LCA shows full picture on energy use

By Robin Taylor

As the world focuses on ways to reduce the use of fossil fuels and curb dangerous greenhouse gas emissions, the minerals industry is seeking ways to become more environmentally sustainable. However, obtaining a true picture of the energy use and greenhouse gas emissions of different processes and identifying opportunities for improving performance can be difficult.
Producing metals from ores typically involves four stages: mining, mineral processing or concentration, metal production (using either pyrometallurgical or hydrometallurgical routes), and refining. At higher ore grades the metal production stage generally makes the greatest contribution to energy consumption and associated greenhouse gas emissions. As ore grades decline, the energy consumption and emissions associated with the mining and mineral processing stages increases.
Life cycle assessment (LCA) is a method of analysing the environmental impacts of a process or product from ‘the cradle to the grave’, which can be used to optimise environmental performance or to compare the impact of different processes.
It can be used, for example, to determine the true energy consumption required for primary metal production and the greenhouse gas emissions associated with that process, or to measure environmental impacts such as water use, waste generation and waste toxicity.
Terry Norgate and colleagues at CSIRO, working through the Minerals Down Under Flagship, have been using LCA to evaluate the environmental and economic impact of different processes for making metals.
Using this method, they measure the impact from the point where ore is extracted from the ground, through the metal making process, its end use and finally disposal of the product.
Norgate said LCA is an integral part of evaluating new processes or changes to existing processes, such as making various metals.
“Researchers are constantly developing and improving processes for making metals. Using LCA we evaluate how those changes affect the economics and environmental impact compared with the existing process,” Norgate explained.
“It allows us to see which parts of the new process make the biggest contribution to the environmental impact so we can focus our efforts on those stages and improve their performance.”
As well as considering the energy used and greenhouse gas emissions from the process itself, the LCA takes account of indirect emissions such as those from the generation of electricity.
“You can’t just look at part of the life cycle in isolation,” Norgate said. “If you look at a process that uses electricity for heating, for example, you don’t see any greenhouse gas emissions, so compared with a process that uses coal for heating, the first process appears much more greenhouse friendly. An LCA will also consider the emissions from generating the electricity in the first place.”
The LCA also takes account of downstream emissions. In a comparison of aluminium and steel, for example, production of aluminium generates more greenhouse gas emissions per tonne of metal than steel, but in the end-use of the metal as components for motor vehicles, the lighter aluminium can lead to improved vehicle performance in terms of fuel consumption and this also has to be considered.
Norgate and his colleagues are working with some of Australia’s major steel manufacturers to investigate charcoal derived from biomass as an alternative energy source to coal in steelmaking.
Although it is possible that new high-grade ore reserves will be discovered, it is almost inevitable that reserves will deteriorate over time as higher-grade reserves are exploited. As ore grades fall, more energy is required in the mining and mineral processing stages to process the additional material and consequently the level of greenhouse gas emissions of primary metal production processes will increase.
In an LCA comparing a number of metals, the researchers showed that the light metals titanium and aluminium have the greatest energy requirement on a per tonne basis, followed by nickel, while steel and lead have the lowest values. However, on a global basis, steel production consumes by far the largest amount of energy and is responsible for the greatest emission of greenhouse gases.
The results showed that the major opportunities for reducing energy consumption and greenhouse gas emissions from primary metal production lie in the metal extraction (smelting) and refining stages of steel, and to a lesser extent aluminium, production.
This work led to another project investigating the energy consumption and greenhouse gas emissions of nuclear power compared to fossil-fuel based electricity.
“Because we had been looking at the effect of ore grade on greenhouse gas emissions for metal production, we saw an opportunity to do the same thing and measure the impact with uranium ore,” Norgate said.
Proponents of nuclear power describe it as a greenhouse gas friendly alternative to electricity based on coal. But, says Terry Norgate, to get the full picture, you have to do an LCA, not just compare a nuclear power station to a coal-fired power station.
“If you just did that you would say the nuclear power station has no carbon dioxide emissions, but if you look at the life cycle you also consider the energy involved in getting uranium to the power station, mining it and processing it,” he explained.
The researchers carried out an LCA of nuclear power production focusing on energy consumption, greenhouse gas emissions and ore grade, and compared the results with fossil-fuel-based electricity production.
The main factors influencing the life-cycle-based greenhouse gas emissions associated with nuclear power were shown to be the energy intensities of the various stages in the nuclear fuel cycle and the grade of uranium ore used to produce the fuel for the reactor.
At lower ore grades, more ore must be processed to extract the same amount of uranium, leading to an increase in greenhouse gas emissions associated with the nuclear power produced.
“It is conceivable that at some ore grades these greenhouse gas emissions may exceed those from fossil-fuel-based electricity for the same amount of power produced,” Norgate said.
As well as evaluating new processes or changes to existing processes, LCA has also been used to examine the energy consumed during metal recycling.
Many metals, including light metals and steel, can be repeatedly recycled to produce new metal products. Metal produced in this way (known as secondary metal production) requires far less energy than metal produced using primary production processes. For example, making aluminium from recycled aluminium scrap consumes between 90 to 95 per cent less energy than primary aluminium production. The main reason for this reduction is that melting metal requires less energy than is needed for reducing naturally occurring metal oxides and sulfides. Of course, any reduction in energy consumption results in a major reduction in greenhouse gas emissions.
Although LCA confirmed that, in terms of energy consumption, secondary metal production is more sustainable than primary metal production, it also revealed that other factors, including metal type, product and geographical location, need to be considered in order to determine the extent to which metals can be recycled.
For more information contact tel: +61 (0)3 9545 8574 or email: Terry.Norgate@csiro.au

Exergetic Life Cycle Assessment
The ‘useful part’ of energy is commonly known as ‘exergy’. It describes the energy that can be used to produce work. Another type of life cycle assessment, known as exergetic life cycle assessment, considers exergy rather than energy. CSIRO’s Terry Norgate said that exergy is used as a measure of the quality of the energy – whether it is high-grade or low-grade – and is increasingly being used for resource accounting purposes.
“It provides a way of assessing the quality and quantity of a resource,” he explained.
Norgate and his colleagues, in an extension of their work on life cycle assessment, carried out an exergetic life cycle assessment comparing the production of aluminium and steel. The analysis shows that aluminium production suffers a higher exergy loss than steel production because of the inefficiencies associated with electricity generation.
This information feeds into research investigating alternative ways of producing aluminium that do not require such a high use of electricity.

Source: CSIRO PROCESS Magazine - http://www.csiro.au/resources/Process-Magazine.html