| The contradiction between supply and demand of iron ores in China has been tense, which has become the main obstacles to restrict the sustainable development of economy, and the furthest utility of iron ore resources relying on technology has been needed urgently. Aiming at the problem of high cost and low efficient in separation of Anshan style lean hematite ores, a new technique of "ultrafine crushing in high-pressure ginding roller (HPGR)-high intensity magnetic preconcentration(HIMP)-high efficient separation" was presented. The ultrafine crushing tests and HIMP tests of lean hematite ore were studied systematically firstly to get the process flowsheet and optimum process parameters, and then HIMP concentrates were separated efficiently. The material and separation characteristics of comminuted hematite products using HPGR and conventional jaw crusher (JC) were comparatively studied to ascertain the effect of comminution in HPGR on separating performances. On all above basis, fragmentation mechanism of lean hematite ore comminuted by HPGR and force analysis of single particles in magnetic field were investigated deeply.The effects of various factors including specific pressure, roll speed and feed moisture on the performances of comminution in HPGR were studied. The results showed that specific pressure had a significant influence on the product fineness and the specific energy, while roll speed had a important influence on the capacity, and feed moisture had a little influences on the product fineness, specific energy and capacity. The optimum conditions for the lean hematite ore crushed by bench-scale HPGR were obtained at a specific pressure of5.2N-mm-2, a roll speed of0.18m·m-1and a feed moisture range of3%-5%. In the closed circuit operation with edge product recycles, the product fineness increased with the increasing of edge recycles proportion, and coarse particles were crushed better. In the closed circuit operation with3.2mm size, HPGR gave a higher reduction ratio, a lower circulating load than JC, and HPGR products had a more uniform size distribution and contained a greater proportion of fines than JC products.The Bond ball work indexes of HPGR products with the screening size of0.28mm and0.074mm, which were5.48kW-h-f’and13.76kW-h-f1respectively, were28.23%and13.96%lower than that of JC products respectively. In the case of grinding fineness of80%-0.074mm, for the-3.2mm feed and-3.2+0.074mm feed, the relative grindability coefficient between HPGR products and JC products were0.83and0.86respectively. The specific surface area and specific pore volume of HPGR products, which were0.714m2·g-1and4.257X10-3cc·g-1respectively, were75.63%and76.21%higher than that of JC products respectively. The liberation degree of valuable mineral in-0.5mm fraction size of HPGR products was77.15%, being24.46higher than that in-0.5mm fraction size of JC products. As the grinding fineness increased from40%-0.074mm to95%-0.074mm, the liberation degree of valuable mineral in grinding products after HPGR crushing, which was improved from73.27%to95.11%, was higher at a decreasing rate than that in grinding products after JC crushing, and the ratio of increase was reduced from8.19%to1.04%.The crushing mode of HPGR was mainly compression, and abrasion as auxiliary. So, the HPGR products were loose with few edge, having plentiful cracks. The cracks in HPGR products consisted of transgranular cracks, innergranular microcracks and intergranular microcracks products. The crushing mode of JC was mainly impaction and shear. So, the JC products were compacting with obvious edges, having a few cracks. The cracks in JC products consisted of transgranular cracks, innergranular microcracks.The two main crack modes, innergranular microcracks and intergranular microcracks, reflected the two main non-random breakage forms of single particles in HPGR, namely preferential breakage and interfacial breakage, while the fracture of particles bed in HPGR was mainly selective breakage. The fracture of lean hematite ore in HPGR was an interactive dynamic process in which selective breakage, resulting from the interaction between coarse particles of high hardness and fine particles of low hardness, was continuous alternately with the compactness and compacting strength of materials increasing. Within a2.8～4.4N/mm2range, valuable minerals were crushed after preferentially absorbing most of energy, but gangue minerals were not crushed completely and only acted as an pressure transfer medium. Within a4.4～5.2N/mm2range, fine particles of valuable minerals, which had been crushed preferentially, acted as an energy transfer medium of coarse particles of gangue minerals, while gangue minerals were crushed adequately after absorbing energy. Within a5.2～6.0N/mm2range, lean hematite ore was not comminuted further due to the "size effect" in the strength of materials, and the comminution effect of materials was stable.The dry preconcentration of hematite comminuted by HPGR was studied, using the roller magnetic separator. The results showed that as the roll speed increased, the tailings grade and the tailings yield reduced, while the concentrates recovery rose. The suitable feed size in dry preconcentration was-3.2+0.5mm, the concentrate grade was improved by4.75percentage points and concentrate recovery was up to90.97%with a tailing yield of20.43%at the condition of0.22m·s-1roll speed. The wet preconcentration of hematite comminuted by HPGR was studied, using the cylindrical ferromagnetic medium. The results showed that as the rod diameter increased, the rod gap decreased and the background magnetic field intensity (BMFI) increased, the tailings grade and the tailings yield reduced, while the concentrates recovery raised. The performance of preconcentration in the case of parallel rods was better than that in the case of staggered rods. The optimum condition for wet preconcentration in integrated size was obtained at a rod diameter of4mm, a rod gap of4mm and a BMFI of645kA-m"1. After preconcentration, the concentrate grade was improved by5.57percentage points and concentrate recovery was up to91.60%with a tailing yield of25.32%. At the respective optimum condition, compared with performance of classifying preconcentration with a screening size of0.5mm, wet preconcentration in integrated size had a higher concentrate recovery, improving by5.09percentage points, but-3.2+0.5mm size fraction was not separated well and the yield of tailing was relatively low.The force analysis of a single particle in the magnetic field was investigated, the results showed that in magnetic field of a magnetic roll, specific magnetic force of hematite in the roller magnetic field was69.42N·kg-1, and the maximum roll speed to separating-3.2mm hematite particles was2.95m·s-1, which should reduce for the lock-particles. In the high-gradient magnetic field of rod medium, for the same BMFI, the magnetic capturing force on coarse hematite particles increased and that on fine hematite particles decreased with the increasing of rod diameter. For the lean locked-particles, considering the gravity and viscous resistance of particles, rods with a big diameter were beneficial to the capture of coarse particles and had no influence on the capture of fine particles, while rods with a small diameter was good for the capture of fine particles and not suitable for the capture of coarse particles of lean locked-particles. The experiment results matched with theoretical calculation.The process flowsheet of "classifying preconcentration-staged grinding, staged separation-high intensity magnetic separation and reverse flotation" was adopted to separated lean hematite ores. The ultrafine products was screened firstly, and then-3.2+0.5mm size fraction was separated by dry preconcentration and-0.5mm size fraction was separated by wet preconcentration. The dry preconcentration concentrate was ground and separated to get rough concentrate firstly, and then the rough concentrate and wet preconcentration concentrate were mixed to regrinding and separation. After separation of lean hematite ore comminuted by JC, the overall concentrate grade was65.69%with a yield of24.28%and a recovery of65.58%, while after separation of lean hematite ore comminuted by HPGR the overall concentrate grade was65.71%with a yield of25.84%and a recovery of70.76%, and the concentrate yield and recovery of lean hematite ore were improved by1.56percentage points and5.18percentage points respectively in the similarity of concentrate grade.The research by this paper can not only provide a new technique method for high efficient utilization of Anshan style lean hematite ore, but also make a theoretical basis for practical application. |
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