Study On The Synthesis Of Geopolymer From Bayer Red Mud And The Effect Of Fe Species On This Geopolymer

Bayer red mud?RM?is the residue in the production of Al₂O₃ in alumina plant after the Bayer process to produce alumina.The reuse of RM has been a problem for its high alkalinity and complicated compositions throughout the world.The RM used in this study was obtained from CHALCO Shandong Branch,which uses the bauxite imported from Indonesia.The content of Fe₂O₃ in this RM is 30.77 wt%.In this dissertation,three issues have been studied:synthesis of geopolymer from the pretreated RM after the alkali-thermal activation pretreatment process;the recovery of iron from the RM after the reduction-calcination process and synthesis of geopolymer from the magnetic separation residue;the effect of Fe species on the geopolymer properties has also been studied.The main contents in this study are presented as follows:1.Direct synthesis of geopolymer from RM with alkali-thermal activation pretreatment processThe mineral phases in raw RM could be transformed by alkali-thermal activation process and the activity of RM was improved.The effects of calcination temperature,calcination time and contents of Na₂O on the dissolution efficiencies of Al and Si in the pretreated RM were evaluated.The results show that the dissolution efficiencies of Al,Si reached an optimal 40.32%and 9.76%respectively as raw RM was calcined at 800°C for60 min with 15.0 wt%content of Na₂O.The gibbsite decomposed after alkali-thermal activation process and sodium aluminum silicate hydrate was transformed into natrodavyne.The effects of content of Na₂O in alkali-thermal activation process,additional content of Na₂O in pasting process and content of RM on the properties of geopolymer based on the pretreated RM were investigated.The compressive strength of geopolymer cured at 28 d reached maximum 23.86 MPa when 5.0 wt%of Na₂O was added in RM.Additive of Na₂O in alkali-thermal activation process has superior effect on the compressive strength than additional Na₂O added in geopolymer paste.Excessive alkali would damage the structure of geopolymer gels.The suitable addition of alkali-thermal activated RM was 50 wt%in red mud-fly ash?RM-FA?geopolymer.2.Recovery of iron from RM by reduction-calcination process and synthesis of geopolymer based on magnetic separated residueHematite in RM was reduced by pulverized coal ash and recovered by magnetic separation.The effects of calcination temperature,calcination time and content of coal ash on the reduction of iron were conducted respectively.The results show that the iron recovery increased with the increase of calcination temperature,while the total Fe in the concentrate decreased rapidly.With the calcination time prolonged,the iron recovery increased firstly and then decreased due to the weaking of the reducing atmosphere.Suitable content of coal ash would attribute to the reduction of hematite in RM.The results show that the optimized reduction condition for iron recovery of RM was calcined at 900°C for 60 min with 5.0 wt%of coal ash.Under these conditions,the iron recovery and total Fe in concentrate were 49.41 wt%and 56.88 wt%,respectively,which satisfied the requirements for iron ore smelting.The compressive strength of geopolymner based on magnetic separated residue could reach 20 MPa,indicating that the processes of reduction calcination-magnetic separation-geopolymer synthesis are feasible.3.Effect of Fe species on the mechanical strength and microstructure of geopolymerThe effect of oxidation state of iron on the mechanical strength of geopolymer was conducted.The results show that the state of iron in RM had little effect on the strength of geopolymer compared with soluble aluminosilicate.In contrast,the effects of the transformations of Al,Si species on the compressive strength covered the effect of Fe species.M?ssbauer spectra and XPS analysis showed that Fe in the RM existed in hematite and isomorphism of aluminosilicates in the form of Fe^III.Most of hematite in RM was stable and could not participate in the geopolymerization.A portion of active iron generated from isomorphism of aluminosilicates during alkali-thermal activation process,then took part in the geopolymerization and played a role similar to Al³⁺and replaced partial Al³⁺in aluminosilicate structures of geopolymer.Inthispaper,thetechnologyof“Reducedcalcination-Magnetic Separation-Geopolymer synthesis”was proposed and studied,and the influence of iron on the properties of the geopolymer was investigated.The results will provide a feasible approach for the comprehensive utilization of RM with high iron content.