Flow of materials in rotary kilns used for sponge iron manufacture: Part I. Effect of some operational variables
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I.
INTRODUCTION
THE progressive
increase in steel production, gradual depletion of the reserves of coking coal, and the anticipated shortage of scrap have prompted the rapid development of direct reduction technology all over the world. For India, a country with little or no deposits of natural gas, direct reduction technology based on the use of non-coking coal in rotary kiln based processes is of obvious interest. With this in view, the Tata Iron and Steel Company Limited (TISCO) installed a pilot plant with a rated capacity of 12 tonnes per day, t'2 which has been in operation since early 1975. A number of experimental campaigns have been carried out in this pilot plant which has resulted in the development of the TDR (Tisco Direct Reduction) process of sponge iron manufacture. 3 In order to commercialize the TDR technology to plants of 300 to 500 tpd capacity, a need was felt to understand some of the fundamental aspects of material flow in rotary kilns. The present study, on cold models, was undertaken to assess the importance of various parameters influencing the flow of materials in sponge iron rotary kilns.
(iii) a high filling degree is required for maintaining a certain productivity. The optimum filling degree which fulfills the somewhat mutually contradictory demands is in the range of 15 to 18 pct along the kiln length? Some work has earlier been carried out s'6 to study the flow of solids in an inclined rotating cylinder. The rotary kiln is also basically a rotating cylinder, and it has been established that solids move in a kiln because of what is termed 'kiln action',6 an action actually composed of two types of motions, viz., 'sliding' and 'cascading'. As a result of the rotation of the cylindrical surface and the effectivc friction between the refractory lining of the kiln and the solid particles, any particle at point A of the charge bed ACB, shown in Figure 1, 'slides' up along the arc ACB till it reaches position B and then begins to descend due to the effect of
B
II.
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B
BACKGROUND
In rotary kiln based direct reduction processes, it is essential to identify the optimized flow conditions which give rise to an almost flat profile of filling degree along the length of the kiln. For this purpose, it is necessary to bear in mind that (i) efficient and effective heat transfer from the reducing gas to the charge bed demands a low filling degree, (ii) intimate mixing of the different components in the charge is not possible beyond a certain maximum filling degree, and AMIT CHATTERJEE, formerly Joint Director, Research and Development, TISCO, is now Managing Director, Ipitata Sponge Iron Ltd.. Jamshedpur-831001, India; A.V. SATHE, Project Coordinator, and P. K. MUKHOPADHYAY, Assistant Engineer, are both with Research and Development Division, TISCO, Jamshedpur; and M.P. SRIVASTAVA, formerly with TISCO, Jamshedpur, is now Research Engineer, Research and Development Centre, SAIL. Ranchi, India. Manuscript submitted March 20, 1981. METALLURGICAL TRANSACTIONS B
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