Tapping into carbon dioxide storage potential of mine waste

Hitch studies the value of mine waste rock for its CO₂-sequestration potential, or "SP." He says mining companies across Canada will, in future, be able to offset CO₂ emissions with so-named "SP rock," and within 25 years could even be selling emissions credits.

Digging, trucking and processing make mining an energy-intensive industry that emits greenhouse gases. However, mine waste rock that is rich in the mineral magnesium silicate has an inherent ability to react with CO₂ and chemically "fix" it in place as magnesium carbonate -- an ability that can be greatly enhanced with some processing. Hitch and his colleagues note that this capacity for CO₂ fixation can be five to ten times greater than total greenhouse gas production from some mine operations. Nickel, diamond, copper, chromite, platinum, palladium, talc, and asbestos mines could all be contenders. Some large mines, the researchers add, could fix 5 million tonnes or more of CO₂ per year.

"I don't like waste," asserts Hitch. "I like to see efficient use of the resources."

Dynamics of mining operating altered

Instead of using just 1 per cent of the materials from a big mining pit, he explains, a company could receive value from the non-commodity rock. "All of a sudden this material starts having value, and this material starts taking on a position in the company's cash flow as a byproduct," says Hitch, adding, "It really kind of changes the dynamics of the mining operation."

With the global price of carbon emissions credits expected to rise, SP rock could become even more valuable.

However, in order to achieve substantial CO₂ sequestration in SP rock, the somewhat sluggish chemical reactions that naturally fix CO₂ require a jump start.

Teamwork key to research

Hitch is working on this problem alongside researchers Greg Dipple, team lead, and Ulrich Mayer, both of UBC's Department of Earth, Ocean and Atmospheric Sciences, and Gordon Southam, with the University of Western Ontario's Department of Earth Science. The collaboration is being funded by Carbon Management Canada, a Network of Centres of Excellence that funds research to produce the technology, knowledge, and human capacity that will reduce carbon emissions in the fossil energy industry and in other large-scale emitters.

Two of the team's primary goals are to measure the rate of CO₂ fixation in mine waste rock and tailings in a lab setting and to speed up the process. Team members have already observed that CO₂ fixation is greatly accelerated in mine tailings, presumably due mainly to the large surface area exposed and available to react after rocks are crushed into small particles.

Lab findings

Dipple's lab reports that their previous research has demonstrated that CO₂ is trapped in mineral precipitates at rates of up to 50,000 tonnes per year within tailings during mine operations, and continues to be sequestered after mine operations cease. Rates of fixation are limited by the dissolution of CO₂ in water and one area of investigation involves increasing the concentration of CO₂ supplied to a slurry similar in chemical composition to tailings process water. Results show a 200-fold rate of increase over atmospheric weathering of some minerals by increasing the concentration of CO₂ in the air passed through the slurry to 10%.

Hitch's lab is currently grinding rock and pre-treating the material in order to change its physical and chemical properties. Dipple's group will then examine the material's ability to fix CO₂. The collaborating researchers hope to move to field trials in five years.

Role of microbes studied

Meanwhile, Southam's UWO research group is studying the role of microbes in fixing CO₂ and precipitating carbonate minerals, in particular as sedimentary rock. They are also working on methods to accelerate this process.

Another important goal is to use computer modeling to predict the sequestration potential of rocks at specific mining sites. To that end, Hitch is designing a way to use mining planning software to put a dollar value on the amount of SP rock that could be obtained at particular locations. These data, coupled with Mayer's modeling of CO₂ uptake in mine wastes at the environmental and climate conditions of specific mine sites, could allow for comprehensive evaluation of CO2 fixation capacity and rate for individual mine sites around the world.

"None of this (work) is done in isolation," notes Hitch, adding that carbon management is not an easy solution. "It requires lots of different perspectives and lots of different skill sets," he says.

The safe storage of CO₂ in mine waste and tailings for thousands of years is an exciting idea that could refresh the public image of the mining industry. One day, Hitch and his colleagues add, research findings from mining could even be applied to CO₂ sequestration projects underground and in marine basins.