Feb. 4, 1969 W. C. HAZEN ETAL 3,425,799
RECOVERY OF PHOSPHATE VALUES FROM PHOSPHATIC SLIMES
Filed Aug. 14, 1964 Sheet ---L- of ~\
PHOSPHATE SLIMES
~
H2S04--------~)oo LEACHING
~
FI LTRfTfONL-! I.:,:-A.:,:-I=LS=__~
. FILTRATE
t H2S04·-------~ CATION EXCHANGE---'''-METAL SULFATES
~
NH
3
,----- rSOLVENT EXTRACTION
STJ1'PtG· I 1
AMMONIUM , LJ SULFATE _..l----- rSOLVENT EXTRACTION
NH, s4+:J j
~~~S~~~T~--..t---- RAFFINATE
150
w
(fJ 12.5
<t
:I: a.
u 10.0
Z
<t
c:>
0:: 7.5
0....
"-
r5'
a.N
01
5.0 7.5 10.0 12.5 15.0 17.5 20.0
9 P20s/1 AQUEOUS PHASE
9
INVENTOR.
WAYNE C. HAZEN
ANGUS V HENRICKSON
PABLO HADZERIGA
av~~'~
ATTORNEYS
Feb. 4, 1969 W. C. HAZEN ETAL 3,425,799
RECOVERY OF PHOSPHATE VALUES FROM PHOSPHATIC SLIMES
Filed Aug. 14, 1964
~4
Sheet -il.- or 3
~5
INVENTOR.
WAYNE C. HAZEN
ANGUS V. HENRICKSON
PABLO HADZERIGA
B'!-d~a-J~
ATTORNEYS
Feb. 4, 1969 W. C. HAZEN ETAL 3,425,799
RECOVERY OF PHOSPHATE VALUES FROM PHOSPHATIC SLIMES
Filed Aug. 14, J.964
~_ 6
~8
Sheet 3 of 3
~7
~10
INVENTOR.
WAYNE C. HAZEN
ANGUS V HENRICKSON
PABLO HADZERIGA
!!4~~~
ATTORNEYS
United States Patent Office
1
3,425,799
Patented Feb. 4, 1969
2
3,425,799
RECOVERY OF PHOSPHATE VALUES
FROM PHOSPHATIC SLIMES
Wayne C. Hazen, Denver, Angus V. Henrickson, Golden,
and Pablo Hadzeriga, Arvada, Colo., assignors, by
mesne assignments, to Hazen Research, Inc. a corporation
of Colorado '
Filed Aug. 14, 1964, Ser. No. 389,750
U.s. CI. 23-107 . 7 Claims
Int. CI. C01b 25/26
ABSTRACT OF THE DISCLOSURE
The process is peculiarly adapted to the recovery of
phos~hate values from colloidal slimes and comprises
leachmg the phosphate containing slimes with sulfuric
acid under conditions which form crystals of calcium sulfate
large enough to function as a filtering aid in filtering
out colloidal clay, silica and other solid foreign material
when the leach slurry is filtered after the addition
of sulfuric acid. Large crystals of calcium sulfate are
formed by the combined steps of adjusting the solids
content of the slimes to about 6-14 percent and adding
the sulfuric acid over a period at least one hour at a
slurry temperature of about 50° C. to 80° C.
This invention relates to a method for the recovery of
phosphate values from waste phosphatic slimes, more
~art~cularly, it relates to such a method utilizing crystallIzatIOn
and solvent extraction techniques.
In the United States today there are two major methods
for manufacturing phosphoric acids from phosphate
ore. These two methods are the electric furnace method
and the wet process.
In the electric furnace method the phosphate ore is
reduced in an electric arc furnace at a high enough temperature
to vaporize elemental phosphorus. The elemental
phosphorus is condensed and converted by oxidation and
treatment with water to phosphoric acid of high purity.
Because of this high purity the phosphate product can
be used in the detergent and food industries. This higher
purity product is produced at considerably higher cost
than phosphoric acid made by the wet process.
In the wet process the phosphate ore is treated with
sulfuric acid which converts the calcium phosphate· in the
ore to phosphoric acid solution and calcium sulfate. The
calcium sulfate is removed by filtration, after which the
solution of phosphoric acid can be evaporated to make
concentrated acid for fertilizer or other use. Because of
the impurities in the wet process phosphoric acid, such
as iron, alumina, fluorine and other contaminants, fertilizer
grade phosphoric acid is unsuitable for those products
which find their way into the detergent or food industries.
Its major outlet is for the productionbf triple
superphosphate, ammonium phosphate, or other phosphate
fertilizer products.
In the fertilizer business there is. an increasing interest
in the use of ammonium phosphate and the consumption
of this material for fertilizer is increasing more swiftly
than consumption of other phosphate products. Diammonium
phosphate is made by reacting phosphoric acid
and ammonia and crystallizing the resultant diammonium
phosphate.
For the manufacture of phosphoric acid and also to
produce phosphate rock which can be used as a starting
point for making superphosphate, phosphoric acid and
triple superphosphate, an upgrading process is used on the
phosphate ore as mined. In Florida this upgrading process
consists of washing and removing the slime clay fraction,
followed by a benefication process of the phosphate
roc~ i~ order to remove the impurities such as silica. The
deslImmg process is also practiced on Tennessee phosphate
ore.
During the desliming, from one-third to one-half· of
5 the contained phosphate values are discarded with the
slimes. These slimes are removed, not only because they
are refractory toward presently known upgrading processes,
but they also interfere with the operation of the
flotation on the remainder of the material. These slimes
10 represent a tremendous loss of raw material as well as
constituting an expensive nuisance since they must be impounded
to prevent stream pollution.
Because the slime material is all -150 mesh in size
and predominantly composed of particles less than 10
15 microns in diameter, it is exceedingly difficult to handle
b~ any presently known technique, such as filtration, set~
tlIng, etc. There is sufficient clay contemt so that the slimes
will not settle to avery high density and they retain many
of the disagreeable aspects of colloids. It has been found
20 that several years are required before the solids will settle
to a density as high as 20 percent solids. These slime
materials constitute a very large tonnage of phosphate
which has already been mined and is available. They have
been the subject of a vast amount of thought and experi-
25 mental research. To date the only method of treating the
slimes which has held out much hope is to pond them
in such a way that over a period of years they will drain
and the land can perhaps be reclaimed for agricultural
use. This does not solve the problem of producing a sale-
30 able phosphate product for the slimes.
If ordinary methods of producing wet process phosphoric
acid are attempted using this slime as a feed material,
there are a number of problems which prevent
economic recovery of phosphoric acid. First the material
35 is so difficult to filter or thicken that there is no present
commercial way to make the separation between the clay
solids remaining after acid leaching and the phosphoric
acid in the solution produced by the sulfuric acid addition.
Secondly, even if methods were available, the high
40 water content of the feed slimes would mean that the
phosphoric acid would be exceedingly dilute. This means
a prohibitive evaporation cost to concentrate this dilute
phosphoric acid to a point where it can compete with the
acid produced from the higher grade phosphate rock.
45 It is therefore an object of this invention to provide a
method for economically recovering a saleable phosphate
product from phosphate bearing materials in general
including waste colloidal slimes from conventional
processes for the recovery of phosphate values from phos-
50 phate rock.
It is another object of this invention to provide a process
for the recovery of phosphate values from waste phosphate
slimes by which phosphate rock particles are effectively
separated from colloidal clay, silica and other solid
55 foreign material.
Another object of the invention is to provide an effective
method for separating calcium sulfate from phosphate
values in leach solutions of phosphatic slimes.
Another object of the invention is to provide an ef60
fective method [or recovering phosphate values from leach
solutions in which they exist in low concentrations.
A further object of the invention is to provide a method
for solvent extraction of phosphate Vlalues from dilute
solutions and effective stripping thereof from the solvent.
65 In 'accordance with the invention, the process comprises
leaching phosphate containing materials including waste
phosphatic slimes with sulfuric acid under conditions
which form crystals of calcium sul£ate large enough to
70 function as a filter aid, filtering the leached slimes to
remove most of the solid foreign material, treating the
leach liquor with a cation exchange agent to remove iron
3,425,799
3
and aluminum, removing phosphate values from the leach
liquor by solvent extmction with an amine solvent, and
stripping the phosphate values from the solvent with ammonia
to recover them as diammonium phosphate. To
form crystals of the required size the slimes are reduced
to a solids content of not more than about 6-14 rercent
and the required amount of sulfuric acid added with agitation
over a period of at least one hour at a temperature
between about 50° C. and 80° C. peferablyfollowed by
agitation for a period of about one hour. A modification
of the invention is the recycling of a portion of the
leached slimes to the leach circuit before the next filtration
to provide seed crystals of gypsum. A further modification
is adjustment of the concentration and pH of the
stripping solution by use of countercurrent stripping
stages to provide for selective recovery of the desired
phosphate by crystallization.
The invention is illustrated herein by its application to
the recovery of phosphate values from waste phosphate
slimes but it is by no means restricted in application to
this starting material as it is likewise applioable to phosphate
containing materials in general, and particularly, to
low grade phosphate materials.
The invention will be explained by reference to the accompanying
drawings in which,
FIG. 1 is a fiowsheet illustrating the process of the
invention;
FIG. 2 is a photomicrograph of solid material of unleached
phosphatic slimes;
FIGS. 3-8 are photomicrographs showing crystals of
calcium sulphate formed by the treatment of phosphatic
slimes with sulfuric acid under conditions, in which
FIG. 3 shows the effect of rapid addition of acid on
cryst,al formation;
FIG. 4 shows the effect of temperature on the formation
of crystals;
FIGS. 5, 6 and 7 show the effect of the time period
over which acid is added on crystal formation, and
FIG. 8 shows the effect of seeding on crystal formation;
FIG. 9 is a plot of isotherms obtained with a variety
of solvent and diluent combinations, and
FIG. lOis a photogr,aph of a magnification of 26 X of
ammonium phosphate crystals produced by stripping an
amine solvent with ammonia after extraction of phosphate
values from leach liquor with the solvent.
Reference is now made to the fiowsheet of FIG. 1
which illustrates the process of the invention applied to
phosphate slimes resulting from desliming in the treatment
of phosphate rock by conventional processes to recover
phosphate values.
The slimes are leached with sulphuric acid, the conventional
leaching agent for the treatment of phosphate
rock to recover phosphoric acid. Since phosphoric acid is
removed from the leach liquor by solvent extraction, there
is no particular requirement in the acid leaching that the
pulp density during leaching be such as to produce a concentrated
phosphoric acid. Accordingly, an ordinary agitation
leach comparable to that used in the uranium industry
for dissolving uranium with sulfuric ,acid was used.
In order to test the process it is important to know the
acid consumption requirements of the slimes being tested.
The sulfuric acid which is consumed during the leaching
of phosphate rock is a function of the PzOs content of the
rock and of the other acid soluble constituents. The presence
of calcium carbonate results in very high acid consumption.
In the case of the Florida slimes which were
investigated rather intensively, the carbonate content is
small, but the proportion of alumina and iron present is
high. Since this has an important effect on the acid consumption,
tests were made to determine the extraction
of PzOs from the Florida phosphate slimes as a function
of the quantity of sulfuric acid added.
The following procedure is typical of that used to determine
acid consumption: Material from a sample of
Florida phosphate slimes having a PzOs content (dry
4
basis) of 15.4 percent and weighing 453.8 grams, at a pulp
density of 10 percent solids, was heated to 60° C. while
being gently agitated. To this pulp, 14.8 grams of sulfuric
acid was added over a period of two hours. This is a
ratio of 2.0 pounds of acid per pound of PzOs percent in
5 the heads. When the addition was complete the slurry was
stirred for an additional hour, after which it was filtered.
The cake was washed with water until no more acid was
in the filtrate, dried, weighed and assayed. The final pH
10 of the pulp prior to filtration was 1.8. Additional tests
were made using ratios of 2.4, 2.6 and 2.8 pounds of
sulphuric acid per pound of PzOs contained in the slime
sample. The percent extraction of the phosphate contained
in the slimes was plotted as a function of the sulfuric
15 acid to PzOs ratio. The plot showed that a maximum
extraction of 92 percent of the PzOs was obtained when
2.6 pounds of sulfuric acid were 'added for each pound of
PzOs contained in the heads.
As mentioned previously, one of the ,big problems in
:20 recovering phosphate values from waste phosphatic slimes
is the separation by filtration or otherwise, of solid calcium
sulphate ,from the phosphate values in solution.
Filtration ,rates of slimes leached by addition of sulfuric
acid in the conventional manner are impossibly slow. It
25 was found that if sulfuric acid is added under conditions
which result in the formation of proper size crystals of
calcium sulfate, the crystals act as a filter aid and successful
filtration is effected.
The reaction between the phosphate rock and sulfuric
30 acid causes about three pounds of gypsum crystals to be
precipitated for each pound of PzOs which is dissolved
from the rock. The net result is that after leaching, the
solids remaining are nearly 50 percent gypsum by weight.
By using the appropriate crystallization technique these
3.., gypsum crystals can be caused to grow large enough so
that they effectively trap the clay slimes during filtration
,and the net result is that ordinary filters become economical.
A set of conditions for leaching Florida slimes with
40 sulfuric acid was developed which causes the growth of
gypsum crystals to such large size that they constitute a
satisfactory "filter aid" for filtration of the leach residue
with filter capacities of 300-400 pounds of filter cake (at
40% solids) per square foot of filter area per 24 hours.
The 40 percent solids filter cake is dry enough so that it
45 can be handled on a conveyor belt and stacked in piles.
The acid consumption during a standard leaching of the
Florida slimes is 2.8 pounds of sulfuric acid consumed
per pound of PzOs dissolved.
In accordance with prior art processes, phosphoric acid
50 may be solvent extracted from leach solutions formed
by the treatment of phosphate rock with hydrochloric
acid, the extracting agent being tri-butyl phosphate in one
method and 4-5 carbon chain aliphatic alcohols in another
method. While both of these methods are operable
55 for extracting phosphoric acid from high cWoride content
solutions, neither seems to be operable on dilute phosphoric
acid solutions obtained by the treatment of phosphate
rock or phosphate slimes with sulfuric acid.
Long chain organic amines will extract anions, includ-
60 ing sulfate, and in fact, these reagents were developed in
the uranium industry for the extraction of the anionic
form of uranium which exists in sulfate solution. Since
these amines are chemically considered to be bases they
would be expected to react with phosphoric acid to form
65 the or,ganic amine phosphate.
The preferred amine extractants are those in which the
alkyl substituents have six or more carbon atoms in the
chain, including branched chain alkyl radicals. Examples
70 are tri-laurYI amine which is a tertiary C1Z straight chain
amine, tri-caprylyl amine, a tertiary amine, di-Iauryl
amine, a secondary straight chain amine, do-decenyl-tri
alkyl methyl amine, a homologous mixture containing 2427
carbon atoms, a secondary amine, and tri-alkyl methyl
75 amine, a homologous mixture containing 18-24 carbon
3,425,799
30
6
Although this does introduce an additional step, it is not
a serious one and it permits the recovery of some of the
excess sulfate whi'ch was used in the leaching.
One of the major advantages of this solvent extraction
system is that in stripping with ammonia gas it is possible
5 to operate under circumstances such that the solubility
of ammonium phosphate salts is exceeded in the stripping
circuit and the crystalline final product :can be produced
directly during stripping without the. necessity of having
10 a separate crystallizer. By using the appropriate number
of stages and proper control it is possible to produce a
mixture of mono-ammonium and di-ammonium phosphate
if desired, or a relatively pure di-ammonium phosphate
alone. As the various phosphates crystallize at dif-
L:; ferent pH values, the required number of countercurrent
stages can be used to provide saturation of the stripping
solution and the required pH for the selective recovery ot
the desired phosphate. Since the system provides for the
removal of aluminum and other cations and the sulfate
20 content 'can be controlled, it is possible to produce a diammonium
phosphate of almost any desired purity.
The results showed that the loaded solvent from the extraction
circuit can be stripped completely of its phosphate
content with gaseous ammonia under circumstances which
25 produce a strip liquor of such high concentration that
ammonium phosphate crystals are produced during the
stripping operation. There was no entrainment of organic
material on the solid crystals produced and they settled
completely into the aqueous phase without emulsion.
The process can be used on 'Ore without desliming or
flotation for upgrading because it is not necessary to have
a high concentration of phosphoric acid produced during
the leaching operation.
Leaching and filtration
A number of examples are included below which show
the results of tests made to determine the optimum process
limitations for forming the required type of crystals of
calcium sulfate which are effective as a filtration aid. The
following table lists the available analyses for the samples
40 of phosphatic material used.
5
atoms, a primary amine. The chemical !Compounds or
compositions represented by trade names used herein are
as follows: "Alamine" is the trade name for tri-capryl
amine. "AmberIite LA-I" is the trade name for dodecenyl-
trialkylmethylamine. "Dowex 50W-X8" is the trade
name for a cation exchange agent, sold by the Dow Chemical
Company of Midland, Mich., which is a strongly acid
sulfonated styrene divinyl benzene. The cation exchange
agent or material is one whieh contains what might be
referred to as a solvent interfering cation. The material
contains H+ as the 'cation which is exchanged or replaced
by cations of iron, aluminum and other metals which exist
as impurities. There are a number of equivalent cation
exchange agents available on the market having this general
composition which can be used.
The problem in connection with the use of amine solvents
is not so much from the extraction standpoint, but
rather the difficulty of stripping the phosphate from the
loaded solvent. Since they are salts (amine phosphates)
stripping them with water by a hydrolysis reaction yields
solutions of phosphoric acid that are very dilute. Stripping
may be accomplished by using storage acids but this results
in a mixture of the stripping acid with the phosphoric
and presents the problem of converting the amine after
stripping to the free-base form for return to the extraction
circuit. However, if the final product can be ammonium
phosphate rather than phosphoric acid, then it is
possible to use ammonia for stripping, thereby removing
the phosphate from the solvent and at the same time converting
the solvent to the free-base form for recycle.
Experiments proved that the extraction was not as
straight forward as expected due to the formation of
emulsions from the presence of cations, such as, aluminum
and iron, which had to be removed before complete extraction
of the phosphate was achieved without emulsion 35
formation.
A numper of solvent extractants were tested, and while
a low distribution coefficient and low loading were experienced
for some of the solvents, they were effective in
general for extraction. Some solvents were more effective
Percent Percent Percent Percent Percent Percent
Solids P,O, AJ,O, FeD F SiD,
Sample:
L_____________________ 16.0
2______________________ 12.7
3______________________ 13.1
4______________________ 9.4 5 _
6" _
7 _
8 _
15.4
9.4
11. 6
7.7
52.0
30.0
52.6
18.3
3.0
0.90
1. 96
1.4
1.0
0.77
1. 52
0.9
1.3
0.9
2.2
1.4
2.3
0.12
0.03
0.07
14.7
Samples 1-4 were Florida phosphatic slimes, Sample 5 was
green phosphoric acid from Utah, Samples 6 and 7 were
diluce and concentrated fertilizer grade phosphoric acid
respectively, Sample 8 was phosphate ore from Wyoming.
55 Filtration tests were performed using a 0.1 square foot
filter leaf. After some experimentation it was decided that
the best filter media to use was a pOlypropylene multifilament
twill cloth. Vacuum used ranged between 15 and 18
inches.
EXAMPLE 1
A blank was run to test the filterability of untreated
Florida slimes. Leaf filtration tests were run on material
from the No.1 sample diluted from the 16% solids to
10% solids content with distilled water. Leaf filtration
tests were also run directly on the slimes. The filtration
rate was so low as to be practically non-existent. The
filtrate which was obtained in all cases was very cloudy
and at best the filter cake was never more than :y[o of an
inch thick no matter how long the leaf was left in the
stirred slurry. The thin slime coating on the filter cloth
could not be blown off with air pressure as it had effectively
impregnated the cloth. Several efforts to improve filtration
of the original slime material through addition of
reagent and changes in procedure were uniformly unsuccessful,
and the conclusion was reache'd that the slimes are
than others, for example, isotherms were obtained on the
extraction of the dilute phosphoric acid which showed
that 98 percent extraction of the phosphoric acid could
be extracted in a four-stage countercurrent circuit using
a 20 percent tertiary amine dissolved in an aromatic solvent,
such as benzene.
The flowsheet shows a preliminary selective stripping
step for removing ammonium sulfate from the loaded
solvent. Since there is inevitably residual sulfate in phosphoric
acid which is produced by the sulfuric acid leach- 60
ing of phosphate rock, this sulfate must be accounted for
in the solvent extraction by an amine extractant. It was
found that sulfate extracts more strongly than phosphate
and therefore will be extracted first. Whether the amount
of sulfate which would be produced as ammonium sulfate 65
with the ammonium phosphate in the stripping circuit will
be detrimental depends upon the specifications of the
product. With many slimes and ores the sulfate stripping
step wilInot be necessary and the ammonium phosphate
can be stripped directly. If the quantity of sulfate present 70
is more than that which can be tolerated in the final product,
then it will be possible to operate a first stage extraction
in which a small quantity of solvent is loaded up
preferentially with sulfate and stripped in a separate stripping
circuit with ammonia to produce ammonium sulfate. 75
3,425,799
Extraction
EXAMPLE 6
A small quantity of "Dowex 5OW-X8" cation resin in
the hydrogen form was placed in a I-inch diameter tube
and leach liquor obtained by leaching a sample of
Florida slime and filtering was passed through at a rate
of 10 m!. per minute. The analyses of these solutions
showed .9 gram per liter of aluminum ion and .3 gram
of ferric iron per liter in the feed. The phosphoric acid
concentration was 16 grams per liter P20 5• The solution
issuing from the ,bottom of the column contained. only
traces of aluminum and iron. The pH of the inflqepce
solution was 1.7 while that of the discharge was 1.3, indicating
that exchange had taken place between the cations
and hydrogen ion on the resin. The solvent extraction
work on the liquor issuing from the ion exchange
column was free of emulsions, indicating that the removal
of cations had overcome this problem. The cation
exchange resin was eluted with sulfuric acid,after Which
it was ready for re-use.
As the flowsheet shows, the final extraction step follows
the cation removal step. Isotherms showing the extraction
of phosphoric acid from relatively concentrated
pure solutions using a C8-ClO straight chain tertiary
8
circumstances of the leach were such that the gypsum
crystals were very large in size, filtration rates of between
300 and 400 pounds of wet filter cake (40-45% solids)
per square foot of filter cloth area per 24 hours were
obtained. Tests showed that the solids content of the slimes
5 is an important factor in the formation of the proper size
crystal. When the slimes are at a solids content between
about 6 and 14 percent the agitation can be performed
smoothly under circumstances which permit growth of
larger gypsum crystals than are obtained by a more violent
10 type of agitation. When the pulp density was much higher,
the agitation intensity had to be increased meterially<in
order to provide a reasonable distribution of th'e sulfuric
acid as it was added. A preferred solids content fbr typical
15 Florida slimes is 10 percent. It was not possible to provide
smooth and uniform agitation of the slini.es at solids content
higher than about 14 percent, therefore a preferred
10 percent solids pulp density was used throughout the
test work. Because of the marked variation in clay type
and content of slimes from different areas, the maximum
20 of 14 percent solids will not hold for all cases. As stated
above, an operative range for the solids content is between
about 6 and 14 percent.
EXAMPLE 5
It was found that crystal growth induced by slow addition
of sulfuric acid at 60° C., and proper pulp density
is improved even further if a portion of the leached
slimes before filtration is recycled to the leach circuit to
provide seed crystals of gypsum. FIG. 8 shows a photo-
30 micrograph of the crystals which were produced by recycling
10% of the total weight of a leached slurry of
Florida slimes to another batch of fresh slimes. The crystals
have a maximum size of 48 mesh.
The results of the crystal formation tests showed that
3,} leaching of colloidal phosphatic slimes can be performed
under conditions which result in the formation of crystals
of adequate size to constitute a filtering aid such
that solid calcium sulfate and other solid foreign mate-
40 rial can be effectively separated from the phosphate
values in solution.
As previously mentioned, the preliminary extraction
test work showed that when the phosphoric acid concentration
had been reduced through extraction eventually
various precipitates began to form, which interfered seriously
with the subsequent solvent extraction because, of
emulsion formation. These precipitates are solid phosphates
of aluminum, iron and other cation impurities.
50 The following example was performed to test the efficiency
of a cation resin for the removal of these cations.
EXAMPLE 2
EXAMPLE 3
EXAMPLE 4
7
not filterable in any ordinary commercial sense. FIGURE
2 is a photomicrograph taken of untreated material from
Sample 1, a Florida slime. This figure serves as a blank
for comparison with other figures showing the results of
the application of various process limitations to the filtration
step.
Example 2 was performed to show the effect on formation
of calcium sulfate crystals of the rate of addition
of acid. It was found that if sulfuric acid is added swiftly
to the slimes during agitation the gypsum crystals which
are produced are exceedingly small. The filtration rates
are very low and are nearly the same order of magnitude
as untreated slimes. It was also found that the small
crystals were formed regardless of the temperature or
solids content whenever the sulfuric acid was added swiftly.
In one test, sulphuric acid was added swiftly to material
from Florida Sample No.1 at 60° C., followed by
one hour of agitation after the addition of acid. FIG. 3
is a photomicrograph of a sample of the leached slurry,
the magnification in this photograph being 200 X. The
average crystal size is less than -325 mesh and the
crystals were ineffective as a filter aid. Further tests were
run in which the sulfuric acid was added to the slimes in
the proper amount over a period of ten minutes, followed 25
by periods of agitation for as long as 24 hours at 60° C.
Examination of these leached slurries at various times during
this agitation always showed the same small size
gypsum crystals, and there was apparently no further
growth.
Tests were made to determine the effect of temperature
on crystal formation. Using material from Sample No.1
having a solids content within the operable range, sulfuric
acid was added slowly over a period of two hours, followed
by an additional hour of digestion, all at room
temperature. FIG. 4 shows the gypsum crystals formed,
the average size being about 325 mesh. Tests on the
Florida slimes 1-4 showed that with exactly the same set
of leach conditions an improvement in the growth of gypsum
crystals was obtained by increasing the temperature to
about 50° C. No particular improvement was found until
50° C. was reached, after which improvement was marked
in the temperature range between 60° C. and 70° C. At
80° C. there was a noticeable increase in the viscosity of 45
the acid leached slurry and even at 10 percent solids it
was not possible to achieve a smooth agitation condition.
Above 80° C. it was not possible to make an acid leach
in which the gypsum crystals were of a proper size to
fuction as a filtration aid.
A short series of qualitative tests using the microscope
for examination of the slurry showed that if the sulfuric
acid were added at a constant rate over a period of at least 55
one hour while the slurry was held at 60° C. crystal
growth could be induced. An addition period of at least
two hours is preferred. An additional hour for digestion
after the end of the addition of sulfuric acid was found to
be beneficial. FIGS. 5, 6 and 7 show the increase in size 60
of the gypsum crystals in a leached pulp at various stages
of a leach in which sulfuric acid was added to Florida
slimes from Samples 1-4 over a period of two hours at a
uniform rate, followed by one hour of gentle agitation for
digestion. The leach temperature was 60° C. for this test, 65
and as can be seen, the gypsum crystals had grown to a
large size upon completion of the procedure. FIG. 5 shows
the crystals after about lI:J of total acid had been added,
the gypsum crystals averaging less than 400 mesh in size.
FIG. 6 shows the crystals after the addition of about 2h 70
of total acid, the gypsum crystals having a maximum size
of about 150 mesh. F1G. 7 shows the crystals after total
acid addition of acid followed by one hour of digestion,
the gypsum crystals maximum size being about 100 mesh.
This size is highly suitable as an effective filter aid. When 75
3,425,799
10
25
tion, it is possible to precipitate either the monoammonium
or the diammoniumphosphate.
In the next stage where the solubility of the phosphate
salts need not necessarily be exceeded an excess of am-
5 moniacanbe used to assure complete stripping. The
aqueous phase from this second stage then goes in a
countercurrent fashion to provide the solution for the
first stage and for the crystallization of the ammonium
phosphate.
10 Many batch stripping tests using an excess of ammonia
were performed. FIG. 10 isa photograph ata magnification
of 26X of some of the ammonium phosphate crystals
which were produced by stripping an amine with· ammonia
after extraction with amine solvent from a leach
15 liquor formed f,om Florida slimes.
The stripping tests demonstrated that the phosphate
was completely removed from the amine solvent by treatment
with ammonia gas, that crystalline ammonium phosphatecan
be produced, and that there was no excess
go ammonia carried back to the leaching circuit from the
strip circuit. There was no visible evidence of organic'
solvent clinging to the crystallized salts. The phosphate
crystals settled clearly and cleanly into the a9ueous phase
without entraining any organic material.
The above description and supporting examples illustrate
that a combined process for the ,effective recovery
of phosphate values in saleable form from phosphatic
slimes has been provided. The process provides an effective
method for separating calcium sulfate from phosphate
30 values in leach liquors resulting from the treatment of
phosphatic slimes with sulfuric acid. An effective solvent
extraction procedure is provided for recovering the phosphate
values from the leach liquor, the process being
adaptable for operation in the presence of iron and alumi-
35 num ions. The process includes a final stripping step for
stripping phosphate values from the solvent extractant
and recovering them as the required phosphate. If an
ammonium phosphate is the required mnal product the
concentration of the stripping medium can be adjusted
40 to provide the product in crystalline form.
Although the invention has been illlllstrated and described
with reference to the preferred embodiments thereof,
it is to be understood that it is in no way limited to
the details of such embodiments, but is capable of numer-
45 ous modifications within the scope of the appended claims.
What is claimed is: .
1. The method of recovering phosphate y~lues from a
mixture comprising clay and phosphate slinies having an
average particle size not in excess of 10 microns in diame-
50 ter, which method comprises: adjusting the solids content
of the mixture of slimes to between about 6 to 14
percent by weight; adding to the mixture at a temperature
of about 50° C.-70° C. over a period in excess of
one hour at a substantially constant rate at least the
55 amount of sulfuric acid required to leach the phosphate
values present to form large crystals of gypsum; filtering
the resulting slurry whereby the large crystals of gypsum
trap the clay slimes and serves as a filter aid; extracting
the phosphate values from the filtrate by solvent extrac-
60 tion; and stripping the phosphate values fFOm the solvent.
2. The method of claim 1 in which the solids content
of the mixture of slimes is adjusted to about 10 percent
by weight; and the sulfuric acid is added over a period
of about 2 hours at a temperature between about 60° C.
65 and 70° C. to form large crystals of gypsum.
3. The method of claim 1 in which a portion of the
leached unfiltered mixture of slimes leached in accordance
with the method of claim 1 is recycled to untreated clay
70 and phosphate slime mixture before the untreated mixture
is leached and filtered.
4. The method of claim 1 in which a solvent extraction
system containing an amine solvent is used consisting
essentially of 20 percent amine solvent for the phos75
phate values dissolved in an aromatic solvent.
9
amine (General Mills' "Alamine 336") indica,ted that
98.5 percent extraction could be obtahled in four stages
of extraction with a solvent loading of 40 grams per
liter as long as the initial solution was relatively concentrated'in
phosphoric acid. Because, the leachliq1.Jpr
obtained by leaching .Florida slimes will contain only a
few percent phosphoric acid rather than the 30 perc~nt
obtained from acid leaching of phosphate rock, the solvent
extraction of more dilute phosphoric acid must be
accomplished. Tests showed that solvent systems effective
for stripping from concentrated solutions of phOSphoric
acid are not necessarily effective on dilutesolu-
~L .
In order to improve the extraction coefficient and ultimately
the loading of the solvent, tests were made using
Florida phosphate slimes on the effect of various diluents
on extraction coefficient. FIG. 9 shows isotherms of various
solvent systems.
The solvent systems represented by the various curves
A-F are as follows:
A-20% Alamine-20% Decanol-60% CaHa
B-lO% Alamine-l0% Decanol~80% CHCl3
C-lO% Amberlite LA-I-lO% Decanol-80% CaHa
D-I0% Alamine-l0% Decanol-80% CaHa
E-1O% Alamine-lO% Decanol-80% Solvesso
F-I0% Alamine-90% CaHa
The isotherms show that all of the solvent systems are
operative; however, the very high extraction coefficient
which was obtained when the concentration of the ter,tiary
amine was increased and benzene used as a diluent
demonstrated the effectiveness of this solvent system.
The question of how much sulfate may be permitted
to go along with the phosphate can only be resolved on
the basis of the required specifications of the final product.
In one test a leach liquor containing 11.9 grams per
liter of P205 and 4.58 grams per liter of sulfate was subjected
to successive solvent extraction steps using a
tertiary amine-isodecanol solvent in kerosene diluent. The
first four stages of extraction removed 100 percent ofsulfate
ion and only 13.3 percent of the P20 5• The fact has
been clearly demonstrated in various tests that the sulfate
ion is much more strongly bound by the amine solvent
than is the phosphate and this can ,be used as the basis for
a separation between sulfate and phosphate in the standard
way.
The extraction tests demonstrated that the phosphate
values can be effectivdy extracted with amine solvents
from leach solutions resulting from the leaching of pho~·
phate slimes in accordance with the above described
leading procedure.
Stripping
In accordance with the fiowsheet, the ,final step in the
process is stripping the phosphate values from the loaded
amine solvent. Since the final product desired is ammonium
phosphate, ammonia was used as the stripping
agent. However, the invention is not limited to ammonia
as a stripping agent unless the product required is an
ammonium phosphate. Alkali metal stripping agents, such
as sodium and potassium hydroxides and carbonates may
be used if alkali metal phosphates are the desired final
products. Water is, of course, used as required.
In the preliminary exploratory work the various loaded
solvents were stirpped by agitation with a small quantity
of water while ammonia gas was blown into the mixer.
It was soon found that the ammonia was capable of completely
stripping the phosphate from the amine and if
the quantity of water was low enough so that it would
become saturated with respect to one of the various ammonium
phosphate salts, then crystals of the ammonium
phosphate would form in the aqueous phase when the
mixture was allowed to settle. Because it is possible to
use a countercurrent system in which the pH can be controlled
at any desired value in the first mixer settler stage
where the loaded organic first meets an ammoniacal solu3,425,799
OSCAR R. VERTIZ, Primary Examiner.
15
HOKE S. MILLER, Assistant Examiner.
12
an average size in excess of about 325 mesh and upon
filtering a filter cake of at least 35 percent solids is
obtained.
7. The improvement of claim 6 in which a portion of
the leached unfiltered mixture of slimes is recycled to untreated
clay and phosphate slime mixture before the untreated
mixture is leached and filtered.
23-122, 165, 309
11
5. In the method of recovering phosphate values from
a mixture comprising clay and phosphate slimes having
an average particle size not in excess of 10 microns in
diameter and a P20 S content not in excess of about 15.4
percent in which the mixture of slimes is leached with
sulfuric acid to convert at least a portion of the phos- 5
phates present to phosphoric acid and the formed slurry
filtered to provide a leach solution as a filtrate, the improvement
which comprises: adjusting the solids content
of the mixture of slimes to between about 6 to 14 percent 10
by weight; adding the sulfuric acid at a substantially constant
rate over a period from about 1 to 2 hours at a
temperature between about 50° C.-70° C. to form relatively
large crystals of gypsum to serve as a filtering aid
when the leach slurry is filtered; and filtering the leach
mixture.
6. The improvement of claim 5 in which 2-2.8 pounds
of sulfuric acid per pound of P20 S dissolved from the
mixture of slimes is added to provide about three pounds
of gypsum crystals per pound of P20 S dissolved having 20
896,016
3,811,105
References Cited
FOREIGN PATENTS
5/1962 Great Britain.
4/1963 Japan.
U.S. CI. X.R.