Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B7/00—Halogens; Halogen acids
  • C01B7/19—Fluorine; Hydrogen fluoride
  • C01B7/191—Hydrogen fluoride
  • C01B7/195—Separation; Purification
  • C01B7/196—Separation; Purification by distillation
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B7/00—Halogens; Halogen acids
  • C01B7/19—Fluorine; Hydrogen fluoride
  • C01B7/191—Hydrogen fluoride

Definitions

• This invention relates to the recovery of anhydrous hydrogen fluoride from metallic fluoride salts which contain substantial amounts of phosphate values.
• metallic fluoride salts include fluoride containing material precipitated from a waste stream from a wetprocess phosphoric acid facility such as pond water. For example, sediment obtained from the floor in gypsum cooling ponds can contain up to 25% fluoride. Similarly, fluoride containing solids can be precipitated from gypsum cooling pond water or waste water by controlling neutralization with limestone and/or lime. These precipitates are contaminated with phosphate and metallic materials.
• hydrogen fluoride substantially free of phosphate values is recovered from metallic fluoride salts comprising metallic cations reactive with sulfuric acid, at least 7% by weight on a dry basis fluorine, and from about 4% to about 25% by weight on a dry basis P 2 O 5 .
• the process comprises the steps of introducing such metallic fluoride salts to a digestion zone, and introducing to the digestion zone water and/or steam, and sulfuric acid.
• the digestion zone is maintained at an elevated temperature sufficiently high to release a gas comprising hydrogen fluoride, water, and P 2 O 5 values from the metallic fluoride salts.
• a slurry comprising a residual liquid including introduced water, introduced sulfuric acid, and phosphoric acid generated by the action of sulfuric acid on metallic phosphate salts, and residual solids resulting from the metallic fluoride salts, is formed in the digestion zone.
• the slurry and the released gas are withdrawn from the digestion zone.
• Ci is from about 20 to about 70
• R is from about 1 to 15
• Cf is from about 25 to 90
• Ci amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone after introduction of the sulfuric acid and water to the digestion zone in excess of the amount of sulfuric acid required for reaction with reactive metal cations in the digestion zone, Ci being expressed in units of percent by weight in the liquid phase;
• R Ci/HFi, where HFi is the theoretical concentration of hydrogen fluoride in the liquid in the digestion zone after introduction of the sulfuric acid and water to the digestion zone in units of percent by weight in the liquid in the digestion zone; and
• Cf actual concentration of the sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid.
• the gas withdrawn from the digestion zone has a weight ratio of hydrogen fluoride to P 2 O 5 of at least 100:1.
• the concentration of water and sulfuric acid in the digestion, zone are maintained such that at least 60% of the fluorine in the metallic fluoride salts introduced to the digestion zone are in the released gas withdrawn from the digestion zone, the released gas comprises at least 10% by weight of hydrogen fluoride, and the slurry withdrawn from the digestion zone has a solids content sufficiently low that it is pumpable.
• the released gas is rectified.
• the rectification can occur in a rectification operation comprising at least two rectification zones in series, where in at least one rectification zone, the rectification occurs in the presence of sulfuric acid.
• the released gas can be introduced to a first rectification zone to produce a substantially azeotropic mixture of water and hydrogen fluoride.
• the azeotropic mixture is introduced to a second rectification zone to produce a vapor stream enriched with hydrogen fluoride
• sufficient sulfuric acid is introduced to the second rectification zone such that the vapor stream withdrawn from the second rectification zone comprises at least 80% HF by weight and has a hydrogen fluoride to P 2 O 5 weight ratio of greater than 10,000:1.
• This can be effected by adding sufficient sulfuric acid to the second rectification zone, to yield in combination with the azeotropic mixture introduced to the second rectification zone, a solution comprising from about 40 to about 90% by weight sulfuric acid on a hydrogen fluoride free basis.
• Steam can be introduced to the second rectification zone to enhance yield of hydrogen fluoride.
• FIG. 1 is a flow sheet showing a process embodying features of this invention
• FIG. 2 illustrates graphically the predicted relationship between process parameters and process variables in a digestion zone operated in accordance with principles of the present invention
• FIG. 3 presents graphically the relationship between the hydrogen fluoride to P 2 O 5 mass ratio and percent hydrogen fluoride in the vapor removed from a digestion zone operated in accordance with principles of this invention.
• FIG. 4 schematically shows test apparatus used to demonstrate this invention.
• metallic fluoride salts 10, water and/or steam 12, and sulfuric acid 14 are introduced to a digestion zone 16.
• the reaction between the sulfuric acid and the metallic fluoride salts releases a gas 1 8 containing hydrogen fluoride at a d ilute concentration and produces a slurry 20 consisting of a residual liquid which includes introduced water and introduced sulfuric acid and a solids residual resulting from the metallic fluoride salts, typically predominately calcium sulfate.
• the slurry 20 is passed to a separator 22 to separate the residual solids 24 from the residual liquids 26.
• the dilute hydrogen fluoride containing gas is concentrated to produce anhydrous hydrogen fluoride by rectification.
• the dilute fluoride containing vapors 18 are passed to a first rectification zone 28 from which liquid bottoms 32 and overhead vapors 30 comprising substantially water are withdrawn.
• the liquid bottoms comprise a substantially azeotropic mixture of water and hydrogen fluoride, and also contain phosphate values.
• the azeotropic mixture is then introduced into a second rectification zone 34 along with sufficient sulfuric acid 36 such that an overhead vapor stream 38 withdrawn from the second rectification zone 34 comprises at least 80% hydrogen fluoride by volume.
• steam is introduced from line 35 to the second rectification zone 34 to enhance hydrogen fluoride yield.
• Liquid bottoms 40 from the second rectification zone can be combined with the liquid 26 from the separator 22 for disposal. If necessary, the vapor stream 38 from the second rectification zone 34 can be further rectified in a third rectification zone 42 to produce anhydrous hydrogen fluoride.
• the first rectifier 28 can be bypassed as shown by dashed line 43 in FIG. 1.
• the metallic fluoride salts introduced to the digestion zone 16 comprise metallic cations reactive with sulfuric acid such as Ca ++ , Al +++ , Fe +++ , Mg ++ , and the like; at least about 7% by weight on a dry basis fluorine; and from about 4% to about 25% by weight on a dry basis P 2 O 5 . If the salts contain less than about 7% by weight fluorine, it can be uneconomical to process them. If the salts contain more than about 25% by weight phosphate values it is very difficult to effect an efficient separa tion of HP from the phosphate values.
• the phosphate values present in the metallic fluoride salts usually are not present as "P 2 O 5 ", but typically are present as PO 4 and complexed with fluorine as fluorophosphates.
• standard tests used for analysis of phosphate values yield results in terms of P 2 O 5 . Therefore, it should be realized, that when the P 2 O 5 content of a material is referred to in this specification and the accompanying claims, only an indirect measure of the phosphorus content of the material is being presented.
• the metallic fluoride salts introduced to the digestion zone can be wet or dry. Typically they are introduced to the digestion zone wet, or even as a concentrated slurry, because there is little to be gained by undergoing the expense to dry the solids because water is introduced to the digestion zone. This is an advantage over prior art processes for producing hydrogen fluoride from metallic fluoride salts where it is necessary that the feed be substantially dry because digestion is conducted in a low humidity atmosphere.
• the metallic fluoride salts introduced to the digestion zone can be solid waste obtained from cooling pond waters and/or fluoride containing sludges resulting from phosphate production operations.
• a preferred method for recovering metallic fluoride salts from phosphate operation waste water is described in U. S. Patent Application Serial No. 863,085, filed on December 22, 1977, entitled “Recovery of Calcium Fluoride from Phosphate Operation Waste Water", This application is incorporated herein by this reference.
• the sulfuric acid and water added to the digestion zone can be added separately, or together, as an aqueous solution of sulfuric acid. Steam can be added to the digester 16 as part of or instead of the water added to the digester.
• the steam is added to provide energy required for vaporization of water and hydrogen fluoride in the digester.
• a portion of the energy required for vaporization is provided by the exothermic reactions occurring in the digester.
• other heating means can be used such as circulating a portion of the liquid in the digester through a heat exchanger, steam jacketing of the digester, or electric heating elements around the digester.
• the digester is maintained at an elevated temperature at or near the boiling of the liquid in the digester.
• the sulfuric acid reacts with the metallic fluoride salts to yield hydrofluoric acid and the corresponding metal sulfate.
• metallic fluoride salts containing calcium fluoride the following reaction occurs:
• phosphoric acid is generated in the digester due to the action of sulfuric acid on metallic phosphate salts which are present in the digester.
• the distillate 18 withdrawn from the digester can be maintained substantially free of P 2 O 5 , i.e., the fluorine to P 2 O 5 mass ratio can be maintained greater than 100:1.
• the F to P 2 O 5 mass ratio is maintained greater than 1000:1 in the distillate. This is unlike prior arrangements where, when metallic fluoride salts containing phosphate values are digested in a low humidity atmosphere, liberation of substantial quantities of fluorophosphates results.
• the percent by weight of the fluorine in the metallic fluoride salts introduced to the digestion zone which are recovered in the distillate withdrawn from the digestion zone, Y is at least 60%, and more preferably at least 80%.
• HF v A high concentration of hydrogen fluoride in the distillate, HF v , is desired, because less energy is required to rectify the distillate to produce anhydrous hydrogen fluoride by vaporization of the water in the distillate. Therefore, it is preferred that HF v be greater than about 10% by weight, and more preferably greater than about 20% by weight.
• a mathematical model has been developed to relate the amount of water and sulfuric acid introduced to the digestion zone to US, Y, and HF v .
• This model was developed using equilibrium data regarding the system HF-H 2 SO 4 -H 2 O presented in the report Sulfuric Acid Extractive Distillation Process for the Recovery of Anhydrous Hydrofluoric Acid from By-Product Aqueous Hydrofluoric Acid, R. W. LeGassie and J. P. Termini, NYO-204A, 1954.
• the mathematical model was developed. According to this model, the quantity of water and sulfuric acid introduced to the digestion zone are selected to substantially satisfy the equations:
• HF v concentration of HF in the released gas withdrawn from the digestion zone in % by weight
• Y the percent by weight of the fluorine in the metallic fluoride salts introduced to the digestion zone which are recovered in the released gas withdrawn from the digestion zone, i.e., yield of HF
• W with respect to the metallic fluoride salts introduced to the digestion zone, the mass ratio of fluorine represented as HF to calcium oxide represented as CaSO 4 ;
• Ci amount of sulfuric acid and phosphoric acid theoretically present in the digestion zone after introduction of the sulfuric acid and water to the digestion zone in excess of the amount of sulfuric acid required for reaction with the reactive metallic cations in the digestion zone, where Ci is in units of percent by weight in the liquid phase;
• HFi theoretical concentration of hydrogen fluoride in the liquid in the digestion zone after introduction of the sulfuric acid and water to the digestion zone in units of percent by weight in the liquid in the digestion zone;
• Cf actual concentration of hydrogen fluoride and sulfuric acid and phosphoric acid in the residual liquid in units of percent by weight in the residual liquid.
• NC Substantially No Change
• the change in each of the process variables has a beneficial and adverse effect on the process parameters.
• increasing R decreases US and increases the HF to P 2 O 5 ratio, both of which are beneficial results, but has the adverse effect of decreasing HF v .
• decreasing Ci has the beneficial effects of increasing Y and the HF to P 2 O 5 ratio, but has the adverse effect of decreasing HF v . Therefore, in order to maintain US, Y, HF v , and the hydrogen fluoride to phosphate ratios in the desired ranges, it is necessary to maintain R, Ci and Cf in the ranges presented in Table 1.
• the ratio of HF to P 2 O 5 appears to depend on the concentration of HF in the distillate.
• FIG. 3 presents the experimentally determined correlation between HF and P 2 O 5 in the digester distillate. As shown in FIG. 3, the HF to P 2 O 5 ratio in the distillate increases as the hydrogen fluoride content of the distillate decreases. It is hypothesized that this correlation indicates that the distillate phopshate content is at least partially a result of entrainment.
• a typical distillate represents about a 3,000 fold refinement in the HF to P 2 O 5 ratio present in the digestion zone.
• the contents of the digester are maintained under agitation to assure that the reaction mass in the digester achieves equilibrium conditions.
• the digester can be operated under either batch conditions or continuously.
• the digestion is conducted at an absolute pressure ranging from about 90 mm Hg to atmospheric so that Cf can be maintained from about 25 to about 90. It has been found that reducing the pressure in the digester below atmospheric does not appear to affect either the yield from the process or the distillate composition. However, digester operation under vacuum decreases the operational temperature required, without affecting the equilibrium properties of the system.
• the slurry 20 withdrawn from the digester comprises a residual liquid and residual solids.
• the residual liquid includes introduced water, introduced sulfuric acid, phosphoric acid, and some hydrofluoric acid.
• the residual solids comprise non-reactive components of the feed material 10, and metallic sulfate salts such as calcium sulfate and magnesium sulfate.
• the solids are separated from the liquid in the separator 22 which can be conventional separation means such as drum filters, settling basins, and the like.
• the distillate 18 withdrawn from the digester 16 comprises phosphate values, from about 10 to up to 35% by volume hydrogen fluoride, with the remainder comprising substantially water.
• the hydrogen fluoride to phosphates mass ratio is greater than about 100:1 and is preferably greater than 1000:1.
• the distillate is subjected to rectification to produce anhydrous hydrogen fluoride having a hydrogen flouride to phosphate weight ratio of greater than 10,000:1.
• This can be effected by introducing the hydrogen fluoride containing distillate to the first rectifier 28, in which the distillate is concentrated to about 35 to about 39% by weight hydrogen fluoride at an absolute pressure of 90 mm Hg to atmospheric, with water and carbon dioxide being removed in the overhead vapor 30.
• the liquid bottoms 32 from the first rectifier are substantially an azeotropic mixture of water and hydrogen fluoride. If the distillate has a relatively high hydrogen fluoride concentration, generally greater than about 20% by weight, the first rectifier 28 can be bypassed and the distillate can be directly introduced to the second rectifier 34 via line 43.
• the azeotropic mixture is passed to the second rectifier 34.
• Sufficient sulfuric acid is introduced to the second rectifier so that the overhead 38 from the second rectifier has a hydrogen fluoride concentration of at least 80% by volume, and preferably is substantially anhydrous. This can be effected by introducing to the second rectifier sufficient sulfuric acid to yield a solution comprising from about 40 to about 90% by weight sulfuric acid on a hydrogen fluoride free basis.
• the liquid bottoms 40 from the second rectifier 34 contain sulfuric acid, phosphoric acid, and water.
• the bottoms can be combined with the liquid stream 26 from the separator 22 and passed to disposal.
• the overhead 38 from the second rectifier has a hydrogen fluoride to P 2 O 5 weight ratio of greater than 10,000:1. If it is necessary to remove additional water from the overhead 38, it can be passed to the third rectification zone 42.
• a batch digestion test was initiated by charging the Teflon flask with a weighted portion of the dry feed material followed by the desired amount of water. This mixture was agitated for about thirty minutes to ensure complete wetting of the solid.
• the desired amount of standarized sulfuric acid containing approximately 96% H 2 SO 4 , was added to the digester from a Teflon separatory funnel attached directly to the system to eliminate fluoride vapor losses.
• the temperature normally increased from room temperature to about 120 to 130°C.
• Heat was applied to the Teflon flask via a variable heating jacket. In the case of reduced pressure operation, the digester was air cooled to about 70°C after which the vacuum and heat were applied. The heating rate was carefully controlled to eliminate digester priming by the liberated CO 2 .
• the termination of the digestion period was estimated from a predetermined temperature -liquid composition curve. After a cooling period the mass of the digester contents was determined, and the contents of the digester were vacuum filtered. The sulfuric acid filtrate was recovered for analysis. The filter cake was washed repeatedly with water until a clear filtrate was obtained. The cake was subsequently oven dried at 105°C overnight and weighed.
• the fluoride contents of the feed solids, distillate, and bottoms residue were analyzed with a fluoride electrode using as a buffer medium TISAB, which is available from Orion Research Incorporated of Cambridge, Massachusetts.
• TISAB buffer medium
• the solids were initially fused with potassium carbonate followed by dissolution in water.
• the liquid samples were water diluted.
• One part of diluted sample (1 to 100 ppm F) was combined with nine parts TISAB prior to fluoride electrode analysis.
• the experimental fluoride yield was determined from the quantity of fluoride not evolved from the digester. This was determined from the masses and fluoride contents of the liquid and solid residue phases. The liquid mass was determined from the total residual mass less than that of the washed. and dried solids. Based on the initial system mass (less the volatile CO 2 ), the final mass and the respective fluoride contents, the quantity of the distillate and fluoride content of the distillate were calculated. The yield calculated in this manner agreed well with the yield based on the actual distillate collected. However, since it was difficult to avoid losses of distillate by hold-up and/or evaporation, the difference method was regarded as superior.
• Table IV are tabulated the experimental results obtained from these batch laboratory tests. Results from experiments not tabulated were rejected from consideration for a variety of reasons; for example, inefficient agitation, system priming or boil-over, and too high solids content in underflow.
• Columns 2 through 6 contain the primary variables which influenced the distillate composition. Yield and underflow solids content are shown in columns 11 through 16, respectively. The analyses of the residue components are indicated by columns 7 through 10, while the ratios of HF/P 2 O 5 and HF/SiO 2 in the distillate are tabulated in columns 17 and 18, respectively.
• Ci and R were calculated in accordance with the definition presented above. Exemplary of such calculation are the calculations conducted for Example 10. This calculation does not consider the amount of H 2 SO 4 which reacts with reactive metal cations in the feed material other than calcium and does not consider the sulfuric acid contributed by the SO 4 in the feed material. These amounts are negligible.
• Equations 2 through 5 assumes a total dissolution of the fluoride contained in feed material. Analysis of the dry, washed calcium sulfate residue showed HF contents ranging from 0.5 to 2.0% HF (Column 7). This is not surprising, for the feed material normally contains significant quantities of aluminum, iron, magnesium, etc., which all form insoluble fluoride species. The undissolved fluoride is likely due to acid insoluble aluminum and iron fluorides. This lack of fluoride dissolution is nearly compensated by the lower than expected fluoride concentration remaining in the underflow liquid phase. Columns 8 and 9 show the prediced .and observed concentrations. In every case the experimenta HF content of the liquid is several times less than that predicted.
• distillate composition is more susceptible to the undissolved fluoride since it is based strictly on the solubilized fluoride.
• the effects of reduced pressure on the digester operation are illustrated by Examples 14-17 and 20-23.
• a decrease in operational temperature appears to be the only significant difference. There is no evidence suggesting an increase or decrease in either distillate composition or yield. More important, however, is the fact that digester operation under vacuum decreases the opertional temperature by 50 to 90°C without affecting the equilibrium properties of the system.
• the observed final temperature in the batch tests corresponded very closely (within a few degrees) with the standard boiling point of a sulfuric acid solution of composition Cf. This was not the case for the reduced pressure tests where the observed temperature was generally lower than that predicted for an analogous sulfuric acid solution at the same pressure.
• Distillate ratios of HF/P 2 O 5 are tabulated in Column 17 of Table IV.
• Column 18 of Table IV shows the ratio of HF/SiO 2 in the distillate. The ratio is remarkably constant and represents about 40% transfer of silica in the feed to distillate. This silica is removed from the system in the final rectification zones. The dry, washed underflow solid typicaly analyzed 97.5% CaSO 4 , 0.02% AI 2 O 3 , 0.02% Fe 2 O 3 , 0.3% MgO and 0.7% F.
• An IR examination using a KBr disk medium showed no indication of hemihydrate or dihydrate calcium sulfate but only trace quantities of free water. Repeated equilibrium with water did not alter the crystalline structure of the calcium sulfate. This material can have commodity value based on its form and purity.
• Example 27 Example 27
• Example 27 An experiment similar to Example 27 was conducted except that the rectification was conducted at an atmospheric pressure of about 755 mm of Hg. Equilibrium boiling point was 111 to 112°C. The bottoms contained 36% HF, 3% SiO 2 , 100 ppm P 2 O 5 , with the balance water. The distillate was essentially water containing 4% HF on the average. Control 1
• a fluoride solution such as the azeotrope produced in Examples 27 and 28 is diluted with 98% H 2 SO 4 in the ratio 100 parts fluoride solution to 44 parts sulfuric acid on a weight basis. This produces a final mixture containing 25.0% HF, 29.9% H 2 SO 4 and the balance water and minor impurities. This solution is boiled at atmospheric pressure to 115°C to yield a vapor phase containing about 50% HF plus some silicon fluoride impurities. At this temperature, the sulfuric acid contains about fifty percent of the fluoride.
• Example 29 An experiment similar to Example 28 is conducted except that the HF-H 2 SO 4 -H 2 O mixture contains 4.4% HF, 86.0% H 2 SO 4 , plus water and silicon impurities. Heating the mixture to 204°C renders a vapor containing nearly 99% HF and silicon fluorides. About eighty percent of the fluoride is distilled from the sulfuric acid.
• Example 29 shows that addition of sufficient sulfuric acid to an azeotropic mixture to yield a mixture having a high concentration of H 2 SO 4 produces a vapor having a high HF concentration.
• Example 30 An experiment similar to Example 28 is conducted except that the HF-H 2 SO 4 -H 2 O mixture contains 4.4% HF, 86.0% H 2 SO 4 , plus water and silicon impurities. Heating the mixture to 204°C renders a vapor containing nearly 99% HF and silicon fluorides. About eighty percent of the fluoride is distilled from the sulfuric acid.
• the underflow solids 56 were essentially CaSO 4 containing minor levels of impurities.
• the weak acid condensate 54 was continuously fed via a pump 82 from the tank 80 to a 7.6 cm I.D. Kynar lined column 84 packed with 178 cm of Kynar mesh. The inlet was 76 cm from the packing bottom.
• a Karbate pipe wrapped with chromel wire was used as a thermosyphon reboiler 86.
• Azeotropic hydrofluoric acid 59 was removed from the column bottom by a gravity overflow and collected in an azeotrope tank 86. At equilibrium under good operating conditions, the reboiler temperature was maintained at 112 + 1°C; while water distillate 58 from an overhead condenser 88 contained less than about 500 ppm HF.
• a reflux ratio of about 0.5 to 1.0 was employed.
• the azeotropic hydrofluoric acid 59 typically at ambient temperature, passed by pump 90 from the azeotrope tank 86 and was mixed with concentrated sulfuric acid 60 in a 1 cm I.D. pipe 92 and was fed 61 cm from the bottom of a 244 cm Teflon mesh packed Teflon lined column 94 of 10 cm inside diameter.
• the feed mixture contained 74.3% H 2 SO 4 on a HF free basis and 35.0% HF on a H 2 SO 4 free basis.
• Live 116°C steam 61 was added at the bottom of the column as an energy supplement and to assist in deflourination.
• a wire wound Karbate thermosyphon served as a reboiler 96.
• Spent sulfuric acid 62 at 143 + 6°C was removed from the reboiler 96 by a gravity overflow system.
• Anhydrous hydrofluoric acid 97 exiting the column was condensed in an overhead monel condenser 98 at 4 to 10°C, and one part of product 63 to about nine parts reflux 99 was collected. The water content of the reflux was continuously measured by conductivity.
• the volatiles 64 from the monel condenser 98 were water scrubbed in scrubber 100.
• Typical analysis of the anhydrous HF product was 0.007% H 2 O, 0.0002% H 2 SiF 6 , 0.0005% P 2 O 5 , 0.007% nonvolatile as H 2 SO 4 (includes P 2 O 5 , and 0.04% SO 2 .
• the overall yield of fluoride from the wet solids to anhydrous hydrofluoric acid was typically 83% excluding recycle of fluori ' d values collected in vent scrubbers.
• Sediment collected from a phosphate operation cooling pond is water washed and dried at 105°C.
• the total active metal content (excluding sand SiO 2 ) is used to compute R and Ci rather than only CaO because of the significant contributions made by the silicon and aluminum in this material.
• the solution is boiled to a final temperature of 183°C while the distillate is condensed and collected.
• the weak acid condensate contains 12.3% HF, 5.6% SiO 2 , and the balance essentially water. Eight-eight percent of the fluoride present in the initial solids is collected as a weak acid condensate.
• a weak acid condensate at 27°C was made in a manner similar to that described in Example 30 but contained 22.0% HF, 0.89% SiO 2 , 0.30% P 2 O 5 , 0.17% SO 4 and the balance essentially water.
• One thousand three hundred and sixty-four grams per hour of this solution were added together with 3469 grams per hour of 98% H 2 SO 4 at 80°F into the sulfuric acid column of 116°C steam 61 were added at the bottom of the column 94 to aid in defluorination.
• the underflow contained 70.6% H 2 SO 4 , 0.01% F, 0.010% P 2 O 5 , 0.0098% SiO 2 and the balance essentially water. This stream was withdrawn at about 143°C and at about 4816 grams per hour.
• Example 33 The same weak acid used in Example 33 was fed together with 1.95 times its weight of 98% H 2 SO 4 to the column 94 under the same conditions except that no steam was added at the bottom of the column.
• the underflow at 140°C contained 69.0% H 2 SO 4 , 1.2% HF, other minor impurities, and water.
• the hydrogen fluoride condensate had essentially the same analysis as illustrated in Example 33.
• Comparison of Examples 33 and 34 shows that introduction of steam to the extractive distillation column enhances yield.

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