Dissolution of carbon from coal-chars into liquid iron at 1550 °C

Abstract

Carbon-dissolution studies were carried out on four coal-chars (ash content ranging from 9.04 to 12.61 wt pct), using the carburizer-cover method, and the rates of carbon transfer into liquid iron at 1550°C were determined. A theoretical model was developed for estimating the interfacial area of contact between the chars and the liquid iron. Using a force-balance approach, the partial penetration of the particles was calculated numerically and the total solid/liquid contact area was evaluated for a range of system parameters. The wettability was found to have a very significant effect on the area of contact. An improvement in wetting reduced the upward force due to surface tension, thereby increasing the downward penetration of particles in the liquid and the contact area. While two chars showed a monotonic increase in carbon pickup by the liquid iron, a two-stage behavior was observed for the remaining two chars. Stage I, which corresponds to short times of contact, showed a much higher rate of carbon dissolution, as compared to stage II during later times. The slow rate of carbon dissolution in stage II was attributed to high levels of interfacial blockage by reaction products, which resulted in many fewer areas of contact between the carbonaceous material and the liquid iron. Firstorder dissolution rate constants (ϫ10 3 ms Ϫ1 ) were computed for stage I in all chars, and the observed trend was as follows: 0.01795 (Char 1) Ͼ 0.00954 (Char 4) Ͼ 0.0061 (Char 3) Ͼ 0.00274 (Char 2). These results compare well with the dissolution rate constants quoted in the literature. Char 1, which had the highest rate constant, also had the lowest concentration of reducible oxides (e.g., silica) among all chars. The consumption of solute carbon through silica reduction could affect the carbon levels in the liquid iron. Due to reduction reactions, the experimentally measured rates of carbon dissolution are expected to be slower than the inherent rates of carbon dissolution into the liquid metal. This study shows strong evidence that chemical reactions at the interface play an important role in determining the rate of char dissolution into liquid metal.

Liquid Iron Wetting of Calcium Aluminates

“…In stage II the appearance of a denser CA layer formed reduced the contact between the coke and liquid iron slowing the kinetics. These findings were broadly consistent with what had been previously reported in the literature [7][8][9][10][11][12][13] but the information on the mineral structures formed was new.…”

Formation of a Mineral Layer during Coke Dissolution into Liquid Iron and Its Influence on the Kinetics of Coke Dissolution Rate

“…This would have a significant impact on any mineral layer formed at the interface. [5] Khanna et al [7] also observed a significant change in the rate of carbon dissolution over time in some chars. Chars with high CaO levels displayed a two-stage behavior, whereby after a period of time, the rate of dissolution would decrease significantly.…”

“…Following experiments that used a sessile-drop apparatus to react a drop of iron with a carbonaceous substrate, Wu et al, [19] McCarthy et al, [8][9] and Khanna et al [7] reported on the presence and composition of the ash product at the droplet/carbonaceous-material interface. General observations of the droplet surface in these studies indicate that although silica was contained in the carbonaceous material, there was none present in the ash at the interface.…”

“…[3,4] As coke is dissolved in the liquid iron, there is potential for the insoluble components of this inorganic mineral matter to form a layer at the surface of the coke, inhibiting carbon dissolution. [5][6][7][8][9][10][11][12] …”

Formation of Light-Weight Ferroalloys in the Fe2O3-Al2O3-C System at 1550 °C: Influence of Silica Impurities

“…Due to cost and availability constraints, it is a common practice to use carbonaceous materials such as coals, cokes, natural graphite, etc. as reductants in industrial operations [21]. Silica (up to 60%) is the main mineral impurity present in these materials.…”