# Contents

## Idea

One of the main long-term mechanisms that removes carbon dioxide from the ocean and atmosphere is the natural weathering of rocks. We can vastly accelerate this process by digging up suitable rocks, crushing them into powder and spreading them around. The rock dust then ‘weathers’ by reacting with carbon dioxide.

This process, known as enhanced weathering, serves as a method of carbon capture and storage. Since the chemical reaction involved is exothermic — that is, heat-producing — it can even serve as a source of carbon negative energy. However, it is difficult to harvest this energy, since the reaction is slow. There are also significant challenges involved in carrying out enhanced weathering on a large scale. However, there is a project that seeks to use enhanced weathering to achieve significant carbon capture:

In Without the hot air, David MacKay writes:

Is there a sneaky way to avoid the significant energy cost of the chemical approach to carbon-sucking? Here is an interesting idea: pulverize rocks that are capable of absorbing CO2, and leave them in the open air. This idea can be pitched as the acceleration of a natural geological process. Let me explain.

Two flows of carbon that I omitted from figure 31.3 are the flow of carbon from rocks into oceans, associated with the natural weathering of rocks, and the natural precipitation of carbon into marine sediments, which eventually turn back into rocks. These flows are relatively small, involving about 0.2GtC per year (0.7GtCO2 per year). So they are dwarfed by current human carbon emissions, which are about 40 times bigger. But the suggestion of enhanced-weathering advocates is that we could fix climate change by speeding up the rate at which rocks are broken down and absorb CO2. The appropriate rocks to break down include olivines or magnesium silicate minerals, which are widespread. The idea would be to find mines in places surrounded by many square kilometres of land on which crushed rocks could be spread, or perhaps to spread the crushed rocks directly on the oceans. Either way, the rocks would absorb CO2 and turn into carbonates and the resulting carbonates would end up being washed into the oceans. To pulverize the rocks into appropriately small grains for the reaction with CO2 to take place requires only 0.04 kWh per kg of sucked CO2. Hang on, isn’t that smaller than the 0.20 kWh per kg required by the laws of physics? Yes, but nothing is wrong: the rocks themselves are the sources of the missing energy. Silicates have higher energy than carbonates, so the rocks pay the energy cost of sucking the CO2 from thin air.

I like the small energy cost of this scheme but the difficult question is, who would like to volunteer to cover their country with pulverized rock?

An alternative approach, studied by the Lamont-Doherty Earth Observatory, involves pumping liquid CO2 underground and having it react with minerals there. The heat produced could keep the rocks from getting cold, thus speeding the reaction.

Yet another approach proposed by the Lamont-Doherty Earth Observatory involves pumping CO2-containing seawater underground. The cost of most CO2 capture and storage methods is dominated by the process of CO2 capture at power plants. This would be avoided by using seawater, since surface seawater naturally contains CO2.

See below for further details.

## Olivine

Olivine is a magnesium iron silicate whose chemical formula is (Mg,Fe)2SiO4, meaning that either magnesium or iron can appear at the same lattice site of this crystal, in variable amounts. During major periods of tectonic uplift in the Earth’s past, huge slabs of rock rich in the mineral olivine (mostly peridotite) were pushed up through the Earth’s crust, with some of it being exposed at the surface. The resulting chemical weathering caused or contributed to a significant drop in CO2 levels leading to global cooling.

Gem-quality olivine

Some work on enhanced weathering of olivine has been done by Olaf Schuiling, a professor in geochemistry at the University of Utrecht. Olivine weathers completely within a few years, depending on the grain size. The reaction is ‘exothermic’: it produces heat. In order to recover the heat produced by the reaction to produce electricity, a large volume of olivine must be thermally well isolated. This process is not yet practical at sufficiently large scales to help prevent global warming, but it may be one day.

To understand the chemistry involved, for simplicity let us consider the extreme form of olivine with all magnesium and no iron, known as forsterite. Forsterite reacts with carbon dioxide as follows:

CO2 + $\frac{1}{2}$ Mg2SiO4 $\to$ MgCO3 + $\frac{1}{2}$ SiO2 + 85 kilojoules/mole

If you see the number twelve above, your browser is behaving suboptimally. According to the TeX source, and the laws of chemistry, it should be a half.

or in words:

carbon dioxide + forsterite $\to$ dolomite + silica

while producing 85 kilojoules per mole of carbon dioxide.

A second, related, reaction is:

Mg2SiO4 + 4 CO2 + 4 H2O $\to$ 2 Mg2+ + 4 HCO3- + H4SiO4

G. R. L. Cowan explains the relation between the first reaction and the second as follows. When the solid magnesium carbonate product MgCO3 — also known as magnesite — dissolves in water, more atmospheric CO2 can be taken down:

2 MgCO3 + 2 CO2 + 2 H2O $\to$ 2 Mg++(aq) + 4 HCO3-(aq)

and summing the two processes, we get:

Mg2SiO4 + 4 CO2 + 2 H2O $\to$ 2 Mg2+ + 4 HCO3- + SiO2

If we then suppose the silica gets hydrated with two more waters, that will give the second reaction:

Mg2SiO4 + 4 CO2 + 4 H2O $\to$ 2 Mg2+ + 4 HCO3- + H4SiO4

not that it matters what the silica does.

## Serpentine

Serpentine is about an order of magnitude more common than olivine. Serpentine refers to any one of a group of common rock-forming hydrous magnesium iron phyllosilicate minerals. The chemical formula for these minerals is (Mg, Fe)3Si2O5(OH)4, meaning that either magnesium or iron can appear here. These minerals may also contain minor amounts of other elements including chromium, manganese, cobalt and nickel.

According to a paper by Goldberg et al, serpentine absorbs carbon dioxide in a reaction of this form:

Mg3Si2O5(OH)4 + CO2 $\to$ MgCO3 + 2 SiO2 + 2 H2O + 64 kilojoules/mole

where again for simplicity we consider the form of this mineral containing only magnesium.

## Comparison of olivine and serpentine

In the reaction

Mg2SiO4 + 4 CO2 + 4 H2O $\to$ 2 Mg2+ + 4 HCO3- + H4SiO4

140 grams of olivine will sequester 176 grams of CO2, with the help of 72 grams of water, i.e. rain or seawater. Supposedly all the CO2 that is produced by burning 1 liter of oil can be sequestered by less than 1 liter of olivine. The market value of olivine is US $50 to US$100 per ton depending on quality. Plugging in the larger number then 5 trillion dollars a year of this material would absorb all the CO2 currently produced. But of course this calculation is oversimplified, since the spike in demand would send the price much higher.

We need sources for the numbers behind these claims! We can make the calculations ‘transparent’.

A kilogram of serpentine can dispose about two-thirds of a kilogram of CO2. According to Wikipedia, in 2008 about 31.8 gigatonnes of CO2 were emitted from burning fossil fuels (and more from land use change). So, to counter that we’d need to grind up about 48 gigatonnes of serpentine a year. For comparison, total world cement production in 2009 was about 2.8 gigatonnes. The total amount of material handled by US mines in 2008 was about 5.6 gigatonnes.

Question: What are the worldwide figures on mining?

## Goldberg et al

This paper by Goldberg et al discusses the weathering of olivine and serpentine:

It also discusses technical challenges and the progress made by a team of researchers at the Albany Research Center (ARC), the Los Alamos National Laboratory, the Arizona State University, and the National Energy Technology Laboratory. To quote:

Technical Challenges and Program Goals

The major technical challenge now hindering the use of minerals to sequester CO2 is their slow reaction rate. Weathering of rock is extremely slow. The highest priority is given to identifying faster reaction pathways. Second, the optimized process has to be economical. Although many carbonation reactions are exothermic, it is generally very difficult to recover the low-grade heat while the long reaction time and demanding reaction conditions contribute to process expense. Clearly, the environmental impact from mining minerals and carbonation processes must be considered. The program goals are specifically designed to address these challenges, including

i. identifying favored technical processes,

ii. determining the economic feasibility of each sequestration process identified, and

iii. determining the potential environmental impacts of each process.

Rapid Progress

Although the program only has about two years of history, the working team consisting of Albany Research Center (ARC), the Los Alamos National Laboratory, the Arizona State University, and the National Energy Technology Laboratory has made significant progress.

In striving to accelerate overall reaction rates, the team has identified one very promising reaction pathway and succeeded in achieving dramatically shortened carbonation reaction times employing magnesium silicates such as olivine and serpentine. For example, research at the Albany Research Center (10,13) has focused upon the direct carbonation of olivine. When the program first started, it took 24 hours to reach 40-50% completion of carbonation of olivine. The reaction required temperatures of 150-250 C, pressures of 85-125 bar, and mineral particles in the 75-100 micron size range. Careful control of solution chemistry yielded olivine conversions of 90% in 24 hrs and 83% within 6 hrs. The most recent results show further modifications of the same basic reaction can achieve 65% conversion in 1 hour and 83% conversion in 3 hours.

While the potential to utilize olivine to sequester CO2 is clearly significant, there is approximately an order of magnitude more serpentine than olivine. Consequently, finding a way to use serpentine to scrub CO2 will have greater practical impact than using olivine. Both minerals are valuable feedstocks and progress has been made in direct carbonation using serpentine also. When the program started, tests conducted at Los Alamos National Laboratory only achieved 25% conversion using 100 micron serpentine particles with CO2 even at a very high pressure of 340 bars. Independently, researchers at ARC developed a successful carbonation process for serpentine that utilizes mineral heat pretreatment and carbonation in carbonic acid in aqueous solution. A recent literature review indicated that weak carbonic acid treatments had also been suggested for Mg extraction in the prior literature (12). Carbonation tests performed at ARC employing heat pretreated serpentine have resulted in up to 83 % conversion in 30 minutes under 115 bars (13).

Because the high pressure requirement of the carbonation reaction will certainly lead to high process costs, the team is modifying solution chemistry to allow reaction to proceed at a lower pressure and temperature. The research is guided by the idea that the concentration of HCO3 - in the solution is critical to the reaction rate. The high CO2 pressure will lead increased CO2 absorption in the solution and thus enhance the HCO3 concentration. Adding bicarbonate such as sodium bicarbonate in the solution will significantly increase the HCO3-concentration even at a relatively lower CO2 pressure. Indeed, by increasing sodium bicarbonate concentration the carbonation reaction of serpentine can reach 62% completion under 50 bars.

To support laboratory carbonation tests, researchers at Arizona State are employing an environmental-cell dynamic high-resolution transmission electron microscope to directly image dehydroxylation of Mg(OH)2, an important step in Mg(OH)2 carbonation reactions. They are extending this technique to study the solid gas reaction path using serpentine to provide insights into pretreatment and reaction issues.

In the process development area, the team has completed a feasibility study of a process originally proposed by Los Alamos National Laboratory (9, 11). This process uses HCl solution reacting with serpentine to produce Mg(OH)2 which is subsequently used to sequester CO2. Although the study found the process energy intensive and inappropriate for CO2 sequestration, the analyses of individual steps were useful for developing new processes. Los Alamos National Laboratory is currently pursuing reaction mechanisms that may allow the heat treatment step for serpentine to be bypassed. Progress has also been made in identifying sources of alternative minerals that can be used for CO2 sequestration. In addition to natural olivine and serpentine deposits, researchers at NETL are engaged in a study of using waste streams such as coal ash rich in calcium and magnesium as a potential mineral source to sequester CO2.

## Schuiling et al

Olaf Schuiling, a professor in geochemistry at the University of Utrecht, has done research on olivine weathering:

The abstract of the second paper says:

Weathering and subsequent precipitation of Ca- and Mg-carbonates are the main processes that control the CO2-concentration in the atmosphere. It seems logical, therefore, to use enhanced weathering as a tool to reduce rising CO2-levels. This can be applied as a technology, by reacting captured CO2 with olivine or calcium-silicates in autoclaves. It can also be applied extensively, by spreading fine-powdered olivine on farmland or forestland. Measures to control the CO2-levels of the atmosphere will be adopted more readily if they also serve some broader economic goals. An effective strategy for CO2 control will require many parallel approaches simultaneously.

## Asbestos tailings

Canadian researchers at Université Laval have noted that, “Asbestos mine and mill residues in Quebec’s Eastern Townships represent 700 megatonnes of CO2 storage capacity.”

Although the amount of material in spoil-heaps is fairly small in global terms, the C02 capture may be worthwhile when done in conjunction with remediation. In the case of asbestos, enhanced weathering or conversion to magnesium cement would put harmful fibres out of harm’s way. Similarly it may also be economic to convert slag heaps from steelworks and fly ash from coal-burning power stations while extracting minor constituents such as thorium and other metals.

Carbon Dioxide Capture and Storage: A Compendium of Canada’s Participation, p. 222.

## Lamont-Doherty Earth Observatory

The Lamont-Doherty Earth Observatory has suggested doing enhanced weathering by pumping CO2 underground rather than digging up rocks. This webpage:

explains some of the potential advantages:

Study of in situ carbonation of mantle rocks is relatively new. We’ve made two key observations. First, based on 14C ages, natural carbonation of olivine proceeds much more rapidly than geologists had previously guessed. We sampled a large peridotite exposure in Oman. Our samples of solid carbonate terraces (“travertine”) forming from spring water emanating from mantle rocks, and of carbonate veins that formed within mantle rocks beneath the surface and were later exposed by erosion, have an average 14C age of about 25 thousand years, whereas scientists had guessed that the veins were 95 to 40 million years old.

Second, we found that the olivine carbonation process can be “self-heating”. Carbonation of olivine gives off heat energy – basically because it involves condensation of CO2 gas or liquid to form CO2-bearing solids. If the carbonation reaction is fast enough, the evolved heat can offset cooling due to diffusion from hot rocks into their cold surroundings, and due to flow of cold fluid through the rocks. At elevated pressure, perhaps one kilometer below the Earth’s surface, the carbonation reaction goes fastest at about 185°C. Under such conditions, the rock volume could become “self-heating”: we calculated that cold CO2-bearing fluid can be pumped into the rocks at about 4 centimeters per second without cooling the rocks and slowing the reaction. Thus, we propose that one method of speeding up olivine carbonation would be to “jump start” the process by drilling, fracturing, and heating a rock volume at depth to about 185°C, and then pumping purified CO2 plus water into that rock volume. We calculate that such a process would convert billions of tons of CO2 into solid carbonate minerals per cubic kilometer of rock per year.

Such an in situ process might be less expensive than ex situ olivine carbonation, “at the smokestack”, for several reasons. First, it avoids the cost of quarrying, transporting and grinding rock reactants. Once a rock volume is heated, the energy to sustain high temperature at depth would be provided by the carbonation reaction itself. Sustained pressure would be provided, to some extent, by the presence of overlying rocks. However, the cost of most CO2 capture and storage methods is dominated by the process of CO2 capture at power plants, and this proposal is no exception. Further, a truly globally important process of this type would involve transportation of really large amounts of CO2, comparable to the amounts of oil, gas and coal currently being consumed.

Thus, we are also considering an alternative possibility. In an end-member scenario, seawater, rather than purified CO2, could be used as a fluid to transport CO2. Shallow seawater maintains CO2 exchange equilibrium with the atmosphere, so using seawater could avoid the costs of industrial CO2 capture and transport. Deeper in the Earth, rocks are hotter, so one could drill into rocks at about 185°C. Thermal convection might drive cold water to circulate down one hole as hot water emerges from another. This would be thousands of times less efficient, in terms of kilograms of olivine transformed to solid carbonate per cubic kilometer of rock per year, than using purified CO2. However, in addition to CO2 capture and transport, this end-member scenario would avoid the costs of pre-heating a rock volume, and pumping fluid at high pressure. If drilling holes and fracturing rocks at depth is thousands of times less expensive than CO2 capture and transport, then this idea could be useful. Certainly, this epitomizes our approach to this problem: Understand the processes of natural olivine carbonation, and then do as little as possible to accelerate these processes in order to consume globally significant quantities of CO2.

A key problem with our proposed methods for accelerating carbonation of mantle rocks is that pore space might become clogged with newly formed carbonate minerals, and remaining olivine could be armored by a layer of carbonate, isolated from continued reaction with circulating fluid. However, geologic examples show that this self-limiting situation is not inevitable, since some rocks – known as listwanites – become fully carbonated, with virtually every Mg and Ca atom combined with CO2. We believe this is because, when reaction rates are rapid, the volume changes associated with the carbonation reactions create large stresses that fracture the rock, providing new, unobstructed pathways for fluid transport and creating fresh, unreacted olivine surfaces. Study of listwanites, and of reactive cracking processes, are now a focus of our research.

## References

In addition to the references listed above, for the geochemistry of olivine and serpentine, see:

CO2 + $\frac{1}{2}$ Mg2SiO4 $\to$ MgCO3 + $\frac{1}{2}$ SiO2 + 90 kilojoules/mole
This first reaction produces ‘90 kilojoules per mole’ according to Philip Goldberg et al. Is that moles of CO2, as opposed to Mg2SiO4? Is that what the $1/2$ is for? I.e. if we double the quantities on both sides of this reaction, we get 180 kilojoules per mole of Mg2SiO4?
        -2176.94        -787.02         -2223.38        -910.8568
Mg2SiO4     +   2 CO2   --->    2 MgCO3     +   SiO2