Title of Invention

METHOD OF PRODUCING PHOSPHORIC ACID

Abstract

The invention relates to a method for producing phosphoric acid involving: a formation of a first aqueous slurry containing calcium sulfate dihydrate and phosphoric acid; a formation of a second aqueous slurry containing calcium sulfate hemihydrate and phosphoric acid; a filtering of a mixture comprised of the two slurries through a filter while forming a filter cake simultaneously containing calcium sulfate dihydrate and hemihydrate, and; separating an aqueous phase containing the phosphoric acid.

Full Text

FORM 2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10) TITLE "METHOD OF PRODUCING PHOSPHORIC ACID" APPLICANT PRAYON TECHNOLOGIES Rue Joseph Wauters 144 B-4480 Engis Belgium Nationality: Belgium The following specification particularly describes the nature of this invention and the manner in which it is to be performed METHOD OF PRODUCING PHOSPHORIC ACID The present invention relates to a method for producing phosphoric acid, comprising - forming a first aqueous slip containing calcium sulphate dihydrate and phosphoric acid, - forming a second aqueous slip containing calcium sulphate hemihydrate and phosphoric acid, - filtering through a filter with the formation of a filter cake and separating an aqueous phase containing phosphoric acid. Methods in which a filter cake formed principally from calcium sulphate dihydrate (CaSO4 2H2O) is filtered have been known for a long time (see for example BE-660216 and BE-683739). In these patents a conversion of the calcium sulphate obtained at higher temperature and acidity is provided for in order to purify it and to recrystallize it into calcium sulphate hemihydrate (CaS04.1/2H2O). The British patent 313.036 and US patent 1.902.648 describe the production of phosphoric acid accompanied by the obtaining of calcium sulphate hemihydrate, the latter then being converted into dihydrate with a view to separating it by an operation of filtration and washing on a second filter. US patent 1.836.672 concerns a method in two steps, namely a first step that comprises the reaction of the phosphate and sulphuric acid with the simultaneous production of hemihydrate and concentrated phosphoric acid, the latter then being separated by a filtration operation, whilst, in a second stage, the hemihydrate, still saturated with phosphoric acid, is put back in suspension in a sulphophosphoric medium, under conditions of acidity and temperature such that the calcium sulphate recrystallizes into dihydrate. There is also known a continuous method of preparing phosphoric acid and calcium sulphate comprising, after the filtration of a sulphate in the form of dihydrate or hemihydrate, a crystallization unchanged in the same form or a recrystallization in the form of anhydrite II or hemihydrate or respectively dihydrate containing a certain proportion of anhydrite II (see EP-B-0181029). Finally, there is also known a method of producing phosphoric acid and calcium sulphate by continuous treatment on a filter treating flows coming from several reaction zones, each supplied with an aqueous slip resulting from the attacking of the ore by sulphuric acid (US-A-3911079). These slips can contain as required calcium sulphate dihydrate or hemihydrate. The filter cake obtained therefore consists of superimposed layers of calcium sulphate dihydrate or hemihydrate without the interpenetration of one layer in the other. The aim of this method is to increase the concentration of P2O5 in the phosphoric acid produced in the last of the reaction zones. This relatively complex method, tricky to master, does however have the drawback of a not very economical yield of the installation. In all these prior methods calcium sulphate hemihydrate or dihydrate is always filtered separately. In the conventional dihydrate filtration methods, gypsum represents a by-product that has to be disposed of. Moreover, it can be put in a heap only under clearly determined conditions, at places sometimes remote from the production sites. The gypsum obtained entrains with it large quantities of water that make it difficult to handle and increase the cost of transport dry. In addition, there is a risk of polluting subterranean waters at the discharge site. The wet methods used in industry, which allow the production of concentrated phosphoric acid (more than 40% P2O5) have for a long time been coming up against the difficulties resulting from the simultaneous production of calcium sulphate hemihydrate and operating at high temperature: the working conditions give rise to phenomena of corrosion and incrustation giving rise to additional production costs. The aim of the present invention is to develop a method for producing phosphoric acid that makes it possible to overcome the aforementioned drawbacks whilst permitting the use of a technically simple installation or even an easy adaptation of an installation currently in service. This problem is resolved according to the invention by a method as indicated at the start, this method comprising - before and/or during the said filtration, mixing the said first slip and the said second slip, and - filtering this mixture with the formation of a filter cake consisting of a mixture of calcium sulphate dihyrate and calcium sulphate hemihydrate. Enormous advantages in using this very simple method have been revealed. Compared with the filtration of a cake formed from calcium sulphate dihydrate, the filterability of the cake is improved since, in fact, its porosity is greater by virtue of the presence of hemihydrate crystals whose structure is different (twins) from those of gypsum crystals (acicular). The effect of this is an improvement in the production capacity of the filter already installed in an existing factory for filtration of gypsum. If a new production unit for phosphoric acid is to be constructed, the size of the filter can be decreased, which reduces the investment costs. In addition, and surprisingly, it has become clear that, compared with slips containing only dehydrate, supplying the filter with a mixture containing dihydrate and hemihydrate gives filter cakes whose free water and water-soluble P2O5 contents are the lowest, for a minimum filtration time, which was not predictable and greatly increases the economic profitability of the installation. As stated above, the cakes discharged by the filter therefore contain less free water than the cakes resulting from a filtration of slips containing only dehydrate. After discharging in a heap, the hemihydrate crystals, which are unstable, rehydrate by virtue of free water molecules still present in the discharged cake. As a result the quantity of acidulated water accompanying the cake on its discharge site is reduced, which reduces the danger of pollution of underground water. The filtered cake dries over time, is easily transportable and can be stored in a heap, which makes it possible to reduce the surface area on the ground necessary for discharge. In addition, the global efficiency of a phosphoric acid production installation producing calcium sulphate dihydrate is increased because the recrystallization of part of the gypsum into calcium sulphate hemihydrate releases phosphate ions in the form of phosphoric acid. According to one embodiment of the invention the method comprises, before filtration, mixing the first aqueous slip and second aqueous slip and supplying the filter with the said mixture. This mixing can be carried out simply, for example by connecting the filter supply pipe to the discharge pipes from the tanks where the two slips of the method according to the invention were produced. It is also possible to provide in addition an intermediate mixer where the two slips are mixed. The method according to the invention also provides for simultaneously supplying the filter with the said first slip and the said second slip and mixing these two slips in the filter itself. The aqueous slips can be obtained by any appropriate means. It is possible for example to envisage that one or other of the two may result from a sulphuric attack on phosphate ores. It is also possible to envisage that the hemihydrate slip may, in a manner known per se, be formed from part of the dihydrate slip, for example by modifying the temperature and/or acidity conditions of this drawn-off part of the dihydrate slip. Such a modification requires only the mounting of a single additional tank provided at the discharge from the attack tank of an existing installation. Thus the method then advantageously comprises a sulphuric attack on phosphate ore with the formation of the said first aqueous slip, a drawing off of part of the said first aqueous slip, and treatment of this giving rise to a conversion of the said first aqueous slip into the said second aqueous slip. This treatment can comprise for example an addition of sulphuric acid to the said drawn-off part of the first aqueous slip, or a heating thereof, for example by heating the tank or injecting steam into the slip, or the two processes simultaneously. Conversely, it is also possible to envisage that, in a manner known per se, the dihydrate slip may be formed from part of the hemihydrate slip. In certain cases, the hemihydrate slip is a more acidic medium than the dihydrate slip, that is to say it contains a higher sulphuric acid content. When a mixture of the two slips is fed to the filter, a phosphoric acid is harvested with a sulphuric acid content that is sometimes higher than the specification and it is then necessary if applicable to desulphate the filtrate, for example by adding phosphate ore to the latter and settling the calcium sulphate formed, which will preferably be recycled to the sulphuric attack on the phosphate ore. Other embodiments of the method are indicated in the accompanying claims. An example of an embodiment of an installation implementing a method according to the invention will now be described in more detail, making reference to the accompanying single figure, which is a flow diagram. This installation comprises an attack tank 1 into which there is introduced at 2 ground phosphate ore and at 3 sulphuric acid, as well as, at 4, the aqueous solution of phosphoric acid recycled from the method according to the invention. In this tank the attack conditions are applied in a known manner in order to obtain an aqueous slip containing calcium sulphate dihydrate and phosphoric acid, a slip that is drawn off from the tank 1 through the discharge pipe 5. Part of this slip drawn off from the tank 1 is taken off by the taking-off pipe 6 and taken to a conversion tank 7. In this tank the slip is treated so as to be converted into an aqueous slip containing calcium sulphate hemihydrate and phosphoric acid. The treatment applied in this example of an embodiment consists of introducing additional sulphuric acid at 8 and/or heating the tank and/or injecting steam at 9 into the slip. Any other suitable source of heat can of course be used for this purpose. Under the effect of the increase in acidity and/or temperature, the dihydrate is converted into hemihydrate. The remaining part of the slip drawn off from the tank 1 is, through the pipe 10, taken to a mixing step illustrated by the arrow 11. The slip treated in the tank is taken to this same mixing step through a discharge pipe 12. This mixing step can be performed, as shown, by means of a common feed pipe into which the pipes 10 and 12 open out, but also for example by means of a stirring tank into which these pipes 10 and 12 open out, and from which a mixture is discharged to the filter 13. It is also possible to envisage a simultaneous supply of the two slips through the pipes 10 and 12 leading directly to the filter 13 where the mixing takes place. The filter 13 may be any suitable known filtration device, for example a band filter, a device with filter cells disposed in a carousel, etc. In the example illustrated, the mixture of the two slips is filtered in a first section of the filter with the obtaining of a filtrate at 14 that is the product of the method, that is to say an aqueous solution of phosphoric acid. The method illustrated comprises two steps of washing the cake, after filtration. The second washing is carried out using a washing liquid that is for example water, supplied at 15. The washing product obtained at 16 is recycled at 17 as a washing liquid for the first washing step. The washing product obtained at 18, which issues from the first washing step, is an aqueous solution of phosphoric acid that can be recycled at 4 to the attack tank 1, through a recycling pipe 19. If the filtered phosphoric acid, obtained at the discharge pipe 14, has an excessively high residual acid content, it is possible to make provision for making this pipe 14 open out into a desulphatation tank 20 provided with a stirrer and supplied at 21 for example with an addition of phosphate ore. In this tank there then takes place a formation of calcium sulphate which may, through a pipe 22, be transferred for example to a clarifier 27 where there takes place a separation by settling of the calcium sulphate, which is discharged from the clarifier 25 through the discharge pipe 23, whilst the purified phosphoric acid is recovered at 28. This calcium sulphate can then also be recycled at 24 to the attack tank 1. Finally, at the downstream end of the filter 13, the filter cake is discharged and possibly transported, in accordance with the indication of the arrow 25. It can then be put in a heap at 26. During the discharge, transportation and/or putting in a heap, the hemihydrate is converted little by little into dihydrate by virtue of the presence of residual water in the cake. The cake therefore progressively loses its water content, which facilitates transportation and discharge in a heap. Advantageously, during the mixing step at 11 the mixture of the two slips contains calcium sulphate dihydrate and calcium sulphate hemihydrate in a ratio by weight of 90/10 to 10/90, preferably from 70/30 to 30/70. It is also possible to provide on the filter, upstream or downstream of the filtration of the mixture according to the invention, a supply to the filter of at least one additional aqueous slip containing for example only dihydrate, only hemihydrate or a mixture of these, in particular in proportions different from those provided for in the mixing step 11. The method according to the invention will now be described in more detail with the help of examples of embodiments given by way of illustration and non-limitingly. Example 1 Two series of four tests were carried out. In each series of tests, the slips were fed to the filter in four different ways: 1. supply of a hemihydrate slip (20% by weight) first of all, followed by a gypsum slip (80% by weight) 2. supply of a gypsum slip (80% by weight) first of all, followed by a hemihydrate slip (20% by weight) 3. supply according to the invention of mixed hemihydrate and gypsum slips, in a dihydrate/hemihydrate ratio by weight of 80/20 4. supply of a slip consisting of 100% by weight gypsum. The filtration was carried out at a low pressure of 400 mm Hg on a Buchner cell having a useful surface area of 1 dm2. It is broken down into four successive steps: the formation of the cake or cakes by separation of the phosphoric acid and crystals contained in the slips ("mother liquors" sector in tables 1 and 2 below), a first washing of the cake thus formed by the filtrate from the second washing ("washing 1" sector), a second washing of the cake with water taken from an industrial environment ("washing 2" sector), and finally the drying of the cake with air ("drainage" sector). The two series show clearly that it is the tests where the hemihydrates slip is supplied first (tests 1a and 1b) and those where the slips are previously mixed (tests 3a and 3b) that give the minimum filtration time, with more favourable results for the mixture in test 3b. In addition, supplying the hemihydrate and gypsum slips in a mixture routinely gives the lowest free-water and water-soluble P2O5 contents in the cake. Example 2 Five series of tests were carried out with dihydrate/hemihydrate mixtures having different weight ratios, in comparison with dihydrate alone. The total filtration times, the P2O5 content of the cake, the yield and the free water in the cake after rehydration of the hemihydrate are given in the table below. D = dihydrate H = hemihydrate WS P2O5 = water soluble P2O5 = impregnated phosphoric acid TOT P2O5 = total P2O5 These results amply show the advantages of filtration in a mixture compared with dihydrate alone: reduction in the filtration time, better recovery of P2O5, reduction in the free-water content of the cake after rehydration of the hemihydrate phase. It must be understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made thereto without departing from the scope of the accompanying claims. CLAIMS 1. Method of producing phosphoric acid, comprising - forming a first aqueous slip containing calcium sulphate dihydrate and phosphoric acid, - forming a second aqueous slip containing calcium sulphate hemihydrate and phosphoric acid, and - filtering through a filter with the formation of a filter cake and separating an aqueous phase containing phosphoric acid, characterised in that it comprises - before and/or during the said filtration, mixing the said first slip and the said second slip, and - filtering this mixture with the formation of a filter cake consisting of a mixture of calcium sulphate dihyrate and calcium sulphate hemihydrate. 2. Method according to claim 1, characterised in that it comprises, before the filtration, a mixing of the first aqueous slip and second aqueous slip and feeding the filter with the said mixture. 3. Method according to claim 1, characterised in that it comprises a simultaneous feeding of the filter with the said first slip and the said second slip and a mixing of these two slips in the filter. 4. Method according to any one of claims 1 to 3, characterised in that the said first slip and/or the said second slip is formed by a sulphuric attack on phosphate ore. 5. Method according to any one of claims 1 to 4, characterised in that it comprises a sulphuric attack on phosphate ore with the formation of the said first aqueous slip, a drawing off of part of the said first aqueous slip, and treatment of this giving rise to a conversion of the said first aqueous slip into the said second aqueous slip. 6. Method according to claim 5, characterised in that the said processing comprises an addition of sulphuric acid to the said drawn- off part of the first aqueous slip and/or heating thereof. 7. Method according to any one of claims 1 to 6, characterised in that it comprises discharging the filter cake containing simultaneously calcium sulphate dihydrate and calcium sulphate hemihydrate as well as residual water and a subsequent hydration of the calcium sulphate hemihydrate into calcium sulphate dihydrate in the presence of this residual water. 8. Method according to claim 7, characterised in that the filter cake is discharged in a heap by dry method. 9. Method according to any one of claims 1 to 8, characterised in that the aqueous phase containing phosphoric acid that is separated during the said filtration is subjected to subsequent desulphatation. 10. Method according to claim 9, characterised in that the subsequent desulphatation comprises an addition of phosphate ore to the above-mentioned aqueous phase and separation of the calcium sulphate thus formed. 11. Method according to claim 10, characterised in that the calcium sulphate thus formed is recycled to the formation of the said first aqueous slip. 12. Method according to any one of claims 1 to 11, characterised in that, after the said separation of the aqueous phase containing phosphoric acid, it comprises at least one step of washing the filter cake with a washing liquid. 13. Method according to claim 12, characterised in that it comprises a recycling of the washing liquid issuing from at least one above-mentioned washing step to the said formation of at least one of the said first and second slips. 14. Method according to any of claims 1 to 13, characterised in that the mixture obtained contains calcium sulphate dihydrate and calcium sulphate hemihydrate in a ratio by weight of 90/10 to 10/90. 15. Method according to any one of claims 1 to 14, characterised in that it also comprises, before and/or after the filtration of the said mixture, at least one feeding of the filter with one of the said first and second aqueous slips.

Documents:

4800-CHENP-2006 CORRESPONDENCE OTHERS.pdf

4800-CHENP-2006 CORRESPONDENCE PO.pdf

4800-CHENP-2006 FORM-18.pdf

4800-CHENP-2006 OTHER PCT FORM 09-10-2009.pdf

4800-CHENP-2006 AMANDED CLAIMS 08-10-2009.pdf

4800-CHENP-2006 AMANDED PAGE OF SPECIFICATION 08-10-2009.pdf

4800-CHENP-2006 CORRESPONDENCE-OTHERS 08-10-2009.pdf

4800-CHENP-2006 FORM-3 08-10-2009.pdf

4800-chenp-2006-abstract.pdf

4800-chenp-2006-claims.pdf

4800-chenp-2006-correspondnece-others.pdf

4800-chenp-2006-description(complete).pdf

4800-chenp-2006-drawings.pdf

4800-chenp-2006-form 1.pdf

4800-chenp-2006-form 18.pdf

4800-chenp-2006-form 3.pdf

4800-chenp-2006-form 5.pdf

4800-chenp-2006-pct.pdf