Preparation Of Chromium And Nickel-Chromium Alloy By Electrochemical Deoxidation In Molten Salts

The recent electro-deoxidation,namely the FFC-Cambridge process,of solid metal oxide or oxides offers a simple,low-temperature and cost-saving technology for the preparation of refractory metal and alloys in molten salt,which is initiated and developed by the Cambridge University.Metallic chromium?Cr?and its alloys present good properties like its high hardness,wear resistance and excellent corrosion resistance,which are widely used in metallurgy,aviation,military,automotive and other fields.However,its current preparation process is complex,high cost,and environmental pollution.Based on the clean production of chromate and chromium oxide?Cr₂O₃?products by the sub-molten salt technology of National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology,the research project 'Preparation of Chromium and Nickel-Chromium Alloy by Electrochemical Deoxidation in Molten Salts' was proposed in order to realize the product upgrading and extension.The influence of molten salt system,cathode preparation,electrolysis parameter and the graphite anode on the electro-deoxidation of Cr₂O₃ was thoroughly investigated.Under the optimum conditions,the Cr product with a purity of 99.18%was obtained.The reaction mechanism of the Cr₂O₃ electro-deoxidation in CaCl₂ molten salt was proposed.Based on the proposed mechanism,some measures to optimize the process were taken in order to avoid the adverse effects of the formation of the intermediate product calcium chromite?CaCr₂O₄?,and to improve the electrolysis speed and the current efficiency.By the addition of NiO to Cr₂O₃ powder,the Ni-Cr alloy with different Ni/Cr molar ratio was successfully prepared and its formation mechanism was also explored.The main innovative work is summarized as follows:1.The influence of molten salt system,cathode preparation and electrolysis parameter on the electro-deoxidation of Cr₂O₃ was thoroughly investigated.The obtained Cr product with a purity of 95.78%from the Cr₂O₃ electro-deoxidation in CaCl₂-NaCl molten salt contains 0.53%oxygen.Its particle size is small,about 500 nm,and covered with a thin oxide film.However,in CaCl₂ molten salt,the Cr product with a purity of 99.18%contains 0.17%oxygen.Its particle size is large,the maximum size up to 5 ?m and presents a nodular morphology.Based on the needs,the Cr particle size can be controlled on purpose by selecting different molten salt system and electrolysis temperature.2.The graphite corrosion behavior is greatly related with the initial surface state provided by graphite anode.The reaction between graphite anode and the dissolved O^2-preferentially occurs on the potholes and crack zones of the polished graphite surface,giving out the CO and CO2 gas.The graphite corrosion is attributed to the consumption with O^2-to form the CO and CO2 gas,the graphite particle detachment of the loosely-connected graphite particles on the corroded surface due to the physical attack,and the formation of CO32-owing to the dissolution of CO2 in the molten salt.The as-formed CO32-near the anode area can further diffuse to the cathode and be electro-reduced to carbon nanotube,at the same time releasing O^2-,which causes a vicious cycle in the molten salt.The carbon contamination in the electrolyzed product is most likely due to the reaction between the produced Cr and the carbon originating from the electro-reduction of the dissolved CO32-near the cathode.3.The cathodic reaction pathway in the electrolysis of Cr₂O₃ is summarized as follows:at the early stage,the spherical Cr₂O₃ releases O^2-,forming cubic Cr;then the released O^2-,in combination with Ca2+ and the Cr₂O₃ particles,chemically forms the large platelet CaCr₂O₄ in nearby locations;finally,the formed intermediate product CaCr₂O₄ gradually decomposes to nodular Cr.At the same time,the cubic Cr particles formed from the electrolysis of Cr₂O₃ at the early stage gradually transform to nodular Cr particles with the electrolysis process progresses.Due to the molar volume of CaCr₂O₄ larger than that of Cr₂O₃ and Cr,its formation blocks the pores inside the Cr₂O₃ pellet,inhibits the diffusion of O^2-out of the pellet to the molten salt and result in the easy breaking of the pellet during the electrolysis.4.?-CaCr₂O₄ was creatively synthesized using a molten salt synthesis method,which was further calcined in Ar atmosphere to prepare rod-like ?-CaCr₂O₄.The influence of reaction time on both materials was thoroughly investigated and their respective magnetic property was introduced in journal paper.5.Based on the discovery of the reaction mechanism of the Cr₂O₃ electro-deoxidation in CaCl₂ molten salt,the CaCr₂O₄ as the raw material to prepare the Cr by the electro-deoxidation method was proposed.The results show the electro-deoxidation of CaCr₂O₄,compared to the Cr₂O₃ raw material,proceeds with a faster electrolysis speed and higher current efficiency.That is due to the small molar ratio of 2Cr/CaCr₂O₄,the removal of Ca2+ and O^2-genenrtes a more porous structure to facilitate the diffusion of molten salt and O^2-during the electro-deoxidation of CaCr₂O₄.For that reason,the direct exploitation of calcim-containing species could accelerate the electrolysis speed of other oxides like ZrO2?Nb2O5?Ta2O5,which also encounter the formation of calcim-containing species.The addition of NH4HCO3 pore-forming agent can improve the porosity of the Cr₂O₃ pellet,which will facilitate the the diffusion of molten salt and O^2-and suppress the adverse impact of the formation of densely stacked CaCr₂O₄.The NH4HCO3 addition enhances the electrolysis uniformity and the pellet remains compact after electrolysis.6.On the basis of the elctro-deoxidation process to prepare the Cr,the Ni-Cr alloy with different Ni/Cr molar ratio was successfully prepared by the addition of NiO to Cr₂O₃ powder.And its formation mechanism was also explored.Firstly,the reduction starts with the formation of Ni metal,accompanied with the formation of an intermediate product CaCr₂O₄.Then,the formed intermediate product CaCr₂O₄ is further electro-reduced to Cr,which is directly alloyed with the newly-formed Ni at the early stage to form the target Ni-Cr intermetallic compound.Pure Cr phase is not detected when the Cr content is relatively low in the NiO-Cr₂O₃ mixture...