Recovery of hydrofluoric acid from fluosilicic acid with high pH hydrolysis

Hydrofluoric acid is recovered from fluosilicic acid by reacting fluosilicic acid with sodium hydroxide to form a first slurry having a pH of from about 11 to about 14, the first slurry containing sodium metasilicate and precipitated sodium fluoride. Sodium fluoride is recovered from the first slurry leaving a first solution which is reacted with sodium fluosilicate or fluosilicic acid or both to form a second slurry comprising silica and dissolved sodium fluoride. The reaction occurs under such conditions that the second slurry contains precipitated amorphous silica. The precipitated amorphous silica is separated from the second slurry leaving a second solution of sodium fluoride. Sodium fluoride is recovered from the second solution. Recovered sodium fluoride is reacted with sulfuric acid to produce hydrogen fluoride.

CROSS-REFERENCES 
This application is related to the following co-assigned and co-pending 
U.S. patent applications: 
(1) Application Ser. No. 954,066 filed on Oct. 23, 1978 by Subhas K. 
Sikdar, entitled "Recovery of Hydrofluoric Acid from Fluosilicic Acid with 
High pH Hydrolysis"; 
(2) Patent application Ser. No. 953,801 filed on Oct. 23, 1978 by Subhas K. 
Sikdar and James H. Moore, entitled "Recovery of Hydrofluoric Acid from 
Fluosilicic Acid"; and 
(3) Application Ser. No. 953,803 filed on Oct. 23, 1978 by Subhas K. 
Sikdar entitled, "Recovery of Hydrofluoric Acid from Fluosilicic Acid". 
BACKGROUND 
Beneficiated phosphate rock from Florida typically contains 3 to 4% 
fluorine values. Phosphoric acid can be produced from this rock by 
treating it with sulfuric acid. Part of the fluorine present in the rock 
is evolved as silicon tetrafluoride and gaseous hydrofluoric acid, which 
upon scrubbing with pond water, form a dilute fluosilicic acid. By 
efficient design of scrubbers, it is possible to continuously produce a 
solution containing about 20% fluosilicic acid. The acid is usually 
contaminated with impurities, including 1,000 to 4,000 parts per million 
of P.sub.2 O.sub.5. 
It is desirable to recover the fluorine values present in the fluosilicic 
acid as anhydrous hydrofluoric acid. This is because hydrofluoric acid can 
be an important source of revenue. In addition, the presence of fluorine 
in the pond water can present an environmental pollution problem. 
Many processes have been developed for concentration of dilute fluosilicic 
acid solutions, and preparation of hydrofluoric acid from the concentrated 
fluosilicic acid. Such attempts are described in U.S. Pat. Nos. 3,645,678; 
3,645,679; 3,689,216; 3,855,399; 3,278,265; 3,218,124; 3,256,061; 
3,140,152; 3,914,398; 3,537,817; 3,758,674; German Offen. Nos. 2,035,300, 
2,032,855, and 2,248,149; and French Pat. No. 7,034,470. However, these 
processes suffer from one or more disadvantages. Disadvantages of these 
processes include operation at excessively high temperatures or under 
severe conditions, use of an excessive number of processing steps, 
consumption of uneconomical quantities of raw materials, production of 
undesirable byproducts, production of contaminated hydrogen fluoride, low 
yield of hydrogen fluoride, and considerable expenditure, both in terms of 
operating expense and initial capital investment. For example, U.S. Pat. 
Nos. 3,218,124 and 3,689,216 describe a process where fluosilicic acid 
solutions are treated with concentrated sulfuric acid to liberate silicon 
tetrafluoride and hydrogen fluoride, which are then separated. The silicon 
tetrafluoride is hydrolyzed to fluosilicic acid which is recycled and 
SiO.sub.2 which is removed. Two disadvantages of this process are that a 
large volume of concentrated sulfuric acid is required per unit of 
fluosilicic acid and the splitting of the fluosilicic acid must be carried 
out at relatively high temperatures. This can result in severe corrosion 
of equipment. 
U.S. Pat. No. 3,256,061 describes a process whereby fluosilicic acid is 
neutralized with ammonia, producing ammonium fluoride and silica. The 
silica is separated by filtration, and the ammonium fluoride is 
concentrated to a molten state constituting NH.sub.4 F--NH.sub.4 HF.sub.2, 
which when treated with concentrated sulfuric acid produces hydrogen 
fluoride. Ammonia remains in the sulfuric acid and is sent to a phosphate 
acidulation unit. The chief drawbacks of this process are the requirement 
to recycle ammonia and the failure to remove any P.sub.2 O.sub.5 impurity 
in the fluosilicic acid. A similar process is described in U.S. Pat. Nos. 
3,914,398 and 3,537,817. 
Therefore, there is a need for a simple, high yield process for recovering 
high purity hydrogen fluoride from phosphoric acid plant process streams. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method with the above features for 
recovery of hydrogen fluoride from aqueous solutions of fluosilicic acid 
such as phosphate plant process streams. According to this process, an 
aqueous solution of fluosilicic acid is combined with sodium hydroxide for 
forming a first alkaline aqueous slurry comprising dissolved silica, 
precipitated sodium fluoride, and dissolved sodium fluoride. The pH of 
this first alkaline aqueous slurry is maintained at a value of from about 
11 to about 14 so the first slurry contains precipitated sodium fluoride 
and substantially only dissolved silica. A first crop of precipitated 
sodium fluoride is recovered from the first slurry, leaving a first 
aqueous solution comprising silica and sodium fluoride. 
The first solution is combined with sodium fluosilicate or fluosilicic acid 
or both for forming a second alkaline aqueous slurry comprising silica and 
dissolved sodium fluoride. The sodium fluosilicate can include sodium 
fluosilicate formed in the purification zone and the fluosilicic acid can 
be a phosphate plant process stream. To form precipitated amorphous silica 
in the precipitation zone, the pH of the second slurry is maintained at a 
value greater than 7 and up to about 9, the second slurry is maintained at 
least saturated with sodium fluoride, and the temperature of the second 
slurry is maintained substantially equal to its boiling point. 
If the second slurry contains precipitated sodium fluoride, the sodium 
fluoride is dissolved by adding water to the second slurry. Then the 
precipitated amorphous silica is filtered from the second slurry, leaving 
a second aqueous solution which comprises a sodium fluoride. Sodium 
fluoride can be recovered from the second solution by evaporating water 
from the second solution, thereby precipitating sodium fluoride, and 
separating the precipitated sodium fluoride from the remaining solution. 
Recovered sodium fluoride can be reacted with sulfuric acid to generate 
hydrogen fluoride.

DESCRIPTION 
With reference to FIG. 1, in a process according to the present invention, 
fluosilicic acid feed 10 and sodium hydroxide 24 are combined in a first 
precipitator 23. The reaction which occurs is: 
EQU Na.sub.2 SiF.sub.6 +6NaOH.fwdarw.6NaF.dwnarw.+Na.sub.2 SiO.sub.3 +3H.sub.2 
O (1) 
The fluosilicic acid feed can be a phosphoric acid plant process stream 
such as scrubber liquor. For this process to be economical, the scrubber 
liquor should contain at least 15% by weight fluosilicic acid, and 
preferably it contains at least 20% by weight fluosilicic acid. Such 
scrubber liquor can contain from 1,000 to 4,000 parts per million by 
weight P.sub.2 O.sub.5 and other impurities. 
Preferably the fluosilicic acid feed has few impurities because impurities 
can contaminate the silica and sodium fluoride process. Unlike the process 
described in the aforementioned application by Sikdar entitled "Production 
of Hydrofluoric Acid from Fluosilicic Acid", the process described herein 
does not have a purification step where the feed fluosilicic acid is 
treated with sodium sulfate. 
It is important that the conditions in the first precipitator be maintained 
such that sodium fluoride is precipitated and silica remains in solution, 
so that the sodium fluoride can be separated from the first alkaline 
aqueous slurry. This is effected by maintaining the pH of the first slurry 
at a value of from about 11 to about 14, and preferably from about 13 to 
about 14. At pH values less than about 11, a substantial portion of the 
sodium fluoride can be left in solution and some silica may precipitate. 
The temperature in the first precipitator can be from about 20.degree. to 
about 80.degree. C., and the pressure can be maintained at about ambient. 
At elevated temperatures or reduced pressures in the first precipitator 
23, water vapor 25 can be given off. The water vapor is condensed in a 
condenser 26 with total reflux via line 27. 
The first slurry is passed via line 28 to a first separation zone 29 in 
which precipitated sodium fluoride 30 is recovered, leaving an aqueous 
solution of sodium metasilicate 31. 
The first separator, and all other separators described in this 
application, can be any suitable separation device such as a gravity 
sedimentation unit, a filtration unit, or a centrifuge. For example, the 
first separator 18 can be a cake filter, a pressure relief filter, or a 
vacuum drum filter. 
The sodium metasilicate solution 31 is transferred to a second precipitator 
32, which is agitated, in which it is combined with fluosilicic acid 
solution, sodium fluosilicate, or both 70. The fluosilicic acid preferably 
comprises at least a portion of the fluosilicic acid feed and can include 
a phosphoric acid plant process stream such as scrubber liquor. 
Preferably, the fluosilicic acid added to the second precipitator contains 
at least 15% by weight fluosilicic acid, and more preferably at least 20% 
by weight fluosilicic acid. The sodium fluosilicate added is preferably 
sodium fluosilicate separated in the first separator 29. 
Exemplary of the reactions which occur in the second precipitation zone 
are: 
EQU 3Na.sub.2 SiO.sub.3 +H.sub.2 SiF.sub.6 .fwdarw.6NaF+4SiO.sub.2 
.dwnarw.+H.sub.2 O (2) 
and 
EQU 2Na.sub.2 SiO.sub.3 +Na.sub.2 SiF.sub.6 .fwdarw.6NaF+3SiO.sub.2 .dwnarw.(3) 
It is important that the conditions in the second precipitator or reactor 
be maintained such that amorphous silica is formed in the second 
precipitator, so that the silica can be separated from the sodium 
fluoride. If a silica gel or a silica sol is formed in the second 
precipitator 32, then it is extremely difficult, if not impossible, to 
separate the silica from the sodium fluoride. Amorphous silica is formed 
in the second precipitator by: (1) maintaining the pH of the second 
alkaline aqueous slurry at a value greater than 7 and up to about 9, (2) 
controlling the water content of the second alkaline aqueous slurry 
sufficiently low such that the second slurry is at least saturated with 
sodium fluoride, and (3) maintaining the temperature of the second 
alkaline aqueous slurry substantially equal to its boiling point. All 
three of these requirements must be satisfied, or else amorphous silica is 
not produced. The pH of the second alkaline aqueous slurry is maintained 
between 7 and 9 by controlling the amount of fluosilicic acid added to the 
second precipitator. 
As used herein, by the term "amorphous silica" there is meant silica not 
having a characteristic x-ray diffraction pattern. 
For example, if the pH in the second precipitator is less than 7, colloidal 
silica is formed, which cannot be filtered easily from sodium fluoride 
solution. If the pH of the second alkaline aqueous slurry in the second 
precipitator 32 is greater than or equal to 9 or less than 7, a high 
percentage of the formed silica is in solution, and thus cannot be 
separated from the sodium fluoride. Furthermore, when the pH of the slurry 
is less than 7, the precipitated silica is in the form of a 
difficult-to-filter gel. 
Because the second slurry is maintained near or at its boiling point, water 
vapor is evolved. The water vapor as well as any other gases evolved are 
withdrawn from the second precipitator via line 33. The water vapor and 
other gases pass to a condenser 34 in which at least a portion of the 
water is condensed, and refluxed via line 35 to the second precipitator. 
Non-condensed vapors are withdrawn via line 36, and excess water, if any, 
is withdrawn from the system via line 37. 
The slurry in the second precipitator is maintained under agitation to 
insure intimate mixing between the reactants, and to permit the 
precipitated silica to be withdrawn from the vessel as a portion of the 
slurry in line 39. The slurry in the first precipitator can also be 
maintained under agitation. 
The sodium fluosilicate and fluosilicic acid preferably are added to the 
second precipitator in an amount about equal to stoichiometric. The sodium 
fluosilicate can be added to the second precipitator as a solid, or 
combined with water. 
The residence time in the second precipitator is that amount which is 
sufficient for the reaction between the sodium metasilicate and sodium 
fluosilicate and fluosilicic acid to go to substantial completion. 
Although this method has been described in terms of recovering silica from 
sodium fluosilicate, it is also useful for recovering silica from 
potassium fluosilicate or ammonium fluosilicate with an 
ammonium-containing compound. In general, the first precipitator can be 
used for reacting fluosilicic acid with MOH, where M is selected from the 
group consisting of sodium, potassium, and ammonium, thereby forming a 
first alkaline aqueous slurry comprising M.sub.2 SiO.sub.3 and the 
precipitated fluoride salt, MF. The pH of the first alkaline aqueous 
slurry is maintained at a value between about 11 and 14. Precipitated MF 
is recovered from the first slurry leaving an aqueous solution of M.sub.2 
SiO.sub.3, which is reacted with M.sub.2 SiF.sub.6 or fluosilicic acid or 
both in a second precipitator to form a second alkaline aqueous slurry 
comprising silica and the fluoride salt, MF. The pH of the second alkaline 
aqueous slurry is maintained at a value greater than 7 and up to about 9; 
the water content of the second alkaline aqueous slurry is maintained 
sufficiently low such that the slurry is at least saturated with the 
fluoride salt MF; and the temperature of the second alkaline aqueous 
slurry is maintained substantially equal to its boiling point, so that the 
second slurry contains precipitated amorphous silica. Although the 
following steps of a process according to this invention are described in 
terms of separating silica from a slurry containing sodium fluoride, the 
same steps can be used for separating amorphous silica from a slurry 
containing potassium fluoride or ammonium fluoride. 
The slurry 39 withdrawn from the second precipitator contains precipitated 
amorphous silica and an aqueous solution of sodium fluoride. Generally, 
the slurry 39 also contains precipitated sodium fluoride. This is because 
if the second precipitator does not contain a saturated solution of sodium 
fluoride, amorphous silica is not formed in the second precipitator. 
Therefore, to be sure to avoid formation of non-amorphous silica, the 
second precipitator is conservatively operated so sodium fluoride 
precipitates. 
If the second slurry contains precipitated sodium fluoride, it is desirable 
to dissolve this sodium fluoride to avoid contamination of the silica with 
sodium fluoride and to avoid low yields of hydrofluoric acid. Therefore, 
before separating the precipitated silica from the slurry 39, the slurry 
39 is introduced to an agitated dissolver 40, where it is combined with 
water 42, which is preferably heated. Sufficient water is added to the 
dissolver to dissolve substantially all of the sodium fluoride. Slurry 44 
containing precipitated amorphous silica and a solution of sodium fluoride 
is withdrawn from the dissolver 40 and is introduced to a second separator 
46 from which the amorphous silica 48 is withdrawn as a product. A 
solution of sodium fluoride 50 is withdrawn from the second separator and 
is passed to an evaporator 52. 
The amorphous silica produced by the process is described in the 
above-mentioned application Ser. No. 953,803, Oct. 23, 1978, filed by 
Sikdar and entitled "Recovery of Hydrofluoric Acid from Fluorsilicic Acid" 
is of high purity, generally containing less than 2% by weight fluorine. 
The silica has a surface area of 37 meters squared per gram, a density of 
from 1.59 to 1.82 grams per cubic centimeter, a pore volume of 0.23, and a 
loss on ignition of from 6.7 to 7.9% by weight. The weight average 
particle size has been found to be from about 15 microns to about 24 
microns. Therefore, the silica can be easily separated from the sodium 
fluoride solution by passing the slurry 44 through a filter, which should 
have an average pore size of less than about 15 microns. It is expected 
that silica produced by the process described herein will have the same 
properties. 
In the evaporator 52, water is removed from the sodium fluoride solution 50 
to precipitate the sodium fluoride. This can be done under vacuum. 
Preferably, the removed water 54 is recycled to the dissolver 40 to 
provide the bulk of the water introduced to the dissolver. The remainder 
of the water added to the dissolver is provided by make-up water 56. The 
sodium fluoride slurry 58 is passed from the evaporator 52 to a third 
separator 60, where sodium fluoride solid 62 is recovered. Sodium fluoride 
solution 64 recovered from the third separator 60 can be recycled to the 
second precipitator 32 to recover the fluorine values contained therein. 
Recovered sodium fluoride can be sold as a product, or can be used to 
produce hydrogen fluoride in a hydrogen fluoride generator 64. In the 
generator 64, sodium fluoride recovered in the second separator and/or 
sodium fluoride recovered in the third separator are combined with a 
stoichiometric amount of sulfuric acid 65 to produce hydrogen fluoride 66 
and sodium sulfate 68. 
It is important to avoid the presence of air in the generator to minimuze 
corrosion. The sodium sulfate 68 can be recovered as product. 
The contents of the generator 64 preferably are a paste or thick slurry for 
high yield. 
The sulfuric acid 65 added to the generator 64 can be 80% to 100% sulfuric 
acid. It has been noted that the concentration of the sulfuric acid used 
in this range has little, if any, effect on hydrogen fluoride yield. 
Preferably a stoichiometric amount of sulfuric acid is used in the 
generator because it has been found that an excess of sulfuric acid 
reduces yield. 
The higher the temperature in the generator, the higher the yield of 
hydrogen fluoride. However, as the temperature increases, problems with 
corrosion also increase. The temperature in the generator is maintained in 
the range of from 80.degree. C. to 300.degree. C., and preferably at about 
200.degree. C., for high yield with minimal corrosion. 
It is preferred that the hydrogen fluoride generation reactor 64 be 
operated as a batch reactor to minimize corrosion problems, and to avoid 
leakage of sulfuric acid and/or hydrofluoric acid to the environment. 
The method of this invention has many significant advantages compared to 
other processes available. For example, unlike most other processes 
described in the literature, this process does not depend upon the 
hydrolysis of gaseous silicon tetrafluoride. Therefore, no corrosive gases 
are handled until the hydrogen fluoride generation step. Thus, corrosion 
problems are minimized. 
A further advantage of the present invention is that corrosion of equipment 
is minimized because all of the steps, except for the generation hydrogen 
fluoride, are carried out at relatively low temperatures. 
A further advantage of the process is the last step, where sodium sulfate 
is produced as a by-product. Thermodynamic analysis indicates that the 
reaction between sodium fluoride and sulfuric acid is highly favorable and 
is less endothermic than the corresponding parallel reaction between 
calcium fluoride and sulfuric acid. 
A further advantage of the process is that the precipitated amorphous 
silica is usable as a by-product. 
These and other advantages of the present invention will become more 
apparent with respect to the following examples and controls: 
CONTROL 1 
(Production of silica sol or gel) 
This test shows the importance of controlling pH and controlling reaction 
temperature when reacting sodium fluosilicate with sodium metasilicate to 
produce amorphous silica. 
A series of batch hydrolysis tests are conducted for the reaction between 
sodium fluosilicate and sodium metasilicate. Enough water is initially 
present to dissolve all of the sodium fluoride produced by the reaction so 
that silica formed can be filtered and the sodium fluoride recovered from 
the filtrate. The reactants are added to an agitated vessel in 
installments and the pH is controlled at various values between 7 and 11. 
The reaction chamber is immersed in a constant temperature bath maintained 
at temperatures between 50.degree. C. and 75.degree. C. 
Irrespective of the pH or the temperature maintained in the reaction 
vessel, the precipitate cannot be filtered in a reasonable length of time. 
A silica sol or gel is obtained below a pH of approximately 10. The sol 
goes through filter cloth or paper and the gel plugs the filter pores in a 
matter of seconds. Above a pH of 10, sometimes no precipitate at all 
forms. 
CONTROL 2 
(Post-Precipitation) 
These tests demonstrate the importance of maintaining the pH of the slurry 
in the second precipitator at a value less than 9 when reacting sodium 
metasilicate with sodium fluosilicate. 
Three tests are conducted with stoichiometric ratios of sodium fluosilicate 
and sodium metasilicate. The sodium metasilicate is added in 25%, 20%, and 
15% solutions. The reaction vessel is supplied with a reflux condenser. 
After reaction for two to three hours, water is added to the vessel to 
dissolve any precipitated sodium fluoride. The slurry in the vessel is 
then filtered, with excellent filtration occuring. The precipitate formed 
is amorphous silica. However, the filtrate, when standing overnight, 
becomes milky and a gel settles at the bottom. The pH of the mother liquor 
is around 9 or 10. Thus, at these pH's, the mother liquor becomes 
supersaturated with amorphous silica, and on standing becomes unstable, 
thus leading to the post precipitation. 
EXAMPLE 1 
(Evaporation of water from sodium fluoride solution) 
One thousand grams of saturated sodium fluoride solution are introduced to 
an evaporator. Five hundred grams of water are evaporated from the 
solution, thereby producing 27.5 grams of solid sodium fluoride. 
EXAMPLE 2 
(Evaporation of water from sodium fluoride solution) 
Five hundred cubic centimeters of sodium fluoride solution were introduced 
to an evaporator maintained at a temperature of 100.degree. C. 285 cc of 
water vapor were removed, producing 7.7 grams of solid sodium fluoride and 
215 cc of liquor saturated with sodium fluoride. 
EXAMPLE 3 
(HF generation) 
These tests were conducted to show the feasability of producing hydrogen 
fluoride from sodium fluoride. FIG. 2 shows a schematic of the 
experimental equipment used. Forty-two grams of sodium fluoride were 
pre-heated to a desired temperature in a Monel reactor 102. Nitrogen gas 
was passed from storage tank 104 through a flowmeter 106 into the reactor 
102 to drive out any oxygen present to avoid corrosion problems. The 
reactor was in a sand bath 108 to maintain the desired temperature in the 
reactor. After purging the nitrogen, sulfuric acid was introduced quickly 
into the reactor 102 from a buret 110. The reaction started immediately as 
evidenced by a large (sometimes 40.degree. C.) drop in temperature. The 
product gas was led via line 112 to a water-cooled Monel condenser 114 
into two serially connected caustic soda traps 116 and 118 kept in ice 
water baths 120. The first trap 116 contained a 20% solution of sodium 
hydroxide and the second trap 118 contained a 10% solution of sodium 
hydroxide. In the traps, the hydrogen fluoride reacted with the sodium 
hydroxide to produce sodium fluoride, which precipitated. About 5 minutes 
after the sulfuric acid addition, the nitrogen gas flow was resumed. The 
nitrogen gas assisted in carrying HF from the reactor. The sodium fluoride 
formed in the traps was filtered, washed with water and reagent alcohol, 
dried and weighed. At the completion of the reaction period, the contents 
of the reactor 102 were dissolved in water and analyses were conducted for 
sulfate and fluoride ions. 
Eight tests were conducted. The reaction temperature, time of reaction, 
concentration of sulfuric acid, percent excess of sulfuric acid, flow rate 
of nitrogen, and yield for each test is presented in Table 1. 
The results of the test were analyzed according to the Yates' method. Based 
on this analysis, the following conclusions were reached: 
1. Flow of nitrogen did not have any influence on yield. 
2. Time was not significant with regard to yield. This indicates that no 
further reaction takes place after the first hour of reaction since yield 
obtained was always less than 100%. This also indicates that diffusion 
limits the reaction. Therefore, some form of mixing is necessary to 
increase yield. 
3. The strength of the sulfuric acid appears to have no influence on yield, 
i.e. 80% sulfuric acid seems to produce the same yield as 96.5% sulfuric 
acid. 
4. Excess sulfuric acid appears to decrease yield. 
5. Temperature is by far the most important variable affecting yield, with 
higher temperatures increasing yield. 
TABLE 1 
______________________________________ 
H.sub.2 SO.sub.4 
N.sub.2 Rate 
Reaction Reaction Concen- 
% (Stand- 
Yield 
Temp. Time tration 
Excess 
ard (% by 
Test (.degree. C.) 
(Hr.) (% wt.) 
H.sub.2 SO.sub.4 
cc/min. 
Weight) 
______________________________________ 
3 A 150 1 80 0 100 23.7 
3 B 200 1 80 2 100 32.9 
3 C 150 2 80 2 300 18.9 
3 D 200 2 80 0 300 49.9 
3 E 150 1 95 2 300 24.2 
3 F 200 1 95 0 300 41.1 
3 G 150 2 95 0 100 33.6 
3 H 200 2 95 2 100 40.2 
______________________________________ 
EXAMPLE 4 
(HF generation) 
In one set of two experiments either of 80% or 96.5% H.sub.2 SO.sub.4 at 
100% excess was used. The reaction mixture was prepared at 0.degree. C. in 
an ice bath and then the reactor was assembled. The heating then was 
started and nitrogen flow was turned on. A final temperature of about 
160.degree. C. was obtained after about five (5) hours. The yield with 80% 
H.sub.2 SO.sub.4 was 83.3% while that with 96.5% was 78%. 
EXAMPLE 5 
(HF generation) 
In a variation of Example 4, a condensor was mounted vertically on the 
reactor to act as a reflux condenser. Thus only HF was allowed to leave 
the reactor. Severe corrosion resulted. Nevertheless the yield was 
consistently around 80%. 
EXAMPLE 6 
(HF generation) 
An attempt was made to reach stoichiometric yield. A platinum crucible was 
used to effect the reaction. The yield was calculated from F analysis of 
the solution made by dissolving the solid residue in a definite quantity 
of water. Two experiments were carried out. The yields were 95.6 and 
97.4%. 
Based on the tests with generation of hydrogen fluoride from sulfuric acid 
and sodium fluoride, it was concluded that corrosion can limit the extent 
of reaction and mixing of the reactants can improve yield. 
Although this process has been described in considerable detail with 
reference to certain versions thereof, other versions are possible. 
Therefore, the spirit and scope of the appended claims should not be 
limited to the description of the preferred versions contained herein.