comparison of direct acid leaching process and blank roasting acid leaching process in extracting vanadium from stone coal

1、Comparison of direct acid leaching process and blank roasting acid leaching process in extracting vanadium from stone coalstone coal contained 0.31% (wt) V, 3.42% (wt) Fe2O3and 15.05% (wt)CaO. Zhou et al. (2009) directly leached the stone coal from Hubeiprovince for 8 h using 18% (v/v) H2SO4, 4.8% (

2、wt) NH4F, L/S = 4:1 mL/gat 95 °C. The vanadium leaching efficiency reached 92%. The used rawstone coal contained 0.99% (wt) V, 18.39% (wt) Fe2O3and 0.19% (wt)CaO. Li et al. (2009) leached the stone coal from Guizhou province for 3h using 11% (v/v) H2SO4, 0.8% (wt) FeSO4, L/S = 1.2:1 mL/g at 180

3、 °C.The vanadium leaching efficiency reached at 76%. The used raw stonecoal contained 1.83% (wt) V, 2.70% (wt) Fe2O3and 0.49% (wt) CaO, andthe V(III) only accounted for 34%.It is clear that both these two processes can achieve high vanadiumleaching efficiency and fluoride is an effective aid-le

4、aching reagent.However, the leaching conditions of these two processes for extractingvanadium from stone coal in different regions are different. Especially,the leaching conditions of each process vary for region-different stonecoals. It is widely known that the process chosen is closely related tot

5、he material itself. However, the characteristics of stone coal wereseldom researched, which resulted in difficulties in exploring the rulesof choosing the blank roasting acid leaching process or the direct acidleaching process.This work is to investigate the characteristics of a certain stone coalan

6、d to direct the choice of process for extracting vanadium from stonecoal by comparing the mechanisms of direct acid leaching process andblank roasting acid leaching process.2. Experimental2.1. MaterialsIn this work, the stone coal was obtained from Jiangxi province inChina.Theorewascrushedtoagrainsi

7、zeof03mmbeforetheleachingtests. For direct acid leaching process, the ore continued to be dryground in vibration mill (model XZM-100) to the particle size of?74 m accounting for 75%. The obtained ore was referred to as rawore throughout this work. For blank roasting acid leaching process, thecrushed

8、 ore was roasted in muffle furnace (model KRY-10) at 700 °Cfor 60 min and then dry ground via the same vibration mill to theparticle size of ?74 m accounting for 75%. The roasted and gainedore was referred to as roasted ore throughout this work.Thecalciumfluoride(AR)wassuppliedbyShanghaiShanpuC

9、hemicalCo., Ltd. All other reagents and chemicals used were of analytical reagentgrade.2.2. MethodsFirstly, the characteristics of raw ore and roasted ore can be clearly-determined by Inductively Coupled Plasma Atomic Emission Spectrom-etry (ICP-AES), X-ray Diffractometry (XRD), optical microscope a

10、ndelectronic probe microanalysis (EPMA). Meanwhile, the main changesduring blank roasting can be found. Secondly, the raw ore and roastedore were used to leach vanadium considering the effects of sulfuricacid concentration and leaching time, respectively. As a result, the opti-mal comparative condit

11、ions of direct acid leaching and blank roastingacid leaching were confirmed through comparison of leaching behav-iors. Thirdly, the obtained leachates and leaching residues under theoptimal comparative conditions were used for a series of chemical anal-yses. The characters of leachates and residues

12、can be acquired. In theend, thevariouschemicalreactions duringleachingprocesscanbesum-marized by synthesizing characters of raw ore and roasted ore beforeleaching and the leachates and residues after leaching. Through thecomparison between direct acid leaching process and blank roastingacid leaching

13、 process, thecharacter of each process for the two process-es can be understood. The optimal process of leaching vanadium fromthe stone coal can be determined. Furthermore, the reasonable usingconditions for the processes respectively can be given.2.2.1. Leaching testsLeaching tests of sulfuric acid

14、 concentrations effects on leachingefficiencyofvanadiumwerecarriedoutinmagneticandcontrollingtem-perature stirrer (model SZCL-2A) at liquid/solid ratio (L/S) of 1 mL/g,temperature of 95 ± 1 °C, 5%(w/w) calcium fluoride for 4 h underH2SO4concentration of 5%(v/v), 10%(v/v), 15%(v/v), 20%(v/v

15、) and25%(v/v) respectively. Similarly, leaching tests of leaching times effectson leaching efficiency of vanadium were carried out in the same stirrerat H2SO4concentration of 15%(v/v), liquid/solid ratio (L/S) of 1 mL/g,Table 1The main compositions of raw ore and roasted ore.ElementRaw oreRoasted or

16、eContent/wt.%Content/wt.%VAlTFeKCaMgBaSiCS0.573.392.181.410.310.900.04329.1016.162.580.704.182.691.740.391.110.05335.861.940.59Fig. 1. XRD patterns of a) raw ore and b) roasted ore.Table 2Mineral composition of raw ore through EPMA (wt.%).ElementCoalPhlogopitePyriteCOMgSAlSiKVClFeTotal82.667.3110.03

17、10026.8959.412.702.052.061009.8847.660.3042.1610041F. Wang et al. / International Journal of Mineral Processing 128 (2014) 4047temperatureof95±1 °C,5%(w/w)calciumfluoridefor1h,2h,3h,4h,5hand6hrespectively.Leaching tests without calcium fluoride werecarried out under the same si

18、tuations other than without 5%(w/w)calcium fluoride. Each leaching test consumed 100 g raw ore. Thenthe slurries were filtrated by vacuum suction filter device (modelSHB-III)andtheleachatesandleachingresidueswereobtained.Accord-ing to the leaching efficiency of vanadium, the optimal comparativeleach

19、ing situation can be ensured. After that, the leachates and theleaching residues under the optimal comparative leaching situationswere used for chemical analyses.2.2.2. Chemical analysesTheraworeandroastedorewereanalyzedforchemicalcompositionsthrough ICP-AES method. The ICP-AES analyses adopted an e

20、missionspectrometer (model IRIS Advantage ER/S) from the Thermo ElementalCompany. The main mineral compositions of raw ore and roasted orewere analyzed through XRD and optical microscope. The XRD analysesused a D/max-III diffractometer from the Rigaku Company. The opticalmicroscope analyses adopted

21、equipment (model DM LP) from the LeicaCompany. The vanadium phase was analyzed through sequential extrac-tion procedures (Tessier et al., 1979; Zhang, 1992) and EPMA. The EPMAused an instrument (model JXA-8230) from the Rigaku Company.The obtained leachates were analyzed for chemical compositionsthr

22、ough ICP-AES. Acidometer and combination electrode were used toobtain pH value of leachates. F-existing states in the leachates weredetermined byThe NMR analyses used the instrument (model Avance-III-500) fromthe Bruker Company.The obtained residues were analyzed for mineral compositionthrough XRD m

23、ethod and morphology through Scanning ElectronMicroscope (SEM) method. The SEM analyses used the instrument(model JSM-5610LV) from the JEOL Company.19F liquid Nuclear Magnetic Resonance (NMR) method.Thevanadiumconcentrationintheaqueousphasewasdeterminedbyferrous ammonium sulfate titration using 2-(p

24、henylamino)-benzoic acidas an indicator (Gao, 2003). The vanadium valence state was determinedby potentiometric titration (Hu et al., 2012). The fluorine concentrationwas determined by fluorine ion selective electrode (Wang et al., 2013)and the iron concentration was determined by 1,10-phenanthrolin

25、espectrophotometry (Li et al., 2013). The content of other elements inthe aqueous phase were analyzed using ICP-AES method.3. Results and discussion3.1. Material characterization resultsThe chemical compositions of the raw ore and the roasted orewere shown in Table 1. The raw ore belonged to high gr

26、adevanadium-bearing stone coal (V = 0.57%) and contained massiveSiO2, Al2O3, Fe2O3, K2O and S.In raw ore, the main minerals through XRD (Fig. 1) were calcite,quartz, phlogopite, chlorite and pyrite. Additionally, the optical micro-scope found the coal in the form of particles. EPMA (Table 2) and van

27、a-dium phase analyses (Table 3) indicated that vanadium mainly existedin silicoaluminate minerals in the form of V(III) and phlogopite was themain vanadium-bearing mineral.In roasted ore, the main minerals through XRD (Fig. 1) were cal-cite, quartz, hematite and phlogopite. Furthermore, the microsco

28、pefound residual coal particles. EPMA (Table 2) and vanadium phaseanalyses (Table 3) similarly presented that vanadium mainly existedin silicoaluminate minerals as V(III) and that phlogopite was the mainvanadium-bearing mineral. Nevertheless, the vanadium bound to freeoxides increased.It was worth n

29、oting for roasted ore that there were minerals (Fig. 2)with a zonal texture which had internal and external annules (Fig. 3).The minerals are referred to as annule minerals throughout this paper.EPMA analyzed the internal annule and external annule and foundthat both of them were the same mineral de

30、spite differences in color.It is further confirmed that the mineral belonged to micas (Table 4) bycomparing the chemical compositions of annule minerals and micas.Furthermore, there was a part of hematite surrounded by micas. Basedon these, blank roasting had an influence in breaking micas down andi

31、ncreased the content of vanadium (bound to free oxides), and changedpyrite into hematite.3.2. Leaching testsIn order to acquire the optimal comparative conditions of direct acidleaching and blank roasting acid leaching, a series of leaching tests wascarried out. Fig. 4 indicates that with increasing

32、 sulfuric acid concentra-tion,thevanadiumleachingefficiencyimprovedapparently.Thevanadi-umleachingefficiencywithcalciumfluoridewasalwayshigherthanthatwithout calcium fluoride under the same leaching conditions. On theother hand, for leaching with calcium fluoride, vanadium leaching fromraw ore was e

33、asier than that from roasted ore before H2SO4concentra-tion = 20% (v/v) and the vanadium leaching efficiency for raw orestarted to remain stable when the H2SO4concentration reached 15%(v/v). The vanadium leaching efficiency for roasted ore with calciumTable 3Vanadium phase analyses and its valence s

34、tate in the sample (wt.%).CategoryTotal vanadiumVanadium phaseValence stateBound to free oxidesSilico-aluminateBound to organicsV(III)V(IV)V(V)Raw oreRoasted ore10010011.1622.3787.0476.941.800.6964.8454.4635.1636.8708.67Fig. 2. Annule minerals of roasted ore under microscope.42F. Wang et al. / Inter

35、national Journal of Mineral Processing 128 (2014) 4047fluoride rose sharply and the upward trend did not become slight until15%(v/v)H2SO4.Incontrast,forleachingwithoutcalciumfluoride,vana-dium leaching from raw ore was harder than that from roasted ore andthe gap of vanadium leaching efficiency betw

36、een raw ore and roastedore became bigger with increasing sulfuric acid concentration. In thisregard, 15% (v/v) H2SO4was the optimal comparative sulfuric acid con-centration. However, the leachate pH presented the reverse information(Fig. 5). The leachate pH for raw ore was always lower than that for

37、roastedoreunderthesameleachingconditions.Particularly,theleachatepH for roasted ore without calcium fluoride began to surpass the leach-ate pH for raw ore with calcium fluoride at 15% (v/v) H2SO4, thoughthe pH of leachate with calcium fluoride was always higher than thatwithout calcium fluoride unde

38、r the same leaching conditions. Theseshowed that blank roasting benefited the breakage of vanadium-bearing minerals. However, some minerals in roasted ore other thanvanadium-bearing minerals, for leaching with calcium fluoride, con-sumed massive amounts of acid.Fig. 6 illustrates that with increasin

39、g leaching time, the vanadiumleaching efficiency increased correspondingly. The vanadium leachingefficiency with calcium fluoride was always higher than that withoutcalcium fluoride under the same leaching conditions. For leachingwith calcium fluoride, the vanadium leaching efficiency of roasted ore

40、reached balance rapidly whereas the vanadium leaching efficiency ofraw ore markedly ascended, surpassed that of roasted ore at 2 h andleveled out at 4 h. For leaching without calcium fluoride, the vanadiumleaching efficiency improved all the way and the efficiency from rawore was always lower than t

41、hat from roasted ore. The gap of vanadiumleaching efficiency between raw ore and roasted ore remained relativelyconstant. In this regard, blank roasting can be in favor of vanadiumleaching. By contrast, the pH of leachate presented the opposite informa-tion (Fig. 7). The leachate pH for raw ore was

42、lower than that for roastedore. Meanwhile, the leachate pH for roasted ore increased gradually withincreasing leaching time whereas the leachate pH for raw ore with calci-um fluoride remained stable from 4 h and that for raw ore without calci-umfluorideremainedsteadyfrom5h.Itwasnotablethattheleachat

43、epHforroastedorewithoutcalciumfluoridewasclosetothatforraworewithcalcium fluoride, although the pH of leachate with calcium fluoride washigherthanthatwithoutcalciumfluorideunderthesameleachingcondi-tions regardless of leaching time. Therefore, the minerals not bearing va-nadiumconsumedmassiveamounts

44、ofacidforleachingfromroastedore.4 h would be the optimal comparative leaching time.Therefore,theoptimalcomparativeconditionwas95 °C,4h,1 mL/g,15% (v/v) H2SO4and 5% (w/w) calcium fluoride. Under the fixed condi-tion, the obtained leachates and leaching residues were used for furtherdiscussions.3

45、.3. Comparison of direct acid leaching process and blank roasting acidleaching process3.3.1. Direct acid leaching process with CaF2The main composition of leachate (Fig. 8) showed that most of V, K,Al,MgandlittleSi,Fewereintheaqueoussolution,inwhichtheleachingefficiency of Al, K, Mg, Fe and Si was 5

46、9.02%, 54.48%, 75.40%, 23.24% and0.10%,respectively.Asinterlaminaratoms,KandMgwereleachedeasilyin acid solution. A great deal of V and Al in the leachate showed that thedioctahedral structure should be broken down. Meanwhile, the analysisFig. 3. Annule minerals of roasted ore under EPMA.Table 4Chemi

47、cal compositions of internal annule and external annule through EPMA (wt.%).COMgAlSiSKVFeTotal11.807.6723.8026.241.371.615.246.8251.3051.180.993.383.411.501.820.621.25100100Internal annuleExternal annule43F. Wang et al. / International Journal of Mineral Processing 128 (2014) 4047of valance state of

48、 V showed all released V in leachate was V(IV). Hence,V(III)mustbeoxidizedintoV(IV)duringtheprocessofthebreakageanddissolution of phlogopite. The oxidant should be O2from air. Besides,60.52% F of 5%(w/w) calcium fluoride was into leachate.In order to make clear the existing forms of fluorine in the

49、leachate,19F liquid NMR was analyzed using trifluoroacetic acid as internalstandard. The result (Fig. 9) shows that there are two kinds of existingforms of fluorine (Fluorine NMR Data, 2012). That is to say, fluorineexisted in leachate as SiF62?and AlF52?.In order to investigate the mineral composit

50、ion of leaching residue,theresiduewas analyzedbyXRD (Fig. 10). Comparingthemineral com-position of the raw ore with the mineral composition of the leachingresidue, it was found that the peaks of calcite, chlorite and phlogopitewere removed. By contrast, the intensity of the peak of quartz becamebigg

51、er and the peaks of pyrite almost had no changes. In addition, newpeaks of gypsum and barite appeared. As analyzed above, the leachingefficiency of vanadium reached 92.39%; meanwhile, most of the K, Aland Mg were in the leachate while only little TFe and Si were in theleachate. Therefore, the struct

52、ures of chlorite and phlogopite had beenbroken down completely to make plentiful V, K, Al and Mg in theleachate, and to generate quartz, making the peaks of quartz stronger.The pyrite in essence did not take part in reaction and dissolution. Tofurther confirm the deduction, SEM image of the leaching

53、 residue wasanalyzed as Fig. 11. The analysis of SEM image showed that the rawore had been broken down in terms of morphology. The main mineralsin the residue were grained quartz and fibrous gypsum.Through the analyses of leachate and residue from raw ore, thewhole direct acid leaching with calcium

54、fluoride process can beFig.5. Effects of H2SO4concentrationonpHof leachates under1 mL/g,95 ± 1 °Cand4 hforleaching vanadium from raw ore with 5%(w/w) CaF2; leaching vanadium from raw orewithout CaF2; leaching vanadium from roasted ore with 5%(w/w) CaF2; leaching vanadiumfrom roasted ore wi

55、thout CaF2.Fig.6.Effectsofleachingtimeonvanadiumleachingefficiencyunder15%(v/v)H2SO4,1mL/gand 95 ± 1 °C for leaching vanadium from raw ore with 5%(w/w) CaF2; leaching vanadiumfrom raw ore without CaF2; leaching vanadium from roasted ore with 5%(w/w) CaF2;leaching vanadium from roasted ore

56、without CaF2.Fig. 4. Effects of H2SO4concentration on vanadium leaching efficiency under 1 mL/g, 95 ±1 °C and 4 h for leaching vanadium from raw ore with 5%(w/w) CaF2; leaching vanadiumfrom raw ore without CaF2; leaching vanadium from roasted ore with 5%(w/w) CaF2;leaching vanadium from ro

57、asted ore without CaF2.Fig. 7. Effects of leaching time on pH of leachates under 15%(v/v) H2SO4, 1 mL/g and 95 ±1 °C for leaching vanadium from raw ore with 5%(w/w) CaF2; leaching vanadium fromrawore withoutCaF2;leachingvanadiumfromroastedore with5%(w/w)CaF2;leachingva-nadium from roasted

58、ore without CaF2.44F. Wang et al. / International Journal of Mineral Processing 128 (2014) 4047described as: CaF2reacted with H2SO4to generate weak electrolyteHF and slightly soluble substance CaSO4; calcite was dissolved togenerate CaSO4, CO2and H2O; chlorite and phlogopite were thor-oughly broken

59、down and dissolved to generate SiO2, K+, Mg2+,SiF62?, AlF52?and Al3+; V(III) was oxidized into V(IV) by O2from air to VO2+. The possible chemical reaction equations can bestated as follows (Eqs. (1) to (4).CaF2t 2Htt ?SO4?2? 2HFeaqTt CaSO4e1TCaCO3t 2Htt ?SO4?2? CaSO4 t CO2 t H2Oe2T2KMg3eV;AlTSi3O10e

60、OHT2t 11HFeaqTt 13Htt O25SiO2t 2VO2tt 2Ktt 6Mg2tt ?SiF6?2?t ?AlF5?2?t Al3tt 14H2Oe3TMg5Al2Si3O10eOHT8t 11HFeaqTt 9Ht2SiO2 t 5Mg2tt ?SiF6?2?t ?AlF5?2?t Al3tt 14H2Oe4T3.3.2. Blank-roasting acid-leaching process with CaF2Themaincompositionofleachate(Fig.8)showedthatmostoftheV,K, Al and Mg were in the a