Fundamental Research On Preparation Of Ferronickel From A Low-grade Typical Saprolitic Laterite Ore

Nickel is an important strategic resource,mainly used in stainless steel production.The global nickel consumption increased with the rapid growth of global stainless steel production and the wide application of nickel in emerging industries.The current nickel ore resources available for extracting nickel metal are nickel sulfide and laterite ores.Nickel sulfide ores are becoming depleted and also more difficult to mine.Consequently,increasing attention is being paid to nickel laterite ores.China has the highest production of stainless steel and the most consumption of nickel in the world.However,China is short of nickel laterite ore resource,and has depended on importation for a long period.Recently,as the change of nickel laterite ore policy in Indonesia,the domestic laterite nickel ore in China is in short supply,thus,domestic nickel production enterprise accelerated the overseas industry layout.The mineral composition and structure of various layers in nickel laterite deposit are complex and varied.The treatment for different types of nickel laterite is different,and the existing laterite ore treatment processes are characterized by poor adaptability on raw materials,high energy consumption as well as poor process stability.Direct reduction-magnetic separation process has advantages of low energy consumption and simple process,and is suitable for the treatment of various nickel laterite ores.However,direct reduction-magnetic separation process still has some problems such as low enrichment efficiency of nickel from nickel laterite reduced at low temperature,high levels of additives and slag bond with refractory material at high temperature.Furthermore,the development and utilization of low-grade nickel laterite ore is an important measure to deal with the shortage of high-grade nickel laterite ore resources,especially the development of innovative technology for the preparation of high-grade ferronickel from low-grade nickel laterite ore has attracted more and more attention.In the present work,the mechanism of the carbothermic reduction of nickel and iron oxides was firstly analyzed through thermodynamic analysis.The carbothermic reduction test of nickel laterite was carried out,and the main influencing factors were analyzed and discussed.The reduction kinetics of nickel and iron oxides were investigated by isothermal reduction-chemical analysis method,on the basis of process mineralogy of a low-grade typical saprolitic laterite ore.The carbothermic reduction of nickel laterite is a complex chemical reaction process involving multiphase and multicomponent participation.The main chemical reactions include direct reduction by solid carbon,reduction by CO and gasification of carbon,reduction by CO plays a majoy role in reduction process.The selective reduction of nickel and iron oxides by solid carbon is just a periodic selective reduction when the reduction atmosphere drops during the reduction process.The factors affecting the metallization of nickel and iron oxides include the amount of reducing agent addition,reduction temperature and time.The reduction rate of nickel and iron oxides was faster in 15 minutes than that after 15 minutes,and the reduction rate of iron oxide is faster than that of nickel oxide throughout the reduction process.The current work explores the aggregation and growth behavior of ferronickel particles in self-reduction process of nickel laterite ore,and the effects of Ca F₂ and Ca SO₄ on particle size evolution of ferronickel particles in reduced nickel laterite ore,then the factors influencing the aggregation and growth of ferronickel particles were analyzed systematically.The action mechanism of various additives on the aggregation and growth of ferronickel particles was revealed by analyzing the mineral composition and microstructure of reduced nickel laterite ore under the action of additives.A technical idea of synergetic action of composite additives was put forward,which will reduce additive usage and maximize additive effectiveness.The main factors affecting the aggregation of ferronickel particles in the self-reduction of nickel laterite ore include the migration capacity of ferronickel particles,the restrictions of gangue on the migration of ferronickel particles,and the distribution of ferronickel particles in gangue.The growth behavior of ferronickel particles after aggregation is mainly affected by the surface tension of ferronickel particles.Compact forsterite transformed into loose tremolite under the action of Ca F₂.Ca F₂served as an activating agent for facilitating melted phase formation and decreased the melting point of the gangue.The change of crystal structure and the decrease of the melting point improved the migration environment of ferronickel particles,and reduced the restrictions of gangue on the migration of ferronickel particles.Ca SO₄ decomposes to S₂ and Ca O in a reducing atmosphere,thus,ferronickel particles in the reduced nickel laterite aggregate and grow up under the synergetic action of S₂ and Ca O.A low-melting-point eutectic of Fe-Fe S formed by the reaction of S₂ with Fe O or Fe,reduced the surface tension of ferronickel particles and facilitated the migration of ferronickel particles with a liquid-phase growth mechanism.Ca O transformed forsterite with compact structure to pigeonite,which is conducive to the migration of ferronickel particles in gangue.We developed a new reduction and selective oxidation process for nickel laterite to produce a high-grade ferronickel.The thermodynamics of the oxidation of metallic nickel and iron in CO₂/CO gas mixtures were investigated,and the feasibility of separating metallic iron and nickel by selective oxidation was explored.The self-reduction of nickel laterite followed by selective oxidation in CO₂/CO gas mixtures was performed.The oxidation behavior of ferronickel in CO₂/CO gas mixtures was revealed by analyzing the mineral composition and microstructure of reduced and oxidized products.The oxidation of iron began at 570°C,while the concentration is higher than the equilibrium CO₂ concentration for the oxidation of iron to wüstite,iron will be oxidsized to form wüstite,nickel is stable in CO₂/CO gas mixtures.Thus,the selective oxidation of iron could be realised by controlling the CO₂ concentration.The optimised reduction conditions in this study included C/O=1.2,Ca F₂ of 4 wt%,reduction temperature of 1200°C and reduction time of 25 min.The reduced nickel laterite was oxidised at a CO₂ concentration of 70 vol%for 50 min at 1100°C.A ferronickel product containing 37.2 wt%nickel and 60.4 wt%iron was subsequently produced,with corresponding nickel and iron recoveries of 95.89%and 13.47%,respectively.The results of self-reduction and selective oxidation with CO₂/CO gas mixtures of nickel laterite ore showed that,higher temperature or higher CO₂concentration was conducive to the oxidation of iron,the nickel content of ferronickel oxidised at 1100°C was far greater than that at temperature?1000°C.