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熔盐电脱氧法制备金属铬及镍铬合金
刘政伟
学位类型博士
导师徐红彬
2017-07
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工艺
关键词熔盐电脱氧 氧化铬 金属铬 镍铬合金 亚铬酸钙
摘要

固态金属氧化物熔盐电脱氧(FFC-剑桥工艺)制备金属、合金是由剑桥大学开始并发展起来的具有创新性意义的一种金属、合金制备新工艺,该方法工序简单,反应温度低,节约成本,适于制备难熔金属及合金。金属铬(Cr)及其合金具有高硬度、耐磨性、优异的抗腐蚀能力,广泛用于冶金、航空、军工、汽车等领域,但是其现有制备工艺复杂、成本高、污染环境。在湿法冶金清洁生产技术国家工程实验室亚熔盐法铬盐生产技术制备铬酸盐、氧化铬(Cr2O3)系列产品基础上,为实现产品升级和扩展,提出了熔盐电脱氧法制备金属铬及镍铬合金的研究课题。本论文综合考察了熔盐体系、阴极制备、电解参数及石墨阳极对Cr2O3熔盐电脱氧过程的影响,确定了较优工艺条件,获得了纯度为99.18%金属铬产品;研究了Cr2O3在氯化钙(CaCl2)熔盐中电脱氧过程机理,并基于机理的思考提出了工艺优化措施,抑制中间产物亚铬酸钙(CaCr2O4)生成带来的不利影响,提高了电解速率和电流效率;通过往Cr2O3中添加NiO,成功制备了具有不同摩尔比的镍铬合金并对合金形成机理进行了探讨。本论文主要在以下几个方面取得了创新性成果:(1) 综合考察了熔盐体系、阴极制备及电解参数对Cr2O3熔盐电脱氧过程的影响。发现CaCl2-NaCl混合熔盐中,Cr2O3电解产品氧含量最低为0.53%,金属Cr纯度为95.78%,产品颗粒粒径小,约500 nm,表面存在氧化膜,电流效率60%;CaCl2熔盐中Cr2O3电解产品氧含量约0.17%,金属Cr纯度高,为99.18%,产品呈节支状颗粒,粒径大,最大可达5 μm。可根据需要选择不同熔盐体系、电解温度,有目的地控制节支状金属Cr颗粒粒径。(2) 阳极石墨腐蚀与石墨抛光面初始表面状态密切相关,O2-优先在石墨表面的坑道、缝隙处发生反应释放CO、CO2气体。认为石墨腐蚀的三个主要原因如下:一是与O2-反应产生CO、CO2;二是石墨表面悬浮颗粒直接从石墨腐蚀表面物理脱落;三是CO2溶解在熔盐中形成CO32-,其在阴极电还原得到碳的同时产生O2-,由此引起O2-恶性循环。阴极产品Cr的碳污染最有可能是由于CO32-阴极电还原生成的碳与金属Cr直接接触、反应生成碳化铬导致的。(3) 研究了CaCl2熔盐中Cr2O3电脱氧过程机理及形貌变化,首先球形Cr2O3电脱氧生成立方体金属Cr,释放出O2-;该释放出的O2-扩散到周边区域,与熔盐中的Ca2+和Cr2O3颗粒反应生成片状CaCr2O4中间产物;最终该片状CaCr2O4电脱氧生成节支状金属Cr。同时,电解初期形成的立方体金属Cr逐渐转变为节支状金属Cr。由于CaCr2O4较Cr2O3、2Cr摩尔体积大,其生成堵塞Cr2O3片体内部孔隙,阻止内部O2-扩散到熔盐中,并且导致片体易破碎。(4) 创新性地提出了熔盐合成法制备片状δ-CaCr2O4的新方法,并在Ar中煅烧δ-CaCr2O4成功制备了棒状β-CaCr2O4,系统考察了反应时间对产物的影响并在期刊文章中详细研究了两种晶型CaCr2O4的磁性能。(5) 基于对Cr2O3在CaCl2熔盐中电脱氧机理的思考,提出直接采用CaCr2O4为原料在CaCl2熔盐中电脱氧制备金属Cr的改进工艺,发现,与Cr2O3电解相比,CaCr2O4的电解过程更快、电流效率更高,这是由于2Cr/CaCr2O4摩尔体积比较小,在CaCr2O4电脱氧过程中产生更多孔隙,利于熔盐和O2-及时扩散。鉴于此,预测采用相应的含钙金属盐为原料可以提高其它金属氧化物如ZrO2、Nb2O5、Ta2O5等电脱氧速率,有望形成一种通用方法。添加NH4HCO3造孔剂提高了Cr2O3片体孔隙率,促进了熔盐和O2-在片体内部孔隙中的扩散,为电解过程中致密CaCr2O4的生成带来的体积膨胀留有空间,抑制了CaCr2O4的生成带来的负面影响,改善了片体电解均匀性和完整性。(6) 在Cr2O3熔盐电脱氧制备金属Cr工作基础上,通过向Cr2O3中添加NiO经熔盐电脱氧制备得到了不同摩尔比Ni-Cr合金,并在工艺优化的基础上,研究了NiO-Cr2O3烧结片体电脱氧反应机理,发现NiO-Cr2O3烧结片体首先生成金属Ni和中间产物CaCr2O4,然后中间产物CaCr2O4在新生成的金属Ni表面发生电脱氧、直接生成最终产品物相,过程中未检测到独立的金属Cr物相。 

其他摘要

The recent electro-deoxidation, namely the FFC-Cambridge process, of solid metal oxide or oxides offers a simple, low-temperature and cost-saving technology for the preparation of refractory metal and alloys in molten salt, which is initiated and developed by the Cambridge University. Metallic chromium (Cr) and its alloys present good properties like its high hardness, wear resistance and excellent corrosion resistance, which are widely used in metallurgy, aviation, military, automotive and other fields. However, its current preparation process is complex, high cost, and environmental pollution. Based on the clean production of chromate and chromium oxide (Cr2O3) products by the sub-molten salt technology of National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, the research project ‘Preparation of Chromium and Nickel-Chromium Alloy by Electrochemical Deoxidation in Molten Salts’ was proposed in order to realize the product upgrading and extension.The influence of molten salt system, cathode preparation, electrolysis parameter and the graphite anode on the electro-deoxidation of Cr2O3 was thoroughly investigated. Under the optimum conditions, the Cr product with a purity of 99.18% was obtained. The reaction mechanism of the Cr2O3 electro-deoxidation in CaCl2 molten salt was proposed. Based on the proposed mechanism, some measures to optimize the process were taken in order to avoid the adverse effects of the formation of the intermediate product calcium chromite (CaCr2O4), and to improve the electrolysis speed and the current efficiency. By the addition of NiO to Cr2O3 powder, the Ni-Cr alloy with different Ni/Cr molar ratio was successfully prepared and its formation mechanism was also explored.The main innovative work is summarized as follows:1. The influence of molten salt system, cathode preparation and electrolysis parameter on the electro-deoxidation of Cr2O3 was thoroughly investigated. The obtained Cr product with a purity of 95.78% from the Cr2O3 electro-deoxidation in CaCl2-NaCl molten salt contains 0.53% oxygen. Its particle size is small, about 500 nm, and covered with a thin oxide film. However, in CaCl2 molten salt, the Cr product with a purity of 99.18% contains 0.17% oxygen. Its particle size is large, the maximum size up to 5 μm and presents a nodular morphology. Based on the needs, the Cr particle size can be controlled on purpose by selecting different molten salt system and electrolysis temperature.2. The graphite corrosion behavior is greatly related with the initial surface state provided by graphite anode. The reaction between graphite anode and the dissolved O2- preferentially occurs on the potholes and crack zones of the polished graphite surface, giving out the CO and CO2 gas. The graphite corrosion is attributed to the consumption with O2- to form the CO and CO2 gas, the graphite particle detachment of the loosely-connected graphite particles on the corroded surface due to the physical attack, and the formation of CO32- owing to the dissolution of CO2 in the molten salt. The as-formed CO32- near the anode area can further diffuse to the cathode and be electro-reduced to carbon nanotube, at the same time releasing O2-, which causes a vicious cycle in the molten salt. The carbon contamination in the electrolyzed product is most likely due to the reaction between the produced Cr and the carbon originating from the electro-reduction of the dissolved CO32- near the cathode.3. The cathodic reaction pathway in the electrolysis of Cr2O3 is summarized as follows: at the early stage, the spherical Cr2O3 releases O2-, forming cubic Cr; then the released O2-, in combination with Ca2+ and the Cr2O3 particles, chemically forms the large platelet CaCr2O4 in nearby locations; finally, the formed intermediate product CaCr2O4 gradually decomposes to nodular Cr. At the same time, the cubic Cr particles formed from the electrolysis of Cr2O3 at the early stage gradually transform to nodular Cr particles with the electrolysis process progresses. Due to the molar volume of CaCr2O4 larger than that of Cr2O3 and Cr, its formation blocks the pores inside the Cr2O3 pellet, inhibits the diffusion of O2- out of the pellet to the molten salt and result in the easy breaking of the pellet during the electrolysis.4. δ-CaCr2O4 was creatively synthesized using a molten salt synthesis method, which was further calcined in Ar atmosphere to prepare rod-like β-CaCr2O4. The influence of reaction time on both materials was thoroughly investigated and their respective magnetic property was introduced in journal paper.5. Based on the discovery of the reaction mechanism of the Cr2O3 electro-deoxidation in CaCl2 molten salt, the CaCr2O4 as the raw material to prepare the Cr by the electro-deoxidation method was proposed. The results show the electro-deoxidation of CaCr2O4, compared to the Cr2O3 raw material, proceeds with a faster electrolysis speed and higher current efficiency. That is due to the small molar ratio of 2Cr/CaCr2O4, the removal of Ca2+ and O2- genenrtes a more porous structure to facilitate the diffusion of molten salt and O2- during the electro-deoxidation of CaCr2O4. For that reason, the direct exploitation of calcim-containing species could accelerate the electrolysis speed of other oxides like ZrO2、Nb2O5、Ta2O5, which also encounter the formation of calcim-containing species. The addition of NH4HCO3 pore-forming agent can improve the porosity of the Cr2O3 pellet, which will facilitate the the diffusion of molten salt and O2- and suppress the adverse impact of the formation of densely stacked CaCr2O4. The NH4HCO3 addition enhances the electrolysis uniformity and the pellet remains compact after electrolysis.6. On the basis of the elctro-deoxidation process to prepare the Cr, the Ni-Cr alloy with different Ni/Cr molar ratio was successfully prepared by the addition of NiO to Cr2O3 powder. And its formation mechanism was also explored. Firstly, the reduction starts with the formation of Ni metal, accompanied with the formation of an intermediate product CaCr2O4. Then, the formed intermediate product CaCr2O4 is further electro-reduced to Cr, which is directly alloyed with the newly-formed Ni at the early stage to form the target Ni-Cr intermetallic compound. Pure Cr phase is not detected when the Cr content is relatively low in the NiO-Cr2O3 mixture 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/24232
专题研究所(批量导入)
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刘政伟. 熔盐电脱氧法制备金属铬及镍铬合金[D]. 北京. 中国科学院研究生院,2017.
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