Some of the important aspects involved  in designing a furnace relate generally to (I) robustness of construction

(2) ease of  control, (3) ease  of  maintenance  and (4) maximum utilisation of heat energy produced in the furn- ace. With a view to ensuring improved  performances, the reheating  and  heat  treatment  furnaces  are now being designed as complicated and  precise  units  In place of simple bricklined boxes, hitherto employed. The demand on the quality  of  refractories  for  application  in  the different zones of such furnaces have become increasingly

stringent

Panti, B N

Sircar, N R

English

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Mill house refractories B. N. Panti tt N. R. Sircar Central Glass & Ceramic Research Institute Calcutta-700032 Introduction Some of the important aspects in-volved in designing a furnace relate generally to (I) robustness of cons-truction (2) ease of control, (3) ease of maintenance and (4) maximum utili-sation of heat energy produced in the furnace. With a view to ensuring im-proved performances, the reheating and heat treatment furnaces are now being designed as complicated and precise units In place of simple brick-lined boxes, hitherto employed. The demand on the quality of refractories for application in the different zones of such furnaces have become increasingly stringent. The present paper deals with the development of improved quality of refractories vis-a-vis application In re-heating furnaces and soaking pits. Selection of refractories Refractories used in heat treatment furnaces are usually employed In two general ways ; (I) bricks and shapes, and (2) castables and ramming mix-tures. The selection of refractories for a specific application has to be guided primarily by the abusive action likely to prevail in operation. Since chemical Various aspects of selection as well as the present trend of requirement of refractories for reheating and heat treatment furnaces have been elucid-ated. Developments visa-vis applica-tions of refractories in different zones of the furnaces have also been indi-cated in the context of environment likely to prevail and to be encounter-ed. Characterisations of some of the products developed at CGCRI have been made through conventional test procedures. The properties are quite comparable with those used in for-eign countries. In the absence of any dependable test method for measuring resistance of abrasion, thermal shock and creep behaviour at elevated temperatures, proper eva-luation of these refractories are consi-dered to be extremely difficult. It is felt that standardisation of procedure for determining these properties is essential in order to ensure improved performance in service. Efforts are now being made at CGCRI to de-velop these tests. attack, slagging or melting can com-pletely destroy a refractory material, relative resistance with respect to these aspects needs consideration. Again, the physical stability of a refrac- 76 tory structure is susceptible to applied compressive and • tensile stresses, as also to stress induced by permanent volume changes ?ia the refractory mass at elevated temperatures. A reason-able immunity is accordingly vital with respect to mechanical abrasion, as well as thermal and mechanical shock, Besides, specific heat, thermal expan-sion and thermal conductivity are also important for adjudging the behaviour of refractories as materials of construc-tion. Certain characteristics of both chemi-cal and physical nature can be antici-pated from the phase equilibrium data of the refractory oxide systems. During use however, refractories usually encounter some reactive mate-rials In the form of slag, vapour or furnace charges so as to complicate the physical and chemical reactions. The crystal-liquid relationships In the re-levant systems are of special significan-ce In understanding the ability of the refractory material to maintain Its Identity when heated, or Its stability when stressed. Particularly, the vis-cosity of the liquid phase and the degree of solubility of the cry-stalline components in the liquid phase have a great Influence. An ideal selection of refractories would thus be one that may compre-hend a techno-economic advantage through a reasonable compromise amongst properties of the refractories in relation to conditions in service. Property requirements of refrac- tories In the mill houses, the refractories should have the ability to withstand, without spalling, the operating tem-perature as well as rapid heating and cooling cycles, often, peculiar to the operation. The loss of heat through the furnace walls may be minimised if the refractories have a low thermal conductivity and a low heat capacity. The furnace may then be heated or cooled rather rapidly, restricting thereby the loss of heat due to storing in the refractory structur es, A high thermal capacity of refractory, Is however desirable in case of soaking pit where the roofs is often removed to charge and discharge the ingots. The discharging is usually carried out at high temperatures and repeated opening of the roof-cover may cause chilling of the stock in a furnace of low heat capacity. In such cases, the use of multi layer wall is helpful to meet the requirements of good Insulation and mechanical resistance. The main factors1  influencing the choice of re-fractory material are— (I) 	 maximum operating tem- perature (ii) heating cycle (iii) extent of impingement of flame on refractories and (iv) contact between the charge and the bricks particularly in the hearth region Refractories for different zones There are three main subdivisions in heat treatment furnaces viz (a) hearth 77 or bottom (b) side walls and (c) roof or cover. Hearth: Refractories employed in the construction of hearth must possess the ability to withstand furnace load over a definite range of temperature. They must be wear•resistant against .abrasion and impact of heavy work pieces. They should be inert against corrossive atmosphere and should en-sure in some applications, a fair con-duction of heat. The majority of refractory hearths are constructed with dense, high-duty or super-duty fireclay bricks bonded with mortar. The operational temper-ature being in the range of 1200' to 1450°C, the attack of mill scale with fireclay refractories may be such that the ingots stand in a pool of FeO-A1203.5102  slag Bonded silicon carbide, fused alumina, magnesite and chrome-bricks are employed when special applications warrant the use of the unique properties possessed by these refractories. Of late, magnesite, chrome-magnesite or 70 to 85 percent alumina bricks are found to be most effective In the hearth and lower parts of soaking pit and reheating furnaces for their Inertness to FeO attack. A layer of coke breeze may also be used to protect the refractories from the molten scale. In some furnaces, rammed hearths have been used and in such cases, proper refractory mixtures which become mechanically strong and highly resistant to abrasion must be employed. Usually, In large car-bottom heating furnaces, the hearths are cast with air- . setting refractory mixtures over several courses of hard-fired bricks. Side walls: The class of refractories mostly employed in side walls of oil and gas-fired heating furnaces are fire-clay brick bonded together with mor-tar, and backed with at least one course of Insulating brick, when neces-sary. The type of fireclay bricks de-pends on the temperature range and the kind of fuel used. A dense refrac-tory is recommended for furnaces constructed to store heat, while those requiring quick heating and cooling are usually lined with insulating fire bricks. High Alumina bricks are used In cases where the flames come In contact with the side wall. The working face of the side wall may be built with good quality fire-clay C.  35-42% A1203) bricks Hor with semi-silica bricks. Alumino silicate refractories are preferred If corrosive heating fuel is present. With an alter-nate course of Insulating fire bricks is advisable in case significant amount of carbon monoxide or hydrogen Is pre-sent in the furnace atmosphere. In lower side walls of soaking pits and reheating furnaces where the refractory Is subjected to the corro-sive action of molten oxide scale, chrome magnesite. magnesite or 85% alumina brick or 85-95% alumina plas-tic mass is set Into the walls. The side walls and division walls are available, In recent times, to modular type cons- 78 truction in which the large blocks are secured with prefixed 	 anchors so as to save much time and labour. 	 The lower wall Is o 31 lined with metal encased, chernicaNy bonded magnesite-chrome brick wherever usual mode of brick laying is followed. Of the burnt type of bricks, the hard burned is preferred. Direct bonded basic bricks have also been tried. Even high alu-mina, (70% or above) brick has provi. ded good results in the lower walls of some pits. Burner blocks and parts: 	Burner blocks are rammed to size with cast-able mode of refractory cement and high alumina aggregates for withstand-ing severe conditions. These are placed in position in the furnace as a unit and then bonded to the walls with proper Insulation or high-duty fire brick. The composition in terms of Al2O3 in the refractory cement depends on the in-tensity of heat to which the burner blocks are subjected. Roof or cover r The tops of pits or covers are receiving careful attention since a close seal is required to save the precious fuel. As the covers are frequently drawn aside- to allow the charging and removal of ingots, they are subjected to considerable thermal shock. Volume stability combined with spalling resistance are the most Important factors that are to be taken Into consideration. A'superduty fire-clay plastic coping has found good applications as it will not crack readily. Because of relatively high cost of fuel,  mouldabies and castables combined with proper Insulation are likely to-be used in increasing quantities in these regions. Waste heat recovery In the context of fuel crisis, there is a need for some mode of waste heat recovery. Without It, the waste gases leave at a temperature equal to or higher than that of working pieces causing a loss of energy. The addition of regenerators or recuperators is thus a prudent approach. In regenerators of soaking pit fur-naces, dense high-duty fire clay bricks are preferred for checker chamber and checkers themselves. 	 For flues and flue connections, medium-duty fireclay bricks are used. Certain intricate shapes used in recuperator construction have been made of high alumina fire bricks, although in more recent constructions, silicon carbide shapes are used in the hotter regions R 	 D acitivities at CGCRI Developmental 	 Central Glass & Ceramic Research Institute in the per-suit of improvement in the quality of refractories or import substitution under different projects developed'—lo several refractories that can be utilised satisfactorily in heat treatment furnaces. Properties of some of these products, as shown In Table I, are quite com-parable to those of some selected bricks ( Table II ) that are recommen-ded for soaking pits and reheating furnaces. 79 The role of the processing variables, be it composition, forming pressure or firing temperature is critically impor-tant in attaining a satisfactory combina-tion of desirable characteristics. The under fired bricks made out of rela-tively pure raw materials or the over.. fired products of indifferent composi-tions are equally prone to premature failure. A special care is thus called for towards judicious selection of raw materials and optimum processing thereafter. Quality control : Years' of expe-rience with testing of refractories at the CGCRI indicates an overall Impro-vement in the indigeneous supply. The evaluation, in terms of determination of conventional properties e. g. poro-sity, cold crushing strength and the like, does not adequately ensure satis-factory performance particularly since the operational abuses are not safe-guarded In the excellence of such characteristics. Effors are, therefore, underway towards developing suitable but dependable method of determining resistance towards corrosion, erosion, abrasion, thermal and mechanical shock. The inter relationship of the bond and matrix as may be deciphered in microstructure studies and creep cha-racteristics is considered to bear a significant bearing on ultimate behavi-our In service. A systematic probe is in progress for a comprehensive under-standing of thermo-mechanical proper-ties of refractories so that the extent  of liquid tolerable for a specific appli-cation may be optimised. Such a study is likely to provide a guide towards assessing the quality of the product in terms of spatial distribution of the phases. The role and the sum total of desirable characteristics quan-tified In terms of creep rate. It Is believed that refractories satisfac-torily characterised in terms of the above attributes will be much less prone to failure obviously because the tests simulate closely the operational conditions. Conclusions Refractories at the mill houses are often subjected to abrasive action when these are rubbed at elevated tempera-ture and are in contact with solid In motion, either in the form of steel Ingots or slabs or dust In gases. 	 No dependable test method has yet been adopted for measuring resistance of abrasion, thermal conductivity, creep beheaviour at elevated temperature and specific heat. Proper evaluation thus becomes extremely difficult. The pre-sent mode of evaluation through con-ventional tests appears to be Inadequate towards ensuring satisfactory perfor-mance. For, performance concurrence has to be necessarily base] 0.3 simula-tive conditions. Efforts are now being made at CGCRI towards standardising some of the testing methods. There has how-ever, been a reasonable improvement through years In the quality of indi-geneous availability of mill house re-fractories. The research effort in the 80 country Is also alive in the task of im-provement of product through Innova-tive measures and Judicious utilisation of depleting resources of high grade raw materials. Acknowledgements The authors are thankful to Shri K D Sharma, Director, Central Glass & Ceramic Research {nstitute, for his kind permission to publish this paper. References 1. J. H. CHESTERS, "Refractories for Iron & Steel Making" The Metals Society, Lon-don 371.72 (1974). 2. J. C. BANERJEE Et N. B. CHATERJEE, "Super duty Fireclay Refractories", Bull Cent. Glass Ceram. Res lOstt, 8(3) 109- 18 (1961). 3. J. C. BANERJEE & N, R. SIRCAR, "Stu- dies on altered Sillimanite rocks from Rewa, Madhya Pradesh; Part-II Industrial Utilisation", Trans Ind Ceram. Soc, 29 (4) 109-16 (1961) 4. S. P. BANERJEE & N. R. SIRCAR, "Sin-tered Mullite for high Alumina Refrac- tories, 60-64 (1973) 5. J. C. BANERJEE, N. C. BANERJEE & N.R. SIRCAR, "Study of natural Sillimanite as a refractory material; Part II—Super ref-ractories for Glass tank furnaces", Bull Cent Glas Ceram Res Instt, 9(4) 139-47 (1962). 6. N, B. CHATERJEE & B. N. PANTI, "Chrome - Alumina Refractories", Bull. Cent. Glass Ceram. Res. lnstt, 10(4) 97-101 (1963) and Trans Ind . Ceram. Soc, 23(3) 125-30 (1964) 7. "Synthetic Mullite & Mullite Corundum Products" (Unpublished). 3. J. C. BANERJEE, S. K. DE Er D. N. NANDI "Diaspore as a reftactory raw materials", Trans. Ind. Ceram. Soc, 25(3) 80-84 (1966) 9. J. C. BANERJEE, N B. CHATTERJEE Er B. N. PANTI, "Improvements in and rala-ting to the manufacture of high alumina refractories; (Indian Patent No. 108703) 10. J. C BANERJEE, S. P. BANERJEE & N.R. SIRCAR, "Preparation and properties of magnesite and Chrome-magnesite bricks from Indian Magnesites"„ Bull. Cent. Glass. Ceram. Res. lnstt, 15(2) 21-37 (1968). 81 O M 	 csi in C) Y 0 N 	.c)r.  (NI  ›-•a)  M co' 0 '0 	 N -C2 ..7Z "a• ....c) 0  a 	 c-  cc,  c 	 LC) CO ... ....... cy) 0 •01' O CO N U) O •cr CO CA co C * cst 'n (a) N 1515-1525 1600  +  N 1• co ai 2.1 4-2.32 0.03-0.14 9,785-13,660 20,676-24,250 1460-1530 1600 +  N LO for  applications  in  rehea ting  and heat  treatment  furnaces  C y a UL  O co O CD '".• Ca -0 C-1 • oro o Proper ties  of so me  of the  r efractories  developed a t  CGCRI,  suitable  Cn a. 1470-1 560 1600 +  0 LC) CD CoO   co 1-• 18.2-22.4 0 co 0 a) O to 0) O + + + 0 0 0 'of 	 I 0 	 CO CD CO 	 0 ‘-. 	 T-- 10 tfl CD • O) M O 0) O O M N 05 O N N 0 1580-1635 0 O ID O 	 CO r. _c .c 10,000-25,000 C) O O 0) co co co E _a 2 I0 0 I "Tt = ,C) tO CO Fired  Magnesite  0 (1▪  3 C 0) 2 O 0 LL * Firing  shrinkage  at  1600  °C 

Mill House Refractories

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Mill House Refractories

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