MnFe2O4-based spinels by mechanochemical and thermochemical reaction of siderite and MnO2 powder mixtures

The manganese ferrite has been processed by mechanosynthesis, using reactive siderite and MnO2 powder mixtures, with and without subsequent heat treatment. Taguchi planning was used to assess the impact of milling conditions (rotational speed and time) and subsequent calcination temperature on conversion of precursors, based on integrated intensities of the main XRD peaks of precursor phases FeCO3 (104) and MnO2 (110), and spinel phase (311), using Ni as internal pattern. Thermogravimetry provided additional information on the residual contents of reactants in the as-milled samples. The best conditions for complete conversion to spinel phase of as-milled samples were obtained by milling at 650 rpm, for 6 h (I311,sp /I111,Ni = 2.41), and for calcined samples were obtained after milling at 650 rpm, for 3 h, and calcination at 650 degrees C (I311,sp /I111,Ni = 2.35). The impact of processing conditions on structural features of the spinel phase was accounted by the contributions to variance of intensity of its (311) reflection, which were 90% ascribed to rotational speed and only 4% ascribed to milling time, for as-milled samples, or 58%, 15% and 16% ascribed to rotational speed, milling time and calcination temperature, for calcined samples. Readier conversion of FeCO3 relative to MnO2 occurred during the milling stage, at low or intermediate rotational speed, with impact on Mn : Fe ratio in the spinel phase, as revealed by changes in lattice parameter of as-milled samples, in the range of 0.8424-0.8464 nm. Subsequent calcination minimized or eliminated traces of secondary phases. The incorporation of magnesium oxide and possibly other secondary components of the siderite precursor also induced structural changes in the resulting spinel phase, mainly in calcined samples (ao = 0.8370-0.8434 nm). Partial oxidation also contributed to structural changes in calcined samples, as revealed by differences between samples calcined in inert and oxidising atmospheres.