Carborundum Refractories

S. C. Linbarger
1918 Journal of Industrial & Engineering Chemistry  
Ceramic Engineer, The Carborundum Co., n'iagara Falls, N. Y. Ever since its inception one of the vital problems of the ceramic industry has been the question of suitable refractory materials, both for use in its own factories and as a product which will meet the demands of the other industries which require the highest grade of refractories. These materials must have sufficient strength under normal conditions to carry the weight of the structure of which they are an integral part and must,
more » ... part and must, ftirthermore, have sufficient refractoriness to carry this same load under the extreme heat conditions to which they are subjected. I t has always been customary to regard heat insulation in a refractory material equally desirable with resistance to fusion, probably because these properties are intimately associated in the case of the common type of fire brick or saggar mix of the aluminous silicate type which are highly refractory and are such poor conductors of heat that in comparison with metallic substances they can safely be classed as heat insulators. However, only a superficial study of the problems involved in the burning of ware in saggars or muffle kilns reveals the fact that heat conductivity of the refractories used, besides being highly desirable, is a potent factor in the economical operation of the process. For many years men connected with all branches of the clayworking industry have been seeking a more effective means of burning clay products in a shorter time and with less fuel. Especially is this true, and the need for it was never greater than at the present time when the vital needs of the country must be met with a maximum of fuel economy. Most of the efforts have been along the line of improving kiln design so that the maximum amount of heat is extracted from the gases of combustion before they pass into the stack. In most cases this is accomplished by passing the gases from the hot zone or chamber over ware in other zones which is a t a muchlower temperature, and thereby gradually raising the temperature of the ware by the utilization of the sensible heat of the gases after they leave the combustion zone. All agree that when ware is being burned a t a high maturing temperature one of the vital points of fuel economy is to get as much as is possible of the heat of the gases transferred from them to the ware. In the pottery and allied industries where the ware is burned in saggars or by setting on shelves or bats, a solution of this problem resolves itself into the proper selection and utilization of a refractory material which will more quickly and more easily absorb the heat from the surrounding gases and transmit and deliver it to the center of the bearing structure. The essential physical properties which govern the selection of the best refractory for this purpose are strength, specific heat, thermal conductivity, and emissivity. Of course in connection with these it must have the required refractoriness and be able to withstand the necessary handling without breakage. The element of strength, both transverse and compressive, is one of the properties which is too often overlooked in the selection of the proper refractory material. By using a material of high mechanical strength, both under normal and heat conditions, not only is the loss by breakage materially reduced, but primarily the walls of the building material can be very much thinner, with the consequence that the heat is conducted to the ware much more readily, to say nothing of the saving in kiln space and the smaller amount of heat required to bring the building material up to the maturing temperature of the kiln. The specific heat of the refractory is the important physical property required in calculating the number of thermal units that are really wasted in bringing the large mass of the supporting material from normal temperature up to the finishing temperature of the kiln. In some instances where the weight of the bats and saggers is equal to or more than the weight of the ware which it protects and supports, it means a considerable item in the burning cost. As a concrete example of the exact factors involved in the transfer of heat from the hot kiln gases to the ware, let us consider the specific case of a plant which is burning ware in saggars, the saggars being set in stacks so arranged in the kiln that their entire peripheral surface is exposed to the kiln gases. The spaces between the saggar stacks can be assumed to be chimneys and the velocity of the gases through them will depend upon their size and shape and also upon the draft of the kiln. Assume: I-That the turbulence is such as to make a fairly uniform temperature in the kiln a t any point on one of a system of surfaces which is symmetrical about the gas passage; and a-That the average temperature on these surfaces is the average of the temperature of the gases a t ingress and egress. It is evident that these hypotheses mean that the turbulence is controlled by some finite law and that a graph indicating the temperature gradient through the kiln would be a straight line. These hyp'otheses hold fairly well if the motion of the gases is so slow as to make the turbulence negligible or if the motion of the hot gases is so great as to make the turbulence very great. The quantity of heat then that will pass through the walls of the supporting refractory medium in a given time and be delivered to the center of the saggars will depend upon the excess temperature of the gases over the ware and the thermal conductivity of the refractory and will be a direct function of the emissivity of it. When heat waves strike a body some of them are absorbed and some of them reflected, unless the body be what is knownas a black body, in which case all of the heat rays are absorbed. All other bodies absorb a definite percentage of the heat waves which strike their surfaces and reflect the rest, the exact ratio of conduction and radiation being dependent upon the surface and the character of the body. This ratio for any material is what is known as the emissivity factor of that material. Under like conditions the same ratios hold true for the radiation of heat units from any solid body into a gaseous medium. The emissivity factor for any substance can then be determined experimentally by finding the radiation per second per unit surface area per degree difference in temperature. As the quantity of heat that will cross the boundary plane between the solid and the gas per unit time is dependent upon a factor other than the thermal conductivity of the solid, it is readily observed that the emissivity factor of the refractory which is usually neglected is an important consideration in the absorption of the greatest amount of the sensible heat from the gases which come in contact with it. Crystallized silicon carbide or carborundum, as it is most commonly called, has long been recognized as having unique physical properties which make it peculiarly adaptable in the construction of highly refractory materials. However, up to the present time it has not had a very wide application in this field owing to its high price and also to the lack of sufficient quantities to supply other than the abrasive industry, which of course is its field of primary importance. At the present time there are two types of crystallized carborundum refractories which have been highly developed. The first type which goes under the trade names of "Refrax" and "Silfrax," depending upon whether the crystallization of the aggregate is large or small, is made according to patents which in general cover the silicidizing of mixtures of carbon and silicon carbide or carbon forms and their subsequent conversion into
doi:10.1021/ie50106a034 fatcat:coxvqrys7fayjlalnsbs5kvxyi