Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention
Field of the Invention
This invention relates to a process for reducing the
radioactive contamination in waste product phosphogypsum.
In the industrial production of phosphoric acid by the
wet methods phosphate rock is reacted with concentrated sul-
furic acid for simultaneously producing both phosphoric acid
solutions and calcium sulfate products. The calcium sulfate,
which may be recovered as the dihydrate or phosphogypsum, the
hemihydrate, or the anhydrite is contaminated with most of
the impurities originally present in the phosphate ores. In
the past, these calcium sulfate products were generally dis-
carded as unwanted by-products because of the impurities; and
large ponds and piles of these materials can be found at most
phosphoric acid plants.
In attempts to produ.ce marketable products from the
heavily contaminated gypsums, extensive purification operations
and alteration of phosphoric acid production conditions have
been attempted without practical success. Most of these
attempts have focused on other impurities, and radioacti.ve
contaminants remain even after extensive washing and elutri-
ation and other process modifications. Typical phosphogypsum
from the Prayon and modified Prayon processes contains about
25 picoCuries per gram of radiation, measured as radium 226.
This contamination has become of increasing concern to govern-
mental regulatory agencies and the industry itself. The radium
imparts a slight radioactivity to the gypsum which may be
dangerous even in the minute levels involved in industrial
:~ dm~
~G~
or construction products made from that material. It is
possible that limitations will be placed upon radioactive
levels for further stockpiling of this phosphogypsum. For
example 5 pCi/g has been proposed as an upper limit on phospho-
gypsum by the Environmental Protection Agency of the United
States.
Description of the Prior Art
There has been little prior concern for reducing radio-
activity of phosphogypsum. U.S. Patent 3,949,047 and U.S.
Patent 4,282,192 disclose treatment of the monocalcium phos-
phate ~MCP) solution prior to the precipitation of the calcium
sulfate. In the former, the MCP is treated by addition of
barium compounds to reduce radium contamination, and in the
latter, the MCP is treated with a sequestering agent and a
first calcium sulfate precipitate high in radium is discarded.
U.S. Patent 4,146,568 discloses a process for reducing radio-
active contamination in the phosphogypsum itself by slurrying
it with a dilute sulfuric acid containing barium sulfate ana
separating the solids resultant therefrom into fine and coarse
fractions. It is indicated that the fine fraction predominates
in the radioactive contamination. It is not indicated propor-
tionally how much of the starting phosphogypsum is recoverable,
but losses could be considerable.
Published European Patent Application 12,487 of June 25,
1980 discloses that phosphogypsum can be converted to anhydrite
by treatment with 60~ sulfuric acid at 60C for 20 minutes.
The obtained anhydrite particles are said to be 2.5 micrometers
long and 0.5 micrometers thick.
dm: ~ - 2 -
~6~ 3
From the above, there is still a need in the art for
effective and economical means for removing a substantial
portion of the radium that originates in phosphate rock.
Furthermore, there is a need in the art for making calcium
sulfate products that have acceptable levels of radium such
that they may be utilized in wallboard and other industrial
and construction materials. There is a need at the present
time to provide improved processes for reducing the radio-
active contaminants in waste product phosphogypsum. The process
of the present invention offers a solution to these needs.
Summary of the Invention
It has now been found that phosphogypsum can be
converted to an anhydrite crystal in a very rapid fashion in
a narrow range of sulfuric acid concentra-tions and temperatures;
and further that by utilizing substantial amounts of recycled
gypsum seed crystals with the resultant anhydrite solution
containing radioactive contamination, a second purifying
recrystallization can be achieved rapidly. Finally, coarse,
substantially radiation free gypsum crystals may be separated
in conventional phosphate industry classification equipment
from the relatively small sized anhydrite crystal relics
containing the radioactive contamination.
The present invention broadly comprises reducing radio-
activity in phosphogypsum by first dehydrating that material
to calcium sulfate anhydrite. The radioactivity at this
stage appears to remain in the calcium sulfate lattice. By
then allowing a portion of the calcium sulfate to rehydrate
and recrystallize on the surfaces of purifiea gypsum seed
. dm: ~ _ 3 _
. ~,
crystals large substantially radia-tion free gypsum crystals
are obtained. The radioactivity remains in -the anhydrite
crystals; and by controlling the portion of anhydrite allowed
to hydrate, coarse, substantially radiation free gypsum
crystals and small anhydrite crystal relics containing
substantially all of the radioactive contamination are
obtained and readily separated.
In a preferred embodiment, the present invention
involves, as depicted in Figure 2, two seqùential recrystalli-
r~ations of the phosphogypsum (dehydration and rehydration) and
sequential separations (in the settlers) to separate diluted
sulfuric acid, to separate anhydrite crystal relics in which
radioactivity is concentrated and to separate substantially
radiation-free calcium sulfate dihydrate product.
Brief Description of the Drawlngs
Figure 1 is a plot of sulfuric acid concentration
versus temperature showing the states of hydxation of the
calcium sulfate; Figure 2 is diagrammatic flow chart
illustrating one preferred method of the present invention;
and Figures 3 and 4 are scanning electron microphotographs
of anhydrite particles and gypsum particles producea durin~
the practice of the present invention.
Description of the Preferred Embodiments
-
In the firs-t step for purifying the was~e product
phosphogypsum in accordance with the process of this invention,
a concentrated sulfuric acid is used to digest the gypsum at
elevated temperatures. As part of the present invention, it
has been found that a s-table anhydrite crystal is rapidly
dm~ 4 -
~6~
formed in Region III delineated in bold lines in Fi~ure l. To
the left of the desired area in Region II of this diagram it
has been found that there is an energy level kinetics
problem. A metastable hemihydrate is first formed requiring
extensive energy and/or time for conversion to a stable
anhydrite crystal. ~n the right hand side of the desirable
area, in Region I in Figure l, there is a deleterious acid or
double salt formation. In Region III, between about 55-~3%
H2SO4 and below the boiling line, anhydrite is precipitated
directly through the dehydration of the gypsum.
Thus, generally an aqueous sulfuric acid haviny a
concentration in the rangP from about 55-63% H2SO4 by weight
is employed in the reaction of the invention. Slightly more
dilute solutions may be employed, if desired, to operate at
the borderline to the metastable hemihydrates ~Region II)
through the addition of seed crystals of anhydrite in amounts
of about 5-40% by weight of the solids present in the slurry.
However, the closer one gets to the border or the more one
extends over the border into Region II the risk of incomplete
conversion increases, delaying the process and risking
metastable hemihydrate formation with the potential
difficulty of having it set up in the equipment.
Thus, referring to Figure 2, an aqueous sulfuric acid
of about 50-lO0 weight % sulfuric acid is added to the
dehydrator at temperatures ranging from about 40C to boiling.
Preferably, the sulfuric acid feed will be about 55 to 96
weight %. ~he sulfuric acid is mixed in the dehydrator with
phosphogypsum in proportions to form a slurry of about 10% to
dm: ~ - 5 -
~L6~
about 60~ solids, preferably about 30-50%. Since the phospho~
gypsum contains about 20% water of crystallization and
slurries contain more water, increased amounts of phospho-
gypsum in the mi~ture allows higher concentrations of ~
sulfuric acid to be added~ The phosphogypsum may be fed
directly from a wet process phosphate plant. It may also be
obtained from the gypsum ponds and stockpiles.
The dehydrator may be any mixing vessel or reactor
conventional to the wet process method and may be single stage
or multiple celled digesting tank or attack tanks.
In the first stage crystallization, in the dehydrator
mixer as shown in Figure 2, the phosphogypsum containing the
radïation reacts rapidly with strong sulfuric acid to form
anhydrite. The anhydrite crystals obtained in the dehydrator
will be generally about 1-25 microme-ters in size, preferably
5-15 micrometers. If the crystals are allowed to grow much
larger than about 25 micrometers, difficulties in separation
after the second stage recrystallization may occur with con-
ventional separation equipment. Crystals less than about 1
micrometer will tend to end up as particles of contaminated
reiic adhering to the dihydrate crystals in the second stage
recrystallization.
It has been found that the phosphogypsum may contain
considerable quantities of silica sand. After reaction, as
shown in Figure 2, the slurry is optionally passed to
conventional industry separation equipment such as a hydro-
cyclone or settler. The silica sand is quite coarse and is
readily separated from tbe sulfuric acid anhydrite aqueous
dm: ~ ~ - 6 -
slurry by conventional classification means, preferably after
separation of the about 60% sulfuric acid product and optional
dilution of the anhydrite slurry to about 10~ sulfuric acid
with water. Thereafter, the slurry passes through a second
classification means to separate the anhydrite crystals
containing the radioactive contamination from the now diluted
sulfuric acid solution. The dehydration of the phosphogypsum
to anhydrite is an endothermic reaction thereby cooling the
slurry, and the release of water of recrystallization dilutes
the concentrated sulfuric acid. As a result the conventional
dilution cooler for the sulfuric acid feed and the flash
cooler in the attack tank may be eliminated in the phosphoric
acid plant. The sulfuric acid is generally diluted to
about 50-60% making it suitable for direct addition to phos-
phate rock attack tanks.
The separated anhydrite crystals, containing some
sulfuric acid, thereupon pass to the second stage crystalliza-
tion, the rehydrator as shown in Figure 2. This again is a
conventional attack (reactor) tank and is operated at about
0-42C, generally at ambient temperature. Optionally, the
anhydrite may first be washed with water counter-currently
to dilute it to the desired sulfuric acid concentration for
rehydration of the anhydrite, and the wash water may be sent
to the phosphoric acid plant for further dilution of the
separated sulfuric acid stream.
In the dehydrator, the anhydrite crystals are mixed
with an aqueous slurry of gypsum seed crystals and optional
additional soluble sulfate(s) hydration accelerator. About
dm: ~ 7
10% sulfuric acid not washed free from the anhydrite in the
settler(s) acts as accelerator. After steady state continuous
operation is established, the slurry is recycled with the
gypsum stream and any necessary make up accelerator added to
the anhydrite slurry. The slurry is mixed with anhydrite to
have a total suspended solids of about 10-60 weight ~,
generally 30-50 weight %. The gypsum seed crystals are added
in a weight proportion of about 10-90, and preferably 50-75
parts of gypsum seed to 90-10, and preferably 50-25, parts of
anhydrite. The gypsum seed will be of coarse particle size
about 30-100 or more micrometers and preferably about 40
micrometers for rapid hydration and subsequent separation.
The soluble sulfate(s) hydration accelerator is for the most
part provided by the sulfuric acid residual with the anhydrite
It is preferred that about 1-20 weight ~ solutions of a
soluble sulfate accelerator such as sulEuric acid~ sodium
sulfata, sodium bisulfate, potassium sulfate, magnesium
sulfate, ammonium sulfate, aluminum sulfate, ferrous sulfate,
ferric sulfate, zinc sulfate and the like and mixtures thereof,
be utilized. Mixtures are preferred and Darticularly
preferred mixtures are from about 1-10% of sulfuric acid with
about 1 10 %and especially 1-5% of sodium sulfate.
During the second stage crystallization, calcium and
sulfate ions are leached out of the anhydrite crystal
containing radioactive contamination and the calcium and
sulfate ions recrystallized upon the purified gypsum seed
crystals to grow into larger, coarser calcium sulfate dihydrate
crystals. By not converting all of the anhydrite crystal,
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the radium contamination remains ~ehind on the anhydrite
crystal relic. Provided the anhydrite crystal relic remains
of a size on the order of about 1-25 micrometers, preferably
greater than 1 or 2 micrometers with 5-15 being particularly
preferred, there is little or no adhering of the radiation
contaminated anhydrite crystal relic on the surface of the
purified gypsum crystals in the ensuing separation. Thus, it
is preferred that about 5-70% and more preferably, depending
upon the level of radioactive contamination that may be
tolerated, on the order of about 50% of the anhydrite not be
converted to gypsum. The anhydrite relic acts as a good
concentrator for the radium and is processed for disposal in
the usual manner for low level radioactive waste.
As further shown in Figure 2, the slurry Erom the
rehydrator is passed through conventional separation equipment
to separate the anhydrite relic first and then preferably in
a second separator to separate a recycle stream and a
purified calcium sulfate dihydrate product. By using about
40 micrometers as a minimum si~e for the seed crystals, the
growing dihydrate on the seed crystals in the rehydrator
produces a purified gypsum granule readily separated in
conventional phosphate industry separation equipment.
Recycling about 30-75% of the purified gypsum product, and
preferably 50-75%, as the gypsum seed crystals results in
a very rapid rate of conversion.
Example
A phosphogypsum from a modified Prayon phosphoric acid
manufacturing plant using Florida phosphate rock was obtained.
dm: ~
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An activity count on the material showed it to have 26+3 pCitg
radium 226. The material also had the following analysis:
28.7% CaO, 42.8% SO3, 5.1% SiO2, 0.37% Fe2O3, 0.11% A12O3,
0.2~% P, less than 0.1% F and 19.4% combined H2O.
600 parts by weight of the phosphogypsum ana 1189 parts
by weight of 67% sulfuric acid at 106C were charged to the
dehydrating zone. During 45 minutes of mixing in the dehydrat-
ing zone, the temperature dropped to 51~C and the phosphogypsum
was dehydrated to anhydrite. The resultant slurry was passed
to the first separation zone and the sulfuric acid proauc-t
filtrate was drawn off. The product sulfuric acid was
analyzed to be 59.5% sulfuric acid and passed on to
acldulation of phosphate ore. The filter cake containing 358
parts of anhydrite plus some gangue from the phosphoric acid
operation was washed with several volumes of water and
screened through a 270 U.S. Standard mesh (53 micrometers)
screen to remove silica sand. The anhydrite filtQr cake was
then found to have a count of 31+3 pCi/g of radium 226 and
had the following analysis: 37.9% CaO, 54.8% SO3, 0.12% SiO2,
less than 0.05% Fe2O3, less than 0.05% A12O3, 0.04% P, less
than 0.01~ F and 0.95% free H20. Scanning electron micro-
photographs, Fig. 3, showed the anhydrite crystals to be
just a few micrometers in size. This anhydrite was split into
two portions for conversion to high purity, low-radioactivity
gypsum.
To a rehydrating mixing vessel was charged 180 parts
by weight of the anhydrite containing radioactive contamination
and a dilute slurry containing 600 parts by weight of water,
~f dm f~ - 10 -
~.~. ,..~.
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31 parts by welght of sulfuric acid accelerator and 20 parts
by weight of gypsum seed crys-tals. Most of the sulfuric acid
accelerator was present for acceleration from the anhydrite
in the first phase, which was not completely washed before
addition to ~his phase. The mixture in the rehydrator was
stirred at ambient temperature Eor 47 hours. Sampling at
that time showed 87% oE the anhydrite had converted to
gypsum. The slurry then was passed to the second separation
zone and, with wet screening and water washing, classified
into the following fractions:
none greater than 50 U.S. standard mesh (297 micrometers)
9.8 weight ~ retained in U.S. standard 100 mesh (150
micrometers)
20.6 weight % passing 100 mesh and retained on 200 U.S.
standard mesh (74 micrometers)
0.2 weight % by weight passing 200 mesh and retained on
270 mesh (53 micrometers)
and 69.4 weight % by weight passing 270 mesh (53
micrometers).
The ~ine fraction, passing 53 micrometers and shown in Fig.
3, was found -to be gypsum crystallites and crystal relics of
anhydrite and upon counting contained 30+3 pCi/g of radium
226. The coarser fractions greater than 74 micrometers,
shown in Fig. ~, were found to be mostly large gypsum
particles but with some particles of anhydrite relic fragments
on the surface, and counted to have 2.3 + 0.09 pCi~g of radium
226. The 53-7~ micrometers fraction was suitable for recycle
seed crystals.
dm:f~ ~ - 11 -
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~` Another 100 parts by weight of the anhydrite obtained
above, 600 parts by weight of water, 60 parts by weight of
sodium sulfate accelerator and 100 parts by weight of the -~53
micrometers gypsum as seed crystal (that was found to contain
some 3 parts by weight calcium carbonate) was charged to the
rehydra-tor and mixed at ambient temperature for 47 hours.
Analysis showed 71% of the anhydrite converted to gypsum.
The slurry was then passed to the second sepa.ration ~one, wet
screened and water washed to the following fractions:
no parts greater than 50 mesh;
22.8 weight % passing 50 mesh and retained on 100 mesh;
62.3 weight % passing 100 mesh and retained on 200
meah;
2.3 weight ~ passing 200 mesh and retained on 270 mesh;
and 12.6 weight % passing 270 mesh.
The ~200 mesh t+7~ micrometers) fraction was air screened to
remove some of the anhydrite relic from the gypsum and, on
analysis was found to give 1.7 + 0.2 pciJg of radium 226. Had
larger anhydrite crystals been grown in the dehydrator section,
as by optionally including a small amount oE anhydrite seed
crystals tlO-50% of the solids by weight), changin~ the total
solids, time, temperature or sulfuric acid concentration of
that slurry in the dehydrator would result in less anhydrite
particles fra~ment on the gypsum and/or less of the anhydrite
containing radioactive contamination being converted to
gypsum, thereby providing lower radioactivity levels in the
coarse gypsum fraction, e.g. less than 1 pCi/g of radium 226.
Further, more complete washing and classification as by
. . ,~ ' .
'~ dm~ 12 -
-~- increased washes, greater separations in particle sizes or
higher efficiency separators or centrifuges would lead to
less anhydrite particle d~sting on the purified gypsum.
The anhydrite relic filter cake may be disposed of.
One means is to admix for example about 10-40 weight ~ of the
radioactive anhydrite with 10 to 40~ phosphatic clay slime
from the phosphate ore beneficiation plant containing about
80% water. This sufficiently dilutes the material that
radioactivity is not a concern. In about 3-6 weeks the water
from the clay slime hydrates the anhydrite and it
crystallizes to form a gypsum binder to set up the mass
forming a strong stable material. This material, and
mixtures cf it with up to about equal weight of sand tailings
from the phosphate ore beneficiation plant may be used for
land reclamation of mined-out areas; road base stabilization
or the like.
The gypsum filter cake is of a purity and particle
size for ready deagglomerating and use as gypsum raw material
source for calcination to industrially usable forms such
as building plasters or gypsum wallboard.
,`J dm~ 13 -
