Date Published: December 01, 2019
Publisher: International Union of Crystallography
Author(s): Volker Kahlenberg, Hannes Krüger, Valerie Sue Goettgens.
Synthesis experiments in the system CaO–SiO2–Al2O3–Fe2O3–MgO resulting in the formation of SFCA-III are reported. SFCA-III is a new n = 2 member of a polysomatic series of M14+6nO20+8n compounds based on pyroxene-spinel modules which are of relevance to iron-ore sintering. Single-crystal diffraction studies using synchrotron radiation revealed that the compound occurs in two polytypes representing maximum degree of order structures which explains the observed allotwinning of the sample.
Silico-Ferrites of Calcium and Aluminum compounds (so-called SFCA’s) are major constituents of iron-ore sinters. Sintering is an important step in the iron-producing process, where loose iron ore fines (< 6 mm) are transformed at temperatures between 1250 and 1350°C into a mechanically stable composite that can be used as a feedstock for the blast furnace (Lu & Ishiyama, 2015 ▸). In the European Union, about 130 million tons of ore have been recently sintered per annum (Fernández-González et al., 2017 ▸), making iron-ore sinters one of the most produced inorganic materials. During the sinter process a moving strand is continuously charged with a mixture of ore (sinter feed), fine coke or anthracite (fuel), limestone (flux), other solid additives as well as water. The charge on the strand is ignited by burners using natural or coke oven gas. After a short ignition time a narrow combustion zone (flame front) is sucked downwards through the bed. In a series of high-temperature reactions a semi-molten porous material – the sinter – is formed including the so-called SFCA phases representing the polycrystalline bonding agent between the different particles (Lu & Ishiyama, 2015 ▸). According to our preliminary studies on the quinary oxide system we focused on samples with a bulk chemistry of 10.45 wt% CaO, 5.49 wt% MgO, 69.15 wt% Fe2O3, 13.37 wt% Al2O3 and 1.55 wt% SiO2. Starting materials for a total of 3 g were Fe2O3 (> 99.997%, Alfa Aesar), γ-Al2O3 (99.997%, Alfa Aesar), MgO (99,998% Alfa Aesar), CaCO3 (99.995% Merck) and amorphous SiO2 (99.99%, Alfa Aesar). The reagents were dried at 400°C for 24 h and checked for impurities using X-ray powder diffraction (XRPD). Before weighing on an analytical balance, the educts were stored at 110°C in a drying cabinet. A planetary mill operated at 600 rpm was used for homogenization for 45 min under ethanol. The resulting slurry was dried for one day at 50°C to remove the alcohol completely, manually re-homogenized in an agate mortar for 30 min and finally transferred to a desiccator. Before high-temperature treatment, about 0.6–0.8 g of the educts were pressed into pellets each having a diameter of 12 mm and a thickness of about 2 mm. Firing was performed in a resistance heated furnace with an external S-type thermocouple placed next to the open platinum crucibles containing the pellets. The samples were heated from 300°C (with a ramp of 90°C h−1) to about 100°C below the respective maximum temperatures (1100, 1200, 1250 and 1300°C). For the last 100°C, a slower rate of 30°C h−1 was employed to avoid over-heating of the sample. After a total experimental time of 72 h the samples were immediately quenched in a water bath. Weight losses were determined from weight differences before and after heating.
The crystal structures of triclinic and monoclinic SFCA-III are closely related. For their description we will start with their common features. The differences between them will be addressed later on in this section.