Preparation Of Fly Ash-Based Whisker Mullite Ceramic Membrane And Its Application In Oil-Water Separation

The presence of oil-in-water（O/W）emulsion in the secondary effluent of oily wastewater and the massive accumulation of fly ash are environmental problems that need to be solved urgently.Mullite ceramic membrane is considered as a microfiltration membrane suitable for treating O/W emulsion wastewater.Therefore,the synergistic utilization of Al₂O₃ and Si O₂ in fly ash to prepare mullite ceramic membrane and its application in oily wastewater can realize"treating waste with waste",which has important ecological and environmental benefits.At present,preparation of mullite ceramic membrane derived from fly ash has some main problems,such as low aluminum to silicon ratio,high sintering temperature,poor whisker morphology.Oxide doping method,a preparation method between pure solid-phase reaction and pure liquid-phase reaction,has the advantages of simple process,excellent whisker morphology and friendly environmental,which can realize in-situ growth of mullite whiskers.Therefore,this study proposes a technical route of“aluminum-silicon ratio regulation-aids system construction-dry pressing-high temperature sintering”for facile preparation of fly ash-based whisker mullite ceramic membrane,which is applied to deep treatment of oily wastewater.It can not only realize controlled preparation of fly ash-based mullite ceramic membrane,but also recycle oily wastewater,reaching resource strategy of"treating waste with waste".The main research contents and conclusions of this paper are as follows:（1）The sintering aids used in the oxide doping method were selected.The effect of preparation conditions on the structure and properties of ceramic membranes was systematically studied,and the growth process and mechanism of mullite whiskers were revealed.The results show that Mo O₃ is more suitable for preparation of fly ash-based whisker mullite ceramic membrane.The addition of Mo O₃ could improve the apparent porosity and bending strength of the ceramic membrane,but excessive molten liquid phase produces more glassy phases.The increase of sintering temperature promotes the anisotropic growth of mullite in liquid phase.When the temperature is higher than 1300°C,the densification of the ceramic membrane is enhanced,and the whiskers bend,coarsen and bond to each other.The ball milling of fly ash is beneficial to formation of small and uniform pore channels.The optimum conditions for preparation of fly ash-based mullite ceramic membrane by oxide doping method are as follows:average particle size of 6.36μm,Mo O₃addition of 5 wt.%,sintering temperature of 1300℃.Under the conditions,the apparent porosity of ceramic membrane is as high as 46.04%,the bending strength is 32.11 MPa,and the average pore size is 0.190μm.In addition,the formation of mullite is controlled by the mechanism of"dissolution-precipitation reaction",and the growth of whiskers conforms to the L-S growth mechanism.（2）Based on content of impurity components in fly ash and solid solution amount in mullite,the effect and mechanism of Ca O,Fe₂O₃ and Ti O₂ on structure and properties of ceramic membrane were systematically studied.The results show that the maximum solid solution amounts of Ca O,Fe₂O₃ and Ti O₂ in mullite are 2 wt.%,10 wt.%and 4 wt.%,respectively.The excessive Ca O reacts with mullite to form anorthite,which is not conducive to apparent porosity and bending strength of ceramic membrane.The excessive Fe₂O₃ and Ti O₂ can exist in the form of hematite and rutile,which have little effect on apparent porosity and bending strength of ceramic membrane.Moreover,Ca²⁺,Fe³⁺and Ti⁴⁺enter the mullite lattice in different ways.Ca²⁺enters the interstitial structure sites of mullite.Fe³⁺not only replaces Al³⁺in[Al O₆]of mullite,but also a small amount of Al³⁺in[Al O₄]is replaced.When Ti⁴⁺replaces Al³⁺in[Al O₆]of mullite,a part of Al³⁺in[Al O₄]replaces Si⁴⁺to compensate for excessive positive charge.（3）The performance and application of the prepared fly ash-based whisker mullite ceramic membrane in deep treatment of oily wastewater were investigated.The results show that the ceramic membrane（the apparent porosity of 46.04%,the bending strength of 32.11MPa,and the average pore size of 0.190μm）prepared under the optimum conditions exhibits excellent permeation flux and oil rejection when treating 250 mg·L^–1 of O/W emulsion wastewater.The stable permeation flux is 391.10 L·m^–2·L^–1,and the oil rejection is as high as 97%.Ceramic membranes have serious membrane fouling problem in separation process of O/W emulsion wastewater,and the membrane fouling is mainly controlled by cake filtration mechanism.After physical and chemical cleaning,the permeate flux recovery rate of ceramic membrane is 81.64%and 91.38%,respectively,which presents excellent regeneration performance.At the same time,as-prepared ceramic membrane sample exhibits strong acid and alkali resistance in solutions with wide pH value（pH=1～14）.