Chapter One INTRODUCTION

Transcription

1 Chapter One INTRODUCTION

2 1. Introduction The development of refractory materials has gone through an enormous evolution during last few decades largely due to following factors 1. Newer processes of steel making involving secondary metallurgy, vacuum degassing and continuous casting technology 2. The constant demandfor reduction of refractory consumption, rapid repairing programme and prolonged service life of the furnaces 3. stringent requirement for energy savings in all high temperature applications The fast changing technology for inclusion-free steel making practice using Linz- Donawitz (LD) converter, Open Hearth(OH) furnace, Kaldo converter and Electric Arc Furnace(EAF)followed by continuous casting technology has exclusively demanded a new generation superior refractory materials which are capable of withstanding not only to the hostile operating conditions such as higher temperature ofmelting,stronger agitation, rigorous refining technique and longer holding time for secondary metallurgy but also to ensure longer service life of the furnaces. Earlier direct bonded and spinel bonded magnesia and magnesia-carbon refractories were used. In coming days, dense monolithic castable refractory appears to be most promising and reliable lining materials for such applications. Infact, the worldwide growth pattern and specific consumption of castable refractory has now been increased to a new height. Refractory castables are generally composed of carefully graded refractory aggregate and binder system with or without additive. Castables or monolithic refractory lining, free from joints, are now extensively used for the construction of various high temperature furnaces and regenerators for ferrous and non-ferrous metallurgy, for the lining of pottery,glass, refractory,cement manufacturing furnaces, basic oxygen furnaces,steel ladles and tundishes with higher heat guarantee and lesser down time. 1

3 Monolithic refractories consist of a continuous bonding phase and discontinuous aggregates and the strength of the monolithic refractories depends largely on bonding phase.however, with variation in the nature of binder, the bonding methodand the bonding mechanism remarkably changes which results in the different structural properties of monilithics. Bonding agents used in monolithic refractories can be classified according to bonding methods as (i) hydraulic (ii) chemical (iii) ceramic (iv) adhesive and (v) coagulation bonding respectively. (i) Hydraulic bonding: The hydraulic binder includes calcium aluminate cements. Hydraulic bonding and strength development arises from the hydration ofdifferent mineralogical phases of high alumina cementat room temperature i.e, by setting and hardening reaction. (ii) Chemical Bonding: Chemical binder includes phosphoric acid or phosphate, sodium silicate and phenolic resin with hardener. (iii) Ceramic bonding: Ceramic bonding is referred to as low temperature sintering bonding. When refractories are associated with fluxes or metallic powders, it can reduce the sintering temperature. The solid-liquid reaction at low temperatures will be accelerated and sintering bond derived. (iv) Adhesive bonding: Mainly organic binder such as dextrin, sulphite lye, carboxy methyl cellulose, poly vinyl alcohol, vinyl polymer and phenolic resin belongs to the adhesive bonding. Most of the binders are burnt out by the heating while othersremain permanent and forms carbon bond as a result of carbonization. (v) Coagulation bonding: The binders producing coagulation bonding include fine clay powders, ultrafine oxide powders like alumina, titania, chromic oxide, alumina sol, silica-alumina sol, Mg- 2

4 Al hydroxides gel. Coagulation bonding results from the close contact of ultra fine colloidal particles. The commercial availability of calcium aluminate cement provides a good bonding system for conventional refractory castable formulation. Most serious limitation of traditional castables lies in their strength retrogression behavior particularly the significant loss in thermomechanical strength in the intermediate range of temperature in between ºC when hydraulic bonding of cement binder gets destroyed but ceramic bonding is yet to develop. At present, several types of refractory castables such as low cement/ultra low cement/zero cement high alumina castables, sorel cement bonded basic castables expand the horizons of monolithics, although still demanded vast improvement in their performance behavior in respect ofgeneration of drying cracks, porous structure and major strength retrogression in intermediate range of temperature from o C which may lead to the failure of castable structure during its in situ firing operation. Low cement/ultra low cement high alumina castables are usually prepared by blending properly graded sintered or fused alumina, synthetic mullite or calcined bauxite as aggregate and 2-5% of high alumina containing calcium aluminate cement as binder with incorporation of some active powders,while basic castables are produced by mixing properly graded dead burnt magnesite, fused magnesia, dead burnt sea-water magnesia, chrome-magnesite, mag-chrome, sintered dolomite and forsterite as aggregate and 15-20% magnesium-oxychloride or oxy-sulphate cement as binder.the physical properties of castables depend mainly on the packing of the different ingredients in the castables. The density, porosity, strength and thermal stability are dictated by the compactness of the refractory castables. The unique method of fabrications like vibratory casting, ramming, fettling and latest technology like gunning and endless lining provides monolithic refractory castables an extra edge and superiority over pre-fired bricks. The installations of monolithic refractories become quicker and easier, advantages of complex and fully anchored constructions, repairing and maintenance done locally in hot conditions, better thermal shock resistance and flexibility in design. Most importantly, the castables owing to concept of 3

5 insitu firing result in huge savings of energy and reduction of manufacturing cost. Thus, refractory consumers are benefited immensely by the use of such new technology. Traditionally, basic refractory castables are also fabricated by using hydraulic binder like high alumina cement and/or chemical binder likemagnesium oxy salts,alkali phosphate etc 1-4. In both the cases, the high hot strength property decreases rapidly on heat treatment.when the conventional high-alumina cement is used, it forms low melting phases like monticellite or merwinite. While chemical bonding agent is used, the formed low melting phases tend to release P 2 O 5 and SO 2 at higher temperatures. Technically, new gel bonding system of basic aluminum oxy-chloride and active spinel precursor likemgo.al 2 O 3 /MgO.Cr 2 O 3 double hydroxides hydrogel may contribute positively toimprove the green strength of the basic castable system at the low temperature rangevia coagulation bondingand then mat act as sintering aidfor consolidation of basic castable through formation of reactive spinel precursors at elevated temperature 5.The properly gel bonded castables are characterized by very cohesive and self flowing properties requiring no vibration for installation, shorter drying time, free from drying defects, better thermal properties and moreover no formation of low melting phases. The unshaped basic refractories are particularly reliable at the bottom and walls of basic Open Hearth, L.D converter and in the sintering zone of rotary kiln. The better service life of basic castable refractory is mainly due to its high melting point, good thermal shock resistance, excellent resistance to slag corrosion and mechanical erosion in the corrosion prone area of slag-metal zone.a great deal of researches for the development of new generationlow cement/zero cement basic castable is in progress worldwide focussing special attention to the following points: 1. Selection of right aggregates, cement free bonding agents, dispersing agents and reactive additive precursors. 2. Improvement of high temperaturecharacteristics of castable by reduction of cement binder and moisture content. 3. Enhancement of flowability of castable mix by the use of deflocculant and special agents. 4. Generation of advance matrix microstructure in refractory castables. 4

6 Through tremendous researches are going on for development of basic castable with superior properties in respect of high hot strength, high thermal shock resistance and improved microstructure, the literature regarding the basic castable is still scarce and most of them are either closely guarded or patented. In twenty first century, the innovative development of a super quality magnesia/dolime based basic refractory castable has posed a real challenge to the ceramic technologists.introduction of nanosize materials are thought to open up new possibilities towards advancement of basic castables. Therefore, for production of superior basic castable, apart from proper choice of aggregate, the selection of the cement free binder system, control of moisture addition,, incorporation of ultrafine compatible precursor areabsolutely essential which will not only arrest the strength retrogression at elevated temperature but also generate a reactive compatible precursor for accomplishing a relatively low temperature sintering of the castable body. In situ formed microfine particles are likely to promote better bonding characteristics and also to assist in deriving a advance microstructure in the basic castable. The study on self-flow castables advocates for rejection of expensive vibration technique of fabrication through an accurate rheological control of the castable mix by providing proper particle size distribution (distribution modulus) and microfine additives which willcontrol the MPT (Maximum Paste Thickness) parameter as well as higher matrix concentration 6. Thus, an innovative composition for preparing cement free superior magnesia based basic castables may be coined from the right choice of high magnesia aggregate with controlled particle size distribution, cement freenon-conventional binder system, use of deflocculant, anti-oxidant and ultrafine reactive precursor. This unique idea completely eliminates harmful lime containing high alumina cement binder from the castable formulation, thereby allows easy degassing, self flowing property into intricate shapes without application of external vibration and lesser quantity of water for casting. Such a sophisticated cement free composition of high magnesia based basicrefractory castable will be unique in terms of uniform dispersion of finer particles into the matrix, arresting of the strength retrogression in the entire intermediate temperature range ( ºC) and 5

7 augmenting the formation of advance matrix microstructure during the higher temperature of sintering ( o C) through generation of reactive micro additive. To this effect, the binder system must ensure enough fluidity at low water content, good dry and cured strength for monoliths at the lower and intermediate temperature region.in addition, upon heat treatment by transforming into an in situ defective oxide structure, it is likely to function as sintering aid to induce lower temperature sintering of the basic castable. The investigation on zero cement high alumina refractory castable using % basic aluminum oxy-chloride as binder and 4.5 % water for casting showed significant strength retention property in the intermediate range of temperature from 500 o C o C and also yielded a well interlocked, advance matrix structure with good thermomechanical properties during sintering at a relatively low temperature of 1500ºC 7. The study on zero cement high alumina castable showed that the combined influences ofaluminum oxy chloride as binder and microfine boehmite gel as additive on the compilation of tabular alumina based no-cement high alumina refractory castable had imparted a good flowability with low casting water, marked improvement in thermomechanical properties and coherent composite microstructure to the high alumina castable composition in all the temperature ranges 8. M. R. Ismael and co-workers 9 reported that the activated alumina addition not onlyyields better strength during intermediate heating temperature but also enhances the process of low temperature mullitization. The investigation on MgO-C-spinel refractories for application insteel ladle slag zone by R. Misra and co-workers 10 highlighted that basic refractories based on 64% fused magnesia, 10% sintered magnesia, 5% fused alumina, 14% graphite and 7% resin by weight was tempered at 200ºC for 20 hours which exhibited controlled expansion on repeated firing, improved slag corrosion resistance and enhanced spalling resistance on field applications and resulted in longer ladle life. N. Ryosuke and co-workers 11 developed a superior spinel-magnesia castable for steel ladles which had high corrosion resistance and high penetration resistance by use of coarse spinel.it exhibited excellent performance in relation to high temperature with- 6

8 standing capacity, processing times and resistance towards high CaO/SiO 2 slag than conventional alumina magnesia castable. Li and co workers 12 studied the effect of calcium aluminate cement on the properties of the corundum based castables using white fused alumina as aggregates and calcium aluminate cement, ultra fine alpha alumina and alpha bond 300 as binders. It was reported that with increasing calcium aluminate cement addition from 0.75 to 3.75 %, the flowability of the castables deteriorates, though the hot strength of the castables heat treated at 110 o C and 800 o C gradually enhanced. The MOR, thermal shock resistance of the corundum based castables also improved during heat treatment at 1100, 1400 and 1600 o C with the increasing content of the calcium aluminate cement in castables. Zhang et al 13 investigated the role of ultra fine alumina and alpha bond on the properties of zero cement castables. It was reported that the bridges had significant effect on the casting and mechanical properties of castables after firing. The optimum addition of 11% ultrafine alumina and 4% alpha bond exhibited great improvement of hot MOR of the castables at 1400 o C or 1600 o C. Li et al 14 demonstrated the effect of addition of microsilica and alumina micro powder on properties of mullite-corundum castables. They used fused mullite in the size range of 5-1 mm as aggregate and corundum powder up to 0.4 mm, fused mullite powder 0.04 mm, calcium aluminate, alpha alumina micro powder 2.11 micron and micro silica 1.07 micron as matrix. The micro silica addition and alpha alumina micro powder additions were kept in the range of (2-4.5)% and (2-6)% respectively. When the micro silica was added alone, the density, strength and slag resistance of the castables were improved, while porosity and thermal shock resistance decreased with the increase in microsilica content. Addition of 2-6% microalumina powder to specimens with 3% microsilica clearly improved the properties but no improvement in properties with increasing addition of microalumina powder alone. Considering all the parameters cited above, the present investigation has been designed to study initially the function of basic aluminum oxychloride as bond for MgO based basic castable with variation of carefully graded fused magnesia and dead burnt sea water magnesia respectively as aggregate. Furthermore, the role of sol-gel derived micro- 7

9 fine spinel precursor like Mg-Al double hydroxides and Mg-Cr double hydroxides on magnesia based cement free aluminum oxy-chloride bonded basic castable composition will be studied systematically in terms of dopant dosages and sintering temperature. Since large additions of sintering aid may adversely affect the end properties of the product and its microstructure, the addition of each precursor to the basic castable formulation has been restricted to small range only. 8