DETAILS · Developing enhanced weathering methods in mine tailings for CO2 sequestration

Developing enhanced weathering methods in mine tailings for CO2 sequestration

Throughout geological history, the Earth has naturally moderated atmospheric CO2 through chemical weathering of rocks, a process that converts atmospheric CO2 into hydrogen carbonate and carbonate ions, known as alkalinity, or as precipitated solid carbonate minerals. These processes typically occur over thousands of years or longer. Based on these natural principles, one CO2 removal method that aims to capitalise on the Earth’s natural processes is through enhanced weathering of rocks. The aim is to speed up this natural weathering process so that instead of it happening over thousands of years, it happens over tens of years, to combat climate change. The principle of the method is to weather the minerals that are contained within the rocks. Silicate minerals which also contain calcium and magnesium can react with CO2 and water in the atmosphere. There are ways to speed the reactions up. For example, by grinding the rocks to smaller grain sizes, we achieve better exposure of the rock to CO2. We can also speed up the reactions by using more concentrated CO2 gases, increasing the temperature, using acidic solutions for the reactions or by using microbial communities that can help break down the minerals. Since small grain sizes, such as sand size grains or finer, are beneficial for enhanced weathering, one potentially significant material for CO2 removal is the unwanted rock wastes that are stockpiled at mine sites. We call these wastes mine tailings, and they are the remaining crushed up rock powders that are left behind after the metals or economic minerals have been extracted. Mine wastes are useful because they are already fine grained, as mine companies require significant rock size reductions in order to harvest the metals and minerals that they are targeting. Tailings are essentially stored as powders on the site. There is also lots of it available, as society needs a lot of mined resources to function. We investigated the different waste streams at the mine site to identify the best minerals and best mine wastes for CDR purposes. We looked at the chemistry and mineral content of the different streams through chemical and mineralogical analysis. After we identified our potentially useful waste streams, we tested them directly, separating out the best individual minerals to test their reactivity with water and CO2. Once potential for CO2 removal through any of the produced wastes was identified, we determined methods to speed up the reaction, either by harnessing high temperatures, high pressures, concentrated CO2, develop acidic conditions through the use of microbial materials, better exposure to the atmosphere (e.g., spreading) or any further crushing and/or separation that can be realistically achieved. We then looked to scale up the experiments, taking the work beyond the laboratory scale up to a big enough scale to use on the mine site. By taking this approach, we were able to select the most appropriate materials and strictly monitor the changes to the mine wastes and calculate the CO2 removal along the way. If we needed to change the system, we did so in real time.

Data: CORDIS, © European Union