Mathematical model of the SL/RN direct reduction process
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IN
recent y e a r s there has been a growing interest in direct reduction as an alternative route t o obtaining iron. 1-6 This development has been hastened by a number of factors: the dwindling supply of coking coal, high s c r a p prices, the increasing technological f e a s i bility of d i r e c t reduction processes, and the ability of these processes t o operate economically on a considerably s m a l l e r s c a l e than blast furnaces. The SL//RN process is one of the d i r e c t reduction processes operated on a c o m m e r c i a l basis.7-1° It is distinguished from the other direct reduction proc e s s e s by its use of solid coal as the reducing agent r a t h e r than a gaseous reductant, and by its use of a rotary kiln as the r e a c t o r r a t h e r than a vertical shaft. The importance of this process r e s t s in the fact that the coal, which is preferably noncoking, is used without treatment; and is therefore less expensive than reducing gas which is obtained either by reforming natural gas or by gasifying c o a l . In the development of a complex process like the S L / R N t h e r e are obvious advantages in b e i n g able to r e l a t e kiln behavior to process variables in quantitative t e r m s with the aid of a mathematical model. At the pilot plant s t a g e this facility can assist in the interpretation of test results and in the selection of new operating conditions. This in turn can lead t o a more rapid achievement of optimum operation, and further to the establishment of r e l i able scale-up criteria. T h e s e were the p r i m e motivating factors for the present work, which was undertaken in c l o s e cooperation with the Steel Company of Canada, to develop a mathematical model of the S L / RN process. In this study considerable effort has been expended to e n s u r e the reliability of the model. The p r o g r a m V. VENKATESWARAN is GraduateStudent and J. K. BRIMACOMBE is Associate Professor, Department of Metallurgy, University of British Columbia,Vancouver,B.C. V6T IW5 Canada. Manuscript submitted August 12, 1976. METALLURGICAL TRANSACTIONS B
of work has therefore involved not only mathematical formulation, but also experimental measurements-in the laboratory t o supply rate constants and in the pilot plant to provide data for comparison to model predictions. In the present paper the development and testing of the m o d e l are described together with model predictions of kiln behavior under a variety of operating conditions. The use of the model in scale-up is also explored. DESCRIPTION OF THE PROCESS A schematic d i a g r a m of the S L / R N process is given in Fig. 1. The c h a r g e t o the S L / R N kiln consists of iron oxide pellets, coal, recycled char and dolomite if sulfur needs to be scavenged from the coal. Air is blown with s h e l l fans into the countercurrently flowing freeboard gas through b u r n e r tubes located at s e v e r a l positions along the length of the kiln. The kiln is usually operated with a slightly positive p r e s sure t o prevent the intrusion
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