Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing inorganic compound
Reexamination Certificate
2000-06-05
2001-05-01
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing inorganic compound
C205S762000, C204S290140
Reexamination Certificate
active
06224740
ABSTRACT:
The invention relates to a process for the parallel operation of amalgam electrolysis units and membrane electrolysis units with a common brine circuit using a mercury-resistant oxygen consumable cathode in the membrane electrolysis unit.
The oxygen consumable cathode for use in NaCl electrolysis is known in principle from the literature. For its operation, for example in a pressure-compensated arrangement, as described in DE 19622744 C1, brine in the conventional membrane cell quality is employed. In order to protect the cathode activation, this brine is kept free from mercury.
The mercury contamination of the NaCl brine known for chloro-alkali electrolysis by the amalgam method is typically from about 10 mg/l to 400 mg/l in normal operation or as a peak value after shut-down of the unit.
It is known of common membrane electrolysis units that mercury, in particular in the above-mentioned high concentration, results in relatively rapid passivation of the cathode coating (cathode material) by mercury ions migrating through the membrane from the anode space. This results in an irreversible increase in the voltage for operation of the electrolysis unit and requires greater energy input. Parallel operation of classical amalgam electrolysis units and membrane electrolysis units with a common brine circuit is therefore not possible, apart from the alternative of carrying out complex mercury removal (precipitation) from the brine intended for the membrane electrolysis unit or alternatively constructing a separate, mercury-free brine circuit. Both variants are associated with high complexity.
Attempts to develop mercury-resistant cathode activations have not brought the hoped-for success, and consequently mercury-free brine must continue to be used as the starting point for full utilization of the energy saving. This is usually carried out via separate brine circuits or mercury precipitation using Na
2
S. Both routes are complex processes.
A further aspect plays an important role in the case of step-wise conversion from amalgam electrolysis to the membrane method: if the energetically less favourable, mercury-resistant cathode activation is to be used during parallel operation of amalgan and membrane methods, with the aim, after complete refitting, of changing over to the optimum, but mercury-sensitive cathode activation, the entire brine and lye circuit must first be rendered totally mercury-free, which causes enormous problems, especially as some of the mercury in the lye circuit may be in metallic form.
The object was therefore, based on the known prior art, to provide an electrolysis process in which an amalgam electrolysis and a membrane electrolysis, preferably using an oxygen consumable cathode, can be operated in parallel with the same brine circuit. The process is to have the advantages of known processes with oxygen consumable cathodes.
The object is achieved in accordance with the invention by the use in a membrane electrolysis process of oxygen consumable cathodes which are resistant to the effects of mercury. The object is furthermore achieved by the use of a Ca/Mg ion exchanger which reduces the Ca/Mg content, even in the case of mercury-containing brine, to <20 ppb, which is necessary in order to ensure the full service life of the membranes.
REFERENCES:
patent: 5028302 (1991-07-01), Rutherford
patent: 5746896 (1998-05-01), Shimamune et al.
patent: 35 43 379 (1987-06-01), None
patent: 196 22 744 (1997-07-01), None
Journal of Applied Electrochemistry vol. 22 (month unavailable) 1992, pp. 699-704, F. Hine, “Combination of the Amalgam Cell and the Membrane Cell Process for Chlor-Alkali Production”.
Gestermann Fritz
Pinter Hans-Dieter
Ziegler Helmut
Gill Joseph C.
Phasge Arun S,.
Whalen Lyndanne M.
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