One-step glass-ceramics production process using iron smelting slags of spent automotive catalysts

The most important secondary resources of platinum group metals (PGMs) are spent automotive exhaust catalysts, which are called “mobile PGM mines.” Low-temperature iron-capture technology is a promising technology for recovering PGMs due to its high efficiency and low pollution. Because of the content of aluminosilicates and toxic heavy metals (Cr, Ba, Ni, and Mn), the disposal of iron-capture smelting slag is necessary. This paper is devoted to the solidification of heavy metals and the resource utilization of iron-capture smelting slag. Glass-ceramics were made by a one-step method using aluminosilicates as network formers. Heavy metals and CaF₂ are employed as nucleating agents in pickling sludge. According to the analysis of differential scanning calorimetry, the glass transition temperature and crystallization temperature of samples are in the range of 650 ℃–700 ℃ and 800 ℃–920 ℃, respectively. The gap between the glass transition temperature and crystallization temperature of samples decreased from 211 ℃ to 150 ℃ when increasing the amount of pickling sludge from 7% to 28% (mass fraction). The devitrification activation energy decreased from 321.8 to 303.5 kJ·mol⁻¹, while the Avrami index increased from 1.7 to 3.7. It demonstrates that pickling sludge can reduce the temperature difference between nucleation and crystallization, which is beneficial in realizing the one-step process. The effects of pickling sludge and heat treatment systems on glass-ceramics were investigated. The diopside phase is the main crystalline phase of glass-ceramics. Nepheline and Magnetite phases were detected when the amount of pickling sludge (mass fraction) reached 28%. The physical properties of the glass-ceramics were improved with the increase in heat treatment temperature and time. When the addition amount of pickling sludge (mass fraction) was 21%, the glass-ceramics prepared by heat treatment at 900 ℃ for 1.2 h had the best properties; namely, the density was 3.04 g·cm⁻³, the water absorption (mass fraction) was 0.11%, and the Vickers hardness and flexural strength were 742.72 HV and 119.32 MPa, respectively. The Toxicity Characteristic Leaching Procedure (TCLP) leaching standard was met by heavy metals such as Cr, Ba, and Ni in the toxicity test. Glass structure analysis revealed that the pickling sludge increased the nonbridging oxygen content in the base glass while reducing the degree of glass network polymerization, resulting in an enhanced crystallization tendency. The pickling sludge proved to have potential as an inexpensive nucleating agent in the preparation of glass-ceramics with excellent performance. The glass-ceramics with these unique properties are promising to be applied as building materials.