Magnetic purification of slurries

Abstract: A process which is especially useful for effecting magnetic separation of magnetically attractable impurities from an aqueous clay slurry or suspension includes passing the suspension (42) through a porous, ferromagnetic matrix while applying a magnetic field to the matrix (10), on which impurities are collected, and thereafter regenerating the matrix by flushing collected impurities therefrom with a flush liquid (26), e.g. water. The improvement comprises, after discontinuing the passage of the suspension through the matrix, but before passing flush liquid therethrough, admitting a pressurized gas (68) e.g. compressed air, into the separator to displace suspension retained therein, and recovering the displaced suspension. In addition, flush liquid retained in the matrix after the flushing step may be displaced therefrom by the compressed air. By displacing the retained suspension from the matrix with compressed gas instead of flush liquid the retained suspension is not diluted and is recovered instead of sewered as in the prior art processes.

Description: The present invention relates to removal of magnetically attractable impurities from suspensions of solids in liquid vehicles by wet magnetic separation, e.g. high intensity magnetic separation of magnetically attractable impurities from aqueous clay suspensions. In one preferred aspect all of the steps are carried out using dispersed clay slurries having a high concentration of clay. In the processing of clay materials, it has been common practice in the art to utilize high intensity magnetic field separation for the removal from aqueous clay suspensions of paramagnetic (weakly magnetic) colorant impurities, e.g. iron-bearing titania and ferruginous impurities. An inherent characteristic of wet magnetic separation as presently practiced is that the magnetically beneficiated slurry undergoes substantial dilution with water during the magnetic treatment. Dilution occurs both when the residual clay slurry in the separator is displaced with water and when residual flush water dilutes the incoming charge of feed slurry. Due to the volumes of flush water required, it is uneconomical to recycle the effluent flush water for processing to recover the clay solids therein, because the resultant high degree of dilution of the process stream would impose an uneconomic dewatering burden on the process. US-A-3838773 discloses magnetically separating particles from a fluid stream by a process which involves [a] passing the stream through a porous, ferromagnetic matrix while applying a magnetic field to the matrix; [b] while continuing to apply magnetic field to the matrix, discontinuing said passage of the stream through the matrix and therafter passing gas downwardly through the matrix; and [c] removing from the matrix magnetically attractable particles collected thereby. The only disclosure concerning treatment of a liquid stream requires the liquid stream in step [a] and air in step [b] to be passed in the same rather than opposite directions through the matrix, does not pressurise the air in step [b] and uses it to dry the matrix rather than to displace retained suspension therefrom, and then uses air rather than liquid to flush said particles from the matrix. US-A-4191591 discloses a carousel system for the same purpose wherein a liquid stream is passed downwardly through the matrix in step [a], and a flush liquid is used to wash it from the matrix in step [b]. The invention provides a method for the wet magnetic separation of magnetically attractable particles from a suspension of solids in a liquid vehicle, the method comprising [i] passing the suspension containing such particles upwardly through a porous, ferromagnetic matrix while applying a magnetic field to the matrix; [ii] while continuing to apply magnetic field to the matrix, discontinuing said passage of the suspension upwardly through the matrix and thereafter passing pressurized gas downwardly through the matrix to displace retained suspension therefrom; [iii] flushing the matrix with flush liquid to remove therefrom magnetically attractable particles collected thereby; and repeating steps [i] to [iii], displaced suspension from step [ii] being recovered. Step [iii] of the method according to the invention preferably involves discontinuing application of magnetic field to the matrix while passing flush liquid through the matrix, and thereafter passing pressurized gas downwardly through the matrix to displace retained flush liquid therefrom. The magnetic separation procedures according to the invention can be applied with advantage to clay feed suspensions of high solids content (e.g. at least 50%, preferably at least 60% solids). Kaolin dilution is avoided, in accordance with the present invention, by controlled injection of a gas stream, preferably air, into the magnetic separator to displace retained clay slurry prior to introduction of water to flush out impurities, and preferably also to displace subsequently introduced flush water prior to introducing new feed slurry. The first gas purge is applied when the magnet is energized, whereby residual clay slurry is selectively removed with minimal rejection of impurities which are retained in the matrix of the HGMS unit. During the preferred second gas purge the magnet can be deenergized. The or each gas purge is carried out by passing gas downwardly through the matrix using controlled gas pressure. Gas pressure is controlled to minimize two-phase flow (water hammer) in order to prevent damage to the matrix. The separators are operated with feed passed upwardly through the matrix. The HGMS procedure according to the invention is applicable as part of the wet processing of kaolin clay in which all processing steps including HGMS are carried out with the feed clay slurry at high solids, above 50%, and the magnetically purified clay slurry is preferably also recovered at high solids, above 50%. The processing steps can include blunging crude kaolin clay at high solids, optionally fractionating the blunged clay to provide one or more fractions of clay of desired particle size, physical removal of the colored impurities by wet magnetic separation according to the invention and, optionally but preferably, bleaching. The brightened, beneficiated clay product is obtained as a dispersed aqueous suspension having a solids level which is ideally not lower than that of the feed suspension to the initial processing steps. One aspect of the invention comprises wet processing clay using the following steps. The crude kaolin clay is first blunged in water, preferably in the presence of a dispersant, at high solids - above 50 or 55%, preferably above 60%, and most preferably at about 68 to 72% solids, and typically at 70% solids. The slurry of blunged clay is then degritted to remove coarse oversize by means well known in the art and is optionally fractionated by centrifugation or gravity to recover one or more fractions of degritted fractionated clay containing the desired concentration of particles finer than 2 microns equivalent spherical diameter (e.s.d). The aqueous clay slurry is in dispersed state throughout these steps. The recovered fine particle size fraction(s) of clay may be, for example, a No. 2 coating clay fraction which is about 80% by weight finer than 2 microns (e.s.d.) or a No. 1 coating clay grade which is about 90% finer than 2 microns (e.s.d.). The dispersed slurry of coating clay fraction is recovered. A secondary quantity of dispersant is then preferably added to the fraction to assure that the slurry is in overdispersed state when it is charged to the magnet unless the slurry is already overdispersed. Especially preferred for achieving overdispersion is to introduce an alkali metal polyacrylate salt, such as sodium polyacrylate, to the clay to achieve minimum Brookfield viscosity and then add a predetermined quantity of alkali such as soda ash or caustic soda until pH rises and Brookfield viscosity undergoes a dramatic increase. The overdispersed, fractionated aqueous clay slurry is then charged to the HGMS unit containing a matrix of stainless steel wool which is housed in a canister. The slurry is passed upwardly through the matrix while applying a sufficiently strong magnetic field to the matrix to set up regions of high gradient in the matrix. After a suitable period, passage of the slurry through the matrix is discontinued. While continuing to apply the magnetic field to the matrix, a stream of pressurized air is passed downwardly through the matrix to displace retained clay suspension which is recovered. The matrix at this point is laden with paramagnetic impurities. These are removed by passing flush water upwardly, downwardly or both through the matrix while deenergising the matrix. Retained flush water is removed by passing a stream of pressurized air downwardly through the matrix. The magnet operation is carried out on a semi-continuous basis. After retained flush water is removed, clay slurry is again charged to the magnet. This clay slurry can be composed in whole or in part of recycle slurry (that has previously undergone magnetic purification). The dispersed slurry of magnetically purified clay is then optionally bleached with a reducing bleach, preferably a dithionite (hydrosulfite) salt, without flocculating and filtering the slurry prior to or subsequently to bleaching, thus avoiding filtration steps. The dispersed brightened clay product, which has a solids content of at least 55% is then dried or formed into a high solids slurry for shipment. Dry clay can be added to the slurry of beneficiated clay to build up solids to produce slurries having a solids content of the order of 70%. Alternatively, thermal evaporation can be employed to increase solids to a high level desired for shipment. It may be desirable to build up solids by adding dry clay which imparts desired rheological or other properties such as enhanced brightness. For example hard kaolin can be added to beneficiated slurries of soft clay to improve rheology. Most preferably, the process of the invention is carried out without introducing soluble salts other than dispersants and while avoiding thickening, filtration and washing steps which are required in conventional kaolin processing to minimize the level of soluble salts which are detrimental to clay viscosity. Deleterious soluble salts are introduced when dispersed slurries are flocculated with acid, usually sulfuric alum,or combinations of acid and alum, forming undesired sulfate salts which must be removed by washing filter cakes. In the preferred process of the invention, all steps are carried out at high solids. Floccing is not practiced. By - product salts are not formed. When bleaching is not practiced, no salts other than dispersants are introduced during production. We have found that wet magnetic separation of high-solids clay suspensions is enhanced by avoiding the use of phosphates, which may have an adverse effect on product rheology and may increase viscosity of the suspension. Although silicate dispersants are useable, we prefer alkaline organic dispersant, preferably polyacrylate/sodium hydroxide dispersant, as such dispersants appear to promote better brightening results in the wet magnetic separation treatment of the clay. Typically, the dispersant may comprise 3.5 parts by weight sodium polyacrylate and 1.0 part by weight NaOH and is used in proportions (dry basis) 1.25 to 1.5 g. of dispersant per ton of clay (2.5-3 lbs/ton) or more, up to about 2.5 g/kg (5 lbs/ton). The amount of dispersant required depends on a number of factors, including the percent solids of the suspension and the type of clay being dispersed. The effectiveness of wet magnetic separation of a high-solids clay dispersion in enhancing brightness is enhanced by over-dispersing the clay, as explained more fully above. The invention is illustrated, by way of example only, by the following more detailed description with reference to the accompanying drawings, in which : FIGURE 1 is a simplified, schematic block diagram of one embodiment of the process of the present invention. For the sake of simplicity the schematic illustration Fig.1 omits numerous necessary and conventional items, such as pumps, bleed lines, controls and the like, the use of which is well known and description of which is not necessary for explaining the present invention. A magnetic separator schematically indicated at 10 is of conventional canister-type design. A commercially available form of porous matrix, a stainless steel wool pad, is packed within the canister such that about 92% to 96% of the volume of the pad is interstitial voids, the steel filaments occupying only about 4 to 8 vol.%. Such porous matrices of considerable interstitial void volume act somewhat in the nature of a sponge and tend to retain the liquid, e.g. water or the suspension, therein for at least a time after cessation of pumping or otherwise passing the suspension or liquid therethrough. Typically, the magnetic separator equipment is operated at a magnetic field intensity of about 5 to 30 kilogauss, say about 8.5 to 20 kilogauss, e.g. about 16 kilogauss. Superconducting magnets operate at higher field strength, typically at 50 kilogauss or higher. It is within the scope of this invention to utilize a magnetic separator of the superconducting type which includes a porous matrix, e.g. a stainless steel matrix. The pressurized gas, e.g. compressed air, is used to displace retained suspension and retained flush water from the porous matrix. The gas pressure can be administered in a controlled fashion such that the liquid head (which results from the elevation of the matrix canister and the vessel to which the product is being purged) is overcome. Both static and dynamic pressure drop can be accounted for in such a manner that displaced product slurrry is maintained in a laminar flow regime. Design of the system, accounting for the total calculated air pressure in this manner, will assure an assentially plug flow displacement of product from the matrix, thus avoiding water hammer which, if it occurred, could result in damage to the matrix. In the process equipment used in the accompanying illustrative examples, the pressurized gas, compressed air, was maintained at 110-225 kPa (8-18 psig), preferably 170-205 kPa (10-15 psig) and ideally 190 kPa (13 psig). At these pressures reasonably rapid displacement of retained suspension was attained without dislodgment into the suspension of collected impurities from the matrix, or at least without an unacceptably high degree of such dislodgment. In order to reduce or avoid such dislodgment it is helpful to introduce the pressurized gas into the suspension-retaining matrix and pass it therethrough in a non-pulsating manner. Limiting the pressure of the pressurized gas as above, and gradual opening of the air valve, helped to control the initial impact. Passing the pressurized gas in a continuous stream, i.e. without significant pulsations or pressure variation, avoids pressure fluctuation impacts on the matrix. Communicating with the outlet end 10a of the magnetic separator 10 is manifold conduit 12 joined to a sewer line 14 containing control valve 16 and communicating with a sewer or other disposal means. A product line 18 having control valve 20 is in communication with manifold conduit 12 to convey purified product to further processing or storage. A flush water line 22 having a control valve 24 connects manifold conduit 12 to a source of flush liquid such as flush water inlet 26. A pressurized gas source, in the illustrated embodiment a compressed air source 68, is connected via line 70 to manifold conduit 12 and has a control valve 72. At the inlet end 10b of the magnetic separator 10, a manifold conduit 28 has connected to it a discharge line 30 which is fitted with a control valve 32 and in turn connects to sewer line 14, thereby connecting the inlet end 10b of magnetic separator 10 to sewage or other disposal. A second flush water line 34 has a control valve 36 and connects flush water inlet 26 via manifold conduit 28 to the inlet end 10b of magnetic separator 10. A feed source 38 supplies a clay feed, such as a 60% solids aqueous dispersion of kaolin clay particles, containing magnetic colorant impurities. Especially in the case of a high-solids feed, the feed is preferably dispersed with a particular class of dispersant, which may be broadly referred to as an alkaline organic dispersant which is capable of satisfactorily dispersing the clay suspension not only for preliminary wet processing steps such as blunging, degritting and fractionating, but for magnetic treatment in accordance with the present invention. The clay solids are passed from feed source 38 to feed tank 42 via a feed supply line 40 having a control valve 41. A feed inlet line 44 leads from feed tank 42 and has a control valve 46 for the controlled introduction of feed into manifold conduit 28. Plug flow is the normal mode of operation during the cycle in which clay slurry is passed upwardly through the magnet. A return line 48 from manifold conduit 28 branches into a feed tank return line 50 which has a control valve 52, and a recovery tank line 54 which has a control valve 56. Feed tank return line 50 connects to feed tank 42 and recovery tank line 54 connects to a recovery tank 58. A transfer line 60 has a control valve 62 and connects to feed tank 42. A secondary product line 64 has a control valve 66 and connects return line 48 to product line 18. In operation, aqueous clay suspension containing magnetic impurities flows from feed source 38 via feed supply line 40 into feed tank 42 in which a suitable inventory of feed is retained. From feed tank 42, the clay suspension is passed through feed inlet line 44, control valves 20 and 46 being open and the other valves closed, except for valve 41 which is opened as needed to keep a sufficient inventory in feed tank 42. The feed slurry flows through manifold conduit 28 and then through magnetic separator 10, entering inlet end 10b, passing through the porous stainless steel matrix (not shown) within separator 10 and exiting via outlet end 10a. Magnetic impurities, under the influence of the magnetic field applied to the matrix in magnetic separator 10, are retained on the matrix which, as described above, comprises a suitable porous ferromagnetic body, such as a body of stainless steel wool. The resultant magnetic impurities-depleted slurry flows via manifold conduit 12 into product line 18, to further processing or product storage. The passage of aqueous clay suspension through the magnetic separator 10 is continued with the power source associated with the magnetic circuitry of the separator being continuously energized to maintain the magnetic field continuously applied to the matrix. When a predetermined time has elapsed, or when the matrix has become saturated with collected magnetic impurities or has accumulated a sufficient quantity of such impurities that removal efficiency of the separator 10 has been reduced to a minimum acceptable level, the matrix is regenerated, i.e. cleaned, by removal of collected impurities therefrom. The length of treatment time before cleaning of the matrix becomes necessary will be a function of the clay suspension being processed, the configuration and characteristics of the magnetic separator, the process conditions such as volumetric flow rate of the clay suspension through the separator, and the type and concentrations of the magnetically attractable particles present in the clay suspension being processed. The magnetically attractable impurities commonly associated with kaolin clays may comprise, for example, one or more of iron, titanium and their oxides, e.g. ferruginous and titania minerals, including colored titania minerals such as iron-stained anatase. When it becomes necessary to clean the porous matrix, the passage of the clay suspension through the magnetic separator 10 is terminated by closing valves 46 and 20 but maintaining the magnetic field circuitry energized. Valves 72 and 52 are then opened, with all other control valves being closed, in order to introduce a continuous stream of compressed air from source 68 through line 70 into manifold conduit 12 and thence into magnetic separator 10 downwardly through the matrix thereof to displace clay suspension retained in the porous matrix. It is a characteristic of the porous ferromagnetic matrix, such as a bed of stainless steel wool, to retain therein a considerable body of suspension or liquid, e.g. the suspension of clay solids being treated or flush water used to clean the matrix, after flow of the suspension or liquid through the matrix is terminated. Such retained suspension of the clay solids being treated is forced by the compressed air through manifold conduit 28 and feed tank return line 50 into feed tank 42. The magnetic field is maintained during discontinuation of the suspension flow therethrough and the pressurized gas displacement of retained suspension, in order to hold the magnetically attractable impurities in place on the matrix. The suspension which was retained in the matrix upon discontinuation of the flow of suspension therethrough is thus recovered and recycled to feed tank 42 for eventual reconveyance to separator 10 for treatment. Alternatively, valve 52 may be closed during all or a selected stage of such pressurized gas displacement while either or both of valves 56 and 66 are open, so that the displaced suspension is fed via recovery tank line 54 into recovery tank 58, and/or via secondary product line 64 to product storage or further treatment. Most, if not all, of the magnetically attractable impurities from the displaced suspension are retained on the matrix, the magnetic field having been maintained during the displacement step. Therefore, it may be economical to incorporate all or part of the displaced suspension into the product (via secondary product line 64). On the other hand, all or part of the displaced suspension may be sent to recovery tank 58 from which it is transferred to feed tank 42 in desired proportions with fresh feed and recycled for treatment in magnetic separator 10. Regardless of the specific routing (to feed tank 42, recovery tank 58 or product line 18) of the recovered solids-containing suspension, because it was displaced from the matrix of magnetic separator 10 by compressed air and not by flush water, the recovered suspension is not diluted. Following the recovery of the displaced suspension, valve 72 (and/or valves 52, 56 and/or 66) is closed, the magnet is de-energized and valves 36 and 16 are opened and all other valves closed to forward-flush the porous matrix in magnetic separator 10 by passing flush water through separator 10 in the same upward (as viewed in Figure 1) direction as the suspension flows during treatment. The flush water and particles of impurities displaced by it from the porous matrix of separator 10 are discharged via manifold conduit 12 and sewer line 14. After a period of such forward flushing, valves 36 and 16 may be closed and valves 24 and 32 opened (with all other valves closed) to back-flush the matrix of magnetic separator 10 by passing flush water downwardly (as viewed in Figure 1) therethrough. During such back-flushing, which may be followed by another period of forward-flushing, flush water and magnetic impurities displaced by the flush water from the matrix of separator 10 flow through the manifold conduit 28, discharge line 30 and sewer line 14. After flushing has been completed, valve 24 is closed and valve 72 is opened so that compressed air from source 68 flows into magnetic separator 10 through line 70 and conduits 12 and 28, downwardly through the matrix of separator 10 to displace from it retained flush water. The displaced flush water flows through discharge line 30 and sewer line 14 to sewer disposal. After retained flush water has thus been displaced, valves 72 and 32 are closed, the magnetic circuitry is again energized, and valves 46 and 20 are reopened to reinitiate passage of clay suspension through the magnetic separator 10 to start a fresh treatment cycle. In the description of the Exemplary Method below, the lines and valves described correspond to the numbered items of Figure 1 as follows: the "feed valve" corresponds to valve 46; the "product line valve" corresponds to valve 20; the "water valve" corresponds to valve 36 for forward (upward) flush through separator 10, and to valve 24 for back (downward) flush through separator 10; the "sewer valve" corresponds to valve 16 for sewering during forward upward) flow, through separator 10, and to valve 32 for sewering during back (downward) flow through separator 10; the "compressed air valve" corresponds to valve 72; and the "recycle valve" corresponds to valve 52. Exemplary Method (in accordance with the invention) For test runs in accordance with present invention, the following technique was employed for both low-solids and high-solids aqueous clay suspensions. The tests were run in the same installation using either a 213 cm. (84 in.) or a 305 cm. (120 in.) diameter PEM high intensity magnetic separator connected to suspension feed, flush water and compressed air lines as indicated by Fig.1. A magnetic field of 16 kilogauss was applied to the porous matrix of the separators. The porous matrices were cylindrical shaped beds of stainless steel wool of the above diameters and 51 cms. (20 in.) deep and the steel wool was packed in the canisters so that about 94% of the matrix volume was voids and about 6% was stainless steel. The smaller matrix was encased in a canister of 1630 l (430 gal.) capacity and the larger one a canister of 3255 l (860 gal.) capacity. 1. Feed Treatment Period . Energize the magnet, close the water and air valves, and open the feed and product line valves to pass the feed of aqueous clay suspension upwardly through the matrix while a 16 kilogauss magnetic field is applied to the matrix. The feed rates were about 1135-1895 l/min (300-500 gal./min) for the smaller matrix and about 2270-3785 l/min (600-1000 gal/min) for the larger. 2. Clay Recovery By Compressed Air While maintaining the magnet in an energized condition, simultaneously close the feed and product line valves, and open the compressed air and recycle valves, to provide a continuous compressed air force at 190 kPa (13 psig) to displace suspension retained in the matrix back into the feed tank or into the recovery tank. (It should be noted that suspension in the matrix has been magnetically processed at the time of its displacement from the matrix. Thus, additional overall brightness improvement can be obtained by sending it back to the feed tank for eventual recycle through the magnetic separator. However, should the alternate procedure of sending the displaced suspension to the recovery tank be chosen, brightened product can be obtained directly from the recovery tank.) 3. Matrix Flush . Close compressed air and recycle valves, de-energize the magnet, and open flush water and sewer valves to pass flush water upwardly through the matrix to flush magnetically attractable impurities from the matrix to sewer. The flow rates were about 4540-5680 l/min. (1200-1500 gal/min.) for the larger matrix and about 7570-8325 l/min. (2000-2200 gal./min) for the larger. Reverse direction of flow of flush water after an initial period to back-flush matrix and then finish flush with additional period of forward flow (upwardly) through matrix. 4. Displace Water . Close flush water valve and open compressed air and sewer valves to continuously pass compressed air at 190 kPa (13 psig) downwardly through the porous matrix to force flush water retained in the matrix to the sewer. In the treatment of low-solids clay suspensions, the compressed air was applied for 45 seconds and in the treatment of high-solids clay suspensions the air was applied for 120 seconds. 5. End Cycle . Close compressed air and sewer valves and repeat step 1 to initiate another treatment period. Step 4, the "Displace Water" step, was carried out for only 45 seconds when treating low-solids clay suspensions because it was deemed that the greater production rate (tons of clay processed per cycle) thereby attained made acceptable the higher flush water dilution that ensued. Higher solids clay suspensions give higher production rate, and more time is devoted to Step 4 to remove more flush water from the matrix and correspondingly reduce dilution of the high solids suspension feed to the matrix in the next cycle. The optimum time devoted to Step 4 will depend on the economics in a given case. In any case, for high-solids suspensions, Step 4 is carried out to remove most if not all (e.g. at least two-thirds, preferably at least three-quarters or at least nine-tenths) of the retained flush liquid. All references to particle sizes in this specification and claims are to sizes as determined by use of a SEDIGRAPH 5000 particle size analyzer and are reported on the basis of maximum equivalent spherical diameter of a stated weight percentage of the material. Similarly, all references to GE brightness refer to GE brightness as measured by the Technical Association of the Pulp and Paper Industry (TAPPI) Standard T452-M-58. All gallons referred to herein are US gallons, all tons are short tons (2000 lbs), and all mesh sizes are Tyler Series. The following Examples treat low-solids clay suspensions in order to provide a uniform basis for comparing the Exemplary and Comparative Methods; for the reasons noted above, use of the Comparative Method is not feasible with high-solids clay suspensions. The Comparative Method was similar to the Exemplary Method but used upward flow of water instead of downward flow of compressed air for removing retained suspension, and did not remove flush water with compressed air before passing new feed suspensions to the matrix. EXAMPLE 1 An aqueous suspension of dispersed kaolin clay particles having an average feed solids of 32.0 percent was treated in a performance test of the Comparative Method using the above-described smaller matrix. The clay suspension had a nominal particle size of 80% by weight finer than 2 microns equivalent spherical diameter. The performance test took place over a period of fifteen consecutive days monitored during three of the fifteen operational days for product brightness and yield. A similar clay suspension having an average solids content of 32.2% and a nominal particle size of 80% by weight finer than 2 microns equivalent spherical diameter was the treated in a performance test of the Exemplary Method as described above over a period of fourteen consecutive days and was monitored for two of the operating days. Both performance tests were carried out in the same equipment and in the Exemplary Method, Step 4 was carried out for only 45 seconds, to enhance productivity, accepting concomitant increased dilution of the brightened clay product. The solids content of the products are shown in Table I. Average Solids Method Clay Feed Purified Clay Product Purified Clay Product Yield Comparative 32.0% 25.9% 92.8% Exemplary 32.2% 30.7% 97.4% Table I shows that even when the Exemplary Method is operated in a production-enhancing and dilution-accepting mode, it provided a significantly higher yield than the Comparative Method. As shown by the data of Table I, the method of the present invention provides a suspension of magnetically purified clay having considerably higher solids, and also provides an increased yield of purified clay. The clay suspension treated by the Exemplary Method sustained significantly less dilution by flush water as compared to that treated by the Comparative Method. The reduced percent solids of the product in both cases results not only from dilution of the product with flush water, but also from losses of clay and the removal of magnetically attractable impurities from the clay suspension. If one assumes that an average of 725 kg (16,000 lbs.) of dry clay solids are treated during a single treatment cycle, the 4.6 percent improvement (97.4% - 92.8%) in yield of the Exemplary Method over the Comparative Method shown in Table I represents an increase of 334 kg.(736 lbs.) of dry product per cycle of operation. At a typical cycle time of 18 minutes, this is more than 1180 kg.(2,450 lbs.) of additional clay product (dry basis) per hour of operation. The extent of flush water dilution sustained by the Comparative Method as compared to the Exemplary Method may be calculated with respect to the data of Table I as follows. Flush Water Dilution Sustained by Comparative Method A feed composition of 32.0% solids has 0.85 kg/l (7.05 lbs./gal) of water and 0.4 kg/l (3.32 lbs/gal) of clay. A product composition of 25.9% solids has 0.88 kg/l (7.34 lbs/gal) of water and 0.31 kg/l (2.57 lbs/gal) of clay. Flush Water Dilution Sustained by Exemplary Method A feed composition of 32.2% solids has 0.84 kg/l (7.04 lbs/gal) of water and 0.4 kg/l (3.35 lbs/gal) of clay. A product composition of 30.7% solids has 0.85 kg/l (7.11 lbs/gal) of water and 0.37 kg/l (3.11 lbs/gal) of clay. Calculations show a marked reduction in dilution of the product by flush water - a reduction of 3925 l (1,037 gal.) per cycle provided by operating in accordance with the present invention. EXAMPLE 2 The smaller matrix equipment of Example I was used to treat, by the Exemplary Method of the present invention, a high-solids, coating clay fraction of two Wilkinson County, Georgia kaolin clays as follows: two parts by weight of a Klondyke coarse, soft kaolin clay and one part by weight of L.D. Smith fine, hard low viscosity clay. The clay was dispersed with approximately 2.5 g/kg. (5 lbs/ton) (dry basis) of a dispersant of sodium polyacrylate and sodium hydroxide in a 3.50:0.75 weight ratio (dry basis) and had a size range of 82% by weight of the particles finer than 2 microns equivalent spherical diameter. This amount of dispersant is in excess of the amount required to obtain optimum Brookfield viscosity. (Such over-dispersal of the suspension has been found to be advantageous in wet magnetic separation of high-solids clay suspensions.) The fractionated, degritted clay feed to the magnet contained 61% solids and had an average GE brightness of about 80.3. The magnetic treatment provided a 56% solids product having a brightness improvement of 3.0 GE. The treated product was recycled and identically treated a second time, and a further brightness improvement of 1.7 GE was attained in a product having 51% solids. The present invention is applicable to the treatment of a wide variety of clay materials which contain magnetically removable impurities. For example, the process can be applied to domestic and foreign clay crudes of the soft and hard types. The treatment also can be utilized with mechanically delaminated kaolins. For example, a crude clay or a fraction of a clay crude containing particles amenable to mechanical delamination can be mechanically delaminated before or after charging the material to the magnet. It is also within the scope of the invention to include processing steps other then blunging, optional fractionation, optional delamination, magnetic treatment, and optional bleaching with a hydrosulfite. By way of example, the clay can be subjected, while in the form of a high solids system, to mechanical work in excess of that required for effective blunging. The mechanical work may be used to achieve at least one of the following benefits: viscosity reduction; increase in the liberation of impurities to facilitate degritting, fractionation, magnetic treatment, increase in the yield of a desired fine size fraction of clay, or change in the particle size distribution of clay fractionated to a predetermined cut point. It is also within the scope of the invention to subject the clay to the action of oxidizing agent such as ozone at any stage during processing of the high solids clay water system. A particle size fractionation step is included in the process of the invention in those cases in which it is desired to recover a fine particle size fraction of a crude clay. Fractionation can be carried out before or after magnet treatment.

Claims: A method for the wet magnetic separation of magnetically attractable particles from a suspension of solids in a liquid vehicle, the method comprising repeatedly [i] passing the suspension containing such particles through a porous, ferromagnetic matrix while applying a magnetic field to the matrix, [ii] while continuing to apply magnetic field to the matrix, discontinuing said passage of the suspension through the matrix and thereafter passing gas downwardly through the matrix, and [iii] removing from the matrix magnetically attractable particles collected thereby, characterised in that the suspension in step [i] is passed upwardly through the matrix, in that the downwardly passed gas in step [ii] is pressurised so as to displace retained suspension from the matrix, in that the magnetically attracted particles in step [iii] are removed from the matrix by flushing the matrix with flush liquid, and in that displaced suspension from step [ii] is recovered. A method according to claim 1 wherein flushing step [iii] comprises discontinuing application of magnetic field to the matrix while passing flush liquid through the matrix, and thereafter passing pressurized gas downwardly through the matrix to displace retained flush liquid therefrom. A method according to claim 1 or 2 wherein the matrix comprises a body of filamentary ferromagnetic metal. A method according to any preceding claim wherein the flush liquid is water. A method according to any preceding claim wherein the pressurized gas is air. A method according to any preceding claim wherein the pressurized gas is at a pressure of from 8 to 18 psig. A method according to any preceding claim wherein the intensity of the magnetic field applied to the matrix is from 5 to 30 kilogauss. A method according to any preceding claim wherein the pressurized gas is applied as a continuous non-pulsating stream. A method according to any preceding claim wherein at least some displaced slurry from step [ii] is combined with product slurry from step [i]. A method according to any preceding claim wherein at least some displaced slurry from step [ii] is recycled to step [i]. A method according to any preceding claim wherein the suspension is an aqueous suspension of clay. A method according to claim 11 wherein the aqueous clay suspension feed in step [i] has a solids content of at least 50 dry wt.%. A method according to claim 12 wherein the aqueous clay suspension feed in step [i] is dispersed with alkaline polyacrylate dispersant. A method according to claim 12 or 13 wherein the aqueous clay suspension feed in step [i] is overdispersed. A method according to any of claims 12 to 14 wherein the aqueous clay suspension feed in step [i] is formed by blunging crude clay in water to form a slurry, removing grit from the slurry, and optionally fractionating the slurry to recover one or more fractions of clay of desired particle size, and wherein the magnetically purified product clay suspension is optionally subjected to a bleaching step. A method according to any of claims 12 to 15 wherein the magnetically purified clay suspension has a solids content of at least 50. A method according to claim 16 wherein the aqueous clay suspension has a solids content of at least 55 dry wt.% for step [i] and of 65 to 70 dry wt.% during preceding treatment. A method according to claims 15 and 16 wherein the blunged clay has a solids content of above 70 dry wt.%, and the solids content is maintained as high as possible for each succeeding step whilst ensuring adequate fluidity for each step, dilution of the aqueous suspension being minimised during all steps. A method according to claims 15 and 16 wherein the clay is blunged and degritted at a solids content of 55 dry wt.% or higher, and the aqueous suspension is introduced to step [i] at a solids content of 55 dry wt.% or higher. A method according to any of claims 12 to 19 wherein the aqueous clay suspension is maintained at a solids content of from about 55 to about 72 dry wt.% throughout all treatment steps. A method according to any of claims 16 to 20 wherein the magnetically purified suspension is spray dried. A method according to any of claims 16 to 21 wherein sufficient dry kaolin clay is mixed with the magnetically purified clay suspension to form a slurry having a dry solids content of 70% or higher.

Inventors: HEMSTOCK GLEN ALTON, WILLIS MITCHELL J, HEMSTOCK, GLEN ALTON, WILLIS, MITCHELL J.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1987
Main classification: B03C1
IPC codes: B03C1