EP0108178B1

EP0108178B1 - Removal of alkali metals and alkaline earth metals from molten aluminium

Info

Publication number: EP0108178B1
Application number: EP82305965A
Authority: EP
Inventors: Ghyslain Dubé
Original assignee: Alcan International Ltd Canada
Current assignee: Rio Tinto Alcan International Ltd
Priority date: 1982-11-09
Filing date: 1982-11-09
Publication date: 1987-07-22
Also published as: NO162028B; DE3276823D1; NO162028C; ES8504267A1; NO834081L; AU562966B2; AU2107783A; JPS59100231A; EP0108178A1; ES527104A0; CA1215237A

Description

This invention relates to the removal of contaminant quantities of alkali metals and alkaline earth metals from molten aluminium by reaction with aluminium fluoride.
U.S. 3,620,716 discloses a method of removing magnesium from molten aluminium by fluxing with a cover flux containing AIF₃. According to this document, the flux is stirred into the melt and the melt is then left for at least a few hours to allow reaction between the flux and the contaminant.
A method of treating molten aluminium with a particulate aluminium fluoride-yielding material has been described in EP-A-0065854. This publication is comprised in the state of the art as defined by Article 54(3) and (4) EPC. In that method a charge of contaminated metal is placed in a vessel, in which it is stirred to establish a vortex and flow currents having both downward and lateral components at the bottom of the vortex and upwardly spiralling currents at the periphery of the vessel. The particulate aluminium fluoride-yielding material is supplied to enter the vortex and become entrained in the molten aluminium and the stirring of the molten aluminium is continued until the alkali metal and alkaline earth metal content is reduced to a desired low level.
The vortex was preferably generated by means of a multi-blade impeller having blades inclined to its axis of rotation and in the already described process this was preferably arranged somewhat eccentrically in relation to the axis of a cylindrical vessel having a vertically arranged axis. It was found that optimum results were achieved by careful dimensioning of the impeller in relation to the dimension of the treatment vessel and also to the depth of molten metal to be contained in the vessel, generally a vessel for transferring a body of molten metal from a reduction cell to a casting station.
The preferred relationships were established as follows:
In the above 8 designates the pitch angle of the impeller blades, d is the overall diameter of the bladed portion of the impeller, h is the height of the impeller blades, y is the vertical distance from the bottom of the crucible interior to the midpoint of the impeller blades, H is the vertical distance from the bottom of the crucible interior to the quiescent level of molten metal in the crucible, D is the internal diameter of the crucible. The impeller was preferably eccentrically located at a distance of 0.10.25D and more preferably at a distance of 0.25-0.7 d in relation to the axis of the vessel.
In the method, as described in our said European Patent Application No. 82,302448.4 (EP-A-0065854) the vortex is generated by a stirrer in a mass of molten metal, which is essentially cylindrical in the unstirred condition. It has now been found that the process can be effectively performed on bodies of molten metal which are initially non-cylindrical (in the non-stirred condition). Solid particulate material can be entrained in and reacted with alkali metal contaminants in molten aluminium by generation of a vortex in such non-cylindrical masses with similar efficiency to that achieved when the metal is contained in an upright cylindrical crucible, thus permitting the process to be employed in all forms of transfer crucible, particularly crucibles having an essentially continuous, rounded sidewall surface free from recesses or abrupt angles, which would result in locally stagnant zones within the upwardly spiralling molten metal flow in the peripheral region of the vessel.
Referring now to the accompanying drawings.

Figure 1 is a diagrammatic plan view of the surface of a body of molten metal contained in a cylindrical crucible tilted with respect to the vertical.
Figure 2 is a similar view of a crucible having parallel sides and semi-circular ends.
Figure 3 is a view of a crucible having a downwardly tapering interior arranged at a substantial tilt angle in relation to the vertical, with a stirrer rotating about a vertical axis.
Figure 4 is a similar view to Figure 3, but with the stirrer rotating about an axis parallel to the crucible axis.

In performing the process of the present invention in the crucibles illustrated in Figures 1-4 the stirrer is constructed and arranged to operate in the same way as in our co-pending European Patent Application No. 82.302448.4. The stirrer is supported by a lid (not shown) and a duct is arranged on the lid for the supply of a particulate aluminium fluoride-yielding material (which expression embraces compounds such as KAIF₄ on to the surface of the molten metal in the vessel).
During the treatment of molten aluminium with AIF₃ powder, alkali and alkaline earth metals react preferentially with AIF₃ (compared to aluminium) to form mixed alkali cryolithionite compounds, e.g. Na₅Al₃F₁₄, Na₃LiAlF₃, and Li₃Na₃Al₂F₁₂.
These compounds, having a relatively low melting point (compared with pure cryolite), can easily be agglomerated or stick to the crucible walls or float to the melt surface where they react with metal oxide or particles of electrolyte always present after the siphoning of electrolytic cells. During subsequent metal transfer from the crucible by siphoning, most of these compounds will remain inside the crucible.
As previously stated, according to the present invention, it is possible effectively to treat contaminated molten Al metal with solid particulate AIF₃ and maintained in a non-cylindrical mass by a crucible or metal-confining vessel which has a configuration other than cylindrical and/or has an orientation other than vertical. For example, the vessel may be shaped as shown in Figures 2 and 3, and may be axially tilted with even an axially tilted stirrer (Fig. 4), although a vertical axis of stirrer rotation is preferred because of the effect of gravity on vortex generation.
We have found that the preferred dimensional and positional relationships already stated for an upright cylindrical vessel can be more broadly defined with reference to the geometric axis of the vessel, the axis of impeller rotation, and the plane of impeller rotation (viz. the plane, containing the midpoint of the impeller blades, perpendicular to the axis of impeller rotation). Thus, the diameter D is the minimum internal diametrical dimension (i.e. measured through the geometric axis of the vessel) of the metal-containing vessel in the plane of impeller rotation. Blade diameter d and eccentricity x are also measured in the plane of impeller rotation, while blade height h, distance y, and distance H are all measured along the axis of impeller rotation, and pitch angle 8 is measured with reference to the axis of impeller rotation.
With the foregoing definitions, all the relationships and ranges of values (i.e. d/D, h/H, etc.) already given, hold true, except for the outside range of eccentricity x. In general, the eccentricity is limited only by the requirement that the minimum distance C from the axis of impeller rotation to the internal wall of the crucible vessel (again measured in the plane of impeller rotation) is at least equal to D/4.
Figure 1 represents the surface, having perimeter 10', of a body of molten metal contained in a cylindrical crucible tilted with respect to the impeller axis. In Figure 1 G is the geometric axis of the elliptical surface of the molten metal D is the short diameter of the ellipse, and L is the long diameter of the ellipse. The axis of impeller rotation can intersect the plane of impeller rotation (viz. the plane of the drawing) anywhere within the area enclosed by broken-line ellipse Ewhich is spaced inwardly from the crucible wall by a constant distance C_o=D/4. In the illustrated eccentric position of the impeller shaft 18, the eccentricity x is clearly greater than D/4, but the axis of impeller rotation is spaced from the crucible wall by a distance C greater than D/4.
In Figure 2, the cross-sectional configuration of the crucible 10" in the plane of impeller rotation is bathtub-shaped, having semicircular ends and straight parallel sides spaced apart by a distance equal to the diameter of the semi-circular ends. The minimum diameter D through the geometric axis G in this plane is the distance between the parallel sides (viz. the diameter of the semi-circular ends), while the long diameter L is equal to A+D, where A is the spacing between the centres of the semi-circles. The broken line E, defining the outer limit of eccentricity of the axis of impeller rotation, is again spaced inwardly from the crucible wall by a constance distance C_°=D/4. The impeller shaft 18 shown in an illustrative eccentric position within this outer limit, having an eccentricity x.
The crucible 10"' shown in elevational cross-section in Figures 3 and 4 has a geometric axis G tilted with respect to the vertical. In Figure 3 the axis of rotation of the impeller 14 is vertical; hence y is a vertical distance (from the midpoint of the impeller blades to the point on the crucible floor vertically beneath that midpoint) and eccentricity x is measured in the horizontal plane P of impeller rotation, which is at an oblique angle to G. In Figure 4, the axis of rotation of the impeller 14 is tilted to the vertical so as to be parallel to G; y is again measured along the impeller axis, now at an oblique angle to the vertical, and x is measured in rotational plane P which is perpendicular to G but at an oblique angle to the horizontal. The pitch angle 8, as indicated, is in each case measured with reference to the axis of impeller rotation. In both Figures 3 and 4 the crucible 10'" tapers so that its cross-sectional diametrical dimensions decrease in a downward direction; the value of diameter D which determines the various aforementioned dimensional relationships is, in each instance, measured in the plane P.
These different arrangements permitted the removal of Li contamination from AI metal, withdrawn from electrolytic reduction cells, to be achieved within reasonable time, but not in all cases with the same process time as could be achieved with a stirrer eccentrically related to the axis of an upright vertical vessel.
It will be appreciated that a mass of molten metal, held in a tilted cylindrical crucible, assumes an essentially non-cylindrical shape, having an elliptical surface. The top surface of the body of metal in the tilted, tapering crucible of Figures 3 and 4 is also ellipsoidal in shape.
We have found that a vortex can be generated satisfactorily where the impeller is tilted by an angle of up to 15° to the vertical, but with decreasing efficiency as the tilt angle is increased. It is however preferred that the tilt angle of the impeller should not exceed about 10° to the vertical. The tilting of the axis of the crucible is of less importance. However, increase of the tilt angle of the crucible will decrease its molten metal-holding capacity. In all cases however improved results are obtained when the crucible (of whatever shape) is maintained in an upright condition and confines the molten metal to a non-cylindrical mass.
It should be noted that the treatment material may comprise aluminium fluoride, possibly containing up to 50% inert material, such as aluminium oxide. Alternatively aluminium fluoride may be added in chemically-bound form, such as a fluoaluminate of sodium or potassium. Conveniently it may be added as NaF/AIF₃ having a low NaF/AIF₃ ratio by weight, for example 0.6-0.7/1.
Aluminium fluoride may also be added admixed with other alkali metal fluorides or chlorides or alkaline earth metal fluorides or chlorides. As an alternative to aluminium fluoride, any active fluorine-containing compound could be added, i.e. any such compound which on addition to molten aluminium will liberate a fluoride which is reactive towards alkali or alkaline earth metal contaminants and does not introduce other undesirable contaminants into the molten aluminium. KBF₄ and K₂TiF₆ are examples of such compounds.
All such materials are for convenience in the appended claims considered as being embraced by the terms "aluminium fluoride" or "aluminium fluoride-yielding material".

Claims

1. A method of removing contaminant alkali metals and alkaline earth metals from molten aluminium comprising:-

(1) stirring a mass of molten contaminated aluminium in a vessel by means of an impeller also contained in the vessel, the axis of which impeller being located eccentrically in relation to the axis of the vessel and the blades of which impeller being pitched downwardly in relation to the axis of rotation, under conditions to establish a vortex therein and flow currents in said molten aluminium having both downward and lateral components at the bottom of said vortex and upwardly spiralling currents in the region of the periphery of the said vessel

(2) supplying particulate aluminium fluoride-containing material to the surface of said molten aluminium for entry into said vortex

(3) continuing the stirring of the molten aluminium until the alkali metal and alkaline earth metal content is reduced to a desired low level, and

(4) separating the molten aluminium from the alkali and alkaline earth metal fluoaluminate reaction products, wherein the vessel is non-cylindrical and/or has its axis inclined to the vertical such that it confines the molten metal to a non-cylindrical mass.

2. A method according to claim 1, wherein the molten metal is treated with powdered AIF₃ or NaF. AIF₃ having a weight ratio of NaF:AIF₃ in the range of from 0.6-0.7:1.

3. A method according to claim 1, further characterised in that the containing vessel is non-cylindrical and has a minimum internal diameter D, and is filled with the molten body of metal to a height H, and the impeller has a diameter d and a blade height h, such that the ratio d/D is between 0.1 and 0.6 and the ratio h/H is between 0.1 and 0.7, D and d being measured in the plane of impeller rotation and H and h being measured along the axis of impeller rotation.

4. A method according to claim 3, further characterised in that the minimum spacing between the axis of impeller rotation and the vessel wall is D/4, measured in the plane of impeller rotation.

5. A method according to claim 3, further characterised in that the midpoint of said blades is spaced above the bottom of said vessel by a distance y, measured along the axis of impeller rotation, said distance y being 0.25H and 0.75H.

6. A method according to claim 3, further characterised in that the axis of impeller rotation is eccentric in relation to the vessel axis by a distance, x, having a value of 0.25d-0.7d in the plane of impeller rotation.

₇. Apparatus for mixing particulate aluminium fluoride-yielding material with molten aluminium to remove dissolved contaminant alkali metals and alkaline earth metals from the molten aluminium said apparatus comprising

(a) a non-cylindrical vessel, having a vertical geometric axis and minimum internal diameter D, for containing a body of molten aluminium to a height H above the floor of the vessel; said vessel being free from internal baffles and having a generally rounded interior surface;

(b) a cover for said vessel supporting a multi-bladed impeller and means for driving said impeller about a vertical axis and means for rotating the impeller, said impeller having a diameter, d, and its blades having a height, h, the midpoint of said blades being spaced above the floor of the vessel by a distance, y, the axis of impeller rotation being spaced from said geometric axis by a distance x, and said blades having major surfaces pitched downwardly at an angle 8 to the vertical;

(c) the values of d, D, h, H, x and 8 being such that d/D is between 0.1 and 0.6, h/H is between 0.1 and 0.7, x is between 0.1 D-0.25 D, y is between 0.25H and 0.75H, and 8 is greater than 0° but not greater than 45°;

(d) the minimum spacing between the axis of rotation of the impeller and the vessel, measured in the plane of impeller rotation, is D/4, the values of D, d and x being measured in the plane of impeller rotation, the values of H and h being measured along the impeller axis of rotation.

8. Apparatus according to claim 7, further characterised in that d/D is between 0.15 and 0.40, h/H is between 0.2 and 0.40, x is 0.25d-D.7 d, y is between 0.4H and 0.6H, and 6 is between 30° and 40°.