The production of aluminium is an energy intensive process—so much so it is sometimes referred to as ‘solidified energy’. Carbon emissions are produced in aluminium smelting from the required electricity generation, along with the direct emissions from the reduction of alumina with a carbon anode.
Consumers are increasingly looking at the full value
chain of materials with an ESG lens and governments are looking broadly across
industries to meet global commitments. The reduction of emissions in alumina
refining will be expected, with failure to act potentially penalised.
While smelting
produces a large proportion of carbon emissions and is often the focus of
decarbonisation efforts, alumina refining is still an energy intensive process.
It uses high pressure steam to produce the heat input required to process
bauxite into alumina. It also uses fossil fuels in its calcination processes.
Alumina is then converted to aluminium in a smelting process, which releases a
large amount of carbon.

The Scale of the Problem
Aluminium can have a carbon emission intensity with an
industry average of ~11.5 metric tonnes of CO2 per tonne of
aluminium (Scope 1+2). This ranges from <5tCO2/tAl—for production utilising
hydropower as an energy source—up to a more typical ~15-20tCO2/tAl if utilising
coal power. Alumina refining by comparison has a typical carbon emission
intensity of 1.5MtCO2/per tonne of alumina. However, with alumina production
forecast to continue growing to 160Mtpa by 2030, the sector will remain a
significant polluter without changes.
The opportunity for reduction of emissions in the
refining industry varies. In Australia, the second largest producer and largest
exporter of smelter grade alumina, the refining industry accounts for around
24% of the country’s Scope 1 manufacturing carbon emissions (~14Mtpa).
A World of Opportunity
The primary source of carbon
emissions at an alumina refinery is the boilerhouse, where steam is produced
for the process. This accounts for an estimated 70% of fossil fuel consumption at
a refinery, with the remainder predominately coming from the calcination
process (~24%) which requires temperatures of over 1000°C to strip chemically
bonded water from alumina hydrate. Initiatives being implemented and explored to
reduce carbon emissions have been outlined by major producer Alcoa’s proposal
for the ‘Refinery of the Future.’ They include:
Fuel Conversion –
a fuel switch is a key enabler for several refineries in meeting increasingly
prevalent sustainability ambitions across the industry. A common action,
already being implemented, has been the replacement of fuel oil with natural
gas. While not eliminating carbon emissions, this conversion does significantly
reduce a site’s emissions.
Norsk Hydro’s Alunorte is converting their
calcination process and some steam generation from fuel oil to natural gas. This
is expected to reduce the refinery’s CO2 emissions by 700ktpa. To eliminate
carbon emissions, at least initially from the calcination process, Rio Tinto is
studying the potential use of hydrogen to replace fossil fuels.
Mechanical Vapour Recompression
(MVR) – MVR is considered a potential replacement for fossil-fuel
consuming boilers to produce steam. Electricity produced from renewable sources
would be used to power compressors to turn waste vapour into the steam used to
provide refinery process heat.
Alone, MVR has the potential to reduce a
refinery’s footprint by 70% and reduce water usage by 35%. MVR has been proposed by Alcoa, which mostly
operates low-temperature refining processes for low silica gibbsitic bauxite.
MVR’s effectiveness at high temperature plants processing boehmite bauxites
remains to be seen.
Electric Boilers
– Alunorte has recently installed an electric boiler for supplying its process
and offsetting coal usage in the boilerhouse—reducing 100ktpa of CO2 emissions.
The boiler is to operate with energy purchased from the market. Investments in
renewable energy and long-term purchase agreements will assist with the plant’s
emission reduction efforts through the installation.

Electric Calcination
– Electric calcination directly replaces traditional direct-fired calcination
plants. Fossil-fuels currently power calcination plants, but can be replaced by
renewables in an electric calciner. Electric calcination also allows steam to
be captured and reused, reducing water wastage. In conjunction with MVR, it has
been suggested carbon emissions from a refinery could be reduced by 98% and
water usage by 70%. Further, incorporation of a thermal storage system enables
the refinery to act like a battery to help support stabilisation of the
electricity grid.

Residue Disposal
– whilst not reducing carbon emissions, innovative solutions for management of
the significant volumes of bauxite residue produced by the process is also
being investigated. Measures include residue filtration and reducing water
content—and returning water to the process—allowing easier handling and more
efficient storage. There has also been investment in research to repurpose
residue to commercially marketable products.
Investigations and studies into these
potential solutions have already started being announced, particularly in
Australia where industry majors Alcoa and Rio Tinto operate significant
capacity.
In May 2021, Alcoa received $11.3m
from the Australian Renewable Energy Agency (ARENA) to demonstrate technology
that can electrify the production of steam utilising renewable energy—MVR—at
its West Australian refineries, starting with Wagerup.
In June 2021, Rio Tinto received
an ARENA grant for a feasibility study into its hydrogen conversion of
calcination at its 3.4Mtpa Yarwun refinery in Queensland. The company is also
investigating the development of a hydrogen pilot plant at the site in a JV
with Sumitomo.
Walking the Walk Ain’t Cheap
Currently, the ‘greenest’ alumina
refineries utilise natural gas in both the boilerhouse and the calcination operations
and utilise dry stacking of residues to minimise footprint and water usage.
Significant capital will need to
be invested to bring down carbon emissions at alumina refineries. Traditionally,
all sites have a boilerhouse to raise steam and burn fossil fuels to do so. Conversions
from fuel oil to natural gas have been undertaken, though generally to save
money rather than to actively reduce emissions.
The reduction in associated
carbon emissions is increasingly being highlighted as a driver of these
initiatives. Adapting all six Australian refineries to replace steam raising
with MVR has been estimated to cost US$2-5bn and will require 1,200MW of
renewable power to operate—and this is assuming the technology is successfully
proven.
Norsk Hydro’s project to convert
its Alunorte plant from fuel oil to natural gas has been connected to a US$200m
‘sustainability-linked loan.’ The pricing of the loan is linked to the
company’s greenhouse gas emission reductions. This could be an emerging theme used
to incentivise emission reduction to secure financing as financial institutions
look to reduce their own exposure to carbon intensive projects. Norsk has also
invested US$8.3m in its electric boiler installation at Alunorte with plans for
two more by 2024.
The capital costs associated with
undertaking these projects are often not contributing to additional production,
raising the cash cost floor of sites. However, costs may arise for producers
not trying to reduce emissions. The European Union has flagged carbon border
tariffs which would impose a cost on failure to act. In many cases, change may
need to be induced by government incentive or other constraints.
A Drop in the Ocean?
These projects are
largely occurring in Western countries. However, China is the largest producer
of aluminium—accounting for ~51% of global production—and efforts
will need to be made in China to meaningfully reduce emissions across the
sector.
Chinese
refineries are typically heavily reliant on coal as a fuel, both in the
boilerhouse and calcination—with estimates of over 80% of capacity reliant on
coal. As such, carbon reduction projects
outside China may not contribute meaningfully to overall reduction in the
sector. The country’s energy efficiency and provincial consumption targets—if
persisted with or tightened—may see an uptake of modifications which could lead
to lower emissions from the domestic sector.
However, the question may be whether
China’s domestic consumers—the downstream market for its refineries—are worried
about ‘greening’ supply lines without being compelled to by its government. A
lack of incentive may see a very slow uptake of the greener production methods.
Across
the aluminium value chain of mining, alumina refining and aluminium smelting,
the contribution of mining to total emissions is negligible and refining
accounts for less than 20%. The major gains to reduce emissions across the
industry remains within the smelting sector. This does not mean that it is not
worth pursuing emission reductions in the alumina sector, and there are several
avenues which can potentially be pursued.