A Study Of The Process And Mechanism Of Pelletizing PFC Specularite By Using High Pressure Roller Grinding

In recent years, the demand for imported iron ores has been greatly growing with the quick development of the Chinese steel industry, amounting to from 6 percent in 1993 to more than 55 percent in 2006 used as burden for blast furnace. Especially, the imported amount of pellet feed is growing gradually for serious shortage. Though Brazilian specularite is characterized by poor ballability and refractory roasting behaviours but high iron grade, abundant reserves, low price. Therefore, it will be of far-reaching significance to solve the tackle shortage by effective using PFC as pellet feed.In this paper, Brazilian specularite was pretreated by using high pressure roll grinding(HPRG) for improving the technology of pelletizing. Under the optimized parameters conditions of HPRG, two close circuits of HPRG to pretreat iron ore concentrates have been developed. The test results showed that the optimum conditions are as follows: the rotation rate 68 r/min, the roll pressure 1MPa, 8.0% moisture of mixed feed, balling for 15min, balling at 8.0% moisture, and at 71% product circulation load in closed circuit. After preheated by this condition, the two suitable pellet feeds (comprising 40% specularite and 60% magnetite, 70% specularite and 30% magnetite, respectively) achieve the best performance in ballibality, preheating and roasting. The former, under the conditions of bentonite 2.1%, the compressive strength, drop strength and thermal shock temperature is 10.02 Newton per one, 5.9 times from 0.5m and 463℃, respectively. Which is meets the demand of blast furnace. The latter is inferior to the former one, the compressive strength and drop number is 10.25 Newton per one and 4.5 times from 0.5m with bentonite 2.4%, respectively. All of which is much higher than that of the unpretreated pellet feed. The green balls are hardened by preheating at 1050℃for 10min and firing at 1280℃for 12min, fired pellets are manufactured with compressive strength of 3560.88N per pellet and 2690.88N per pellet, respectively, assaying 65% iron. The study of mineralogy and metallurgic performance results show that by pretreating PFC by HPRG, the two types of fired pellets can meet the product requirement of blast furnace, with a ration of PFC as high as 70%.The mechanism of HPRG has been revealed that, after PFC pretreated by HPRG , finer size fractions is increased greatly, higher specific surface areas are achieved and surface wetting heat is elevated, due to higher surface activity by forming unsmootheness face, sharp corner and crest edges, leading to better balling performance.In addition, adding magnetite improves the preheating and roasting behaviours of single hematite pellets, newly formed Fe₂O₃ crystal from oxidation reaction has higher ability in transference and can bond each other in lower temperature, then, the strength of pellet is improved. In the other side, after the HPRG pretreated, material mixed again, magnetite can adequate distribute to hematite profitable, the crystal bonding each other was better during the preheat and boast process, the pellet strength was enhance.