| High-carbon ferrochromium slag (HCFS) is a by-product from the production of ferrochrome. Producing one ton of ferrochrome can generate 1.3 tons of HCFS. At present our country has accumulated up more than 20 million tons of slag, with the growth of 3 million tons per year. Lots of waste is piled up, the HCFS not only occupies the land, but also pollutes the environment. Hence, a clean, low cost and quantities utilization of HCFS is explored in the work. The valuable elements, such as Cr, Fe, Mg, Ca etal., are firstly separated from HCFS by magnetic separation, and then a certain valuable material of glass-ceramics have been prepared from HCFS and tailings after the magnetic separation, respectively..In order to improve the utilization of high-carbon ferrochromium slag, it is firstly used to prepare glass-ceramics. The CaO-MgO-SiO2-Al2O3-Na2O system glass-ceramics is determined by the phase diagram calculation, and the contents of each element is as followed, Al2O3:7.31%-11.68%, CaO:12.03%-13.41%, MgO: 13.06%-19.96%, and SiO2:37.51%-46.72%, Na2O:7.83% to 5.91%. The percentage of each material is determined on the basis of the raw materials (high-carbon ferrochromium slag and waste glass as the main raw materials, small amount of limestone, soda ash as the chemical composition regulator and fluorite as the flux). The mass ratio of the high-carbon ferrochromium slag to the waste glass (R(H/W)) is in the range of 0.60-1.67, and the utilization of HCFS achieves 50%. The Raman spectra shows that the number of bridging oxygen bonds in the parent glass decreases and the degree of polymerization decreases with the decrease of R(H/W) in the raw materials. The XRD result of the prepared glass-ceramics shows that the pyroxene-nepheline-composite glass-ceramics have successfully been prepared.The crystallization kinetics analysis has been subsequently carried out to determine the optimum heat-treatment conditions. It is shown that the stability of the parent glass decreases with the increase of the R(H/W) value, and the activation energy decreases from 253.41 to 183.52 kJ/mol. The crystallization growth index (n) is calculated by the Augis-Bennett equation. It is found that with the increase of R(H/W), n reaches its maximum value 5.94. When the R(H/W) is greater than 0.78, the bulk crystallization happens for all the samples. The optimum heat treatment conditions are obtained, namely, the heating rate is 5℃/min, the nucleation and crystallization temperature are 627.1℃,820.9℃, respectively.The microstructure and mechanical properties of the high-carbon ferrochromium slag-based glass-ceramics prepared from different R(H/W) values are further detected. It is indicated that when the R(H/W) increases, the particle size decreases and the porosity increases slowly. When the R(H/W) is more than 1.29, the porosity increase drastically. The microhardness of glass-ceramics increases with the increase of R(H/W), and reaches its maximum value of 9859.78MPa at R(H/W)= 1.67. The flexural strength of the glass-ceramics increases first and then decreases. When the R(H/W) is 1.29, the flexural strength reaches the maximum value of 104 MPa.The toxic leaching performance (TCLP) of high-carbon ferrochromium slag glass-ceramics is systematically studied. The maximum concentration of chromium element in the TCLP test is 0.257mg/L, which is far lower than the international standard of toxic emission of chromium (5.0mg/L). After 140 days long-term toxicity leaching experiments, it is found that the leaching content of chromium in glass-ceramic is as lower as 0.2mg/L, indicating that HCFS-based can be safely used in daily life. Extreme leaching tests of intense agitation at rotation speed of 30-2000rpm and strong acid-base conditions for 0-14 pH range are carried out. It is illustrated that the leaching concentration of chromium element increases with the increase of stirring intensity, and reaches the maximum of 0.396mg/L at the stirring intensity of 2000rpm. The smaller the pH value is, the higher the leaching content of chromium is. When the pH is 0, the leaching amount the total chromium is the largest of 0.413mg/L.In this work, the high-carbon ferrochromium slag is separated to magnetic concentrates and tailings by magnetic separation. The effects of different current and particle size on magnetic separation are investigated. It is shown that the enrichment ratio of chromium in the concentrate is 13.9% when the current intensity is 3.2A and the particle size is less than 48um. Increasing intensity current and particle size can improve the extraction ratio of chromium. When the current intensity is 2.4A and the particle size is 150um, the extraction ratio reaches its maximum of 24.9% of the total chromium content, which fully meets the requirements of raw materials as high-carbon ferrochrome production. After magnetic hysteresis loop detection, it is found that saturation magnetization of the magnetic concentrate is as higher as 76emu/g, indicating that it is possible to be used as magnetic materials.On the basis of the preparation and properties investigations of high-carbon ferrochromium slag-based glass-ceramics, the tailings after magnetic separation are continuously prepared to glass-ceramics, and the influence of different tailings to glass ratio (R(T/W)) on the properties of glass-ceramics is finally investigated. The results show that when the R(T/W) value increases, the crystal grain size decreases and the porosity increases. When the R(T/W) is more than 1.29, the porosity increases sharply. When the R(T/W) value increases, the micro-hardness of glass-ceramics increases, when R(T/W) is 1.00, it reaches a maximum of 9187.89 MPa. However, the flexural strength increases firstly and followed decreases, when R(T/W) is 1.29, it reaches the maximum value of 112MPa, which is corresponds to the trend of porosity. TCLP detection shows that the maximum leaching concentration of chromium is 0.035mg/L when R(T/W) is 1.67, and the detection result is far lower than the international standard of toxic emission of chromium (5.0mg/L).In this work, after the magnetic separation of high-carbon ferrochromium slag, the obtained concentrate can be used as raw materials return back to the electric furnace for high-carbon ferrochromium, and the high-carbon ferrochromium slag and tailing can be also made into glass-ceramics in large quantities, low cost, and clean use. Compared with the ordinary metallurgical slag-based glass-ceramics, HCFS-based glass-ceramics shows the lower nucleation and crystallization temperature, crystallization activation energy, even higher flexural strength and microhardness, and its total chromium leaching concentration is far lower than the international standard. Therefore, the utilization of high-carbon ferrochromium slag developed in this work not only has important theoretical and practical significance considering from energy saving, emission reduction and environmental protection, but also provides a new idea for the clean utilization of other heavy toxic metallurgical slags. |
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