Research On Preparation Of Chromium Oxide Via Electro-oxidation Of Ferrochrome In Ammonium-alkali System

Chromium oxide?Cr₂O₃?is a high value chromium salt product,which can be used as catalyst,thermal spraying material,metallurgical raw materials,abrasives and so on.However,the traditional production method is long process flow and serious environmental pollution.It is an inevitable trend to develop a new technology of Cr₂O₃.In 2013,Qinghai Salt Lake Institute of Chinese Academy of Sciences had developed a new production technology for preparing chromate by ferrochromium electro-oxidation.Sodium chromate?potassium chromate?was prepared by electro-oxidation under normal temperature and pressure with ferrochromium as sacrificial anode and sodium alkali?potassium alkali?as electrolyte.The technology has the advantages of less discharge of chromium-containing wastes,mild reaction conditions,controllable process and high economic value of by-products,and which has excellent application prospect.On this basis,the new technology of electro-chemical preparation of chromium salt was applied to ammonium salt system,and a two-step process for preparing Cr₂O₃was designed.The first step was to prepare ammonium chromate by electro-oxidation of ferrochromium,and the second step was to prepare Cr₂O₃by thermal decomposition of ammonium chromate.This paper starts from two aspects.Firstly,the electro-oxidation process of ferrochromium in ammonium-alkali system,including the optimization of electro-oxidation parameters,the exploration of crystallization method of ammonium chromate solution and the speculation of electro-oxidation mechanism.Next,the preparation process of Cr₂O₃by thermal decomposition,including the optimization of thermal decomposition process parameters and the influence of additives on the preparation process of nano-Cr₂O₃.The results are as follows:?1?With the current efficiency as the reference index,the effects of the molar ratio of chromium to ammonia,current density and circulating flow rate on the electro-oxidation of ferrochromium were investigated by single factor experimental system.The optimum conditions were as follows:the ratio of chromium to ammonia is 0.2:1,the current density was 140 A/m²,the circulation flow rate was 12 L/h,and the current efficiency was 63%.The polarization process of ferrochromium electrode in ammonia water was studied by steady-state linear polarization curve method.The data showed that high carbon ferrochromium exhibited the characteristics of activation-inhibition-reactivation in ammonia water.?2?The crystallization process of ammonium chromate solution was preliminarily explored.The effects of different crystallization methods on the crystalline form,morphology and color of the product were investigated.It was found that ammonium chromate crystals could not be obtained by evaporation crystallization of ammonium chromate solution,and only orange ammonium dichromate crystals could be obtained.The electrolyte was frozen and consolidated by freeze-drying method,and then dehydrated by low-temperature sublimation equipment.Finally,the bright yellow and pore ammonium chromate crystals were obtained.Comparing with the products of ammonium chromate produced by Aladdin,the ammonium chromate obtained by freeze-drying showed better purity and color.?3?Optimizing the thermal decomposition conditions of ammonium chromate and ammonium dichromate to prepare Cr₂O₃.From the thermal analysis results of ammonium chromate and ammonium dichromate,the chemical reaction process of thermal decomposition of ammonium chromate and ammonium dichromate was inferred.The results showed that the thermal decomposition process of ammonium chromate and ammonium dichromate follows the reduction process of Cr⁶⁺?Cr⁴⁺?Cr³⁺.The effects of thermal decomposition temperature,holding time and heating rate on the crystal form,morphology and chroma of Cr₂O₃were investigated.The bright green spherical Cr₂O₃with good crystallinity was synthesized by ammonium chromate and ammonium dichromate under the conditions of pyrolysis temperature 850?,holding time 0.5 h and heating rate4?/min and 5?/min,respectively.?4?The effects of additives in pyrolysis process on the preparation of Cr₂O₃were studied.The results showed that when ammonium nitrate was added to ammonium chromate and the molar ratio was 0.625:1,the Cr₂O₃particles with average particle size of 84 nm can be obtained.When ammonium bicarbonate was added to ammonium dichromate and the molar ratio was 1:1,the Cr₂O₃particles with average particle size of 42 nm can be obtained by thermal decomposition.In this paper,a new technical route of ferrochromium-ammonium chromate?ammonium dichromate?-chromium oxide was developed.It provides basic data and technical reserves for the future industrial production of Cr₂O₃prepared by ferrochromium electro-chemical method.It also promotes the development of clean production technology of chromium electro-oxidation in a certain extent.