| Calcium carbide(CaC2)is produced in industry from CaO and coal derived char in an electric-arc furnace at temperatures of 2000-2200? following the reaction of 3C+CaO=CaC2+CO.Besides the by-product CO the process also produces off-gas dust and ferroalloy in calcium carbide(CCF).The process requires large-size granular feed,around 5-30 mm in size,to facilitate the continued release of CO.Although the reaction temperatures are lower than the melting point of CaC2,the CaC2 formed melts and discharges from the reactor in the form of CaO-CaC2 eutectics.The energy consumption of the process is high,approximately 3250 kW·h per ton of calcium carbide with a purity of approximately 80wt.%.The calcium carbide is used by reacting with water to generate acetylene that is a feed stock for syntheses of vinyl chloride,acetic acid vinyl ester and acrylonitrile.The calcium carbide residue(CCR)generated from the reaction of calcium carbide and water is composed of mainly Ca(OH)2,around 85wt.%,and a by-product CCF around 1-2wt.%.The annual production of calcium carbide in China tripled in past ten years,reached 27.3 million tons in 2016,which was accompanied with the generation of 1.3-2.2 million tons of dust and 31-33 million tons of CCR,including 0.3-0.6 million tons of CCF.The high temperature operation and the electrical heating made the electric arc calcium carbide technology of high energy consumption while the large amount of off-gas dust and CCF resulted in high pollution.These drawbacks limit further application of the CaC2 technology.It has been reported recently by our group that biomass derived char is more active in CaC2 production than coal derived char,and fine char and CaO particles result in a low reaction temperatures,less than 1500? for example.Furthermore,autothermal heating can greatly reduce the energy consumption,especially when the CaC2 production is carried out at temperatures below 1700? in the solid-state.In this regard,this dissertation studies the morphological behavior and reaction kinetics of CaC2 production from a biomass(corncob)derived char(CC-char)and CaO at temperatures lower than 1700 0C,and then invented a novel autothermal rotary kiln process for the production of CaC2 from bio-char.The feed for the production of CaC2 usually contains some amounts of Si,Al,Fe and Mg oxides,they as well as CaO react at the high CaC2 production temperature to form various byproducts.Some of the byproducts are discharged along with the molten CaC2-CaO eutectics while some others are emitted along with the off-gas.At present,there are little studies can be found in the literature on analysis and application of these byproducts.In this regard,this dissertation also studies the separation and utilization of these byproducts.The main findings and conclusions obtained are summarized below.(1)The reaction of fine CC-char and CaO for the production of CaC2 can be achieved at temperatures below 1700? in solid-state.For 22 ?m size feed the CaC2 formation starts at temperatures as low as 1393?.With a heating rate of 5?·min-1 the reaction completes in 60 min with the maximum temperature of lower than 1700?.The reaction proceeds through diffusion of CaO toward and into CC-char.The calcium carbide formed binds the particles together and,therefore,the average particle size of the reacting matter increases with an increase in CaO conversion.(2)A kinetic model is developed for solid state synthesis of CaC2 at temperatures lower than 1700?.The unique feature of the model is that it,for the first time,uses activation energy that varies with the conversion to reflect the morphological change of the reacting matter during the course of reaction.The model also considers the slight change of reaction rate with the particle size of CC-char and CaO by introducing a particle size-dependent parameter.The changes in activation energy is 406.3-420.7 kJ-mol-1 for CaO conversion of 0.1-0.9.The overall process is under the chemical reaction control and fits well to the 3-D contraction model R3.(3)Besides Ca(OH)2,CCR mainly contain Si and Al compounds,such as SiC,Ca9Al6O18,Ca3Al2(OH)12,Ca2SiO4 and Ca3SiO5,for examples,with particles mainly in the range of less than 74 ?m.(4)The CCR contains approximately 1.3wt.%SiC that may be categorized into two fractions.One is in a large particle size range of 25.7-97.1 ?m which includes mainly SiC and FeSi containing SiC.Another one is in a small particle size range of 6.6-26.5 ?m,which is enclosed in Ca9Al6O18 particles.(5)The CCR also contains FeSi and CaSi2 which are fused together into granules of greater than 2 mm in size.CaSi2 can react with halogen acids to yield siloxene and FeSi.At HCl concentrations of 1-11.6 mol·L-1 and 0-70?,the siloxene yield is found to increase with an increase in temperature over time,with the optimum yield at an HC1 concentration of 4 mol-L-1.The siloxene products produced from different CCF differ in yield but all are layered matters with similar chemical properties and particle sizes smaller than those prepared from commercial CaSi2.(6)The off-gas dusts from the calcium carbide furnace mainly contain MgO,CaO and carbon fines.The formation of these matters is attributed to reactions of CO with elemental Mg and Ca in the gas phase during cooling.The leaching of the duct by HCl and HF yields carbon fines which are mainly hollow spheres of 0.5-3.5 ?m in diameter.Approximately 18wt.%of the carbon spheres are filled with Fe3C,which can be separated out magnetically.The carbon fines and magnetic carbon fines are able to adsorb Congo red from water quickly to adsorption capacities of 296 and 216 mg·g-1,respectively.The magnetic carbon fines can be easily reused in adsorption/desorption cycles of Congo red due to their easy magnetic separation from water. |
PDF Full Text Request