一種新型的煙片復(fù)烤機加濕方法*

王兆鐵

(湘西鶴盛原煙發(fā)展有限責(zé)任公司，湖南 吉首 416000)

摘要：傳統(tǒng)KG型煙片復(fù)烤機加濕效果差，加濕噴嘴容易堵塞與損壞，回潮效率低，回潮區(qū)溫度超標，嚴重浪費蒸汽與水.采用新型氣水二流體霧化噴嘴，在噴孔外端加裝調(diào)節(jié)帽，實現(xiàn)二次霧化.回潮效果明顯提高(達到4%以上)，回潮區(qū)溫度可控制在70℃左右，節(jié)約蒸汽30%以上.

關(guān)鍵詞：葉片復(fù)烤；噴霧加濕；新型加濕

文章編號：1007-2985(2015)06-0049-04

中圖分類號：TS43文獻標志碼：A

DOI:10.3969/j.cnki.jdxb.2015.06.012

收稿日期：*2015-05-20

作者簡介：王兆鐵(1973—)，男，湖南吉首人，湘西鶴盛原煙發(fā)展有限責(zé)任公司工程師，主要從事機械設(shè)備設(shè)計及制造研究.

DOI:10.3969/j.cnki.jdxb.2015.06.013

KG型煙片復(fù)烤機是打葉復(fù)烤生產(chǎn)的主要設(shè)備，其功能是將煙葉農(nóng)產(chǎn)品變?yōu)楣I(yè)原料.煙葉經(jīng)過復(fù)烤機的干燥、冷卻和回潮處理，可以調(diào)整煙葉水分(一般調(diào)到11%～13%)，去除煙葉中的雜質(zhì)和雜氣，殺滅煙葉中的害蟲和病菌，使煙葉更適合于貯存、醇化和人工發(fā)酵，確保煙葉質(zhì)量規(guī)范一致.干燥、冷卻均為回潮作準備，回潮性能的高低處決于復(fù)烤機的工藝性能與能源消耗水平.目前使用的加濕回潮噴嘴霧化效果差、易堵塞、可靠性差，導(dǎo)致回潮蒸汽浪費多、回潮率低.因此，解決復(fù)烤機的加濕問題成為目前研究的熱點.

1KG型煙片復(fù)烤機加濕存在的問題

1.1 煙片復(fù)烤機的吸濕機理

圖1　煙葉等溫等濕吸濕效果

煙草是一種毛細多孔固體物料，在烘烤干燥過程中，煙草毛細管脫水，空氣進入其中.只有當空氣中的蒸汽分壓力大于煙草表面的蒸汽壓力時，煙草才能吸濕.環(huán)境溫度越高、濕度越大，則空氣中的蒸汽分壓力越大，煙草吸濕越快.[1-2]在增溫增濕初始階段，提高環(huán)境溫度或濕度均能增加煙葉的吸濕速率.隨著處理時間的延長，環(huán)境溫度越高，煙葉吸濕變化趨近于0的時間越短(圖1).從圖1可知，提高煙葉的吸濕能力，其主要途徑是提高環(huán)境的溫度和濕度，但是溫度達到一定值時，其吸濕效果并不明顯.

1.2 傳統(tǒng)加濕方法存在的問題

煙片經(jīng)復(fù)烤機的干燥區(qū)干燥后，從冷卻區(qū)進入回潮區(qū)時溫度和水分都很低，因此必須將它加熱和回潮到打包所需的溫度和水分.保證煙片在一定的時間內(nèi)快速、均勻地吸收水分，就必須有一個較高溫度和濕度的環(huán)境.KG型煙片復(fù)烤機采用的加濕方法為，利用循環(huán)風(fēng)將適當?shù)恼羝退F通過煙片，煙片吸收霧化和蒸汽凝結(jié)水，以此連續(xù)地加濕加熱煙片.傳統(tǒng)霧化水的方式為高壓噴嘴霧化和汽水混合噴嘴霧化.

高壓噴嘴霧化需要的水壓高達5～7 MPa，其霧化效果在15 μm以下.該霧化水產(chǎn)生的方式采用變頻控制泵的轉(zhuǎn)速，從而控制水分的多少，但存在頻率降低、霧化效果下降、控制精度不高等缺陷.另外，高壓泵的機械故障率非常高，噴嘴噴孔極小，易堵塞.

汽水混合噴嘴霧化分2種形式.一種為內(nèi)混式霧化，即汽水在噴嘴內(nèi)混合后噴射.此種霧化噴嘴為減少內(nèi)部結(jié)垢堵塞，噴孔尺寸往往較大、射程較遠，多數(shù)噴霧顆粒直徑在50 μm以上.另外，汽水比例與各自壓力要求較高.水壓過大霧化效果達不到要求，汽壓過大則水無法噴出.另一種為外混式霧化，即汽水在噴嘴外混合噴射.此種噴嘴利用高速汽流對水進行摩擦產(chǎn)生霧化，具有運行穩(wěn)定、無需維護等優(yōu)點，但大多數(shù)霧化顆粒直徑在50 μm以上，不能被循環(huán)風(fēng)帶走，加濕能力不足且水浪費嚴重.

煙片復(fù)烤機回潮區(qū)的循環(huán)風(fēng)風(fēng)速小于0.65 m/s，其目的是防止煙葉被吹起形成風(fēng)洞，影響均勻性，同時避免15 μm以上的水滴被風(fēng)帶到煙片上形成水漬煙.

由上述分析可知，煙片復(fù)烤機加濕能力不足的原因是霧化水顆粒過大，無法被風(fēng)機帶走，從而產(chǎn)生加濕量不足現(xiàn)象.

2煙片復(fù)烤機加濕方法的改進

針對傳統(tǒng)噴嘴霧化效果差、調(diào)節(jié)不便等問題，筆者提出一種新型氣水二流體壓力式噴嘴，選用優(yōu)質(zhì)不銹鋼材質(zhì)精密加工而成.采用氣水內(nèi)混壓力式霧化，并在噴孔外端加裝調(diào)節(jié)帽實現(xiàn)二次霧化.這種噴嘴霧化后，大多數(shù)水滴直徑在10 μm以下，最大水流量為12 kg/h.多個噴嘴組合使用后，煙片復(fù)烤機的加濕能力達到4%以上，回潮區(qū)的溫度可控制在70 ℃左右，成品煙箱箱芯溫度可控制在30 ℃左右，節(jié)約蒸汽達30%以上.

2.1 氣水二流體壓力式噴嘴的原理

壓縮空氣經(jīng)噴嘴內(nèi)腔沿軸向垂直撞擊水流,將水初步霧化,然后噴出，噴射流繼續(xù)向前運動與噴嘴調(diào)節(jié)帽的頂針碰撞進行二次霧化.噴嘴利用壓縮空氣產(chǎn)生的射流與水在噴嘴出口處混合，將水破碎成非常微小的水滴，水滴直徑為5～10 μm.小水滴進入空氣后蒸發(fā)，完成加濕過程.噴嘴的徑向裝有水量調(diào)節(jié)閥，可根據(jù)壓縮空氣的流量與壓力手動調(diào)節(jié)水量，達到最佳霧化狀態(tài)(圖2).

二流體主要由空氣和液體二部分組成.水霧噴頭有2個進口(分別接入氣體和水)，水進入水霧噴頭后，通過氣壓將水打散形成很細的霧.如果要求霧化粒細小，就必須加大空氣的壓力；如果要增加霧的流量，就加大液體的壓力.在水霧噴嘴中，水與空氣之間正常壓力比為1∶1.2.

2.2 氣水二流體壓力式噴嘴的應(yīng)用

以湖南省湘西鶴盛原煙發(fā)展有限責(zé)任公司KG型煙片復(fù)烤機為例，在煙片復(fù)烤機回潮區(qū)的每個室中安裝2組噴組，每組噴嘴控制方式如圖3所示.

圖2　氣水二流體噴嘴示意

1—噴嘴；2—過濾器；3—電控系統(tǒng)；4—氣動薄膜閥； 5—氣源過濾與減壓閥；6—電磁切斷閥；7—壓力檢測裝置. 圖3　氣水二流體噴嘴元件集中控制布置安裝

壓縮空氣經(jīng)過氣源過濾與減壓閥、電磁切斷閥分別進入每個噴嘴(提供霧化氣體)，軟水經(jīng)過濾器、氣動薄膜閥分別進入每個噴嘴，與壓縮空氣混合，然后從噴嘴噴出而霧化.軟水壓力與壓縮空氣的壓力均由壓力監(jiān)測裝置實時檢測.根據(jù)最佳霧化效果所需的氣水壓差1.0～1.5 kg/cm2，由電氣控制系統(tǒng)自動調(diào)節(jié)氣動薄膜閥的開度，從而控制軟水的流量，使水壓與氣壓能在任何情況下自然控制，以提供最佳的霧化效果.同時在軟水管道上設(shè)有自動排污裝置，與氣動薄膜閥工作時間相反，便于排除管道中的污垢.當水分儀檢測出煙片復(fù)烤機出口煙葉的水分超出設(shè)定值，可由電氣控制系統(tǒng)關(guān)閉1組或多組噴嘴管路上的電磁切斷閥，從而關(guān)閉壓縮空氣.減少回潮區(qū)的霧化水量，則可以降低煙葉的水分.當壓縮空氣壓力為0 N時，則自動關(guān)閉氣動薄膜閥，從而切斷軟水.

3加濕方法改進效果

3.1 工藝指標的改善

(1)煙箱的包芯溫度明顯下降.在保持總加濕量不變的情況下，增加霧化水量則必然減少蒸汽量，從而降低回潮區(qū)的環(huán)境溫度，最終降低煙片復(fù)烤機出口煙片溫度.經(jīng)過實測回潮區(qū)溫度，由原來的80 ℃降到70 ℃，煙片復(fù)烤機出口煙片溫度由原來的55 ℃降到45 ℃，煙箱的包芯溫度由40 ℃降到32 ℃.

(2)回潮能力明顯增加且水分偏差降低.為了保證煙片復(fù)烤機出口煙片的水分質(zhì)量分數(shù)穩(wěn)定在11%～13%，先將干燥煙片的水分調(diào)至臨界點(質(zhì)量分數(shù)8%)，然后回潮達到要求.若回潮能力未能達到4%以上，則不可能將煙片干燥到質(zhì)量分數(shù)為8%左右.文中用標準偏差σ度量水分的離散程度：

改進前后的實時水分檢測結(jié)果如圖4，5所示.由圖4，5可知，改進前打包水分質(zhì)量分數(shù)在11%～12.5%范圍內(nèi)均勻分布.改進后，打包水分質(zhì)量分數(shù)大部分分布在11.5%～12.5%之間.

圖4　改進前連續(xù)4 d的煙箱水分檢測結(jié)果分布

圖5　改進后連續(xù)4 d的煙箱水分檢測結(jié)果分布

打包水分分析如下：2014年10月10—14日，均值為11.7%，出口水分標準偏差為0.38.10月23—27日，均值為11.95%，出口水分標準偏差為0.3.由此可知，改進前后，標準差下降幅度約為21%，均值提升0.25%.

3.2 蒸汽耗量降低

連續(xù)實測改造前10 d的平均小時耗汽為5.64 t，連續(xù)實測改進后10 d的平均小時耗汽為3.97 t.每小時節(jié)約1.67 t蒸汽.實測數(shù)據(jù)如圖6所示.按照復(fù)烤廠每年生產(chǎn)180 d計算，節(jié)約蒸汽量1.67 t/h×24 h×180 d=7 214.4 t,能量節(jié)約30%.

圖6　改造前后蒸汽耗量實測數(shù)據(jù)

4結(jié)語

將多個新型氣水二流體壓力式噴嘴與電氣控制組合后，KG型煙片復(fù)烤機的水分調(diào)節(jié)精度與加濕能力得到明顯的提升，降低了打包煙箱的水分離散度與包芯溫度.改進后的KG型煙片復(fù)烤機回潮效果達4%，回潮溫度可控制在70 ℃左右，節(jié)能效果明顯(節(jié)約蒸汽30%以上).

參考文獻：

[1]顧中鑄，吳薇.烤煙煙葉的等溫吸濕和解濕特性.南京師范大學(xué)學(xué)報：工程技術(shù)版,2004，4(4)：32-34.

[2]增強，李斌，閆亞明，等.增溫增濕過程中煙葉吸濕速率的變化.煙草科技,2006(1)：15-17；32.

[3]金仁喜，袁江濤，楊立，等.壓力噴嘴常溫下霧化特性實驗研究.海軍工程大學(xué)學(xué)報,2012，24(3)：52-56.

[4]鄭捷慶，羅惕乾，張軍，等.氣力霧化噴嘴最佳耗氣率的實驗.農(nóng)業(yè)機械學(xué)報，2007，38(10)：195-197；200.

[5]中國煙葉公司.YC/T146—2010煙葉打葉復(fù)烤工藝規(guī)范.北京：中國標準出版社，2010.

[6]曹顯奎，孫志剛，許建良，等.液體射流在同向氣流中的破裂.化學(xué)工程報,2007，35(5)：29-31.

[7]唐虎，成竹，蔣軍亮，等.空氣助力霧化噴嘴特性實驗研究.裝備環(huán)境工程,2012，9(2)：38-41；45.

NewMethodofTobaccoRedryingMachineHumidification

WANGZhaotie

(XiangxiHeshengTobaccoDevelopmentLLC.Jishou416000,HunanChina)

Abstract:The traditional methods of KG-type tobacco redrying machine humidification are not very effective,nozzle gets easily blocked,and the temperature of moisture regaining parts exceeds standards,resulting steam and water waste.The new steam-water atomizing nozzle with adjusting cap can realize re-atomization,thus increasing the moisture regaining effect by over 4%.With this method,the temperature of moisture regaining parts can be controlled within 70 ℃,and the steam can be saved by over 30%.

Keywords:tobaccoredrying;mistspraying;balancedmoisture;energysaving;atomizingnozzlewithsteam-waterfluid;newhumidification

(責(zé)任編輯陳炳權(quán))

ArticleID：1007-2985(2015)06-0053-06

CombustionMethodtoSynthesizeLiFePO4and Combustion Mechanism*

Abstract：Many traditional methods used to synthesize LiFePO4 (LFP for short) have the disadvantages of long reaction time or unavoidable oxidation of Fe2+.In this work,the combustion method was developed to facilely synthesize LFP by using ferrous sulfate,diammonium hydrogen phosphate and lithium nitrate as main raw materials.The synthesized LFP was characterized by XRD and TG-DSC.The results showed that the combustion method could facilely synthesize purer LFP at 600 ℃ in a short period without the protection of inert gas.The studies on combustion mechanism indicated that from 120 ℃ to 290 ℃ the organic citric acid in the dry gel was combusted which could lead to a self-propagating combustion reaction and bring in carbon preventing the oxidation of Fe2+ simultaneously.And φ=n(CA)/n(LFP)=2 is the best amount of citric acid to synthesize LFP materials.

Keywords：combustionmethod;synthesize;LiFePO4;battery

CLCnumber：O614.111Documentcode：B

*Received date：2015-04-16 Foundation item：National Natural Science Foundation of China (51374155);National Key Technology R&D Program (2013BAB07B01);Hubei Province Key Technology R&D Program (2014BCB034);National Natural Science Foundation of Hubei Province of China (2014CFB796) Biography：CHEN Juan(1992—),female,was born in Gong’an County,Hubei Province,graduate student for master degree;research area is function materials Corresponding author：HUANG Zhiliang(1964—),male,was born in Wangjiang County,Anhui Province,prorfessor,doctor;research area is inorganic mineral material.

1Introduction

Li-ionbatteryissecondarybatterywithtwodifferentlithiuminsertioncompoundsaspositiveandnegativeelectrodes.AndthestructureofcompoundscouldinsertandemergeLi+reversibly.Li-ionbatteryisoneofthewidelystudiedgreenenergybatteries.TheresearchoncathodematerialsofLi-ionbatteryhasbecomeahottopicsincecommercialization.AmonglotsofthecandidatesforcathodematerialsofLi-ionbattery,olivine-typeLithiumironphosphateisregardedasthemostpotentialoneforapplicationsowingtoitsuniqueadvantages,includingrelativelyhightheoreticalspecificcapacity(170mA·h/g),highandstablecharge-dischargevoltage(about3.4V),goodcyclingperformance,goodthermalstability,wideoperatingtemperaturerange,cheaprawmaterials,highenvironmentalcompatibility,etc.[2-4]

Many methods have been developed to synthesize LFP,mainly including the high-temperature solid-phase method,the sol-gel method,the microwave method,the hydrothermal method and the co-precipitation method.For the high-temperature solid-phase method,high reaction temperature needs to be provided,and great amounts of nonuniform large size particles are unavoidable which limit the electrochemical performances of synthesized LFP.Padhi A K et al used a two-step solid-phase method to successfully synthesize LFP by using Li2CO3,Fe(CH3COO)2andNH4H2PO4asrawmaterials;however,thesynthesizedLFPhadarelativelylowinitialdischargecapacityof110mA·h/g.Asforthesol-gelmethod,thereareremarkableadvantagesinsmallanduniformparticlesize,lowreactiontemperatureandfacilesynthesisprocess,butalongreactiontimeisneededandtheheavyshrinkingeasilyoccursduringdrying.Bythismethod,ScroceFetalsynthesized a kind of Cu/Ag-incorporated LFP cathode materials which enhanced the initial discharge capacity to 140 mA·h/g at room temperature.The reaction of the microwave synthesis is fast and thermally even,but it’s difficult for large-scale production to a certain extent.Despite the advantage of direct synthesis of LFP,easily controlled crystal form and grain size of sample,the hydrothermal method spends much on production facilities which should be heat-resisting and high pressure-resistant.Prosini P P et al used this method to synthesize LFP with a average size of 3 μm and a discharge capacity of 100 mA·h/g at 120 ℃.In the co-precipitation method,Fe2+is oxidized to FePO4solublesaltprecipitationandthenFePO4isreducedtoLFP.Inref. [10],Mg2+is easily incorporated into LFP to improve its conductivity by this method.To sum up,the above-mentioned methods all have the difficulty in fully avoiding the oxidization of Fe2+into Fe3+during reaction despite the protection of inert gases.Therefore,it is urgent to develop a facile,low-cost and fast synthesis method.

Herein,we developed a novel combustion method to synthesize LFP cathode materials,and investigated the combustion mechanism.

2Experiment

2.1ReagentsandInstrumentation

FeSO4·7H2O,(NH4)2HPO4,citric acid and LiNO4were used in this experiment and were of analytical grade.The instruments used were as follows:X-ray powder diffraction (model XD-5A，Japan,Cu target,λ=0.154 056nm,scanningrangeof10°～70°),batterytestsystem(chargeanddischargevoltagerangeof2.5～4.2V,rateof0.2C,1C=170mA·h/g),electrochemicalworkstation(scanningvoltagerangeof2.4～4.2V,thescanningspeedof0.1mV/s).

2.2ExperimentalProcedure

ThesyntheticrouteofLFPisshowninfig. 1.

Fig. 1　Synthetic Route of LFP

FeSO4·7H2O,(NH4)2HPO4and LiNO3(mole ratio 1∶1∶1) were made into saturated solution respectively at room temperature,then mixed,and some turquoise precipitate emerged immediately.Part of the precipitate was washed,filtered,dried and dark green powder was obtained (sample 1).Adding a certain amount of citric acid into the other part of the sediments,the precipitate disappeared.Then the clear liquid was put into a vacuum drying oven (70 ℃) to form dry gel.Finally the dry gel was combusted at 600 ℃ for 6min and some dark green powder was obtained (sample 2).The gases released in the process were discharged into the alkaline solution.In this paper,the amount of citric acid indicated that the φ=n(CA)/n(LFP).Tostudytheeffectthatamountofcitricacidhasonelectrochemicalpropertiesandstructureofthesyntheticmaterials,φ=1,1.5,2,3 (markedas①,②,③,④).

3ResultsandDiscussion

3.1AnalysisofSamples

Fig. 2　XRD Patterns of the Synthetic LFP

Fig. 2showstheXRDpatternsofthesyntheticLFPsamples.Itindicatesthatsample2ismainlycomposedofLFP(JCPDS,No. 401499)whichhasaspacegroupofpnmb[10].Theunitcellparametersarea=0.601 8nm,b=1.034nm,c=0.470 3nm.PeaksofFe3O4and Fe2O3are not found in diffraction pattern of sample 2,indicating that Fe2+wasnotoxidizedtoFe3+incombustionprocess.Therearenoobviouscharacteristicpeaksofcrystallinephaseinsample1,indicatingsample1isamorphous.Itexplainsthatolivine-typeLFPcouldn’tbepreparedwithoutthecomplexationofcitricacid.

XRDpatternsofthesyntheticLFPunderdifferentamountofcitricacidareshowninfig. 3.①ispurephaseLFP,and②stillincludesimpurityphase,③and④arepurephaseLFP,andhaveintensediffractionpeakandenhancedcrystallinity.

Fig. 3　XRD Patterns of the Synthetic LFP Under Different the Amount of Citric Acid

Fig. 4　 Cyclic Voltammetry Curves of the Synthetic LFP Under Different the Amount of Citric Acid

Fig. 4isCVcurvesofthesyntheticLFPunderdifferentamountofcitricacid.Thecurveshowsthatallthematerialsemergedapairofredoxpeaksincirculation.Peakcurrentincreasesfirstly,andthendecreaseswiththeincrementoftheamountofcitricacid.①istheminimum,and③themaximum.Peakcurrentsizetosomeextentreflectsthelithiumionsdiffusionrateinthematerial;andthereforein③,Lithiumiondiffusionrateismaximum.Inaddition,thecurveshowsthepotentialdifferenceoftheredoxpeakwiththeamountofcitricacidvaries,thereinto,thepotentialdifferenceofthematerial②istheminimum(approximately0.26V).Thesmallerthepeakpotentialdifference,thesmallerpolarizabilityofthematerial,andthebettertheelectrochemicalreversibility,indicatingthat③hasthecharge-dischargecycleperformance.Experimentalresultsareconsistentwiththis(asshowninfig. 5).Besides,thesizeoftheredoxpeakareareflectsthepolarizedstateofinternalelectrodeandutilizationofactivematerial[11],thehigherthesamplecapacity,thegreaterthepeakarea,indicatingthat③hasthebestmaterialcharge-dischargecapacity.

Fig. 6isthefirstdischargecapacitycurvesofthesyntheticLFPunderdifferenttheamountofcitricacidat0.2C.Itshowsthatwiththeamountofcitricacidincreasing,thedischargecapacityincreases,and③hasmaximumdischargecapacityof121.8mA·h/g.

Insummary,inordertoimprovedynamiccharacteristicsoftheelectrodematerial,andtoenhancethecapacityoftheLFPcathodematerialonthemaximum,undertheexperimentalconditions,φ=2isthebestamountofcitricacidtosynthesizeLFPmaterials.

Fig. 5　 Cycling Performance Curves of the Synthetic LFP Under Different the Amount of Citric Acid at 0.2 C

Fig. 6　 The First Cycling Performance Curves of the Synthetic LFP Under Different Amount of Citric Acid at 0.2 C

3.2PreliminaryStudiesonCombustionMechanism

3.2.1CombustionMethodThereactionofsynthesizeLFPcathodematerialsbycombustionmethodincludesearlysol-gelreactionandredoxreactionduringthepowdersynthesisprocess.Asolcanbeformedbycoordinationcomplexationbetweencomplexingagentandmetalioninsol-gelreaction.Afterthedryingprocess,thedrygelprecursorisobtained.RedoxreactionisperformedinthecombustionprocessofdrygelgeneratedLFPpowder.Sol-gelmodeofsyntheticLFPbycombustionmethodisshowninfig. 7.

a—solution;b—sol;c—gel;d—aging;e—dry gel. Fig. 7　Sol-Gel Mode of Synthetic LFP by Combustion Method

Fig. 8　 TG-DSC Pattern of the Synthetic Sample in Combustion Process

3.2.2ReactionProcessDSC-TGanalysisiscarriedoutonthedrygelinordertostudythecombustionprocessintheconditionofnitrogenatmosphere.Thetestingtemperaturerangeis20～900 ℃andtheheatingrateis20 ℃/min.Thetestresultsarepresentedinfig. 8.

Fromfig.8,wecanseetheweightlossofthedrygelisdividedintothreeparts,accompaningwiththreechangesinthequantityofheat.From20 ℃to120 ℃,thefirstweightlossofdrygelisabout14%ofthetotalmass.Thedeviationmaybecausedbytheoverlapreactionofcombustion.DSCcurveshowsthisprocessisendothermic,resultingfromtheevaporationofthefreewaterfromthedrygel.Afterthen,thedrygelshouldcontainLiNO3,(NH4)2HPO4,FeSO4and citric acid.

From120 ℃to290 ℃,theTGcurveshowsthesecondweightlossisabout54%oftheoverallweightandtheDSCcurvedisplaysanexothermicreactioninthisstep.Theorganiccitricacidabsorbsheattoreachignitiontemperatureatfirstandthencombusts,releasingalargenumberofpost-combustionheat.Afterweightloss,thereshouldbeLiNO3,(NH4)2HPO4and FeSO4.

From290 ℃to800 ℃,theTGcurveshowsthethirdweightlossisabout9%oftheoverallweight.ItmaybeinferredthattheformationofLFPisderivedfromthedecompositionofnitrateandsulfate.TheweightlosscanbetheliberationofNH3,SO2and NO2(the wt.% of released gases is coincident with the third weight loss 9%).The reaction equation is as follow:

2LiNO3+2(NH4)2HPO4+2FeSO4+2C6H8O7+9O2→

2LiFePO4+2NH3↑+2SO2↑+2NO2↑+12CO2↑+14H2O

3.2.3ComplexationMechanismofCitricAcidCitrateisastrongcomplexingagentwhichcanformstablecomplexwithvariousmetalionsespeciallyinacidiccondition.Inthisexperiment,thecomplexationmechanismappliestotheformingprocessofthesol-gel.Metalions(Fe2+，Li+) and citric acid complex by half-and-half mole ratio to form sol[2-14]:

C6H8O7+Fe2+=C6H6O7Fe+2H+,C6H8O7+Li+= C6H7O7Li+H+.

Aftercomplexation,twokindsofcitratecomplexmoleculesareassociatedwithhydrogenbondingtoformgel.Buthydrogenbondingisnotstableanddisconnectedinheatingormoistatmosphere.Theobtaineddrygeliseasytoabsorbwater，whichwillleadtodeliquescence,sothedrygelshouldbeplacedinthefurnaceof600 ℃immediately.Otherwise,itwillhavethehardaggregateafterbeingcalcinedathightemperature.

3.2.4RedoxReactionTheredoxreactionmechanismappliestothecombustionprocessofdrygel.Citricacidrootisakindofstrongreductantion[11-12,15].Withtheincreasingofthecontentofcitricacid(n(citricacid)∶n(Fe2++Li+)≥1),the mixture decomposition changes from multistep decomposition to one-step decomposition with a sharp strong exothermic peak.It is a self-propagating combustion synthesis including two kinds of oxidation reduction reaction in the process:

2C6H8O7+9O2=12CO2↑+8H2O,C6H8O7=3C+3CO+4H2O.

Withtheconsumptionofoxygenincombustionreaction,somecitricacidcan’tcompletelycombust.TheycrackandformacarbonnetscoveringtheparticlewhichpreventstheoxidationofFe2+.Therefore,thefirstincompletecombustionredoxreactionandthesecondcarbonizationreactionistoprovidereducingatmospheretopreventthegeneratedLFPfrombeingoxidized.

4Conclusions

Ferrousnitrate,diammoniumhydrogenphosphateandlithiumnitrateareusedasmainrawmaterialstoprepareLFPbyauto-combustionmethod.TheresultsshowthatLFPissuccessfullypreparedbycombustionmethodwiththeadvantagesofshortreactiontime(6min),highpurity,nooxidationandnoneedtopassintotheinertgas.ThebestamountofcitricacidtosynthesizeLFPmaterialsisφ=n(CA)/n(LFP)=2.Citricacidactsasreductantandcomplexingagentinthewholereaction.AndthecrackingofcitricacidalsobringsincarbonpreventingtheoxidationofFe2+.

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