Patent GB-975150-A (GB)
Title: Process for electrolytically purifying dielectric liquids
IPC: B01D57/02
Filed: 1962-04-23
Abstract: A dielectric liquid is purified by the successive steps of (1) immersing in the liquid two or more pairs of electrodes, (2) applying D.C. across one or more but not all the pairs until the ionic impurity content of the liquid has been reduced, (3) removing the used electrodes from the liquid before escape from the electrodes of the adhering ionic impurities, and (4) repeating in one or more cycles the procedure of steps (2) and (3) using the remaining pairs of immersed treating electrodes. Preferably the used electrodes are removed from the liquid while the voltage is still applied but may be reduced from that applied during treatment to avoid arcing danger. However, an example is given of de-energizing the electrodes before removal (Example 4). A large number of liquids which may be so treated are listed in the Specification, as are suitable electrode materials which may be of any metal or alloy thereof except the alkali and alkaline earth metals, copper, selenium and tellurium but including magnesium, silicon, arsenic and nickel-copper alloys, or carbon but are preferably of nickel or aluminium.  The treating vessel may be of stainless steel, nickel, silver, aluminium, glass or P.T.F.E. and before introducing the liquid may be purged with dry nitrogen, argon or helium.  Gaseous dielectrics may be cooled and viscous or solid dielectrics heated to a maximum temperature of 250 DEG C. to permit treatment.  Prior to the electrolytic treatment, the liquids may be treated with an absorbent clay.  Electric fields of up to 10 kv./cm. may be used, the initial and final currents being in the range 10-0.1 m A, and a protective inert atmosphere over the liquid is continuously maintained.  The liquid is preferably continuously stirred as by an immersed P.T.F.E.-covered magnetic stirring bar.  Examples are given involving the use of 2, 3, or 4 pairs of treating electrodes together with a pair of electrodes for measuring the dissipation factor, the electrodes in use being withdrawn when no further change in this is observed.  The electrodes may comprise (a) two concentric cylinders held apart by three P.T.F.E. spacers 8 connected by leads which pass through a P.T.F.E. plug in openings in the top of the vessel, or (b) four concentric cylinders of which the first and third are connected to one pole and the second and fourth to the other pole. A storage drum provided with dry batteries in its cover is provided with two pairs of purification electrodes and a pair of storage electrodes which remain connected to the batteries after a two cycle purification.  Similarly a tank car is provided with storage batteries, six pairs of purification electrodes arranged in two sets spaced by half the car length, and used two pairs at a time in parallel, one from each set, to give three purification cycles, and three storage electrodes disposed one at each end and one in the middle of the car.  These nine pairs of electrodes are each formed of two sheets of aluminium wire screening interleaved with two larger pieces of open mesh P.T.F.E. screening and rolled to produce two spiral metal electrodes.