Linus Perander

The use of XANES to clarify issues related to bonding environments in metakaolin: a discussion of the paper S. Sperinck et al., “Dehydroxylation of kaolinite to metakaolin-a molecular dynamics study,” J. Mater. Chem., 2011, 21, 2118–2125

We provide a discussion of some issues raised by a recent paper published in Journal of Materials Chemistry regarding the local structure of metakaolin. Furthermore, we show using synchrotron X-ray absorption near-edge spectroscopy (XANES) that tri-coordinated aluminium sites can exist in metakaolin, providing new evidence regarding the coordination environment of aluminium in metakaolin.

Towards Redefining the Alumina Specifications Sheet - the Case of HF Emissions

For smelting applications, alumina quality is typically defined in terms of chemical and physical properties, with emphasis on impurity elements, surface area, moisture content, particle size distribution and attrition index. However, these properties fail in prediction of the true HF generation potential, as well as the real capacity for HF removal in the dry scrubbers. Using plant measurements and additional laboratory characterization of a number of alumina samples a broadening of how alumina quality is specified is argued for. Measurements of the residual gibbsite/boehmite content and the pore size distribution, coupled with characterization of the alumina microstructure, can be used to predict and understand the generation of HF during feeding and dissolution as well as the ability to capture HF in the dry scrubbers.

Relationships Between Smelter Grade Alumina Characteristics and Strength Determined by Nanoindentation and Ultrasound-Mediated Particle Breakage

The mechanical strength of smelter grade alumina (SGA) is of considerable practical significance for the aluminum reduction process. Attrition of alumina during transportation and handling generates an increased level of fines. This results in generation of dust, poor flow properties, and silo segregation that interfere with alumina feeding systems. These lead to process instabilities which in turn result in current efficiency losses that are costly. Here we are concerned with developing a fundamental understanding of SGA strength in terms of its microstructure. Nanoindentation and ultrasound-mediated particle breakage tests have been conducted to study the strength. Strength of SGA samples both industry calcined and laboratory prepared, decrease with increasing α-alumina (corundum) content contrary to expectation. The reducing strength of alumina with increasing degree of calcination is attributed to the development of a macroporous and abrasion-prone microstructure resulting from the ‘pseudomorphic’ transformation of precursor gibbsite during the calcination process.

Alumina Calcination: A Mature Technology Under Review from Supplier Perspective

Calcination is the last step in the production of alumina from Bauxite. In modern refineries this step is carried out in stationary calciners, such as Circulating Fluidized Bed (CFB), Gas Suspension (GSC) or Fluid Flash (FF) Calciners. These technologies have been available for over 40 years, and are thus very far matured. The technologies have developed substantially and many boundaries have been pushed, sometimes close to the theoretical limit. Yet the development has not stopped and new concepts and technologies are being explored. In this paper the authors discuss, from a supplier perspective, what was driving the design in the past, at present and possibly in the future, and also what the challenges typically encountered are.

The Impact of Alumina Quality on Current Efficiency and Energy Efficiency in Aluminum Reduction

Current efficiency and energy efficiency are the two most important metrics for assessing the overall performance of a given potline. The influence of alumina quality on these parameters is however poorly understood although likely influential. This interplay is considered here with the construction of multiple regression models from daily data of test groups of 60 cells each for a number of important parameters, pertinent alumina properties from detailed materials characterization of judiciously chosen aluminas and relevant weather parameters for approximately 20 months of effective sampling. The concentration of α-Al2O3 in the fines is found to be the single most influential parameter affecting current efficiency and energy efficiency. Models of superheat and noise indicate that this is predominantly due to issues associated with alumina dissolution. Energy efficiency is also correlated with gibbsite content of the fines, likely due to the added energy required for the phase transformation from hydroxide to sesquioxide.

Two Perspectives on the Evolution and Future of Alumina

Over the 125 year history there have been a number of step-changes in the Hall-Heroult process, despite a remarkable adherence to the original concepts of the invertors. In addition to the steady increment in scale, most noteworthy perhaps have been the introduction and impending disappearance of Soderberg technology, the introduction of magnetically compensated cell design, changes in dynamics of alumina feeding and the introduction of dry-scrubbers for HF control and fluoride recovery.

Balancing Sodium Impurities in Alumina for Improved Properties

As there are direct and indirect impacts of feed material purity on the aluminum production process and metal grade, there is a high demand on the so-called pure smelter grade alumina (SGA)—the main feedstock for aluminum production. In this work, impurities within the precursor gibbsite used for SGA production and SGA are studied using NanoSIMS and XPS with a focus on sodium—the most abundant impurity. Although the industry trend is towards minimizing sodium due to the well-known negative impacts on the process, high sodium is also correlated with relatively attrition-resistant calcined products. Here, we show that this relationship is indirect and arises from sodium’s role in inhibiting α-alumina formation. Alpha alumina formation in SGA has previously been demonstrated to induce a macro-porous and therefore attrition-prone microstructure. Sodium distribution within the precursor gibbsite and its migration during the calcination process are proposed to be most likely responsible for the spatial distribution of α-alumina within the calcined product grain. This in turn determines the behavior of the product during its transportation and handling (i.e., attrition). Therefore, tolerance of a certain amount of sodium within the precursor material does demonstrate a net benefit while balancing its negative impacts on the process.

Increased Operational Flexibility in CFB Alumina Calcination

Operational flexibility with regards to production rates and fuel usage in alumina calcination is becoming increasingly important in today’s market. Outotec has a range of technologies, which can be retrofitted in old plants, or offered as options in new developments, that are aimed at improving the plant flexibility and performance, while maintaining product quality.

Observation of a Collapsing Layer Structure in Mg/Al Layered Double Hydroxides with Interlayer Hydrogen Phosphate by High Temperature In Situ X-Ray Diffraction with Synchrotoron Radiation

The collapsing layer structure of hydrogen phosphate intercalated Mg/Al-layered double hydroxides (LDH-HPO4) was investigated by high temperature in-situ X-ray diffraction with synchrotron radiation (in-situ XRD). The thermal behavior of the layer structure of LDH-HPO4 showed the same behavior in both in-situ and ex-situ XRD up until 109 °C as previously reported. The basal spacing of LDH-HPO4 continuously decreases above this temperature and finally the layer structure of LDH-HPO4 collapses. Another diffraction peak, d110, which gives a lattice constant a0, decreases at 62 °C, keeps constant to 120°C, increases up to 176°C and again keeps constant until the layer collapse. In this study, the different thermal behavior is observed between two reflections by in-situ XRD measurement, which explains the mechanism of the collapse of the layer structure in LDH-HPO4.