(62c) Reductive Calcination: Process Integration in Mineral Processing

Authors: 
Baldauf-Sommerbauer, G., Graz University of Technology
Lux, S., Graz University of Technology
Siebenhofer, M., Graz University of Technology

Reductive Calcination: Process
Integration in Mineral Processing

Georg Baldauf-Sommerbauer, Susanne Lux, Matthäus
Siebenhofer, Institute of Chemical Engineering and Environmental Technology,
Graz University of Technology, NAWI Graz, Graz, Austria

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.