Biofuel crops growing on tailings at Xstrata Nickel’s facility near Onaping, Ontario | Courtesy of Xstrata Nickel
In 1991, Ontario implemented laws requiring mining, milling and smelting operations to design and fund long-term closure plans. At the same time, Xstrata
Nickel’s mine and mill complex near Onaping was ahead of the curve, exploring innovative ways to go beyond legal obligations. One of the first mining
operations to replace tailings impoundments with desulphured covers, Onaping has now gone a step further, and Xstrata Nickel’s Sudbury operations have
transformed what was once a liability into a source of revenue by seeding the cover with biofuel crops. With its eye on the future, the company has started
modelling the potential effects of climate change to ensure stability of the closure site over the long term.
Joe Fyfe, superintendent of environment and sustainable development systems at Xstrata Nickel’s Sudbury operations, says his company takes a broad interest
in innovation. “We participate in a large number of research areas,” he says, “trying to move the yardsticks forward.”
Biofuels: the latest reward
Because Xstrata Nickel mines a copper-nickel sulphide ore at its Sudbury facilities, it must prevent the acid runoff that occurs when sulphides are
oxidized. Xstrata Nickel’s technical specialists are working with Canmet Mining, Mirarco and Laurentian University to study the use of biosolids from
municipal food and yard waste composts as a growth medium for biofuel crops, with additional benefits as an oxygen-blocking top cover. As part of the Green Mines Green Energy (GMGE) initiative launched by Canmet Mining, three northern Ontario sites – owned by Xstrata Nickel, Vale and Goldcorp – have proven the
technical feasibility of growing biofuel species and have now moved on to assessing the economics.
Phase one of the project determined whether canola, corn, switchgrass, sunflowers and willow could grow in a biosolids plot. Xstrata Nickel used municipal
compost trucked from Toronto to build a metre-deep, 0.5-hectare plot over part of its Strathcona tailings facility in 2008. Between 2009 and 2011, the plot
was tripled in size. Researchers sampled the soil and biomass for metal contamination, measured each crop’s growth level, and fine-tuned the mix.
Accumulation of tailings metals in either the soil or biomass did not appear to be a problem. A low water table prevented Xstrata Nickel’s original
groundwater well installations from gathering meaningful data, but Fyfe says the site’s monitoring wells are being upgraded for deeper penetration.
That first phase was a success, with a few caveats. At Xstrata Nickel’s Sudbury operations, yields improved year-on-year as the immature compost responded
to fertilizer and the perennial switchgrass reached maturity. The corn was a new dwarf variety and contained too little sugar to be economically viable as
a biofuel, and due to competition with weeds, the willows’ growth was “not spectacular,” says Fyfe. Switchgrass and sunflowers, however, performed well. In
2011, the two switchgrass varieties produced a respective 11 and 9.6 tonnes per hectare (dry weight), enough to serve as potential fuel feedstock. “The
energy balance is more favourable with the perennial type of crop, so we’re leaning that way,” Fyfe points out.
The future of Xstrata Nickel’s biofuel crop depends on the next phase of the GMGE assessment. Phase two will gauge the commercial viability of the project,
addressing issues like the availability of biosolids and the relative efficiency of transforming each crop into fuel. The GMGE consortium has put together
research proposals, but the timeline for this phase is still open-ended, says Daniel Campbell, director of the environmental monitoring and rehabilitation
group at Mirarco.
Canmet Mining has rough estimates of the potential revenues, suggesting that 11 tonnes of switchgrass growing on one hectare could net $468, assuming a
value of $56 per tonne and a production cost of $148 per hectare per year. For canola, net revenue could reach $905 per hectare per year. However, those
figures omit the significant cost of transporting and spreading the compost. As a result, participants are considering experimenting with lowering their
biosolid use by spreading shallower layers or by mixing compost directly into the tailings, which could mitigate both the cost and the lack of local
compost availability. The Strathcona tailings site currently has 65 hectares of slimes cover eligible to grow biofuels crops.
According to Bryan Tisch, senior environmental scientist at Natural Resources Canada (NRCan), GMGE was the first study to introduce biofuel crops onto mine
tailings. Potential biofuel partners and other Canadian sites have already expressed an interest, but few other sites appear to have mounted trials.