Migration And Removal Laws Of Trace Elements In Coals During Coal Washing Process In Guizhou Province

To study the migration and removal laws of trace elements in coal during coal cleaning process in Guizhou Province,four coal preparation plants named WJZ,TC,ZZC,LYS were selected as the research objects,and the migration and removal laws of trace elements from coal into cleaning products and water were studied systematically and comprehensively via combination of sampling in the coal cleaning plants and conducting bench-scale experiments,including screening tests,float-andsink tests,flotation experiments and static release tests.The resukts of proximate analysis shown that the WJZ coal is high-ash and highsulfur l/3 coking coal,TC coal is high-ash and low-sulfur fat coal,ZZC coal is mediumash and low-sulfur fat coal,LYS coal is high-ash and low-sulfur l/3 coking coal.XRF and XRD analysis indicated that the main chemical compositions of the coal samples are Al,Si,Fe,Ca,and quartz,kaolinite are the main minerals in the coal samples.Modes of occurrence of trace elements in coal were studied via correlation analysis,screening tests and float-and-sink tests.Correlation analysis shown that there are significant correlations between the content of S and Hg,which also means that Hg may mainly associated with sulfur-containing composition in coal;the contents of Be and Ad in TC,ZZC,LYS coal samples also have a significant negative correlation which indicated that Be has a strong inorganic affinity.Besides,there are also significant correlations between the concentration of Pb and Cr.According to the results of screening test,trace elements in coal tend to enriched in-0.038 mm size fraction or 0.5-0.25 mm size fraction,and trace elements are mainly distributed in +0.5mm size fractions.Float-and-sink tests demonstrated that trace elements in coal are mainly enriched and distributed in high density fractions while the change tendencies of the content of Hg,Cr in different density fractions of TC coal abnormal.More than 50% of the total trace elements in coal can be removed when the separation density is 1.8.g/cm3 for pre-removal of gangue.Detection results of trace elements in cleaning products samples in the four coal cleaning plants suggested that trace elements have a tendency to enriched in gangue and the magnetic separation tailings of fine gangue.The migration laws of trace elements in coal during cleaning process are significantly affected by the modes of occurrence of trace elements in coal.Enrichement of trace elements in middlings was found in ZZC plants,which is definitively because of the low distribution of trace elements in +1.8 g/cm3 density fraction while higher distribution in middle density fractions.Detection results of the water samples collected from coal cleaning plants shown that there are release of the As,Se,Pb from coal into water during coal cleaning process and Be,Hg in coal are relatively stable.Dosages of collector and frother greatly influence the removal of trace elements during coal flotation.The yield of clean coal increase with the increase of the dosages of collector and frother while the relative removal rate of trace elements(except Cr)decrease.The clean coal yields in the 0.25 th,and 2rd min during the flotation are relatively high,which also cause a large amount of trace elements float up.Kinetic analysis demonstrated that the classical first order flotation dynamic model fit well with the flotation process of trace elements in coal.For the WJZ coal,Hg float up fastest with Cr float up lowest,as for the TC coal,As has the highest float speed while Cr hold the lowest.Contact time and pH value have a remarkable effect on the release of trace elements from coal into water during the static release tests,a strong acid/ alkaline environment is beneficial to the release of trace elements in coal.Dynamic analysis indicated that the pseudo-second-order model fit the Se release process best,which also suggest that the release process is mainly controlled by chemical reaction.The Se release enthalpy changes are 18.314?3.533kJ/mol when the pH value of solution are 7,11,respectively,that means the release process is a endothermic reaction.