IL29380A - Manufacture of phosphoric acid - Google Patents

Description

of phosphoric acifi 7823 relates to the production of phosphoric acid process and to the of calcium sulphate as a In the oanufacture of phosphoric acid by the process9 it is customary to react rock with sulphuric and phosphoric under such conditions of temperature and concentration that calcium sulphate commonly as is If other conditions of temperature and ionic concentration are employed then it is possible to precipitate calcium sulphate and subsequently to cause this to recrystallise to gypsum merely by altering the conditions of ionic concentration and temperature in the appropriate It ia to cause precipita to by suitably adjusting the conditions of ionic concentration and temperature of the mother Conditions of higher ionic concentration and temperature are required to cause calcium sulphate to crystallise as heiaihydrate than are required to bring about the crystallisation In Dutch Specification corresponding to British Patent there is described process ia which phosphate rock reacted with sulphuric acid under as to produce calcium sulphate dihydrate and reaction product from this is subsequently treated with further sulphuric acid to result la a mixture a content in the range 15 to by weight and a sulphuric acid content in the 2 to by weight at a temperature in the to thereby bringing about of the residence time of the reaction the of the process artificially now found that if the conditions of the of such reaction are controlled so that the temperature is raised only to about which can be accomplished by the addition of sulphuric acid of concentration by to the mixture obtained in the first more stable reaction system ia Shis permits one to be able to such residence times as are in order to obtain desired stable conditions it is necessary to employ rather more sulphuric acid than is necessary simply to any calcium ions left in the system to it w normally desirable to carry out a further treatme the phosphoric acid obtained after separation of the sulphate in order to lower its sulphate Accordingly the present invention provides a for the production of phosphoric and calcium sulphate hydrate which phosphate rock to react with acid at a temperature in the range of to under such conditions of ionic concentration that gypsum is transferring the product of this to a second vessel where it is further nised with suff cient sulphuric acid of concentration of at least by weight to result in a temperature of about thereby causing the recryetallisat of the present to calcium sulphate heiaihydratej filtering the precipitated calcium sulphate the 2b phosphate rock to least some liquid separated sulphate to precipitate substantially all of the sulphate values therein in form of gypsums and gypsum eo formed from the phosphoric acid recycling the gypsum produced to either stage stage of the also surprisingly that calcium sulphate formed be uoed in production of sulphuric acid and Portland by uhne this is described in Engineering Chemistr 9 1204 has hitherto is not use o in particulars gypsum produced as a tho manufacture of p Our discovery that calcium sulphate produced in the form of as a in phosphoric acid manufacture can be used in the process is of some economic bearing in mind the difficulty of disposing of the gypsum obtained in the conventional phosphoric acid One reason for being able to employ the hemihydrate in this process not gypsum is believed to be that the half molar proportion of in the can be tolerated in kilns used in the process whereas the two molar proportions in gypsum cannot because an excessive amount of fuel needed to drive them When the is obtained as a from phosphoric acid manufacture the P 0c content is too high to enable the clinker obtained to be a satisfactory raw material for We have that calcium sulphate hemihydrate produced by the process of the present has a considerably lower content of of the order of and the clinker obtained from it is quite suitable for making We believe that this lower content in calcium sulphate hydrate produced by the process of the invention as compared to that in gypsum produced as a in processes for the production of phosphoric acid is as a result of the comparative ease with which ions will replace ions in the gypsum crystal this being a result of the close relationship between the and crystal as compared with its replacing the ion in the crystalographically unrelated Sulphuric acid produced by treating the hemihydrate by the be used to carry out the attack on phosphate rock in the first stage of the process of the from a further aspect the present invention provides a process for making phosphoric acid with cement as a in which phosphate rock and sulphuric acid are caused to react in such conditions of ionic concentration and temperature that gypsum is the resulting slurry is transferred to another the temperature of the slurry is is ground to make cemen and the is for example by a process known as per into sulphuric In performing the process of the invention various methods for producing suitable conditions for the gypsum to to calcium sulphate hemihydrate may be For example simply heating the vessel in which the is to however the addition of sensible heat is liable to be too costly and We have found that it is possible to generate the necessary amount of heat in the slurry by the additii of concentrated sulphuric acid which will liberate its heat of solvation in Such acid should be concentrated to at leat by The amount of sulphuric acid required does not render the product phosphoric acid unsatisfactory for certain in fertiliser manufactui According to a feature of our we remove most of the sulphate content of the both that carried over from the original reaction with phosphate rock and that introduced by the process of the present by the addition of phosphate rock to the acid after removal of the hemihydrate This results in the precipitation of gypsum whic is separated and to either the initial reaction vessel or the vessel in which recrystallisation of the gypsum Such a separation of the gypsum from the acid product may be carried out in any convenient for example by by or by allowing the crystals to settle down and then decanting the It will be observed that since the amount of gypsum at this stage is comparatively small and can all be a good separation of gypsum from the product is not If it is desired that the calcium sulphate hemihydrate crystals formed by the process of the invention be converted into sulphuric acid and Portland this calcium sulphate is mixed with coke and shale and heated first to a temperature at which is liberated from the calcium this commencing at about and subsequently to a higher temperature to bri is in the opposite direction the the feedstock to flush out sulphur sulphur produced say be converted to sulphuric acid in convenient fashion this is frequently carried out the contact in which sulphur to by passing it ouch as with air and then dissolving the product in moderately concentrated sulphuric 9 other methods of the process if in particular known for converting into sulphuric aci be Specific for carrying out of the invention will described with roferenee to accompanyin 1 and 2 are flow diagrams for processes for production of phosphoric acid and sulphate 1 and 2 sulphuric acid and phosphate rock into a reactor together with recycled in the reactor as produce a slurry containing phosphoric crystals residual sulphuric She acid fed to the reactor concentration of at least and the temperature maintained and In process of the invention it preferred to operate so that the product contains about about and 33 to 400 of slurry ia then to a second vessel 1 to which sulphuric acid of at of concentration is added in a quantity necessary to raise the temperature of the slurry t at which is converted into Under th third of that required for the initial reaction on a weight If heat losses are the heat due to the addition of sulphuric acid is supplemented by sensible by passing the slurry through a heat exchanger or by g in live sulphuric acid may be in the form of oleum for above equivalent resulting slurry contains about of and percentage of the latter having been reduced by water liberated from the slurry is to a filter to separate the phosphoric acid from the precipitated calcium sulphate hemihydrate and the crystals of the latter are that there little or no reversion of the hemihydrate to on the filter 3 that are easily contain oay about of solid material is washed xith water and the washings recycled to the initial reaction and the crystals collected in a vessel the amount of water used is such to maintain correct concentration of acid in the bearing in the initial strength of the sulphuric She filtrate 1B then passed to a vessel this vessel there is added to the filtrate the necessary quantity of phosphate rock to the that is the stoichiometric quantity baaed on of of the amount o present may be carried out in any convenient but have it particula ly useful to employ a process described in wherein a metered flow of phosphoric acid substantially free from suspended solid and a metered flow iron and the resulting solution passed through turbidiaetric of s In of of phosphoric acid and crystals is to a filter to filter off According to 1 crystals to t initial reaction and product filtratej contains about to According to process 2 gypsum are recycled to the soeond tho product acid is passed to a product acid rook TO fed into a the as sufficient sulphuric weight to 5 per litre tho product acid with phosphoric produced by the at a of of She of such a that 4 slurry of in phosphoriG acid to a maintained at sulphuric added a proportion such that 5 of for 90 ml ia tho slurry of converted to a slurry of calci a of approximately and a liquid phase containing 33 time the slurry in this vessel was The slurry resulting from the treatment in the second vessel was then filtered and the strong filtrate passed on to the next The calcium sulphate hemihydrate crystals separated were washed to remove any phosphoric acid and the washings recycled to the first reaction The calcium sulphate crystals produced contained about insoluble soluble making a total content of about which was low enough to enable the calcium sulphate hemihydrate formed to be used in the production 10 of and cement clinker which was of good strengh and high The strong filtrate from the filtration stage which contained about P and was treated in a vessel maintained at with the 4 necessary amount of Morrocan phosphate rock to precipitate the present as The residue time in this vessel was 2 The gypsum was then 15 filtered off without washing and recycled to the first reaction The acid filtrate contained and was suitable for the manufacture of fertilisers and alkali metal phosphates and for any other use for which wet process phosphoric acid is normally 9 insufficientOCRQuality

Claims (10)

1. A process for the production of phosphoric acid and calcium sulphate hemihydrate which comprises: a) causing phosphate rock to react with sulphuric acid at a temperature in the range of 60° to 70°C under such conditions of ionic concentration that gypsum is precipitated; b) transferring the product of this reaction to a second vessel where it is further mixed with sufficient sulphuric acid of a concentration of at least 93 by weight to result in a temperature of about 75°C thereby causing the recrystallisation of the gypsum present to calcium sulphate hemihydrate; c) filtering the precipitated calcium sulphate hemihydrate from the liquid; d) adding sufficient phosphate rock to at least some of the liquid phase separated from the calcium sulphate hemihydrate to precipitate substantially all of the sulphate values therein in the form of gypsum; and e) separating the gypsum so formed from the phosphoric acid and recycling the gypsum so produced to either stage (a) or stage (b) of the process.

2. A process according to Claim 1, wherein the sulphuric acid is employed in stage (a) in a concentration of at least ll by weight.

3. A process according to any of the preceding claims, wherein the ratio of sulphuric acid used ih;, - 10 - 29380/2 stages (a) and (b) is 3:1 by weight.

4. A process according to any of Claims 1, 2 and 3» wherein the slurry produced in stage (a) contains about 3Qo ^2° about 0.5% SO^ and about 33 - Oo of suspended solids.

5. A process according to any of Claims 1 and 3» wherein the slurry produced in stage (b) contains 7 of

6. A process according to any of Claims 1 to 5, substantially as described herein with reference to the Example and the accompanying drawings.

7. A process according to any of the preceding claims, wherein cement is produced as a by-product and wherein the calcium sulphate hemihyd-fcate crystals which have been separated from the phosphoric acid are converted to SQ^ and cement clinker, the clinker is ground to make cement, and the SO2 is converted into sulphuric acid.

8. A process according to Claim 7, wherein the sulphuric acid produced is used to react with phosphate rock as claimed in Claim 1.

9. A process according to either of Claims 7 and 8, wherein the calcium sulphate hemihydrate crystals are converted to SO^ and cement clinker by heating the crystals in a kiln with shale and coke to a temperature of 450°C which is subsequently increased to at least 1400°C and the cement - 11 - 29580/2 clinker formed is subsequently ground to form cement and the S02 formed is converted to sulphuric acid. 11. Phosphoric acid whenever produced by the process of any one of Claims 1 to 6. 12. Portland cement whenever produced by the process of any of Claims 8 to

10. PC/rb