Zhihong Peng

Recovery of alumina and ferric oxide from Bayer red mud rich in iron by reduction sintering

A great amount of red mud generated from alumina production by Bayer process not only threatens the environment but also causes waste of secondary resources. High-iron-content red mud from Bayer process was employed to recover alumina and ferric oxide by the process of reduction-sintering, leaching and then magnetic beneficiation. Results of thermodynamic analyses show that ferric oxide should be reduced to Fe if reduction of ferric oxide and formation of sodium aluminate and calcium silicate happen simultaneously. Experimental results indicate that alumina recovery of Bayer red mud can reach 89.71%, and Fe recovery rate and the grade of magnetite concentrate are 60.67% and 61.78%, respectively, under the optimized sintering conditions.

Thermodynamics of chromite ore oxidative roasting process

To explicate the thermodynamics of the chromite ore lime-free roasting process, the thermodynamics of reactions involved in this process was calculated and the phrases of sinter with different roasting times were studied. The thermodynamics calculation shows that all the standard Gibbs free energy changes of the reactions to form Na2CrO4, Na2O·Fe2O3, Na2O·Al2O3 and Na2O·SiO2 via chromite ore and Na2CO3 are negative, and the standard Gibbs free energy changes of the reactions between MgO, Fe2O3 and SiO2 released from chromite spinel to form MgO·Fe2O3 and MgO·SiO2 are also negative at the oxidative roasting temperatures (1 173–1 473 K). The phrase analysis of the sinter in lime-free roasting process shows that Na2O·Fe2O3, Na2O·Al2O3 and Na2O·SiO2 can be formed in the first 20 min, but they decrease in contents and finally disappear with the increase of roasting time. The final phase compositions of the sinter are Na2CrO4, MgO·Fe2O3, MgO·SiO2 and MgO. The results indicate that Na2CrO4 can be formed easily via the reaction of Na2CO3 with chromite ore. Na2O·Fe2O3, Na2O·Al2O3 and Na2O·SiO2 can be formed as intermediate compounds in the roasting process and they can further react with chromite ore to form Na2CrO4. MgO released from chromite ore may react with iron oxides and silicon oxide to form stable compounds of MgO·Fe2O3 and MgO·SiO2, respectively.

Thermodynamic model for equilibrium solubility of gibbsite in concentrated NaOH solutions

Abstract The thermodynamic properties of the most important NaOH–NaAl(OH) 4 –H 2 O system in Bayer process for alumina production were investigated. A theoretical model for calculating the equilibrium constant of gibbsite dissolved in sodium hydroxide solution was proposed. New Pitzer model parameters and mixing parameters for the system NaOH–NaAl(OH) 4 –H 2 O were yielded and tested in the temperature range of 298.15–373.15 K. The results show that the proposed model for calculating the equilibrium constant of gibbsite dissolution is applicable and accurate. The obtained Pitzer model parameters of β (0) (NaAl(OH) 4 ), β (1) (NaAl(OH) 4 ), C Φ(NaAl(OH) 4 ) for NaAl(OH) 4 , the binary mixing parameter of θ(OH − Al(OH) 4 − ) for Al(OH) 4 − with OH − , and the ternary mixing parameter of ψ(Na + OH − Al(OH) 4 − ) for Al(OH) 4 − with OH − and Na + are temperature-dependent. The prediction of the equilibrium solubility of gibbsite dissolved in sodium hydroxide solution was feasible in the temperature range of 298.15–373.15 K.

Reaction behavior and mechanism of anatase in digestion process of diasporic bauxite

Abstract Based on the study of influence of temperature, digestion time, amount of CaO added and composition of aluminate solution on reaction behavior of pure anatase in high-pressure digestion process of bauxite, reaction mechanism of anatase was preliminarily determined. Anatase first reacts with caustic soda to produce Na 2 TiO 3 , then the resultant Na 2 TiO 3 reacts with 3CaO·Al 2 O 3 ·6H 2 O resulting from the reaction of CaO with sodium aluminate solution to produce CaO·2TiO 2 ·H 2 O which eventually converts into CaTiO 3 . Higher temperature, concentration of free Na 2 O k (the caustic soda uncombined with aluminate anions in the form of Na 2 O) and molar ratio of CaO to TiO 2 are favorable to the conversion of CaO·2TiO 2 ·H 2 O to CaTiO 3 . And Al(OH) 4 − shows the function of a catalyzer in the reaction of anatase with caustic soda with or without CaO added during the digestion process of diasporic bauxite.

Interaction of sulfur with iron compounds in sodium aluminate solutions

Abstract Reaction behaviors of sulfur and iron compounds in sodium aluminate solutions were investigated. The results show that iron compounds can remarkably remove the S 2– but cannot get rid of and in sodium aluminate solutions. The removal efficiency of S 2– using ferrous compound and ferric compound can reach 86.10% and 92.70% respectively when the iron compounds were added with a molar ratio of 2:1 compared with the sulfur in liquors at 100 °C. Moreover, several same compounds are formed in those two desulfurization processes with ferrous or ferric compounds, including erdite, hematite, amorphous ferrous sulfide, polymerized sulfur-iron compounds and ferric sulfate. The major difference between these two processes is that the erdite generated from ferrous compounds at the initial reaction stage will convert to a sodium-free product FeS 2 at the subsequent stage.

Removal of S2− ion from sodium aluminate solutions with sodium ferrite

Abstract The synthesis of sodium ferrite and its desulfurization performance in S 2− -bearing sodium aluminate solutions were investigated. The thermodynamic analysis shows that the lowest temperature is about 810 K for synthesizing sodium ferrite by roasting the mixture of ferric oxide and sodium carbonate. The results indicate that the formation process of sodium ferrite can be completed at 1173 K for 60 min, meanwhile raising temperature and reducing Na 2 CO 3 particle size are beneficial to accelerating the formation of sodium ferrite. Sodium ferrite is an efficient desulfurizer to remove the S 2− in aluminate solution, and the desulfurization rate can reach approximately 70% at 373 K for 60 min with the molar ratio of iron to sulfur of 1:1–1.5:1. Furthermore, the desulfurization is achieved by NaFeS 2 ·2H 2 O precipitation through the reaction of and S 2− in aluminate solution, and the desulfurization efficiency relies on the generated by dissolving sodium ferrite.

Relationship between Al(OH)3 solubility and particle size in synthetic Bayer liquors

Surface tension of sodium aluminate solution and the contact angle between Al(OH)3 particles and aluminate solution were measured, then the dependence of Al(OH)3 solubility on its particle size was calculated and thus the variation of the critical nucleus sizes was determined based on the Ostwald ripening formula. The results show that the Al(OH)3 solubility in sodium aluminate solution decreases with the increment of particle size, and the critical nucleus sizes increase with the rise of alkali concentration, caustic ratio and precipitation temperature. The results also imply that the presence of small particles in seeded precipitation system is an important factor to limit the depth of precipitation.

Conversion of ferric oxide to magnetite by hydrothermal reduction in Bayer digestion process

Digesting aluminum-bearing minerals and converting ferric oxide to magnetite simultaneously in Bayer digestion process is crucially important to deal with high-iron diasporic bauxite economically for alumina production. The reaction behaviors of hydrothermal reduction of ferric oxide in alkali solution were studied by both thermodynamic calculation and experimental investigation. The thermodynamic calculation indicates that Fe3O4 can be formed by the conversion of Fe2O3 at proper redox potentials in alkaline solution. The experimental results show that the formation ratio of Fe3O4 either through the reaction of Fe and Fe2O3 or through the reaction of Fe and H2O in alkaline aqueous solution increases remarkably with raising the temperature and alkali concentration, suggesting that Fe(OH)3− and Fe(OH)4− form by dissolving Fe and Fe2O3, respectively, in alkaline aqueous solution and further react to form Fe3O4. Moreover, aluminate ions have little influence on the hydrothermal reduction of Fe2O3 in alkaline aqueous solution, and converting iron minerals to magnetite can be realized in the Bayer digestion process of diasporic bauxite.

Variation of soda content in fine alumina trihydrate by seeded precipitation

Abstract High soda content in fine alumina trihydrate(ATH) limits its application and increases the soda consumption. The variation of soda content in the fine ATH by seeded precipitation was determined by detection of electric conductivity of solution, soda content in ATH, measurement of particle size distribution and microscopic analysis. The results show that high concentration of sodium aluminate solution, ground circulative seed, low temperature or fast initial precipitation rate increases the soda content in ATH. Soda mainly exists in lattice soda and less soda in desilication product (DSP) exists in the fine ATH precipitated from sodium aluminate solution with concentration of Al2O3 (ρAl2O3) more than 160 g/L and mass ratio of alumina to silica (μSiO2) of 400, and lattice soda decreases with increasing initial precipitation temperature, aging seed, and low precipitation rate and precipitation time. Results also imply that Na+Al(OH)4− ion-pair influences lattice soda content in ATH on the basis of electric conductivity variation.

Phenomena in late period of seeded precipitation of sodium aluminate solution

Abstract Aiming at seeded precipitation of aluminate solution with high caustic ratio(α k >2.4), corresponding to the late period of seeded precipitation, the influence of different types of seed on precipitation ratio was explained with respect to solution structure in the interface of seed and the evolution of Al(OH) 3 growth units in this layer. The effects of solid content and seed size on agglomeration were determined by calculating the particle number of product. The results imply that the solution structure in the interface of seed imposes a notable significance on the process in the late period of seeded precipitation. Agglomeration still exists in this period. However, the agglomeration bodies break in the case of prolonging precipitation due to the mechanical effect, which results in the increase of particle number.