Fundamental And Applied Studies On Preparing Oxidized Pellets From Mixed Iron Ore Concentrates

In this paper, based on the KISC practical situation for the oxidized pellets production by grate-kiln process, feasibility and related techniques by using many kinds of hematite and magnetite bulk concentrates of Yunnan province and surrounding areas to produce oxidized pellets were studied, also the basic theory about iron ore concentrates ballability, thermal hardening and ring-forming of rotary kiln and so forth were studied. The contents include:1. The ballability of single iron concentrates and mixed iron concentrates was studied. The results show that the ballability of single iron concentrates is not good. The ballability of five kinds of iron ore concentrates are ranked from bad to good as follows:Mannankan> Xiaohongshan> Dahongshan> Youjing> Brazil. The results show that rational ore blending can improve quality of pellet and decrease the bentonite addition. Considering the ballability, using iron concentrates of Yunnan province and surrounding areas instead of imported hematite was feasible.2. The preheating and roasting properties of single iron ore concentrates were studied. The preheating properties of iron ore concentrates are ranked from good to bad as follows:Youjing> Brazil> Xiaohongshan> Dahongshan> Mannankan; The roasting and preheating properties are ranked from good to bad as follows:Dahongshan> Xiaohong-shan> Youjing> Brazil> Mannankan. The preheating and roasting properties are close under different scheme of ore blending.The thermal hardening mechanism of mixed magnetite-hematite concentrates in Kunsteel shows that activity of Fe2O3 in the secondary hematite is higher than Fe2O3 in the original hematite. Therefore, during roasting process, either in preheating stage or in roasting stage, the exsiting magnetite promotes the recrystallization of Fe2O3, and can be beneficial to improve pellet strength and reduce roasting temperature.3. Based on the study of characteristics of ring-forming substances in rotary kiln of Kunming Steel and the simulation experiments in laboratory, the ring-forming mechanism of rotary kiln was found. The results showed:the ring-forming substances is dominantly attributed to solid agglomeration of the Fe2O3 phase recrystallization, and partly to liquid phase agglomeration of low melting point substances, which is accordant to that of oxidized pellets. Under the normal operation temperature, the ring-forming substances and iron ore concentrates would not soften and smelt. The deforming and softening temperatures changes with the raw materials structure or the compositions and contents of gangue were not obvious. Therefore, liquid phase agglomeration was not the main reason of ring formation. It has been proved by experiments: Powder is the material basis for ring-forming. The fluctuations of operating duty were the direct cause of ring formation. Alkali metals promote formation and development of ring-forming. During the practical production of oxidized pellets some effective countermeasures have been pertinently adopted to control and prevent the generation of ring formation, such as adjusting raw materials structure, enhancing balling process to improve the quality of green balls, optimizing the process of preheating and roasting, standardizing the blow in/out operation and so on.4. On basis of the above researches, the problems that lower granulating rate in the initial operation period and the serious ring-forming in rotary kiln were solved well, by adjusting raw materials structure, enhancing balling process, optimizing the process conditions, standardizing operation. The output and quality of pellets were increased evidently. The granulation ratio was raised from 34.32% to 66.32%. The pellet compression strength was raised from 1988N to 2497N. ISO tumbler indes was raise from 93.08% to 97.46%. Since the self-produced pellets replaced imported Brazil pellets, the technical and economical index of blast furnace was improved:the blast furnace production of iron increased by 1155t/d. The utilization coefficient of BF increased by 0.563 t/m3·d. The rate of the first class products increased by 11.69%, fuel ratio decreased by 18.32 kg/t-Fe. The total production cost per ton pig iron decreased by 56.82 Yuan. An annual benefit of 94.90 million Yuan can be achieved..In this paper, taking mixed hematite magnetite concentrates as raw material, the ballability, the preheating and roasting properties, the consolidation mechanism and the mechanism of the ring-forming were studied, which has a higher theoretical value and academic value. The industrial practices, taking mixed hematite magnetite concentrates as raw material, using grate-kiln pelletizing process for producing pellet, can provide the theory instruction for some domestic pelletizing Plant.