AU2006100894B4 - Improved method for tapping metal from an electrolytic cell - Google Patents

Description

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2. Description and Specification.

O 2.1 Background of the Invention: This invention relates to the tapping of liquid metal from electrolytic reduction cells. In one respect the invention relates to the tapping of liquid aluminium from electrolytic reduction smelting cells so as to avoid simultaneous tapping of electrolyte and/or sludge with the aluminium.

Aluminium is produced by electrolytic reduction of alumina in a liquid electrolyte. The 00 aluminium produced commercially by this process is referred to as primary aluminium.

0 The electrolytic cell is depicted pictorially in Figure 1, which is a cross sectional end elevation through a typical cell.

The cell consists of a rectangular open reinforced steel shell which is lined with a series of carbonaceous blocks which form the cathode of the process The cathode blocks have steel conductors ,1 imbedded in them to conduct current from the cell Carbon anodes are suspended above the cell body by an insulated superstructure which supports a longitudinal anode beam to which the anode rods are clamped.

The superstructure also serves to support alumina bins and their associated feeders During operation, direct current electricity is transferred from the anodes and then into the liquid electrolytic bath Powdered alumina is fed into the bath on a regular basis to maintain a more or less constant concentration of dissolved alumina.

During the electrolytic process, positively charged aluminium ions from the dissolved alumina migrate towards the negatively charged cathode, where they form a pool of liquid aluminium The negatively charged oxygen ions from the dissolved alumina migrate to the positively charged anodes where they react with the carbon in the anode and leave the cell as carbon dioxide.

The electrolytic process described above is a continuous operation, so that the depth of the pool of liquid aluminium progressively increases. The process takes place at a temperature of approximately 9600 C.

It is important to note that the specific gravity of liquid aluminium is approximately 2.3 whereas the specific gravity of the liquid electrolyte, at cell operating temperature, is approximately 2.1.

This explains why the pool of liquid aluminium is below the less dense layer of electrolyte.

The typical heights (depths) of the electrolyte and the metal layers are both of the order of 18-20 cm.

To remove the liquid aluminium, which has been produced by the electrolytic process, the industry has universally adopted a system of tapping some of the metal approximately once per day.

Figure 2 is a longitudinal cross section of a typical reduction cell and tapping crucible. In the tapping process an enclosed insulated crucible (10) is bought alongside the cell, and its integral tapping spout (11) is inserted through the layer of electrolyte and some way into the metal pad, as depicted in the figure.

After the tapping spout has been inserted into the metal pad some predetermined depth, a vacuum is then created in the tapping crucible, usually by means of a simple compressed air eductor (12).

The external atmospheric pressure applied to the cell surface then forces liquid aluminium up the tapping spout and into the evacuated crucible. After the required mass of metal has been tapped, the compressed air CK1 is turned off, the vacuum collapses and the crucible is then raised and moved to the next cell to continue the tapping operation.

C.)

O The foregoing Background explanation of the electrolytic process describes an ideal situation, and no mention has been made of some of the practical realities of a real life operation which result in "tapped bath", or "tapped sludge" being removed from the cell along with the liquid aluminium.

The term tapped bath is synonymous with tapped electrolyte.

Sludge is a layer of liquid muck which may be present in the cell below the metal pad. It arises 00 as a result of over feeding the cell with alumina.

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It has been noted above that the specific gravities of liquid aluminium and liquid bath at cell temperature (960C) are 2.3 and 2.1 respectively. As it happens, the physical properties of the metal and bath change significantly as these 2 liquids cool.

Aluminium has a solidus temperature of 660 0 C and a solid specific gravity of 2.7, whereas bath has an exceptionally high solidus temperature of approximately 840C and a solid specific gravity of 2.9.

The result of this is that the bath starts to solidify in the crucible in a relatively short time as the crucible loses temperature, and as it solidifies it starts to sink through the liquid aluminium with serious consequences for subsequent process steps.

Some of these detrimental issues are: Liquid floating bath can be partially removed from the crucible by manual or mechanised skimming. This is a time consuming costly process and the skimmed material has to then be treated and recycled.

Residual or unskimmed bath will solidify on the wall and bottom of the crucible causing a loss of volumetric capacity. The consequence of this is that more crucibles have to be placed in service to account for ones removed for frequent mechanical cleaning.

Many smelters employ a TAC process which involves injection of AIF 3 into the crucible to reduce sodium concentration in the liquid aluminium. Residual bath causes inefficiencies in the TAC process, meaning that sodium levels are not reduced to desired targets.

Frequently plants equipped with mechanised bath skimmers are rendered dysfunctional because frozen bath around the rim of the crucible prevents the skimmer from operating.

As floating bath solidifies it can form loose lumps which sink to the bottom of the crucible. When the crucible is taken from the reduction section to the casthouse the aluminium is transferred to holding furnaces by either cascade pouring or by syphoning. Loose lump bath seriously compromises the safety and efficiency of either method of metal transfer.

Some bath species may still be liquid or mushy as the metal is transferred into the holding furnaces and this substance fouls the furnace walls and hearth. This reduces furnace capacity and necessitates expensive furnace downtime for cold mechanical cleaning.

Any bath transferred to the holding furnaces is incorrectly accounted for as metal, thereby causing errors in material balance reporting.

Any bath in furnaces processing magnesium alloyed aluminium will cause sodium to be liberated from the bath into the melt and causes the product to be off composition.

All of these issues and more are well known to those in the industry and arise because the crucibles are contaminated with tapped bath as a result of the rudimentary cell tapping process.

,l Tapped electrolyte and/or sludge occurs for a variety of reasons, but most commonly because the tapping process is a wholly manual operation and relies on compliance with S.O.P's (Standard 0 Operating Practices) for success, in lieu of robust technology or automation.

0 The reasons the aluminium becomes contaminated with bath during tapping specifically include: The tapping operator incorrectly applies vacuum to the crucible before the spout goes into the metal pad and will obviously tap bath as the spout lowers through the bath layer.

Notwithstanding the point above, the tapping spout goes through the bath layer and 00 therefore unavoidably has some 18-20 cm of bath in the spout to begin with.

The operator positions the spout incorrectly in the metal pad. If too low then sludge _will be tapped from below the metal pad. If the spout is too high bath will be tapped even though the spout may be submersed in the metal pad due to vortexing at the Ielectrolyte/aluminium liquid interface.

A cracked spout may cause bath to be tapped.

C1 The operator raises the spout through the electrolyte layer after tapping, without waiting for the vacuum to collapse.

The crucible and spout may change orientation during the tapping process as the crucible gains weight.

o 2.2 Specification of the Invention

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The Invention separates liquid aluminium from liquid electrolyte at the tapping point, making it _virtually impossible to tap sludge or bath.

This is achieved by providing a large diameter tapping port inserted into the cell at the tapping point into which the tapping spout is subsequently inserted.

The Invention is depicted pictorially as Figures 3, a, b, and c.

00 The features are that: ID* Fig 3a shows the bottom entry of the port is at an exact, fixed height above the cathode, providing optimal depth in the metal pad to eliminate sludge or electrolyte tapping. The assembly is supported at the top from the cell deck plate and superstructure columns xl and at bottom by being seated on the cathode.

The large diameter reduces any tendency of vortexing because inflow velocities are substantially reduced compared to those in the crucible spout.

Fig 3b shows that any electrolyte in the port initially can be discharged into the cell by applying a slightly pressurised gas cap to the top of the port prior to tapping so as to expel the bath and metal downwards out of the port and into the cell Upon removal of the gas pressure pad the port refills only with aluminium, providing a deep well into which the tapping spout can then be inserted (14) as depicted in Fig 3c.

The outstanding feature of the Invention is that the depth of liquid aluminium in the tapping port is double the existing metal pad height since the metallostatic head of the electrolyte elevates it. This provides a substantial margin for error in tapping spout insertion depth but in any case, makes it impossible to tap electrolyte because of the physical separation from the bath provided by the port body.

A number of variants of the port invention are possible, including inter alia: A fixed in situ port.

o A removable port, inserted in the cell only during the tapping operation.

o A port which is an integral part of the crucible tapping spout.

The Invention eliminates the possibility of tapping bath, and thereby obviates all of the serious issues identified in the Background.

The invention has three further benefits: The fixed in situ option eliminates the need for operators to manually break open a tap hole through the frozen crust prior to every tapping operation.

Secondly, there is much less area of liquid electrolyte exposed to the cell atmosphere, thereby significantly reducing hydrogen fluoride emissions from the cell. The exposed surface inside the tapping port is aluminium which does not emit volatile pollutants.

Thirdly, the Invention is applicable for all aluminium reduction cell sizes and technologies whether the cells are Prebake, Soderberg, end for end, or side by side configuration.

2.3 Further Detailed Specification of the Invention Figure 4 depicts a sectional arrangement of the tapping port mounted in a typical side by side cell as well as a plan view. A similar mounting arrangement would be required for end to end cells.

The actual tapping port (15) would have a typical outside diameter if225m.m. to fit between rows of adjacent anodes which are normally spaced about 250m.m. apart. The inside diameter of the tapping port would be typically 200m.m. thereby having a wall thickness of typically 12.5m.m.

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O0 The port includes an integral support frame device (16) towards the top of the tapping port to afford accurate vertical and horizontal positioning and location of the assembly as it is inserted in the cell, and a tyne bracket (17) to further facilitate installation and removal as well as to resist buoyancy during INO electrolyte expulsion.

SThe port has a bell shaped top section (18) to facilitate entry of the crucible spout as the spout is N lowered into the port, as well as to form a low pressure gas seal surface.

The port is placed in situ using a small fork truck or bobcat or other means, and is designed to be self aligning by virtue of the configuration of the integral support frame. The assembly is not physically bolted or otherwise fixed to the cell structure.

An extension piece (19) which forms part of the port, rests on the cathode and so accurately locates the bottom of the port and sets the angle of declination to match the angle of the crucible tapping spout. The height of this extension is approximately 6 cm, thereby locating the port bottom opening 6 cm above the cathode surface.

Figure 5 depicts a cross section arrangement of the tapping port in operation and shows the crucible and its tapping spout inserted into the port.

Provided that the spout is inserted approximately half way into the metal in the port, it is obvious that no electrolyte or sludge can be tapped with this arrangement. The actual tapping performance is quite insensitive to insertion distance, unlike the prevailing technology without the provision of the port.

2.4 Summary of Invention The method of tapping liquid aluminium using the present Invention is useful for preventing tapped bath and/or sludge from entering the tapping crucible and contaminating the aluminium. This is achieved by provision of an isolated deep well of liquid aluminium, devoid of electrolyte, into which the crucible tapping spout can be inserted for syphoning uncontaminated metal.

Claims (4)

1. A tapping port for aluminium electrolysis cells comprising: An inclined tubular body with the top open end having a bell shaped entry orifice, the bottom end having a cylindrical or similar shape metal entry orifice, and the bottom end furthermore having an integral support lug; and the tubular body attached to an upper support/location frame.

2. The tapping port of Claim 1 wherein said inclined tubular body and support frame includes a 00 means for insertion and removal of the port to an electrolytic cell, typically achieved by a fork truck with a single tyne engaging in a bracket on the support frame.

3. The tapping port of Claim I wherein the bell shaped entry orifice is designed to receive a manually applied low pressure gas sealing plug, as well as to facilitate smooth entry of a crucible tapping spout (-i

4. The tapping port of Claim 1 wherein the inclined tubular body preferably comprises cast steel alloy or a non-metallic material or a combination of both materials. An improved method of tapping aluminium from electrolysis cells to eliminate tapped bath (tapped electrolyte), comprising: providing a metal tapping port having an assembly consisting of an inclined tubular body, a bell shaped top entry orifice, an integral bottom end support lug, a means for insertion and removal of said assembly from a cell and a means for removing electrolyte from the port prior to tapping; upon insertion of the port in a cell the port assembly largely self positions so it is centrally located; so that it is the correct distance from the cathode; and so that it is angled to receive the declined crucible tapping spout; the port will be manually gas pressurised to expel the contained metal and electrolyte downwards and out of the port into the metal pad where the electrolyte will float towards the metal/electrolyte interface so that when the gas pressure is removed the port will quite naturally refill with metal only, thus providing a depth of pure metal almost twice the depth of the metal pad; the crucible tapping spout may then be inserted into the tapping port and will commence tapping only metal because of the physical absence of electrolyte created by the port body. Peter Whiteley B.E. F.I.E.Aust 1 October 2006