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        優(yōu)化堿酸法提純石墨

        2024-09-29 00:00:00劉云澤孟繁榮崔學民王林杰何振全李仁濤蓋國勝
        中國粉體技術 2024年3期

        摘要:【目的】優(yōu)化天然石墨提純效果,降低酸在提純過程中的過多使用對環(huán)境造成的影響,滿足各行各業(yè)對高品質石墨的需求,實現(xiàn)更環(huán)保、高效的石墨提純?!痉椒ā恳憎[片石墨為原料,采取NaOH-HCl-HF聯(lián)合處理的工藝對石墨進行提純研究,提高石墨的固定碳含量(質量分數(shù),下同),降低石墨中的主要雜質元素如Si、Fe、Al、Cu等的含量,詳細考察NaOH的用量以及焙燒溫度2個關鍵因素對該工藝提純效果的影響;通過掃描電子顯微鏡(SEM)觀察石墨形貌特征,X射線熒光光譜儀(XRF)和電感耦合等離子體原子發(fā)射光譜儀(ICP)測定提純處理前后石墨的雜質含量,X射線衍射儀(XRD)確定石墨及其灰分的晶體結構?!窘Y果】當焙燒溫度為500℃、焙燒時間為2.5 h,HCl的體積與石墨的質量比為2∶1,氫氟酸的體積與石墨的質量比為2∶1時,石墨的平均固定碳含量從原來的95.3%提高到99.93%;當NaOH與石墨的質量比分別為0.5∶1和0.6∶1時,石墨的平均固定碳含量為99.91%和99.93%;考慮到成本效益等因素,確定當NaOH與石墨的質量比設定為0.5∶1時為理想工藝條件;經(jīng)過提純處理后的石墨層結構并不會出現(xiàn)明顯的變化,基本性能不變,提純處理后的石墨相比于提純處理前的石墨,雜質含量明顯地降低。【結論】該堿酸工藝不僅能有效地去除石墨中的雜質,鹽酸和氫氟酸的組合還可以顯著地提升提純效果,可有望應用在石墨提純處理和新能源材料領域。

        關鍵詞:石墨;堿酸法;加堿焙燒;固定碳;純化

        中圖分類號:TQ127;TB4文獻標志碼:A

        引用格式:

        劉云澤,孟繁榮,崔學民,等.優(yōu)化堿酸法提純石墨[J].中國粉體技術,2024,30(3):76-87.

        LIU Y Z,MENG F R,CUI X M,et al.Optimizing alkaline-acid method for graphite purification[J].China Powder Science and Technology,2024,30(3):76?87.

        石墨是戰(zhàn)略性稀缺資源,制備高純石墨是石墨應用的前提和基礎[1]。石墨是一種主要由電子經(jīng)sp2雜化形成共價鍵結合的碳原子層通過層間電子形成π鍵結合的非金屬礦物,廣泛分布于自然界的變質巖和火成巖之中[1-2]。石墨具有較好的導熱以及導電性能[3-4],可以耐受高低溫,抗化學腐蝕。石墨結構中具有一定的規(guī)則剛度和強度,即使在將近4 000℃時仍然可以保持其堅固性和強度[5]。隨著科學技術的不斷發(fā)展,普通的高碳石墨產(chǎn)品已經(jīng)不能滿足各行各業(yè)的要求,需要進一步提高石墨的純度。石墨提純主要采用物理法[6]和化學法,若要將石墨的含碳量(質量分數(shù),下同)提升至95%~99%或更高,單純使用物理法無法達到此目標,因此需借助于化學法[7-11]?;瘜W法一般包括堿酸法、氫氟酸法、混合酸浸法、氯化焙燒法以及高溫焙燒法等[12-17]。其中,堿酸法具有生產(chǎn)成本較低、產(chǎn)品純度較好和操作流程簡單等優(yōu)點,已成為我國石墨提純生產(chǎn)中最廣泛使用的方法。石墨中的硅酸鹽和石英與氫氧化鈉反應生成可溶性物質,導致雜質去除;石墨中氧化物和堿熔工藝中形成的氫氧化物隨后在酸浸過程中被去除[18]。根據(jù)李玉峰等[19]的研究,通過采用多組正交實驗設計方法得知,影響石墨提純因素按作用程度從小到大依次為HCl使用量、焙燒時間、NaOH濃度、NaOH與石墨的質量比、焙燒溫度??捉ㄜ姷龋?0]以黑龍江鶴崗某地石墨精粉為原料,通過堿酸法工藝獲得固定碳含量為99.92%的提純石墨。李常清等[21]在堿熔過程中創(chuàng)新性地引入偏硼酸鈉作為助熔劑,顯著降低反應溫度并有效提升了除雜效率,從而取得較好的提純效果。前人的研究肯定了調節(jié)焙燒溫度、NaOH與石墨的質量比等因素可以提高石墨純度[22-26],但是傳統(tǒng)的堿酸法所需的焙燒溫度較高(>600℃),能源和物質消耗多,效果差。林祖德等[27]采用氫氟酸法提純廢棄石墨電極,把石墨固定碳含量提升至99.05%。單純的堿酸法或氫氟酸法提純石墨效果不佳且用量過多對環(huán)境造成不良影響,而且利用不同酸的組合來進行后續(xù)酸浸處理的方法鮮有報道,因此,本文中采用堿酸法與氫氟酸洗滌結合的工藝,采用較低溫度(450~550℃)條件下NaOH-HCl-HF聯(lián)合提純法對鱗片石墨進行提純研究,以求在低能耗條件下達到一個良好的提純效果。

        1材料與方法

        1.1試劑材料和儀器設備

        試劑材料:氫氧化鈉(質量分數(shù)為96%,天津市大茂化學試劑廠);鹽酸(質量分數(shù)為36%~38%,國藥集團化學試劑有限公司);氫氟酸(質量分數(shù)為40%,北京北化精細化學品有限責任公司)。

        儀器設備:HH-8型數(shù)顯恒溫水浴鍋(江蘇金壇榮華儀器制造有限公司);101型電熱恒溫干燥箱(余姚上通溫控儀表廠);TDL-5-A型大型離心機(上海安亭科學儀器廠);HTP312型電子天平(上?;ǔ彪娖饔邢薰荆?;DX-2700B型X射線衍射儀(XRD,丹東浩元儀器有限公司);KSW-5-12A型智能纖維電阻爐(天津市中環(huán)實驗電爐有限公司);KYKY-EM6200型掃描電子顯微鏡(SEM,北京中科科儀股份有限公司);ARL Perform'X型X射線熒光光譜儀(XRF,美國賽默飛世爾科技公司);5110(ICP-OES)型電感耦合等離子發(fā)射光譜儀(美國安捷倫科技公司)。

        試樣為鱗片石墨,根據(jù)石墨化學分析國家標準GB/T 3521—2008《石墨化學分析方法》[28]測定鱗片石墨原料的揮發(fā)分(質量分數(shù),下同)為1.20%,灰分(質量分數(shù),下同)為3.50%,固定碳含量為95.30%。

        圖1所示為鱗片石墨原料的SEM圖像。由圖可以看出,鱗片石墨多呈片層結構,片層長度可達100μm以上。圖2所示為石墨原料的XRD譜圖。由圖可以看出,鱗片石墨在2θ為26.6°、54.8°、87.3°處有比較明顯的衍射峰,分別對應鱗片石墨的(002)、(004)、(006)晶面的特征峰,以及存在著峰強較低的雜質衍射峰。圖3所示為鱗片石墨原料灰分XRD譜圖,由圖可以看出灰分中的硅元素主要以SiO2形式存在,鐵元素主要以Fe2O3形式存在。

        石墨原料灰分通過XRF測定的元素組成如表1所示。原料石墨灰分的主要成分為SiO2、Fe2O3、Al2O3、CaO、MgO、K2O以及少量的TiO2、CuO等氧化物。其中SiO2的含量最多,達到45.32%,F(xiàn)e2O3含量達到23.38%,Al2O3的含量達到17.52%,與天然石墨礦中伴生有許多其他礦石有關,如石英(SiO2)、鐵礦石(Fe2O3)、高嶺石、云母石、鋁硅酸鈣(CaO·Al2O3·4SiO2·5H2O),故純化過程主要除去Fe、Si、Al等雜質元素。

        1.2石墨提純

        用電子天平稱取10 g石墨,放入標號的容積為50 mL的坩堝中,NaOH固體顆粒質量分4、5、6 g 3個水平用量加入,用玻璃棒攪拌混合均勻后將坩堝置于可調溫箱式電阻爐中,分別在溫度為450、500、550℃條件下,研究不同NaOH用量及焙燒溫度對鱗片石墨提純的影響。焙燒時間對石墨提純的影響研究則設置在較適宜的焙燒溫度和NaOH用量比例條件下,分析焙燒2.0、2.5、3.0、3.5 h酸洗后的石墨提純效果差異。焙燒反應完畢后,將坩堝用去離子水多次洗滌,將洗滌形成的渾濁液轉移到離心管中,離心分離棄去上清液。將20 mL的鹽酸加入到上述堿熔洗滌的石墨中,然后在80℃的恒溫水浴中加熱并攪拌3 h。之后過濾并用去離子水洗滌直到pH接近中性,然后將濾餅置于溫度為120℃的恒溫干燥箱中干燥3 h,得到第1次酸洗提純后的樣品。取出5 g上述酸洗提純后的石墨到200 mL燒杯中,加入10 mL氫氟酸。在50℃的恒溫水浴中加熱攪拌45 min,然后進行過濾、洗滌直到pH接近中性,最后把濾餅置于溫度為120℃的恒溫干燥箱中干燥3 h得到提純樣品。

        圖4所示為NaOH-HCl-HF聯(lián)合法提純石墨的工藝流程,將2次干燥后的石墨樣品進行含碳量等表征測試,以便對不同提純工藝的效果進行分析和評價。石墨經(jīng)過提純后產(chǎn)生的廢水主要通過化學沉淀、混凝沉淀等方法處理[29-30]。

        2結果與分析

        2.1不同焙燒溫度對HCl洗滌后石墨純度的影響

        不同焙燒溫度對HCl洗滌后石墨固定碳含量的影響見表2所示。不同焙燒溫度下HCl洗滌后的石墨平均固定碳含量見圖5所示。稱取10 g原料石墨,在焙燒時間為2.5 h條件下,焙燒溫度為450℃時,提純石墨的平均固定碳含量為97.35%;焙燒溫度為500℃時,提純石墨的平均固定碳含量達到最大值97.98%;當溫度繼續(xù)增加到550℃時提純效果反而下降到97.86%。這可能是因為焙燒溫度過高時,NaOH與Al2O3、SiO2之間發(fā)生反應形成溶解性較差的鋁硅酸鹽,這種鋁硅酸鹽具有很強的耐酸性,因此難以通過酸浸來使其溶解[31-33]。從上述結果看,焙燒溫度為500℃時提純石墨具有最高的固定碳含量,作為最優(yōu)的焙燒反應溫度。

        2.2 NaOH與石墨不同質量比對HCl洗滌后石墨純度的影響

        不同NaOH用量下HCl洗滌后的石墨平均固定碳含量見圖6所示。由圖可知,當NaOH與石墨的質量比分別為0.4∶1、0.5∶1、0.6∶1時,石墨的平均固定碳含量分別為97.52%、97.55%、98.13%。當NaOH與石墨的質量比超過0.4,石墨固定碳含量的提升效果不大,考慮到環(huán)境、材料以及能源消耗等因素,因此選取NaOH與石墨的質量比為0.4∶1比較合適。其次還對焙燒時間為2.5 h,焙燒溫度為500℃,NaOH與石墨的質量比為0.4∶1和0.5∶1的2種提純樣品進行雜質成分分析,結果見表3。由表3可知,經(jīng)過HCl提純后,SiO2的含量分別降到0.26%和0.62%,F(xiàn)e2O3的含量分別降到0.07%和0.2%,Al2O3的含量分別降到0.21%和0.15%,CuO的含量則降到0.001%和0.002%,說明經(jīng)過堿酸法提純石墨,石墨中的雜質含量大為降低。

        2.3不同焙燒時間對HCl洗滌后石墨純度的影響

        不同焙燒時間對HCl洗滌后石墨固定碳含量的影響見圖7所示。當焙燒溫度為500℃,NaOH與石墨的質量比為0.4∶1條件下,焙燒時間為2 h時,石墨的碳含量為98.21%;在焙燒時間為2.5 h時,石墨的碳含量達到最大值98.26%;當時間繼續(xù)增加到3和3.5 h時,石墨的碳含量分別下降為98.23%、97.92%。這其中的原因可能是焙燒時間過長,少量石墨被氧化,所以使得石墨中固定碳含量下降,因此最佳焙燒時間選擇2.5 h為宜。

        2.4 HCl提純石墨的形貌和晶體結構分析

        HCl提純樣品的石墨在掃描電鏡下的形貌見圖8所示。由圖可以看出,經(jīng)HCl提純處理后,石墨樣品的片狀結構在酸洗提純過程的各個階段保持完整,分層結構的邊緣沒有因高溫加熱而出現(xiàn)卷曲;石墨層結構保持不變,其基本性能保持不變,表明堿酸法提純過程不會改變石墨原有的結構,與Fei等[34]的研究結果相一致。

        在焙燒溫度為500℃條件下,對不同堿用量條件下經(jīng)過HCl處理階段的樣品進行了XRD測試比較,結果如圖9所示。由圖可看出,經(jīng)過焙燒處理與HCl洗滌后的石墨,XRD譜圖中出現(xiàn)的2θ為26.6°、54.8°、87.3°處石墨碳衍射峰的峰強和峰寬與原料石墨比較基本沒變化,但雜峰的變化較為明顯,表明在焙燒酸洗過程中HCl溶解了石墨中的雜質,提高了鱗片石墨的固定碳含量。即使改變了堿和石墨的質量比,也未對石墨晶體結構產(chǎn)生顯著影響。

        2.5不同焙燒溫度對HF二次洗滌后石墨純度的影響

        不同焙燒溫度對HF二次洗滌后石墨固定碳含量的影響見表4所示。稱取5 g經(jīng)過NaOH-HCl提純后的石墨進行試驗。圖10所示為第2次氫氟酸洗滌階段焙燒溫度對石墨純度的影響。從圖可以看出,在焙燒溫度為450、500℃時,HF二次洗滌后石墨平均固定碳含量顯著提升,且含量隨溫度升高而提高,并且在500℃達到最大值99.93%。當焙燒溫度為550℃,HF二次洗滌后的平均固定碳含量下降為99.89%。從二次洗滌效果上看,考慮到能源消耗等因素,選取焙燒溫度為500℃為宜。

        2.6 NaOH與石墨不同質量比對HF二次洗滌后石墨純度的影響

        不同NaOH用量下HF二次洗滌后的石墨平均固定碳含量見圖11所示。從圖可以看出,NaOH用量對提純效果影響顯著。NaOH與石墨質量比為0.4∶1時,石墨的平均固定碳含量為99.73%;當NaOH與石墨的質量比為0.5∶1時,石墨的平均固定碳含量為99.91%;當NaOH與石墨的質量比為0.6∶1時,此時石墨的平均固定碳含量為99.93%??紤]到成本等因素,采用NaOH與石墨的質量比為0.5∶1為理想工藝。利用氫氟酸進行二次清洗提高了固定碳含量,與Jara等[35]的研究結果一致,但本研究提升量較高。

        2.7 HF提純石墨的形貌和晶體結構分析

        焙燒溫度為500℃時HF提純樣品的顆粒微觀形貌見圖12所示。由圖可以看出,經(jīng)HF提純處理后,石墨樣品的片狀結構基本上保持完好,石墨層結構保持不變,基本性能不變,表明進一步使用HF提純不會改變石墨原有的結構。

        為了確定HF處理前后石墨中的晶體結構是否產(chǎn)生較大變化,對經(jīng)過HF處理階段的樣品進行了XRD測試,氫氟酸處理階段不同堿固比樣品的XRD譜圖如圖13所示。由圖可知,在經(jīng)過HF提純后石墨2θ為26.6°、54.8°、87.3°處的特征峰的峰強和峰寬與原料石墨相比差異極微,這說明HF與石墨中的雜質反應提高鱗片石墨的固定碳含量,同時加堿焙燒過程和二次酸洗過程對其晶體結構影響較小。

        3結論

        1)采用堿酸法結合HF清洗的NaOH-HCl-HF聯(lián)合提純法對鱗片石墨具有較好的提純效果,與傳統(tǒng)堿酸法相比具有反應溫度低、提純效率高的特點,且不改變原有的石墨結構。

        2)當焙燒溫度為500℃時,石墨的平均固定碳含量提高到99.93%,繼續(xù)升溫則會導致固定碳含量下降;當堿和石墨的質量比為分別為0.5∶1和0.6∶1時,石墨的平均固定碳含量由原料95.3%分別提高到99.91%和99.93%??紤]到成本因素,堿和石墨的質量比以0.5∶1為宜,繼續(xù)提升堿的比例,固定碳含量提升效果不明顯。

        3)通過SEM、XRD的測定和提純效果實驗的結果都表明,堿酸法提純石墨不會改變石墨本身的晶體結構,且提純效果良好。

        利益沖突聲明(Conflict of Interests)

        所有作者聲明不存在利益沖突。

        All authors disclose no relevant conflict of interests.

        作者貢獻(Author’s Contributions)

        劉云澤、蓋國勝和崔學民進行了方案設計,劉云澤、蓋國勝、王林杰、何振全、孟繁榮和李仁濤參與了論文的寫作和修改。所有作者均閱讀并同意了最終稿件的提交。

        The study was designed by LIU Yunze,GAI Guosheng and CUI Xuemin.The manuscript was written and revised by LIU Yunze,GAI Guosheng,WANG Linjie,HE Zhenquan,MENG Fanrong,LI Rentao.All authors have read the last version of paper and consented for submission.

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        Optimizing alkaline-acid method for graphite purification

        LIU Yunze1,MENG Fanrong2,3,CUI Xuemin1,WANG Linjie4,HE Zhenquan 5,LI Rentao5,GAI Guosheng 2,3,4

        1.School of Chemistry and Chemical Engineering,Guangxi University,Nanning 530004,China;

        2.Wuxi Institute of Applied Technology,Tsinghua University,Wuxi 214100,China;

        3.Shandong Province Powder Material Pilot Demonstration Base,Dongying 257061,China;

        4.School of Chemical Engineering,Shandong Institute of Petroleum and Chemical Technology,Dongying 257061,China;5.Zibo Qingda Powder Materials Engineering Co,Ltd,Zibo 255086,China

        Abstract

        Objective To enhance the purification efficiency of natural graphite and reduce the environmental damage caused by excessive acid use,while meeting the increasing demand for high-quality graphite,a more environmentally friendly and efficient graphite purification process is required.

        Methods A combined NaOH-HCl-HF treatment process was used to purify flake graphite by increasing its fixed carbon content and reducing impurities such as Si,F(xiàn)e,Al,and Cu.The experiment investigated the influence of sodium hydroxide dosage and roasting temperature on the purification process.The study utilised scanning electron microscopy(SEM)to observe the morpho?logical characteristics of graphite.X-ray fluorescence spectrometry(XRF)and inductively coupled plasma atomic emission spectrometry(ICP)were used to detect impurity content in graphite before and after purification.Additionally,the crystalstruc?ture of graphite and its ash was determined using X-ray diffraction(XRD).

        Results and Discussion The majority of the graphite structure retained its flaky form,with lengths over 100μm and relatively thin layers.After purification with HCl and HF,the flake structure of the graphite sample remained intact,with the edges of the layered structure unaffected by high-temperature heating.In XRD spectrum,its diffraction peaks for graphite carbon appeared at 2θ=26.6°,54.8°,and 87.3°,while its peak intensities and widths remained essentially unchanged compared to raw graph?ite.This indicated that the alkali-acid purification process did not alter the intrinsic structure of the graphite itself.Whenroast?ing for 2.5 hours at 450℃,500℃,and 550℃,and with a hydrochloric acid(mL)to graphite(g)ratio of 2∶1,the NaOH-HCl method resulted in an average fixed carbon contents of 97.35%,97.98%,and 97.86%,respectively.If the roasting tempera?ture was excessively high,NaOH would react with Al2O3,SiO2,and other substances to form aluminosilicates,which had poorsolubility and exhibited strong resistance to acid,making it difficult to dissolve through acid leaching.The average fixed carbon content of graphite increased with NaOH graphite ratio,reaching 97.52%,97.55%,and 98.13%at ratios of 0.4∶1,0.5∶1,and 0.6∶1,respectively.However,its fixed carbon content only improved marginally at ratios beyond 0.4∶1.Therefore,consid?ering cost and energy consumption,a mass ratio of 0.4∶1 was recommended when roasting at 500℃.The fixed carbon content of graphite increased gradually with longer roasting time,reaching a maximum value at 2.5 hours before gradually decreasing.At this point,the carbon content of graphite was 98.26%.The fixed carbon content in graphite may decrease due to excessive roasting and minor graphite oxidation.The NaOH-HCl purification method was used to reduce SiO2 content to 0.26%and 0.62%,F(xiàn)e2O3 to 0.07%and 0.2%,Al2O3 to 0.21%and 0.15%,and CuO to 0.001%and 0.002%,respectively.The results indicated that the impurity content of graphite significantly decreased after purification.Average fixed carbon contents were 99.91%and 99.93%for NaOH graphite ratios of 0.5∶1 and 0.6∶1.However,considering factors such as cost and efficiency,it was determined that a mass ratio of 0.5∶1 for NaOH and graphite was optimal,which met the ideal process conditions.

        Conclusion The alkaline-acid process can effectively remove impurities in graphite and reduce the environmental harm caused by excessive use of hydrofluoric acid.Additionally,the combination of hydrochloric acid and hydrofluoric acid can significantly improve the purification effect.This method is expected to be used in the fields of graphite purification treatment and new energy materials.

        Keywords:graphite;alkali-acid process;alkaline roasting;fixed carbon;purification

        (責任編輯:王雅靜)

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