Pre-Oxidation of New Zealand Ironsand - Where does the Titanium go?

The interplay between iron- and titanium oxides has been studied, for a better understanding of how different amounts of titanium affects iron ore, for relevance in downstream ironmaking processes. The oxidation of NZ ironsand as well as synthetic titanomagnetite (TTM) of three different compositions has been studied in air up to 1320°C. In-situ monitoring of mass gain and thermal response during oxidation (TGA-DSC), as well as ex-situ analysis of crystals phases (powder XRD) and microstructure (SEM) of samples after quenching from select oxidation temperature at deliberate time intervals. Crystal phases in the samples were mapped spatially using SEM- EBSD, and chemical composition using (SEM-EDS). This has enabled a detailed evaluation of the relationship between the growth of specific crystals and their relative chemical compositions. Characterization of NZ ironsand revealed an overall homogenous ore body with a majority (90%) of homogenous (TTM) particles. However, a distribution of compositions was found between particles, with regards to Ti (0-42%), Al (1-5%) and Mg (1-4%). Approximately 1% of particles where homogeneous titano-hematite (TTH) particles of significantly higher Ti content, and 10% of particles where partially oxidised two-phase particles, containing both TTM and TTH. The two-phase particles showed higher Ti content in the TTH phase and higher Al- and Mg content in the TTM phase. Isothermal oxidation of Ironsand pellets at 960 °C,1100 °C and 1310°C, found that pseudobrookite (PSB) was formed within 3 min of oxidation time at these temperatures and that the amount of PSB formed decreased with increasing temperature in the range. Development of SEM-EBSD analysis enabled spatial mapping of crystal phases within the samples, showing that PSB formation was occurring preferentially at locations of higher Ti -content, such as two-phase and TTH particles.

Temperature and Ti- content significantly affects the resulting phases and morphology after oxidation. To isolate and elaborate on the effects of Ti content, the use of synthetically prepared single- phase TTM samples, where each sample is homogeneous in composition was proposed. A novel synthesis methodology for producing such single-phase TTM samples was developed, where thermal decomposition of ferrous oxalate mixed with rutile nano powder, in a controlled atmosphere, at 1200°C produced samples with Ti/(Ti+Fe) = 8, 15 and 20%, in a reduced time frame. TGA-DSC analysis of synthetic samples revealed four distinct oxidation events, associated with distinct oxidation mechanisms and temperatures, and the effects of Ti content on them. The first (~350°C) related to gradual transformation to titanomaghemite (TMH), which manifested as a shortening of the lattice parameter (a). The second (~550°C) related to partial formation of TTH, and was preferential towards higher Ti samples, with more Ti in the TTH phase. The third (~660°C) related to the formation of PSB, and was also preferential to higher Ti samples, driving the segregation of Fe and Ti further. The fourth (~720°C) was related to oxidation of the remaining (Ti poor) TTM phase to TTH. The third and fourth events showed considerable overlap, due to the sequential nature of the phase changes (TTM → TTH → PSB). Isothermal experiments with synthetic TTM samples were done at 500°C, 960°C, 1100°C and 1310°C. The gradual formation of TMH at 500°C was monitored through measurements of the lattice parameter (a) which was shown to decrease linearly with oxidation degree (z) for all three sample compositions, with a steeper slope with increased titanium content. At higher temperatures (960°C, 1100°C and 1310°C) the formation of TTH and PSB was monitored. The amount of PSB increased with increasing Ti content but decreased with increasing temperature. The synthetic samples of similar composition formed more PSB than ironsand, indicating that other cations in ironsand depresses the formation of PSB.

The morphology of samples showed small, finely dispersed, lamellar features at 960 °C, and larger elongated globular features at the higher temperatures (1100°C and 1310°C), reaching ~5 μm after 15 min oxidation at 1310 °C. The reaction progressed through the samples in a shrinking core fashion. Progression of reaction fronts varied with temperature and composition. At 960 °C TTH primarily formed at grain boundaries, and then progressing along parallel crystal planes. At 1100°C and 1310°C, reaction fronts were less ordered with broader features. A clear (10-20 μm) gap between two distinct reaction fronts was observed for high Ti samples, illustrating the sequential oxidation (TTM → TTH → PSB).

In general, increasing Ti-content promotes oxidation, and segregation of Fe -Ti, both at high and low temperatures. Increasing the oxidation temperature decreases the amount of PSB formed and thus the Fe- Ti segregation. This was found in ironsand and subsequently validated using synthetic TTM samples.