DE102010020030A1 - Reducing an electrical contact resistance or voltage drop between a connection unit and a pre-combusted carbon anode for the melt-flow electrolytic aluminum extraction, comprises shrinking the liquid cast iron to the mold parts - Google Patents

Abstract

The method for reducing electrical contact resistance/voltage drop between a connection unit made of steel nipple and cast iron sleeve and a pre-combusted carbon anode for melt-flow electrolytic aluminum extraction, comprises shrinking the liquid cast iron during its solidification and cooling to the mold parts, which are made of combusted carbon of the anode block and are provided in a nipple hole, based on shaping of nipple hole and during casting process of the nipple, where a vertical slip of the connection unit of steel nipple/cast iron sleeve does not occur longer in the nipple hole. The method for reducing electrical contact resistance or voltage drop between a connection unit made of steel nipple and cast iron sleeve and a pre-combusted carbon anode for the melt-flow electrolytic aluminum extraction, comprises shrinking the liquid cast iron during its solidification and cooling to the mold parts, which are made of combusted carbon of the anode block and are provided in a nipple hole, based on the shaping of the nipple hole and during the casting process of the nipple, where a vertical slip of the connection unit of steel nipple/cast iron sleeve does not occur longer in the nipple hole. The connection unit of steel nipple/cast iron sleeve performs a contact pressure on the bottom of the nipple hole by shaping the nipple hole and during the insertion of the anode block into an electrolysis cell. The nipple hole bottom is implied in the power transmission from the connection unit of steel nipple/cast iron to the carbon anode in a targeted manner. The annular grooves are formed in a cylindrically slightly conically shaped nipple hole wall of the green carbon anode and are then filled later with cast iron in the combusted carbon anode. The nipple hole bottom has a round sharply limited carbon socket, which is cast with cast iron and serves for placing the cylindrical steel nipple. An independent claim is included for a device for shaping a nipple hole.

Description

The invention relates to a method and apparatus for designing and forming the nipple hole in carbon anodes for the molten aluminum electrowinning to significantly reduce the electrical contact resistance or voltage drop between the steel nipple and the "pre-baked" carbon anode.

In the electrolytic cells for the molten-aluminum electrowinning at a temperature of about 960 ° C are used as anodes mostly prefired carbon body. "Prebaked" in this case means that the shaped "green" carbon blocks consisting of, for example, calcined petroleum coke and binder pitch are subjected to a firing process of up to nearly 1200 ° C. before being used in electrolysis. The carbon anodes not only serve to supply power to the electrolytic bath, but are also consumed by oxidation to CO ₂ and some CO, because oxygen from the electrolytic decomposition of alumina is deposited at the anode. If the carbon anode is consumed to a 15-20 cm thick remainder, this anode residue is replaced with a new carbon anode.

In the electrolytic cell, the power supply from the horizontally mounted and adjustable aluminum anode bar to the carbon anode via the vertical anode bar made of aluminum (or copper), then via the cross bar made of steel (or cast steel) and then to the associated steel nipples in the carbon anode. The usually cylindrical steel nipples are anchored in the nipple holes of the carbon anode by means of cast iron.

Between the cylindrical steel nipple and the smooth inner wall of the nipple hole there is an annular space filled with liquid cast iron. The cast iron shrinks during its solidification and cooling to ambient temperature on the round nipple and thus gives a tight fit. In the wall of the nipple hole are inclined grooves, usually 6 or 8, which are also filled with cast iron and represent in the solidified state of the cast iron bevels, which in combination with other nipple holes (2-4) carry the anode. The solid cast iron piece between the cast-in nipple part and the nipple hole with oblique grooves is called a cast sleeve.

The configuration of the nipple hole with inclined grooves has, with few exceptions, largely become established for connecting the nipple to the carbon anode in the aluminum smelters. The nipple holes are formed by appropriately designed Nippellochformer, which are rotatably mounted in the cover weights of the vibratory compactors or the upper dies of the swaging presses in the green anode blocks. When lifting the deck weights or upper punch, the Nippellochformer turn out of the nipple holes. 1 shows an example.

The majority of known nipple holes have a depth of 100 to 130 mm. The clear width of the nipple hole depends on the nipple diameter, which ranges from 110-180 mm. All nipple holes have a conical shape from top to bottom. The taper as a difference between the upper and lower diameters is in the range of 4-8 mm. The oblique grooves are slightly wedge-shaped. They taper, for example, from top to bottom by 2-4 mm. The conical shapes allow a smooth and secure extraction of the hole former from the green anode block.

There are also different views on the average steepness of the oblique grooves, which is measured in angular degrees with respect to the vertical. You will encounter steepnesses of 5-15 °. The cross sections of the oblique grooves are usually slightly trapezoidal and narrower outside than inside. But there are also triangular and roundish groove cross sections. For the depth of the oblique grooves, design dimensions of 10-19 mm were chosen.

Another important design dimension is the annular distance between nipple and nipple hole wall near the bottom and at the top. For this distance dimensions of 8-12 mm and 11-15 mm are known. A wide annular space means fewer problems in practice, but has the disadvantage that a larger shrinkage gap is formed due to increased Gusseisenschwindung between the cast iron sleeve and the carbon anode.

Since the Nippellochformer must rotate in any position out of the molded nipple hole, is provided at its upper edge an extended annulus for the pouring of cast iron. This filled with cast iron annulus is later a thick collar of the cast sleeve, which almost does not contribute to the power transmission from the nipple to the anode. This is due to the fact that between the bead-shaped collar and the anode, a relatively wide shrinkage gap is formed. In this gap, the cast iron can be oxidized by air access stronger and thus become electrically less conductive. In addition, fine, non-conductive dust of the anode cover material can trickle into the gap. The temperature is lower at the upper nipple hole edge than at the bottom of the nipple hole. Therefore, there is the thermal expansion of the composite unit nipple / cast sleeve less and less sufficient to close shrink gaps and build contact pressure.

The bottom of the nipple is mostly flat and the bottom of the nipple is usually not provided. Only with an anode yoke with new nipples the nipples are close to the bottom of the nipple holes. The nipples may be elongated by hot flow of the steel at high temperature and high circulation, so that only one nipple touches the bottom of the nipple and the other nipples are raised in the anode. The latter can then be poured with cast iron more or less thick. The shrinkage of the cast iron causes a slight slip of the nipple with the cast iron sleeve stuck to it in the vertical direction, so that little contact pressure is exerted on the Nippellochboden. As a result, the current transfer from the nipple through the bottom of the nipple hole into the carbon anode is negligibly small. This assumption is supported by model calculations.

The cast iron used to infuse the nipples normally shrinks by about 2%, with 1% attributable to the transition from liquid to solid and 1% to the thermal contraction upon cooling from the solidification temperature to the ambient temperature.

Measurements of the voltage drop between the nipple and the casting sleeve have shown that this is relatively small with cleanly emitted nipples coated with a thin graphite layer and is only about 7-14 mV. The main voltage drop occurs between the cast iron sleeve and the carbon anode and is in the final state in the temperature range between 650 and 950 ° C at least ten times as large as the voltage drop between the nipple and cast sleeve. After replacing a new carbon anode in the electrolytic cell, the voltage drop nipple anode is particularly high, namely Ca. 150-300 mV, because the previously mentioned shrinkage gaps are still predominantly present and have not yet been compensated by the thermal expansion of the nipple / cast iron composite unit, which is three times greater than that of the carbon anode. The anode essentially hangs with its oblique grooves on the top of the helical ribs of the cast iron sleeve and has mainly electrical contact there. With increasing warming, the contact pressure between nipple / cast iron and carbon anode increases and the voltage drop decreases accordingly. Only at contact temperatures of 500-600 ° C does the voltage drop asymptotically approach a final level.

The current transmission from the nipple to the anode is predominantly via the lower cylindrical half of the nipple hole, because there on the one hand, the distance between the nipple and anode because of the slightly conical shape of the nipple hole is lower than in the upper half, and consequently also the radial shrinkage of the cast iron. On the other hand, the temperature in the lower half of the nipple hole is much higher. The higher temperature causes a greater thermal expansion of the composite unit nipple / cast iron and thus also a stronger contact pressure. This state of power transmission was confirmed by model calculations.

It also emerged from operating experiments with different Nippellochgestaltung that for the height of the voltage drop, the contact pressure is more crucial than the contact surface.

There is a tremendous leap in electrical resistivities in the interface between the cast iron sleeve and the carbon anode. In the temperature range of 100-900 ° C, for example, the specific electrical resistances of the nipple steel change from 0.2 to 1.2, the cast iron from 0.75 to 1.5 and the carbon anode from 50-55 to 42-45 μΩm. The ratios of the specific resistances of steel and cast iron to the carbon anode are at temperatures below 500 ° C at values of over 60 and at 900 ° C still between 30 and 40. This means that the current density distribution in the above-mentioned interface in addition to the contact resistance predominantly from electrical resistance of the carbon material around the nipple hole is determined.

The constructive and material-specific conditions of the known, conventional nipple-anode compounds have therefore been described in detail in the foregoing in such detail and meticulous to clearly illustrate the significant differences to the configuration of the nipple hole according to the invention and its advantages.

• – Der Spannungsabfall zwischen Stahlnippel und Kohlenstoffanode wird im Durchschnitt über die Standzeit der Anode in der Aluminium-Elektrolysezelle um rund 50 mV oder mehr gesenkt. Dieser Spannungsgewinn kommt entweder der Zellenspannung oder dem Abstand Anode – Kathode zugute.
• – Der extrem hohe Spannungsabfall zu Beginn des Anodeneinsatzes in der Elektrolyse auf Grund der Gusseisen-Schrumpfung und Spaltenbildung zwischen Gusseisenhülse und Kohlenstoffwand des Nippellochs wird beseitigt. Es wird praktisch ein nahezu gleichgroßer Spannungsabfall zwischen Nippel und Kohlenstoffanode während der gesamten Anoden-Zykluszeit gewährleistet und die stark temperaturabhängige Stromverteilung über drei oder vier Nippel in der Anode deutlich verbessert, d. h. gleichmäßiger gemacht. Diese Vergleichmäßigung hat einen günstigen Einfluss auf den Gesamtspannungsabfall zwischen dem Anodenjoch und der elektrochemisch aktiven Unterseite der Kohlenstoffanode.
• – Es wurde eine konstruktive Lösung gefunden, einen vertikalen Schlupf der Verbundeinheit Stahlnippel/Gusseisenhülse im Nippelloch völlig zu verhindern, d. h. dieser Schlupf tritt erfindungsgemäß nicht auf.
• – Die Bodenfläche des Nippellochs, die etwa ein Drittel der zylindrischen Stromtransferfläche des Nippellochs darstellt, wird in die Stromübertragung vom Nippel zur Anode durch gezieltes Untergießen des Nippelendes mit einbezogen. Die Unterbindung des Nippel-Vertikalschlupfes ermöglicht die erforderliche Ausübung eines Kontaktdruckes auf den Nippellochboden. Des Weiteren ist auf dem Nippelboden ein runder Sockel des Anodenkörpers vorgesehen, der nicht nur zum Aufsetzen des Stahlnippels dient, sondern auch ein Aufschrumpfen des Gusseisens und damit eine anfängliche Stromeinleitung in den Nippelboden gestattet.
• – Es ist ohne großen Aufwand möglich, den erfindungsgemäßen Nippellochformer in seiner Höhe und seinem Durchmesser so auszulegen, dass sich mit diesem bereits betrieblich vorhandene Nippellochformer bekannter Konstruktion (Siehe 1) in den Anlagen zur Abformung grüner Kohlenstoffanoden unmittelbar ersetzen lassen. Der erfindungsgemäße Nippellochformer kann außerdem in derselben Position montiert werden, wo vorher der an sich bekannte Nippellochformer installiert war, z. B. unter dem Deckgewicht eines Vibrationsverdichters. Das bedeutet, dass durch den Einsatz des erfindungsgemäßen Nippellochformers im gesamten Prozessablauf der Anodenfabrik sowie in der Schmelzflusselektrolyse des Aluminiums keine weiteren konstruktiven Veränderungen erforderlich sind.
• – Ein vorteilhafter wirtschaftlicher Aspekt ist, dass im Falle einer Umstellung auf den erfindungsgemäßen Nippellochformer in einer Aluminiumhütte mit einer Produktion von 600000–700000 t Primäraluminium pro Jahr beispielsweise nur 12 dieser Einheiten benötigt werden.

The individual objects of the invention are as follows:

• - The voltage drop between steel nipple and carbon anode is reduced on average over the life of the anode in the aluminum electrolysis cell by about 50 mV or more. This voltage gain benefits either the cell voltage or the anode - cathode gap.
• - The extremely high voltage drop at the beginning of the anode use in electrolysis due to the cast iron shrinkage and gap formation between the cast iron sleeve and carbon wall of the nipple hole is eliminated. It is virtually guaranteed an almost equal voltage drop between nipple and carbon anode during the entire anode cycle time and the strong temperature-dependent current distribution over three or four nipples in the anode significantly improved, ie made more uniform. This homogenization has a favorable influence on the overall voltage drop between the anode yoke and the electrochemically active underside of the carbon anode.
• - It has been found a constructive solution to completely prevent vertical slippage of the composite steel nipple / cast iron sleeve in the nipple hole, ie this slip does not occur according to the invention.
• - The bottom surface of the nipple hole, which represents about one third of the cylindrical current transfer surface of the nipple hole, is included in the current transfer from the nipple to the anode by deliberate pouring the nipple end. The suppression of the nipple vertical slip allows the required exercise of a contact pressure on the Nippellochboden. Furthermore, a round base of the anode body is provided on the nipple bottom, which serves not only for placing the steel nipple, but also allows shrinking of the cast iron and thus an initial current introduction into the nipple floor.
• - It is easily possible to design the Nippellochformer invention in height and diameter so that with this already operational existing Nippellochformer known design (See 1 ) can be replaced immediately in the facilities for molding green carbon anodes. The Nippellochformer invention can also be mounted in the same position where previously the known Nippellochformer was installed, z. B. under the cover weight of a vibratory compactor. This means that no further structural changes are required by the use of the Nippellochformers invention throughout the process flow of the anode factory and in the electrolysis of aluminum fused.
• - An advantageous economic aspect is that in case of a switch to the invention Nippellochformer in an aluminum smelter with a production of 600000-700000 t primary aluminum per year, for example, only 12 of these units are needed.

After describing these method features according to the invention, reference will now be made to an example and with reference to the drawings in 2 to 8th the constructive realization of the inventive concept will be explained.

In the 2 - 8th the following details are shown:

Fig. 2: vertical section through the middle of the Nippellochformers with the lateral mold in impression or outer position

• 1 Dickwandiges, zylindrisches Rohrstück des Nippellochformers
• 2 Bodenplatte des Nippellochformers
• 3 Runde Aussparung in der Bodenplatte 2
• 4 Formkolben aus Stahl oder Bronze
• 5 Außenkranz des Formkolbens zum Einformen einer Ringnute
• 6 Pleuelstange
• 7 Kugelköpfe der Pleuelstange
• 8 Haltescheibe für den Kugelkopf im Formkolben
• 9 Befestigungsschraube
• 10 Mittellager
• 11 Oberteil des Mittellagers
• 12 Unterteil des Mittellagers
• 13 Löcher für Verbindungsschrauben des Ober- und Unterteils 11 und 12
• 14 und 15 Aussparungen für Schwenkbewegung der Pleuelstange 6
• 16 Unterseitiger Ausschnitt aus dem Deckgewicht des Vibrationsverdichters
• 17 Durchgangsloch durch das Deckgewicht
• 18 Kolbenstange des doppeltwirkenden Hydraulikzylinders (eingebaut im Deckgewicht)

In this (if not specially specified, the components are made of steel):

• 1 Thick-walled, cylindrical pipe section of the Nippellochformers
• 2 Base plate of the Nippellochformers
• 3 Round recess in the bottom plate 2
• 4 Molded piston made of steel or bronze
• 5 Outer ring of the piston for forming a ring groove
• 6 connecting rod
• 7 Ball heads of the connecting rod
• 8th Holding disc for the ball head in the piston
• 9 fixing screw
• 10 center bearing
• 11 Upper part of the center bearing
• 12 Lower part of the center bearing
• 13 Holes for connecting screws of the upper and lower part 11 and 12
• 14 and 15 Recesses for pivotal movement of the connecting rod 6
• 16 Lower section of the cover weight of the vibratory compactor
• 17 Through hole through the cover weight
• 18 Piston rod of the double-acting hydraulic cylinder (installed in the cover weight)

Fig. 3: Vertical section through the center of the Nippellochformers with the lateral mold piston in the retracted inner position

• Name and meaning of the items as in 2

Fig. 4: Horizontal section through half the height of the Nippellochformers with the lateral piston in molding or outer position

• 19 Bohrungen im dickwandigen zylindrischen Rohrstück 1 für die Formkolben 4
• 20 Bohrungen für Schraubenbolzen zur Befestigung des Nippellochformers unter das Deckgewicht 16

In this mean:
parts 1 to 15 as in 2 and 3

• 19 Holes in the thick-walled cylindrical pipe section 1 for the pistons 4
• 20 Bolt holes for mounting the nipple punch under the cover weight 16

Fig. 5: cone-shaped additional element on the Nippellochformer for molding a sprue for liquid cast iron

• 21 Einguss-Formsegment
• 22 Vertikalschnitt durch das Formsegment 21

In this mean:

• 21 Sprue-mold segment
• 22 Vertical section through the mold segment 21

Fig. 6: Cylindrical steel nipple, which is cast with cast iron in the invention Nippelloch in the carbon anode

• 23 Zylindrischer Stahlnippel
• 24 Gusseisen
• 25 Kohlenstoffanode

In this mean:

• 23 Cylindrical steel nipple
• 24 cast iron
• 25 Carbon anode

Fig. 7: Alternative movement device for the piston, shown as a central horizontal section with the piston in impression or outer position

• 26 Drehachse (Betätigung in oder oberhalb des Deckgewichts 16)
• 27 Drehwinkel (ca. 55°)
• 28 Drehscheibe
• 29 U-förmiger Anker aus Rundstahl
• 30 Drehlager für den U-Anker
• 31 Halteriegel für den U-Anker im Formkolben

In this mean (in addition to 2 ):

• 26 Rotary axis (actuation in or above the cover weight 16 )
• 27 Rotation angle (about 55 °)
• 28 turntable
• 29 U-shaped anchor made of round steel
• 30 Swivel bearing for the U-anchor
• 31 Retainer for the U-anchor in the piston

Fig. 8: Alternative movement device for the mold piston, shown as a central horizontal section with the piston in the retracted inner position

• Name and meaning of the items as in 7

The movement mechanics for the pistons 4 is after 2 / 4 and 7 designed so that the pistons during the vibration compaction on the connecting rods 6 or over the U-shaped anchors 29 can record high pressure pulses in a straight, rigid orientation.

The shaping of the nipple holes according to the invention into the green carbon anode takes place as follows:
First, the empty molding box is clamped on the vibrating table of the so-called vibrating or vibrating compactor. Thereafter, a weighed amount of the green, loose pourable carbon or anode mass is filled evenly in the mold box. The nipple shapers (usually three, possibly 2 to 6) are under the heavy cover weight 16 attached to the vibratory compactor. The pistons 4 the nipple-hole shaper are in retracted position as shown in 3 or 8th , The top weight floating above the mold box is lowered onto the filled green anode mass, and the nipple shapers sink into the green anode mass. Subsequently, the cover weight for the vibration is released, and the piston mold 4 the nipple shaper are pushed into their outward or molding position by means of the above described moving devices.

Then, the vibratory compactor is started and densification of the green anode mass into a stable shaped body, the green anode block, is performed for a fixed period of vibration.

When the molding process for the green anode block is complete, the pistons become 4 withdrawn and the cover weight is subsequently lifted with the Nippellochformern of the green anode block.

It should be mentioned for the sake of completeness that in practice better compaction conditions of the green anode mass are known in which with the aim of higher density and improved structure of the green anode both vacuum and pneumatic pressurization of the cover weight be applied. These additional applications in the shaping of dense, green anode blocks by vibration have, in principle, no influence on the sequence of the shaping of the nipple holes according to the invention.

Claims (8)

A method for reducing the electrical contact resistance or voltage drop between the composite unit of steel nipple and cast iron sleeve on the one hand and the "pre-burned" carbon anode on the other hand for the Schmelzfluss-electrolytic aluminum production, characterized in that due to the design of the nipple hole, the molten cast iron during its solidification and cooling during Pouring process of the nipple on the provided in Nippelloch molded parts of burnt carbon of the anode block shrinks and a vertical slip of the composite steel nipple / cast iron sleeve in the nipple hole no longer occurs. A method according to claim 1, characterized in that the composite design steel nipple / cast iron sleeve exerts a contact pressure on the bottom of the nipple hole when using the anode block in the electrolytic cell, and thus the Nippellochboden in the power transmission from the composite steel nipples / cast iron to the carbon anode is deliberately involved. A method according to claim 1 and 2, characterized in that in the cylindrical, slightly conical Nippellochwand the green carbon anode annular grooves are formed, which then later be filled in the burned carbon anode with cast iron. A method according to claim 1 and 2, characterized in that the Nippellochboden has a round, steeply limited carbon base, which is cast around with cast iron and serves for placing the cylindrical steel nipple. Device for shaping the nipple hole according to claim 1-3, characterized in that are used for the formation of the lateral annular grooves piston mold with outer rim, which are mounted horizontally movable in the thick wall of the Nippellochformers. Apparatus according to claim 5, characterized in that the pistons of the Nippellochformers be moved by means of connecting rods with ball heads via a common connecting bearing of a built-in cover weight, double-acting hydraulic cylinder between a defined outer and inner position back and forth. Apparatus according to claim 5, characterized in that the piston of the Nippellochformers means of U-shaped rotatably mounted armature via a common, pivotable turntable which is fixedly connected to a vertical, guided by the cover weight axis of rotation, between a defined outer and inner position and to be moved. An apparatus for shaping the nipple hole according to claim 1-7, characterized in that the Nippellochformer invention are mounted in the same places under the cover weight of the vibratory compactor, as they were previously occupied by the conventional Nippellochformern with bevels.

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