It is expected that for the upcoming years, the demand for energy will substantially increase, which will need to be reconciled with the rising demand for fossil fuels. This means that we need to develop eco-friendly and innovative technologies capable to capture, transport and storage CO2. The development of such technologies allow us to separate CO2 from different gaseous waste streams, transport it to storage locations and make CO2 long-term isolation from the atmosphere. It is expected that these type of technologies can contribute up to 55% of the cumulative global climate change mitigation effort [1]
A number of technologies already exist for each phase: capture, transport and storage, which consider the use of organic solvents, membranes and/or chemical processes with re-circulation for storing. Meanwhile, transporting can be made by using pipelines, rail and road tankers.
CO2 capture (or also known as CO2 sequestration) by mineral carbonation is a methodology based on the process of natural rock weathering where carbonic acid, H2CO3, formed during the dissolution of CO2 in water, is neutralised with high pH minerals to form stable carbonates, such as CaCO3 and MgCO3. The products remain as solids and there is no possibility of CO2 to be released after sequestration. The concept behind mineral carbonation is shown in the diagram: CO2 from the industry or power plants is transported to a carbonation reactor, combined with some silicate compounds from a nearby mine and held at the appropriate reaction conditions until the desired degree of carbonation is reached. The products of the reaction, which might be slurry of carbonated minerals and residues in aqueous CO2, are separated. The CO2 is recycled, useful materials are collected and the carbonated materials and residue are returned to the mine site.
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Mineral carbonation can be done directly or indirectly. The main difference between them is the number of step needed in each case. Thus, while direct carbonation requires only one step, indirect carbonation requires two or more steps. However, despite that both two routes have demonstrated quite good results in laboratories, their application at industrial scale are still being evaluated in terms of cost and benefits.
Currently, several industries must deal not only with CO2 emissions, but also with solid waste products (e.g., metallurgical slags, incineration ashes, mining tailings, asbestos containing materials, bauxite residues and oil shale processing residues), which represents a significant environmental liability for the companies. In most of the cases, such residues must be deposited in special dumps, which must be isolated from the ground and/or be able to treat with an excess of water. After the limit capacity, the deposit must be neutralized and stored, which requires great investment and strict security and environmental policy. During the last decades, scientists have demonstrated with great success the use of mineral carbonation when it is applied to such residues. Nevertheless, despite of the great number of research already developed, still there exist an uncertainty regarding the application of such technology at industrial scale, as it is not possible to compare the results obtained with different waste materials due to differences between chemical, mineralogical and morphological properties. However, the method offers a permanent sequestration for CO2, and the solid products can be used in applications ranging from land reclamation to iron- and steelmaking.
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