GB2217700A - Potassium nitrate - Google Patents

Abstract

Initially, a reaction stage is carried out in which the potassium chloride is put into contact with a solution of dilute nitric acid coming from a stage of recovery of the acid by re-extraction and with a solution of concentrated nitric acid, all the potassium chloride supplied being thus dissolved and avoiding any crystallization of the potassium nitrate formed in order to, subsequently, carry out a stage of separation of the insoluble impurities which accompany the potassium chloride from the reaction mixture obtained, leaving this completely clarified and free from solids. With these improvements complete separation of the insoluble impurities of the potassium chloride can be carried out immediately after the reaction stage thus producing a white potassium nitrate of high purity.

Description

IMPROVED PROCEDURE FOR OBTAINING POTASSIUM NITRATE FROM POTASSIUM CHLORIDE AND NITRIC ACID British Patent Application No. 8521772 refers to a procedure for obtaining potassium nitrate from potassium chloride and nitric acid in which, with the aid of a predissolution stage of the denser fractions of the potassium chloride in a stream of dilute nitric acid, it is possible to start out with potassium chloride from any source (whether of marine origin or coming from sylvinite flotation).

In this procedure and based on the aforementioned predissolution and at a reaction temperature of between 10 and 35 C, all the potassium chloride becomes dissolved in the reaction medium, initially composed of potassium chloride, dissolution of the dense particles of this in dilute nitric acid (3 to 10% by weight) and concentrated nitric acid (60% by weight).

However, these conditions are not sufficient to keep all the potassium nitrate formed in solution, as a result of which part of this crystallizes in the reactor itself. In this way part of the product remains bonded to the insoluble impurities, generally of a clayey nature, which accompany the starting potassium chloride, especially if this is of mining origin.

If, in accordance with one of the options envisaged in the specifications of GB 8521772 this product crystallized at the reaction stage itself is subsequently separated, it will inevitably be accompanied by impurities, which in addition to colouring it, will render it unsuitable for use in the development of explosives and for use as a soluble fertilizer, as the said impurities will cause blockage of the irrigation nozzles.

If, on the other hand, the reaction stage is followed by a crystallization stage with a cold organic phase and subsequent extraction of the mother liquor with an organic solvent to recover the unreacted nitric acid, as is also described in the application GB 8521772, the impurities still give rise to problems.

-In effect, on the one hand, and although to a smaller extent than in the option mentioned before, impurities will accompany the crystallized product, colouring it, on the other hand, the insolubles will also impede the crystallization and extraction stages as they show a certain tendency to accumulate in the aqueous solution-organic phase interphase favouring the formation of emulsions and thereby rendering difficult the separation of phases.

So, the purpose of the present invention patent is to bring various improvements to the procedure, according to which complete separation of the insoluble impurities of the potassium chloride can be made immediately after the reaction stage, totally eliminating the problems which these give rise to and producing a white potassium nitrate of high purity suitable for any use.

In accordance with the aforementioned improvements, the procedure for obtaining potassium nitrate from potassium chloride and nitric acid to which the present invention patent refers consists of the following stages: I) Reaction between the starting potassium chloride and an aqueous solution of nitric acid coming from the addition of a stream of concentrated nitric acid and a stream of dilute nitric acid coming from the re-extraction stage indicated below. This stage is carried out in such a way that not only is all the potassium chloride dissolved in the reaction, but there is no crystallization of part of the potassium nitrate formed either.

II) Separation of the insoluble impurities from the reaction medium.

III) Crystallization of the potassium nitrate by contact of the brine clarified in the previous stage with an immiscible, inert and cold organic phase, then going on to separate, wash and dry the potassium nitrate product thus obtained.

IV) -Extraction of the mother liquor of the previous crystallization with an organic solvent to give an aqueous solution of hydrochloric acid free from nitrate ions and an organic extract containing the unreacted nitric acid.

V) Re-extraction of the organic extract of the previous stage by contact with water to give a solution of dilute nitric acid which is taken to the reaction stage and a regenerated organic solvent which can be reused in the previous stage.

The reaction stage is carried out without predissolution of the dense fractions of the potassium chloride being necessary by means of putting in contact at temperatures of 55 to 65 C of this, a solution of concentrated nitric acid of around 60% by weight and a solution of dilute nitric acid of 3 to 10% by weight coming from the re-extraction stage and which involves approximately 5 to 10% of the stoichiometric amount.

In this operation, the total nitric acid which is added is in slight excess.over the stoichiometric amount.

Under these conditions, total dissolution of the potassium chloride supplied, with the exception of the impurities, is achieved while at the same time keeping all the potassium nitrate formed in solution.

After that it is possible to go on to a stage of separation of the impurities at the same temperature as the reaction. The said separation can be carried out by conventional techniques, the use of a decanter into which a flocculant solution is incorporated being particularly suitable, the sludge can be removed through the bottom of the said decanter and then filtered or centrifuged to recover the aqueous solution which accompanies it.

With the aid of this separation stage total elimination of the impurities present is achieved, resulting in a completely clarified brine.

From this moment, the procedure described in application GB 8521772 is practically altered as a result of the improvements introduced by the present invention.

The clarified brine is then taken to the crystallization stage in which it is put in contact with an immiscible and inert organic phase which has been previously cooled. The temperature which it is necessary to reach in the aqueous phase for complete crystallization of the potassium nitrate ranges between 10 and -15 C.

This temperature can be achieved by means of one or more thermal drops by contact with progressively colder organic phases working in countercurrent in a battery of mixing-settling machines.

Kerosene, decane or naphthas can be used advantageously as the cold organic phase.

In this way the crystals of potassium nitrate, which now require only washing and drying, are obtained and separated, for example in hydrocyclones. Washing these crystals with basic solutions (for example potassium carbonate) is particularly suitable in order to eliminate the residual acidity which accompanies the product due to the presence of hydrochloric acid.

Drying can be done by conventional methods, hot air or direct combustion gases on a fluid bed being suitable. The product so obtained is of a white colour and presents a purity greater than 99.7% by weight.

The crystallization mother liquor is then subjected to an extraction stage with an organic phase made up of or containing an extraction solvent.

During the said stage the nitric acid contained in the crystallization mother liquor passes to the organic phase giving an aqueous solution of hydrochloric acid practically free from nitrate ions and of a concentration of 10 to 20% by weight.

As organic phase an organic solvent is used which may optionally be combined with an inert diluent. Suitable solvents which can be used are ketones of 5 to 10 atoms of carbon, -polyalkyleneglycols, phosphoric, phosphonic and phosphinic acid derivatives and linear or cyclical polyesters.

Tributylphosphate (TBP) is particularly suitable.

Amongst inert diluents it is possible to use kerosene, decane and naphthas.

This extraction stage, like the re-extraction stage which follows it, can advantageously be done working in countercurrent in a battery of mixing-settling machines.

The organic extract charged with nitrate ions obtained in the extraction stage is taken to a re-extraction stage by contact with water, such that an aqueous solution of nitric acid of a concentration of 3 to 10% by weight is obtained, which in its turn is taken to the reaction stage. The organic solvent is now regenerated after this re-extraction operation and can be used again in the extraction stage.

In figure 1 a diagramatic representation is made of the principal stages and streams involved in the procedure: Stages I Reaction.

II Separation of insolubles.

III Crystallization.

IV Extraction.

V Re-extraction.

Streams 1. Dissolution of nitric acid recovered in the re-extraction stage.

2. Potassium chloride.

3. Concentrated nitric acid.

4. Reaction mixture containing potassium nitrate, hydrochloric acid, unreacted nitric acid and insoluble impurities.

5. Insoluble impurities.

6. Reaction mixture free from impurities.

7. Crystallized potassium nitrate.

8, Cold inert organic phase.

9. Reusable inert organic phase prior cooling.

10. Crystallization mother liquor containing hydrochloric acid and nitric acid.

11. Dissolution of hydrochloric acid.

12. Regenerated organic phase containing the extraction solvent.

13. Organic phase charged with extracted nitric acid.

14. Water.

Example A quantity of 100 kg/h of commercial potassium chloride of 96% purity coming from sylvinite flotation is fed to a reactor together with a stream of 50.7 Kg/h of nitric acid of 8% by weight and a stream of 143 Kg/h of concentrated nitric acid of 60% by weight.

The temperature in the said reactor is kept at 60 C, by means of direct steam injection, full dissolution of the potassium chloride being achieved with only the clayey impurities which accompany it remaining insoluble.

Separation of the reaction mixture so obtained from the said insoluble impurities in a decanter is then prbceeded with, also at 60 C. In this way, once they are centrifuged to recover as far as possible the aqueous solution which accompanies them, 4.3 Kg/h of impurities are obtained and then discarded.

The clarified reaction mixture is then put into contact with kerosene at -20 C until a final temperature of -12 C is reached. Practically the whole of the potassium nitrate contained in the aqueous solution which is separated from the mother liquor by means of a centrifuge is crystallized, until a percentage of moisture of 4 % is reached. It is then dried to give 127 Kg/h of potassium nitrate of 99.79% purity, which is equivalent to a yield of 97. 6% in relation to the potassium chloride.

The crystallization mother liquor is in turn taken to a battery of mixing-settling machines in which it is put in contact at room temperature with a mixture at 25/75 by volume of TBP/kerosene. 333 Kg/h of an aqueous solution of hydrochloric of 13. 7 X largely free from nitrate ions is thus obtained.

The organic extract which is separated from the said solution of hydrochloric acid is re-extracted with water in a new battery of mixing-settling machines, 58.7 Kg/h of a solution of nitric acid at 8% being obtained, which is taken to the reactor and a regenerated organic phase, reusable in the extraction stage.

Claims (5)

1.- Improvements in patent application GB 8521772 for a procedure for obtaining potassium nitrate'from potassium chloride and nitric acid, characterised in that initially, a reaction stage is carried out in which the potassium chloride is put into contact with a solution of dilute nitric acid coming from a stage of recovery of the acid by re-extraction and with a solution of concentrated nitric acid, all the potassium chloride supplied being thus dissolved and avoiding any crystallization of the potassium nitrate formed in order to, subsequently, carry out a stage of separation of the insoluble impurities which accompany the potassium chloride from the reaction mixture obtained, leaving this completely clarified and free from solids. And, finally, proceeding with the subsequent stages of crystallization, extraction and re-extraction according to application GB 8521772.

2.- Improvements, as in Claim 1, characterised in that the temperature at which the stages of reaction and separation of insoluble impurities are carried out is between 55 and 65 C.

3.- Improvements, as in Claims 1 and 2, characterised in that the potassium nitrate obtained is of a white colour and a purity greater than 99.7% by weight.

4. Improvement as in Claim 1 and substantially as herein described.

5. Improvements as described in any of the specific examples.