Stefaan Simons

Low energy synthesis of cement compounds in molten salt

Abstract Abstract The world cement industry is responsible for >5% of the total anthropogenic carbon dioxide emissions blamed for causing global warming. The production of cement clinker minerals by precipitation from a molten salt solvent offers a potential route to energy reduction in cement manufacture. Molten salt synthesis of the major cement compounds β-dicalcium silicate (β-Ca2SiO4, β-C2S) and tricalcium silicate (Ca3SiO5, C3S) has been attempted in fused sodium chloride (NaCl). The synthesis of β-Ca2SiO4 was carried out by the reaction of CaCO3 with SiO2 in molten NaCl (CaCO3-SiO2-NaCl mole ratios 2∶1∶19·2 at 908°C, and 2∶1∶20·4, 2∶1∶13·5, 2∶1∶10·3 and 2∶1∶8 at 1140°C). The product was characterised by powder X-ray diffraction, Raman scattering and scanning electron microscopy. For the synthesis of Ca3SiO5, reactants with a molar ratio of 3∶1 were used (CaCO3–SiO2–NaCl 3∶1∶19·8 at 908 and 1000°C, and 3∶1∶20, 3∶1∶14, 3∶1∶9·9 and 3∶1∶8·1 at 1140°C). In all cases β-Ca2SiO4 was the principal product, with the CaO phase still present and (if any) only small quantities of Ca3SiO5. These observations, coupled with previous studies on the solid state synthesis of Ca3SiO5, indicate that β-Ca2SiO4 is an intermediate compound, requiring in excess of 1140°C to react with CaO in order to form tricalcium silicate.

Purification of pigment grade titanium dioxide by processing in molten salts

Abstract An alternative method of purifying the mineral rutile (TiO2) is reported. This offers potential savings in process energy and reduced waste when compared with current technology. The work reported here focused on refining rutile sand (95%TiO2) containing transition metal oxides (Fe2O3, SiO2, ZrO2, Cr2O3, V2O5 and Al2O3), which impart a strong colour to the mineral concentrate. The objective was to remove the colour giving oxides to produce rutile of over 99% purity (which is white in colour). Complete dissolution in molten salt (alkali chloride–fluoride) at 750°C allowed electroseparation of the transition metals between a graphite anode and a stainless steel cathode. The voltage maintained across the cell ensured removal of transition metal ions from the solution, with minimal loss of titanium. In this process, dissolution of TiO2 was enhanced by partial replacement of chloride by fluoride in the melt to allow the complex ion TiF2−6 to dominate the titanium speciation. This had the additional advantage of minimising losses of Ti as volatile TiCl4.