Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1994-06-21
1995-06-13
Valentine, Donald R.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204279, C25C 314
Patent
active
054239680
DESCRIPTION:
BRIEF SUMMARY
This invention relates to apparatus for the controlled supply of alumina or other solid materials to an electrolytic cell in which the alumina is converted to aluminum.
In the electrolysis of alumina, solid alumina is dissolved in a tank or pot containing molten electrolyte such as cryolite and it is desirable to maintain the alumina concentration in the electrolyte within a predetermined range. In current practice for the electrolysis of alumina, the alumina is fed in successive doses of predetermined size into one or more holes which are made in the electrolyte crust so that the alumina can be admitted when required. As the electrolysis of the alumina proceeds continuously, it would be desirable if the alumina consumed in the electrolysis process could be continuously replaced so as to maintain the optimum alumina concentration in the electrolyte. However, the optimum operating conditions are such that the electrolyte crust continuously reforms on the surface of the electrolyte, making it difficult to supply alumina continuously to the molten electrolyte beneath the crust.
For this reason, known alumina feeding procedures involve the use of a crust breaker which is operated intermittently to break the electrolyte crust and form a hole through which the solid alumina can be fed. However, the action of the crust breaker is necessarily such that the crust breaking mechanism, such as a pneumatically operated shaft with an appropriate chisel means (hereinafter referred to as a plunger) at its free end, will be moved in and out of the hole formed by the plunger.
In one known feeding procedure, a single pneumatic mechanism is used to operate the crust breaking mechanism, and the discharge of alumina from a storage device is co-ordinated with the downward movement of the crust breaker. In this procedure, the alumina charge is thus released when the crust breaker is through the crust so that the alumina is not free to enter the hole in the crust until the crust breaker is retracted. While this procedure has the advantage of a single pneumatic system, it is obvious that not all the alumina will be able to pass through the hole into the electrolyte immediately when the crust breaker is retracted.
It will also be apparent that with this form of mechanism, the plunger must travel through the crust each time a charge of alumina is to be introduced into the electrolyte. This not only involves use of sufficient air to drive the plunger but also may involve dipping the plunger into the electrolyte with each stroke. It is desirable to reduce the number of times the plunger contacts the electrolyte as far as possible so that wear of the plunger can also be reduced.
Accurate alumina flow control is made difficult by the required relationship between the plunger movement and movement of the flow control valve controlling alumina discharge. It will be understood that the force available and speed necessary for plunger movement must be sufficient to achieve crust breakage. This force, speed and the amount of air needed to produce them are far greater than that needed to move the alumina flow control valve and it is therefore necessary to cushion the final stages of the pneumatic mechanism against the considerable force available for plunger movement. Because of the interaction between the pneumatic mechanism and the control valve, this necessary cushioning action affects addition accuracy.
In another procedure, two separate pneumatic systems are employed, one operating the crust breaking mechanism and the other operating the alumina feeding system. In this procedure, it is possible for the mechanism operating the crust breaking mechanism to form the necessary hole in the electrolyte crust and retract the crust breaker so that the feeding system can then be operated to place a charge of alumina into the hole formed by the crust breaker. There is less air usage as the dosing cylinder of the alumina feeding system is of smaller capacity and may operate more often than the crust breaker as it is independent of the crust breaker me
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Portland Smelter Services Pty. Ltd.
Valentine Donald R.
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