Rodolfo Marin Rivera
It is well known that emissions of carbon dioxide (CO2), originating mainly from burning oil and coal, contribute enormously to the so-called greenhouse effect. Due to its increase in the atmosphere during the last two and a half centuries, it has been estimated that by 2100, atmospheric CO2 concentrations could reach approximately 1150 ppm, resulting in a global temperature increase about 5.5 °C  … It doesn’t look so much, eh? However, such minor increase of temperature will have harmful effect on water and food availability, human health, ecosystem, coastlines and biodiversity. Therefore, if we are not able to perceive the danger of this situation, we will not be able to react to this threat on time… as the frog that is slowly cooked to death once the water is heated.
Today there is a worldwide concern about such drastic implications and, governments have made commitment to reduce their greenhouse gas emissions. The EU has committed to reduce its greenhouse gas emissions by 80-95% by 2050. Moreover, in countries like Sweden, Norway, Netherlands, Denmark, Finland, Italy, UK and Ireland have been implemented regulations in the form of CO2 taxes. However, despite of these efforts, it is estimated that the emissions of CO2 can only be reduced up to 30% by reducing the amount of carbon energy sources and the use of non-fossil energies. For instance, whatever is the way to have under control the greenhouse effect, it is essential to concentrate our efforts on the reduction of both the emissions and the atmospheric CO2 level.
If we consider the increase in demand for energy and the fact to reconcile the rising demand for fossil fuels, we have to start developing technologies that fall under the concept of Carbon Capture and Storage (CCS). With the development of such technologies, it will be possible to separate CO2 from gaseous waste streams, transport CO2 to storage locations and make its long-term isolation from the atmosphere. It is expected that the development of such technologies may contribute up to 55% of the cumulative global climate change mitigation effort .
A number of technologies exist for each phase of CCS, which consider the use of sorbents, membranes and/or chemical-looping for storing, while transporting can be made by using pipelines, rail and road tankers.
CO2 “sequestration” by mineral carbonation is a technology based on the process of natural rock weathering where carbonic acid, H2CO3, formed during the dissolution of CO2 in water, is neutralized 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 the 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.
The carbonation process 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 represent 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.