Chemistry: electrical and wave energy – Processes and products
Patent
1988-12-02
1990-04-03
Tung, T.
Chemistry: electrical and wave energy
Processes and products
204105R, 204222, 204273, 204284, B01D 5940, C25C 100, C25C 700
Patent
active
049137794
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process and an installation for electrolysis by percolation across one or several porous volumic electrodes, in order to achieve an electrochemical reaction. The invention applies in particular to the recovery of metals from dilute ionic solutions.
The carrying out of electrochemical reactions by electrolysis of a circulating solution across a conductive bed of solid particles, polarized negatively or positively according to the desired reaction, has been known for some time. This bed forms an electrode generally termed a "porous volumic electrode," which offers high specific surfaces and permits in particular the treatment of dilute ionic solutions, subjected to low current densities. One could refer, for example, to the following prior document which describes examples of such electrolyses: French patent No. 80.07039.
The fundamental deficiency of this type of electrolysis resides in the rapid plugging or clogging of the bed of particles forming each porous volumic electrode. This clogging causes, first, the appearance of preferential passages with different circulation speeds which disturb the activity, then rapidly block the operation. In the case of a reduction reaction (for example the recovery of metals), this clogging results from the appearance of solid bridges which are formed in the pores between particles.
This deficiency is accentuated on the periphery of the bed, in the vicinity of one or several polarized counter electrodes, at the sites where the activity is the most intense. When a separation membrane is provided, it may be impregnated with micro-crystals and be inflated to the point of rupture.
It should be noted that electrolyses of this type have been carried out across beds of particles placed into movement, periodically, in order to suppress the phenomenon of clogging. However, the electrical conductivity at the center of the bed is then carried out under very poor conditions and much lower electrolysis current densities that in a fixed bed, leading to mass transfers which are very insufficient for making the process applicable on an industrial scale.
For example, French patent No. 2,020,055 describes a process consisting of confining the bed of particles between two grills and mentions that it is possible to assure a fluidization by impulses in order to bond the bed in the upper part with a high circulation speed and to leave it fixed in the lower part at a lower speed. U. S. Pat. No. 3,966,571 describes an analagous process in which the bed of particles is only polarized in the top part, the displacement toward the lower part serving to unclog the bed. However, the material transfers are very mediocre and remain quite inferior in this type of process to those of an equivalent fixed bed. Thus, an unclogging of the bed is achieved, but only at the expense of a notable decline in efficiency.
The present invention proposes to provide a solution to the problem set forth above of the clogging of porous volumic electrodes.
The essential object of the invention is to suppress the phenomena of clogging, while considerably improving the mass transfer.
Another object is to obtain the effects described above without disturbing the selectivity of the reaction viv-a-vis the deposited specie.
To this effect, the electrolysis process according to the invention comprises electrically polarizing each volumic electrode, formed by a conductive bed of solid particles, causing to circulate through said volumic electrode a liquid electrolyte at an average flow speed Vo, and causing a pulsation of the electrolyte circulating through the volumic electrode. The process according to the invention is characterized in that a periodic pulsation is superposed on the circulation of the electrolyte, the pulsation having an amplitude -a- and a frequency -f- satisfying the following conditions:
in the case of an ascending or upward circulation: ##EQU1##
in the case of a descending or downward circulation: ##EQU2## in which Vo, a and f are the respective arithmetic values of the flowing velo
REFERENCES:
patent: 3966571 (1976-06-01), Gagnon et al.
Dutton, Jr. Harold H.
Institut National Polytechnique de Toulouse
Ryser David G.
Tung T.
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