Significant improvement of refractoriness of Al2O3–C castables containing calcium aluminate nano-coatings on graphite
Introduction
The fast pace of technological advancement has urged for the demand of more robust and stringent property specifications for refractories and monoliths. It provides longer campaign life in ladles, converters, blast furnace hearth, incinerators, boilers, rotary kiln sintering zone, etc. Various castable systems e.g. Al2O3, SiO2, MgO, Al2O3–MgO, Al2O3–SiC, Al2O3–MgAl2O4 etc have gained popularity but carbon containing and nano-bonded castables are the latest substitutes for both ferrous and non-ferrous industries [1], [2].
Carbon in the form of natural graphite or from pitch, resin, carbon black and coke can be present to the extent of 4–30 wt% in castable and refractory systems. Incorporation of carbon in the form of liquid binders like tar, resin and pitch has been known since decades [3]. But due to liberation of hazardous pyrolysis products, choice of solid form of carbon is nowadays more intended. Graphites are, however, much appreciated owing to their chemical, mechanical, thermal stability and other appreciable characteristics. Flaky graphite possesses some drawbacks if its content is increased. Increase in carbon content leads to risk of higher carbon pick up from castable affecting the quality of steel. Also chances of deformation of steel vessel exist due to increased shell temperature as a result of higher thermal conductivity of carbon [4]. Thus to reduce the energy consumption per unit of steel produced, an optimum carbon content is to be selected.
Though graphite offers several advantages to the castable matrix, its main limitations are low water-wettability and poor oxidation resistance. The main challenge is to disperse flaky graphite in a water-based castable batch with minimum casting water so as to obtain a dense structure after firing. A secondary problem of atmospheric pollution arises due to emission of carbon monoxide (CO) and/or carbon dioxide (CO2) gases because of prevailing oxidizing atmosphere at the application site. Surface modification of graphite has thus been adopted by many researchers with a desire to reduce water content of batch and minimize rate of emission of carbonaceous products by environmentally benign manner [5], [6], [7]. The use of novel forms of carbon materials e.g. carbon nanotubes, graphene platelets and nanosheets in refractory ceramics recently opened up newer possibilities for carbon containing refractories [8], [9], [10], [11], [12], [13].
Various scientists have tried to modify the surface with different types of coatings though each one has its own distinct merits and demerits [14], [15], [16]. Sol–gel methods, in this regard, had been and are still being practiced, stemming from its operational simplicity, moderate reaction parameters and reproducible coating quality. However, only a handful of reports are available that takes the full advantage of this method [17], [18], [19].
In spite of cost factor, sol–gel based coatings with binary oxides has been dealt with by some researchers. Selective thin coating, prepared by a hybrid set of precursors and applied over flaky graphite can make a balance between cost factor and coating quality [20], [21]. In other words, an optimization between utilization of inherent graphitic properties as well as a stronger linkage of graphite with composite matrix can be realized.
Calcium aluminate based coatings, in this context, needs further exploration. Calcium aluminate cement, a crucial hydraulic material to bind aggregate and matrix in conventional, low and ultra-low cement castables has its own merits. Development of calcium aluminate forming bond over the hydrophobic graphitic surface has been investigated in detail in our previous publications [22], [23]. The nanostructured Ca-doped γ-Al2O3 phases possessing Lewis acidic characteristics are sporadically distributed over graphite flake thereby improving its hydrophilicity. Heat treatment of this graphite under oxidizing conditions (at controlled pH) leads to development of some graphite oxide regions containing hydrophilic functional groups like carboxylic (–COOH), hydroxyl (–OH) etc. Zeta-potential test confirmed that it rendered better miscibility of castable matrix fines with minimum quantity of water. Raman spectroscopy studies revealed that partial exfoliation of graphite helped in the development of sp2 hybridized small sheets of graphene-like regions [24]. These were responsible for better intercalation of Ca-ions and boehmitic nano-crystallites in between the sheets. It provided better connectivity of hydrophilic Lewis acidic sites with the adjacent aromatic ring structure via the functional groups present on surfaces and edges. The nanostructured phases of Ca-doped γ-Al2O3 also improved the surface area and intimately bonded the graphene sheets through the defects, fine pores and cleavages present. As a result graphite is well retained within the alumina based castable batch via better bonding of the matrix with the coating constituents.
Flaky graphites (both coated and uncoated) to the extent of 5.0% has been incorporated in a high alumina castable batch in this work. A comparison between the physical properties, thermal shock resistance and slag resistance of above mentioned castables had already been reported in our last papers [7], [22], [23], [24]. However it is worth discussing the other refractory characteristics in detail to extrapolate the possibility of including increasing amount of coated graphites in refractories. The present investigation entails the mechanism of dramatic improvement of refractoriness, e.g. RUL and oxidation resistance by incorporation of only 5.0% of coated graphite. The matrix part of monolithic mass containing the surface-modified graphite has been separately characterized. The role of partially formed cross-linked nanoplatelets of graphite oxide in the refractory matrix, in this context, has been emphasized. In addition, the coating characteristics and phase evolution in castable have also been studied in depth to substantiate the refractory performance.
Section snippets
Experimental
Refractory grade natural flaky graphite having 97% fixed carbon and surface area 1.82 m2/gm has been selected for this investigation. The sol–gel synthesis of calcium aluminate and preparation of that coating on graphite flakes had been discussed elsewhere in detail [22], [23]. Stoichiometric calcium nitrate and aluminum-sec-butoxide were the chief ingredients to prepare calcium aluminate precursor. Thermal analysis study of graphite has always been considered as an important parameter,
Results and discussion
The DSC plots of coated and uncoated graphites (Fig. 1a) up to 1200 °C conspicuously show the advantageous effect of coating on graphite. In DSC the equipment is designed to allow a quantitative measure of enthalpy change as a function of either temperature (or time). In coated graphite control of graphite oxidation is obvious due to the sol–gel selective calcium aluminate nanocoating formation over graphite. Although both of them show high exothermic peaks from 650 °C to 1100 °C the peak maxima
Conclusions
Sustaining graphite in the castable system for a longer period had been seriously attempted by refractory scientists. However, the commercial potential towards the utilization of the calcium–aluminate coated graphite in steel plants has not yet been given any attention. In order to fill this knowledge gap, an attempt has therefore, been made in the present work to utilize the coated graphite in unshaped refractories. This laboratory scale investigation gave a strong indication on the enormous
Acknowledgments
The authors thankfully acknowledge Prof. T.K. Parya and Dr. P.K. Maiti of Department of Chemical Technology, Calcutta University, for their help extended during the course of this investigation.
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