US2885154A - Method of and apparatus for grinding solid materials by fluid energy - Google Patents

Description

y 1959 DU BOIS EASTMAN ET AL 2,885,154

METHOD OF AND APPARATUS FOR GRINDING SOLID MATERIALS BY FLUID ENERGY 2 Sheets-Sheet 1 Filed Aug. 17, 1954 MQQNUXb y 5, 1959 Du BOIS EASTMAN ET AL 2,885,154

METHOD OF AND APPARATUS FOR GRINDING SOLID MATERIALS BY FLUIDENERGY 2 Sheets-Sheet 2 Filed Aug. 17. 1954 hwy NEQ Q METHOD OF AND APPARATUS FOR GRINDING SOLID MATERIALS BY FLUID ENERGY Du Bois Eastman and Frank E. Guptill, Jiu, Whittier, 'Calif., assignors to The Texas Company, New Yorir, N.Y., a corporation of Delaware Application August 17, 1954, Serial No. 450,506

9 Claims. (Cl. 241-) The present invention relates to a novel method of and apparatus for employing fluid energy for grinding particles of solid materials.

Recently, there has been developed a novel method of grinding solid particles by first making up a slurry of the solid particles in a vaporizable liquid such as water, and then pumping this slurry through an elongated tubular heating zone wherein the liquid is vaporized by heating the slurry. The resulting dispersion of solid particles in vapor flows turbulently at a high velocity in excess of 25 feet per second, and preferably in excess of 100 feet per second, through the elongated tubular zone so that the particles impinge forcibly against one another and are disintegrated. The disintegrated solid particles in vapor then pass to a centrifugal cyclone separator or other separating device wherein the vapor is separated and passes off the top while the ground solid particles are taken oii the bottom as product. Reference is made to Patent 2,735,787, issued February 21, 1956, for a more complete description of this grinding procedure.

While considerable grinding of the solid particles takes place when the dispersion simply flows at high velocity and turbulently through a long unobstructed tube, it has been found that erosion difficulties are minimized by flowing the dispersion at a relatively low velocity through such a tube and then increasing the turbulence and absolute velocity of particles, or their relative velocities with respect to one another, in a localized zone. One way proposed for locally increasing absolute velocity is to pass the dispersion through a convergent-divergent nozzle at supersonic velocity while the velocity upstream of the nozzle is maintained relatively low, such as 100 feet per second or less, to prevent erosion. Another 'procedure for locally increasing relative velocities is to divide the slurry stream or the flowing dispersion stream into two parts, and then to impinge the resulting two flowing streams of dispersion against one another at high velocity by passing them through a pair of nozzles which are opposed to one another at 180, more or less.

While the above-described procedure has been operated successfully in a pilot plant, it has only recently been adapted to a commercial scale. The present invention deals with novel techniques and designs of apparatus which have been devised to make possible the conversion from a pilot plant scale to a commercial scale of operations.

Among the objects of the present invention are: to provide a novel procedure for starting the operation of a grinding plant designed in accordance with the invention when it is initially at or near ambient temperature; to provide novel automatic control apparatus and procedures whereby the flows of liquids in various parts of the plant, are correlated with one another so as to assure continuous, safe and economical operation; to provide a novel and economical procedure for preheating; the liquid used for making up a slurry of the solid par ticles to be ground; to assure a substantially constant 2,d85,l54 Patented May 5, 1959 proportion of solid particles in the slurry being fed to the grinding apparatus; to recover any fine particles of product which may have been carried over with the steam separated in the cyclone; to recover useful energy from the steam separated out in the cyclone, thus improving the economy of operation; to provide for economical heating of the slurry to vaporize the slurry liquid; to provide for removing oversized particles from the solid product recovered in the cyclone; and to prevent clogging of the system by the accumulation of solid particles therein.

The above and other objects and the novel features of the invention will become apparent from the following detailed description, having reference to the drawings wherein:

Fig. 1 is a how diagram of an arrangement of apparatus for performing the method of the present invennon;

Fig. 2 is a fragmentary vertical sectional view of the grinding head as seen from the left in Fig. 1;

Fig. 3 is a schematic diagram, parts being in vertical section, showing apparatus for indicating and automatically controlling the proportion of solid particles in a slurry to be fed to the grinding apparatus; and

Fig. 4 is a schematic sectional View, parts being in elevation, of the specific gravity meter shown in Fig. 3.

Many solid materials can be ground by the method and apparatus of the present invention such as coal, talc, barite, oyster shells, limestone, clay, chalk, gypsum, marl, and metals such as aluminum and magnesium. Their particle size generally should be less than one third the pipe diameter, and desirably is less than M. inch in diameter as received from the mechanical crusher. Also, numerous liquids can be used for making up the slurry, such as water, oil or kerosene. The proportion of liquid mixed with the solid should be sufiicient that the slurry exhibits the general flow properties of a liquid in that it will flow through a conduit under the influence of gravity. Generally it is less than 2:1 by volume, often being 1:1, and usually more than 1:2. For simplicity, the following description will refer to talc as representative of solid materials, and to water as representative of volatilizable slurry liquids, but it is apparent that the principles also apply to other solids and other liquids.

Assuming that the plant is down and the apparatus is cold, a slurry of talc and water is made up in a slurry tank 11 by opening valve 13 at the bottom of a tale feed bin 15 and by operating one or both three-way valves 17 and 18 so that the straight-through ducts of the T-shaped passages connect lines 19 and 21 to the slurry tank. Generally the proportions of tale to water are about 1:1 by weight, and maintenance of these proportions is assured by occasionally taking samples and determining their specific gravity so that the operator can tell whether water or talc should be added at a greater rate. The tale particles in the feed bin 15 may be the product from a Hardinge ball mill averaging about 20 microns in diameter, or the product from a secondary gyrator cone crusher which is l4 mesh (US. Standard) in size. Solid particles are held suspended in the water of the slurry by a motor-operated agitator 22 in the tank, and by continuous circulation through a side arm in a manner to be described hereinafter.

When therslurry tank 11 has been filled, continuous circulationof the slurry through a closed side arm system is begun by turning the valve 18 so that slurry flows from the tank 11 into a conduit 23 and is pumped by pump 25 through a specific gravity meter 27 and a conduit 29 back into the slurry'tank. This assures thorough mixing of the slurry in tank 11 while at the same time keeping track of the-slurry proportions as shown by the specific gravity.

entrance to an exhaust stack 48 so as to preheat the water almost to its boiling point. Tubes of the preheating section are kept free of soot by periodically blowing blasts of compressed air over them from a plurality of nozzles on a soot blower tube 46.

From the preheating section 43 the water then flows out of the heater through a conduit 47 into a vaporizing section 49 comprising a pipe coil arranged near the peripheral wall of the heater in the hottest section, wherein the water is heated still higher to vaporize it and form high pressure high temperature steam. This steam then leaves through a conduit 51, passes through a two-way valve 53 to a conduit 55, then through a two-way valve 57, and is discharged through a conduit 59. Alternatively, valve 57 can be connected to a conduit 60 leading to a scrubber 78t'or delivering startup steam and solids to the latter so that the solids can be returned to tank 11 until operating conditions have been attained.

Overheating of the piping of the coil 49 is prevented by providing a heat resistant cone 50 centered within the coil near its top. Cone 50 is suspended from four threaded rods 52 (only one being shown), and it can be raised or lowered to the most suitable position by operating nuts 54 on the rods.

Concurrently, a grinding device 61 (to bedescribed morein detail hereinafter) is preheated to a high temperature by introducing hot air through a valve-controlled conduit 63, after which the hot air leaves through the large discharge conduit 65 and enters a cyclone 67 to preheat the latteralso. Condensation of steam in the grinder and subsequent piping is thus avoided, thereby preventing possible clogging with solids. Hot air leaving the top of cyclone 67 flows to scrubber 78.

When the heater 45, the tubular sections 43 and 49, the grinding device 61, the cyclone 67,and the scrubber 78 have reached the normal operating temperature, say around 750' F., a switch-over is then made to slurry operation. This is accomplished by turning the control valve 17 so that the connection between water line 19. and conduit 31 is broken and a connection is completed between the conduit 31 and slurry tank 11; and by opening a closed valve 39 to connect a conduit 77 with tank 11 to return any slurry in excess of that taken in by the suction of pump 37. Slurry from tank 11 is then pumped by booster pump 33 and main pump 37 into the sections 43 and 49 of the elongated heating zone in the same manner described previously for water. The settling of solids from the slurry in the lines from tank 11 to pump 37 is prevented by purposely feeding slurry therethrough at a rate considerably greater than the demand of pump 37 and allowing the excess to return to the tank line 77.

In the heating zone the flame from oil burner 69 heats the water content of the slurry, vaporizing it and forming a flowing dispersion of the solid particles in steam which passes by way of conduit 51 and valve 53 (now turned so as to disconnect line 51 from line 55 and to connect line 51 with a conduit 71) to the grinder 61.

Conduit 71 is alternatively and selectively connectable through a two-way valve 72 to two grinding heads 73 and 74 supplied with dispersion by two pairs of ducts 75 and 76, the ducts of each pair being connected to opposite sidesoflagrinding head so as to deliver opposed jets of dispersion at extremely high relative velocity against one another from l80'opposed pairs of nozzles 68 and 70 within the grinding heads, thereby disintegrating solid particles. Nozzles 68 and 70 should have relatively small diameter, such as inch. A gate valve is provided 4 I t F for isolating grinding head 73 when its repair is necessary. From grinder 61 the resulting dispersion of ground solid particles in steam flows at relatively low velocity through conduit 65, which is of relatively large diameter such as 12 inches, into the cyclone 67. A sampling cyclone 64 is connected into conduit 65 by a valved duct for keeping a check on product quality.

Cyclone 67 may be of any conventional construction,

as is well known for centrifugal separators. Steam leaves the top of the cyclone through a conduit carrying with it a small amount of extremely fine talc which escapes recovery with the product at the bottom of the cyclone. Product talc is taken off the bottom of the cyclone periodically through a valve 81, enters a conventional mechanical separator 82 (such as the Raymond Whizzer Scparatorshown on pages 9-34 of Taggart's Handbook of Mineral Dressing (1945) within which any oversized particles are removed, passes through a conventional 30 mesh vibratory screen 86 designed to remove any feed material which may have entered the product stream due to some process upset or equipment failure, passes through a conventional Fuller-Kinyon solids pump 84 comprising a feed screw and air jet (as shown on pages 18-54 of Taggart), and then is removed as the ultimate product. Oversize particles from separator 82 are returned to slurry tank 11 by way of a conduit 88 for recycling. l

The steam and fines carried over at the top of the cyclone pass by way of the conduit 80 to a point near the bottom of a scrubber and feed water preheater 78, while fresh feed water is supplied at the top through a conduit 79 having an automatically operable control valve 83 set for normally passing water at a substantially constant rate,

and a manually controlled bypass valve 85 around the automatic valve.

Scrubber 78 is a tall column of relatively small diameter having a plurality of staggered trays 87 spaced vertically from one another along the column so that the water-flows downwardly in sheets along a tortuous path burner 69. Any desired pressure can be kept on the system by means of an adjustable back pressure regulator 92 in conduit 89. While atmospheric pressure is satisfactory from the grinding standpoint, higher back pressures are desirable when energy is to be recovered from the steam. For example, high pressure clean steam can be used for operating pumps, compressors, generators or the like. a a

The water is preheated as it flows down the column and the tale fines are scrubbed out of the steam so that a pool 95 of preheated water and talc fines accumulates in the bottom of scrubber 81. A level-responsive device such as a float 97 operates a pneumatic controller 98 in an air line 100 for actuating an automatically operable valve 99 located in a discharge conduit-101 leading to a surge tank 103. When the level of pool 95 reaches a predetermined high point, float 97 actuates controller 98 and causes valve 99 to open, whereupon preheated water and talc fines flow into surge tank 103. When the level of pool 95 drops to a predetermined low point float 97 again actuates thecontroller to close valve 99.

Surge tank 103 includes a float 105 responsive to the level of liquid in the surge tank and controlling a switch which in turn controls the automatic water regulating valve 83 in conduit 79 so that when the level of the water in the surge tank reaches a predetermined low point the switch is opened and valve 83 is opened further to increase water flow above the normal rate;

and when the level reaches a predetermined high point excess of water in the surge tank above that required to make up slurry in the slurry tank 11 is allowed to overflow through a pipe 107'.

Flow of water from surge tank 103 into the slurry tank 11 is controlled automatically in accordance withthe liquid level in the slurry tank by means of a float 109 in the slurry tank operating a switch which in turn operates an automatic valve 111 in a conduit 113. Whenthe level in tank 11 is at a predetermined high point the float 109 causes valve 111 to be closed and the flow of water reduced, and when the level reaches a predetermined low point the float 109 causes valve 111 to be opened and increase the flow of water.

Occasionally some difficulty may arise from clogging of the heater tube 49 with deposited solids. Such clogging is most likely to occur in the zone where the water content of the slurry is evaporated to steam, due to the low steam velocity. To prevent clogging it is desirable to interrupt the flow of slurry periodically and pass plain water through the system without solids for sulficient time to remove any deposited solids. This can be done by operating the valve 17 to connect water conduit 19 to the conduit 31 leading to pump 37.

Flushing with water may be done, for example, for five minutes after eight hours of operation on slurry. Advantageously the same flow rate of water by weight is maintained as of slurry, thus assuring that liquid water will be present in the zone of solids deposition, rather than just steam. The water which later is vaporized passes off through the cyclone separator and scrubber as usual.

After the water flushing period, the operation is switched back to slurry by means of valve 17.

A number of controls and safety devices are embodied in the improved grinding plant described above.

Inasmuch as water from surge tank 103 and talcfrom feed 'bin 15 are almost continuously being added to slurry tank 11, there is provided in line 23 an automatically operable specific gravity meter 27 to tell the operator how to vary manually the feed of talc particles into the slurry tank to preserve the proper ratio of talc to water. Specific gravity meter 27 also may be operatively connected to the outlet from feed bin 15 to regulate automatically the feed of tale to tank 11 in accordance with variations in specific gravity of the slurry, as will be described in detail hereinafter.

The supply of fuel oil to heater 45 is subject to a safety shut-01f valve 121 in line 93 which is responsive to the conditions of a pressure gauge 123 in conduit 41 or a photocell 125 within the heater. When pressure in conduit 41 rises too high as the result of a stoppage in the heater tubes, gauge 123 causes valve 121 to close and shut 011 the flame. Further pressure increase actuates a pressure relief valve 127 to blow olT slurry through a conduit 129. If the flame in the heater 45 goes out. the'photocell 1'25 responds and shuts valve 121.

Pressure relief also is provided by rupture discs at sensitive points, such as the discs 1'31 and 132.

Temperature in the heater 45 is controlled automatically by coacting temperature controllers 133 and 135, respectively responsive to the temperature in the stack of heater 45 and the temperature of the dispersion leaving the heater through conduit 51, and regulating the fiow of oil through burner 69 by means of an automatic valve 137.

in addition, ordinary pressure gauges are provided throughout the system, as at 139,141, 143, 145 and 146 to assist in controlling operations.

Another important safety factor is the provision of an auxiliary air-operated pump 149 for automatically pumping water into the system if a power failure should cause pump 37 to shut down. Pump 149 .is operated by compressed air from a high pressure air receiver 151 which is connectedto the pump by a conduit 153 having a pressure reducing valve 155 and a normally closed solenoid valve 157 therein. The solenoid of valve 157 is connected into the power line supplying pump 37 so that, on power failure, valve 157 opens automatically. Auxiliary pump 149 then pumps water through conduit 159 and check valve 161 into conduit 41 to flush the heater tubes 43 and 49, thus preventing plugging of the heater tubes with talc and overheating of the tubes due to stoppage of flow.

An automatic control system is shown in Figs. 3 and 4 for continuously maintaining a substantially constant proportion of solid particles in the slurry fed to the grinding system.

Slurry from tank 11 passes out through conduit 23 having a pump 25 therein, and enters horizontal U-shaped section or loop 27 having opposite ends connected into the conduits 23 and 29 by flexible hose couplings 171 and 173 which permit section 27 to move in response to variations in the weight of slurry therein. In a typical design, loop 27 is a 1 /2 inch aluminum pipe 25.7 feet long bent into a'loop 12.6 feet long, with its legs 10.5 inches apart.

Loop 27 is supported by a fixed knife-edged fulcrum 175 adjacent the open end 177 of the loop so that the closed end 179 will move up and down as the weight of slurry in the loop fluctuates.

A specific gravity meter 181 comprises a torque tube 183 which is adjustably connected to the mid point of a balancing lever 185 by a nut 186. One end of lever 185 is connected by a pivoted link 187 to the closed end of loop 27. The other end of lever 185 carries balancing counterweights 189. An indicator hand 191 operated by torque tube 183 traverses a scale to indicate the specific gravity of the slurry as reflected by the weight in loop 27. Obviously other means may be employed for converting movement of loop 27 to specific gravity signals, for example a pair of induction coils linked by a core on loop 27, or a photo-electric device responsive to changes in position of the loop.

In calibrating the mechanism the lever 185 and indicator hand 191 are set at zero while flowing plain water fills loop 27, after which nut 186 is tightened. Thereafter, deviations of the indicator hand from zero are proportional to the specific gravity of the slurry.

Torque tube 183 also is connected by a rod 192 and lever arm 194 to a valve 196 (see Fig. 4) to provide a pneumatic signal by varying the pressure of compressed air supplied from a supply line 193 to a conduit which leads to a conventional chart recorder and controller 197. Controller 197 in turn operates a valve connecting a conduit 199 to an air supply line 200 to regulate the pressure in conduit 199 proportionally to the signal from meter 181, thus activating the diphragm of an air motor 201 to increase or decrease the speed of a variable speed electric motor 203 by actuating a plunger 204 connected to a speed regulator such as a rheostat (not shown).

Motor 203 is operatively connected to a feed device 205 at the bottom of feed bin 15 to carry solid particles out of the feed bin to the tank 11. Feed device 205 can be of any conventional type such as a worm screw or paddle wheel.

In operation, controller 197 is set to regulate motor 203 at a normal constant speed when the slurry in tank 11 has the desired composition and specific gravity meter 181 is at a position indicating a specific gravity corresponding to the desired composition.

If for any reason the feed rate of solid particles from feed bin 15 is insufficient to maintain the desired composition, the specific gravity of the slurry drops off with the result that the'weight of material in loop 27 decreases, the closed loop end 179 rises and the speed of feed device 205 increases to increase the particle feed rate and bring the specific gravity and composition back to normal.

Conversely, if the feed rate is too great, the specific gravity increases, the closed loop end 179 falls, and the 1 lpeedoffeeddevicezwisdecreasedautomaticallyto decrease the particle feed rate.

The method and apparatus ,for automatic control de-. scribed above are especially advantageous where the feed rate of water from. water tank 103 may be variable, as changes in water feed require compensating changes in solids feed.

In an example of how the method described above is performed, talc of 20 microns average Particle size flows at a rate :of 3 tons per hour from feed. bin 15 into slurry tank 11 where it is mixed with unequal weight of hot water (212' F.) to provide a slurry which is pumped through 116 inch pipe 41 at a pressure of. 1600 pounds per square inch (p.s.i.) and a rate of 16.3. gallons per minute (g.p.m.) to heater tube 49, wherein the water is vaporized and a dispersion formed of talc particles in steam at a temperature of 750' F. and a pressure of 930 p.s.i.

The dispersion then, flows through 0.391 inchbore nozzles 8 and 7. to form two iets flowing atsonic velocity and impinging against one another at 180?. Talc are greatly reduced in size as a result of the impact, to the point where 80% is finer than 11 microns and 50% is even finer than 3.5 microns.

The dispersion then fiows through a 12 inch pipe 65 to cyclone 7 wherein steam is separated and flows "off" the topat p.s.i..and 650' F. while product tale is removed at the bottom.

The 08 steam enters the bottom of scrubber 78 and flows upwardly while cold water initially at 70' F. enters through conduit 79 at a rate of 18 g.p.m. Inpassage the water is heated to 212' F. and then enters surge, tank 1.3. From the surge tank water flows at 12 g.p.m. through conduit 113 into slurry tank 11.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and, therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

l. A method of pulverizing particles of a solid material comprising the steps of preparing a slurry of such particles in a vaporizable liquid; passing a stream of a vaporizable liquid through an elongated tubular heating zone and heating said liquid therein to above itsiboiling point to form a flowing stream of hot vapor; concurrently preheating a fluid energy grinder to a high temperature by passing a hot gas therethrough; when said elongated tubularheating zone and said fluid energy grinder have attained. a desired preheat temperature discontinuing the flows of said hot gas to said grinder and of said va orizable liquid to said tubular heating zone; immediately thereafter starting the flow of said slurry through said preheated tubular heating zone, vaporizing the liquid thereof and forming a flowing dispersion of solid particles in vapor, and then delivering said dispersion to said grinder for disintegrating such particles of solid material; thereafter separating the vapors of said dispersion from the solid particles so disintegrated, collecting said solid particles as product, passing the separated vapors in direct heat exchange contact with a streamof said vaporizable liquid to preheat said liquid and recover any suspended solid particles from said vapors; and then feeding the liquid ao preheated and any solid particles therein to form said slurry.

2. A method of pulverizing particles of a solid material compriaingthestepsofpreparingaslurryofsuchparticles in a vaporizable liquid; passing a stream of a vaporizable liquid-through an elongated tubular heating zone and heating said liquid thereinto above its boiling point to form a flowingrstream of hot vapor, thereby preheating said tubular heating zone; concurrently preheating a fluid energy grinder to a high temperature by passing a hot gas therethrough; when said elongated tubular heating none and said fluid, energy grinderhave attained a desired preheat temperature discontinuing the flows of said hot gas to said grinder and of said vaporizable liquid to said tubular heating zone; and immediately starting the flow of said slurry through said preheated tubular heating zone, vaporizing the liquid thereof and forming a flowing dispersion of solid particles in vapor, and then delivering said dispersion to said grinder for disintegrating such particles of solid material.

3. A method in accordance with claim 2 also comprising continuously circulating said slurry in a closed circuit while preheating said elongated tubular zone and said grinder, whereby it is possible to change instantaneously from the preheating operation to the slurry feeding operation.

4. A method for pulverizing particles of a solid material comprising the steps of preparing a slurry of such solid particles in water; passing said slurry through an elongated tubular heating zone while heating said slurry to vaporize said water and form a dispersion of solid particles in steam; disintegrating said solid particles while flowing in said dispersion; separating steam from the disintegrated solid particles; scrubbing said separated steam free from any fine particles carried therewith by passing water in contact therewith, thereby transferring heat from said steam to said water and transferring said fine particles to said water; feeding said preheated water and fine particles to a pool before said slurry; thereafter feeding the water so preheated together with said fine particles from said pool to form said slurry; and automatically controlling the flow of cooling water in said scrubbing step in accordance with the level of the liquid in said pool, said flow normally being substantially constant but being increased automaticallywhen said level reaches a predetermined low point.

5. A method for pulverizing particles of a solid material comprising the steps of preparing a slurry of such solid particles in water; passing said slurry through an elongated tubular heating zone while heating said slurry to vaporize said 'water and form a dispersion of solid particles in steam; disintegrating said solid particles while flowing in said dispersion; separating steam from the disintegrated solid particles; scrubbing said separated steam free from any fine particles carried therewith by passing water in contact therewith in a scrubbing zone, thereby transferring heat from said steam to said water and transferring said fine particles to said water; thereafter feeding the water so preheated together with said fine particles to form said slurry; automatically controlling the feeding of said water and fine particles outof said scrubbing zone in response to changes in the liquid level in said zone; and automatically controlling the rate of feeding particles to form said slurry in accordance with variations in the specific gravity of said slurry, such feed rate being automatically increased when the specific gravity decreases below normal, and being automatically decreased when the specific gravity increases above normal.

6. In a method of pulverizing particles of a solid material by preparing a slurry of said particles in a liquid, and passing said slurry through an elongated tube while heating said tube and the slurry therein to vaporize said liquid and form a flowing dispersion of said solid particles in vapor, the improvement which comprises preventing the plugging of said elongated tube with deposited solids by periodically stopping the passing of slurry therethrough and passing plain liquid therethrough, and thereafter resuming the passing of slurry therethrough.

7. Apparatus for pulverizing particles of a solid mate rial comprising, in combination, a tank for containing a supply of a slurry of said solid particles in liquid; a heater containing an elongated tube having an inlet and an outlet, for heating said slurry and converting the liquid thereof to vapor to form a flowing dispersion of solid particles in vapor; means for transferring slurry from said tank to said inlet; means operable for alternatively supplying liquid alone or slurry to said elongated tube whereby liquid alone can be passed through said elongated tube during the starting period of a grinding operation, after which a switch to slurry may be effected; a grinder communicating with said outlet of said tube; a separator communicating with said grinder for separating vapor from said solid particles; a scrubber communicating with said separator for receiving said separated vapor and any solid particles carried therewith; means for passing liquid through said scrubber to be preheated by said vapor and to remove any solid particles therefrom; and means for passing said preheated liquid and separated solid particles to said slurry tank.

8. Apparatus in accordance with claim 7 also comprising means for circulating slurry from said tank through a closed circuit and back to said tank during said starting period while liquid alone is being pumped through said system.

9. Apparatus for pulverizing particles of a solid material comprising, in combination, a tank for containing a supply of a slurry of said solid particles in liquid; a heater containing an elongated tube having an inlet and an outlet, for heating said slurry and converting the liquid thereof to vapor to form a flowing dispersion of solid particles in vapor; means for transferring slurry from said tank to said inlet; a grinder communicating with said outlet of said tube; a separator communicating with said grinder for separating vapor from said solid particles; a scrubber communicating with said separator for receiving said separated vapor and any solid particles carried therewith; means for passing liquid through said scrubber to be preheated by said vapor and to remove any solid particles therefrom; means for feeding said liquid and accompanying removed solid particles to said tank; and means responsive to the liquid levelin said scrubber acting automatically to control the discharge of liquid and accompanying removed solid particles therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 1,493,579 Walter May 13, 1924 2,332,953 Tromp Oct. 26, 1943 2,552,603 Tanner May 15, 1951 2,560,807 Lobo July 17, 1951 2,596,352 Wuensch May 13, 1952 2,612,320 Croft Sept. 30, 1952 2,704,635 Trost Mar. 22, 1955 2,735,787 Eastman et al Feb. 21, 1956 FOREIGN PATENTS 315,229 Great Britain Pub. 1931 683,318 Great Britain NOV. 26, 1952

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