Process for producing hydrogen fluoride

A method of recovering fluorine, e.g. as hydrogen fluoride, from wastes of aluminum electrolysis furnaces, chemisorbents, adsorbents or absorbents, etc. in which the fluorine-containing material is subjected to pyrohydrolysis in an expanded fluid bed and the HF-containing gas is subjected to condensation or scrubbing for the removal of the HF therefrom. According to the invention, the exhaust gases from the fluidized bed are cooled by direct contact with solids which can be circulated in a separate cycle and are themselves cooled in a cooler, e.g. by contact with gas which is to be fed to the expanded fluidized bed. The circulated solids thus allow recovery of the sensible heat of the gas without diluting it.

FIELD OF THE INVENTION 
Our present invention relates to the production of hydrogen fluoride and to 
the recovery of fluorine in the form of hydrogen fluoride from 
fluorine-containing mineral and like materials, especially waste materials 
from electrolytic aluminum furnaces and materials which have been used to 
absorb or adsorb fluorine compounds from gases as part of a gas treatment. 
More particularly, the invention relates to a method of producing HF which 
eliminates dilution of the gas from which the HF is to be ultimately 
recovered. 
BACKGROUND OF THE INVENTION 
The problem of recovery of fluorine from mineral or other wastes of 
metallurgical processes, e.g. electrolytic refining of aluminum, is one 
which arises because of economic considerations and environmental or 
social considerations and hence has been the subject of considerable 
investigation. Fluorine-containing compounds are present in waste 
materials from such furnaces and these materials can include walls, 
linings and the like. Such materials also arise, as described below, from 
the removal of fluorine-containing compounds from waste gases of 
metallurgical and other processes utilizing absorption and adsorption 
techniques. 
It is known that fluorine-containing compounds release hydrogen fluoride in 
the presence of water vapor at elevated temperature (pyrohydrolysis) and 
that the released hydrogen fluoride can be recovered in a higher 
concentration by a condensing or scrubbing step. 
The pyrohydrolytic release of hydrogen fluoride has become significant 
particularly in the processing of waste materials which become available 
at various stages in the electrolytic production of aluminum. For 
instance, in the production of aluminum by fused-salt electrolysis, in 
which cryolite or similar fluorine-containing fluxes are usually employed, 
fluorine-containing constituents enter the lining of the electrolytic 
cell. This lining must be renewed from time to time and the old lining 
material which has been broken out is then available and may contain 10 to 
15% by weight of fluorine, depending on the mode of operation of the cell 
and on the time for which the lining has been in operation. 
Another fluorine-containing material will be obtained when hydrogen 
fluoride is removed from exhaust gases from fused-salt electrolysis by dry 
scrubbing. Where alumina is used as a sorbent, a chemisorption agent will 
become available, which is laden with hydrogen fluoride in dependence on 
the scrubbing conditions and must be processed because it cannot be fed to 
the fused-salt electrolysis as it contains other impurities formerly 
contained in the exhaust gases, such as carbon, sulfur, iron, silicon, 
phosphorus and/or vanadium. 
It has been found that such waste materials can be processed by 
pyrohydrolysis (Opened German Specification Nos. 2,346,537 and 2,403,282), 
which may be combined with the recovery of additional valuable substances, 
such as aluminum or alkali metal (U.S. Pat. No. 4,113,832). In the 
last-mentioned process, the pyrohydrolytic treatment is carried out within 
a temperature range of about 1110.degree. to 1350.degree. C., e.g. in an 
expanded fluidized bed, in the presence of adequate quantities of water 
vapor. Alkali fluoride and hydrogen fluoride are removed from the exhaust 
gas. The solid residue from the pyrohydrolytic treatment is leached with 
an alkaline solution, and hydrated alumina is formed. Before the alkali 
fluoride and the hydrogen fluoride are removed from the gas, the latter is 
cooled by being sprayed with water or by being mixed with cold gas or by 
an indirect cooling. 
A disadvantage of this process is that the sensible heat of the exhaust gas 
is wasted and the gas rate is considerably increased when the exhaust gas 
is cooled by a spraying of water. Similar remarks apply to the cooling by 
an admixing of cold gas, which involves particularly an undesired dilution 
of the gas, which apart from this contains only a small percentage of 
hydrogen fluoride. Whereas these disadvantages can be avoided by indirect 
cooling, this can be controlled only with difficulty because problems of 
corrosion and erosion arise and because the deposition of dust on the 
cooling surfaces decreases the coefficient of heat transfer so that a high 
structural expenditure is involved (for cleaning) and/or large exchange 
surfaces are required. 
OBJECTS OF THE INVENTION 
It is, therefore, the principal object of the present invention to provide 
a process for producing hydrogen fluoride in which the disadvantages of 
earlier techniques are avoided and the energy utilization is maximized. 
Another object of the invention is to provide an improved method of 
recovering fluorine-containing materials at low cost and with little 
likelihood of corrosion damage or erosion damage to the recovery 
apparatus. 
Still another object of our invention is to provide an improved method of 
operating a plant for production of hydrogen fluoride or for the recovery 
of fluorine as hydrogen fluoride from fluorine-containing material. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained in accordance with the present invention in a method of 
recovering fluorine as hydrogen fluoride from fluorine-containing 
materials and materials which contain fluorine compounds by subjecting 
these materials to pyrohydrolysis at a temperature in a range of 
1000.degree. to 1400.degree. C. in an expanded fluidized bed, thereby 
producing a hot exhaust gas containing HF and with which particles of the 
bed material are entrained out of the pyrohydrolysis reactor, separating 
the hot gas containing the HF from the entrained solids, cooling the hot 
gas after the separation or during the separation by direct contact of the 
gas with particles of solids at a lower temperature and cooling the 
particles of solids thus heated by circulating same in a cooling cycle 
which includes a cooler. Thus, instead of diluting the HF-containing gas 
for cooling or using indirect heat exchange which raises a corrosion 
problem, direct cooling with circulated solids is used in accordance with 
the invention. 
Thus the objects of the invention are attained in that the exhaust gases 
are cooled by a direct contact with solids which are circulated in a 
separate cycle and which are recooled in a cooler with utilization of the 
sensible heat. 
The process according to the invention can be used to process waste 
materials which become available in the electrolytic production of 
aluminum. Fluorspar (CaF.sub.2) or other fluorine-containing inorganic 
materials from which hydrogen fluoride can be released by pyrohydrolysis 
can be treated too. 
Depending on the nature of the feedstock, a fuel must be used for heating 
to the required reaction temperatures, which lie between about 
1000.degree. and 1400.degree. C., as usual. Liquid, gaseous but also solid 
fuels may be used for this purpose and are directly introduced into the 
expanded fluidized bed. If the feedstock has a sufficiently high carbon 
content, as will usually be the case with broken-out lining material, 
there will be no need for separate addition of fuel. 
The most suitable and most simple method of cooling the exhaust gas 
comprises contacting the exhaust gas with recooled solid in at least one 
suspension-type heat exchanger. 
The solids are preferably recooled in a fluidized bed cooler, which may 
comprise a plurality of stages. Its mode of operation and design will 
mainly depend on the nature of the feedstock. If the same has a fuel 
content which is so high that the temperature conditions required in the 
expanded fluidized bed can be achieved, the solid can be recooled with 
utilization of the sensible heat, e.g. with generation of water vapor. The 
resulting hot exhaust gas is desirably recycled into the fluidized bed 
reactor. If it is essential to add fuel separately or if the 
pyrohydrolitic process is just self-sustaining, then it will be desirable 
to recool the solids in a fluidized bed cooler having a plurality of 
cooling chambers, which are traversed in succession and in which 
oxygen-containing gas is heated and subsequently fed to the fluidized 
reactor, particularly as a fluidizing gas. 
In both embodiments, the fluidized bed cooler may be succeeded by a cooling 
chamber, which is supplied with cold water and in which additional heat is 
extracted from the solids. That cooling chamber may form a structural unit 
with the fluidized bed cooler or may be separate from the latter. In the 
latter case the exhaust gases from the cooling chamber may be used for 
other purposes. 
The expanded fluidized bed used in accordance with the invention differs 
from an "orthodox" fluidized bed, in which a dense phase is separated by a 
sudden change in density from the overlying gas space, by having a density 
distribution with no defined phase interface. There is no sudden change in 
density between a dense phase and the overlying gas space but the solids 
concentration in the reactor decreases gradually from bottom to top. 
In a particularly advantageous technique, the oxygen-containing gas 
required for the combustion is supplied to the fluidized bed in two 
streams on different levels and the solids which have been entrained by 
the exhaust gas and separated from the latter are recycled to the lower 
portion of the fluidized bed. This mode of operation will result in a weak 
combustion in two stages so that hot spots and a formation of NO.sub.x 
(nitrogen oxide) gases are precluded and the recirculation of the solids 
separated from the exhaust gases will result in a highly constant 
temperature in the system consisting of the fluidized bed reactor, the 
separator and the return duct. 
Virtually any gas which will not adversely affect the nature of the exhaust 
gas may be used as a fluidizing gas. Suitable gases include inert gas, 
such as recycled flue gas (exhaust gas), nitrogen and steam. In order to 
intensify the combustion it will be desirable to feed part of the required 
oxygen-containing gases to the fluidized bed reactor as fluidizing gas. 
It is apparent that the invention may be carried out in the following 
modes: 
1. Inert gas is used as a fluidizing gas. In that case the 
oxygen-containing combustion gas must be fed as secondary gas on at least 
two levels. 
2. Oxygen-containing gas is used as fluidizing gas. In that case, secondary 
gas may be fed on only one level although the secondary gas may also be 
fed in a plurality of levels. 
A plurality of inlets for the secondary gas are advantageously provided on 
each level. The volume ratio of fluidizing gas to secondary gas should be 
in the range of 1:20 to 2:1. 
The secondary gas is suitably fed to the fluidized bed reactor on a level 
which is spaced by up to 30% of the height of the fluidized bed reactor 
and by at least 1 m over the level on which the fluidizing gas enters the 
reactor. If the secondary gas is fed on a plurality of levels, the limit 
of 30% will be applicable to the level of the uppermost secondary gas 
inlet. That level ensures that there will be an adequate space for the 
first combustion stage so that there will be an almost complete reaction 
between the combustible constituents and the oxygen-containing gas, 
whether it consists of the fluidizing gas or of a secondary gas fed on a 
lower level. On the other hand, a zone which is sufficiently large for a 
complete combustion will be provided in the upper reaction space above the 
secondary gas inlet means. 
The gas velocities in the fluidized bed reactor above the secondary gas 
inlet means are usually higher than 5 m/sec. and may be as high as 15 
m/sec. 
The ratio of the diameter to the height of the fluidized bed reactor should 
be selected so that gas residence times of 0.5 to 8.0 seconds, preferably 
1 to 4 seconds, are obtained. 
The feedstock should have an average particle diameter of 30 to 250 
micrometers. This will ensure good fluidizing conditions as well as 
sufficiently short reaction times. 
The mean density of the suspension to be maintained in the fluidized bed 
may be varied within wide limits and may be as high as 100 kg/m.sup.3. In 
order to minimize the pressure loss, a mean density of the suspension in 
the range from 10 to 40 kg/m.sup.3 should be maintained above the 
secondary gas inlet means. 
If the Froude and Archimedes numbers are used to define the operating 
conditions, the following ranges will then be obtained: 
##EQU1## 
In the above expressions, 
F.sub.r Froude number; 
Ar Archimedes number; 
u relative gas velocity in m/sec; 
.rho.g density of gas in kg/m.sup.3 ; 
.rho.k density of solid particle in kg/m.sup.3 ; 
d.sub.k diameter of spherical particle in m; 
.nu. kinematic viscosity in m.sup.2 /sec.; 
g acceleration due to gravity in m/sec..sup.2. 
The feedstock is fed into the fluidized bed reactor in the usual manner, 
most suitably through one or more lances, e.g. by pneumatic blowing. Owing 
to the good transverse mixing, a relatively small number of feed lances 
and in small fluidized bed reactors even a single lance will be 
sufficient. 
The eminent advantage afforded by the invention resides in that the use of 
recooled solids results in a shock-like cooling of the exhaust gases so 
that corrosion is substantially avoided, and that dust deposits which 
would adversely affect the heat transfer obviously cannot be formed. In 
the preferred embodiment using a fluidized bed cooler, the solids are 
recooled under conditions which permit a high heat transfer to the cooling 
fluid. 
Because the cooling solids are handled in a cycle which is separate from 
the solids which are subjected to pyrohydrolysis, said cooling solids will 
not be laden with HF, except for the starting period, and no HF will be 
lost in the recooling step. Besides, the exhaust gas is cooled in such a 
manner that a dilution of the exhaust gas will be precluded or will be 
minimized if the cooling by solids is supplemented by an addition of water 
at a low rate, e.g., in a stage which succeeds the addition of the 
recooled solids.

SPECIFIC DESCRIPTION 
Feedstock as well as water, possibly in the form of steam, and fuel, if 
required, are fed through lances 4, 5 and 6 to a cycle which consists of a 
fluidized bed reactor 1, a cyclone separator 2 and a return conduit 3. 
After a sufficiently long residence time, a pyrohydrolyzed residue derived 
from the feedstock is withdrawn via duct 7 and is discarded or may be 
leached for a recovery of valuable substances. 
The exhaust gas from the fluidized bed reactor 1 enters the suspension-type 
heat exchanger 8, in which it is subjected to a first cooling step by 
being contacted with circulated solids, which are fed via duct 9. The gas 
and solids are separated in a succeeding separator 10. The gas enters a 
second suspension-type heat exchanger 11, which is fed with recooled 
solids via a pneumatic conveyor 12. When the gas has thus been cooled 
further, it is separated from the solids in another separator 13. Dust is 
then collected from the HF-containing gas in an electrostatic precipitator 
14, from which the gas is delivered in duct 15 to an absorbing -or a 
condensing unit, not shown. 
The solids collected in the separator 10 are fed via duct 16 to the 
fluidized bed cooler 17, in which they first flow through four cooling 
chambers. In said chambers, the solids deliver a substantial part of their 
heat content by indirect heat exchange to oxygen-containing gases, which 
flow countercurrently to the solids and are free from dust as they are 
subsequently fed via duct 18 as fluidizing gas to the fluidized bed 
reactor 1. The solids are then finally cooled in two succeeding cooling 
chambers, which are cooled, e.g. with water, and from which the solids are 
fed to the pneumatic conveyor 19. 
The oxygen-containing fluidizing gas extracts a substantial amount of 
additional heat from the solids in the fluidizing bed cooler 17 and is 
then passed through the separator 20 for dust collection. From the 
separator 20, the gas is fed to the fluidized bed reactor 1 via duct 21 as 
secondary gas. The dust collected in the electrostatic precipitator 14 is 
recycled to the fluidized bed cooler 17 in duct 22. 
When solids have become enriched or depleted in the cycle consisting of the 
fluidized bed cooler 17, duct 12, suspension-type heat exchanger 11, 
separator 13, suspension-type heat exchanger 8, separator 10 and conduit 
16, solids are transferred from said cycle via duct 23 to the circulated 
fluidized bed or may be fed via duct 24 to the cycle which includes the 
cooler 17. 
Ducts 25, 26 and 27 serve to supply fluidizing or conveying gases. 
SPECIFIC EXAMPLES 
Example 1 
The feedstock consisted of dry lining material which had been broken out 
from an electrolytic cell used for fused-salt electrolysis in the 
production of aluminum and had been ground to an average particle size of 
100 to 200 micrometers. The feedstock had a bulk density of 1.1 kg/l and 
contained 
26% by weight of carbon and 
15% by weight of fluorine (calculated as F). 
Owing to the high carbon content, the pyrohydrolysis was self-sustaining, 
i.e., there was no need for additional fuel. The gas quantities which will 
be stated hereinafter are based on standard conditions. 
The broken-out lining material was fed to the fluidized bed reactor 1 via 
duct 4 at a rate of 5000 kg/h together with water at 20.degree. fed via 
conduit 6 at a rate of 3.1 m.sup.3 /h. At the same time, the reactor 1 was 
fed via duct 18 with fluidizing air at 300.degree. C. at a rate of 3000 
m.sup.3 /h and via duct 21 with secondary air at 400.degree. C. at a rate 
of 9500 m.sup.3 /h. The fluidizing air and secondary air had previously 
been preheated in the fluidized bed cooler 17. As a result of the selected 
fluidizing conditions and operating parameters, the solids were circulated 
in the cycle consisting of the fluidized bed reactor 1, cyclone separator 
2 and recycling duct 3 in such a manner that the suspension in the 
fluidized bed reactor 1 had a mean density of about 100 kg/m.sup.3 below 
the secondary gas duct 21 and about 20 kg/m.sup.3 above the secondary gas 
duct 21. To ensure a satisfactory recycling of solids into the fluidized 
bed reactor 1, the solids in the recycling duct 3 were fluidized with air 
at a rate of 200 m.sup.3 /h. The temperature in the cycle was about 
1100.degree. C. 
After a residence time of 1 hour, treated residue was withdrawn via duct 7 
at a rate of 3000 kg/h, equivalent to the feed rate, and was cooled in a 
separate cooler. The treated residue had a bulk density of 1 kg/l. Its 
residual contents of fluorine and carbon were below 1% by weight and 0.1% 
by weight, respectively. 
The exhaust gas from the fluidized bed reactor 1 was cooled with solids 
that had become available in the process itself. For this purpose, the 
exhaust gas leaving the cyclone separator 2 at 1100.degree. C. was cooled 
to 590.degree. C. in the suspension-type heat exchanger 8 by means of 
solids at 280.degree. C., which were fed via duct 9 at a rate of 50,000 
kg/h and were thus heated to 590.degree. C. The solids were then fed from 
separator 10 via duct 16 to the fluidized bed cooler 17. 
In the second suspension-type heat exchanger 11 in the gas-flow path, the 
exhaust gas from the separator 10 was contacted with solids at 80.degree. 
C., which had been pneumatically conveyed via duct 12 from the fluidized 
bed cooler 17 at a rate of 50,000 kg/h. Gas and solids were then separated 
in the separator 13, in which solids at 280.degree. C. became available. 
The gas that became available at a rate of 19,500 m.sup.3 /h in the 
separator 13 at 280.degree. C. was fed to the electrostatic precipitator 
14 and from the latter to the plant for recovering hydrogen fluoride. The 
exhaust gas had the following composition in % by volume: 
______________________________________ 
CO.sub.2 
12.8 
O.sub.2 
3.3 
HF 4.4 
N.sub.2 
61.5 
H.sub.2 O 
17.8 
______________________________________ 
Solids from the separator 10 were fed at a rate of 50,000 kg/h via duct 16 
to the fluidized bed cooler 17 and were cooled therein in four cooling 
chambers by a heat exchange with air for the fluidized bed reactor 1 and 
in two cooling chambers supplied with water. The fluidized bed cooler 17 
was supplied with fluidizing air at a rate of 9500 m.sup.3 /h and air at a 
rate of 3000 m.sup.3 /h was supplied for an indirect heat exchange. The 
two air streams were withdrawn via ducts 21 and 18, respectively. 
The water-cooled chambers of the fluidized bed cooler 17 were supplied with 
water at a rate of 95 m.sup.3 /h. In said chambers the cooling water was 
heated from 40.degree. to 90.degree. C. whereas the solids were cooled to 
80.degree. C. The cooled solids were recycled to the suspension-type heat 
exchanger 11 with the aid of conveying air, which was at 60.degree. C. and 
a pressure of 500 mbars above atmospheric pressure and was supplied via 
duct 27 at a rate of 2500 m.sup.3 /h. 
Example 2 
Hydrogen fluoride was recovered from fluorspar which had an average 
particle size of 100 to 200 micrometers, a density of 1.2 kg/l and a 
calcium fluoride content of about 95% by weight. Different from Example 1, 
additional fuel was required in this case. 
The fluidized bed reactor 1 was supplied via conduits 6, 4 and 5, 
respectively, with 
1210 kg/h coal (29,260 kJ/kg) 
1540 kg/h fluorspar (calculated as CaF.sub.2) 
3100 l/h water at 20.degree. C. 
The reactor 1 was supplied with fluidizing air at 400.degree. C. at a rate 
of 3000 m.sup.3 /h via duct 18 and with secondary air at 550.degree. C. at 
a rate of 7000 m.sup.3 /h via duct 21. Both air streams came from the 
fluidized bed cooler 17. 
In the fluidized bed reactor 1, the suspension had a mean density of 100 
kg/m.sup.3 below the secondary gas duct 21 and of 25 kg/m.sup.3 above the 
secondary gas duct 21. The reactor was operated at 1120.degree. C. As in 
Example 1, a satisfactory recycling of solids into the fluidized bed 
reactor 1 was ensured by a supply of air at a rate of 200 m.sup.3 /h into 
the recycling duct 3. A residence time of 90 minutes was maintained. 
Treated residue at a rate of 1230 kg/h, equivalent to the feed rate, was 
withdrawn from the cycle via duct 7. The residue had the quality of burnt 
lime and could be used in the building industry. 
By contrast with Example 1, the exhaust gas was cooled with extraneous 
solids consisting of alumina. The gas and solids were conducted along the 
same paths as in Example 1. 
Solids at a rate of 40,000 kg/h were circulated through the conveyor duct 
12, the suspension-type heat exchanger and separator and the fluidized bed 
cooler 17. Conveying air at 60.degree. C. was supplied at a rate of 2100 
m.sup.3 /h and a pressure of 500 mbars above atmospheric pressure. The 
exhaust gas and solids temperatures obtained in the suspension-type heat 
exchanger and separator stages amounted to 290.degree. C. (11/13) and 
610.degree. C. (8/10). 
A gas from which hydrogen fluoride could be recovered became available in 
duct 15 at a rate of 16,750 m.sup.3 /h and had the following composition 
in % by volume: 
______________________________________ 
CO.sub.2 
14.8 
O.sub.2 
2.1 
HF 5.0 
N.sub.2 
58.0 
H.sub.2 O 
20.1 
______________________________________ 
When the solids fed via duct 16 were cooled in the fluidized bed cooler 17, 
air which had been indirectly heated and for this reason was free from 
dust became available in duct 18 at a rate of 3000 m.sup.3 /h and directly 
heated air at a rate of 7000 m.sup.3 /h became available in duct 21. These 
air streams were fed to the fluidizing bed reactor 1 as fluidizing air and 
secondary air, respectively. 
The water-cooled chambers of the fluidized bed cooler were supplied with 
water of 40.degree. C. at a rate of 67 m.sup.3 /h, which was heated to 
90.degree. C. whereas the solids were cooled to 80.degree. C. in said 
chambers.