Leaching Characteristics And Environmental Impact Of Harmful Elements In Fushun Oil Shale And Ash

During the accumulation of oil shale and its products in the open air,a large number of harmful elements will be leached out and transfer to the environment due to natural rainfall,resulting in pollution.This paper takes Fushun oil shale and its pyrolysis products as the research object.The leaching characteristics and environmental impact of harmful elements Cr,Mn,Co,Ni,Cu,Mo,Cd,Pb and Sr under different leaching conditions are studied by simulating natural rainfall.Firstly,the basic characteristics of oil shale and its products,the background content of harmful elements and the occurrence state are analyzed by means of precision instruments,microwave digestion and chemical step-by-step method.On this basis,through the design of orthogonal column leaching test,the influence of pH,flow velocity and particle size of leachate on the leaching characteristics of elements in ash are analyzed.And the leaching characteristics of elements in oil shale under different pH are analyzed by simple comparison method.Finally,the environmental effects of elements in oil shale and ash are evaluated.The results show that after pyrolysis of oil shale,Aad,FCad and Sad increase.Cr,Mn,Ni,Sr,Mo and Cd are enriched in the products of pyrolysis.Mn is the highest content in oil shale and pyrolysis products,Cd is the lowest.The content order of elements in oil shale and semi coke is as follows:Mn>Cu>Cr>Ni>Sr>Co>Pb>Mo>Cd,and the order in each particle size ash is:Mn>Cu>Cr>Ni>Sr>Pb>Co>Mo>Cd.Most of the elements in the oil shale exist in the residue state and the sulfide combination state.After low temperature pyrolysis,the residual content of most elements increase.After high temperature pyrolysis,the content of sulfide binding state of most elements decrease,but it still exist mainly in residue state or sulfide binding state with weak activity,and the occurrence state of the elements in each particle size ash residue is equivalent.The potential leaching rate of Sr,Mo,Cd,Co in oil shale and Sr,Cu,Co in ash are all over 10%,which have strong activity.The total leaching rate of Sr is the highest and that of Pb is the lowest.Mn,Co,Ni and Sr have similar leaching characteristics,and they all have higher leaching rate under the condition of neutral low flow rate.Due to the resolution and dissolution of the potential leaching state,most of the elements except Pb peak at the initial stage of leaching,and then decrease to become stable with the extension of time.The order of the external leaching conditions affecting the leaching of Cr and Pb in the ash is:pH>d>v;the order of affecting leaching of Mn,Co and Cu is:pH>v>d;the order of affecting leaching of Ni,Sr and Mo is:v>pH>d;the order of affecting leaching of Cd is:d>v>pH.The total leaching rate of Mo is the highest and that of Co is the lowest in oil shale.Cr,Mn,Cu,Sr,Cd,and Pb are more likely to leach out under acidic leachate,Ni is more leachable under neutral leachate,Co and Mo are more leachable under alkaline leachate.The leaching rate of most elements increase and then decrease with time.Under the same leaching conditions,the leaching rate of Mn,Co,Ni and Sr in the ash is higher,while that of Mo and Cd in the oil shale is higher.The leaching rates of most elements in oil shale and ash are similar,both tend to stabilize more quickly under acidic and neutral conditions,but the leaching of elements in oil shale is more volatile.The comparison between the total leaching concentration of elements and the class III standard of groundwater environmental quality shows that the harmful elements in oil shale have little impact on the environment.The maximum total leaching concentration of Mn in ash is more than 20 times of the standard value,and the maximum total leaching concentration of Sr also reaches 1.83 mg/L.It is necessary to pay attention to the possible environmental pollution caused by ash accumulation.