Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for Rapid Conversion of Fluoroanhydrite to Gypsum
Bac~ground of the Invention
Field of the Invention
This invention relates to a process for producing indus-
trially usable gypsum from a commercial by-product, in par-
ticular fluoroanhydrite~ More particularly, this invention
relates to an improvement in processes for the production of
hydrogen fluoride and waste product fluoroanhydrite.
In the industrial production of hydrogen fluoride,
fluorspar is reacted with concentrated sulfuric acid in an
externally heated reaction vessel to co-produce hydrogen
fluoride and fluoroanhydrite. For every ton of fluorspar
consumed, approximately 1.75 tons of anhydrite is produced.
After reaction~ the hydrogen fluoride is drawn off and con-
densed while the fluoroanhydrite is generally slurried with
water, neutralized with lime or limestone and pumped to dis-
posal ponds.
The fluoroanhydrite is contaminated with excess sulfuric
acid and fluoroaluminate impurities. Historically, because
of the contaminants, the fluoroanhydrite has not been com-
mercially useable and has been allo~ed to hydrate naturally
over a several year period of time. Thus large tonnages of
mixed fluoroanhydrite-fluorogypsum materials have been accu-
mulated in these ponds over the years.
Description of the Prior Art
-
U.S. patent 3,825,655 discloses an improvement in hydro-
gen fluoride processes said to produce a coarse-grained sul-
fate co-product in either anhydrite or gypsum form said to be
industrially useful~
However, it has been shown that certain fluoroaluminum
impurities still present, e.g. l(hlF5H20)] cause
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deleterious effects in attempts to manufacture gypsum pro-
ducts from the fluoroanhydrite. These impurities inhibit the
hydration of the fluoroanhydrite to gypsum. These impurities
raise the calcining temperature during conversion of fluoro-
gypsum to calcium sulfate hemihydrate. Further, they inhibit
the setting of the produced hemihydrate; and finally, they
result in producing gypsum products such as industrial plas-
ters, building plasters and gypsum wallboard of poor qua-
lity.
From the above, it is apparent that there is a need in
the art for effective and economical means for removing a
substantial portion of the impurities that originate in the
hydrogen fluoride process. Furthermore, there is a need in
the art for making calcium sulfate products that may be used
in wallboard and other industrial and construction materials.
The process of the present invelltion offers a solution to
these needs.
Summary of the Invention
These and other objects and advantages are reali~ed in
accordance with the present invention wherein floroanhydrite
obtained directly from a conventional hydrogen fluoride pro-
duction process is rapidly hydrated to gypsum in a dilute
acidic water slurry to produce a gypsum free of harmful
soluble and syncrystalli~ed impurities and which is accept-
able fo~ gypsum board manufacture. The process comprises
adding small amounts of fluoroanhydrite to a fluid reaction
medium containing large quantities of small gypsum seed cry-
stals (about 40-90~ by weight of the solids of the medium)
and about 0.5-~0% by weight of a soluble sulfate accelerator.
It has been found that ~he hydra~ion reaction of fluoroanhy-
drite is very rapid in the presence of large quantities of
s~
small sized gypsum seed crystals and these seeds magnify the
effect of soluble sulfate hydration accelerators such as sul-
furic acid or sodium sulfate, especially in the earlier
stages of reaction. As hydration proceeds to build up large
sized gypsu~ particles, the medium is filtered to separate
about 15% of the solids of the medium as purified gypsum
product; and it is preferred to recycle the remaining 85%
solids for Eurther processing.
Brief Description of the Drawings
Figure 1 is a diagrammatic flow chart illustrating one
preferred method of the present invention and Figures 2-5 are
plots of percent conversion of fluoroanhydrite to gypsum ver-
sus time under various conditions of the present invention.
Description of the Preferred Embodiments
The process comprises a continuous, rapid conversion of
fluoroanhydrite, produced by a hydrogen fluoride plant, to
gypsum by adding small amounts of the fluoroanhydrite to a
fluid reaction medium containing a soluble sulfate ~prefer-
ably sulfuric acid and sodium sulfate) and large amo~nts of
small gypsum seed crystals. The fluid reaction mixture has a
total solids content of about 20-60% by weightl with prefer-
ably about 50-90~ of the total solids being small sized
gypsum seed crystals. The reaction mixture also contains
about 0.5-20~ soluble sulfate accelerator and fluoroanhydrite
is added to the mixture in amounts less than the amount of
gypsum seed crystals. The fluoroanhydrite is hydrated to
large sized gypsum particles on the gypsum seed crystals.
About 10-30~ o the gypsum particles in any one cycle is re-
covered as large size gypsum crystal product. rhereafter~ the
separated, large crystal gypsum product is passed on to scpa-
rate processing for conversion to building plas~er products
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such as wallboard. The remaining reaction mixture suspension
contains a major proportion of small size gypsuM crystals and
is recycled in the process to act as a supply of seed cry-
stals for processing subsequent additions of fluoroanhydrite.
The fluoroanhydrite feed may be produced by any of the
conventional hydrogen fluoride production processes, prefer-
ably the cooled, dry product directly from the hydrogen
fluoride reactor. This material has a particle siæe of less
than 10 micrometers, generally less than 5 micrometers, as
obtained from the reactor. Stockpiled fluoroanhydrite cake
may be easily broken up into suitable particle sizes for
processing in accordance with the present invention.
The sulfuric acid is conveniently the same as that used
as feed in the hydrogen fluoride reactor, diluted to a weight
concentration of about 1% to about 20~, preferably about
5-15% and most preferably about 7~5~. The necessary soluble
sulfate moiety may be provided in whole or in part by resi-
dual acid in the fluoroanhydrite from the hydrofluoric acid
reactor. Any sulfuric acid added beyond that coming in with
unwashed fluoro2nhydrite should be reaction grade~ The
accelerator, preferably soluble sulfate may further be pro-
vided in whole or in part by soluble sulfate salts, such as
sodium, potassium, magnesium, aluminum, iron, zinc and other
soluble sulfates. Combinations of accelerators are preferred
such as mixtures of ammonium and potassium sulfate combined
with sulfuric acid. A particularly preferred combination is
about 7~ sulfuric acid and 5-15~ sodium sulfate in the liquid
solution of the reacting slurry. Above about 20a or below
about 1~ sulfuric acid are generally unsatisfactory; since in
the former solubility of calcium sulfate decreases and the
resultant gypsum product is unstable and reguires too much
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neutralization for use in gypsum construction products, and
the latter requires uneconomical conversion times.
The gypsum seed material may be any natural ground gyp
sum or high purity synthesized calcium sulfate dihydrate of
about 5-60 micrometers size and preferably 15 to 30 micro-
meters particle size. For the start-up of the process of
this invention, the gypsum seed may be either ground,
naturally occuring gypsum or prepared from industrial pro~
cesses yielding a high purity gypsum of the appropriate
particle size, such as from titanium dioxide preparation/
citric acid preparation, flue gas desulfurization processes
and the like. Once steady state is achieved, conveniently the
gypsum seed is obtained as recycle of the fluid reaction
medium after having separated larger size dihydrate particles
as gypsum productO Such larger particles of gypsum are of
convenient size for gypsum board or plaster production.
In implementing the process, it is important that there
be a sufficient quantity of small sized, pure gypsum seed
crystals and sufficient soluble sulfate present in the re-
action medium to achieve a rapid oonversion of the fluoroan-
hydrite to large gypsum particles, to keep impurities in the
fluoroanhydrite solubilized, and to maintain easy separation
of the highly pure gypsum product from the recycle seed cry-
stals. Thus, generally 50-90~ by weight, and more preferably
60-75~, of the solids of the reaction slurry should be small
sized, pure gypsum seed crystals. About 5-20% concentration,
and more preferably 5-10~, of sulfuric acid and water soluble
sulfate salt in the reacting slurry will provide optimum
rapid conversion.
The process i!s carried out, as illustrated in Figure 1
in one preferred embodiment by metering cooled and dried
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fluoroanhydrite powder (1) into a recycling reaction medium
in a first mixing zone (2). All of the vessels used in the
process are conventional. At start-up all of the materials
are fed to the first mixing ~one from an outside source, but
when steady state operation has been achieved, the reaction
medium is supplied with recycled materials from the separator
(3) plus any necessary makeup of water, sulfuric acid and
soluble sulfate accelerator. Upon steady state operation,
very little makeup of soluble sulfate and sulfuric acid is
necessary except to compensate for losses and purged bleed
off (6) to avoid buildup of soluble fluorides in the system.
While four hydrating tanks are illustrated in Figure 1, con-
ventional mixing vessels or compartmentalized cells, at am-
bient temperatures, from a single through a half dozen units
may be utilized depending upon the scale of operations. The
hydrating tanks may be operated at from about 0C to about
42C, although ambient temperatures of about 20-22C are pre-
ferred.
The hydration of the fluoroanhydrite proceeds progres-
sively as the material advances from one compartment to the
next. Numerous tests have shown that if fluoroanhydrite con-
taminated with sulfuric acid is hydrated directly with a
major proportion of recycle gypsum seed crystals, the con-
version takes place in a matter of minutes. The hydrating
mixture seems to be acidic enough to keep complex fluoro-
aluminum species from precipitating out on the surface of the
growing gypsum crystals. Otherwise, the fluoroaluminum im-
purity would inhibit the crystal grow~h and co-crystallize
with the growing gypsum particles thereby rendering ~hem un-
suitable for production of building construction material
products. If there is sufficient proportion of small size
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seed crystals in the reaction mixture, hydration i5 very
rapid as shown in Figures 2 and 3. Furthermore, the degree of
conversion can be controlled so as to aid in maintaining any
desired recycle proportion as shown in Figures 4 and 5.
Under usual operating conditions, the fluoroanhydrite
is very rapidly converted (generally 10 minutes to 2 hours)
to sufficiently large sized gypsum particles for convenien~
separation by conventional,ls~para~ion equipment e.g., one or
more hydrocyclones, centriuges, fixed or moving bed filters
and the like. In the preferred operation, a first separation
is made to classify and separate very small sized gypsum cry-
stals for recycle in the reaction by a hydrocyclone or the
hydraulic separator (3) illustrated in Figure 1. The larger
gypsum particles are then passed to a further separator such
as bed filter (4) for separation of very large size product
gypsum particles. The acidic filtrate from the filter (4)
can be partly neutralized by lime or limestone addition (7)
to immobilize the fluoroaluminum impurities for purging from
the system (6) as their concentration builds up to an inter-
fering level. The product gypsum underflow from filter (4)
may be washed (not shown) to remove residual filtrate. The
filtrate and wash waters May then be combined to reconstitute
the fluid reaction medium for recycle. Optionally, a certain
amount of lime treated fluoroanhydrite from waste piles may
be used to neutralize the acidic filtrate. Alternatively, the
acidic filtrate can be used for washing hydrated, lime
treated fluorogypsum from waste piles to remove some of the
fluoroaluminum impurities from tha~ gypsum making it suitable
for use as seed crystals.
E~ample 1
Fluoroanhydrite from a manufacturer of hydrofluoric
acid is treated as set forth in Figure 1.
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Table la
Process
Point A B C D E F G
Gypsum30.037.534.8 54.181.1 0 32.2
Anydrite10.0 4.1 3.8 5.9 8.9 0 3.5
Sodium
Sulfate 3.0 3.0 3.2 2.1 0 2.8 3.2
Sulfuric
Acid6.0 6.0 6.3 4.1 0 8.5 6.4
Water51.049.451.9 33.8lO.Q 88.7 54.7
Total
wt. ~100.0100.0100.0100.0100.0100.0100.0
_
AEter ohtaining steady state conditions, 25.7 short tons
(51400 lbs., 23315 kg) per hour of fluoroanhydrite from the
hydrogen fluoride reactor are introduced into a mixing appa-
ratus such as that shown schematically in Figure 1 which com-
prises 4 hydration compartments. 11.25 short tons of sodium
sulfate and 11.25 short tons of 100~ sulfuric acid are intro-
duced into the first compartment before reaching steady state
to provide on reaching steady state conditions, a reaction
medium as set forth in Table la~ the slurry having a specific
gravity of 1.37 grams per milliliter and flowing at a rate of
1143.8 gallons (4329.75~ liters) per minute. A~ various
points in the process data are taken of the materials at the
points identified in Figure 1 and set forth in Table la, the
points corresponding to letters in the figure.
Table lb
Process
Point A B C D E F G
Gypsum32.6 41.332.6 8.7 8.7 -- 32.6
Anydrite 9.3 2.5 1.9 0.5 0.5 -- 1.9
Sodium
Sulfate4.3 4.3 3.7 0.6 ~ 4.3
Sulfuric
Acid 7.8 7.8 6.8 1.0 -- -- 7.8
Water61.0 59.251.0 8.2 1.0 -- 51.0
Total
gms115.0 115.096.0 19.0 10.2 --- 117.6
Relative Weight flows, g~min.
Solids 41.0 43.7 34.5 9.2 9.2 -- 34.5
Liquids 73.1 71.3 61.5 9.8 1.0 -- 73.1
Product P~operties
Vicat Set
Mortar Cube Strengths of 47.7 p.5.f. cubes: 863 psi dry
compressive strength; 517 humid~
Wallboard Bond Load Capability at humid2 conditions: 9.8
po~nds compressive strength
Elapsed time from mixing 5Q g standard accelerated
plaster water mix to when a 300 9 Vicat needle will no~
penet~ate more than half way into the setting slurry.
Humidified for 16 hours at 90~F and 90~ relative
humld lty .
s~
The run was repeated with fluoroanhydrite from a dif~
ferent manufacturer. After achieving steady state condi-
tions, 7.4 grams per minute of the fluoroanhydrite were
metered into the process as set forth in Figure 1 with a flow
rate through the reactor vessels (2~ of 115 grams per minute.
Data taken at various process points is set forth in Table
lb. The product gypsum was calcined to hemihydrate and pro-
perties of gypsum products made from it are also set forth in
Table lb. As set forth in Table lb, acceptable gypsum board
and plaster product properties were obtained.
Example 2
A fluoroanhydrite from hydrogen fluoride production was
analyzed as follows:
Wet Chemical Analysis (in weight %)
Loss on Ignition,40-230~C 2.79%
CaO 38.35
SO 57.29
Mg~ 0.03
SiO 0. 1~
Fe ~ 0.12
A12O3 0.05
co2 3 0.05
F 2 0.20
Excess Loss on Ignition,
230C-950C 1.39
pH 1.70
Water Soluble Salts (in ppm)
Potassium 118
Sodium
Magnesium 25
Chloride ~o
Fluoride 5
The rate of hydration to gypsum with varying proportions
of dihydrate seeding and accelerators was evaluated. For
this, 300 gram aliquots containing the appropri~te amount of
fluoroanhydrite and gypsum were combined with 600 ml aliquots
of water containing the accelerator, stirred for 24 hours,
and samples taken for hydration reaction analysis at 2, 4~ 6
and 24 hours.
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Increasing the amount oE gypsum seeding has a direct
influence on the rate of conversion of fluoroanhydrite to
gypsum as shown in Figure 2. For example, at 3 hours the
percent conversion for a mixture of fluoroanhydrite with 10%
gypsum seed and 1% sodium sulfate at 22C increased about 5
times when using a 50% gypsum seed in the system. It is dif-
ficult to reach a conversion much above 95~ since the surface
area of the anhydrite becomes a limiting ~actor. A way
around this is to hydraulically classify, as at separator
(3), the mixture after a suitable residence time and recycle
the small anhydrite crystals for further conversion. In this
manner a gypsum product having a purity greater than 95% is
produced.
Figure 3 shows the ef f ect of seed crystal surface area
on the rate of conversion. In this evaluation, although the
seed crystals were provided as 50~ of the total solids, the
surface area of the seed was varied by using crystals of dif-
ferent sizes. Again, there appeared to be a direct depen-
dence of conversion rate as a function of surface area. For
example, at 2 hours a 50~ seeding went from a conversion of
16~ to about 78~ by using 7 micrometer seeds having a surface
area of 6100 square centimeters per gram in contrast to 32
micrometer seed having a surface area of 900 square centi-
meters per gram . Thus, a 6.7 fold increase in surface area
provided an increase of about S times in conversion rate. In
this case, the sodium sulfate concen~ration becomes a
limiting factor because of greater ion pair formation and
tying up of reactant water through hydration o~ the salt
ions.
The soluble sulfate salt concentration was ~aried as
shown in Figure 4, for a 50~ seed system. Again, there
seemed to be a direct correlation be~ween the s~l~
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concentration at the lower levels and the conversion rate.
For example, at 3 hours the rate was approximately doubled by
increasing the sodium sulfate ~oncentration by a factor o~ 2.
However, this relationship was complicated by the 1-2~ sul-
furic acid present as an impurity in the fluoroanhydrite.
The effect of sulfuric acid concentration is shown in
Figure 5. Sulfuric acid is not as effective an accelerator
as the soluble cationic sulfate ~alts. This is because its
sulfate ion concentration is probably only 1~'10th mole per
cent in comparison to that of sodium sulfate or potassium
sulfate. Also, the rate of conversion becomes much slower
above about 60~; however, this may be augmented by mixtures
of sulfuric acid and the sulfate salts. For example, it is
believed that combinations of sulfuric acid having a concen-
tration between about 5-10~ with sodium sulfate haYing a con-
centration between about 0.5-3~ are at least additive in hy-
dration; and thus, mixtures are preferred.
Example 3
An extended run for 16 hours continuous using 75~
seeding with 10~ sulfuric acid and 5~ sodium sulfate acceler-
ators in the liquid phase was performed to evaluate conver-
sion and fluoride buildup on recycle as follows.
A fluoroanhydrite fr~m a hydrogen fluoride reactor oper-
ation was obtained. The material had the following analysis:
38.98% CaO, 54.79~ 5O3, 0.03% MgO, 0.08~ SiO2, 0.10%
Fe2O3, 0.10~ A12O3, 0.13% P and 3.21% P. Loss on
ignition between 40C and 230C was 3%; and X-ray diffraction
analysis showed only anhydrite as the calcium sulfate ~orm
present.
Fl~oroanhydrite (150 parts by weight) and 99~ purity
gypsum seed crystals (450 parts by weight and 7 micrometers
S~ 713
particle size) were added with stirring to 1200 parts by
weight of a solution containing 10% by weight of sulfuric
acid and 5% by weight of sodium sulfate. This mixture formed
a 33% solids hydrating media with the seed crystals consti-
tuting 50% based on the total solids of the media. The slur-
ry was continuously stirred for a 16 hour interval. Every 3-4
hours a 370 parts by weight portion of the slurry was di-
verted, filtered and washed, and analyzed for gypsum and
fluoride content; and a fresh portion of 100 parts by weight
of the fluoroanhydrite plus 270 parts by weight of the 10%
sulfuric acid and 5~ sodium sulfate solution was ball milled
to reduce gypsum crystal size then returned to the mixing
slurry. Analysis of the sampled portions was as follows:
Time Interval ~ Gypsum Content PPM Fluoride Particle Size
4 97.8~ 18 ppm 13.32
8 97.5% 22 ppm 17.21
12 96.9% 35 ppm 18.58
16 96.8% 119 ppm 20.82
From the above, it can be seen that periodic purging of
fluoride from the recycle is desirable. From this and ear-
lier examples it is seen that conversion rate is greatest in
about the first hour or so. Most practical operations will
require only about 60-90g conversion in order to recycle some
anhydrite. It should be noted that in this and the foregoing
examples, the results are expressed in weight ~ of fluoro-
anhydrite converted. Since, for example, most of the
slurries utilized 50~ gypsum solids present by the seeding,
at a 50~ total suspended solids slurry, a 78~ conversion of
the anhydrite present in the first hour or less provides a
90~ gypsum content.
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Example 4
~ composite of the samples from Example 2 which con-
tained over 70~ gypsum on hydration in 24 hours, was calcined
to calcium sulfate hemihydrate plaster and compared with com-
mercial plaster made from natural gypsum rock.
Standard plaster slurries were made with water and
set accelerators, with results of setting properties as
follows:
Table 2
Setting
Plaster Agent Time of Temperature Rise Set
Source Addition Vicat Set Minutes ~F F/min.
Natural Rock
plaster -- 32 minutes 37.5 110
0.1 9 5.75 15 113 6.7
Fluoroanhydrite source
plaster -- 8 15 118 8.6
0.1 3 7.5 11~.8 10.6
Additional fluoroanhydrite source plaster samples were
blended in about equal weight proportion with natural rock
plaster and then blended with conventional additives for gyp-
sum board manufacture with results as follows;
stucco color: gray;
pH 7.2;
reground Blaine surface area : 6600cm2/g
mortar cube strength of 43.81 lb/ft.3 density c~bes:
720 psi dry compressive
wallboard bond load capability, on 16 hour conditioning at
90F/90~ relative humidity: 13.53 pounds
Satisfactory plaster, stucco and wallboard properties
were obtained from calcination of the fluoroanhydrite source
plaster.
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