| dc.description.abstract | In the production of ferromanganese the gas-solid reactions taking place in the furnace shaft (the so-called pre-reduction zone, where higher manganese oxides in the manganese ore are reduced to MnO) are to a large extent decisive for the total consumption of both coke and electricity of this process. In particular the reduction of Mn3O4 to MnO may take place by one of two reaction paths. It may be reduced either by CO(g) (Mn3O4(s) + CO(g) = MnO(s) + CO2(g), an exothermic reaction), or this reaction may take place in conjunction with the Boudouard reaction (C(s) + CO2(g) = 2 CO(g), a very endothermic reaction). In the latter reduction path the CO2(g) produced by the reduction of Mn3O4 becomes a reactant in the Boudouard reaction, producing CO(g) which in turn reduces Mn3O4 to MnO. The overall reaction then becomes Mn3O4(s) + C(s) = 3MnO(s) + CO(g). This “direct” reduction of Mn3O4 is very energy consuming, and it is thus evident why it is of major importance that the higher manganese oxides are fully reduced to MnO before the Boudouard reaction becomes of significance. This is believed to be the case first at temperatures above 1000°C and with more than 30% CO2 in the gas-atmosphere. However alkalis have been known to catalyse the Boudouard reaction and studies have shown that alkalis accumulate in the ferromanganese furnace. Thus the Boudouard reaction is of significance also at temperatures below 1000°C. It is of course desirable to keep the consumption of coke (and electricity) to a minimum thereby contributing to cost efficiency and also because of the present focus on CO2(g) as the main greenhouse gas. Therefore a better understanding of the actual mechanisms of the above mentioned reactions is very important.
In this present work the influence of alkalis on the Boudouard reaction and the reduction of Mn3O4 to MnO have been studied in a thermobalance apparatus. The latter experiments were carried out in order to establish the rate controlling mechanism in the reduction of Mn3O4 to MnO.
In the Boudouard reaction experiments the reactivity of both ordinary metallurgical coke and coke impregnated with potassium were measured at various temperatures (800-1000°C). From the results of these experiments it is clearly evident that potassium does catalyse the Boudouard reaction, indicating that the Boudouard reaction is of significance at temperatures well below 1000°C when alkalis are present (possibly at temperatures down to 850°C), thereby increasing both the coke and energy consumption in the process of ferromanganese production. The potassium-impregnated coke displayed a much larger weight loss than the ordinary coke at both 900 °C and 1000°C, resulting in more than a tenfold increase in the reaction rate for the experiments carried out at 1000°C. In addition both ordinary and impregnated coke displayed increasing reaction rate with increasing temperature, but this effect of temperature was observed to be greater for the impregnated coke.
The manganese ore reduction experiments were performed using two size fractions of ore particles (2.4-4.8 and 6.7-9.5 mm) at various temperatures (900-1100°C), with either coke or CO(g) or both used as reductants. In addition both the reactant gas flow and the composition of the gas (100 % CO(g) and 83 % CO(g) + 17 % CO2(g)) were varied in the experiments. The manganese ore material used was Comilog ore. This choice of Comilog ore was made despite of its inhomogeneity because it is used by the Norwegian ferromanganese industry, and the results of the experimental investigations are thus of relevance to this industry. In advance the ore material, originally consisting for the most part of MnO2, was either calcined (decomposed by heating) at 1100°C to mainly Mn3O4 or pre-reduced to Mn3O4 in 100 % CO(g) at approximately 290-300°C (only a few experiments), so that the reduction of Mn3O4 to MnO could be studied separately. From the experimental result it was clear that both reaction temperature and particle size (of manganese ore) did influence the rate of reduction, to varying degrees depending on the experimental conditions. The reaction rate increased with increasing temperature and decreasing particle size. This indicates that both diffusion and chemical reaction contributes to controlling the rate of the reduction. External mass transport between bulk gas and particle surface does however not seem to limit the rate of reaction. Thus it is believed that a mixed control of gaseous diffusion and chemical reaction limits the reduction of Mn3O4 to MnO in the temperature range of 900-1100°C. Assuming that both diffusion through the product layer and chemical reaction at the interface between MnO and Mn3O4 are rate controlling, the data was tried both in a shrinking unreacted core model and in a grain model using regression (the grain model gives the best physical description of the process since the solid reactant is a porous media). Using the grain model the effective diffusivity was calculated to be in the 1,5-4,0• 10-5 m2/s range, with the pore size measured to be mainly between 0.1 and 10 µm.
The results of the Mn3O4 reduction experiments and the analysis of these gave indications of a change in mechanism taking place between 950°C and 1000°C, believed to be caused by sintering effects in the product layer of the reacted manganese ore particles. Sintering at reaction temperatures from 1000°C and above coarsens the microstructure of this MnO layer, producing larger pores. This is consistent with the results of the porosimetric analyses showing a difference in the pore size distribution of the samples from experiments performed at 900-950°C and samples from experiments carried out at 1000-1100°C, thus supporting the theory of a change in mechanism due to sintering effects. The structure of the solid reactant is also observed to vary according to the preparation of the manganese ore material (if the ore has been calcined or pre-reduced to Mn3O4), and these variations in both total porosity and especially the specific surface (much larger for pre-reduced Mn3O4 than for calcined Mn3O4) in turn affected the rate of reduction. Experiments carried out using pre-reduced Mn3O4 at 950°C displayed a significantly larger rate of reduction than experiments carried out using calcined ore material at this temperature. This is most probably due to a larger specific surface enhancing the rate of chemical reaction. However at 1000°C the preparation of the ore material (and thus the variations in specific surface and total porosity) did not seem to have any effect on the reduction rate. This is believed to be due to the total porosity of the samples from experiments done using pre-reduced ore was measured to be lower than the total porosity of samples from experiments performed with calcined ore. Thus diffusion through the product layer is probably slower for the pre-reduced material. At 1000°C the effect on the reduction rate due to this decrease in diffusion is believed to be of the same magnitude as the increase in chemical reaction due to larger specific surface. Consequently the effects of increased chemical reaction and decreased diffusion add up and result in the same rate of reduction at this temperature for pre-reduced and calcined ore material. | nb_NO |
