IL26524A - Phosphoric acid process - Google Patents

Description

PATENTS AND DESIGNS ORDINANCE SPECIFICATION Phosphoric acid process I STRUTHERS SCIENTIFIC AND INTERNATIONAL CORPORATION corporation organized under the laws of the State of 630 Fifth New New York do hereby declare the nature of this invention and in what manner the same is to be to be particularly described and ascertained in and by the following statement relates to improvements in the preparation and concentration of phosphoric acid the wet More the invention is the concentration purification of phosphoric the evaporative impurities during the concentra on step and crystallization of impurities b subsequent to the evaporation ste Xn the wet process for the preparation phosphoric from phosphate bearing by the reaction such material sulphuric acid the principal products calcium sul ate precipitate a relatively phosphoric acid is saturated not only in gypsum but in other undesirable impurities impurities tend to crystallize late in the process as minute crystalline the relatively dilute phosphoric acid produced by the initial reaction with sulphuric acid is freed by filtration or centrifugation of crystallized gypsum and then concentrated by ation of solvent water at elevated During this the phosphoric acid supersaturated with respect to both gypsum and other This supersaturation is due to decreased amount of solvent and thus crystallization takes place at the elevated temperature during the evaporation the concentrated phosphoric remains supersaturated with respect to temperature even after removal from the tion There are several major problems inherent in described the calcium sulfate and other impurities which separate from the process during evaporative concentration are often very to remove because are of relatively small particle very calcium sulfate removal is accompanied by a large loss of concentrated product phosphoric In when the concentrated phosphoric acid is allowed to cool to near ambient a relatively large quantity of extremely fine crystalline material tends to precipitate as a This sludge cannot be easily reraoved from the phosphoric acid without loss of product and is a deterrent to the subsequent use of the acid in the preparation of such products as the particle size of the impurity crystals which develop in the concentrated acid upon cooling never exceeds 15 microns and t erefore very difficult to either filter or centrifuge from the phosphoric In it is often the case that the fine particulate sludge requires two to ten days to develop in the acid as it cools from 190 or to ambient It is therefore an object of this invention to provide an improved process for the preparation of concentrated phosphoric further object of this invention is the provision of a process for the concentration and cooling of process phosphoric acid in which the concentrated product is not only relatively free of particulate but is not supersaturated with respect to impurities present at ambient In the drawings accompanying this Figure I is a representation of apparatus employed in the cooling crystallization of phosphoric and Figure II is a schematic diagram of the process and apparatus employed in the evaporative concentration of phosphoric The above and other objects of the invention in accomplished by a process for the controlled cooling of concentrated phosphoric acid which has been prepared by the evaporative concentration of dilute phosphoric acid which in the wet process of treating phosphate rock with sulfuric acid to form solid calcium and dilute phosphoric The process of this invention comprises cooling relatively concentrated phosphoric acid to near ambient temperature by establishing a body cooled concentrated acid containing a suspension of and once the body has been continuously a relatively large circulating stream the adding to the circulating stream a minor portion of hot concentrated acid to be thereby orming a combined stream at above the temperature of the body stream is cooled and thus supersaturated with respect to crystals in a heat exchanger om the bod combined stream is then with the body of cooled acid so that the stream passes through a suspension of impurity crystals and and is relieved in the combined by precipitation of impurities the crystals in the effluent concentrated phosphoric acid containing crystals is withdrawn the body at a substantially equal to the feed rate of hot phosphoric acid to the circulating The concentrated phosphoric acid is cooled in such a that no precipitate orms in the cooled This is accomplished by mixing the hot phosphoric acid with previously cooled acid which has been relieved of supe saturation by passage over a bed of amount of hot feed is always small in proportion to the of already cooled liquid being the crystal during the of the phosphoric acid never highly supersaturated and can gradually be relieved by passage over an already established This the effect of growing larger crystals than ba grown by simple gradual cooling o the bulk phorphorie acid in holding Tho crystals grown by the process of thi3 invention can have particle a minimum of about 40 microns whereas when tho hot is allov7ed to cool gradually without recirculation and without the establishment of a bed of impurity the crystal siae almost nevor exceeds about 15 microns and is usually broadly distributed in the range of 2 to 15 and thus very difficult to handle and separate from the Referring now to Figure a cooling shown generally at 10 has an inlet duct 11 extending through the top and into the body of the crystallize The inlet duct may terminate with a fluted section 12 near the bottom portion of the relatively diameter 13 from the upper portion of the crystallizer Through this duct recirculating slurry of phosphoric acid and impurity crystals is conducted through the heat exchanger 14 by means of pump In the heat exchanger the slurry conducted through a series of tubes 16 and returns to the 10 through duct feed from the phosphoric acid concentration step to be described later is added the recirculating stream in the duct 13 through line 17 prior to the recirculating stream being conducted through the heat exchanger 14 Cooling preferably is admitted to the space 18 around the tubes in the heat exchanger 14 through the coolant Coolant circulates in an upward direction through the hea 14 is withdrawn through the lino In order to control the temperature in tho heat a portion of the coolant is recirculated through the line 21 and portion of the coolant is rejected through line and fresh coolant is supplied through the 2 a representative the material entering line 17 is at a of between 175 the bulls inside the 10 can be maintained at between 150 and recirculating in duc before introduction of the fresh is at the same temperature aa the in 10 and ia heated the introduction of the hot Conditions in the heat exchanger are controlled such that the coolant effluent frcia the exchanger has a temperature of approximately the bulk temperature of the In starting supplied through line 17 as rapidly possible to fill the without withdrawing any through the effluent lines and 26 or the duct without regard the crystalliser 10 is full of recirculation is begun the duct 13 and cooling is initiated gradually to prevent the precipitation of on the internal surface of heat exchanger tubes 1β the bulk in crystalliser 10 approaches the operation cooling through the heat exchange and the addition of fresh hot food through line 17 adjusted to the conditions previously In this a slurry of impurity crystals in the concentrated phosphoric acid is developed in the crystallizer The warm feed to line 17 comprise concentrated phosphoric acid having a concentratio between and per cent in terms of material may be essentially free of solids or may contain to about 30 per cent by volume to about 15 per cent by calcium sulfate and impurity solids formed as a result of It is to properly initiate that the feed material contain to about per cent solid raaterial by since additional crystallisation takes place in crystalliser there may be up to in of 30 per cent solids by volume per cent by in the crystalliser The main course of circulating tlirough the apparatus is as followsa Material enters the crystalliser 10 through the duct travels down the entire length of the duct tlirough the fluted section 12 then up the body of crystalliser 10 to the recircula duct A small amount of the raaterial is withdrawn through either near the of the 10 or line 26 at the top of the crystalliser 10 to be conducted to the second stage through the line via pump The material in the line now for about 3 per cent solids by and which is at a of about ia subjected to a second stage of cooling tion in the crystalliser which is operated in essentially the same manner as crystalliser 10 but at a temperature of in the line 7 is admitted to the recirculating duct 31 along with a raajor proportion of raaterial recirculated from the crystalliser Pump 3 feeds the tlirough the heat exchager 33 back into the crystallizer 30 through the duct which to near the bottom of the crystallizer as shown in crystalliser Heat exchanger 33 also has the coolant feed line coolant reject recirculating line cooling crystalliser 3 is operated a temperature of the coolant temperature maintained in the heat 33 is approximately In the amount of feed is reduced in temperature to about and desupersaturated upwardly through a bed of impurity Since conditions in the duct such that the material is slightly supersaturated with respect to the and in the presence of impurity is little tendency or the formation of new supe ion relieved gradually while the material in upward direction through the cr Effluent material is withdrawn through the line 39 as shown in conjunction with the may one near the the other near the of the cooling During the cooling in the c of impurity have grown in and total magnitude and the ratio of to liquids in line 39 somewhat higher than in line 27 shown in Figure the process is conducted in throe stages so that the material in lino 39 conducted into a third cooling 40 where the process is repeated at a temperature approximately below the temperature in the material in line 39 conducted through pump 41 and blended with stream of desupersaturated material in the duct The combined which has a minor proportion of feed from line 3 is conducted from pump 3 to the heat exchanger The same basic arrangement of circulating circulating line e line and coolant feed line 48 is found in conjunction with the heat In order to prevent crystallisation in the heat e changer the coolant in heat exchanger 44 about and operating temperature of within the in the duct 42 is hot line 39 stream is to about in tho heat O becoming Tho supersaturated is returned 40 the duc and flows an upward direction through the while is to tho present Product which not saturated at is withdrawn as a slurry through which raay in practice be two product lines to the top and bottom of crystalliser 40 this the average solids tion in the crystalliser 40 and effluent line in the range of to 40 per cent volume or about 2 to 20 per cent by product concentrated phosphoric which is now relatively free of dissolved can then freed o the crystalline by either cent or raade possible by fact tha crystals in the ef product have a particle size range sufficiently large to permi cooling of the concentrated phosphoric acid according to the present invention can be in a single stage in which the bulk temperature of the slurry is at or it is preferred to carry cooling and consequent impurity precipitation in a plurality of two to ten stages can be is generall found that excellent results are obtained if the cooling crystallisatio is carried ou in two to of the stage is upon the average i nt of locality in whic phosphoric acid In it is found that final stage of 85 to a practical lower t to the ready availability of cooling water capable of obtaining this final when cooler water is available for use in heat lower temperatures be in final difference in temperature between successive stages can he regulated up to or it is preferred that the inter stage temperature difference be no than about fewer and larger crystals may be grown under these each stage of the ng operation the ratio of feed to recycle can vary from about to about This the quantity of material flowing in the outlet duct 13 is fron to as great sg the fresh feed material admitted to the process through the feed line In this manner the cooling in heat only ver slightly supersaturates the combined and the efficient of relatively is preferred ratio of t recycle is in the range of to excellent results are obtained i temperature of concentrated acid cooled according to the process this invention an initial of about to about and the choice of the of cooling stages and the stages will in part depend on both the initial of the hot acid and its concentration and impurity Although the cooling of this inventio has bee described wi to concentrated phosphoric it is equally to with th t to Thus the cooling process this invention is applicable to phosphoric acid having a about to about per Another of this invention is process for the controlled concentration of dilute phosphoric acid to relatively concentrated phosphoric acid followed by the controlled cooling of the acid as described above In this of the the hot dilute phosphoric acid is evaporated to higher concentration under carefully controlled conditions so that the formation of and hard to separate sulfate and crystals is prevented during the concentration As the acid is concentrated at elevated it supersaturated w to impurities and unless special are taken during the evaporative the crystals resulting from ration are extremely and difficult to of the invention generally concentrating the dilute phosphoric acid at elevated and under relatively high vacuum by establishing a body of relatively concentrated hot phosphoric acid containing a suspension of continuously ing the body a circulating streaia of hot phosphoric contaixing the adding to the circulating a po of dilute phosphoric acid feed to form a heating the combined and subjecting the streaia to evaporation so as to water therefrom and concentrate the combined to the tion of the thus supersaturating the stream with respect to sulfate and other concentrated combined is passed into the existing body of relatively concentrated acid and t e body and thus passes through suspension impurity saturation of the ined is relieved fcy precipitation of the in suspension evaporative concentration is conducted in one or a plurality of staged and the relatively phosphoric acid then to the cooling process described in conjunction with Figure By thi3 combination of concentrating and cooling processes under carefully controlled a concentrated phosphoric acid produc is produced from the impurity crystals are easily oeparated because o their relatively large with the cooling process described the concentration step of this invention Toe conducted in a series of From ona to ten stages may be in om two to five stages dilute phosphoric acid be concentrated to to about 60 per cent production of relatively large impurity Referring to Figure the evaporative for concentrating phosphoric acid according to this invention has an evaporator section 101 and a crystallization section inlet duet into the or evaporation section which is to the through the central pipe which extends to near the bottora of ting clurry of phosphoric acid enters the evaporator section 101 through the duct 103 and a liquid level ia maintained in the evaporator By raaintaining a vacuum above the liquid level in the evaporator section solvent water ia caused to evaporate from the alurry and is t e unit the a not Slurry the central pipe 104 into the crystallizer section 10 and is recirculated to the evaporator section through the outlet duct The material circulated through the outlet duct is combined with a portion of fresh feed through 107 This fresh feed is prepared by treating phosphate rock with sulfuric acid to form relatively dilute phosphoric acid and solid calcium sulfate in a production unit not shown and can have a composition varying 20 to about per cent phosphoric acid as contain to 10 per cent impurity crystals of by The combined stream of fresh feed and recirculating slurry pumped through the 108 to the heat exchanger 109 where it is heated while passing through the tubes The tubes are heated by injection of through the line and cooled steam or condensate is withdrawn from the space from around the tubes through the line The heated mixture of fresh feed and recycled slurry as pointed out conducted through the inlet duc the evaporation section 101 where it loses water by evaporation and is slightly cooled by the evaporation and thus becomes supersaturated with respect to calcium sulfate and the other impurities The circulating slurry continuously moves down through the central pipe and is released by flowing over an established bed of impurity crystals in the crystallisation section Concentrated product is removed via line 113 to a subsequent stage of the evaporative concentration or the process of Figure after th last stage of evaporative the slurry may centrifuged or ltered to remove impurity it is preferred that after the last stage of evaporative concentration the material flowing into line Figure contain crystals of calcium sulfate or other a typical operation of the portion of the invention illustrated Figure the evaporative concentration is conducted in three unit is supplied through line 107 for feed material containing cent a temperature o approximately this initial ture can vary from between and depending on the source of the phosphoric After the crystallizer section 102 completely filled with the slurry to be the ratio of through line recycled slurry through the duct 106 is adjusted to a ratio of between from 1 to 25 and 1 to The combined is then heated to temperature of approximately it passes through the heat exchanger 110 and then passes into inlet duct The amount o steam supplied through the steam inlet adjusted so that a temperature difference between the inside and outside of the tubes 110 never exceeds about Water is caused to evaporate from slurry in the evaporator section 101 by maintaining the vapor space at the of the evaporator a vacuum of about inches of The temperature the crystallisation 102 is maintained the product effluent in line 113 contains from to per cent phosphoric By continuing the tion to concentrated phosphoric acid in the formation of undesirable calcium sulfate crystals and other crystals is prevented and the crystals which form are more readily removed from the phosphoric acid by filtration or In by maintaining a high ratio of recycle slurry relatively large amounts of to fresh feed material in the heat exchanger tubes fouling of the heat exchanger tubes deposition calcium sulfate is prevented for long pe iods of operatio apparatus as shown in II is the first stage of a evaporative concentration the second stage e for to produce respectively to er cent phosphoric acid solution in the second stage and to per cent phosphoric acid in th third In this second stage operation is conducted with a vacuum of about inches of Hg absolute in the evaporative section and a temperature of from to the crystallization The material effluent from the second stage can contain from 2 to 8 per cent by weight solid crystalline which may approximate volume of from to 20 pe The third then be operated at a vacuum of about inches of Hg absolute in the evaporator section and a temperature of from to in the crystallisation The effluent from the third stage will then contain from about to cent by weight calcium sulfate crystals having an approximate volume of from to 30 per cent and the phosphoric acid product can be between and per cen calcium and other iiapurity crystals are in the particle size range of at least about microns as opposed to much smaller and much more dif icult to separata crystal particles produced by othe evaporative processes to prior conducting the evaporative concentration according to the process of this it is desirable that the temperature rise between the duct 106 and uct 103 he controlled in temperature range of from 3 to for maximum of in first stage if the material passing through the 108 has a temperature of the material in the inlet duct 103 should have a temperature no higher than In concentration of dilute phosphoric the dilute that acid which is prepared by the of phosphate rock with sulfuric or other and ordinarily has a concentration of from 20 to about 32 per cent phosphoric acid expressed in terras of the dilute from the preparation step is at an elevated temperature of from 130 to when fed to the tion but the initial temperature of the acid to be concentrated is not a part of the The initial quantity of calcium sulfate and other impurity crystals in the phosphoric acid solutio subjected to the above evaporative concentration procedure may vary from zero to as high as 5 cent or higher by Depending on the particle these crystals may occupy up to about 15 per cant of the volume of the concentrated phosphoric acid effluent from the last of and before the controlled cooling described above will ordinarily contain from 40 to 56 per cent phosphoric acid as P lower and higher concentrations are and contain from 2 to about cent by weight to 30 per cent by of impurity The temperature the inal product depends on the inal concentration and the vacuum applied in the final stage of As pointed out using dilute acid of about per cent and concentrating to to cent in three final effluent temperatures can bo adjusted in the range of 175 to As used throughout this and the accompanying the expression treating phosphate rock with sulphuric acid to solid sulphate and relatively dilute phosphoric refers to a variety of process variations in which a phosphate bearing material is treated with sulphuric many of these processes both the phosphate bearing and the sulphuric acid are added to an already sulphate and phosphoric acid containing calcium In another process the calcium phosphate is first dissolved in phosphoric acid and this dissolution step is followed by treatment with sulphuric Illustrative of many possible this process inter following 0 and The process of this invention is equally applicable to the preparation of phosphoric acid by the of phosphate bearing material nitric acid by the crystallization of calcium alternate process employed as an alternate to the of phosphat rock with sulphuric acid and has the advantage that the calcium nitrate produced is a more material than the calcium sulphate produced front the sulphuric acid insufficientOCRQuality

Claims (10)

- 17 - 26524/2

1. · A process for t e preparation of concentrated phosphoric acid comprising: (1) concentrating dilute phosphoric acid by evaporation of water therefrom at elevated temperatures to form a more concentrated hot phosphoric acid solution; (2) establishing a body of cooled concentrated phosphoric acid and a suspension of impurity crystals therein, (3) continuously withdrawing a first circulating major stream of cooled solution from said body, (4) adding to said circulating major stream a minor stream of feed from Step (1) to form a combined stream at a temperature above that of said body, (5) cooling and supersaturating said combined stream, (6) passing said combined stream into said body so that said combined Btream mixes with said body of cooled solution and passes through said suspension of impurity crystals to relieve super-saturation by precipitation of impurities on said suspension of impurity crystals* (7) withdrawing deeupersaturated phosphoric acid and impurity crystals from said body at a rate substantially equal to the rate of addition of feed in Step (4) and (8) separating the impurity crystals from said desuper-saturated phosphoric acid.

2. fne process according to Claim 1 wherein the rate of flow of the first circulating major stream in Step (3) is from 25 to 150 times the rate of the minor stream added thereto in

3. Step <4). 5· The process according to Claim 2 wherein, in Step (5) the combined stream is cooled in an indirect contact heat ex 18 26524/2 ¾r

4. · The process according to Claim 3 wherein said combined stream in Step (5) is cooled to the temperature of the body established in Step (2)·

5. e process according to Claim 4 wherein* in Step <5) the indirect heat exchanger is a shell and tube heat exchanger, the coolant flowing therefrom being less than 10°? below the temperature of the body established in Stqp(2).

6. · A process a¾ claimed in Claim 1 wherein said hot concentrated acid contains from 40 to 60 percent by weight of phosphoric acid, expreesed as

7. · A process as claimed in Claim 6 wherein said hot concentrated acid is at a temperature of om 175°* to 195°?*

8. A process as claimed in Claim 7» wherein said acid is cooled in from 1 to 10 stages to a temperature between and 85°*.

9. · A process as claimed in any of the preceding claims wherein said concentrated phosphoric acid contains from 2 to 15 per cent by weight of impurity crystals.

10. A process as claimed in any of the preceding claims comprisingI {X) concentrating said dilute phosphoric acid at elevated temperatures by (a) establishing a first body of concentrated hot phosphoric acid and a first suspension of impurity crystals therein, b) continuously withdrawing from said first body a first circulating major stream of hot phosphoric acid containing impurity crystals* (e) adding to said first circulating stream a portion of dilute feed comprising phosphoric acid more dilute than in said body to form a first combined stream, (d) heating first combined stream, (e) subjecting said first combined 19 26524/2 stream to evaporation so as to concentrate said first combined stream to the concentration of said first body thus supersaturating said first combined stream with respect to impurities, (f5 passing said first combined stream from Step (1)