DK168326B1 - Process for Continuous Separation of Electrically Charged Fine Inorganic Particles in Aqueous Medium by Electrophoresis and Electrosmosis - Google Patents

Abstract

The pulverised solids are in the form of a suspension in an electrophoresis and electroosmosis cell; the process is characterised in that a fraction of the catholyte (13) is removed and then treated (15), part of it being treated with an acidic, preferably gaseous, agent (19), the treated part being reintroduced into the cathode compartment (14) while the other part (18) of the fraction removed is eliminated (17, 18). The acidic treatment (19) is servo-controlled by the continuous measurement of the pH (20) of the fraction removed from the catholyte (13). <IMAGE>

Description

in DK 168326 B1

The present invention relates to a method for continuously separating electrically charged fine inorganic particles in an aqueous medium by electrophoresis and electroosmosis. The invention thus relates to a process 5 for continuous separation using electrophoresis and electroosmosis of electrically charged solid materials present in powder form. It relates to solid materials which, in suspension in an aqueous medium and when subjected to the action of an electric field, behave as electronegative ions, their electronegative nature being either natural or applied using appropriate agents.

Numerous methods developed in the inorganic industry, especially for the treatment of kaolin material, require the concentration of suspensions containing very fine or even colloidal particles. The methods of dewatering generally use classical treatments such as filtration, sedimentation, centrifugation, cyclone treatment, drying under thermal action, and these methods are either too ineffective in that they do not allow a sufficiently dried solid to be obtained. or they incur a prohibitive cost financially. It must be acknowledged that for 25 particles with a diameter not exceeding 10 µm, the electrofiltration is an effective means of concentration, in order to obtain a corresponding final result, it uses an amount of energy equal to only one tenth of that energy. , which is needed in a thermal drying treatment.

The operating principle of electrofiltration is known. The treatment consists in subjecting the suspension containing the solid particles which it is desired to separate from the liquid carrier to the effect of an electric field formed between two electrodes. This presupposes, first, that the suspension to be treated is conductive to the electrical current. If the solid particles have an electronegative character, they will move under the influence of the electric field towards the anode, at which point they will tend to settle. The liquid released by the actual displacement of the solid moves in the opposite direction and thus migrates with the electropositive ions present in the suspension towards the cathode. The offsets in opposite directions of the solid particles 10 and of the liquid produce a separation of the two phases, the solid being concentrated by deposition on the anode.

Obviously, the effectiveness of this treatment is contingent on a certain number of parameters to mention the most important: - the dispersion of the particles in the liquid mass surrounding them, this dispersion being a function of their density and of their electrokinetic potential, also called the Zeta potential, - the mobility of the solid particles, which is a function at one time of the Zeta potential of the electric field applied, and of the viscosity of the liquid in which they move and - the electrical resistance associated with the medium which determines, for a given electric field, the intensity of the circulating current,

The emergence of an electronegative character and the importance of this character, which particularly affects the dispersion of the particles in the liquid phase, may be linked to the use of inorganic or organic water-soluble additives. These additives, which are introduced in very small amounts, fixate by adsorption on the solid particles, thereby imparting to them a more or less pronounced electronegative character and thus promoting their dispersion in the liquid. They can further modify the liquid's own ionic state and thus affect the resistance of the medium.

5

Other parameters related to the concept and to the performance of electrofiltration cells affect the efficiency of the function of these cells. Among these are mention of the shape of the electrodes, their location, their distance and the nature of the materials forming these electrodes.

The problems encountered by those skilled in the art of any electrofiltration treatment are generally those relating to the obtaining of deposits on the anode of solids sufficiently dry to be readily collected and those relating to the removal. of liquid which is too crowded in the concentration of the deposition of solid materials.

Over time, the shape of the anode has been developed to increase the efficiency and collection of these solids. It has been known for many years, in particular, through US Patent No. 1,133,967, the use of rotary anodes with drum shape, which made it possible at the same time to continuously perform the deposition and removal of the cake of solids. The drum, whose shaft is horizontal, is immersed in half in a bath containing the suspension to be concentrated. The solids are collected continuously by scraping by means of a suitable device (knife, thread, iron) on the surface of the anode as it exits the suspension due to its rotation.

It is also known (US Patent Nos. 3,972,799,35 and 4,107,026 and French Patent Nos. 2,552,096) the use of anodes in the form of discs, which significantly increase the active surface of one and 4 DKs. 168326 B1 the same volume of the apparatus. These anodes are placed vertically on a horizontal arm and are halfway immersed in a container which is fed to the suspension to be treated. Between each anode and at a constant interval, partitions which are inseparably connected to the container and which are electrically insulated for the latter and connected to the negative pole of a power source are fixed.

The person skilled in the art has been led by the oxidation phenomena which take place on the anode upon oxygen release and which may cause the anode to break down through corrosion: 15 - either to protect the anode by placing a semiconductor in its immediate vicinity. permeable membrane defining a low-volume anodic chamber on which the cake of the solids is deposited so as to avoid contamination of the deposited solids from the anode's corrosion products (U.S. Patent No. 4,048,038 and Supplementary Publication No. 2,423,257 to French Patent No. 2,354,802), or to use non-attackable anodes and 25 which consist of noble materials (e.g. tantalum), or of materials covered by electro-deposition of these nobles materials (platinum titanium) or of metal oxides that are insensitive to corrosion.

However, a difficulty encountered by one skilled in the art is related to the continuous removal of the water released by the migration and electrodeposition of the solids to the anode while maintaining a constant concentration in the suspension. located in the electro-separation cell.

The first cells did not include semi-permeable membranes, which allowed an anode chamber and a cathode chamber to be delimited.

According to U.S. Patent No. 1,132,967, the 5 cathodes used consisted of metal elements in the form of rods or plates which formed spaces for the passage of the liquid phase. Other inventors used as an electrode a metal tissue rolled up over a drum constituting the cathode (US Patent No. 1,335,886). The anode made using a single-piece roll-mounted band came into contact with the solid-state cake deposited on the drum whose curvature it rested. Obtaining a concentrated solid deposit by continuously removing liquid in the belt was preferred, either by applying a contact pressure between the belt mounted over rollers and the drum, or by bringing the drum under vacuum. Thus, in order to increase dehydration efficiency, an extraction of the liquid by electroosmosis and by filtration was combined either under the influence of an external pressure applied or under the influence of a vacuum.

In continuation of this, the search for better efficiency has led the skilled artisan to use semi-permeable membranes to form two clearly separated chambers, an anode chamber and a cathode chamber. The roles of the semi-permeable membranes are to allow the liquid to pass while remaining impermeable to the passage of the solids.

As the suspension to be treated is introduced into the anode chamber and as the electronegative ions move toward the anode, the membrane plays a role as a filtering medium relative to the cathode. As the liquid, whose volume corresponds to the volume of the solids, moves towards the anode and is displaced under the action of so-called Drag forces in the opposite direction towards the cathode, it flows through the filtering medium and thus passes into the cathode chamber. The passage of this liquid through the medium is dependent on a number of factors which, on the one hand, are linked to the actual composition of this medium (the nature of the material, the porosity, the permeability, etc.) / and on the other hand. to the liquid medium (the viscosity of the liquid that plays the role of electrolyte and in which water constitutes the main constituent).

The passage of the fluid through the medium in the direction of the cathode chamber can be facilitated, as previously indicated, by bringing this chamber under a partial vacuum (US Patent No. 4,003,849), which is equivalent to increasing the moving pressure of the osmotic flow through the medium. The regulation of the extraction of liquid by regulating the level of the catholyte is disclosed in U.S. Patent No. 4,107,026. It is known that the amount of water circulating through the medium is bound to the loss of charge of the flow. Since the intended purpose is for the solid particles to move in the direction of the anode, there is a value for the electric field referred to as "the critical value" beyond which the electric force exerted on the particle is equal to the so-called Drag Kraft. Thus, the intensity of the applied electric field should be greater than this "critical value" to avoid the progressive accumulation on the cathodic filter medium of the smallest particles entrained by the displacement of the liquid. This accumulation, which acts against the flow through the medium, makes the passage of the aqueous phase more difficult.

The entry of water into the cathode chamber upon passage through the medium has the effect of causing a dilution of the electrolyte (which is still called a catholyte) and present in this chamber. The need to maintain a certain concentration level in the catalytic converter to ensure the passage of the electrical current from the cathode towards the suspension to be treated has led some researchers to remove diluted electrolyte in order to renew it using direct removal into the cat-i-r-1 ^ - ---- 7 DK 168326 B1 decoder.

French Patent Specification No. 2,354,802 and Additional Patent Specification No. 2,423,254 disclose a regulation of the amount of liquid extracted from the cathode chamber. The extraction of the aqueous phase from the cathode chamber is controlled by continuously measuring the liquid level of the catholyte, either independently or simultaneously affecting the following two parameters: the negative pressure prevailing over the liquid level in the cathode chamber, and the electrical current density circulating through the electro-separation cell.

Indeed, the passage of fluid through the cathodic filtration medium is facilitated by permanently maintaining a vacuum above the catholyte level. The partial vacuum over the cathode is controlled by a vacuum pump which is controlled by the measurement of the vacuum. The formation of a light deposit of fine solid particles on the filtration medium or the collection of these particles in its immediate vicinity can be further accelerated or delayed by modifying the intensity of the electric field and thus the current density. An increase in current density induces a corresponding increase in the migration rate of the solid particles towards the anode. Consequently, a decrease in current density will lead to the formation of a deposition of particles on the filtration medium which acts against the passage of the liquid. Thus, while simultaneously working with these two parameters (the negative pressure prevailing above the level and current density of the catholyte), the skilled artisan has achieved an equilibrium function of the cell for a given amount of extraction filtrate from the cathode chamber.

35 Experience, however, has shown that over a period of time after a certain number of hours of operation, a regulation that includes only the above two only parameters is found to be very insufficient to maintain an efficient function of the electro-separation cell, since at the same time observes a progressive decrease in deposits on the anode, a decrease in the amount of filtrate flowing through the cathodic medium, and an increase in pH of the catholyte approaching very high alkaline values.

Those skilled in the art have used the introduction of acidic additives into the cathode chamber to increase the efficiency of the separation of solids. Thus, in certain previously mentioned U.S. patents (Nos. 3,980,547, 4,003,811, and 4,048,038), the use for this purpose of inorganic acids such as hydrochloric acid, sulfuric acid or phosphoric acid is mentioned. According to these patents, the pH of the catholyte is forced to be between 2 and 7, with the concentration of the acid solution supplied ranging from 0.1 to 10% by weight (US Patent No. 3,980,547) or from 0.1 to 1% by weight. -% (US 4,003,811 and 4,048,038).

20

As the aqueous phase emanating from the anode compartment penetrates the membrane of the cathode chamber by electroosmosis, the resulting dilution of the catholyte causes a change in its electrical characteristics (especially resistance). In order for the electric current transport to be maintained at an effective level, it is necessary that the properties of the catholyte and, as a result, its composition, do not vary beyond certain limits. In order to extract the aqueous phase as indicated above and to maintain a permanent current passage of known intensity values, one skilled in the art has been led (U.S. Patent Nos. 3,980,547, 4,003,811 and 4,048,038) to ensure a continuous open circulation of the catholyte through the cathode chamber. Fresh catholyte is continuously introduced directly into the cathode chamber and the spent catholyte is extracted from this same chamber. As previously stated, an acidic solution is added to the fresh catholyte in a manner such that the pH of the catholyte in the cathode chamber is continuously maintained within a value range of 2 to 7.

According to other embodiments of the prior art, certain electroseparation devices further comprise another semi-permeable membrane located around the anode defining a dense space surrounding the anode chamber. The role played by this membrane (which also constitutes a filter medium) on which the solid 10 particles deposit by electrophoresis is to avoid contamination of this deposit with corrosion products from the anode (U.S. Patent No. 4,048,038).

The electrolyte present in the anode compartment, referred to as an anolyte, contains inorganic salts, some of which are dissociated under the action of electrochemical reactions located to the immediate vicinity of the anode. This change in the electrical properties resulting therefrom, just as is the case for the catholytes, necessitates a renewal of these anolytes in order to maintain their composition in the vicinity of constant values.

French supplementary patent specification no. 2 423 254 discloses 25 the presence of devices which make it possible to secure this renewal. The construction comprises an apparatus system located in a closed loop of the anode chamber.

The withdrawn anolyte first passes into a degassing chamber, in which separation is effected between the liquid phase and the gases formed by the electrochemical reactions associated with the decomposition of the electrolyte in contact with the anode. The liquid phase flows with gravity to a buffer vessel before being returned to the anode chamber by means of a pump. The spent anolyte is taken directly from this buffer vessel. Similarly, the fresh anolyte contained in an intermediate storage reservoir is introduced at a circuit point before the buffer vessel. However, if the withdrawal of spent electrolyte is effected directly through a fixed overflow threshold of the buffer vessel flowing through the recycled anolyte flow towards the anode chamber, however, as described, the device can only function effectively if the supply of new anolyte to the circuit and the extraction of used anolyte are carried out independently and in a discontinuous manner. As a result, the composition of the electrolyte in closed circulation over the anode cannot hold constant, as described in the aforementioned supplementary patent, and it varies between limit values which have not been specified.

Since the anolyte is a solution of NaCl, 15 is further formed by electrolytic reaction chlorine which is released at the anode. This chlorine, as stated in French Appendix Patent Specification No. 2 423 254, should be removed or possibly reinserted into the cathode chamber. However, the handling of gaseous chlorine and the reintroduction into this chamber exhibit very serious drawbacks. In the case where the introduction of the chlorine takes place in an atmosphere of the hydrogen which is released into the cathode chamber, an explosive mixture may result. In the case of direct injection of chlorine into the catholyte, this injection leads to the formation of low dissociation chlorine sub-acid, which does not counteract the gradual evolution of the electrolyte towards strongly alkaline pH values. The hypochlorites formed in the catholyte are further aggressive compounds which cause destruction of the cathode by corrosion and contamination of the catalyst, and therefore the fraction withdrawn cannot be discarded in the natural environment.

Independently of the solutions described in the prior art for the treatment of the catholyte, i.e. injection of chlorine or introduction of solution of inorganic acids (HCl, SO2, H2 PO2), this catholyte thus contains foreign elements such as sulfur, chlorine and phosphorus - · - - - --- - 11 DK 168326 B1 phor, which was not originally present in the suspension to be treated. The fraction of the catholyte withdrawn from the cathode chamber constitutes a waste stream which, prior to each discarding, requires appropriate treatment in order to satisfy the anti-pollution regulations only.

On the other hand, it is scientifically established that the volume of the aqueous phase removed corresponds to the volume of the aqueous phase displaced by electromosis over the cake of solid particles of bed right on the anode; and for this reason, a continuous function of electroseparation without interruptions at once necessitates perfect regularity with regard to the passage of this aqueous phase through the filtration medium and a high regularity with respect to its removal.

Further research has now shown the importance of the role played by hydroxyl ions (-0H) formed near the cathode in the electrolysis reaction. These 20 ions moving towards the anode encounter during their displacement the barrier formed by the cathodic filter medium and counteract the passage of the aqueous phase in the opposite direction through this medium.

Thus, the accumulation in the cathode compartment of hydroxyl ions adversely affects the function of the cell, constituting an obstacle to the passage of the aqueous phase towards the cathode and, as a result, interfering with the concentration by electroosmosis of the solid particles which are deposited on the anode. This accumulation induces a modification of the pH of the catholyte, which develops, if no precaution is taken, towards stronger alkaline values. As a result, after a certain number of hours of operation, it causes a very disturbed function of the cell with a risk of blocking, regardless of the control values used for the other parameters affecting the electro-saving ring (the electric field). , current density, media resistance, suppressed cathode space, etc.)

Thus, the present invention relates to a continuous process for separation by electrophoresis and electrosmosis of electronically charged inorganic particles in suspension in an aqueous medium, wherein the process of treating the catholyte is performed continuously by an agent which, while modifying between the normally present types of ions ensures an efficient electroseparation of solids by continuous and regular formation of a deposition of particles on the anode which increases the efficiency of the osmotic passage of the aqueous phase through the cathodic filter medium making it is possible to maintain the pH of the catholyte within a range of values close to the neutralization point, and which does not introduce into the catholyte any foreign element which modifies the composition of that catholyte and is not already present in the suspension to be treated; and which therefore gives it for the purpose of removal making a water phase in excess of the cathode chamber's aqueous phase non-polluting.

The present invention thus relates to a method of the kind referred to in the preamble of claim 1 which is characterized by the characterizing part of claim 1.

30 Numerous experiments have now demonstrated the importance of the yield of the water extraction, which is defined as the ratio of, on the one hand, the amount of water flowing across the filtration medium and being definitively removed, and on the other hand the theoretical amount of water QT. which is displaced by electroosmosis during the formation of the solid particle deposition on the anode, the magnitudes referring to a unit of time. Experience 13 DK 168326 B1 shows that a condition which is absolutely necessary for a continuous and regular functioning of the electroseparation cell is that this yield is at least equal to 1, that it is desirably greater than 1 and that it is preferably 5 say in a value range between 1.01 and 1.50.

It is thus possible, knowing the dry matter content of the suspension to be treated and determining the dry matter content of the cake to be recovered at the anode, to determine the amount of QT per time unit of liquid displaced by electroosmosis in the cake, thus determining the value of the amount of liquid per minute. time unit that definitely needs to be removed.

Thus, when the fraction Q1 is completely defined, in the cathodic compartment, an amount of Q time unit which is greater than 2.5 QT. This unit is then divided into two quantities and Q2, the quantity Q1 being definitively removed while the quantity Q2 is intended to be processed.

The fraction Q2 intended to be reintroduced into the cathodic compartment after treatment is regulated in such a way that the ratio Q2 / QT is at least equal to 1.5, desirably at least equal to 2.5, and preferably between 4 and 8 to promote the diffusion and mixing of Q2 in the interior of the catholyte located in the cathode compartment.

At the same time, the pH of the fraction is measured and the measured value is compared with a determined value. Comparison of the two pH values makes it possible to control the introduction of CO 2 gas into the Q2 fraction in such a way that in the cathodic compartment a pH value close to or equal to the pH desired for the fraction is obtained. value.

The pH of the fraction Q 1 to be removed is determined by the user in a general way and allows the value of pH to be defined as the determined value. The pH of this fraction Q1 is preferably selected and to satisfy the anti-pollution standards in the range of 6.5 to 8.

In order to promote the passage of fraction through the filtration medium, a vacuum is established in the known manner in the cathodic compartment. This negative pressure is generally _310 set to a value at least equal to 6.5x10

Pascal (corresponding to 50 mm mercury column) and preferably -3 -3 to values between 13 x 10 Pascal and 47 x 10 Pascal (corresponding to 100-350 mm mercury column).

The electrical characteristics of the function of the electro-separating cell are in a good embodiment of the method according to the invention as follows: - the intensity of the electric field between the two electrodes is selected in the range 1-25 volts x cm ~ \ preferably in the range 5 -15 volts x cm 2 and - the electrical current density is regulated in the range 1-20 _2 milliamps x cm, preferably between 5 and 16 milli - 2 25 amps x cm

According to the invention, the suspension to be treated, which contains fine inorganic particles of electronegative nature obtained naturally or artificially, generally exhibits an electrical resistance in the range 250-2500 Ω x cm, preferably between 500 and 1800 D x cm. to allow the passage of a current of sufficient intensity, while limiting the consumption of electrical energy.

The present invention will be better understood by the description referring to enclosed Scheme 35 (Figure 1) illustrating the method by way of example.

In accordance with Figure 1, this device in use consists of an electro-separation cell comprising a container (1) provided with a piping for approach (2) which is itself connected to a storage container (3) which contains the suspension to be processed and a dispenser (4) connected to a receiving container (5) such that the cell operates at a constant level of the suspension and excess supply of material ends automatically in the receiving container (5). ). In the interior of the original container (1) is arranged one or more series of cathode electrodes and anode electrodes arranged alternately with regular spacings between each other and whose flat surfaces are parallel to each other. The anodes (6), which are connected to the positive pole of an external power source (7), may: - be either rectangular in shape and it is then possible to displace them in a vertical plane in a downward and upwardly parallel movement by themselves, so that they can leave the electrolyte vessel in which they are immersed to collect the cake of solids deposited during the course of the electrolysis, or they may preferably be in the form of circular discs which are moved with a rotating motion around their horizontal shaft? and in this case, the removal of the cake of solids is performed continuously on the portion of the disc emerging from the suspension using any means (knife, thread, iron knife, etc.), as will be well known to those skilled in the art. .

These electrodes are made of precious metal, such as titanium, coated with electro-alloy with a thin layer of platinum or any other metal whose metal oxide is resistant to corrosion by the gases formed at the anode. , or of the H + ions associated with the gaseous effluent during the electrochemical reaction.

The cathodes (8) consist of solid members and, in accordance with one embodiment of the present invention, may be in the form of semi-circular sectors of stainless steel wire tissue which are completely immersed in the suspension and which are associated with the negative surface. 10 t pole of the current generator (7). Each cathode is provided with a filtering medium (9) defining an enclosed volume (10) referred to as the cathode compartment and containing the catholyte.

The cathodic compartment is: - connected, on the one hand, by a conduit (11), which in its upper part opens by a vacuum pump (12), which enables the negative pressure above the level of the catholyte 20 to be regulated around a predetermined value. a function of the actual measurement (21) of this suppression in the cathodic compartment, - on the other hand connected by wires (13) 25 and (14), which culminate in the compartment below the catholyte level, with a device (15) which ensures the continuous and regular injection of the treating agent (carbon dioxide CO2) into the closed loop constituted by these two lines, the injection device (15) and the cathodic compartment (10).

Continuous circulation of the catholyte is ensured by means of a pump (16) in the loop thus defined. Injection of the treating agent is carried out by any means known to those skilled in the art, which makes it possible to achieve effective contact between the agent and the liquid. The device should be adapted as a function of the agent used and of the flow rate Q2 of the circulating catholyte.

In the case where the treating agent is carbon dioxide 5 (CO 2) and for low capacity apparatus, the CO2 injector may be a sintered glass disc. For larger liquids and gas volumes, it may be recommended to use devices which will be known to those of ordinary skill in chemical engineering, such as filled columns or dispersion columns. The rate of treating agent is controlled as a function of a measurement (20) of the pH of the catholyte which is continuously applied to the catholyte extracted from the cathodic compartment. The measurement of this pH is compared to a determined value, after which an automatic valve (19) which allows the speed of the agent introduced into the loop to be modified to permanently maintain a pH value in the cathode compartment at values close to the value chosen by the user for pH.

20

An amount of Qg of the catholyte calculated per time unit from the compartment (10) by the pipeline (13). This volume is set to a value much greater than QT. On the other hand, volume Q1 is taken out of the total amount of catholyte flowing out of the compartment (10) and evacuated under the circuit by the valve (17 ) and the pipeline (18) in the case where the treating agent is CO2 and where the removed fraction constitutes a waste stream in which the pH is continuously controlled and the content of chemical constituents complies with official anti-pollution regulations, only the difference in quantity is treated. Q3-Q1, ie Q2 in the device (15) for introducing the agent and this is fed to the lower part of the cathode compartment 35 (10) by means of the pump (16) and of the return line (14).

In the case where the treating agent is CO 2, the use of a suitable device (15) for injecting it allows a homogeneous dispersion of the gas in the liquid fraction ζ ^. The return to continuous circulation of this fraction in the lower portion of the cathode compartment as well as the extraction of the fraction at the top of this compartment ensures a complete diffusion of the catholyte which is processed within the catholyte contained throughout the height of the cathode compartment. The rate of circulation Q2 is such that it allows an absorption of gaseous CO2 to be produced in sufficient quantity 10 to neutralize the hydroxyl ions formed in the cathode compartment and to maintain in this compartment a pH value e.g. . not more than 8. Taking into account the low alkalinity of the catholyte withdrawn from the cathodic compartment and, consequently, the very low solubility of CC> 2 in the catholyte, the ratio of the mass velocities of the circulating liquid to for the gaseous CO 2 be very large to ensure the required absorption of gaseous CO 2.

The process of the invention as described herein can be used to treat any suspension containing fine particles of electronegative solid materials for separating these particles from the liquid phase where they are maintained in suspension. Such suspensions may contain as diverse materials as carbohydrate, kaolin, silicates, titanium oxides, alumina, calcium phosphates, gypsum, etc. This concentration of the solids content of the cake obtained by electrospraying, for example, can reach at values as high as 75% for 30 concentrations of the suspension subjected to the treatment comprising the range of 20-50% by weight.

In order to more precisely illustrate the possibilities provided by the invention, two examples are given below, one of which relates to the electro-separation of a suspension of calcium carbonate and the other to the electro-separation of a suspension of kaolins.

EXAMPLE 1

A suspension of calcium carbonate (CaCO 3) with a solids concentration equal to 49.6% is used, in which an organic polymer (sodium polyacrylate) has been added in concentrations of 0.3-0.4% by weight expressed as dry matter. ratio of calcium carbonate. This polymer, which is adsorbed on the calcium carbonate particles, has the property of negatively charging the solid particles. The specific resistance of the aqueous phase is close to 600 Ω cm.

The electric field applied between the electrodes is 11.7 volts x cm-1, which, taking into account a distance of 6 cm between the electrodes, corresponds to a total power of 70 volts. The current density is in the vicinity of 16 mil-2 lamps x cm. The negative pressure applied above the level 3 of the liquid in the cathode compartment is 26 x 10 Pascal (i.e., about 200 mm of mercury column).

20

For a feed rate of suspension to the electrospraying cell equal to 765 kg xh ~ and for a collected mass of 506 kg x h- ^ filter cake in which the solids concentration of CaCOg is 75%, the specific amount of Q is of liquid removed from the cell and wasted from the · * · -1 cathode compartment 263 1 xh. The theoretical amount of water displaced by electroosmosis during the deposition of the solid materials on the anode is 259 1 x h. Thus, the extraction yield with respect to water is in the vicinity of 30 of 1,015. The total amount of Q_ extracted from the -1 cathode compartment is 1763 1 x h. The amount of catholyte Q0 recycled to the cathode compartment is 1500 1 x h corresponding to a ratio Q2 / QT equal to 5.79. The pH of the extracted catholyte is 7.9. The injection of CO2 is regulated such that, taking into account a fluid flow in recirculation Q ^ of 1500 1 x h ~ \, the pH of this fraction Q ^ can be lowered to 7.3. The amount of gaseous CO2 is less than 0.4 kg x h- *, which corresponds to a consumption of CO 2 quite a bit more than one gram (ie 0.51 1 under the normal pressure and temperature conditions). per. kg of CaCO

5

The wastewater extracted from the cell has the following chemical composition under the operating conditions described above: 10 Na + 1.2 g x l "1 HCOg- 3.61 ST x 1_1

Ca ++ 34.5 mg x 1 ^ DCO 80 mg x 1 ^

Mg ++ 0.7 mg x 1 ^ pH of (as previously indicated) is 7.1. Due to the use of a filtering medium defining the cathode compartment, in the form of a thin pore beam membrane and made of polyvinyl chloride, the solids content present in the waste stream is less than 50, -1 mg x 1

Chemical composition, pH and solids content are such that the liquid effluent has a non-contaminant nature and can be discarded arbitrarily without supplementary treatment intended to bring it into compliance with the anti-pollution wastewater standards.

EXAMPLE 2 This example relates to the separation of kaolin particles present in an aqueous suspension with a concentration of solids equal to 24.6% introducing a dispersant consisting of a copolymer of sodium acrylate and of acrylamide at a concentration of 0.6 35% expressed as wt% in dry state over dry state relative to kaolin.

21 DK 168326 B1

The apparatus used is the same as that used in Example 1, although there is a distance between the electrodes which has been reduced to 5 cm instead of 6 cm. The voltage across the current generator terminals is in the vicinity of 56 volts, which corresponds to an electric field of the order of 11.2 volts x cm-1. The current density is maintained at 8 milliamps x cm. In addition, suppression above the catholyte level in the cathode compartment was 13 x 3 10 Pascals (i.e., about 100 mm of mercury column). Under the 10 thus-defined operating conditions and at a rate of supply to the cell of suspension in the order of 489 kg xh, the amount of moist kaolins deposited on the anode was equal to 261 kg xh, the concentration of these kaolins with respect to dry matter was 46.9 %. The amount of 15 Q- of liquid extracted from the cathode compartment and definitively 1 -1 wasted was 300 1 x h for a theoretical quantity of water Q «· * · displaced by electroosmosis of 228 1 x h. Thus, the extraction yield with respect to water was equal to 1.315. The amount of CO 2 injected into fraction Q2 of catholyte 20 while recirculating over the cathodic compartment was adjusted so that the pH of the cathodic compartment varied within a range ranging from 6.7 to 7.3.

The aqueous fraction Q 2, which was extracted from the cathodic compartment and in which the eliminated portion constitutes a waste stream, had a pH value very close to neutral. Moreover, the content of the elements calcium and sodium in this waste stream was as follows:

Na +: 700 mg x in "*" * · and Ca 2+: 21 mg x l ”1 DCO was between 20 and 40 mg x.

As was the case in Example 1, the amount of solids in suspension in the waste stream was lower than 35 50 mg x 1. EXAMPLE 3 - Comparative Example

The role played by CO 2 is shown in the following example, in which the same suspension of kaolin 5 (in English: "China Clay") is treated during the same time period of the experiment in a laboratory cell, successively in the presence and then without the presence of an injection of gaseous CO 2 · 10 The operating conditions of the cell were as follows:

Electric field 11.5 volts x cm ~ ^

Distance between the electrodes 5 cm _2

Current density 8 milliamps x cm 15

Suppression above the catalytic level in the cathode compartment: 3 13 x 10 Pascal (i.e., about 100 mm of mercury column).

The treated suspension of kaolin had an initial solids concentration of 25.9%. A dispersant consisting of a copolymer of sodium acrylate and acrylamide having a concentration of 0.6% by weight expressed as dry matter versus dry matter relative to kaolin was introduced into this suspension.

25

Two tests were carried out, one without the presence of CO 2 and the other according to the invention with an injection of CO 2 regulated in such a way that a pH value of 8 was maintained in the cathode compartment; these 30 two trials each had a duration of 12 hours.

In Table I below is collected for each experiment in the presence of or without the presence of gaseous CO 2 and for respectively. the first, third, sixth, and twelfth hours of operation of the cell values: - the amounts of solids expressed as dry matter phrase pared on the anode, - the amounts of water extracted from the cathode compartment, expressed in liters per liter. water, 5 - the yields for water (Q1 / QT).

10 15 20 25 30 35 DK 168326 B1 24 (l- co ω ^ x) 2 N ^ w 44 3 y. -> x C C3 - · - ~ - xi ro \

3> r4 3 CJ

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Claims (9)

25 DK 168326 B1 It is of interest to note from this table that in the absence of injections C02, the yield of water in the cell as well as the amount of urogenic material deposited on the anode increases with the passage of time, the loss of productivity 5 being on the order of 13% of 12 hours, ie a little more than one percentage point per while in the presence of an injection of CO 2 the yield is water and the amount of inorganic material deposited on the anode remains constant with time. 10 A method for continuously separating electrically charged fine inorganic particles in suspension in an aqueous medium by electrophoresis and electroosmosis comprising: - continuously introducing the suspension of the solid 20 having an electronegative character into an electro-separation cell provided with an anode and a cathode between which an electric field is maintained, which cathode is provided with a filtration medium permissible for the only liquid phase, thereby defining a cathode compartment containing a catholyte, while the space between the filtering medium and the anode constitutes the treatment zone for the suspension from which the excess of the suspension is discharged by a suitable device; 30. the displacement of the solid particles by the action of an electric field; deposition thereof in the form of a cake on the anode surface and discharge of the cake outside the treatment zone of the suspension; opposite direction towards the cathode section of the liquid phase and filtration thereof through the filtration medium under the influence of a vacuum and removal thereof from said compartment, characterized in that in order to continuously and simultaneously carry out a regular deposition of the solid phase on the anode and a regular extraction of the liquid phase with a water yield at least equal to 1, this yield being defined as the ratio of the amount of water Q 2 flowing through the cathodic filtration medium to the theoretical amount of water Q . a) takes in the cathode compartment an amount of Q3 catholyte per b) separates the quantity into two fractions and Q2, is greater than QT and Q2 is greater than 1.5 QT, and removes Q1 'c) continuously processes the fraction Q2 into a processing device and d) continuously re-injecting the treated fraction Q2 into the cathode compartment to modify in the catholyte the ratio of ionic types present, thereby making the catholyte suitable for a discharge into nature. Process according to Claim 1, characterized in that the ratio Q 1 of Q 1 is between 1.01 and 1.5. 25 Method according to claim 1 or 2, characterized in that the ratio Q2 / QT is between 4.0 and 8.0. Process according to any one of claims 1-3, characterized in that the pH is continuously measured in the extracted fraction Q 2 and that the introduction of CO 2 gas is controlled by the measured pH and the chosen pH. A method according to any one of claims 1-4, characterized in that a partial vacuum of at least 6.5 x 10 Pasc Pascal is carried out in the cathode compartment, preferably between 13 x 10 og and 47 x 10 Pasc Pascal. . DK 168326 Pg 27 Process according to any one of claims 1-5, characterized in that, by introducing a cationic agent, the specific resistance of the introduced suspension is controlled between 250 and 2500 Ω x cm, preferably between 5 500 and 1800 Ω x cm. Method according to any one of claims 1-6, characterized in that the electric field between the two electrodes has a value between 1 and 25 volts x cm, preferably between 5 and 15 volts x cm. Method according to any one of claims 1-7, characterized in that the current density is between 1 and 20 -2 -2 milliamps x cm, preferably 5-16 milliamps x cm Process according to any one of claims 1-8, characterized in that the ratio Qg / QT is greater than 5 and equal to or less than 9.5. 20 25 30 35