FR2523467A1 - ELECTRODE FOR ELECTRICALLY ACCELERATED VACUUM FILTRATION - Google Patents

Abstract

THE INVENTION CONCERNS THE ELECTRODES USED FOR DEHYDRATION OF SUSPENSIONS. IT RELATES TO AN ELECTRODE 15 INTENDED TO CONSTITUTE PREFERABLY THE ANODE OF AN APPARATUS FOR DEHYDRATION OF A SUSPENSION 9, FOR EXAMPLE OF CLAY. ACCORDING TO THE INVENTION, THE ELECTRODE 15 HAS A CHAMBER CONTAINING ELECTRODE ELEMENTS AND CLOSED BY WALLS INCLUDING AN ION EXCHANGER MEMBRANE, PREFERABLY CATIONS. THIS MEMBRANE IS ADVANTAGEOUSLY FORMED FROM A POLYMER OF PERFLUOROSULPHONIC ACID. APPLICATION TO THE DEHYDRATION OF SUSPENSIONS BY ELECTROFILTRATION.

Description

The present invention relates to an electrode

  developed for use in dehydration

  suspending colloidal or finely divided solids in a liquid vehicle, for example a suspension of kaolin in water, by application of a vacuum, the vacuum dehydration being accelerated by an electric field created by circulation of a current in the suspension, by implementing two

  remote electrodes immersed in the suspension.

  U.S. Patent Nos. 4,168,222

  and 4,207,158 describe dewatering methods and apparatus

  According to these patents, electrodes

  autonomous digs are normally immersed in the

  However, these hollow electrodes have two types of wall surface, ion-permeable walls for electrodes of a first polarity and liquid-permeable walls for electrodes. electrodes of the other polarity The surfaces of the electrode walls

  include a chemically and electrically

  qually neutral or porous permeable membrane carried by a grid, so that they have a surface

plane of electrode.

  In operation, when the electrodes of both types are immersed in the suspension, a source of a vacuum is connected to the interior of the electrodes which have the liquid-permeable walls so that an adjustable pressure difference is formed and causes the circulation of the liquid vehicle through the filtration surfaces, solids migrating in the opposite direction, under the influence of the electric field, and depositing under

  shape of a cake on the electrodes having permeable walls

  The liquid forming the filtrate, i.e.

  say the liquid vehicle free of solids, is extracted or pumped from inside the electrodes

  hollow filled with liquid, with adjustable flow.

  As indicated above, the cake is deposited on the hollow electrodes which have the ion-permeable surfaces; these electrodes are filled with an electrolyte and have an electrode element, immersed in the electrolyte and isolated against direct contact with the suspension. The electrolyte is specially selected for

  its high conductivity and its good compatibility with

  The "compatibility" indicates that the electro-

  lyte is relatively non-corrosive under the conditions that usually prevail inside the hollow electrode As decomposition products or secondary

  and heat emerges at the level of the elec-

  trode, inside the hollow electrode permeable to

  ions, the electrolyte can flow to the elec-

  trode so that these foreign products and heat are drawn out of the chamber, the composition of the electrolyte

  remaining relatively constant at a predetermined value.

  The wall permeable to the ions of the electrode of

  these known structures is a chemical filtration material

  electrically neutral or a porous permeable membrane which, when of the type of a film or the like requiring a support, may be carried by a chemically or electrically neutral grid so that a surface

  Filtration plane is presented to the treated slurry.

  As the cake is formed on this electrode during the

  trofiltration and must be removed by contact with scraper blades, a friction cage or a device

  spacing may be used to protect the

  filtration device against direct contact with scraping blades The friction cage comprises an open and thin grid of a relatively hard material, covering the filter material and intended to be in contact with

  scraping blades, and the spacing device may

  take strips of plastic material, for example of "Delrin" acetal resin, having a configuration similar to a frame and whose thickness is sufficient to prevent contact between the scraper blade and the filtration medium. From the recovery of the cake, the electrode can be lifted out of the suspension, the layer

  collected solid material or cake adhering to it

  the electrolyte remains in the electrode when

  In the raised position, a depression is applied inside the electrode so that it reduces the pressure exerted on the filter material and thus prevents its breaking when the electrode is immersed.

  during operation, the depression applied to the in-

  interior allows the extraction of gaseous products such as chlorine or carbon dioxide

level of the electrode element.

  The ion-permeable electrodes of known type, used for the dehydration of clay, pose certain problems of implementation As the clay particles of the feed material have a colloidal particle size, a large amount of these particles passes through the filter material This clay accumulates in the electrode chamber, contaminates the electrolyte

  circulates in the clay and limits or prevents the circulation

  electrolyte in the final electrode chamber.

  Electrode should be switched off, disassembled, cleaned and reassembled, according to an operation which is long and expensive. Moreover, it is found that the electrolyte is pumped from the chamber of the electrode to the bath. matter in the bath actually harms

  properties of the dehydrated product There is no elec-

  trode that works without presenting these disadvantages.

  The invention relates to an improved electrode

  intended for use in electrofilter operations

tration.

  Other features and advantages of the invention

  tion will become more apparent from the following description,

  made with reference to the accompanying drawing in which

  FIG. 1 is a diagram of an electrical apparatus

  filtration comprising the electrode according to the invention; Figure 2 is a detailed elevation with torn parts of the electrode of the invention; and FIG. 3 is a section along the line 3-3

of Figure 2.

  The electrode according to the invention, intended to be immersed in a suspension of solids, generally comprises a chamber having at least one wall formed of an ion exchange membrane, with at least one electrode element placed in a the room and immersed

in an electrolyte.

  In the following description, the electrode is

  called "anode" for convenience, but it should be noted that it can also be cathodic in some dehydration operations In the case of an anode, the membrane is cation exchange type whereas, if the electrode is cathodic, the membrane is of the type

anion exchange.

  FIG. 1 schematically represents an electrically-accelerated vacuum dewatering apparatus of the type concerned by the invention, comprising a tank containing a bath 9 of suspended solids, two cathodes 31 and 32 being immersed in the bath 9 of part and of Another of an electrode 15 The reservoir 10

  The bowl shape has a suspension supply connection 11.

  The latter may be a suspension of clay or a suspension of uniformly dispersed solids, finely divided, negatively charged, of colloidal size The required height of suspension in the tank is determined at any time by a discharge edge 12 associated with a channel 13 of the overflow, so that the electrodes placed in the tank are totally immersed. Thus, the feed suspension reaches a flow rate such that an excess is constantly overflowing.

  of the tank, and the suspension mass is renewed

  In addition, a pump 14 for circulating

  connected to the reservoir as indicated by 14a and 14b, maintains the contents in displacement so that the solids remain properly dispersed in the suspension, and that the cathode surfaces and

  anodic exposure to the suspension in the reservoir

  properly and uniformly.

  The cathode and anode surfaces of autonomous structures of planar configuration, parallel to each other, are thus produced and arranged in such a way that

  can be lifted vertically from their location

  up to a position which is outside the suspension, and which allows their return to the suspension

by lowering.

  In the case considered, that is to say suspended solids such as clay bearing a negative charge, the autonomous anode structure 15 of positive polarity and hollow construction occupies the

  center as previously stated.

  A vertical guide arrangement, not shown

  allows this electrode to be moved vertically

  hold, in its own plane, towards a position which is

  outside the suspension, and his return to

  evacuation devices also allow the sepa-

  and transporting the cake material formed on the anode surfaces from the slurry during the descent which returns the electrode in place In one embodiment, these evacuation devices, such as

  schematically represented, have two blades of

  17 and 18, arranged symmetrically and intended to pivot about horizontal axes between a neutral position and a cake separating position. The cake thus removed can be driven by the transport devices.

  represented in the form of conveyor belts 19 and 20.

  Obviously, the removal devices can also be made and arranged so that the removal and transport of the cake are made during the recovery of the electrode in the output position.

  2 and 3 in detail, the anode 15 has a

  truction hollow and comprises a rectangular member 21 forming a frame and two walls 22 having a membrane 22 formed of a negatively charged ion exchange resin,

  mounted on the frame 21 The latter has a U-shaped section

  singing outwards, allowing the walls to be fixed

  permeable to ions Each wall 22 is a multi-unit

  layer comprising a cation exchange member 22a, a support grid 22b and a protective cage 22c, and the negatively charged solids from the suspension are intended to be deposited therein in the form of a

layer or cake.

  Two brackets 15a of support are fixed to the ex-

  upper end of the frame 21 and are intended to

  connect and support the electrode 15 in the tank 10.

  A positive polarity terminal is formed by a vertical rod 26 penetrating inside the electrode and connected thereto to the electrode element 27 while the upper end portion of this rod which is exposed is a 26 connection connection

a cable.

  It should be noted that the frame 21 and the walls 22

  of the electrode 15 are electrically neutral and in

  sequence are formed of non-conductive material, for example of plastic material, or are isolated from the contact of the element

  27 and electrodes 26 and 26 a.

  In addition, the inside of the electrode can be filled with a suitable electrolyte (anolyte). A new electrolyte current circulates in the electrode so that the composition of the electrolyte remains relatively constant during operation of the vacuum filter. accelerated

  electrically The device that maintains this electricity supply

  trolyte, in its simplest form, can be a system

  gravity feed in which a feed tank

  high-level electrolyte is connected to the electri-

  trode through a conduit 28 supply and a conduit 29 discharge from the electrode and joined for example a waste tank The gas that is released at the anode is driven with the electrolyte exhausted The electrolyte flow can be achieved In some cases, it is desirable that the gases of the anode be separated from the anolyte and injected into the catholyte (so that they regulate the pH or in a

other purpose).

  The ion exchange resin membrane used in the electrode according to the invention is impervious to

  mass circulation of solids and liquids.

  As a result, clay particles can not

  penetrate inside the anodic chamber and can not

  In addition, the anolyte can not penetrate the bath (However, under the action of the electric field applied during the electrofiltration, an electro-osmosis causes the introduction of a part of the anolyte in the bath This effect is at least a factor 2 and very often a factor of 10 at the

  circulation of the anolyte observed with the known membranes).

  There are commercially a number of membranes

  cation exchangers, but the perfluorinated acid membranes

  rosophonic sold by E I Pont de Nemours & Company,

  under the trademark "Nafion" are particularly suitable

really well.

  In one example, we modify an anode of electro-

  filtration by fixing a "Nafion" membrane tensioned on one side of the anode and by plugging on the other side of the anode This membrane was previously treated in NaCl at 48.90 C for 1 h The feed is a clay slurry having a particle size of 5 μm, with ammonium polyacrylate constituting a dispersing agent. The dehydration is continued for nearly 4 hours under the conditions indicated in Table I.

  and there is a relatively constant flow of filtrate (desiccated

dration).

TABLE I

  Time Voltage, V Intensity Depression A mbar

8:20 60 6.0 300

8:55 60 6.0 300

9:25 58 6.6 300

: 05 60 6.0 200

: 35 60 6.0 230

11:10 59 6.2 270

  11:30 11:40 Temperature Flow rate of the filtrate bath 31 C 37 C 37 C C 43 C 1010 ao 3/20 min 1000 cm 3/20 min 6.4

41.5 C

  12:00 Ended The following table II shows pH values, conductivity and percentage of solids measured in the feed solution, the filtrate, the

  bath and cake, during this dehydration operation.

TABLE II

  Sample Time p H Conductivity Materials (m.mho / cm) solids,% feed 8:20 8.5 2700 36.3

filtrate 10:45 12.2 6300 -

  bath 11:15 9,1 2800 35,7 cake 11:30 8,9 * 710 * 75,0 bath 11:40 9.2 2700 36.0

filtrate 11:40 12.2 5950 -

cake 11:40 74.0

anolyte 12:30 1,3 117,000 -

  * the cake is resuspended at 13% sodium

lides, with deionized water.

  The comparison of the conductivity of the bath (2700 to 2800 m mho / cm) during the test with that of the anolyte (117 000 m mho / cm) shows that the membrane of

  cations prevents the introduction of the anolyte into the bath.

  The characteristics given by an anode having a cation exchange membrane ("Nafion") are compared to that of an anode having a known ion-permeable membrane "Dynel" for the dehydration of a suspension

  of 5 gm clay containing ammonium polyacrylate

  dispersant, as shown in Table III.

TABLE III

  Known membrane cation exchange membrane Temperature 25.5 C 41.5 C

Food -

  solids,% by weight 36.3 36.3 conductivity (m mho / cm) 2700 2700 p H 8.5 8.5 Dry Flow (t / h) at 9876 A 2.50 3.40 wet (m 3 / h) 5.38 7.31 Filtrate solids (wt.%) 0.0 0.0 p H 12.5 12.2 flow (m 3 / h) to 9876 A 3.56 4.77 Solid product solids ,% by weight 75 74.5 flow rate (t / h) to 9876 A 2.50 3.40 Bath solids,% by weight 38.0 36, 0 p H 9.4 9.2 Conductivity (m mho / cm ) 2690 2700 Continuous current required product (k Wh / dry t) 172 174 filtrate (k Wh / m 3) 121 124 The results in Table III show that the characteristics of the cation exchange membrane are

  comparable in general to those of known membranes.

  Although the flow rate of solid product obtained with the cation exchange membrane is greater than that obtained with the known membrane, this difference can be attributed to

  in large part unlike the operating temperatures

  However, we observe that it does not pass

  in the electrode, contrary to what happens inva-

  reliably when using known membranes.

  Moreover, contrary to the usual losses

  when using known membranes, the introduction of

  Anolyte production in the cake is low or zero.

  It is understood that the invention has been described and shown only as a preferred example and that

  may provide any technical equivalence in its

  constituent elements without departing from its scope.

Claims (6)

  Electrode for electrically accelerated vacuum filtration of a suspension of solids in a liquid vehicle, characterized in that it comprises a non-conductive frame (21) supporting the walls of an electrode chamber,   a device (28, 29) for circulating an electri-   high conductivity trolyte in the electrode chamber, at least one electrode element (27) placed in the chamber and electrically connected to a voltage source external to the chamber, this electrode element being immersed in the electrolyte, and at least one of the walls (22) comprising a ion exchange membrane.   Electrode according to Claim 1, characterized   in that the solids suspension is a sus-   clay board, and the electrical connection is such that the electrode is anodic.   3 electrode according to one of claims 1 and   2, characterized in that the ion exchange membrane   is a cation exchange membrane.   4 electrode according to one of claims 1 and   2, characterized in that the ion exchange membrane   is formed of a perfluorosulfonic acid polymer.   Electrode according to Claim 1, characterized   in that the connection is such that the electrode is cathodic dique.   6 electrode according to one of claims 1 and   , characterized in that the ion exchange membrane   is an anion exchange membrane.