Lab-scale DC electric arc furnace design and thermodynamic slag optimization for metal recovery from slag phase

Abstract

Entrapment of metals in slag phase cause a drastic economical loss in various metallurgical slags due to inefficient metal/slag seperation. Copper slags, display a substantial copper (Cu) presence, ranging between 0.5% and 3.7%, comparable to or exceeding the copper content observed in standard copper ores. Moreover, treatment of secondary sources such as printed circuit boards (PCBs), spent auto-catalytic converters (SACs) and low grade jewelry waastes could be treated by the very same principles. E-wastes are found to contain notable concentrations of copper (Cu) and gold (Au), with levels approximately 13–26 times and 35–50 times higher, respectively, than those typically present in the minerals conventionally employed for the extraction of these metals. In SACs, Platinum group metals (PGMs) such as Pt, Pd, Rh is typically present in larger quantities with concentrations ranges spanning from 100-3000 ppm, 5-1700 ppm and 5-240 ppm respectively while PGM ore deposits contains only 2.8 to 0.7 ppm. Low-grade jewelry waste including floor sweepings, polishing dusts, rags and tissues, emerge as significant reservoirs of gold (Au) and silver (Ag), exhibiting concentrations as high as 848 ppm Au and 7812 ppm Ag was reported. In contrast, the gold concentration in conventional gold ores typically falls within the range of 1 to 15 ppm. This highlights the potential value in extracting precious metals from seemingly less significant waste materials, underscoring the importance of exploring diverse sources for resource recovery. In this study, a pyrometallurgical process and slag optimization methodology were designed for the efficient separation of metals from slag phase. Particularly, a direct current (DC) electric arc furnace (EAF) capable of reaching a temperature of 1800 °C in a short time was designed and constructed for the recovery of metals trapped in various sources such as Cu flash furnace slag, jewelry waste slag and spent auto-catalyst (SAC) containing platinum group metals (PGMs). Slag optimization studies were made through thermodynamic calculations and thermal investigations. During the construction of the DC-EAF involved several critical components: A hearth design was devised through heat transfer calculations, ensuring the inner crucible's resilience to temperatures reaching a maximum of 1800 °C. Consequently, the inner hearth was crafted using high-purity Al2O3 castable, while the outer hearth was constructed using MgO bricks and Al2O3-ZrO cement. Heat transfer calculations indicated that the outer steel casing could potentially attain temperatures as high as 425 °C, where the incorporation of a cooling system for both electrodes and the steel casing is required. To facilitate electrode motion during smelting, an electrode motion system featuring two step-motors connected in two axes was designed. Continuous arc formation during melting is essential to reach high temperatures as quickly as possible. However, due to the variation in electrical resistance of the melting environment with temperature, presence of solid and liquid phases, arc formation was controlled using an automation system that could adjust both voltage and electrode-electrode distance (or arc length). Continuous arc formation was investigated using current-time graphs and it was observed that the arc consistently formed except for brief short circuits. A power source incorporating an AC to DC rectifier and an electrode system was constructed to provide the requisite energy for arc smelting. Additionally, a mechanical tilting system was implemented to facilitate the straightforward tapping of the system. The recovery of metals from slag phase involved optimizing the melting of slags since the homogeneous melting of all phases in the slag and its connection to viscosity were crucial. Therefore, the optimization of the slags to be melted was carried out through thermodynamic calculations and thermal behavior measurements. Initially, slag samples were characterized using X-ray diffraction (XRD) and X-ray fluorescence (XRF). The SAC phase contained high amounts of SiO2, Al2O3 and MgO thus, located in the cordierite phase region. Cu flash furnace slag contained fayalite slag with FeO.SiO, while jewelry waste slag was rich in PbO due to the cupellation process used to collect the metal phase in lead. In this study, binary phase diagrams of SiO2, Al2O3, MgO and FeO, along with flux additions (CaO, Na2O, B2O3), were systematically examined, revealing crucial insights into low-temperature phases, eutectics and low liquidus temperatures. The phase-equilibrium diagrams for the spent auto-catalyst (SAC) phase, augmented with Na2O, CaO and B2O3 flux additions, were meticulously calculated. Notably, SAC phase demonstrated a melting point of 1453 °C and Na2O addition resulted in the formation of NaAlO2, NaAlSiO4 and Na2SiO3, at 0.75 SiO2/Na2O+SiO2 and (0.05 Al2O3/Na2O+Al2O3 achieving an overall slag formation temperature of 1100 °C. The introduction of B2O3 at a ratio of 0.5 MgO/B2O3+MgO led to the creation of Mg3B2O6 at an exceptionally low temperature of approximately 500 °C. Moreover, a mere 30% (w/w) B2O3 addition remarkably decreased the slag temperature to 900 °C, demonstrating the pronounced influence of B2O3 on reducing viscosity through its interaction with the SiO2-Al2O3 network. Conversely, the incorporation of CaO ((0.6 SiO2/CaO+SiO2) and (0.354 Al2O3/CaO+Al2O3)) formed calcium-based silicate and aluminates, with specific phases transforming at temperatures of 1350 °C, 1200 °C, 1180 °C and 1150 °C. Gibbs free energy calculations elucidated the feasibility of reactions between fluxes and slag phases. When the formation energy of CaB2O4 surpasses that of calcium silicate and aluminates, it does not contribute to a reduction in the melting point of these oxides. On the other hand, Na2O together with B2O3 prove useful as the Na6Si8O19, NaAlO2 target phases forms rather than sodium borate species resulting a decrease in melting temperature and lowered viscosity. The study further explored the thermal behavior using Differential Scanning Calorimetry (DSC), revealing endothermic peaks corresponding to slag formation temperatures and viscosity calculations demonstrated that B2O3 induced a drastic reduction in slag viscosity, even at low concentrations (5%-10%), reaching as low as 125 Pa.s. This comprehensive investigation provides valuable quantitative insights into the thermodynamic and kinetic aspects of the studied pyrometallurgical process, paving the way for enhanced efficiency in metal recovery from diverse sources. When determined flux additions from thermodynamic calculations, (0.75 SiO2/Na2O+SiO2, 0.05 Al2O3/Na2O+Al2O3, 0.5MgO/MgO+B2O3 and 30% B2O3) into SAC and (0.22 FeO/FeO+Na2O and 30% B2O3) into Cu flash furnace slag and only 30% B2O3 added to jewelry slag, recovery efficiencies were determined. The results indicated attainment of <%92.94 Pt and Pd recovery for SAC, %83.16 Cu recovery for Cu flash furnace slag and %98.48 Ag and %84.77 Cu recovery for jewelry slag. This research strives to pioneer the creation of a comprehensive approach for extracting metals from slag phases. It integrates high-temperature smelting and introduces a slag optimization methodology to reduce melting temperature and viscosity. The objective is to incorporate a minimal flux volume, a critical element for improving recovery efficiency and streamlining the industrial adoption of the process.