Study of Enhanced Fine Coal De-sulphurization and De-ashing by Ultrasonic Flotation

Journal of China University of Mining TechnologyVol 17 No. 3Availableonlineatwww.sciencedirect.comSCIENCEDIRECT.J China Univ mining& Technol 2007, 17(3): 0358-0362Study of Enhanced Fine CoalDe-sulphurization and De-ashing byUltrasonic flotationKANG Wen-ze. XUN Hai-xin, Chen Jun-taoSchool of Resource and Environment, Heilongjiang Institute of Science and Technology, Harbin, Heilongjiang 150027, ChinaAbstract: The feasibility of using ultrasound to enhance the performance of de-sulphurization and de-ashing duringslime flotation was investigated. The Setaram C80 calorimeter, the contact angle gauge DCAT2I and an electrophoresisapparatus were used to study the surface nature of coal, pyrite and refuse before and after ultrasonic conditioning. Theield, ash and sulfur contents of equally sized coal slimes were also measured before and after ultrasonic conditioningThe results show that ultrasonic conditioning can drive the separation of pyrite and refuse from coal. After ultrasonicconditioning the hydrophobicity of coal and hydrophilicity of pyrite and refuse increase. The perfect index of flotationthe perfect index of de-sulphurization and the percentage of de-sulphurization increase by 22.51%, 25.36% and 2.49%respectively. This study shows that ultrasonic conditioning can enhance the performance of de-sulphurization andde-ashing of coal flotation methodsey words: ultrasonic; separating; hydrophobicity; de-sulphurization; de-ashingCLC number: TD 9239: TB 5591 Introductiontill unclear so it investigation is of great significandIn thisCoal is the major source of energy in China and the minerals by means of ultrasonic excitation of pulprational processing of it is the basis for its safe and may be one possible route to improve flotationefficient utilization. But the high percentage of sulfur selectivity and efticiencyand dust in coal hinders its utilization, in particularsulfur severely pollutes the environment.2 Experimentalstudies focusing on de-sulphurization and de-ashing 2.1 Mineral sample and agentare being carried out in many countries. Normalgravity separation can effectively de-sulphurize coalThe coal was a sample of No 10 layer lump coalhaving larger pyrite particles (>0.5 mm). But for fine from the Xiazhuang Coal Mine, Shandong Zibohigh-sulfur coal particles (<0.5 mm)ideal de- Bureau of Mines. The coal sample is a high-sulfursulphurization can not be achieved using ordinary lean coal. The experimental sample was obtainedflotation. Hence methods to enhance de-sulphuriza- after the lump coal had been crushed. The sampletion and de-ashing through slime flotation are a majorlysis was Aad=9. 36%(ash content), S,d =3.82%aspect of research carried out in various countries-71.(total sulfur content), Sp=2.38%(pyrite sulfur),Among numerous methods for enhancing de- So=0.90%(organic sulfur content)and Ss=0.54%sulphurization and de-ashing, ultrasonic technology is (sulfate sulfur content). A pure sample of pyrite wasideal because it is a clean and mild method that can hand selected from the sample. The pyrite isbe applied at normal atmospheric temperatures and paragenetic with the coal and has a size range frompressures. Many scholars have studied this 30 tonethod-l. The mechanism by which ultrasonicTo中国煤化工 were crushed andflotation enhances de-sulphurization and de-ashing is grourCNMHGesize. They wereReceived 05 February 2007; accepted 15 May 2007Projects 50274036 supported by the National Natural Science Foundation of China and 1055G039 by the Mainstay Teacher Originality Innovation PlanCorrespondingauthorTel+86-451-88036448:E-mailaddresskwz010@163.comKANG Wen-ze et alStudy of Enhanced Fine Coal De-sulphurization and De-ashing bythen put into brown reagent bottles and kepthermetically sealed until needed. Kerosene was used3as the collector and secondary octyl alcohol served asfrother during the separation experiments. Theproportion of collector to frother was 3: 12.2 Slime ultrasonic-conditioning device andprincipal analytical instrumentsFig. 1hown in Fig. 1. The principal analytical instruments4. Support; 5. Ultrasonic power sourceused in the experiments are listed in Table 1Table 1 Test instrumentsModelMeasuring accuracyFrance Setaram CompanyContact angleDCAT21±0.0lmgGermen Physical Data CompanySurface potential measuringJS94GShanghai Zhongchen Technology Company3 Results and Discussionultrasonic energy at a frequency of 20 kHz for 10 minwith a power of 200 w. The sample was then wet3.1 Ultrasonic conditioning drives the separation screened. In a similar way, 100 g of noof pyrite and refuse from coalconditioned coal was wet screened as aA sample of the Zibo coal weighing 100 g was results of the screening are listed in Tablemixed with 300 mL of water and processed withTable 2 Coal sample screening results before and after ultrasonic conditioningBefore ultrasonic conditioningAfter ultrasonic conditioningSulfur0.5000.2500.1250018.525.108990.63007500451272-6.15Total100.As can be seen from Table 2, after ultrasonic separation of pyrite and refuse from coal andconditioning the particle yields of grades +0.5 mm about conditions that assist de-sulphurizationand 0.5-0.25 mm are obviously lower than before the de-ashing by flotation of slimeultrasonic conditioning. The sulfur and ash content ofthese sized coals are higher than before ultrasonic3.2 Ultrasonic conditioning can change the naturetreatment. Conversely the particle yields for gradesof the mineral surface0.125-0.075 mm and 0.075-0.045 mm are higher 3.2.1 Effects of ultrasonic conditioning on contactafter conditioning than before while the sulfur andangles of coal and pyash content of these smaller sized coal particles isCoal and pyrite were ground to under 0. 2 mm inlower after treatment. There is also a change in the size and then mixed with 70 mL water. This pulp wasgrade yields, the sulfur content and the ash contentprocessed for 3 min at an ultrasonic frequency of 20for the size range 0. 25-0. 125 mm. The grade yields,kHz with a power of 200 w. The processed pulp wasthe sulfur content and the ash content of the -0.0dewatered and dried in the air. The contact angles ofmm fraction all increase. These changes show that the coal and the pyrite particles were measured by aultrasonic treatment has two effects. One is the contact angle gauge DCAT21. The test results arecrushing effect on the particles, which reduces the listed in Table 3rough grade. The other is that ultrasonic conditioningcan drive separation of pyrite and refuse from theTable 3 Contact angle of coal and pyriteoal. Some of the separated pyrite and refuse become中国煤化工rite ()ery fine and fall into the -0.045 mm sized fraction.making yield, sulfur content and ash content of thisCNMHGfraction increaseHence ultrasoniconing can assist theournal of China University of Mining TechnologyVoL.17No.3From the data shown in Table 3 it appears that thecontact angle of unconditioned slime is 87. 32 andTable 5 Wetting heat of pyrite or refuse by water orkerosene before and after ultrasonic conditioning /g)that the average value after ultrasonic conditioning isRefuse89.64. Ultrasonic conditioning increased the contactPyriteWetting heatangle by 2.32. The contact angle of unconditionedconditioned Conditioned NonConditionedpyrite was 49.63 and theaverage valueultrasonic conditioning was 39. 43. Ultrasonic1.3701189783.98077.7010.2%. Hence ultrasonic conditioning increases the Wetting eat with 3.6235230352903626628hydrophobicity of slime and decreases hydropho-bicity of pyrite.As shown in Table 4, the wetting heat for each3.2.2 Effects of ultrasonic conditioning on wetting grade of coal wetted by water decreases by 17.72%heats of mineral with water keroseneafter ultrasonic conditioning. Conversely the wettingWetting heat is another criterion for measuring theheat of coal wetted by kerosene increases by 45.85%wettability of mineral surfaces. Changes in the heat of after conditioning. This confirms that the coalwetting show to what extent the liquid and mineral particles hydrophilicity decreases after conditioningaceor that the oleophilicity increases, a result inSamples were selected and processing under theagreement with the wetting angle experimentssame conditions as in section 3. 2. 1. because the sizeportedistribution of the slime changes after ultrasonic Ultrasonic conditioning increases the wetingconditioning the coal samples before and afterith water but decrultrasonic conditioning were wet screened into five of wetting of pyrite, or refuse, by kerosene. This alsogrades:0.5-0.25 mm,0.25-0. 125 mm, 0.125-0.075 agrees with the contact angle measurements/4.mm,0.075-0045 mm and -0.045 mm. This helps Hence ultrasonic conditioning increases the hydro-isolate the effect of change in particles size on phobicity of slime i. e, the oleophilicity of the slimemeasured wetting heat. Comparing the wetting heats increasesof equal sized coal particles with the liquid, beforeAs shown in Table 5, the wetting heat of pyrite, orand after ultrasonic conditioning, shows the change in refuse, with water increases by 38.52% and 93.66%,the coal particle's surface accurately. The heats of respectively. after ultrasonic conditioning. Thewetting for different grades of coal wetted with water wetting heats with kerosene decrease by 36.43%andor kerosene, both before and after ultrasonic 8.92%, respectively. These results show thatof pyrite wetted with water or kerosene are listed in ultrasonic conditioning causes pyrite and refuse toTable 5. Note that the size of the pyrite particles is become more hydrophilic and oil-repellent, which0.2 mm and that of refuse is -0.045 mmhelps improve de-sulphurization and de-ashing duringslime flotationTable 4 Wetting heat of different size coal with water or 3.2.3 Effects of ultrasonic conditioning on the electrokerosene before and after ultrasonic conditioning o/g kinetic potentials of mineral surfaceWetting heat of coaletting heat of coalCoal particles and pyrite samples of the Zibo mineconditioned Conditioned conditioned Conditionedwere ground to a size smaller than 0.075 mm or 0.0455000.2500.86930.2500.1251.723.4267samples were processed with the ultrasonic treatment,0.1250075147180075-00454.55784.0326following the procedure of section 3. 2. 1. The s3.89525.7719electric potentials were measured. The results are261542.15193.33024.8572shown in Fig. 220中国煤化工。CNMHG(a)Zibo coal surface electro kinetic potentialsurtace electro kinetic potentialig 2 Surface electro kinetic potential for different mineral before and after ultrasonic conditioningKANG Wen-ze et alStudy of Enhanced Fine Coal De-sulphurization and De-ashing byAs can be seen from Fig. 2, the change in electro averages of three individual trials)kinetic potentials of the surface of Zibo coal isTable 6 Results of flotation testing of slime (%)biggest when the pH value is 24. The change of thepyrite surface is biggest when the pH value is 5-8Evaluating indicatorSlime flotation is usually carried out using neutralpulps, so there is a practical significance toconditionin39321134545856301359measurements made at a pH value of 7. The changes Non-conditioning 55.76 4.48 1.98 32.07 30.9471.10in surface potential of the coal and pyrite samplesDifference19.53-1.27064225125.36249before and after ultrasonic conditioning are shown inFig. 3.As shown in Table 6, the yields of clean coal afterultrasonic processing increased by 19.53%; the sulfurNon-conditonineand ash content of the clean coal decreased by 0.64%0a Conditioning 3 minand 1.27%; the perfect index of flotation, the perfectindex of de-sulphurization and the percentage ofde-sulphurization increased by 22.51%, 25.36% and2.49%, respectively. Flotation of slime processed byultrasonic energy not only increased the yield of cleanMineTal nanecoal but also showed increased selectivity comparedFig. 3 Change of surface electro kinetic potential ofto untreated samples. This was true when equivalentcoal and pyrite when pH value is 7amounts of flotation agent were used in the flotationHence ultrasonic conditioning can enhanceThe greater the absolute value of the negative the performance of de-sulphurization and delectric charge on a mineral surface the stronger theby coal flotation.hydration of that surface will be. As shown in Fig 3after ultrasonic conditioning for 3 min the absolute4 Conclusionsvalue of the electro kinetic potential of the Zibo coalsamples decreases from 24.18 mV to 23.27 mV(by1)After studying yields, sulfur and ash contents of3. 76%)while the absolute value of the potential of the same grade coal before and after ultrasonicthe pyrite increases from 24. 47 mV to 33. 28 mV(by conditioning it appears that ultrasonic conditioning36%). This suggests that hydration of the coal surface has a crushing effect on coal particles and canwill decrease, and hydration of the pyrite surface will enhance the separation of pyrite, and refuse, fromcrease, after ultrasonic conditioning. This helps coalelucidate the difference between coal and pyrite2) The heat of wetting measured on each grade ofsurfaces, which difference helps to improve de- coal using water as the wetting agent decreased bysulphurization and de-ashing during slime flotation. 17.72% after ultrasonic conditioning. Conversely, theThese observations are in agreement with the contact wetting heats using kerosene as the wetting agentangle and wetting heat results reported aboveased by 45.85%. This means that ultrconditioning causes coal to become more3.3 Test of de-sulphurization and de-ashing hydrophobic and oleophilic.Ultrasonic conditioningduring slime flotationincreased the wetting heat of pyrite, or refuse, byPulps with a solids content of 50 g/L were prepared water by 38.52%b, or 93.66%. However the wetting byand treated with ultrasonic conditioning(Fig. 1). The kerosene showed a decreased heat of 36.43% for theconditions of the ultrasonic treatment were the samepyrite, or 8.92% for the refuse, showing that theas in Section 3. 2. 1. The ultrasonic processed pulps surface of these substances becomes morewere cooled to room temperature and then treated by hydrophilic and oil-repellent after treatment.flotation. The samples and flotation agents are shown3) The absolute value of the electric charge on thein Section 2.1. The consumption of flotation agent coal surface decreased by 3. 76%, and that of pyritewas 1.3 kg/t of dried slime. The flotation device was increased by 36%, after ultrasonic treatment, whicha model XFG with a capacity flotation groove of 100 increased the potential difference between these twomL. The operational procedure for flotation followed types of surfacethe standard GB/T4757-2001. The perfect index of4)On one hand, ultrasonic conditioning can drivespecified in M omputed following the formula physical separation of pyrite and refuse from coal.Onflotation was180-1988 and the perfect index of the her hand ultrasonic conditioning can changede-sulphurization,and the actual percentage of the中国煤化工 articles. Ultrasonicde-sulphurization,was computed with the formula condspecified by MT/T623-1996. The results of the andCNMHG refuse For a 13flotation tests are shown in Table 6(These results are kg/t flotation agent to coal ratio, the perfect index ofoumal of China University of Mining TechnologyvoL 17 No. 3flotation, the perfect index of de-sulphurization and Acknowledgementthe percentage of de-sulphurization after ultrasonicFinancial support from the National Naturalprocessing increased by 22.51%, 25.36% and 2.49%0, Science Foundation of China(50274036)andrespectively. This study shows that ultrasonicconditioning can enhance the performance of Mainstay Teacher Originality Innovation Plande-sulphurization and de-ashing coal flotation Foundation of the Education Department ofmethodsHeilongjiang Province (1055G039)are gratefullyacknowledgedReferences1] Juszczak a, Domka F, Kozowski M, et al. Microbial de-sulfurization of coal with thiobacillus ferrooxidans bacteria. Fuel1995,74(5):725-728[2] Celik M S. Effect of ultrasonic treatment on the floatability of coal and galena. 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