Smelter Grade Alumina Research Articles

Feasibility Analysis of Smelter Grade Alumina (SGA) Project Development at PT. X

Abstract: Indonesia has a wealth of promising natural resources in the form of minerals and coal such as bauxite. Considering that the government is planning to stop bauxite exports and increase domestic processing or downstreaming. So it is necessary to carry out a factory financial feasibility analysis for processing bauxite into alumina or Smelter Grade Alumina (SGA) at PT.X. Feasibility analysis using a reference capacity of 1,000,000 tons of alumina. The parameters used to determine the financial feasibility analysis in alumina production at PT. X are Net Present Value (NPV), Internal Rate Return (IRR), Payback period (PP). Based on the results of the analysis research, it was obtained that the total cost of CAPEX (Capital Expenditure) that had to be spent was Rp. 7,057,514,558,158 and the total OPEX (Operational Expenditure) costs that must be incurred is Rp. 1,479,733,686,519 with a Net Present Value of Rp. 208,752,623,349, the Internal Rate of Return is 15%, and the Payback Period is 8 Years and 3 Months, so the project is feasible to run.

The Recycling of Coal Fly Ash: A Review on Sustainable Developments and Economic Considerations

The recycling and utilization opportunities for coal fly ash (CFA) have increased in the past two decades. However, limited commercialization of the material is still reported, while disposal and management remain major concerns. CFA utilization is currently commercially feasible in the building and construction industry. Other alternative uses that are being explored involve the extraction of valuable metals and the purification of wastewater. The CFA-produced adsorbent material utilized in wastewater purification processes should be able to generate water that meets the legal quality requirements for reutilization in alternative applications. On the other hand, in the recovery of metallic components such as smelter-grade alumina, high recovery and high purity products are only achievable through the processing of CFA using expensive and energy—intensive processes. Furthermore, most of the current CFA recycling processes tend to generate secondary solid residues (SSR), which can cause environmental pollution, thus requiring further downstream processing. In this context, this paper reviews and discusses current research on CFA recycling methods, challenges and opportunities associated with resource recovery from CFA, and the acceptability of the value-added products, and it therefore proposes sustainable processes for CFA utilization. This review further suggests that to successfully compete with bauxite for production of smelter-grade alumina, other saleable value-added products such as Ti, Fe and the REEs should be recovered by engineering an integrated process design. The generated SSR in each process must also be characterized, recycled and re-used to reduce waste production and advance the circular economy concept. The review concludes that for CFA to become considered as a more attractive commercial resource, there is need for its complete and holistic utilization in high volumes and in different applications to offset its low value.

A Review: Understanding the Science and the Impacts of Impurities upon the Electrolytic Bath of Hall–Héroult Reduction Cells

The aim of this article is to present an overview of impurities of alkali metals and alkaline earth elements included in smelter-grade alumina and other raw materials that primarily report to the electrolyte, i.e., bath, and to suggest pathways for improved control at primary aluminum smelters. Comments are included on how these impurities affect intermediate subprocesses, metal products, and cell condition. The authors have worked to create a reference that not only reports on the scientific aspects of how impurities react in a bath of molten salt under electrolysis but also covers the practical matters of concern regarding impurities that affect the electrolyte. By combining scientific knowledge with process engineering techniques, readers should be able to better manage impurities and related impacts in aluminum production processes.

Recycling of spent refractory materials to produce Al–Si master alloys via the aluminum reduction cell

Abstract This paper explored a process for preparing Al–Si master alloys by recycling spent refractory material via aluminum electrolytic cells. The feasibility of spent refractory material electrolysis in the electrolytic cell was evaluated through thermodynamic analysis. The dissolution rates of alumina and spent refractory material were compared to obtain the appropriate mixing ratio. The effect of the addition of spent refractory material on the process was studied. The results show that the cryolite ratio, cathode voltage drop, and bath level were generally stable and had little negative effect on electrolysis. The current efficiency for the production of Al–Si master alloys was about 4% lower than that for the production of primary aluminum, which increased power consumption. However, the alumina consumption per ton of metal produced was reduced by 100 kg, which reduced the consumption of smelter grade alumina. Adding spent refractory material as a raw material to the aluminum electrolytic cell, fluorides were recycled to the electrolyte, whereas alumina and silica were electrolytically reduced to form liquid aluminum and silicon at the cathode. The process proposed shows immense advantage of generating no waste as slag, water or gas, and achieving cleaner production.

Revealing cracking and breakage behaviours of gibbsite particles

Mitigating gibbsite particle cracking and breakage during industrial alumina production can increase the quality of smelter grade alumina product by reducing the ultrafine particle content. Therefore, it is essential to investigate the particle cracking during static calcination and the breakage of calcined gibbsite particles under external force. In this work, we investigated the impact of the calcination ramping rate and the crystallite size on gibbsite particle cracking during static calcination. A slow ramping rate and a large pristine crystallite size tend to increase particle cracking. Apart from the study of particle cracking behaviour, we also investigated the breakage of calcined gibbsite particle under external force. Cracks on the particle surface can initiate breakage within the crystallite and along the grain boundary under external force. The breakage within crystallite occurs as the cleavage of the crystallite, while the breakage along the grain boundary leads to the shedding of a whole crystallite. We further explored the factors influencing the strength of calcined gibbsite particles. With increasing calcination temperature, the strength of particle increases when gibbsite converts to boehmite, and then decreases when boehmite converts into amorphous alumina. Particles containing smaller crystallites and calcined with fast ramping rates exhibit higher resistance to breakage.

The Adsorption Behavior of Moisture on Smelter Grade Alumina during Transportation and Storage—for Primary Aluminum Production

Smelter grade alumina (SGA) plays multiple roles in the Hall–Héroult process for primary aluminum production. Given its very porous nature, one major role of SGA is to adsorb toxic hydrogen fluoride (HF) in the dry scrubber. However, also because of its porous nature, SGA inevitably adsorbs ambient moisture. This paper discusses the influence of alumina properties, including pore size distribution and specific surface area, on the physical adsorption of water vapor on SGA, as well as the adsorption kinetics. The result shows that the adsorption enthalpy of moisture on SGA is in the range of 4–13 kJ/mol. The adsorption capacity increases significantly with the particle specific surface area and total pore volume. A higher adsorption temperature indicates a much faster adsorption rate but corresponds to a lower equilibrium adsorption capacity.

Cracking behaviour and mechanism at grain boundary of gibbsite during calcination

Limiting the cracking of gibbsites during calcination can reduce the content of ultrafine particles in the smelter grade alumina product and increase the quality of products. However, the investigation on the cracking behaviour and mechanism of gibbsites during calcination remains a challenge. In this study, we focus on the cracking behaviour and mechanism of gibbsite grain boundary during calcination. We first observe the cracking behaviour of the gibbsite grain boundary during calcination and propose a cracking mechanism. Based on the observed crack evolution at the gibbsite grain boundary, we found that the cracking of gibbsite grain boundary is initiated on the surface of particles, and then the cracks grow into the inner side of particles with the calcination. We also investigate the impact of the crystallite size and the c-axis angle between crystallites on gibbsite grain boundary. As a result, the increased crystallite size and the c-axis angle between crystallites can result in increased crack size at the gibbsite grain boundary.

Innovative Process for Boehmite Precipitation in Bayer Circuit

JNARDDC is intensely working toward the development of boehmite product hydrate as an intermediate product to reduce energy consumption in the calcination step. The innovative process of modifying the gibbsite seed property to boehmite and using it in precipitation has been thoroughly explored in the study. The investigations have shown that it can be produced in the precipitation circuit using boehmite seed hydrate under conditions different than that of gibbsite precipitation. Statistical design of experiments was applied, and liquor productivity achieved was 45–55 g/L at optimal parameters. Temperature and seed ratio are the most important facilitating factors for boehmite precipitation. Experimental results have confirmed that boehmite can be precipitated at 90–110 °C and with a caustic solution having a concentration of pregnant liquor of 145 g/L. Upon calcination at 600–700 °C, boehmite product hydrate generates alumina having properties very close to smelter-grade alumina. Characteristics of boehmite seed and product hydrate and alumina obtained by calcination of boehmite product hydrate have been explained in detail as well.

Cracking Behavior and Mechanism of Gibbsite Crystallites during Calcination

AbstractCracking is one of the major contributors to the breakage of gibbsite particles during calcination, which generates superfine particles causing detrimental effects to the quality of product in the downstream smelter grade alumina production. Therefore, understanding the cracking behavior and mechanism can provide a solution to avoid the generation of superfine particles. In this study, for the first time, the phase transformation and structural evolutions from gibbsite to boehmite then to amorphous alumina during calcination via using focus ion beam–scanning electronic microscopy and in situ transmission electron microscopy are investigated. After detailed and real‐time observations, it is illustrated that the crack formation on gibbsite particles during calcination can be attributed to the stress introduced by crystal shrinkage inside the crystallites during the phase transformation. Moreover, cracks in calcined gibbsite crystallites are found to initiate as pores inside the crystallites, which grow toward the surface and then form open cracks. Studies will open the possibility to tune the breakage of gibbsite particles.

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.

Sedimentation behavior of high-temperature concentrated colloidal suspension based on potassium cryolite

ABSTRACTThe paper describes the sedimentation behavior of concentrated high-temperature oxide-fluoride slurries with alumina particles volume fraction range 0.24 ≤ φ ≤ 0.32 at ∼700 °C. The behavior is of interest due to perspectives of the non-Newtonian fluids usage in the future aluminium reduction technology. To characterize sedimentation behavior several techniques were used: density analysis, X-Ray diffraction, microphotography and electron scanning microscopy. Sample with −63 µm dispersed phase as well as smelter-grade alumina which contains particles in a range of 10–150 µm has been examined. It has been found that particle settling occurs with the initial velocity 0.97·10−5 m·s−1 at φ = 0.24 and gradually reduces reaching zero at φ close to 0.32 which was considered as a maximum packing fraction. MS−1 sedimentation mode has been identified, the type of sedimentation was found to be changed from type 1 to type 3 in the sample with φ = 0.24. The alumina partial density distribution in suspension as a function of time was determined. Alumina average motion velocity during sedimentation was calculated to be 154·10−3 kg·m−2·s−1 at φ = 0.24 in the top layer of the sample within the first 150 s of sedimentation. It had been reducing drastically before it reached zero after 300 s. An alumina flow was higher in the top layers. Some of the rheological properties (such as relaxation time, which increased from 7.2·10−3 to 17.7·10−3 s after the increase of φ from 0.24 to 0.28) has been determined.

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.

Towards the Utilization of Fly Ash as a Feedstock for Smelter Grade Alumina Production: A Review of the Developments

The major commercial technology for the production of smelter grade alumina has conventionally been the Bayer process which converts bauxite ore, an aluminum containing ore which typically contains about 50 % Al2O3, into smelter grade alumina that contains over 99.0 % Al2O3. Another potential source of aluminum that is receiving renewed attention is coal fly ash (CFA), which is a by-product of coal combustion during electricity generation in power stations. Most coal fly ash found around the world contain typically between 25 and 35 % Al2O3, which is substantial to make CFA a potential alternative source of alumina. There has been some considerable research effort, in the last two decades, aimed at extracting aluminum from this semi-waste product. Much emphasis from the different workers has been on the leaching of the contained aluminum, while the purification and recovery of alumina from the liquor solution has received less attention. Notwithstanding that several patents on the processes to recover alumina from CFA exist, limited commercialisation of the processes has been reported. This paper seeks to review and discuss the hydrometallurgical extraction processes that have been formulated and tested by different workers and then assesses the available opportunities and the way forward on the prospects of commercially extracting alumina and other valuable components from this abundant resource.

The Formation and Dissolution of Crust Upon Alumina Addition into Cryolite Electrolyte

The properties of the alumina agglomerate/crust formed upon smelting grade alumina feeding and the corresponding dissolution rates in cryolite electrolyte were studied using a suspended weighing device. A series of experiments was designed to investigate the dissolution rate of the crust at various temperatures and the effects of initial alumina concentration in the electrolyte on the formation process and properties of the crust. Initial results showed that under experimental non-stirring conditions, a large proportion as high as 54–64% of the fed alumina participated in the formation of the crust upon feeding at 1233–1238 K (960–965°C). The dissolution rate of the crust decreased greatly with the increase of initial alumina concentration and the decrease of operating temperature. The mass of the crust increased significantly at high initial alumina concentrations and decreased temperature, while the alumina content of the crust showed a slight decreasing trend. Other parameters of the crust such as volume density and gas-filling fraction were also measured. The influence of physical stirring on alumina dissolution rate was also discussed.

Adsorptive capacity and evolution of the pore structure of alumina on reaction with gaseous hydrogen fluoride.

Brunauer-Emmet-Teller (BET) specific surface areas are generally used to gauge the propensity of uptake on adsorbents, with less attention paid to kinetic considerations. We explore the importance of such parameters by modeling the pore size distributions of smelter grade aluminas following HF adsorption, an industrially important process in gas cleaning at aluminum smelters. The pore size distributions of industrially fluorinated aluminas, and those contacted with HF in controlled laboratory trials, are reconstructed from the pore structure of the untreated materials when filtered through different models of adsorption. These studies demonstrate the presence of three distinct families of pores: those with uninhibited HF uptake, kinetically limited porosity, and pores that are surface blocked after negligible scrubbing. The surface areas of the inaccessible and blocked pores will overinflate estimates of the adsorption capacity of the adsorbate. We also demonstrate, contrary to conventional understanding, that porosity changes are attributed not to monolayer uptake but more reasonably to pore length attenuation. The model assumes nothing specific regarding the Al2O3-HF system and is therefore likely general to adsorbate/adsorbent phenomena.

Dynamic flow modelling in precipitator vessels – A study of turbulence modelling approaches

Precipitation is a fundamental unit process within the Bayer circuit for the production of smelter-grade alumina, with the predominant technology used in this unit process being the mechanically-agitated draft-tube precipitator. Previous physical modelling studies have identified dynamic behaviour in these vessels which could potentially impact fluid residence time and the ability to obtain adequate solids suspension. In the current study the commercial CFD code ANSYS CFX14 is used to simulate the dynamic, single phase flow behaviour in a laboratory-scale replica of a full-scale precipitator. Simulations are conducted to investigate the impact of mesh refinement and turbulence model selection, with two-equation, Reynolds Stress and the Scale Adaptive Simulation (SST–SAS) models being investigated. The impact of the different modelling choices on the accuracy of the simulations is assessed through comparison of the CFD results with high-quality Laser Doppler Velocimetry (LDV) data obtained in a laboratory-scale vessel.

An innovative approach for pseudoboehmite precipitation from seeded sodium aluminate solutions

Precipitation of boehmite from seeded sodium aluminate (SA) solutions for production of smelter grade alumina by calcination is in the early stage of development. However, the reported boehmites are usually well crystallized and actually most boehmites for activated aluminas are poorly crystallized and less ordered forms, which is also called pseudoboehmite. In the present study, an attempt has been made to find the appropriate precipitation conditions of pseudoboehmite from seeded SA solutions by inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that pure pseudoboehmite showing minor morphology difference with the pseudoboehmite seed can be easily obtained under hydrothermal conditions at 373 K not less than 10 h or at 398 K not less than 3 h. The hydrothermal seed precipitation is a dynamic inhibited process that approximately consists of three stages, namely a fast precipitation stage, a slow precipitation stage and an analogously apparent equilibrium stage. However, the seeded precipitation under circulation reflux at approximate 0.1 MPa is not suitable for preparing pure pseudoboehmite because of by-products formation, such as gibbsite, nordstrandite, diaspore, etc. This work demonstrates that pseudoboehmite can be efficiently precipitated from seeded SA solutions under certain hydrothermal conditions.

The nature and impacts of fines in smelter-grade alumina

Fines in smelter-grade aluminas are recognized as a significant process problem in aluminum smelting. However, understanding the nature of this fine material and how it impacts the reduction process are less clearly understood. The combination of new analytical methods such as variable pressure scanning electron microscopy and very high field solid state nuclear magnetic resonance provide new insights into the phases present and their spatial distribution within aluminas, and suggest how such fine materials are generated, particularly during calcination in the alumina refinery.

Characterization of metallurgical-grade aluminas and their precursors by 27Al NMR and XRD

The structure of metallurgical- or smelter-grade aluminas (MGAs) is complex and poorly understood. Ultra-high-field solid-state 27Al NMR results on industrial as well as on laboratory-prepared aluminas are discussed in relation to XRD results. It is demonstrated that high-field NMR can effectively be used to quantify the proportion of the thermodynamically stable alpha-alumina phase in these materials. The results demonstrate that 27Al NMR is a vital adjunct to XRD methods to quantify the transition alumina phases that invariably dominate the MGAs. The nature of the disorder in these materials, determined by 27Al NMR, is also compared with literature data, such as XANES and EXAFS studies, on typical laboratory-prepared materials. The utility of 27Al NMR studies to provide new insight into the structural complexity of metallurgical aluminas is shown.Key words: solid-state magic-angle-spinning NMR, metallurgical-grade alumina, transition alumina, gamma-alumina, coordination number.

Sequential leaching for the recovery of alumina from a Canadian clay

Clays are abundant minerals containing from 15 to 40% w/w of alumina that can be extracted using various processes. Laboratory tests were conducted to verify if alumina can be produced from a Canadian illite/chlorite type clay. Results show that smelter grade alumina can be produced, although with low alumina recoveries, using a standard sulphuric acid processing flow sheet that requires clay to be initially heat treated. The application of a sequential sulphuric/hydrochloric leach to the clay increases alumina recovery by more than 10% and can be almost equivalently applied to heat treated or crude clay.