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PROCESS DESIGN AND CONTROL Material Balance of Vanadium in Aluminum Reduction Cells V. Daneˇ k,† A. Silny´ ,† M. Bocˇ a,*,† M. Stasˇ ,‡ and M. Koniar‡ Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-845 36 Bratislava, Slovak Republic, and Slovalco a.s., SK-965 48 Z ˇ iar nad Hronom, Slovak Republic
The material balance of vanadium in the Slovalco a.s., related to the production of 1000 kg of aluminum, was performed and the distribution of vanadium between bath, aluminum, and anode gases was evaluated using statistical analysis. In the Slovalco a.s., the 230-kA pots with prebaked anodes, point feeding, and dry scrubber technology are used. The content of V2O5 in primary and secondary alumina, aluminum fluoride, and fresh and crushed bath as well as of the V content in prebaked anodes, anode butts, and the aluminum produced in the period 2001-2003 was followed. For every material, the average content of V2O5, or V, and its standard deviation were calculated for every year as well as for the whole investigated period. The total vanadium material balance (inputs - outputs) in all years is near zero, which refers to the right calculation of the material balance. It was found that the main sources of vanadium are primary alumina and prebaked anodes. Both materials participate on the input vanadium almost equally. On the other hand, almost the whole amount of vanadium introduced by raw materials leaves the process through aluminum. The effect of other materials on the vanadium transport is negligible. Introduction In general, impurities are introduced into an electrolyte during electrolysis through alumina, anode materials, and the next input raw materials used as additives. Because of improvements of the Hall-He´roult process during the last 2 decades, especially at substitution of the So¨derberg anodes by the prebaked ones and recycling of materials, the accumulation of impurities in cells becomes one of the most serious problems. The presence of impurities affects the electrolysis in different ways. Because of reactions with electrolyte components, they may cause a change in the electrolyte composition, and because of, in general, lower decomposition potentials, they decompose on a cathode, thus lowering the current efficiency of the electrolysis and the purity of the aluminum produced. The content of some impurities may be dramatically increased by the use of alumina going through a dry scrubber, in which the substantial part of gaseous exhalants is gathered. Phosphorus, iron, silicon, and vanadium belong to the most frequently followed impurities. While phosphorus and iron were the matter of interest in prevailing papers dealing with impurities, data on vanadium are, however, very scarce in the literature. To the most harmful impurities belong phosphorus and vanadium. Phosphorus in aluminum makes it brittle, and when its content in the metal surpasses 8 ppm, the metal cannot be used in the car industry for engine block casting. 1 Vanadium is the undesirable electrolyte admixture because its presence in the bath lowers the current efficiency of the * To whom correspondence should be addressed. Tel.: ++421(0)2-59410490. Fax: ++421-(0)2-59410444. E-mail:
[email protected]. † Slovak Academy of Sciences. ‡ Slovalco a.s.
electrolysis, and when present in aluminum, it lowers its electrical conductivity.2 The survey of vanadium as well as of other impurity impacts on the aluminum electrolysis was given by Grjotheim and Matiasovsky.2 According to these authors, the impurity of metal dissolves in cryolite as an oxide, forming the fluoride of this metal, which is then electrochemically reduced and transferred to aluminum. Bratland et al.3 studied the solubility of vanadium pentoxide in the cryolite alumina melts. They found that the solubility of V2O5 decreases linearly from about 1 mass % in pure cryolite to zero in the melt saturated with alumina. Vanadium is distributed between the metal and bath according to the thermodynamic data. Similar results concerning the solubility of vanadium pentoxide in cryolite were found also by Rolin and Bernard4 and Belyaev et al.5 Many impurities such as vanadium may also escape from the bath by evaporation, by entrainment on solid particles in the off-gas stream, and/or by evaporation from raw materials. While the contribution of each mechanism is unclear, Sparwald6 introduced the distribution factor R, defined as the ratio of the impurity content in the pot gas and that in raw materials. The distribution factors for some impurities were summarized by Augood.7 Vanadium was found to be very volatile, with the distribution factor being in the range of 0.66-0.85. Distribution of vanadium between aluminum and the bath and between the bath and air was studied in laboratory-scale tests by Goodes and Algie.8 These studies showed that the transfer between the bath and metal of impurities, with lower decomposition than that of aluminum, was not complete. The experimental evidence supported the existence of relatively stable, nonreduced impurity complexes in the bath. Measure-
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Figure 1. Content of V2O5 in primary (O) and secondary (0) alumina in the period 2001-2003. Solid lines: average content in individual years. Dashed lines: trend for the whole period.
Figure 2. Content of vanadium in aluminum fluoride in the period 2001-2003. Solid lines: average content in individual years. Dashed lines: trend for the whole period.
Figure 4. Content of vanadium in prebaked anodes in the period 2001-2003. Solid lines: average content in individual years. Dashed lines: trend for the whole period.
Figure 5. Content of vanadium in anode butts in the period 2001-2003. Solid lines: average content in individual years. Dashed lines: trend for the whole period.
trends in the vanadium content in input and output materials and to understand the reasons for these observed trends. Statistical Analysis of Vanadium
Figure 3. Content of V2O5 in fresh (O) and recycled (0) electrolytes in the period 2001-2003. Solid lines: average content in individual years. Dashed lines: trend for the whole period.
ments at the bath/air interface indicated significant volatilization of vanadium, iron, and nickel. High-conductivity metal must contain minimal levels of trace elements, such as vanadium, that have a large impact on the conductivity. Among the most harmful impurities with respect to the current efficiency are vanadium and phosphorus.9-11 Cole et al.12 found that there is also a correlation between the contents of vanadium and iron in metal. The recovery of vanadium varies with the form and method of iron introduction. Iron added to the cell as an oxide causes an increase in the vanadium content in metal, while metallic iron has no effect on vanadium recovery in metal. To contribute to better knowledge on the behavior of the vanadium impurity in the aluminum electrolysis, the content of vanadium in input and output materials used in Slovalco a.s., Zˇ iar nad Hronom, Slovakia, in the period of 2001-2003 was determined in the present work by means of the statistical analysis method. On the basis of the data obtained, the total material balance of vanadium in all 3 years was made in order to evaluate
The samples for analysis were removed at three different time intervals: almost every day, once a week, and once a month. To obtain the unique time axis in the interval of 0 < t < 1, the relative time of analysis was calculated as the ratio of day, week, or month and their sequence number in year. For every input and output material, the average V2O5 (or vanadium) content and the standard deviation of the median for every year were calculated. Besides that, the trend over the whole followed time period was calculated using the linear regression analysis. This trend represents the tangent in the V2O5, or V, content versus the relative time plot. The average content in every year is shown in figures by a solid line, while the dashed one shows the trend in the whole time period. All needed data but the vanadium content in exhausting gases were available. The average content of V2O5, or V, in the individual materials and the trends in the 3-year period are graphically shown in Figures 1-6 and summarized in Table 1. The calculated amounts of vanadium in input and output materials related to 1 ton of the aluminum produced in 2001-2003 are given in Tables 2-4, respectively. The content of V2O5 in primary alumina in 2001 was 14.2 ( 4.4 ppm, in 2002 was 17.4 ( 5.1 ppm, and in 2003 was 19.6 ( 4.6 ppm (Figure 1). The trend during the 3 years shows a slow, but insignificant increase by 2.9 ppm/year because the standard deviation in all 3
Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004 8241 Table 2. Amounts of Consumed Input Materials and the Calculation of the Vanadium Amount in Input Materials Related to the Production of 1 ton of Aluminum in the Period 2001-2003 material primary Al2O3 AlF3 covering
Figure 6. Content of vanadium in the aluminum produced in the period 2001-2003. Solid lines: average content in individual years. Dashed lined: trend for the whole period.
anodes total V input
years is much greater than the statistical increase. The dispersion of values is obviously caused by a producer. The average V2O5 content in the secondary alumina in 2001 was 15.6 ( 5.0 ppm, in 2002 was 18.1 ( 3.7 ppm, and in 2003 was 21.0 ( 3.7 ppm (Figure 1). The trend in the 3-year period was an increase by 2.5 ppm/ year, which is again below the standard deviation and thus insignificant. The total vanadium amount difference per 1 ton of the aluminum produced between the primary and secondary alumina in 2001, 2002, and 2003 is only 1.5, 0.7, and 0.5 ppm V2O5, respectively. Such low values refer to the fact that only a very small amount of vanadium escapes from the pot and is gathered in the dry scrubber. If some gaseous vanadium compounds originate in the bath because of the reactions of vanadium compounds, introduced by raw materials, with bath components, they could be rather gathered by covering. The average V content in aluminum fluoride in 2001 was 3.2 ( 2.2 ppm, in 2002 was 5.4 ( 3.8 ppm, and in 2003 was 7.9 ( 4.3 ppm (Figure 2). The statistically unimportant annual increase by 2.6 ppm/year is below the standard deviations and is thus not important. Because the consumption of aluminum fluoride is very low, the whole amount of vanadium introduced by it is negligible. Delivered data on the content of V2O5 in the fresh bath are in all 3 years almost the same, 10.1 ( 0.9 ppm V2O5 (Figure 3). Because of this fact, the trend in the followed period is also negligible (0.09 ppm/year). We could not affect the precision of these data because we obtained the analyses already on the end of the project. The substantially higher V2O5 content in the crushed bath in 2001, 16.8 ( 4.7 ppm, in 2002, 20.6 ( 2.5 ppm, and in 2003, 21.5 ( 5.3 ppm (Figure 3), is caused most probably by the absorption of gaseous vanadium compounds in the crust near the anode covering, as was mentioned above. The annual increase by 2.6 ppm V2O5 may be regarded from the statistical point of view as
secondary Al2O3
2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003
m/kg
w(V2O5)/%
w(V)/%
m(V)/g
1930 1930 1930 13.5 13.5 13.5 317 312 319 517 518 518
0.001 42 0.001 75 0.001 96
0.000 795 0.000 978 0.001 098 0.000 367 0.000 500 0.000 775 0.000 488 0.000 578 0.000 648 0.003 260 0.003 596 0.004 030
15.3 18.9 21.2 0.05 0.07 0.10 1.5 1.8 2.1 18.6 18.6 20.8 35.3 40.1 44.8 16.9 19.6 21.7
1930 1930 1930
0.000 872 0.001 032 0.001 157
0.001 56 0.001 81 0.002 01
0.000 874 0.001 014 0.001 125 6
already significant and refers to the slow increase of the vanadium content in the bath caused by its circulation. The average vanadium content in prebaked anodes in 2001 was 32.6 ( 5.0 ppm, in 2002 was 35.9 ( 6.7 ppm, and in 2003 was 40.7 ( 5.4 ppm (Figure 4). The annual increase by 4.8 ppm is just on the level of the standard deviations and may be thus treated as statistically significant. The average content of vanadium in anode butts is, within the error of analysis, the same as that in prebaked anodes; i.e., in 2001, it was 32.4 ( 3.9 ppm, in 2002, it was 36.1 ( 8.3 ppm, and in 2003, it was 40.9 ( 7.4 ppm (Figure 5). The annual increase by 5.6 ppm is comparable with the standard deviations in individual years and may be thus treated as statistically significant. The average content of vanadium in metallic aluminum in 2001 was 31.1 ( 3.1 ppm, in 2002 was 35.8 ( 3.3 ppm, and in 2003 was 37.4 ( 3.7 ppm (Figure 6). The increase by 3.1 ppm V/year is comparable with the statistical deviations and is thus regarded as statistically important. This is not a favorable trend in the vanadium balance and follows the increased content of vanadium in all input and output raw materials. It was supposed that 95% of the difference of the vanadium oxide content in primary and secondary alumina is adsorbed in the dry scrubber and 5% escapes as emissions. When anode gases transport the vanadium content in primary and secondary alumina and when 95% of it is adsorbed in the dry scrubbers, the concentration of vanadium in anode gases can be estimated. Supposing that the total volume of anode gases is 94 500 m3/1000 kg of the aluminum produced,
Table 1. Summary of the Statistical Analysis of the Vanadium Content in the Period 2001-2003 material
2001
w(mean)/ppm 2002
2003
w′(trend)/(ppm/year) for 2001-2003
primary alumina (V2O5) secondary alumina (V2O5) aluminum fluoride (V2O5) fresh electrolyte (V2O5) crushed electrolyte (V2O5) prebaked anodes (V) anode butts (V) aluminum produced (V)
14.2 ( 4.4 15.6 ( 5.0 3.2 ( 2.2 10.1 ( 0.9 16.8 ( 4.7 32.6 ( 5.0 32.4 ( 3.9 31.0 ( 3.1
17.4 ( 5.1 18.1 ( 3.7 5.4 ( 3.8 10.1 ( 0.9 20.6 ( 2.5 35.9 ( 6.7 36.1 ( 8.3 35.8 ( 3.3
19.6 (4.6 21.0 ( 3.7 7.9 ( 4.3 10.1 ( 0.9 21.5 ( 5.3 40.7 ( 5.4 40.9 ( 7.4 37.4 ( 3.7
2.9 2.5 2.6 0.09 2.6 4.8 5.6 3.1
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Table 3. Consumed Amounts of Output Materials and the Calculation of the Vanadium Amount in Output Materials Related to the Production of 1 ton of Aluminum in the Period 2001-2003 material aluminum emissions anode butts covering total V: output difference: in - out exhausting gases
c(V)/(g/m3)
m/kg 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003 2001 2002 2003
c/(g/m3)
w(V2O5)/%
1000 1000 1000 95 130 93 593 95 726
0.000 908 0.001 089 0.001 233
Table 4. Daily Amounts of Input and Output Materials Used for One Average 230-kA Pot in Individual Years material
2001
2002
2003
Al produced/(kg/day) consumed alumina/(kg/1 ton of Al) exhausting gases/(m3/day) consumed AlF3/(kg/1 ton of Al) covering for one anode/(kg/day) crushed electrolyte primary alumina no. of exchanged anodes gross anode consumption/ (kg/1 ton of Al) net anode consumption/ (kg/1 ton of Al)
1766 1930 168 000 13.5
1765 1930 168 000 13.5
1755 1930 168 000 13.5
300 260 1 517
300 260 1 518
300 260 1 518
410
411
411
then the vanadium concentration in anode gases is approximately 0.007 mg/m3. This value seems to be very probable. Material Balance of Vanadium The material balance of vanadium was performed on the basis of the results of vanadium oxide and vanadium analyses in input and output materials. Basic information needed for the material balance calculation for one average 230-kA pot consisted of the following items: (i) amount of aluminum produced (kg), (ii) amount of exhausting gases (m3), (iii) covering for one anode consisting of (a) the amount of recycled electrolyte (kg) and (b) the amount of primary alumina (kg), (iv) number of the exchanged anodes, (v) mass of the average anode (kg), (vi) amount of consumed alumina (kg), and (vii) amount of consumed AlF3 (kg). These basic input and output data were delivered by Slovalco a.s., Zˇ iar nad Hronom, Slovakia. They changed only a little bit from year to year. The amounts of input and output materials for the daily production of 1000 kg of aluminum in 2001for one average 230-kA pot are schematically shown in Figure 7. The mass of covering was calculated as the sum of the amount of recycled electrolyte and primary alumina per 1000 kg of the aluminum produced divided by the daily aluminum production in tons. The average content of vanadium in the covering was calculated as the weighted amount of vanadium in both input materials. This value was estimated from the vanadium difference
m(V)/g 31.0 35.3 37.2 1.5 0.7 0.03 3.5 3.9 4.4 1.6 1.9 2.2 36.2 41.1 43.8 -0.8 -1.0 1.0 0.1 0.1 0.5
0.000 016 0.000 007 0.000 000 3
107 107 107 317 312 319
95 130 93 593 95 726
w(V)/% 0.003 10 0.003 53 0.003 72
0.000 000 8 0.000 000 4 0.000 005 6
0.003 24 0.003 62 0.004 08 0.000 509 0.000 610 0.000 690
in primary and secondary alumina and the difference in the total material balance. It was assumed that the whole difference of the vanadium amount in primary and secondary alumina was transported by exhausting gases to the dry scrubbers, where 95% was adsorbed and 5% went out into the atmosphere as emissions. To relate the material balance to 1000 kg of the aluminum produced, the amount of exhausting gases and the covering of one anode have to be divided by the daily aluminum production in tons. The number of exchanged anodes multiplied by their average mass gives the anode income. The anode consumption was calculated as the difference between income and anode butts. The daily amounts of input and output materials used for one average 230-kA pot in individual years are given in Table 4. The total vanadium material balance in individual years, i.e., the difference between the vanadium amounts in input and output materials, should be equal to or near zero if the balance has been made correctly. In 2001, the total vanadium material balance was equal to -0.8 g of V, in 2002, it was -1.0 g of V, and in 2003,
Figure 7. Material flow in the 230-kA pot related to the production of 1000 kg of aluminum.
Ind. Eng. Chem. Res., Vol. 43, No. 26, 2004 8243
it was 1.0 g of V, which with regard to the statistical deviation in the individual average values refers to the correctness of the vanadium material balance. From Table 3, it follows that the sum of the amount of vanadium in primary alumina and in prebaked anodes equals approximately the amount of vanadium in the aluminum produced. Other sources of vanadium are far beyond these values and are thus unimportant. This means that the whole amount of vanadium introduced into the process leaves it by the aluminum produced. This is the most important knowledge on the vanadium circulation in the aluminum electrolysis.
the vanadium amount in the aluminum produced. The other sources of vanadium are unimportant. This means that the whole vanadium introduced into the process leaves it via the aluminum produced. Acknowledgment The work presented was financially supported by the Scientific Grant Agency VEGA of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences under Grant 2/1032/01. Literature Cited
Conclusions From the results of the statistical analysis of the vanadium content in input and output materials used in Slovalco a.s., Zˇ iar nad Hronom, Slovakia, in the period of 2001-2003 follow the steady slow increasing content of vanadium in the aluminum produced as well as in recycling materials, caused most probably by its accumulation in the pots. On the other hand, the content of vanadium in fresh and crushed electrolytes almost does not change. This observation refers to the already steady-state process. The content of vanadium in prebaked anodes increases by the average value of 4.8 ppm/year with relatively large dispersion up to (5 ppm. It is interesting, however, that approximately the same values concerning the amount and trend have been found also in anode butts. This means that no vanadium is trapped in the anodes during their preparation and work in the pot. From the statistical point of view, the content of vanadium in the aluminum produced in the period of 2001-2003 increases systematically with the average trend of 3.1 ppm/year, which is not very significant because of the average dispersion value of 3.4 ppm. A very important conclusion follows from the total vanadium balance in the period of 2001-2003 concerning the trends in the vanadium mass flow. The difference between input and output covering refers to some amount of vanadium, which may be probably absorbed in the covering during electrolysis. These values are, however, very small, being only 0.1 ppm. This means that practically no vanadium gathers in the covering. The most important knowledge on the vanadium circulation in the aluminum electrolysis is the observation that the sum of the vanadium amount in primary alumina and in prebaked anodes equals approximately
(1) Daneˇk, V.; Chrenkova´, M.; Silny´, A.; Stasˇ, M.; Koniar, M. Long-Termed Material Balance of Phosphorus in Aluminium Reduction Cells. Chem. Pap. 2001, 55, 209. (2) Grjotheim, K.; Matiasovsky´, K. Impurities in the Aluminium Electrolyte. Aluminium 1983, 59, 687. (3) Bratland, D.; Jose del Campo, J.; Kangjo, C.; Grjotheim, K.; Thonstad, J. The Solubility of Vanadium Pentoxide in Molten Cryolite-Alumina and the Aluminothermic Reduction of Vanadium Pentoxide. In Light Metals; Andersen, J. E., Ed.; Kaiser Aluminum & Chemical Corp.: Chalmette, LA, 1982; p 325. (4) Rolin, M.; Bernard, C. Solubilite´ des oxydes dans la cryolithe founde. Bul. Soc. Chim. Fr. 1963, 1035. (5) Belyaev, A. I.; Rapoport, M. B.; Firsanova, L. A. Elektrometallurgia Alyuminiya, Metallurgizdat: Moscow, 1953. (6) Sparwald, V. Beitrag zur Verflu¨chtigung der Begleitelemente bei der Aluminium-Schmelzfluβelektrolyse. Erzmetal 1973, 26, 529. (7) Augood, D. R. Impurities Distributions in Alumina Reduction Plants. In Light Metal; McMinn, C. J., Ed.; Reynolds Metals Co.: Sheffield, AL, 1980; p 413. (8) Goodes, C. G.; Algie, S. H. The Partitioning of Trace Impurities Between Aluminium, Cryolite and AirsA Laboratory Study. In Light Metals; Campbell, P. G., Ed.; Alumax of South Carolina: Coose Creek, SC, 1989; p 199. (9) Thonstad, J.; Nordmo, F.; Rolseth, S.; Paulsen, J. B. On the Transfer of Some Trace Elements in Dry Scrubbing Systems. In Light Metals; Miller, J. J., Ed.; Alumax, Inc.: San Mateo, CA, 1978l; p 463. (10) Kerouanton, A.; Badoz-Lambling, J. Rev. Chim. Miner. 1974, 11, 223. (11) Deininger, L.; Gerlach, J. Stromausbeutemessungen bei der Aluminiumoxid-Reduktionselektrolyse in Laboratoriumszellen. Metall 1979, 33, 131. (12) Cole, D. L.; Terrell, M.; Wood, S. T. Effect of Iron Source on Vanadium Recovery in Pot Metal. In Light Metals; Evans, J. W., Ed.; University of California: Berkeley, CA, 1995; p 213.
Received for review August 4, 2004 Revised manuscript received September 24, 2004 Accepted September 28, 2004 IE049296Y
