EP4039854A1

EP4039854A1 - Method for baking a cell bottom of an aluminium electrolyzer

Info

Publication number: EP4039854A1
Application number: EP20867862.3A
Authority: EP
Inventors: Il'ya Ivanovich PUZANOV; Andrej Vasil'evich ZAVADYAK; Vitalij Vladimirovich PLATONOV
Original assignee: Rusal Engineering and Technological Center LLC
Current assignee: Rusal Engineering and Technological Center LLC
Priority date: 2019-09-24
Filing date: 2020-08-26
Publication date: 2022-08-10
Also published as: CA3154865A1; EP4039854A4; CN114502777B; CN114502777A; RU2717438C1; CA3154865C; WO2021061015A1

Abstract

The method comprises coating a cell bottom with an electrically conductive material, placing thereon prebaked anodes connected to anode buses of an anode busbar of an electrolyzer, passing an electric current through the electrically conductive material and controlling the current load on the anodes during baking; conditions for even baking are provided by selecting the amount of electrically conductive material under the anodes; more particularly, the amount of electrically conductive material under the anodes is chosen such that there is less material under the anodes located in the middle of the electrolyzer than under the anodes located next to the outermost edge anodes, and less electrically conductive material under the anodes located next to the outermost edge anodes than under the outermost edge anodes. The technical result consists in providing uniform heating of the cell bottom of an aluminium electrolyzer throughout the baking time, safe commissioning and an increased service life of the aluminium electrolyzer.

Description

Technical area

The invention refers to the non-ferrous metallurgy, in particular to electrolytic reduction of aluminium, namely to pot bottom preheating methods for aluminium pots with prebaked or inert anodes.

Technical level

Some of processes used in the aluminium industry require significant amount of thermal energy necessary for preheating of equipment before its start-up. In past, the lined equipment preheating process was often neglected, which for example resulted in cold starts of pots and reduction of their life. Before start-up of a pot, its cathode lining should be thoroughly and uniformly preheated to minimise potential damage from excessive temperature differences.
High-temperature differences and application of crude bottom ramming paste upon bath pouring to the pot may result in a heat shock, cracking of a cathode block, leaking, and finally to reduction of the pot life.
There are two basic pot bottom preheating methods:

Electrical preheating;
Preheating using gas or liquid fuel.

During preheating using gas or liquid fuel, it is difficult to control generated amount of thermal energy and heat distribution on the cathode surface / cathode lining thickness. It is also difficult and maybe even impossible to properly heat the side and end walls, when necessary. There is probability of non-uniform temperature distribution on the cathode surface with excessive overheating of some areas, as well as quite significant temperature differences throughout the cathode lining.
The electrical preheating methods are based on current supply from anode bars to the cathode through a coke bed for the pot heating by means of electrical conductance and heat radiation.
There is a well-known aluminium pot bottom preheating method including: placement of prebaked anodes on the pot bottom; attachment of the prebaked anode rod assemblies to buses of the anode busbar; raising of the prebaked anodes; pouring of liquid aluminium for submersion of the prebaked anodes into it; and connection of the pot to the electrical circuit (G. Wolfson, V. Lankin. Aluminium production in pots with prebaked anodes. Moscow: Metallurgy, 1974, p.55, p.56).
A disadvantage of the well-known aluminium pot bottom preheating method is that pouring of liquid aluminium exposes the pot bottom to a heat shock, which may result in formation of cracks in cathode blocks and breakdown upon further operation of the pot. Another great disadvantage is direct contact of the pot bottom with liquid aluminium, which has low viscosity and melting point. Aluminium may penetrate deeply inside the pot bottom before solidification, react with insulation, break it, or create a thermal shunt.
There is also another well-known aluminium pot bottom preheating method (patent # RU 2215825 , IPC C25C 3/06) including: covering of the pot bottom made of cathode blocks and end peripheral joints with a layer of carbon fill; placement of prebaked anodes on it so that their soles come into contact with the carbon fill layer throughout its area and rods of the anode rod assemblies adjoin anode buses of the pot anode busbar; attachment of the prebaked anode rod assemblies to anode buses of the pot anode busbar; passage of electrical current through the prebaked anodes, carbon fill layer, and cathode blocks; and control of current load on the prebaked anodes via their controlled disconnection.
A disadvantage of the well-known aluminium pot bottom preheating method is that up to 50% of all prebaked anodes can be attached to anode buses of the pot anode busbar using basic locks (rigidly). Upon the pot bottom heating via natural burning of the carbon material, the anodes attached using flexible elements will go down, while the rigidly attached anodes will stay in place, which will result in local overheating spots of the pot bottom.
The pot bottom preheating method for aluminium pots with prebaked anodes (patent # RU 2526351 , IPC C25C 3/06), which is the closest method by technical substance to this application, includes: covering of the pot bottom made of cathode blocks and steel bars with an electrically conductive material; placement of prebaked anodes with stubs on it; connection of the installed prebaked anode rod assemblies to anode buses of the pot anode busbar; passage of electrical current through the electrically conductive material; and control of current load on the prebaked anodes. At that, the electrically conductive material is graphite fill placed as truncated pyramid rows located in projections of stubs throughout the prebaked anode length with height of each row set in inverse proportion to passed amperage and connection of all installed prebaked anode rod assemblies to anode buses of the pot anode busbar using flexible elements.
A disadvantage of this aluminium pot bottom preheating method is that the graphite material is filled as rows in projections of stubs throughout the length on all prebaked anode blocks. This filling method of the graphite material does not allow uniform heating of the pot bottom in the first half of the preheating process, since current will flow to the pot middle as heated if the graphite material under the anode has the same section. As a consequence, the pot ends will be heated more slowly, which will result in a significant temperature gradient.

Invention disclosure

The task of the proposed invention is to ensure uniform heating of the aluminium pot bottom throughout the preheating process.
The technical result is solution of this task, safe start-up, and extended life of the aluminium pot.
The technical result achieved upon implementation of this application also consists in non-uniform current distribution in the pot bottom to get its uniform heating up to 900°C for less than 60 hours as during gas-flame preheating.
The invention substance is explained by drawings, where:

Fig. 1 shows geometry of the electrically conductive material (graphite 'bed') - top view using the example of a pot with 24 paired anodes;
Fig. 2 shows a stencil for knurling of the graphite 'bed' up to 200 kA;
Fig. 3 shows a stencil for knurling of the graphite 'bed' over 200 kA;
Fig. 4 shows knurling of the graphite 'bed' at the end anodes;
Fig. 5 shows knurling of the graphite 'bed' at the anodes located near the end anodes;
Fig. 6 shows knurling of the graphite 'bed' at other anodes;
Fig. 7 shows a temperature pattern of the pot bottom before the pot start-up with non-uniform heating of the pot bottom due to non-optimal filling of the graphite material;
Fig. 8 shows surface temperature of the pot bottom in 1 hour before the pot start-up;
Fig. 9 shows amperage measured using 'pliers' on the end anodes (1, 12, 13, 24) throughout the pot preheating process with modification of the graphite material configuration (refer to Fig. 1);
Fig. 10 shows a heating trend of the pot bottom at check points;
Fig. 11 shows the proposed flexible elements for connection of anode rods with the anode bus to carry out independent preheating;
Figs. 12, 13 show alternative flexible elements.

Invention implementation

The graphite 'bed' is installed (Fig. 1) using one of the proposed stencils (Figs. 2, 3) depending on the pot amperage.
Knurling of the graphite 'bed' at the extreme end anodes is carried out as follows.
The stencil (Fig. 4) is placed onto the pot bottom in the knurling area of the graphite 'bed' on the anode projection (arrangement of bars: #1 - side-anode; #10 - row spacing). The graphite material is filled up to the upper face (flush) to the space between rails. The graphite material is levelled without ramming using the bar edges as supports. The excessive graphite material is removed, for example, using a levelling scraper. The stencil is demounted from the pot bottom and the excessive graphite material is removed.
Knurling of the graphite 'bed' at the anodes located near the end anodes is carried out as follows.
The graphite 'bed' is installed using one of the proposed stencils (Figs. 2, 3) depending on the pot amperage. The stencil is placed onto the pot bottom in the knurling area of the graphite 'bed' on the anode projection (arrangement of bars: #1 - side-anode; #10 - row spacing). The graphite material is filled up to the upper face ('flush') to the space between rails. The raw material is not filled to the space between the 7th and 8th bars of the stencil (Fig. 5). The graphite material is levelled without ramming using the bar edges as supports. The excessive graphite material is removed, for example, using a levelling scraper. The stencil is demounted from the pot bottom and the excessive graphite material is removed.
Knurling of the graphite 'bed' at other anodes is carried out as follows.
The graphite 'bed' is installed using one of the proposed stencils (Figs. 2, 3) depending on the pot amperage. The stencil is placed onto the pot bottom in the knurling area of the graphite 'bed' on the anode projection (arrangement of bars: #1 - side-anode; #10 - row spacing). The graphite material is filled up to the upper face ('flush') to the space between rails. The raw material is not filled to the space between the 3rd, 4th, 7th, and 8th bars of the stencil (Fig. 6).The graphite material is levelled without ramming using the bar edges as supports. The excessive graphite material is removed, for example, using a levelling scraper. The stencil is demounted from the pot bottom and the excessive graphite material is removed.
After installation of all anodes, the start-up charge (cryolite, crushed hard bath, soda) is loaded to the side-anode space and the anode body is covered with cryolite on top.
All installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar, for example, using a set of flexible aluminium tapes and full electrical current is passed through the graphite material layer. Current load of the prebaked anodes is controlled via disconnection of anodes with high load or local bottom overheating.
Fig. 7 shows a temperature pattern of the pot bottom before the pot start-up with non-uniform heating of the pot bottom due to non-optimal filling of the graphite material. It is clear that the pot middle is heated up to 800-750°C, while the pot ends have temperature below 400°C. The ends are heated in the second half of the preheating process via heat transfer from the middle and, as a consequence, uniform temperature of the pot bottom is achieved at the end of the preheating process.
Fig. 8 shows surface temperature of the pot bottom in 1 hour before the pot start-up. Fig. 9 shows amperage measured using 'pliers' on the end anodes (1, 12, 13, 24) throughout the pot preheating process with modification of the graphite material configuration (refer to Fig. 1), i.e. it shows a trend of amperage on the end anodes. It is clear from the diagram (Fig. 9) that current on these anodes is higher by 20-25% of the nominal value due to more strips of the graphite material (Fig. 1).
It is clear from Figs. 8, 9 that the new graphite fill geometry allows us to:

1) Uniformly heat the pot bottom surface up to the target values for 48 hours;
2) Redistribute current to the end anodes.

Fig. 10 shows a heating trend of the pot bottom at check points. It is clear that the average pot bottom surface temperature is achieved at the check thermocouples located as follows:

1. In the row spacing - 949°C (target - over 900°C);
2. At the 1st stub on the 'inlet' and 'outlet' sides - 808°C (target - over 800°C);
3. At the pot ends - 736°C (target - over 550°C).

So, the proposed pot bottom preheating method for aluminium pots with prebaked anodes includes: covering of the pot bottom with an electrically conductive material; placement of prebaked anodes on it; their connection to anode buses of the pot anode busbar; passage of electrical current through the electrically conductive material; and control of current load on the anodes for preheating as inherent for the pilot model. At that, uniform preheating is ensured via proper selection of the electrically conductive material quantity under the anodes. Namely, quantity of the electrically conductive material under the anodes is selected so that the material quantity is less under the anodes located at the pot middle than under the anodes located near the extreme end anodes, while the material quantity is less under the anodes located near the extreme end anodes than under the extreme end anodes. The electrically conductive material is preferably graphite with fraction from 0.1 mm to 10 mm. It is reasonable to set height and length of each row of the electrically conductive material under the anodes in inverse proportion to passed amperage. The installed prebaked anode rod assemblies are usually connected to anode buses of the pot anode busbar using flexible elements (Fig. 11).
The 'anode bus - anode rod' flexible element features the following design solutions distinguishing it from alternative options:

Section of contacts, area, and holddown pressure ensure current density: for contact of parts - not more than 0.6 A/mm²; for flexible conductors - not more than 1.2 A/mm²;
Overall and connection dimensions allow unhampered installation and disconnection of the flexible element;
Size and thread pitch of nuts are unified; design of screws allows using a traversing mechanism (wrench) for tightening of anode locks.

After installation of all anodes, the start-up charge (for example, cryolite, crushed hard bath, soda) is loaded to the side-anode space and the anode body is covered with cryolite on top. At that, all installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar using a set of flexible aluminium tapes and electrical current is passed through the graphite material layer. Current load of the prebaked anodes is controlled as well via disconnection of anodes with high load or local bottom overheating.
It should be noted that in connection with the current economic situation at present, smelters should take measures to detect and eliminate operational / general production costs affecting production cost of commodity products at all production stages without reduction in quality of products. One of aspects directly affecting production cost of primary aluminium is relining and process maintenance of metallurgical equipment via preheating.
The pot preheating stage before connection and start-up is one of the most important operations during their operation. The pot life, quality of produced aluminium, and key performance indicators to a large extent depend on quality of the preheating operation. During preheating, it is important to ensure uniform and smooth heating of the pot cavity and cathode.
Requirements to the pot preheating before its start-up consist in the following:

Ensure smooth transition from the cold state to the reduction temperature conditions;
Exclude thermal 'shocks' including those during bath pouring;
Achieve minimum thermal pressure on the cathode in both vertical and planar directions;
Ensure proper baking of bottom ramming paste;
Ensure full drying of the pot bottom pedestal after its lining using liquids.

In the world practice, the following three basic pot preheating methods are used depending on the heating principle:

1. Preheating with electrical current, where heat radiation is defined by Joule-Lenz's law:
- 1.1. On both fine-dispersed and coarse carbon materials;
- 1.2. On liquid metal or aluminium chips;
- 1.3. With moulding of a new anode (Soederberg);
2. Thermal preheating, where heat-transfer medium is natural gas or oil product;
3. Start-up without preheating with bath and metal pouring immediately to a cold pot.

Before 1995, the pot preheating operation was carried out at the Sayanogorsk Aluminium Smelter with two methods:

On S-175M2 pots - via flame preheating (by a preheating unit designed by VAMI);
On S-255 pots - with electrical current on carbon grits ('seeds'), where anodes after their placement on the 'seed' layer were rigidly pressed to the anode busbar with standard clamps.

Since 1995, the following measures were implemented at the Sayanogorsk Aluminium Smelter under the pot life extension programme for optimisation of the preheating process:

Flexible connections of anode rods to anode buses for independent preheating with electrical current on all pot types;
Improved control of supplied pot power via disconnection of rheostat shunts with increased quantity of disconnection steps from 2-3 to 6-8, which significantly improved heating quality;
Arrangement of a specialised team for pot preheating and start-up operations.

During electrical preheating on carbon grits, anodes were rigidly pressed to the anode busbar with standard clamps. After 1995, flexible connections of anode rods to anode buses were applied. The main disadvantages of this electrical preheating on coke are as follows:

Problems with control of heating rate (disconnection of rheostat shunts);
Non-uniform pot bottom heating due to applied raw material (coke) and non-uniform adjoining of anode soles (knurling, design of connection tapes);
High labour intensity during pot start-up (coke removal).

Since 2004 and at present after adjustments of RA-300 technology and start-up of the Khakas Aluminium Smelter, all pots at the Sayanogorsk Aluminium Smelter are preheated with the gas-flame method. The existing preheating and start-up procedure for RA-300 and RA-400 pots is schematically represented as follows:
Gas-flame preheating => Bath pouring => Pot connection to circuit without potline disconnection => Adjustment of parameters to target values
The disadvantages of the gas-flame preheating method are as follows:

1) For lining preheating in volume and achievement of target temperatures, preheating duration should be increased from 72 hours to 96 hours (topical in the cold season).
2) Insufficient quantity of burners in the Hotwork unit for long pots. Small quantity of temperature check points. Problems with operation of the unit in magnetic fields and during heavy frost.
3) No data on temperature of the pot bottom during preheating - temperature of gas-air environment is measured.
4) Troubled operations of connection/start-up at full amperage:
- Personnel safety;
- High probability of unscheduled current drops;
- Long start-up duration (pouring of more bath), high-voltage anode effects during start-up.

The obtained experience from start-up of pots in the RA-400 pilot area shows that gas-flame preheating does not meet the process requirements in the cold season (longer preheating is necessary to achieve the minimum required pot bottom temperature). This fact is not acceptable for quick commissioning of the Taishet Aluminium Smelter, since Taishet (Irkutsk Region, Russia) has a negative average monthly ambient temperature in seven months per year according to the climatic parameters. The main condition for connection of pots at the Taishet Aluminium Smelter taking into account its production capacity is also connection of RA-400 pots to the circuit without disconnection of the potline process load to exclude high loads on the Siberian energy system.
The main technical solution allowing excluding the above-listed disadvantages is to substitute gas-flame preheating of pots with electrical current preheating. Application of the electrical preheating process will allow us to:

Reliably connect a pot to the circuit without the potline disconnection or current drop;
Exclude costs for expensive preheating equipment and fuel (exclude the limiting factor for quick commissioning of the smelter and environmental impact);
Reduce duration of the pot preheating operation.

The achieved key performance indicators are as follows:

1. Reliable and safe connection of a pot at full current of the potline.
2. Reduction of the pot preheating duration from 72 hours to 54 hours.
3. Exclusion of costs for expensive preheating equipment and fuel (reduction of environmental impact).

The fundamental distinctions of the proposed technical solution are as follows:

1) Preheating at full amperage without rheostat shunts;
2) Application of graphite materials;
3) Differentiated knurling of the graphite 'bed';
4) Optimal design of flexible contact parts:
- Anode freedom in three directions (X, Y, Z);
- Immediate control via current distribution on anodes;
5) Automated temperature monitoring.

Taking into consideration the above description of the method, examples, and distinctions, the scope of legal protection under the formula is solicited in the following limits:

1. Pot bottom preheating method for aluminium pots with prebaked anodes including: covering of the pot bottom with an electrically conductive material; placement of prebaked anodes on it; their connection to anode buses of the pot anode busbar; passage of electrical current through the electrically conductive material; and control of current load on the anodes for preheating. This method is distinguished by the fact that uniform preheating is ensured via proper selection of the electrically conductive material quantity under the anodes. Namely, quantity of the electrically conductive material under the anodes is selected so that the material quantity is less under the anodes located at the pot middle than under the anodes located near the extreme end anodes, while the material quantity is less under the anodes located near the extreme end anodes than under the extreme end anodes.
2. The method described in Item 1 distinguished by the fact that the electrically conductive material is graphite with fraction from 0.1 mm to 10 mm.
3. The method described in Item 1 distinguished by the fact that height and length of each row of the electrically conductive material under the anodes are set in inverse proportion to passed amperage.
4. The method described in Item 1 distinguished by the fact that the installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar using flexible elements ensuring the anode freedom in three directions (X, Y, Z).
5. The method described in Item 1 distinguished by the fact that after installation of all anodes, the start-up charge (for example, cryolite, crushed hard bath, soda) is loaded to the side-anode space and the anode body is covered with cryolite on top.
6. The method described in Item 1 distinguished by the fact that all installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar using a set of flexible aluminium tapes and electrical current is passed through the graphite material layer.
7. The method described in Item 1 distinguished by the fact that current load of the prebaked anodes is controlled via disconnection of anodes with high load or local bottom overheating.

Claims

Pot bottom preheating method for aluminium pots with prebaked anodes including: covering of the pot bottom with an electrically conductive material; placement of prebaked anodes on it; their connection to anode buses of the pot anode busbar; passage of electrical current through the electrically conductive material; and control of current load on the anodes for preheating. This method is distinguished by the fact that uniform preheating is ensured via proper selection of the electrically conductive material quantity under the anodes. Namely, quantity of the electrically conductive material under the anodes is selected so that the material quantity is less under the anodes located at the pot middle than under the anodes located near the extreme end anodes, while the material quantity is less under the anodes located near the extreme end anodes than under the extreme end anodes.
The method according to claim 1 distinguished by the fact that the electrically conductive material is graphite with fraction from 0.1 mm to 10 mm.
The method according to claim 1 distinguished by the fact that height and length of each row of the electrically conductive material under the anodes are set in inverse proportion to passed amperage.
The method according to claim 1 distinguished by the fact that the installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar using flexible elements ensuring the anode freedom in three directions (X, Y, Z).
The method according to claim 1 distinguished by the fact that after installation of all anodes, the start-up charge (for example, cryolite, crushed hard bath, soda) is loaded to the side-anode space and the anode body is covered with cryolite on top.
The method according to claim 1 distinguished by the fact that all installed prebaked anode rod assemblies are connected to anode buses of the pot anode busbar using a set of flexible aluminium tapes and electrical current is passed through the graphite material layer.
The method according to claim 1 distinguished by the fact that current load of the prebaked anodes is controlled via disconnection of anodes with high load or local bottom overheating.