Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Involving measuring – analyzing – or testing during synthesis
Patent
1998-04-14
2000-01-04
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Involving measuring, analyzing, or testing during synthesis
205775, 2042431, 204294, C25C 306, C25C 320
Patent
active
060106115
DESCRIPTION:
BRIEF SUMMARY
FIELD
This invention relates to a method and an apparatus for selective electrochemical measurement of components of an electrolyte. It relates to measuring the concentration of oxide ions (ore) dissolved in a bath (a reduction cell or pot) based on molten halides. In particular it relates to the measurement of the concentration of dissolved aluminium oxide in smelter pots (where the electrolyte consists partly of wholly of fluoride salts) for the production of aluminium.
BACKGROUND
In industrial aluminium reduction cells or pots, where aluminium oxide is dissolved in a bath based on molten halides, one of the main concerns of the operator and also the main difficulty is to monitor the proportion of oxide ions (dissolved alumina) in the electrolyte. An excess of oxide which is soluble to about 7-8% results in the formation of sludge in the bottom of the crucible with highly prejudicial consequences in regard to progress of the electrolysis operation, while impoverishment of oxide ions results in a polarisation action, referred to as an "anode effect" which interferes with the operation of the cells. The anode effect is well known in aluminium smelters. It is related to current density or, in effect, applied voltage and causes the cell resistance to exhibit a sudden large increase above a critical current density
Direct and indirect methods and apparatus for determining the amount of dissolved oxide ions have been proposed. Indirect methods include those disclosed in U.S. Pat. No. 4,814,050 (Alcoa) and U.S. Pat. No. 5,025,219 (Pechiney). Direct methods are disclosed in U.S. Pat. No. 3,471,390 (Reynolds; inventors Kibby et al), U.S. Pat. No. 345,063 (Reynolds) and U.S. Pat. No. 4,935,107 (Pechiney). U.S. Pat. No. 3,471,390 discloses apparatus including a probe or test electrode having a cathodic graphite tip, a boron nitride spacer, and an anode, serving as a miniature reduction cell within which a rising voltage will elicit a rising current until such time as the "anode effect" appears, causing a rise in resistance and hence a drop of current. This effect depends primarily on the concentration of alumina in the electrolyte. Kibby's electrode is a plane surface on the end of a rod, having a graphite centre surrounded by an insulating ring. Kibby et al prefer to apply voltage changes in discrete steps over a predetermined period of time, and data is taken from the instrumentation by reading the dials of voltage and current meters. However, the extended duration of a single reading tends to result in significant electrode consumption and therefore the calibration drifts with time. The direct methods disclosed in the other patents referred to above also lack reproducibility and are therefore are not widely used.
STATEMENT OF INVENTION
In a first broad aspect the invention provides a method for measuring the concentration of alumina in a molten electrolyte in a bath, using the characteristic "anode effect" produced by an electrode assembly means having a cathode and an anode and which assembly means is placed in the bath, wherein at least one rapid cyclic voltammetry sweep is applied to the electrode assembly means by means of a voltage-controlled source of electric power connected to the electrode assembly, and the current passing through the electrode assembly means is monitored during the or each sweep in order to detect the anode effect, and the voltage at which the anode effect is observed is recorded, in order to derive information capable of providing a measurement.
In a related aspect the invention provides that the or each rapid cyclic voltammetry sweep comprises a substantially linear change in voltage and is applied to the anode of the electrode assembly means at a rate of voltage change of between +5 to +100 volts per second during a first measurement phase, to reach a potential which is more than sufficient to produce an anode effect, and then is returned at a rate of between -50 to -1000 volts per second during a second, return phase prior to a rest phase.
In a further related aspect the inv
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Haverkamp Richard Gerard
Welch Barry John
Auckland UniServices Limited
Valentine Donald R.
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