Influence of acid leaching and calcination on iron removal of coal kaolin

International Journal of Minerals, Metallurgy and MaterialsVolume 21, Number 4, April 2014, Page 317DO:10.1007/s12613-014-0911-zInfluence of acid leaching and calcination on iron removal of coal kaolinPei-wang Zhu, Wei-giang Zeng, Xiu-lin Xu, Le-ming Cheng, Xiao Jiang, and Zheng-lun ShiState Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China(Received: 23 October 2013; revised: 17 December 2013; accepted: 19 December 2013Abstract: Calcination and acid leaching of coal kaolin were studied to determine an effective and economical preparation method of calcinedkaolin. Thermogravimetric-differential thermal analysis(TG-DTA)and X-ray diffraction(XRD) demonstrated that 900 C was the suitabletemperature for the calcination. Leaching tests showed that hydrochloric acid was more effective for iron dissolution from raw coal kaolin(RCK), whereas oxalic acid was more effective on iron dissolution from calcined coal kaolin( CCK ). The iron dissolution from CCK28.78wt%, which is far less effective than the 54.86wt% of RCK under their respective optimal conditions. Through analysis by usingMossbauer spectroscopy, it is detected that nearly all of the structural ferrous ions in RCK were removed by hydrochloric acid. However,iron sites in CCK changed slightly by oxalic acid leaching because nearly all ferrous ions were transformed into ferric species after firing at900C. It can be concluded that it is difficult to remove the structural ferric ions and ferric oxides evolved from the structural ferrous ionshus, iron removal by acids should be conducted prior to calcinationKeywords: kaolin; iron removal; calcination; acid leaching; extraction; MOssbauer spectroscopyused as raw industrial materials. Beneficial uses of coal1. Introductiongangue include power generation [7] and original substancesfor construction materials such as cement, bricks, and conCoal kaolin, coal gangue dominated by kaolin, is widely crete [8-9]. However, the utilization rate of coal gangue indeposited in China, and it is estimated that the amount up to cement as an admixture is always lower than 15% because11.09 billion tons has been deposited in reserves [1]. Be- of its weak cementitious capability [10]. In addition, a largecause coal gangue is generated simultaneously with the amount of valuable mineral resources such as SiO2, Al2O3,production of coal, the strong demand for coal in China over Fe2O3, CaO, and other oxides existing in the coal ganguethe last decades has resulted in a considerable increase in can be extracted as beneficial materials for value-addedcoal gangue waste [2]. It has been estimated that 4.5 billion products. Mesoporous Al2O3, calcined kaolin/TiO2 compostons of coal gangue are stockpiled at 1700 waste dumps oc- ite particle material( CK/TCPM), and porous ceramics havecupying 15000 hectares of land [3]. Consequently, the waste been successfully synthesized from coal kaolin [11-13takes up large areas of land, which results in water and soilFor large-scale utilization of coal gangue, kaolin productspollution, soil erosion, and other environmental problems have been examined by many[4-5]. The disposal of this coal wastes has turned out to be kaolin products, the coal gangue must meet the requiredan increasing economic and environmental burden because thermal stability and whiteness [14]. Therefore, research hasof the increasing regulatory laws and the increasing costs of been conducted on two procedures: calcination and iron reTo minimize the waste of resources and to protect theCheng et al. [15] investigated the thermal transformationenvironment, many researchers have focused on alternative of coal kaolinite by thermogravimetric analysis-mass specutilization of coal gangue. The main ingredients of coal trometry (TG-MS)and infrared emission spectroscopygangue are carbonaceous and clay minerals, which can be Results show that after calcination, some kaolinite structuresponding author: Zheng-lun Shi2 Sprringero University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2014中国煤化工CNMHG318Int J. Miner. Metall. Mater., VoL 21, No. 4, Apr. 2014break down, and the iron-containing minerals are converted (ICP-OES)analysis. It is evident that the major phases ofto free oxides that impart color to the product. The mini- RCK are kaolinite and quartz minerals; however, the quanmum brightness occurs from 500oC to 600C for kaolin tity of iron present demonstrates that RCK is highly con-samples, whereas a sharp improvement in optical properties aminated. The valence states of iron were tested by Beijingfor all the samples from 1000C to 1 100C agrees well with General Research Institute of Mining and Metallurgythe crystallization of mullite, as indicated by the XRD studies[16]. In addition, after calcining coal kaolin from Jiaozuo Table 1. Chemical and mineral characteristics of the coalat 900 C for 4 h with a 5wt% reducing agent and a 2wt% kaolin samplechlorinating agent, the whiteness of the kaolin product in-creased from 45.84wt% to 88.68wt% and satisfied the demands for rubber filling and moderating paper coating [17ALO3The main contaminant in clay and kaolin minerals is iron3.44which is negatively related to lightness [18]. Leaching stud0ies have been performed for iron removal that employedChemical compositionMgo0.71various chemicals such as oxalic acid, hydrochloric acid,0.38citric acid, carbohydrates, and ascorbic acid [19-21]. Foroxalic acid, Ambikadevi and Lalithambika [22]reported thatthe optimum conditions required for achieving brightnessLoss on80%are a temperature of 100C, an oxalic acid concentra-31.24tion of o1 mol/l, and a reaction time of 90 min OxFe-tin RCK7186ic-sulphuric acid treatments showed better iron extractionFe in rckValence state of irond alone [21Fe" in CCKOn the other hand, physical methods such as flocculationFe in ccK8261and high-gradient magnetic separation also achieved goodMineralsperformances [23-25In these methods, chemical processing to remove the Note: *Calcining at a temperature of 900Ccontaminated iron oxides from clay and silica minerals wasconsidered as the most cost-efficient method [26]. In theHydrochloric acid (37wt%)was obtained by the Hangresent investigation, iron dissolution from coal kaolin by zhou Chemical Reagent Company. Other analytical gradehydrochloric and oxalic acids is studied before and after cal- reagents including oxalic acid, sodium acetate, glacial aceticcination. Factors such as acid concentration, temperature, acid, hydroxylamine hydrochloride, and phenanthrolinereaction time, and solid-to-liquid ratio(S/L) are examined to were obtained by Sinopharm Chemical Reagent Co, Ltdachieve reasonable conditions. The residues of coal kaolin2.2. Thermal analysisare characterized by XRD andFe Mossbauer spectroscopyto probe the impacts of acid leaching and calcination on iron To observe the changes in the physical properties of rCKremoval and the phase of ferrum that were not presented in during thermal treatment, the rCK sample was placed in anprevious studies and to provide insight into future coal kao- STA449 F3 thermal analysis system for TG-DTA. Thelin processing10-mg sample underwent thermal treatment with a linearheating rate of 10%C/min, from 20C to 1300 C, in a flow2. Experimentaling air atmosphere(60 mL/min). Typical temperature points2. 1. Raw materialsof600,700,800,900,1000,1100,and1200° C were se-lected to analyze the mineral composition by calcining coalRaw coal kaolin(RCK)used in this study was obtained kaolin with a muffle roaster. Each sample was heated for 2 hfrom northeast China. The sample was crushed to pass and then cooled down in a dryerthrough a 0.074 um sieve for analysis, and moisture was2.3. Leaching studiesremoved by placing the sample in a desiccator after dryingat 105C and self-cooling, table i shows that the chemicalAs indicated in Table 1. due to calcination the Fe andand mineralogical compositions were tested by XRD and Fe" contents中国煤化工 es in rck andinductively coupled plasma optical emission spectrometer calcined coal kaYHCNMHGtests were conP W. Zhu et al., Influence of acid leaching and calcination on iron removal of coal kaolin319ducted by two different methods. The first method involved exothermic and showed a significant mass loss of up tothe use of hydrogen chloride to leach iron from RCK, and 29wt% of the sample. The mass of the sample then becamethe other involved the use of oxalic acid to leach iron from stable. this major mass loss and intense exothermicitCCKbecause of the combustion of organic matter and carbonLeaching tests were conducted in a 50-mL glass beaker2.0that was placed on a magnetic hot plate for heating and stirring. The speed of agitation was constant to ensure suspension of the particles. The leaching agents were prepared atdesired concentrations by dissolving known masses or vol-umes of acids in distilled water. Thus, the variables that af-DTAfect coal kaolin bleaching and iron leaching are temperature,acid concentration, S/L, and duration of the process52. 4. Characterization and evaluationThe iron leached from the kaolin was measured by using020040060080010001200he 1, 10-phenanthroline colorimetric method. Other elements including Si and Al were tested according to the FigThermogravimetric-differential thermal analyzerChina Standard: GB/T 14563--2008 specification and test (TG-DTA)curves of coal kaolin under air atmosphere.method of kaolin clay. X-ray analysis was performed by theFig. 2 shows that the structure lost its crystallographicRigaku D/Max-2550pc powder diffractometer, using Cu Ka order between 500@C and 900oC when compared with RCK,(=0. 154059 nm)radiation at 40 kV and 250 mA. The the mineral phases of which are kaolinite, quartz, and dickitescans were run from 5 to 85.0%(20), with an increasing step Moreover, new diffraction peaks occurred because of thesize of 0.02 and a scan rate of 5/min Data were processed generation of crystals such as hematite and silicon. Theby using MDi-Jade version 7.0 softwaredominant mineral in RCK altered to metakaolin. DehySFe Mossbauer spectra were recorded at room tempera- droxylation continued up to 963, and the gradual oola-ture with a standard constant acceleration spectrometer by tion of the metakaolin generated a complex amorphoususing aCo/Pd source. The velocity scale(10 mm/s)was structure of aluminum-silicon spinel (Si3ALO12)[33]. Thesecalibrated with a standard a-Fe foil. and all isomer shifts reactions contributed to a weak endothermic effect. with anwere given with respect to the center of the a-Fe spectrurexothermic effectThe 1. 5-g samples were placed in a 2-mm-thick holder and reappeared because the amorphous substance nucleated andwere prepared as absorbers. The entire spectral Mossbauer formed mullite(3Al2O3 2SiO2) crystals and highly crystalparameters were fitted to Lorentzian lines by a least-squares line cristobalite(Sio2 ). However, the formation of thesemethod that used the ISo fit programme [27]. The various minerals is a disadvantage to the product of calcined kaolinphases of iron in the samples were determined by comparing Therefore, the coal kaolin calcined at 900oC was selectedthe mossbauer parameters with the previous results [28-31. for leaching with oxalic acidQuantitative phase identification of iron in the samples wascalculated with reference to the Huffman method [28]3.2. Leaching rck by hydrochloric acidPrevious orthogonal experiments indicated that hydrogen3. Results and discussionable for leaching iron from rCK3.1. Mineral and thermal characterizationLeaching tests with rCK by hydrochloric acid were con-ducted with various acid concentrations, reaction tempeThe weight loss TG-DTA curves as the functions of the tures, time, and S/L. The effect of one factor on iron retemperature of RCK are illustrated in Fig. I. In the first moval was obtained while keeping other factors constantstage of the endothermic reaction, the mass loss from 20C Fig. 3(a) shows the effect of acid concentration on iron disto 100oC was 3wt% because of the loss of surface water, solution while maintaining 20oC for 2 h and an S/L of 1 gbound water. and void water From 100C to 275 C. a net 4 mL. It was observed that with the increase of acid concerweight gain of lwt% was recorded, which is attributed to tration, the dissolution of iron increased until the acid concentration wasbustion [32. The second stage from 275C to 654C was effect was lessTH中国煤化工 centration,theCN rate of320Int. J. Miner. Metall. Mater., voL. 21, No. 4, Apr. 20144843wt% was achieved as the maximum value. It can be higher cost and does not always lead to better performancesuggested that an increase in hydrochloric acid results in of the iron leaching rateDHHQHQ QHoQ Q 500CDRaw material700°CQ D51015202530354045505560657075808551015202530354045505560657075808520/(°0/(°Fig. 2. X-ray diffraction patterns ofcoal kaolin calcined at various tem-MMperatureskaolinite, Q: quartzl100°CD: dickite. S: silicon H: hematite. M:mullite, and C: cristobalite)51015202530354045505560657075808550(a)624582号8550Hydrochloric acid concentration/(mol- L-)Temperature/°C56(d)56789101:6Time/hSolid to liquid ratio /(g mL-)Fig 3. Effects of hydrochloric acid on iron dissolution from raw coal kaolin(rCK):(a) effect of acid concentration at 20C and anL of 1 g: 4 mL for 2 h;(b)effect of temperature at 2.0 mol/L and an S/L of 1g:中国煤化工 ction time at2.0mol/L, 40oC, and an S/L of 1 g: 4 mL;(d)effect of S/L at 2.0 mol/L and 40C for 2 hCNMHGP W. Zhu et al., Influence of acid leaching and calcination on iron removal of coal kaolinFig. 3(b) illustrates the increase in the iron leaching rate process were achieved, which are 2.0 mol/L hydrochloricwith an increase in temperature. The constant factors were acid at 40C with a 2 h reaction time and an S/L of 1 g: 3 mL2.0 mol/L hydrochloric acid, 2 h, and an S/L of 1 g: 4 mL3.3. Leaching cck by oxalic acidResults show the dissolution of iron was slow and variedbetween 40C and 60C. Two stages of rapid increase in theBefore leaching CCK by oxalic acid, a comparativeiron leaching rate were observed including that from 20C leaching test was conducted on CCK by hydrochloric acidto 40C and from 60oC to 80C. These results indicate that under the suitable conditions achieved in the previous sec-he increased temperature contributes to a swelling effect tion. The results show that the dissolution of iron was onlyoves the 11.76wt%, which is undesirable. However, ferric ion canreaction activity of the iron leaching. However, the compo- form a stable water-soluble chelate with oxalic acid. Lee etdissolution at 60C is 6.47wt%, which is significantly higher solvent reagent because of its acid strength, good comp anent analysis of the leaching residues indicates that aluminum al. [34] reported that oxalic acid was the most promisingthan 1.07wt% corresponding to that of 40oC. These condi- ing characteristics, and high reducing power over those oftions are harmful to the kaolin product; therefore, 40 C was other organic acids. Hence, oxalic acid was examined to deselected for the temperature in the following leaching teststermine an optimum condition of higher iron dissolutionFig 3(c)shows the dissolution of iron at various times from CCKwith 2.0 mol/L hydrochloric acid at 40C and an S/L of I gFirst. tests were conducted to establish the iron dissolu4 mL. During the first half of the experiment time, the dis- tion with oxalic acid concentration at room temperaturesolution of iron increased rapidly, and a maximum iron (20C)and an S/L of 1 g: 3 mL Fig 4(a)shows the resultsleaching rate of 53. lwt% at 2 h was achieved. The slow de- of dissolution over a period of 16 h. The iron dissolutioncrease in iron leaching rate indicates that the particle surface was independent of the oxalic acid concentration from 0.05may have been blocked by a product layer. It can beto I mol/L, although the dissolution rate did increase whencluded that 2 h is suitable for iron leaching from RCK by the oxalic acid concentration increased during the early stagehydrochloric acidHowever, the trends in iron leaching rates varied with timeChanges in the S/L had a limited effect on the dissolution From 0.05 to 0.5 mol/L, the iron leaching rate first increasedof iron from RCK, as shown in Fig 3(d). Tests were con- with an increase in reaction time. When the reaction timeducted under the conditions of 2.0 mol/L hydrochloric acid reached 8 h, the maximum value was obtained, and the ironat 40oC with a 2 h reaction time. In the range from 1 g: 2 dissolution rate then decreased. This result indicates that amL to 1 g: 6 mL, the dissolution of iron had a small change long reaction time causes side effects in iron dissolution. Atwithin 6wt%. However, the iron leaching rate affected by 1.00 mol/L, the iron dissolution continued to decrease overS/L appeared to show a clear trend, such that the dissolution 16 h, which demonstrates that lengthy reaction time andof iron increased with S/L after the first cut increased When high oxalic acid concentration are harmful to iron dissoluS/L was I g: 3 mL, the maximum iron leaching rate was tion from CCK. The reasonable conditions at room tempera-54.86wt%ture were determined to be 0.1 mol/L oxalic acid concentraIn this work, suitable conditions for the RCK iron leaching tion at 8 h for the maximum iron dissolution of 28.78%30F(a)-0.05 mol151014810121416Time/hTime/hFig 4. Effects of oxalic acid on iron dissolution from calcined coal kaolin(CCK): (a)of temperature at 0.1 mol/L acid concentration中国煤化工20c;(b)emetCNMHG322Int. J. Miner. Metall. Mater., voL. 21, No. 4, Apr. 2014ig. 4(b) shows the effects of temperature on iron disso- 49.84wt%, which is less than that by hydrochloric acid atlution over time while maintaining an oxalic acid concentr4.86wt%, and confirms that hydrochloric acid is better fortion of 0. 1 mol/L. At lower temperatures below 60oC, the iron dissolution from RCKremoval of iron increased with an increase in reaction timeefore gradually decreasing. The maximum point of iron3.4. Product analysis by Mossbauer spectroscopy studiesdissolution always occurred from 1 to 16 h. At 20C, theFour samples were investigated to demonstrate the efmaximum point of iron dissolution was 28.78wt%at 8 h. fects of calcination, hydrochloric acid, and oxalic acid onFor 40C, it was 28.75wt% at 4 h, and for 60C it was iron distribution in kaolin through Mossbauer spectroscopy17.93wt% at 4 h. At higher temperatures above 60oC, iron which provided information on iron site geometry and iron[35].RCels with an increase in reaction time. The results observed in drochloric acid(CKHA) and CCK processed by oxalic acidFig 4(b) illustrate that high temperature enhanced iron dis-( CCKO) obtained from the above experiments were usedlution, causing the maximum iron dissolution shift to aFig 5 shows the MOssbauer spectra of the four sampleslower reaction time with temperature increaseat room temperature. In Fig. 5(a), the MOssbauer spectrumIn these tests, the effect of iron dissolution at 40C in 4h of rcK consists of a doublet and a singlet with two differentwas approximately the same as that at 20C in 8 h. Both are isomer shifts(IS)and quadrupole splitting(QS). The dousuitable conditions for iron removal by 0. 1 mol/L oxalic blet with IS==1.002 mm/s and Qs =2.123 mm/s can be asacid.Moreover, leaching of CCK by oxalic acid obtained an sociated with Fe, which may substitute for Alin kaoliniteiron leaching rate of 28.78wt%, which is better than that by The singlet, with Is= 1. 16 mm/s and Qs=2.85 mm/s, cor-hydrochloric acid at 11. 76wt%responds to the dominant part of iron and could be related toFinally, a contrast test was conducted to examine the ef- structural ferrous ions. The iron contents of the four samplesfects of iron dissolution from rCK by oxalic acid under the Table 2)were calculated from the relative surfaces of theirdetermined suitable conditions of 40.C and 4 h reaction respective subspectra, which are referred to as atom quanti-time. The results show that the iron leaching rate reached ties1.005+(a)1门婷1004E0.9900.9940.9700.965-1010Velocity /(mm:s")Velocity /(mm:")1.0041.0021.0000.99509960.9850.9880.9800.984Velocity/(mm:")Fig. 5. Mossbauer spectra of kaolin samples at room temperature: (a) raw coalchloric acid(CKHA);(c) calcined coal kaolin(CCK); (d)calcined coal kaolin procemH中国煤化工 essed by hydro-CNMHGP W. Zhu et al., Influence of acid leaching and calcination on iron removal of coal kaolinTable 2. Mossbauer parameters of iron-bearing phases in four samples at room temperatureQS/(mm.sMHF /TAssignment0.34164.0641.0022.1235936CKHA0.2120.584CCK0.99448.7290.11251.993Iron oxides54606CCOFig 5(b) shows the Mossbauer spectrum of CKHA It is mine a suitable temperature for calcined kaolin preparationcomposed only of one doublet with Is=0.212 mm/s and Qs Two chemical methods were then proposed for iron removal0.584 mm/s, which corresponds to Fe. That is, after to study whether acid leaching before or after calcinationleaching by hydrochloric acid, the Fe species in RCK was was more effective. Reasonable conditions for each processeffectively removed. However, the Fe in kaolinite is not were obtained through many experiments with the variableseasily dissolved by hydrochloric acidof concentration, reaction temperature, and time. The rawAfter heating at 900C for 2 h, additional sextets ap- materials and products were analyzed by Mossbauer speccared in CCK(Fig. 5(c)) that led to a magnetic hyperfine troscopy to determine the effects of calcination and acidfield(MHF)of 51.923 T with Is =0.256 mm/s and Qs= leaching on the iron distribution of rCK. The following-0 116 mm/s. The ferrous cells trapped within the normal conclusions were drawndioctahedral aluminum structure then transformed into Fe+(1)900C is a suitable temperature for the calcination ofspecies. When the Fe disappeared, the hematite and mag- RCK. Below 900 C, conditions are unfavorable for decarnetic irons increased according to the magnetically split burization; those over 900C cause the formation of mullitesextets. Theoretically, the content of Fe will decrease be- and cristobalite, which is detrimental to the kaolin productcause high temperature leads to a partial collapse of the kao-(2)With regard to RCK leaching by hydrochloric acid,linite crystal, which contributes to the transformation of Fe the maximum iron leaching rate(54.86wt%)was achievednto iron oxide. In fact, Table 2 shows an increase. Thus, under the conditions of 2 mol/L hydrochloric acid at 40oCsome octahedral Fe must have transformed into Feocta- with a 2 h reaction time and an S/L of 1 g: 3 mLhedral species, which increased the difficulty of iron re-(3)For CCK, the effect of iron dissolution(28.75wt%)at40C in 4 h was approximately the same as that at 20C foAfter processing by oxalic acid, the chemical treatment 8 h(28.78wt%). 0.1 mol/L oxalic acid was used for bothdid not significantly influence the line profiles of the Moss- conditionsbauer spectrum(Fig. 5(d). The iron distributions of CCKO(4) Mossbauer spectroscopy showed that nearly all of thealso changed slightly compared to those of CCK. From Ta- structural ferrous ions in RCK were removed by acidsDle 2, it can be inferred that the relative content of Fe in However, iron sites in CCK altered slightly after processingCCKo decreased compared to that of Fe in CCK. This re- by oxalic acid because nearly all ferrous ions were trans-sult indicates that at least small parts of Fewere dissolved. formed into ferric species after firing at 900%The spectroscopy study demonstrates the adverse effects(5)The structural ferric ions and ferric oxides evolvedof calcination on iron removal. Because of the calcination from ferrous ions by acids were difficult to remove; there-process, nearly all ferrous ions changed into ferric species, fore, iron removal by acids should be achieved prior to calwhich caused lower iron dissolution by chemical treatment. inationFor this reason, the iron leaching rate from CCK(28.7%wt%)is significantly less effective than that of RCK(54.86wt%) Acknowledgementsunder their respective optimal conditionsThe study is financially supported by Zhejiang NaturalConclusionsScience Foundation(No. Y1080393)and Opening Foundtion of stateTG-DTA and XRD analysis were conducted to deter- (No. ZJUEDUuYH中国煤化x! y UtilizationMr. Tong JianCNMHGInt. J. Miner. Metall. 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