CC BY
14Journal of Siberian Federal University. Engineering & Technologies, 2016, 9(5), 617-631
УДК 669.7.018.672
Development and Experimental Research of Materials for Forming Stable Bottom and Side Ledges
Gennadiy V. Arkhipov*a, Peter V. Poliakovb, Evgeniy R. Shaydulina and Yuriy O. Avdeeva
a«RUSAL ETC» LTD 37 Pogranichnikov, Krasnoyarsk, 660111, Russia bSiberian Federal University 79 Svobodny, Krasnoyarsk, 660041, Russia
Received 07.03.2016, received in revised form 22.05.2016, accepted 14.07.2016
The main purpose of this paper is to find a solution for saving energy in the cell room. Another purpose of the paper is to find materials suitable for stable side and bottom ledge formation, research the properties of such materials, and study their behavior in cryolite-alumina melts, including selection of the most suitable material. Moreover, the paper is concerned with energy consumption reduction by forming stable side and bottom ledges. Furthermore, as mentioned earlier, several materials have been selected to form stable side and bottom ledges during the post-start-up period. The properties of the selected materials have been analyzed.
Keywords: Electrolysis, electrolytic cell, start-up of electrolytic cell, forming stable bottom and side ledges.
Citation: Arkhipov G.V., Poliakov P.V., Shaydulin E.R., Avdeev Y.O. Development and experimental research of materials for forming stable bottom and side ledges, J. Sib. Fed. Univ. Eng. technol., 2016, 9(5), 617-631. DOI: 10.17516/1999-494X-2016-9-5-617-631.
© Siberian Federal University. All rights reserved
Corresponding author E-mail address: Gennadiy.Arkhipov@rusal.com
*
Разработка и экспериментальные исследования материала для формирования устойчивой настыли и гарнисажа
Г.В. Архипова, П.В. Поляков6, Е.Р. Шайдулина, Ю.О. Авдеева
ООО «РУСАЛ ИТЦ» Россия, 660111, Красноярск, Пограничников, 37/1 Сибирский федеральный университет Россия, 660041, Красноярск, пр. Свободный, 79
Целью данной работы является разработка технических решений для повышения энергоэффективности действующих алюминиевых электролизеров, включающая подбор материалов для создания искусственной настыли и экспериментальные исследования свойств выбранных материалов, их поведения в криолитоглиноземных расплавах с последующим выбором материала с оптимальными свойствами, а также снижение удельного расхода электроэнергии за счет создания искусственной настыли и гарнисажа. По результатам работы установлены два вида материалов для создания искусственной настыли: на основе SiС-SiN композиции, на основе магнезита. Оба вида материалов показали хорошую стойкость в расплаве электролита и могут быть рекомендованы для создания искусственной настыли.
Ключевые слова: электролиз, электролизер, пуск электролизера, образование устойчивой настыли и гарнисажа.
Introduction
During the post-start-up period, all the process parameters have to reach the desired values, and protective side and bottom ledges have to be formed. It is clear that the shorter the post-start-up period is, the less it takes time for the cell to start operating at the full performance level. The limiting stage of the post-start-up period is the process of bottom and side ledge formation, which makes the cell work at a higher target voltage level to preserve its energy balance.
Faster bottom and side ledge formation could shorten the post-start-up period and reduce the target voltage quicker, which, in turn, could lead to a reduction in the specific power consumption.
This paper deals with the development of solutions for optimizing currently operating cells, including selection and testing of materials for forming artificial ledges; and investigates the behavior of these materials in cryolite-alumina melts (for the purpose of choosing the most suitable material.) This paper also discusses the possibility of reducing the specific power consumption by means of forming artificial bottom and side ledges.
Development of technical solutions for reducing the anode-to-cathode distance in aluminum electrolysis cells
Selecting new materials for ledge formation
Silicon carbide and magnesite blocks were selected as promising materials for ledge formation. In the aluminum industry, silicon carbide blocks have been used as a side lining material for the aluminum reduction cell cathode for a long time. Magnesite blocks are used as a side lining material
Table 1. Composition and some physical features of silicon carbide blocks and magnesite blocks
Components and features Silicon carbide blocks Magnesite blocks
SiC, % mass 72.90
SiO2, % mass 0.99 1.86
Al2O3, % mass 1.20 8.28
Si3N4, % mass 24.50
N2, % mass 9.80
MgO, % mass 87.00
CaO, % mass 1.20
Density, g/cm3 2.63 - 2.66 2.60 - 2.80
Porosity, % 12.30 17.10 - 19.30
in the aluminum refining cell (three-layer refining.) The composition and some physical parameters of silicon carbide and magnesite blocks are listed in Table 1.
Thermal conductivity of silicon carbide
and magnesite blocks
The coefficient of thermal conductivity (X) of silicon carbide blocks is one of the crucial properties which define side ledge formation. Carbon blocks were superseded by silicon carbide blocks mainly because of a high thermal conductivity of the latter. According to the Siacatec 75 Quality Certificate, the coefficient of thermal conductivity of the blocks lowers as the temperature rises. Figure 1 shows X vs. (temp.) within the temperature range of 20 to 900 °C, which falls quite well on the straight line described by the following equation:
X = 45.153 - 0.0285 t, W/m-K, (1)
where 't' stands for the temperature measured in °C.
Figure 2 shows X vs. (temp.) within the temperature range of 300 to 1,200 °C, according to [1]. The dependency becomes non-linear at t>800 °C.
During cell operation, the coefficient of thermal conductivity of the blocks decreases. According to [2], a reduction in the coefficient of thermal conductivity may reach 38 % and 77 %. Figure 3 shows an even more severe decrease, which was seen during the research period [3]. For instance, for a block taken out from a 270-day-old, shut-down cell, the coefficient of thermal conductivity of the material contacting the bath, at 550 to 700 °C (the temperature of the block during cell operation; the cell with the side ledge), was 3 times lower than the initial value (28 W/m-K), which is equal to the coefficient of thermal conductivity of carbon blocks. It is clear that a decrease in thermal conductivity leads to side ledge melting.
The coefficient of thermal conductivity decreased significantly only at the areas of contact with the bath, whereas in the gas-phase area the coefficient of thermal conductivity barely changed. Such a significant decrease in the bath area may be caused by the formation of sodium silicate, which is characterized by a lower thermal conductivity.
•I
u
*
- k. 30 IS ^
£
■c H
0.0285x + 45.1530 R2 = 0.9894
Temperature, °C
Fig. 1. Thermal conductivity vs. temperature for silicon carbide blocks
40
35
20
0
400
600
800
1 200 400 600 800 1000 1200 1400 h Temperature , °C
Fig. 2. Thermal conductivity vs. temperature for silicon carbide blocks, according to [3]
o jw joo an eoo low imo
Temperature. °C
Fig. 3. Thermal conductivity vs. temperature for magnesite blocks, according to [4]
Figure 3 shows thermal conductivity vs. temperature for magnesite blocks, which is described by the following equation (according to [4]):
I = 4.7 - 0.0017 t, W/m-K, (2)
where 't' stands for temperature measured in °C.
Figure 4 shows thermal conductivity vs. temperature for magnesite blocks, which is described by the following equation (according to [5]):
I = 5 + 0.0015 t, W/m-K, (3)
where 't' stands for the temperature measured in °C.
- 620 -
Since the thermal conductivity measurements described in the literature differ significantly, several independent experiments were conducted during this research to define the coefficient of thermal conductivity of magnesite blocks.
Specific electrical resistivity
Figures 5 and 6 show specific electrical resistivity, p vs. temperature for silicon carbide and magnesite blocks [5].
As shown in the above graphs, the specific electrical resistivity of both silicon carbide and magnesite blocks decreases significantly when the temperature rises. Within the temperature range of 25 to 800 °C, electrical resistivity vs. temperature is described by the following linear equation:
£
E
b 15 ■
~ o itm jiic «jo sm ickm ii«i
I em per a tare, °C
Fig. 4. Thermal conductivity vs. temperature for magnesite blocks, according to [5]
Temperature, "C
Fig. 6. Specific electrical resistivity vs. temperature for magnesite blocks [5]
p = 10,242 - 10,716-t, Ohm-cm, (4)
where 't' stands for the temperature measured in °C.
Experimental research of silicon carbide and magnesite blocks for ledge formation purposes
The following materials were provided for testing:
- periclase-spinel refractory based on sintered periclase and aluminum spinel. Block type: PShGS-64 (nmrC-64), manufactured by Magnesit, LLC, the city of Satka, Russia. Block size: 600x230x90 mm;
- silicon carbide block, manufactured by the VolzhskiyAbrasive Works, block size: 350x350x65 mm. Samples (035x90; 040x50; 065x50 mm) were taken from the above materials by drilling and
lathing.
The following characteristics of the materials were defined:
- spinning wear in the bath and molten aluminum;
- thermal expansion within the temperature range of 20 to 950 °C; and
- thermal conductivity within the temperature range of 100 to 950 °C.
Spinning wear of silicon carbide and magnesite blocks in the bath and molten aluminum
The tests were run using an erosive wear measurement system in accordance with the erosive wear testing methodology for spinning samples in the bath and molten aluminum. Figure 7 shows the sample appearance.
Silicon carbide blocks are difficult to lathe, so it was impossible to make a cylindrical sample. Silicon carbide and magnesite samples were made in the form of octagon, which was the closest to the form of a cylinder. The dimensions of the samples are listed in Table 3.
The samples were mounted (Fig. 7), put into the chamber filled with molten aluminum and the bath and, then, were spun at a certain speed. The aluminum level was 25 mm, and the bath level was
Fig. 7. Mounted samples, before spinning in the bath and aluminum: a - magnesite block; b - silicon carbide block
b
a
25 mm. Table 2 shows the experimental composition of the bath. During testing, the bath temperature was 980 °C.
Silicon carbide blocks. The test results are listed in Table 3 and shown in Fig. 8-9 demonstrates the cross-section of the sample at the aluminum/bath interface after spinning in the bath/Al system for 12 hours.
The appearance of the samples and the cross-section of Sample 2 did not change as a result of the tests (as shown in Fig. 8, 9) This means that the material did not react with the bath. The change in the sample dimensions as a function of time / height is shown in Fig. 10 and 11.
The graphs show that the maximum wear of the silicon carbide samples is observed on the edges both in the area of contact with aluminium and in the aluminum/bath interface area after 12 hours. The distance between the longitudinal edges decreased by 0.36 mm and 0.44 mm, respectively.
Magnesite blocks, PShGS-64 (nfflrC-64). The appearance of the samples before and after the test is shown in Fig. 12 After 12-hour and 24-hour experiments, the samples were cross cut at a height
Table 2. Experimental bath composition
NajAlF6 + AlF3 (CR = 2,0) NaF, Analytical grade Al2O3, Analytical grade CaF2, Analytical grade CR
77, % by mass 14, % by mass 4, % by mass 5, % by mass 2,65
CR - cryolite ratio.
Table 3. Test results: measuring the spinning wear of silicon carbide blocks in the bath-aluminum system
Test No. Testing time, hours Sample spinning speed, cm/s Average sample dimensions and mass Before test After test Difference
Dpl*, mm at the bottom 33.400 33.400 0.000
Ded.**, mm 35.520 35.450 -0.070
Dpl*, mm at the height of 33.530 33.450 -0.080
1 6 16 Ded**, mm 25 mm 35.880 35.760 -0.120
Dpl.*, mm at the height of 33.212 33.175 -0.037
Ded.**, mm 50 mm 34.920 34.780 -0.140
Height, mm 88.700 88.700 0.000
Mass, grams 208.200 208.200 0.000
Dpl.*, mm at the bottom 32.275 32.250 -0.025
Ded.**, mm 34.300 33.940 -0.360
Dpl.*, mm at the height of 33.025 33.175 0.150
2 12 32 Ded.**, mm 25 mm 34.580 34.140 -0.440
Dpl.*, mm at the height of 32.775 32.550 -0.225
Ded.**, mm 50 mm 34.840 34.620 -0.220
Height, mm 88.900 88.900 0.000
Mass, grams 195.500 195.500 0.000
Dpl.* - distance between the planes; Ded.**- distance between the edges.
abc
Fig. 9. Sample 2, silicon carbide: a - before the test; b - after the test; c - cross-section when cut at the bath/ aluminum interface
below
: 0.080 height - 50 mm
: 0.000 0.015
-0.037 """ -♦ -0.025
-0.225
5 7 9 11 13
The while of the rotation, hours
Fig. 10. Change in the distance between the faces of the silicon carbide samples vs. spinning time
of 15 and 35 mm. Fig. 13, 14 demonstrate cross-sections of the sample in three areas: a) aluminum area; b) bath area; and c) area above the bath.
As seen in the above Figures, the sample was impregnated the most within the bath area. The test results are listed in Table 4.
: -0.07B below height - 25 mm
:-0.14G height - 50 mm
, -0.220
- -0.340
—1— -0.440 -1-
5 6 7 8 9 LO 11 12 13
The while of the rotation, hours Fig. 11. Change in the distance between the edges of the silicon carbide samples vs. spinning time
a b c d e
Fig. 12. The appearance of magnesite samples: a - before the test; b - after 4 hours; c - after 8 hours; d - after 12 hours; e - after 24 hours of spinning in the bath and aluminum
Fig. 14. Sample cross-section after 24 hours of spinning in the bath and aluminum: a -area; c - area above the bath
aluminum area; b - bath
Table 4. Test results: measuring the spinning wear of magnesite blocks in the bath-aluminum system
Test No. Testing time, hours Sample spinning speed, cm/s Average sample dimensions and mass Before test After test Difference
1 2 3 4 5 6 7 8
Dpl*, mm at the bottom 27.375 27.750 0.375
Ded.**, mm 29.320 29.260 -0.060
Dpl.*, mm at the height of 27.800 27.700 -0.100
1 4 16 Ded.**, mm 25 mm 29.500 29.500 0.000
Dpl*, mm at the height of 27.700 27.875 0.175
Ded.**, mm 50 mm 29.480 29.320 -0.160
Height, mm 89.900 0.000
Mass, grams 178.800 18.800
Dpl*, mm at the bottom 32.875 33.400 0.525
Ded.**, mm 35.340 35.440 0.100
Dpl*, mm at the height of 32.900 33.025 0.125
2 8 16 Ded.**, mm 25 mm 35.280 35.340 0.060
Dpl*, mm at the height of 32.875 33.400 0.525
Ded.**, mm 50 mm 35.220 35.140 -0.080
Height, mm 89.750 0.000
Mass, grams 253.000 25.900
Dpl.*, mm at the bottom 33.300 33.525 0.225
Ded.**, mm 35.560 35.740 0.180
Dpl.*, mm at the height of 33.300 33.575 0.275
3 12 9 Ded.**, mm 25 mm 35.580 35.900 0.320
Dpl.*, mm at the height of 33.250 33.400 0.150
Ded.**, mm 50 mm 34.660 35.280 0.620
Height, mm 89.340 0.000
Mass, grams 255.900 24800
Dpl.*, mm at the bottom 33.350 33.275 -0.075
4 24 9 Ded.**, mm 35.900 36.000 0.100
Dpl.*, mm at the height of 33.300 33.375 0.075
Ded.**, mm 25 mm 35.440 35.600 0.160
Table 4. Continued
1 2 3 4 5 6 7 8
Dpl*, mm at the height of 50 mm 33.375 33.55 0.175
Ded.**, mm 35.520 35.460 -0.060
Height, mm 89.400 0.000
Mass, grams 260.100 31.100
- distance between the planes;
- distance between the edges.
Fig. 15. Change in the distance between the faces of the magnesite samples vs. spinning time
E °-s
CD
9.4
^ 0.3 <0
-C 0.2
o
8 »■'
w 0.0
-JA
J.62 —a ifiigft: - JS.wn j
\
0.32
0.1 0.18
0 \ 6.l|
; -o.oa
-0.08 ,J
6 S 10 12 14 15 18
The while of the rotation, hours
Fig. 16. Change in the distance between the edges of the magnesite samples vs. spinning time
It is shown that all the samples became heavier after the tests, which makes it impossible to define the extent of spinning wear of these blocks in the bath-aluminum system.
Figures 15 and 16 show the change in the sample dimensions as a function of time and height.
The graphs and pictures demonstrate that the samples were impregnated to a great extent and became bigger. The change in the dimensions was less significant at the later stage of the test, so the PShGS-64 (nfflrC-64) samples were supposedly subject to wear, but it was difficult to define the degree of wear.
The tests showed that neither of the materials was destroyed by the bath and aluminum after 24 hours of testing, so both of them can be used for forming artificial ledges, because the walls of the cell are usually fully protected by a layer of molten aluminum in 18 - 26 hours after the start-up.
D
pi
D
Coefficient of Thermal Expansion (CTE) for magnesite blocks
The rated CTE of silicon carbide blocks is 4.6-10-6 K-1 (according to the quality certificate). The CTE of magnesite blocks was not defined by the manufacturer, so it was necessary to measure it. The IK-4 (HK-4) measurement system was used. The experiment was run according to the methods of CTE experimental measurement in the temperature range of 20 to 950 °C.
The graph in Fig. 17 shows the results of the measurement within the temperature range of 20 to 950 °C. The vertical axis represents the relative change in the height of the sample defined as:
e = Ah/V100 %, (5)
where e stands for the relative change measured in %; Ah stands for the absolute change measured in mm; h stands for the initial height of the sample measured in mm.
The Coefficient of Thermal Expansion (CTE) was counted as:
a = e / AT (6)
where a stands for the CTE measured in 0K-1; stands for the relative change in the height of the sample heated from the room temperature T0 to T = (950 ± 10) °C; AT stands for the change in temperature from the room temperature T0 to T = (950 ± 10) °C.
The measurement results for PShGS-64 (nfflrC-64) are listed in the Table 5.
The experiments showed that the magnesite block's CTE ranged within 11.1T0-6 to 11.2-10-6 in the temperature range of 20 to 950 °C. This means that magnesite is a suitable material for artificial ledges, since the internal stress will not break it when the temperature rises.
sampl 3 1
sample 2 ....1..............
O 100 200 300 400 500 600 700 800 900 1000
Temperature, UC
Fig. 17. Thermal expansion of the PShGS-64 (nfflTC-64) block vs. temperature
Table 5. Characteristics of the PShGS-64 (nfflTC-64) magnesite samples before and after the experiment
Experiment No. m, g d, mm h, mm Density, grams/cm3 CTE, °K-1
1 Before the experiment 205.40 42.92 50.67 2.80 11.210-6
After the experiment 205.00 41.65 50.89 2.96
2 Before the experiment 204.10 41.51 50.47 2.99 11.110-6
After the experiment 203.60 42.00 50.78 2.89
Thermal conductivity (TC)
of silicon carbide and magnesite blocks
The rated TC of silicon carbide blocks is 28 W/m-К (T=300 °C) and 20 W/m-К (T=950 °C) (according to the quality certificate). The TC of magnesite blocks was not defined by the manufacturer, so it was necessary to measure it. The TEP (ТЭП) measurement system was used. The experiment was run according to the methods of thermal conductivity experimental measurement within the temperature range of 100 to 950 °C.
The results of the measurement within the temperature range of 100 to 950 °C are shown in Fig. 18 as TC/temperature dependency graphs. The measured TC values at different temperature levels for magnesite blocks are listed in Table 6.
These results can be used for mathematical modeling when estimating the thermal balance of the
cell.
г J .238
1.1S6 (11Ï
) f>i ■> in ,2.422
■
♦ 1 ■ Sample 2 * № "
:
.1 MM :. h.........................."MM .....Л ■ I ......
Temperature, "C
Fig. 18. Thermal conductivity of the PShGS-64 (ПШГС-64) magnesite block
Table 6. Thermal conductivity of the PShGS-64 (ПШГС-64) magnesite block
Experiment No. Temperature, °C TC, W/m^K Apparent density before the experiment, g/cm3 Apparent density after the experiment, g/cm3
1 103.28 2.469 2.79 2.78
303.84 2.397
506.25 2.616
716.32 2.963
918.71 3.303
963.41 3.437
2 103.28 2.078 2.79 2.78
303.84 2.041
506.25 2.229
716.32 2.548
918.71 2.920
963.41 3.040
Properties of silicon carbide and magnesite blocks for mathematical modelling
Apparent density p, g/cm3
- PShGS-64 magnesite block (experimental value) - 2.79
- Silicon carbide block (rated value) Coefficient of Thermal Expansion a, °K-1
- PShGS-64 magnesite block (experimental value)
- Silicon carbide block (rated value) Thermal conductivity X, W/WK
- PShGS-64 magnesite block (experimental value T,°C 100 300 500 700
■-I
- 2.58
- 11.2-10-6 - 4.6-10-6
800 2.90
900 3.07
950 3.21
X, W/WK 2.25 2.20 2.40 2.73
- Silicon carbide block (rated value) T,°C 300 950
X, W/m^K 28 20
20
The tests showed that neither of the materials was destroyed when exposed to the circulating bath melt for 24 hours. Neither material was subject to wear when exposed to the circulating aluminum melt. Thus, both materials can be used for artificial ledges, because the walls of the cell are usually fully protected by a layer of molten aluminum in 18 - 26 hours after the startup.
Experiments showed that the magnesite block's CTE ranged within 11,H0-6 to 11,2T0-6 in the temperature range of 20 to 950 °C. This means that magnesite is a suitable material for artificial cell ledge formation, since the internal stress will not break it when the temperature rises.
The following goals were achieved by this research:
- the thermal conductivity of silicon carbide and magnesite blocks was investigated;
- temperature vs. electrical resistivity of silicon carbide and magnesite blocks was analyzed;
- the thermal expansion coefficient for magnesite blocks was defined;
- the properties of silicon carbide and magnesite blocks were investigated for mathematical modelling purposes;
- wear-resistance tests of the materials were conducted.
As a result of the research, two materials were chosen for artificial ledge formation:
a) SiC-SiN based material; and
b) magnesite based material.
Both materials demonstrated high resistivity against the bath melt and can be recommended for artificial ledge formation.
References
[1] Appendix 1 to the production certificate for silicon carbide blocks.
[2] Production certificate № 32010575 ot 07.10.2014 for magnesite blocks.
Conclusions
[3] Chunxia Liu, Zhigang Huang, Guohua Liu, Zhiping Zhang. Excellent Cryolite Resistence and High Thermal Conductivity SiC Sidewall Material For High Amperage Aluminium Reduction Cells, Light Metals, 2010, 889-894.
[4] George E., Marguni O., Temme P. Use of nitride-bonded silicon carbide bricks as a refractory material for aluminum cells, Siberian Aluminum: collection of scientific articles. Bona Company. Krasnoyarsk, 2003, 203-208.
[5] Wang Zh., Skybakmoen E., Grande T. Spent Si3N4 Bonded SiC Sidelining Materials in Aluminium Electrolysis Cells, LightMetals, 2009, 353-358.
[6] Чиркин В.С. Теплофизические свойства материалов ядерной техники. М.: Атомиздат, 1967, 474 c. [Chirkin V.S. Thermophysical properties of nuclear engineering materials. Moscow, Atomizdat, 1967, 474 p. (in Russian)]
[7] Казанцев Е.И. Промышленные печи. Справочное руководство для расчетов и проектирования, 2-е изд., доп. и перераб. М.: Металлургия, 1975, 368 с. [Kazantsev E.I. Industrial furnace. Reference guide for calculations and design, Moscow, Metalurgiia, 1975, 368 p. (in Russian)]
