GB2211440A

GB2211440A - A method for dewatering an aqueous coal slurry

Info

Publication number: GB2211440A
Application number: GB8824760A
Authority: GB
Inventors: Sr Edward Harris Greenwald
Original assignee: Individual
Current assignee: Individual
Priority date: 1987-10-22
Filing date: 1988-10-21
Publication date: 1989-07-05
Anticipated expiration: 2008-10-21
Also published as: CA1330427C; FR2622203B1; FR2622203A1; GB2211440B; DE3836064A1; NZ226661A; IN170718B; AU2407988A; GB8824760D0; AU617698B2

Description

2,211440 METHOD AND APPARATUS FOR SEPARATING CLAY FR6M

XN-D-YH-EN-TE-WATERYN-G UMYRA -FINE COAL

BACKGROUND OF THE INVENTION

1. Field of the Invention: This invention relates to a method and apparatus for dewatering ultra fine coal and more particularly to dewatering an aqueous coal slurry after separating peptized clay from coal particles in a slurry thereof.

2. Description of the Prior Art: In United

States patent 4,537.599 there is disclosed a process for removing sulfur and ash, particularly clay and pyrite from the surface of coal particles. A peptized slurry of coal particles is treated to separate clay and pyrite from the coal and weaken chemically bonded contaminants on the coal surface. The clay and pyrite particles are dispersed as a colloid In an aqueous medium of the slurry. To maintain the colloidal suspension, the pH of the slurry is adjusted by the addition of a normalizer. The slurry is then beneficated in a centrifuge and in froth-flotation cells to recover coal particles greater than two microns. Thereafter, an aqueous coal slurry is,again formed and the pH is adjusted to maintain contaminants as colloids in the aqueous medium of the slurry during treatment with sonic energy and ozone. Thereafter, the aqueous coal slurry with the aqueous medium C 2 containing colloidal contaminants is classified to again separate coal particles from contaminants The cleaned coal product recovered from this process can be used for diverse purposes, one use as disclosed in U.S. Patent No. 4,662,894, is as a feed stock for the preparation of a coal water mixture.

The cleaned coal product is first separated so that coal particles of different size classifications form two or more coal feed streams which are delivered to separate surge vessels in a liquid medium. The feed stream comprised of coal particles having the smallest size are again classified to discard a minus two micron fraction which is comprised mainly of contaminants, 15articularly clay, and thereafter selected quantities of each of the coal feed streams are mixed together in the presence of a dispersing agent to form a coal-water mixture.

It is a time consuming and costly, particularly-in terms of energy requirements, to reduce the water content of a mass of coal particles, particularly in instances where the coal particles after cleaning to remove clay and pyrite are t&be used in the form of a feed stock having a low moisture content for any of diverse purposes such as for a coal-water mix. The moisture content of the cleaned M 3 coal after classification in the centrifuge according to the process of U.S. Patent No. 4,537,599, is.

usually about 32% to 36% by weight and no significant moisture reduction occurs even after several days 1 residence in storage. When an aqueous coal water slurry is dewatered in a conventional belt press, it was found that the moisture content was about 36%.

SUMARY OF THE INVENTION

It is an object of the present invention to provide a method and apparatus for economically reducing the water content of an aqueous coal slurry to a greater extent then heretofore possible without using sensible heat.

It id a further object of the present invention to provide a method and apparatus for economically dewatering a mass of ultra fine coal particles by treating the coal particles as part of a aqueous slurry wherein the coal particles are rendered hydrophobic and clay contaminants stripped from the surface of the coal particles are treated so as to flow freely from the mass of coal particles in the aqueous medium.

It is a further object of the present invention to provide a method and apparatus for dewatering an aqueour, coal slurry wherein the slurry 4 is subjected to high shear forces in the presence oi a peptizing agent so that hydrophilic clay particles stripped from the coal particles are rendered readily separable with the aqueous medium from the coal particles by treatment of the aqueous coal slurry with vibratory energy which can take the form of an air stream to drive aqueous medium from the coal particles.

More particularly, according to the present invention, there is provided a method of dewatering an aqueous coal slurry wherein the method includes imparting high shear forces to the aqueous coal slurry. in the presence of a peptizing agent to render the coal particles'dialant and hydrophobic by the stripping of clay contaminants from the coal particles, the stripped clay being peptized in the aqueous medium of the slurry, separating the aqueous medium including the peptized clay from the coal particles, and vibrating the coal particles at a frequency sufficient to drive aqueous medium from the mass of the coal particles.

In the apparatus of the present invention there is provided a vessel wherein coal particles in an aqueous medium are subject to high sheer forces as by mixing, in the presence of a peptizing agent which 4 is added to the aqueous medium in the vessel, and means for imparting vibratory energy to the aqueous coal slurry recovered from the vessel while allowing an aqueous medium to drain from the coal particles.

These features and advantages of the present invention as well as others will be more fully understood when the following description is read in light of the accompanying drawings in which:

Figure 1 is a schematic illustration of one arrangement of apparatus of the present invention which Is also suitable to carry out of the method thereof; Figure 2 is a schematic illustration of a second embodiment of the apparatus of the present invention which is also suitable to carry out the method of the present invention; Figure 3 is a schematic illustration of the third embodiment of the present invention.

Figure 4 is a schematic illustration of a fourth embodiment of the apparatus of the present invention which is also suitable to carry out the method thereof; Figure 5 is an enlarged view in section of the coal withdraw portion of a classifier included in the apparatus of Figure 4; and 6 Figures 6, 7 and 8 are graphs illustrating the moisture reduction effect on dialant and hydrophobic coal particles according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to Figure 1, there is illustrated a mixing vessel 10 into which there is added a coal feed stock, which is indicated by reference numeral 12, the feed stock may be freshly mined coal or coal salvage from silt pbnds or other suitable sources. The feed stock can be an aqueous slurry. as for example, the underflow from a static thickener or other thickening device common to the practice of codl processing. Untreated ultra-fine coal may comprise the feed stock for the process and apparatus of the present invention. Such coal generally has constituent components of: coal of varying purity; high ash impure coal or bone; clay and clay shales; pyrite/marcasite; and various other high ash carbonous and non-carbonous rocks and minerals.

If desired. the feed stock can be cleaned coal particles derived from other coal cleaning process after treatment for removal of any hydrocarbon containment from the surfaces of coal particles that are to be peptized. Preferably the feed stock is made 7 up of coal particles typically 100 mesh or less but can be 4 mesh or less, Tyler series. The vessel.10 also receives water. If required. which is introduced by line 14 to form an aqueous slurry In the vessel to which there is also added a peptizing agent by way of line 16. Preferably the aqueous coal slurry contains 25% to 35 solids by dry weight. The peptizing agent is added as one step in the forming of the aqueous coal slurry. Any one of a n er of substances can form the peptizing agent, an example of one peptizing agent is sodium hexametaphosphate which is effective in a pH range of about 6.8 to about 8.0. Another peptizing agent which is not pH sensitive is Praestaminol which is available from Stockhausen.

In the forming of the peptized coal slurry, a motor 18 is energized to rotate a mixer blade 20 that is submerged in the tank. The mixer is operated for a period of time, usually at least 5 minutes under operating conditions that impart high shear forces to the coal water slurry. During the high shear mixing/peptizing period, a high degree of aeration or air entrainment takes place including solubilizing of air in the water of the slurry. Thus there is solubilized air in the water on the surface of the coal particles. The application of high shear forces 8 breaks the adhesion and ionic bonds which bond clay particles. The mixing of the slurry in the pepi;izing vessel, peptizes the clay particles and deagglomeration occurs which renders the individual coal impurities, other than clay, and clay particles into a state of discreteness. The individual clay particles fall within a typical size range of between 0.68 to 2.0 micrometers and interact with a peptizing agent to effect an ionic exchange thereby imparting to the clay particles a strong negative electrokinetic charge. The peptized clay particles become discrete and become suspended as a colloid in the associated water of the slurry. Under these conditions, the individual coal particles attain a state of discreteness from clay and other coal impurities released from the face surfaces of the coal particles.

Once free of adhered clay. the coal particles are rendered hydrophobic. In the event the selected peptizing agent requires a pH adjustment to the aqueous slurry, then according to the demands of the particular peptizing agent a suitable neutralizing agent is introduced into the vessel to bring about the required pH adjustment.

9 The treated aqueous slurry is then discharged from the vessel 10 by line 22 to a wet screening apparatus 24 having three tandem arranged vibrating screens 26, 28 and 30 where the aqueous phase along with peptized clay particles and other undersized contaminants drain from the coal particles which remain on the screens. After the initial drainage of a predominant liquid part of the aqueous phase, the vibrating screen, imparts energy to the coal particles which are now in a state of dilatancy.

The energy causes an expulsion of surface moisture from the coal particles along with clay particles associated therewith. It can be expected that the moisture contefit of the overflow product will be reduced from an initial 70-75% moisture by weight down to 30-314 percent moisture content when discharged from the vibrating.screens. The reduced moisture content Is significantly less than the usual 40% which Is obtained without peptization and attendant clay desliming.

The wet screening apparatus illustrated in Figure 1 is of the type well known in the art and made by Derrick Manufacturing Company of Buffalo, New York.

The three screen panels 26, 28 and 30 are mounted on a frame which is excited by a high speed vibrating motor t- 32. The screen panels preferably have screens having a 400 mesh (37um) size. Typically, the screen pAnels vibrate at 3600 cycles per minute which is a high frequency vibration having a low amplitude to rapidly expel aqueous liquid including clay, which may be still present, from the coal particles. The aqueous coal slurry is distributed across the width of the first screen panel 27 by a head box 34. The screen panels are inclined to the horizontal in a manner such that the screening surfaces extend downwardly to a terminal end where the top sczeen product passes to an underlying collector 36 which directs the top screen product to discharge line 38. The aqueous liquid medium, peptiz6d clay, other undersized contaminants and undersized coal particles pass through the screens and are collected in an inclined underlying tray 40 for discharge from the machine by conduit 42.

The solids of the underflow are -37um fractions which can be discarded or when desired the residual coal content can be recovered by treatment in froth flotation cells or in a high speed centrifuge.

The peptized condition of the underproduct greatly enhances the separation process by froth cells. The coal is more amenable to the action of the frothing agent and the efficiency of coal recovery is improved.

When a high speed centrifuge is used, a cut should be made at about 2, um because the clay is predomin ' antly contained in the -2, um fraction. To be economical feasible the plus 2, um. of the -37 um fraction must be mainly coal. The burden on the centrifuge is reduced due the initial separation by the vibrating screens. In Figure 1 conduit 42 is illustrated schematically in which clay and other contaminants conducted by the conduit are discharged into a silt pond or a waste area generally indicated by reference numeral 44. The screen over product recovered from the vibrating screens is delivered by line 38 to a vibrating hopper 46 or if desired the top screen product may be'delivered first to a belt press 48.

When the coal mass is delivered by line 38 from thevibrating screens to the vibrating hopper 46, the coal particles collect in a hopper chamber 50 wherein vibrating tubes 52 are arranged as fingers as shown in Figure 1, extending along the height of the coal mass and angularly at the bottom portion to exit sites in one of two stopping bottom wall sections. The tubes 52 are perforated and packed with long grained filter media. A motor 54 coupled through an eccentric 56 to vibrator frame imparts low frequency large amplitude vibration to the tubes 52. The tubes 12 deliver liquid collected in the hopper chamber to drainage ports 58 which communicate with a drain box 60. The hopper chamber 50 has an opening in the other slopping bottom walls which communicate with a discharge chamber 62 having helical flight attachments 64 on a shaft 66 driven by a motor 68. Coal particles are carried by the flight attachments from the bottom of the hopper chamber to a discharge header box 70 for delivery by line 77. The incoming coal particles ifi line 38 typically have a moisture content of 30-31% by weight. After about 29 minutes treatment time in the vibrating hopper, the moisture content is reduced to 24% by weight.

Enercjy can be imparted to the mass of coal in the hopper chamber 50 by other means than mechanically operative vibrator device. Such other means can take the form of perforated tubes extending in the coal mass in a manner similar to tubes 52. The perforated tubes connected with a header by which compressed air can be delivered to the tube and exhausted through the perforated openings to the coal mass. Such streams of compressed air permeating the coal mass transfers sufficient energy to drive moisture from the surface of the coal particles.

13 When it is desired to speed up the dewatering process, the coal particles in line 3.8 are first mixed with a small quantity. typically A ligninsolfunate by weight of coal particles in a ribbon type mixer 74 and then the admixture of coal particles and ligninsolfunate are feed to the hopper chamber 50. After about 10 minutes treatment in the vibrating hopper, the moisture content of the coal is reduced to about 244 by weight when discharged from the hopper. No further moisture reduction will occur beyond 10 minutes treatment time with the ligninsolfunate in the hopper 46. The ligninsolfunate has an affinity for carbon and therefore drives adhered moistute on the coal from the coal particles.

A surprising result resides in the discovery that the removal of the clay renders the resulting coal particles particularly suitable for extruding operations to produce a product which can be more easily handled and shipped. Ultra fine clay containing coal without dewatering including desliming, according to the present invention, cannot be extruded without a moisture reduction to bel o.'w about 10% by weight and the addition of expensive lubricants and binders. The extruder is identified by reference numeral 76.

14 The feed stream in line 38 can be fed alternatively to a chamber 78 of the belt press 48 and discharged under gravity onto a first endless belt 80 which carries the coal water burden beyond a roller 82A to a second endless belt 84. The belts 80 and 84 are sieve belts made of synthetic fiber so that liquid, particularly water. can freely pass from the coal particles on and between the belts on a horizontal drainage section 86 and in a roller pressing section 88B. Liquid draining from the belts is collected in a container 90. The coal and liquid mixture between the belts entering section 88A is subject to high pressures and shearing forces as the belts pass alofig a tortuous path formed by rollers 88B which are connected to a suitable drive. Other rollers 88C and 88D as well as roller 88A are removably mounted to control tensioning of the belts by actuators. The dewatered feed stream is discharged from between the belts at 88E. The coal mass recovered from the belt press is ready for use.

The hydrophobic characteristic of the mass of recovered coal particles from the vibration hopper or belt press even without the addition of ligninsolfunate undergoes an accelerated reduction to the moisture content. Upon exposure to the atmosphere, a constant rate drying period is initiated during which the surface of the coal mass is entirely wet. Evaporation takes place at whatever rate is dictated by the surrounding conditions of air temperature, humidity and replacement rate of air and coal particle moisture. An upward change in the temperature of the cake whether induced internally or by external means, results in an expansion of the solubilized air. This results in creation of internal pressure on the entrained moisture in capillaries and interspaces between coal particles. Net effect of this is to accelerate the capillary migration, or wicking of the internal moisture toward the surface of the coal mass dnd in turn, the reduction of time required for the evaporative process to be totally effective.

The wet screening apparatus is surprisingly effective and produced a dramatic reduction to the water content. to the top screen product. Prior to the present invention, it was not possible to reduce the water content of a clay contaminated mass of -100 mesh (-150, um) coal particles recovered from an aqueous slurry below about 40 by weight. Moreover, a more surprising result is thedramatic decrease of the ash content of the various fractions of the minus -100 16 mesh (-150, um) screen over-product when compared with a wet sieve analysis of a peptized head sample of the feed to a Derrick screen equipped to separate at a nominal 400 mesh or 37.um. The results are given in Table 1.

TABLE I

Weight Dry Ash Over- Over- Mesh Head SanDle Product Head Sample Product glv"z-e -V- - Cum. V-Cum. % Cum. V-Cum 10OX325 32.1 32.1 66.0 66.0 15.9 15.9 8.2 8.2 325X400 5.1 37.2 10.6 76.6 17.2 16.1 11.6 8.7 40OX500 5.7 42.9 6.0 82.6 20.3 16.6 12.9 9.0 - 500 57.1 100.0 17.4 100.0 45.6 32.1 39.8 14.4 Wet sieve analyses show the transfer of the higher ash clay and other minus 37,u.m. impurities to the screen under product by way of the passage of the aqueous solution through the screen openings. This can be seen by comparison of the dry ash content of the screen over product fractions with those of the under product fractions. The results are given in Table II.

TABLE II

Weight Dry Ash Over- Under- Over- Under Mesh Pr-oWSEE Product ProdUct -Pr-wu-ct W1 'ze --- T_ Cum. V_Cum. %_ UUM. V_Cum.

10OX325 66.0 66.0 0.1 0.1 8.2 8.2 11.7 11.7 325X400 10.6 76.6 0.6 0.7 11.6 8.7 13.4 13.1 40OX500 6.0 82.6 5.7 6.4 12.9 9.0 20.7 19.8 - 500 17.4 100.0 93.6 100.0 39.8 14.4 45.8 45.4 17 In this test the cumulative recovery of coal values in the plus 25, um fraction of the feed as calculated on an ash balance basis was found to be 99.2%.

In Figure 2 there is Illustrated a further emb odiment of the present invention, in which vessel 10, like the embodiment of Figure 1,, receives coal feedstock, water and a peptizing agent from supply lines 12, 14 and 16, respectively. The feed stock is mixed under high shear forces by motor 18, and the mixing process is carried out for a sufficient period of time to peptize clay particles which are driven from the surface of the coal particles. The mixture Is delivered from the vessel 10 to a head box of a vibrating sievd bend 94 where the aqueous phase including peptized clay and undersized solids drain to a collection pan 96 from the mass of coal particles on curved screen assembly 98. The screen assembly is vibrated by drive motor 100 at a high frequency. e.g.

3600 cycles per minute at a low amplitude motion. The under product including an aqueous phase is carried away by a drain line 102. The over-product from sieve bend 98 is delivered by way of a hopper collector 104 to a pug mill 106. The fraction in drain line 102 is delivered to a vibrating screen assembly 108 which can be the same as screen assembly 24 described 18 hereinbefore and shown in Figure 1. The screen over product is delivered by line 110 to the pug mill. 106.

This arrangement of apparatus has the advantage of offering a greater residence time by the underflow in line 102 on a vibrator screen 108 so that the moisture on the face surfaces of the coal particles can be acted upon for a longer period of time as the coal particles pass along the vibrating screens. The product fed to the pug mill 106 is mixed with ligninsolfunate, a binder or other agent delivered by line 112 to the pug mill and mixed therein with the coal particles. The product discharged from the pug mill is delivered by line 114 to vibrating hopper 116.

After dewaterihg in hopper 116. the coal particles form into suitable extruded shapes by extruder 118 for shipment. Instead of extruder 118, if desired, the product from the vibrating hopper can be feed to a rotating peltatizum drum, disc or pin mixer where the coal particles are agglomerated into pellets.

In Figure 3, like the embodiments of Figures 1 and 2, vessel 10 receives coal feedstock, water, and a peptizing agent from supply lines 12, 14 and 16, respectively. Mixing occurs under high shear force conditions produced by blade 20 driven by motor 18.

The coal slurry is delivered by line 22 to a vibrating 19 screen assembly 120 which is the same as screen assembly 24. The screen under products can be discarded by line 122 and the screen over product is delivered to a pug mill 124 which includes a helical mixing blade 126. Ligninsolfunate is mixed with the coal particles in the pug mill to speed up the dewatering process. Aqueous liquid drains from the coal slurry in the pug mill by line 130. The coal particles can then be treated according to embodiments of Figures 1 and 2 downstream of assembly 24.

In the embodiment of the invention shown in Figures 4 and 5, like the embodiments of Figures 1, 2 and 3, a vessel 10 receives coal feedstock, water and peptizing agent in lines 12, 14 and 16 respectively.

The feedstock, initial pulping water and peptizing agent are introduced into the peptizing tank to produce a slurry having from 10% to 45% dry solids. The slurry is mixed for a period of time, under conditions imparting high shear forces to the coal particles. Usually a mixing time of at least 5 minutes is required. Should the peptizing agent be pH sensitive, then a pH adjustment is made to the aqueous coal slurry by the addition of a suitable neutralizing agent before the addition of the peptizing agent. This is particularly true when sodium hexametaphosphate is used. Other peptizing agents which are not pH dependent are well known. and can be selected for use. As explained previously, the mixing of the slurry in the peptizing vessel, peptizes the clay particles and deagglomeration occurs which renders the individual coal impurities other than clay and clay particles into a state of discreteness. The individual clay particles fall within a typical size range of between 0.68 to 2.0 micrometers and interact with a peptizing agent to effect an ionic exchange thereby imparting to the clay particles a strong negative electrokinetic charge. The peptized clay particles become discrete and become suspended as a -colloid in theassociated water of the slurry.

The peptize slurry after mixing, is transferred by line 22 to a dilution tank 132 wherein the percent of dry solids making up the coal water slurry is adjusted by lowering the percent to permit unhindered settling of unpeptized particles larger than the point of classification size as well as unhindered upward migration of water, clay and such other particles of coal and associated unpeptized material that are smaller in particle size than the chosen point of classification. Typically, it is desired to adjust the solid content of the slurry with 21 the addition of water from supply line 134 whereby the dry solid content is a maximum of 10 percent but.not less than 2.5 percent. Solids are kept in solution in dilution tank by means of stirring blades 136 driven by motor 138. The slurry from the dilution tank is then delivered by a flow line connected to a metering pump 140 into an upcurrent classifier 142. The point of entry in the classifier by the diluted coal water slurry is at the site of dispersion dispenser 144.

The dispenser is a geerally conical member having the apex thereof directed upward whereby a slurry entering the classifier proceeds against the inter conical wall of the disperser so that the slurry rebounds and is directed toward the truncated conical bottom 146 of the upcurrent classifier. The dispenser can take the form of a rotating perforated arm driven by the velocity of thd aqueous slurry exiting from the perforations.

The slurry introduction procedure provides guiding for the particles of coal to settle in the manner of a sediment. Water with peptized clay as a colloid therein along with small coal particles and other minerals of lesser size migrate to a point of classification along a reverse coarse of travel and pass upwardly towards the discharge opening 148 at the 22 top of the classifier. The classifier is constructed so as to siphon off the effluent in a quiescent.and controlled manner. Since the settled coal particles and non-clay mineral particles are rendered dilatant and hydrophobic in the peptization process as opposed to a non-dilatant, hydrophilic properties of clay and of a mixture of clay, coal and others associated minerals. The settled particles in the truncated conical bottom 146, of the classifier arrange themselves in a manner conducive to the exclusion of the presence of water and its accompanying peptized clay load imposed by the overlying column of liquid.

Low frequency, high amplitude vibrations, generated by a vibrator 150'driven by motor 142 are imposed on the conical bottom 146 which serves as an injucement to the resistance of the introduction of water into the settled material at the conical bottom. To eliminate the possibility of rat holing, it may be necessary in some instances to install an inverted cone in the interior of the conical bottom so as to create an annular opening between the cone and conical bottom 146 of a proper width through which settled particles can pass in their downward movement towards a discharge conveyor screw assembly 154 which draws off quantities of coal particles from the classifier.

23 As best shown in Figure 5, an upset cone 156 is fit into the interior of the conical bottom of the separator in a spaced relation from the conical bottom 146 to form angular opening between the two cones.

The width of the opening Is selected so that cone particles can pass in their downward movement towards a discharge assembly 154. As the settled particles become compacted in the annular gap, free water migrates to the interface between the compacted mass and the surface of the conical bottom. Further, dewatering is achieved by a series of horizontally arranged annular rings 158 and 160 which are installed as shown in Figure 5 on the cone 156 and annular bottom 146. respectively, to interrupt the continuity of the flow path. The rings may be packed with non corroding metal wool so as to provide an unobstructed free passage for captured water to exit at the ring through a portal opening In the side wall of the classifier. Each ring will produce a constant discharge of water from the classifier. Point size of classification is a function of the vertical distance

between the point of dispersion/distribution and the velocity of the upward noving column of water and its load of colloidal clay and ultra fine particles. In turn, velocity is a 24 function of the area dimensions of the vessel, percent of dry solids in the slurry and volumetric rate.of introduction of slurry into the up-current classifying vessel. Since the characteristic of each raw feedstock and the market requirements for the end product to be recovered from it are site specific and the largest size clay platelet is smaller that 2,um, a classification point of 2,um for most coals should prove desirable. However, higher points of classification, such as 1Oum and even as large as 25, um, may be found more suitable to effect the desired degree of ash reduction to be attained in the process.

Ash teduction is limited to the total of the ash of the clay minerals that can be peptized. In the majority of coals, this should be sufficient to reduce the ash in the finished product to a point acceptable to the market place. Since pyrite occurs as circular platelets in-all sizes in ultra-fine raw coal, all free pyrite in the peptized slurry of a finer size than the point of classification will report to the tailings and the sulfur content of the recovered product will he commensurately reduced.

The effect of rendering the recovered particles dilatant and hydrophobic along with compaction through vibration allows removal of the product from the classifier at an Initial moisture level of about 27 percent as compared to a moisture content of about 34 percent should the product be repulped and then dewatered in a high speed centrifuge. Recovery by centrifuging of an unpeptized -100 mesh (-150, um) feedstock produces a cake having a minimum of 40 percent moisture and more likely in the range of 50 percent to 60 percent moisture. By way of comparison, the moisture content of the underflow from an up-current classifier feed of an ultra-fine unpeptized raw coal slurry will rarely be less than 65 percent.

Within the initial h6ur after discharge from the vessel, natural drainage will eliminate all of the free water. During this period and continuing thereafter until all of the unbound water has been eliminated, the evaporation phenomenon is in effect.

For example,. without thermal assist the moisture content of a layer of product 1/211 to 3/411 thick will be reduced at the rate of 1.2 percent per hour until air dry equilibrium is reached.

If the product is mixed with a small amount of ligninsolfunate and then exposed to the ambient atmosphere in a layer of similar thickness without 26 moving air, the natural evaporation phenomenon results in a reduction of the remaining unbound moisture' at the rate of 3.6 percent per hour.

fine It is necessary to understand that ultra coal that has been stripped of all clay through the peptization process has entirely different physical characteristics than those of a normal coal/clay association. In addition to the dewatering phenomenon, this change in characteristics permits the reforming of ultra-fine coal into larger, more marketable forms by extrusion, pelletizing or pressing. Generally, this is not presently economically p6ssible with unpeptized ultrafine coal.

In the graph of Figure 6, a drying test is demonstrated by a comparison between peptized and unpeptized centrifuge cakes about 3/411 in thickness without the addition of ligninsolfunate in a 115 degrees F forced air atmosphere. As can be seen from Figure 6, graph line 162 indicates the per cent reduction of moisture in a processed cake comprised of dilatant coal particles and graph line 164 show a reduction to moisture contact of a raw cake comprised of coal particles that are not dilatant. A dramatically increased amount of moisture is drawn 27 from the dilatant coal particles as can be seen from a comparison of graph lines 168 and 170. The percent of the moisture content in the processed cake was found to decrease as shown by graph line 168 to about 2 percent within about 3 1/2 hours after processing according to the present invention. An unprocessed filter cake did not obtain a corresponding percent moisture content until about 14 hours after treatment as shown by graph line 170.

Figure 7 illustrates, by two sets of graph curves. the percent reduction to the moisture content of a peptized classifier coal product in a drying test wherein the ambient temperature was 75 degrees F.

average in static atmospheric air. Graph line 172 shows the dramatic reduction to the percent of the moisture content in the classifier product which is mixed with one percent by weight ligninsolfunate as compared with graph line 174 which shows the percent reduction tothe moisture content in the same classifier product but without the addition of ligninsolfunate. There was about a 47% moisture reduction to the treated peptized product within the first five hours as compared with only about a 13% reduction during the same time period of an untreated peptized product. Similarlyi the percent of moisture 28 It in the classifier product treated with ligninsolfunate dropped rapidly as can be seen by graph line 176. as compared with graph line 178 which shows the percent of moisture content of the classifier product without the addition of ligninsolfunate.

In Figure 8, the effect of heated forced air at 360 degrees F. verses 90 degrees F. ambient still air on the moisture reduction of a peptized extruded classifier product is shown by pairs of graph lines.

In the drying test depicted by the graph lines of Figure 8, classifier product samples are treated with one percent ligninsolfunate. One sample was exposed to forced air at 350 degrees F, while another sample was exposed tostill air at 90 degrees F. ambient.

The percent reduction of moisture content in the forced air classifier product is shown by graph line 180, and it can be seen that within the first 10 minutes, a very dramatic moisture reduction occurs as compared with the moisture reduction of the classifier product in still air as shown by graph line 182. The effect on the moisture content of the classifier product under forced air condition shows that within about 15 minutes as depicted by graph line 184 there was a percent moisture reduction that could not be achieved in still air until about a lapsed time of 30 29 or more minutes as depicted by graph line 186. Thus, as can be clearly understood by those skilled in the art. the effect of ligninsolfunate greatly enhances the speed at which dewatering of the mass of coal particles occurs.

While the present invention has been described in connection with the preferred embodiment of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.

Claims

Claims:

1. A method of dewatering an aqueous.coal slurry including the steps of:

stripping containments including clay from the surface of coal particles in the presence of a peptizing agent in the aqueous coal slurry to produce hydrophobic coal surface on dilatant coal particles; separating aqueous medium including peptized clay from the dilatant coal particles of the aqueous coal slurry; and dewatering the dilatant coal particles by draining aqueous medium from the hydrophilic surfaces of the coal particles.

2. The method according to claim 1 is including the further step of forming a mass of coal particles after said step of dewatering and allowing unbound moisture to pass from the hydrophobic surfaces of coal particles.

3. The method according to claim 1 wherein said step of dewatering includes vibrating the coal particles at a frequency sufficient to drive moisture from the hydrophobic surfaces of the coal particles.

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4. The method according to claim 3 wherein said step of dewatering includes discharging a compressed air supply toward said dilatant coal particles.

5. The method according to claim 1 wherein said step of dewatering includes vibrating said dilatant coal particles at a high frequency with low amplitude and at a low frequency with a high amplitude.

6. The method according to claim 1 wherein said step of dewatering includes treating the coal particles in a vibrating screen separator.

7. The method according to claim 6 including the further step of treating the tailings comprising the aqueous medium, clay and other particles recovered from said separating screen to recover coal particles greater than 2,um.

8. The method according to claim 1 wherein said step of-separating includes feeding the aqueous medium including goal particles.and peptized clay into an upcurrent classifier and withdrawing coal particles from the bottom of the said classifier and withdrawing aqueous medium including peptized clay from the upper portion of said classifier.

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9. The method according to claim 1 including the further step of mixing ligninsolfunate with the recovered coal particles, and forming an agglomerated extruded mass of coal particles after admixture with said ligninsolfunate.

10. The method according to claim 1 including the.further step of recovering dilatant coal particles after said step of dewatering; and agglomerating the recovered dilatant coal particles to form an agglomerated mass of dilatant coal particles having a desired shape and size.

11. The method according to claim 10 including the further step expelling moisture from the recovered dilatant coal particles before said step of agglomerating.

12. The method according to claim 11 Including the further step of allowing moisture to pass from the agglomerated mass of dilatant coal particles after said step of agglomerating.

13. A method of dewatering an aqueous coal slurry including the steps of:

imparting high sheer forces to an aqueous coal slurry in the presence of a peptizing agent to render the coal particles hydrophobic by the stripping of clay contaminants from the dilatant coal particles, 33 the stripped clay being peptized in the aqueous medium of the slurry and the coal particles being rendered dilatant with hydrophobic surfaces; separating the aqueous medium including the peptized clay from the coal particles; and dewatering aqueous medium from the hydrophobic surfaces of the dilatant coal particles.

14. The method according to claim 13 including the further step of forming a mass of coal particles after said step of dewatering and allowing unbound mollsture to pass from the surface of the coal particles.

15. The method according to claim 13 wherein said step of dewatering includes treating coal particles recovered from said step of separating in a vibrating hopper chamber

16. The method according to claim 13 wherein said step of dewatering includes vibrating the coal particles at a frequency sufficiently to drive moisture from the hydrophobic surfaces.of the dilatant coal particles.

17. The method according to claim 16 wherein said step of dewatering includes discharging a compressed air supply toward said dilatant coal particles.

34

18. The method according to claim 14 wherein said step of dewatering includes vibrating said coal particles at a high frequency with a low amplitude.

19. The method according to claim 13 wherein said step of dewatering includes vibrating said coal particles at a high frequency with low amplitude motion and at a low frequency with high amplitude motion.

20. The method according to claim 13 including the further step of treating the tailings comprised of the aqueous medium, clay and other particles to recover coal particles greater than 2,um.

21. 'The method according to claim 13 wherein said step of separating includes feeding the aqueous medium including coal particles and peptized clay into an upcurrent classifier, withdrawing coal particles from the bottom said classifier, and withdrawing aqueous medium including peptized clay from the upper portion of said classifier.

22. The method according to claim 13 wherein said step of dewatering includes treating the coal particles on a vibrating screen separator.

z 1 3,

23. The method according to claim 13 wherein said step of dewatering includes treating the coal particles in a belt press.

24. The method according to claim 13 including the further step of mixing a binding agent with the recovered coal particles and forming an agglomerated mass of coal particles after admixture with said binding agent.

25. The method according to claim 24 10.-wherein said binding agent includes ligninsolfunate.

26. The method according to claim 13 wherein said step of separating includes treating the aqueous coal slurry on a sieve bend.

27. 'The method according to claim 26 wherein the under screen flow from said sieve bend is treated on vibrating screens.

28. The method according to claim 26 including the further step of mixing the screen overproduct from the sieve bend and the screen over product from the vibrating screen.

29. The method according to claim 28 wherein said step of dewatering includes the mixed product recovered from said step of mixing in a hopper to drive moisture from the dilatant coal particles.

36

30. The method according to claim 13 wherein said steps of separating and dewatering.

include treating the aqueous coal slurry on vibrating screens.

31. The method according to claim 13 including the further step of recovering dilatant coal particles after said step of dewatering; and agglomerating the recovered dilatant coal particles to form an agglomerated mass of dilatant coal particles having a desired shape and size.

32. The method according to claim 31 including the further step expelling moisture from the recovered dilatant coal particles before said step of agglomerating.

33. The method according to claim 32 - including the further step of allowing moisture to pass from the agglomerated mass of dilatant coal particles after said step of agglomerating.

34.. Apparatus for dewatering an aqueous coal slurry, said apparatus including a vessel for coal particles in an aqueous medium, means for imparting a hydrophobic characteristic to the face surfaces of coal particles by subjecting the coal particles to high shear forces in an aqueous slurry in 25 said vessel, means for introducing a peptizing agent 37 to the aqueous medium in the vessel to peptize clay displaced from the face surfaces on the coal particles in the vessel. means for separating aqueous medium including peptized clay from the coal particles, and means for draining aqueous medium from the hydrophobic surfaces of the coal particles.

35. The apparatus according to claim 34 wherein said means for separating and said means for draining include a vibrating screen separator.

36. The apparatus according to claim 34 wherein said means for separating and said means for draining include an upcurrent classifier.

37. The apparatus according to claim 36 wherein said upburrent classifier includes means for vibrating coal particles collected in the bottom of the classifier.

38. The apparatus according to claim 36 wherein said upcurrent classifier includes annular rings to interrupt the flow path of coal particles.

39. The apparatus according to claim 34 further including means for agglomerating the coal particles having said hydrophobic surfaces after discharge from said means for separating.

38

40. A method of dewatering an aqueous coal slurry substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 or Figures 4 and 5, and Figures 6, 7 and 8, of the accompanying drawings.

41. Apparatus for dewatering an aqueous coal slurry substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 or Figures 4 and 5, and Figures 6. 7 and 8, of the accompanying drawings.

PWed 1989 atThe Patent Office, State House, 66171 HighHolbuT-T4LondcrWr, 1R4TP. Further copiesmaybeobtamedfrom The Patent Office. Wes Branch, St Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1187