TECHNICAL FIELD

[001] This invention relates to improved systems and methods for the

purification and recovery of potash.

BACKGROUND ART

[002] Potash is important for agriculture because it improves water retention, yield, nutritional value texture and disease resistance of food crops. It has wide application to fruits and vegetables, rice, wheat and other grains, sugar, corn, soybeans, palm oil and cotton, all of which benefit from the nutrient’s quality enhancing properties. Economic growth in Asia and Latin America has greatly contributed to the increased use of potash-based fertilizers.

[003] Because potash is a fertilizer for the above-mentioned plants, agricultural plant wastes become a reservoir of potassium from which potash can be recovered by extraction from the residue (ashes) which are left from the burning of such agricultural plant wastes. In particular, the agricultural plant wastes which are burned to ashes and from which potash is extracted preferably are cocoa pod husks, plantain (and banana) peels and cola nut husks. Thus, potash may be recovered by extraction from the residue (ashes) left from the burning of the above preferred agricultural plant wastes.

[004] There are many patents that deal with the purification of potash from solutions of potash. Among them are the following:

[005] US Patent No. 9,017,426, issued April 28, 2015, to GC Technology Limited for "Interconnected System and Method for the Purification and Recovery of Potash" by leaching impure potash from agricultural wastes and concentrating and carbonating the leached potash before crystallizing and heating to provide potash.

[006] US Patent No. 7,892,298, issued Feb 22, 2011, to Toagosi Co Ltd for "Method for Producing High Purity Caustic Potash" through crystallization by bringing an aqueous solution of caustic potash into a high temperature zone.

[007] US Patent No. 7,041,268, issued May 9, 2006, to Council of Scientific and Industrial Research for "Process for Recovery of Sulphate of Potash" from sulphate-rich bittern through the use of lime by fractionation of the bittern to obtain kainite type mixed salts and then reaction with muriate of potash to produce crude sulphate of potash.

[008] US Patent No. 5,456,362, issued Oct 10 1995, to The University of British Columbia for "Flotation Process for the Flotation of Coarse Fraction of Potash Ores". By using a column flotation device in which air bubbles are generated by a sparger that utilizes high intensity shearing.

[009] US Patent No. 4,787,506, issued Aug 30 1988, to Kali und Salz

Aktiengesellschaft for "Electrostatic Treatment of Milled Crude Potash Salts Containing Kiesserite" by conditioning sequentially with two conditioning agents and feeding the crude potash salt to an electrostatic free fall separator.

[010] US Patent No. 4,198,288 issued Apr 15 1980 to Celanese Polymer

Specialties Company for "Desliming of Potash Ores", by treating pulped potash ore with a polygalactomannan gum flocculant, then with a polyamine collector and then subjecting it to froth flotation.

SUMMARY

[Oil] One technical problem to be solved was that the extraction/purification of potash from crude potash required evaporation of a large amount of water. A further technical problem to be solved was that the extraction and purification used a large amount of consumable material.

[012] The applicant has discovered that this problem may be solved by way of methods that include the steps of discharging impure potash from whatever source, e.g. agricultural waste ashes, into a leaching zone to provide leached potash. The leached slurry is then passed through at least one thickener zone, e.g., through a cascade of thickener zones connected in series, to provide a partially clarified potash solution. A portion of this solution is returned to the leaching zone, allowing the amount of fresh water used to be reduced, leading to an increase in the dissolved potash content in the leachate solution without the need for an evaporation step. The thickened potash solution is carbonated in a carbonization zone to convert the potash into potassium bicarbonate. The pH of the potash solution in the carbonization zone is reduced by introducing C02. The carbonated solution is filtered and passed through an ion exchange column, then sent to an activated carbon bed to remove dissolved organic matter. The potassium

bicarbonate is crystallized and the potassium bicarbonate crystals are filtered. The filtrate is returned to the crystallizer. A portion of the filtrate is used to regenerate the ion exchange column and a separate portion of the filtrate is added to the purge from the ion exchange column, thereby increasing the volume of the purge to keep the concentration of other impurities below the concentration at which they will precipitate in the crystallizer. Potassium carbonate is then regenerated from the potassium bicarbonate crystals in a heating zone. Finally the potash is ground to provide ground potash

[013] Thus by one broad aspect of the present invention, a method is provided for the purification of impure potash comprising discharging the impure potash in an aqueous solution into a leaching zone, and leaching potash out of the impure potash, thereby producing leached potash slurry; passing the leached potash slurry through at least one thickener zone to provide a thickened potash solution; carbonating the concentrated thickened potash solution in a carbonization zone, thereby producing potassium bicarbonate; filtering the solution from the carbonization zone in a first filter zone; passing the filtered solution through an ion exchange column to remove bivalent cations; passing the product from the ion exchange column to an activated carbon bed; crystallizing the potassium

bicarbonate in a crystallization zone into potassium bicarbonate crystals;

separating the potassium bicarbonate crystals from the supernatant solution in a second filter zone, thereby producing separated potassium bicarbonate crystals and separated supernatant solution; and regenerating the potassium carbonate from the separated potassium bicarbonate crystals in a heating zone, thereby producing purified potash.

[014] By a further broad aspect of the present invention, an interconnected system for the purification of impure potash is provided, the interconnected system comprising a leaching tank for containing impure potash to be purified; a solids inlet into the leaching tank for introducing impure potash to be purified; a water inlet line for introducing water into the leaching tank; a rinse water inlet line from a belt filter into the leaching tank; a thickener; a conduit for leading sludge of the impure potash from the leaching tank into the thickener; a carbonation column for converting the concentrated potash solution into potassium bicarbonate; a conduit for leading the concentrated potash solution from the thickener into the carbonation column; a first filter for filtering the potassium bicarbonate solution; a conduit leading from the carbonation column to the first filter; an ion exchange column to remove bivalent cations from solution of carbonization zone; a conduit for leading the potassium bicarbonate solution from the first filter into the ion exchange column; an activated carbon bed to remove colour imparted by dissolved organic matter; a conduit for leading the potassium bicarbonate solution from the ion exchange column into the activated carbon bed; a crystallizer; a conduit for leading the concentrated solution of potassium bicarbonate from the activated carbon bed into the crystallizer; a second filter; a conduit for leading a slurry of crystallized potassium bicarbonate into the second filter to provide crystallized potassium bicarbonate which is substantially free of supernatant solution; an oven for converting the crystallized potassium bicarbonate which is substantially free of supernatant solution into crystallized potassium carbonate; and a conveyor for conveying the crystallized potassium bicarbonate into the oven. BRIEF DESCRIPTION OF THE DRAWINGS

[015] FIG. 1 is a block flow diagram of the system and method for the production of potash from agricultural waste ashes.

DETAILED DESCRIPTION

[016] As seen in FIG. 1, the system for the production and purification of potash, preferably from the ashes of agricultural wastes, such as the ashes of cocoa pod husks, plantain (and banana) peels and cola nut husks, includes a leaching tank 500 (BLOCK A). In a preferred embodiment the leaching tank 500 includes an ash introduction line 501, a water inlet line 503 and a potash slurry outlet line 507 leading to a thickener 502 (BLOCK B).

[017] Thickener 502 is provided with an overflow line 509 to lead a dilute potash solution to a carbonation column 508 (BLOCK C). A portion of the dilute potash solution from the Thickener 502 is returned to the Leaching Tank 501 through a return line 511. A sludge outlet line 513 from the Thickener 502 sends the sludge to a device such as a belt filter 521 (BLOCK D). A rinse water inlet line 523 introduces water to rinse the sludge before the rinsed sludge is disposed through disposal line 519, and the rinse water solution is returned to the leaching tank 500 through a rinse water return line 513.

[018] The solution pH in the carbonation column 508 is reduced by providing a C02 stream 543 from both oven 526 (BLOCK J) and boiler 510 exhaust gas. The potassium bicarbonate solution which is formed in carbonation column 508 exits carbonation column via bicarbonate feed line 545 and is fed into first Filter 513 (BLOCK E). The filtered solution is sent through a filter outlet 516 to Ion Exchange Column 517 (BLOCK F). [019] The product from the ion exchange column 517 is sent via a column line 511 to an activated carbon bed 518 (BLOCK G). The carbon bed line 512 leads the filtered clear solution from the activated carbon bed 518 to the Crystallizer 561 (BLOCK H).

[020] A slurry of crystallized potassium bicarbonate then exits from crystallizer tank 561 and is fed into second filter 524 (BLOCK I) via slurry line 547. The solid potassium bicarbonate exits second filter 524 and is conveyed by conveyor 549 into oven 526 (BLOCK J), wherein the potassium bicarbonate is converted to potassium carbonate.

[021] A filtrate return line 548 leads from the second filter 524 to a crystallizer return line 549, to return filtrate back to the crystallizer 561. Filtrate return line 548 also returns a portion of filtrate via line 551 to regenerate the ion exchange column 517. Purge line 560 carries purge solution from the ion exchange column 517. Another portion of the filtrate solution from the filtrate return line 548 is directed by filtrate line 550 to mix with the ion exchange purge solution in purge line 560.

[022] In the oven 526, the potassium bicarbonate decomposes into crystalline potassium carbonate and effluent gases comprising carbon dioxide, nitrogen and steam are released. In an embodiment of the present invention, these effluent gases exit oven 526 and are fed into condenser 558 (BLOCK K) via gas line 567. Condenser 558 is fed with water from thickener return line 511 and rinse water return line 513 to condense the steam to water, thereby heating recycled clear solution from the thickener 502 and rinse water from the belt filter 521 to increase the leaching of potash in the leaching tank 500. The condensed gases, nitrogen and carbon dioxide, exit condenser 558 and are fed into compressor (BLOCK L) 514 through gas line 557. Nitrogen gas is vented through vent 541, and recovered gaseous carbon dioxide exits compressor 514 to add carbon dioxide into carbonation column 508 through carbon dioxide line 543. [023] The crystalline potassium carbonate (potash) from oven 526 exits onto oven conveyor 550 to be conveyed to grinders 528 (BLOCK P) where it is ground. The ground potassium carbonate (potash) exits grinders 528 onto grinder conveyor 561 to be conveyed to packaging machine 530 (BLOCK Q). The packages so formed may be supplied for local uses or may be sent to export 532 (BLOCK R) via export line 563.

[024] In a further embodiment, a portion of the potash solution can be removed from the process and used for commercial processes as is. The solution could be withdrawn prior to the carbonation stage and the commercial solution would be a potassium carbonate solution. Alternatively, the solution could be withdrawn after the carbonation stage and the commercial solution would be a potassium bicarbonate solution. The removal of solution prior to the crystallization step reduces the volume of high purity crystals produced, but may provide local benefits.

[025] In a further embodiment, Boiler 510 (BLOCK L) is preferably fueled with a mixture of liquid petroleum gas and methane through fuel line 525. Steam effluent from boiler 510 exits through main steam line 527 to crystallizer 561. Flue gases, i.e., carbon dioxide, nitrogen and steam, pass from gas outlet of boiler 510 to condenser/economizer 558 (BLOCK K) through main gas line 533.

Condenser/economizer 558 is fed with water through return water line 511.

Water condensate from condenser/economizer 512 is recycled from

condenser/economizer 512 back to boiler 510 (not shown). Effluent gases from condenser/economizer 558 exit condenser/economizer and are carried via second gas line 557 to compressor 514 (BLOCK L) for further use as described above, ie nitrogen gas is vented through vent 541, and recovered gaseous carbon dioxide exits compressor 514 to add carbon dioxide into carbonation column 508 through carbon dioxide line 543.

[026] Water vapor from crystallizer 561 may be collected through water vapor line 571 and condensed using the water from the return line 511 and the rinse water return line 513, thereby heating the leaching solution to provide increased dissolution of the potash in the leaching tank 500.

[027] As previously described in FIG. 1, the first step in the

extraction/purification of the potash, which may be organic potash produced by controlled combustion of agricultural wastes, for example the ashes of cocoa pod husks, plantain (and banana) peels and cola nut husks, consists of discharging the ashes, containing typically about 77% potash, into water in preferably one or more stainless steel leaching tanks connected in series, which are of the CSTR type.

[028] Next, the leached slurry is sent through a thickener to remove un-dissolved matter and to form a clarified potash solution. The ash waste is then thoroughly washed until it contains practically no potash.

[029] The clarified potash solution is sent to a carbonation column, where the potash is carbonated to potassium bicarbonate.

[030] The potassium bicarbonate solution is filtered and the filtered solution is sent to an ion exchange column to remove bivalent cations. The resultant potassium bicarbonate solution is mixed, preferably in a stainless steel tank, with an adsorbent, preferably activated carbon. The activated carbon adsorbs ferric ion complexes and clarifies the solution.

[031] Next, the potassium bicarbonate solution is sent into a crystallizer operating from a maximum of about 90°C down to a minimum of 30°C, from where it is sent to another filter to separate the crystals from the mother liquor. The mother liquor is returned into the crystallizer while the bicarbonate crystals obtained are sent to an oven to regenerate the potash and to release carbon dioxide. The carbon dioxide may be returned to the carbonation column for reuse.

[032] The potash which is obtained is ground into powder for market. This final product potash is widely used in the fertilizer, soap, petrochemical, glass, food and pharmaceutical industries among others. The purified potash may be of about 90% purity, or may be of about 99% purity.