Mining waste is fertile ground
Finding a new use for a by-product of aluminium production
Last updated: 3 November 2022
In Canada’s Quebec region, a by-product of aluminium production is helping local farmers correct soil acidity and provide the nutrients their crops need.
The product, CHAC, offers local farmers a lower-cost local soil treatment, while also finding use for a waste material.
Making the most of waste
It’s one example of the work our scientists and engineers are doing to create new products from the waste created when producing minerals and metals.
CHAC is a by-product of making anodes, carbon rods that carry an electric current and create the chemical reaction needed to turn alumina into liquid aluminium.
Anodes are essential to making aluminium, but their production creates a waste gas, sulphur dioxide, that can be harmful to people’s health and the environment.
To prevent the sulphur dioxide being released into the air, special filters called scrubbers capture and “clean” potentially harmful emissions, like gas and dust.
Normally the scrubbing process involves adding a mix of different materials to the gas to create a solid, which is then carefully treated and stored as landfill. Instead, our scientists developed a patented process to combine lime with the sulphur dioxide, causing a chemical reaction that creates CHAC – a safe and effective soil treatment.
Around 17,000 tonnes of CHAC is recycled every year for use in construction and industrial applications. While we have more work to do to remove all waste from our operations, products like CHAC are helping us reduce our impact.
“Giving a second or third life to a material is the way we need to work for the future,” says François, Manager, By-Product Valorisation.
“If we don't move forward in this way, and if we don't change our way of doing things, we will exhaust our resources.”
Ingredients essential for plant growth
CHAC – chaux hydratée aqua-catalysée or aqua-catalysed hydrated lime – is a mixture of a sulphur compound called anhydrite (calcium sulphate) and hydrated lime (calcium hydroxide) – both important ingredients for plant and soil health.
"Calcium is important for the structure of plants' membranes and cell walls," says Julie Élize, an R&D Scientist at our Arvida Research and Development Centre. "And sulphur is needed for the synthesis of sulphur amino acids that make up proteins. Both are essential for plant growth.”
"The lime in CHAC helps correct soil acidity, which helps farmers optimise their crop yields."
Before it could be used, CHAC was rigorously tested across different soil types and on 15 different crops for performance and safety. It also complies with the “Liming material from industrial processes” Bureau de normalisation du Québec (BNQ) standard.
"In the past, cash crop producers in the Saguenay-Lac-St-Jean region of Canada may have been reluctant to lime their soils, as the materials available on the market have often had to come from outside the region, making them very expensive," says Julie Élize.
"Our industrial-scale trials allowed farmers to validate whether they could use our product, at the right dose, with their equipment to offer an alternative local, lower-cost, quality product.”
We recently entered a 5-year partnership with Viridis Environnement to moisten 5,000 tons of CHAC per year from the Arvida Plant at their Éco-Luzerne plant in Hébertville-Station. They’ll then sell it to farmers, who can use it as a soil treatment in their fields – a locally grown and sourced option.