RU2418104C1 - Procedure for preparing samples of calcium containing electrolyte in aluminium production for analysis of composition with rpa (roentgen-phase analysis) method - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: procedure for sampling electrolyte consists in cleaning crust from alumina, in local fracture of electrolyte crust, in cleaning surface of electrolyte from crust and alumina lumps, in removing carbon foam, in electrolyte mixing, in sampling into thick-wall conic pan-sampler preliminary heated over surface of melt, in extraction of sample and in placing it into process basket with cells, amount of which corresponds to amount of baths in case. Before supply of selected samples to a line of sample preparation, where there is determined composition by the method of RPA, the samples are subjected to thermal treatment in the furnace at temperature 480-520C during 20-40 minutes. ^ EFFECT: thermal treatment ensures balanced phase contents and good crystallisation of phases in sample which is required for method of roentgen phase analysis. ^ 3 dwg, 1 ex, 1 tbl

Description

The invention relates to the electrolytic production of aluminum and can be used to determine the composition of the electrolyte to regulate the process parameters.

Monitoring the composition of the electrolyte is an important process in the electrolytic production of aluminum. During the operation of the electrolysis bath, the composition and properties of the electrolyte change. In this regard, the electrolyte is analyzed approximately every three days, on the basis of which the composition of each bath is corrected. Among the controlled characteristics of the composition are the cryolite ratio (hereinafter referred to as KO) - the ratio of total sodium fluoride to aluminum fluoride (NaF / AlF ₃ ), total calcium fluoride, magnesium fluoride, and in some cases lithium fluoride. The cryolite ratio determines such important electrolyte parameters as crystallization temperature, alumina solubility, electrical conductivity, viscosity and some others. The composition is determined by the method of quantitative x-ray phase analysis (XRD) on crystallized electrolyte samples taken from the baths. The required accuracy for determining the CO is Δ = ± 0.03 and the relative error in the determination of CaF ₂ is Δ = ± 10%.

In crystallized electrolyte samples, the simultaneous presence of four, and sometimes five, phases is observed, which indicates nonequilibrium crystallization conditions. During crystallization, the main components of NaF and AlF ₃ form two phases: cryolite Na ₃ AlF ₆ and chiolite Na ₅ Al ₃ F ₁₄ . Calcium fluoride forms three phases: CaF ₃ (fluorite), NaCaAlF ₆ and Na ₂ Ca ₃ Al ₂ F ₁₄ (calcium cryolites). Since calcium fluoride also binds aluminum and sodium fluorides, calcium must be determined not only as a chemical component, but also to measure the content of each calcium-containing phase (NaCaAlF ₆ and Na ₂ Ca ₃ Al ₂ F ₁₄ ). Unfortunately, the NaCaAlF ₆ phase has insufficient crystallization, which reduces the accuracy of its measurement by the XRD method. Meanwhile, the ratio between the calcium-containing phases in the sample during sampling may vary (Kirik S.D., Yakimov I.S., Golovnev N.N. et al. // Abstracts of the International Conference “Aluminum of Siberia 2002”, Krasnoyarsk, 2002, p. 400-404). A clear understanding of the reasons why this is happening is lacking.

Due to the difficulty of determining the concentration of calcium fluoride exclusively by the XRD method, the X-ray analysis scheme is supplemented by an X-ray fluorescence measurement of total calcium (Combined XRD / XRF System for Potflux Analysis PW1760 / 10. Instructional Manual 9499-303-01711, 840127. Almelo, Netherlands). These data, however, do not allow taking into account the contribution to the cryolite ratio of sodium and aluminum fluorides associated with calcium. In practice, when calculating the final values, it is conditionally assumed (Lossius LP, Hoie H., Pedersen HH et.al. // Light Metals, 2000, p. 265-270) that calcium with a constant ratio is distributed between possible calcium-containing phases (NaCaAlF ₆ and Na ₂ Ca ₃ Al ₂ F ₁₄ ). The real situation, however, differs significantly from this, which leads to a distortion of the analysis result. For example, with the composition of the electrolyte, expressed through the main components: Na ₃ AlF ₆ - 72% wt. and AlF ₃ - 14% wt. and CaF ₂ - 8% wt.), the assumption that calcium crystallized as the NaCaAlF ₆ phase gives KO = 2.19, and in the case of the Na ₂ Ca ₃ Al ₂ F ₁₄ phase, KO = 2.27. The resulting difference in results (0.08 units of KO) significantly exceeds the permissible error. The accuracy of the analysis can be improved if you use samples obtained by equilibrium crystallization, when the x-ray phase is suitable for quantitative measurement. This can be achieved by improving the stages of sampling and / or sample preparation, during which the substance selected for analysis will contain well-crystallized phases.

The known technology of sampling electrolyte for scrap, which consists in the fact that metal scrap is immersed for a short time in the melt and separate the sample of electrolyte adhering to scrap (Standard of the company "Sampling of electrolyte" STP 04-01-01-96, Krasnoyarsk aluminum plant ) The disadvantage of this method is that when sampling for scrap as a result of rapid cooling, amorphization of a part of the electrolyte occurs. Quantitative X-ray diffraction cannot be applied to such a sample.

The known technology of sampling electrolyte in a double-circuit mold, including local destruction of the crust of the electrolyte, heating the double-circuit mold in the molten electrolyte to red heat, heating a metal spoon over the surface of the electrolyte for 3-4 minutes, scooping up the electrolyte and pouring it into the mold, first into the ring, then into a glass (Standard of the company "Electrolyte Sampling" STP 04-01-01-96, Krasnoyarsk Aluminum Plant). The disadvantage of the method of sampling in a massive metal mold is the duration of the sampling process.

Known technology for sampling electrolyte, including local destruction of the crust of the electrolyte, removing solid residues of the crust, mixing the electrolyte, sampling the sampler previously heated to 200-250 ° C, and then to the temperature of the electrolyte and pouring the liquid sample of the electrolyte without residue from the sampler into the mold and keeping the sample in the mold for at least 2.5 hours (Patent RU No. 2284377). The disadvantage of this method is the long duration of sampling. For this reason, this technique is currently practically not used.

The following technology for sampling electrolyte from an aluminum electrolyzer is known and used at aluminum plants, including cleaning the crust from alumina, punching the pulmonary opening in the electrolyte crust, cleaning the surface of the electrolyte from pieces of crust and alumina, sampling a thick-walled conical mold pre-warmed above the melt surface (sampler ), extracting the sample and placing it in the technological basket with cells (the number of cells corresponds to the number of baths in the housing) (Instruction “Outflow electrolyte breakdown "and 8-34-2002, Krasnoyarsk Aluminum Plant). This method is adopted as a prototype.

The disadvantage of this method is that when using this method it is difficult to maintain the same sampling schedule. To speed up the operation, in practice, the stages of heating the sampler often do not perform; they reduce the cooling time of the sample in the sampler. As a result, the crystallization of calcium-containing phases proceeds under nonequilibrium conditions, which complicates the application of the XRD method.

The technical result, the achievement of which the present invention is directed, is the reproducible preparation of the analyzed samples based on the use of heat treatment, which allows to achieve an equilibrium phase composition and good crystallization of the phases in the sample, which is necessary when applying the XRD methods.

The desired technical result is achieved by the fact that crystallized samples taken from the baths are subjected to additional heat treatment. To do this, the basket with the selected samples is placed in a furnace heated to a temperature of 480-520 ° C, and kept in it for 20-40 minutes. After this, the sample basket is removed and cooled in air.

The essence of the method is that during the heat treatment of the crystallized sample at 480-520 ° C for 20-40 minutes, the calcium-containing NaCaAlF ₆ phase, which has insufficient crystallization, is transformed into well-crystallized Na ₂ Ca ₃ Al ₂ F ₁₄ . What is happening can be described by the following reactions:

The compound NaAlF ₄ released as a result of reaction (1) interacts with Na ₃ AlF ₆ to form Na ₅ Al ₃ F ₁₄ :

Thus, the balance of the system is not upset. The result is a well-crystallized Na ₂ Ca ₃ Al ₂ F ₁₄ phase, suitable for X-ray diffraction measurements.

In the course of the studies to determine the composition of the electrolyte for regulating the technological parameters of the process, it was found that the optimal conditions for heat treatment of samples are a temperature of 480-520 ° C and annealing time of 20-40 minutes.

An increase in the heat treatment temperature of samples over 520 ° C leads to the fact that the compound NaAlF ₄ formed by reaction (1), having high volatility, leaves the sample, changing its composition. Sample heat treatment temperatures of less than 480 ° C require an unnecessarily long process time.

It was experimentally established that when the heat treatment time of the samples is less than 20 minutes, the decomposition of the NaCaAlF ₆ phase does not occur completely. Heat treatment of samples for more than 40 minutes is impractical due to the increase in the analysis duration, since 40 minutes is more than enough to complete the phase decomposition processes and alignment of the sample microstructure.

Examples of the method

As test materials, we used electrolyte samples taken from various industrial baths in a conical mold. Heat treatment conditions and data on the composition of electrolytes, determined by various methods, are shown in table 1.

Example 1

Electrolyte samples taken in a conical mold from an industrial bath were calcined in an open crucible at certain temperatures for a predetermined time. The mass of the sample before and after calcination was controlled on an analytical balance with an accuracy of 10 ^-3 g. The mass loss was less than 0.1% wt. The electrolyte composition was monitored using XRD (X'pert Pro diffractometer (PANalitical, Netherlands)). An X-ray spectral analysis method (Axios Advanced X-ray fluorescence spectrometer (PANalytical, Netherlands)) was used as an arbitration method for monitoring the electrolyte composition. Figure 1-3 shows the x-ray of the original electrolyte and electrolyte, heat-treated.

Experiments show that during the heat treatment of electrolytes (samples 1_1, 2_1), which have a poorly crystallized NaCaAlF ₆ compound in their initial composition, this compound decomposes to form a well crystallized compound Na ₂ Ca ₃ Al ₂ F ₁₄ . The thermal treatment of the electrolyte (sample 3_1), which does not have NaCaAlF ₆ in its initial composition, does not lead to a change in the phase composition. X-ray spectral analysis reveals that heat treatment does not change the elemental composition of the sample.

The results of experiments on the heat treatment of industrial electrolytes allow us to recommend this method when preparing samples for analysis. Heat treatment should be carried out at a temperature of 480-520 ° C for 20-40 minutes. These conclusions were made for electrolyte samples taken at the KRAZ plant. The mass loss during heat treatment of samples under these conditions did not exceed 0.1% wt.

As follows from the above examples, the use of heat treatment of samples allows us to achieve reproducibility of the analyzed samples, the equilibrium phase composition and good crystallization of the phases in the sample, which is necessary when applying the XRD methods.

Table 1
Electrolyte composition data determined by various methods and heat treatment conditions Experience number sample name heat treatment modes CaF ₂ ,
% wt. Data x-ray analysis, wt. share CO (RFA) Data of x-ray spectral analysis, wt. share T, ° C t min 1_1 42_i Initial electrolyte sample, no heat treatment applied 8.19 Calcium is in three calcium-containing phases: 2.54 Na - 0.26 NaCaAlF ₆ - 0.20 Al - 0.12 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.03 F - 0.52 CaF ₂ - 0.01 Ca - 0.04 O - 0.03 1_2 42_500_30 500 thirty 8.17 NaCaAlF ₆ phase completely disintegrated 2.66 Na - 0.26 NaCaAlF ₆ - 0.00 Al - 0.12 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.14 F - 0.52 CaF ₂ - 0.01 Ca - 0.04 O - 0.04 1_3 42_450_30 450 thirty 8.06 NaCaAlF ₆ phase not completely disintegrated 2.61 Na - 0.26 NaCaAlF ₆ - 0.03 Al - 0.12 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.14 F - 0.52 CaF ₂ - 0.01 Ca - 0.04 O - 0.04 1_4 42_600_30 600 thirty 8.11 The NaCaAlF ₆ phase did not completely disintegrate; NaAlF ₄ took off 2.67 Na - 0.26 NaCaAlF ₆ - 0.05 Al - 0.12 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.08 F - 0.52 CaF ₂ - 0.02 Ca - 0.04 O - 0.04 1_5 42_500_15 500 fifteen 8.13 NaCaAlF ₆ phase not completely disintegrated 2,58 Na - 0.26 NaCaAlF ₆ - 0.03 Al - 0.12 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.15 F - 0.52 CaF ₂ - 0.01 Ca - 0.04 O - 0.03 1_6 42_500_45 500 45 8.08 NaCaAlF ₆ phase completely disintegrated 2.66 Na - 0.26 NaCaAlF ₆ - 0.00 Al - 0.12 Na ₂ Ca ₃ AlF ₁₄ - 0.14 F - 0.52 CaF ₂ - 0.01 Ca - 0.04 O - 0.04 2_1 2062_i Initial electrolyte sample, no heat treatment applied 7.93 Na ₃ AlF ₆ - 0.64 2,36 Na - 0.26 Na ₅ Al ₃ F ₁₄ - 0.18 Al - 0.15 NaCaAlF ₆ - 0.10 F - 0.53 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.09 Ca - 0.04 CaF ₂ - 0.00 O - 0.02 2_2 2062_500_30 500 thirty 7.89 Na ₃ AlF ₆ - 0.63 2,38 Na - 0.26 Na ₅ Al ₃ F ₁₄ - 0.22 Al - 0.15 NaCaAlF ₆ - 0.00 F - 0.53 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.14 Ca - 0.04 CaF ₂ - 0.01 O - 0.02 3_1 P3_i Initial electrolyte sample, no heat treatment applied 7.02 Na ₃ AlF ₆ - 0.47 2.14 Na - 0.25 Na ₅ Al ₃ F ₁₄ - 0.35 Al - 0.13 NaCaAlF ₆ - 0.00 F - 0.53 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.18 Ca - 0.04 CaF ₂ - 0.00 O - 0.03 3_2 P3_500_30 500 thirty 7.02 Na ₃ AlF ₆ - 0.47 2.14 Na - 0.25 Na ₅ Al ₃ F ₁₄ - 0.35 Al - 0.13 NaCaAlF ₆ - 0.00 F - 0.53 Na ₂ Ca ₃ Al ₂ F ₁₄ - 0.18 Ca - 0.04 CaF ₂ - 0.00 O - 0.03

Claims (1)

A method of preparing electrolyte samples from an aluminum production electrolyzer for determining the electrolyte composition, including cleaning the crust from alumina, local destruction of the electrolyte crust, cleaning the electrolyte surface from pieces of crust and alumina, removing carbon foam, mixing the electrolyte, sampling the sampler previously heated above the melt surface in in the form of a thick-walled conical mold, sample extraction and its placement in a technological basket with cells, the number of which corresponds to the number baths in the case, characterized in that before the sample is sent to the sample preparation line for X-ray phase analysis (XRD), the sample is thermally processed in a furnace at a temperature of 480-520 ° C in the furnace for 20-40 minutes.

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