Sintering behaviour and hydration resistance of reactive dolomite
Introduction
Dolomite is a basic refractory material with the ideal composition [MgCa(CO3)2]. Doloma (MgO·CaO) produced from dolomite consists of a phase mixture of lime and periclase. They have extremely high melting point with a eutectic temperature of 2370 °C. Varying amounts of impurities including SiO2, Al2O3 and Fe2O3 are present in dolomite [1], [2]. The amounts and types of these impurities may have a large effect on the extent of densification. Dolomite was traditionally used as fettling material for the hearth of furnaces (Bessemer converter) [3]. Hydration of dolomite refractory is a major problem due to the presence of CaO phase. To minimize this problem, at earlier times stabilized doloma was produced with the reaction of dolomite with silica or iron oxide materials [4]. Sintering allows the formation of phases, like dicalcium ferrite, tetracalcium aluminoferrite, which stabilize the microstructure against hydration.
After the introduction of basic oxygen furnace (BOF) more attention was drawn to develop better quality highly dense and corrosion resistant doloma. The development of highly dense well shrunk particles is essential not only to resist hydration but also to hinder the slag penetration into the material [5], [6]. Originally in BOF, pitch/tar bonded dolomite/magnesite refractories were being used [7]. With the development of MgO–C refractory, the BOF converters are lined only with this refractory. Today world wide Argon oxygen decarburisation (AOD) process is standard route for the production of stainless steel with EAF–AOD route being the most common one [8], [9]. Here the primary process involves preblowing of oxygen to oxidize the carbon of steel followed by inert gas blowing to revert oxidized component like Ni, Mo into the bath. The conditions of AOD converters are temperature above 1700 °C, turbulence of liquid metal and gas and corrosion of slag having basicity 1.5–2.0. Dolomite is a thermodynamically stable material in the steelmaking environment. By virtue of its chemical nature, dolomite does not get affected by reducing conditions generated during stainless steel manufacturing process.
Ghosh et al. reveal [10] that when the starting material is in the submicron range (0.5 μm) dolomite may be densified by single firing technique at 1650 °C. The crystal size of CaO and MgO in doloma is controlled by the temperature and impurity content [11]. Increasing these impurities would increase the crystallite size. Wong and Bradt [12] observed that when dolomite in original carbonate form is sintered, a classical interpenetrating cluster microstructure occurs. During the sintering of dolomite, it was observed that the CaO grains adhered to each other grow faster than MgO grains and as a result bigger CaO grains are formed [13].
Baldo and Bradt [14] experimented in details the growth of individual CaO and MgO in dolomite. They explained that the activation energy for grain growth of CaO is 333 kJ/mol, which is similar to that being reported for Ca2+ diffusion in CaO. Since CaO is the continuous matrix in doloma, the result appeared reasonable. But the same for MgO is 437 kJ/mol is due to the discontinuity of MgO phase in dolomite and the Mg2+ must diffuse through CaO.
The reactivity of oxide derived from hydroxide decomposition at 550 °C is much greater than that formed by the decomposition of original carbonate at 1000 °C. It is thus seemed reasonable to consider that the MgO and CaO produced at lower temperatures from its precursors should be better suited for sintering [15].
In this work the sintering behaviour of dolomite was studied in relation to their reactivity and the microstructure and hydration resistance of doloma were evaluated.
Section snippets
Preparation and characterisation of precursors
The starting material used in this investigation was a natural dolomite obtained from Jabalpur, India. Raw dolomite was crushed to get −300 BS mesh size. Chemical composition of dolomite was determined by standard wet chemical method. Two hydroxide precursors were made by precalcination followed by hydration in two different ways. In the first case dolomite was calcined at 1000 °C for 2 h, allowed to cool inside the furnace, taken out from the furnace at room temperature and poured water into it.
Results and discussion
The chemical composition of natural dolomite selected for the study is shown in Table 1. Result indicates the dolomite is relatively pure with an impurity content of around 2 wt.%.
Conclusions
Dolomite converted to its hydroxide shows better densification than that from its original carbonate form. For two stage calcination process the oxides derived from its hydroxide at 600 °C has smaller particle size, which is responsible for the better densification. The hydroxide sample obtained by air quenching show superior densification compared to furnace cooled sample. Air quenched sample experiences thermal shock, which creates small particle with defective structure and improves the bulk
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