Two problems in refractory heat treatment

Abstract

The various forms of refractory heat treatment are considered: drying, initial firing, and heat treatment proper. It is possible to mechanize the treatment completely starting with the molded refractories in ring tunnel ovens with single-row shrinkers. There is a discussion of stages and causes for unsatisfactory working in Russian ultrahigh-temperature shaft ovens for making superdense products from magnesium oxide.There are three distinct heat-treatment processes in the current production of refractories: drying, initial heating, and heat treatment proper. The first two are traditional aspects, while the last has appeared relatively recently in the production of refractories with special properties, in particular ones containing carbon.Drying is the removal of water, while initial firing produces the required properties and stabilizes the geometry, while the heat treatment proper produces the final properties of the product. All the processes are conducted in the temperature pattern of a circulating working body, which consists of the combustion products from hydrocarbon fuel, as mixed with air, or gas mixtures of special composition, and air.The heat treatment is given by changing the thermal state (enthalpy) in the temperature pattern of the working body. The decisive processes in the heat treatment are the heat and mass exchange between the product and the working body, whose roles vary in the different stages. In drying, the decisive process is mass exchange, while on preliminary firing it is heat exchange, and in heat treatment proper both heat and mass exchange are involved. In these processes, the properties of the finished product are generated or else those of a semifinished product for subsequent processing.The three processes are characterized by individual modes of treatment which are based on the dependence of the temperature and concentration patterns in the product or working body on time and the specifications for the composition of the latter. The modes of treatment are determined by the physicochemical nature of the product, but the heat-treatment method, the geometrical parameters of the product, and the design of the oven have substantial effects on the intensity, energy consumption, and product quality.The three processes are conducted in heat-technology plant (HTP) termed dryers and ovens. Current HTP are fairly complicated systems consisting of reactors in the form of drying or oven chambers; transport facilities; energy supply systems; facilities ensuring industrial and ecological safety and process control. The main element in any HTP is the reactor, where the process is conducted with heat and mass exchange between the product and the working body.At this organization in the half-century of its existence we have dealt with a wide range of HTP for treating powder and molded refractories. The HTP developments have taken two lines for traditional and new forms of heat treatment. In the development of HTP for traditional forms, much use has been made of experience in operating existing pla...