNING POINT
The melting point or temperature of melting of a pure mineral can be determined, but it is impossible t o find a melting point in a mixture of minerals or even in such relatively simple mineral mixtures as refractory fire clays. This is due not alone to the viscosity of the melt but also t o the range between the melting points of the components or compounds present. Rather than a melting point, we have .a melting range or period. You cannot intelligently speak of fusion of clays in terms of degree of temperature. A clay, which when molded into the standard cone shape will squat a t 1600' C., most likely will have had its fusion well progressed a t IOOO' C. This melting range, of course, has some relation t o the rate of fusion, but neither can be stated in definite temperature terms, and neither can be closely correlated with the chemical analysis of the mixture. To judge clay refractories solely on basis of temperature of fusion or on chemical analysis, or both, as valuable as such evidence certainly is when considered with data on other properties, will always lead one astray. THE ESSENTIAL REQUIREMENTS Many refractory installations must sustain a heavy load at relatively high temperatures; others must be impregnable to gases; others must offer the least opportunity for collection on their surface of fusible solids such as coal ash; others must resist corrosion of molten glass, slag, or metal. In nearly all cases i t is desirable, and in most cases essential, that the refractories maintain a constancy in permanent volume, and in most installations the refractory must be resistant to abrasion and withstand sudden temperature changes. Now what will give the essential combination of these properties in a given case? High ultimate fusibility, structural strength, suitable density, and low permanent volume change are essential in all cases of superior refractory installation. Notice, please, that neither chemical characteristics, heat conductivity, nor electrical resistance are mentioned here, although in a few special cases these are essential considerations. The frequently used classification of refractories as acid, basic, or neutral has a value in but very few cases, such as basic and acid processes for steel, and even in these there is evidence that too much stress has been placed on having the chemical nature of the refractory the same as that of the slag. One need but make a few slag tests on bricks to learn that the effects of basic and acid slags have practically the same corrosive fusion effect through a very long range in composition of either the slag or the refractory. The emphasis that is now placed on basic and acid refractories will be found as much a bugaboo in the vast majority of refractory installations as the old notion that it is essential to have a course of neutral brick between the acid silica crown and the basic magnesite linings. The refractory most suitable for a stoker furnace cannot be determined by consideration of balance between the chemical nature of the coal ash and that of the refractory. Bear in mind, please, that special cases are not being considered here; it is only the general, but very much the largest number of cases, requiring superior refractories that we should consider when attempting to determine the best superior refractory. It is recognized that there are exceptional cases where the chemical characteristic of the refractory is a very essential consideration, but why, I am arguing, should we "pick on" the 99.9 per cent of the cases and apply to them the chemical considerations that hold only for the other 0.1 per cent of the refractory cases.
Dec., 1919
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
1153
The incoming secondary air enters the recuperators a t atmospheric temperature and leaves them a t a temperature of about 760' C. The retorts themselves are charged or filled with coal a t from IO- to 12-hr. intervals. This means that the internal surfaces of the retort are subjected to a very considerable variation in temperature, due t o their filling with a large quantity of cold material. The retorts are about 23 f t . long and have a t the top a heavy iron casting forming the upper mouthpiece. This involves a rather considerable load per square inch of cross section a t their lower portions, where they are subjected to the highest temperatures, so that, in addition t o their refractory qualities, their resistance to deformation under load must also be considered. I n order to secure the most economical materials under the rather diverse conditions that exist in a modern retort bench, it has been necessary t o use both silica and fire-clay materials. This gives rise to an extremely complicated problem in the design and construction 01 these benches, on account of the different coefficients of expansion of the two classes of material. The average silica material will expand about a / ~ e in. per ft., while the average fire-clay material, under the same temperature, will probably expand about in. Another consideration, especially in heating up the setting for the first time, is the fact that silica develops the greater part of its expansion at 3m'to 350' C. while fire clay expands gradually up to about 1100' C. The bench structure may be looked upon roughly as a rectangular column of silica material, floating on a cast-iron base, NORTON COMPANY WORCESTBR, MASSACHUSETTS which a t the existing temperature has about the same expansion as the fire clay, surrounded by a retaining wall of fire-clay material and enclosed in a heavy steel framework. REFRACTORY PROBLEMS OF THE GAS INDUSTRY These requirements may be more clearly illustrated by a series By W. H. FULWEILBR AND J. H. TAUSSIQ of slides showing a modern vertical setting. There are two general processes in use for the manufacture of Fig. 2 shows the steel framework and lower cast-iron plates inuminating gas. The one, which is known as the coal-gas process, involves the distillation of coal in refractory vessels called which support the retorts and the upper structure of the bench. Fig. 3 shows the combustion chamber where the initial comretorts a t a comparatively high temperature, i. e., 1000' t o 1200' C., and the other, known as the water-gas process, consists in bustion of the producer gas and secondary air takes place. These decomposing steam by passing i t through a highly heated quan- arches are double and are of the highest grade.silica material tity of carboniferous material, i. e., generally, anthracite coal in order to resist the extremely high temperature, and also to or coke and in carbureting this gas with petroleum oil, which support a load which may amount t o 25 Ibs. per sq. in. is injected into heated vessels along with the water gas. Fig. 4 shows the retorts which are of silica and also the recuperators which are of fire-clay material. COAL GAS I n the operation of these recuperators, the heating waste gases We niay take as a typical example of a modern coal-gas installation, a system now coming into general use, which is known pass back and front in their downward travel through the large horizontal flues, while the secondary air. which is being heated, as the vertical retort system. Fig. I giv& a sectional elevation of such an installation. It rises around these flues through the rather small spaces t o be will be noted that there are a number of retorts located in the seen alongside the blocks forming the horizontal flues. Fig. 5 shows the upper portion of a bench nearing completion combustion chamber. These are heated by means of producer gas generated in the producer and burned with a preheated and illustrates how the upper sections of the retorts are connected with the iron castings, forming the upper mouthpiece. supply which is technically known as secondary air. Fig. 6 illustrates the large mass of iron work with the many The products of combustion pass downward through recuperators or heat interchangers, in which the incoming supply joints which connect the upper portion of the retorts to the gascollecting system. of secondary air is preheated to the desired temperature. It will be apparent that the question of expansion, and parProbably a more efficient type of apparatus, and the one being used in the most recent installations, is one in which the ticularly the differential expansion, is one that requires a very considerable amount of thought in both the design and actual producer gas is made in external producers. The coal gas as it passes out a t the tops of the retorts is con- work of construction, since provision must be made for the iron ducted through appropriate piping to the condensing, scrubbing, plates t o move as they expand on the lower framework, and t o follow the expansion of the fire-clay walls so as to keep the reand other purifying apparatus. The temperature in the combustion chamber is 1 4 5 0 ~ torts in line with the iron mouthpiece and to prevent the reto 1550" C., in the lower portion of the retorts, 1350'to 1500' C. mainder of the silica structure being displaced relative to the This temperature gradually decreases throughout the re- fire-brick construction. This is especially important in view of tort chamber and at the top, where the gases enter the recupera- the large number of joints that must be kept tight. tors, is about 850' C. It gradually decreases through the reIt may be said that in the usual type of construction, the cuperators and the gas leaves them a t a temperature between benches are built in stacks of about go f t . in length. Expansion 480' and 530' C. springs are provided on the longitudinal tie members, so that an SUPERIOR REFRACTORIES OF THE FUTURE
For all installations where the cost will justify the expense, the superior refractory of the future will be made of fused alumina, silicon carbide, crystallized sillimanite, fused spinels, sintered magnesia, or other very refractory mineral substances that have been so fused or sintered as to have attained completed cliemical change and have come to a constancy in volume. Which of these refractory substances will be most suitable in a given case will be much more dependent on physical than on chemical conditions, and the desired physical properties can be obtained with much more certainty with such refractories than with such materials as clay, bauxite, or calcined magnesite in which the physical and chemical reactions and alteration have not been carried to stability, i. e., the physical-chemical reactions are only partially completed in the fabricated refractory ware. In the case of the superior refractories under discussion, there need be either no bonding material used, or, t o produce the maximum strength, there need be but a small per cent of very fusible material such as silicate of soda, very fusible clay, or stoneware glaze. A strong, highly refractory article can be thus made of any of these materials, and have any structural characteristic desired. The present-day demands for special superior refractories will be met by very refractory materials which have been fused or sintered t o a physical and chemical constancy; and so fabricated, as here described, as t o have the essential physical and mechanical properties.
