Adamantia Lazou

The utilization of bauxite residue (BR) was experimentally studied through hydrogen reduction, focusing on iron oxide reduction and the formation of leachable alumina phases. Three different types of pellets were made: one from bauxite residue, and the other two via calcite addition with varying calcite amounts. All pellets were isothermally reduced by H2 gas in a thermogravimetry furnace at constant temperature under fixed hydrogen flow rate and reduction times. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) coupled with Energy dispersive spectroscopy (EDS), were used for the phase and microstructural analysis. The hydrogen reduction rate is influenced by iron-bearing oxides formed during heating. Bauxite residue pellets exhibit a higher initial reduction rate than bauxite residue–calcite pellets as brownmillerite formation occurs in the latter one, which hinders iron oxide phase reducibility. The reduction kinetics are affected by the reduction temperature and calcite quantity added, while dominant phase formation during hydrogen reduction depends on their combined effect. There is no mayenite (Ca12Al14O33) phase formation in reduced BR pellets, while with the addition of calcite, small amount of mayenite phase starts to appear during reduction, which increases with more calcite addition.

Titanium oxide is a major component of bauxite residue (BR) with a high value, but it is often an unwanted element in common BR reuse options such as cement or iron production. Conventional carbothermic reduction smelting of BR produces a slag still containing a large amount of Ti. This study investigates an aluminothermic process for producing an FeTi alloy by combining BR, ilmenite ore, and fluxes. Based on thermodynamic calculations and batch experiments, the amounts of aluminum (reductant) and fluxes were investigated to achieve the optimum alloy production in parallel with a slag that could be further valorized in the cement industry. The mineralogical and chemical analysis of the metallic and slag phase agreed with the thermodynamic calculations. The results obtained by this study can lead to the development of a new process for the complete valorization of BR, paving the way for scaling up aluminothermic processes for producing ferroalloys from all iron-rich residues.

This work evaluates the characteristics of calcium aluminate slag and pig iron samples obtained from the smelting of calcined and reduced diasporic bauxite ore. The study is conducted in the Pedersen process framework, which is a method to produce alumina from low-grade resources. Parameters such as the effect of crucible type, lime addition, and atmospheric conditions are studied considering the characteristics of the product pig irons and calcium aluminate slags for further uses. The behavior of the bauxite and distribution of the species between slag and metal was assessed based on the applied analytical techniques and thermodynamic calculations. Iron was reduced and separated from the slags in the presence of carbon (graphite crucible) for both the reduced and calcined bauxite. Si and Ti were mainly concentrated in the slags. Iron was separated from the slag in the absence of carbon (alumina crucible) for the H₂-reduced bauxite. The results show that slags with increased lime additions are composed mainly of 5CaO.Al₂O₃ and CaO.Al₂O₃, that are considered highly leachable compounds. An optimum CaO/Al₂O₃ mass ratio of 1.12 was suggested. The presence of O₂ and/or OH^- in the furnace atmosphere will result in the formation of 12CaO.7Al₂O₃.

The Pedersen process is a method to produce alumina from Al-containing sources, and it is a more material-efficient method than the current commercial Bayer process, since the formation of bauxite residue (red mud) is avoided, and the bauxite can be holistically consumed. The smelting reduction (SR) part of the Pedersen process yields pig iron and a calcium aluminate slag, and the latter is a feedstock material for alumina extraction via alkaline leaching. In the present study, three different bauxite ores (Greek, Turkish and Jamaican) were smelted with lime to ease the process and control the slag chemistry and coke for the carbothermic reduction of iron oxides. The slags produced were analyzed with XRD, XRF, and EPMA to identify the phases and chemical compositions. According to the results, the slags composed of Al-containing leachable phases. Moreover, it is shown that the amount and distribution of both the leachable and non-leachable phases in the slags depend on the ore chemical composition. The results are discussed regarding the characteristics and potential leachability of the slags. Standard leaching tests were performed to examine the actual leachability.

Metallurgical grade alumina is produced worldwide through the well-known Bayer process, which unavoidably generates bauxite residue (BR, also known as red mud) in almost equal amounts to alumina. This study aims the valorization of BR through a smelting-reduction process to obtain calcium aluminate slags that can be a proper feed for alumina recovery via the Pedersen process. It investigates the thermodynamics and characteristics of the slags and pig iron produced from mixtures of BR, a bauxite beneficiation byproduct, and lime. In this context, the evolution of the different phases in the slags is studied with advanced analytical techniques and thermodynamic calculations. According to the results, a CaO/Al₂O₃ mass ratio within 1.3 to 1.4 in the slags can yield more Al₂O₃-containing leachable phases, such as CaO·Al₂O₃ and 12CaO·7Al₂O₃. The cooling dictates the amount and the characteristics of these phases, and the slower cooling rate yields improved slag characteristics. The distribution of the elements between the slag and metal phases shows that iron is separated, and the majority of the P, Cr, Ni, and V are distributed in the produced pig iron, while S, Ti, and Si are mostly concentrated in the slags.

The Bayer Process is the dominant industrial method to produce alumina from bauxite ore. Due to the generation of large amounts of Bauxite Residue (red mud), an alternative method, called the Pedersen Process, is of our interest. This process makes use of a combination of pyrometallurgical and hydrometallurgical techniques in order to prevent the Bauxite Residue generation. In the conventional Pedersen Process, iron in the bauxite is separated in the form of pig iron through a carbothermic smelting-reduction step which has a CO₂ emission similar to that during conventional iron production. In order to eliminate the CO₂ emission of this step, the focus of the present work is to reduce the iron oxides of bauxite ore by hydrogen gas (H₂) prior to smelting and minimizing the use of solid carbon materials for the reduction. The thermochemistry and the kinetics of reactions during calcination and direct reduction of a bauxite ore were studied by a thermogravimetric technique and in situ analysis of the gaseous products. The onset temperatures for the decomposition of bauxite components during calcination were determined. The kinetics of the reduction of hematite to metallic iron was studied and it is shown that the reduction of iron oxides to metallic iron starts at temperatures below 560 °C and it accelerates at higher temperatures. Moreover, it is indicated that the formation of hercynite (FeAl₂O₄) phase retards the complete reduction at temperatures higher than 760 °C.