The impact of raw material selection on damring formation and pre-reduction during ferrochrome production
Electricity consumption is the largest cost component in the production of ferrochrome (FeCr). Currently the pelletised chromite pre-reduction process (solid-state reduction of chromite) is the process option with the lowest specific electricity consumption (MWh/ton). In this process, composite chromite pellets are pre-reduced at approximately 1300 °C in a rotary kiln. Excessive damring formation (material build-up) in the rotary kiln requires routine shutdowns to remove it, which cause damage to the kiln refractory and result in loss of revenue due to the break in the production of pre-reduced pellets. Damring formation can be caused by: i) melting of the ash of the pulverised fuel (PF) coal, which is used to fire the kiln, and/or ii) partial melting of the chromite pellets and/or pellet fragments. Ash fusion temperatures (AFT) of twenty different carbonaceous samples were evaluated to assess the temperature at which the PF coal ash will start to contribute to damring formation. The softening temperature (Tsoft), as determined with AFT analysis, was assumed to be the lowest temperature at which PF coal ash could start contributing to damring formation. The results indicated that the reducing and oxidising Tsoft of the carbonaceous materials differ substantially from one another, with many of these being below the typical material temperature in the prereduction kiln (~ 1300 °C). Therefore, PF coal ash can contribute significantly to damring formation. Multiple-linear regression (MLR) analysis was used for derive optimum MRL equations that could be used to relatively accurately calculate/predict reducing Tsoft and oxidising Tsoft. The equations will enable FeCr producers to select PF coals, in order to limit damring formation. The mathematical information obtained by the MLR analyses were also converted into a chemical context, by considering the relatively importance of the independent parameters included in the optimum MLR equations. This indicated that the PF ash composition, which is currently not considered by FeCr producers when selecting PF coals, is very important to minimise damring formation. Sessile drop tests were used to assess the softening behaviour of seven different fine chromite ores, as well as the softening behaviour of composite chromite pellets (containing the afore-mentioned ores) and the other pellet components (a carbonaceous reductant and a clay binder). The first sign of deformation detected during the sessile drop tests (deformation temperature), was taken as the lowest temperature at which the pellets and/or pellet components could start contributing to damring formation. The results proved that the composite pellet mixtures had significantly lower deformation temperatures than the ores alone. However, the deformation temperatures of the ores and pellet mixtures were above the typical material temperature expected in a pre-reduction rotary kiln (~ 1300 °C), therefore these materials are expected to contribute less significantly to damring formation than PF coal ash. Actual damrings were also analysed using scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS), which indicated that the damrings contained significant amounts of Cr and Fe (chromite or chromite derived particles). However, the matrix that bound these particle together most likely originated from PF coal ash. Interestingly, the sessile drop results also demonstrated that UG2 ore will not necessarily contribute more to damring formation than metallurgical grade chromite ore (which is currently assumed by FeCr producer). In order to assess the possible contribution of ores to damrings, the liberation of gangue minerals need to be considered, rather than the chemical composition of the ores. In addition to investigating damring formation, it was also attempted to correlate thermodimensional changes of composite chromite pellets to pre-reduction. This was done by first designing, constructing, commissioning and testing (to verify the accuracy thereof) a large mass thermo-gravimetric analyser (TGA). This was necessary, since the gas environment inside the prereduction rotary kiln is partially oxidising to allow for PF combustion, but also partially reducing due to the partial positive CO pressure inside the composite pellets (creating a reducing atmosphere inside the pellets themselves and in the pellet bed). This complex environment could not be recreated using a small mass commercial TGA. The results from the large mass TGA compared very well with the thermo-mechanical analysis (TMA) measurements, with both instruments indicating maximum rate of Fe and Cr reduction at 780 and 1380 °C, respectively. Therefore, it was concluded that thermo-dimensional changes can indeed be used to follow chromite prereduction.