Electrolytic cell arrangement for production of aluminum

Details Electrolytic cell arrangement for production of aluminum Electrolytic cell arrangement for production of aluminum

2001-10-24

2003-04-22

Valentine, Donald R. (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S247100

Reexamination Certificate

active

06551473

ABSTRACT:

DOMAIN OF THE INVENTION
The invention relates to the production of aluminum by igneous electrolysis according to the Hall-Héroult process, and more particularly to methods and means of implementing it industrially. In particular, the invention relates to lines of electrolysis pots laid out crosswise, in other words with their long sides perpendicular to the centerline of the line.
1. State of the Art
Metal aluminum is produced industrially by igneous electrolysis, namely by the electrolysis of aluminum in solution in a molten cryolith bath called an electrolysis bath according to the well known Hall-Héroult process. The electrolysis bath is contained in a pot including a steel shell lined on the inside with refractory and/or insulating materials, and a cathodic assembly located at the bottom of the pot. Anodes made of carbonaceous material are partially immersed in the electrolysis bath. The pot and the anodes form what is frequently called an electrolysis cell. The electrolysis current that passes through the electrolysis bath and the liquid aluminum layer through the anodes and cathodic elements, brings about alumina reduction reactions and also keeps the electrolysis bath at a temperature of the order of 950° C. by the Joule effect.
In order to maintain the profitability of a plant, efforts are made firstly to reduce investments and operating costs, and secondly to obtain the highest possible current intensities and current efficiencies at the same time, while protecting and even improving operating conditions of the electrolysis cells.
Consequently, the most modern plants contain a large number of electrolysis cells laid out in line in “electrolysis” pot rooms electrically connected in series using connecting conductors in order to optimize the occupancy of factory floors. The pots, that are almost always rectangular in shape, are usually laid out side by side, in other words with the long sides perpendicular to the center line of the line (it is also said that they are laid out “crosswise”) but they may also be placed head to head (in this case they are said to be laid out “lengthwise”). The pots are usually arranged to form two or several parallel lines that are electrically connected to each other by end conductors. The electrolysis current thus passes in cascade from one cell to the next. The length and weight of the conductors are as small as possible in order to limit the corresponding investment and operating costs, particularly through a reduction of Joule effect losses in conductors. Furthermore, bringing electrolysis pots closer together and increasing the intensities of electrolysis current has led to the development of conductor configurations capable of compensating for the effects of magnetic fields generated by the electrolysis current.
With the same objective, it is known that pots, or lines of pots, can be provided with sophisticated regulation means that enable good control of the electrolysis process. In particular, French application FR 2 753 727 filed by the applicant proposes a detailed temperature regulation process that can give high values of the current efficiency.
Electrolysis pots are usually controlled such that they are in thermal equilibrium, in other words the heat dissipated by each electrolysis pot is globally compensated by the heat produced in the pot, which originates essentially from the electrolysis current. Thermal equilibrium conditions depend on the physical parameters of the pot such as the dimensions and the nature of the materials from which the pot is made, and the pot operating conditions, such as the electrical resistance of the pot, the bath temperature or the intensity of the electrolysis current. The pot is frequently made and operated so that a ridge of solid bath is formed on the sidewalls of this pot, which in particular inhibits attack of the linings of the said walls by the liquid cryolith. The thermal equilibrium point is usually chosen such that the best operating conditions are achieved both technically and economically.
French patent FR 2 552 782 (corresponding to American patent U.S. Pat. No. 4,592,821) in the name of the applicant describes a line of electrolysis pots that can operate industrially at current intensities exceeding 300 kA and with current efficiencies exceeding 90%.
2. Statement of the Problem
The continuous improvement in the performances of electrolysis plants, both technically and economically, has led the applicant to search for global solutions for increasing the cost effectiveness of plants, particularly by allowing for the possibility of a range of pot operating intensities. The possibility of making deliberate variations to operating conditions, which may be quite different from nominal conditions, is often useful in the management of an electrolysis plant. For example, an attempt can be made to vary the power of the series of electrolysis pots as a function of an electrical energy contract.
The applicant has found that electrolysis pots have temperature heterogeneities and more precisely a dispersion of temperature values within the liquid mass which, although relatively small, tend to be constant over time, in other words some differences between local temperatures and the average temperature of the pot are not cancelled by averaging over in time. In particular, these heterogeneities have the disadvantage that they limit the accuracy of the temperature regulation of the pots. Known regulation processes can control temperature fluctuations in time, but do not necessarily limit the dispersion of temperatures over the entire pot. Furthermore, zones in which the temperature is below the said temperature encourage material deposits at the bottom of the pot and the formation of extending ridges (in other words part of the ridge partially covers the cathode) that increase the cathodic voltage drop and are the cause of pot instabilities, and zones in which the temperature is higher than the set temperature, tend to reduce the protective solidified bath ridges on the sides of the pot and possibly lead to non-uniform wear of the linings.
Therefore, the applicant searched for means of reducing the temperature dispersion and temperature fluctuations in electrolysis pots that would overcome the disadvantages of prior art while remaining satisfactory for the general pot design, particularly concerning floor occupancy and investment in operating costs, and for operation of the pots.
3. Purpose of the Invention
The first object of the invention is an arrangement of electrolysis pots laid out crosswise for the production of aluminum by igneous electrolysis according to the Hall-Héroult process.
Another object of the invention is an electrolysis plant including an arrangement of pots according to the first object of the invention.
DESCRIPTION OF THE INVENTION
According to the invention, the arrangement of electrolysis pots for the production of aluminum by igneous electrolysis according to the Hall-Héroult process with an electrolysis current with intensity To includes at least one first line of electrolysis pots forming a first electrical circuit and at least one second electrical circuit located at a determined average distance from the said first line, the said first line including N pots arranged crosswise and connecting conductors to transmit the said electrolysis current Io from a pot in the first line called the upstream pot, to the next pot in the said line called the downstream pot, each pot including a metal shell, internal lining elements, anodes and cathodic elements, the said cathodic elements being provided with cathodic connection ends projecting on the upstream side and the downstream side of the shell of each pot, a first part Im of the current Io being output through the cathodic ends projecting from the upstream side of each pot, a second part Iv of the current Io being output through the cathodic ends projecting from the downstream side of each pot, the said connecting conductors including rising conductors called “risers”, the current Io output from all cathodic elements in an upstre

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