| The reserves of tin-bearing iron ores, mainly distributed in Guangdong, Inner Mongolia, Hunan, Guangxi, and Yunnan province, are relatively rich in China. In tin-bearing iron ores, taken from Huanggang in Inner Mongolia Autonomous Region, magnetite is the main mineral and cassiterite is the chief mineral of tin. Cassiterite in iron ores in the forms of irregular fine monomer and fine grains embedded in magnetite. Cassiterite can not be efficiently separated from the tin-bearing iron concentrate by using physical separation methods. Relatively complete tin separation can be achieved by weak reduction roasting process for tin-bearing iron concentrate pellet by using grate-kiln, which was developed by Central South University.On the basis of the novel process of weak reduction roasting by using grate-kiln, the fundamental theories on the reduction roasting of tin-bearing iron concentrate were studied in this paper, including the reduction mechanism of stannic oxide and tin separation from tin-bearing iron concentrate by reduction roasting.The results of the reduction of stannic oxide showed that the reduction mechanism of stannic oxide was closely related to the reduction temperature and atmosphere. At temperatures between 975℃and 1100℃, according to the volume fraction of CO in CO+CO2, three categories of the reduction mechanisms of stannic oxide can be summarized as the following:(1) SnO2+CO=SnO(g)+CO2At certain temperature, when the volume fraction of CO is below a specific value, the stannic oxide is only reduced to gas stannous oxide which is volatilizing, but not be further reduced to metallic tin. When the temperature is between 975℃and 1050℃, the specific value of CO content is 16.25%, and when the temperature varies from 1075℃to 1100℃, the specific value is 15%.(2) SnO2+2CO=Sn(1)+2CO2 and Sn(1)+SnO2=2SnO(g) At certain temperature, when the volume fraction of CO is above a specific value, the stannic oxide is reduced directly to metallic tin and the reaction of Sn(1)+SnO2=2SnO(g) occurs concomitantly to some degree. With the increase of temperature, the specific value is increased from 45% at 975℃to 60% at 1100℃.(3) SnO2+CO=SnO(g)+CO2, SnO2+2CO=Sn(1)+2CO2 and Sn(1)+ SnO2=2SnO(g)At certain temperature, when the volume fraction of CO is higher than the specific value in the first category and lower than the specific value in the second category, the reduction of stannic oxide include the above three reactions, but which is the primary reaction is dependent on the temperature and the volume fraction of CO.Kinetics study on the reaction of SnO2+CO=SnO(g)+CO2 indicated that with the increase of volume fraction of CO, the activation energy of this reaction decreased remarkably, from 328.79kJ/mol at CO 10.0% to 248.13kJ/mol at CO 12.5%. The investigations displayed that the reaction was matched with the shrunk solid model without hole of the reagent and without solid production layer. The reaction was controlled by interfacial chemical reaction.The research on tin separation from tin-bearing iron concentrate pellet by reduction roasting showed that the separating effect increased at first and then decreases as the CO content increased, and it reached the maximum when the content of CO reached 50%. Other conditions being equal, the effect of tin sepretation from preheated pellets is significantly weaker than that non-preheated ones.An analysis of the pellets after reduction roasting was performed by optical microscopy, SEM and EDS. The results proved that the critical factor in effective tin separation from tin-bearing iron concentrate pellet was that the products of Fe-Sn alloys and metallic tin must be decreased.Study on tin separation from tin-bearing iron concentrate powder by reduction roasting indicated that, when the content of CO was between 22.5% and 40%, and the time of reduction process was above 30min, a significant tin separating effect can be obtained at 1075℃, tin volatilization rate came up to above 98.9%, and the content of tin of the reduced powders was lower than 0.01%.
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