r
United States Patent Office 3,359,067
Patented Dec. 19, 1967
1 2
Accordingly, it is an object of this invention to provide
an economically feasible method for producing usable
phosphates from 'phosphate ores, in which sulfurous acid
is used as the leaching agent.
It is another object of this invention to provide a
method for recovering usable phosphates from low-grade
phosphate ore utilizing sulfurous acid as the leaching
agent in which the sulfur dioxide used to form the sulfurous
acid can be .recovered and cycled for reuse in the
10 process.
It is another object of this invention to provide a process
for recovering usable phosphates from low-grade phosphate
are in which suHur dioxide is used as the leaching
agent to form sulfites and phosphates in solution which
15 can be separated and isolated and the sulfur dioxide recovered
from the sulfite for reuse in the process.
The above objects are accomplished by the method of
this invention which, for the purposes of explanation, can
be considered to comprise three steps, namely, a leaching
20 step, a separation step and a sulfur dioxide recovery step.
In the preferred modification of the leaching step, sulfur
dioxide is bubbled through a water slurry of the ore at
a 15: 1 dilution for 24 hours at a temperature of 4 0 centigrade
and a pH of about 1 to form a solution which is
25 essentially mono-calcium phosphate and calcium bi-sulfite,
Alternatively, dilutions of at least 5: 1 of ore to slurry
may ·be used accompanied by successive extractions. In
the separation step, the usable phosphate in the form of
phosphoric acid is separated ,by solvent extraction by
30 means of an organic solvent or other conventional process.
The sulfite remaining after removal of the phosphate
is recovered by conventional methods and heated to recover
sulfur dioxide for reuse in the process in accordance
with well known procedures.
The following reaction is postulated as occurring in
the reaction between phosphate ore and sulfurous acid:
Caa(P04)2+4S02+4H20=Ca(H2P04)2+2Ca(HSOg)2
The Ca(HSOah is broken down with heat as follows:
Ca(HSOg)2+heat=2CaO+4S02+2H20
By recycling the sulfur dioxide for ,reuse, it is apparent
that reagent consumption is reduced to a minimum.
The invention is illustrated by the treatment O'f Wyo-
45 ming high-lime phosphoria formation rock. The rock is
low in phosphate content and high in lime which makes
the use of ,a standard sulfuric acid process for producing
phosphoric acid or phosphate fertilizers higher in cost
than if a commercial 31% P205 ore were available as a
50 feed. The lime content of the are, as well as the final
form in which the phosphate is produced, determines the
net acid consumption. The rock used analyzed 21.2%
P205'
As to the leaching step, early experiments established
55 that the phosphate ote is readily attacked by acids even
at a low temperature if the pH is low. If the ,acid which
is used to promote this attack also converts the calcium
to a compound which can be decomposed by heat, it
offers a possibility of cheap recovery of the acid for
60 reuse. An acid which offers this possibility is sulfurous
acid. If it is strong enough to attack the phosphate, it
should form mono-calcium phosphate or phosphoric acid
and at the same time calcium sulfite or calcium bi-sulfite.
The sulfite compounds can then be separated from. the
65 phosphates and decomposed by heat to form lime and
sulfur dioxide which can be recycled for reuse in the
process.
Product: Pounds H2S04 required per unit P20 5
Phosphoric acid 41.5 40
Mono-calcium phosphate 27.8
Di-calcium phosphate 13.9
3,359,067
METHOD FOR THE RECOVERY OF CALC~
PHOSPHATES FROM HIGH LIME CONTENT
PHOSPHATE ORES
Angus V. Henrickson, Golden, and Emo L. Coltrinari, 5
Arvada, Colo., assignors, by mesne assiguments, to Susquehanna
Western, Inc., Denver, Colo., a corporation
of Wisconsin
No Drawing. Filed Dec. 17, 1962, Ser. No. 244,930
11 Claims. (CI. 23-109)
This provides an indication of the amount of sulfuric
acid required to produce the various phosphates.
While the cost of sulfuric acid necessary for the standard
sulfuric acid process is not 'prohibitive when used on
a commercial grade ore containing around 31% phosphorous
pentoxide, it is prohibitive for low grade-low
phosphate ores existing in quantities in various parts of the
country having a P20 5 content of around 21 %.
The above emphasizes the importance of developing a
method for producing usable phosphates from low-grade
phosphate are, which is cheaper than a process utilizing
sulfuric acid. A method which has been extensively tested
in the past involves the use of sulfurous acid in the leaching
step to dissolve the tri-calcium phosphate in the phosphate
are. Past attempts to use this acid have been economically
unsuccessful mainly because the production of
usable phosphate involves the addition of prohibitively
high cost materials and no feasible expedient was availa,
ble for the reuse of the sulfurous acid or the sulfur dioxide
from which it was made, and no economically feasible
method was available for separating the usable
phosphate from the other components of the solution
formed by the reaction of sulfurous acid on the tri-calcium
phosphate.
This invention relates to a method for the recovery of
usable phosphates from high lime content phosphate ores.
More particularly, the invention relates to such a method
in which the phosphates in the ore are brought into solution
in the leaching step by the use of sulfurous acid.
One method for the production of usable phosphates
from phosphate rock involves the use of sulfuric acid
in an amount sufficient to convert the phosphorous content
to a predominantly monocalcium phosphate solution,
Le., a solution having a minor but appreciable quantity
of phosphorous present as phosphoric acid. The cost of
the acid used is an important consideration as respects
the economic feasibility of the process; it follows also
that the amount of the acid used is an important factor
in the cost. The phosphate constituent of phosphate rock
is predominantly tri-calcium phosphate. The reaction between
the tri-calcium phosphate and sulfuric acid may
be looked upon as a successive addition of acid to phosphate.
The addition of one hydrogen ion per phosphate
radical to the tri-calcium phosphate would result in the
formation of di-calcium phosphate, the addition of two
hydrogen ions per phosphate would give mono-calcium
phosphate and the addition of three hydrogen ions results
in phosphoric acid. The theoretical sulfuric acid requirements
per unit of phosphorous pentoxide to produce 35
various phosphates from tri-calcium phosphate are as
follows:
,
3,359,067
18.3
18.8
12.7
13.7
14.5
17.5
Solution Assay
P,O"g.p.!.
1.2
1.0
1.0
0.9
0.9
1.0
1.0
1.0
pH
200 21.5% ---_
1,160 15.4g.p.L____ 82.0
25.6 17.9%_________ 18.0
RESULTS
4
TABLE II
TABLE III
TABLE II-Continued
DETAILED LEACH DATA
o
2.5
6.0
9.0
14.0
23.0
27.0
31. 5
46.5.
Agitation Time;
Hours
TEST NO. 2
SO, Leaching at 10: 1 Dilution, Temperature 3° C.
Single Stuge
CONDITIONS
100 grams 01 -100 mesh ore
Temperature, 3° C. Dilution, 10: 1
Mechanical agitation in 2 liter flask
Agitation time 48 hours total
SO, bubbled through pUlp continuously
CONDITIONS
66;7 grams 01-100 mesh ore
Temperature, 3° C. Dilution 15: 1
Mechanical agitation in 2 liter flask
Agitation time 44 hours total
SO, bubbled through pulp continuously
DETAILED LEACE: DATA
TEST NO.3
SO, Leaching at 15: 1 Dilution, Temperature 3° C.
Single Stage
Heads _
Filtrate plus WasIL _
Tails _
TABLE I 45
3
In the leaching of the ore with S02 a slurry of finely
ground ore and water is formed and diluted to about 15
parts of water to 1 of ore. The high dilutions were required
because it was discovered that there is a limit to
the solubility of phosphate in solution in the presence 5
of high concentrations of calcium bi-sulfite. The pH of
the solution is maintained at about 1 the temperature at
about 4° centigrade and the 802 is bubbled through the
slurry fora peliod of about 24 hours.
The phosphate from Wyoming ore is relatively easy 10
to dissolve if there is sufficient ,acid present to hold the
pH of the solution down to about land at least below
1.5. The important point in obtaining high leach extraction
is to have enough free acid present. Since sulfur
dioxide gas in water forms sulfurous acid which is a 15
weak acid, the concentl1ation of sulfur dioxide in solution
must be quite high in order to have enough free hydrogen
ions to dissolve the phosphate. This high concentration
of 8°2.is maintained by conducting the process at
a low temperature. 20
The leaching procedure by which the experimental
data given in Tables I-VI below was produced is as
follows: The ore used as heads for all the sulfur dioxide
leaching was the Wyoming high lime content phosphate
rock described above. 'Portions of this ore were ground 25
to 100 mesh size in a pulverizer. The particle size, however,
is not critical. Weighed amounts of the finely
ground material were mixed with an appropriate amount Weight, Ass,y Distribution
of distilled water in a 2 liter "resin flask" equipped with Grams P,O, ~~ic~~t
a propeller agitator and set in a cold water bath.· The 30 ----------1---1------1----temperature
of this bath was kept low by mechanical
refrigeration. During leaching, gaseous 100% 802 was
bubbled through the pulp via a glass frit at a slow rate.
At proper time intervals a sample of the pulp was with·
drawn and allowed to settle while maintained at the 35
reduced temperature. A portion of the supernatant
liquor was decanted for assay and the rest of the sample
returned to the leach slurry. At the completion of leaching,
the entire pulp was filtered, washed with water and
the solids dried, weighed and analyzed for P205' The 40
filtrate plus wash. solution was measured and analyzed
for P205' The data from the tests is presented in
Tables I-V below:
Assay
P,O,
66.7 22.1% • __
1,100 13.4g.p.L____ 98.9
13.6 1.3%__________ 1.1
RESULTS
Weight,
Grams
TABLE IV
Agitation Time, pE: Solution Assay
Hours P,O"g.p.l.
---
0 ------- 0
3.5 1.0 9.52
19.0 1.4 13.35
23.0 1.0 13.42
27.0 1.0 13.40
44.0 1.0 ----~----------
Heads _
Filtrato plus wash _
Tails •__
55
50
Distribution
oIF,O"
60 1 1 1 percent_
65
Agitation Time, pH Solution Assay
Hours P,O"g.p.!.
0 8.7 0
1.2 1.7 4.0
5 1.7 7.2
13 1.4 8.7
17 1.7 10.2
21 1.7 10.6
25 1.7 11.1
39 1.7 10.8
DETAILED LEACH DATA
TEST NO. 1
SO, Leaching at 5: 1 dilution, Eoom Temperature
Single Stage
CONDITIONS
200 grams 01 -100 mesh ore
Temperature, 18° C. Dilution, 5: 1
Mechanical agitation in 2 liter flask
Agitation time, 39 hours total
SO, bubbled through pulp continuously
RESULTS
75
200 21.5% _
1,050 10.89 g.p.L___ 26.3
E:eads _
Filtrate plus wasIL _
Weight,
Grams
Dlstrlbntion
oIF,O" 70
percent
TEST NO.4
SO, Leaching at 5: 1, Temperature, 3° C.
2 Stages
CONDITIONS
200 grams 01 ~100 mesh ore
'femperature, 3° C. Dilution 5: 1
Mechanical agitation in 2 liter flask
Agitation time, first stage 47 hours, second stage 22
hours, 69 hours total
SO, bubbled through pulp continuously
5
TABLE IV-Continued
DETAILED LEACH DATA
3,359,067
6
TABLE V-Continued
RESULTS
TABLE VI.-SUMMARY OF S02 LEACH TESTS
1Calculated on assumption that P,O, in the solid sample from .1st stage
would have contributed its contained P,O, during 2d stage leaching.
The following Table VI gives a summary of the S02
leach tests:
20
22.1% _
19.4% _
17.7 g.p.!..__ 48.0 _
15.6 g.p.L___ 150.6 98.6
20..14%g.p.!..__ 1.1 99.7_
L7% _
.3%_______ .3 _
Extraction,
Percent
Distribution of
P,O,
Percent Cum.
Percent
No. of
Stages
Assay
P,O,
Dilution
per Stage
Weight
Grams
Test No.
5
15
Heads (Assayed) _
10 Heads (Calculated)______ 200
Filtrate 1st stage_________ 1,050
Filtrate 2d stage_________ 1,420
Filtrate 3d stage__ __ _____ 1,000
Toils 1st stage _
Tails 2d stage _
Final Tails______________ 38
RESULTS
Agitation Time,
I
pH
I
Solution Assay
Hours P,O"g.p.1.
First Stage
0.0 8.7 0
1.0 1.5 4.10
5.5 1.8 9.91
9.5 1.4 9.98
13.5 1.4 10.15
17.5 1.4 14.68
21.5 1.1 17.00
24.5 1.4 16.82
39.0 1.4 15.32
47.0 1.3 --.------------
Second Stage
0.0 .------ ---------------
3.25 1. 07 10.60
11.00 1. 00 12.20
2L50 1.20 -------------- .
..
Weight
Assay
, Assay
P,O,
Distribution of
P,O,
Percent Cum.
Percent
5:1
10:1
15:1
5:1
5:1
111
2
3
26.3
82.0
98.9
95.9
99.6
DETAILED LEACH DATA
TESTNO.5
SO, Leaching at 5:1 Dilution, Temperature, 3" C.
Three Stages
Agitation Time, I pH I Solution Assay
Hours P,O" g.p.1.
CONDITIONS
200 grams of -100 mesh ore
Temperature, 3" C. Dilution 5:1
Mechanical agitation in a 2 liter flask
Agitation time stage 1, 48 hours, stage 2, 44 hours,
stage 3, 24 hours, total 116 hours
SO, bubbled through pulp continuously
30
The test for which results are shown in Table I was
made at' room temperature and at as: 1 dilution. The
extraction of phosphate in solution (26.3%) was so low
that it was not considered necessary even to analyze the
35 solid tails. This test indicated that there was a limiting
solubility of phosphate in solution which affected the
extraction. The tests of Tables II and III were performed
at low temperature with successively higher dilution.
Since the extraction increased from 82% at 10: 1 dilution
40 to ,98.9% at 15: Ldilution, it was, obvious that the concentration
of phosphate in solution is a controlling factor
in leach extraction. This .is an important feature of the
invention. 'The, 15: 1 dilution for the tests of Table III
provided the most. effective leach extraction. The results
45 of Tables IV and V show that successive extl'actions at
low dilutions have results comparable to a single stage
leach at higher dilutions. In the tests producing the
results of Table IV, two successive 5: 1 dilution leaches
gave a 95.9% extraction (compared with 82% for a
50 single stage leach at a 10:'1 dilution) . In the test giving
the test results of Table V, three successive 5: 1 leaches
gave 99.6% extraction compared to 98.9% for a single
stage at 15: 1 dilution.
As the results tabulated illustrate, for the high lime
55 content are a large ratio of water to ore ora large
volume of solution is required to keep the phosphate dissolved.
This condition is 'achieved by leaching at a ratio
of water to ore of 10 or more to 1, or by successive
leaches starting with lower ratios of water to ore. The
60 ratio of water to ore in the final leach to achieve a high
recovery must be 10 or more of water to 1 of ore. The
above tests established that sulfur dioxide treatment of
phosphate ores under the above conditions dissolves
essentially all the phosphate from its ores and is a com-
6fi mercially feasible process.
The leaching step, of course, is not restricted to the
specific type ore used for the above tests but may be
used for lower or higher grade ore.
However, the invention provides a commercially feasi-
70 ble process for recovery of over 98% of phosphate values
from high lime content ores in which the P20S content
is less than about 30%. As stated above there was no
commercially feasible process for recovery of phosphate
from these high lime content ores prior to this invention.
75 The following is an assay of Wyoming phosphate ore
0.27
0.27
o
6.82
11. 75
15.25
15.25
15.50
19.4
14.1
17.1
17.6
18.2
20.9
2L5% , _
19.2g.p.L___ 56.2
14.3 g.p.L__ 39.4 95.6
4.4%_______ 4.4 _
L3
Ll
1.1
1.0
L5
LO
LO
LO
1.0
8.5
1.4
L2
LO
LO
0.9
LO
LO
1.0
First Stage
Third Stage
Second Stage
TABLE V
200 '
1,250
1,165
43.8
o
4.5
7.5
23.25
0.0
4.0
20.0
24.0
28.0
29.0
44.0
o
, 2.5
6.0
9.0
14.0
23.0
27.0
3L5
47.5
",
Heads _
Filtrate plus wash #L _
TFialitlrsate plus wash #2 __
3,359,067
7
(Stambaugh Creek sample), the type ore used for the
tests set forth herein:
Assays
Percent
P
2
0
S
--____ 22.4
CaO 421
F 1.4
CO
2
._____________________ .8.1
Si0
2
16.9
AI
2
0
s
2.4
Fe20S 1.5
MgO 1.2
SOs 2.2
L.O.I. (1000° C.) ~____________ 9.5
Insol ,______________ 16.7
UsGs -__________ 0.02
V
2
0
S
0.004
Mo ~______________________________________ nil
g
of pure phosphoric acid solution and their distribution
coefficients determined. The results. of these tests are set
forth in Table VII below. The distribution constant, indicated
by Ka 0, is defined as Ka °= gm. HSP04 per liter of
5 organic phase divided by grams of HSP04 per liter of
aqueous phase. In performing the test, 100 milliliters of
the solvents were equilibrated with 100 ml. of a .5 molar
(49.2 g.p.I.) H3P04 solution in a cylindrical graduated
separatory funnel. The mixture was agitated vigorously
10 with a mechanical stirrer for ten minutes and allowed to
separate into two phases. The volume of each phase was
measured and the phosphoric acid remaining in the aqueous
phase was titrated with standardized sodium hydrox-
15 ide and the concentration of each phase calculated..The
resulting data as well as the distribution coefficient for
each solvent at the particular phosphoric acid concentration
is given in Table VII:
TABLE VII.-SCREENING TEST
[Extraction of H,PO, from ,5 M H,PO, solutlonl
Volume of- H,PO,ln-
Organic Aqueous Organic Aqueous
N-butanoL _______________ .. __ .- _-- -- 111 89 6.48 47.2 0.14
N-amylAlcohoL. __ •_.. ___ •__ -- _---- 104 96 1.15 49.0 0.02
2-octanoL___________ •_________ -- _.. _. 100 100 0 49.3 0
tri·Butyl Phospbate___ .... ___ .... ___ 108 92 5.74 47.1 .0.12.
tri·Capryl Amine (10% in kerosene) __ 103 97 17.6 37.3 0.47
Calculated composition
Cas(P04h Per4c9en.0t
CaF
2
~ __-_________ 2.9
Si0
2
- .______ 16.9
CaCO
s ~__________________________ 21.0
CaS04 3.8
MgCO
s
_ 2.6
Al
2
0
s
2.4
Fe20S 1.5
Water and organic 1.4
The preferred concentration range for the slurry is lO
or more parts of water to 1 of ore by weight. The pH
value of the solution must be less than about 1.5. The
temperature at which the sulfurous acid is reacted with
the slurry is preferably from just -above the freezing point
of the solution up to about 10° C. The freezing point of
the solution would always be below 0° C.
The solution formed by the reaction of sulfurous acid
on the ore contains essentially mono·calcium phosphate
and calcium bi-sulfite as solutes with lesser amounts of
other phosphates andsulfites. The method contemplates
the separation of the phosphates and sulfites in such a
way as to produce a relatively pure phosphate product
and calcium sulfite. Calcium sulfite is then roasted to
liberate sulfur dioxide for recycling to the leaching circuit.
The two components can be separated by crystallization
using techniques well known in the art. A crystallization
process for the separation of the sulfite and phosphate
components of the leach solution is disclosed in
U.S. Patent 2,899,271. Solvent extraction tests are briefly
described below which indicate that the phosphate values
can be separated as HSP04 by this method also; with refinement
the method might be commercially feasible.
It is well known that there are many organic liquids,
immiscible in water, which will dissolve phosphoric acid.
Accordingly, to use these liquids it is necessary to convert
the phosphates present to HSP04. If the leach solution is
kept at a high concentration of sulfurous acid, the calcium
phosphate salt is converted to phosphoric acid.
Five organic.. liquids were tested by batch extractions
It is noted. from Table VII. that the distribution coeffi35
cients of N-amyl alcohol and 2-octanol are too low to
justify their use for commercial application.
The distribution constants of the other three show that
they can be used for the extraction of phosphoric acid
from pure phosphoric acid solutions. Further tests of nor-
40 mal butyl alcohol as a solvent showed that the presence
of sulfurous acid in relatively high concentrations did not
noticeably interfere with the extraction of phosphoric acid
although significant amounts of S02 were adsorbed by
the solvent. It appears, therefore, that the solvent extrac-
45 tion method, with proper refinement, might offer attractive
possibilities for commercial adaptation in separating the
phosphate values as HSP04 from the leach solution.
The calcium sulfite remaining in the aqueous phase
after solvent extraction of HSP04 by whatever method is
50 used can be recovered by evaporation and crystallization
by well known techniques. It is then decomposed by
roasting into calcium oxide and sulfur dioxide, the latter
being returned to the leaching circuit for Jreatment of
fresh ore. Accordingly, the overall process of recovering
55 calcium values from high lime content ores can be used
either as a batch process or as a continuous process.
It is thus seen that the invention prOVides a novel
process for the recovery of usable phosphates from high
lime content ores having a phosphate content. based on
60 P20s as low as 21.2%. The improved leaching step in the
process, as described herein, effecting over 99% solution
of ore phosphate is a significant contribution to the commercial
feasibility of the overall process.
Although the invention has been illustrated and de-
65 scribed 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 numerous
modifications within the scope of the appended claims.
What is claimed is:
70 1. The process for recovering phosphate. values from
phosphate ores which comprises: forming a water slurry
of the ore in which the ratio of water to ore by weight is
greater than about 10 parts of water to 1 part of ore,
leaching the slurry at a temperature between the freezing
75 temperature of the slurry arid about lOO-c;-with concen,
3,359,067
References Cited
UNITED STATES PATENTS
4/1.891 Lisenberg 23-189 X
5/1915 Stewart 23-109
1/1918 Blumenberg 23-109
450,243
1,137,806
1,251,741
40 OSCAR R. VERTIZ, Primary Examiner.
L. A. MARSH, 0. F. CRUTCHFIELD,
Assistant Examiners.
10
the temperature of the slurry during leaching between
the freezing point of the slurry and about '100 C. to form
calcium sulphites, calcium phosphates and phosphoric
acid.
9. The process of claim 1 in which the recovered suIphites
are heated to produce sulphur dioxide, and the
sulphur dioxide recovered is recycled for reuse in the
process.
10. The process of claim 4 in which the phosphoric
10 acid is recovered by solvent extraction with an extractant
selected from the group consisting of N-butanol, tri-butyl
phosphate and tri-capryl amine.
11. A process for recovering phosphoric acid from
high lime content ores of calcium phosphates having a
15 P20 S content between about 20 to 30 percent, which comprises:
forming a water slurry of the ore in which the
ratio of water to ore by weight is from about 10: 1 to
about 15:1, leaching the slurry at a temperature between
the freezing point of the slurry and about 40 C. with con-
20 centrated sulfur dioxide at a pH between about 1 and
about 1.5 to maintain the leach solution at a high concentration
of sulfurous acid and form calcium sulfites and
phosphoric acid, extracting the phosphoric acid from the
slurry by solvent extraction with a substantially water im-
25 miscible organic extractant medium, concentrating the
slurry by heating it, roasting the formed concentrate of
calcium sulfites to produce sulfur dioxide, and recycling
the sulfur dioxide for reuse in the process, whereby about
98 percent of the original P20 S content of the ore is re-
30 covered and a portion of the sulfur dioxide necessary for
the leaching step is conserved.
9
trated sulphur dioxide at a pH between about 1 and 1.5
to maintain the leach solution at a high concentration of
sulfurous acid and form a solution containing calcium
sulphites, calcium phosphates and phosphoric acid, recovering
sulphur dioxide from the sulphites and recycling 5
it for reuse in the process, and selectively removing the
sulphites, phosphates and phosphoric acid from the slurry.
2. The process of claim 1 in which the step of recovering
and recycling sulphur dioxide is performed after selectively
removing the phosphates and phosphoric acid
from the slurry.
3. The process of claim 1 in which the ratio of water
to ore in said slurry is about 15 to 1 by weight and the
temperature of the slurry during leaching is maintained
at about 40 C.
4. The process of claim 1 in which the ratio of water
to ore in the final slurry is achieved by starting at a
ratio of water to ore of about 5 to 1 followed by successive
extractions of the leach solution to result in a total
cumulative ratio of water to ore by weight greater than
about 10 to 1.
5. The process of claim 1 in which a high concentration
of sulphurous acid formed by the S02 treatment is maintained
in the slurry to convert at least a portion of the
phosphates to phosphoric acid, and the phosphoric acid
is separated from the sulphites in the solution by solvent
extraction.
6. The process of claim 1 in which the phosphates are
selectively removed from the slurry by crystallization.
7. The process of claim 1 in which the phosphate content
of the ore based on P20S is less than about 30 percent.
8. In the process for recovering phosphate values from
phosphate ores in which the leaching is performed by introduction
of sulfur dioxide into the slurry of water and
ore to form sulfurous acid and the formed phosphoric 35
acid and phosphates are selectively removed from the
slurry, the improvement which comprises forming a slurry
in which the ratio of water to ore is at least about 10 parts
of water to 1 part of ore by weight, maintaining the pH
of the slurry between about 1 and 1.5 during the leaching
by the introduction of sulfur dioxide, and maintaining