阅读说明：本技术 一种计算铝电解质体系电导率的方法 (Method for calculating conductivity of aluminum electrolyte system ) 是由 韩硕 范培育 白晓伟 黄涌波 池君洲 刘大锐 闫淑君 李伦 原铎 徐宏雷 吴永峰 于 2020-07-02 设计创作，主要内容包括：本发明公开了一种计算铝电解质体系电导率的方法,所述铝电解质体系包括NaF-AlF<Sub>3</Sub>基电解质及添加剂,所述铝电解质离子结构包括配位数分布和氟类型分布,所述配位数分布为铝电解质中四配位铝氟团簇[AlF<Sub>4</Sub>]-、五配位铝氟团簇[AlF<Sub>5</Sub>]<Sup>2-</Sup>和六配位铝氟团簇[AlF<Sub>6</Sub>]<Sup>3-</Sup>的含量分布,所述氟类型分布为铝电解质中桥接F、终端F和自由F<Sup>-</Sup>离子含量分布。本发明基于铝电解质中Al-F原子对强烈共价相互作用特性,结合简单自由离子体系计算电导率的Nernest-Einstein公式,提出改进的公式来计算铝电解质体系的电导率,为铝电解质熔盐体系电导率计算提供了一种精确的理论计算方法。(The invention discloses a method for calculating the conductivity of an aluminum electrolyte system, wherein the aluminum electrolyte system comprises NaF-AlF 3 A base electrolyte and an additive, wherein the ionic structure of the aluminum electrolyte comprises a coordination number distribution and a fluorine type distribution, and the coordination number distribution is four-coordination aluminum fluorine cluster [ AlF ] in the aluminum electrolyte 4 ]-, pentacoordinate aluminum fluorine cluster [ AlF ] 5 ] 2‑ And hexacoordinated aluminum fluorine clusters [ AlF ] 6 ] 3‑ The fluorine type distribution is bridging F, terminal F and free F in the aluminum electrolyte ‑ Ion content distribution. The invention provides an improved formula for calculating the conductivity of an aluminum electrolyte system based on the strong covalent interaction characteristic of Al-F atom pairs in the aluminum electrolyte and combined with a nernestest-Einstein formula for calculating the conductivity of a simple free ion system, and the invention is the aluminum electrolyteThe conductivity calculation of the molten salt system provides an accurate theoretical calculation method.)

1. A method for calculating the conductivity of an aluminum electrolyte system is characterized by comprising the following steps: the aluminum electrolyte system comprises NaF-AlF₃A base electrolyte and an additive, wherein the ionic structure of the aluminum electrolyte comprises a coordination number distribution and a fluorine type distribution, and the coordination number distribution is four-coordination aluminum fluorine cluster [ AlF ] in the aluminum electrolyte₄]-, pentacoordinate aluminum fluorine cluster [ AlF ]₅]^2-And hexacoordinated aluminum fluorine clusters [ AlF ]₆]^3-The fluorine type distribution is bridging F, terminal F and free F in the aluminum electrolyte^-(ii) ion content distribution; the total conductivity sigma of the aluminum electrolyte system and the local conductivity sigma of each ion_iThe formula is adopted to calculate and obtain:

in the formula, K_BBoltzmann constant, equal to 1.38 × 10^-23J/K, T is the temperature of the molten salt system of the aluminum electrolyte, n_iRefers to the volume concentration of ion i, i.e. the number of ions i per unit volume, D_iIs the diffusion coefficient, q_iIs the charge of ion i; wherein ion i comprises M^k+、[AlF₄]^-、[AlF₅]^2-、[AlF₆]^3-、And C^h-I.e., cations, four-coordinate clusters, five-coordinate clusters, six-coordinate clusters, free fluoride ions, and anions;and

2. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: the additive comprises LiF, KF and MgF₂、CaF₂And NaCl.

3. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: the four-coordination cluster [ AlF ] is obtained by performing first principle molecular dynamics calculation on an aluminum electrolyte system₄]^-Five-coordinate cluster [ AlF ]₅]^2-Six coordinate cluster [ AlF ]₆]^3-Corresponding coordination number distribution, free fluorine content distribution and diffusion coefficient D of each ion in the system_i。

4. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: the above-mentionedLocal conductivity calculationWhen the cation M is^K+One or more, i.e. calculated

5. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 4, wherein: the cation being K⁺、Na⁺Then q is_iIs + 1; the cation being Mg²⁺、Ca²⁺Then q is_iIs + 2.

6. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: the above-mentionedAnd

7. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1 or 6 wherein: the four-coordinate cluster [ AlF ]₄]^-Five-coordinate cluster [ AlF ]₅]^2-And a six-coordinate cluster [ AlF₆]^3-Charge q of the radical_iRespectively-1, -2 and-3.

8. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: what is needed isThe above-mentioned

9. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: self-diffusion coefficient D of simple free ions including one or more of K, Na, Al, F, Mg, Ca_iThe trajectory data obtained by molecular dynamics simulation calculation is obtained by combining a mean square displacement function.

10. The method of calculating the electrical conductivity of an aluminum electrolyte system of claim 1, wherein: the above-mentionedWhen the term conductivity is calculated, wherein the anion Cl^-The conductivity of (b) is preferably 0.

Technical Field

The invention relates to the technical field of metallurgy, in particular to a method for calculating the conductivity of an aluminum electrolyte system.

Background

Currently, the industrial aluminum electrolyte is NaF-AlF₃Electrolyte base (CR 2.1-2.5), which contains LiF additive to improve system conductivity. The conductivity is one of the most critical physical and chemical properties in aluminum electrolysis, the electrolyte conductivity directly determines the electrolyte resistance and the cell voltage, further influences the heat balance of the electrolytic cell, and finally relates to key technical and economic indexes such as energy consumption, current efficiency and the like in the aluminum electrolysis process, so that the maintenance of the proper conductivity of the aluminum electrolyte is particularly important for the aluminum electrolysis process; in the aluminum electrolysis process, due to the introduction of impurities into the raw materials, volatilization of electrolyte components and other reasons, the components of the aluminum electrolyte are constantly changed, and the electrolyte is constantly added in the industry according to the needs; the current commercial ton aluminum electrolyte consumption is about 18kg of fluoride, i.e. about 18kg of fluoride needs to be replenished to the cell for each ton of aluminium produced. If the type and amount of the electrolyte to be supplemented are to be accurately controlled so as to maintain constant electrolyte conductivity and ensure efficient operation of the electrolytic production process, a method for accurately obtaining the conductivity of the molten salt electrolyte is required. In addition, in the future development of low-temperature electrolysis technology, the electrolyte conductivity needs to be accurately calculated, and a method capable of accurately obtaining the conductivity of the molten salt electrolyte is also needed.

At present, there are two main methods for obtaining the conductivity of the molten salt electrolyte, one is experimentally measured, and the other is theoretical calculation. Due to the high temperature volatility of fluoride molten salt electrolyte and the precision problem of high temperature instruments, the conductivity error of experimental measurement is large, instrument loss is serious under the conditions of high temperature and fluoride corrosion, and the measurement cost is high. The method for theoretically calculating the conductivity of the aluminum electrolyte avoids high experimental error and high cost, and is widely concerned. The method for theoretically calculating the conductivity of the molten salt is only known by a Nernest-Einstein formula which is adopted in many documents to calculate the conductivity of an aluminum electrolyte system, however, when Einstein deduces the formula from Brownian motion in 1905, Einstein assumes that all solute ions in the system are simple elastic collisions, but ions in the aluminum electrolyte molten salt are not only simple elastic collisions, but also strong coulombic interaction and Al-F bond covalent interaction, which does not accord with the Einstein assumption, and the Nernest-Einstein formula is not theoretically suitable for the aluminum electrolyte system.

Therefore, it is important to find a new method for accurately calculating the conductivity of the aluminum electrolyte system.

Disclosure of Invention

The invention provides a method for calculating the conductivity of an aluminum electrolyte system to make up for the defects of the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme: a method of calculating the conductivity of an aluminum electrolyte system comprising NaF-AlF₃A base electrolyte and an additive, wherein the ionic structure of the aluminum electrolyte comprises a coordination number distribution and a fluorine type distribution, and the coordination number distribution is four-coordination aluminum fluorine cluster [ AlF ] in the aluminum electrolyte₄]-, pentacoordinate aluminum fluorine cluster [ AlF ]₅]^2-And hexacoordinated aluminum fluorine clusters [ AlF ]₆]^3-The fluorine type distribution is bridging F, terminal F and free F in the aluminum electrolyte^-(ii) ion content distribution; the total conductivity sigma of the aluminum electrolyte system and the local conductivity sigma of each ion_iThe formula is adopted to calculate and obtain:

in the formula, K_BBoltzmann constant, equal to 1.38 × 10^-23J/K, T is the temperature (K) of the molten salt system of the aluminum electrolyte, n_iRefers to the volume concentration of ion i, i.e. the number of ions i per unit volume, D_iIs the diffusion coefficient, q_iIs the charge of ion i; wherein ion i comprises M^k+、[AlF₄]^-、[AlF₅]^2-、[AlF₆]^3-、

And C^h-I.e., cations, four-coordinate clusters, five-coordinate clusters, six-coordinate clusters, free fluoride ions, and other anions;andeach represents a cation M^k+Tetra-coordinated cluster [ AlF ]₄]^-Five-coordinate cluster [ AlF ]₅]^2-Six coordinate cluster [ AlF ]₆]^3-、And C^h-Local electrical conductivity of.

Can see and build bodyThe total conductivity of the system is cation M + and four-coordinate cluster [ AlF₄]^-Five-coordinate cluster [ AlF ]₄]^2-Six coordinate cluster [ AlF ]₆]^3-The sum of the local conductivities of free F-ions and other simple anions (e.g., Cl-).

In the method of the present invention, the additive comprises LiF, KF, MgF₂、CaF₂And NaCl.

In the aluminum electrolyte system, all Al exists in the form of aluminum fluorine groups due to strong covalent interaction between Al-F atom pairs, including four-coordinated aluminum fluorine clusters [ AlF [ ]₄]^-Five-coordinate aluminum-fluorine cluster [ AlF ]₅]^2-And hexacoordinated aluminum fluorine clusters [ AlF ]₆]^3-And F exists in aluminum fluorine cluster and also exists in free F^-In the presence of ions, cations M in the system^k+It is present entirely in the form of simple ions. In a specific embodiment, a Castep program can be used to perform first principle molecular dynamics simulation on each system in sequence to obtain a four-coordinate cluster [ AlF ]₄]^-Five-coordinate cluster [ AlF ]₅]^2-Six coordinate cluster [ AlF ]₆]^3-Corresponding coordination number distribution, free fluorine content distribution and diffusion coefficient D of each ion in the system_i。

In the method of the present invention, theWhen local conductivity is calculated, cation M^K+Possibly one or more, i.e. calculated

May be the sum of one or more terms, q in each term_iThe value of (A) is different depending on the cation; if the cation is K⁺、Na⁺Then q is_iIs + 1; and the cation is Mg²⁺、Ca²⁺Then q is_iIs + 2.

In the method of the present invention, theAnd

diffusion coefficient D of three groups when calculating local conductivity_iAll adopt aluminum atom diffusion coefficient D_AlTetra-coordinated cluster [ AlF ] in the system₄]^-Five-coordinate cluster [ AlF ]₅]^2-And a six-coordinate cluster [ AlF₆]^3-Number of ions of cluster or volume concentration n of ions_iObtained from the coordination number distribution, namely multiplying the content distribution of each group by the total aluminum atom number; the four-coordinate cluster [ AlF ]₄]^-Five-coordinate cluster [ AlF ]₅]^2-And a six-coordinate cluster [ AlF₆]^3-Charge q of the radical_iRespectively-1, -2 and-3.

In the process of the invention, the self-diffusion coefficient D of each ion (e.g. simple free ions such as K, Na, Al, F, Mg, Ca, etc.)_iThe trajectory data obtained by molecular dynamics simulation calculation can be obtained by combining the mean square displacement function with the trajectory data obtained by calculation by using a castep program, for example.

In the method of the present invention, theFree F when calculating the local conductivity of the item^-Number of ions or volume concentration n of ions_iObtained by calculation of the type F distribution, i.e. free F^-Ion content fraction multiplied by total F ion number, charge q_iIs-1, having a diffusion coefficient D_iMay be replaced by the F atom diffusion coefficient.

In the method of the present invention, theOther anions are often referred to as Cl when calculating the term conductivity^-But when the additive does not contain NaCl, this term is 0.

The method of the invention can accurately calculate the conductivity of the aluminum electrolyte system, and further accurately control the type and the quantity of the supplemented electrolyte, so as to maintain the constant electrolyte conductivity and ensure the efficient operation of the electrolytic production process, such as: when the conductivity of the system measured by the method is higher, aluminum fluoride can be added, and when the conductivity is lower, lithium fluoride or sodium fluoride can be added.

Compared with the prior art, the invention has the following advantages:

the method is based on the strong covalent interaction characteristic of Al-F atom pairs in the aluminum electrolyte, combines a nernests-Einstein formula for calculating the conductivity of a simple free ion system, provides the calculation formula for calculating the conductivity of the aluminum electrolyte system, and provides an accurate theoretical calculation method for calculating the conductivity of the molten salt system of the aluminum electrolyte. The method can accurately calculate the conductivity of the aluminum electrolyte and can be used for calculating the conductivity of the industrial aluminum electrolyte and designing the components of the electrolyte.

Drawings

FIG. 1 shows 1.3(KF + NaF) -AlF calculated by the method of the example of the invention₃The ionic conductivity of the system is changed along with the content of NaF and is compared with the values in the literatures 1 to 3 to show a schematic diagram.

Document 1: electric conductivity of the (KF-AlF)₃)-NaF-LiFmolten system with Al₂O₃additions at low cryolite ratio[J].ECSTransactions,2009。

Document 2: dedyukhin A. Electrical Conductivity of the KF-NaF-AlF₃MoltenSystem at Low Cryolite Ratio with CaF₂Additions.Light Metals 2011.Lindsay,S.J.Cham:Springer International Publishing:563-565。

Document 3: conductivity of KF-NaF-AlF₃System Low-temperatureElectrolyte.Light Metals 2013.Sadler,B.A.Cham:Springer InternationalPublishing:689-693)。

Detailed Description

The present invention will be further described with reference to the following examples and the accompanying drawings, but the present invention is not limited to the examples listed, and shall include equivalent modifications and variations of the technical solutions defined in the claims appended to the present application.

For the molecular ratio [ CR ═ (KF + NaF)/AlF₃]NaF-KF-AlF at 1.3 and 1100K (827 ℃ C.) temperature₃And (3) constructing five simulation systems by using the modeling software packmol when the mass content of NaF is respectively 10.22 wt.%, 14.72 wt.%, 18.42 wt.%, 22.23 wt.% and 26.14 wt.% according to the base electrolyte system, sequentially carrying out first-principle molecular dynamics simulation on each system by using a Castep program to obtain a corresponding coordination number distribution and a free fluorine content distribution according to the base electrolyte system shown in the table 2, and obtaining each ion diffusion coefficient of the system shown in the table 3.

TABLE 1 simulated 1.3(KF + NaF) -AlF₃Atomic number and density of low temperature electrolyte

TABLE 2 1.3(KF + NaF) -AlF at different NaF contents₃Coordination number distribution and free fluorine content of electrolyte

TABLE 3 1.3(KF + NaF) -AlF at different NaF contents₃Ionic diffusion coefficient of electrolyte

The diffusion coefficient, coordination number distribution and fluorine atom type distribution of each ion can be directly obtained by first-principle molecular dynamics calculation, and then the electric conductivity can be calculated using the following formula given in the example of the present invention, in combination with the diffusion coefficient of each ion and the initial conditions (the number of atoms and the density of the system).

In the formula, K_BBoltzmann constant, equal to 1.38 × 10^-23J/K, T is the temperature (1100K) of the molten salt system of the aluminum electrolyte, n_iFinger separatingVolume concentration of i, i.e. number of ions i per volume, D_iAs diffusion coefficient (see Table 3), q_iIs the charge of ion i; in this embodiment, ions i include K⁺、Na⁺、[AlF₄]^-、[AlF₅]^2-、[AlF₆]^3-、

Namely K ions, Na ions, four-coordinate clusters, five-coordinate clusters, six-coordinate clusters and free fluorine ions;

each represents a cation K⁺、Na⁺Tetra-coordinated cluster [ AlF ]₄]^-Five-coordinate cluster [ AlF ]₅]^2-Six coordinate cluster [ AlF ]₆]^3-Andlocal electrical conductivity of.

Wherein D is_iCan be obtained from the mean square displacement function, diffusion coefficient D_iCalculation of referable: LV X, XU Z, LI J, ethyl. molecular dynamics integration on structure and transport properties of Na₃AlF₆–Al₂O₃molten salt[J]Journal of Molecular Liquids,2016,221:26-32, D in this example_iThe calculation results are shown in Table 3.

In the present example, the coordination number distribution, free fluorine content distribution and ion diffusion coefficient D obtained as described above_iThe method of the embodiment of the invention is used for calculating the conductivities of five systems by combining the initial conditions of a simulation system (including the atomic number and the density shown in the table 1, the atomic number used in the calculation in the formula and the like), as shown in the figure 1, and compared with the conductivities measured by the instruments in the documents 1 to 3, the document values are found to have larger differences, and the conductivities calculated by the method of the invention are between the document values, which illustrates thatThe method of the invention is used for calculating the applicability of the conductivity of the aluminum electrolyte.

It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. Not all embodiments are exhaustive. All obvious changes or modifications of the technical solution of the present invention are within the spirit of the present invention.