Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
  • C25C3/18—Electrolytes

Definitions

• This invention relates to the production of aluminum by the electrolytic reduction process involving passage of electric current through a molten electrolyte which consists chiefly of cryolite and in which the alumina is dissolved.
• the invention relates to an improved electrolyte for the reduction process, having certain advantages, especially in economy, and to methods of aluminum reduction employing such electrolyte.
• Metallic aluminum is conventionally produced by passing electric current of large value through so-called pots or reduction cells wherein one or more carbon anodes project downwardly into a fused, i.e. molten bath, contained in a rather shallow, refractory-walled vessel which has a carbon bottom with means for electrical connection thereto.
• the molten bath is chiefly cryolite (sodium aluminum fluoride) or a similar mixture of sodium and aluminum fluorides, maintained at an appropriate temperature of the order of 1000" C. by the passage of cur- Purified alumina is dissolved in this molten electrolyte, in amounts usually ranging from about 2% up to the limit of its solubility, say 10% or so.
• the effect of the electric current is to deposit aluminum at the submerged cathode, i.e. the carbon bottom, where it collects in molten form and where it may be tapped or siphoned oil from time to time.
• the carbon anode combines with the oxygen of the alumina to form principally carbon monoxide and carbon dioxide so that the electrodes are progressively consumed and must be replaced from time to time, either by physical substitution or by use of socalled continuous electrodes.
• a typical electrolyte composition comprises cryolite (or a similar mixture of sodium and aluminum fluorides) 88% to 90% and calcium fluoride 10% to 12%, the latter component being introduced for the purpose of lowering the melting point of the bath.
• Alumina is added to the bath as needed, and it is also necessary to make up the cryolite content from time to time because of loss of fluorine, such make-up being mostly effected by adding aluminum fluoride.
• an electrolyte consisting of from 6% up to 9% sodium chloride (i.e. in the range below or less than 9%, but very preferably at least about 6%), from 6% to 9% of calcium fluoride, and the balance cryolite, provides definite advantage in several ways.
• electrolyte composition herein is given by weight, and that in operation, the electrolyte also includes, of course, alumina dissolved therein, in the usual purified form suitable for reduction by the electrolytic process, and likewise very minor quantities, ordinarily totaling less than one percent, of other compounds such as magnesium fluoride.
• cryolite in strict contemplation is a specific composition of sodium aluminum fluoride, viz. Na A1F it may be broadly understood as meaning, in electrolytes of this type, either the specific compound or equivalent or like mixture of sodium and aluminum fluorides.
• the composition has a lower melting point than conventional electrolytes, i.e. lower even than electrolytes that contain calcium fluoride which is commonly used (in larger amount) for the purpose of reducing melting point.
• a lower melting point the operation is readily performed at a lower temperature. This result is not only of general convenience, but indicates lower specific energy consumption, and also probably involves a reduction in the re-oxidation of cathodic products, so as to reduce current losses occasioned by such re-oxidation.
• the electrolyte has a higher electrical conductivity, with the direct result that the process can be performed with a higher electrical efiiciency, as to power or current or both.
• the electrolyte may have a somewhat reduced solubilty for alumina, it does not appear that the reduction is suflicient to impair the convenience of the operation or the working capacity of the cells or pots, nor is it sufficient to require extra attention or supervision by attending personnel.
• the anode gases i.e. the gases released from the cell (and usually drawn 01f through appropriate covering or other arrangements), may be a little more irritating, but no serious detriment has been found in practical test.
• the present, improved electrolyte has been found to have no unusual sensitivity and to present no serious difficulty from the standpoint of released gas.
• the reduction process involving the improved electrolyte (including sodium chloride from 6% to less than 9%) has specifically been found, in long tests, to be satisfactory for use in regular commercial aluminum production.
• electrolytes containing sodium chloride and containing calcium fluoride 6% to 9% also appear to offer some realization of the advantages of the invention with even a little lower percentage of sodium chloride, i.e. above 4%.
• the principal constituent of the electrolyte is the conventional cryolite, such term being here employed to include equivalent sodium fluoride-aluminum fluoride mixtures.
• references herein to sodium and aluminum fluoride material will be understood to mean cryolite and similar compositions as just stated and as conventionally employed in aluminum reduction cells.
• the proportion of sodium fluoride to aluminum fluoride, i.e. by weight (or by molecular weight in the specific complex compound) is conventionally called the bath ratio.
• 3NaF.AlF this ratio is therefore 1.50 to 1, conveniently expressed as 1.50. It is found that some variation in this ratio is permissible or indeed often desirable, particularly with respect to inclusion of larger relative quantities of aluminum fluoride, and hence may range from. 1.10 'to 1.60, with operation usually controlled to approximate the'ratio of 1.50 or, preferably in some cases,
• the range 1.20 to 1.40 seems to be especially desirable from both operating and economic viewpoints, in the practice of the present invention.
• the electrolyte wasmade up to include sodium chloride and calcium fluoride along with the major, conventional cryolite component.
• the sodium chloride content was kept between6% and 9%, i.e. inside such range, and most usually at about 7% (a special, preferred feature of invention being a sodium chloride content of 6% to about 8%
• the calcium fluoride was kept within the range of 6% to 9%, generally at about 7%.
• Alumina was supplied to the bath in essentially the same manner and amount as for conventional procedure, i.e. so as to maintain an alumina content of at least about 2% it being found that the periodic additions could be sufficient to raise the content to as high as 10%. Special observation was maintained throughout the period but no undue sensitivity was noted, nor was it concluded that extra supervision would be necessary for successful operation. As compared with corresponding pots with conventional electrolyte, it was found that these cells showed 6% lower carbon consumption and a 20 C. lower operating temperature. The indication was that the current efficiency was higher by 1.2%. 1 Thus a positive and significant improvement was achieved in economy of carbon and electric current, with no detrimentto results in the reduction and recovery of metallic.
• the cells were force to the point of anode effect at regular intervals i.e. somewhat more often than the usual occurrences.
• the anode effect is a situation occurring when the alumina content drops too low, i.e. usually below 2%, whereupon the voltage across the cell rises to an abnormally very high value, indicated by the conventional signal, and the operation of the cell deteriorates, especially in exhibiting a sudden increase in energy and anode consumption.
• no unusual circumstances accompanied the anode effect conditions and correction by addition of alumina was readily accomplished as in the case of conventional pots.
• the stated alkaline-earth fluoride component may consist in part of magnesium fluoride and in part (preferably the major part) of calcium fluoride.
• An electrolyte for an aluminum reduction cell consisting essentially of sodium chloride in amount from 6% to less than 9%, calcium fluoride in amount from 6% to 9% and the balance sodium and aluminum fluoride material wherein the ratio of sodium fluoride to aluminum fluoride by weight lies between 1.10 to l and 1.60 to 1, with alumina dissolved therein as a source of aluminum upon passage of current through said electrolyte in molten form.
• the method of producing metallic aluminum from alumina which comprises dissolving alumina in a molten electrolyte consisting essentially of. sodium chloride in amount from above 4% to less than 9%, calcium fluoride in amount from 6% to 9%, and the balance sodium and aluminum fluoride material, and passing electric current through said electrolyte.
• the method of producing metallic aluminum from alumina which comprises dissolving alumina in a molten electrolyte consisting essentially of sodium chloride in amount from 6% to less than 9%, calcium fluoride in amount from 6% to 9% and the balance sodium and aluminum fluoride material wherein the ratio of sodium fluoride to aluminum fluoride by weight lies between 1.10 to 1 and 1.60 to 1, and passing electric current through said electrolyte.
• the method of producing metallic aluminum from alumina which comprises dissolving alumina in a molten electrolyte consisting essentially of sodium chloride in amount from above 4% to less than 9%, material in amount from 6% to 9%, selected from the group consisting of calcium fluoride and magnesium fluoride, and the balance sodium and aluminum fluoride material, and passing electric current through said electrolyte.
• An electrolyte for an aluminum reduction cell consisting essentially of sodium chloride in amount from above 4% to less than 9%, material in amount from 6% to 9%, selected from the group consisting of calcium fluoride and magnesium fluoride, and the balance sodium and aluminum fluoride material, with alumina dissolved therein as a source of aluminum upon passage of current through said electrolyte in molten form.

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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)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)

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Citations (3)

• 1957
  • 1957-06-12 US US665132A patent/US2915443A/en not_active Expired - Lifetime
• 1958
  • 1958-06-09 GB GB18406/58A patent/GB831930A/en not_active Expired
  • 1958-06-11 CH CH6046358A patent/CH372470A/de unknown
  • 1958-06-11 DE DEA29675A patent/DE1125190B/de active Pending
  • 1958-06-12 FR FR1208457D patent/FR1208457A/fr not_active Expired

Patent Citations (3)

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