US2042818A - Apparatus for effecting dissolution or lixiviation - Google Patents

Description

June 2, 193s. w. H, ALLEN ET AL 2,042,818

APPARATUS FOR EFFECTING'-DISSOLUTION OR LIXIVIATION 3 Sheets-Sheet l Filed Oct. 28, 1933 `une 2, 1936; w. H. ALLEN ET Al.

APPARATUS FOR EFFECTING DISSOLUTION OR LIXIVIATION Filed Oct. 28, 1933 3 Sheets-Sheet 2 311m@ 2, 1935 w. H. ALLEN Er AL 2,042,818

APPARATUS FOR EFFECTING DISSOLUTION OR LIXIVIATION Fled Oct. 28, 1953 5 Sheets-Sheet 5 .5" uids are immiscible.

Patented .lune 2, i936 TATES FATENT OFFICE APPARATUS FOR EFFE C'NG DISSOLUTION OR LIXIVIATHON Application October 28, 1933, Serial No. 695,644

8 Claims.

This invention relates to an apparatus for effecting the dissolution or lixiviation of solids, and immiscible liquids, and has particular reference to an apparatus for producing solu- '5 tions of solutes at a denite temperature and having denite concentration of such solutes.

The apparatus of the present invention is of particular Value in rapidly producing saturated solutions of a solute, free from -any excess or undissolved particles: of solute. Heretofore, when it Was desired to produce a concentrated solution of a solid, or of a liquid in another liquid in which it is immiscible, particularly a saturated solution free from any particles of undissolved --15 matter, it has been generally necessary to effect the solution in a batch process, in Which the solvent is brought to equilibrium with the solute by agitation, after which the contents are permitted to' settle before a clear solution can be Withdrawn. '2.0 In prior practice, it has been necessary, if it was desired to produce a saturated solution of a solid in a continuous manner, to use two connected pieces of apparatus, in the rst of which the solvent and solute are agitated together continuous- 25 ly to produce a. saturated solution. From such apparatus it has been necessary to withdraw a sludge of the saturated solution and undissolved solute to a second apparatus, such as a thickener, to separate the solution from the excess solute, 30 which, latter is then returned to the mixing and dissolving apparatus.

It is the primary object of the present invention to provide an apparatus for producing solutions of solutes and especially solids by which a concentrated or saturated solution may be rapidly obtained and from which a clear solution may be withdrawn free from any of the solute While the mixing operations are still beingperformed in the apparatus. 40 It is a further object of the present invention to provide an apparatus in which a saturated solution of a solute may be continuously produced and withdrawn from the apparatus free fromany undissolved material.

The apparatus of the present invention has been particularly designed for forming saturated solutions of a salt, but it may also be used in the manipulation of sludges or mixtures of solutions and solids for other purposes. For example, the apparatus may be employed in the preparation of concentrated solutions of solids by lixiviation or leaching of salts, ores, or the like. Also, the apparatus may be utilized for the preparation of solutions of liquids in solvents in which the liq- Herein, the description is given principally by reference to the preparation of solutions of solids.

In the manufacture of many crystalline salts, it is often Idesired to refine crude salts byrecrystallization. This requires dissolution of the crude 5 salt. In many instances, it is also desired to produce a solution having a definite concentration of the constituent. For example, in the production of borax from Searles Lake brine, crude salts containing minor impurities are crystallized from 10 a concentrated solution also containing other salts. These crude salts are rened by dissolving them to form a solution of borax and recrystallizing the pure borax from this solution. In the recrystallization of the borax it is desired to produce single nucleus crystals of good size. The cryst-allization of borax to produce single nucleus crystals of good size is dependent, among other things, upon controlling the number of crystal seeds present. The control of the number of crystal nuclei present ispreferably accomplished by starting with a solution free of solid particles of borax. The apparatus of the present invention enables the production of a concentrated solution of crude salts containing no solid particles, which solution may be utilized for recrystallization of the salt in the desired form of crystals.

In the handling of many solids, it is desired to produce a solution of a definite concentration, 3o which, however, is not saturated at the final temperature at which it is to be used. Such a solution may be prepared in the apparatus of the present invention by first carrying out the dissolution of the solid at a temperature at which the saturation value of the salt in the solvent is equal to the desired concentration. This saturated solution is then rendered unsaturated by changing its temperature to the desired extent. When the solute possesses a solubility which varies directly with the temperature, the desired unsaturated condition is accomplished by heating. When the solubility is 4an invert one, cooling of the saturated solution is utilized to obtain an unsaturated solution. Consequently, an unsaturated solution may be easily prepared in accordance with the features of our invention, both when the solubility curve increases with increasing temperature and when it decreases therewith.

As applied to the refining of borax, for example, it has been found desirable to eliect recrystallization of the borax from a solution containing approximately 16% NazBiOv at a temperature of about 180 F. In preparing such a solution, the crude borax is dissolved in the apparatus of the present invention at a temperature of about F. to 145 F. At this temperature the saturation concentration is approximately 16% NazBiO'i. 'I'he saturated solution may then be heated to F. to provide a clear, solids-free solution, containing the desired amount of borax at the desired temperature.

The preparation of controlled concentration Vsolutions in accordancewith out invention involves a regulation of the temperature of the solution during the digestion operation. When the substance being disolved possesses a low rate of solution, a large heat of solution (i. e. negative heat of crystallization), or both of these properties, we have found that careful regulation of the temperature during digestion is often difcult to accomplish. If the solute requires heat for solution, i. e. possesses a negative heat of solution, heat must be supplied in suicient quantity to 4both elevate the temperature of the solution to the desired point and to compensate for the heat taken up during the dissolution process. Suitable cooling means are employed when heat is given 01T during the dissolving process, and in some cases the heat required for elevation of the temperature of the solution may be supplied by the heat given off during the solution process. We provide means for conveniently accomplishing a careful regulation of the temperature of the saturated solution. We thereby obtain solutions having accurately controlled concentrations.

The apparatus of the present invention is designed to prepare a saturated solution of a solid, which may be continuously withdrawn in a clear state by the provision of a chamber, in one portion of which the solvent and solute undergo agitation together in order to effect rapid solution, while in another portion of the chamber, partially separated therefrom by a suitable di- -vision wall or walls, the same body of solutions which has reached substantially the saturation value may become quiescent to permit the continuous withdrawal o-f a clear saturated solution without the necessity of withdrawing any of the free or undissolved solute. Temperature control in the dissolver tank or container is accomplished by subjecting the mixture of solution and excess solute to a primary heat treatment pre- Vious to its introduction into the dissolver and to a secondary heating under careful control in the dissolver. In particular, we found that ease of regulation of the solution temperature is enhanced when the secondary heating is accomplished through the use of steam injected into the solution during the dissolving process. 'Iime lag of control is thereby reduced and balancing of rapid temperature fluctuations is improved.

The present invention, together with various further objects and advantages thereof, will best be understood from reference to a particular form or example of an apparatus embodying the invention. For this purpose, We have hereinafter described a preferred form or example of the apparatus. y q In the drawings:

Figure 1 represents a vertical section.

Figure 2 is a section along the line 2 2 of Figure 1.

Figure 3 is a sectional elevation of the impeller employed.

Figure 4 is a plan view of the impeller.

Figure 5 is a vertical sectional view of another form of settler.

Referring to Figures 1 and 2 ofthe drawings, the apparatus comprises an outer shell I, preferably of a cylindrical cross-section, having top and bottom closures 2 and 3 which together provide a chamber in which the desired solutions are to b-e produced. I'he bottom closure 3 is preferably dish-shaped and provided near its center with inlet pipe 3a, through which a mixture of solution and undissolved solute may be introduced into the apparatus. Within the ap-` paratus there is provided suitable means for effecting continued agitation, preferably by internal circulation of the sludge in order to effect the desired solution of the solute. Said agitating means preferably include a centrifugal impeller 4 and a vertical circulating well. The circulating well is preferably formed by a cylindrical shell 5 fixed concentrically within the outer shell I by suitable means, such as rectifying vanes 6. The rectifying vanes 6 are disposed in the annular space between the inner and outer shells,

preferably four of the vanes being utilized. The

vanes are in the form of vertically positioned plates so as to serve the dual purpose of holding the inner shell 5 in position and of reducing swirling or vortex action of the solution during circulation.

The impeller 4 is preferably fixed to a drive-1 shaft 1 journalled in bearings 8 and 9 and sup-y ported by the top and bottom plates 2 and 3.

The production of saturated solutions in rapid manner is facilitated by the provision of means for securing a positive circulation of the solute and solution in such a manner as to maintain the undissolved solute continuously in suspension in the solution during the dissolving action. In order to best attain this end, the diameter of the inner shell 5 is preferably made such that the cross-sectional area of the well will be about onehalf the total cross-sectional area of the outer shell I. Thus, there is no difference in the velocity of the liquor throughout the circulating path formed by the inner shell 5. In this manner, the apparatus avoids the formation of quiescent areas of liquid in which settling of the solids might occur. The relative areas or diameters of the inner and outer shells, however, may be varied without in some cases greatly affecting the e'iciency of the apparatus. The inner shell 5 is made to terminate sufficiently above the bottom plate 3 of the apparatus so as to provide a space for passage of fluid from the well into the annular space between vthe shells.. The vertical cross-sectional area of this space is preferably substantially equal to the area of the center well provided by the inner shell 5. The cross-sectional area of the circulating path is thereby maintained substantially constant throughout.

The impeller 4 is preferably disposed in this space provided below the inner shell. In its preferred form the impeller is a large volume, slow speed unit capable of imparting substantially stream-line movement to the solution. By "stream-line movement is meant circulation in which each unit volume of solution or sludge maintains a more or less constant relation in position to every other unit volume of solution or sludge. Thus, swirling is not stream-line movement and is preferably a minimum in the circulation provided. Swirling and vortex action tend toappreciably dissipate the forcel imparted by the impeller. The. impeller 4 is also preferably designed to discharge the liquor substantially entirely in a radial direction. The design ill() i ofstheiimpellerfor-accomplishing .theseAA purposes Y.isrbestl illustrated inxFigures Band 4.

'The impeller comprises impelling vanes I0 mounted vertically upon a base plate II at an angleof approximately 45 with diameters pass- Ling through the inner edges of the vanes. The vaneslll are illustrated as short straight pieces, but they may be of convolute contour. .The convolute contour, however, is practically straight Vnear the periphery of ythe impeller, and since 'theiimpeller vanes Ill are utilized. only at near the periphery of the impeller, the straight and .convolute contours are substantially equivalent.

:The'base plate II is an annular plate tapered .upwardly where it meets the vanes IB at an angle corresponding to the taper on the bottom plate 3 of the apparatus. For supporting the base ,plate I`I Aand impeller vanes Il) on the shaft '1, thereis provided a spider I2, having upwardly and -inwardly extending braces I3, connecting at one `end. with the base plate II and at their other end with a hub It for the impeller which is keyed to the shaft In order to prevent `undue liquor slippage through the impeller, an :annular cover plate I5 is mounted on the tops .o'ffthe vanes I0. This top plate is preferably formed at a taper parallel to that of the bottom plate II.

'The diameter of the impeller is preferably equivalent to that of the well 5 in order to facilitate the maintenance of stream-line movement throughout the system. For vthe same reason, the height of the vanes Ill is such that the area 'defined by the outer edges of the plates II and I5 is equivalent to the cross-sectional area of .the center well provided by the inner shell 5. In this way, the cross-sectional area of the circulating path through the impeller and around the well may be made substantially constant.

'I'he mixture of solution and undissolved solute introduced through the inlet 3a circulates directly through the impeller and into the circulating body of the solution. Thorough mixing of the fresh ingredients with the solution and solute in-the dissolver is thereby accomplished. Also, byzspaoing Ythe annulus II only a short distance .from the bottom 3, a stream of solution is co-nytinuously by-passed from the main circulation stream at the edge of the impeller into the space beneath the impeller and back into the main stream. The likelihood of deposition of undissolved solute in the dead space below the irnpeller is thereby decreased.

The rate of upward iiow created by the impeller shouldpreferably be sufficient to prevent vv.packing'o'f settled particles in any part of the system and to maintain all particles in suspenfsion in the solution. In some cases it may be :desirable to employ a rate of flow substantially lequal Yto or even greater than the settling rate `of the largest particles in suspension in the solution. We have found that under certain coniditions and particularly when dense sludges are .present a rate less than the settling rate of the largest suspended particles can be employed.

'The apparatus is also provided with means for baffling oi a portion of the mixture of solution and suspended solute from the main body thereof vundergoing agitation or circulation. This baflling .means permits solution which has reached the .desired concentration to become quiescent so that suspended undissolved solute may settle therefrom. This may be accomplished by the provi- .sionof acone I6 disposed above the inner shell The diameter o-f the within theouter shell flows over the upper edge 110 of the cone. It is supported concentrically within the inner shell I by suitable means, such as the .straps Il. `At its lower end it is preferably braced from the inner shell 5 by braces I8. The main body of the settler or baffle cone I6 preferably has'a slope somewhat greater than the angle of repose of the settled particles. At the bottom the cone is provided with an outlet connecting Vwith the short nipple I9, which nipple I9 is vof greater diameter than that of the shaft l. 'Ihe shaft 'I is preferably provided with a helical screw section 2S where the shaft extends through the nipple i8. The mechanism for operating the shaft I with the screw section 2B and the impeller-i may be any suitable drive means, not shown, preferably connected to the upper end of the shaft. The rotating helical screwsection 2G is preferably of large pitch and. of a diameter suiiicient to provide a close t in the nipple I9.

It serves to forcibly eject any solids which may settle in the conical settler I6 into the circulating zone existing therebelow.

The apparatus also preferably comprises suitable means, such as the outlet pipe 2|, for withdrawing the liquor from a point near the axis of the apparatus, and preferably slightly above Vthe cone I6. This is directly opposed to the conventional settler operation in which the mixture of solids and liquids is introduced near the middle at the top and the clarified solution withdrawn as an overflow at the periphery. Two zones of localized disturbance exist in a settler, one at the inlet area and the other at the outlet area. In the conventional settler, the location of the inlet at an axial point at the top creates an area of great localized disturbance and turbulence in the center of the settler. Settling is therefore diminished until the liquor iiows beyond this area of localized disturbance. In the apparatus of the present invention the intensity of the disturbance caused by the introduction of the liquor or suspension of solids in liquor into the settling cone is greatly minimized by distributing the incoming liquor' around the periphery ,of the settler. Settling then occurs 'quickly and the settled solids soon come in contact with the walls of the settler and pass down to the outlet orifice I9. By so introducing the suspension of undissolved solids at the periphery of the settler and locating the outlet conduit near the middle and the top thereof, we are enabled to continuously draw off saturated solution which is substantially free of suspended solute.

In the operation of the apparatus, the solution and excess undissolved solute are continuously introduced through the inlet 3e. The impeller 4 is continuously operated to effect circulation of the solids and solution around the inner shell 5, and the clear saturated solution is continuously I Withdrawn through the outlet 2|. In operation, the apparatus may, if desired, be operated entirely full of liquid. The circulation of solute and solution over the top of the internal shell 5 vheating within the dissolver.

draws most of the suspended solids down'through the shell, and only a small portion of undissolved solute overflows the edges of the baille and settling cone I5. This small portion will settle out of the liquor within the settling cone I6, if the dimensions of the unit are correct for the particular materials treated.

The rate of Vremoval of the solution and its temperature are so controlled that the liquor is retained in contact with suspended solute'until saturation or substantial saturation of the solution with respect to the solute is reached. 'Ihe control of the temperature of the solution being prepared involves two major factors. Suflicient heat must be supplied or withdrawn to counterbalance the heat of solution of thesolute in the solvent and to raise or lower the temperature to the desired point for producing the saturated solution. When the solubility increases directly with temperature, heat will be taken up during solution and also a high temperature must be used for the production of a concentrated solution. Thus for this class of salts, considerable heat will be required during the process. If the solute possesses an invert solubility curve, heat will be given off during the solution process and more concentrated solutions will exist at the lower temperature. Consequently cooling or refrigeration will be required for the dissolution of such solutes.

A careful control of the temperature in thedissolver is often diiiicult to accomplish. This is particularly true when the solute is slow to dissolve and when a large heat of solution. is involved. Borax, for example, possesses a comparatively slow rate of solution and a heat of solution of approximately 26,000 calories per gram-mol. We have found that the heat required for maintenance of a uniform temperature within the dissolver will Vary considerably.

In accordance with another feature of Athis invention, we accomplish the heating in two steps. We have found that a close. control of the final temperature is facilitated by effecting a portion of the total heating required in a primary operation and the remainder in a secondary The distribution of the heating between the primary and secondary treatments will Vary with various materials being dissolved. The heating required in the dissolver is desirably accomplished by injectingA stream under close control. We have found that close control is particularly well accomplished when this secondary heating is by means of steam injected under control into the solution in the dissolver. We supply steam nozzles or injectors 22 which inject steam preferably into the path of liquid in the annular space between the twoshells and 5 of the apparatus. The mechanism for controlling the amount of steam introduced may be any suitable accurate thermostatic control device operating against the solution in the dissolver or in the outlet conduit.

The heating accomplished by steam may if desired be provided by the use of an external lheater in parallel with the dissolver. A portion of the solution is then continuously withdrawn,

-passed through the heater and returned to the dissolver. When using an external heater in parallel with the dissolver for temperature control purposes, the circulation effected through the withdrawal of solution and solute and its return may conveniently replace the combination of 'internal well and centrifugal impeller described'herein. Thatis, an outside pump transferring solution and solute from the dissolver through the heater and returning it to the dissolver can be used to serve as a circulating means.

Thermostatic control of the heating to be employed or effected in an external heater is also desirable, but close control is often more dithcult to effect than with heating by steam injection.

The production of aclear liquor is dependent upon the settling area provided in the settler i8 and to some extent upon the presence of a zone of quiescence in the tank and just below the rim of the settler. The amount of settling which will result in any given settling zoneis dependent among other factors, upon the degree of quiescence or conversely upon the presence or absence of influences which 'tend to cause turbulence or liquor disturbance in the settling area. In the present instance, the settler is preferably located sufficiently above the circulation well that the greater part of the velocity head present in the circulation path is dissipated before the solution reaches the inlet rim of the settler. The actual settling area formed by the settler per se is then located in an area removed from the disturbing influence of the circulation effected in the lower part of the dissolver. Some additional settling may be accomplished by increasing the distance ben tween the settler rim and the top of the circulation well to thereby increase the extent of the quiescent area.

The creation of this quiescent area between the circulation well and the rim of the settler may be enhanced by the use of suitable baiile plates 30 preferably stream-lined to possess a concave surface on their lower faces and attached to the inner surface of the dissolver'shell l. These baffles will be preferably so disposed as to divert the upward direction of flow of the vliquor in the circulation path toward the interior of the circulation well. The velocity head of the liquor moving toward the rim of the settler is then appreciably reduced and the production of the desiredquiescent area enhanced.

In accordance with another feature of our invention we also provide another form of settler which provides greater settling area. Referring to Figure 5 of the drawings there is shown a duplicate dissolver unit of that shown in'Figure l, the same numerals referring to like parts in both gures. of Figure 1 is supplemented by an additional superimposed cone settling unit 26. It may be of similar design to the cone i6 and its settling area may have any desired relation in size to that of the lower cone i6. Herein, we have shown the conical units i6 and 26 as being equivalent in design and size. In the operation of this unit the liquor level is carried at or above the rim of the top settler 26 and will be determined by the location of the outlet conduit 2l. The mixture of solution and undissolved solute above the top of the inner shell 5, first sus the semer le, and then sus the semer is. Clear liquor is drawn off simultaneously through outlets 2i and 2l. The rate of combined withdrawal will determine the duration of the retention period for any particular piece of equipment. The relative rates of withdrawal through the two outlets are preferably regulated so that the volumes withdrawn will be in direct proportion to the settling capacities (i. e. capacities for In this Figure 5, the settler I6 producing clear liquor) of the units I6 and 26. With an apparatus similar to that described herein, it will usually be desirable to control the withdrawal of clarified liquor so that it is divided substantially equally between the conduits 21| and 2l. This control may be accomplished in any suitable manner, as by valves or by using conduits of predetermined cross-section which will permit passage of only the desired ilow.

Undissolved solute which settles in the lower unit i6 is passed, asin Figure 1, into the circulating stream near the top of the well l. Solute removed from the upper cone 26 is preferably returnedto the circulating stream in some manner which will not disturb the liquor in the lower cone. We accomplish this b'y providing a restricted section 28 from the bottom of the cone 26 to a point in the lower cone i6, but below the Zone of quiescence within which clear liquor is produced. In this manner we produce clear liquor in each settler and return all the excess solute to the circulating stream at-the top of the circulation well 5. When particles settling in the upper cone are passed into the lower cone, the outlet orifice i9 of the latter is made large enough to permit the continuous withdrawal of all the settled particles.

Movement of the excess or settled solute through the several settlerV orifices I9 and 33 is expedited by the usev of a helical screw 29 similar to that described for Figure 1. By locating the helical screw 29 around the impeller shaft 9 from a point belowthe lower settler to above the orifice of the upper settler, continued movement of solids-through both settlers is conveniently expedited. The use of such a screw section also improves movement through the leg 28.

When using a multiple cone settler, as shown in Figure 5, the lower settler is preferably arranged suilciently above the well 5 to provide a quiescent zone in which dissipation of the velocity head of the upward moving liquor takes place. Bailie plates 3B, as previously described, may be employed if desired to assist the dissipation of this velocity head.

To assist in the maintenance of the quiescent Zone within the lower settler I6 and to prevent turbulence therein as a result of settling particles in the liquor between the two settlers, we provide a skirt 3l which extends from the rim of the upper settler 26 to a level close to the. rim of the lower settler. lower edge of the skirt 3l and the rim of the lower settler is made suiiicient to permit entrance of that volume of liquor which it is desired to settle in the lower settler. Suitable vents as shown at 32 are provided near the juncture of the skirt with the upper settler to permit the escape of air present between the upper settler and the skirt when the apparatus is lled.

While we have described our invention by reference to the preparation of solutions having definite concentrations of dissolved constituents, it should be obvious that it is also applicable in many instances where the dissolution of solids in a solvent is encountered. For example, lixiviation processes and leaching processes may be carried out in the present apparatus either where a particular concentration of solution is desired or not. It is also obvious that in case of a leaching or lixiviation process provision may be made for withdrawal of solids. The use of our apparatus in multiple units with solids and liquors passing concurrently may also be desirable in leaching The distance between thel or lixiviation processes. It is also apparent that the solute and solventi may be separately introduced in the apparatus in place of being introduced through the common inlet 3a.

While the particular form of the apparatus here described is well 'adapted to carry out the objects of this invention, it is to be understood that various modifications and changes may be made without departing from the principles thereof, and that this invention includes all such modications and changes as come within the scope oi the appended claims.

We claim:

1. An apparatus for dissolution or lixiviation, comprising means for forming a dissolving chamber for solute and'solvent having an inlet and an outlet, means for dividing the lower portion of said chamber into concentric portions of substantially equal cross-sectional area through which solvent and solute may be circulated, a centrifugal impeller for circulating solvent and solute therethrough, an inverted conical baille plate providing a settling zone in the upper portion of said chamber into which solution may overflow and having a discharge orifice for the return of settled particles to the lower portion of said chamber, and means for forcibly passing particles through said discharge orifice.

2. An apparatus for preparing solutions, cornprising means for forming a chamber for holding solute and solvent, a circulating well dividing the lower portion of said chamber into up'and down circulation paths of substantially equal cross-sectional area, a centrifugal impeller rotatably mounted insaid chamber to effect circulation of solute and solvent through said path, means for forming a settling zone in the upper portion of said chamber into which solution and solute may overflow from said circulation path, means for withdrawing a clarified solution from said settling zone, and means for returning solute from said settling zone to said circulation paths.

3. An apparatus for dissolution or lixiviation, comprising means for forming a chamber for holding solute and solution, an inverted conicalshaped baie plate dividing said chamber into an upper settling zone and a lower agitating Zone, said bale plate being disposed to permit influx of solution around its upper periphery and having a lower discharge orice, the agitating Zone of said chamber being provided with means for dividing the same into concentric up and down circulatin'g paths, an impeller for circulating solvent and solute down the central path and up the annular return path, the shaft for said impeller being provided with means for forcibly ejecting settled particles through the orifice of said baiile plate.

4. An apparatus for dissolution or lixiviation, comprising means forming a chamber for holding a solute and solvent, a downwardly contracting baie means in said chamber for dividing the same into an upper settling zone and a lower zone for agitation of solvent and solute, said baie means permitting an influx of solute at its upper periphery and its lower end being opened for the return of settled particles to the agitating Zone, an inlet into the agitating zone, an outlet for solution near the upper central portion of the settling Zone formed by said baffle plate, a balile in said agitating zone dividing the same into up and down circulating paths, and means for forcing the solute and solvent to pass through said circulating paths, the lower end of the rst-mentioned baille being not substantially lower than the upper end of said second-mentioned baille.

5. An apparatus for effecting dissolution or lixiviation, comprising means for forming a dissolving chamber, a downwardly contracting baille in the upper portion of said chamber for forming a settling zone, a baille in the lower portion of the chamber for forming a circulating agitating zone, the first-mentioned baille permitting an inilux of solute at its upper periphery at la point spaced substantially from the circulating path of solute and solvent in the lower portion of said chamber, said baille at its lower end being opened for return of settled particles to the agitating zone, an inlet into the agitating zone, and an outlet for solution near the upper portion of the settling zone formed by said first-mentioned baille.

6. An apparatus for eilecting dissolution or lixiviation, comprising means for forming .a dissolving chamber, a downwardly contracting baille in the upper portion oi said chamber for forming a settling zone, a baille in the lower portion of the chamber for forming a circulating lagitating zone, the first-mentioned baille permitting an iniluX of solute at its upper periphery at a point spaced substantially from the circulating path of solute and solvent in the lower portion of said chamber, said baille at its lower end being Opened for return of settled particles to the agitating Zone, an inlet into the agitating zone, an outlet for solution near the upper central portion of the settling zone formed by said rst-mentioned baille, and an impeller for forcing solute and solvent to circulate through said circulating path..

7. An apparatus for effecting dissolution of lixiviation, comprising means for forming a dissolving chamber, a downwardly contracting baille in the upper portion of said chamber for forming a settling zone, a baille in the lower portion of the chamber for forming a circulating agitating Zone, the ilrst-mentioned bale permitting an influx of solute at its upper periphery at a point spaced substantially from the circulating path of solutel and solvent in the lower portion of said chamber, said baille at its lower end being opened for return of settled particles to the agitating Zone, an inlet into the agitating zone, an outlet for solution near the upper portion of the settling zone formed by said first-mentioned baille, and means for forcing settled particles through the open end of said first-mentioned baille into the lower portion of said chamber.

8. An .apparatus for eilecting dissolution or lixiviation comprising means forming a dissolving WILLIAM H. ALLEN. ELIOT UNDERHILL. CHARLES F. RITCHIE.

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