| With the fast development of the steel industry in China, the production of pellets has been further expanded. Pellets are used as a high quality charge for blast furnaces. However, the steel industry in China has been confronted with a serious shortage of domestic pellet feed due to the low iron grade of Run-of-Mine ore in spite of massive iron ore reserves. Therefore, more pellet feed, mainly hematite, has been im5ported for pellet production due to its high iron grade and low impurities. However, VALE hematite fines, especially the specularite type, is refractory for pelletising. It is of significance to improve the pelletising of VALE hematite fines for supplying of hematite type pellet feed with Chinese pellet industry.In this thesis a study of the improvement of the pelletization of VALE hematite fines using grate-kiln process was carried out, and pretreating of VALE hematite fines by high pressure roller grinding, elevating pellet basicity and designing of blends comprising VALE hematite fines and magnetite concentrates as pellet feed were developed in small scale and pilot scale tests. Pretreating process and full flowsheet parameters were optimized. The corresponding mechanisms were demonstrated by measuring the microstructure and surface properties of iron ores, properties of green balls and fired pellets, metallurgical performance and mineralogy of fired pellets. And industry application of the improvement measures was carried out successfully.It is shown that VALE hematite fines, assaying grade of66.79%Fe, low silica and alumina, and minor impurities, are good pellet feed in chemistry. However they possess poor ballability and are refractory for pelletising due to low specific surface areas of520cm2/g and poor ballability index of0.15in spite of a fine size of91.14%passing0.074mm and61.59%passing 0.043mm. Even the hematite fines are finely ground by ball milling up to1320cm2/g, the compressive strength of preheated and fired pellets are only369and1365Newton per pellet when preheating at1150℃for15min and firing at1330℃for15min, far below the target of2500Newton per pellet.Three technological solutions have been developed to improve the pelletization of VALE hematite fines, such as blending30%magnetite concentrate with hematite fines as pellet feed, pretreating of VALE hematite fines by ball milling of hematite to1140-1320cm2/g from520cm2/g, followed by pressing through HPRG up to1500-1700cm2/g, and elevating the basicity of fired pellets to0.2-0.3basicity made of100%hematite fines with natural0.04basicity. In all cases, the compressive strength of preheated and fired pellets are higher than the target of500and2500Newton per pellet, respectively.HPRG is the most effective way to improve the pelletising of hematite fines technically and economically. Two types of roller press circuits are recommended, one is the open circuit with the pellet feed passing through two separate rollers continuously, the other is the closed circuit with only one roller pressing machine operating at certain recirculating load. The improvement for pelletising of blends consisting of30%magnetite and70%hematite is achieved as follows:●Specific surface areas of hematite pretreated by HPRG at210%recirculation load and in open circuit with two-pass reached1495cm2/g and1294cm2/g from1100cm2/g, resulting in statistic ballability index of0.798and0.628from0.15, respectively. The specific surface areas are used to assess the ballability of iron ores.●HPRG not only elevates improve balling, but also decrease the dosage of bentonite. The optimum conditions for balling are as follows:0.7%-1.0%bentonite,9.0%●0.5%moisture, balling for6to10min. The drop numbers and compressive strength of green balls are over5time from 0.5m and15Newton per pellet, respectively.●Fired pellets made of feed pretreated by HPRG with two-pass have good firing performance at a relative low induration temperature. The compression strength of preheated and fired pellets can reach over771and2759N/pellet when preheating at1100-1130±10℃for10to13min and with air flow rate of2.2m/s, firing at1280±10℃for13min. In contrast, for firing of pellets made of hematite pretreated by HPRG at210%recirculation load, the compression strength of preheated and fired pellets can reach over707and2745N/pellet when preheating at1130±10℃for12min and with air flow rate of2.2m/s, roasting at1300±10℃for15min, respectively.●The functions of HPRG are probably explained that the hematite particles are mechanically activated by transferring the mechanical energy into chemical energy on the surface, leading in more cracks formed in the particles and higher lattice deformation and defects. All of which result in the improvement of higher surface hydrophilicity, better ballability of hematite fines, excellent firing behaviours of green balls and metallurgical performance of fired pellets.The firing behaviours of green balls made of100%hematite fines with suitable specific surface areas are dramatically enhanced by elevating the basicity of fired pellets. Especially the compressive strength of fired pellets is elevated from1100Newton per pellet to over2500Newton per pellet from natural basicity of0.04to0.15-0.3basicity. Which is ascribed to the formation of calcium ferrite at low temperature, enhancing the solid diffusion reactions and consolidation of pellets.The metallurgical performance of fired hematite pellets are significantly improved by HPRG, blending magnetite concentrates and elevating basicity, respectively or in combinative patterns, as follows:●The size distributions of product pellets are over90%between8and16mm, assaying65-66%iron,0.5%FeO and very impurities. Which means that VALE hematite fines can help to elevate the iron grade of fired pellet.●Blending more magnetite into hematite pellet feed can control the reduction swelling. Pellet reduction swelling index drops to20.36%when30%magnetite2is blended. The fired pellets made of100%hematite possess strong temperature reduction degradation, with high values of RDI+3.15of51.45%and RDI.0.5of44.81%. When30%magnetite are blended into hematite feed, resulting in RDI+3.15elevated up to over97%and RDI.0.5dropped to less than2.5%.●Elevating the basicity of fired pellets from natural to0.2-0.3basicity is similar to that by blending30%magnetite with hematite fines. The reduction swelling index ranges between21%to26%and RDI+3.15is beyond97%.●Pellets, made of blends comprising80%hematite and20%magnetite concentrate2pretreated by HPRG in closed circuit at210%recirculation load and open circuit with two-pass, possess the best metallurgical performance, such as RI of67.18%and77.54%, RSI of13.08%and6.64%, RDI+3.15of81.74%and75.21%being achieved.All of the above achievements are proven by the industry practice in Zhuhai YPM pellet plant with an annual capacity of1.20million tons of fired pellets, where pellet feed comprises70-100%VALE hematite and some0.2basicity. Also they have been spreading to Joint Venture Plant of Anyang Steel, WISCO Minerals, etc., the compressive strength is over2500Newton per pellet, and all the metallurgical performance index meet the requirement for blast furnace. The achievements are included in the project "Key Techniques Development and Their Application of Large Scale Grate-kiln Pelletising of Hematite", which was granted the Second Class Prize of China Science and Technology Progress Prize in2010. |
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