Waterstofreductie van bauxietresidu voor duurzaam metaalherwinning

Bauxite residue (BR), a red-colored waste material from the Bayer process used for alumina extraction, poses a significant environmental challenge due to its large-scale generation and storage. Over 160-170 million tons of BR are produced annually worldwide, with global stockpiles of this material exceeding 5 billion tons. These large quantities of BR are often stored in specially designed impoundments or tailing ponds, creating significant environmental risks, including contamination of surrounding ecosystems, water resources, and air quality. The vast amounts of BR, coupled with its high alkalinity, fine particle size, and complex mineral composition, make its management a critical issue for the alumina industry. BR is primarily composed of iron oxides, aluminum oxyhydroxides, perovskites, calcite, complex phases including desilication products, and trace amounts of valuable metals such as scandium and rare earth elements (REEs). Despite its potential as a polymetallic resource, less than 3% of BR is currently utilized, hindered by technological, social, and economic challenges. Given the growing global demand for metals and the urgency to address environmental concerns, advanced metal recovery technologies are essential to transform BR from waste into a valuable resource. This shift is critical not only to reduce the environmental footprint of the alumina industry but also to promote a sustainable, circular economy. Existing disposal practices pose significant challenges, and many process routes remain confined to the lab scale. Developing efficient and scalable methods for recovering metals from BR has become increasingly important. To address these challenges, this PhD work explores several innovative processes aimed at recovering valuable metals from BR. The focus is on maximizing metal recovery while simultaneously minimizing waste generation. The first method explored in this research involved the hydrogen reduction roasting of BR at relatively low temperatures, followed by combined water leaching and wet magnetic separation. This process was designed to maximize the formation of magnetite (Fe3O4) and sodium aluminate (NaAlO2), enabling efficient recovery of Fe (as magnetite), Al, and Na (as sodium aluminate-rich leach liquor). The reduction roasting experiments were conducted within a temperature range of 400-700 °C, durations (30-120 min), and NaOH concentrations (10-20 wt%), under controlled H2 atmospheres. At 500 °C, complete conversion of hematite to magnetite was achieved, while temperatures up to 700 °C facilitated enhanced sodium aluminate formation. The roasted material underwent combined water leaching at 60 °C for 60 min, with wet magnetic separation at a 0.29T magnetic block. The optimum process parameters, including NaOH concentration, reduction temperature, and reduction time, were identified using response surface methodology (RSM). The optimized parameters identified as 600 °C, 120 min, 20 wt% NaOH, and 5 vol% H2 (20 L/h flowrate) enable the concurrent recovery of Fe (74%), Al (81%), and Na (88%). The magnetic fraction's magnetite content was 66 wt% (with Fe grade of 48%) at optimum conditions. The Fe grade of raw BR is 26.6%. The Fe-rich magnetic fraction potentially can be used for additive in Fe sintering applications, pigments, and energy storage, and it needs further investigation for various market applications. The non-magnetic fraction (contained 16 wt% SiO2, 17 wt% CaO, 29 wt% Fe3O4, 12 wt% TiO2) required further study to assess its use as a cement filler, an additive for vitrified BR, or a source of Ti recovery (as perovskite). A pilot-scale campaign was conducted using a modified 200 kW tilted rotary induction furnace with a batch capacity of 50 kg. The process followed the sequential steps of H2 reduction roasting, followed by wet milling, filtration, and wet-magnetic separation. In addition to the parameters identified in the lab scale (temperature, time, and NaOH addition), a variety of process parameters, including H2 flow rates, furnace inclination angles, H2 pressure, and reduction time (in pilot scale) were investigated. The optimized conditions were found to be 600 °C, 20 wt% NaOH, and 5 vol% H2 at 20 L/min for 4 h. The roasted pellets were subjected to wet milling in a 5-liter attritor mill for 30 min, followed by wet magnetic separation using a 0.15 T magnetic field in the lab scale. This process achieved a maximum recovery of 71% Fe with a magnetite content of 55 wt% (with a Fe grade of 40%). The leach liquor had an Al recovery of 68% (with an Al2O3 concentration of 160 g/L), and a Na recovery of 66% (with a Na2O concentration of 139 g/L). The limited (compared to lab-scale process) recoveries of Fe, Al, and Na were primarily attributed to the formation of sodium aluminum silicate phases and ultra-fine magnetite particles, which were challenging to separate. In addition, the magnetite phase consisted of both solid solutions, with trace elements such as Sc and 232Th substituting into the crystal structure of Fe3O4, and pure Fe3O4 crystals present within multiphase grains. Impurities like sodium, aluminum silicate, and perovskite were also embedded in the matrix, leading to limited Fe grades and recoveries. Overall, in the initial process, Fe separation was limited, achieving a maximum recovery of only 75-77.8% with a magnetite content of 55-63.6%. Additionally, the magnetite phase contained impurities, including perovskite in the magnetic fraction, which could complicate downstream processing. These limitations highlighted the need to explore alternative approaches to improve Fe recovery and purity. In another approach to improve the recovery rates and purities, a combined H2 reduction roasting of BR + 20 wt% NaOH, followed by water leaching and a two-stage wet magnetic separation, was employed to recover metals at higher temperatures (up to 900 °C) in the form of metallic Fe and sodium aluminate. The objective was to achieve efficient recovery of metallic Fe, Al, and Na while generating non-magnetic residue enriched with Ca, Si, and Ti. Under optimized conditions (900 °C for 120 min with 20 wt% NaOH, 30 L/h H2 flowrate), the recovery rates were 77% for Fe, 86% for Al, and 88% for Na. The non-magnetic fraction was enriched with perovskite, calcium silicate, and calcite, along with some amorphous phases. Compared to the initial process, this approach achieved higher Fe recovery, and Fe grade, and improved Al and Na recovery. The magnetic fraction presents a high-grade Fe source, suitable as an additive in sintering, blast furnace pellets, or self-fluxing pellets for Fe production in an EAF. Although the Fe grade reached 69.1%, further refinement is needed to meet the DRI standard (>88%) for EAF steelmaking. Additionally, the sodium aluminate solution can be further processed to extract pure alumina and sodium carbonate through wet carbonation, as there is high liquid generation. The non-magnetic fraction, enriched in Ca, Si, and Ti, holds potential for use as a filler material in construction applications or for Ti extraction. The previous processes generated a highly diluted sodium aluminate-rich liquid, which required additional steps to recover aluminum trihydrate. Additionally, Fe recovery/purity was limited, and the presence of impurities in the magnetic fraction made it unsuitable for direct use in the iron/steel-making process. The final process developed in this work combines an integrated approach of a low-temperature H2 reduction roasting of BR with 20 wt% NaOH at 600 °C, 120 min, 5 vol% H2 - 20 L/h. This is followed by water leaching and carbonation to separate aluminum trihydrate. The remaining insoluble solids are then smelted with biocarbon, producing pig iron and slag (enriched in Ca, Si, and Ti). The optimum conditions for carbonation (80 °C, 120 min) were found to yield an Al recovery of 96.5% with an alumina grade of 98.8%. The remaining insoluble solids were smelted at 1650 °C with 6 wt% CaO and 20 wt% bio-carbon, resulting in a 98.7% Fe recovery and 92.6% Fe grade, ensuring efficient slag-metal separation and pig iron production. While this integrated process offers a comprehensive, sustainable solution for BR utilization, its reliance on high-temperature smelting presents certain limitations. The processes developed in this PhD work offer advanced approaches for metal recovery from BR. Among them, one process was demonstrated at a higher TRL, highlighting its potential for scaling up. The metal recovery processes developed in this work provide a foundation for future research for utilizing BR for metal recovery using hydrogen or other environmentally friendly additives, pilot-scale validation, and potential industrial adoption. These strategies may also be applicable to other secondary residues or tailings with similar chemical properties, contributing to resource efficiency and supporting the sustainable development goals established by the United Nations.