Climate Action - Assisting business towards carbon neutrality

Sharif Jahanshahi, Sustainable Processing Theme Leader, CSIRO 

Australia is blessed with both mineral and environmental richness. One of the most important challenges for the Australian minerals industry is to ensure that using our mineral resources does not come at the expense of the environment. CSIRO, Australia’s national science agency, is focusing on energy usage and greenhouse gas emissions in the mineral sector to ensure sustainability.  

To harness the economic opportunities of Australia’s mineral richness, the industry consumes large quantities of energy and water and produces considerable amounts of residues, emissions and wastes. Although significant engineering and operational improvements are underway to improve environmental sustainability, only R&D-driven transformational and systemic solutions can have the necessary scale of impact without compromising the economics of the minerals industry.

CSIRO, Australia’s national science agency, is working with other R&D bodies to ensure sustainability of Australian industries. Through CSIRO initiatives such as the Light Metals, Minerals Down Under, Energy Transformed, and Climate Adaptation National Research Flagships, we are focusing on science to underpin response strategies to climate change. Energy usage and greenhouse gas (GHG) emissions in the minerals sector are key components of this push.

 

Identifying major opportunities for reducing emissions

About 1.5 billion tonnes of greenhouse gas emissions are produced annually from metal production globally, representing about five per cent of the world’s total GHG emissions from fossil fuels (Baumert, K, Herzog, T and Pershing, J (2005). Navigating the Numbers: Greenhouse Gas Data and International Climate Policy. World Resources Institute.

Working through the Centre for Sustainable Resource Processing and using life cycle assessment methods, CSIRO has calculated energy use in metallurgical processes and related GHG emissions, and identified the major opportunities in Australia and globally for reducing both.

Producing metals from ores typically involves four stages: mining, mineral processing or concentrating, metal production and refining. Both pyrometallurgical (heat based) and hydrometallurgical (solvent based) processing routes are used in the metal production stage. The choice as to which processing route to use is invariably based on economic considerations, which are strongly influenced by factors such as ore grade (ie metal content) and mineralogy.

CSIRO’s assessment revealed that, at higher ore grades, metal production generally makes the greatest contributions to energy consumption and associated GHG emissions of the four stages. The contributions of the mining and mineral processing stages become more significant as the ore grade falls (see Figure 1).

Figure 1: Gross energy requirement (GER) for the production of various metals

We also found that the light metals aluminium and titanium have the highest life cycle-base energy consumption and GHG emissions per unit mass of refined metal. But when the annual production amounts of the various metals are accounted for, steel is responsible for the greatest global amount of energy consumed and GHGs emitted from primary metal production, followed by aluminium (see Figure 2).

Figure 2: Global annual greenhouse gas emissions for various primary metals

This suggests the major opportunities for substantial energy and GHG reductions both globally and in Australia lie in the metal extraction stages of steel and aluminium production. Ore grades will inevitably deteriorate over time, however, so opportunities in the mining and mineral processing stages will become more significant, particularly in the ore crushing, or comminution, step.

Developing technologies to harness these opportunities

CSIRO and other organisations are researching various technologies to address these opportunities. Some of the most promising emerging technologies are described below.

Bath smelting: HIsmelt

Rio Tinto is developing the HIsmelt process for direct ironmaking at Kwinana in Western Australia. HIsmelt produces molten iron using fine iron ores (and other iron-bearing fines) and non-coking coals, avoiding the traditional two-step process that uses sinter plants and blast furnaces for smelting iron ores to produce pig iron, which is then refined into steel. Advantages claimed for the HIsmelt process over conventional technologies include lower operating and capital costs, and greater raw material and operational flexibility. Based on data reported by the HIsmelt team, we have estimated that this direct, bath-smelting technology could reduce energy consumption and GHG emissions per tonne of steel by 2.2 Giga joules (GJ)and 0.17 tonnes of carbon dioxide equivalents (t CO2e), respectively. This equates to an annual global GHG reduction of 188 million t CO2e.

Charcoal from biomass

Reducing GHG emissions from metal production processes in the longer term will require gradual substitution of fossil fuel-based reductants and fuels with suitable materials derived from renewable sources such as biomass. The term biomass is usually used to describe biologically produced material that readily burns or can be converted into char, and includes wood and wood wastes, and agricultural crops and their waste products.

Metallurgical coke is used in the blast furnace both to remove oxygen from oxide ores and as a source of thermal energy. Coke (or its original source coal) contributes about 75 per cent of steel production’s GHG emissions. Replacement of coke with biomass-derived charcoal would reduce these emissions, as the biomass comes with a GHG credit because of the carbon dioxide sequestered during its formation.

Brazilian iron producers have been using charcoal as heat source and reductant for years. The current cost of biomass-derived charcoal in Australia, however, compares unfavourably with coal, but the Minerals Down Under Flagship and its partners are investigating other potential sources of biomass that may yield lower-cost charcoal. These include municipal paper wastes, biosolids, and rotationally harvested mallee trees grown in the Western Australian Wheatbelt region to ameliorate salinity. We have also made substantial progress towards meeting the technical challenges presented by substituting coke and coal with biomass in metallurgical processes.

We estimate that substituting only 20 per cent of coke with charcoal in traditional steel production could reduce GHGs by about 0.48 t CO₂e/t steel. On a global scale this translates to over 500 million t CO₂e per annum, or close to the total GHG emissions of countries such as Australia.

Dry granulation of slag

Pyrometallurgical processes produce molten slag as a by-product product, which is sometimes quenched using water to produce glassy granules that can be used in cement. Several research groups, including CSIRO are trying to advance a process first proposed in the UK and Japan in the 1980s to dry granulate blast furnace slag using a spinning disc or cup and air.

This would overcome the environmental problems and other disadvantages associated with wet granulation, including substantial water use, acid mist creation, loss of high grade heat energy and the energy requirement of drying the wet product. We estimate that avoiding the energy input for slag drying could save 0.5 GJ and 0.04 t CO₂e per tonne of metal. The use of dry granulated slag as feed stock for cement production could boost the savings in energy and GHG emission to 1.6 GJ and 0.27 t CO₂e per tonne of steel, respectively. This equates to potential annual global GHG reduction of up to 300 million t CO₂e.

Drained cathodes

Rio Tinto is working with the Light Metals Flagship to develop a commercially viable technology that reduces the energy requirements of aluminium production. The use of drained cathode cells (DCC) in the conventional Hall–Héroult process offers industry a way to cut the associated GHG emissions by reducing energy consumption.

DCC technology works by shortening the distance between the anode and cathode in a cell, thereby reducing the electrical resistance and, consequently, energy consumption. 

A reduction in anode-cathode distance of two centimetres could lower electrical energy consumption in the electrolysis stage by 16 per cent, reducing GHG emissions by 2.28 t CO₂e/t metal.

Waste heat recovery

Metallurgical processes operate over temperatures ranging from ambient up to 1600 ^oC or more. This offers many opportunities to recover heat from the various streams exiting the processes and from other internal sources. The most likely candidates are product and waste streams (eg metal and slags), exhaust gases and equipment heat losses. CSIRO in collaboration with industry and other organisations is actively exploring opportunities to capture and re-use of the high grade waste heat from such streams.

Cooling a typical blast furnace slag from 1400 ^oC to 200 ^oC with 80 per cent efficiency could yield about 0.4 GJ/t steel. This corresponds to about 0.02 t CO₂e/t steel (if replacing natural gas) or 0.04 t CO₂e per/t steel (if replacing coal).

Similarly, recovering waste heat potentially available from aluminium production cells could generate electricity and GHG credits of 830 kilowatt hours and 0.83 t CO2e/t aluminium (based on black coal-based electricity).

Developing alternative processes

While considerable effort is being devoted to improving the energy efficiency of existing metallurgical processes, CSIRO and other organisations are also developing alternative, less energy-intensive process stretching across the metal supply chain, including:

•          Direct (single-stage) production of aluminium and magnesium using carbon
•          Low temperature production of aluminium using ionic liquids
•          Direct smelting of stainless steel
•          Nitrogen-free smelting processes
•          Microbiological extraction and processing of minerals
•          Direct bath smelting of low-grade ores. 

These processes are in various stages of development, with some still in their infancy, so it is too early to usefully calculate their potential energy and GHG savings. CSIRO has estimated, however, that direct production of aluminium using carbon would cut energy and emissions by about 20 to 25 per cent over the conventional route.

Conclusion

At CSIRO, we are working with our partners to develop transformational technologies, strategies and practices to dramatically reduce the minerals sector’s environmental impact.

Achieving a sustainable balance between wealth creation and environmental responsibility, requires everyone – industry, industry bodies, government agencies, community groups, regulatory authorities, NGOs and R&D providers – to sit at the table together.

By looking to the future expectations of both the community and the minerals industry, our efforts will help transform the way Australia uses its mineral resources, address our nation’s research priorities, deliver benefits to all Australians for many generations, and have substantial global impact.

  

Author

Dr Jahanshahi completed a Bachelor of Science in metallurgy and a Doctorate in extractive metallurgy at the Imperial College of Science & Technology (United Kingdom). He leads much of CSIRO’s research in sustainable mineral processing and manages the CO2 Breakthrough Technologies program at the Centre for Sustainable Resource Processing.

Organisation

CSIRO is a powerhouse of ideas, technologies and skills for building prosperity, growth, health and sustainability. It serves governments, industries, business and communities across Australia. CSIRO initiated the National Research Flagships to provide science-based solutions in response to Australia’s major research challenges and opportunities, by forming multidisciplinary teams with industry and the research community. CSIRO is also a foundation member of the Global Research Alliance, which is working towards achieving the United Nations’ Millennium Development Goals.

Enquiries

Dr Sharif Jahanshahi
Sustainable Processing Theme Leader
Minerals Down Under National Research Flagship
Tel: +61 3 9545 8621
Fax: +61 3 9562 8919
E-mail:

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