Prereduction Behavior in Induction Furnace
Extent of prereduction of manganese ores
Experiments were conducted in a setup that simulates the industrial furnace process with temperatures on top of the coke bed held at 1200 °C, 1300 °C and 1400 °C and ores were unmixed and mixed with carbon on parallel studies. Figure 2 shows the extent of prereduction of Comilog, Nchwaning and UMK ores measured as the variation of O/Mn ratio as a function of temperature from samples excavated from the induction furnace. For ores not mixed with coke, UMK prereduces completely i.e., reaches O/Mn ration equal to 1.0 at temperatures lower than both Nchwaning and Comilog. Nchwaning and Comilog reaches an O/Mn ratio of 1 at around 950 °C, and UMK at 100 °C lower temperature. As such, UMK shows the highest extent of prereduction of the three ores. The variation of O/Mn ratio for excavations from experiments of ores mixed with coke show an increased extent of prereduction for all ores.
The prereduction of ores is affected by CO concentration in the reducing gas atmosphere. Several studies[10,11,19,26,27] have reported an increased reduction rate of manganese ores with increasing CO concentration in CO–CO2 atmosphere. The concentration of CO in the reducing gas atmosphere is generated from the metal producing reaction (reaction ) and/or the oxidation of C by CO2. In the induction furnace set up, no significant metal producing reaction was expected for Comilog, Nchwaning and UMK since the highest temperature peak of 1400 °C is below smelting temperatures of these ores.[28,29,30] Hence, less CO is produced from the metal producing reaction compared to pilot or industrial furnaces. In comparison to Nchwaning, the higher extent of prereduction in UMK is mainly because UMK contains a higher CO2 content as shown in Table I, and will release more CO2 during heating which oxidizes C thereby producing CO for prereduction of manganese oxides. The reason for the poor extent of prereduction for Comilog is mainly due to of a lack of CO. Since Comilog is not a carbonate ore, the lack of CO is due to the less CO2 produced that can react according to the Boudouard reaction and produce CO. The increased extent of prereduction when ores are mixed with solid carbon is due to increased oxidation of C by CO2 in the presence of coke, thereby increasing the rate of reduction by CO. Gasification of carbon in coke has been previously reported to be catalyzed by Mn3O4 and determines the overall rate of reduction.
Porosity development and decomposition of carbonates
The porosity results shown in Figure 3 are for excavated ores, which were unmixed and mixed with coke and their corresponding residual carbonate content (given as CO2 content) as a function of temperature. The trendlines were calculated using a Lowess filter with a smoothing parameter of 0.35. It can be mentioned that the porosity is measured for single particles, and hence it is expected a large variation as the properties from particle to particle may be quite large.
Porosity development during heating and reduction shows that ore particles become more porous with increase in temperature as shown in Figure 3. This is in agreement with previous findings.[17,31] The peak porosity of Comilog and Nchwaning during reduction is around 800 °C to 1000 °C, whereas for UMK it is at 800 °C to 900 °C. Upon reaching a peak porosity, a downward trend for the porosity measurements with further increase in temperature is observed for UMK ore. This decrease in porosity is assumed to be due to the observed sintering of the particles. The presence of coke is expected to increase the porosity at lower temperatures, because of better prereduction, but the effect is unnoticeable considering the heterogeneity of the ores.
The carbonates present in the ores decompose endothermically. Several studies[10,32,33] have shown that the decomposition of carbonates is highly dependent on temperature and CO2 content in the reducing atmosphere, with increasing temperature and lowering the partial pressure of CO2 resulting in a greater extent of decomposition. The remaining CO2 content in the samples analyzed as a function of temperature is shown in Figure 3. It can be seen in seen that almost no CO2 content was observed from Comilog ore (Figure 3(a)), since the ore does not contain carbonate minerals and the analyzed CO2 could be from coke dust contaminating the ore particles. UMK ore (Figure 3(c)), shows a clear trend of decrease in CO2 content due to carbonate decomposition with increase in temperature up to 900 °C, beyond which carbonate decomposition is complete. This trend is also observed in Nchwaning ore, with some outliers since the initial content in Nchwaning is around 4.45 wt pct. Minerals distribution in Nchwaning ore is heterogeneous and the ore is in general characterized by very low porosity.[22,34] Therefore, the high carbonate content in some particles at elevated temperatures, as seen in Figure 3(b) could be pockets of undecomposed carbonates resulting from this heterogeneity. The rapid increase in porosity for UMK is believed to be due to decomposition of carbonates leaving voids in the ore particles.
Prereduction behavior in 70/30 CO/CO2 atmosphere
The experimental weight loss data acquired from the TGA setup are reported and utilized in calculation of change in O/Mn ratio and its first derivative as reaction rate (wt pct/s) all as functions of temperature. The variation of weight loss with temperature for non-isothermal reduction of Comilog, Nchwaning and UMK is shown in Figure 4(a). The corresponding O/Mn ratio calculated through the mass loss as a function of temperature is shown in Figure 4(b). The mass loss follows the increase in sample temperature. Comilog ore which has a higher initial O/Mn ratio of 1.93 forms a “Z-shape” in the weight loss vs temperature curve. This is due to exothermic reduction reactions, which cause the temperature to increase above the programmed temperature and is characteristic of MnO2–ores.[17,19] Some deviation is observed in Figure 4(a), where the “Z-shape” curve for Comilog ore at 6 °C/min reaches a higher temperature than that at 9 °C/min. At 9 °C/min the exothermic reactions initiate at lower temperatures compared to 6 °C/min, thus more MnO2 has been consumed and this results in a lower exothermic peak temperature. Nchwaning and UMK are Mn2O3 based ores and their prereduction starts from an initial O/Mn ratio of 1.47 and 1.45, respectively. Figure 4(a) shows that UMK has the highest weight loss followed by Comilog and lastly, Nchwaning. This trend is in accordance with theoretical weight loss shown in Table II. A greater part of the weight loss from UMK is from carbonate decomposition, which is approximately 4 times the weight loss contribution from prereduction as shown in Table II, whereas in Comilog its mainly from prereduction reactions and evaporation as it contains higher manganese oxides MnO2 and around 3.8 wt pct water content. Nchwaning is an Mn2O3 ore, with lower carbonate content compared to UMK has weight loss contribution from prereduction and carbonate decomposition.
The very exothermic reactions observed when prereducing Comilog ore occurs very close to 500 °C at heating rates 6 °C/min and 9 °C/min as seen in Figure 4. This exothermic peak rapidly increases the temperature close to or exceeding 800 °C. At a heating rate of 3 °C, the exothermic peak is absent as the furnace is able to tune the temperature with less exothermic reactions per minute. Weight loss in Comilog ore starts at approximately 200 °C and Comilog ore has an O/Mn ratio very close to 1 at 800 °C. Between 600 °C and 800 °C, one can see that the lower heating rate gives a lower O/Mn ratio at a given temperature. Nchwaning ore shows a clear distinction of the effect of heating rate on prereduction, with a lower heating rate of 3 °C/min giving rise to a higher extent of prereduction. The prereduction of Nchwaning ore commences at about 350 °C and the prereduction is complete by 1000 °C. On the other hand, the prereduction of UMK ore commences at slightly higher temperature at 500 °C compared to Nchwaning ore and is completely prereduced by 900 °C. The corresponding reaction rate (wt pct/s) curves as a function of temperature for all the three heating rate scenarios in the studied ores is shown in Figure 5. In general, the reaction rate is highly influenced by heating rate, with decreasing heating rate resulting in longer time at each temperature. The rate curves for Comilog ore in Figure 5(a) shows that at 6 °C/min, the exothermic peak reaches a higher temperature compared to 9 °C/min heating rate. The rate curves in Figure 5(b) for Nchwaning ore are characterized by the highest peak occurring at about 900 °C. The lower peaks at temperatures less than 900 °C become less pronounced with decreasing rate. As shown in Figure 5(c), the prereduction of UMK ore occurred with two major distinctive steps, the first one mostly occurring at about 700 °C, and the second one at the higher temperature 900 °C. It is also observed that the peaks in UMK reduce in size with decreasing heating rate, due to the longer time at each temperature.
An interesting phenomenon that could affect the TGA weight loss data and subsequently the rate curves, is carbon deposition. This may be difficult to detect from weight behavior when prereduction reactions occur simultaneously. According to thermodynamics, the Boudouard (CO2 + C = 2CO) reaction is shifted to the left below 700 °C resulting in carbon deposition, whereas at higher temperatures it will proceed to the right. A high CO/CO2 ratio enhances carbon deposition. Larssen conducted experiments with quartz in 80/20 CO/CO2 atmospheres and found that carbon deposition was initiated at temperatures close to 400 °C and continued with increase in temperatures. At temperatures above 780 °C gasification of the deposited carbon was observed. As such, it is believed that the initial peaks observed in the reaction rate curves for mostly Nchwaning ore and UMK ore, which increase with increasing heating rate, as shown in Figures 5(b) and (c) are due to carbonate decomposition. Prereduction reactions in Comilog ore start at relatively low temperatures as shown by the weight loss curve in Figure 4(a). This causes CO to be consumed at low temperatures, thus the driving force for the carbon deposition is lower in this case. Therefore, it is believed that prereduction of Comilog ore is not affected by carbon deposition. No carbon was observed in the reduced samples in all experiments; hence any deposited carbon had been gasified during the experiments. However, gasification of carbon is expected to occur simultaneously as the higher reduction rates in these experiments and would be obscured by the reduction reactions.
Evaluation of the reactions occurring at the distinctive peaks in Nchwaning and UMK was done by conducting non-isothermal experiments at 9 °C/min in 70/30 CO/CO2 gas atmosphere, with final temperatures at the peak regions. The final temperatures were 700, 800 and 900 °C for UMK whereas for Nchwaning the experiment was ended at 900 °C. The chemical composition of samples from these experiments in addition to the initial experiments are shown in Table III. The chemical composition shows significant decrease in carbonate content for Nchwaning and UMK at the investigated peak temperatures. Therefore, the percentage carbonate decomposed from Nchwaning and UMK was calculated from initial values and the results are shown in Figure 6. These results show that the described distinctive peaks in UMK and Nchwaning are mainly due to decomposition of carbonates in addition to prereduction reactions.
Analysis of the constituents of TGA rate curves
The change of mass loss during TGA experiments corresponds to removal of oxygen from higher manganese oxides and iron oxides i.e., prereduction reactions, decomposition of carbonates and water evaporation. As such, there are several reactions involved and it is not straightforward to isolate effects of these individual reaction components. Therefore, to establish the contributions of these components, Fityk a general purpose peak or model fitting software was used to decompose the rate curves.
The constituents contributing to the overall rate of weight loss in Comilog ore are water evaporation and prereduction reactions shown in Figure 7, at different heating rates. Temperature profiles are indicated on the secondary axis as a function of time. When looking at the decomposed rate curves for 3 °C/min, the water evaporation happens virtually at the same time as the main reduction rate peak. Considering all the heating rates, integration of the water loss curves gives total weight of 3.24, 3.88 and 3.84 wt pct at heating rates of 3, 6, 9 °C/min, respectively. These values show a high accuracy in relation to 3.8 wt pct water content in the ore as presented in Table II. The temperature range for evaporation is 200 to 300 °C, which is in agreement with previously reported evaporation temperature range. At 6 and 9 °C/min, the reduction reaction can be seen starting at low temperatures and the crucible temperature increases in accordance with exothermic reactions, but it is not until the water is completely evaporated that the main bulk of the reaction occurs, giving a very exothermic peak.
The constituents contributing to the overall rate of weight loss in Nchwaning ore are decomposition of carbonates and prereduction reactions shown in Figure 8, at different heating rates. Temperature profile indicated on the secondary axis as a function of time show an endothermic dip in the sample temperature at 900 °C, which corresponds to where the carbonate contribution peaks are. The expected weight loss from decomposition of carbonates is 4.45 wt pct as presented in Table II. Integration of the rate curves of the carbonate contribution gives total weight loss of 4.21, 3.91 and 3.47 wt pct at heating rates of 3, 6, 9 °C/min, respectively. Therefore, the weight loss due to carbonate contribution decreases with increasing heating rates. The majority of the reduction, and exothermic reactions, occur at temperatures below 800 °C as shown by reduction constituents.
Figure 9 shows the contributions of decomposition of carbonates and prereduction reactions to the overall rate of weight loss in UMK ore and their changes with heating rate. Temperature profiles for UMK indicated on the secondary axis as a function of time show two endothermic dips in the sample temperature at 700 °C and 900 °C. The height of the first peak at 700 °C is shown to decrease with increasing heating rate and is attributed to both prereduction and decomposition of carbonates. As previously shown in Figure 6, about 20 wt pct carbonates are decomposed at this first peak, and the O/Mn ratio is 1.29 as presented in Table III. Therefore, in the first peak reduction reactions happen in parallel with the carbonate decomposition reaction. On the other hand, the height of the second peak at 900 °C appears to be unaffected by the heating rate. At this peak, a greater proportion of carbonates is decomposed as shown in Figure 6 and the prereduction reactions are complete as O/Mn ratio is 1.0 as shown in Table III. The curves in Figure 9 reveal that UMK is fully reduced by 900 °C and the carbonates decompose in two steps and this is supported by the chemical analysis in Table III and variation of the decomposition of carbonates with temperature shown in Figure 6. Based on the chemical analysis of UMK ore in Table I, the carbonates are assumed to be dolomitic and previous studies[32,33] have shown dolomite to decompose in two distinct steps with temperature zones depending on the type of mineral existing around it. Integration of the rate curves of the carbonate contribution gives total weight loss of 14.7, 14.0 and 13.8 wt pct at heating rates of 3, 6, 9 °C/min, respectively. As seen from Table II reduction of iron and manganese oxides in UMK ore is expected to contribute about 5 wt pct and decomposition of carbonates about 16 wt pct to the overall weight loss. The curves presented have been verified to match the total weight loss expected from their constituents and the slight variations could be attributed to heterogeneity of ores and uncertainty in chemical analysis. In conclusion, the two endothermic peaks observed in the rate of weight loss curves in UMK are essentially due to decarbonation of dolomite and reduction reactions for the first peak and mainly decarbonization of calcite in the second peak.
Decrepitation of ores
Figure 10 shows the particle size distribution for Comilog, Nchwaning and UMK ores heated to 1000 °C in 70/30 CO/CO2.
The Nchwaning ore particle size distribution curve has a similar shape as the UMK ore compared to the Comilog ore curve when heated in CO/CO2. However, in the finer size fractions below 4.75 mm Nchwaning ore generates more fines compared to UMK ore. The distribution below 10 mm is shallow for Nchwaning ore, meaning that the particles are distributed more equally between the finer mesh sizes. This signifies that when Nchwaning ore decrepitates it produces more fines than medium sized particles whereas when UMK ore decrepitates it tends to crack into larger particles, as opposed to generating fines. There is no apparent correlation between heating rate and decrepitation for Nchwaning ore. The effect of heating rate on decrepitation on UMK ore shows that an increase in heating rate results in increased decrepitation. On the contrary, Comilog ore shows that increase in heating rate results in decreased decrepitation. Though there is a difference on the effect of heating rate on decrepitation when comparing UMK ore and Comilog ore, the significance of heating rate on decrepitation of UMK ore is not high. From Figure 10, the fraction of particles with size range lower than 10 mm are 75 to 90 pct, 35 to 42 pct and 48 to 60 pct for Comilog, Nchwaning and UMK ores, respectively. There is a significant difference in size distribution for all the three ores in CO/CO2, which shows that Comilog ore produces the most fines followed by Nchwaning ore and lastly UMK ore. Higher decrepitation of Comilog in comparison to other ores is in agreement with previous investigations.[22,31] Comilog ore has been reported to have lower mechanical strength and high initial porosity in the range 30 to 50 pct and Nchwaning ore is characterized by a higher mechanical strength and very low porosity (0 to 1 pct). As such Comilog is expected to decrepitate the most. Larssen measured the decrepitation of Comilog and Nchwaning ores, and showed the same correlation between lower heating rates and an increase in decrepitation for Comilog ore. In another study, Biørnstad studied the decrepitation for UMK, Nchwaning and Comilog and showed that Nchwaning decrepitates more than UMK, which is contrary to findings in this work, however they are both in the same area.