The calcium carbide slag used in the experiment was obtained from Shandong province, China, and the coking coal was obtained from Shanxi province, China. The chemical composition of the calcium carbide slag is presented in Table 1, and the proximate analysis and the ultimate analysis data of coking coal are presented in Table 2. The slag composition was measured using X-ray fluorescence (XRF). Proximate analysis was performed following the Chinese standard methodology GB/T 212–2008, and ultimate analysis data was obtained from an elemental analyzer.
CaO SiO2 Al2O3 MgO SO3 Fe2O3 Cl 89.88 5.23 2.78 0.38 0.57 0.33 0.43
Table 1. Chemical composition of calcium carbide slag
wt% Proximate analysis Ultimate analysis Aad Vdaf FCdaf Cad Had Oad Nad Sad 10.99 12.46 87.54 76.47 4.25 4.52 1.36 2.41 Note: ad—air dry basis; daf—dry ash-free basis; FC—fixed carbon; A—ash; V—volatile matter.
Table 2. Proximate analysis and ultimate analysis of coking coal
The calcium carbide slag and coking coal were mixed in a certain proportion in a mortar and grinded. A 4-g mixture was placed into a stainless-steel mold (diameter: 13 mm) and pressed vertically with a pelletizing machine (XQ-5, Xiangtan Xiangke Instrument Co., Ltd., China) under 75 MPa for 3 min to make green pellets with a size of ϕ13 mm × 20 mm. The green pellets were dried at 80°C for 24 h, and then they were calcined at various temperatures in the range of 700–900°C in a muffle furnace for 30 min to convert to the different types of calcium coke (Ar atmosphere). Fig. 1 shows the green pellets and the formed calcium cokes after the calcination.
The thermal weight-loss characteristics of the green pellets were determined using an SDTQ600 thermogravimetric analyzer. The thermogravimetric conditions were as follows: a sample mass of 5–10 mg, an atmosphere of pure Ar, a pyrolysis temperature interval of room temperature to 1360°C, a temperature increase rate of 20°C/min, and a gas flow rate of 200 mL/min.
The thermal strength of the calcium coke samples formed from various precursors were measured using an automatic high-temperature strength testing device, GKY-II (Xiangtan Xiangke Instrument Co., Ltd., China). The testing device needed to be heated up from room temperature to 1000°C at a heating rate of 10°C/min and held for 5 min before the strength test was started.
To study the influence of the pyrolysis temperature and the coking coal content on the thermal strength of calcium coke, the green pellets were pyrolyzed for 30 min in Ar atmosphere at 700 and 900°C, respectively. The obtained calcium coke was placed in a high-temperature compressive tester at constant temperature for 5 min to test its thermal strength. Each sample was tested three times, and the average value was taken as the thermal strength of calcium coke.
The calcium coke was prepared from green pellets via pyrolysis at 700°C (30 min), and it was then cooled to 300°C (industrial exhaust temperature) under N2 atmosphere. Afterward, N2 was shifted to CO2 (500 mL/min) for different holding times of 0, 5,10, 15, and 30 min, respectively. The calcium coke after carbonation was obtained for the hydration resistance test.
The sample composition was detected by X-ray diffraction fluorescence (XRF, AXIOS-MAX, Japan). The phase composition of the specimen was detected using X-ray diffraction (XRD, X'Pert PRO MPD, Holland) with Cu-Kα radiation in the 2θ range of 10° to 90°. The microstructure of the sample was examined using field-emission scanning electron microscopy (SEM, JSM-7001F, Japan) and energy-dispersive spectrometry (EDS, INCA X-MAX, Oxford Instrument, UK). The particle size of CaO with calcination was measured by Nano Measurer 1.2 coupled with a SEM. The pore structure parameters of the sample were determined using an automatic mercury porosimeter (Autopore IV 9500, Micromeritics, USA).
Development of calcium coke for CaC2 production using calcium carbide slag and coking coal
13 January 2020
Revised: 21 March 2020
Accepted: 24 March 2020
Available online: 26 March 2020
Abstract: A type of calcium coke was developed for use in the oxy-thermal process of calcium carbide production. The calcium coke was prepared by the co-pyrolysis of coking coal and calcium carbide slag, which is a solid waste generated from the chlor-alkali industry. The characteristics of the calcium cokes under different conditions were analyzed experimentally and theoretically. The results show that the thermal strength of calcium coke increased with the increase in the coking coal proportion, and the waterproof property of calcium coke also increased with increased carbonation time. The calcium coke can increase the contact area of calcium and carbon in the calcium carbide production process. Furthermore, the pore structure of the calcium coke can enhance the diffusion of gas inside the furnace, thus improving the efficiency of the oxy-thermal technology.