The single-particle model of calcium carbide production and strengthening mechanism of the novel pelletizing process

Abstract

Calcium carbide (CaC2) is an important chemical raw material and China is the world's largest producer and consumer of CaC2, with more than 90% of the world's total production capacity. The equipment for producing CaC2 by electric arc method is commonly known as submerged arc furnace (SAF). In the traditional production process, lime (CaO) and coke (C) are charged into SAF in lump form and finally melted into liquid CaC2, which consumes a large amount of electric energy. In a newly proposed production process, the lump materials are replaced by lime powder and low-rank coal powder. The powder materials are first pelletized and dehydrated, and then pyrolyzed in a rotary hearth furnace. The hot pellets after pyrolysis are used as the substitutional material for SAF. Nowadays, researchers usually study the CaC2 smelting process by thermodynamic calculation, kinetic experiment and numerical simulation. However, the detailed information on the evolution of a single particle in the SAF smelting process is still insufficient. In this paper, a transient model of single-particle transportation and reaction is established for both traditional and novel pelletizing processes. The temperature, component and phase state of falling materials are calculated by numerical method. The traditional process model takes coke particle as the object and assumes that solid-solid diffusion is carried out from lime particle to coke particle. The novel process model takes pellet particle as the object and ignores the solid-solid diffusion between powders. In both models, complicated heat transfer process in the arc zone is simplified, and the electric heating phenomenon in the whole furnace is described in the form of Joule heat source. The contracting volume model (R3) is used in the kinetic model. The simulation results show that a heat retention stage occurs in both models after the particle reaches the reaction temperature, during which the temperature of the particle rises slowly. The heat retention stage accounts for about 45%-50% of the total smelting process, and the heating rate of the particles in this stage is only 2-3 K/min under the influence of intense reaction decalescence. In the traditional process, the solid-solid diffusion significantly retards the reaction rate, and the smelting process is jointly controlled by diffusion and reaction. In the novel process, the material smelting is only controlled by the reaction because the pellets are formed by uniformly mixing fine powder and then pressing. In this way, the reaction time of the traditional process is 14 min longer than that of the novel process. Under the same input electric power and productivity conditions, due to the hot charging of pellets into the furnace, the outlet temperature of the new process is 858 K higher than that of the traditional process, which can provide space for high yield and energy saving. In order to avoid the waste of overheat, when the material descending speed of the novel process is increased by 11%, the product temperature can be consistent with that of the traditional process. The final result suggests that the novel process in SAF has an obvious energy substitution effect via replacing coke with coal and replacing electricity with heat, and may have important implications in developing more energy-efficient and productive CaC2 production technology by the means of pelletizing and hot charge.