Study On Inorganic Composite Coagulant Derived From Iron And Aluminum Reovered From Red Mud And Its Performance

Red mud is a solid waste produced in the alumina production process.In recent years,the recovery of metal elements in red mud has been extensively studied,but how to further quantify and use high-value metals after metal recovery is a technical challenge.In this paper,recovering Fe and Al from red mud to obtain a high-valent cation composite coagulant by acid leaching,and combining residue as a skeleton builder to condition the sludge was studied.In addition,the Fe and Al separated by leaching were produced as polymeric aluminum ferric sulfate,and the coagulation performance of the advanced sewage treatment was discussed.The main research and results of this thesis include the following:1.Acid leaching method to recover Fe and Al from red mud to prepare inorganic composite coagulant and residues to jointly condition sludge and dewatering performanceFor the characteristics of high Fe and Al contents in the red mud,sulfuric acid was used to directly leaching Fe and Al in red mud to prepare a liquid inorganic composite coagulant.The experimental results showed that as the liquid/solid ratio,H₂SO₄ concentration,reaction time,and temperature were 7:1 m L/g,10 wt%,5 h,and of 70 ^oC,the recovery rates of Fe and Al in red mud were 16.82%and 66.91%,respectively.When the liquid inorganic composite coagulant obtained from the separation of acid leaching red mud is used for sludge conditioning,the optimized dosage of trivalent cations was 19.22 mg/g DS calculated as dry sludge(including 9.10 mg Fe³⁺/g DS and 10.12 mg Al³⁺/g DS),and the leaching residue was 420 mg/g DS as skeleton builder.The water content of conditioned sludge could reduce to 58.13%.When the inorganic composite coagulant and residue were combined to condition sludge,the sulfuric acid in the coagulant dissolve part of the S-EPS into the sludge liquid phase.At the same time,Fe³⁺and Al³⁺can coagulate the sludge flocs,increase the Zeta potential and the sludge particle size,and reduce the bound water content of sludge.The residue acts as a skeleton builder during the dewatering processconstructing a filtration channel,and promoting the removal of water from the sludge.2.Roasting pretreatment improves the recovery efficiency of Fe and Al in red mud and its effect on the performance of inorganic composite coagulant conditioning sludgeThe red mud is calcined at high temperature and reduction roasting,and the effect of roasting pretreatment on the recovery rate of Fe and Al in the red mud is explored.The experimental results show that as the mixing amount of coal powder to into red mud,roasting temperature,leaching temperature,and sulfuric acid concentration t were 25 wt%,900 ^oC,70^oC,and 20 wt%,the leaching rate of Fe was 69.01%and the leaching rate of Al was 56.07%.The inorganic coagulant prepared by acid leaching of red mud pretreated by reduction roasting has a better effect on conditioning sludge.When the optimized dosage of trivalent cations was 34.89 mg/g DS(including 26.54 mg Fe³⁺/g DS and 8.35 mg Al³⁺/g DS),and the leaching residue was 420 mg/g DS as skeleton builder.,The water content of the dewatered sludge cake could reduce to 65.62 wt%,which is lower than the sludge conditioned by inorganic composite coagulant prepared by directly leaching the red mud(73.08 wt%)and commercially available agents(68.82 wt%)are more effective.The inorganic composite coagulant and solid residue prepared by acid leaching of red mud pretreated by reduction roasting were applied to the pilot test of Sanjintan wastewater treatment plant,and under the optimized dosage,the water content of the dehydrated cake is 64.17 wt%.It is proved that the preparation of inorganic composite coagulant combined with solid residue conditioning sludge after red mud reduction roasting pretreatment and acid leaching is a feasible way for the comprehensive utilization of red mud,realizing the full quantitative utilization of red mud.3.Study on the effect of preparing polyaluminum ferric sulfate and coagulant by recovering Fe and Al from red mud by acid leaching methodAfter oxidation,hydrolysis,and polymerization of the red mud leach liquor of reduction roasting,a polyaluminum iron sulfate coagulant is obtained.The experimental results show that H₂O₂ is added to the red mud acid leach liquor at a molar ratio of H₂O₂/Fe²⁺at a ratio of1.2,and Na OH is used to adjust the p H as 1.6,the PAFS coagulant obtained after aging for 12h at 80 ^oC has the best performance.The prepared coagulant was used to treat simulate wastewater and compared with commercially available polyferric sulfate and polyaluminum sulfate.When the prepared PAFS is used to treat simulate wastewater,the dosage of 200 mg/L has the best coagulation effect.It has a wider p H range than polyferric sulfate and polyaluminum sulfate,and the settling time is between PFS and PAS.FTIR test analysis suggest that the self-made coagulant PAFS also contains Fe-OH and Al-OH of the commercial PFS and PAS.XRD results show that the self-made coagulant PAFS are similar in amorphous structure,and the SEM results show that it has the mixture structural characteristics of PFS and PAS.4.Study on the coagulation mechanism of polyaluminum iron sulfate coagulant based on red mud recovery for advanced sewage treatmentWhen the self-made polyaluminum iron sulfate is used for the advanced treatment of the effluent of the secondary settling tank,TP and COD are used as the removal object.When the dosage of PAFS is 200 mg/L,p H is between 6-9,and the settling time is 20 min,the removal rate of TP is above 75%,and the removal rate of COD is above 70%.Compared with commercial coagulants,the self-made coagulant PAFS has the advantages of a wide range of p H and fast settling speed.The use of high-speed camera technology proves that in the process of coagulation,the electric double layer is compressed and the adsorption electric neutralization occurs first,and then the adsorption bridging effect occurs,and the floc growth is completed under the common action of several mechanisms.