Dynamic Leaching Test Of The Jurassic Coal Seam And Roof At Xiaojihan Coal Mine

As an important coal resource and coal chemical industry base in China,Yulin City of Shaanxi Province is one of the key development areas of coal industry.However,the hydrogeological conditions in the western region are complex,water resources are scarce,and the ecological environment is fragile.Coal mining has seriously damaged the ecological environment of the mining area.Therefore,it is particularly important to study the dynamic changes of hydrogeochemical characteristics in coal mining areas.This paper takes Xiaojihan Coal Mine as an example to carry out dynamic leaching test.The test samples were taken from the 2² coal roof of Xiaojihan Coal Mine.First,the mineral composition and chemical composition of the samples were determined by XRF and XRD.Then,the hydrogeochemical characteristics of the Jurassic coal roof of Xiaojihan Coal Mine were studied by using standard Humidity Cell Test dynamic leaching device for 13 weeks.Main results and conclusions:?1?XRD and XRF measurements show that the main mineral components of roof core samples include plagioclase,quartz,albite,potassium feldspar,gypsum,calcite and clay minerals.The main chemical compositions are SiO₂,Al₂O₃ and Fe₂O₃.The main mineral components of coal samples include quartz,sulphide,silicon and a large number of organic matter.The main chemical components are SiO₂,S and Al₂O₃.?2?Physicochemical characteristics of leachate:?1?the initial pH value of roof leachate is less than 6,and after week3,the pH value fluctuates around 7;the pH value of direct roof leachate gradually rises from 3.51 to 6.85;the pH value of coal leachate fluctuates between 3 and 7,with an average pH value of 5.26;and?2?the average EH value of roof leachate and coal leachate are in the range of 200-220 mv,and the maximum value of mixed roof leachate exceeds 800mv,with an average value.The DO value of roof and leachate of coal sample increased gradually in the range of5-10 mg/L,and the TDS of roof and leachate of coal sample decreased gradually.The TDS values of direct roof leachate were all greater than 1000 mg/L,and the TDS values of coal leachate finally fluctuated around 200 mg/L.?3?Variation of leachate water quality types:?1?The water quality types of No.1,2 and 4 leachate samples are all SO₄-Ca.Na is transformed from SO₄-Ca.Mg to SO₄-HCO₃-Ca.Mg?2?No.3,No.5 direct roof samples,No.6 mixed samples and No.7 coal samples are all SO₄-Ca.?4?Analysis of the change of metal cation concentration in leachate:?1?Na concentration in roof leachate gradually decreased from 111.93 mg/L to less than 10mg/L,Na concentration in coal leachate averaged 8.35 mg/L;?2?K concentration in roof leachate gradually decreased from 10.22 mg/L to about 1 mg/L,K concentration in coal leachate averaged 0.12 mg/L;?3?Ca concentration in roof leachate gradually decreased,while that in direct roof leachate was 0.12 mg/L.Ca concentration in leachate is stable at about 500 mg/L,Ca concentration in leachate of coal sample fluctuates below 100 mg/L;?4?Mg concentration in roof leachate gradually decreases from 41.69 mg/L to 5-10 mg/L;Mg concentration in direct roof leachate gradually decreases from 23.59 mg/L to about 2 mg/L;Mg concentration in leachate of coal sample is low,with an average of 0.61 mg/L;?5?Al concentration in roof leachate and coal sample leachate is small,after the test.The period was less than 1 mg/L.?5?By analyzing the dynamic changes of the concentration of conventional metal cations in leachate,it is concluded that Na in leachate mainly comes from the dissolution of albite and zeolite in core samples;K in leachate mainly comes from the dissolution of potassium feldspar in samples;the release of Fe in leachate is related to the oxidation of sulfides;Ca is the main cation in leachate,and Ca in leachate mainly comes from carbonate.The dissolution of similar sediments and gypsum-bearing sediments,as well as the weathering dissolution of calcium-bearing minerals;Mg in leachate mainly comes from Magnesium-bearing carbonate deposits,in addition,it also comes from the weathering dissolution of Magnesium-bearing minerals;Al ions are relatively high in the samples,but less released.?6?Through the analysis of the dynamic change of trace element concentration in leachate,it is concluded that the concentrations of trace elements Ni,Pb,Mn and Zn in groundwater exceed the Hygienic Standard for Drinking Water?GB5749-2006?during the weathering of Xiaoji Khan seam roof.Ba,Co,Cu,Mo,Cr,Sr and Ti all appeared dissolution phenomenon,but it did not endanger the general health of human body.It is speculated that groundwater pollution may exist in the process of water-filling and weathering of the roof of Xiaoji Khan coal seam.?7?Through the analysis of correlation between ions,it is concluded that Al in leachate of roof core samples mainly comes from the dissolution of aluminosilicate,Ti in leachate mainly comes from the dissolution of rutile in aluminosilicate,Ca in leachate of roof core samples mainly comes from the dissolution of gypsum and calcite,and Fe is released with the dissolution of pyrite and other minerals in roof core samples.Ions and Mn also leached out gradually.It can be inferred that Mn in roof core mainly occurs in pyrite and other minerals.From the above conclusions,it can be predicted that during the weathering period of Xiaoji Khan seam roof,the mine water environment was weak alkaline and strongly oxidized in the later stage;Ca and SO42-were the main cations and anions in groundwater respectively;trace elements Ni,Pb,Zn and Mn exceeded the Sanitary Standard for Drinking Water?GB5749-2006?,which may cause groundwater pollution problems;and Xiaoji Khan Coal Mine.Well water is formed after roof leaching;Na and SO42 pollution in mine water of Xiaojihan Coal Mine mainly comes from roof.