Smelter Grade Alumina from Bauxite

This book centers on the production of purified alumina (Al₂O₃) that is used as a feedstock to smelting cells that convert alumina to purified aluminum (Al). The smelting process used universally around the world for the production of aluminum is known as the Hall-Heroult process. This process was independently discovered and patented in1886 in the United States by Charles Martin Hall and in France by Paul Heroult. For the process to be commercial, however an inexpensive supply of high purity alumina was necessary. Fortuitously, in 1888, the Austrian chemist, Karl Joseph Bayer patented a simple caustic based chemical process for recovery of purified alumina using bauxite as a raw material.

Bauxite turned out to be a very abundant raw material, although, primarily in the tropical regions of the world. As a result, the combination of these two processes remain to this date the most economical method of producing purified aluminum metal.

This Chapter gives an overview of the Bayer Process as a lead into the following chapters where all aspects of the history and improvements to the original process are discussed. Also in this chapter is a brief discussion of alternative aluminum containing raw materials and processes that have been researched and, in a few cases, commercially used to recover alumina.

In 2020 the production of Smelter Grade Alumina (SGA) reached 126.7 Mton with China accounting for 53.3%. The corresponding global average energy intensity was about 10.7 GJ/ton SGA. The global demand growth for primary aluminum in 2021 is estimated to increase about 7% per annum, driving the demand for SGA subject to changes in stored inventory and any oncoming new capacity commissioned.

By the end of 2020, the price of SGA was 305 US$/ton, up from 275 US$/ton the year earlier. The decoupling of SGA prices as a percentage of the 3-month LME primary aluminum price continues.

The focus on the Environmental Footprints is changing from best practice bauxite residue management to reduction of “green-house” gases, notably CO₂, which appear to be a tremendous challenge for the global alumina industry for many years to come.

In this chapter the Authors summarize the typical features of bauxite which are believed to play roles in mining, beneficiation and mainly in alumina processing, that is, the industrial value of the raw material. Geological curiosities are neglected. General physio-chemical conditions are outlined in order to understand the ore formation being determinant of the quality (chemistry and mineralogy) and quantity of the bauxite ore. Special emphasise is given to its chemical and mineralogical composition make up and their alterations are shown in selected examples, indicating the possible inhomogeneity of the mine product processed in the refineries. In deposit geology laterite, karst and sedimentary/paralic deposits are distinguished requiring significant differences in mining methods and technology applied in processing. In laterite deposits three main types are introduced namely: (1) plateau type deposits developed on morphological terraces and low land interfluves, (2) whale back plateaus and (3) dome shaped hills confined by hilltops and bauxites on hill tops with associated slope bauxites. For estimating the possible supply furnishing of operating refineries or for establishment of green-field plants an overview is given on global bauxite reserves and resources with their grade and mineralogy, amended with their further potential established with the aid of remote sensing technics introduced by the Author in 2006. Principles of physical bauxite beneficiation of the Run of Mine (ROM) bauxite is presented together with the most common unit operations applied. A flowchart example is shown integrating many of the unit operations. To minimize transportation cost and be able to supply more than one refinery, the beneficiation takes place at the mine site before the beneficiated bauxite is exported to the refinery. Principles of thermo-chemical bauxite beneficiation/activation is presented for three distinctive purposes: (1) Reduce organic carbon in the ROM bauxite before feeding the refinery and thus reducing the organic carbon input to the Bayer process liquor before digestion; (2) Activate the content of boehmite in the ROM bauxite to enable low temperature digestion with the side benefit of significantly reducing the organic carbon in the ROM bauxite, and (3) Pyro-genic attack or sintering of the ROM bauxite with limestone and/or sodium carbonate, when the bauxite is composed of mainly mono hydrate bauxites (boehmite or diaspore) and low to high silica content in order to maximize yield of alumina, recovery of caustic and reduced energy consumption. Only the soda-lime sintering process has gained commercial status over time, and especially in China. All the thermo-chemical bauxite beneficiation/activation processes are planned to be built at the refinery site, and in the case of the soda-lime sintering process integrated with the Bayer process in the overall process flowsheet.

The most common process step to feed an alumina refinery with bauxite is sizing of the raw bauxite material that is extracted from the mine. The first crushing of the bauxite may often take place at the mine before transport to the alumina refinery if located nearby, or to the shipping port for export. At the refinery further crushing and/or washing may take place before the crushed bauxite is subjected to a grinding operation for final sizing before digestion. This chapter present some of the various crushing, washing, and grinding equipment available in the market as well as the various options for designing a grinding flowsheet.

This Chapter provides an introduction to the Bayer process and how the mineral composition of bauxites affects the process variants and the principal parameters to be selected. The Chapter covers the rational of sizing of principal equipment (such as heat recovery system and reactors) and also of calculating the energy (heat) requirement of the digestion unit operation. Beside the processing behaviour of the main constituents of bauxite such as hydrated alumina and silica minerals, the Chapter covers the impact of other constituents, such as titania, iron compounds, organics, and other impurities. The use of lime in the Bayer process and the chemistry behind that is also discussed.

Post-digestion, the solids and aluminum bearing liquor need to be separated, and the solids washed to remove and recover the bulk of the liquor. Sedimentation and filtration equipment are commonly used for this. High temperatures, high scaling rates, and very fine particle size solids make these applications challenging. De-sanding equipment technologies and combinations thereof are described which frequently have been installed to protect flat bottom thickeners from high loads of coarse solid material. Today’s thickener technology providing relatively small diameters, steep bottom cones and a high mud bed mostly are operating without sand separation units. Flocculation of the fine solids into larger agglomerates is a key aspect of thickening, along with diluting the feed to an optimal concentration for the flocculation reaction. Good flocculation allows both the production of both clean overflows and high underflow densities that result in better recoveries and washing efficiencies in the CCD circuit. Scale growth is a major operating issue and eventually requires equipment shut down and descaling, so all thickeners are designed to be bypassed during the descaling operation. Historically, descaling was done largely by hand, but newer equipment is frequently designed for use with mechanical descaling arms that improve operator safety. Filtration technologies are described that have been developed by various companies for pregnant/green liquor filtration along with their specific features. A novel filter design is described that is operated completely without the addition of tri-calcium aluminate (TCA) as a filter aid. That specific characteristic is achieved due to a unique filter media cleaning technology comprising an automated hydro jet system.

The chapter on bauxite residue (red mud) comprehensively reviews all aspects of bauxite residue from separation, management and disposal through to the ever-increasing use of the Material. Mud washing and the optimum recovery of soda and alumina values is presented. Dewatering options, filtration theory is described in detail and the benefits that can be achieved using various filter types. The management and storage of bauxite residue is then reviewed from a historical perspective together with the trends and changes in disposal methods. Topics covered include sea water neutralisation, costs of disposal, dry mud stacking, filtration and as well as recent developments in the remediation of closed bauxite residue areas. Some current best practices in Europe and Australia are discussed with a detailed review of mud farming methods and rehabilitation trials that have been carried out at Aughinish Alumina in Ireland. Alcoa’s vision for more sustainable management of bauxite residue in their Australian operations is described together with a historical overview of best practices covering their move into thickened slurry disposal, solar drying through to filtration. The mineralogy and chemical composition data for bauxite residue are discussed prior to discussing possible uses. The very wide spectrum of uses is reviewed from laboratory studies that have been undertaken through to current significant industrial uses. Areas covered include use of bauxite residue in cement, iron and steel extraction, materials recovery (particularly rare earths), landfill capping, heavy metal and phosphate removal, manufacture of construction products, heat storage medium, and fire retardants. Some of the recent extensive work programmes being undertaken under EU HORIZON 2020 funding are also reviewed.

The precipitation area is an area of a Bayer plant that is mostly responsible for the production and product quality. The control of the circuit is seen by many as art rather than a science because of its complexity and the many aspect of the process that are difficult to understand easily at first. Another factor that is particular to this area is the long inertia and slow response of the circuit, partly due to the large inventory of seed, equivalent to more than a week of production for most circuit. This chapter covers the science to help understand this area in order to control and optimise it better. Classification of the precipitated hydrate is an integral part of the precipitation circuit operation and the presentation is based on hydrocyclones as primary and secondary classification devices which have replaced conventional thickeners in modern refineries. Fine seed thickening is performed in a conventional thickener. 3-stage classification flowsheets are discussed versus 2- and 1-stage and the influence on final strength of alumina is discussed as a consequence of the number of classification stages. Disc filters with large diameter discs are the standard equipment for coarse and fine seed hydrate filtration. These filters have replaced other vacuum filters, mainly disc filters with smaller disc diameter, due to their significantly higher filtration capacity which is a result of a significantly increased hydraulic capacity of the filter components in combination with a short cycle time. For product hydrate filtration horizontal pan filters are the state of the art technology, allowing for effective cake washing with up to three counter current wash stages, in combination with the application of steam for reducing the cake moisture.

Impurities introduced into or generated by the Bayer alumina production process can have a detrimental effect on the efficiencies or productivity of industrial alumina production, and/or alumina hydrate or alumina product quality. Significant complexity and cost can be added to the Bayer process to manage them. This chapter looks at the main impurities and the common strategies and industrial processes for management of their impact on smelting grade alumina’s production cost, quality and environmental footprint.

This chapter aims to illustrate and discuss the inputs and outputs for both water and energy in the context of the Bayer process, highlighting throughout the sections their interconnectedness. It breaks down these inputs and outputs in light of the unit operations of the Bayer process and discusses the reasons as to the variations in consumption figures, based on refinery design and other raw material considerations. The section on water shows how different technological developments, in conjunction with the bauxite to be processed, can drive much of the Bayer loop water balance. Explanations are given as to why boehmitic bauxites have the potential to drive somewhat different water balances compared to gibbsitic bauxites, owing to higher digestion temperatures and the removal of water within their precipitation circuits. Going through the history of the Bayer process and the changes brought out by modern equipment, examples show how the alumina refining process has undergone significant change over time. From a design standpoint, examples discussing caustic washing systems for precipitators highlight the significant impact this critical system can have on both equipment availability as well as the plant water balance. Beyond the Bayer loop, the greater water balance of the refinery is considered, showing how much of this balance is a slave to the climatic conditions of the refinery, but also highlighting how residue disposal technology has significantly changed the net balance through adjusting the areas for water catchment versus evaporation. Within energy, a significant focus is placed on the importance of precipitation yield of alumina in the Bayer liquor, in determining a plant energy efficiency, and how this can perhaps help to explain the variability in different plant energy numbers. The quality of the bauxite is also discussed, including the analysis of energy requirements in high temperature digestion refineries, or boehmitic/diasporic bauxite operators. The subsequent section on clarification reinforces the importance of yield further, emphasizing how residue handling solutions in different parts of clarification have both hindered and helped yield and recovery. The sections on heat interchange and precipitation show how modern design improvements such as plate heat exchange between green and spent liquor as well as inter stage cooling have unlocked even more potential in energy efficiency.

This chapter covers the changes in calcination technology, energy consumption, air pollution control and quality of SGA developed over time since the end of the 2nd World War and up until today. The demand for SGA with high specific surface area ranging from 50 to 80 m²/g (BET method) originated from the alumina smelter customers driven by the increasing need to capture HF gas emitted from the smelter cell’s by so-called Dry-Scrubbing with virgin alumina in Gas Treatment Centers (GTC) located at the alumina smelter. The physio-chemisorbed HF on the virgin alumina collected in the GTC as secondary alumina, was then fed to the alumina smelter cells, or pots, via pot feeding systems. This development starting around 1970 made the Floury type SGA, with very low specific surface area around 5 m²/g, obsolete, and Floury SGA was gradually replaced with so-called Sandy type SGA. The oil crisis around 1972 accelerated the development, scale-up and commercialization of stationary calciners for production of SGA initiated by Alcoa in the mid 1950 ties. The 25–30% reduction in specific thermal energy consumption in Rotary Kilns, when compared to Stationary Calciners was a very strong driver. The retention time in the Rotary Kiln was reduced from hours to minutes in Fluidized Bed calciners and from minutes to seconds in Gas Suspension or Flash calciners. Simultaneously with the above development several Rotary Kilns were retrofitted with pre-heater cyclones, and in one case also with a calciner furnaces, to reduce the specific thermal energy consumption and increase the production capacity of SGA. The largest Rotary Kilns for production of SGA had a design capacity of 1400 tpd SGA. This capacity was exceeded several times at Queensland Alumina, Australia, where the new Gas Suspension Calciners installed in 2002–2004 was designed for 4500 tpd SGA, driven by economy of scale. Today, the preferred design capacity of stationary calciners is around 3500 tpd SGA to match the single train production capacity of the Bayer Process circuit in modern alumina refineries. However, the above technology shifts have not come without new challenges to the Bayer process design. One such major challenge is to produce a hydrate quality, which upon calcination to SGA in stationary calciners, results in a particle size distribution and strength as SGA, that meets the specification of the smelters. Driven by environmental requirements of reduced dust emission Queensland Alumina, Australia, as the first in the alumina industry, decided in 2002, to install Baghouses or Fabric filters on their new Gas Suspension Calciners instead of Electrostatic Precipitators (ESPs), in order to avoid excessive dust emission during a power failure. Up until today heavy fuel oil, natural gas and coal gas (CO + H₂) is used as fuel in stationary calciners. But the challenges laying ahead with a predicted increase of the global warming of mother earth, makes hydrogen produced by electrolysis powered with renewable energy the preferred fuel for the not so distant future with potential to make the production of SGA CO₂ free.

The pneumatic handling of Primary Alumina, Secondary Alumina as well as Crushed Bath, Anode cover material and Fluoride (AlF3) is very challenging, caused by the very different and extreme bulk characteristics. Large capacity storage facilities as well as huge transport capacities over long conveying distances are required at the harbor areas and the smelter plants all over the world. The most important State-of-the-Art technologies and components are shown and described in this chapter. The right selection of a competitive system for a smooth and highest reliable pneumatic handling of the different bulk materials is our daily challenging business. Long term experiences and a strong performance are the keys for satisfied clients worldwide, our best partners since decades. The selected references are taken from the list of hundreds of plants worldwide.

The health and safety of personnel working in industrial facilities must be secured by reasonable protection measures. Likewise, the protection of the environment from the operation of the industrial facilities must meet the emission standards set by the authorities to obtain and maintain the License to Operate these industrial facilities. The prime focus of this chapter considers the Health, Safety and Environmental (HSE) issues from a process point of view. This means that protecting personnel health is limited to a description of the major chemicals that the personnel can be exposed to. Thus, leaving it up to the refinery management to ensure that adequate procedures and protection measures to protect the health and well-being of their personnel during their working hours is enforced. Several cases of hazardous events in alumina refineries, including the bauxite residue storage area, introduces some of the process safety issues that have occurred in the past. From there, an introduction to Functional Safety is made representing a systematic approach to deal with this subject in today’s alumina refinery design and capacity expansions. Safety aspects of operating high pressure and temperature equipment in the digestion area are touched upon as well. Sources of the major gas borne emissions to the Environment from the alumina refinery operation within allowable local emission limits is discussed for NO_x, CO, SO_x, VOC, CO₂, and particulate matter. Time has shown that authorities in general tend to tighten the allowable gas borne emissions to the environment and in many places rightfully so. Anticipating that this trend will continue in the future, it is important to understand current days technology with a view to future possibilities for acceptable solutions. In the near-term Natural Gas will be replacing HFO to lower SO_x emissions and the “Carbon Footprint” of Alumina Refineries. In the medium-term, the ESP may be gradually outplaced with Fabric Filters (FF) or ESP/FF hybrids using “Tailor made” catalyst to reduce emissions of NO_x and VOC’s and eliminate emissions of dust plumes when a power failure happens. In the long-term we may expect to see a gradual replacements of fossil fuels with “Green Hydrogen” to reduce the “Carbon Footprint” of future Alumina Refineries to almost ZERO.

The aim of this chapter is to show the intricate relationship between bauxite resource, refinery technology, sustainability, and economics of (Bauxite and) Smelter Grade Alumina projects, and by doing so generate a better understanding of the underlying factors controlling the technical and economic success of these projects. The chapter opens with an overview of key aspects of project economics, including evaluation criteria, sensitivities, main scope items, project development stages and some of its main economic drivers. It also defines general and economic, and project specific assumptions of a reference project which is used throughout the chapter. The following two sections provide more in-depth analyses of capital and operating costs of a (Bauxite and) Smelter Grade Alumina project, including cost build-up aspects, and a detail breakdown of capital and operating costs of the reference project (based on economic conditions as at end 2016). The chapter concludes with a section covering plant and process capital and operating cost saving opportunities, e.g., through plant productivity and energy savings, Bayer Loop simplification and alternative plant design.