| Nickel slag is the industrial waste residue in the nickel metallurgical process,which contains iron,nickel,cobalt,copper and other valuable metal elements,and total Fe(TFe)content is up to 40%on average.The production 1 ton nickel accompanies generally with 6-16 ton nickel slag.Because of low utilization rate of nickel slag,a large quantity of nickel slag has been accumulated in slag dump,which not only occupies land and pollutes the environment,but also causes enormous waste of metal resources in the slag.If the iron resources in nickel slag can be availably recycled and utilized,the comprehensive utilization of nickel slag will be improved significantly and the industrial chain of the metallurgical enterprises also be extended.The iron-bearing phase in nickel slag is mainly fayalite phase,and its structure is stable resulting in its difficult recovery.At present,the recovery of iron resources from nickel slag by wet leaching or deep reduction has not yet formed industrial scale due to the problems of process,energy consumption,environment pollution,product reuse and so on.Based on the development trend of high-efficiency comprehensive utilization of metallurgical waste slag and the research status of extraction iron resource from nickel slag,this paper proposes the modified molten oxidation(MMO)method to recover iron resources from nickel slag,which can not only recover iron resources as magnetite,but also effectively reduce the energy consumption and environment pollution,thus providing a new approach for the comprehensive utilization of nickel slag.In this paper,the thermodynamic simulating computation and experimental performance have been both used to carry out the basic research on the iron enrichment in nickel slag by MMO method with the water quenching nickel slag of a company as raw material.The main research contents include:analyzing the thermodynamic conditions of MMO process of nickel slag;investigating the melting characteristics and viscosity of modified nickel slag;analyzing the influence of alkalinity,oxidizing atmosphere,oxidation temperature and oxidation time on the morphology and crystallization of magnetite phase;building oxidative macro-kinetic model to explore the enrichment behavior of iron;establishing isothermal crystallizing dynamic model of magnetite grains to study the nucleation and growth mechanism of magnetite crystals and to explore the crystallization behavior of magnetite grains.The specific conclusions are given as follows.(1)The iron resources in the nickel slag exist mainly as fayalite phase and the hortonolite phase with partial solid solution of MgO;the activity of FeO(aFeO)computation results show that aFeO in molten slag is related to alkalinity,FeO content and MgO content,and aFeO increases linearly with the increase of w(FeO)at 1500℃.With the increase of alkalinity,aFeO increases first and then decreases,and reaches the maximum value at alkalinity of 1.10.Moreover,aFeO increases with the increase of w(MgO)when w(CaO)/w(SiO2)is 0.20,while it decreases with the increase of w(MgO)when w(CaO)/w(SiO2)is in the range of 0.80～1.10.aFeO becomes nearly unchangeable in the condition of w(CaO)/w(SiO2)of 0.50,implying that w(MgO)has slight inflence on aFeO in this condition.Therefore,in order to increase aFeO in molten slag,the suitable alkalinity range is 0.90-1.10,and w(FeO)is necessary to increase,and MgO needn’t be added.(2)The addition of CaO can promote fayalite decomposition to release free FeO in molten slag,which can be oxidized into Fe3O4 in the subsequent oxidation process.The value of aFeO in modified nickel slag is much higher than that of magnetite precipitated in air atmosphere.The spinel solid solution mainly composed of Fe3O4 is first precipitated in the slag during the molten oxidation of CaO-FeO-MgO-SiO2 slag in the air when the alkalinity is 0.60.With increase of alkalinity,the spinel phase region first increases and then decreases.The Fe2O3 phase region gradually increases with the increase of oxygen partial pressure.Therefore,in order to promote the enrichment of iron in magnetite phase,the optimal conditions are alkalinity of 0.60～0.90 and oxygen partial pressure of 21kPa(air).(3)The effect of alkalinity on melting characteristics and viscosity of modified nickel slag is significant,and the experimental results are in good agreement with the results of software computation.When the alkalinity is in the range of 0.38～1.50,due to the influence of CaO on the structure of silicate and the phases of molten slag,the melting temperature of modified nickel slag decreases first and then increases with the increase of alkalinity,and its viscosity in melting state is in the range of 0～0.50Pa·s.When alkalinity is 0.90,the softening temperature,the hemispherical temperature,the flowing temperature and the viscosity of modified nickel slag all reach minimum values 1282,1326,1440℃,and 0.20Pa·s,respectively.Therefore,in order to improve the conditions of oxidation reaction and mass transfer kinetics of molten slag,the suitable treatment conditions are temperature of 1350～1450℃ and alkalinity of 0.90.(4)It can be found that Fe,Ni,Co and Cu are enriched into magnetite phase after the oxidation of modified nickel slag.The phase morphology and crystallization of magnetite can be influenced observably by alkalinity,gas supply mode,oxidation temperature and oxidation time.The crystallization amount of magnetite increases first and then decreases with the increase of alkalinity.If the alkalinity is too low or too high,the crystallinity of magnetite phase is poor,showing as dendritic and skeleton shapes.When the alkalinity is 0.60 and 0.90,the magnetite crystals exhibit granular shape with the average particle size of greater than 50μm,preferable crystallinity,large crystallization quantity,and uniform distribution.When the slag is oxidized naturally in air,the magnetite crystals display dendritic structure with low crystallinity and crystallization amount of 18.52%.While air gas is introduced into the slag for oxidation,the magnetite crystals exhibit complete particles with better size uniform distribution,more preferable crystallinity and larger crystallization amount of 33.52%.As the oxidation temperature increases,the magnetite particle size increases obviously.With the extension of oxidation time,the magnetite crystals gradually change from fine dendritic shape to granular shape.The formation rate constant and apparent activation energy of magnetite crystals is calculated to be 0.0183 and 76.49kJ/mol,respectively.Therefore,in order to facilitate the subsequent magnetic separation,the suitable conditions are alkalinity of 0.60～0.90,oxidation temperature of 1450℃,and air gas supply for 30min.(5)The morphology of magnetite crystals is significantly affected by cooling rate,holding temperature and holding time.When the cooling rate is 3℃/min and 5℃/min,the magnetite particles are relatively complete with the average particle size more than 50μm.As the cooling rate decrease,the crystallinity of magnetite crystals in slag increases and the iron content in matrix phases decreases obviously.With the extention of the holding time,the crystallization amount of magnetite phase increases gradually,but the crystallization rate slows down.The crystallization amount of seems unchangeable after holding for 90min,implying the crystallization reaction approaches to equilibrium.The crystallization amount of magnetite increases gradually with the decrease of holding temperature.In the range of 1350～1450℃,the JMAK equation can be used to express the isothermal temperature crystallization kinetics of magnetite grains in molten slag,in which n and apparent activation energy E are about 0.48 and-192.37kJ/mol,respectively.The negative E indicates that the crystallization behavior of magnetite grains in molten nickel slag accords with the anti-Arrhenius law.(6)Under suitable conditions,the oxided nickel slag can be magnetically separated,resulting in the magnetic materials of about 65%and the non-magnetic materials of about 35%.TFe is about 51.04%among the magnetic materials.89.6%Fe deposites in the magnetite crystals,in which Fe3+accounts for 50.8%,Fe2+ accounts for 38.8%,and the remaining 10.4%exists as hedenbergite.The recovery rate of iron is up to 84.85%. |
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