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US20080202939A1 - Process For the Electrolysis of Aluminiumsulfide - Google Patents

Process For the Electrolysis of Aluminiumsulfide Download PDF

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Publication number
US20080202939A1
US20080202939A1 US10/550,678 US55067804A US2008202939A1 US 20080202939 A1 US20080202939 A1 US 20080202939A1 US 55067804 A US55067804 A US 55067804A US 2008202939 A1 US2008202939 A1 US 2008202939A1
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US
United States
Prior art keywords
bath
process according
electrolysis
cryolite
anode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Abandoned
Application number
US10/550,678
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English (en)
Inventor
Dietrich Willem Van Der Plas
Steven Christian Lans
Markus Andreas Reuter
Mpia Cyril Roger Mambote
Anthonie Van Sandwijk
Yanping Xiao
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Novelis Koblenz GmbH
Original Assignee
Corus Aluminium Walzprodukte GmbH
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Filing date
Publication date
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Assigned to CORUS ALUMINIUM WALZPRODUKTE GMBH reassignment CORUS ALUMINIUM WALZPRODUKTE GMBH ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: REUTER, MARKUS ANDREAS, MR., MAMBOTE, MPIA CYRIL ROGER, MR., VAN SANDWIJK, ANTHONIE, MR., LANS, STEVEN CHRISTIAN, MR., XIAO, YANPING, MR., VAN DER PLAS, DIETRICH WILLEM, MR.
Publication of US20080202939A1 publication Critical patent/US20080202939A1/en
Abandoned legal-status Critical Current

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    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25CPROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/18Electrolytes
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25CPROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium

Definitions

  • the invention relates to a process for the electrolysis of Al 2 S 3 , using a bath of molten salt, preferably a bath of molten chloride salt, in which Al 2 S 3 is dissolved.
  • Hall-Héroult process consumes about 13-15 MWh of electrical energy per tonne of aluminum.
  • the anodes are consumed during the process and have to be changed periodically.
  • the Hall-Héroult process gives rise to green house emissions such as CF 4 and C 2 F 6 , which should be removed from the off-gas in compliance with environmental legislation.
  • aluminium oxide is converted in a sulfidation step into aluminium sulfide Al 2 S 3 by a reaction with carbonsulfide CS 2 .
  • CAPP Compact Aluminium Production Process
  • the aluminum metal can be extracted from Al 2 S 3 by electrolysis, producing sulfur gas at the anode, preferably a graphite anode. The sulfur gas will be collected and recycled to produce CS 2 , which is used in the sulfidation step, which is of particular advantage in combination with the CAPP-process.
  • the simplified reactions (assuming no complexions) of the electrolysis process are:
  • FIG. 1 shows the decomposition potential of various aluminium compounds to produce aluminium by electrolysis and shows immediately that the electrolysis of Al 2 S 3 is very advantageous with regard to energy consumption, i.e. it has the lowest decomposition potential.
  • the first bar is a theoretical value for comparison.
  • the second bar represents a process wherein Al 2 O 3 is converted into AlCl 3 which is decomposed.
  • the fourth bar represents the alternative sulfide process and the third bar represents the actual Hall-Héroult process.
  • the theoretical value of the decomposition potential is determined by:
  • E 0 ⁇ - ⁇ ⁇ ⁇ G 0 nF
  • n ⁇ the ⁇ ⁇ valency ⁇ ⁇ of ⁇ ⁇ the ⁇ ⁇ ion ⁇ ⁇ ( 3 ⁇ ⁇ for ⁇ ⁇ aluminium )
  • F ⁇ Faraday ' ⁇ s ⁇ ⁇ constant ( 4. )
  • a problem with the sulfide process is the low current density that can be achieved in the known molten chloride bath.
  • the eutectic composition of a MgCl 2 —NaCl—KCl mixture (50-30-20 mole %) has been proposed previously as an appropriate electrolyte for the electrolyses of Al 2 S 3 (see N. Q. Minh, R. O. Loutfy, N. P. Yao, “The Electrolysis of Al 2 S 3 in AlCl 3 —MgCl 2 —NaCl—KCl Melts”, J. Appl. Electrochem, Vol 12, 1982, 653-658; R. O. Loutfy, N. Q. Ming, C. Hsu, N. P.
  • a limiting current density of 0.3 A/cm 2 at the saturation solubility of Al 2 S 3 ( ⁇ 3 wt %) and of 0.2 A/cm 2 in the MgCl 2 —NaCl—KCl eutectic composition containing about 2 wt % Al 2 S 3 was measured.
  • the current efficiency i.e. the percentage of the current that is actually used for the electrolysis was determined to be about 80% at a current destiny of 0.2 A/cm 2 , a cell potential of about 1.5 V and an interelectrode gap between anode and cathode of 3 cm.
  • the normal current density at which the Hall-Héroult process is carried out is about 0.8 A/cm 2 .
  • the achievable current density in the electrolysis of Al 2 S 3 in a eutectic MgCl 2 —NaCl—KCl bath is about 0.3 A/cm 2 . This means that the cell area, when applying the sulfide process should be about three times larger then required for the Hall-Héroult process. This makes the sulfide process not an attractive alternative despite the drawbacks associated with the Hall-Héroult process.
  • AlCl 3 allows current densities of up to 2 A/cm 2 , but the use of AlCl 3 is not a viable alternative. Even though the eutectic temperature of a MgCl 2 —NaCl—KCl mixture is relatively low, the high vapour pressure of AlCl 3 causes a considerable amount of AlCl 3 to volatise.
  • AlCl 3 was used to enhance the electrowinning process. Since AlCl 3 is readily volatilized from the melt and has to be separated from sulfur downstream to recycle it to the electrowinning process, it was discarded as being impractical.
  • the present inventors have found that the solubility of Al 2 S 3 in a bath of molten salt having an appropriate composition is not the limiting factor in the achievable current density.
  • the cell potential of an electrolysis cell is built up of thermodynamic, kinetic (activation potential and mass transfer limitations) and ohmic contributions.
  • the inventors have taken a different approach and found a nearly linear relationship between current density and cell potential which indicates that the electrolysis process, at least above a minimum concentration of dissolved Al 2 S 3 , is not diffusion controlled, but has ohmic limitations.
  • an increased solubility of Al 2 S 3 does not result in a substantial enhancement of the cell performance.
  • the relationship between cell potential and current density is nearly linear, which means that this relation is determined by an ohmic relation. Consequently the allowable current density can be increased by improving the electrical conductivity of the bath.
  • the conductivity is improved in an embodiment of the invention in which the measures comprise adding an additive to the bath.
  • the additives are selected so as to increase the overall electrical conductivity in the bath of molten salt. As an additional effect the additives may increase the activity of both aluminium and sulfur and also the solubility of Al 2 S 3 . As described above AlCl 3 is not a preferred addition.
  • a preferred embodiment of the process according to the invention is characterised in that the additive comprises, preferably mainly consists of a fluoride compound.
  • This embodiment is based on the insight that the amount of fluoride has a positive effect on the electrolysis process resulting from a higher activity of AlF n m ⁇ than AlS + species. Also, as complexing of aluminium with fluor is favoured over complexing of aluminium with sulfur, the concentration of sulfur ions is higher when fluoride is added, favouring the anodic reaction.
  • a further preferred embodiment of the process according to the invention is characterised in that the fluoride compound is cryolite.
  • cryolite shows a larger improvement of the conductivity than the addition of other fluorides such as NaF, although the specific conductivity of NaF is much higher.
  • cryolite has a high melting point (1012° C. and therefore much higher than the boiling point of AlCl 3 ) and volatilization of cryolite at the normal operating temperature of the electrolysis cell is assumed to be negligible.
  • cryolite is relatively small and operating temperatures are only about 700° C., compared to about 950° C. for the conventional Hall-Héroult process. Thus the vapor pressure of fluorides will be very low.
  • the anode effect can be avoided, because sulfur reacts at the anode.
  • the electrowinning can be carried out in a closed system, providing improved off-gas capturing.
  • concentration of the cryolite is in the range of 5-30 wt %, preferably 7-15 wt %, more preferably about 10 wt %. Test have shown that relatively low concentrations of cryolite are sufficient to obtain the desired increase in conductivity with an optimum concentration of about 10 wt %.
  • the process according to the invention is also improved in a embodiment which is characterised in that the measures comprise enhancing the effective area of an anode extending into the bath by reducing the amount and/or size of gas bubbles covering the anode.
  • a preferred embodiment of the process according to the invention is characterised in that the bath of molten salt mainly comprises alkali metal chlorides, preferably KCl and NaCl.
  • a particular advantageous embodiment of the process according to the invention is characterised in that the bath of molten metal is substantially free of earth alkaline chlorides.
  • the bath of molten salt is substantially free of earth alkaline chlorides.
  • the electrolysis is carried out at a bath temperature of between 600° C. and 850° C., preferably between 700° C. and 800° C.
  • MgCl 2 is added to the bath of molten salt to increase the solubility of Al 2 O 3 and to lower the melting temperature of the bath so that AlCl 3 can be added to increase solubility.
  • the melting temperature of the bath is increased, but that is acceptable since the melting point of cryolite is much higher than the proposed bath temperatures. Further it has to be recognised that the melting temperature of the NaCl—KCl eutect is still substantially lower than proposed bath temperature.
  • a still further preferred embodiment of the process according to the invention is characterised in that the electrolysis is carried out in a multi-polar electrolysis cell.
  • the interelectrode gap can be reduced and kept constant and a multi-polar cell operation is possible, which will increase productivity, reduce energy consumption and reduce capital costs
  • FIG. 1 shows the decomposition potential of aluminium compounds to produce aluminium by electrolysis.
  • FIG. 2 shows a schematic view of an experimental electrolysis cell.
  • FIG. 3 shows a plot of cathodic current density as a function of the cell potential for the electrolysis of aluminium from Al 2 S 3 in a MgCl 2 —NaCl—KCl electrolyte of 50-30-20 mole % at 725° C. using cryolite as an additive (flux).
  • FIG. 4 shows a plot of cathodic current density as a functions of the cell potential for the electrolysis of aluminium from 4 wt % Al 2 S 3 in a MgCl 2 —NaCl—KCl electrolyte of 50-30-20 mole % at 725° C. using different amounts of cryolite as an additive (flux).
  • FIG. 5 shows a plot of the cathodic current density as a function of the cell potential for the electrolysis of aluminium from 4 wt % Al 2 S 3 in a MgCl 2 —NaCl—KCl electrolyte of 50-30-20 mole % of 725° C. using NaF as an additive (flux).
  • FIG. 1 has been described above.
  • the electrolysis of aluminum from aluminum sulfide is carried out in a two electrode system.
  • a schematic view of the experimental cell is depicted in FIG. 2 .
  • the cathode is a pool of molten aluminum ( 1 ) (effective area 8.1 cm 2 ), which is polarized by a graphite block ( 2 ) connected by a rod of stainless steel ( 3 ) shielded by a quartz tube ( 4 ).
  • the anode is constructed of a graphite block ( 5 ) of 1 cm 2 , 5 cm high, which is immersed 2 cm into the electrolyte and is connected by a stainless steel rod ( 7 ).
  • the interelectrode gap is 2 cm.
  • the anode acts as the reference electrode, thus the cell potential is measured during the electrolysis.
  • the electrochemical cell is constructed of sintered Al 2 O 3 (Alsint) walls ( 10 ).
  • the melt is protected by an inert Ar atmosphere.
  • Argon is introduced through inlet ( 8 ) and leaves the cell through outlet ( 9 ).
  • the cell is externally heated by a 2100 W cylindrical furnace equipped with heating elements (not shown).
  • the maximum operating temperature is 1400° C.
  • the temperature is measured and controlled by type S thermocouples and a control unit (not shown).
  • the potential is measured with a potentiostat/galvanostat, which was used in combination with a current booster, to enable a high current throughput (20 A range).
  • the electrochemical measurement system is fully computer controlled.
  • FIG. 3 shows the major improvement of the electrolysis performance, because of the addition of Na 3 AlF 6 .
  • the electrolyte composition changes to a quaternary mixture of 48-29-19-4 mole % of MgCl 2 —NaCl—KCl—Na 3 AlF 6 .
  • the current density is more than 3 times larger at a given cell potential.
  • the Nernst equation indicates that the activity of Al 2 S 3 in a melt with cryolite addition approaches unity. Further increase of the Al 2 S 3 concentration in the melt did not produce a significant effect (compare experiments C and D).
  • FIG. 4 depicts a graph showing the influence of the amount of cryolite added to the melt. There seems to be an optimum at about 10 wt. % cryolite addition.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
US10/550,678 2003-03-31 2004-03-31 Process For the Electrolysis of Aluminiumsulfide Abandoned US20080202939A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
EP03075948 2003-03-31
EP03075948.4 2003-03-31
PCT/EP2004/003625 WO2004088000A2 (fr) 2003-03-31 2004-03-31 Procede d'electrolyse de sulfure d'aluminium

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US20080202939A1 true US20080202939A1 (en) 2008-08-28

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US (1) US20080202939A1 (fr)
EP (1) EP1625245A2 (fr)
JP (1) JP4975431B2 (fr)
AU (1) AU2004225794B8 (fr)
CA (1) CA2520798C (fr)
NO (1) NO20055027L (fr)
RU (1) RU2341591C2 (fr)
WO (1) WO2004088000A2 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20130289329A1 (en) * 2012-04-25 2013-10-31 Korea Atomic Energy Research Institute Decontamination method of cladding hull wastes generated from spent nuclear fuel and apparatus thereof
WO2019094921A1 (fr) * 2017-11-13 2019-05-16 Chromalox, Inc. Réchauffeur de sel fondu à moyenne tension et système d'accumulation d'énergie thermique à sel fondu comprenant celui-ci

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
ATE466974T1 (de) * 2005-03-31 2010-05-15 Aleris Aluminum Koblenz Gmbh Verfahren und vorrichtung zur produktion von aluminium
US7867373B2 (en) 2005-04-01 2011-01-11 Aleris Aluminum Koblenz Gmbh Method and apparatus for the production of aluminum
JP2014237873A (ja) * 2013-06-07 2014-12-18 住友電気工業株式会社 溶融塩の製造方法、溶融塩及びアルミニウムの製造方法
RU2567429C1 (ru) * 2013-07-09 2015-11-10 Общество с ограниченной ответственностью "Объединенная Компания РУСАЛ Инженерно-технологический центр" Электролит для получения алюминия электролизом расплавов

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2939824A (en) * 1957-07-26 1960-06-07 Kaiser Aluminium Chem Corp Method and apparatus for the production of aluminum
US4133727A (en) * 1977-05-17 1979-01-09 Aluminum Company Of America Method for extracting heat from a chamber containing a molten salt
US4265716A (en) * 1979-06-14 1981-05-05 The United States Of America As Represented By The United States Department Of Energy Method of winning aluminum metal from aluminous ore
US4464234A (en) * 1982-04-01 1984-08-07 The United States Of America As Represented By The United States Department Of Energy Production of aluminum metal by electrolysis of aluminum sulfide

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB484014A (en) * 1936-12-04 1938-04-29 Daniel Gardner Improvements in or relating to electrolytic processes for the manufacture of aluminium
GB1483193A (en) * 1974-11-29 1977-08-17 Courtaulds Ltd Process for the manufacture of carbon disulphide
JPS54114410A (en) * 1978-02-27 1979-09-06 Shirou Yoshizawa Production of metal aluminium
DE3412114A1 (de) * 1984-03-31 1985-10-10 Brown, Boveri & Cie Ag, 6800 Mannheim Verfahren zur herstellung von aluminium
DE69927605T2 (de) * 1998-12-18 2006-08-03 Corus Aluminium Walzprodukte Gmbh Verfahren zur erzeugung von aluminium aus aluminiumoxid via aluminiumsulfidprozess

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2939824A (en) * 1957-07-26 1960-06-07 Kaiser Aluminium Chem Corp Method and apparatus for the production of aluminum
US4133727A (en) * 1977-05-17 1979-01-09 Aluminum Company Of America Method for extracting heat from a chamber containing a molten salt
US4265716A (en) * 1979-06-14 1981-05-05 The United States Of America As Represented By The United States Department Of Energy Method of winning aluminum metal from aluminous ore
US4464234A (en) * 1982-04-01 1984-08-07 The United States Of America As Represented By The United States Department Of Energy Production of aluminum metal by electrolysis of aluminum sulfide

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20130289329A1 (en) * 2012-04-25 2013-10-31 Korea Atomic Energy Research Institute Decontamination method of cladding hull wastes generated from spent nuclear fuel and apparatus thereof
WO2019094921A1 (fr) * 2017-11-13 2019-05-16 Chromalox, Inc. Réchauffeur de sel fondu à moyenne tension et système d'accumulation d'énergie thermique à sel fondu comprenant celui-ci

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Publication number Publication date
AU2004225794A1 (en) 2004-10-14
WO2004088000A2 (fr) 2004-10-14
RU2005133440A (ru) 2006-06-10
EP1625245A2 (fr) 2006-02-15
CA2520798C (fr) 2009-06-16
JP2006522220A (ja) 2006-09-28
RU2341591C2 (ru) 2008-12-20
JP4975431B2 (ja) 2012-07-11
NO20055027L (no) 2005-10-28
WO2004088000A3 (fr) 2005-01-13
AU2004225794A8 (en) 2009-10-22
AU2004225794B8 (en) 2009-10-22
AU2004225794B2 (en) 2009-08-20
CA2520798A1 (fr) 2004-10-14

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