US2905535A - Phosphoric acid concentration - Google Patents

Description

United States Patent PHOSPHORIC ACID CONCENTRATION Sydney Atkin, Springfield, N..l., and Sidney S. Prince, New York, N.Y., assignors to Chemical Construction Corporation, New York, N.Y., a corporation of Delaware Application December 19, 1955, Serial No. 553,840

4 Claims. (Cl. 23-165) This invention deals with the concentration of aqueous phosphoric acid contaminated with compounds of silicon and fluorine, such as the weak acid obtained by digesting ground phosphate rock with sulfuric acid or mixtures of sulfuric and phosphoric acids. Phosphoric acid obtained from this and similar sources is in wide commercial use for the production of superphosphate fertilizer, for which purpose it must be concentrated to a P content of about 50% or higher. It is a principal object of the invention to provide an improved concentrating process for such acid wherein losses due to volatilization of phosphoric acid during the concentrating step are largely avoided. A further object is the provision of an efiicient method for recovering the siliconand fluorine-containing compounds volatilized from the phosphoric acid during its concentration as commercially pure potassium or sodium silicofluoride.

Aqueous phosphoric acid is usually concentrated by contacting it with a stream of hot gases in a drum concentrator. This procedure utilizes fully the heat content of the gases for evaporating water and also removes a substantial part of the siliconand fluorine-containing impurities from the acid, but some of the phosphoric acid is also vaporized or entrained and lost from the concentrating process. The present invention provides a method whereby this loss of phosphoric acid in the stack gases from the concentrator is avoided so that the overall efliciency of the concentrating process is materially improved.

In its broadest aspects the process of our invention consists essentially in scrubbing the stack gases from a drum-type phosphoric acid concentrator with a stream of strong aqueous phosphoric acid of relatively high P 0 content, whereby the volatilized phosphoric acid is recovered from the stack gases, and introducing a portion of the scrubbing liquid into the concentrator. By this procedure substantially all of the phosphoric acid that would otherwise be lost in the stack gases is recovered in the product acid from the concentrator. We have found that two very important advantages are obtained when the stack gases are scrubbed with a relatively strong phosphoric acid; the siliconand fluorine-containing impurities in the gases are dissolved in such a strong acid to a much smaller extent than in Water or weak phosphoric acid, and the hydrofluosilicic acid that is present in the strong acid can be more completely precipitated by the addition of Water-soluble sodium and potassium compounds thereto.

The solubility characteristics of hydrofluosilicic acid in aqueous phosphoric acid solutions must be understood for an accurate comprehension of our invention. It is known that hydrofluosilicic acid is quite soluble both in water and in relatively dilute phosphoric acid having a strength up to about 19 percent H PO We have found, however, that in stronger phosphoric acid the solubility of hydrofluosilicic acid decreases rapidly as the phosphoric acid content is increased, so that when an acid having 21 P 0 content of 20% is used the fluorine content cannot be more than about 9%. With still stronger acid the hydrofluosilicic acid solubility is still further decreased, and a phosphoric acid of 35% P 0 content saturated with hydrofluosilicic acid contains only about 6% of fluorine.

As a result of this discovery it became evident to us that a strong aqueous phosphoric acid would function as a selective scrubbing agent for removing substantially all of the volatilized phosphoric acid from the stack gases of a phosphoric acid concentrator while dissolving only a minor proportion of the siliconand fluorine-containing impurities therefrom. Introduction of the resulting scrubbing liquid into the concentrator would return practically all of the entrained phosphoric acid to the process while increasing only slightly the content of compounds of silicon and fluorine in the product acid. We found, however, that these impurities could be almost completely removed from the scrubbing liquid prior to its introduction into the concentrator, if desired, by a simple treatment with potassium chloride, sodium chloride or other Watersoluble alkali metal salt and subsequent filtration or centrifuging.

The invention will be further described with reference to the accompanying drawing, the single figure of which is a flow sheet illustrating a preferred embodiment of the process and showing the apparatus used therein. On this drawing reference numeral 1 indicates generally a phosphoric acid concentrator of the drum type; 2 is a mist agglomerator and entrainment separator for partially separating entrained phosphoric acid from the stack gases and returning it to the concentrator; 3 is a scrubber for removing phosphoric acid mist from the gases; 4 is a cyclonic separator for disengaging the scrubbing liquid from the gas, and 5 is a water scrubber wherein residual siliconand fluorine-containing impurities may be removed from the stack gases before they are discharged to the atmosphere.

In the concentrator 1 a body of aqueous phosphoric acid 10 is concentrated by the passage of a stream of hot gases therethrough. These gases are injected into the acid through a refractory-lined pipe 11 which may be connected to any suitable furnace. In the embodiment illustrated a burner 12 is mounted directly over the concentrator 1 and the products of combustion are introduced directly into the concentrating drum, but it will be understood that a separately mounted furnace maybe used if desired. The hot gases from sucha furnace are frequently diluted with air to obtain an inlet gas temperature in the pipe 11 of about l200l400 F., but it is an important advantage of the invention that inlet gas temperatures of 1500 F. and higher, and preferably within the range of l5002200 F. may be used. Such high temperature gases are not ordinarily suitable for drum concentrators as they cause excessively high losses of acid through mist formation, but with the mist-recovery process of the present invention they may be employed with a corresponding increase in the thermal efliciency of the concentrating process.

The moisture-laden gases from the concentrator leave through the stack 13 at a temperature which is ordinarily Within the range of about 225-275 F., the exact temperature depending on the strength of the product acid. These gases are discharged tangentially into the lower portion of the mist agglomerator 2 which is an unpacked cylindrical tower wherein the gases ascend through a spiral path while entrained liquid phosphoric acid separates out and is returned to the concentrator through line 14. During the passage of the gases and vapors through this vessel the average size of the mist particles is increased by agglomeration, which is an important factor in the removal of these mist particles from the gases in the subsequent scrubbing and separating steps.

lFliomqthe aggle nerator lithe gases pass through pipe 15 into the inlet of a scrubber 3. The scrubbing apparatus illustrated on the drawing is a venturi scrubber of the type shown and described in US. Patent No. 2,604,-

"1.85 ;.inth-is apparatus the gases areaccelerated to a rela .tively high linear .velocityina contraction-section .16 and are-then passed into athroat section 17.provided withjets .for the injectionof-streams-of scrubbing liquid transverse to the direction of flow of the gases therein. It will be understood that scrubbing equipment of other types may .be used in practicing theinvention, but a venturi scrubber .is preferred as it operatesat high efficiency with a minimum quantitytof scrubbing liquid.

The mixture .of gases and scrubbing liquid from the .throat 17 of :the venturi3 is decelerated in an expansion section 18 and then passes through an outlet pipe 19 into itheseparatorA. :Inthis separator, which is an unpacked cylindrical chamber having a tangential gas inlet, the .gases .and vapors again pass upwardly through a spiral path while the scrubbing liquid containing dissolved phosphoric acid and impurities settles out by gravity and is .drawn off :through ;line 20. The gases leaving the separator through pipe 21 may be.discharged directlyto the .atmosphereor they may be .further scrubbed with water inthefinal scrubber 5 by means of Water sprays 21 before being discharged through stack 22.

Ordinarily no water -is.condensed from the stack gases when they are scrubbed with aqueous phosphoric acid solution because of the relatively high temperature of the entering gases. The phosphoric acid concentration in the scrubbing liquid is therefore increased by the acid which it removes from the gas. When the desired high P content has beenattained the scrubbing liquid is preferably diluted with a sufficient quantity of water to maintain a constant concentration of phosphoric acid and the excess scrubbing fluid is withdrawn and introduced into the concentrator. By this procedure the phosphoric acid obtained from the stack gases is recovered in the product acid.

Referring again to the drawing, the scrubbing liquid from the separator 4 is passed through line 20 into a recirculating tank 25,.from.which it is elevatedto the scrubber 3 by pump 26 and line 27 and reintroduced into the throat of the scrubber through line 28. This recycling procedure is continued until a phosphoric acid concentration of at least 20% P 0 and preferably about 25- 30% P 0 has been attained after which water is introduced through line 29 and a corresponding quantity of the scrubbing liquid is withdrawn through valve 30 and line 31 into the storage tank 32 which feeds the drum concentrator. The quantity of water introduced through line 29 is just suflicient to maintain a constant P 0 content in the'recirculating scrubbing liquid, and by thisprocedure the quantity of phosphoric acid returned to the concentrating step ,isexactly equal to that scrubbed from thegases.

The above-descn'bed procedure is the best embodiment of the process of our invention for concentrating plants operating on weak phosphoric acid obtained by digesting ground phosphate rock with sulfuric acid, since this acid usually contains considerable quantitiesof calcium sulfate in solution or as finely divided crystals. When a pure aqueous phosphoric acid from other sources containing at least 20% P 0 is being concentrated the gas scrubbing process can be modified, if desired, by injecting a stream of said acid from the storage tank 32 into the throat 17 of the venturi scrubber instead of the recirculating stream of acid entering through the line 28. In this case the separated scrubbing liquid in line 20 would be returned to the storage tank 32, or would be sent directly to the concentrator 1 or, .if desired, it could be introduced into the recirculating tank 25 and returned one or more times through the venturi scrubber .whilea side stream is introduced into the concentrating system through line 31.

These and other modifications to adapt the process to --the requirements of varying types of feed acid will readily be suggested to those skilled in the art by the foregoing description, and are included within the scope of the invention.

When it is desired to purify the vw'thdrawn scrubbing liquid before passing it'to the concentrator, the valve 30 is closed and a side stream is withdrawn from the tank 25 throughline 33 and pumped by pump 34 intothe tank 35. In this tank the acid is mixed with a quantity of water-soluble potassium or sodium salt suflicient to carry out the reaction This reaction goes to-substantial completion by reason of the very low solubility of potassium or sodium silicofluoride in phosphoric acid having a P 0 content of 20% or higher, and therefore the treated phosphoric acid is substantially completely free from dissolved hydrofluosilicic acid. The stream of purified acid and precipitateis-drawn ofi through line 36 and passed to a filter or centrifuge 37, from which the potassium or sodium silicofiuoride is recovered as a solid product of good purity. The'phosphoric acid, which is removed as filtrate through line 38, has a very low fluorine content and therefore may be returned to the concentrator 1 to be recovered in the concentrating process which materially ,decreases the-fluorine content of the product acid.

From the foregoing description it will be seen thatthe process of the invention provides animportant improvement in the concentration of aqueous phosphoric acid. By scrubbing the stack gases from a drum concentrator with aqueous ,phosphoric acid having a P 0 content of 20% to 35% or higher a substantially complete recovery of the volatilized phosphoric acid in the gases is obtained as a solution containingonly about 69% of hydrofluosilicic acid. This solution can beintroduced into the concentrator in quantities corresponding to the phosphoric acid removed therefrom .in the stack gases with only a slight increase-in the fluorine content of the product acid and with a very substantial increase in the overall efficiency of the concentrating process. If desired, however, the hydrofluosilicic acid can be more easily and completely removed from this scrubbing liquid by treatment with water-solublesodium or potassium salts than is possible with a more dilute scrubbing liquid, and therefore the purificationof the scrubbing liquid and the recovery of alkali metal silicofluorides therefrom is also improved.

The advantages obtained by the present invention are further demonstrated ,by thefollowing specific examples.

EXAMPLE 1 The process of the invention was tested in a plant in which dilute phosphoric acid is concentrated to a P 0 content of 50-55% and then reacted with ground phosphate rock for the production of superphosphate fertilizer. The concentrating drum was designed to evaporate '100 tons of water per 24 hour day from a feed acid of about 22-25% P 0 content using hot gases from a separate furnace which entered the drum at about 2000 F. and at a rate such as to produce a How of approximately 10,000 cubic feet perminute of gas at about 255 F. in the inlet pipe 15 of the scrubber. The linear gas speed in the venturi throat 17 was from 275 to BOO-feet per second and from 40 to 60 gallons per minute of scrubbing liquid were injected into this throat through line 28; the temperature of the gas and scrubbing liquid in pipe 1? was about F.

Using .a feed acid to the concentrator having a P 0 content of 23.5% and a fluorine content of 1.7% the concentration of hydrofluosilicic acid in the scrubbing liquid was determined asits P 0 content increased by continued recirculation and the specific gravity and fluorine content were found to be as follows:

Acid in Scrubbing Liquid Specific Percent Gravity Fluorine Percent Percent Table I Feed Scrubber HaPOl Weir H3PO4 Drain Product Box (Line 20) Percent Percent Percent Percent 22. 6 22. 8 57. 8 23. 3 1. 75 7. 53 1.19 4. 2 21. 8 22. 8 50. 3 22. 8

The plant was also operated for several days with water as the scrubbing liquid in the line 28 and with no admixture of scrubbing liquid with the feed acid. With this procedure the concentrated acid analyzed 53.0% P 0 and 1.12% F on one typical day of operation and 51.0% P 0 and 1.04% F on another day, but the overall loss in phosphoric acid from the concentrating process increased from practically nothing to about 5-6%. It is evident, therefore, that the process of the invention increases materially the efficiency of the phosphoric acid concentrating process while increasing only very slightly the fluorine content of the product acid.

The following analytical figures show that the quality of the superphosphate fertilizer is not adversely affected when acid concentrated by the new process is used for reaction with ground phosphate rock.

Triple superphosphate Made From When the process is operated for recovery of the hydrofiuosilicic acid in the recirculating scrubbing liquid the valve 30 is closed and the pump 34 is operated to withdraw a stream of the solution into the tank 35 at a rate equal to the addition of water through line 29. The solution in tank 35 is treated by adding the amount of potassium chloride, sodium chloride or other watersoluble potassium or sodium salt theoretically necessary to react with the hydrofluosilicic acid in solution according to the equation The resulting slurry is passed through line 36 to a centrifuge or filter 37 where the purified acid is separated from the precipitate and returned to the concentrating drum by admixture with the feed acid entering this drum.

When a scrubbing liquid containing 25% of P 0 and a quantity of hydrofluosilicic acid corresponding to 7.0% fluorine was treated with potassium chloride and filtered the filtrate was found to contain only 0.05% of fluorine. It is evident, therefore, that when the concentrated phosphoric acid is to be used for purposes other than the manufacture of superphosphate fertilizer the hydrofiuosilicic acid can be completely removed from the withdrawn stream of scrubbing liquid before it is returned to the concentrator. By this procedure a substantial increase in concentrating efliciency is obtained without increasing the fluorine content of the product acid or, if desired, the purified scrubbing liquid can be separately concentrated.

What we claim is:

1. A method of concentrating a dilute impure aqueous phosphoric acid containing compounds of silicon and fluorine as impurities to a product phosphoric acid while simultaneously eliminating said impurities which comprises establishing in a drum-type concentrator a body of dilute phosphoric acid having a 2035% P 0 content, passing a stream of hot gas through the dilute impure phosphoric acid in a concentrating step whereby hot gas introduced into the body of said dilute phosphoric acid produces a product phosphoric acid having a P 0 content of at least 50% and a stack gas containing said impurities along with volatilized phosphoric acid and water vapor, said stack gas leaving the phosphoric acid being concentrated at a temperature of between 225 F. and 275 F., thereafter scrubbing said stack gas with an aqueous phosphoric acid having a P 0 content of at least 20% and not over 35% thereby removing said volatilized phosphoric acid from said stack gas while dissolving limited quantities only of siliconand fluorine-containing compounds therein, and recycling a portion of the resulting scrubbing liquid to the concentrating step.

2. A method according to claim 1 in which the hot gas being delivered to the concentrating step is at a temperature within the range of about 1500-2200 F.

3. A method according to claim 1 in which the gas stream from the concentrating step containing water vapor and impurities and at a temperature of between 225275 F. is accelerated and there is simultaneously injected into the accelerated gas a scrubbing liquid which is a recirculating stream of aqueous phosphoric acid having a P 0 content of at least 20% and not over 35%.

4. A method according to claim 1 in which a portion of the scrubbing liquid, having dissolved therein limited quantities only of siliconand fluorine-containing compounds, is purified by adding thereto a water soluble compound of a metal of the group consisting of potassium and sodium, removing the resulting precipitate, and passing purified acid to the concentrating step.

References Cited in the file of this patent UNITED STATES PATENTS 1,133,840 Collett Mar. 30, 1915 1,264,510 Hechenbleikner Apr. 30, 1918 2,611,681 Bellinger Sept. 23, 1952 2,655,431, Allen et a1. Oct. 31, 1953

Claims (1)

1. A METHOD OF CONCENTRATING A DILUTE IMPURE AQUEOUS PHOSPHORIC ACID CONTAINING COMPOUNDS OF SILICON AND FLUORINE AS IMPURITIES TO A PRODUCT PHOSPHORIC ACID WHILE SIMULTANEOUSLY ELIMINATING SAID IMPURITIES WHICH COMPRISES ESTABLISHING IN A DRUM-TYPE CONCENTRATOR A BODY OF DILUTE PHOSPHORIC ACID HAVING A 20-35% P2O5 CONTENT, PASSING A STREAM OF HOT GAS THROUGH THE DILUTE IMPURE PHOSPHORIC ACID IN A CONCENTRATING STEP WHEREBY HOT GAS INTRODUCED INTO THE BODY OF SAID DILUTE PHOSPHORIC ACID PRODUCES A PRODUCT PHOSPHORIC ACID HAVING A P2O5 CONTENT OF AT LEAST 5/% AND A STACK GAS CONTAINING SAID IMPURITIES ALONG WITH VOLATILIZED PHOSPHORIC ACID AND WATER VAPOR, SAID STACK GAS LEAVING THE PHOSPHORIC ACID BEING CONCENTRATED AT A TEMPERATURE OF BETWEEN 225* F. AND 275* F., THEREAFTER SCRUBBING SAID STACK GAS WITH AN AQUEOUS PHOSPHORIC ACID HAVING A P2O5 CONTENT OF AT LEAST 20% AND NOT OVER 35% THEREBY REMOVING SAID VOLATILIZED PHOSPHORIC ACID FROM SAID STACK GAS WHILE DISSOLVING LIMITED QUANTITIES ONLY OF SILICON- AND FLUORINE-CONTAINING COMPOUNDS THEREIN, AND RECYCLING A PORTION OF THE RESULTING SCRUBBING LIQUID TO THE CONCENTRATING STEP.

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