ELECTROLYTIC CELL

FIELD OF THE INVENTION

The present disclosure relates to an anode assembly for aluminium electrolysis cells used for aluminium production. More particularly, the present disclosure relates to a reduced voltage drop rodded stepped stub anode assembly for an aluminium electrolytic cell.

BACKGROUND OF THE INVENTION

Aluminium has turned out to be the wonder metal of the industrialized world. No other single metal is so versatile in use and economics. Aluminium's growth rate is the highest amongst the major basic metals today. However, the aluminium production is highly energy consuming. The major cost components of aluminium production are DC Power, Alumina and Carbon anode. Aluminium is conventionally produced by the Hall-Heroult process in which alumina is dissolved in molten cryolite and electrolyzed with an intense direct current (DC). The cryolite bath is contained in electrolytic cells lined with carbon cathodes while carbon anodes hang from rods connected to an electrical bus bar. The alumina is reduced with oxygen being deposited on the anodes and forming mostly carbon dioxide while the molten aluminium is deposited on the cell bottom and periodically tapped. The reduction process is performed in a large aluminium reduction cell which includes a container or "pot" lined with refractory and carbon. Within the pot is a molten mixture of alumina dissolved in cryolite and other materials, such as various fluorides, which are generally referred to as "bath". The pot and a metal pad of molten aluminium, which collects at the bottom of the pot, form an associated cathode.

A voltage potential is applied between the carbon anodes and the pot, resulting in a large current flow from the anodes, through the molten bath mixture, to the pot. The electrical current passing through the bath mixture reduces the alumina into its aluminium and oxygen components. The aluminium drops to the bottom of the pot, forming the metal pad. The oxygen combines with the carbon from the anodes and escapes as carbon dioxide gas, which is vented from the pot. As the alumina is consumed, more alumina is added to the bath in the cell. During the aluminium reduction process, the carbon anodes are consumed, therefore, the spent anodes are routinely replaced by fresh anodes so that the aluminium production can continue in an efficient manner. The decomposition voltage of alumina to aluminium is 1.60 V but the normal voltage required for smooth pot operation is 4.40 V. This excess power consumption during smelting is due to various drops :-

Anode rod to stub drop - Depends on welded joint quality,

Stub to carbon drop - Surface area and contact quality between stub and Carbon block,

Cathode lining drop - Cathode construction,

Clamp Drop - Quality of surface contact & tightening of Clamps,

Fixed drops - Bus Bar dimensions & current density,

Electrolyte Drop - Depend on electrolyte chemistry.

Higher resistance at stub-carbon interface leading to heat generation, restricting increase of pot line current thereby limiting the avenues of productivity / efficiency improvement. The anode connection has to be replaceable, tight and resistance to deterioration. A loose connection causes high voltage drops, while a cramped connection causes crack in the anode block. Moreover, the aggressive environment of the aluminium production cell causes continuous stub deterioration and stub bending. The known prior arts of anode assembly have the drawbacks of high voltage drop at stub-anode connection and have high power consumption and involve high cost.

The non-patent disclosure titled as "Low resistance anode assembly using steel stubhole conductors across the cast iron to carbon interface (Edward Williams, W. Berends, S. Haley and M. Gagnon) relates to anode assemblies which suffer a significant electrical contact resistance across the cast iron to carbon interface that is inversely dependent on contact pressure and area. Industry efforts have incrementally reduced this electrical resistance by increasing stub diameter, changing iron chemistry and by improving the stubhole shape. The additional use of multiple steel conductors to bridge across the cast iron to carbon interface provides a means to further reduce the electrical resistance. The function of the conductors is independent of the iron to carbon contact pressure, the stub temperature, iron chemistry, and the stubhole shape. The steel conductors are tightly driven into the carbon anode at one end, with the other end bonded into the cast iron. This paper includes in-pot performance testing results which demonstrate the reduced resistance when using stubhole conductors. However, in this disclosure nails (conductor) are provided with the help of pneumatic tools in the anode carbon block & current is passed through these multiple nails to avoid the air-gap between carbon block & cast iron. WO2016/141475 titled "anode assembly for aluminium electrolysis cells and method for manufacturing anode assemblies" discloses an anode assembly for an aluminium electrolysis cell. The anode assembly includes a baked anode block, a plurality of elongated connection elements each having an anode block contact surface and an electrical connection surface, at least one electromechanical crossbar connector covering the electrical connection surfaces of the elongated connection elements, and a crossbar electrically connected to the elongated connection elements. The method of manufacturing an anode assembly includes the steps of forming a block of green anode paste, inserting a plurality of elongated connection elements in the green anode paste, baking the green anode, positioning a crossbar above the electrical connection surfaces of the plurality of elongated connection elements, and covering the electrical connection surfaces and at least partially the crossbar with a surface-conforming electrically-conductive material. However, during green anode manufacturing of carbon block, the conductor have been forged with plurality of elongated connection elements and stub has not been used. Also, the elongated structure has to be baked along with in baking furnace for strengthening.

EP2006419 titled "reduced voltage drop anode assembly for aluminium electrolysis cell" discloses an anode assembly for aluminium electrolysis cells comprising carbon anodes with stubholes and an anode hanger having stubs, whereas the anodes are fixed to the anode hanger by cast iron, characterized in that the stubholes are fully or partially lined with an expanded graphite lining. Further, mechanical stresses in the stubhole area are reduced. By further forming a collar from the lining, the spilling of cast iron over the anode surface is prevented and optionally a protective shot plug or a protective collar prevent direct contact of the hot electrolyte bath with the stub and the cast iron. However, this prior art discloses the graphite lining in which baked anode block is coated with graphite to reduce the voltage drop.

WO 2015089654 titled "Low resistance electrode assemblies for production of metals" discloses an electrode assembly for use in a reduction cell for the production of metal such as aluminium. The electrode comprises an electrically conductive carbon electrode block with an electrically conductive metal member connected thereto. The connection provides an improved electrically conductive connection between the carbon electrode block and the conductive metal member, with reduced resistance. The insert may provide a direct connection between the electrode block and the metal member, or the connection may be provided through a layer of cast iron or other metal element provided between the electrode block and the metal member. Generally, in conventional aluminium smelter electrical connection is done by copper / aluminium rod with stub and in Rodding Shop molten cast iron is poured in the hole / carbon block through which current passed to the Pot. Therefore, in smelter, high Stub to Carbon drop has an impact on specific power consumption, cost of aluminium production and life of Anode rod assembly.

OBJECTIVES OF THE INVENTION

An objective of the present disclosure is to reduce the voltage drop at the anode connection by increasing contact surface area between anode block and stub.

Another objective of the present disclosure is to reduce stub to carbon drop in a smelter.

Still another objective of the present disclosure is to reduce heat dissipation through the stub. Still another objective of the present disclosure is to reduce the cost of aluminium production.

SUMMARY OF THE INVENTION

An aspect of the present disclosure is to provide a rodded anode assembly for an aluminium electrolytic cell, the anode assembly comprising: - an anode block, the anode block having a stubhole;

a stub having a first end portion and a second end portion, the first end portion of the stub accommodated in the stubhole in the anode block;

a copper bar electrically and mechanically connected through a stiffener by welded joint to the second end portion of the stub;

Characterized in that

the first end portion of the stub has an enlarged diameter and the second end portion of the stub has a reduced diameter, the first end portion abutting the second end portion;

the dimensions of the first end portion and second end portion being configured such that the first end portion accommodated in the stub hole has maximized surface area thereby reducing the voltage drop between the anode and the first end portion of the stub.

Another embodiment of the present disclosure provides that the stub is made of mild steel.

Still another embodiment of the present disclosure provides that the anode block is made of carbon. Still another embodiment of the present disclosure provides that clearance between the stubhole and the first end portion of the stub has cast iron filling forming a thimble around the first end portion of the stub.

Still another embodiment of the present disclosure provides that dimensions of the stubhole are configured in proportion to dimensions of the first end portion of the stub.

Another embodiment of the present disclosure provides that the stub is mechanically, thermally and electrically connected to the anode block by casting molten iron.

Another embodiment of the present disclosure provides that an elongated copper bar is connected to the stub through stiffener made of mild steel and it helps in penetrating copper bar in mild steel stub to strengthen the weld joint.

These and other features, aspects and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows rodded anode assembly with stepped stub for an aluminium electrolytic cell (pot cell);

Figure 2 shows stepped Stub anode assembly in the electrolytic cells (pot cells) of a smelter; Figure 3 shows the stepped stub accommodated in stubhole;

Figure 4 (a) shows the stepped stub according to the present disclosure;

Figure 4 (b) shows normal stub as per the prior arts;

Figure 5 (a) shows graph for voltage of an electrolytic cell (pot cell) with stepped stub;

Figure 5(b) shows graph for voltage of an electrolytic cell (pot cell) with normal stub.

DETAILED DESCRIPTION OF THE INVENTION

Industrial production of aluminium has very high energy consumption, therefore, improvements in energy efficiency can be made by voltage saving at the anode connection. The voltage drop between stub and carbon anode is an essential part of the overall voltage drop at the anode and has a detrimental impact on the electrolytic process. Smelter Power consumption accounts for approx. 40% of the cost of aluminium production thus, any saving in Power consumption has high impact on the cost of aluminium production.

Primary aluminium is produced by electrolytic reduction of alumina, the process is commonly known as Hall-Heroult Process, alumina is dissolved in molten cryolite bath and electrolysis is carried out in specially designed aluminium electrolysis cells commonly known as 'pots'.

An electrolytic cell consists of the following components :- Anode (the +ve terminal) - Prebaked, made of CP coke and coal tar pitch;

Cathode (the -ve terminal) - made of cathode carbon blocks in which steel collector bars are embedded to make electrical connection;

Electrolyte - Medium for carrying out electrolysis, consisting of mainly, cryolyte, aluminium fluoride and calcium fluoride in which alumina is dissolved for electrolysis;

Electrolytic cells are connected in series along with certain bus bar configuration for passage of current constitutes a potline, spread over in two rooms.

In an electrolytic cell, when DC current is passed from anode to cathode through molten electrolyte, alumina dissolved in electrolyte is dissociated into aluminium & oxygen. Aluminium being positively charged goes to the cathode while oxygen being negatively charged goes to anode, aluminium continues to deposit at cathode, which is tapped out periodically. Oxygen reacts with anode carbon forming C02 & CO, which ultimately goes out of cell as pot gases. The consumed anodes are replaced in certain sequence periodically with new anode to continue the process.

Reduction Plant, in particular comprises of Carbon Plant, and Pot rooms. In Carbon Plant, carbon electrodes (anodes and cathode Paste) are produced which are utilized during electrolytic process of reducing alumina powder to molten aluminium metal. The metal produced in pot rooms is finally converted into value added products viz. Properzi wire rods, Slabs, Billets, ingots. Further value on these products is added in Fabrication Plant.

Anode Manufacturing for Aluminium Production: -

Carbon anode is one of the major raw material in aluminium smelter operation. It takes active part in electrolysis and gets consumed in the process. On completion of its scheduled stay in pots, the anode is replaced by a new one and the remaining portion known as butt is processed together with calcined coke and Binder Pitch for fresh anode production.

The carbon anode is manufactured in three stages: GREEN ANODE

The ingredients in anode manufacturing are Calcined Petroleum Coke (C.P.coke), High softening point Coal Tar Pitch and Pot room returned butts.

CP. Coke consignment received from various sources is stored in coke silos. For the production of anodes, coke is passed through primary and secondary crushers and screened. Potroom return butts are fed to jaw crushers and crushed in 4"-6" sizes. The same is passed through hammer mill and crushed into -3/4" particle size. This material is stored in butt silo and transferred to bins as butt fraction. Pitch is stored holding tanks from where it is transferred to buffer tank then weighing scale for consumption.

Dry aggregate constituting 5 fractions coarse, medium, fine, BMP and filter dust fractions are separately weighed, as per the standards specified, and charged into preheater. The dry aggregate is mixed and heated up in preheater and then transferred to buss mixer where measured quantity of molten pitch is added to the mixer and prepared paste. Hot paste is then transferred to surge hopper. From surge hopper, the weighed quantity of paste is feed to transfer trolley which finally feeds to vibroforming mould and form anode subsequently into green anodes. Formed anodes are water cooled and unloaded on skips (carrier for carrying green anodes). These anodes are checked for their physical appearance, height, weight & density and only good anodes are transferred for baking.

BAKED ANODE

The good green anodes from paste plant are packed in empty pits of Baking Furnace. All the anodes are covered by CP. coke all around. The top of the last layer anode is covered with CP. Coke as blanket material. These anodes are baked at a temperature around ~1100 deg. C (min) in the pits and the total baking cycle takes about 15-20 days. During the baking process the binder pitch gets converted into pitch coke. The released volatile matter contributes in baking of anodes. The firing system in baking furnace consists of computerized oil (F/Oil) fired burners regulated by temperature feedback of flue thermocouples. The baking process improves anode quality interns of strength, reactivity, thermal and electrical conductivity etc.

On completion of baking process and cooling, anodes are unpacked from the sections and cleaned properly. Only good anodes are loaded on conveyor belt for further cleaning and subsequent rodding. RODDED ANODE

Baked anodes are fixed with Cu-steel stub assembly by pouring molten cast iron around the steel stub, in the anode hole. On solidification the Cu - stub assembly gets fixed with the carbon anode. A reference mark is marked at about 2.2 meters from the bottom of the anode, on copper bar. The ready anode is then sent to Potroom for replacing the old consumed anodes in pots. The rodding shop has the induction furnaces where the cast iron charge is prepared. A rodded assembly with anode block comprises of copper bar and stub welded in house the length of the individual material i.e. Cu bar stub and an anode block may varies from one assembly to another therefore the marka (painted line) is marked from bottom reference these marka line helps for purposed of anode setting.

In the aluminium smelter factors like cost of stub material, weight of stub material, cast iron quantity, heat Loss, weld strength depend on the stub size. Stub to carbon drop has significant contribution in overall power consumption and depends on Stub dimension, Cast Iron quality, Stubhole design and Pouring Parameters.

The present disclosure provides a rodded anode assembly with stepped stub for an aluminium electrolytic cell wherein stub is designed in such a fashion that the contact surface area inside the hole is kept maximum to reduce the stub to carbon drop for anode thereby minimizing the cost involved.

Referring to figure 1 , which shows rodded anode assembly with stepped stub.

The rodded anode assembly ( 1) comprising a carbon anode block (2), an electrically conductive stub (4) having a first end portion (Al) and second end portion (A2), the first end portion (Al) of the stub (4) is accommodated in the stubhole (3) of the anode block (2), a copper bar (5) electrically and mechanically connected through stiffener by welded joint to the second end portion (A2) of the stub. The clearance between the stubhole (3) and the first end portion (Al) of the stub has cast iron filling (6) forming a thimble around the first end portion (Al) of the stub. Referring to figure 2, which shows the stepped Stub anode assembly in pot cells of the smelter. Various components of the assembly are described as below: -

• Anode block (2) - It is a consumable material comprising of calcined petroleum coke and hard pitch. When current passes through it to perform electrolysis reaction it gets consumed. · Stepped Stub (4) - it is made of mild steel and conduct electric current from copper bar to Anode Block through cast iron. • Copper bar (5) - It is a rectangular section of copper for conducting electric current anode ring bus to carbon block.

Figure 3 shows the stepped stub accommodated in a stubhole and it is a cross sectional view of anode hole with stepped stub.

Referring to figure 4(a), which shows the stepped stub designed in such manner that total length remains constant in comparison to normal stub shown in figure 4(b), however, total cross sectional area of the stub is increased i.e. (A< A1+A2), (D2<D<D1); whereas L=L1+L2 (Total length remains same), W=W1+W2 (Almost Same or marginally increased), L, A, W are total length, area and weight of normal stub.

Based on the theory Κ=ζ*\/Α, Where ζ and 1 are constant,

R inversely proportional to A, therefore, increase in area result decrease in resistance.

Figure 5 (a) and Figure 5(b) therein illustrated graphical view of average voltage of pot cell with stepped stub and with normal stub over the specific period of time. EXAMPLES

Experiments have been conducted to optimize the stub dimension and cast iron thickness around stub, with various diameters of stub (ranging from 160 mm to 220 mm) for anodes with different stub hole dimensions and cast iron weight shows how much to increase the stub dimension to a maximum which could be accommodated in anode for maximum voltage saving.

Although Stub to Carbon drop reduced substantially from 90 mV for 160 mm diameter stubs to 60 mV for 200 mm diameter stepped stub but consumed anode (Butt) cracking inside the pots at the time of anode replacement also increased substantially. Anode failure rate of 20% was observed. Stub hole is designed to minimize butt cracking during anode replacement as well as maximum gain from reduced Stub to Carbon drop. The possible causes of the increased butt cracking in the pots was attributed due to high thermal stresses developed in higher diameter stubs in pots at operating temperature of 955°C.

It is a known fact that Steel expands on heating whereas Cast Iron expands on solidification. Keeping in view the above material properties, the Steel Stub is preheated to 300°C (approx temp of stub in pots) before pouring molten cast iron around the stub placed in anodes. This was done to develop residual thermal stresses in the stub due to contraction at room temp, as well as to maintain sufficient gap for allowing expansion of cast iron after solidification without impacting the structure of Carbon Portion around the Stubs and its strength. However, butt cracking persisted with the same rate. This is due to improper Cast Iron thickness resulting in unbalanced stresses/forces on Steel stub, Cast iron ring and Carbon Material.

To overcome above mentioned problem, Cast iron thickness was increased in steps and then optimized to keep a balance between the rate of butt cracking & molten cast iron required for Pouring in Rodding Shop without affecting normal supply of anodes (Nos.) to potrooms.

Following table shows the possible causes & remedial steps to reduce stub to carbon drop:-

TABLE- 1

A detailed comparative study of the data for different set of experiments conducted on pilot basis with different diameters of stubs, use of flat/slanted contours along with the different Cast Iron thickness around stub in rodding shop was performed.

Comparative study :-

D- 19 is experimental Pot which have 200 mm stub diameter.

D-21 is Reference Pot which have 160 mm stub diameter.

Referring to the Table-2 of D- 19 and D-21 avg volt column in which the avg voltage of D- 19

(Exp Pot) from Dec- 15 to Jul- 16 is 4.31 1 V whereas Pot D-21 (Ref Pot) avg Voltage is 4.312 V.

Experiment was started from 1 ^st Aug 16 and 1st changeover was completed on 31 ^st Aug 16 on

D- 19. As evident from August- 16 figure, the pot Voltage got reduced from 4.31 V to 4.25 V of D- 19. This was achieved by process improvements on account of higher stepped stub diameter.

TABLE-2 (Stepped stub Vs Normal Stub drop)

Attribute to the savings due to reduced voltage drop or increase in Pot line Amperage in proportion to the DC power saved with respect to reduced stub to carbon drop for a constant DC Power feed to Pot line. This also opens avenues of increasing Pot line amperages on account of increased heat dissipation from larger surface area stubs, thereby enhancing the productivity & efficiency of Pots with the existing Pot line infrastructure. Also, the cost is reduced in change over as minimal infrastructure changes required since the gross weight of anode assembly has not been changed i.e. in rodding shop existing equipment can be utilized with minor modification.

Followings saving were realized with above diameter of stubs :-

• It reduce stub carbon drop

• Reduce heat dissipation from stub

• Reduce heat dissipation from alumina crust

The stepped step anode assembly has been implemented in 44 pots and if replicated in all 2138 pots of a smelter unit, it provides voltage drop of 40-50 mV/ pot, which results in cost saving of approximately 140 million INR based on the production 405000 ton/annum and power saving of 126 units/ton, wherein energy unit charges have been taken as INR 2.8 /unit.

24 152 198 13 239 253 24 175 228 13 194 269

20 199 278 9 155 263 20 194 299 9 141

16 173 5 195 268 16 173 5 202 276

12 172 1 198 242 12 174 1 199 279

2 240 280 15 187 280 2 170 267 15 146

6 230 282 19 249 250 6 188 276 19 123

10 125 23 193 10 146 23 163 220

14 240 234 3 176 14 140 245 3 171 231

18 113 7 171 254 18 144 7 189 274

22 237 224 11 160 249 22 180 233 11 113

191.1 259.1 198.5 260.8 167.5 260.9 163.8 262.1

Comparative temperature data for normal and stepped stub

Stall no are the position of individual anode mounted on pots. If there are 24 anodes then 24 stall are there in a particular pot and if there are 20 anodes then 20 stall in a particular pot.

TABLE-3

To further validate the results, this experiment has been extended in all 44 pots (Dl to D44) and it has been concluded that pot voltage has been reduced by 40 to 50 mV in three months' time with this improvement.

ADVANTAGES OF THE PRESENT INVENTION

• Reduced carbon stub drop.

• Reduced energy consumption.

• Reduced cost in change over.

• Reduced heat dissipation from stub.

Some of the components, used in an electrolytic cell (pot cell) are defined as below:-

Stub: To conduct electric current from copper bar to Anode Block through cast iron and made of

Mild Steel.

Cast Iron: Filling material for fixing of stub with carbon block. Also it carries current from stub to carbon block.

Pot Cover: These are made of aluminium to cover the pots there by reducing the dusting and also arresting toxic gasses such as HF and C02. Stiffener: To strengthen the welding joint between copper bar and steel stub and made of Mild Steel.

Copper bar: It is a rectangular section of copper for conducting electric current anode ring bus to carbon block.

Carbon Block: It is a consumable material comprising of calcined petroleum coke and hard pitch. When current passes through it to perform electrolysis reaction it gets consumed.

Bath: It is an electrolyte solution, work as a catalyst for electrolysis.

Cathode Block: It is made of carbon and used for electrolysis reaction as a -ve potential.