Considerable work has been done and is continuing on the potential of using industrial minerals to sequester anthropogenic carbon dioxide (CO2).
The minerals may be reacted in situ by drilling into their formations and pumping down CO2 in liquid or gas form.
Or, the minerals may be mined and ground to particle size less than 100 microns to expose a high surface area for faster reaction. The ground mineral may be transported to the CO2 generating facility or the gas or liquid may be pumped to the mine site.
The products of the reaction may then be returned to the pit although the volume will be much greater than the original ore.
The net reduction in the CO2 burden on the atmosphere may also be effected by replacing minerals like limestone and magnesite that are processed to their final products by calcining off the contained CO2.
Minerals that contain magnesium and calcium as silicates are the primary candidates.
Another process is to use minerals to enhance biological activity in the oceans that sequesters CO2.
The majority of the worlds CO2 is emitted by the developed or developing countries outlined in the panel The Magnitude of the problem.
Minerals capable of sequestering CO2
The minerals that are capable of sequestering CO2 are those that contain the alkaline earths magnesium oxide and calcium oxide.
Weathering of these rocks and conversion to carbonates are important processes that control CO2 concentration in the atmosphere. If this weathering can be accelerated, it creates a means to reduce CO2 levels in the atmosphere. It is known that olivine, once crushed to expose additional surface area, breaks down in a few years to form carbonates.
This suggests that olivine and other similar minerals be crushed and exposed to high levels of anthropogenic CO2. This exposure can be in an autoclave or other contacting vessel, in a dry or slurry form.
The mineral could also be crushed and spread on fields, where it will pick up carbon dioxide from the atmosphere and supply essential magnesium, calcium, silica and iron to the growing plants.
Minerals proposed for carbon sequestration
||TheoreticalTonnes CO2/tonne mineral|
MgO v CaO
Magnesium oxide (MgO) is more effective in reacting with CO2 than calcium oxide (CaO). This is best demonstrated when comparing the theoretical tonnes of CO2 reacted per tonne of oxide: MgO captures 1.0-9 tonnes while CaO lower sequestrating 0.78 tonnes.
When analysing the table, Minerals proposed for carbon sequestration, brucite would seem to be the best choice. But the requirement to sequester all the CO2 would be 36,000m. tonnes.
In the USA, brucite was until recently produced by only one company, Applied Chemical Magnesias Corp. of Van Horn, Texas. Applied Chemicals production was about 25,000 tpa.
An additional consideration, most of the economic brucite deposits appear to be hosted by marbles.
The fibrous variety of brucite, nemalite, is common in ultramafic rocks, where it coexists with chrysotile, however these ores are undesirable because of the possibility of asbestos contamination.
Carbonate-hosted brucite deposits of economic significance are found in Wakefield, Quebec, Canada; Kuldur in Russia; Granase in Norway; Gabbs magnesite-brucite deposit, Nye County, Nevada, USA; and Marble Canyon, Culberson County, Texas, USA.
Undeveloped or exhausted brucite deposits occur in Arizona, British Columbia, Canada, United Kingdom, Ireland, China, and North Korea.
An advantage of brucite is its white colour. Providing the carbon dioxide used is clean, the magnesium carbonate that is produced should be white and thus suitable for use as a white extender pigment to compete with calcium carbonate.
Magnesium carbonate is at least equal in brightness to calcium carbonate.
See IM October 2007, p.74, Brucite Bonanza, for a review of the worlds brucite production capability.
A considerable body of work has been done on olivine because of its abundance, and on serpentine, because of the large amount of already pulverised rock available in asbestos tailings.
In 2008, olivine was produced in the following countries: Norway 2.5m. tpa, Japan 2m. tpa, Spain 1m. tpa, and Greenland 600-700,000 tpa (For a review of the global Olivine industry, see IM November 2008: Olivines future in flux).
Global production of olivine is presently hovering around 8.5m. tpa. Production of dunite (90% olivine) is also a contender for CO2 capture.
Focusing on the USA, a more likely candidate to invest in CO2 sequestration technology, there are only two producers the country. Industrial minerals group, Unimin Corp., mines from an open pit in Hamilton, Washington state and operates a mine in Green Mountain, North Carolina. The other major producer is Olivine Corp. which, together with Unimin, accounts for around 100,000 tpa production.
US consumption of olivine is predominately the foundry industry (87%) followed by refractories sector (7%) and abrasives (6%). Imports of olivine into the country are in the region of150-200,000 tpa mainly from Norway; 50% of this is for slag conditioner in the steel industry. Total US consumption is 200-250,000 tonnes.
Flue gas remediation
The utility power plant category accounts for 90% of the flue gas desulphurisation (FGD) market. In the US, electrical power generation sector, coal supplies about 50% of generating capacity and is the largest producer of greenhouse gas, followed by natural gas. Almost one third of all CO2 emitted in the US comes from coal burning power stations.
Current treatment of flue gas from fossil fuel-burning power stations is designed to remove acids from the gaseous effluents. The acids are mainly sulphuric and sulphurous, due to burning fuels with high sulphur contents; some hydrochloric acid may be present.
Sulphur trioxide is also generated as a by-product from Selective Catalyst Reduction (SCR) used in some power plants to control nitrogen oxide (NOx).
Coal has the highest sulphur content, followed by oil, which may be high in sulphur, like Venezuelan crude. Natural gas can be `sweet` with low sulphur or can be high in hydrogen sulphide.
Limestone and lime are used to sequester the sulphur acids as gypsum and is use in large amounts to removed sulphur from flue gases.
In 2007, the US used 14.5m. tpa of limestone in FGD, at an average price of $5.63/tonne and about 6.4m. tpa of CO2 were released into the atmosphere due to FGD limestone use.
US consumption of lime in the FGD sector is 3.73m. tpa at a price of $74.80/tonne.
The use of lime does not produce CO2 at the power plant, but the gas is evolved at the lime kiln, so there is a net emission of 1.6m. tpa CO2 owing this. This CO2 production would not happen if silicate minerals were substituted for limestone and lime.
Total CO2 emissions in 2007 due to the use of limestone and lime in US FGD operations totaled about 8m. tpa
Typical flue gas composition of a coal burning power station
||% by weight|
Olivine in FGD
The requirement for olivine to replace the limestone for sulphur removal would be about 10m. tpa. But olivine would also sequester the CO2 produced, which is about 20 times that of the sulphur oxides.
Total requirement for flue gas remediation sulphur and carbon oxides at the current level would be of the order of 100m. tpa
Wastes from olivine use
The olivine would be converted into a mixture of magnesium sulphate from the sulphur, magnesium carbonate from the CO2 and silica in the form of a gel. Assuming the olivine is used in slurry form, the magnesium sulphate would be in solution, the rest would be solids.
Magnesium sulphate could be recovered by crystallisation as Epsom salts. There is a limited market for this: US consumption in 2007 was 44,000 tonnes at an average price of $274/tonne.
The balance, a solid slurry mixture of impure magnesium carbonate and silica gel would need to be wasted, however it is unlikely that a high value commercial end use can be found, since olivine contains iron which will colour the waste products.
Since the solids will be in finely divided form, they will occupy about 3.5 times the volume of the original mined ore.
In contrast, the waste from the use of limestone is usable as synthetic gypsum. Around 8m. tpa of gypsum FGD was consumed in the US in 2008. Therefore, it is unlikely that direct use of olivine is viable in flue gas remediation.
Indirect use of olivine
Researchers at the Icelandic Research Council have used olivine as a source of magnesium for preparation of magnesium metal. They dissolved the olivine in hydrochloric acid and recovered the magnesium chloride. This was either electrolysed to form the metal, or spray roasted in air to form magnesia.
The chlorine was recovered and re-converted to hydrochloric acid and a by-product was a porous silica with surface area as high as 300-400 m²/g.
Since magnesia is produced by the roasting of magnesite, this non-carbonate route to magnesium metals reduces the amount of carbon dioxide liberated. About 14.5m. tonnes of magnesite were produced worldwide in 2004. The CO2 released by its calcination was about 8m. tonnes.
This does not include CO2 release due to calcination of dolomite used in the production of magnesia from seawater and brines.
| The two ways minerals can trap carbon dioxide|
| Residual or structural sequestration captures gas between the pores of a rock whereas mineral or chemical sequestration traps gas through a chemical reaction with the rock.
Source: IEA Greenhouse Gas R&D Programme, UK
Vermont Asbestos Groups mine on Vermonts Belvedere Mountain closed in 1993 after almost a century in operation and left 75m. tonnes of serpentinite tailings, although there are concerns about asbestos, arsenic and nickel contents.
In Quebecs asbestos mining region, tailings cover 5.5 km² and are estimated at 200m. tonnes; asbestos contents of up to 10% have been found.
Attempts were made by Noranda to extract the 24% magnesium content from the tailings by hydrochloric acid, at Danville, to make magnesium metal.
Wollastonite potential in FGD
Theoretically, wollastonite is only about half as efficient as brucite in sequestering CO2, being only 70% as effective, but it is much more widely available.
The main reason for considering wollastonite as a viable sequestration mineral is that the it is high in purity and the by-products of sequestration should be marketable and of reasonably high value.
Wollastonite reacts with sulphuric, carbonic and hydrochloric acids. The products are calcium sulphate (gypsum), calcium carbonate (calcite) and calcium chloride, respectively. The first two are insoluble; calcium chloride is soluble and will pass out in the liquid wastes. Markets for calcium chloride are in excess of 2m. tpa in the US.
Wollastonite can remediate the flue gas as to sulphur, chlorine and carbon emissions.
If wollastonite was used to capture the sulphur, the amount needed would be about 17m. tonnes annually.
Wollastonite would also be used to remove CO2 . The amount of wollastonite needed in the US to remove all the CO2 generated by coal burning power plants total would be over 5,000m. tonnes.
World production of wollastonite in 2007 was 780,000 tonnes, reserves in China are reported to be 130m. tonnes, while the sole Indian producer, Wolkem India, reports reserves of 3m. tonnes and has an output in excess of 160,000 tpa.
There are two wollastonite producers in the USA NYCO and R.T. Vanderbilt with combined capacity of about 190,000 tpa, and reserves of 7m. tonnes. Canadian Wollastonite is about to start-up, near Kingston, Ontario, with planned capacity of 20-80,000 tonnes and reserves 9.6m. tonnes.
These suppliers are well situated to feed the industrial centers of the US and Canada, where most of the carbon dioxide emissions are located.
In Sonora, Mexico, NYCO has a subsidiary with capacity of 240,000 tpa, which could supply US west coast emitters.
If the sulphur and other contaminants were removed by prior limestone treatment, and ash and unburned carbon particles were removed from the gas stream by electrostatic precipitators or filters, before reaction with wollastonite, the product would be a mixture of pure precipitated calcium carbonate and pure silica.
Since wollastonite is pure and white, this mixed filler would also be white. The mixed product would have characteristics imparted by its constituents and could be marketed in paint, plastics, paper, rubber, Portland cement etc.
The combination of calcite and silica should make such a composition interesting to cement manufacturers and for agriculture. Rubber manufacturers are increasingly using precipitated silica to replace carbon black in tires; calcium carbonate is also used in rubber.
If an acid process were used in the remediation process, the precipitated calcium carbonate and silica would emerge as separated products, each of which has well-defined markets (markets for precipitated calcium carbonate are discussed in Paper).
Wollastonite's use in CO2 capture
||Carbon Dioxide emissions, USA 2007|
||Wollastonite required m. tonnes|
Precipitated silica markets
The silica from wollastonite use in sequestration should be similar in properties to silica precipitated from sodium silicate.
Precipitated silica demand is forecast to hit 1.5m. tonnes by 2010, with an average price of $1,000/tonne.
Rubber, especially tires, is the largest market with 600,000 tonnes consumed in 2003, forecasted to reach 800,000 tonnes in 2010. Silica has been replacing carbon black as the reinforcement in green tires; in 2006, the consumption in Europe was $127m.
Precipitated silica is a reinforcing mineral. These effects are used in paper, paint, plastics and rubber (see IM November 2008, p.50, Silicas clean up).
Other FGD targets
In view of the huge quantities of minerals required for CO2 sequestration from power generation, it is likely that smaller emitters will be targeted. Of these, the cement and paper industries seem the most likely potential consumers.
Portland cement consumption in the US in 2007 was 110.3m. tonnes. Canadian consumption in 2006 was 14.02m. tonnes, (NRC) with Ontario the largest consumer, followed by Quebec.
The cement industry could use both calcium carbonate and silica by-products from flue gas treatment with wollastonite. Limestone is a vital ingredient 79.3m. tonnes were used in cement manufacture in the US in 2004.
Silica fume, 84-98% silica, with a surface area of 20m²/g is produced as a by-product from the preparation of silicon and ferrosilicon alloys. It is gray in colour, but some white grades are available.
It is used in concrete as a pozzolan to react with the excess lime improving compressive and bond strength, abrasion resistance, sulphate resistance and reduces permeability to salt that causes corrosion of reinforcing bars.
About 110-132,000 tpa is produced in the USA and about 100,000 tpa in Europe, with prices ranging from $200-1,000/tonne.
Paper production presents an opportunity for the use of wollastonite for carbon dioxide sequestration in that the mineral replaces the lime used to prepare precipitated calcium carbonate for use as paper filler and coating ingredient.
Precipitated calcium carbonate (PCC) is made conventionally by the reaction of lime slurry with CO2. This is done near a lime plant where CO2 is available from the lime kiln.
Alternatively, lime is shipped to satellite plants adjacent to paper mills, to generate PCC for captive use.
World consumption of PCC in 2007 was 13m. tonnes, 33% in North America. SMI alone produces over 4m. tonnes. PCC consumption is forecasted to grow to 16m. tonnes by 2012, with 50% in China.
Consumption in paper was 5.5m. tpa in 2007, with 1.5m. tpa in North America; 1.6m. tpa in Europe and 1.8m. tpa in Asia. Plastics consumed 3.5m. tonnes of PCC, 85% of which was in Asia.
The largest market for PCC is in coating fine paper, for copy paper and lightweight coated paper used in flyers and mailers. A smaller amount is used as a paper filler.
PCC has displaced kaolin as a filler in paper coatings, because it has superior whiteness, and opacity and can be used at higher loadings, thus lowering cost by about 10%.
Lime, often in slurry form is shipped to the paper mill where it is reacted with cleaned CO2 from the incineration of wastes and the recovery of process lime. Since the lime was made by driving off the CO2, there is no net change.
Other uses are in paint, caulks, sealants plastics and rubber.
Wollastonite for Lime?
Researchers in Finland and Japan have looked at substituting wollastonite for lime and have found that using it in CO2 sequestration will result in a net gain in removal.
EU Emissions Trading regulates CO2 emissions and limits limestone and paper production at emissions of 50 tpd. The penalty for amounts over this limits is 100/tonne ($ 127.76/tonne), thus, there is an incentive to replace lime.
Finland has 7 paper plants that produce 420,000 tonnes of PCC for internal use and also used 1.15m. tonnes of ground limestone (GCC).
Complete replacement of the 235,000 tpa of lime needed in 2002 for Finnish paper mill PCC production would require 487,000 tpa of wollastonite and would produce 252,000 tpa of precipitated silica.
| Engineering company, The Weir Group Plcs flue gas desulphurisation installation which are becoming increasingly commonplace in coal and oil fired power plants. Courtesy The Weir Group.|
Alkaline steel slag has been shown to behave similarly to wollastonite in the sequestration of CO2. Wollastonite and steel slag convert CO2 at only 10-40 bar pressure, whereas 100 bar is required for olivine.
Steel slag is more alkaline and has been shown to have a higher conversion rate at high temperatures, however olivine and steel slags are impure and coloured.
The PCC and silica produced by them is likely to be contaminated, possibly with toxic elements and will be unacceptable in colour. A cost of disposal is incurred, rather than the creation of marketable products, as is the case for wollastonite.
Iron ores in seawater sequestration
CO2 is absorbed by seawater in solution. The rate depends on its initial concentration and on temperature the balance is delicate. Too much CO2 depresses further absorption, increases acidity and diminishes the coral reefs that host rich marine life.
It is calculated that one-third of anthropogenic CO2 is absorbed by the oceans. Some organisms in seawater also absorb CO2.
Phytoplankton absorb CO2 in the same way as land plants. Phytoplankton growth depends on sunlight and the presence of minerals. An important mineral element, often in short supply, is iron.
Dissolved iron is a necessary micronutrient for phytoplankton, the tiny aquatic plants that serve as food for fish and other marine organisms, and also reduce CO2 levels in Earths atmosphere via photosynthesis. Dissolved iron in the oceans has been shown to be decreasing.
Phytoplankton carry out almost half of Earths photosynthesis even though they represent less than 1% of the planets biomass.
This represents about 180,000m. tpa tonnes of CO2 absorbed by phytoplankton, compared to 6,050m. tpa of anthropogenic CO2 emissions from the US and 27,246m. tpa worldwide.
Experiments have shown that seeding the ocean with iron-containing minerals, such as hematite, increases phytoplankton growth rate. The haematite is ground to micron sizes to aid bio-availability. It has been suggested that the absorption is improved by acidification of the mineral to create soluble iron.
Other minerals that are needed include: nitrates, phosphates and silicon. This suggests that fertiliser run-off may not be totally harmful.
There is a danger of toxic plankton blooms (red tides) may harm other marine life, declining water quality due to overgrowth, and increasing anoxia in areas, harming other sea-life such as zooplankton, fish, coral, etc.
A test in 2002 in the near Antarctica found that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water. This suggests that to sequester the annual US anthropogenic CO2 emissions of 6,000m. tonnes by stimulation of phytoplankton growth would only require seeding by 50,000 to 500,000 tonnes of finely ground and possibly acidified magnetite.
In addition, 225,000-2.25m. tpa of hematite would cancel out the entire worlds anthropogenic CO2 emissions problem.
This means that application of iron nutrients in select parts of the oceans, at appropriate scales, could have the combined effect of restoring ocean productivity while at the same time mitigating the effects of human caused emissions of CO2 to the atmosphere.
World production of iron ores in 2008 was 2,200m. tonnes, 54m. tpa was produced in the USA, mostly low iron (20-30%) taconite, at an average price of $66/tonne. The USGS estimates world reserves at 800,000m. tonnes, containing 230,000m. tonnes of iron.
Thus, if the research is correct and no bad side effects are found, the entire worlds problem of anthropogenic CO2 could be solved for the cost of $14.80 to 148m. plus transportation and application costs.
Portions of this paper were taken from a market study on wollastonite commissioned from the author by Canadian Wollastonite, a division of 2005948 Ontario Limited. The information is included with their permission.
Contributor: George Hawley, of George C. Hawley & Associates, Canada, is an industrial minerals consultant.
Magnitude of the problem
The worlds developed or transition economies are the worst offenders when it comes to CO2 emissions. This can be seen when analysing the two tables. The USA has been used as a case study to highlight which areas of industry CO2 is generated in.
Primary CO2 emitting countries
||Carbon dioxide emitted, 2007 (m. tonnes)|
||( source CDIAC for the UN)|
Major CO2 emitters in the USA in 2007
||Carbon dioxide emitted (m. tonnes)|
|Electric Power Generation|
|Residual Fuel Oil
|Petroleum Coke Production
|Natural Gas, CO2 content
|Natural Gas flaring
Source: Adapted from Energy Information Administrations Emissions of Greenhouse Gases Report 2007
Reacting to emissions: wollastonite v lime
Finnish researchers have developed a process whereby wollastonite is reacted with acetic acid, and the insoluble silica is removed by thickeners:
CaO.SiO2 + 2CH3.COOH = Ca++ + 2CH3.COO- - + H2O + SiO2
The calcium acetate solution is then reacted with paper mill stack gas carbon dioxide to produce PCC, with particles size less than 1 micron for use internally by the paper mill. Acetic acid is regenerated and is recycled:
Ca++ + 2CH3COO- - + H2O + CO2 = CaCO3 + CH3COOH
The calcium carbonate is precipitated in the solution and at the surface of the wollastonite this layer ultimately flakes off and leaves a pure silica surface.
The controlling step is the leaching of the calcium ion from the wollastonite. This is increased by increasing surface area by reducing the particles size. Catalysts include acetic acid and sodium salts chloride, bicarbonate and nitrate.
Conversion of wollastonite was 40% at 25°C at CO2 pressure of 1 bar and 75% at 30 bars.
The extraction rate depends on wollastonite particle size: 30% in 30 min at 33 microns and 10% in 30 min at 125 microns.
Gerdemann et al at the Albany Research Center found 70% efficiency in one hour at 185°C and 152 bars of CO2 pressure, for wollastonite with particle size d50 < 2-4 microns.
Another researcher found that 70% carbonation of 38 micron wollastonite occurred in 15 minutes at 200°C and 20 bar CO2 pressure. Maximum conversion rate is reported to be at 200°C. The reaction is exothermic 87 kJ/mole.
Researchers working with olivine have found that the reaction rate is speeded up by fluidising the mineral with 2mm. bead grinding media to remove the reaction products from the surface as they are produced. This may also work with wollastonite.
It has also been shown that a similar process using hydrochloric acid, in place of acetic acid, is feasible.
Comparative costs of storage
|Comparative costs of storage Remediation Process
||Cost , /tonne of CO2|
|Wollastonite + acetic acid
|Wollastonite + hydrochloric acid
|Molten salt storage
|Underground storage, CCS1
1CCS Carbon Capture and Storage: this involves capturing CO2 as it is generated pumping to a suitable site and down a well. Industry estimates CO2 trading value under the ETS will have to go to $45-116/tonne to make this technique economical.
Each tonne of CO2 would produce 1.36 tonnes of precipitated silica. In 2003, the average price of precipitated silica was $880/tonne. Thus the use of wollastonite has the potential to create a net return.