Fundamental Research On Staged Reduction-Efficient Separation Of Boron-containing Iron Concentrate

Boron bearing iron concentrate is the ore dressing production of paigeite ore, containing B2O34～6% and accounting for 30% of the total boron amount of raw ore. If this part of boron resources cannot be utilized as raw material for the boron industry, then the recoveries of the boron will be less than 70%. Hence the research on the separation between boron and iron from boron bearing iron concentrate has an important theoretical and practical significance.According to the differences in material properties of boron bearing iron concentrate ore, a new technique combined with beneficiation and smelting, called staged reduction- efficient separation was put forward in this thesis, breaking through the conventional concept of mineral processing. By staged reduction, iron minerals in boron bearing iron concentrate ore was fully reduced into metallic iron particle, while other minerals, such as boron and magnesium could not be reduced. After that, the increasing temperature made the metallic iron connect together and form the big sized grains, finally fully reduced product was separated effectively. Iron and boron in the ore were separated and enriched effectively by systemic experiments. The mechanism of staged reduction was also studied.In this thesis, systemic experiments were carried out to investigate the effects of reducing temperature, reducing time, heating rate, dosage of Na2CO3 and CaO on reduction. The results were evaluated by chemical analysis technique, with metallization rate of reduced materials as index, The results showed that separation efficiency mainly depended on metallization rate, which was determined by reducing temperature, reducing time and so on. The optimum conditions for the first stage reduction determined by experiments were that reducing temperature was 1125℃, reducing time was 150 min, heating rate was 5K/min, the dosage of Na2CO3 and CaO both were 5wt-%(mass ratio of 5.00g sodium carbonate to 100 g solid carbon). Reduced materials with iron grade 72.17wt-%, content of metallic iron 64.17wt-% and metallization rate 88.92% were obtained under this condition. Then under the conditions of grinding size below 0.074 mm 68wt-% and magnetic field intensity of 0.008T, a good index with iron powder grade 92.71wt-%, recovery 95.11wt-% and boron concentrate containing B2O314.51wt-%,recovery 88.69wt-% was obtained.Due to the finding that the boron oxides would melt and become liquid phase at high temperatures, hindering the reduction of iron oxides but promoting the growth of iron grains, so the seconde stage reduction was carried out based on the first stage reduction. The optimum conditions for the seconde stage reduction determined by experiments were that reducing temperature was 1250℃, reducing time was 60 min. Reduced materials with iron grade 73.8wt-%, content of metallic iron 67.25wt-% and metallization rate 91.12% were obtained under this condition. Then under the conditions of grinding size below 0.074 mm 68wt-% and magnetic field intensity of 0.008T, a good index with iron powder grade 93.72wt-%, recovery 96.24wt-% and boron concentrate containing B2O314.55wt-%,recovery 93.57wt-% was obtained.Optical microscope, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy was used to analyse the mineralogical properties of the materials both before and after reduction, including chemical and mineral composition of and crystal size. Combining with the study on dynamics of nucleation and growth of metallic iron grain, the mechanism of staged reduction was investigated thoroughly. The iron minerals were reduced in the sequence of Fe3O4â†'FeOâ†'Fe. The main reaction was the indirect reaction between carbon and iron oxide.The research on reduction properties of other element showed that, szaibelyite (Mg2[B2O4(OH)(OH)]) was decomposed into water vapor (H2O) and suanite (Mg2B2O5),then most part of it combined with magnesia (MgO), the decomposion of magnesite (MgCO3), to generate kotoite (Mg3B2O6). The serpentine ore, the main gangue minerals, was decomposed into forsterite (MgSiO4, quarts (SiO2) and water vapor.The activity of B2O3 test by different means of cooling the reduced materials down showed that the activity of B2O3 could reach higher than 81.23% when the reduced materials was cooled to room temperature by water quenching.In summary, the new process-staged reduction-efficient separation can realize the utilization of boron bearing iron concentrate under the optimum conditions. And the mass fraction of B2O3 in boron-rich slag is about 14wt-%, which is good raw material for sodium borate production. The final product (metallic iron powder) containing about 94wt-% Fe after briquetting can be used as the burden for steel making by electric arc furnace (EAF) to replace scrap steel.The research by this paper can improve the utilization rate and value in use of boron-bearing iron concentrate, which has a guiding significance for the recovery of boron from the boron-bearing iron concentrate.