SEM image processing as an alternative method to determine chromite pre-reduction

Abstract

Ferrochrome (FeCr) is a crude alloy containing chromium (Cr) and iron (Fe). FeCr is mainly used for the production of stainless steel, which is an important modern-day alloy. FeCr is produced from chromite ore through various smelting methods. In this study, the focus was on the pelletised chromite pre-reduction process, which is also referred to as the solid state reduction of chromite. In this process, fine chromite ore, a clay binder and a carbon reductant are dry milled, agglomerated (pelletised) and pre-reduced (solid state reduction) in a rotary kiln. The pre-reduced pellets are then charged hot, immediately after exiting the rotary kiln, into a closed submerged arc furnace (SAF). This production process option has the lowest specific energy consumption (SEC), i.e. MWh/ton FeCr produced, of all the FeCr production processes that are commercially applied. Other advantages associated with the application of the pelletised chromite pre-reduction process are that it eliminates the use of chromite fines, has a high Cr recovery, and produces low sulphur- (S) and silicon (Si)-containing FeCr. The main disadvantage of the pelletised chromite pre-reduction process is that it requires extensive metallurgical control due to the variances in the levels of pre-reduction achieved and carbon content of the pre-reduced pelletised furnace feed material. This implies that the metallurgical carbon balance has to be changed regularly to prevent the process from becoming carbon deficient (also referred to as 'under coke') or over carbon (also referred to as 'over coke'). The analytical technique currently applied to determine the level of chromite pre-reduction is time consuming, making it difficult and expensive to deal with large numbers of samples. In an attempt to develop a technique that would be faster to determine the level of chromite pre-reduction, a new analytical method using a combination of scanning electron microscopy (SEM), image processing and computational techniques was investigated in this study. Metallurgical grade chromite (<1 mm), anthracite breeze (<1 mm), and fine FeCr (<1 mm) that were used to prepare pellets in the laboratory, as well as industrially produced pre-reduced pellets that had already been milled in preparation for the determination of the pre-reduction level with wet chemical analysis were received from a large South African FeCr producer. These laboratory prepared pellets and the industrially produced pellet mixtures were considered in this investigation. Samples were moulded in resin and polished in order to obtain SEM micrographs of the polished cross sections. Elements with higher molecular weights are indicated by lighter greyscale, while elements with lower molecular weights are indicated by darker greyscale in SEM micrographs. This basic principle was applied in the development of the new analytical technique to determine the level of chromite pre-reduction, with the hypothesis that the pixel count of white pixels (representing metallised particles), divided by the combined pixel count of white (representing metallised particles) and grey (representing chromite particles) pixels would be directly related to the level of chromite pre-reduction determined with the current wet chemical method. This hypothesis can be mathematically expressed as: The newly-developed analytical method was validated by correlating the white pixel% calculated with the chromite pre-reduction levels (%) determined with wet chemical analysis of laboratory prepared and industrially produced pellet mixtures, which had R2 values of 0.998 and 0.919, respectively. This suggests that the method can be used to determine chromite pre-reduction accurately.