No abstract

Coke was produced from a Waterberg semi-soft coking coal using microwave heating and selected microwave susceptors. Waterberg semi-soft coking coal is poorly susceptible to microwave heating, especially below 500°C, and therefore requires microwave susceptors. Susceptors were selected from ferroalloy fines and their respective ores.Various batch experiments were performed on compressed discs utilizing a resonant microwave cavity at a constant 915 MHz frequency to heat a batch of about 5 to 7 kg of the semi-soft coking coal (sscc) to obtain coke. Materials were characterized using Inductive Coupled Plasma (ICP) analysis and coke strength tests.Dielectric property results showed that chrome and manganese ores, as well as their respective high carbon ferrochrome and ferromanganese alloys, are suitable microwave susceptors to enable rapid coke formation during microwave heating. Coke formation was completed within 2 to 3 hours up to 1 100°C compared to 21 hours for a commercial plant since microwave heating reduces the "cold centre" in a coke oven. Obtained coke strengths were slightly lower than for a commercial coke but still of a high quality. It was also shown that the admixture of chrome ore resulted in its partial reduction which will be advantageous to the ferrochrome industry since this method allows for recycling of fines without additional pelletisation. The results also showed that microwave energy has the potential to be employed during commercial coke formation, either on its own or as a hybrid technology.

Prior to operating a non-recovery coke pilot plant, it was critical to ascertain coal cake stability during the loading of a 1 m 3 coal cake into the oven. Various compaction parameters were verified and established on a small laboratory-scale compaction machine to obtain a coal cake of acceptable stability. These parameters include cake density, cake surface moisture, transverse strength (force applied perpendicular to the original compacted coal cake layers), and applied force to the coal cake. This work determined the behavioural characteristics of the coal while being compacted either with a full-sized or a 1 /3-sized compaction plate in a 9 kg capacity mould. Two different coals were evaluated, namely Waterberg semi-soft coking coal (sscc) and Oaky North hard coking coal. The target wet cake density of 1 100 kg/m 3 (79% of 1 400 kg/m 3 relative density) was achieved for Waterberg sscc, with a particle size d 50 varying between 0.6 mm and 1 mm, utilizing the 1 /3-sized compaction plate in the laboratory-scale setup , with 11.6% surface moisture and 92.2 t/m 2 commercial equivalent applied force. For Oaky North hard coking coal, a wet cake density of 1 189 kg/m 3 (85% of 1 400 kg/m 3 relative density) was achieved at a surface moisture content of 12.3% and at a lower applied force than that for Waterberg sscc, i.e. 78.5 t/m 2. Coal cakes of acceptable strength, and therefore sufficient stability for further processing, were obtained for all materials evaluated during this study. Further studies should be conducted to determine the effect of zeta potential during the compaction of coals.

No abstract

Prior to operating a non-recovery coke pilot plant oven, it was essential to ascertain coal cake stability during the loading of a 1 m 3 (1 m × 1 m × 1 m) coal cake into the cold pilot-plant oven by a pusher mechanism. Previously established laboratory-scale compaction parameters, to obtain coal cakes with sufficient stability, were employed during the loading tests of the coal cakes. Two different coals were evaluated, namely Waterberg semi-soft coking coal (sscc) and Oaky North hard coking coal. It was demonstrated that lower applied compaction forces (14%) can be utilized during 1 m 3 compaction, in comparison to laboratory-scale tests, to obtain similar wet cake densities, owing to differences in the effect of frictional forces. Wet cake densities for Waterberg sscc improved slightly (3%) to 1.13 t/m 3 when utilizing 79 t/m 2 applied force for a 1 m 3 coal cake, in comparison to laboratory-scale tests. Similar results were obtained for Oaky North hard coking coal, with a 6% density increase to 1.27 t/m 3 at a similar applied force. Porosities decreased by 13% and 32% for Waterberg sscc and Oaky North coal, respectively, when 1 m 3 compaction was compared to laboratory-scale tests. An opportunity exists to further decrease the porosities of Waterberg sscc during compaction, whereas acceptable compaction parameters were achieved for Oaky North hard coking coal. Most major vertical cracks and the partial collapse of coal cakes were mitigated, and high-stability coal cakes were obtained that could be successfully loaded into the pilot-plant coke oven by a pusher mechanism. Factors contributing to the instability of the coal cake during loading into the oven, such as vertical cracks, are (i) the cubic geometry (1 m 3) of the coal cake, (ii) friction exerted by the oven floor, and (iii) mechanical problems, such as a misalignment between the pusher machine feeder plate and the oven floor.