Wei Gao

Blast furnace slag (BFS), steel slag (SS), and flue gas desulfurized gypsum (FGDG) were used to prepare metallurgical slag-based binder (MSB), which was afterwards mixed with high-antimony-containing mine tailings to form green mining fill samples (MBTs) for Sb solidification/stabilization (S/S). Results showed that all MBT samples met the requirement for mining backfills. In particular, the unconfined compressive strength of MBTs increased with the curing time, exceeding that of ordinary Portland cement (OPC). Moreover, MBTs exhibited the better antimony solidifying properties, and their immobilization efficiency could reach 99%, as compared to that of OPC. KSb(OH)₆ was used to prepare pure MSB paste for solidifying mechanism analysis. Characteristics of metallurgical slag-based binder (MSB) solidified/stabilized antimony (Sb) were investigated via X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). According to the results, the main hydration products of MSB were C-S-H gel and ettringite. Among them, C-S-H gel had an obvious adsorption and physical sealing effect on Sb, and the incorporation of Sb would reduce the degree of C-S-H gel polymerization. Besides, ettringite was found to exert little impact on the solidification and stabilization of Sb. However, due to the complex composition of MSB, it was hard to conclude whether Sb entered the ettringite lattice.

The disposal of nonferrous metal tailings poses a global economic and environmental problem. After employing a clinker-free steel slag-based binder (SSB) for the solidification/stabilization (S/S) of arsenic-containing tailings (AT), the effectiveness, leaching risk, and leaching mechanism of the SSB S/S treated AT (SST) were investigated via the Chinese leaching tests HJ/T299-2007 and HJ557-2010 and the leaching tests series of the multi-process Leaching Environmental Assessment Framework (LEAF). The test results were compared with those of ordinary Portland cement S/S treated AT (PST) and showed that the arsenic (As) curing rates for SST and PST samples were in the range of 96.80–98.89% and 99.52–99.2%, respectively, whereby the leached-As concentration was strongly dependent on the pH of the leachate. The LEAF test results showed that the liquid–solid partitioning limit of As leaching from AT, SST, and PST was controlled by solubility, and the highest concentrations of leached As were 7.56, 0.34, and 0.33 mg/L, respectively. The As leaching mechanism of monolithic SST was controlled by diffusion, and the mean observed diffusion coefficient of 9.35 × 10⁻¹⁵ cm²/s was higher than that of PST (1.55 × 10⁻¹⁶ cm²/s). The findings of this study could facilitate the utilization of SSB in S/S processes, replacing cement to reduce CO₂ emissions.

To dispose of arsenic-containing tailings with low carbon and high efficiency, sodium sulphate (Na₂SO₄), sodium hydroxide (NaOH), calcium nitrate Ca(NO₃)₂ and calcium hydroxide Ca(OH)₂ were independently added to metallurgical slag-based binder (MSB) solidification/stabilisation (S/S)-treated tailings (MSTs) to enhance the MST arsenic S/S performance. Results showed that only Ca(OH)₂ could increase the unconfined compressive strength of MST from 16.3 to 20.49 MPa and decrease the leachate As concentration from 31 μg/L to below 10 μg/L. Na₃AsO₄·12H₂O and NaAsO₂ were used to prepare pure MSB paste for mechanism analysis. The results of microstructure analyses showed the high specific surface area and amorphous properties of calcium–sodium aluminosilicate hydrate facilitated the adsorption or solid-solution formation of As(V) and As(III). As(V) formed an inner-sphere complex in ettringite, whereas As(III) formed an outer-sphere complex, and the relatively larger size and charge of As(V) compared with SO₄²⁻ restrict substitution inside channels without affecting the ettringite structure under high loading of As(V). The added Ca(OH)₂ promoted the hydration reaction of MSBs and facilitated the formation of a Ca–As(V) precipitate with low solubility, from Ca₄(OH)₂(AsO₄)₂·4H₂O (K_sp = 10^−27.49) to Ca₅(AsO₄)₃(OH) (K_sp = 10^−40.12). This work is beneficial for the application of cement-free MSB in the S/S process.

Steel slag is alkaline in aqueous solution, and this is a typical feature for solid wastes that contain large amounts of basic oxides. Therefore, this parameter is used for solid waste hazard assessments. Special attention needs to be paid to the chemical parameters of steel slag to improve its comprehensive utilization, disposal, and storage. The corrosiveness of converter slag samples from different steel plants was evaluated according to the method stipulated by the Chinese national standard GB5085.1–007 to identify whether they would be classified as alkaline hazardous waste. The results showed that steel slag leaching solution had a certain corrosivity, and the pH value of the leaching solution increased with a decrease in the particle size of the steel slag and a decrease in temperature. However, the pH value of the short-term and long-term deposited slag leaching solution was not more than the standard limit value, and hence was not a hazardous waste. It was also found that the pH value (12.66 and 12.60) of the leachate of the grounded products of some of the steel slag was close to the pH of saturated Ca(OH)₂ at 16°C. The measured pH value entered the pH range for a hazardous waste because it was obtained from the grinding of slag powder products. It was not in the original state of steel slag commonly produced by the steel slag industry. Therefore, it cannot be said that this pH directly indicated that the steel slag produced by these steel plants is hazardous waste. This study provides an analytical method that can be used as a basis for the safe storage and comprehensive utilization of steel slag.