Investigation On The Enhancement Mechanism Of Additives To The Oxidative Roasting Process Of Chromite Ore With Sodium Carbonate

Chromium is an important strategic resource.Chromium compounds and chromium bearing materials have been widely applied in extensive fields.As the primary intermediate product for preparing other chromium chemical products,sodium chromate is industrially manufactured through the oxidative roasting of chromite ore with sodium carbonate.During the roasting reaction,the low-melting-point materials will form the liquid phase.A remarkable amount of chromite ore processing residue(COPR)is recycled and mixed with the raw materials in order to dilute the proportion of liquid phase and ensure the steady operation of rotary kiln.However,the addition of COPR partly reduces the heat utilization and industrial production efficiencies.At present,it is a technical problem to keep the operation of roasting equipment with reduced amount of COPR during the oxidative roasting process of chromite ore with sodium chromate.In this thesis,additives were introduced instead of COPR in order to reduce the volume of reaction liquid phase and increase the chromium conversion rate.The leaching rates of chromium and aluminum,as well as the sintering characteristics of the roasted mixture,were taken as the key indexes for investigating the roasting process.Based on the study of the influence of different addtives on the roasting process,the best additive and the optimized processing parameters were selected.The effect and mechanism of the selected additive on the oxidative roasting process of chromite ore with sodium chromate were further investigated in detail.The main progress of this thesis was summarized as follow;(1)The thermodynamics of the oxidative roasting process of chromite ore in the medium of Na2CO3 with different additives were investigated.The values of the Gibbs free energy for the possible chemical reactions among the main components,including FeCr2O4,MgCr2O4,FeAl2O4 and MgAl2O4,of the chromite ore and Na2CO3 were calculated with different additives,including Fe2O3,MnO2,TiO2,Co3O4,CuO and NiO,during the roasting process.According to the results of calculation,the thermodynamic trends of reactions were definited,and the possible reaction paths and phase changes of the roasting process with additives were clarified.Moreover,the introduction of additives would promote the conversion of Cr3+ from Cr-bearing spinel to Na2CrO4 and hinder the formation of Na2O·Al2O3 from Al-bearing spinel.(2)The effects of different additives on the oxidative roasting of chromite ore with sodium carbonate were studied in the temperature range from 850 ? to 1000 ?.The results showed that Fe2O3 could promote the leaching process of chromium and inhibit the leaching process of aluminum.MnO2 would simultaneously impede the leaching process of chromium and aluminum.TiO2 can block the leaching process of aluminum and enhance the leaching process of chromium at the roasting temperature below 900 ?.Co3O4,CuO and NiO exhibited different effects on the leaching process of chromium and aluminum depending on the temperature.As for the COPRs obtained from both the calcium-free roasting process of chromite ore with sodium carbonate and the aqueous solution oxidation process of chromite ore with caustic alkali,although their Fe2O3 content is as high as 55%to 70%,their addition can not substitute for the addition of the Fe2O3 additive in the oxidative roasting process of chromite ore with sodium carbonate.(3)After adding the Fe2O3 additive,the processing parameters of oxidative roasing process of chromite ore with sodium carbonate was optimized and the related mechanism was also studied in details.Based on the leaching rates of chromium and aluminum as well as the sintering characteristics of the roasted mixture in the single factor experiment,the optimal operation parameters were found as:alkali ratio of 1.1,additive dosage of 30%,and roasting temperature range from 900 ? to 950 ?.After a roasting time period of 2.5 hours to 3.0 hours,the leaching rates of chromium and aluminum were 97%and 24%,respectively;the content of Cr2O3 in the COPR was as low as 1%;a loose and unsintered roasted mixture was obtained under the optimal conditions.The reaction mechanism of the roasting process with the Fe2O3 additive was investigated by thermogravimetric analysis and phase analysis.In the initial stage,no reaction occurred between chromite ore,Fe2O3 additive and Na2CO3.However,Fe2+ in the chromite ore was converted to Fe3+ resulting in the lattice distortion of spinel and then enhanced the activity of chromium,aluminum,iron components.In the middle stage,Na2CrO4,Na2O Al2O3 and Na2O Fe2O3 formed in the reaction among chromium,aluminum,iron components in spinel,Fe2O3 additive and Na2CO3.Because Na2O Fe2O3 could reduce the diffusion of Na+,the formation of Na2CrO4 and Na2O Al2O3 was impeded.In the final stage,Na2O Al2O3 and Na2O·Fe2O3 reacted with Cr-bearing spinel to form Na2CrO4 and ferroaluminite spinel.Fast decomposition of Na2O Al2O3 and Na2O·Fe2O3 and quick generation of ferroaluminite spinel increased the diffusion of Na+ and O2,thereby improving the formation of Na2CrO4.