EP0136969B1 - Cell for the electrolytic refining of aluminium - Google Patents

Description

The invention relates to a thermally insulated cell for the electrolytic cleaning of aluminum.

• - unedlen Komponenten (z. B. Natrium, Kalium, Lithium, Magnesium, Calcium) der eingesetzten Legierung zwar im Aluminium anodisch auflösen, aber an der Kathode nicht abgeschieden werden können, und
• - die edlen Komponenten (beispielsweise Kupfer, Silizium, Eisen, Titan) sich nicht anodisch auflösen und somit unter Bildung von Seigerkristallen im Anodenmetall zurückbleiben.

The electrolytic cleaning of aluminum is based on the fact that it is relative to aluminum

• - Dissolve base components (e.g. sodium, potassium, lithium, magnesium, calcium) of the alloy used in the aluminum anodically, but cannot be deposited on the cathode, and
• - The noble components (for example copper, silicon, iron, titanium) do not dissolve anodically and thus remain in the anode metal with the formation of Seiger crystals.

• - Die unterste schwere Schicht, die üblicherweise aus einer AI-Cu-Si-Fe-Legierung besteht und deren Oberfläche zugleich die Anode ist.
• - Die Elektrolytschicht, bestehend aus den Fluoriden und/oder Chloriden von Alkali- und Erdalkalimetallen.
• - Das raffinierte Aluminium, die dritte, oberste Schicht, wobei deren untere Fläche die Kathode bildet.

The three-layer aluminum refining cells known since the beginning of this century contain three molten layers:

• - The bottom heavy layer, which usually consists of an Al-Cu-Si-Fe alloy and whose surface is also the anode.
• - The electrolyte layer, consisting of the fluorides and / or chlorides of alkali and alkaline earth metals.
• - The refined aluminum, the third, top layer, the lower surface of which forms the cathode.

When the direct electrolysis current is applied, the aluminum is oxidized to trivalent aluminum ions at the anode, these ions migrate through the electrolyte layer to the cathode, where they are reduced again to metallic aluminum.

The forehearth of the cell, which has a lower temperature than the 700 to 800 ^° C. customary for the refining of aluminum, removes the crystallized impurities, in particular intermetallic products of Al, Cu, Fe and Si, which are known as Seiger crystals.

The energy consumption of conventional three-layer aluminum refining cells is relatively high. Typical values for the cell voltage are around 5.5 V with a current yield of around 75-97%. This results in an energy consumption of around 16-18 kWh / kg of refined aluminum.

• - es werden Elektrolyten mit höherer elektrischer Leitfähigkeit eingesetzt, und/oder
• - die Interpolardistanz, d. h. die Dicke der Elektrolytschicht, wird erniedrigt.

From a physical point of view, there are two development directions for reducing energy consumption:

• - Electrolytes with higher electrical conductivity are used, and / or
• - The interpolar distance, ie the thickness of the electrolyte layer, is reduced.

However, the 10 to 15 cm thick electrolyte layer in conventional three-layer electrolysis cells cannot be reduced in size as desired without the risk of mechanical contamination of the refined aluminum layer due to contact with the anodically connected aluminum alloy.

Recently, diaphragms which are in the form of vessels or can be displaced in the vertical direction have been used, which help to reduce the high energy consumption.

EP-A-0 049 600 discloses a cell for the electrolytic cleaning of aluminum, in which the metal to be cleaned flows through the cell, the cell being equipped with a separator grill. This cell can be used to produce relatively pure metal, but not pure aluminum. In addition, Seiger crystals cannot be removed.

The inventor has set himself the task of creating a cell for the electrolytic cleaning of aluminum which, in addition to low energy consumption, has high metallurgical efficiency and which can be implemented with low investment costs.

The object is achieved according to the invention by the characterizing features of claim 1. Further preferred features result from claims 2 to 7.

In order for the lowest possible voltage drop to be ensured via the diaphragm plate, its material must be readily wettable by the electrolyte, and the aluminum ions must be able to migrate from the interior to the surface of the diaphragm with the least possible voltage drop. On the other hand, the diaphragm plate must be absolutely impermeable to the metallic aluminum, i.e. not be wettable.

If the aluminum to be cleaned is filled into the cavity of the electrode unit, between the graphite wall and the diaphragm plate, a static pressure increases with the filling level. If the value is critical, it becomes so large that the molten aluminum is pressed through the open-pored channels of the diaphragm plate, even though the material of the diaphragm plate is not wetted.

For example, the electrode units used in an industrial setting have a cross-sectional area of 2 x 2 m. If these are used in a vertical or almost vertical position, the open-pore structures can no longer be made so fine that the static pressure does not push the unpurified aluminum through the diaphragm plate. The electrode plates are therefore divided with partition walls made of graphite, preferably with a square or rectangular grid, the side length of which is between 5 and 30 cm. Each of these sub-elements formed by the partition walls has a separate diaphragm plate and a feed of aluminum to be cleaned.

However, the sub-elements of an electrode unit can also be designed as separate units, the wall to wall joined together and held together by a graphite frame. Such electrode units composed of building blocks have the advantage that individual sub-elements can be replaced. Of course, here too each sub-element has its own porous diaphragm plate and a feed line for the aluminum to be cleaned.

Window-shaped recesses can be made in the partition walls or in the joined walls. The molten aluminum does not only circulate in one sub-element, neighboring chambers are in the Metal flow included. However, the dimensions of the recesses had to be so small that the static pressure on the porous diaphragm plate remains below the critical value discussed above. When designing the cell, care must be taken to ensure that the thickness of the diaphragm plate, the material of which is the density of the electrolyte, the clear width of the open-pore channels, the dimensions of the sub-elements and the window-shaped recesses in the partition walls are matched to one another in such a way that this increases cleaning molten aluminum cannot penetrate into the pores of the diaphragm plate.

Aluminum oxide, magnesium oxide, oxynitrides of silicon or oxynitrides of aluminum and silicon are preferably used as materials for the open-pore diaphragm plate. The porosity is preferably between 60 and 90%. In general, the ceramic filters for cleaning liquid metal of CH-PS 622 230 can also be used as porous diaphragm plates if they are dimensioned appropriately for the graphite frame. In practice, 3-15 mm thick diaphragm plates are used for the electrolytic cleaning of aluminum.

The electrode units are used in thermally insulated cells with a steel tub embedded in a wall, which in turn is lined with magnesite stones or refractory material containing nitride. The electrode units form one or more rows inside the cell. All electrode units are arranged parallel to the terminal anode and the terminal cathode. The interpolar distance between the inside of the anodic diaphragm plate and the outside of the cathodic graphite frame is preferably 10-25 mm.

• - eine Horizontalanordnung, bei welcher die Elektrodeneinheiten vertikal oder nahezu vertikal eingesetzt sind,
• - eine Vertikalanordnung mit horizontal oder leicht geneigt eingesetzten Elektrodeneinheiten.

Depending on the arrangement of the electrode units, a distinction is made between two bipolar cell types for the electrolytic cleaning of aluminum:

• a horizontal arrangement in which the electrode units are inserted vertically or almost vertically,
• - A vertical arrangement with horizontally or slightly inclined electrode units.

The space surrounding the electrode units is filled with electrolyte material which is molten at the working temperature. The level of the electrolyte in the cell is practically not subject to fluctuations and lies above the uppermost part of the electrode units. The electrolyte preferably consists of a mixture of lithium chloride, potassium chloride and sodium chloride, it having a particularly favorable effect if a smaller amount of alkali metal fluoride is added. All of these electrolyte compositions are known and can be found in the specialist literature.

The aluminum to be cleaned is introduced into the cell for electrolytic cleaning via forehearths. These foreheads also serve to separate out the Seiger crystals. They consist of intermetallic compounds of aluminum, iron, silicon, titanium etc. As a rule, the Seiger crystals do not contain copper, as is the case with rotating layer electrolysis. Because the aluminum to be cleaned is separated from the high-purity aluminum by the diaphragm plate, the density of the anodic metal does not have to be controlled or increased. This means that the density of the electrolyte is irrelevant, which makes it easier to select an electrically highly conductive material.

The electrical direct current is conducted to the terminal anode via at least one anodic electrode rod, and is conducted bipolarly via the electrode units and the electrolytes through the cell to the terminal cathode, where the electrical direct current is in turn discharged through at least one cathodic electrode rod. In principle, the electrolysis with a bipolar cell, as in the three-layer process, where the aluminum is dissolved from the contaminated metal, passes through the electrolyte (as a result, the electrolyte material located in the open-pored channels of the diaphragm plate and the electrolytes located between the electrode units) and is deposited on the cathode becomes. In the present case, the cathodic surface is the back wall of the graphite frame. With bipolar cells, the electricity and investment costs can be significantly reduced compared to three-layer cells.

The separated high-purity aluminum flows from the cathodic graphite frame into a scoop channel, which is arranged in the electrically insulating part of the cell bottom and from where the high-purity aluminum can be drawn off with a suction pipe.

• Fig. 1 einen Vertikalschnitt durch das kathodische Ende einer bipolaren Zelle zur elektrolytischen Reinigung von Aluminium
• Fig. 2 ein Detail des unteren Bereichs von Fig. 1
• Fig. 3 eine vertikal einsetzbare Elektrodeneinheit in perspektivischer Darstellung
• Fig. 4 einen Vertikalschnitt durch eine bipolare Zelle mit zwei Reihen von vertikal angeordneten Elektrodeneinheiten
• Fig. 5 einen Vertikalschnitt durch eine horizontal einsetzbare Elektrodeneinheit
• Fig. 6 eine bipolare Zelle mit horizonal eingesetzten Elektrodeneinheiten.

The invention is explained in more detail with reference to the drawing. The schematic sections show:

• Fig. 1 shows a vertical section through the cathodic end of a bipolar cell for the electrolytic cleaning of aluminum
• FIG. 2 shows a detail of the lower area of FIG. 1
• Fig. 3 is a vertically insertable electrode unit in a perspective view
• Fig. 4 is a vertical section through a bipolar cell with two rows of vertically arranged electrode units
• 5 shows a vertical section through a horizontally insertable electrode unit
• 6 shows a bipolar cell with electrode units inserted horizontally.

The bipolar cell shown in FIG. 1 shows five vertical electrode units 10 with a graphite frame 12, the full-area recess of which is closed in the direction of the terminal graphite cathode 14 with a porous diaphragm plate 16. The vessel-shaped cavity of the electrode units 10 is filled with the aluminum 18 to be cleaned, which is present in molten form at an operating temperature of 700 to 800 ° C. In the bottom of the cell lining made of refractory masonry, drainage channels 20 are cut out for the pure aluminum. The electrolyte 22 is arranged between the electrode units and above the ultrapure aluminum.

If direct electrical current is conducted to the terminal anode, not shown, of the cell, it flows through the electrode units to the terminal cathode, from where the circuit via cathodic electro, also not shown the bars are closed. In the working phase, the side of the aluminum to be cleaned facing the porous diaphragm plate acts as an anode, the rear wall of the next electrode unit arranged in the direction of the terminal cathode acts as a cathode. All electrode units arranged in the cell therefore work bipolar. The interpolar distance d corresponds to the shortest distance of the aluminum to be cleaned from the rear wall of the next graphite frame, in other words the thickness of the porous diaphragm plate plus the thickness of the electrolyte layer.

2 shows the freshly deposited aluminum 26 which bubbles down along the cathodic rear wall of the graphite frame 12 and collects in the drainage channel 20. The interpolar distance d extending across the electrolyte 22 and the porous diaphragm 16 in the horizontal direction is also clearly visible.

For the sake of simplicity, the partitions dividing the interior of the electrode unit are omitted.

The electrode unit 10 shown in FIG. 3 and intended for vertical use consists of four sub-elements 28 which are held together with a graphite rim 30. Each sub-element 28 has a graphite frame 12 with a full-area opening which is closed by the porous diaphragm plate 16. This diaphragm plate is expediently already used with electrolyte material in the open-pore structure.

Each sub-element 28 has its own forehearth 32, which communicates via an opening 34 with the interior of the sub-element. The foreheads provided for separating the Seiger crystals and for introducing aluminum to be cleaned are also offset horizontally to make operation easier. The clear height H of a sub-element may only be so great that the static pressure for the passage of aluminum through the open-pore structure is not reached. In the present case, H measures approximately 30 cm.

The electrode units 10 of FIG. 2 are used in two rows in the cell according to FIG. 4. A steel trough 36 is inserted into the wall 24 and is closed by means of a corrosion-resistant, double-walled cover 38 made of steel using a seal 40. On the inside, the steel trough is lined with magnesite stones 42, which are resistant to both the molten electrolyte and the molten aluminum.

A steel base plate 44 supports the entire cell and offers additional insulation thanks to the air chambers 46.

The cover 38 of the steel trough 36 is penetrated by an insulating tube 58, which on the one hand allows the level 50 of the electrolyte 22 to be kept above the electrode units 10 by material replenishment and on the other hand evolving gases. A special device 48 is connected during cell operation in order to extract any gases that may arise.

A siphon 70 allows the cleaned aluminum to be sucked off the trough 20 once or several times a day. The level 52 of the pure aluminum, however, must always be below the electrode units 10.

The electrode unit 10 shown in FIG. 5 corresponds essentially to FIG. 3, but is intended for the horizontal arrangement of a cell. The forehearth 32 and its opening 34 are arranged accordingly. The vertical section through the electrode unit 10 runs through a partition wall 54 made of graphite, which has window-shaped recesses 56 on its underside. This enables the circulation of the aluminum to be cleaned between neighboring sub-elements. The height H of about 25 cm is chosen so that the static pressure of the aluminum to be cleaned is not sufficient to press the aluminum into the pores of the diaphragm 16.

5 are inserted into a cell of the type of FIG. 6, with the diaphragm plate 16 down, in a horizontal position. The bottommost graphite cathode 14 is equipped with three cathodic electrode rods 62, the uppermost terminal graphite anode 60 with three anodic electrode rods 72.

In addition to the inner steel pan 36, the cell is equipped with an outer steel pan 64, with individual refractory stones acting as spacers 66. The space formed by the steel tubs is covered with a light insulation material 68, such as. B. rock wool, filled.

Claims (7)

1. A heat-insulated cell for the electrolytic purification of aluminium, comprising a trough having a steel tub (36) which is embedded in brickwork (24) and which is clad with electrolyte- and high temperature-resistant material (42) and is closed by a cover (38), having an electrolyte (22) on the base of alkali chlorides, forehearths (32) for the addition of the aluminium (18) to be purified and the separation of the liquation crystals, a supply line (58) for electrolyte material, which is designed also as a gas drain, and a collecting and discharge system for the high- purity aluminium (20), and connected electrically in series in the interior of the cell bipolar electrode units (10) immersed in the electrolytes (22) and each having:

- a graphite frame (12) of vessel-shaped construction open over its whole area in the direction of the graphite cathode (14) fixed at the end;

- partitions (54) of graphite for subdivision into at least three subelements (28);

- a diaphragm plate (16) which closes off and seals the opening to each sub-element (28) in the graphite frame (12) and is wettable by the molten electrolyte (22) but on the contrary not wettable by the aluminium and the open-pore structure of which is filled with electrolyte material; and

- a separate forehearth (32) for each sub-element (28) for the feeding in of aluminium (18) to be purified and the separation of liquation crystals the height (H) of the volume formed between the graphite frame (12) and the diaphragm plate (16) for the aluminium (18) to be purified being dimensioned in such a way that with the volume filled the static pressure is lower than the critical value for flow through the porous diaphragm plate (16).

2. A cell as in Claim 1, characterized in that the sub-elements (28) are made as separate units and held together by a graphite enclosure (30).

3. A cell as in Claim 1 or 2, characterized in that the partition walls (54) serving for the mechanical stabilization of the diaphragm plate (16) or respectively the joined-together walls of the sub-elements (28) have openings (56) in the form of windows.

4. A cell as in at least one of the Claims 1 to 3, characterized in that the diaphragm plate (16) has a porosity of 60-90%.

5. A cell as in at least one of the Claims 1 to 4, characterized in that the interpolar distance (d) between the inside of the anodic diaphragm plate (16) and the cathodic graphite frame (12) amounts to 10-25 mm.

6. A cell as in at least one of the Claims 1 to 5, characterized in that the electrode units (10) are built in vertically or nearly vertically side by side with diaphragm plates (16) arranged at the side.

7. A cell as in at least one of the Claims 1 to 5, characterized in that the electrode units (10) are arranged one above the other between horizontal and slightly sloping, with the diaphragm plates (16) downwards.

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