Resource Utilization Of Electrolytic Manganese Dioxide Residue Based On Geopolymer Preparation

Electrolytic manganese dioxide residue(EMDR)is a solid waste generated from the pressure filtration process after acid leaching of manganese carbonate ore or reduced pyrolusite during the industrial production of electrolytic manganese dioxide.EMDR mainly contains silica and alumina,and also contains some soluble heavy metal ions,such as Mn(?),Cu(?),Zn(?)and Pb(?).Currently,almost all EMDR is dumped into landfill sites without any treatment,which can cause serious pollution in the surrounding soil and groundwater.Therefore,there is an urgent need to develop a method for EMDR utilization with high efficiency,low cost and high value-added.Geopolymer is a kind of inorganic polymer cementing material with a tridimensional structure and excellent mechanical,chemical and thermal properties.Furthermore,the geopolymer synthesis route is simple and well-suited to produce value-added materials that incorporate high-volumes of waste.Based on the investigation of the physical properties,chemical properties and reactivity of EMDR,this paper studied three ways to utilize EMDR,including preparation of phosphoric-acid-based geopolymer,water-glass-based geopolymer and ceramic materials.In addition,the reaction mechanism of iron phosphate based geopolymer was studied.The main research contents and conclusions are showed as follows.(1)The particle size,specific surface area,chemical composition,phase,morphology,TG/DTA,leaching toxicity and reactivity of EMDR were studied.The results show that EMDR has a median diameter(D50)of 10.52 ?m and a specific surface area of 44.72 m2/g.It mainly contains aluminum sulfate hydrate(3AL2O3·4SO3·8H2O),magnetite(Fe3O4)and quartz(SiO2).The pH value and Mn(?)leaching concentration of the EMDR are 4.53 and 2129 mg/L,respectively,and other measured elemental concentrations are all within the limits of Integrated Wastewater Discharge Standard in China.EMDR has little reactivity with water and water glass,but it is highly reactive with phosphoric acid and could be cured to form geopolymer gel materials.In addition,the calcination treatment could not increase the reactivity of EMDR.(2)The main influencing factors,leaching toxicity,thermal stability and corrosion resistance of phosphoric-acid-activated EMDR geopolymer were studied.The results show that increasing the concentration of phosphoric acid,decreasing the amount of phosphoric acid,moderately increasing the curing temperature and adding appropriate amount of river sand were beneficial to increase the compressive strength of phosphoric-acid-activated EMDR geopolymer.The optimum preparation conditions are as follows:phosphoric acid concentration of 55%-65%,phosphoric acid/EMDR mass ratio of 0.8-1.0,the river sand/EMDR mass ratio of 0.8,and the curing temperature of below 80'C.The 2-day compressive strength of phosphoric-acid-activated EMDR geopolymer could reach up to 62.0 MPa under the conditions of phosphoric acid concentration of 65%,phosphoric acid/EMDR mass ratio of 0.8,river sand/EMDR mass ratio of 0.8,and curing temperature of 80?.The high compressive strength was due to the amorphous and dense reaction products of phosphoric acid and magnetite in EMDR.Pretreatment of wet EMDR with CaO could effectively improve the manganese stabilization efficiency in the phosphoric-acid-activated EMDR geopolymer.When CaO to wet EMDR mass ratio was 0.10,the manganese stabilization efficiency could reach 95.4%.In addition,phosphoric-acid-activated EMDR geopolymer showed good resistance to high temperature and aggressive environment except strong acid.(3)The evolution process of chemical composition,microstructure and macroscopic properties of iron phosphate based geopolymer with Fe3O4 and Fe2O3 as raw materials were respectively investigated,and then the reaction mechanism of iron phosphate based geopolymer was obtained.The results show that the chemical composition of final products was controlled by Fe/H3PO4 molar ratio of raw materials.When the molar ratio of Fe/H3PO4 was above 3/2,Fe3O4 reacted with phosphoric acid to form amorphous Fe(H2PO4)2 and Fe(H2PO4)3 gel initially,and then gradually transformed into amorphous FeHPO4 and Fe2(HPO4)3 gel.Finally,the gel was converted into crystal Fe3(PO4)2(OH)2,and the geopolymer samples almost lost the strength.During the evolution process of the gel products,the pH value of the geopolymer matrix changed from acidic to weakly alkaline,meanwhile,the local enrichment of the gel phase lead to higher porosity and lower strength of the geopolymer matrix.Consequently,the Fe/H3PO4 molar ratio must be controlled below 3/4.(4)The apparent morphology,mechanical property,phase,microstructure,leaching toxicity,thermal stability and corrosion resistance of water-glass-activated EMDR-metakaolin geopolymer and water-glass-activated EMDR-slag geopolymer were studied separately.The results show that increasing the amount of EMDR enhanced the efflorescence degree of the alkali-activated geopolymer samples.So the addition amount of EMDR should not exceed 60%.Under this condition,the 28-day compressive strength of the metakaolin-based geopolymer and slag-based geopolymer could reach up to 46.7 MPa and 42.6 MPa,respectively.The strength of both kind of geopolymer was provided by water glass and the amorphous gel product resulted from the reaction of water glass and aluminosilicate(metakaolin or slag).Both kind of geopolymer showed excellent solidification/stabilization effect on EMDR,and all measured elemental concentrations met the Integrated Wastewater Discharge Standard.Compared with water-glass-activated EMDR-metakaolin geopolymer,water-glass-activated EMDR-slag geopolymer exhibited better resistance to high temperature,water,acid,alkali and salt solution.Interestingly,the compressive strength of water-glass-activated EMDR-slag geopolymer increased with the increase of calcination temperature,and reached up to 148.5 MPa after calcination at 1100? for 1 h,which was due to the whiskers produced in-situ during the calcination process.(5)The main influencing factors and leaching toxicity of EMDR-based ceramics were studied.The results show that the sintering temperature and sintering time were the main factors that affected the performance of EMDR-based ceramic materials.The optimum preparation parameters are as follows:molding pressure of 2?6 MPa,heating rate of 4?6?/min,sintering temperature of 1200? and sintering time of 1.0 h.The compressive strength of the ceramic material prepared under this condition could reach up to 204.0 MPa,and the main phase were maghemite(?-Fe2O3)and glass phase.And the glass phase mainly contained Si,Al,Fe,Mn,Na,K,Ca and P.The apparent porosity and compressive strength of the porous ceramic material prepared with 20%calcium carbonate were 45.59%and 23.6 MPa,respectively.The main crystal phases of the porous ceramic were Ca0.15Fe2.85O4,CaSiO3,Caa4Al6O12SO4 and ?-Fe2O3.The leaching concentrations of harmful metal ions such as Mn,Cu,Zn,Pb,Cr and Cd in EMDR-based ceramic and porous ceramic materials were all within the limits of Integrated Wastewater Discharge Standard.