Reductive Calcination: Process Integration in Mineral Processing
Inorganic carbonates are mainly found in minerals and brines. They have been mined, purified and used by mankind for hundreds of years. Many of the carbonates are used as raw materials for the production of metal oxides which are used as bulk products (e.g. cement, refractories) or fine chemicals (e.g. catalysts). Processing of carbonate based minerals preferably starts with their decomposition in oxidizing atmosphere. From the thermodynamic point of view the decomposition of carbonates is driven by temperature (and pressure). The by-product of thermal decomposition (=calcination) is carbon dioxide. This carbon dioxide is emitted into the atmosphere, leading to commonly acknowledged greenhouse gas problems. Depending on the type of mineral relatively high temperatures have to be applied. The temperature is maintained by combustion of fossil fuels, leading to even more carbon dioxide emissions.
Our approach is to combine the CO2-emitting step with methanation in a reactor concept, enabling single step conversion of hydrogen with process carbon dioxide to methane. This would lead to less carbon dioxide emissions by lowering the operation temperature of the calcining step, and by coupling the endothermic calcining with the exothermic methanation reaction. A thermodynamic analysis of the data for the decomposition of several carbonates in hydrogen atmosphere to produce methane was performed in the temperature range of 20-500 °C. This analysis revealed that from the first and second group elements only magnesium carbonate provides negative standard free energy of calcination in the range of -55 kJ/mol. Several transition metal carbonates (Mn, Fe, Co, Ni, Zn, Cd, Pb, Cu, Ag) show a negative standard free energy of calcination in the range of -50 to -120 kJ/mol. Analysis of thermodynamics revealed that for all carbonates the standard free energy of calcination is less negative with increasing temperature, thus facilitating the reaction at lower temperatures.
Experimental validation of calcination in hydrogen atmosphere confirmed the technology of single step calcination plus conversion of carbon dioxide into methane and carbon monoxide. The product gas composition and conversion can easily be adjusted by altering the temperature and pressure set point. The calcination step draws significant advantages from reduction of the carbon dioxide partial pressure with the heat carrier hydrogen, while conversion of carbon dioxide to methane and carbon monoxide can be performed in an optimum temperature window.