Research On Characteristics Of Red Mud Slurry Flow In Carbonation Reactor

Red mud,the Bayer process residue,is generated from alumina industry.Approximately 1 t～1.5 t of red mud is generated per ton of alumina.Over 120 million tons of red mud with a high alkalinity is generated annually throughout the world,therein over 50%was generated in china.This consumes a large amount of space,causes air pollution,destroys the peripheral soil,and contaminates the source of groundwater due to the alkaline slurry.So that it is important for red mud processing.In this paper,the process of calcification-carbonation method for red mud processing and the Characteristics of red mud slurry flow in carbonation reactor were studied.Using this method,the red mud was calcified with lime to transform the silicon phase into hydrogarnet,and the alkali in red mud was recovered.Then,the calcified residue(hydrogarnet)was decomposed by CO2 carbonation,affording calcium silicate,calcium carbonate,and aluminum hydroxide.Alumina was recovered using an alkaline solution at a low temperature.The effects of the new process were analyzed by thermodynamics analysis and experiments.The extraction efficiency of the alumina and soda obtained from the red mud reached 49.4%and 96.8%,respectively.The new red mud with<0.3%alkali can be used in cement production.Using a combination of this method and cement production,the Bayer process red mud can be completely utilized.In the study of thermodynamics,based on the available data of simple binary compounds and pure oxide,an approach was proposed to predict Gibbs energies and enthalpies of formation for complex minerals.The approach assumes that complex minerals are formed from binary composite oxides,which in turn,are formed from individual pure oxides.By taking the reaction with the largest reaction free energy in any pair pure oxide between the two composite compound instead of the polymerization reaction between the two composite compound,the unknown variable of reaction free energy between two composite compound can be detemined.The validity of this approach is examined by comparing the calculated values of Gibbs energies and enthalpies against the experimentally measured ones reported in literature.The results show that for typical complex minerals with available experimental data,the calculated results exhibit an average residual of 0.51%for Gibbs energies and 0.52%for enthalpies,compared to the experimental results.This new approach thus correlates well with experimental approaches and can be applied to most of the complex minerals.Based on the available data of pure oxides and binary composite oxides,the Gibbs free energies and enthalpies of sodium aluminosilicate hydrate and hydrogarnet with different silicon unsaturated coefficient were estimated by this method,and utilized in analysis of calcification-carbonation process.The characteristics of the fluid of gas-liquid and red mud slurry-gas-liquid phases in the Venturi carbonation reactor(jet-flow reactor)were investigated by a water model experiment,mainly the effects of superficial gas and liquid velocities and the effective height-to-diameter ratio(H/D)of the reactor for gas holdup,mixing time,bubble refining,and CO2 absorption.The results show that the maximum CO2 utilization rate(η)was achieved,i.e.,η=83.3%when the H/D value of the reactor was 4 and superficial gas and liquid velocities were 10.62 m·s-1 and 0.89 m·s-1 respectively.The characteristics of the fluid of gas-liquid phase in a novel Venturi carbonation reactor was numerically simulated by commercial software ANSYS 16.0/CFX.The effect of operation parameters such as superficial gas velocity on velocity field and gas holdup in the reactor was simulated and investigated.It shows that high liquid velocity benefits gas-liquid mass transfer and reaction,but the gas holdup should not be too large.The simulation results of average gas holdup agree well with the measured ones.The gas flow in reactor swing with liquid flow periodically,this favor for the homogeneous distribution of gas holdup.A homogeneous distribution of gas bubbles may promote the efficiency of carbonation reaction.The bubble size decreased with the increase of amount of liquid flow.The minus pressure at the inlet of Venturi pipe vary periodically and the flow swing periodically which was caused by flow compress.The results provided a theoretical basis for the design and application of venturi carbonation reactor under a high temperature and a high pressure.