A Study On Mechanism And Process Of Direct Reduction Of Pellets Made From Concentrate And Composite Binder

In recent years, with the rapid development of steelmaking by EAF process, direct reduced iron (DRI) has gained world-wide recognition as an indispensable charge to replace scrap for quality steel production. DRI technology has been making great progress meanwhile global DRI output has been surging year by year. In China, there are abundant resources of high grade magnetite concentrate and noncoking coal, which provides advantages to develop direct reduction process of pellets by coal-based rotary kiln. In comparison with the traditional direct reduction of fired pellets (two-step process), direct reduction of pellets made from concentrate and composite binder (one-step process) possesses such advantages as: shorter flowsheet, lower capital investment, greater economic profits and better quality of DRI. One step process has been finding more application in China. Miyun DRI plant with a capacity of 62 ktpa and Luzhong DRI plant with a capacity of 50 ktpa have been under operation successfully, and a brand-new plant with 150 ktpa DRI using one-step direct process will put into operation in Xinjiang in July 2007. However, no systematic and mechanism study have been carried out on strength change, reduction behaviors, variations of macrostructure and microstructure of pellets made from concentrate and composite binder during reduction in huge grate-kiln. In this paper, Xinjiang magnetite and imported hematite concentrate are chosen as pellet feed, a study on mechanism and process of direct reduction of pellets made from concentrate and composite binder was conducted, which enhances the spread and application one-step process for DRI production.The effects of concentrate types and pellet blends, pretreating methods, types and dosage of binder, and internal coal on green ball making, pellet preheating concretion and direct reduction behavior have been studied systemically. It was shown that damp milling can reduce fines ratio of dry pellets to eliminate potential kiln accretion. Using composite binder CB can dramatically increase compressive strength of dry pellets, enhance the porosity of preheated pellets and accelerate the reduction.The isothermal and non-isothermal kinetics of the reduction of pellets with CB as binder and fired pellets with bentonite as binder were investigated respectively. Results from isothermal reduction kinetics show that the reduction rate of CB pellets is controlled by a mixed mechanism of chemical reaction and diffusion when the reduction temperature ranges from 800 to 1050℃. However, the reduction rate of fired pellets is primarily controlled by chemical reaction in the range of 800 ~ 900℃, but by both chemical reaction and diffusion from 1000 to 1050℃. In the range of 800 to 1000℃, the chemical reaction rate and effective gas diffusivity of CB pellets are much bigger than that of fired pellets. When the temperature is above 900℃, CB pellets can be reduced rapidly, while high-speed reduction of fired pellets only happens at temperatures higher than 1000℃.Results from non-isothermal reduction research showed that the reduction rate of CB pellets is twice as fast as that of fired pellets. Based on the outcomes of reduction kinetics, one-step direct reduction of pellets made from concentrate and composite binder is significantly improved by using preheated hot pellets as feed and reducing at high temperatures higher than 900℃inside whole kiln.During the reduction, an investigation of strength changes of two kinds of pellets was carried out. It is shown that CB pellets present the valley of compressive strength (less than 500 Newton per pellet) for 30 mins during reduction. However, the valley of compressive strength lasts for 70 mins during reduction of fired pellets. Even more, CB pellets can maintain as whole pellets after reduction finished, but 50% of fired pellets broke into many fragments. The compressive strength of CB pellets increases dramatically when reduction proceeds beyond the valley, while that of fired pellets increases slowly. DRI products of CB pellets possess firm structure and smooth surface, with an average compressive strength 2570 Newton per pellet. However, cracks were bestrewn on the surface of DRI products of fired pellets and some pellets even turned into many pieces, with compressive strength of 775 Newton per pellet.During the reduction, there are some differences in compressive strength and structure changes between two types of pellets, which is determined by microstructure and macrostructure variations. Microstructure development of pellets during the reduction was studied by using microscope, XRD, SEM and image analysis software. Thermal properties of two types of compact were measured by using thermal dilatometer and thermal conductivity measurer. The mineral crystal microstrain of two types of pellets was investigated by employing XRD and MDI Jade software. The mechanism of stress accumulation and releasing was unveiled for two kinds of pellets and for the first time, the essential reasons to explain the difference on compressive strength and reduction behavior between two kinds of pellets were discovered. It is demonstrated that the strength difference is caused by different crack generation and extension inside pellets. During the reduction, the concentric cracks occurred inside CB pellets as a result of only phase change stress. After the appearance of concentric cracks, CB pellets are enwrapped, constricted by shell of metallic iron. In the latter stage of reduction, cracks can be self-cicatrized with pellet volume shrinking, which makes DRI pellets present whole structure, smooth surface and high strength. However, inside fired pellets, some radial cracks appeared at early stage of reduction due to coupling stress of phase change and thermal stress. The layer of metallic iron formed after pellets structure has been destroyed by radial cracks. Furthermore, in the latter reduction, the direction of pellet shrinking was vertical to the direction of cracks, resulting in wider radial cracks instead of cracks close. So the DRI products of fired pellets possess many cracks with low compressive strength.The main technical and economic results of scale-up test for one-step direct reduction process of CB pellets in a rotary kiln were achieved as follows: total iron content of DRI 91.48%, metallic iron content 87.05%, metallization degree 95.16%, Fe recovery 93.58%, ore concentrate consumption 1.442 t/t·DRI, dry coal consumption 0.854 t/t·DRI, magnetic fines ratio (-3 mm) 0.3%, dust 161 Kg/t·DRI. In contrast, the results of for direct reduction process of fired pellets were as follows: total iron content of DRI 89.24% , metallic iron content 83.22%, metallization degree 93.25%, Fe recovery 90.66%, ore concentrate consumption 1.508 t/t·DRI, dry coal consumption 0.889 t/t·DRI, dust 223 Kg/t·DRI. Direct reduction of CB pellets has such advantages as: higher total iron and metallic iron grade, higher metallization degree, less dust, higher recovery of iron and higher compressive strength of DRI compared with direct reduction of fired pellets. Through the scale-up tests, the direct reduction of Xinjiang DRI plant is using one-step process certified as feasible, and process parameters were utilized for designing the plant.One-step direct reduction process of pellets made from hematite concentrate and composite binder was also optimized in scale-up tests for the first time. It is shown that Indian hematite can be used to produce metallic pellets by one-step direct reduction process when mixing with magnetite concentrate at any ratio. High quality metallic pellets with iron grade of 87.83% and metallization degree of 93.30% were manufactured using 100% Indian hematite as feed. Which further extend the raw material of direct reduction feed and enhance the applicability of one-step direct reduction process.