Research On Microwave-enhanced Carbothermal Reduction Of Low-grade Pyrolusite Pellets

Manganese has a very important strategic position in the development of national economy and society.It can be seen that manganese plays an important role in both industrial production and human production and life.Meanwhile,the demand for manganese is particularly large.The grade of manganese carbonate used in China has been reduced to 13%-15%.On the other hand,a large amount of pyrolusite with the grade between 20%-25%has not been fully utilized because of the high cost of reduction process or serious environmental pollution.Therefore,an efficient low-grade pyrolusite intensified reduction process will be of great strategic significance to alleviate the contradiction of manganese resources shortage and ensure the sustainable development of manganese materials industry.In view of the urgent demand for low-grade pyrolusite treatment,a process of microwave reduction of low-grade pyrolusite pellets was proposed in this paper,which integrated the advantages of microwave heating and pelleting.The dielectric constant,dielectric loss and dielectric loss tangent of the mixture of low-grade pyrolusite and coal at room temperature are 5.854,0.146 and 0.0249,respectively,with 4.314,0.0771 and 0.0179 at 600?C.Although the dielectric properties of the mixture of low-grade pyrolusite and coal decrease at high temperatures,it still has good microwave absorption properties.So under low microwave power?400W?,the material?22g?can be heated to 800?C in 200 seconds.In microwave carbothermal reduction of low grade pyrolusite,aiming the reduction rate is controlled above 90%and the content of ferrous is controlled about 4%,when the coal content is 10%,the low grade pyrolusite need to be roasted for 40 min at700?C,50 min at 800?C for conventional carbothermal low grade pyrolusite-coal pellets.In microwave carbothermal carbothermal reduction of low grade pyrolusite-coal pellets,the reduction rate can reach 92.2%and the content of 0 is about4%when the carbon content is 10%and roasted at 600?C for 40 minutes,which fully meets the requirements of industrial production that the reduction rate is more than90%.XRD analysis showed that iron began to exist in the form of Fe₃O₄ after roasting at 500?C,and Mn₂SiO₄ was found in the samples after roasted at 600?C.Compared with the low grade pyrolusite raw material,SEM showed that there were many cracks and holes in the sample surface after microwave roasting.This phenomenon can be attributed to three reasons:Firstly,due to the selective heating of microwave,secondly,gases were produced in the reaction,and there were certain volatiles in the coal powder.Thirdly,the product layer was crisp and porous.According to particle size analysis and SEM,the sample size after microwave roasting was significantly lower than that of raw materials,which indicated that the characteristics of microwave selective heating and rapid heating were conducive to the reduction of particle size and the opening of inclusions.The optimum technological parameters of microwave carbothermal reduction of low-grade pyrolusite pellets were also analyzed by response surface methodology.Based on the central combination design,the regression equations of reduction rate?Y ₁?and ferrous content?Y₂?were fitted:Y₁%=66.34+21.32A+14.30B+15.85C+0.99AB-0.18AC-3.58BC-2.96A²+2.81B²+2.96C²-2.00ABC-4.72A²B-1.58A²C-11.52AB².Y₂%=Sqrt?Y₂?+0.04?=0.31+0.22A+0.39B+0.33C+0.13AB+0.12AC+0.23BC+0.098A²+0.11B²+0.21C².The optimum technological parameters for microwave-enhanced carbothermal reduction of low-grade pyrolusite pellets were given as follows:roasting temperature 600?C,roasting time 41.01 min,and carbon content 11.04%.Under the optimum technological conditions,the reduction rate of manganese was 90.33%,and the content of ferrous was 1.07%.