WO1998001214A1 - Method of recovering fluorine compounds from vapors exiting wet process phosphoric acid evaporators - Google Patents

Abstract

A method and apparatus for removing fluorine compounds from solutions formed in the concentration of 'wet-process' phosphoric acid is disclosed, characterized by contacting the exit vapor stream from a phophoric acid concentrating stage (2) with a scrubbing liquor that is sprayed into the vapor exit duct (42) of the phophoric acid concentrator, draining a liquid fraction from the exit duct that contains condensed fluorine compounds, and passing the remaining exit vapor stream/sprayed scrubbing liquor mixture into a cyclonic-type entrainment separator (4) where the entrained scrubbing liquor and condensed fluorine compounds are separated from uncondensed vapors, with the uncondensed vapors passing to a barometric condensor (70) where additional separation of impurities by condensation is effected. In one embodiment, identified as 'single stage FSA recovery', the vapor exit duct from the acid concentrator is connected to an entrainment separator and the scrubbing liquor is contacted with the evaporator exit vapor stream by spraying the liquor into the vapor exit duct via a series of spray heads.

Description

Method Of Recovering Fluorine Compounds From Vapors Exiting Wet Process Phosphoric Acid Evaporators

This invention relates to scrubbing methods and apparatus and more particularly to removal and recovery of gaseous compounds from vapors by scrubbing same with a scrubbing liquor. A specific application of the invention comprises recovering gaseous fluorine compounds produced in the concentration of "wet process" phosphoric acid.

BACKGROUND OF THE INVENTION

The "wet process" method for producing phosphoric acid involves acidulation of phosphate rock with an inorganic acid such as sulfuric acid. In this process ground phosphate rock is pre-mixed with weak phosphoric acid to form a slurry, and that slurry is then introduced into a reactor where it is contacted with sulfuric acid. Phosphoric acid with about 25- 33% P₂0₅ and a calcium sulfate precipitate are separately recovered from the reactor slurry. The phosphoric acid is usually concentrated to about 54% P₂0_5, but there are times when a concentration greater or less than 54% is desired. The acid may be concentrated in a single evaporation stage but two or more evaporation stages are often employed for reasons of heat and equipment economy.

Almost all phosphate rock contains fluorine compounds. During acidulation of the rock, and also during concentration of the dilute phosphoric acid, some fluorine is volatilized, mainly in the form of silicon fluoride (SiF₄) and hydrogen fluoride (HF), along with water vapor. These fluorine compounds need to be removed and recovered.

Subsequently the vapors produced in the concentration of phosphoric acid are scrubbed with a selected scrubbing liquor to recover the fluorine compounds, and in that scrubbing phase the SiF₄ and HF react to produce a stoichiometric amount of fluosilicic acid (H₂SiF₆) in the scrubbing liquor. Hereinafter, fluosilicic acid is sometimes referred to as "FSA". In general two moles of evolved HF are required for each mole of SiF to produce a stoichiometric amount of fluosilicic acid (H₂SiF₆ ) in the scrubbing liquor. However, in multiple staged evaporation of phosphoric acid wherein the acid is concentrated to a P₂0₅ content of less than about 50% in the first stage, more than one mole of SiF₄ per two moles of HF tends to be evolved in the first stage. The stoichiometric excess SiF₄ hydrolyzes in the following scrubber and tends to precipitate silica. The reaction is believed to be as follows:

3SiF₄ + 2H₂O > 2H₂SiF_β +Si0₂

However, in the second stage of evaporation, more than about two moles of HF per mole of SiF₄ is evolved and consequently no precipitation of silica is encountered.

U.S. Patent No. 3,273,713, issued 20 September 1966 to W.R Parish for "Removal of Fluorine Compounds From Phosphoric Acid", describes a multi-stage method of removing fluorine compounds from phosphoric acid which avoids precipitation of silica during recovery of the fluorine compounds. More specifically that patent discloses a method of removing fluorine compounds from phosphoric acid solutions by scrubbing the fluorine-containing vapors recovered from an acid concentrating evaporator with a scrubbing liquid that contains dissolved hydrogen fluoride, with the improved method being characterized by the fact that the phosphoric acid is concentrated in two stages using two vacuum evaporators, and the vapors recovered from each concentration stage are contacted with a scrubbing liquor in separate scrubber stages Using a scrubbing liquor that is an aqueous solution of hydrogen fluoride, or an aqueous solution of hydrogen fluoride and other fluorine compounds such as SιF₄, the recovered fluorine compound product is fluosilicic acid

The invention described and claimed in said U S Patent No 3,273,713 is based on the concept of transferring the second stage scrubbing liquor, which is enriched in HF, to the first stage liquor (enriched in SιF₄) so that the excess SιF will not precipitate out silica Since the scrubbing liquor in the second stage is less rich in Si, it is possible, by introducing the second-stage liquor into the first stage, to prevent siiica from precipitating in the first stage Consequently by mixing the second-stage product into the first stage product, one can obtain a clear product with no silica precipitation in either stage The reaction wherein the silica is solubilized by the hydrogen fluoride is as follows Sι0₂ + 6HF > H₂SιF₆ + 2H20

When the scrubbing liquor, enriched in HF, from the second stage is transferred back to the first stage scrubbing liquor (which contains excess SιF₄), the hydrogen fluoride reacts with the SιF₄ to produce fluosilicic acid according to the following equation SιF₄ + 2HF > H₂SιF₆

U S Patent No 3,273,713 also suggests that an outside source of aqueous hydrogen fluoπde may be added to the first stage scrubbing liquor either in place of or in conjunction with the second-stage scrubbing liquor The important point of the invention of U S Patent No 3,273,713 is that hydrogen fluoride, whether from an outside source or from the second stage scrubbing liquor, must be added to the scrubbing liquor used in the first stage in order to prevent silica precipitation

A disadvantage of the system disclosed by said U S Patent No 3,273,713 is that relatively large and expensive scrubber towers are required in order to effectively scrub out the fluorine compounds Additionally those scrubber towers need a demister pad above the spray chamber to remove entrained particles, and the presence of that demister pad necessitates a lower vapor velocity through the scrubber, which in turn limits the throughput of the system

OBJECTS AND SUMMARY OF THE INVENTION

A first primary object of this invention is to provide an improved scrubber apparatus for use in various processing systems that require a vapor effluent to be scrubbed by a suitable scrubbing liquor for the purpose of absorbing volatile compounds that are soluble in the scrubbing liquor

A second primary object of this invention is to provide an improved method and system for removing fluorine compounds from phosphoric acid solutions

Another object is to provide an improved apparatus for scrubbing vapors recovered from a vacuum evaporator

Another object of the invention is to provide an improved method and apparatus for treating a dilute phosphoric acid so as to remove entrained materials and produce a higher strength phosphoric acid A more specific object of this invention is to provide an improved apparatus for use in scrubbing the vapor effluent from a phosphoric acid evaporator.

Another specific object of this invention is to provide an improved method of concentrating dilute "wet process" phosphoric acid comprising the steps of vacuum concentrating the dilute acid in a vacuum- evaporating vessel so as to a produce a first vapor stream comprising volatile fluorine compounds and a second liquid stream constituting more concentrated acid, and then scrubbing the vapor stream in a novel high velocity apparatus using a selected scrubbing liquid so as to effect removal of the fluorine compounds.

A further specific object of this invention is to provide an improved method and apparatus for removing fluorine compounds from the vapors emanating from a phosphoric acid concentrator, said improved method and apparatus being characterized by spraying a scrubbing liquor into the vapor exit duct of the concentrator.

In the preferred embodiment of the invention, these and other objects are achieved by (1 ) contacting the exit vapor stream from a phosphoric acid concentrating stage with a scrubbing liquor that is sprayed into the vapor exit duct of the phosphoric acid concentrator, and (2) passing the exit vapor stream/sprayed scrubbing liquor mixture into a cyclonic-type entrainment separator where the scrubbing liquor and condensed fluorine compounds are separated from uncondensed vapors, with the uncondensed vapors passing to a barometric condenser where additional separation by condensation is effected. Any remaining uncondensed gases or vapors are passed out of the condenser for disposal or further treatment. In one embodiment hereinafter identified as "single stage FSA recovery" the vapor exit duct from the evaporator is connected to an entrainment separator and the scrubbing liquor is contacted with the evaporator exit vapor stream by spraying the liquor into the vapor exit duct via a series of spray heads. In a second preferred embodiment hereinafter identified as the "dual stage FSA recovery", the entrainment separator is replaced by a combination scrubber/entrainment separator vessel and a second array of spray heads is used to introduce additional scrubbing liquor into the scrubber/entrainment separator above the vapor inlet. Although the invention is described and illustrated in connection with a single stage acid concentrating process system, it is to be appreciated that it also may be utilized in a multiple stage acid concentrating process system, e.g. , in a system using two or more vacuum evaporators.

Other features and advantages are set forth in or rendered obvious by the accompanying drawings and the following detailed description.

THE DRAWINGS

Fig. 1 illustrates schematically a phosphoric acid concentrating system comprising a single evaporator stage coupled to a single stage FSA recovery system embodying the present invention;

Fig. 2 illustrates schematically a phosphoric acid concentrating system comprising a single evaporator stage coupled to a dual stage FSA recovery system embodying the invention;

Fig. 3 illustrates schematically a phosphoric acid concentrating system comprising two evaporator stages each coupled to a single stage FSA recovery system embodying the invention; Fig 4 illustrates schematically a preferred embodiment of the invention comprising two evaporator stages each coupled to a dual stage FSA recovery system, and

Fig 5 illustrates how a conventional scrubber of the type previously used in recovering volatile fluorine compounds can be adapted to utilize the present invention

The drawings are not to scale and are intended merely to illustrate the general aspects of the invention Hence, unless otherwise stated herein, the relative sizes and proportions of the various components shown in the drawings are not to be construed as fixed or limiting the invention in any way. In the several drawings, like parts are identified by like numerals, e g , elements 68 and 68A-D are alike in construction and/or function Also in certain of the drawings (such as Figs 1 and 2) certain conduits such as ducts 42 and 60 are illustrated by double lines to indicate mat they have a substantially large internal diameter, while in other drawings (such as Figs 3 and 4) the same components are represented by a single line

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to Fig 1 , the illustrated system includes a vacuum evaporating unit 1 which may take various forms but preferably comprises an evaporator as disclosed by U S Patent No 4,375, 386, issued 1 March 1983 to Donald M Windham for "Cyclonic Entrainment Separator For Evaporator" The teachings of that patent are well known and are incorporated herein by reference thereto

More specifically with respect to Fig 1 , the illustrated evaporating unit comprises an evaporator vessel 2 that is closed at its top end, while its bottom end has an outlet to which is connected a downleg conduit or pipe 6. The latter is connected via a pump 8 and a return pipe 10 to a liquid feed conduit 12 that is connected to an inlet port in vessel 2. A fresh dilute acid feed pipe 14 is connected to liquid feed conduit 12. Vessel 2 has two side outlets provided above and below the level of inlet pipe 12 to which are connected lines 18 and 20 respectively. The side outlet connected to line 18 is intended to serve as an overflow vent and is located above the desired level 24 of boiling liquid in the evaporator, while the second outlet to which line 20 is connected is located below level 24. Although not shown, it is to be understood that vessel 2 is provided with heating means, preferably associated with return pipe 10, for maintaining the liquid in the vessel at a boiling temperature. Also it is to be understood that the vessel is operated under a vacuum, as further described hereinafter.

Side outlet lines 18 and 20 are connected to an acid product line 26 which leads either to (1 ) a phosphoric acid product storage tank (not shown) if the evaporator is the sole or last evaporator stage or (2) to the next evaporator stage if the evaporator system has more than one evaporation stage and vessel 2 is not the last stage. The level of the junction of line 26 with lines 18 and 20 relative to the two side outlets determines the liquid level 24 in vessel 2. A pump (not shown) may be provided in line 26 for facilitating transfer of concentrated liquid acid product to storage or another evaporation stage.

The space 32 above the liquid level 24 functions as a flashing chamber. The vapors evolved in that flashing chamber are removed from the vessel and are then treated for recovery of fluorine compounds. In the conventional evaporator, the vapors are removed from the flashing chamber via a port in the upper end of the evaporation vessel. Although such conventional evaporators may be used in practicing the present invention, it is preferred to utilize an evaporation unit having an entrainment separator 4 for separating entrained liquids from the evolved vapors (which may comprise water vapor and fluorine compounds) and thereby reduce contamination of the concentrated phosphoric acid recovered from the system. Separator 4 is located at the upper end of the space 32. Various types of known entrainment separators may be used in conjunction with vessel 2, but it is preferred to use a cyclonic entrainment separator of the form shown in said U.S. Patent No. 4,375,386. Since entrainment separators are well known, and since the form of the evaporator unit is not critical to the invention, specific structural details of entrainment separator 4 and evaporator vessel 2 are not illustrated in the drawings. However, to assure a full disclosure and understanding of the present invention, the teachings of Windham U.S. Patent No. 4,375,386 relating to entrainment separators are incorporated herein by reference thereto.

Nevertheless, to assure full understanding of entrainment separator 4, Fig. 1 schematically shows that the entrainment separator comprises a vertical pipe section 34 which is mounted inside and located along the axis of the vessel. The bottom end of pipe 34 is connected by an elbow pipe section 36 and a horizontal pipe section 38 to an exit vapor duct 42. Pipe sections 34, 36, and 38 form part of the entrainment separator, and correspond to the components 21 , 23 and 22 respectively shown in the drawings of said U.S. Patent No. 4,375,386. The top end of vertical pipe section 34 located in vessel 2 is open and serves as a vapor intake port for exit duct 42. The entrainment separator further includes a helical spin plate, represented schematically at 44, which corresponds to the spin plate 26 shown in said U S Patent No 4,375,386 Spin plate 44 surrounds vertical pipe section 34 and is secured to that pipe section and also to the wall of the vessel

The separator has a relatively large vapor inlet which is the opening formed between the lower end edge of the spin plate and the portion of the spin plate that lies directly above and is spaced from that lower end edge, plus a relatively large vapor outlet which is the opening formed between the upper end edge of the spin plate and the portion of the spin plate that lies directly below and is spaced from that upper end edge of the spin plate

As viewed in cross-section , the spin plate may extend radially at a right angle to the axis of vessel 2 and pipe section 34 Preferably, however, it is inclined in a radial direction relative to the axis of vertical pipe section 34, so that gravity will tend to cause the liquid on the spin plate to move not only downwardly on the spin plate toward the separator's relatively large vapor inlet but also radially away from that pipe section toward the inner surface of the vessel wall

Spin plate 44 includes a helical trough (not shown) that (a) is located at the outer edge of the spin plate adjacent to the inner surface of the wall of vessel 2, and (b) corresponds to the trough 28 described in said U S Patent No 4,375,386 The bottom end of the trough terminates in communication with a side port 46 that is connected to an external dram pipe 48 that corresponds to the drain pipe 34 shown in said U S Patent No 4,375,386 Drain pipe 48 is connected to downleg conduit 6 so that liquid collecting in the trough of the entrainment separator will be returned to the vessel at a point below liquid level 24 Further details of the construction of entrainment separator 4 are provided by said U S Patent No 4,375,386

As noted above, fresh dilute phosphoric acid (typically about 26- 32% P₂0₅) is introduced to the vessel 2 via an acid feed pipe 14, and simultaneously recycle phosphoric acid liquid is supplied to inlet conduit 12 via pump 8 and return pipe 10

Exit duct 42 is connected to an inlet port of an FSA recovery tower or vessel 50 provided in accordance with this invention Mounted inside of vessel 46 is an entrainment separator 52 for separating entrained liquids from the vapors fed via duct 42 Various types of entrainment separators may be used, but it is preferred that the separator be a cyclonic entrainment separator like the one disclosed in said U S Patent No 4,375,386 Consequently separator 52 comprises a vertical pipe section 54 which is located along the axis of vessel 50, an elbow pipe section 56, and a horizontal pipe section 58, which correspond to the components 21 , 23, and 22 shown in said U S Patent No 4,375,386 The upper end of vertical pipe section 54 is open and serves as a vapor intake port The horizontal pipe section 58 is located above the level of duct 42 and its outer end is connected to an exit vapor duct 60 which in turn is connected to a vapor inlet port of a barometric condenser 70 The latter may take various forms known to persons skilled in the art

Separator 52 also comprises a helical spin plate 62 which corresponds in structure and function to the helical spin plate 26 shown in said U S Patent No 4,375,386 Spin plate 62 surrounds pipe section 54 and is secured to that pipe section and also to the surrounding wall of vessel 50 The separator has a relatively large vapor inlet which is the opening formed between the lower end edge of its spin plate and the portion of that spin plate that lies directly above and is spaced from that lower end edge, plus a relatively large vapor outlet which is the opening formed between the upper end edge of its spin plate and the portion of that spin plate that lies directly below and is spaced from that upper end edge of the spin plate

As viewed in cross-section, the spin plate 62 may extend radially at a right angle to the axis of vessel 50 and pipe section 54 Preferably, however, spin plate 62 is inclined in a radial direction relative to pipe section 54, so that gravity will cause liquid on that spin plate to move not only downwardly on the spin plate toward the relatively large vapor inlet of the separator but also radially away from center pipe section 54 toward the inner surface of the surrounding wall of vessel 50. Spin plate 62 also has a helical trough (not shown) that is located at its outer edge adjacent to the inner surface of the wall of vessel 50 and corresponds to the trough 28 described in said U.S. Patent No. 4,375,386. The bottom end of the trough terminates in communication with side discharge port 64 that is connected to a drain pipe 66 that corresponds to drain pipe 34 shown in said U.S. patent and is arranged to discharge condensate into an FSA recirculation tank 68 Further details of the construction of separator 52 are provided by said U S. Patent No 4,375,386

The vapors delivered by exit duct 42 undergo rapid expansion in vessel 50 This rapid expansion occurs because of the relatively large space for expansion offered by vessel 50 and also because vessel 50 is operated at a reduced pressure due to its being connected to barometric condenser 70. As a consequence of such expansion, the vapors undergo adiabatic cooling with a consequent condensation of water vapor, fluosilicic acid and other fluorine compounds Some liquid condensate is collected in the bottom end of vessel 50 and removed at a selected rate via a bottom port in the vessel to a recirculation tank 68 via a drain line 80 Additionally liquid condensate recovered in the spin plate trough in cyclonic separator 52 is fed by drain line 66 to tank 68

In accordance with this invention, a plurality of spray heads 72 are mounted within exit duct 42 In this connection it is to be understood that in a typical commercial scale acid-concentrating plant the exit vapor duct from each evaporator has a substantial size, typically having an internal diameter in the range of 3 to 6 feet Each spray head 72 consists of one or more spray nozzles (not shown) that are oriented so as to direct a spray in a direction counter to the flow of vapors exiting the evaporator unit The number of spray heads and the number and arrangement of nozzles in each spray head may be varied without departing from the principles of the invention The spray heads are connected via a feed line 74 to a pump 76 that in turn is coupled to an outlet port of FSA recirculation tank 68 Pump 76 operates to deliver liquid from tank 68 to spray heads 72 at a selected rate, whereby that liquid is sprayed into duct 42 to perform a vapor scrubbing function The system also includes a delivery line 82 for adding makeup water to tank 68 and a line 84 connected to line 74 for withdrawing FSA product from the system

Barometric condenser 70 is of conventional design, and includes a liquid inlet port 86 at its top end for introduction of cooling water, a vapor inlet port 87, an exit vapor port 88 that is located between the vapor inlet port and the liquid inlet port, and a liquid drain port 90 at its bottom end that is connected to a line 94 leading to a suitable disposal facility Exit vapor port 88 is connected by a line 92 to a vacuum pump (not shown) Preferably the vacuum is created by a steam-jet type vacuum pump Line 92 also serves to remove uncondensed vapors from the condenser and deliver same to a suitable disposal facility.

Operation of the above described system will now be described. Condenser 70, and hence the evaporator unit and the entrainment separator vessel as well, are operated at a reduced pressure, i.e., less than the existing atmospheric pressure. The reduced pressure in the evaporator unit will cause the dilute acid to be concentrated by flash vaporization of water and fluorine in the evaporator vessel. The vapor contains droplets of phosphoric acid and water vapor. This vapor enters the relatively large vapor inlet of separator 4 and follows a helical path as it moves rapidly over the upper surface of spin plate 44 around center pipe section 34 to the relatively large separator vapor outlet. The vapor with entrained droplets moves through the separator at a high velocity in a cyclone-type action, producing a centrifugal force on the entrained droplets which causes them to be thrown radially toward the inner surface of the vessel wall and the edge trough mentioned above, thereby separating them from the cyclonically moving vapor which reverses direction on reaching the upper end of the separator and moves down into center pipe section 36. The vapor is discharged from the evaporator vessel via duct 42 under the suction effect of the vacuum maintained in tower vessel 50 and condenser 70 by the steam jet type vacuum pump (not shown). Some of the droplets separated by the cyclonic action that collect on the surface of the spin plate may travel down along the spin plate to the entrainment separator inlet where they tend to fall back into the vapor chamber 32.

As the vapor stream moves along duct 42 it is sprayed with scrubbing liquor supplied by pump 76 and directed by spray heads 72. The scrubbing liquor comprises an aqueous solution consisting primarily of fluosilicic acid, plus phosphoric acid and other scrubbed fluorine- containing compounds. That resulting vapor/scrubbing liquor mixture passes into FSA recovery tower 50 where the mixture is separated by cyclonic action, with a liquid fraction comprising mainly the scrubbing liquor and dissolved fluorine-containing compounds being recovered and delivered to FSA recirculation tank 68 via lines 66 and 80 and a vapor fraction being recovered by way of exit duct 60 and delivered to condenser 70. In condenser 70, the cool water supplied via liquid inlet port 86 condenses any remaining water vapor and also any other uncondensed fluorine compound vapors. The condensed substances are delivered to a disposal facility via drain port 90 and line 94. Any remaining uncondensed vapors or gases are removed from the system via vapor exit port 88.

In FSA recovery vessel 50 vapor from the evaporator unit enters the relatively large entrainment separator inlet (i.e., the opening formed between the lower end edge of spin plate 62 and the portion of that spin plate that lies directly above that lower end edge) and follows a helical path as it moves rapidly over the upper surface of the spin plate around center pipe section 54. The vapor moves through the separator 52 at a high velocity in a cyclone-type action, producing a centrifugal force on the entrained droplets which causes them to be thrown radially toward the inner surface of vessel 50 and its helical edge trough mentioned above, thereby separating those droplets from the cyclonically moving vapor which reverses direction on reaching the upper end of the separator and moves down into center pipe section 54. Depending on the velocity of the vapor, some droplets also may deposit on the upper surface of the spin plate under the influence of gravity Those deposited liquid droplets, and also any droplets collecting on the inner surface of the vessel, tend to drop by gravity into the edge trough or back into the vapor chamber located below the separator instead of being allowed to move up to the level of the inlet end of center pipe section 54, with the result that the entrained substances separated by the cyclonic action cannot recombine with the vapor as the latter reaches the upper inlet end of center pipe section 54

As a consequence of the foregoing operation, a substantial portion of the fluorine compounds (primarily FSA) in the vapor stream in exit duct 42 are removed from that vapor stream as a liquid condensate via entrainment separator vessel 50, while uncondensed fluorine compounds are removed from the system via line 92

Fig 2 shows how the invention may be modified to provide multistage scrubbing The system shown in Fig 2 is identical to that of Fig 1 , except that provision is made for (1 ) recovering a fjrst stage FSA liquid product from exit duct 42 and recycling that product for use in scrubbing other new vapors in that exit duct, and (2) recovering another (second stage) FSA product from FSA tower vessel 50 and using that FSA product as a scrubbing liquor in that vessel Optionally some of the second stage FSA product (i e , the FSA product stream recovered from FSA tower vessel 50) is returned to the first stage FSA recirculation tank for mixing with first stage FSA product used to scrub the vapors in exit duct 42

More specifically, at a point between the spray heads 72 and the vapor inlet port of FSA tower 50, a line 100 is connected from the exit vapor duct 42 to an FSA recirculation tank 68A (for purposes of the embodiment of Fig 2, tank 68A is hereinafter referred to as the "first stage recirculation tank"). Line 100 removes from exit duct 42 a liquid stream that comprises absorbed condensed vapors of fluosilicic acid and other fluorine compounds, plus recovered scrubbing liquor introduced via line 74 (a "first stage FSA product"), and delivers same to tank 68A. Tank 68A is connected to pump 76A. The latter operates to remove liquid from tank 68A and deliver it to spray heads 72 via line 74 as a scrubbing liquor.. A portion of the liquid removed from tank 68A by pump 76A is removed as a final FSA product via a line 84.

In this Fig. 2 embodiment, one or more spray heads 102 is (are) added to FSA tower vessel 50. Each spray head may comprise one or more spray nozzles. If each spray head comprises more than one spray nozzle, it is preferred that the nozzles be arranged in a circular array so as to assure intimate liquid/vapor contact across the entire cross-section of the vessel. If more than one spray head is used, the several spray heads are located at different levels in vessel 50. The number of spray heads, and also the number of spray nozzles in each spray head, may be varied without departing from the principles of this invention. The spray heads are located in the space between the separator 52 and the level of the inlet port to which duct 42 is connected. The liquor supplied by spray heads 102 is introduced into vessel 50 in countercurrent flow to the upwardly flowing vapors supplied via duct 42.

A liquid fraction is recovered from the bottom port of vessel 50 and is delivered via line 80 to a second recirculation tank 68B (the "second stage recirculation tank"). That liquid fraction comprises scrubbing liquor introduced via spray heads 102 and liquid condensate formed in vessel 50. Also condensate collecting in the edge trough of separator 52 is delivered via line 66 to tank 68B. Makeup water also may be supplied to tank 68B via line 82 The second tank 68B is connected by a second pump 76B and a line 104 to secondary stage spray heads 102, whereby liquid accumulating in tank 68B is recirculated to spray heads 102 for use in scrubbing vapors in vessel 50 As an optional feature, some of the liquid pumped by pump 76B is delivered via a line 108 back to the first stage recirculation tank 68A Phosphoric acid product and FSA product are recovered via lines 26 and 84 respectively

Figs 3 and 4 illustrate how the invention is applied to phosphoric acid concentrating systems that utilize two evaporator stages In these drawings, no attempt is made to illustrate spray heads mounted in the evaporator vapor exit ducts 42A and 42B, but it is to be understood that spray heads like spray heads 72 are mounted in those ducts Also, certain other components, such as pump 8, are omitted only for the purpose of simplifying the drawing However, it is to be understood that the apparatus shown in Figs 3 and 4 includes essential operating components such as pump 8 or means (not shown) for heating the- liquid acid recirculated via lines 6 and 10, and condensers like condenser 70 and a vacuum pump like the one to which condenser 70 is connected

Fig 3 schematically illustrates an acid concentrating process that utilizes two evaporator vessels 2A and 2B and two single stage FSA recovery systems embodying the present invention

Preferably evaporators 2A and 2B have entrainment separator sections at their top end like the one embodied in evaporator 2 shown in Fig 1

In the system of Fig 3, the liquid acid product recovered from first evaporator 2A is delivered via line 26A to the second evaporator 2B for further concentration The vapor produced by flash evaporation in the first evaporator 2A is fed to an entrainment separator vessel 50A via a vapor exit duct 42A Some liquid in duct 42A is drained off and fed by line 100A to a recirculation tank 68C Liquid from entrainment separator vessel 50A is delivered via lines 66A and 80A to recirculation tank 68C, and liquid is recovered from that tank by a pump 76C and fed as a scrubbing liquor via line 74A to spray heads (not shown) disposed in duct 42A in the manner described above in connection with spray heads 72 The vapor from separator vessel 50A is fed to a condenser (not shown) like condenser 70

The concentrated acid produced in evaporator 2B is recovered via line 26B and sent to storage The vapor stream from evaporator 2B is scrubbed in exit duct 42B en route to a second entrainment separator vessel 50B Some liquid in duct 42B is drained off and fed by line 100B to a recirculation tank 68D Liquid condensate is recovered from separator 50B via lines 66B and 80B and sent thereby to recirculation tank 68D Liquid in tank 68D is fed by pump 76D and line 74B to spray heads (not shown) disposed in duct 42B in the manner of spray heads 72 where it serves as a scrubbing liquor The remainder of the liquid in tank 68D is transferred to tank 68C via line 84B to enrich the fluorine content of the scrubbing liquor that is fed to duct 42A FSA product is recovered from tank 68C via line 84A

Fig 4 schematically illustrates an acid concentrating process like that of Fig 3 except that a dual stage FSA recovery system is associated with each evaporator In this case, tanks 68E and 68G correspond to tank 68A of Fig 2 while tanks 68F and 68H correspond to tanks 68B of Fig 2 Similarly, pumps 76E and 76G correspond to pump 76A of Fig 2 and pumps 76F and 76H correspond to pump 76B of Fig 2 The liquid accumulating in recirculation tank 68F is fed by pump 76F and a line 104A to one or more spray heads 102A in entrainment separator vessel 50A, while the liquid accumulating in tank 68H is fed by pump 76H and a line 104B to one or more spray heads 102B in entrainment separator vessel 50B As with the embodiment of Fig 3, final FSA product is removed from tank 68E and acid product is recovered from evaporator 2B via lines 84A and 26B respectively

In the embodiments of Figs 1-4, the scrubbing liquor comprising an aqueous solution of fluorine compounds, primarily fluosilicic acid but also including other fluorine compounds such as hydrogen fluoride, is maintained at a temperature between about 100-150 degrees F , depending upon the amount of vacuum employed in the system In a system comprising two evaporator stages, it is common to operate the first and second stages at different levels of vacuum and different temperature ranges due to the difference in P₂0₅ content of the acid being concentrated In any event, it is preferred that the temperature of the scrubbing liquor in each stage be substantially the same as that of the vapors which are being scrubbed The temperature of the scrubbing liquor is elevated sufficiently to minimize the amount of condensation of water vapor that takes place in ducts 42, 42A and 42B, yet the fluorine compounds in the vapors are sufficiently absorbed The vacuum employed is usually greater than 22 inches of mercury vacuum Use of a vacuum in the range of 22-28 inches of mercury vacuum requires the temperature to be about 100 - 150 degrees F

Usually, it is advisable to keep the concentration of the dissolved fluorine compounds in the scrubber liquor below about 25% (as FSA) to obtain good scrubbing efficiency if only one scrubbing stage is used in FSA recovery systems, e.g., the systems of Figs. 1 and 3. If the concentration of the fluorine compounds such as fluosilicic acid or its salts in the recycle scrubbing liquor becomes too high, the vapor pressure of the fluorine compounds in the liquor becomes sufficiently great so that the amount of fluorine removed in the single scrubber stage is diminished. Generally, a balance is maintained between highly concentrated recycle liquor and very dilute liquors so that large volumes of dilute aqueous fluorine compound solutions need not be handled and, on the other hand, an excessive amount of fluorine compounds will not be lost in the condenser. However, it is recognized that in some cases it may be desirable, even though fluorine recovery is not as efficient, to use an aqueous scrubbing liquor having more than about 25% fluosilicic acid. It is preferred to use the dual stage FSA scrubbing system, shown in Figs. 2 and 4, since that system makes it possible, to achieve fluorine compound recovery in the 90-95% range.

Fig. 5 illustrates a further embodiment of the invention for use in a conventional FSA recovery system like the one shown in said U.S. Patent No. 3,273,713. In this case, a conventional scrubber tower or vessel used to separate FSA product from the vapor emanating from a first or second stage evaporator, e.g., the scrubber tower 17A or 17B shown in said U.S. Patent No. 3,273,713, is modified by coupling it to a cyclonic entrainment separator that is constructed and performs like the one described above in connection with Fig. 1. More specifically, a conventional scrubber vessel 140 (like those shown at 17A and 17B in said U.S. Patent No. 3,273,713) has its top vapor port coupled to an entrainment separator vessel 144 that is like the vessel 50 shown in Fig. 1. Scrubber vessel 1 0 has one or more spray heads 1 6 (comparable to spray heads 102) that are positioned to spray a scrubbing liquor into the vapor chamber that is directly above the vessel's vapor inlet port to which vapor duct 42 is connected. Scrubbing liquor is recovered from vessel 140 via a line 148 that leads from the vessel's bottom port to a recirculation tank 68. Liquid in that tank is recycled by a pump 76 to spray heads 146 via a line 150 and FSA product is recovered via a line 152. If the scrubber is part of the second stage of a two evaporator system as shown in said U.S. Patent No. 3,273,713, the line 152 may be used to deliver liquor from tank 68E to the scrubber associated with the first stage evaporator for the purpose of reducing the possibility of silica precipitation, as taught in that patent.

In the arrangement shown in Fig. 5, the vapors remaining after counter-current scrubbing in scrubber tower 140 are fed to entrainment separator vessel 144 where they are subjected to cyclonic separation of entrained liquids. Vessel 144 has a spin plate/edge trough arrangement 154 like that associated with entrainment vessel 50 shown in Fig. 1. Entrained liquid condensate collecting in the edge trough surrounding the spin plate in entrainment separator vessel 144 is recovered and returned via a line 156 to tank 68. The separated vapor is passed from the entrainment separator vessel 144 via an exit duct 158 to a barometric condenser (not shown) like the one shown at 26A, 26B in said U.S. Patent No. 3,273,713.

The embodiment of Fig. 5 is advantageous in applying the present invention to existing phosphoric acid concentration systems that recover FSA using conventional scrubbers as shown in said U.S. Patent No. 3,273,713. Retrofitting conventional scrubbers of existing FSA recovery apparatus with entrainment separators so as to provide an arrangement as shown in Fig 5 offers the advantage that it permits higher vapor velocities to be employed in the existing scrubbers without any overall sacrifice in FSA recovery efficiency

The following examples are presented to illustrate the invention These specific example should not be viewed in any manner as limiting the invention as defined in the appended claims Concentrations are expressed on a weight basis

EXAMPLE 1

In this example, an FSA recovery system as shown In Fig 1 is employed Dilute phosphoric acid (approximately 27% P₂O₅ and 2 5% fluorine) is continuously introduced into evaporator 2 operating at a pressure of about 3 inches absolute mercury and at a temperature of about 185 degrees F In the evaporator the dilute phosphoric acid is subjected to flash evaporation, with the result that two streams are produced (a) a vapor stream that exits the evaporator 2 via exit duct 42 and contains up to about 3 5% fluorine compounds, and (b) a concentrated (about 54% P₂0₅) acid product stream that exits the evaporator via line 26 Exit duct 42 has a diameter of about 6 feet and the vapors in that duct have a velocity in the range of 150 to 250 feet/second The vapors in exit duct 42 are scrubbed in that duct with a scrubbing liquor in the form of an aqueous solution (comprising primarily fluosilicic acid and lesser amounts of other fluorine compounds such as SιF₄ and HF) that is supplied via line 74 and spray heads 72 The scrubbing liquor supplied via line 74 has a fluorine compound concentration of about 25% (measured as FSA) and is supplied at a rate of about 1 100 gallons/minute The scrubbed vapors entering entrainment separator vessel 50 have a reduced fluorine concentration of about 0 8% Entrainment separator vessel 50 is operated at a pressure of about 2 5 inches absolute mercury and has a height of 36 feet and an inside diameter of 16 feet The vapor stream exiting entrainment separator 52 and entering barometric condenser 70 contains about 0 8% fluorine The liquid in tank 68 has a temperature of about 120 degrees F and makeup water is added as required to provide in tank 68 a fluorine concentration (measured as FSA) of about 25% Some of that 25% FSA product is recovered via line 84, while the remainder is recycled to spray heads 72 in duct 42 It should be noted that little or no phosphoric acid is contained in the aqueous liquid supplied to lines 74 and 84 About 75% of the fluorine content of the vapors (measured as FSA) leaving the evaporator via duct 42 are recovered via the sprays and the entrainment separator

EXAMPLE 2

In this example, a dual FSA recovery system as shown In Fig 2 is employed The operating conditions of the evaporator and entrainment separator are the same as in Example 1 Also the P₂O₅ concentration in the phosphoric acid feed, the phosphoric acid product, the fluorine compound concentration in the FSA product recovered from tank 68A, the fluorine concentration in the vapors discharged into duct 42 from the evaporator vessel, and the fluorine concentration in the vapors delivered by duct 42 to entrainment separator vessel 50 are the same as in Example 1 However, the vapors fed from the entrainment separator to condenser 70 contain about 0 1 % fluorine and the fluorine concentration in lines 104 and 108 is about 15% measured as FSA and the liquid in tanks 68A and 68B have a temperature of 120 and 1 10 degrees F. respectively. About 95% of the fluorine compound content of the vapors entering duct 42 (measured as FSA) are recovered via the sprays and the entrainment separator.

EXAMPLE 3

In this example, an FSA recovery system as shown In Fig. 3 is employed. Evaporators 2A and 2B are of like size. Exit ducts 42A and 42B also are of like size. Dilute phosphoric acid (approximately 27% P₂0₅ and 2.5% fluorine) is continuously introduced into evaporator 2A operating at a pressure of about 6.5 inches absolute mercury and at a temperature of about 185 degrees F. The second evaporator 2B is operated at the same temperature but a pressure of about 3.0 inches absolute mercury. Entrainment separators 50A and 50B are of like size and are operated at pressures of about 6.0 and 2.5 inches absolute mercury respectively. Two exit streams are recovered from evaporator 2A: (1 ) an inlet vapor stream to duct 42A that contains about 2.7% fluorine, and (2) a more concentrated (40% P₂O₅) acid product stream that exits the evaporator via line 26A. Exit duct 42A has a diameter of 6 feet and the vapors in that duct have a velocity in the range of 150 to 250 feet/second. The vapors in exit duct 42A are scrubbed in that duct with a scrubbing liquor in the form of an aqueous fluosilicic acid solution that is supplied via line 74A and sprayed by spray heads disposed in that duct like spray heads 72. The scrubbing liquor supplied via line 74 has a fluorine concentration of about 25% (measured as FSA). After removal of liquid via line 100, the scrubbed vapor entering entrainment vessel 50A has a reduced fluorine content of about 1 .5%. Vessels 50A and 50B each have a height of 36 feet and an inside diameter of 16 feet The vapor stream exiting entrainment separator 50A and entering barometric condenser 70A contains about 1 5% fluorine The liquid condensate recovered from separator vessel 50A via drain line 66A and bottom line 80A and stored in tank 68C has a fluorine compound concentration of about 25% FSA and has a temperature of about 140 degrees F Makeup water is added to tank 68D as required so as to maintain the concentration of FSA in the scrubbing liquor fed to spray headers 74A and the FSA product at about 25% The 40% acid product from evaporator 2A is concentrated in evaporator 2B to about 54% P₂O₅ The vapor exiting evaporator vessel 2B via duct 42B contains about 5 0% fluorine, but after contacting scrubbing liquor supplied by line 74B in duct 42B, the vapors are reduced to a fluorine concentration of about 0 15% The vapor stream from condenser 70B has a fluorine content of about 0 15%o The liquid recovered in tank 68D has a fluorine content of about 15%) FSA Some of that liquid is used as a scrubbing liquor in duct 42B and the remainder is fed by pump 76D to tank 68C About 70% of the fluorine content of the vapors from evaporators 2A and 2B is recovered via the sprays and entrainment separators 50A and 50B, with the product stream in line 84A having an FSA concentration of about 25%

EXAMPLE 4

In this example, a dual evaporator and a dual FSA recovery system as shown in Fig 4 are employed The two evaporators and the two entrainment separators are operated under the same conditions as in Example 3 Also the sizes of ducts 42A and 42B and vessels 2A and 2B, 50A and 50B are the same as in Example 3 Dilute phosphoric acid (approximately 27% P₂0₅ and 2 5% fluorine) is continuously introduced into evaporator 2A The vapor stream recovered from evaporator 2A via exit duct 42A contains about 2 7% fluorine and the acid product stream recovered via line 26A has a phosphoric acid concentration of about 40% P₂0₅ The scrubbing liquor supplied to exit duct 42A via line 74A has a fluorine compound concentration of about 25% FSA The vapor remaining after removal of liquid via line 100 has a fluorine content of about 1 5% The vapor stream exiting entrainment separator vessel 50A and entering barometric condenser 70A contains about 0 55% fluorine The liquid condensate recovered from separator vessel 50A via drain line 66A and bottom line 80A and contained in tank 68F has an FSA concentration of about 20% and a temperature of about 140 degrees F Some of the liquid in tank 68F is recycled to the spray nozzles 102A in separator vessel 50A via line 104A and the remainder is recycled to tank 68E which collects liquid form duct 42A via line 100 Some of the liquid in tank 68E is recycled via pump 76E and line 74A to the spray heads in duct 42A for scrubbing the vapor stream in that duct, while the remainder is drawn off by line 84A as FSA product

A phosphoric acid product containing about 54% P₂O₅ is recovered from evaporator 2B via line 26B The vapor entering duct 42B from evaporator 2B contains about 5 0% fluorine After scrubbing in duct 42B the vapor that enters entrainment separator vessel 50B has a fluorine content of about 0 15% The liquid condensate recovered in tank 68H from entrainment separator vessel 50B has an FSA concentration of about 1 0% and is at a temperature of about 1 10 degrees F Some of the liquid in tank 68H is fed back to the internal spray heads 102B of vessel 50B, while the remainder is returned to a tank 68G where an FSA concentration of about 12.5% is maintained at a temperature of about 120 degrees F. A portion of the liquid in tank 68G is supplied to spray heads in duct 42B, while the remainder is fed back to tank 68F via line 84B. About 90% of the fluorine content of the vapors from evaporators 2A and 2B are recovered as FSA product.

A primary advantage of the invention is that use of an entrainment separator to facilitate recovery of FSA makes possible higher vapor velocities and hence greater vapor throughput in the FSA recovery stage(s). Also FSA recovery systems like the one described by said U.S. Patent No. 3,273,713 require large scrubber vessels that are costly and also limit the velocity at which the vapors can be scrubbed. The present invention overcomes that limitation by replacing those large scrubbers with smaller and less costly entrainment separator. By way of example, the present invention makes it possible to replace a typical FSA scrubber vessel (e.g., a vessel as used in the process disclosed by Parish U.S. Patent No. 3,273,713) having an overall height of 42 feet and a diameter of about 22.5 feet with an entrainment separator vessel having a height of about 36 feet and a diameter of 16 feet, to achieve a comparable FSA recovery. More specifically, the invention makes it possible to recover FSA at least as efficiently as with the Parish process, but with lower capital equipment costs. Use of a dual stage FSA recovery system as shown in Figs. 2 and 4 is preferred since it materially increases the amount of fluorine compounds that are recovered. A further advantage of the invention is that it teaches (Fig. 5) that existing FSA recovery systems having scrubbers may be modified by coupling the vapor output of each scrubber to an entrainment separator of the type described, with the result that the vapor velocity (and hence the throughput) through the scrubber may be increased without any decrease in the percentage of fluorine compound recovery (in fact the total amount of fluorine compounds recovered may be is increased by utilizing an entrainment separator in combination with a conventional scrubber in the manner shown in Fig. 5). Obviously many other modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof. For example, the dotted line 100 shown in Fig. 1 indicates that a drain line may be connected between duct 42 and tank 68 to enhance recovery of scrubbing liquid. Also, the combined scrubber/entrainment apparatus shown in Figs. 2, 4 and 5 may be used in processes other than those involving phosphoric acid or fluorine compounds. Accordingly, only such limitations should be imposed as are indicated in the appended claims.

Claims

WHAT IS CLAIMED IS:

1 In a method for concentrating "wet-process" phosphoric acid wherein said acid is concentrated to produce vapors containing water vapor and fluorine compounds mainly in the form of HF and SιF₆, and further wherein said vapors are recovered and contacted with a selected liquor for absorbing said fluorine compounds, the improvement comprising passing said vapors to a cyclonic entrainment separator via an exit duct, injecting said selected liquor into said exit duct in contact with said vapors, and separately recovering from said entrainment separator a vapor fraction with a reduced fluorine compound content and a liquid fraction enriched with absorbed fluorine compounds

2 A method according to claim 1 wherein at least part of said selected liquor comprises said liquid fraction

3 A method according to ciaim 1 wherein said cyclonic entrainment separator is part of a vessel that also has a vapor scrubbing section in communication with said entrainment separator and said vapors are passed by said exit duct to said vapor scrubbing section, and further wherein the vapors in said vapor scrubbing section are scrubbed by injecting another selected liquor into said vapor scrubbing section in contact with said vapors

4 A method according to claim 1 further including the step of recovering a portion of said liquid fraction

5. A method according to claim 4 further comprising the steps of recycling another portion of said liquid fraction to said exit duct for use as or part of said selected liquor.

6. In a method for concentrating "wet-process" phosphoric acid wherein said acid is concentrated under vacuum in a concentrator to produce vapors containing water vapor and fluorine compounds mainly in the form of HF and SiF_G, and further wherein said vapors are scrubbed with a liquid for absorbing said fluorine compounds, the improvement comprising passing said vapors from said concentrator to a cyclonic entrainment separator via an exit duct, injecting a selected liquor into said exit duct in contact with said vapors whereby said fluorine compounds are absorbed into said liquor to form an enriched liquor, and separately recovering from said entrainment separator a first stream of unabsorbed vapors and a second stream of said enriched liquor, and utilizing some of said enriched liquor to further scrub said vapors.

7. An apparatus for scrubbing a vapor stream so as to remove selected compounds from said stream, said apparatus comprising: a vessel having an upper end and a lower end and defining a chamber; a first port in said vessel for introducing into said chamber a vapor stream containing selected compounds that are to be removed by a scrubbing action, said first port being at a first selected level between said upper and lower ends of said vessel; a second port in said vessel for removing a gas stream from said chamber, said second port being at a second selected level between said upper end of said vessel and said first level; a third port in said vessel for removing a liquid stream from said chamber, said third port being located at a third selected level below said first port; spray means within said chamber between said upper end of said vessel and the level of said first port for introducing a liquid scrubbing medium into said chamber in counter-current flow relationship with the vapor stream introduced via said first port so as to effect scrubbing of said selected compounds; and means defining an entrainment separator between said spray means and said second port for effecting separation of said liquid scrubbing medium and any of said selected compounds that are entrained in the vapor that rises in said chamber above the level of said spray means, said entrainment separator comprising means providing (i) a vapor discharge passageway connected to said second port for removing vapor from said separator and (ii) a liquid downflow passageway connected to said third port whereby liquid entrained in said vapor but separated therefrom by action of separator is removed from said vessel via said third port.

8. In a system for concentrating phosphoric acid wherein a vapor effluent from an acid-concentrating evaporator unit contains uncondensed fluorine compounds, an apparatus for scrubbing said effluent so as to remove selected compounds from said effluent, said apparatus comprising: a vessel having an upper end and a lower end and defining a chamber, a first port in said vessel for introducing into said chamber said vapor effluent, said first port being at a selected level between said upper and lower ends of said vessel, a second port in said vessel for removing a vapor stream from said chamber, said second port being at a second selected level between said upper end of said vessel and said first level, a third port in said vessel for removing a liquid stream from said chamber, said third port being located at a third selected level below said first port, spray means within said chamber between said upper end of said vessel and the level of said first port for introducing a liquid scrubbing medium into said chamber in counter-current flow relationship with the vapor stream introduced via said first port so as to effect scrubbing of said selected compounds, and means defining an entrainment separator between said spray means and said second port for effecting separation of said liquid scrubbing medium and any of said selected compounds that are entrained in the vapor that rises in said chamber above the level of said spray means, said entrainment separator comprising means pi oviding (i) a vapor discharge passageway connected to said second port for removing vapor from said separator and (n) a liquid downflow passageway connected to said thud port whereby liquid entrained in said vapor but separated therefrom by action of sepai ator is removed from said vessel via said third port

9. In a method for concentrating a dilute phosphoric acid produced by the "wet-process" wherein said dilute acid is concentrated under vacuum in a first concentrator to produce a first more concentrated acid stream and a first vapor stream containing water vapor and fluorine compounds mainly in the form of HF and SiF₆, and further wherein said first more concentrated acid stream is concentrated under vacuum in a second concentrator to produce a second higher concentration acid stream and a second vapor stream containing water vapor and fluorine compounds, and further wherein said first and second vapor streams are scrubbed with a liquid for absorbing said fluorine compounds, the improvement comprising passing said first and second vapor streams through first and second exit ducts to first and second entrainment separators respectively, injecting first and second scrubbing liquors into said first and second exit ducts into contact with said first and second vapor streams respectively, recovering from said first entrainment separator a first stream of unabsorbed vapors and a first stream of liquid containing condensed fluorine compounds, recovering from said second entrainment separator a second stream of unabsorbed vapors and a second steam of liquid containing condensed fluorine compounds, recycling some of said first stream of liquid to said first exit duct for use as said first scrubbing liquor, and recycling some of said second stream of liquid to said second exit duct for use as said second scrubbing liquor.

10. A method according to claim 9 wherein some of said second liquid is admixed with said first liquid for use in forming said first scrubbing liquor.

1 1. A method according to claim 9 wherein said first entrainment separator is part of a first vessel that is connected to receive vapors via said first duct from said first concentrator, and further including the step of injecting a third scrubbing liquid into said first vessel to effect further scrubbing of the vapors from said first concentrator before they enter said first entrainment separator.

12. A method according to claim 1 1 wherein said second entrainment separator is part of a second vessel that is connected to receive vapor via said vessel duct from said second concentrator, and further including the step of injecting a third scrubbing liquid into said second vessel to effect further scrubbing of the vapors from said second concentrator before they enter said second entrainment separator.