Anode retraction device for a Hall-Heroult cell equipped with inert anodes

A compressed air-powered retractor for an inert anode in a Hall-Heroult cell is actuated after power interruption by a time delayed solenoid actuating a valve controlling the air supply to a cylinder connected to the anode support structure.

BACKGROUND OF THE INVENTION 
Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina 
in molten cryolite, using conductive carbon electrodes as anodes. During 
the reaction the carbon anode is consumed at the rate of approximately 450 
kg/mT of aluminum produced under the overall reaction 
##STR1## 
The problems caused by the consumption of anode carbon are related to the 
cost of the anode consumed in the above reaction and to the impurities 
introduced into the melt from the carbon source. The petroleum cokes used 
in manufacturing the anodes usually have significant quantities of 
impurities, principally sulfur, silicon, vanadium, titanium, iron and 
nickel. Sulfur is oxidized to its oxides, causing particularly troublesome 
workplace and environmental pollution. The metals, particularly vanadium, 
are undesirable as contaminants in the aluminum metal produced. Removal of 
excess quantities of the impurities requires extra and costly steps when 
high purity aluminum is to be produced. 
If no carbon were consumed in the reduction the overall reaction would be 
2Al.sub.2 O.sub.3 .fwdarw.4Al+30.sub.2 and the oxygen produced could 
theoretically be recovered, but more importantly no carbon would be 
consumed at the anode and no contamination of the atmosphere or the 
product would occur from the impurities present in the coke. 
The aluminum industry has long sought to develop an inert ceramic anode to 
replace the consumable carbon anode used in Hall-Heroult electrolysis. In 
recent years the development effort has been accelerated and, although no 
instance has been reported where aluminum is produced commercially with 
inert anodes, significant strides have been made toward the realization of 
this goal. The cost of the anodes, due almost entirely to the cost of the 
materials from which they are made, exceeds by an appreciable amount that 
of baked carbon anodes now in use. However, the longer lifetime of a 
ceramic or cement anode, one to two years, results in a net savings in 
anode cost per unit of aluminum produced. From an operational viewpoint, 
the increased capital worth of a cell equipped with new inert anodes 
versus one equipped with less expensive carbon anodes justifies taking 
additional precautions to prevent the anodes from being damaged. 
The inert anode materials disclosed to date all contain metal oxides as 
their principal constituent for the reason that oxides are stable to the 
oxygen anode product. However, most metal oxides are chemically reduced by 
liquid aluminum at high temperature forming Al.sub.2 O.sub.3 and a metal 
ion which, in a Hall-Heroult cell, is co-deposited with the aluminum ions 
at the cathode to contaminate the aluminum product. Attack of the anode by 
aluminum does not occur to any appreciable extent during normal 
electrolysis because the oxygen gas produced at the surface of the anode 
acts as a protective barrier to aluminum attack and additionally 
stabilizes the oxide-based anode material. It is during periods when 
electrolysis is interrupted, e.g., during an extended power failure, that 
the anodes are susceptible to chemical reduction and the aluminum metal in 
the cell to subsequent contamination. Power outages for periods as little 
as five minutes may be sufficient to produce these effects, the actual 
time being dependent on the electrode material and the operating 
anode-cathode spacing. In the worst case, when power cannot be restored 
for several hours, the cells are subject to freeze-up which would be 
catastrophic to the anodes resulting in a serious financial loss. 
The invention described herein, an anode retraction device, is capable of 
sensing a power interruption and withdrawing the anodes from the melt to 
preclude or minimize such damage. 
SUMMARY OF THE INVENTION 
The anode retraction device of the invention comprises one or more 
pneumatically operated actuators affixed to the busbar/anode drive 
mechanism and controlled by an electrical solenoid valve which is capable 
of sensing a power loss. The actuators function such that when power to 
the solenoid valve is interrupted, compressed air stored in an accumulator 
is admitted to the actuators to raise the anode assemblies to a 
pre-determined locked position. The direct current solenoid valve is 
normally energized with the valve in the closed position. To prevent the 
anodes from retracting as a result of a momentary power interruption, a 
separate battery-operated circuit is employed in conjunction with a 
mechanical or electrical time delay device to maintain the solenoid valve 
in the energized state until the pre-set delay time has expired. Although 
two actuators per cell should be adequate to perform the task, the actual 
number is dependent on the cell design, the weight of the anodes/busbar, 
the size of the actuators, the accumulator pressure and other factors. The 
electrical sensing equipment, time delay circuitry, and gas accumulator 
can be constructed so as to be common to many, if not all, cells. It is 
understood that appropriate design modifications will be required. 
The selection and placement of the actuators should provide the necessary 
flexibility to raise the anodes to any desired position. It may not be 
necessary in some cases to retract the anodes from the melt completely but 
rather only to remove them from the immediate proximity of the liquid 
aluminum pad. For example, an inert anode is expected to operate at an 
anode-cathode spacing of 1.9-3.8 cm which places the active surface of the 
anode at a depth of 10-12 cm in the melt. Withdrawing the anodes to a 
spacing of about 7.5 cm may be sufficient to prevent chemical attack by 
aluminum. If necessary to remove the anodes completely from the cell, for 
example, when bath freeze-up is imminent, they should be retracted to a 
point just above the melt yet still within the hard protective cell crust. 
In this position thermal shock to the anodes is minimized as are heat 
losses from the cell which would otherwise hasten melt freeze-up. Once 
power is restored, return of the busbar and anodes from the locked 
position to the normal operating position is accomplished by pneumatic, 
hydraulic, or mechanical means.

DETAILED DESCRIPTION OF THE INVENTION 
An anode retraction device similar to that shown in the FIGURE was 
installed on each of two laboratory reduction cells used for inert anode 
test purposes. 
The cylinder was obtained from BIMBA Manufacturing Company, model no. 
MRS-313-XP. An electrical solenoid control valve produced by Skinner 
Precision Industries, model V53A-DB2-2100, was used to sense a power loss 
and to activate the cylinder by admitting compressed air to the actuator. 
The retraction devices functioned as designed over a two year period to 
provide protection for the test anodes. 
The invention is not limited to usage in a Hall-Heroult aluminum reduction 
cell but may also be useful in other cells in which the electrodes or the 
cell itself may be damaged if the electrodes are allowed to remain in the 
electrolyte when the power supply is interrupted.