GB2154224A - Clay dewatering process - Google Patents

Abstract

A two-stage method for dewatering aqueous, low-solids kaolin clay slurries comprises carrying out initial dewatering of the slurry on a mechanical filter to produce a wet filter cake of no more than 57% by weight solids, which is a lower than usual solids content. Discharge of the wet filter cake from the filter cloth is facilitated by use of a suitable organic polymeric flocculating agent, preferably an aminoplast resin. The wet filter cake is then dispersed by addition of a suitable deflocculant and is filtered by an electrofilter to form a concentrated slurry of about 65 to 80% by weight solids. The electrofilter is preferably an electrically augmented vacuum filter having perm-selective membrane-equipped anodes to control or prevent anion and gaseous vacuum product contamination of the filter cake which provides the concentrated slurry. <IMAGE>

Description

SPECIFICATION Clay dewatering process The present invention is concerned with dewatering refined clays and, more specifically, is concerned with a two-step dewatering process. Generally, in the manufacture of materials such as calcined clay for use as pigments, coatings, fillers and the like, the crude clay is refined by being "wet processed" to remove grit and other impurities and sometimes to carry out other operations such as delaminating.

Wet processing typically includes forming an aqueous suspension of the clay for which purpose dispersing chemicals such as sodium silicate, sodium hexametaphosphate, tetrasodium pyrophosphate or the like may be employed, and treating the resultant clay slurry by blunging and degritting it to remove undesirable foreign matter and recover a refined clay fraction. Centrifugation of the dispersed, i.e., chemically deflocculated, aqueous suspension of degritted clay may also be carried out to segregate a fraction of desired particle size range and the clay suspension or slurry may also be bleached to enhance its brightness.Since the wet processing of crude hydrated kaolin ores results in very dilute or low-solids slurries which often contain less than 25% by weight solids, conventionally, the kaolin slurry is thickened to approximately 25% solids before being bleached or otherwise further treated.

The clay slurry is typically obtained from the bleaching or other treatment at a solids content of less than 50% by weight solids, for example, from about 25 to 40% by weight solids. In recent years there has been an increased demand for hydrated (uncalcined) kaolin clay in the form of a slurry of about 70% by weight solids. The market also demands pulverulent clay materials, for example, calcined kaolin clay pigment. It is therefore necessary to dewater or dewater and thermally dry the low-solids slurry obtained from the wet processing in order to produce a salable product. Dewatering of the slurry, which may contain a flocculant, is typically carried out on a suitable filter, such as a rotary vacuum filter, to produce a partially dewatered slurry of about 58 to 60% by weight solids.The partially dewatered slurry may then be thermally dried to produce a pulverulent product which may be used as is or mixed with additional dewatered slurry to produce, e.g., a 70% solids product.

Dewatering of low-solids clay slurries to produce high-solids slurry or pulverulent clay products, such as calcined clay pigments, accordingly requires the removal of large quantities of water from a wet processed clay, which is usually comprised of extremely fine particles. The task is, therefore, a difficult, expensive and energy-consuming one. Conventionally, mechanical filters such as rotary vacuum filters or plate and frame filters are employed for the initial dewatering, and the product may thereafter be spray dried by being injected into direct contact with a stream of hot gases.

U.S. Patent 4,246,039 (Mixon), assigned to the assignee of this application, discloses in the manufacture of white calcined kaolin clay pigments from dilute dispersed aqueous suspensions of wet processed hydrated kaolin clay, the utilization of an electrofilter such as the electrically augmented vacuum filter described in U.S. Patent 4,107,026 (Freeman). In the Freeman patent the electrofilter is utilized to prepare, from a dilute aqueous clay suspension of, e.g., 45% by weight solids, a spraydryable slurry of about 60% by weight solids.The Freeman patent, for example, discloses at column 20, lines 23-46, a process in which a flocculated clay slurry is dewatered on a rotary vaccum filter and a dispersing agent is added to the discharged filter cake to provide a dispersed slurry of about 60% solids concentration, which is then dewatered on an electrofilterto provide a 70% solids colloidal clay suspension. U.S. Patents 4,170,529 (Freeman) and 4,168,222 (Freeman) issued on applications related to the application which matured into the 4,107,026 patent.

U.S. Patent 4,312,729 (Wills) discloses improvements in electrically augmented vacuum filter apparatus such as that disclosed in the aforesaid Freeman patents, and in a process of dewatering a suspension of particulate solids such as hydrous kaolin clay wherein a perm-selective membrane is associated with the hollow anodic structure of the electrically augmented vacuum filter. The perm-selective membrane is so associated with the anode as to permit passage of cations from and through the anolyte (the liquid contained within the anode) through the perm-selective membrane and into filter cake deposited on the exterior of the anodic structure. The cations pass through the filter cake and into the suspension of solids being filtered.At the same time, the perm-selective membrane substantially prevents the passage of anions and gaseous electro-chemical reaction products from the anolyte and into filter cake and the slurry being dewatered. As explained below, preventing the escape from the hollow anode compartment and into the slurry of anions and gaseous electro-chemical reaction products is particularly advantageous in the dewatering of clay slurries, as such anions and products contaminate the slurry being processed.

The following U.S. Patents are also of interest. An electro-osmotic/phoretic process for concentrating clay is disclosed in U.S. Patent 4,110,189 to Kunkle et al. An electroflocculation cell is disclosed in U.S.

Patent 4,048,038 to Kunkle, and an electrokinetic cell is disclosed in US Patent 3,980,547 to Kunkle.

U.S. Patent 3,753,902 (Stettler) discloses the use of certain water-soluble nitrogen-containing compounds as filter aids, more specifically the use of amino group-containing compounds in conjunction with cation-active polyamines and/or polyamide amines. Among the amino group-containing compounds are the condensation product of formaldehyde and certain amines and the condensation product of an aliphatic amine with cyanamide, dicyandiarnide, dicyandiamidine or guanidine. These compounds are stated to be useful for producing a water suspended swellable mass of inorganic materials of improved filterability, particularly for crude cement slurries but including silicates and silicoaluminates, e.g., bentonite, montmorillonite and kaolinite, among others.The cationic polyamines and polyamide amines, if used singly, are stated to be effective only if the solids content of the slurry is not in excess of 25% whereas, when used in conjunction with guanidine derivatives, effective filter aid action is shown for suspensions and slurries of up to 65% solids.

U.S. Patent 2,981,630 (Rowland) discloses the addition of a hydrophilic colloid, e.g., a urea formaldehyde resin, prior to filtering a deflocculated (dispersed) pulp of a heterogeneous mineral mixture to carry out selective flocculation with the hydrophilic colloids to separate constituents of the mixture. The patent is especially directed to the separation of crude kaolin clays into clay fractions of different viscosity properties. The use of other polymeric flocculants, such as polyethylene oxide polym#ers, as a filter aid is also known.

Generally, the present invention provides for a two-stage clay slurry dewatering process, the first stage of which comprises a mechanical filter and the second stage of which comprises an electro-filter, preferably, an electrically augmented vacuum filter.

The mechanical filter is operated at a high rate to filter a slurry containing an organic polymeric flocculating agent to produce a wet filter cake of good water-permeability from a slurry of low-solids content. The discharge of the resultant wet filter cake from the filter medium is facilitated by the use of the polymeric flocculating agent. As used herein and in the claims, "filter medium" means any suitable filter cloth, screen or the like used in a mechanical or vacuum-assisted mechanical filter to collect the filter cake. The wet filter cake discharged from the mechnical filter is deflocculated, i.e., dispersed, by addition of a dispersant, and is further dewatered by an electrofilter, the combination of steps providing a highly efficient dewatering process.

In accordance with the present invention, there is provided a method for dewatering an aqueous, low-solids clay slurry, for example, a kaolin clay slurry, comprising the following steps: adding an organic polymericflocculating agent to the lowsolids slurry in an amount as defined below; filtering the low-solids slurry on a filter medium to produce a wet filter cake containing not more than about 57%, preferably about 50 to 57%, by weight solids; discharging the wet filter cake from the filter medium, the amount of flocculating agent added to the low-solids slurry being at least sufficient 15 facilitate discharge of the wet filter cake from the filter medium, and preferably, comprising at least about 0.02%, more preferably, about 0.3 to 15%, still more preferably about 0.02 to 2% by weight of dry clay solids; adding a dispersant to the discharged wet filter cake and forming therefrom a dispersed slurry which contains not more than about 57% by weight solids; and filtering the dispersed slurry on an electrofilterto produce a concentrated slurry of at least about 65%, preferably about 65 to 80% by weight solids.

In a preferred aspect of the invention, the electrofilter is an electrically augmented vacuum filter.

In one aspect of the invention, the flocculating agent is selected from the group consisting of polyethylene oxide polymers and hydrophilic aminoplast resins, for example, an N-methoxylated amino compound.

In other aspects of the invention, at least a portion of the concentrated slurry is dried to pulverulent solids; at least a portion of the concentrated slurry is diluted to form a diluted slurry of not more than about 65% by weight solids and the diluted slurry is spray-dried to pulverulent solids; andior the electrofilter may be an electrically augmented vaccum filter.

Yet another aspect of the invention provides that the polymeric flocculating agent may be added in the amount of about 0.1 to 0.5 by weight of the weight of dry solids.

Another aspect of the invention provides for the use of an electrically augmented vacuum filter which includes a hollow electrode structure containing electrolyte therein and submerged within the slurry, the electrode structure having a perm-selective membrane associated therewith in a manner such as to substantially prevent passage of anions and gaseous electro-chemical reaction products from the electrolyte into the slurry.

In another aspect of the invention, the electrically augmented vacuum filter includes a hollow anodic electrode structure containing anolyte therein and submerged within the slurry, the anodic structure having a perm-selective membrane associated therewith in a manner such as to permit passage of cations from the anolyte and through the permselective membrane thence into the slurry through any filter cake accumulated on the anodic structure, while substantially preventing passage of anions and gaseous electrochemical reaction products from the anolyte into the slurry and any filter cake accumulated on the anodic structure.

The sole figure of the drawings is a schematic flow sheet illustrating a preferred embodiment of the method of the invention.

Referring now to the drawing, a source 10 of a low-solids clay slurry supplies a clay slurry, for example, a kaolin clay slurry of about 25 to 40% by weight solids, via line 14 to a mechanical filter 12 which may be a rotary drum vacuum filter of the type commonly employed for initial dewatering of clay slurries. Alternatively, mechanical filter 12 may comprise any suitable mechanical filter apparatus such as a plate and frame type filter. Preferably, however, mechanical filter 12 is a rotary drum vacuum filter of the type wherein slurry is deposited onto the filter cloth of the filter, and a vacuum is imposed on the underside of the filter cloth to draw water through the filter cloth as rotation of the drum causes the filter cloth to travel over the vacuum section of the apparatus. As the drum continues to rotate, the accumulated filter cake is discharge by gravity from the filter cloth and fresh slurry is deposited onto the cleared portion of the cloth.

A suitable flocculating agent as described below is added via line 16 to slurry transfer line 14 prior to introduction of the low-solids slurry onto mechanical filter 12. The use of flocculating agents is, of course, well known in the art. Filtrate water is discharged from mechanical filter 12 via discharged line 18 and a wet filter cake is discharged from mechanical filter 12 via filter cake discharge line 20. A deflocculating agent or dispersant, such as tetrasodium pyrophosphate, is added via line 22 to the discharged wet filter cake in line 20. The discharged wet filter cake is transported via discharge line 20 to mixer 24 wherein it and the dispersant are thoroughly mixed to form a deflocculated or dispersed slurry, which contains not more than about 57% by weight solids, at least a portion of which is then transported via line 26 to an electrofilter 28.

Electrofilter 28 may be any suitable type of filter, preferably an electrically augmented vacuum filter, which utilizes an electric field to cause solids to migrate relative to a liquid in which the solids are dispersed so as to form a layer or cake of the solids on a portion of the structure and to permit the liquid portion of the slurry to be removed, whereby separation of the solids from the liquid may be accomplished. One commercially employed type of such filter is disclosed in the aforesaid Patents 4,168,222, 4,170,529 and 4,107,026 to Mark P. Freeman, the entire disclosures of which are incorporated by reference herein and which described the workings of an electrically augmented vacuum filter.

As used herein and in the claims, the term "electrofilter" means any suitable apparatus, such as an electrically augmented vacuum filter, which utilizes an electrical field in the course of, or as an aid in, separating particulate solids from a liquid containing the solids.

The electrofilter may comprise an electrically augmented vacuum filter of the type disclosed in the aforesaid Freeman patents. Such apparatus comprises a suitable tank in which the slurry is received and within which a pair of opposed electrode structures is positioned to provide a cathode and an anode, and includes means to form between the electrodes a controllable electric field. One of the electrode structures is adapted for the formation thereon of a layer of the suspended solids under the influence of the electric field. Means are provided to withdraw carrier liquid from the tank as a filtrate separated from the solids which migrate towards one of the electrodes under the influence of the applied electric field.The means for withdrawinging the filtrate may comprise, for example, forming the electrode opposite that towards which the solids migrate as a hollow body having a liquid pervious wall through which the filtrate liquid will pass. A vacuum may be utilized to help withdraw the filtrate liquid, for example by connecting a vacuum supply to the hollow body of the second electrode whereby to apply a pressure differential to help effectuate passage of the carrier liquid from the tank. Generally, under the influence of the applied electric field, the solid particles, which may be either negatively or positively charged, and the carrier liquid will tend to move in opposite directions, towards respective ones of the oppositely disposed electrode pairs.

Gaseous electrochemical reaction products may be formed at the electrodes. The finely divided solids will accumulate upon and be somewhat compacted onto one of the electrodes and, when a sufficient amount of solids has accumulated, may be withdrawn by removing that electrode from the tank and removing the solids from the electrode by means of a doctor blade or the like. The electrode may then be returned to the tank. As disclosed in the Freeman patents, the equipment may be designed to provide for a continuous input of slurry into the tank and may contain a plurality of electrode pairs with means for temporarily removing from the tank selected electrodes to recover the accumulated solids therefrom.

As used herein and in the claims, an "electrically augmented vacuum filter" means an apparatus, such as that described in this paragraph, which employs an electric field and an applied vacuum in the course of, or as an aid in, separating solids from a liquid containing the solids.

The filtrate from electrofilter 28 is discharged via filtrate discharge line 30. The separated solids are discharged from electrofilter 28 in the form of a concentrated slurry via discharge line 32. At least a portion of the high-solids slurry withdrawn via discharge line 32 may be passed via line 34 to a mixer 36 in which the concentrated slurry is mixed with a portion of the dispersed wet filter cake withdrawn from line 26 via line 38, to provide in mixer 36 a slurry of not more than about 65% by weight solids. For example, if the concentrated slurry withdrawn from electrofilter 28 via line 32 contains more than 65% by weight solids, it may be diluted somewhat by addition of the dispersed wet filter cake in a proportion, relative to the concentrated slurry, as is necessary to adjust the solids content to a level appropriate for spray drying or other treatment.Generally, for spray drying, a solids content of not more than about 65%, usually about 50 to 60% by weight, is desired in order to maximize the efficiency of the spray drying operation. Accord tingly, a diluted slurry of reduced solids content is withdrawn from mixer 36 via line 40 and introduced into spray dryer 42. The diluted slurry is contacted in spray dryer 42 directly with hot gases, such as heated air and combustion gases, and substantially all the moisture is removed therefrom so that dried pulverulent solid clay particles such as a dried pulverulent kaolin, is withdrawn from spray dryer 42 via discharge line 44. The dried pulverulent solid may itself comprise a saleable product or may be further treated, such as by calcination, depending upon the end use for which it is desired.

A portion of the dispersed or deflocculated slurry withdrawn from line 26 may also be passed via line 46 into admixture with the slurry withdrawn from electrofilter 28 via line 32, then passed into mixer 48 in order to adjust the solids content of this portion of the slurry withdrawn from electrofilter 28. For example, as mentioned above, a clay slurry such as a kaolin clay slurry comprising about 70% by weight solids is a saleable article of commerce. Therefore, a selected proportion of the dispersed slurry feed to electrofilter 28 may be bypassed around electrofilter 28 by being withdrawn from line 26 via lines 38,46, and mixed with the concentrated slurry withdrawn via line 32 from electrofilter 28, as required to reduce the solids content of the feed to mixer 48. The mixed, diluted slurry is withdrawn from mixer 48 via line 50 as a product high-solids slurry.

The present invention provides a process in which the mechanical filter may be utilized to produce a partially dewatered slurry in the form of a wet filter cake of lower than usual solids content, i.e., to produce a wet filter cake which is mixed to form a discharged slurry having a solids content of not more than about 57% by weight solids. Normally, it is not possible to discharge such a high-water, low-solids clay slurry, i.e., a wet filter cake, from the filter medium of conventional mechanical filters such as rotary drum vacuum filters. In order to enable the discharge of the wet filter cake from the mechanical filter, a polymeric flocculating agent is added to the slurry prior to introduction thereof onto the mechanical filter. Suitable flocculating agents are hydrophilic, ie., water-soluble or waterdispersible, aminoplast resins of the type well known in the paper coating art.These resins are thermosetting resins comprising Nhydroxymethylated amino compounds and may be formed by condensing formaldehyde with urea or polyamines such as an aminotriazine. Among such resins are methylol ureas (urea formaldehyde resins, e.g., dimethylol urea), N-methylol melamines, e.g., N-dimethylol melamine and N-trimethylol melamine, N-methylol acetoguanamines, N-methylol benzoguanamines, methyl or ethyl esters thereof such as methylated methylol melamines, e.g., trimethyoxy trimethylol melamine, and copolymers of N-methylolacrylamide. A method for making suitable urea formaldehyde resins is described in U.S. Patent 2,428,752 to P.S. Hewett. melamine formaldehyde resins having a molal ratio of formaldehyde to melamine (triamino triazine) of at least 2 to 1, preferably 3 to 1, may be employed as the flocculating agent.Methods for preparing melamine formaldehyde and various methylol guanamine condensates are described in U.S. Patent 2,811,500.

Dimethylol melamine and trimethylol melamine resins are commercially avialable under trademarks PAREZ 611 and PP 4009, respectively. Methylated trimethylol melamine resins are sold under the trademarks Aerotex Resin M-3 and PAREZ Resin 613.

These resins are capable of further condensation in acid media and are slightly cationic in nature.

Strongly cationic melamine formaldehyde resins, which may be produced by the acid-condensation methods described in U.S. Patent 2,345,543 to H.P.

Wohnsiedler et al, are also useable.

Polyethylene oxide polymers found to be suitable for use in the present invention include those having molecular weights of about 100,000 to 5,000,000.

One commercially available material found to be suitable is sold under the trademark POLYOX by Union Carbide Company.

The amount of organic polymericflocculating agent used may be limited to a certain extent by the intended end use of the clay, although the amount used, generally, about 0.02 to 15% by weight of the weight of dry clay solids, is small in comparison to the amount of clay to be filtered. When insufficient resin is used, discharge of the wet filter cake from the filter medium is hampered. On the other hand, the use of an excess amount of resin as flocculant may result in adhesion of the resin to the fiter cloth of the mechanical filter. The use of too much resin may impair the rheology of paper coating grades of the clay being processed and make the clay coatings undesirably absorptive towards printing inks.A preferred quantity for use with such paper coating clays is in the range of about 0.02 to 2%, based on the dry clay weight, more preferably in the range of about 0.1 to 0.5% of the dry clay weight. With filler grades of kaolin, especially kaolins intended for use in filling rubbers or plastics, the amount of resin used may be within the range of about 0.03 to 15%, preferably 0.05 to 5% by weight of the weight of dry clay solids. If the end use constrains the amount of flocculant which may be used, it may be necessary to use the mechanical filter with a slurry of a higher solids content within the specified range of about 50 to 57% by weight solids, in order to facilitate discharge of the wet filter cake.Addition of the flocculating agent in amounts of at least 0.1% by weight on a dry clay solids basis is preferred to provide effective discharge of the low-solids clay slu, ries from the filter medium.

Generally, any suitable organic polymeric flocculating agent which will facilitate discharge of the low-solids slurry from the filter cloth of the mechanical filter may be employed. Typical inorganic flocculating agents such as sulfuric acid are not effective to attain the permeability and filter medium-dischargeable qualities of the low solids filter cake obtained on the mechanical filter in accordance with the present invention. It is believed that the organic polymeric flocculants, particularly the aminoplast resin e.g., melamine-formaldehyde resin act not only by providing charged particles to agglomerate the clay particles, but by serving as bridges to physically collect individual clay particles.

However, aminoplast resins, particularly Nmethoxylated amino compounds and most preferably melamine-formaldehyde resins are preferred, at least for the preparation of clays for use as paper coating clays. Such compounds, particularly melamine-formaldehyde resins, used in effective amounts have been found not to unduly adversely affect low-shear (Brookfield) viscosity of kaolin paper coating clays, which affects the coatability of the clay. If the low-shear viscosity is excessively high, slow and unsatisfactory coating results are obtained.

The efficacy of the invention is demonstrated by the following example.

EXAMPLE A. An aqueous slurry of kaolin particles containing from about 20 to 27% by weight kaolin has a pH of from about 2.5 to 3.5. The slurry is mixed with a flocculating agent comprising a melamineformaldehyde resin sold under the trademark PAREZ 613 by American Cyanamid Company. The flocculating agent is used in an amount of 0.25% by weight flocculating agent, based on the weight of dry clay sol,ds. The rotary vacuum filter is operated at a rotational speed higher than that normally employed and therefore produces a filter cake containing a higherthan usual water content and lower than usual solids content of about 50% by weight solids.

The presence of the flocculating agent enables discharge of the low-solids wet filter cake from the rotary vacuum filter.

B. The cake discharged from the rotary vacuum filter has added thereto 0.3% by weight, based in the dry weight of clay solids, of a deflocculant comprising tetrasodium pyrophosphate. The pH of the slurry is adjusted by the addition of sodium carbonate in an amount sufficient to provide a pH of 7 + 0.5. The resultant deflocculated or dispersed slurry is then passed to an electrofilter comprising a Dorr-Oliver electrically assisted vacuum filter of the type described in U.S. Patent 4,107,026. A dewatered, concentrated slurry of kaolin discharged from the electrofilter contains 75% by weight solids + 5%.

One portion of the dewatered kaolin slurry from the electrofilter is diluted with an amount of the feed kaolin slurry to the electrofilter to provide a product slurry of about 70% by weight kaolin solids. Another portion of the dewatered, high-solids slurry of kaolin from the electrofilter is diluted with feed slurry to the electrofilter in an amount sufficient to provide a solids content of about 65% by weight in the slurry.

This slurry is then passed to a flash dryer wherein it is directly contacted with heated gases and dried to provide a pulverulent kaolin solids product.

Operation of the mechanical filter at a high production rate of course increases the efficiency of that operation and provides a wet filter cake of a solids content which is well adapted to be treated by an electrofilter to efficiently further dewater the feed provided by the wet filter cake. An electrofilter such as an electrically augmented vacuum filter as described above, operates efficiently with a clay slurry feed of about 50% or so by weight solids. However, it is not economically feasible to operate the electrically augmented vacuum filter with a clay slurry feed of about 60% or more by weight solids because the limited amount of water to be removed from such a feed is insufficient to warrant use of the electrofilter.

On the other hand, clay filter cakes of significantly less than the usual 58 to 60% by weight solids are too wet to be efficiently discharged from the filter medium of a mechanical filter, such as a rotary vacuum filter. However, by utilization of a flocculating agent as called for by the present invention, a low-solids or wet filter cake can be efficiently discharged from the mechanical filter. This enables the filter to be run at a higher rate for a given feed as less filtrate must be removed, and the resultant wet filter cake provides a slurry which, at about 50 to 57% by weight solids, provides a feed which is sufiicient- ly dilute to enable efficient operation of an electrofilter, especially an electrically augmented vacuum filter.The method of the present invention generally provides a higher production rate for the mechanical filter equipment and reduces thermal energy requirements for spray drying. The filtration efficiencies thus obtained make it not only feasible but economical to produce from the electrofilter a concentrated slurry of very high-solids content which may be diluted as necessary to provide a high-solids product slurry suitable for sale as such for further treatment, such as spray drying, to provide a pulverulent solids clay product.

As described in the aforesaid Wills Patent 4,312,729, an electrically augmented vacuum filter (EAVF) such as that disclosed in the Freeman patents essentially comprises anode and cathode structures which are submerged in a slurry of solid particles to be dewatered. The structure usually includes a tank containing the slurry and means for lifting the anode structure with the filter cake deposited thereon from the tank, and means to scrape the filter cake from the anode structure before re-submerging it into the slurry for further deposition of filter cake thereon.

Thus, one embodiment of an EAVF described in U.S.

Patent 4,168,222 comprises a tank within which a series of parallel and alternating cathode and anode electrode structures are submerged within a circulating slurry of clay solids suspended in a carrier liquid in the tank. A direct current is applied to the electrodes to deposit solids as a filter cake on the walls of the support structure of the anode. Each anode is a self-contained hollow structure defining a compartment and having planar surfaces and comprises an anode, i.e., a positive electrode, means for circulating an electrolyte (the anolyte) into and out of the compartment, a support grid, a filter medium and a protective cage. When the anolyte is sodium chloride solution, chlorine gas is generated at the anode by virtue of electrochemical reaction. The extant of the reaction will depend upon the electric current density.A vacuum is applied to the anode and the recirculating anolyte is effective for removal of the majority of the generated gas from the anode compartment.

Each cathode is similarly a self-contained hollow structure having liquid-pervious filter cloth (which is substantially impervious to the passage of clay particles) covering a supporting grid or grid-like structure. Cloth similar to that used on the anode support structure may be used as the filter medium.

A source of vacuum is connected to the hollow pianar cathode and causes the liquid phase of the slurry, i.e., the filtrate, to fill the hollow interior of each cathode between the electrode and the support structure. Filtrate is pumped from the filtrate-filled hollow cathodes at a controlled rate and, under the application of the electric current, the filtrate, which contains various ionic species and water, will react at the cathode to produce primarily hydrogen gas and sodium hydroxide solution.

As further disclosed in the aforesaid U.S. Patent 4,312,729, membranes suitable for use as the permselective membrane are materials which are permeable to cations and substantially impermeable to anions, gases, water and other liquids. Useful permselective membranes are perfluorosulfonic acid polymeric materials sold under the trademark NA FION by E.I. DuPont de Nemours Inc., although other suitable perm-selective membranes can be used.

NAFION brand membranes are permeable to positively charged ions (cations) and are impermeable to negatively charged ions (anions). The perm-selective membrane may be placed onto the anode compartment between the support directly adjacent to it and the filter medium so that the latter acts to minimize the possibility of clay solids clogging the permselective membrane. When the cathode structure is equipped with a perm-selective membrane, the membrane must be positioned on two inner support grids which also constitute the cathode electrode, thereby forming a center compartment which is adapted to contain the catholyte, i.e., the liquid within the cathode.

In addition to sodium chloride, sodium carbonate and sodium hydroxide solutions may be used as the anolyte, among others. As explained in the aforesaid U.S. Patent 4,312,729, utilization of the permselective membrane in dewatering clay suspensions controls the migration with hydroxyl and other anions which are deleterious to the slurry, thereby preventing or minimizing contamination of the slurry. In the case of kaolin slurries, anion contamination of the filter cake increases the viscosity of kaolin/ water slurries thereby rendering the kaolin unsuitable for certain intended uses such as for paper coatings and fillers in paints.

While the invention has been described in detail with respect to specific preferred embodiments thereof, it will be appreciated that numerous variations to those embodiments may occur to those skilled in the art upon a reading and understanding of the foregoing, and it is intended to include such variations within the scope of the appended claims.

Claims (1)

1. A method for dewatering an aqueous clay slurry which comprises adding organic polymeric flocculating agent to the slurry, filtering the flocculated low-solids slurry on a filter medium to produce a wet filter cake containing not more than about 57% by weight solids, adding dispersant to the discharged wet filter cake and forming therefrom a dispersed slurry containing not more than about 57% by weight solids, and filtering the dispersed slurry on an electrofilterto produce a concentrated slurry of at least about 65% by weight solids, the amount of flocculating agent added to the initial slurry being at least sufficient to facilitate discharge of the wet filter cake from the filter medium.

2. A method according to claim 1 wherein the wet filter cake contains about 50 to 57% by weight solids.

3. A method according to claim 1 or 2 wherein the concentrated slurry contains about 65 to 80% by weight solids.

4. A method according to claim 1, 2 or 3 wherein the concentrated slurry contains more than about 65% by weight solids, the method further including diluting at least a portion of the concentrated slurry to form a diluted slurry of not more than about 65% by weight solids and spray drying the diluted slurry to pulverulent solids.

5. A method according to any preceding claim wherein the flocculating agent is selected from polyethylene oxide polymers, hydrophilic aminoplast resins, condensation products of formaldehyde and urea, and condensation products of formaldehyde and polyamines.

6. A method according to claim 5 wherein the flocculating agent is selected from N-methyoxylated amino compounds and melamine-formaldehyde resins.

7. A method according to any preceding claim wherein the flocculating agent is added to the initial slurry in an amount of about 0.02 to 15% of the weight of dry clay solids.

8. A method according to claim 7 wherein the flocculating agent is added to the initial slurry in an amount of about 0.5 to 5% of the weight of dry clay solids.

9. A method according to claim 8 wherein the flocculating agent is added to the initial slurry in an amount of about 0.1 to 0.5% of the weight of dry clay solids.

10. A method according to any preceding claim wherein the electrofilter is an electrically augmented vacuum filter.

11. A method according to claim 10 wherein the electrically augmented vacuum filter includes a hollow electrode structure containing electrolyte therein and submerged within the slurry, the electrode structure having a perm-selective membrane associated therewith in a manner such as to substan tally prevent passage of anions and gaseous electro-chemical reaction products from the electrolyte into the slurry.

12. A method according to claim 11 wherein the electrically augmented vacuum filter includes a hollow anodic electrode structure containing anolyte therein and submerged within the slurry, the anodic structure having a perm-selective membrane associ ated therewith in a manner such as to permit passage of cations from the anolyte and through the perm-selective membrane thence into the slurry through any filter cake accumulated on the anodic structure, while substantially preventing passage of anions and gaseous electrochemical reaction products from the anolyte into the slurry and any filter cake accumulated on the anodic structure.

Amendments to the claims have been filed, and have the following effect: *(b) New or textually amended claims have been filed as follows:

1. A method for dewatering an acidic aqueous clay slurry which comprises adding organic polymeric flocculating agent to the slurry, filtering the flocculated low-solids slurry on a filter medium to produce a wet filter cake containing not more than about 57% by weight solids, discharging the wet filter cake from the filter medium, adding dispersant to the discharged wet filter cake and forming therefrom a dispersed slurry containing not more than about 57% by weight solids, and filtering the dispersed slurry on an electrofilter to produce a concentrated slurry of at least about 65% by weight solids, the amount of flocculating agent added to the initial slurry being at least sufficient to facilitate discharge of the wet filter cake from the filter medium.