CA1054970A - Treatment of aluminium reduction cell linings - Google Patents

Abstract

A B S T R A C T
The alumina linings of electrolytic reduction calls become contaminated with carbides, nitrides and fluorides with continued use of the cell.
A procedure for the recovery of such used alumina linings comprises subjecting the crushed used lining to the action of dry steam in a pressure vessel to effect break down and removal of carbides and nitrides. The thus treated material, which is partially hydrated, is then charged to the electrolyte of an electrolytic reduction cell to recycle both the fluoride and alumina content of the used alumina lining.

Description

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r~he present invention relates to the treatment of used linings of aluminium reduction cells and to methods of recovering usable materials therefrom.
In an aluminium reduction cell the electrolyte, consiætin~ essentially of alumina 9 dissolved in molten cryolite 7 is co~tained within a cell, having an outer steel shell, lined with carbon. ~he floor of the cell forms the cathode and co~sists of carbon blocks~ Between the cathode floor blocks and the steel shell it is customary to include a layer of highly compactea alumina~ which acts as a thermal and electrical insulation between the cathode blocks and the steel shell.
It i5 well known that the carbo~ lining of a reduc-tion cell becomes increasin~ly permeated with the c~mponents of the electrolyte, including undesired impurities, which build u~
in the electrolyte during prolonged use. In addition to contamination of the carbon lining, the alumina underlayer beGomes permeated by the electrolyte. r~he concentration of the electr~lyte materials in the alumina progressively increases t~wards the i~terface between the alumina and the carbon blocks.
The insulating properties of the alumina layer deteriorate as a result of this permeation by electrolyte materials and, in conse-. ~ .
; ~uence, it is usual to remove and discaxd this alumina layer whenever the carbon lining of the cell is removed for renewal~
~he discarded alumina material is unsuitable for re-use in the , ,.: .
reduction cell lining and, because of its high content of impurities, is unsuitable for direct addition to the electrolyte of another reduction cell, although in additio~ to -the alumi~a, it contains a substantial amount of valuable fluorides.
The principal objection to the addition of the dis-3 carded alumina to a reduction cell is the strong odours which arise when -the material i8 heated after it is added to the solid crust of a reduction cell. We have now discovered that this , ., ~ 54~7~
difficulty may be overcome by subjecting the alumina material to hydration by dry steam under elevated pressure~ ~his has the effect of breaking dow~ the carbides and/or nitrides contained ~ thereinO ~he steam treatment ~ay be continued until the evolu-; 5 tion of hydrocarbons and/or ammonia has ceased. ~ypically a layer of alumina removed from a reduction cell includes up to

2~/o ~ in the form of flu~rides and up to 10% of unwanted car~
bides and/or nitrides~ ~hese unwanted carbides and nitrides may be substantially comp]etely eliminated by subjecting the alumina material, after grinding to an appropriate size, to dry steam at a temperature in the range of 120 to 175C and a gauge pressure of 1 to 7 kgs/cm2 in a pressure vessel for about 7 hours. The treated material undergoes a gain in weight of about 7% as a result of hydration of the alumina.
~he th~s treated, partially hydrated material con-; taining valuable fluorides, is suitabLe for use as the alumina feed to a reduction cell either on its o~m or mixed with regular calcined alumina.
In o~e example the alumina insulating material ; 20 removed from a reduction cell was crushed coarsely to particles below 1 cm in size and was treated in open tra~s in a thermally in~ulated pressure vessel, equipped with a pressure gau~e, thermometer and a bleed valve to exhaus-t ammo~ia and hydro- t car~ons. ~he alumina lini~g material was charged in an am~unt ~f about 500 kgs. ~ive steam at a pressure of 105 kg/cm2 was admitted to the vessel and the pressure was maintained u~til the - presence of ammonia or hydrocarbons was not detectable by smell ~; or pe~iodic sampling of the blow-off gases released by opening the bleed valve. The time required to reach this stage is about 7 hours. In this period the content of aluminium carbides and nitrides are substantially completely broken down and a small proportion o~ alumina is hydrated to give a weight increase of _2-' ,:
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about ~/o, as noted above.
~ he treated material is then suita~le for direct addition to the electrolyte of an electrolytic reduction cell without any further treatment and in this way the valuable fluoride content of the used alumina lining is recovered in a simple manner.
~ he treatment time required to achieve the required degree of breakdown of carbides and nitrides varies with process conditions 7 particularly with the particle size of the matexial charged to the pressure vessel, the depth of crushed material in the tra~s, the pressure and temperature o~ the steam (which may be to some extent superheated), as well as on the content o~
contaminants in the insulation material. ~owever, appropriate control of process time is extremely simple, since the treatment . .
is effectively completed when the release o~ steam via the bleed valve is ~ longer accompanied by objectionable odours.
In general the treatment requires about 4 to 10 hours, according to circumsta~ces.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A procedure for the recovery of valuable components from the used alumina insulation of an aluminium reduction cell which comprises the steps of crushing the used insulation, subjecting the used insulation to dry steam at a pressure in the range of 1 to 7 kgs/cm2 and a temperature of 120 to 175°C in a pressure vessel for a time sufficient to effect substantially complete breakdown of contained carbides and nitrides and adding the thus treated partially hydrated alumina insulation material to the electrolyte of an aluminium reduction cell without further treatment whereby the alumina and fluoride content of the used alumina insulation is recycled.