Experiment of Coal Leaching and Study of the Separation of Trace Elements

Vol. 74ACTA GEOLOGICA SINICAJune 2000Experiment of Coal Leaching and Study of theSeparation of Trace ElementsLIU GuijianDepartment of Earth and Space science, University of science andTechnology of china, No. 96 Jinzhai road, Hefei 230026, AnhuiAbstract Leaching is an important way of separation of trace elements from coal. Based on leachingexperiments, the content of trace elements separated out from coal is related to their existing state andcontent in coal, the duration of leaching process, temperature of leaching liquor and its acidity and basicityThe higher the temperature of leaching liquor and the longer the leaching time, the higher the contents ofseparated trace elements will be In the course of leaching, the ph values of leaching liquors change anddiffe rent trace elements are affected differently by the ph valuesKey words: Leaching experiment, trace elements, influence factors, separation of trace element1 Introductionelements during leaching and possible harms tothe environment in the futureLeaching is an important way for trace elementsto be separated out from coal and gangues. With 2 Leaching Experimentthe action of leaching some trace elements willdissolve completely or partly and are separated 2.1 Experiment preparationout from coal and gangues. For instance, affectedby rich dissociation oxygen in rainfall, oxidation collected from Coal 17 of the Taiyuan Formationf pyrite in coal occurs at first to produce in the Yanzhou mining area as the research object[Fe2(SO4)31 and vitriol. On the one hand, water Coal samples were ground into pieces of around Iliquors are strongly acidified to make carbonate mm in size and then mixed thoroughly. Sampleheavy metals and hydroxides dissolve; on the pieces smaller than 0. 25 mm were prepared forother hand, the rainfall is a direct oxidizer of such leaching. The coal core number is tMi and thetrace elements as Hg, Pb, Zn and Cu, thus ulti- gangue number is TG17. The contents of the tracemately making the heavy metals in coals move as elements in original coal and gangues are referredsulphate with the liquorsto TableThe leachincommonly used inLeaching liquor, pH-value acidometer, vesselsthe study of the separation of trace elements from diluted nitric acid and tested elements should alsocoal and gangues and is one of the best ways to be preparedgive good explanations to studied problemsIn order to research the existing state of trace 2.2 Testing methodslements in coals, Wang Yunquan (1994)and The tested elements include mainly Cu, Pb, ZiZhao Fenghua (1997) did leaching experiments As, F, Cl, Hg and Cr. The flame atomic absorptionfor bottom and flying ashes of the coal combus- method was used for testing Cu and Zn; the dition outcome. The aim of the leaching experiment pheny -carbonylic spectrophotometer for Cr; thedescribed in this paper is to study the mechanism graphite stove atomic absorption method for Pb,and influence factors of the separation of trace the ion-selecteTH中国煤化工F, the specCNMHGCoal Leaching and Separation of Trace elemeLiu guijianTablel Contents of trace elements in coal and gangues (ppmHPb2.310.984.3910.098.123815.310.67124.20232.55Table 2 Contents of trace elements separated out from coal and gangues (ppmLeaching testSampleCZnFeratureTMI0.561.421.60.230.0210.031TG170.681.871.121.511.380.260.0320.0252NormalTG171.961.983.240.70.080.030.132TG170.19TGI71.540.0340.06lTG170.530,890.0300.010TG172.723.134.874.00,420.0520.251SecondTG171.14l.620.690.0210.096ThirdTG10.710.580.70.060.080.0070.008trophotometer for As, and the ammonium thio- elements in the leaching liquor were measuredcyanate colourimetry for CITable2)Sixteen hours later, the sample was tested the2.3 Experiment proceduresame way as the last time util the third test wasEffects of time, temperature and acidity-basicity done. Table 2 gives detailed results of each teston the separation should be considered whendesigning leaching experiments, therefore TG17 3 Separating Factors of Trace Elementsas tested for different times different tempera-tures and different pH valuesThere are many factors influencing the separationFirstly, all vessels were dipped in diluted nitriof trace elements from coal. They are either n-acid for more than 24 hours and washed with dtrinsic ones, e.g. existing forms of elements intilled water. The analyser then put TMI7 and coal and gangues, or extrinsic ones, e.g. changesTG17, weighing 200 g for each, into a 2-litre of conditions during the leaching process. Bothglass bottle respectively, added I litre of distilled intrd extrinsic factors influence the moy.ater(pH=7), mixed them up, shake them con- ing and separating features of trace elementstinually for 8 hours with an motor-driven oscillator and lie it for 16 hours. A 500-mL pumping and 3.1 Existing state and concentration of traceleaching bottle was used to decompress the elements and their separation-outleaching liquor to produce clean liquor and con- Existing state of trace elements mainly controlcentrations of trace elements were determined for quantity of elements separated out from coalthe two samples (table 2)gangues. Their contents in coal also dominate theThe next step is to mix distilled water with separating concentration. Every element has twodiluted nitric acid so that pH=2.0, 4.0, 6.0 and.0. possible existing states: organic and inorganic. IfA new TG17 sample was taken to repeat the steps trace elements are combined in inorganic stateabove to determine the contents of trace elements they are hardly separated out, but if they are richin the leaching liquor (Table 2)in coal and gangue, then they will have relativelyAt this step, TG17 sample was used again. It high concentration of separation. For example, Fwas heated up after mixed with distilled water of and Cl exist mainly in inorganic state and haveoH=7 while shaking, the temperature being kept very high contents in coal and gangues, thus aaround 80C for 8 hours; then contents of trace relatively larghe separated中国煤化工CNMHG388Vol. 74 No, AACTA GEOLOGICA SINICAJune 2000out; while Pb exists in organic and inorganic state during leaching. The ph values of all the leachinth an equal probability, but it in organic usually liquors were measured after each test was finishedforms fairly steady crystal textures although the (table 3)concentration of pb in coal is up to hundreds ofFrom Table 3 one can see that the ph value ofPpm(10). However, the separated quantity of Pb the liquor reduces after leaching. The main reasonunder the normal temperature is still less than is that there are high contents of sulphides in the2x10. Because the content of As is low in coal newly mined coal gangues. Chemical reactionsand gangues, the separated quantity is usually less take place for sulphides in coal and gangues durthan 0. Ix10, but separated quantities of Cu, Pb, ing leaching, especially pyrite is oxidized andZn, Cl and F are relatively high under the same dissolved during leaching to produce part ofcondition because of their high contents in coal vitriol to increase the H ion concentration andand gangues (huang et al., 2000)reduce the ph value after leaching. Other sul-phates, sulphides and inorganic sulphur in3.2 Acidity-basicity related to the separation gangues also acidify the leaching liquor. Theof trace elementshigher contents of sulphides in coal and gangues,There is a close relationship between acidity- the stronger acidification of the leaching liquorbasicity and the separated contents of trace ele- will be. The oxidization and dissolution processesments in coal and gangues. Under the normal of Fes, during leaching are as followstemperature, there are three cases. (1)With theFeS2+O,→Fe2O3+SO,SO2+H2O一H2SOncrease of pH value of leaching liquor, the sepaH2SO4→>SO2+Hrated quantity reduces. This is mainly for eleFrom the reaction above one can know thatments Cu, Zn and Cl. There is an obvious inverse compositions of coal and gangues have effects onproportion between the separated quantity and the the acidity-basicity of leaching liquors. In generalpH value.(2) With the increase of pH value of there are many sulphides and sulphates in gangues,leaching liquor, the separated quantity increase, which usually make the leaching liquor acidictoo. This is mainly for elements F and Cr. (3) during a long period of leaching of rainwater. ForWith the increase of ph value, the separated instance, there is a large amount of pyrite in rocksquantity changes little, that is, the ph value hasn the coal roof and floor and coal-seam stonelittle effect on the migration of trace elements. bands in the Taiyuan Group in the Y anzhou min-This is mainly for elements Pb and Hg. Although ing area. Acidified liquor flows into its sur-As is separated out very slightly, with the increase rounding space to acidify soils and does greatof its ph value the separation also has a reducing harm to the growth of plantsendency. This shows that acidity-basicity ofleaching liquor has a significant effect on theseparating-out rate oftrace elements duringthe course of migrationle 3 ph values of leaching liquors before and after leachingpH value after leachand separationTemperatureLeaching testThe leaching liquorsTMI6.82for the experiment werefirst confected artifi-TG171.75NormalFirst3,38cially for proper acTG17basicityChemicalFirstreactions with coal orTGI80Secondld chacidity-basicity中国煤化工CNMHGCoal Leaching and Separation of Trace ElemenLiu guijian3. 3 Temperature related to the separation of 3. 4 Time related to the separation of tracetrace elementselementsTemperature has an effect on the activity of tex- Time is also one of the significant factors influiral bodies of molecules and ions that constitute encing the separation of trace elements. Manymatter. The energy and activity of molecules of trace elements in coal and gangues will belots of inorganic compounds or adducts, organic solved and separated out from coal and ganguescompounds will increase under relatively high after some chemical reactions during long-termtemperatures, and the molecules will be reorgan- affection by water and other media, though theyrace elemenIts usually dissociate when monot easy to be separated outlecules have changes. Trace elements are usually Three leaching tests were carried out with thesolved and separated out in the midst of water same sample (tG17) at 80C. The total separateddium. The concentration of trace elements quantity of all three leaching tests is far higherseparated out under normal temperatures is less than that of the first test under the same conditionthan that as heated up to 80C (Table 4). Elements (Table 5)of Cu, Pb, Zn, Cl and F are separated out remar k- It is clear in Table 5 that the separated quantityably and the separation concentration of As, Hg increases with time at the same temperature andand Cr is comparatively high under a heating-up that the total separated quantity of all threecondition although they are separated out insig- tests>the total quantity of the first and secondnificantly in general(Zhang et al 1999)tests>the separated quantity of the first test. HowTable 4 shows the ratio of separating-out quan- ever, if time lasts beyond a certain limit, the sepatities of TG17 sample under normal temperature rated quantity reduces greatly. For example, theand as they are heated up to 80Cseparated quantity of Zn is 4.87x10 during theThe separated quantity of As when heated is first 24 hours; 1. 1310- for the second 24 hours10.04 times that under the normal temperature and only 0.58x10- for the third 24 hours. The(Table 4); Cu and Zn are the next, whose separat- first separated quantity is 8.4 times that of theed quantities are 4 and 4.35 times that under the third one. In the case of As, the separated quantitnormal temperature respectively. The separated of the first time is 31. 4 times that of the third timequantity of any one of the trace elements under a 26.3 times that of the first time for F. Trace eleheating-up condition is 1+x(x is a positive num- ments will not be separated out any more withber )times that under the normal temperature. This time further increases, that is, the separation peakshows that temperature has significant effect on of trace elements appears mainly in the primarythe separated quantity of trace elementsperiodTable 4 Ratios of separated contents under normal and heating-up temperatureconditionsContent l: content 21.621.63otes: Content I for heating-up condition; Content 2 for the normal temperatureTable 5 Total separated contents and corresponding ratios of different leaching tests24h4.874.021.580.420.0520.253,864.750.570.0730.34748hB: A1.421.52441.3672h1.681.66otes: A for the content of first test: B for total content of first and second tests: C for total content of all three tests中国煤化工CNMHGVol. 74 NoACTA GEOLOGICA SINICAJune 2000chemistry of Trace Elements in Coal in Yanzhou4 ConclusionsMining-Area(Ph D dissertation). China University ofMining and Technology (in Chinese)Liu Guijian, Wang Guiliang and zhang Wei, 1999. StudThe above experiment shows that the separatedon the Environmental Geochemistry of Trace Elementsquantity of trace elements is not only related toin Coal. Xuzhou: China University of Mining andtheir existing state and concentration, but also inTechnology Press, 58-92(in Chinese)of cl and f in coalacidity-basicity of the leaching liquor has differCoal Geology and Exploration. 24(1):9-11 (in Cht effects on different trace elthe ph value of the leaching liquor changes dur-Wang Yunquan and Ren Deyi, 1996. Study on leachingexperiment of trace elements in coal combustion res it sci(1): 16-19(in Chinese)pile up on the ground for a long period of time, so Zhang Junying, Ren Deyi, Xu Dewei et al.,1999.Thetrace elements will be separated out and enter intodistribution of mercury in major associated mineralsthe air-water-soil environmental system underfrom coal beds in southwestern Guizhou. Geologicalcomplex conditions especially in acid-rain areasReview, 45(5): 539-542(in Chinese with English abstract)This will affect the atmosphere quality, deterio-Zhao Fenghua, 1997. Study on the mechanism of disrate waters, destroy the function of soils, blocktributions and occurrences of hazardous minor andnormal growth of plants, and finally influence thetrace elements in coals and leaching experiments oflife and existing environment of human beings. Socoal combustion residues(Ph D dissertation ). Chinathe coal industry should pay more attention to theUniversity of Mining and Technology (in Chinese)comprehensive utilization of gangues and theimprovement of coal-cleaning technology. This is About the authoralso the necessity of the environmental develop-Liu Guijian Male: born in 1966; graduatedmentfrom the department of Cola geology, HuainanMCollege in 1988 and awarded M.s. at nManuscript received Jan. 2000vironmental geology in 1966. He obtained hisedited by Liu Xinzhu Ph. D at China University of Mining and Technology in 1999. Now Dr Liu works on environmen-Referencestal geochemistry in the postdoctoral station of theHuang Wenhui, Y ang Qi, Tang Dazhen et al., 2000. RareDepartment of Earth and Space Science, Univer-earth element geochemistry of Late Palaeozoic coals in sity of Science and Technology of China. Tel: 86NorthChina.ActaGeologicaSinica(Eng.ed.),74(1):551-3603714,3607091,E-mail:Igj@ustc.edu.cn7483Liu guijian, 1999. Study on the Environmental geo中国煤化工CNMHG