CA1145115A - Method of recovering slurry mother liquor in the recovery of salts from aqueous solutions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is an improved method of recovering a first said from an aqueous solution containing the first salt and a second salt, the first salt having a solubility increasing more with increasing tempera-tures than the solubility of the second salt. The solution is passed through an evaporation zone thereby concentrating the solution with respect to the first salt and precipitating the second salt, which precipitated salt is withdrawn from the evaporation zone in a slurry. In the improved method the withdrawn second salt slurry is introduced into the upper level of an elutriation column wherein the precipitated second salt slurry mother liquor is displaced with a relatively dilute first salt aqueous solution introduced into a lower level of the column, while the second salt crystals are removed from the lower level of the column in a relatively dilute first salt aqueous solution slurry ant while the relatively concentrated first salt mother liquor displaced from the slurry is removed from the upper level of the column for recycle back to the evaporation zone.

Description

il45115 , ~

, ~ACKGROUND OF THE INVENTION
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This invention relates to recovery of a salt from its aqueous solution by concentrating the solution with respect to the salt while precipieating other salts from the solution. More particularly, this invention relates to the removal of the precipitated salts from the con-, centrated solution while avoiding loss of the concentrated solution.

Salts can be produced from solution mined ore. Invariably, this .

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.. . ' :- ' .. -source contains salts other than the salt desired to be produced. Hence, a separation is necessary. In cases where there are two predominate salts, one of which having a solubility increasing more with increasing tempera-tures (hereinafter called "first salts") than the solubility of the other salt (hereinafter called "second salts"), the separation can be effected by passing a solution containing the salts through an evaporation zone wherein the solution is concentrated with respect to the first salt while precipitating the second salt. This is made possible by choosing tempera-tures at which evaporation is carried out so that the second salt becomes supersaturated and consequently precipitates while the first salt remains in solution because of the flrst salt's greater solubillty at the evapo-ration temperature~ The solution may be evaporated at a very high tempera-ture thereby depleting the solution to low second salt content while con-centrating the solution to high first salt content. The second salt depleted solution is subsequently treated for first salt production.
In one method of producing potassium chloride ( first salt) from solution mined ore containing sodium chloride (second salt~ and salt impurities such as magnesium chloride, magnesium sulfate, and calcium sulfate, for example, the solution iB passed through multiple effect evaporators operated at progressively higher temperatures. The salution is heated countercurrent to the direction of feed, often described as backward feed. That i8, the flrst evaporator effect is heated by an external source, while the subsequent evaporators a.e heated by the vapors from their preceding evaporator. The raw feed solution is fed into the coldest evaporator effect (last evaporator effect) from which mother liquor effluent overflow is forwarded to the subsequent hotter evaporator effect and so on to the first evaporator effect. First effect evaporator mother liquor overflow effluent is forwarded to a potassium chloridè

~l~S~5 recovery zone wherein the solution can be cooled or treated otherwise to precipitate crystalline potassium chloride. Sodium chloride is pre-cipitated in each evaporator effect until the invariant composition is reached, that is, until the solution is saturated with respect to sodium chloride and potassium chloride, before the solution is f~rwarded to the subsequent stage or zone.
During the removal of water in an evaporation zone in which a solution is continuously treated by concentration with respect to a first salt and precipitating a crystalline second salt, the second salt crystals must be continuously removed. To facilitate removal of the second salt, the second salt is precipitated into large enough crystals having a density to enable the crystals to settle readily through the solution by gravity thus forming a bed at the bottom of the evaporation vessel while mother liquor effluent overflow from the vessel may be for-warded to another stage,~step, or zone essentially free of the second ; salt crystals. Large crys'tal size is produced by methods well known in the art of crystallization. Such metbods include maintaining a bed of suspended crystals within a zone in a vessel. This suspension is main-tained by same manner of upward flow of liquid in the vessel. Control of nuclei production by controlling the rate at which supersaturation occurs, thus limiting the extremely large number of sites upon which crystal growth occurs, is another method.
The level of the bed in the vessel is maintained by removing the second salt crystals in the form of a slurry at the same rate the :~ second salt precipitates. Large amounts of second salt crystals may be removed at fast rates wbereby the only restraining factor is the ability o a pump or other device to remove high density solids slurries. Con-~equeDtly, slurry with dgDsities qf about 35-70 percent solids by volume 1~45115 is expediently removed from a vessel continuously. It is therefore a desideratum that the large volumes of slurry mother liquor necessary to remove solids from the vessel be recovered since the slurry mother liquor is relatively concentrated with first salt, e.g., saturated with first salt. It is difficult to recover all of the mother liquor since much of it is absorbed in and adsorbed on the second salt crystal.
Several methods known in the art are used, such as gravity sedimentation operations, filtration, and centrifugation. These methods and refinements thereof are efficient in separating liquids and solids, however, the more refined the methods become the greatér the maintenance costs are associated therewith. For example, use of a mechanical conveyor centrifuge such as a helical-conveyor conical-bowl continuous centrifuge, or cylindrical-conical helical-conveyor centrifuge require a delicate balance between the frictional force of the solids on the conveyor and the frictional force on the bowl wall. Additionally, these centrifuges must be run by highly powered motors for production on a large scale.
Consequently, frequent breakdowns are inherent in the use of these type devices and similarly in other devices of the art. Such breakdowns are extremely costly to large scale operations, especially when alternate or replacement systems are not available. Moreover, with these methods, solution adhering to solid surfaces is not recovered. When the solids are relatively small in size, an extremely large surface area retains large volumes of solution.

SUMMARY O~ THE INVENTION

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It has been found that in a method of recovering a first salt from an aqueous solution containing the first salt and a second salt where the solution is treated by passing the solution through an evaporation . ~ , .

zone wherein water is removed from the solution to concentrate the solu-tion with respect to the first salt while precipitating the second salt, there is a novel improvement. An lmproved means has been found for sepa-rating second salt slurry mother liquor from second salt crystals by treating second salt slurry withdrawn from the evaporation zone so that the second salt slurry mother liquor can be recycled back to the evapo-ration zone~at a compatible location from the point of view of first salt concentratlon and temperature.
In accordance with this improvement, en elutriating llquid comprising a relatively dilute first salt aqueous solution is introduced into the lower level of an elongated tubular column in such a manner to establish a rising liquid level in the column. ~lle second salt slurry from the evaporation zone i8 introduced into the upper level of the column where means is provided for second salt crystals to settle~ Withdrawn from the upper level of the column is a solution relatively concentrated with first salt and essentially free of second salt crystals. The second salt crystals settle to the lower level of the column where they are withdrawn in a slurry comprising relatively dilute first salt solution introduced into the lower level of the column as an elutriation liquid.
The solids content of the slurry withdrawn from the lower level of the column is designed so that there is a resultant low net upward flow of relatively dilute flrst salt solution, thus llttle dilution of relatively concentrated first salt solutlon occurs and second salt crystals are allowed to settle.
Thls lnventlon allows the recycling of relatively concentrated flrst salt solutlon used as second salt slurry mother liquor while avoiding a substantial loss of first salt and heat content therefrom. Nore impor-tantly, since this lnventlon requires a simple vertical column lnto which and from which llquids are pumped, virtually no maintenance such as is ~1~5115 required for mechanical-conveyor centrifuges or similar devices is neces-sary.
This invention is particularly useful for treating solution mined potassium chloride containing ore since with solution mining it is important to maximize the amount of potassium chloride product obtained from each pass of solution through an above earth surface process. Also it is important that the process be continuous and unencumbered with breakdowns thereby limiting shutdown and start-up costs, since it can be an expensive operation to stop a solution mining operation. Further, since upon initial solution mine cavity creation large quantities of relatively dilute potassium chloride solution are available, use of the relatively dilute potassium chloride solutions as elutriating liquids in the method of the present invention is facilitated. Additionally, as improved methods of depleting solutions to low potassium chloride content are developed, these potassium chloride depleted streams may be used as elutriating liquids by the method of the present invention.
, BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more clear including xespects to further advantages and ob~ects ~rom the detailed description thereo~
~; 20 made below with reference to the drawings in which: ~

; ` Figure 1 is a diagram of a column used by the present invention `~ wherein its functional characteristics are illustrated; and Figure 2 is a schematic diagram illustrating the relationship between a column used in the practice of the present invention and four multiple effect evaporators. ~ ~

l~S~15 DESCRIPTION OF THE INVENTION

According to the present invention a first salt is separated from an aqueous solution containing the first salt and a second salt.
The first salt May be any salt having a solubility increasing with in-creasing temperatures within a given temperature range. Accordingly, first salts include salts such as potassium chloride, mangesium chloride, sodium carbonate, and sodium chlorate. The second salt may be any salt which does not form a compound with the first salt and which has a solu-bility increasing less with increasing temperatures than the first salt within the temperature range at which the evaporation is to be carried out and within the temperature range at which the solubility of the first salt increases with increasing temperatures. ~y having a solubility in-creasing less with increasing temperature, no significant increase in solubility occ~rs with increasing temperatures so that an increase in - solution temperature will increase the solubility of the first salt much greater than the increase in temperature will increase the solubility of the second salt. Therefore, it is contemplated within the scope of this invention that-second salts include salts that have a solubility which is relatively unchanged with increasing sQlution temperatures as well as salts whose solubility decreases with increasing solution temperatures.
Accordingly, second salts include sodium chloride, calcium sulfate, and magnesium sulfa~te.
The golutlon is subjected to a= evaporation zone so that the ~ solutlon is concentrated with respect to the first salt-while precipitating ; ~ the second salt. Conse~uently, the second salt is removed fro~ the evapo-ration zone to avoid accumulation therein. rhe second salt is removed in th~ form of a slurry~of high 601ids content to minimize the amount of i 1~5115 relatively concentrated first salt solutlon removed from the evaporation zone.
In a preferred embodiment of the present invention a potassium chloride-sodium chloride solution is sub~ected to an evaporation zone comprising multiple effect evaporation operated at progressively higher effect temperatures in the direction of the flow of the solution passed through the evaporators, often described as being backward fed. The solu-tion is concentrated with respect to potassium chloride while precipitating sodium chlorlde in each evaporator effect. A 20 to 50 percent sodium chloride solids content by weight is maintained in the evaporatlon effects to insure that the size of the sodium chloride crystals is sufficient to cause the crystals to settle through the evaporating solution to the bottom of each evaporator. A sodium chloride slurry is withdrawn from each evapo-rator effect and either forwarded to a subsequent evaporator, recycled to a preceding evaporator (Canadian application ~.N. 323,340 filed 13 March 1979), or purged from the evaporation zoneO The evaporator effect from which withdrawn soaium chloride slurry is purged from the evaporation zone should not concentrate the solution therein to 100 percent saturation with respect to potassium chloride lest potassium chloride be precipitated and subsequently lost with the precipltated sodium chloride. The sodium chloride slurry purged from the evaporation zone can be forwarded to a sodium chloride separation zone for the production of sodium chlorlde.
The sodlum chlorlde-depleted, potassium chloride-rich solution i9 withdrawn from the evaparation zone and forwarded to a p~tas61um chloride recovery step.
In the practice of this invention, second salt slurry withdrawn and purged from the evaporation zone is introduced in the upper level of an elongated tubular column. The size of the column may vary to accommodate r, , ~45115 the volume of slurry being treated. The column should be large enough so that liquid flow therein is not turbulent, i.e., have a Reynolds nu~ber less than about 2000. Preferably, the liquids are as near plug flow as practicable. In a preferred embodiment, the bottom section of the column has a smaller cross sectional area than the upper section so that when there is a net upward liquid flow within the column the upward velocity of the liquid at the bottom of the column is greater than the upward velocity at the top of the column. A relatively concentrated first salt solution free of second salt crystals is withdrawn from the upper level of the column, while a relatively dilute first salt solution is intro-duced into the lower level of the column thereby creating a concentration gradient within the column. A second salt slurry comprising second salt crystals and relatively dilute first salt solution is withdrawn from a '~ lower level of the column. By the bottom section of the column having a small cross sectional area, upward flowing liquid fluidizes the crystals therein, inhibiting plugging of the bottom of the column.
Means is provided within the column so that relatively concen-trated first salt solution withdrawn from the upper level of the solution in the column is substantially free of second salt crystals. Such means may be an extension on the end of the condult through which evaporator second salt slurry iY introduced into the column. The e~tension may or may not have a cross sectional area larger than that of the conduit but will extend~down through the surface of the liquid level in the column below the polnt~at which the~relatively concentrated first salt solution is withdraw~. The second salt crystals will then settle through the liquid in the column as long as the upward liquid velocity created by the withdraw-ing of the relatively concentrated first salt solution is not greater than ~ ~ .
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. _g_ :~45115 the settling velocity of the-second salt crystals. This means that the cross sectional area of the annular space created by the outer surface of the extension and the inner surface of the column must be large enough to allow the passage of a large enough volume of solution at a low enough upward liquid velocity. Similarly, baffles can be placed in the column to confine a ~one in which the second salt crystals can settle without disturbance from solution withdrawn from the upper level of the column.
A relatively dilute first salt solution is introduced into the lower level of the column to establish a ris~ng liquid level within the column. Preferably, the solution is saturated with respect to the second salt. The solution should be as saturated with respect to the first salt as is economically practical (since the solution may be disposed in a tailings pond and the first salt content thereof lost as a consequence) to avoid as much as possible the dilution of tne relatively concentrated first salt solution withdrawn from the upper level of the column. The temperature of the solution should be no less than about 60C, below the temperature of the evaporator second salt slurry introduced into the column.
Typically salt solution become more dense at lower temperatures. Therefore, the temperature should not be so low that the relatively dilute first salt solution is too dense for second salt crystals to settle therethrouæh. -A sufficlent volume of relatively dilute first salt solution is introduced into the lower level of the column so that while a part of the relatively diIute first salt solution is being withdrawn with the second salt slurry withdrawn from the lower level of the column, there is still a low net upward flow of relatively dilute first salt selution. The upward velocity of the rising liquid in the upper part of the column should be less than the settling velocity of the second salt. Preferably, the up-ward velocity Or the rising liquid up through the column is greater than the -~ ~, :' . :

11~511S

~velocity of diffusing first salt ions created by the concentration gradient of first salt in the column. Thus, the net diffusion of first salt ions would be upward.
A second salt slurry comprising second salt crystals and rela-tively dilute first salt solution is withdrawn from the lower level of the column. The solids content of the second salt slurry withdrawn may be varied along with the variance of the amount of relatively dilute first salt solution introduced into the lower level of the column to control the net upward liquid flow mentioned above. Preferably, the solids content of the slurry is as high as possible, 1.e., as high as that which can be efficiently pumped, e.g., about 50 to 70 weight percent solids density.
Second salt crystals are allowed to form a bed at the lower level of the column after which the height of the bed depth is maintained at a constant level. The top of the bed depth is defined as the interface between high concentration second salt crystals and low concentration second salt crystals since all of the crystals are partially fluidized. The height o~the~bed of crystals can be important in that the temperature interface ;between~the top~and the bottom of~the column will always be near the top of the crystal bed.~ The temperature interface is near the top of the bed of crystals because of the high heat content of the crystals. It is therefore pre~erred that the bed height be high enough so that by the time the second salt crystals are withdrawn from the lower le~el of the column the~aecond salt crystaIs would have been leached of relatively concentrated flrst salt solutio~n and would have 109t its heat content to the upward flowing relat1vely dllute ~irst salt solution which eventually is withdrawn ;and returnèd to the evaporation zone. ~hus part of the heat imparted to the crystal6 ls r6turned to the evaporation zone. The crystal bed should not be too~high so that relatively concentrated first salt solution wit~h-~ ' , ~

dra~ from the top of the column is reduced in temperature thereby. Thus, by adjusting the height of the salt bed the temperature of the withdrawn relatively concentrated first salt solution can be maximi2ed. The salt bed also facilitate displacement of the relatively conc~ntrated first salt solution by the relatively dilute first salt solution by minimizing fingering or channeling of the relatively concentrated solution down through the relatively dilute solution. Consequently, the upward flow of the relatively dilute first salt solution is near plug flow.
The relatively concentrated first salt solution is withdrawn from the upper level of the column and recycled back to the evaporation zone. The first salt content of this solution can be as low as about 95 percent of the first salt content of the evaporator second salt slurry mother liquor. When the first salt content is below thisiamount~ the expense of lost evaporator economy can approach the high maintenance cost of using devices such as mechanical conveyor centrifuges, depending on the volume of slurry handled. The liquid level in the column can be maintained constant by controlling the amount of relatively concentrated , :
flrst salt solution withdrawn. However, the withdrawing rate cannot be so great that the liquid upward flow from the bottom of the second salt crystals settling zone to the point from which the solution i9 withdrawn i9 greater than the settling rate of the second salt crystals. The tempera-ture of solution withdrawn from the upper level of the column should be no less than 5 percent less than the evaporator second salt slurry feed temperature for the aforesaid reason of compared cost with use of other devices. The solution withdrawn from~the upper level of the column is substantially free of second salt crystals. ~y maintaining the upward liquid velocity less than the aettling rate of the crystals, the withdrawn solution is substantially free of salt crystals, e.g., maintaining the 11~5115 upward liquid velocity below 20 percent of the settling rate of the crystals, the withdrawn relatively concentrated first salt solution will contain less than about 0.5 percent solids by volume when the feed contains about 35 percent solids by volume. The withdrawn solution should not contain solid densities so high that recirculating the salt crystals through the evapo- -rator would affect evaporator heat economy.
In a preferred embodiment of this invention, a relatively con-centrated potassium chloride solution is separated from sodium chloride crystals. Reference is now made to Figure 1 which illustrates a column which is utilized in the practice of the present invention for separating relatively concentrated potassium chloride solution from sodium chloride crystals. A sodium chloride slurry is introduced into inlet 2. Baffle 4 allows the sodium chloride crystals to settle through settling zone 8 without being withdrawn with relatively concentrated potassium chloride solution withdrawn from outlet 6. Sodium chloride crystals settle into crystal bed 5. The bottom section of the column 3 is smaller in diameter so that the upward liqùid flow rate through the salt bed 9 is greater ~; ~ than the upward liquid flow rate at the upper section of the column. This allows enough ~luidization of the salt in the bed so that the bottom of the column 3 does not become plugged with salt. The fluidization also aids in the sodium chloride slurry removal through outlet 7. A relatively dilute potassium chloride solution saturated with sodium chloride is intro-duced into the bottom of the column through inlet 1 and through dispersion plate 7, Reference is now made to Figure 2 which illustrates how the ` -above described column is used with multiple effect evaporators comprising four evaporator effec~s. The evaporator effects 1, 2, 3, and 4 are heated by steam and vapors from subsequent evaporators via streams 1, 2, 3, and 4, respectively. Raw feed potassium chloride-sodium chloride solution is fed into the 4th effect evaporator via stream 10. ~other liquor over-flow effluent from evaporator effects 4, 3, 2, and 1 are fed into evapo-rator effects 3, 2, and 1 and the potassium chloride recovery zone, re-spectively, via streams 11, 12, 13, and 14, respectively. Sodium chloride slurries withdrawn from evaporator effects 1, 3, and 4 are fed into evapo-rator effects 2 and 3 via streams 20, 22, and 23, respectively. The sodium chloride slurry withdrawn from the second effect evaporator is forwarded to the upper level of elutriating column 40 via stream 21. Potassium chloride-rich solution withdrawn from the upper-level of the elutriation column 40 is forwardea to evaporator effect 2 via stream 30. Crystalline sodium chloride is removed from the bottom of column 40 in a relatively dilute potassium chloride solution via stream 33 and forwarded to a tail-ings pond. Potassium chloride depleted liquor from the potassium chloride recovery zone can be mixed with relatively concentrated potassium chloride solution from the top of the column via stream 31 to make up enough liquid to elutriate the second effect evaporator. The remainder of stream 16 may be forwarded to solution mines.
The following example illustrates the manner in which the present invention may be practiced.
' EXAMPLE

Using an apparatus as depicted in Figure 1, a crystalline sodium chloride slurry withdrawn from multiple effect evaporators, wherein a solu-tion is concentrated with respect to potassium chloride while precipitating sodium chloride, is introduced into coIumn 3 via inlet 2. Leg 8 of column 3 is 4.5 meters long and o.6 meters in diameter. The sodium chloride 1~45~1S

slurry, having a crystalline sodium chloride solids density of 46 percent by volume, comprises along with the solids, a mother liquor saturated with respect to sodium chloride and 90 percent saturated with respect to potassium chloride and is introduced into the column at a rate of 86.3 liters per minute. The temperature of the slurry feed was 65C.
A potassium chloride solution saturated with respect to sodium chloride and saturated with respect to potassium chloride is introduced into the bottom of column 3 via inlet 1 at a temperature of 20C. This solution is introduced at a rate of 56.8 liters per minute. The solution introduced into inlet 1 serves to displace the s7urry mother liquor intro-duced into inlet 2 toward the top of the column 3 while crystalline sodium chloride is allowed to settle to the bottom of the column to establish a bed 3.6 meters in height.
A crystalline sodium chloride slurry is withdrawn from outlet 7 at the botto~ of the bed about .3 meters from the bottom of the column.
This slurry has a crystalline sodium chloride solids density of 60 percent by volume and has a~mother liquor saturated with respect to sodium chloride ~and saturated with respect to potassium chloride (this mother liquor is substantially, if not all, comprised of the solution introduced into inlet 1). The slurry ~s withdrawn in such a manner to maintaln a constan~ bed height.
A greater volume of solution is introduced into inlet 1 than is withdrawn as slurry mother liquor through outlet 7 so that there is a net liquld upward flow of .15 meter per minute to displace slurry mother liquor introduced into inlet 2 while sodium chloride crystals are allowed to settle at a rate of .34 meter per minute. These crystals sèttle in the settling zone 8 which e~tend 2 meters below the surface of the solution ln the column. The net upward liquld flow serves to fluidize the sodium ~145115 chloride crystals then facilitating plug flow displacement of liquor toward the top of the column 3.
A potassium chloride solution made up substantially of slurry mother liquor displaced toward the top of column 3 is withdrawn through outlet 6 and recycled back to the evaporators. This solution has a sodium chloride solids density of less than .5 percent by volume. The solution is saturated with respect to sodium chloride and about 90 percent saturated with respect to potassium chloride. The solution is removed at a rate of 7.6 liters per minute thereby maintaining a constant liquid level in the colulmn. ~ -Thus,,it can be seen that sodium chloride crystals may be separated from a relatively concentrated potassium chloride solution by using a column wherein the sodium chloride crystals are washed with a relatively dilute potassium chloride solution. The separated relatively concentrated potassium chloride solution is not greatly reduced in tempera- -ture nor greatly reduced in potassium chloride content. In the above example the potassium chloride content of the recycled solution is only 2 :: ~
~ parts per 100 parts water less concentrated than the slurry mother liquor .
lntroduced into the column.

Whlle the invention has been described with reference to details ~: of specific embodiments and illustrations, it is not intended that it should be construed as limited thereto except to the extent such details are set : ~:
forth.in the Claims.

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Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method of recovering a first salt from an aqueous solu-tion containing the first salt and a second salt, the first salt having a solubility increasing more with increasing temperatures than the solubility of the second salt, by passing the solution through an evaporation zone wherein water is removed thereby concentrating the solution with respect to the first salt and percipitating the second salt into crystals having a sufficient size and density to enable the crystals to settle through the solution by gravity, withdrawing a second salt slurry from the evaporation zone, separating second salt slurry crystals from the second salt slurry mother liquor, recycling the second salt slurry mother liquor to the evapo-ration zone and forwarding first salt concentrated solution from the evapo-ration zone to a first salt recovery zone, an improvement which comprises:

a. introducing into the lower level of an elongated tubular column a relatively dilute first salt aqueous solution thereby establishing a rising liquid level within the column, the relatively dilute first salt aqueous solution having a density sufficiently low to enable second salt crystals to settle therethrough;

b. introducing in the upper level of the column second salt slurry withdrawn from the evaporation zone, the upper level of the column having a second salt crystal settling zone so that substantially all of the second salt crystals will settle to the bottom of the column;

c. withdrawing from the lower level of the column a second salt slurry of second salt crystals and relatively dilute first salt aqueous solution, the second salt slurry withdrawn from the lower level of the column having a solids content so that a low net upward flow of relatively dilute first salt aqueous solution exists within the column to allow the second salt crystals to settle and so that there is a bed of second salt crystals at the lower level of the column;
d. withdrawing solution from the upper level of the solution in the column to maintain a constant liquid level with-in the column, the withdrawn solution being substantially free of second salt crystals; and e. forwarding the solution withdrawn from the upper level of the solution in the column to a region of the evapora-tion zone wherein the first salt concentration and the tempera-ture are compatible with the first salt concentration and temper-ature of the withdrawn solution being forwarded thereby avoiding the loss of first salt contained in the second salt slurry mother liquor withdrawn from the evaporation zone.

2. The method of Claim 1, wherein the first salt is potassium chloride and the second salt is sodium chloride.

3. The method of Claim 1, wherein the evaporation zone is multiple effect evaporators comprising four evaporators operated at progressively higher temperatures, wherein the multiple effect evaporators are backward fed, wherein the first effect is the hottest and wherein the second salt slurry with-drawn from the evaporation zone is withdrawn from the second effect evaporator and the solution withdrawn from the upper level of the solution in the elongated tubular column is recycled to the second evaporator effect.

4. The method of Claim 1, wherein mother liquor effluent from the first salt recovery step is the relatively dilute first salt fluid introduced into the bottom of the column.

5. The method of Claim 1, wherein the evaporation zone is multiple effect evaporators comprising five evaporators.

6. In a method of recovering potassium chloride from an aqueous solution containing potassium chloride and sodium chloride by passing the solution through multiple effect evaporators operated at progressively higher temperatures, wherein the multiple effect evaporators is backward fed, wherein the first effect evaporator is the hottest and wherein the solution is concentrated with respect to potassium chloride and depleted of sodium chloride by precipitating sodium chloride crystals having a suf-ficient density and size to enable the crystals to settle through the solution by gravity, withdrawing sodium chloride slurry from each evaporator effect, forwarding withdrawn sodium chloride slurry to a sodium chloride separation zone and forwarding mother liquor effluent from the multiple evaporators to a potassium chloride recovery step, an improvement which comprises:

a. introducing into the lower level of an elongated tubular column a relatively dilute potassium chloride aqueous solution thereby establishing a rising liquid level within the column, the relatively dilute potassium chloride aqueous solution having a density sufficiently low to enable sodium chloride crystals to settle therethrough;

b. introducing into the upper level of the column sodium chloride slurry from the multiple effect evaporators, the upper level of the column having a sodium chloride crystals settling zone so that substantially all of the sodium chloride crystals will settle to the lower level of the column;

c. withdrawing from the lower level of the column a sodium chloride slurry of sodium chloride crystals and rela-tively dilute potassium chloride aqueous solution, the sodium chloride slurry withdrawn from the lower level of the column having a solids content so that there exists a low net upward flow of relatively dilute potassium chloride aqueous solution within the column to allow sodium chloride crystals to settle and so that there is a bed of sodium chloride crystals at the bottom of the column;
d. withdrawing solution from the upper level of the solution in the column to maintain a constant liquid level within the column, the withdrawn solution being substantially free of sodium chloride crystals; and e. forwarding the solution withdrawn from the upper level of the solution in the column to an evaporator of the multiple effect evaporators wherein the potassium chloride con-centration and the temperature are compatible with the potassium chloride concentration and temperature of the withdrawn solution being forwarded, thereby avoiding the loss of potassium chloride contained in the withdrawn sodium chloride slurry mother liquor from the multiple effect evaporators forwarded to the column.

7. The process of Claim 6 wherein the solution withdrawn from the top of the elongated tubular column is forwarded to the evaporator of the multiple effect evaporators from which sodium chloride slurry was withdrawn and forwarded to said column.

8. The method of Claim 6, wherein the loss in steam economy is less than 2 percent.

9. The method of Claim 6, wherein the column has a smaller cross section at the bottom than at the top.

10. The method of Claim 7, wherein the withdrawn relatively concentrated potassium chloride solution from the column is within 95 percent of the concentration of the with-drawn sodium chloride slurry mother liquor forwarded to the column.

11. The method of Claim 6 or 7, wherein the sodium chloride crystals settling zone comprises a concentrically located baffle extending above the liquid level and below the point at which relatively concentrated potassium chloride solu-tion is removed from the top of the column and wherein the baffle creates an annular space between the outer surface of the baffle and the inner surface of the column and wherein sodium chloride slurry is introduced into the inner column created by the baffle and relatively concentrated potassium chloride solution is withdrawn from the annular space.

12. The method of Claim 6 or 7, wherein the with-drawn chloride slurry mother liquor is saturated with respect to potassium chloride.

13. The method of Claim 7, wherein the multiple effect evaporators comprises four evaporators.

14. The method of Claim 6 or 7 wherein the multiple effect evaporators comprises five evaporators.

15. The method of Claim 13, wherein the sodium chloride slurry forwarded to the column is withdrawn from the second effect evaporator.

16. The method of Claim 13, wherein the sodium chloride slurry forwarded to the column is withdrawn from the third effect evaporator.

17. The method of Claim 13, wherein the sodium chloride slurry forwarded to the column is withdrawn from the first effect evaporator.