Limit to erosion rate in a high-current carbon arc

Abstract

A mathematical madel of the thermal processes in an anode has shown that there is a maximum rate at which a specified feedstock can be vaporized in a hlgh-intensity arc reactor. For a hlghquaiity graphite anode this is about 7.8 g s-', and this is thought to be the maximum carbon vaporization rate for any pure carbon feedstock. Experimental results support the use of the model. The energy requirements for carbon vaporization are discussed. IntroductionA high-intensity carbon arc can vaporize very refractory electrode material, and because of this it has been considered for a number of diverse processing applications. For example, acetylene has been produced from solid carbon and hydrogen (Baddour and Iwasyk, 1962;Baddour and Blanchet, 1964; Abrahamson, 1971; Ward, 1974); a high-intensity arc reactor has been used in the carbothermic reduction of rhodonite (Harris et al., 1959); several arc processes for reducing refractory ores have been proposed and patented (for example, Sheer and Korman, 1952, 1962; and a high-intensity arc reactor has been used in research on the production of particulate boron carbide (Wickens, 1976).The scale of the work reviewed above is small, and we have not found any examples of production plants using high-intensity arc reactors. Satisfactory conversions of feedstock have been reported, but the energy required to make unit mass of product was in all cases high and often several times that needed by alternative process routes to the same product. For instance, Baddour and Iwasyk (1962) used 6.35 mm diameter anodes at feed rates up to 0.15 g s-l. Up to 52% of the carbon feedstock was converted into hydrocarbons, mainly acetylene. The specific energy requirement for carbon vaporization in an electric arc, (SER),, estimated from the published results, decreased with increasing ablation rate, and was about 26 kWh kg-l when the ablation rate was 0.15 g s-l. At a carbon conversion of 50%, this is 48 kWh kg-' of acetylene. Industrially, the production of one kilogram of acetylene from gaseous and liquid feedstocks requires about 9.9 kwh (Lobo, 1961). Harris et al. (1959) used 50 mm diameter composite anodes containing 1618% carbon and 82-84% rhodonite and decomposed the rhodonite in the anode feedstock to MnO and SiOz. The feed rate was only 2.5 g s-l and the energy required was 8.5 kwh kg-' of product, which is about six times as much energy as required by an alternative process that melted the rhodonite in a conventional arc furnace (Fuller et al., 1964). Expressed in terms of the carbon in the feed, energy consumed was 34.5 kWh kg-l of carbon vaporized, i.e. (SER), = 34.5 kwh kg-'.Though some authors have expressed optimism that performance could be improved by increasing the scale of operation, low throughput and low energy efficiency appear to be inherent properties of high intensity arc processing, and increase in scale does not bring much benefit. We have modeled the behavior of a carbon anode in a highintensity arc reactor and have found that there is a max-* 01964305f83f 1 l22-0226$0...