Study On The Preparation Of Iron Powders With Molten Semisteel By Rotary Cup Atomizer

The main product and by-product of titania slag-smelting process of ilmenite concentrate with an electric furnace are high titania slag and semisteel.The high titania slag produced by the smelting process is mainly used as a raw material in the sulfate process to produce titanium dioxide with a high added value.The main chemical compositions of semisteel are low carbon(approximately 1.50 wt.%)and high sulfur(approximately 0.35 wt.%),resulting in that the semisteel cannot be widely used in the steel-making process.The market value of semisteel produced by the smelting process with an electric furnace is consequently low.Therefore,the high value-added utilization of semisteel has an important effect on the economy of the entire electric furnace smelting process.Meanwhile,high-quality iron powders are facing the situation of demand exceeds supply in the fields of powder metallurgy and chemical industry.Nowadays,iron powders are mostly manufactured through the Hoganas and water atomization processes in China.From the perspective of molten semisteel,the water atomization process is a suitable iron powder production process.However,the granulation process by high-pressure water consumes large quantities of electricity and water.Difficulties also exist in separating cooling water from fine iron granules efficiently and in heat recovery.Based on the above-mentioned problems,a novel iron powder granulation technique combining a rotary cup atomizer(RCA)with water curtain was proposed in this study.The size control of iron granules,the disintegration mechanism of moleten semisteel,the cooling behavior of the iron droplets and the oxidation behavior of the iron powders in hot vapor were investigated.A modified correlation that considers the influences of slope angle and cup depth was proposed.Moreover,a simplified model of iron droplets was established to elucidate traveling trajectory and heat transfer with different cooling media during atomization.All of these researches provide a good insight for the high value-added utilization of semisteel,as well as heat recovery.The effect of atomizer configuration and operation parameter on the particle size of the iron granules were investigated through granulation experiments.The results indicated that the median diameter of the obtained iron granules decreases with an increase in rotating speed,cup diameter,slope angle,and cup depth.The optimal conditions for the granulation of semisteel in the present study are a rotating speed of1800 rpm and a cup diameter of 150 mm.Under such conditions,the accumulative fraction of the obtained iron granules that measured less than 0.45 mm reaches a maximum of 93.12 wt.%when the slope angle and cup depth were 45°and 50 mm.The atomization mechanism of the molten semisteel was calculated to follow the ligament formation mode based on the current granulation experiments.A modified correlation that considers the influences of slope angle and cup depth on the thickness of liquid semisteel was proposed in the present study to accurately predict the median size of granules with an average relative error of 11.43%.A simplified model was established to elucidate traveling trajectory and temperature distribution of iron droplets during passing through different cooling media.As the flow rate and thickness of water curtain increase,the cooling process of iron droplet is accelerated,and the latter workes better.An increasing rotating speed increases the flying velocity of droplets and shortens the time before crossing the water curtain.The convective heat transfer coefficient increases with an increase in rotating speed and accelerates heat transfer between iron droplets and water.However,the residence time of droplet in water curtain also decreases with the increase of rotating speed,which is not conducive to the heat transfer between iron droplets and water.The oxidation of iron powders is due to the reaction between iron powders and vapor during the cooling process.The results indicated that the oxidation rate increases with the increasing temperature and decreasing powder size.The entire isothermal oxidation process includes the following two stages.The kinetic mechanism of first stage consistent with the linear law,and the rate-controlling step is a chemical reaction;whereas the kinetic mechanism of second stage consistent with the integrated law of line and parabola,and the rate-controlling step include internal diffusion and chemical reaction.For iron powders in the range of 0.1–0.15 mm,0.3–0.35 mm and 0.5–0.55 mm,the reaction activation energy at the first oxidation stage is 40.03,61.28 and 49.92KJ/mol;the reaction activation energy at the second oxidation stage is 107.52,118.75and 111.97 KJ/mol.The effect of operation process parameters on the oxidation degree of iron granules were investigated.It can be found that the oxygen content of obtained iron granules decreases with the increase of rotating speed and water flow rate.For iron granules in the range of 0.3–0.55 mm,the oxygen content of iron granules is 21.53–13.78%obtained using RCA without a water curtain when the rotating speed is 1200 rpm;whereas the oxygen content of iron granules is 12.01–7.83%and 10.67–5.23%when the the flow rate of the water curtain increases to 0.125 L/min and 0.25 L/min.The oxygen content of iron granules obtained at 0.25 L/min is 8.73–4.35%when the rotating speed changes from 1200 rpm to 1800 rpm.Small iron granules present a good spherical morphology,and the surfaces of iron granules are smooth and are covered with a layer of iron oxides,including Fe₃O₄,Fe O,and relatively small amounts of Fe₂O₃.The oxygen content of the oxide layer increases along the radius direction of iron granules from inside to outside.Therefore,it should agree with the unreacted shrinkage core model.After reduction,the main phase of the obtained iron powders is Fe.The iron powders have dramatically rough surfaces and large specific surface areas,which benefit their reaction as reducing agents in the sulfate process.