WO2003074424A1

WO2003074424A1 - A cyclic process for continuously producing potassium sulphate and potassium sulphate produced thereby

Info

Publication number: WO2003074424A1
Application number: PCT/IN2002/000035
Authority: WO
Inventors: Visweswara Rao Abburi
Original assignee: Visweswara Rao Abburi
Priority date: 2002-03-07
Filing date: 2002-03-07
Publication date: 2003-09-12
Also published as: AU2002247943A1

Abstract

This invention relates to a cyclic and continuous process for the production of potassium sulphate and simultaneous recovery of hydrochloric acid formed during the reaction. Potassium chloride and sulphuric acid are reacted at ambient temperature in stoichiometric ratio to produce potassium bisulphate. This reaction is carried out in the presence of mother liquor containing bisulphate ions recycled from a subsequent step when the continuous reaction is in progress. Hydrochloric acid is removed by evaporation and calculated quantity of water is added to the residue to produce a double salt of potassium sulphate and potassium bisulphate. Said potassium sulphate is separated from this and the mother liquor is recycled to the first reaction stage. A calculated quantity of water is added to the residue and potassium sulphate is crystallised and separated therefrom and the mother liquor obtained is recycled to the double salt separator stage. This processor is carried out at ambient temperature.

Description

A CYCLIC PROCESS FOR CONTINUOUSLY PRODUCING

POTASSIUM SULPHATE AND POTASSIUM SULPHATE

PRODUCED THEREBY

Technical field:

This invention relates to a cyclic process for continuously producing potassium sulphate and potassium sulphate produced thereby. This process also includes simultaneous recovery of hydrochloric acid which is a byproduct. The process according to the invention is cost effective since the energy requirement is reduced and does not require complex and expensive plants and machinery.

Background ofthe invention:

Potassium sulphate is produced by reacting potassium chloride and sulphuric acid according to the following reaction: 2KC1 + H₂S0₄ → K₂S0₄ + 2HC1 Depending upon the ratio ofthe reactants certain amount of potassium bisulphate is also produced during this reaction: KC1 + H₂S0₄ → KHSO₄ + HC1 Mannheim process is most widely used for commercial production of potassium sulphate. In this method, the reaction between potassium chloride and sulphuric acid is carried out at a temperature range of 400 to 500°C. P T/IN02/00035

Hydrogen chloride gas is recovered and potassium sulphate is obtained as a residual solid. This process has several disadvantages. It is energy intensive. Construction of the plant for manufacturing the product commercially is also complex, since hydrogen chloride at high temperatures is extremely corrosive in nature.

Potassium sulphate is also obtained by indirect processes. US patent No.3998935 discloses such a process which results in the production of 30 to 50% excess of potassium bisulphate.

US patent No.5,045, 543 discloses yet another method in which nearly 10 tons of water per ton of potassium sulphate produced has to be evaporated. Evaporation and removal of water to obtain solid potassium sulphate consumes large amount of energy and it is time consuming. Concentration of hydrochloric acid obtained as a by- product according in this process is only about 5%.

US patent No.4,588,573 describes a process for producing potassium sulphate from potassium chloride and sulphuric acid by sequential decomposition of the potassium sulphate salts. This process consists of adding potassium chloride to sul&ric acid in stoichiometric ratios to produce potassium sulfate in aqueous solutions recycled from process steps. This process is an improvement over US patent No.4,045,543. About 3 tons of water per ton of potassium sulphate produced need to be evaporated in this process as well. The steps involved in this process is carried out at different temperatures and include recycling of at least three mother liquors. The process is based on ternary phase diagram of K₂S0₄, H₂S0₄ and H₂0. the process calls for filtering potassium salt from a solution consisting of hydrochloric acid and hence, truly, the product can not be totally free from chloride. Hence, the present invention addresses these objectives of preparing a potassium sulfate salt free from chlorides and also does not use any energy for evaporating any water from the process steps.

Disclosure ofthe invention:

The object of this invention is to develop cost effective process which will have the least energy requirement for the production of potassium sulphate. The reaction steps are carried out at ambient temperature in the range of 25 to 30°C. Potassium sulphate produced according to this process is substantially free of potassium chloride. Hydrochloric acid obtained as a by-product is of about 25% concentration and therefore commercially usable without further concentration. The reaction stages are simple and easy to operate. Yet another advantage of this invention is that evaporation is avoided or minimised at various stages of this process due to optimized use of water at different process stages. This results in the production hydrochloric acid of a concentration not less than 25% which may be used directly without further concentration.

The present invention is based on the following chemical reactions: Step 1: Consists of producing potassium bisulfate

KC1 + H₂S0₄ = KHSO₄ + HC1 Step 2: the second step consists of removing one mole of H₂S0₄ from two moles of KHSO₄ to produce potassium sulfate - K₂S0 2 KHS0₄ = K₂S0⁴ + H₂S0₄ The process consists of removing the sulfuric acid according to the above step and recycling to the first step according one of the embodiments of the invention or treat it separately according to variations in the process invented.

In accordance with the present invention potassium chloride and sulphuric acid are reacted in the stoichiometric ratio required for the production of potassium bisulphate. This reaction may also be earned out in the presence of recycled aqueous solution containing potassium bisulphate obtained from a subsequent process step, once the cyclic process is in progress. The reaction mixture is then evaporated to remove water and hydrogen chloride which is recovered by known methods like condensation or absorption. It is possible according to the invention to produce only potassium bisulfate by reacting KCl with H₂S0₄ in stoichiometric ratios and treat the recycle solution differently.

The bisulfate obtained after evaporation is treated with a calculated amount of water to separate substantial quantity of bisulphate therefrom in solution. This separation step may also be carried out in the presence of an aqueous recycled solution from a subsequent step once the cyclic reaction commences. The residue obtained thereafter contains a double salt of potassium bisulphate and potassium sulphate but consisting of a larger percentage of potassium sulphate. This double salt is further treated with adequate quantities of water, to yield substantially pure potassium sulphate free of potassium chloride and potassium bisulphate. The filtrate obtained after separation ofthe pure salt is recycled to the double salt separator stage.

The process according to this invention uses advanced techniques to determine the equilibrium between solution and precipitation conditions so as to optimise the use of water in the reaction steps based thereon, such that the final process steps minimises the boiling out of water. A phase diagram depicting the solubility of double salt, potassium bisulphate and potassium sulphate together with the boundary precipitation conditions helps in deciding the quantity of water to be added at different stages. Hydrochloric acid by product obtained by this method is of 25 to 30% w/w concentration. The precipitation conditions are decided through simulation and optimization method from the phase diagram.

It is also found that the process steps could be even reduced to only one recycle and eliminate total energy usage, by not recycling the filtrate from double salt crystalliser to the bisulfate stage. It has been found that the solubility of K₂SO in sulfuric acid is the least at a concentration of 52 to 56%. According to one embodiment of the invention, the recycle soilution is exposed to a stream of dry air in a suitable equipment. This way, the dry air absorbs the water and the solution becomes concentrated gradually to the desired level. The bisulfate precipitated from the solution is recovered and the filtrate consisting of about 0.8 to 1% K₂SO₄ is used elsewhere where acid is required.

Hence, the present invention can operate on one recycle or at option on two recycles, minimise the energy requirement, give a chloride free potassium sulfate and operates at one temperature.

Brief description ofthe drawings:

Figure 1 depicts the flow diagram of the cyclic process for the production of potassium sulphate according to this invention. Figure 2 illustrates a phase diagram showing the relationship between K₂S0₄ and H₂S0₄ as percentage in the gross product.

Detail description with reference to the accompanying drawings:

Stoichiometric quantities of potassium chloride and sulphuric acid required for producing potassium bisulphate is fed to bisulphate reaction chamber (1). After completion ofthe reaction, hydrogen chloride and water produced during the reaction are removed by evaporation. This is absorbed in a convention absorption towers or condensed in a known manner to produce hydrochloric acid of concentration of about 30% w/w. Also, it is possible to produce bisulfate and anhydrous hydrogen chloride gas by only reacting potassium chloride with sulfuric acid in stoichiometric ratios and treat the recycle solution separately. Solid reaction product is conveyed to a double salt crystalϋzer (2) and optimum quantity of water is added thereto to remove major portion of bisulphate in solution. A double salt separator (3) is used to separate solubilized potassium bisulphate leaving a residue containing a major portion of potassium sulphate. To the sulphate crystallizer (4) to which the residue obtained from the double salt separator (2) is transferred, optimum quantity of water is added thereto to remove bisulphate therefrom and to crystallize potassium sulphate present therein, which is filtered in the sulphate separator (5). Aqueous solution containing potassium bisulphate obtained in the double salt separator (3) is recycled to bisulphate reaction chamber or treated separately (1) while the filtrate obtained after purification and separation of potassium sulphate is recycled to the double salt separator (4). It is therefore clear that the reaction schemes involves only two recycling steps ofthe mother liquor or even only to one recycling depending upon the facility to use recycle acid elsewhere. This minimises the quantity of water in the reaction mixture.

Figure 2 illustrates a phase diagram showing the relationship between K₂S0₄ and H₂SO₄ as percentage in the gross product containing water, at different isotherms depicting the boundaries which define the existence of bisulphate, a mixture of potassium bisulphate and potassium sulphate and a zone indicating the existence potassium sulphate. This diagram also indicates the solubility lines of the double salt and potassium sulphate. The amount of water to be added to solubilize potassium bisulphate is calculated on the basis of this phase diagram.

The invention provides a cyclic process for continuously producing potassium sulphate with simultaneous recovery of hydrochloric acid which comprises the steps of : (a) reacting potassium chloride and sulphuric acid in stoichiometric ratio to produce potassium bisulphate, evaporating said reaction mixture to substantially recover the hydrochloric acid produced thereby; (b) adding a calculated quantity to the residue and water to a recycle solution obtained from potassium sulfate crystalliser to produce a double salt containing potassium bisulphate and potassium sulphate; (c) separating double salt from the aqueous suspension thus produced and recycling the mother liquor to step (a), (d) purifying and separating said potassium sulphate by crystallizing from water and recycling the mother liquor after said separation to step (b), wherein the quantity of water added in step (b) and step (d) are regulated and optimized to produce about 30% w/w of hydrochloric acid in step (a) and repeating said steps sequentially and continuously to produce potassium sulphate.

In the continuous process described above potassium bisulphate and potassium sulphate are produced depending upon the concentration of H₂S0 in the gross material.

The total quantity of water added in step (b) and step (d) may be equivalent to produce 25% w/w of hydrochloric acid in step (1).

The total quantity of water added to the double salt is equivalent to the total quantity of water required to produce 25% w/w of hydrochloric acid in step (a) minus the quantity of water added in step (d) to crystallise pure potassium sulphate. The reaction mixture is heated to 80 to 130°C to substantially remove hydrochloric acid therefrom. This temperature can be lowered, if the reaction is carried out under sub-atmospheric conditions.

The double salt formed during the cyclic process contains about 50 to 55% of potassium sulphate.

This invention also includes potassium sulphate when prepared by a process as described hereinabove.

The recycle solution from the double salt crystalliser can be circulated in a tower or a suitable equipment with dry air in contact with the solution. The concentration of H₂S0₄ in the solution progressively reaches to about 55%, and the solution can be filtered to remove almost the entire potassium salt, thereby leaving sulfuric acid of about 55% concentration and still containing about 0.8 to 1.2% of potassium salt. This solution could be used elsewhere where acid is required, such as in a fertiliser unit or for producing other sulfates of ammonia, zinc, magnesium etc. This saves further energy requirement and also allows for recovering HCl in anhydrous form in the process step (a). The following examples illustrate the process according to the invention.

Example 1 :

A mixture of potassium sulfate and potassium bisulphate in a ratio of 59:41 is prepared with a total weight of 1520 gms. The mixture is added with an amount of 1200 gms of water. The mixture is thoroughly stirred and heated to about 60°C. The contents cooled and filtered. The precipitate consisted of 990 gms (wet) and 940 gms (dry) of potassium sulfate of 98.88% purity. The filtrate weighed 1660 gms. The acidity of the filtrate was found to be 11.6%.

This shows that when a double salt of potassium sulfate and bisulfate are treated with adequate quantity of water, it is possible to selectively precipitate pure potassium sulfate.

Example 2:

The filtrate from Example 1 consisting of solution of 1660 gms is added with potassium bisulfate of 995 purity weighing 1820 gms. To the above, water of 700 gms was added and the contents thoroughly mixed. On filtration, the precipitate yielded a double salt of 1520 gms (wet) and 1470 gms (dry) with content of potassium sulfate and potassium bisulphate in a ratio of 60.5 : 39.5. The filtrate consisted of 2620 gms and acidity of 23.99%. This shows that in an aqueous solution of potassium sulfate, water and acid, we can add potassium bisulfate to crystallize a double salt of potassium sulfate and bisulfate.

Example 3:

Filtrate of 1400 gms from the above experiment is collected in a flask and water is evaporated by circulating the solution through a tower,through which dry air at a rate of 45 CUM/hr and 120 mm wc pressure is passed. The solution is continuously filtered to remove bisulfate salt precipitating during the process. The filtered precipitate weighed 288 gms and the solution weighed 468 gms with an acidity of 54% and K₂0 content of 0.88%.

This shows mat the filtrate from example 3 can be treated separately and concentrated by exposing to dry air and recover substantial portion of potassium.

Example 4:

700 gms of filtrate from Example 2 is collected and evaporated to remove 210 gms of water. To the contents, added potassium chloride of 192 gms and sulfuric acid of 98% concentration of 126 gms. The contents are heated to about 96°C in a vacuum and the gases condensed in a condenser. The heating continued till all the solution is dried out and residue collected. The residue yielded 497 gms of dry potassium bisulfate of 99 % purity. The condensed cooled acid consisted of 324 gms with hydrochloric acid concentration of 29%.

This shows that it is possible to produce potassium bisulfate by adding potassium chloride and sulfuric acid to an aqueous solution.

Obvious equivalents and modifications known to persons skilled in the art not excluded from the spirit and scope of this invention and that of the appended claims.

Claims

CLAIMS:

1. A cyclic process for continuously producing potassium sulphate with simultaneous recovery of hydrochloric acid comprises the steps of:

(a) reacting potassium chloride and sulphuric acid in stoichiometric ratio to produce potassium bisulphate, evaporating said reaction mixture to substantially recover the hydrochloric acid produced thereby;

(b) adding a calculated quantity of water to the bisulfate in an aqueous solution from a previous process step to produce a double salt containing potassium bisulphate and potassium sulphate;

(c) separating double salt from the aqueous suspension thus produced and recycling the mother liquor to step (a) and

(d) purifying said double salt and separating potassium sulfate by crystallizing from water and recycling the mother liquor after said separation to step (b); wherein, the quantity of water added in step (b) and step (d) are regulated and optimized to produce about 25% w/w of hydrochloric acid in step (a) and repeating said steps (a) to (d) sequentially and continuously to produce potassium sulphate.

2. The process as claimed in claim 1, wherein one of the recycle solutions from the double salt step to a. stream of air to increase the concentration of the solution to about 55% to recover a substantial portion of potassium from the solution. The process as claimed in claim 1, wherein the reaction mixture obtained from step (a) is heated to 80°C to 130°C to substantially remove hydrogen chloride and water therefrom.

The process as claimed in claim 2, wherein the recycle solution is exposed to dry air to concentrate the solution to a degree where the solubility of potassium in sulfuric acid reaches the least.

The process as claimed in claim 3, wherein the hydrochloric acid produced is recovered by absorption or by condensation in a known manner.

The process as claimed in claim 1, wherein said double salt produced in step (b) contains upto 55% of potassium sulphate.

The process as claimed in any one of the claims 1 to 5, wherein the reaction between potassium chloride and sulphuric acid is earned out at 80 to l30°C.

The process of crystallisation of double salt of potassium and crystallisation of potassium sulfate at one temperature 25 to 30°C. Potassium sulphate whenever prepared by a process as claimed in any one ofthe claims 1 to 6.

A cyclic process for continuously producing potassium sulphate substantially as herein described.

Concentrating any aqueous solution consisting of a salt to a stream of dry air in any arrangement to concentrate the solution.