| In 2015, more than six million tons of Cr-containing steel slag (CCSS) were generated in china. CCSS is considered as hazardous waste (HW21) because its heavy metal leaching concentration is 3 to 10 times of the limited values in GB5085.3-2007. CCSS is mainly used in cement, but it has a risk of high leaching concentration of heavy metals. Therefore, it is urgent to develop a technology to dispose CCSS harmlessly with high values. Glass-ceramics method has the advantages of excellent solidifying effect on heavy metals and the products have high value. Meanwhile, the composition of CCSS is suitable for preparation of glass-ceramics. Hence, glass-ceramics method is an optimal way to dispose CCSS environmentally friendly and reutilize CCSS valuably. In this research, the formula, process route, crystal phase composition, microstructure and properties of CCSS glass-ceramics were studied systematically. One-step heat treatment mechanism, nucleation and crystal growth mechanism were clarified. Simultaneously, one-step heat treatment process was developed. These research results provide theoretical and technical supports for the eco-friendly disposing and valuable utilizing of CCSS. It contributes to breaking through the technological bottleneck of hazardous waste disposal, and to realizing the sustainable development of stainless steel industry.Crystal phase composition and microstructure have important effects on the properties of glass-ceramics. In this paper, the effects of binary basicity (BB) and nucleation agent (TiO2) content on the crystallization ability and mechanical properties of CCSS glass-ceramics were studied. When the BB increased from 0.79 to 1, the length-diameter ratio of microcrystal grains decreased, with columnar crystal (>100 μm) transforming into equiaxial crystal (2-3 μm). At the same time, density, hardness and bending strength of CCSS glass-ceramics increased. When the BB was greater than 1, the density and hardness decreased with the precipitation of akermanite and nepheline. The optimal BB was 1 for CCSS glass-ceramics. Adding TiO2 into the CCSS glass-ceramics (BB=1) induced the formation of perovskite, restrained the precipitation of akermanite, promoted grain refinement, and improved the hardness and bending strength of CCSS glass-ceramics. The optimized content of TiO2 was 7 wt.%. The main crystal phase of glass-ceramics (BB=1, TiO2=7 wt.%) was diopside with a particle size of 2 μm. The hardness and bending strength of CCSS glass-ceramics were 7.3Gpa and 150.8Mpa respectively, reaching the standard of JC/T 2097-2011.In order to cut down the cost, pickling sludge (PS) was reused as nucleation agent for the first time in this research. The pickling sludge contained CaF2,Fe2O3 and Cr2O3 with a total quantity of 76 wt.%. CaF2 was fluxing agent. Fe2O3 and Cr2O3 induced phase separation and nucleation, promoted grain refinement, and increased the properties of glass-ceramics. Adding 14 wt.% PS into the CCSS glass-ceramics (BB=1) enhanced the hardness by 2.69 Gpa, decreased the water absorption by 1.00 wt.%, and improved the density by 0.395 g/cm3. The Cr and Ni leaching concentrations of CCSS glass-ceramics (Cr=0.13 mg/L, Ni=0.04 mg/L) were below the limited values in GB5085.3-2007. It provided a new method for pickling sludge disposing.This research clarified one-step heat treatment mechanism, nucleation and crystal growth mechanism by building a fast diffusion layer model. Based on the model, one-step heat treatment process was developed. During the phase separation of parent glass, Na+ and nucleation agent ions (such as Cr3+, Fe3+) were separated, increasing the nucleation driving force of diopside. Meanwhile, a Na+-rich fast diffusion layer was formed around diopside crystals, decreasing the temperature difference between nucleation and crystallization, which contributed to achieving one-step heat treatment. The CCSS glass-ceramics (BB=1, PS=14 wt.%), obtained by one-step heat treat at 780℃ for 1h, had the bending strength of 126.5 Mpa, the water adsorption of 0.04%, the acid resistance of 1.82%, and the alkali resistance of 0.23%. Comparing with two-step heat treatment, one-step heat treatment process shortened half of the technological route and reduced energy consumption by 50%. |
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