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J. Cent. South Univ. 2012 19 2611−2619 DOI 10.1007/s11771-012-1318-5 Mechanism of high pressure roll grinding on compression strength of oxidized hematite pellets FAN Jian-jun范建军1,2 , QIU Guan-zhou邱冠周1, JIANG Tao姜涛1, GUO Yu-feng郭宇峰1, HAO Hai-zheng郝海正1, YANG Yong-bin杨永斌1 1. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China; 2. Taiyuan Iron and Steel Group Co. Ltd., Taiyuan 03003, China Central South University Press and Springer-Verlag Berlin Heidelberg 2012 Abstract The mechanism of high pressure roll grinding on improvement of compression strength of oxidized hematite pellets was researched by considering their roasting properties. The results indicate that oxidized hematite pellets require higher preheating temperature and longer preheating time to attain required compression strength of pellets compared with the common magnetite oxidized pellets. It is found that when the hematite concentrates are pretreated by high pressure roll grinding HPRG, the compression strengths of preheated and roasted oxidized hematite pellets get improved even with lower preheating and roasting temperatures and shorter preheating and roasting time. The mechanism for HPRG to improve roasting properties of oxidized pellets were investigated and the cause mainly lies in the increase of micro-sized particles and the decrease of dispersion degree for hematite concentrates, which promotes the hematite concentrate particles to be compacted, the solid-phase crystallization, and finally the ation of Fe2O3 bonding bridges during subsequent high temperature roasting process. Key words hematite concentrate; pellet; high pressure roll grinding HPRG; specific surface area; lattice defect; dispersion degree; micro-sized particle 1 Introduction Over the last several years, compared with damp-grinding, high pressure roll grinding HPRG has received considerable popularity in mineral processing industry because of its advantages of low energy consumption [1−4], high productivity and easy maintaining [5−7]. Its potential to improve balling perance also attracts much attention in pellets industry, especially for the treatment of coarse-sized iron concentrates. It was reported [8−9] that particle morphology of iron concentrates was altered and balling perance got better when iron concentrates were pretreated by HPRG. The corresponding studies attributed this function to the improvement of specific surface area and wetting heat [10], so it is confirmed that coarse-sized iron concentrates become easy to be agglomerated into green pellets when HPRG is applied properly, which results in the reduction of bentonite consumption rate and the improvement of total iron content of pellets correspondingly. Other studies also revealed that the compression strength of preheated and roasted pellets increased when magnetite concentrates were pretreated by HPRG, for which the mechanism was investigated and the causes were proved to be the reduction of apparent activation energy, lattice deation, increase in micro-sized particles, huge specific surface area, and greatly increased oxidation reaction rate during magnetite oxidation process, which finally enhance the ation and crystallization of Fe2O3 lattices during high temperature roasting process [11−13]. For hematite pellets, compared with magnetite pellets, the roasting property is quite different. The findings show that the structure of hematite pellets always keeps their original state if the roasting temperature is less than 1 200 C. The Fe2O3 grains are not enlarged, nor the Fe2O3 lattice crystallized until the temperature is over 1 300 C. At higher temperature, initial connecting bridges are ed between crystal grains and re-crystallized grains of Fe2O3 [14]. Therefore, it is evident that thermal requirements for hematite pellets are higher than those for magnetite pellets. The related research also covered the effect of HPRG on improving the compression strength of preheated and roasted pellets [15]. However, there is little specific study focusing on the mechanism of this effect, and the cause is just based on the assumption that new cracks on particle surface facilitate the ation of bonding lattices. Since magnetite and hematite pellets exhibit Foundation item Project50725416 supported by the National Natural Science Funds for Distinguished Young Scholars of China Received date 2011−07−18; Accepted date 2012−05−18 Corresponding author FAN Jian-jun, PhD Candidate; Tel 86−15935102760; E-mail fanjj1 J. Cent. South Univ. 2012 19 2611−2619 2612 different roasting features, in order to reveal the function of HPRG on the compression strength of hematite pellets and use hematite concentrates properly in the production system of rotary kiln pellets, a series of experiments were conducted on pellets for hematite pretreated by HPRC. The results showed that the compression strength for preheated pellets reaches over 400 N only with roasting temperature being higher than 1 070 C, so it is difficult for such preheated pellets to withstand the mechanical collision during the initial roasting process in rotary kiln. However, it was found that the compression strengths of preheated and roasted pellets increase even at lower preheating and roasting temperature for shorter time after the hematite concentrates are subjected to HPRG. Consequently, the present work focuses on the effect of HPRG pretreatment on the changes of specific surface area, dispersion degree, lattice defects, microstructure and the micro-sized particles etc, and more emphases are concentrated on the mechanism of HPRG on the improvement of compression strength of pellets by comparing HPRG pretreatment with ball mill grinding. 2 Experiment and 2.1 Working principles of high pressure roll grinding The HPRG was designed initially according to the theory of “Inter-particle comminution [16]”, while the conventional crusher was based on the theory of “Impact breakage”. Instead of being broken as single pieces of particle by conventional crusher, the material particle is broken or comminuted as a compacted material bed or cake in HPRG process, so the material particles squeeze with adjoining particles and many micro-cracks and edges are caused subsequently under high pressure compression. The schematic diagram of HPRG is shown in Fig. 1. Fig. 1 Schematic diagram of HPRG 2.2 Basic studies on iron concentrates 2.2.1 Chemical composition of iron concentrates The chemical compositions of iron concentrates are listed in Table 1. Table 1 Chemical compositions of iron concentrates mass fraction, Total Fe FeO SiO2 Al2O3 65.03 6.13 4.15 0.38 CaO MgO S Ig 0.21 0.26 0.023 0.94 Table 1 shows that the iron ore is hematite since FeO content is only 6.13. 2.2.2 Size distribution and specific surface area of iron concentrates The size distribution and specific surface area of the hematite concentrates are listed in Table 2. The specific surface area was determined by using Brunauer-Emmett- Teller . Table 2 Size distribution and specific surface area of hematite concentrate Size distribution mass fraction/ 0.074mm0.074−0.044 mm 0.044 mm Specific surface area/cm2g−1 2.34 5.87 91.79 1723.0 The hematite concentrates are very fine with a 91.79 portion being less than 0.044 mm in size, and the specific surface area is as high as 1 723.0 cm2/g. 2.3 s of experiment The hematite concentrates prepared for experiments were ground by HPRG and ball mill grinding, respectively. The specification of high pressure roll is d 250 mm120 mm, while the grinding compression pressure is 1 MP and the moisture content of iron concentrates is 8.0. In order to get the required specific surface area, recycled grinding was carried out. The specification of ball mill is d 500mm 500 mm with 12 of media occupancy at 25−40 r/min. Green pellets were prepared using a laboratory balling disc with a diameter of 600 mm, an edge height of 200 mm and a tilting angle of 45 at 25 r/min. After balling, green pellets were screened with 10 mm and 12.5 mm screens to get 10−12.5 mm pellets. The roasting of pellets was finished in an corundum crucible set in an electric tube furnace with 35 mm in diameter, where the temperature profile was divided into three continuous zones preheating oxidizing at 600− 1 000 C, roasting at 1 150−1 300 C and cooling at 600−900 C. The parameters of roasting process include preheating temperature and time, and roasting temperature and time. The quality indices of pellets are expressed by compression strength of preheated pellets and roasted pellets for an individual pellet. J. Cent. South Univ. 2012 19 2611−2619 2613 3 Results and discussion 3.1 Effect of HPRG on compression strength of preheated and roasted pellets The experiments were first focused on the effect of HPRG on compression strength of preheated pellets and roasted pellets. In order to uate the effect of HPRG on compression strength of pellets, the lower limit of required compression strength for preheated and roasted pellets are stipulated to be over 400 N and 2 500 N, respectively. The hematite concentrates were ground by HPRG and the specific surface area increased from 1 723.0 cm2/g base case to 2 201.1 cm2/g. Then, the green pellets were prepared and roasted at preheating temperatures of 950, 970, 1 000, 1 020 and 1 070 C, roasting temperatures of 1 180, 1 200, 1 220 and 1 250 C, and preheating and roasting time of 10 min. The effects of preheating and roasting temperatures on compression strength of pellets are presented in Figs. 2 and 3. The effects of preheating and roasting time on the compression strength of pellets are illustrated in Figs. 4 and 5. Fig. 2 Effect of HPRG on compression strength of preheated pellets Fig. 3 Effect of HPRG on compression strength of roasted pellets Fig. 4 Effect of preheating time on compression strength of preheated pellets Fig. 5 Effect of roasting time on compression strength of roasted pellets Figure 2 indicates that, for the hematite concentrates, without being pretreated base case, the compression strength of preheated pellets improves slowly with the increase of preheating temperature. The compression strength of pellets reaches 404 N only when temperature reaches 1 070 C, so the preheating temperature is higher than that of pellets made from common magnetite concentrates, for which the preheating temperature is always in the range of 850−1 000 C. Therefore, it is difficult for such preheated pellets to withstand the mechanical collision during the initial roasting process in rotary kiln. However, when the hematite concentrates are ground by high pressure roll, the specific surface area reaches 2 201.1 cm2/g, and the compression strength for preheated pellets improves sharply with the increase of preheating temperature and reaches 482 N when preheating temperature is only 1 000 C. Figure 4 indicates that the compression strength for preheated pellets even reaches 425 N when preheating time decreases to 8 min with preheating temperature being 1 000 C. Therefore, for HPRG-pretreated hematite concentrates, the preheating temperature and time of J. Cent. South Univ. 2012 19 2611−2619 2614 pellets decrease by 70 C and 2 min, respectively, compared with those for the untreated concentrates. As for the compression strength of roasted pellets, in the base case, Fig. 3 indicates that it is only 1 658 N when pellets are roasted at 1 200 C and it reaches 2 707 N when roasting temperature reaches 1 250 C. According to the requirements for pellets making processes, the lower limit of required compression strength for roasted pellets should be higher than 2 500 N. Therefore, it is confirmed that hematite pellets require higher roasting temperature. However, when the hematite concentrates are ground by high pressure roll, the compression strength for roasted pellets increases sharply with the increase of roasting temperature and reaches 2 846 N when roasting temperature is only 1 180 C. Figure 5 indicates that the compression strength for roasted pellets even reaches 3 055 N when roasting time decreases to 8 min with roasting temperature being 1 200 C. Therefore, for HPRG-pretreated hematite pellets, the roasting temperature and time of pellets have been decreased by at least 50 C and 2 min, respectively, compared with those of the base case. Therefore, for hematite pellets, it is confirmed that HPRG pretreatment improves not only the compression strength of preheated pellets, but also that of roasted pellets even with lower preheating and roasting temperatures and shorter preheating and roasting time. 3.2 Mechanism of HPRG pretreatment on pellets compression strength A series of experiments were conducted to determine the effect of changes in hematite concentrates properties after HPRG on compression strength of pellets. The concerned changes were focused on specific surface area, lattice defects, content of micro-sized particles, dispersion degree and microstructure. In order to uate the effect of HPRG pretreatment on the compression strength of pellets, the hematite concentrates were pretreated both by ball mill grinding and HPRG, so as to get the concentrates with close specific surface area. Here, the obtained specific surface area of the hematite reached 2 191.6 cm2/g and 2 201.1 cm2/g after being pretreated by ball mill grinding and HPRG, respectively. The green pellets were prepared and roasted with the same operating parameters. 3.2.1 Effect of specific surface area on compression strength of pellets In order to uate the effect of specific surface area changes on compression strength of pellets, the experiments were conducted with preheating temperatures being 950, 970, 1 000 and 1 020 C, roasting temperatures being 1 180, 1 200, 1 220 and 1 250 C, and preheating and roasting time being 10 min. The results are shown in Figs. 6 and 7. Fig. 6 Effect of different pretreatment measures on compression strength of preheated pellets Fig. 7 Effect of different pretreatment measures on compression strength of roasted pellets Figures 6 and 7 indicate that when the hematite concentrates are pretreated by ball mill and HPRG and reach nearly same specific surface area, the compression strength of preheated and roasted pellets is improved compared with that of base case, but the effect from HPRG appears to be greater than that of ball mill grinding even with the same preheating and roasting parameters. For example, in the case of preheating temperature and time being 1 000 C and 10 min, roasting temperature and time being 1 250 C and 10 min, respectively, when the hematite concentrates are pretreat
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