894
INDUSTRIAL A N D ENG1ATBERIiVGCHEMISTRY MELTING
Ordinarily, the fusion point of a fire brick would appear to 'be its most important characteristic, but in most cases failures will occur from plastic deformation before fusion occurs. The same precautions as used in the case of plastic deformation will help materially to prevent melting of the fire brick. SPAWLING When expansion or shrinkage occurs within a short temperature range, if the change in volume is considerable or the temperature unevenly distributed, then failure by spawling is apt to occur. If more than one surface of the brick is exposed to the heat, especially portions of the four adjacent sides, that part of the brick which is so exposed becomes hotter than the balance of the brick and this causes internal stresses which result in spawling. Bricks adequately protected with a good refractory cement do not take on or give up heat so quickly as exposed brick, so the temperature range of expansion and contraction is increased. Besides, as the heat enters the brick from one face only, the temperature gradient is even and there is less probability of inlernal stress. If spawling occurs or plastic deformation takes place, even though the bricks are bonded together with a suitable refractory cement, additional protection can be had by applying a l/* to '/&xh coating of super-refractory cement to the surface of the combustion chamber. The refractory cement used must have a high melting point, be dense, and have practically no shrinkage. CRACKING AND BULGING Expansion or contraction of fire brick will cause a setting to crack or bulge. When fire clay is used for setting up the brick, attempts are often made to overcome its inability to stay in the joint, by reducing the thickness of bond. This method throws all the stress due to expansion and contraction on the brick itself and causes more severe cracking or bulging. High-grade refractory cements have the necessary amount of elasticity to cushion this strain. SLAGGING The erosive action or slagging of fire brick by molten ash is very prevalent where coals are used, the ash of which softens at a comparatively low temperature. The flames carry particles of molten ash which enter the cracks or pores in the brickwork. This combination of ash and fire brick has a lower melting point than the fire brick itself. The deeper the molten ash enters the brickwork, the more severe will be the action. If the joints of the settings are well protected with a good bond, and the pores and minor cracks in the firebrick filled with a refractory cement applied as a wash coating, this slagging is less likely to occur. CLINKERING
It is practically impossible to prevent clinkering, but it can be retarded if the setting is made as smooth as possible. This can be accomplished by using a good bonding cement and then wash-coating the entire setting with a fine, high temperature cement designed for this purpose. The greatest trouble occurs in removing the clinker. Unless the fire bricks are strongly bonded together, which is not the case when fire clay is used, bricks are apt to be pulled out with the clinker. When parts of fire brick or even whole bricks have been pulled out of the setting with the clinker, the setting can be readily patched with a suitable high temperature cement.
Vol. 15, No. 9
ARCHES A great deal of trouble is experienced with arches where replacement is difficult. Excess expansion or contraction and spawling give the most trouble. Arches bonded with a good refractory which hold the brick in place, cushion the effect of expansion and contraction, and protect the brick from spawling, will give better service. Suspended arches are rather expensive, but they are dependable, and separate bricks which have failed can be replaced with a minimum of time and cost. CEMENTS Whenever possible it is preferable to use a dry, high temperature cement rather than a plastic cement, not only from a standpoint of economy, for plastic cements contain a large percentage of water, but also because of greater satisfaction in the results obtained. Plastic cements contain large quantities of fluxes, such as silicate of soda, which increase shrinkage and lower the fusion point of the cement. A high-grade, machine-mixed, dry, refractory cement will prove most economical. When preparing dry refractory cements for use as a bond between the brick, sufficient water should be added to bring the mixture to the proper mortar consistency, which depends on the porosity of the brick. An excess of water should be avoided, and the mortar should be thoroughly mixed and free from lumps. The mortar should be battered OF troweled on the brick already in place, the brick set on top and tapped uiitil the joint between the brick is about inch thick and cement squeezes out between the edges. Instead of cutting off this cement that squeezes out between the brick, it should be plastered back over the face of the joint and brick so that it protects the edges. In other words a T-joint construction is recommended. When using refractory cements for wash-coating, the cement should be thoroughly mixed with sufficient water to bring it to a thin grout consistency and then applied to the face of the brickwork with a stiff brush or broom. In treating a fire box with a plaster coating of a superrefractory cement to resist spawling, plastic deformation, and melting, all clinkers and loose pieces should be removed, glazed surfaces chipped away, and sufficient notches cut in the wall to provide a key for the cement. The cement should be mixed to a stiff consistency and plastered over the setting to a depth of from l/* to l/z inch, depending on the condition. One refractory cement cannot be a panacea for all ills. Conditions are so varied and requirements of a refractory so diversified that it is necessary to have different cements. with different properties. Guesswork or haphazard treatment of refractory cement problems will mean early failure and expense that could be avoided. It, is safer to realize the importance of using the correct material in the correct manner and call on the refractory manufacturer who has trained engineers available for this service. Hammermill Paper Company Fellowships The Hammermill Paper Company has offered a fellowship t o Yale University for the purpose of developing a research work on cellulose chemistry, The recipient of this fellowship is Tohn I,. Parsons, and he will work under Dr. Hibbert during the coming year. Another fellowship has been granted to the New York State College of Forestry to aid this institution in the development of a course in pulp and paper manufacture. Burton L. Kassing, who graduated this summer, will work for another year under the direction of Professors Wise and Libby on some fundamental work in connection with pulp and paper making.
