Although the global warming
potential of carbon dioxide (CO2)
emissions has always been acknowledged by
the scientific community and governments worldwide,
CO2 has been historically undervalued in the market
place, which did not encourage companies to reduce emissions.
In the European Union (EU), prices dropped from $45/tonne of
CO2 in 2008 to $4-10/tonne of CO2 in
2013, due to the global economic crisis.
However, the US Interagency Working
Group raised its estimate of the social cost of CO2
to $65/tonne of CO2 in 2010 and the UK added a
carbon price floor of about $30/tonne of emitted CO2
in order to prop up carbon pricing.
As regulations on CO2
emissions become increasingly strict, and market forces move
CO2 prices higher, it is expected that there will be
a push for companies to become more efficient and develop
technologies for carbon sequestration and storage.
CO2 sequestration in
minerals, also known as mineral carbonation (MC), is one of the
options considered by research centres and industries for
mitigating the impact of power plant and transport emissions on
MC is based on the reaction of
CO2 with metal oxide bearing minerals to form
insoluble, chemically stable carbonates; calcium (Ca) and
magnesium (Mg) being the most reactive metals.
This process occurs spontaneously
in silicates but is a very slow process.
Suitable minerals for MC include
Mg-rich silicate rocks and minerals, such as serpentine and
olivine, Ca-rich minerals, such as wollastonite, and alkaline
industrial residues, such as slag from the steel industry or
The process can take place in-situ,
by injecting CO2 in silicate-rich geological
formations, in alkaline aquifers or in an ex-situ chemical
processing plant after mining and pre-treating the
MC offers two main advantages to
other carbon sequestration options, such as geological or ocean
storage, having the advantage of abundant metal oxide bearing
materials, in particular natural silicates, and the permanent
storage of CO2, which rules out any risk of leakage
and cuts monitoring costs.
The reaction of CO2 with
serpentine and olivine produces stable magnesium carbonate,
silica and heat, while the reaction with wollastonite produces
heat, calcium carbonate and silica.
According to the Intergovernmental
Panel on Climate Change, (IPCC), rocks containing magnesium
silicates have the highest estimated storage capacity of 0.55kg
of CO2/kg of rock, while calcium-rich basaltic rocks
have a theoretical storage capacity of 0.08kg of
CO2/kg of rock.
Accelerated mineral carbonation (ACM)
One of the main characteristics
being investigated by scientists and industries is the
possibility of accelerating the MC rate, either by increasing
the temperature (up to 6500 C for serpentine) or by
ultrafine grinding of the mineral, which has been used for
olivine and wollastonite.
The reaction is speeded up at
higher temperatures and pressures. This is exactly our research
[at the University of Leuven]. Most companies involved in
mineralisation are working in these kinds of process
conditions, Pol Knops, research scientist at KU Leuven
spin-off company, Innovation Concepts BV, told
Locations with an acidic
environment and erosion sensitivity also show much faster
reaction rates, but if no water is present, the reaction
stops, he added.
Research has been focusing on
speeding up metal extraction from the input materials by using
additives and catalysts in water solution, such as acetic acid
and citric acid.
The dissolution rate is a
function of surface area, thus small grains are
preferred, Jens Hartmann, professor from the Institute
for Biogeochemistry at the University of Hamburg, Germany, told
Hartmann explained that low pH
(acidic conditions) and longer residence time - time during
which a water is in contact with the mineral surface - also
increase the MC rate.
The highest chemical
weathering rates from volcanic areas have been reported. This
could reach 250 tonnes CO2/km2 per year
in very warm and wet environments, with high soil water
permeability, Hartmann added.
Australian Mg and Ca-based
fertilisers and building materials producer, Calix, bought a
processing technology from UK-based Novacem in 2012, which uses
accelerated carbonation of magnesium silicates at temperatures
of 1800C and a pressure of 150 bar to produce a
magnesium oxide and hydrated magnesium carbonate-based
Other companies which are
developing a pilot plant to study ACM include Australia-based
Greenmag, and US-based Skyonic.
IPCCs figures show that AMC,
using ground olivine dissolved in an aqueous solution of sodium
chloride and sodium bicarbonate, and in contact with high
pressure CO2, recovered a maximum of 81%
CO2 in one hour, at a temperature of
185oC and a CO2 partial pressure of 150
The IPCC said that 1.6 tonnes of
olivine could fix one tonne of CO2, producing 2.6
tonnes of solid material for disposal, assuming 90% carbonation
conversions and 10% losses.
Costs and feasibility ACM implies
operations at the mines, including crushing, grinding, milling,
some mechanical separation, such as magnetic extraction of
magnetite and transportation.
A study from the US Department of
Energy (DoE) on energy and economic costs on high-temperature
and high-pressure flow-loop reactors, shows that costs range
from $55/tonne of CO2 stored for olivine with
standard pre-treatment, to $64/tonne of CO2 stored
for activated wollastonite, with a percentage of CO2
conversion after one hour varying between 61% and 82%.
According to the DoE, the cost of
the most realistic technologies falls in the range of
$50-100/tonne of CO2 stored. Energy requirements
have been estimated to be about 305 kWh/tonne of
According to the IPCC, energy costs
for thermal and mechanical activation are about 300 kWh/tonne
and 70-150 kWh/tonne respectively.
We calculated the
CO2 efficiency for mineral sequestration in mafic
and ultramafic rocks, considering mining, crushing, grinding,
transportation and application for most agricultural areas,
globally. If the closest locations of identified sources are
used, the CO2 balance is always positive,
Hartmann told IM.
He added that the estimates do not
include rates calculated in the CO2 budget, and did
not include further beneficial processes from the use of the
material to increase biomass and crop productions.
Carbon capture in power
plants and tailings
An AMC process was tested at the
Jim Bridger 2120 MW coal-fired power plant in Wyoming, US,
which is owned by PacifiCorp.
The study, which was published this
year by the University of Wyoming and the Brigham Young
University, demonstrated the mineralisation of fly ash
particles at 700C with CO2 emissions at
600C, and at a pressure of about 21 kPa (0.58 bar)
with 16% moisture in the fluidised bed reactor (FBR).
Carbonation of the fly ash
particles produced calcium carbonate at the plant, which
increased from 3.11% in weight after 10 minutes to 3.86% in
weight after two hours.
Scientists are planning to study
the process on other coal-fired power plants under a wide range
of conditions and evaluate the economic feasibility of the AMC
process. They are also studying the use of carbonated fly ashes
as a replacement for Portland cement in construction.
Researchers from The University of
British Columbia, Canada, and from The University of
Queensland, Monash University and Bond University, Australia,
have recently explored the possibility of sequester
CO2 in mine tailings through MC.
By studying the natural MC at the
abandoned Clinton Creek asbestos mine, the active Diavik
diamond mines in Canada, and the active Mount Keith nickel mine
in Australia, scientists found that different conditions of
temperature and humidity, hydrology and tailings management
practices impact differently in carbon sequestration.
While passive carbonation rates at
the dry tailings at the Clinton Creek asbestos mine were
estimated to be 6.2kg/m2 of CO2 per year,
tailings at the Diavik mine captured only about
400g/m2 CO2 per year, despite similar
climatic conditions; the majority of tailings at Diavik are
submerged in a pond.
MC rates from tailings at Mount
Keith are about 2.4 tonnes CO2/m2 per
year, which is a faster rate than at the Diavik mine due to a
warm, dry climate, a saline water environment and a design of
the tailing storage which favours CO2
Scientists are investigating
possible strategies for AMC in the tailings, such as
bioleaching of the ultramafic rocks, using acidophilic bacteria
and increasing the number of tailings deposition points at the
mine to enhance exposure to atmospheric CO2.
Scientists estimated that complete
carbonation of 11m tonnes of tailings, generated annually at
Mount Keith, would sequester about 3.6 tpa of CO2,
accounting for about 11% of the total greenhouse gas emissions
from the mine.
According to the study, the
enhanced carbonation process at Mount Keith would have a value
of between $1m/year and $4.7m/year, depending on the carbon
price floor, and would have the potential to offset up to 74%
of annual CO2 emissions at the mine.
Added values of mineral
Knops said that carbon
sequestration in olivine could also provide combined benefits,
such as the use of magnesium carbonate as a replacement for
constructing materials, or to release alkalinity and
The construction industry is
rather conservative, but more and more civil applications are
being developed, such as replacement in roofing materials and
flooring applications, Knop said.
Netherland-based company Olivine
Group BV is also producing an olivine-based greenSand product
for pH stabilisers in soil, acting as a source of bicarbonates
to counteract acidification and delivering magnesium, manganese
and iron plant nutrients.
Jens Hartmann, professor from the
University of Hamburg, Germany, told IM that
not only olivine, but mafic to ultramafic volcanic rocks are
more suitable for CO2 removal at a global scale.
Mafic volcanic rocks contain,
in addition to basic cations, geogenic nutrients, such as
phosphorous, calcium and magnesium, which are beneficial for
biomass increase and crop production, Hartmann said.
Harmann explained that increasing
forestation in South America and Africa could be helped by soil
carbon improvement and nutrients supply through deeply
weathered rocks in laterite areas.
If other rocks are used, clay
minerals will form in the soil and can have a stabilising
effect on soil carbon. Also, clay minerals can be useful as
cation exchange pools in the soil, Hartmann told
Hartmann said that due to an ever
increasing population to feed, limited agricultural areas and
the exhaustion of large phosphate mines within the next
two-four decades, a long-term strategy to guarantee crop
production needs to be considered, and this might include the
use of enhanced weathering of volcanic rocks.
It would be necessary to
identify the best rock source by its composition and the ground
rock would be the basis for a new soil or would be ploughed
into existing soils, Hartmann said.
If enhanced weathering is to
be applied as a large scale method in CO2 management
and food production, a new global infrastructure needs to be
built up, Hartman told IM.
We want to establish contacts
with mining companies, not only with olivine producers, but
also with suppliers of volcanic rocks, in order to establish a
cooperation which will be useful to analyse the life cycle of
the minerals, he added.
Mining companies are slowly
realising that this could be a new, additional outlet for
[industrial minerals such as] olivine, Knops said.
But, as there are no existing
markets it is hard for them to anticipate this market, he
Innovation Concepts BV has started
contacting companies to establish partnerships with, for
example, Sibelco, Pasek and Nuova Cives.
*Conversion made July