GB2080779A - Process for purifying potassium chloride particles - Google Patents

Abstract

The process comprises (a) introducing particulate potassium chloride containing minor contaminating amounts of sodium chloride into and near the top of a leaching column (20), (b) contacting the particles counter- currently with an aqueous leaching salt solution that is saturated with respect to potassium chloride and contains less than 45 gm of sodium chloride per liter of solution, (c) removing aqueous effluent from the top of the leaching column, (d) removing a slurry of potassium chloride particles substantially reduced in sodium chloride content from the bottom of the leaching column, and (e) separating leached potassium chloride particles from the mother liquor of the slurry of step (d) and recycling the mother liquor to the leaching column as leaching salt solution. <IMAGE>

Description

SPECIFICATION Process for purifying potassium chloride particles The present invention relates to a process for purifying particulate potassium chloride containing inorganic salt impurities, e.g., sodium chloride. More particularly, this invention concerns substantially reducing the minor contaminating amount of inorganic metal salts found in industrially produced potassium chloride by leaching same with an aqueous salt solution saturated with respect to potassium chloride and unsaturated with respect to said contaminating inorganic salts.

Potassium chloride is obtained commercially by the mining of ores containing the salt and subsequently separating the potassium chloride from the other ore constituents. Typically, potassium chloride ores contain other inorganic salts of which the most common are sodium chloride, calcium chloride, magnesium chloride, magnesium sulfate and sodium carbonate. Sodium chloride commonly represents the other principal inorganic salt constituent present in the potassium chloride-bearing ore. Potassium chloride is separated from the other soluble and insoluble constituents present in the ore by methods well known irrthe art, which methods include froth flotation, fractional leaching, selective precipitation and solution mining.

In the froth flotation process, ground potassium chloride-containing ore is mixed with a solution saturated with respect to the ore constituents and to which frothing agents have been added. In this manner, particulate potassium chloride is separated from the other inorganic metal salts and other impurities present in the ore. Potassium chloride solids thereby obtained typically contain relatively high levels (about 2-5 percent by weight) of the other inorganic metal salts found in the ore. These salt impurities are believed to have been incorporated, encapsulated or occluded within the potassium chloride particles during the geological time frame of natural crystal formation.The aforesaid particles often have a reddish discoloration due to traces of iron and are thought to have a characteristically unique distribution of the inorganic metal salt impurities throughout the potassium chloride particle.

In other industrial processes, potassium chloride is crystallized from a salt solution rich in both sodium and potassium chloride. Production of industrially crystallized potassium chloride involves dissolving potassium chloride ore in an aqeous solvent to form a solution containing potassium chloride and other soluble inorganic metal salts that are present in the ore. Subsequently, the potassium chloride is crystallized from or separated, e.g., by precipitation, from the solution. A minor contaminating amount of the inorganic salts, principally sodium chloride, is incorporated, encapsulated or occluded within the crystal during its formation. Incorporation of such salt impurities in the potassium chloride crystal obtained is unavoidable since the solution from which the potassium chloride crystals are obtained inevitably contain such salt impurities.

The potassium chloride product obtained by the aforesaid industrial processes is sufficiently pure for many commercial applications. There are, however, some industrial applications which require very low levels of other salt impurities, such as sodium chloride. A method for reducing the sodium chloride content of industrially crystallized potassium chloride is described in copending, coassigned U.S. Patent application 858,866 filed December 8, 1977. In the method described in said application, industrially crystallized potassium chloride, particularly compacted potassium chloride particles, are leached under isothermal conditions with a leaching solution that is saturated with respect to potassium chloride and unsaturated with respect to sodium chloride for a time sufficient to substantially reduce the sodium chloride content of the potassium chloride crystals.

It has now been discovered that leaching of substantially pure potassium chloride particles to substantially reduce the minor contaminating amount of sodium chloride contained therein can be effected in an efficient and facile manner. In this process, industrially produced potassium chloride particles (usually as an aqueous slurry) are fed to the top of a leaching column wherein they are contacted countercurrently with an aqueous leaching solution that is saturated with respect to potassium chloride and unsaturated with respect to sodium chloride. A slurry of the leached particles is removed from the bottom of the column and the entraining liquid phase (mother liquor) of the slurry separated from the leached particles. This mother liquor is recycled as leaching fluid to the leaching column.Liquid aqueous effluent from the top of the leaching column is removed and utilized for the preparation of fresh potassium chloride slurry feed to the leaching column. In this manner, liquor rich in sodium chloride is segregated from liquor lean in sodium chloride within the process, thereby resulting in a substantial reduction of sodium chloride levels in the potassium chloride particles charged to the leaching column. Washing of the leached particles with a saturated potassium chloride aqueous solution containing no sodium chloride or washing with water will reduce the sodium chloride level of the leached particles still further.

The process of the present invention is capable of purifying industrially crystallized potassium chloride to very low levels of contaminating inorganic metal salt impurities, i.e., sodium chloride, calcium chloride and magnesium chloride. It has been found that substantial reductions, i.e., a reduction of greater than 50%, based on the initial contaminating concentration, in the content of such salts can be achieved. It is not unusual, for example, to achieve sodium chloride levels of less than 0.2 weight percent, e.g., less than 0.1 weight percent, by the aforesaid process. In one preferred embodiment, the sodium chloride level of compacted, industrially crystallized potassium chloride has been reduced from about 1 weight percent to less than 0.1 weight percent, e.g., 0.07-0.09 weight percent, with an average residence time in the leaching column of about 6 hours.Calcium and magnesium chloride levels have simultaneously been reduced from about 150 parts per million (ppm) to less than about 60 ppm.

The aforesaid reduction in inorganic salt impurities is achieved by the above-described process, which segregates sodium rich liquors in the system from sodium lean liquors. Thus, the overflow from the leaching column (a sodium rich liquor) is used to repulp the solid potassium chloride feed, which is also rich in sodium chloride compared to the aqueous leaching salt solution and leached potassium chloride product The latter solution and product are lean in sodium chloride and, in accordance with the present process, are kept separate from the process streams rich in sodium chloride, except in the event of inadequate quantities of sodium chloride lean make-up liquor.

The present invention will now be further described with reference to the accompanying drawing which is a simplified schematic diagram of the process of the present invention with certain illustrated preferred embodiments.

In the embodiment illustrated in the drawing, industrially produced potassium chloride from a source (not shown) is forwarded by feed line 6 to feed screw 8 and thence into slurry tank 10 by means of flow line 9.

As the potassium chloride feed to the present process, there can be used industrially produced potassium chloride particles having a potassium chloride content of at least about 95 weight percent. Often, the potassium chloride content will be higher, e.g., from about 96 to about 99.5, e.g., 97.5 to 98.5, weight percent potassium chloride. Accordingly, the potassium chloride feed can contain up to about 5 weight percent sodium chloride. More typically, the feed will contain between about 0.5 and about 2.5, e.g., between about 0.9 or 1 and about 1.5 or 2 weight percent sodium chloride. The calcium and magnesium content of the potassium chloride feed will generally each be less than about 300 parts per million (.030 weight percent).

As the principal metal salt impurity in industrially produced potassium chloride particles is by far sodium chloride, the other metal salt impurities, e.g., those of calcium and magnesium can be and will be disregarded for the most part in this discussion. By potassium chloride "particles" is meant that crystalline body formed by the solidification of potassium chloride having a regular repeating internal arrangement of its atoms, including the product formed by consolidating solid potassium chloride by high pressure, in which case the internal arrangement of the potassium chloride may be distorted.

As indicated, the potassium chloride feed used in the present process is industrially produced potassium chloride, such as an industrially crystallized product, e.g., potassium chloride produced by crystallizing the product from a solution saturated with respect to potassium chloride and containing sodium chloride in the solution. Preferably, the potassium chloride feed used is a compacted industrially crystallized product.

It is preferred that the potassium chloride to be leached in accordance with the present process be so treated before it has been conditioned or treated in a manner that seals the surface of the particles. For example, treatment of dry potassium chloride with small amounts of water to reduce dusting and improve particle competency renders the process of the present invention less effective. Further, it is generally preferred that the potassium chloride to be leached is treated in accordance with the process of the present invention soon after it is produced as the evidence at hand indicates that less sodium chloride is leached from aged potassium chloride product.

Although the particle size of the particulate potassium chloride treated is not critical, it is preferred that the potassium chloride feed be larger then 35 Tyler mesh, e.g., -10, +35 Tyler mesh. While smaller particles than 35 mesh can be used, they can present handling problems caused by their poor flowability and tendency to be dusty. Particles larger than 10 mesh generally find application as a commercial product but such larger particles can also be treated in accordance with the present process. Thus, it is contemplated that granular potassium chloride, which commonly is a compacted product with a size range of about -4, +14 mesh, can be treated by the herein described process. More typically, the compacted particles treated will have a size range of -14, +35 Tyler mesh.

Referring again to the drawing, the potassium chloride feed introduced into slurry tank 10 is repulped therein with an aqueous salt solution that is preferably saturated with respect to potassium chloride and unsaturated with respect to sodium chloride at the temperature of mixing. Slurry tank 10 is equipped with stirring means, e.g., an agitator, to produce the desired slurry. The aqueous salt solution is introduced into slurry tank 10 by means of line 52 and is obtained from the liquid effluent overflow of leaching column 20. At start up, the aqueous salt solution used to repulp the potassium chloride crystals in slurry tank 10 can be unsaturated with respect to potassium chloride and contain from none to greater than 50 grams sodium chloride per liter of solution (gpl), e.g., Oto 45 gpl.The aqueous salt solution will become saturated with respect to potassium chloride at the expense of dissolving product charged to the slurry tank. The sodium chloride content of this salt solution will be controlled during the leaching process by purging the system of sodium chloride rich brine by means of line 54 and valve 56.

Sufficient aqueous salt solution is added to slurry tank 10 to repulp the potassium chloride feed to a slurry containing between about 20 and about 50 volume percent solids. The percent potassium chloride solids present in the slurry will be determined by the pumpability of the slurry and the contact time available in the leaching column. The higher the level of solids in the slurry pumped to the leaching column at a given flow rate, the larger the column required or the longer the residence time required in the column in order to achieve the needed intimate contact of the solids with the leaching solution to accomplish the desired degree of leaching in a continuous process. Commonly, the slurry will contain between about 20 and about 30 volume percent of potassium chloride solids. This slurry is removed from slurry tank 10 ancipurriped by means of pump 12 through line 16 to the top or near the top of leaching column 20. Valve 14 in line 16 regulates the flow of slurry to the leaching column.

As used in the specification and claims, the terms "top", "near the top", "bottom", or "near the bottom" of the column or terms of like import are intended to mean and include the uppermost or bottommost portion, as the case may be, of the column. The exact location of liquid inlet and outlet streams to the column will depend on the design of the column and no specific location (other than a general location) is intended by such terms.

Leaching column 20 is of a size (height and diameter) such that intimate contact of the potassium chloride solids charged thereto with the leaching solution is obtained for a residence time that is sufficient to substantially reduce the sodium chloride content of the potassium chloride solids. It is contemplated that residence times of between about 0.5 and 24 hours, e.g., between 2 and 18 hours, more particularly between 4 and 12 hours, are sufficient to accomplish the aforementioned result. The exact residence time will, of course, depend upon the initial sodium chloride content of the potassium chloride feed and the degree of purification desired.For example, compacted industrially crystallized potassium chloride of a particle size of -14, +35 Tyler mesh and containing a sodium chloride content of about 1 weight percent can be purified to contain a sodium chloride content of less than about 0.2 weight percent in about 6 hours at steady state conditions by the herein described process. By substantially reducing the sodium chloride content of potassium chloride particles is meant that the sodium chloride content is reduced to less than 0.5 weight percent; e.g., less than 0.25 weight percent.

The aqueous salt solution used as the leaching fluid is forwarded from each liquor tank 40 by means of pump 47 and line 46 to or near the bottom of leaching column 20. The rate at which leaching fluid is introduced into the column can be regulated by means of valve 48 in line 46. The leaching fluid is a salt solution, the solute of which is selected from potassium chloride and mixtures of potassium chloride and sodium chloride. The leaching fluid is substantially saturated with respect to potassium chloride and substantially unsaturated with respect to sodium chloride at the temperature of the solution. By "substantially saturated" with respect to potassium chloride is meant that sufficient potassium chloride is in solution so that little, if any, of the potassium chloride particles treated will be dissolved as a consequence of being treated by the process of the invention.The amount of sodium chloride in the leaching fluid can vary; but, should be maintained at less than 45 gpl of sodium chloride in order to provide a significant driving force to permit removal of substantial quantities of sodium chloride from the potassium chloride solids.

Preferably the leaching fluid introduced into the bottom of the leaching column will contain less than 25, e.g., less than 20, gpl sodium chloride. More preferably, the leaching fluid will contain between 5 and 20, e.g., between 10 and 16 gpl of sodium chloride at steady state conditions.

The amount of leaching fluid introduced into the leaching column can vary; however, the amount should be sufficient to provide intimate contact thereof with substantially all of the potassium chloride particles in the column. By intimate contact is meant that enough leaching fluid is used so that the entire surface area of each potassium chloride particle is contacted with leaching fluid for the time required to obtain substantial reduction of the sodium chloride content therein. A ratio of 4000 grams of potassium chloride particles introduced into the leaching column per liter of aqueous leaching solution has been found to be useful.

Variations in the ratio, such as down to 200 or 300 grams of potassium chloride per liter of leaching solution, would not significantly improve results. Thus, a ratio of from about 300 to about 4000 grams of potassium chloride per liter of aqueous leaching solution can be used. Lower ratios can be used also; but, are economically unattractive since it would involve the movement of large quantities of liquid per unit weight of potassium chloride particles leached.

The leaching fluid is introduced into the leaching column countercurrent to the flow of potassium chloride solids in the column and in such a manner as to provide a net upward flow of leaching fluid within the column. The leaching fluid is percolated upwardly through a settled bed of the potassium chloride maintained within the column at a rate that is insufficient to cause fluidization or bridging of the bed. A slurry of leached potassium chloride is removed from near or at the bottom of column 20 by means of line 25 and forwarded to separating means 26 for separating the liquid phase (mother liquor) from the solids. The rate at which leached product is withdrawn from the column can be regulated by means of valve 24 in line 25.Some leaching fluid will be withdrawn with the product from the bottom of the column because of the closeness of the point of product withdrawal and the point of leaching fluid entry. Withdrawal of the leached product can be facilitated, if needed, by dilution with the leaching fluid.

The means by which the entraining liquid associated with the leached potassium chloride product is separated therefrom should be chosen to minimize degradation of the leached particle and to maximize efficient dewatering. Any conventional apparatus which accomplishes such a separation and allows efficient washing or rinsing of the solid product can be used. In the drawing, separating means 26 is shown as a filter for separating the leached potassium chloride withdrawn from the bottom of column 20 from the liquid phase (mother liquor) associated therewith. Examples of apparatus that can be used to separate the leached crystals from its mother liquor include horizontal filters, e.g., belt, pan and table filters, drum filters, disc filters, and centrifuges. Preferably, a centrifuge is not used for leached compacted potassium chloride particles.Efficient dewatering and washing is to be encouraged for the reason that a significant amount of the remaining sodium chloride is derived from the liquor adhering to the leached particles.

The product slurry is deposited on separating means (filter) 26 and the filtrate (mother liquor) is withdrawn by means of line 27 and forwarded to each liquor tank 40 with the assistance of pump 34. The moist filter cake can be rinsed with washing fluid forwarded to the filter cake by means of line 41. The amount of washing fluid used can be controlled by means of valve 44 in line 41. The washing fluid can be water or a salt solution of potassium chloride, or potassium chloride with low levels of sodium chloride. If a salt solution is used, it can be saturated or unsaturated with respect to potassium chloride. It is contemplated that a solution 50 to 60 percent saturated with respect to potassium chloride and containing very low levels of sodium choride be used.The level of sodium chloride present in the washing fluid should be less than the amount of sodium chloride remaining in the leached potassium chloride so that such product is not recontaminated with sodium chloride. Preferably, the washing fluid is water or a potassium chloride salt solution substantially free, i.e., less than .05 weight percent, of sodium chloride. It is contemplated that the filter cake is washed with up to 15 displacements of washing fluid. By "displacement" is meant the amount of water remaining in the filter cake following separation of the mother liquor by the separating means, e.g., filter 26.

The washed potassium chloride product is removed from filter 26, e.g., by a doctor blade, and forwarded by means of line 28 to drier 30 wherein the moisture associated with the product is removed. The final dried product is removed from the drier by means of line 32 and forwarded to storage or packed out.

Washing fluid for rinsing the filter cake is introduced into the system from a source not shown through line 41 and controlled by valve 44. Water for make-up is forwarded by line 42 to leach liquor tank 40. The amount of make-up water introduced into the tank is regulated by means of valve 43 in line 42.

As the leaching fluid proceeds upwardly through the leaching column, it displaces mother liquor associated with and removes sodium chloride from the potassium chloride particles descending countercurrently to it. An aqueous overflow liquor is removed from near the top of leaching column 20 by means of line 22 and forwarded to overflow tank 50. The overflow liquor is saturated with respect to potassium chloride and contains a minor amount of sodium chloride, which amount is slightly higher than the sodium chloride level present in the leaching fluid introduced at the bottom of the column. Also dissolved in the overflow liquor are small amounts of calcium and magnesium salts.Aqueous liquor in overflow tank 50 is forwarded by means of line 52 to slurry tank 10 for utilization in repulping potassium chloride feed, i.e., for preparing further potassium chloride slurry feed to the leaching column. In the event that there is insufficient make-up water available in the system for the process, the liquor in overflow tank 50 can be recycled by means of line 53 and valve 55 to leach liquor tank 40. However, the recycling of this liquor from overflow tank 50 to leach liquor tank 40 is not desired for the reason that it results in forwarding a salt solution rich in sodium chloride to tank 40 which contains a salt solution lean in sodium chloride.

In order to prevent a build-up of sodium chloride and calcium and magnesium salts in the system, a bleed stream is provided in line 53 by means of line 54 and valve 56. Sufficient of the overflow from column 20 is taken as bleed to maintain the desired sodium balance in the system, which in turn depends on the desired purity of the product. Accordingly, the amount of make-up water required to the system will be equal to the amount of liquid withdrawn by means of the sodium chloride bleed stream, the moisture removed at the dryer and for other unaccounted losses of liquid in the system less the amount of water introduced to the system bythefilterwash stream 41.

In operating the leaching column, the slurry of industrially produced potassium chloride crystals (about 20-30 volume percent) is introduced into the top of the leaching column where it slowly descends against the rising countercurrent flow of leaching fluid. A bed of potassium chloride of about 90 percent settled solids is formed in the column through which the leaching fluid percolates. As the leaching fluid rises in the column, it displaces the mother liquor associated with the solids and leaches sodium chloride contained therein, thereby increasing its sodium chloride content. For example, a saturated potassium chloride aqueous solution leaching fluid containing initially about 13 grams per liter sodium chloride will contain about 15-17 grams per liter of sodium chloride when it is withdrawn from the leaching column as overflow.When the slurry of leached potassium chloride solids is withdrawn from the bottom of the leaching column, it is diluted with leaching fluid so that the slurry charged to the filter 26 contains only about 30 to 40 percent settled solids.

The temperature at which the present process is conducted can vary. However, the higher the temperature the more potassium chloride required to maintain the various process streams saturated therewith and the greater the energy requirements. Consequently, moderate temperatures, i.e., from about 20"C. to about 70"C., e.g., from about 25 to about 55"C., are preferred for conducting the process of this invention. Typically, the process is operated isothermally, i.e., without the intentional addition or removal of heat.

In performing the process described hereinabove with respect to the Figure, the system, i.e., the leach liquor tank, leaching column, overflow tank and slurry tank, are filled with a potassium chloride solution.

Potassium chloride is then fed to slurry tank 10 by operating feed screw 8. A stirring system, such as a propellor, within tank 10 induces circulation of the solution in the tank, thereby causing suspension of the particulate potassium chloride charged to the tank. The resulting slurry is forwarded to leaching column 20.

If the solution contained in the system is not saturated with respect to potassium chloride, particulate potassium chloride will dissolve until saturation is reached. Thereafter, a solids bed will build up inside the leaching column. The bed height will depend on the desired retention time. Once the target bed height is reached, product discharge valve 24 is opened and product discharge from the column forwarded to filter 26.

The setting for valve 24 is adjusted to maintain a constant bed height.

Following introduction of product to the filter, filter wash is forwarded to the filter and filtrate pump 34 is started to recycle filtrate to the leach liquor tank. Washed product is forwarded to preheated dryer 30 which removes residual moisture from the product.

The sodium chloride content of the leaching solution forwarded to column 20 through line 46 is determined. If the sodium chloride content of this solution is higher than the desired target concentration, valve 56 in line 54 is opened to purge sodium chloride from the system and make-up solution, which has a lower sodium chloride content, is added through line 42 to compensate for the volume of the sodium chloride bleed stream purged from the system. If the solution has a sodium chloride content less than the target concentration, bleeding is not required and valve 56 is closed. Make-up solution (other than to replace losses) is also not required until the system reaches steady state conditions.

The present process is more particularly described in the following example which is intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art.

Example In the following example, values and operating conditions given are average values for 28 hours of steady state operation. Flow rates are in U. S. gallons per minute (gpm) unless noted otherwise.

Compacted, industrially crystallized potassium chloride having the average sieve analyses reported in Table I was charged to slurry tank 10 at a rate of about 1.73 tons per hour. The potassium chloride feed contained on the average 0.91 weight percent sodium chloride and 0.023 and 0.014 weight percent calcium and magnesium respectively. An aqueous slurry of the aforesaid compacted potassium chloride feed having about 22 volume percent solids (settled solids) was charged at a rate of 60 gpm to the top of a leaching column in which the settled bed was about 21-22 feet.

A saturated potassium chloride solution containing about 13.1 grams per liter (gpl) of sodium chloride was introduced into the bottom of the column as the leaching fluid at about 51.3 gpm. The liquid overflow from the column contained about 15.6 gpl sodium chloride. Retention time in the column was calculated to be about 6.1 hours. Water make-up averaged 6-9 gpm.

Leached potassium chloride was withdrawn from the column and forwarded to a filter. The filter discharge was found to contain about 0.084 weight percent sodium chloride, 0.0036 weight percent calcium and 0.00098 weight percent magnesium. When dried, unrinsed samples of the filter cake were found to contain an average of 0.108 weight percent sodium chloride, 0.0035 weight percent calcium, and 0.0016 weight percent magnesium. Samples of filter cake rinsed for 30 seconds in a potassium chloride saturated solution containing no sodium chloride and then dried were found to contain 0.042 weight percent sodium chloride, 0.0022 weight percent calcium and 0.00055 weight percent magnesium.

The data of the aforesaid example demonstrates that the sodium chloride content of compacted, industrially crystallized potassium chloride can be reduced to low levels, i.e., from about 0.9 to about 0.09 weight percent by the leaching process of the present invention.

TABLE I Tyler Sieve Cumulative Weight, % 14 23.5 16 48.6 20 71.7 28 91.2 35 95.7 48 97.5 Although the present process has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.

Claims (16)

1. A process of purifying industrially produced potassium chloride comprising potassium chloride and minor contaminating amounts of sodium chloride and salts of calcium and magnesium wherein said potassium chloride is leached with a saturated potassium chloride salt solution, said process comprising:: (a) introducing particulate potassium chloride into and nearthetop of a leaching column, (b) introducing sodium chloride lean, aqueous leaching potassium chloride salt solution into the bottom of said leaching column in an amount sufficient to contact said particulate potassium chloride intimately and in a direction to produce an upward flow of leaching salt solution within said column, said leaching salt solution being saturated with respect to potassium chloride and containing less than 45 grams of sodium chloride per liter of solution, (c) removing aqueous liquid effluent from the top of the leaching column, said effluent having a sodium chloride content greater than the leaching salt solution introduced into said column, (d) removing a slurry of leached potassium chloride substantially reduced in sodium chloride content from the bottom of said leaching column, (e) separating leached potassium chloride from sodium chloride lean entraining mother liquor of the slurry of step (d) and recycling mother liquor to the leaching column as leaching salt solution, thereby obtaining potassium chloride substantially reduced in sodium chloride content.

2. A process as claimed in claim 1, wherein the particulate potassium chloride introduced into the leaching column is introduced as an aqueous slurry.

3. A process as claimed in claim 2, wherein liquid effluent from the top of the leaching column is utilized for the preparation of further potassium chloride slurry feed to the leaching column.

4. A process as claimed in any of claims 1 to 3, wherein a portion of the liquid effluent from the top of the leaching column is purged to control the sodium chloride content within the column.

5. A process as claimed in any of claims 1 to 4, wherein the leached potassium chloride of step (e) is washed with a washing fluid selected from water and an aqueous potassium chloride salt solution, said salt solution containing a sodium chloride content no greater than the sodium chloride content of the leached potassium chloride.

6. A process as claimed in claim 5, wherein the potassium chloride washing fluid is substantially free of sodium chloride.

7. A process as claimed in claim 5 or claim 6, wherein the potassium chloride washing fluid is substantially saturated with potassium chloride.

8. A process as claimed in any of claims 1 to 7, wherein the industrially produced potassium chloride is a compacted particulate potassium chloride.

9. A process as claimed in any of claims 1 to 8, wherein the leaching salt solution of step (b) contains less than 25 grams of sodium chloride per liter of solution.

10. A process as claimed in claim 9, wherein the leaching solution contains less than 20 grams of sodium chloride per liter of solution.

11. A process as claimed in any of claims 1 to 10, wherein the residence time within the column is from 0.5 to 24 hours.

12. A process as claimed in claim 11 wherein the residence time within the column is from 2to 18 hours.

13. A process as claimed in claim 12, wherein the residence time within the cblumn is between 4 and 12 hours.

14. A process as claimed in any of claims 1 to 13, wherein the temperature at which leaching is conducted is from 20"C to 70"C.

15. A process as claimed in claim 1 and substantially as hereinbefore described with reference to the Example.

16. A process as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.