Non-melting Metallization Reduction Process And Basic Theory Of Magnesium-based Nickel Oxide Ore

Nickel is an important strategic metal,which is mainly used for the production of stainless steel.In 2015,the apparent consumption of nickel in our country was nearly 960,000 tons,and more than 80%of the raw material was imported from abroad.Domestic nickel reserves are mainly low-grade nickel sulfide ores and nickel oxide ores with high level of basic gangue;statistics reports showed that more than 80%of the overseas nickel resources mastered by China's enterprises were nickel oxide ores.The development of new technologies for processing the aforementioned nickel oxide ores is of great significance to alleviate the huge demand for nickel of our country.Given the current issues,this paper proposes a non-molten metalized reduction technology followed by magnetic separation to beneficiate nickel and iron from nickel oxide ores.Based on systematic experimental studies and theoretical analysis,the reliable theories have be founded and detailed data have be obtained,which are conducive to comprehensively utilizing the abovementioned nickel oxide ores in a low cost and high efficiency way.The topic of this paper has achieved the financial support of the National Natural Science Foundation of China and the main points are as follows:?1?Mineralogical studies of the nickel oxide ores were carried out to determine the main minerals and the distribution of target elements by means of XRD,OM,SEM/EDS,EMPA,TG-DSC,ICP-AES,artificial heavy fraction and some chemical phase analysis methods.The main mineral of the magnesium-rich nickel oxide ore from Yuanjiang of Yunnan Province was serpentine,and iron oxide was the minor.Nickel and iron were regarded as the target elements.Over 87%of nickel embedded in serpentine in the forms of isomorphism or adsorption and presented a uniform distribution,which could not be beneficiated through a sieve classification.The majority of iron was distributed in iron oxides.In contrast,the main minerals of the representative limonitic nickel oxide ore from the Philippines were iron oxides/oxyhydroxides,and clay and gangue minerals were the minor.Nickel,cobalt and iron were regarded as the target elements.More than 90%of nickel and iron concentrated in iron oxides,as well as 50%of cobalt.The rest of cobalt existed in psilomelane and gangue aggregates.The chemical components were closely related to the particle size of constituent minerals,such that the beneficiation of nickel,cobalt and iron could be separately achieved through a sieve classification,but the ratios were not high.?2?The feasibility of metalized reducing nickel oxide ores in a non-molten state using coal as the reductant was analyzed based on the studies of thermodynamics basis and the structure,thermodynamics and reduction features of the nickel oxide ore from Yuanjiang.Then,the metalized reduction of the nickel ore followed by magnetic separation to beneficiate nickel and iron were systematically conducted.The feasibility analysis showed that when the temperature was controlled in the range of 750 to 1250 ?,the non-molten metalized reduction of nickel oxide ores could be achieved.The optimal technological parameters were determined as:1200 ?,60 min,8%of coal,7%of CaF₂,grinding duration of 8 min?D97=34.32 ?m?and magnetic field intensity of 150mT.Under these conditions,the magnetic concentrate with the nickel and iron grades of 5.7%,70.8%,respectively,could be produced.Meanwhile,the recoveries of nickel and iron could reach 93.8%and 84.8%,respectively.However,when the accelerant was not used,the recoveries of nickel and iron were only 14.2%and 32.5%,respectively,and whilst the grades of nickel and iron of the concentrate were just 2.6%and 62.6%,respectively.These results suggested that CaF₂ could dramatically promote the metallization of nickel and iron.XRD analysis showed that the main phase in the concentrate was ferronickel alloy and the minor was forsterite;the major in the tailing was forsterite.SEM/EDS analysis showed that nickel compeletly migrated outwards after adding CaF₂,and concentrated in newly generated ferronickel alloys;small amount of scattered alloy particles embedded in the silicate matrix resulted in nickel and iron loss.?3?Inspired by the mechanism of chloride reduction roasting,thermodynamical analysis and an optimal microscope were utilized to investigate the growth of ferronickel alloys.The process of metalization was determined.It should be CO diffusing through micro channels between minerals to the interfaces of nickel and iron comounds and then reacting,not similar to segregation of chlorides.During the process of segregation,nickel and iron chlorides generate and volatilize,then they are adsorbed by carbon particles and metalized.Morphologies of the reduction products under different conditions were observed.In the absence of CaF₂,a part of nickel and iron oxides could be reduced only in situ,and the generated ferronickel alloy particles were almost around 2 ?m.Although more metallic nickel and iron could migrate outwards with the increased temperature,the migration and aggregation were not enough.In the presence of CaF₂,the nickel metallization was greatly improved.Ferronickel alloys generated around the margins of silicate minerals and aggregated as end-to-end band with the size of over 100 ?m.The interfaces between alloys and silicates were obvious,which was beneficial to magnetic separation.?4?Phase transformation behaviors of nickel-and iron-bearing minerals before and after adding CaF₂ under different reaction conditions and interactions between CaF₂ and minerals were investigated.The mechanism of non-molten metalized reduction using coal as the reductant in the presence of CaF₂ was determined.The results showed that CaF₂ could induce chemical bonds of nickel-contained serpentine[?Mgx,Niy?3Si2O3?OH?4]to break,which reduced the temperature of nickel oxide releasing;enforce iron oxides and unstable state nickel oxides[NiO]to migrate towards the cracks and margins of silicates;inhibit nickel-contained magnesium silicate phase?MgNiSi2O6?from generating,which might make nickel inert;promote nickel and iron oxides in well contact with each other to react and generate nickel ferrite spinel owning better reducibility than oxides,which resulted in effective aggragation of nickel and iron before reduction;prompt amorphous forsterite?Mg2SiO4?to be produced early and react with SiO2 to form enstatite?MgSiO3?with incompact structure;promote nickel and iron oxides in bad contact with each other to be reduced separately to metallic state followed by capturing each other and aggregating into large alloy particles.Results of optimizing the crystallization water content,the Fe-to-Ni mass ratio and the SiO2 content confirmed the inference of the mechanism of the non-molten metalizing magnesium-rich nickel oxide ores process.The metalized process was well interpreted with AE0.5 model under diffusions control,"and the apparent activation energy was estimated to be 309.16 kJ·mol-1.?5?The semi-pilot tests were conducted in a rotary kiln of 0.45 m inner diameter and 7.5 m length,in order to verify the stability and feasibility of industrial application.The results showed that an appropriate positive pressure?0.02-0.04 MPa?in the kiln terminal was of benefit to the metals metallization,that the overall recoveries of nickel and iron reached 91.3%and 73.8%,while the nickel and iron grades of the ferronickel concentrate were 7.4%and 69.6%.The kiln ringing did not occur during the nearly 40 days of continuous tests,which consumed approximate 18.5 tons nickel oxide ores.The return ratio of dusts was nearly 8%.Preliminary calculation suggested that the electricity consumption in the non-molten metalized process was only approximate 52.5 kW h/t-ore.