Damring Formation During Rotary Kiln Chromite Pre-reduction: Effects of Pulverized Carbonaceous Fuel Selection and Partial Pellet Melting

Abstract

Electricity consumption is the largest cost contributing factor in the production of ferrochrome. Currently the pelletized chromite pre-reduction process (solid-state reduction of chromite) is the process option with the lowest specific electricity consumption (MWh/ton). In this process, pelletized chromite is fed into a rotary kiln at 1573 K (1300°C), where partial pre-reduction takes place. Damring formation (material build-up) in the rotary kiln causes routine shutdowns, resulting in loss of revenue. The damring formation is possibly caused by melting of the ash of the pulverized coal used to fire the kiln and/or the partial melting of the chromite pellets. Ash fusion temperatures of twenty different samples were evaluated to assess the temperature at which the pulverized coal ash will start to contribute to damring formation. Sessile drop tests were used to assess the softening behavior of different ore types (e.g., UG2, MG, and LG metgrade), as well as softening of composite chromite pellets made from these ores. Actual damrings were also analyzed using scanning electron microscopy with energy dispersive X-ray spectroscopy. Results indicate that it is mainly the pulverized coal ash that will contribute to damring formation, and not ore or pellet softening. Multiple-linear regression was used to derive equations to predict the ash fusion temperatures of the pulverized coal ash, which can be used by ferrochrome producers to optimize pulverized coal selection.

An Overview of Currently Applied Ferrochrome Production Processes and Their Waste Management Practices

“…As opposed to limited damring formation, excessive damring formation, indicated with "b" in Figure 7a, can increase the effective slope that the pellets experience within the furnace. This results in a shorter retention time within the furnace hot zone which reduces the pellet pre-reduction level [80].…”

“…Approximately 30 wt% Cr 2 O 3 is present in damrings. However, the chromite particles themselves could not contribute to damring formation as the temperature at which chromite softening initiates (i.e., 1756 • C) is significantly higher than the ash softening (i.e., 1054 • C) temperatures [80]. Figure 7a shows a cross-sectional and longitudinal view of the rotary kiln used during chromite pre-reduction.…”

Influence of Parameters on the Pre-Reduction Process of Vanadium-Titanium Magnetite Carbon-Containing Composite Pellets in Rotary Kiln

“…In the smelting phase, the obtained hot and pre-reduced pellets are then fed directly from the kiln head into the oxy-coal combustion smelting reduction furnace. Feeding the hot pre-reduced pellets into the smelting reduction furnace could reduce the energy consumption obviously [14]. The smelting reduction furnace is provided with two rows of the tuyere.…”

Contact sorption drying of chromite concentrates

“…Where chromite is mined as the primary product, it is preferable to produce a concentrate known as lumpy, chip, or pebble ore, which varies in size between 6 mm and 150 mm (Beukes et.al., 2017). However, when it is produced as a by-product during platinum group metal (PGM) production, the complexity of the ores necessitates the reduction of chromite particle size below 1 mm to produce a concentrate known as metallurgical grade (van Staden, 2018). This is achieved using primarily spiral and cyclone technology (Murthy, Tripatjy, and Kumar, 2011).…”