| Nickel slag is produced on a large scale in the nickel smelting process.At present,the massive nickel slag,accumulated stock of over 40 million tons in Jinchuan Co.has brought a great threat to the ecological environment of the Qilian Mountains and the upper reaches of the Yellow River,and causes a serious influence on the implementation of the national strategy for ecological protection and high-quality development of the Yellow River basin.The nickel slag is a valuable secondary resource due to its containing 40%total iron(TFe)and some valuable metal elements such as Ni,Co and Cu.The fayalite phase,as a main iron-containing phase in nickel slag,transforms hard into another phase because of its stable structure,resulting in the difficulty of recycling iron resources.Therefore,it is still a technical bottleneck to the comprehensive utilization of nickel slag that the iron resources are extracted efficiently and reused in high-value applications.The solution to the problem would be conducive to the sustainable development of enterprises and bound to have good social benefits.In this paper,Jinchuan’s nickel slag has been taken as a research object,and the iron resources in the slag are efficiently extracted by the molten oxidation method and further designed as microwave-absorbing materials.The oxidation kinetics of molten slag,crystallization and growth of magnetite crystals,the regulation of crystalline morphology and its influence on iron recovery were systematically studied.The electromagnetic characteristics and microwave absorption properties of the magnetite crystals extracted from nickel slag were investigated,and the feasibility of using the valuable components extracted from nickel slag to design microwave-absorbing materials was explored.These studies explored a new idea and a new avenue for the comprehensive utilization of nickel slag.The results can be summarized as follows:(1)The complete molten temperature of nickel slag after CaO modification is 1488℃.When the isothermal oxidation temperature of molten nickel slag increases from 1500℃ to 1575℃,the oxidation reaction ending time can be shortened from 40 min to 20 min,and the mass transfer coefficient of Fe2+ increases from 1.69×10-6 m/s to 3.68×10-6 m/s,and the diffusion coefficient of oxygen rises from 2.02×10-9 m2/s to 4.42×10-9 m2/s.The order and apparent activation energy of the oxidation reaction are 1.36±0.09 and 286.83 kJ/mol,respectively.The isothermal oxidation kinetics of molten nickel slag is controlled by the mixture of the interfacial chemical reactions and component diffusion,and the component diffusion is dominant.The isothermal oxidation kinetics equation of molten nickel slag can be expressed as:ln(k/[k])=-286.86×10-3J·mol-1/RT+15.06(2)The crystalline morphology of precipitated magnetite in molten nickel slag is significantly affected by the melting and oxidation temperature.The magnetite crystals obtained at 1400℃ exhibit granular shapes with an average size of about 30 μm,while they change into skeletal shapes with a uniform distribution in the silicate matrix at 1450℃ and 1500℃The melt viscosity and solute concentration could significantly affect the preferential growth tendency of crystallization,resulting in the different morphology formation of the magnetite crystals in slag.The formation of skeletal shape crystals could be effectively inhibited by aeration and stirring during the cooling process of the oxidized molten nickel slag,and thus granular crystals with an average particle size over 60 μm were obtained.The iron recovery rate of nickel slag can be increased from 77.28%to 86.19%by crystallization control after molten oxidation at 1450℃.(3)The nucleation and precipitation of magnetite crystals in the isothermal crystallization of molten nickel oxide slag are mainly influenced by undercooling degree and nucleation incubation time.When the slag temperature is lower than 1390℃,magnetite crystals can be precipitated directly from the molten slag without incubation time.The growth of magnetite crystals in molten slag is principally affected by the viscosity and the concentration of crystallization ions.The average size and average growth rate of magnetite crystals in the isothermal crystallization process at 1375℃ reach maximum values of 65.56 μm and 0.1 μm/s,respectively.The isothermal crystallization growth index of magnetite crystals is 0.47,indicating that the crystallization growth mechanism is surface crystallization and low-dimensional growth dominated by diffusion control.The isothermal crystallization behavior follows the Anti-Arrhenius law and the apparent activation energy is-172.93 kJ/mol.(4)The non-isothermal precipitation growth of magnetite crystals in molten nickel slag is mainly affected by undercooling,solute diffusion and melt viscosity.The initial precipitation temperature of non-isothermal crystallization is about 1450℃ at the cooling rate of 5～50℃/min.When the cooling rate is not higher than 15℃/min,the change range of melt undercooling and the initial crystallization amount are both small.At this time,the viscosity resistance of the melt increases slowly,which is conducive to the growth of magnetite crystals in the slag,and the average growth rate is up to 0.141 μm/s,causing the formation of larger granular crystals.The Avrami index of the non-isothermal crystallization decreases from 1.43±0.17 to 0.89±0.05 with the decrease of molten slag temperature from 1380℃ to 1300℃,indicating that the crystallization mechanism changes from three-dimensional growth to surface nucleation and low-dimensional growth.When the cooling rate increases from 5℃/min to 50℃/min,the apparent activation energy of the non-isothermal crystallization changes from-581.98±46.86 kJ/mol to-339.36±34.01 kJ/mol.(5)The magnetite crystals extracted from nickel slag exhibit excellent absorbing performance in the C～Ku frequency band.The minimum reflection loss value reaches-29.90 dB,and the widest effective bandwidth is 3.28 GHz(from 5.52 to 8.80 GHz)with a thickness of 3.5 mm.This excellent absorbing performance could be mainly attributed to the good impedance matching generated from the interaction of strong magnetic loss and appropriate dielectric loss.The microwave-absorbing properties of the magnetite crystals extracted from nickel slag have reached the level of Fe3O4 particles prepared by soft chemical methods,indicating the magnetite crystals have a good application prospect. |
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