简介概要

Electrolytic properties and element migration in Ni-TiB₂/Al₂O₃composite cathode

来源期刊：Rare Metals2021年第1期

论文作者：Yu-Dong Liang Li-Jun Wang Deng-Peng Chai Sheng-Zhong Bao Ting-Ting Niu Jun-Wei Wang Ying Liu

摘要：With alumina sol as binder and Ni metal as sintering aids,the Ni-TiB₂/Al₂ O₃ composite cathode material for aluminum electrolysis was prepared by coldpressed sintering.The mechanical properties of the composite cathode material were measured.Its electrolytic properties were identified by a 20-A electrolysis test.Cathode samples before and after electrolysis test were measured by energy-dispersive spectroscopy（EDS）.The migration behavior of various elements in the electrolysis process was studied by phase analysis.The result shows that Ni metal can effectively fill the gap between the aggregate during the sintering process,which can improve the sintering density of the composite cathode material significantly.The voltage of the 20-A electrolysis test is stable.The impurity of aluminum liquid is 0.42%.The aluminum liquid can wet the cathode surface effectively,and the Ni-TiB₂/Al₂ O₃ composite is an ideal wettable cathode material.In the process of electrolysis,the alkali elements in the electrolyte penetrate the electrode,where K goes deeper than Na.Al generated on the cathode surface will also penetrate the cathode through the gap of the composite material,while Ni in the electrode will spread into the aluminum liquid layer.

稀有金属(英文版) 2021,40(01),57-64

Electrolytic properties and element migration in Ni-TiB₂/Al₂O₃composite cathode

Yu-Dong Liang Li-Jun Wang Deng-Peng Chai Sheng-Zhong Bao Ting-Ting Niu Jun-Wei Wang Ying Liu

Rare Metals and Metallurgy Materials Research Institute, General Research Institute for Nonferrous Metals

Zhengzhou Nonferrous Metals Research Institute Co.,Ltd

作者简介：Li-Jun Wang e-mail:lyd_works@126.com;

收稿日期：6 August 2018

基金：financially supported by the Major Science and Technology Projects of Henan Province (No. 131100210700);the Major Science and Technology Programs of CHALCO (No.ZB2013CBBCe1);

Electrolytic properties and element migration in Ni-TiB₂/Al₂O₃composite cathode

Yu-Dong Liang Li-Jun Wang Deng-Peng Chai Sheng-Zhong Bao Ting-Ting Niu Jun-Wei Wang Ying Liu

Rare Metals and Metallurgy Materials Research Institute, General Research Institute for Nonferrous Metals

Zhengzhou Nonferrous Metals Research Institute Co.,Ltd

Abstract：

With alumina sol as binder and Ni metal as sintering aids,the Ni-TiB₂/Al₂ O₃ composite cathode material for aluminum electrolysis was prepared by coldpressed sintering.The mechanical properties of the composite cathode material were measured.Its electrolytic properties were identified by a 20-A electrolysis test.Cathode samples before and after electrolysis test were measured by energy-dispersive spectroscopy(EDS).The migration behavior of various elements in the electrolysis process was studied by phase analysis.The result shows that Ni metal can effectively fill the gap between the aggregate during the sintering process,which can improve the sintering density of the composite cathode material significantly.The voltage of the 20-A electrolysis test is stable.The impurity of aluminum liquid is 0.42%.The aluminum liquid can wet the cathode surface effectively,and the Ni-TiB₂/Al₂ O₃ composite is an ideal wettable cathode material.In the process of electrolysis,the alkali elements in the electrolyte penetrate the electrode,where K goes deeper than Na.Al generated on the cathode surface will also penetrate the cathode through the gap of the composite material,while Ni in the electrode will spread into the aluminum liquid layer.

Keyword：

Aluminum electrolysis; TiB₂; Alumina sol; Cold-pressed sintering; Infiltration;

Received： 6 August 2018

1 Introduction

As a nonferrous metal with the largest output,aluminum is one of the most important pillar metals of the Chinese economy.The current process of aluminum reduction is the Hall-Heroult electrolysis method [ 1] .The main reaction equation is 2Al₂O₃+3C=6Al(l)+3CO₂↑.After more than 100 years of development,this process has been greatly improved in many aspects such as current efficiency,bath life and production automation.However,there are still many technologies far beyond satisfaction.The carbon anode is one among them.The carbon anode consumed for producing each ton of primary aluminum exceeds 400 kg,and the emission of equivalent CO₂ is as high as 6.3 t per ton of aluminum.The anode effect is accompanied by the emission of asphalt flue gas,acid gas and strong greenhouse gas CF_n [ 2] .In addition,the power utilization rate of this process is less than 50% [ 3] .Meanwhile,the power consumption in aluminum industry is quite high.Presently,the average power consumption per ton of primary aluminum in China is 13,900 kWh.The power consumed for aluminum reduction accounts for about 6%of the total power generated in China.On the other hand,Chinese aluminum industry suffered greater stress from the increasingly stringent environment protection regulations,which greatly affects the development of aluminum industry [ 4] .Therefore,a new aluminum electrolysis technology based on inert anode materials and wettable cathode materials has become the research focus of the aluminum industry [ 5, 6, 7, 8, 9] .

At present,the extensively researched inert anode materials are all temperature-sensitive.The high temperature of 950℃for traditional primary aluminum reduction will greatly reduce the service life of inert anode.Therefore,aluminum electrolysis systems based on inert anodes,wettable cathodes and low-temperature electrolyte systems are increasingly favored [ 10, 11, 12] .

Wettability refers to the spreading capability of liquid phase on solid surface.TiB₂ has been proven to be well wetted by molten aluminum.Instead of a certain height of aluminum liquid layer on the cathode surface in traditional process,a thin aluminum liquid layer covering the cathode surface to form a stable cathode system is well served in the new process,which effectively reduces the disturbance of the magnetic field to the aluminum liquid,as well as reduces the electrode distance significantly and energy consumption.Therefore,TiB₂ is a preferred material for wettable cathodes [ 13] .Currently,researches of TiB₂wettable cathode materials are mainly focused on TiB₂ceramic cathode materials [ 14] ,TiB₂/C composite cathode materials [ 15, 16, 17, 18] and TiB₂/metal composite cathode materials [ 19, 20, 21] .However,the strong covalent bonding [ 22] and thermal expansion anisotropy of TiB₂ materials result in the high melting point and low diffusion coefficient,and the TiB₂ materials are difficult for densely sintering.The dense TiB₂ ceramic cathode materials are usually obtained through hot-press sintering.The complicated sintering process,the high cost and the difficulty to prepare a complex-shaped material restrict the popularization and application of TiB₂ ceramic materials.In order to address these issues,the preparation of composite TiB₂materials has been proposed,such as the metal phase can be combined with TiB_2.The metal phase is as a sintering aid to form a small range of liquid phase,which promotes solid-phase particle reaction and reduces sintering temperature.Finch and Tannery [ 23] studied the effects of different Ni contents on the properties of TiB₂,suggesting that a certain amount of Ni can significantly improve the sintering performance of TiB₂,but reduces its erosion resistance to aluminum liquid.According to Sylvie et al. [ 24] ,adding a small amount of metal Ni,NiB and Fe is helpful to the pressureless sintering of TiB₂ at1300-1700℃.When the sintering temperature reaches1500℃,the relative density of the sintered body is over94%.But when the sintering temperature is over high,abnormal particles growth occurs.

Currently,alumina sol is mainly used as inorganic binders to prepare TiB₂ cathode coatings [ 25, 26] .Han and Zhang [ 27] prepared TiB₂/Al₂O₃ composite cathode materials using alumina sol as binder.The effects of sintering temperature,sintering time and sol addition on the relative density,resistivity and compressive strength of the composite cathode were studied.However,the electrolysis performance of the composite cathode was not thoroughly studied.In this paper,the above two methods for preparing TiB₂ composites were combined.Using alumina sol as binder and Ni metal as sintering aids,Ni-TiB₂/Al₂O₃composite cathode material was prepared by cold-pressed sintering method.The microstructure of the cathode material,the electrolytic properties and the interaction between the electrolyte and the electrode were systematically studied.

2 Experimental

2.1 Sample preparation

2.1.1 Alumina sol preparation

The raw material used for preparing the alumina sol is boehmite powder produced by the Zhengzhou Light Metal Research Institute.A certain amount of deionized water was put into a conical flask.The conical flask was placed in a DF-101S-type thermostatically heated magnetic stirrer(Zhengzhou Great Wall Technology&Trade Co.,Ltd.).When the condenser tube was installed and fixed,condensation water was turned on and heated to peptization temperature,and then the temperature was kept constantly.Then the weighed boehmite powder was added,stirring at constant temperature for 50 min.The magnetic rotor speed was controlled at 35 r-s^-1;finally,a certain amount of acetic acid was added,stirring at constant temperature for45 min,and then the speed increased to about 40 r-s^-1 to guarantee a sufficient peptization [ 28] .At last,the alumina sol was taken out at room temperature and set aside.

2.1.2 Preparation of Ni-TiB2/Al2O3 composite cathode

TiB₂ powder (purity greater than 98%,particle size of less than 74μm),Ni powder (purity greater than 99.9%,particle size of less than 74μm),and alumina sol were mixed to form a paste in accordance with the ratio of 85:5:10,and then the paste was put into the oven to dry most of the water.Then the solid material was filled in a special mold,then prepressed under a pressure of 25 kN,and pressed under a pressure of 50 kN for 5 min.Finally,the sample was loaded into corundum crucible,and then the powder was filled in.The crucible was moved into a vacuum sintering furnace and sintered to obtain a Ni-TiB₂/Al₂O₃composite cathode.Based on the research results [ 10] and previous experimental data,the material was sintered at1500℃for 3 h.Microstructure and phase compositions of the sintered samples were analyzed by a scanning electron microscope (SEM,JSM6360LV) equipped with an energydispersive spectrometer (EDS).

2.2 Electrolytic performance test

A NaF-KF-AlF₃ low-temperature electrolyte system was used.The cryolite ratio is 1.42,electrolysis temperature is800-820℃,Al₂O₃ concentration is 5%,and the anode current density is 0.5 A-cm^-2.The electrolytic experimental setup diagram is shown in Fig.1.Finally,the aluminum balls in the electrolyte were collected,the surface electrolyte was removed,and the aluminum product was weighed.The current efficiency (CE) was calculated by the following equation:

where W_act is the weight of aluminum produced in the experiment,0.3356 is the electrochemical equivalent of aluminum,I is the electrolytic current and t is the electrolysis time.The impurity content in the aluminum liquid was analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES,HK-2000).

3 Results and discussion

3.1 Microstructure analyses of sintered samples

Figure 2 shows the microstructure of the sintered composite cathode material.It can be seen that when composite cathode material was sintered at 1500℃,the white Ni alloy phase,the gray TiB₂ ceramic phase and the black Al₂O₃ binder phase were dispersed together and equally distributed,which is in line with the result in Ref. [ 29] .There is an obvious sintering agglomeration phenomenon in local alumina,indicating an over high sintering temperature;Ref. [ 30] shows that alumina can convert toα-Al₂O₃ at from 1200 to 1400℃,and it can be completely transformed into a more stableα-Al₂O₃ at 1500℃,which has denser and smoother apparent morphology.Its physical and chemical properties are more stable compared with other crystalline alumina.The higher strength is a preferred composite mechanical property.

Fig.1 Schematic structure for aluminum electrolysis experiment

In Fig.2a,the phase compositions by EDS is also marked.It can be seen that at 1500℃,the liquid phase Ni fills in the void between the TiB₂ particles.Owing to the capillary force,the metal phase material is forced into the particles gaps and plays a role in bonding.It shows a clear branch shape.It can be seen that due to the addition of the metal phase,part of the TiB₂ matrix is formed as a whole by the bonding of the Ni metal,and the bonding degree is higher.Figure 2b shows the microstructure of the pure sintered TiB₂ material.It can be seen that without the addition of sintering aids,the sample is loose and porous,with low bonding degree.

As stated above,alumina sol and Ni both have effect on improving the density and the bonding strength.The relative density of Ni-TiB₂/Al₂O₃ composite cathode materials measured was measured by wax immersion method(ASTM C373-88,1999),which was compared with TiB₂/Al₂O₃ composites prepared in Ref. [ 27] .It can be seen that the relative density of the composite material increases by1.69%to 94.17%compared to that of TiB₂/Al₂O₃ composite material in Ref. [ 27] through the addition of Ni metal.

3.2 Electrolytic performance test

Figure 3 a shows electrolytic voltage curve of Ni-TiB₂/Al₂O₃ composite cathode for 24 h.The voltage value rises slowly with time.This is due to the constant consumption of the carbon anode,resulting in an increase in the electrode distance and a gradual increase in voltage value from3.3 V to about 3.5 V.The voltage curve shows that the electrolytic voltage is relatively stable,which is a positive basis for obtaining higher current efficiency.Figure 3b shows macro-morphology of Ni-TiB₂/Al₂O₃ composite cathode after electrolysis.In the process,the cathode surface is covered by a dense layer of aluminum liquid,and no obvious crack or falling cuttings are seen.The thickness and density of the liquid layer in the lower part of the cathode are better than those in the upper part.Because of the vertically arranged electrolysis system,the aluminum liquid is generated on the surface of the cathode and continuously thickens,and then agglomerates at the bottom due to gravity,and finally falls into the electrolyte.

Figure 4a shows the cross-sectional image of the aluminum layer of the Ni-TiB₂/Al₂O₃ composite cathode.It can be seen that the cathode surface is covered by dense aluminum,indicating that the Ni-TiB₂/Al₂O₃ composite cathode material is well wetted by the aluminum liquid.Figure 4b shows macro-photographs of aluminum produced by the 20-A electrolysis test.Totally,149.28 g primary aluminum was collected in this test.The calculated current efficiency is 92.7%,the aluminum content is99.58%,and the impurity content is 0.42%,among which Fe content is 0.21%,which may be due to the corrosion and the guide bar falling into the melt under electrolyte atmosphere.Fe impurities can be completely avoided in later experiments by optimizing electrode assembly.Si content is 0.013%,probably resulting from the presence of a small amount of Si in the carbon germanium.The impurity content of Si will gradually decrease as the electrolysis experiment time continues to increase.Ni content is0.187%.Based on the aluminum yield,the nickel is calculated to be 0.277 g.Ni in the aluminum is diffused back into the electrolyte from the cathode and then into the aluminum liquid.The Ni-TiB₂/Al₂O₃ composite cathode can be used as the focus of further research.

Fig.2 SEM images of a sintered composite cathode material and b pure sintered TiB₂ material

Fig.3 20 A electrolysis test of Ni-TiB₂/Al₂O₃ composite cathode:a voltage curve and b cathode surface after test

Fig.4 20 A electrolysis test of Ni-TiB₂/Al₂O₃ composite cathode:a SEM image of section diagram of aluminum liquid layer on surface of cathode and b macrograph of aluminum produced in aluminum electrolysis test

3.3 Change in cathode interface morphology and composition after electrolysis

In normal electrolysis,the precipitation potential of A1 is higher than that of Na by about 250 mV.During the initial electrolysis,the higher Al concentration contributed to a downward precipitation potential of Al.Metal A1 preferentially precipitated on the cathode surface and formed a dense layer of aluminum liquid.This layer of aluminum hindered the precipitation of Na on the cathode surface,ensuring the normal operation of electrolysis.In the electrolysis process,the aluminum liquid layer on the cathode surface was constantly changing.When the thickness reached a certain level,the aluminum liquid started sinking downward to the bottom of the electrolyte.Since the aluminum liquid did not completely wet the carbon material,the extracted solid aluminum was found in a spherical form when the test was finished.

Cross-sectional SEM images of Ni-TiB₂/Al₂O₃ composite cathode after electrolytic test are shown in Fig.5a,b.It can be seen from Fig.5a that the metal aluminum is completely wetted on the surface of composite cathode.Figure 5b shows an enlarged part of Fig.5a.EDS results of Points A-D are shown in Fig.5c-f,respectively.It is known that Ni-TiB₂/Al₂O₃ composite cathode can be well wetted by aluminum liquid.The liquid acts as a barrier,which slows down the penetrating speed against K,Na and electrolyte penetrating the matrix.

In the liquid aluminum layer,a trace amount of Ni is found,indicating that Ni in cathode material reversely penetrated the electrolyte and finally deposited along with the aluminum on the surface of the cathode.In Fig.5b,K and Na are not detected in the TiB₂ aggregate of the composite cathode,but K and Na are detected in the binder phase,and F is also detected.It indicates that the alkali metal penetrates the binder phase with the electrolyte.

Fig.5 a,b SEM images of sample surface and c-f EDS results corresponding to Points A-D in a,b,respectively

The statistical EDS results in Fig.5c-f show that in the binder phase on the cathode surface,besides the O and Al being introduced by Al₂O₃ sol,the statistical F content is large,with certain amount of Na and K.This shows that the electrolyte enters the cathode material through the pores of the cathode material and then penetrates the binder phase.Al and O are found on the TiB2 matrix material,indicating that the TiB₂ matrix material is surrounded by Al₂O_3·No Na is found in the TiB₂ matrix.K content is 1.29%,which may be due to surface contamination.

Figure 6a,b shows cross-sectional SEM images at the middle of the cathode,and Fig.6c-f shows corresponding EDS statistics of points A-d.As Fig.6c shows,no other element is found in the TiB₂ aggregate.Figure 6f shows a trace amount of K in the alumina binder phase,which is about 1.73%,while no Na and F are found,indicating that K has stronger ability of penetration to cathode than Na.Figure 6e shows that in the metal phase,besides Ni,approximately 6.26%Al and 15.22%Ti are found,indicating that Al precipitated on the surface of the cathode enters the material through the cathode pores and then enters the metal phase.Ti comes from TiB₂ aggregate.Finally,Na and K are not observed in TiB₂ matrix at the middle of the cathode cross section.This is closely related to the density of the sintered TiB₂ material and the effective resistance from the aluminum layer.

As stated above,the penetration of Ni-TiB₂/Al₂O₃composite cathode during the electrolysis process can be inferred as follows:The alkali metal first infiltrates the pores of the cathode together with the electrolyte,and then gradually penetrates the binder phase.In the electrolysis process,K penetrates the cathode more deeply than Na.At the same time,metal Al enters Ni metal phase through the voids of the cathode material,and metal Ni diffuses back into Al metal phase on the cathode surface.

Fig.6 a,b SEM images of central cathode and EDS results corresponding to Points A-D in a,b,respectively

The Ni-TiB₂/Al₂O₃ composite cathode cannot prevent alkali metals’penetrating completely,but it acts better than carbon materials to a certain extent.This is closely related to the density of the composite material and its own density.In addition,a key to guaranteeing the efficient and long-term usage of composite cathode material is that the aluminum liquid layer on the cathode surface can be stably generated and maintained.

4 Conclusion

With alumina sol as binder and Ni metal as sintering aids,Ni-TiB₂/Al₂O₃ cathode material was prepared by coldpress sintering.The addition of Ni metal can not only reduce the sintering temperature,but also significantly improve the density.The electrolysis performance of NiTiB₂/Al₂O₃ composite cathode was studied under 20-A electrolysis experiment.The entire electrolysis process voltage is stable.The efficiency reaches more than 92%.The quality of primary aluminum is close to that of commercial aluminum.The impurity content in the aluminum liquid is low,and aluminum and alkali metal elements on Ni-TiB₂/Al₂O₃ composite cathode material mainly penetrate pores of the electrolyte and then enter the binder phase.K penetrates significantly deeper than Na;Al passes inward the voids of the cathode material,while Ni metal in the electrode will spread into the aluminum liquid layer.Liquid can effectively wet the cathode,which indicates that Ni-TiB₂/Al₂O₃ composite cathode is an ideal wettable cathode material.