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Inert anode technology for aluminium smelters

“Final energy consumption in the global aluminum industry in 2007 was estimated to be 93 Mtoe. The industry is highly electricity-intensive. Primary aluminum smelters used just over 50 Mtoe of electricity in 2007, equivalent to about 4% of global electricity consumption. In total, the aluminum industry emits 0.4 Gt CO2-equivalent of greenhouse gases, including process emissions and indirect emissions from electricity production, equivalent to just under 1% of total global greenhouse-gas emissions (IEA, 2010).”

Given that primary aluminum production consumes about 20 times more energy as recycled aluminum and given that electricity consumption constitutes roughly one-third of the production costs, reducing electricity consumption is one of the industries R&D priorities. Aside recycling, several technologies to improve the performance of the aluminum production process are available, such as the use of inert anodes (instead of the conventional carbon anode). Inert anode technology is seen as one of the more promising options for the industry for the years to come.

Introduction top

Production of aluminum is based on the electrolytic reduction of alumina, which is an energy intensive process (about 13-16 kWh per kg aluminum) and produces significant emissions of greenhouse gases (mainly direct process emissions from CO2 and PFCs and indirect emissions from electricity production). The Hall-Heroult electrolytic process that has been developed in the late 19th century still forms the cornerstone of the aluminum production process. Most of today’s aluminum production facilities use carbon anodes that are dipped into an iron vessel or ‘pot’ (that functions as a cathode) that contains cryolite and alumina. The electric energy that runs through the anode starts the smelting process, where molten aluminum is tapped for further processing.

illustration © climatetechwiki.org

Figure 1: Design of a Hall-Heroult electrolytic smelting cell

When carbon (or consumable) anodes are used, the reaction frees up the oxygen present in the alumina, but it immediately reacts with the carbon from the anode to form CO2. The process as such consumes over 400 kg carbon anodes per tonne of aluminum. Using inert (or non-consumable) anodes avoids the formation of CO2, so that only pure oxygen is produced as a byproduct. “If successfully developed and applied, inert anode technology could have significant energy, cost, productivity, and environmental benefits for the aluminum industry worldwide. When combined with other advances in electrolytic cell design, such as wettable cathodes, the technology should achieve even greater benefits. (Inert Anode Roadmap, 1998).”

Feasibility of technology and operational necessities top

The feasibility of the technology depends on multiple factors of which a considerable number are R&D related. Some of the main technological challenges include that of anode material selection, combining inert anodes with advanced electrolytic cell design.

Materials

Conventional carbon anodes have a limited life-span as they as ‘consumed’ during the smelting process. The oxidation (or consumption) of the carbon anode creates greenhouse gas (GHG) emissions. Inert anodes or so-called non-consumable anodes avoids oxidation and thus limits the development of GHGs. However, within the Hall-Herault smelting process, the electroactive surface of an inert anode must be oxide with semiconducting properties, and as all oxide materials have a finite solubility in the fluoride electrolyte (which is highly corrosive), a completely inert anode is unlikely to be found.

Most research on anode materials is therefore focused on finding and developing the right alloys and/or composite materials that possess low corrosion characteristics (i.e. low consumption rates), so that anode lifespan is optimised. Examples of anode materials being researched or under development are, ceramic anodes, cermet anodes, metal (alloy) anodes and various coatings. Inert anode should a.o. possess desirable properties such as ‘1) low solubility in the electrolyte, 2) high resistance towards oxygen produced at the anode, 3) high electric and negligible ionic   conductivity, 4) low oxygen overvoltage, 5) resistance towards electrolytic decomposition of the oxide material, 6) adequate mechanical strength, 7) easy electrical connection, 8) non-polluting in manufacture, use and disposal, 8) acceptable contamination of the aluminum produced and 9) economically attractive (Thonstad, J. et al. 2007).’

Electrolytic cell design

Aside from the anode a basic Hall-Heroult electrolytic cell also comprises a cathode and electrolyte. So-called development and use of wetted cathodes in combination with inert anodes allows for lower anode-cathode distances. This in turn is accompanied by reduced voltages and lower energy consumption (estimates are in the range of 20-30% reduced energy consumption). Other cell design aspects to be dealt with relate to thermal issues (e.g. heat loss), magneto hydrodynamic issues (e.g. impact of magnetic forces on bath circulation and consequences for alumina solution rate) and physical issues (e.g. size, shape and location of sump as well as free space for cell insulation materials). 

Advanced aluminum plant design

In terms of operational necessities, aside the general resource requirements, such as access to bauxite/alumina, sufficient finance/investment and a market for the end-product; integrating inert anodes and advanced electrolytic cell designs in the aluminum plant is the next design step. This aspect is dependent on plant economics. Retrofit solutions only make sense if replacement of outdated process elements is economically due. In general, greenfield plant designs have a higher degree of freedom to optimize its technical design.

Status of the technology and its future market potential top

According to the IEA, 2008, “the ultimate technical feasibility of inert anodes is not yet proven, despite 25 years of research.” Additional fundamental R&D on materials is needed. The following Table provides an overview of the technological status of inert anodes and some other technology options for the aluminum industry. The Table shows that inert anode technology is ready for large-scale demonstration at the plant level, while full commercial deployment is expected from 2015 – 2020 onwards.

Technology R&D needs Demonstration needs Deployment milestones
Wetted drained cathodes Ready for demonstration Deployment to start by 2015 with full commercialization by 2020
Inert anodes Extensive testing at laboratory and batch scale Ready to be demonstrated at plant level Deployment to start in 2015 - 2020 with full commercialization by 2030
Carbothermic reduction Extensive research under way 2020 - 2025 Deployment to start between 2030 and 2040 with full commercialization by 2050
Kaolinite reduction Research under way 2025 - 2030 Deployment to start between 2035 and 2040

Table: Technology options for the aluminum industry, IEA Technology perspectives, 2010.

Within a growing global industry - where electricity consumption makes up about one-third of the production costs - there is significant market potential for most technology options that reduce energy consumption of primary aluminum production, including inert anodes. More locally, the technologies’ market potential depends on local circumstances. Here the lifetime (or age) of the smelting cell or production plant are major determinants of whether inert anodes are used as a retrofit option or as greenfield investments.

Some of the worlds’ most energy efficient smelters are based in China and Africa, but in the case of China the characteristics of its indigenous bauxite deposits (i.e. lower quality) a higher energy input per ton alumina input is required. The potential for energy efficiency technologies, like inert anodes could increase with an increased policy focus on environmental impacts of aluminum smelting. As the aluminum industry consumes large quantities of electricity, the costs and way in which that electricity is produced heavily influences the industry’s needs to preserve energy. Low-cost renewable electricity in the form of hydropower is amongst the most preferred mode of electricity supply in the aluminum industry. In coal-based economies, low-cost electricity is available, but the environmental impact of power production (e.g. air pollutants and GHGs) is relatively high, implying that with stringent environmental policy there is a larger market potential for inert anode (and other energy efficiency) technologies. As an alternative to energy efficiency and fuel switch measures, conventional cell technologies could be combined with Carbon Capture and Storage (CCS) technology.

How the technology could contribute to socio-economic development and environmental protection top

For those economies that are considering implementing, inert anode technology in the aluminum sector, the technology could contribute to (see also Table):

  • an increase in energy efficiency of up to 25% (when coupled with a stable, wetted cathode),
  • a reduction in operating costs of up to 10%,
  • significant reductions of emissions such as CO2 and perfluorocarbons (either via direct process emissions or via indirect reduced emissions related to electricity production),
  • a process productivity increase of up to 5%,
  • lower emissions of polycyclic organic matter (POM) generated during anode manufacture and consumption; hydrogen fluoride (HF) generated during electrolysis, anode effects, and anode replacement; and carbonyl sulfide (COS) generated during electrolysis.
Cost / productivity Process simplification / control Energy Greenhouse gas emissions Safety and health Other environmental benefits
Lower anode cost Eliminate carbon plant More thermally efficient cell Reduce / eliminate CO2-emissions Fewer anode changes Eliminate / reduce polycyclic aromatic hydrocarbons / polycyclic organic matter
Increased space utilization Many fewer anode changes Lower heat losses Reduce / eliminate PFC emissions Improved industrial hygiene Eliminate emissions of carbonyl sulfide
More production per unit volume Control system changes (how to deal with fixed plane anode and moving plane cathode) Energy saved from elimination of carbon anode production Eliminate dry coke scrubbers and anode paste plant, also eliminate scrubbers from bake ovens
Less operating labor Dimensionally stable flat bottom, better control of anode-cathode distance (ACD) Potential for lower ACD when combined with wettable cathode Reduce spent pot liner, possible if combined with new cathode technology
Opportunity to retrofit to work with carbon cathodes Reduce hydrogen fluoride emissions
Higher metal quality, no dissolution of anode component
Increased flexibility in cell design

Table: Potential benefits expected/required from inert anodes, Inert Anode Roadmap, 1998.

Climate top

Two categories of greenhouse gases will be reduced or eliminated by the use of Inert (or non-consumable) anode technology, namely carbon dioxide (CO2) and perfluorocarbons (PFCs; CF4 and C2F6). As a result of the use of the inert anode the process related CO2-emissions from the carbon anode will be reduced or eliminated. The emissions from PFCs are eliminated because the inert anode does not contain carbon. The reduced electricity consumption per unit of output results in a reduction of power production related CO2-emissions.

The Aluminum Sector Greenhouse Gas Protocol (October 2006) published by the International Aluminium Institute (IAI) as an addendum to the Greenhouse Gas Protocol of the World Resource Institute (WRI) and the World Business Council for Sustainable Development (WBCSD) provides a detailed description of how one can monitor and report the direct greenhouse gas emissions associated with aluminum production. 

Under the Clean Development Mechanism (CDM) there is an approved baseline and monitoring methodology AM 0059 for ‘Reduction in GHGs emission from primary aluminium smelters’, which is linked to a project at the Hindalco Industries aluminium smelter in Hirakud, India. The project activity involves replacement of the existing potlines to Point Fed Pre Baked (PFPB) technology, while operating the new pot lines at a pot line current of 85kA instead of 55kA. Although inert anode technology is not used in this CDM-project the methodology could have potential use for inert anode based CDM projects in the aluminum industry.

Financial requirements and costs top

The status of the technology makes that full-scale deployment under competitive conditions is not yet achievable due to the risks and costs associated with advanced cell design. However, individual elements of advanced cells, for instance inert anodes or wettable cathodes could be commercially feasible in the short term.

However, some ball-park figures have been given in recent years stating for instance that the capital costs for cell replacement range from $1-2 million (i.e. retrofit) and $1-2 billion for greenfield projects incorporating inert anode technology. Based on these figures, it was estimated that adoption of inert anode technology could result in a reduction of about 3% in terms of operating costs and a 2% improvement of ROI (return on investment) in case of a greenfield installation.

As a base-case financial consideration, it is important that the overall costs of a new anode type to be competitive is at least equal to or lower than the cost-price of a conventional carbon anode. Conventional carbon anodes usually have a cost-price ranging from $110 to $120 per ton. In those cases where inert anodes provide direct energy savings a slightly higher cost-price for inert anodes might be feasible.

Aside from the general information on estimated cost ranges for inert anode technology, no detailed publicly available sources on cost information on implementation of inert anodes were found (update required).

References top

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