HIGH BULK DENSITY CALCINED CLAY

Abstract: Calcined clay pigments in a form for bulk shipment are prepared by the addition of water to dry particles to form plastic aggregates which are then increased in size and rounded, as by disc pelletizing, and air dried to produce a dust free, free-flowing, high bulk density material.

Description: HIGH BULK DENSITY CALCINED CLAY BACKGROUND OF THE INVENTION Field of the Invention This invention relates to powdered calcined clay pigments in a free-flowing, high bulk density, low moisture product form that is useful for shipments in bulk. The invention also relates to the manufacture of such products by the controlled addition of water to dry calcined clay powders in order to change their consistencies from a non-plastic powder to plastic aggregates which are then formed into round, free flowing pellets, as by the use of a disc pelletizer. Subsequent drying causes compaction and shrinkage of the plastic aggregates which increase its bulk density to twice that of the original powder. Description of Related Art Kaolin pigments have been widely used for many years by the paper industry to coat and fill paper and paperboard products. Such pigments are also used by the paint, plastics and ink industries. These pigments are available in a large number of grades, including low abrasion calcined pigments and hydrous (uncalcined) kaolin products. Hydrous products include delaminated and undelaminated grades. The kaolin product selected by the paper maker is dictated by the particular end use, such as the grade of paper. In the case of paper filler, a relatively coarse hydros product or a calcined kaolin is usually selected. In the case of paper coating pigments, the selected kaolin product may be a #0, #1 or #2 grade, a delaminated kaolin, a calcined kaolin or a mixture. For example, some premium grades of coated paper utilize a mixture of calcined kaolin and hydrous kaolins such as mechanically delaminated kaolin. In the early days of the kaolin industry in the United States, refined hydrous kaolin pigments were exclusively prepared by filtering slurries of wet processed kaolins through filter cloths, forming an acidic filter cake typically containing about 30 to 35% moisture . The cakes were then dried in a rotary or tunnel dryer and milled to break up the cake into small chunks. Subsequently, it became the practice in some plants to prepare filter cakes for drying by extrusion through perforated plates and then drying the putty-like extrudate in a suitable dryer, usually a tunnel or rotary dryer to produce noodle-like pellets. The resulting "lump" type kaolins were shipped in bulk or in bags. A density of 65 pounds per cubic foot was reported in the literature for a Georgia clay (Kaolin Clays and their Industrial Uses, published by J.M. Huber Corporation (1949)) . Some lump grades were pulverized and shipped in bags. Pulverized lump grades had lower densities; values of 25-45 pounds per cubic foot were reported in the 1949 publication. These dry bag or bulk shipments of acid clays were unloaded at the site of the paper mill where the kaolin was mixed with water and clay dispersant to form a slurry which was then formulated into products such as paper coating pigments. High shear energy and addition of dispersant were required for the pigment makedown. In the United States, spray dryers have now replaced the equipment used in the past to manufacture the early dry lump hydrous kaolins. These dryers convert clay slurry to small dry pulverulent spheres called "microspheres." Spray dried production generally utilizes labor and energy more efficiently than lump kaolin production. Nevertheless, costs incurred during the spray drying step represent a significant expense to the manufacturer. One difficulty with spray-dried products, especially spray-dried filler products, is that they are dusty. Environmental laws are placing restrictions on the utilization of spray dried kaolin production because of the dust factor. This may impose a constraint on the shipment of spray dried products. Shipment of spray dried microspheres is widespread in the United States and Japan. Not all paper mills are equipped to handle spray dried production. For example, at present, many paper mills in Europe do not normally utilize spray-dried kaolins. U.S. Pat. No. 5,074,475, Suitch et al . , relates to a process for slurrying and then spray drying calcined clay. However, such spray-dried calcined kaolin products have generally not been found to be economical for bulk shipping. Current commercial practice is to ship calcined kaolins as pulverized powders or aqueous slurries. Production of kaolin pigments in England is frequently carried out by tube and/or pressure filters which remove more water than conventional rotary vacuum or plate and frame filtering equipment. The filter cake from a tube and/or pressure filter is passed to a mill in which the acid filter cake is broken down into small chunks which are then dried to a desired moisture content, usually about 20%. Densities of 56-69 pounds per cubic foot are reported by the supplier. Lump clays produced in rotary or Buell dryers have densities of 56- 69 pounds per cubic foot and a moisture content of about 10%, and paper mills in Europe are equipped to handle these moist chunks. In recent years slurry shipments of kaolin pigments have been widely supplied. A significant factor in the increased use of slurry shipments is that slurries avoid the dust problem associated with dry bulk handling and the wettability problem associated with use of lump clay. However, slurry shipments are limited to situations in which appropriate storing, shipping and handling equipment is available. Facilities required to handle slurry shipments require a significant capital expenditure. The dispersed slurries may contain up to about 30% water in the case of hydrous kaolins and about 50% water in the case of calcined kaolins. The expense of transporting large volumes of water is impractical under some circumstances. Thus, slurry transport is of practical use in limited situations. For these reasons, few European paper mills utilize slurry shipments. Fine particle size, low-abrasion calcined kaolin pigments, such as the products supplied under the trademarks ANSILEX, ANSILEX 93, SPECTRAFIL and LUMINEX, have become widely used by the paper industry. See, for example, U.S. Pat. No. 3,586,523, Fanselow et al . Frequently paper mills mix a calcined kaolin pigment with hydrous kaolins in the paper mill. Commonly used blends contain 20 to 10 parts by weight calcined kaolin and 80 to 90 parts by weight hydrous kaolin, usually mechanically delaminated hydrous kaolin. These calcined kaolin pigments are obtained by pulverizing spray-dried hydrous kaolins, then calcining and repulverizing. As mentioned, the pigments are shipped as a dry pulverized powder or as an aqueous slurry. Calcined kaolin pigments differ significantly from hydrous kaolin pigments in physical properties as well as performance properties. Among the differences between calcined and hydrous kaolins is the significantly lower bulk density and higher porosity of the calcined grades. The differences may be reflected in the enhanced performance of calcined grades in certain commercially important end use applications such as paper making. However, the differences are responsible, at least in part, for unique difficulties encountered in handling and shipping calcined kaolins. Calcined kaolin products are especially difficult to wet, which makes them difficult to slurry and difficult to handle in conventional bulk systems. See the discussion in U.S. Pat. Nos. 4,561,597 and 4,593,860 (both Cook, et al . ) . Cook, et al . refer to unsuccessful or impractical results obtained in attempts to agglomerate calcined kaolins by addition of water using an undefined procedure. Patentees' invention involves dry ball milling calcined powder using a high energy input. Densification apparently takes place by compaction. According to the teachings of these patents, the performance properties of the calcined kaolin in paper are not "impaired to any substantial degree" ('597 patent, col. 4, lines 6-8) . This indicates that a loss in opacification was in fact experienced in practice of the invention. In a preferred embodiment, the ball milled material of Cook, et al . is pulverized to eliminate +325 mesh residue produced during ball milling. Patentees did not seek to eliminate dusting. U.S. Pat. Nos. 4,241,001 and 4,246,220, both Lamond, et al . , relate to a process for making densified pellets of pigmentary material such as kaolin clay (apparently hydrous kaolin) . Patentees seek to provide products of "high bulk density, good handling and bulk flow properties." Lamond, et al . produce hard pellets intended for use by the rubber industry and do not seek to make a readily wettable, dust-free calcined kaolin pigment product sought by the paper industry. It has been observed that gradual addition of small amounts of water to calcined kaolin powders while mixing the powders with high shear mixing resulted in large compact balls, e.g. balls of 1" diameter. These balls were clearly not suitable for subsequent slurrying by a customer utilizing conventional slurry makedown equipment. In another effort to reduce dusting, water was sprayed on spray dried hydrous kaolin while the kaolin was being transported on a conveyor belt. This did not solve the dust problem. Thus, prior attempts to utilize added moisture to aid in the bulk handling characteristics of kaolin products left much to be desired. U.S. Pat. No. 5,328,506, Crumbley, et al . , teaches a technique for providing dustless moist bulk shipments of powdered hydrophilic (water wettable) pigments such as, for example, hydrous kaolin, calcined kaolin pigments or mixtures thereof. In Crumbley, et al . , a dry hydrophilic pigment is mixed with water or an aqueous slurry of pigment, optionally in the presence of a pigment dispersant, and the mixture is mechanically worked by kneading to produce doughlike agglomerates suitable for bulk shipping. The moisture content is generally kept to below about 30%, and no subsequent drying or pelletizing steps are included. When using calcined clays undiluted with appreciable hydrous clay, this process does not sufficiently increase the density of the material to allow for a free-flowing product. Dry clays are non-plastic in nature. That is, they do not retain their shape when shaped or molded without the use of high shear or extrusion force to make pellets. Some moist clays, on the other hand, are plastic, and are capable of being shaped or molded with or without the application of heat . For powdered calcined clay, it has been found that a minimum of about 20% water is needed to transform the non-plastic powder into a plastic material. On the other hand, when the water content exceeds about 48%, then the composition becomes a slurry of the clay in water. Therefore, the clay is generally in plastic form when the water content is from about 20% to about 48%. The exact water percentages for the plastic state may vary depending on the particular clay, but can be readily determined by one skilled in the art. SUMMARY OF THE INVENTION The present invention provides a method for producing spherical pellets of calcined clay adapted for bulk shipment and subsequent unloading in a mechanical conveyor and makedown in water which comprises the following steps: a) spraying enough water on a mass of finely divided particles calcined clay while agitating and lifting said mass to form plastic aggregates of particles of calcined clay, the aggregate having a nonspherical form; b) tumbling the moisturized plastic aggregates of clay to form spherical pellets; and c) air drying the spherical pellets to reduce the moisture content of the clay to less than about 10% by weight and to render the pelletized clay substantially free of dust. PREFERRED EMBODIMENT The process of the present invention is an inexpensive technique for producing a dustless, high bulk density, free-flowing product form for shipping calcined clay powders or calcined clay/hydrous clay mixtures in bulk form. In accordance with this invention, a dry calcined clay pigment is gradually sprayed with enough water while agitating and suspending the particles in air and thus cause the pigment particles to aggregate into plastic particles. This generally requires about 20 to 48% moisture, by weight, when the pigment is a calcined kaolin clay. Good results are obtained using a minimum of about 35% moisture, and particularly good results are obtained when the moisture is from about 35 to 45%. The gradual addition of moisture while maintaining the particles of calcined clay under turbulent, air-suspended condition, changes the powder into plastic aggregates typically ranging in particle size from 50 to 200 mesh (U.S. screen) . For example, particles designated 200 mesh are retained on a 200 mesh screen. Particles designated 50 mesh are retained on a 50 mesh screen. Establishing the plastic condition allows for subsequent pelletizing in conventional pelletization equipment, such as in a disc pelletizer. Pelletization achieves the consolidation of the finer aggregates to form larger aggregates and compaction and rounding of the aggregates including aggregates of all particle sizes. Moist, free-flowing substantially spherical pellets having a minimal content of dust are products. Drying of the plastic pellets to below 10% moisture, preferably below 2% moisture, allows for free- flowing, high bulk density dustless pellets to be formed, The dustless pellets can be easily re-dispersed into water without affecting the final optical properties of the slurry of calcined clay. The products of this invention are pelletized calcined clays adapted for bulk shipment and subsequent unloading in a mechanical conveyor and make down in water. That is, the pellets are sufficiently strong to resist breakdown to dust during shipment, but are capable of being dispersed in water when agitated therein using conventional clay make down equipment. Desirably, in a make down process, the products of the present invention are easily re-slurried to a minimum of 45% and a maximum of about 55% by weight solids, preferably about 50% solids. (All percentages are by weight unless stated otherwise . ) The pellets are substantially spherical and substantially free of dust . To be considered "substantially free of dust", the pellet compositions preferably contain less than about 4% by weight, and more preferably less than about 1% by weight, minus 100 mesh (149 μm) particles. (U.S. Sieve Series screen sizes are used here and throughout this application.) The pellets are dry, preferably containing less than about 2% by weight of water. The average particle size of the pellets, by weight, is preferably between about 40 mesh (0.42 mm) and 10 mesh (2 mm) , more preferably greater than about 20 mesh (0.841 mm) . The bulk density of the calcined kaolin clay product of the present invention is significantly higher than that of standard calcined kaolin clay, preferably at least twice as high, and the materials are free-flowing. Preferably the bulk density of the pellets is at least about 25 pounds per cubic foot (lbs/ft 1 ) , more preferably at least about 30 lbs/ft 3 . BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a schematic representation of the process of the present invention. DETAILS OF PREFERRED EMBODIMENTS The process of the present invention is depicted schematically in Figure 1. Dry calcined clay feed is fed to a mixer 10, where it is gradually mixed with water by spraying water to the clay while agitating and lifting the clay in a mixer such as a pin mixer. This causes the particles of calcined clay, originally in the micron size range, to form aggregate which may range in size from 10 to 100 mesh. Lifting while mixing facilitates consolidation of the powder into discrete plastic aggregates. Typically, average particle size of the aggregates is about 80 mesh. These aggregates contain more fines, e.g., particles small enough to pass through a 200 mesh screen, than are desired in the end product which typically has an average size between 10 and 20 mesh, for example 16 mesh. While the pin mixer is operated, enough water is added to fill the void spaces between the particles of clay and provide surface moisture on the particles, while not adding so much water that would cause the formation of a slurry. This moisturized clay is then considered "plastic", that is, capable of being formed into fixed shapes. The plastic moisturized clay is then fed to a pelletizer 12, also known as an agglomerator, where the aggregates formed in the pin mixer are formed into substantially spherical pellets and small aggregates are consolidated and rounded. At this stage, the material is referred to as "green" pellets. The green pellets are then fed to a dryer, where they are air-dried to a desired moisture level, preferably less than about 2 percent by weight. The air drying also renders the pelletized clay substantially free of dust, as discussed further below. The dry pelletized product is then ready for storage or bulk shipment. The pelletized product of the present invention is free flowing and adapted for bulk shipment and subsequent unloading, such as by a mechanical conveyer, followed by make down in water. The pellets are substantially spherical and substantially free of dust. To be substantially free of dust, the pellet compositions preferably contain less than about 4% by weight minus 100 mesh (149 μm) particles, and more preferably less than about 1% by weight of minus 100 mesh particle. As mentioned, the pellets are dry, preferably containing less than about 2% by weight of water. The average particle size of the pellets (50% by weight finer than and 50% by weight coarser than) is preferably between about 40 mesh (0.42 mm) and 10 mesh (2 mm) , more preferably greater than about 20 mesh (0.841 mm) . The mixer is a batch or continuous mixer, which is capable of providing the intimate mixing of the water and dry clay feed of the type referred to as pin-mixers. In a continuous process feed and product are continually fed and withdrawn. Various types of mixers may be used, but a pin mixer is preferred. One mixer suitable for use in this process is a Bepex Turbulizer ® pin mixer, in which rotating pins or fingers mix and knead the clay while uniformly dispersing water as a spray onto the clay to obtain a plastic product. The pelletizer is an apparatus which molds the plastic moisturized clay into substantially spherical pellets. A suitable pelletizer, which was used in illustrative examples below, is a Ferro-Tech disc pelletizer. The green pellets produced by the pelletizer should be of a size such that the dried product which exits the dryer has an average particle size which is preferably between about 40 mesh (0.42 mm) and 10 mesh (2 mm) , more preferably greater than about 20 mesh (0.841 mm) . A fine mist of water may be added to the charge in the pelletizer to facilitate operation of the equipment. Most of the water however, is added during the mixing stage of the operation. These mixers and pelletizers allow added water to fill the void spaces in typical calcined kaolin clay powders. This allows for surface moisture to be produced which causes the originally non-plastic pigments to become plastic. Mixing time in a 15 inch OD X 24 inch length Bepex Turbulizer ® pin mixer is from about 0.5 to about 2 minutes, with one minute residence time preferred. Residence time in the pelletizer is controlled by the desired final particle size distribution of the resulting pellets . A preferred average pellet size of about 10 to 20 mesh (2 mm to 841 μm) was achieved with a residence time of approximately 20 minutes when using a 10-ft. diameter by 3-ft. deep Ferro-Tech disc pelletizer as the pelletizer. The dryer is used to reduce the moisture level of the green pellets to preferably less than about 2% moisture, by weight, more preferably less than about 1% moisture. Preferably, an air dryer is used which not only dries the pellets, but also carries away substantially all of the dust particles. A preferred type of dryer is a continuous feed fluid bed dryer, with a Carrier vibrating fluid bed dryer being suitable. A rotary bed dryer can also be used, but was found not to be as effective for drying the pellets and removing the unwanted dust . Pigments used in practice of the invention (or at least a significant portion of the pigments) are hydrophilic and may or may not be capable of absorbing water. Examples of pigments include inorganic materials such as clays, especially kaolins (hydrous and calcined) , natural and synthetic alkaline earth carbonates such as calcite and precipitated calcium carbonate, mica, wollastonite, silica, alumina hydrate, smectites and naturally occurring and synthetic zeolites such as ultramarine and chabazite. Mixtures of two or more of the aforementioned are within the scope of the invention. Mixtures include mixtures of different types of hydrous kaolins, e.g., a blend of delaminated kaolin and #1 grade coating clay. Especially preferred are mixtures of calcined and hydrous kaolins. Minor amounts of hydrophobic pigments such as talc or carbon black can be included in mixtures. While products of the invention contain pigment particles and water as essential ingredients, they may also contain conventional additives. Examples are biocides, thickeners such as CMC, as well as conventional dispersants. However, products of the invention do not contain nonfugitive binders such as waxes and polymeric glues. Commercially available sources of calcined kaolin pigments useful in practice of this invention include those supplied by Engelhard Corporation under the trademarks ANSILEX, ANSILEX 93, SPECTRAFIL and LUMINEX and HIOPAQUE P. Generally, these are fine particle size powders (e.g., average size (weight basis) is below 1 micron, e.s.d.) . Such products may be prepared by selectively mining kaolin crudes, wet processing the hydrous kaolin, followed by drying, pulverization, calcination and repulverization. See, for example, U.S. 3,586,523 (supra) . Calcined kaolins are a pigment of choice when high opacification is a requirement . There are four conventional modes of shipping bulk material from the manufacturing site to the port . These include bulk rail hoppers, bulk truck pneumatic trailers, hatch top boxcars, and sea bulk containers. Regardless of the mode of transportation, the material is unloaded using an appropriate mechanical device. At this point, the product is generally conveyed to a silo or hopper as an intermediate point of storage . Direct loading from that point to ships typically takes place; however, with sea bulk containers transportation within that container is also possible. Unloading of the ship is generally done in a reverse fashion for transportation to the final end use point. In a typical process utilized by a paper mill or other end use application, product unloaded from ships is conveyed to a warehouse or storage area and stored in segregated piles. A loading device which could include conventional conveyors or even front end loaders could be used to transport material to appropriate storage bins or silos. At that point in the process make down is generally completed in a batch fashion where water is added to a mixing tank and dry clay iε conveyed for dispersion. The suitable use of chemicals and dispersants to meet the individual demands of the customer varies. Reslurried product is then typically screened to remove any foreign matter or contamination that results from the transportation process. The solids of the slurries vary depending on the final application. Examples would include nominally 30% solids for filling applications and up to as high as 74% solids utilized for some merchant grade paper coating applications. The following examples are given for illustrative purposes. In these examples, the following test procedures were employed. ANGLE OF REPOSE: The equipment used to measure angle of repose consists of a clear Plexiglass sheet bolted to a support board with a funnel attached to the top of the Plexiglass sheet. The funnel opening is large enough to let the largest particles through. To run the test, the sample is poured through the funnel and then, without shaking or vibrating the equipment, the angle of repose is measured with a protractor from the board up to the angle of the clay. In other words, a 35 degree angle of repose is 35 degrees from the horizontal. Generally two measurements are taken on opposite sides of the sample, and identified as LT and RT. BULK DENSITY: One hundred (100) grams of sample are poured into a glass graduated cylinder and the cylinder is observed from the markings on the side and this is recorded. Bulk density is 100 grams divided by the milliliters in the cylinder. The results of the analysis are then converted from g/cm 3 to pounds per cubic foot (#/ft 3 ) . TAMPED BULK DENSITY: The tamped bulk density test uses Stampfvolumeter, model STAV2003, to tamp a sample. 75 grams of sample is weighed and then the level in the cylinder is read after the device is tamped 1500 times. SCREEN SIZE: A Rotap vibrating dry screen tester with US Sieve Series standard screens was employed using 10 mesh (2 mm) , 20 mesh (841 μm) , 40 mesh (420 μm) , 80 mesh (177 μm) , and 100 mesh (149 μm) screens. In some tests, a 50 mesh (297 μm) screen was also used. Results from the test are reflected as plus (or what is retained on those individual screens) . In general, 100 total grams of feed were used for the analysis. The products of this invention are substantially dust free, which is preferably less than about 4% by weight -100 mesh, more preferably less than about 1% -100 mesh. MOISTURE CONTENT: A 1-3 gram sample of the material to be tested is placed on a tared balance pan in the oven maintained at about 130°F of a CEM AVC 80 moisture/solids analyzer. The analyzer is actuated to drive off the moisture, and the percent moisture of the sample is determined by weight loss . EXAMPLE 1 Densified calcined clay pellets in accordance with the present invention were produced in accordance with the process depicted schematically in Figure 1. For this example, mixer 10 was a 15 inch OD X 24 inch length Bepex Turbulizer ® pin mixer. In this production scale process, 4.3 gallons per minute (GPM) of well water was injected into the mixer. The calcined clay which was fed to the mixer was 1.5 tons per hour (TPH) of dry powder Ansilex 93 ® pigment feed having a particle size distribution of about 88-92% ≤ 2 μm. The resulting plastic mass had a moisture content in the range of 31 to 40% by weight. The pelletizer for this example was a 10- ft . diameter by 3-ft. deep Ferro-Tech disc pelletizer. The moisturized material was fed directly from the mixer into the rotating disc pelletizer which was rotating at 8.5 rpm. Residence time was optimized with the pigment feed rate to produce substantially round green pellets with a bulk density of 39 ±4 #/ft 3 , and 32 ±3 % moisture. These green and rounded pellets were conveyed via conveyor belt to a 45 ft. long by 4 ft. diameter direct gas-fired rotary dryer. During this early test run, the equipment was adjusted in accordance with normal operating procedures until steady-state operation waε obtained. During the course of the test run, the equipment was found to dry the pellets to 7 ±7 % moisture. Final bulk density of the dried pellets from the entire experimental test run was 30 ±3 #/ft 3 with a pellet size distribution of 72% ±11 +20 mesh, 19% ±14 +50 mesh, 5% ±3 +100 mesh, and 4% ±3 -100 mesh. A grab sample from this material during steady-state operation had a bulk density of 30.6 #/ft 3 at 9.9% moisture, with a pellet size distribution of 45.12% +10 mesh, 24.33% +20 mesh, 21.96% +40 mesh, 5.52% +80 mesh, 0.90% +100 mesh and 1.18% -100 mesh. The low value of the -100 mesh indicates that this material was essentially dust free. The relatively high moisture content was in part due to the experimental nature of this test run, and the use of a rotary dryer. As shown by Example 3 below, the use of a ' fluid bed dryer is preferred for obtaining the desired low moisture content levels. The angle of repose of this material was 32 LT and 33 RT. The Ansilex 93 pigment feed for producing this product form measured 37 LT and 35 RT, with a bulk density of 15.3 #/ft\ For comparison, two spray dried hydrous kaolin products were run through a 100 mesh screen to determine the amount of -100 mesh dust present in these products. The products were a 90.3 brightness floated clay with a slurried particle size of 91% less than 2 μm, and an 88.3 brightness delaminated clay with a slurried particle size of 79% less than 2 μm. Both of these clays exhibited large amounts of -100 mesh dust after spray drying. The floated clay was 49.0% -100 mesh, and the delaminated clay was 56.0% -100 mesh. EXAMPLE 2 The grab sample and ANSILEX 93 pigment control from Example 1 were made down in a pilot Cowles mixer at solids form 48 to 52% solids. Standard dispersion (2 It/ton of C211 and caustic to pH 6.5 to 7.5) was used in all samples. Results indicated that optical properties of the resulting slurries were not significantly changed by the densification process. Residues were initially high for the densified product, and consisted mainly of bauxite and calcined clay. The calcined clay residues were greatly reduced by increasing the solids of the slurry above 48%. The bauxite residues were merely a result of residual bauxite present in the particular test facility equipment used, and would not normally be present. EXAMPLE 3 In this example, samples of dry pelletized kaolin prepared were tested for flow properties indicative of the requirements for bulk shipping. Tests were run on samples of pellets which had a moisture content of 0.2% at 72°F. Moisture value was determined by drying small samples at 107°C for two hours in a forced convection oven. The loss in weight of each sample, divided by its original weight before drying, is denoted as the moisture. For the pelletized kaolin, these tests were run to determine the cohesive strength of the material (used for arching and rat-holing dimensions) , wall friction angles (used for calculating mass flow hopper angles) , and the bulk density/consolidating pressure relationship. Cohesive strength and wall friction tests were run for continuous flow conditions and for flow after three and seven days storage at rest . Additional tests were run to determine the pellets' tendency to crush under consolidation, as well as after impact on a surface. ANSILEX 93 ® calcined clay was fed into a Ferro- Tech Model 12T35 Turbulator™ mixer at a rate of 96 lbs. per hour (lbs/h) . Water was atomized into the Turbulator at a rate of 96 lbs/h. The product from the Turbulator mixer was discharged directly into a Ferro-Tech Model 036-3' 0" disc pelletizer. An atomized water spray was added to the disc pelletizer to reduce dusting and to control the moisture to approximately 45% moisture. Actual moisture from the Turbulator mixer product was measured at 37%, while the addition of atomized water at the disc pelletizer raised the green pellet moisture to 47%. The green pellets produced above were then fed into a Carrier pilot scale fluid bed dryer which operated with a 700°F inlet temperature and a 284°F outlet temperature. Residence time in the dryer (approximately 117 seconds) was controlled to insure all materials were dried below 1% moisture. Bulk density of the dried pellets from the test were measured and ranged between 28.7 and 30.6 lbs/cubic ft (lbs/ft 3 ) . The yield of the product of this dryer was 96%. The remaining 4% was captured as bag house fines which had a moisture of 7.6%, and a bulk density of 15.2 lbs/ft 3 . These fines were essentially identical to the ANSILEX 93 ® clay feed material in brightness, particle size and bulk density. The dryer product and baghouse fines were then tested to determine the cohesive strength and attrition resistance of the densified pellets. COHESIVE STRENGTH: Test results indicted that the baghouse fines had significantly higher cohesive strength than the densified and pelletized kaolin. For example, at a consolidating pressure of 600 pounds per square foot (psf) , the pelletized kaolin and bag house fines had unconfined yield strengths of 130 and 280 psf, respectively. These results indicate that the pelletized material would not arch in a mass flow bin. However, the high yield strength of the baghouse fines would promote arching (non-flow conditions) in a mass flow bin with an outlet diameter of less than two feet. Also, these test results indicated that the pelletized kaolin is much less sensitive to over-pressure (head pressure) effects that would occur due to vibrations during shipping. This dramatic increase in compaction of the product is shown in Table 1, and shows why such calcined pigments are generally not shipped via bulk rail. TABLE 1 The Effect of Effective Head on the Bulk Density of Pelletized Calcined Clay and Baghouse Fines Note: Fines with effective head of greater than 5 feet would not flow out of the test container. ATTRITION TESTS: Compression and impact attrition tests were conducted on the dried, densified and pelletized calcined clay produced in accordance with the process of Example 3. The compression test consisted of placing a small sample (100 g) in a short cylinder and compressing it under a known consolidation load. The load was then removed and the sample tested by screening to determine the amount of fines generated from the crushing of the sample. Table 2 indicates that the attrition caused by static loads of up to 1800 psf is negligible for the pelletized kaolin. This equates to an effective head of over 47 feet and will allow for the pelletized kaolin to be stored in a standard size industrial silo and transported by standard rail hopper cars without any degradation of the material. TABfrE 2 Effect of Static Compressive Loads on the Pellet Particle Size of Pelletized Calcined Clay The impact attrition of the pelletized kaolin was also tested by dropping a sample down a vertical pipe and onto a horizontal steel plate. The sample was then recovered and screened to determine the amount of fines generated. Table 3 indicates that the test material did not attrit at a drop of over 44 feet. This test further emphasizes the durability of calcined clay pelletized in accordance with the present invention. Table 3 Effect of Drop Height on the Pellet Particle Size of Pelletized Calcined Clay EXAMPLE 4 Densified calcined clay pellets were produced in accordance with the process depicted schematically in Figure 1. The ANSILEX 93 pigment feed for producing this product form measured 37 LT and 38 RT, with a bulk density of 15.8 lbs . /cubic foot. Mixer 10 was a 22 inch OD X 60 inch length Ferro-Tech Turbulator mixer. In this production scale process, 1.2 gallons per minute (GPM) of well water was injected by a spray into the mixer. The calcined clay which was fed to the mixer was 600 lbs . /hour of dry powder ANSILEX 93 pigment feed having a particle size distribution of about 86-90% finer than 2 microns. The resulting plastic aggregates had a moisture content in the range of 38% to 45% by weight. The aggregates ranged in particle size from 50 mesh to 200 mesh. The particles were irregularly shaped with angular sides. The pelletizer for this example was a 8-ft. diameter by 14 inches deep FerroTech Pelletizer. The moisturized material was fed directly from the mixer to a belt conveyor which fed directly the rotating disc pelletizer which was rotating at 17 RPM. Residence time was optimized with the pigment feed rate to produce substantially round green pellets with a bulk density ranging from 40-50 lbs. /cubic foot and 38-45% moisture. The green pellets ranged in size from about % inch to 100 mesh. These green and rounded pellets were conveyed via conveyor belt to a 30 foot long by 3 foot diameter direct gas fired rotary dryer. During this test, the equipment was adjusted in accordance with normal operating procedures until a steady state operation was obtained. The rotary dryer product had a moisture of between 0 and 5% moisture by weight . Final bulk density of the dried pellets from the entire experimental run was between 24 and 30 lbs. /cubic foot with a pellet distribution of : Weight Mesh 57-77% + 20 mesh, 15-25% + 40 mesh, 4-8% + 50 mesh, 3-5% + 70 mesh and 0-2% + 100 mesh. The low value of the + 100 mesh indicates that this material was essentially dust free. The angle of repose of this material ranged from 29 LT to 33 RT.

Claims: CLAIMS We claim: 1. A method for producing a pelletizing powdered calcined kaolined clay adapted for bulk shipment and subsequent unloading in a mechanical conveyor and makedown in water which comprises : a) gradually adding enough water to powdered calcined clay, while agitating and lifting the clay to form a mass of plastic aggregates composed of calcined clay, b) tumbling said mass of aggregates of plastic clay to convert said aggregates into larger substantially spherical plastic pellets, c) and air drying the pellets to reduce the moisture content of the clay to less than about 2% moisture and to render the pelletized clay substantially free of dust . 2. The method of claim 1 wherein in step a) enough water is added to the calcined clay to raise the moisture content to about 20% to 48% by weight. 3. The method of claim 2 wherein the moisture content is raised to at least about 35% by weight. 4. The method of claim 3 wherein the moisture content is raised to about 35% to 40% moisture by weight. 5. The method of claim 1 wherein the air drying step is conducted using a fluid bed dryer. 6. The method of claim 1 wherein the pellets formed in step b) have a bulk density of at least about 25 pounds per cubic foot. 7. The method of claim 1 wherein the pellets formed in step b) have an average particle size by weight, in U.S. Standard sieve sizes, of between about 40 mesh (0.42 mm) and 10 mesh (2 mm) . 8. The method of claim 7 wherein said average particle size is greater than about 20 mesh (0.841 mm) . 9. The method of claim 1 wherein the pellets after air drying in step c) contain less than about 4% by weight particles less than U.S. Standard sieve size 100 mesh (149 μm) . 10. Pelletized calcined clay adapted for bulk shipment and subsequent unloading in a mechanical conveyor and makedown in water, said pellets being substantially spherical and free of dust, containing less than about 2% by weight of water, and having an average particle size by weight, in U.S. Standard sieve sizes, of between about 40 mesh (0.42 mm) and 10 mesh (2 mm) , wherein said pellets are sufficiently strong to resist breakdown to dust during shipment , but are capable of being dispersed in water when agitated therein using conventional clay makedown equipment. 11. The pelletized calcined clay of claim 10 wherein the bulk density of the pellets is at least about 25 pounds per cubic foot. 12. The pelletized calcined clay of claim 11 wherein the bulk density of the pellets is at least about 30 pounds per cubic foot. 13. The pelletized calcined clay of claim 10 wherein said average particle is greater than about 20 mesh (0.841 mm) . 14. The pelletized calcined clay of claim 13 containing less than about 4% by weight particles less than U.S. Standard sieve size 100 mesh (149 μm) . 15. The pelletized calcined clay of claim 14 containing less than about 1% by weight particles less than U.S. Standard sieve size 100 mesh (149 μm) .

Inventors: MUNSTERMAN DAVID H, SUITCH PAUL R, MUNSTERMAN, DAVID, H., SUITCH, PAUL, R.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1997
Main classification: C09C1
IPC codes: C09C1