Iron and Steel Division - Reaction Zones in the Iron Ore Sintering Process

DESPITE almost fifty years of commercial practice, the sintering of iron ore has received little fundamental study. Much of the theoretical work1-'has dealt with the constitution of sinter produced under widely varying conditions. While these studies have broadened our knowledge of the changes that occur in the sintering zone and in the freshly formed sinter during the early stages of cooling, they provide little insight into the changes that precede the formation of sinter. These preliminary changes merit study as a part of the overall process. Hessle. working with beds of Swedish magnetite concentrates, was one of the first investigators to study the sintering process in its entirety. On the basis of temperatures observed at various levels of the bed during sintering, he postulated a number of distinct reaction zones to account for the chemical changes leading to the formation of sinter. A more direct method of attack is that of arresting the sintering zone after it has progressed part way through the bed. A study of a vertical cross section through such a quenched bed provides direct information on the changes taking place at various levels. This method was used by McBriar et al.' to show that several well-defined zones of chemical change existed within beds that were typical of British sintering practice. The same general method of attack was developed independently in the present investigation to study partially sintered beds typical of American practice. Experimental Sintering Equipment The sintering operation was carried out on an experimental scale with the equipment shown in Fig. 1. The refractory-walled sintering chamber A was 11 in. deep and averaged 9 in. in diameter. Air was introduced through a tapered flow section B, which contained the orifice C for accurate metering of the incoming air. This section was located directly above the square ignition housing D, which in turn rested upon the sintering chamber A. The bed was ignited with burner E. The required suction for the operation was furnished by a fan F, which had an air capacity of 500 cfm (stp). Hot exhaust gases from the sintering chamber were cleaned in the dustcatcher G before entering the exhaust fan. In the study of partially sintered beds, it was essential to find some technique for removing the entire charge from the sintering pot without disarranging the unsintered bottom portion. This problem was finally solved by sintering the charge in a removable basket, which snugly fitted the sintering chamber. This basket was constructed of two thicknesses of window screen and was lined with a 3/16-in. layer of asbestos paper. The bottom of the basket consisted of two thicknesses of wire screen, which were fastened to the basket wall. For high fuel mixtures, additional insulation was provided by a somewhat thicker layer of asbestos cement. Preparation of Partially Sintered Mixtures The moist feed was carefully placed in the sintering basket, to prevent segregation of the particles, which varied widely in size and composition. A thermocouple was placed in the center of the basket with the hot junction halfway down, and the mixture was evenly distributed around it. During ignition and throughout the sintering of the upper half of the bed, the hot junction temperature increased very little. When the sintering zone reached the halfway point, as indicated by the sudden increase in the hot junction temperature, the charge was quenched. During quenching the suction was turned off and the orifice was tightly stoppered to prevent further influx of air. At the same time, nitrogen was admitted to the sintering chamber through the orifice tap. As soon as the nitrogen had displaced the air and products of combustion, the charge was removed from the sintering pot for immediate dissection. It is impossible to preserve the exact zone structure of the bed at the instant that combustion is arrested unless the downward transmission of heat is also immediately stopped. Fortunately, heat transfer is very slow in beds containing a stationary fluid, especially if the particle size is small. It follows that the minimum quantity of nitrogen should be used to displace the air and that static conditions be established as soon as possible. A very steep temperature gradient across the combustion zone for some time after the quench was evidence of in-