Chengyan Xu

Study on Phosphorus Removal of High-Phosphorus Oolitic Hematite by Coal-Based Direct Reduction and Magnetic Separation

In this article, mineralogical phase changes and structural changes of iron oxides and phosphorus-bearing minerals during the direct reduction roasting process were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). It has been found that the reduction of hematite follows the following general pathway: Fe2O3 → Fe3O4 → FeO → Fe. The last step of the reduction process contains two side reactions: either FeO → Fe2SiO4 → Fe or FeO → FeAl2O4 → Fe depending on the micro mineralogical makeup of the ore. In the reduction process of FeO → Fe, oolitic structure was destroyed completely and fluorapatite was diffused into gangue while metallic phase is coarsening at temperatures below 1200°C. Therefore, the separation of phosphorus-bearing gangue and metallic iron can be achieved by wet grinding and magnetic separation, and low phosphorus content metallic iron powder can be obtained. However, when the temperature reached 1250°C and beyond, some of the fluorapatite was reduced to elemental P and diffused into the metallic iron phase, making the P content higher in the metallic iron powder.

Study on the strength of cold-bonded high-phosphorus oolitic hematite-coal composite briquettes

Composite briquettes containing high-phosphorus oolitic hematite and coal were produced with a twin-roller briquette machine using sodium carboxymethyl cellulose, molasses, starch, sodium silicate, and bentonite as binders. The effect of these binders on the strength of the composite briquettes, including cold strength and high-temperature strength, was investigated by drop testing and compression testing. It was found the addition of Ca(OH)2 and Na2CO3 not only improved the reduction of iron oxides and promoted dephosphorization during the reduction-separation process but also provided strength to the composite briquettes during the briquetting process; a compressive strength of 152.8 N per briquette was obtained when no binders were used. On this basis, the addition of molasses, sodium silicate, starch, and bentonite improved the cold strength of the composite briquettes, and a maximum compressive strength of 404.6 N per briquette was obtained by using starch. When subjected to a thermal treatment at 1200°C, all of the composite briquettes suffered from a sharp decrease in compressive strength during the initial reduction process. This decrease in strength was related to an increase in porosity of the composite briquettes. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the decrease in strength of the composite briquettes could be caused by four factors: decomposition of bonding materials, gasification of coal, transportation of byproduct gases in the composite briquettes, and thermal stress.

Effects of embedding direct reduction followed by magnetic separation on recovering titanium and iron of beach titanomagnetite concentrate

Embedding direct reduction followed by magnetic separation was conducted to fully recover iron and titanium separately from beach titanomagnetite (TTM). The influences of reduction conditions, such as molar ratio of C to Fe, reduction time, and reduction temperature, were studied. The results showed that the TTM concentrate was reduced to iron and iron-titanium oxides, depending on the reduction time, and the reduction sequence at 1200 °C was suggested as follows: Fe2.75Ti0.25O4→Fe2TiO4→FeTiO3→FeTi2O5. The reduction temperature played a considerable role in the reduction of TTM concentrates. Increasing temperature from 1100 to 1200 °C was beneficial to recovering titanium and iron, whereas the results deteriorated as temperature increased further. The results of X-ray diffraction and scanning electron microscopy analyses showed that low temperature (⩽1100 °C) was unfavorable for the gasification of reductant, resulting in insufficient reducing atmosphere in the reduction process. The molten phase was formed at high temperatures of 1250–1300 °C, which accelerated the migration rate of metallic particles and suppressed the diffusion of reduction gas, resulting in poor reduction. The optimum conditions for reducing TTM concentrate are as follows: molar ratio of C to Fe of 1.68, reduction time of 150 min, and reduction temperature of 1200 °C. Under these conditions, direct reduction iron powder, assaying 90.28 mass% TFe and 1.73 mass% TiO2 with iron recovery of 90.85%, and titanium concentrate, assaying 46.24 mass% TiO2 with TiO2 recovery of 91.15%, were obtained.

Industry Test on Phosphorus Removal and Direct Reduction of High-Phosphorus Oolitic Hematite Ore

Industry test on phosphorus removal and direct reduction of the “Ningxiang type” high-phosphorus oolitic hematite ore has been carried out in a tunnel kiln on the basis of laboratory experiment. The iron grade and phosphorus content of the initial sample are 42.46% and 0.867%, respectively. The results showed that high-phosphorus oolitic hematite could be exploited on industrial scale, with the new process direct reduction roasting – grinding – magnetic separation, and that the final concentrate with an iron grade 92.56%, iron recovery 82.77% and phosphorus content 0.089% was obtained under the optimal conditions. Besides, X-ray diffraction (XRD) and scanning electron microscope with X-ray energy dispersive spectrum (SEM-EDS) were used to analysis the mechanism of phosphorus removal and direct reduction. It was shown that oolitic structure was destroyed, and metallic iron particle coarsening was obvious, besides fluorapatite particles were dispersed in the gangue by diffusion during the reduction roasting process. The liberation of metallic iron and gangue can be achieved by grinding, so high iron grade and low phosphorus content concentrate can be obtained after magnetic separation.

Effect of Na2CO3 and CaCO3 on coreduction roasting of blast furnace dust and high-phosphorus oolitic hematite

Iron was recovered from blast furnace dust and high-phosphorus oolitic hematite in the presence of Na2CO3 and CaCO3 additives. The functions of Na2CO3 and CaCO3 during the coreduction roasting process were investigated by XRD and SEM-EDS analyses. Results indicate that these additives not only hinder the reduction of fluorapatite, CaCO3 also decreases the P content of direct reduced iron (DRI) by increasing the reduction alkalinity. P remains as fluorapatite in the slag, which can be removed by grinding and magnetic separation under optimal conditions. The Na2CO3 promotes hematite reduction and improves the iron recovery (εFe) by replacing the FeO from fayalite, which results in quick growth and aggregation of metallic iron and improvement of εFe in DRI. A DRI with 91.88 mass% Fe, and 0.065 mass% P can be achieved at a recovery of 87.86 mass% under the optimal condition.