PH-dependent Leaching Characteristics And Controlling Mechanism Of Elements From Red Mud

As an industrial by-product of alumina smelting process from bauxite ore,red mud is one of large industrial solid waste in China.On the one hand,due to its large amount of alkaline substances and potential environmental pollutants such as heavy metals and fluorides,the increasing stockpiles will inevitably lead to more and more serious potential environmental risks.On the other hand,red mud is a potential resources of engineering materials,environmental application materials and secondary resources.At present,there is a certain laboratory research basis for the application of red mud resources.However,few studies focus on the leaching characteristics of elements and the leaching control mechanism from red mud,as well as the morphology distribution of trace elements in the red mud.In view of this situation,this research takes the Bayer process and the Bayer-Sintering combined red mud as study subjects,collecting red mud samples five bauxite smelters from Pingguo County and Jingxi County in Guangxi(marked as GX-AB,GX-B-B,Bayer process),Xingyang City in Henan Province(marked as HN-A-C,combined process)and from Zibo City in Shandong Province(marked as SD-AB,SD-B-B,Bayer Process).Research on issues as follows:toxic leaching of potential environmental pollutants elements from red mud using3 types of standard leaching procedure;trace elements morphology distribution in red mud;the leaching characteristics of elements based on p H changes and the relative leaching control mechanism.The achievements would provide theoretical basis for the safe treatment and resource utilization of red mud.Firstly,the paper adopts China's solid waste toxicity leaching method(HJ/T 299-2007)to leach red mud.The results show that:Concentration of main elements Al,Fe,Na and Ti,trace heavy metals As,Cr,Mo,and Se,anion F^-are found exceed the standard of surface water and groundwater quality in China(?-type)from several times to hundreds of times,which has potential environmental risks.Secondly,improved Tessier step-by-step extraction leaching experiment is used to analyze the morphology distribution of valuable trace elements such as rare earth elements and trace valuable metals in red mud.The results show that:(1)Rare earth elements Sc and Y have higher potential extraction from GX and HN red mud,the ratios are 44.86-50.70%and56.35-62.71%,respectively.Lanthanide rare earth elements(except Pm)are enrichment in red mud.The highest content reached 1340.8?g/g(GX-B-B),and the leaching ratio of the five red mud samples is between 18.84-27.11%.(2)Among other valuable trace metals,Ga has a higher leaching ratio in GX and HN red mud(28.14-35.32%).Both of content and leachable ratio of Li in HN red mud(287?g/g,61.43%)are much higher than that in GX and SD red mud.Nb content is relatively higher in GX red mud(277-395?g/g),as well as leachable ratio(4.78-7.68%).V content is high in red mud(353-600?g/g),the maximum potential leaching amount is from GX red mud,and the leaching ratio is as high as 51.7-64.7%.Thirdly,this study use EPA Method 1313 to determine the leaching characteristics of main elements(Al,Ca,Fe,Si),trace elements(Mg,heavy metals,rare earth elements and valuable metals),and toxic anions(Cl^-and F^-)by batch leaching experiment through varied p H values.The results show that:(1)A rapid drop of p H existed right after the acid was added,and a plateau occurred at approximately p H=6.(2)Al,Si,Cr,Cu,Pb,Ga and V show a V-shape amphoteric leaching pattern.Ca,Mg and Ba show a cationic leaching pattern.Mo is oxyanionic leaching pattern.While Fe,Si,Mn and Li show a relatively less obvious leaching pattern.(3)Except for the maximum leaching amount of As,Ga and V in both extreme acid and alkaline conditions,the maximum leaching amount of other elements occur at p H=2.(4)Under strong acidic conditions,the heavy metal elements As,Cr,Pb and Cu in combined red mud(HN-A-C)are more easily leached than that in Bayer red mud,the maximum leaching rates are 33.7%,50.9%,54.5%and 50.4%,respectively.Finally,the work uses Visual MINTEQ model to simulate the leaching control mechanism of the main elements(Al,Ca,Fe),and trace elements(As,Cr,Cu,Pb,Mg,Ba and Mn)in red mud.The results show that:(1)Fe,As,Cr,Cu,Pb and Mn are redox-sensitive elements,Al,Ca,Mg and Ba are less or non redox-sensitive elements.(2)The leaching of Al,Fe,Cr,Cu,Pb and Mn are mainly controlled by the dissolution and precipitation of their(hydr)oxide mineral/solids.The leaching of Ca,Mg and Ba is mainly controlled by the dissolution and precipitation of their sulfate/carbonate mineral/solids.(3)Supersaturation leaching of Fe³⁺is caused by the increase of the dissolution equilibrium constant due to the decrease of Fe contained mineral particle size.Supersaturated leaching of Ca and Mg under alkaline conditions(p H>12)are due to the sulfate compound transfer to form a carbonic acid compound in such p H.(4)The leaching control mechanism of As involves the dissolution of arsenate mineral/solids and the adsorption on mineral surface.The research work shows that:(1)The main elements Al,Fe,Na and Ti,trace heavy metals As,Cr,Mo and Se,anion F^-have potential environmental risks,further measures need to be taken during the stockpile.(2)Rare earth elements(except Pm)and trace valuable metal elements such as Ga,Li and V in red mud have potential recovery value.(3)p H value is an important factor affecting the leaching of elements in red mud.Elements show different leaching patterns with p H changing due to their own characteristics.(4)The dissolution-precipitation control mechanism of elements in red mud are mostly controlled by their(hydr)oxide mineral/solids,excluding Ca,Mg,and Ba are controlled by their sulfate/carbonate mineral/solids.