[II]
[19)
[4S]
[54] MEmOD FOR DEWATERING PHOSPHATE
SLIMES
[75] Inventors: Krishnapillai V. Nair; Ponisseril
Somasundaran.both of Nyack, N.Y.
[73] Assignee: The Trusteesof Columbia University
in tbe City of New York. New York,
N. Y.
[21] Appl. No.: 639,253
[22) Filed:
[51] I.t. a. 4
[52] U.s. a.
Au&. 9. 1984
at2F 1/52
nOnl4; 209/5;
210n17; 210n24; 210n26; 210/907
[58) Field of Search
209/5; 210n02. 714.
210n24. 726, 907. 717
[56]
ReferencesCited
U.S. PATENT DOCUMENTS
3,450,6336/1969 Siemen
3.6M>,698 8/1972 Liu et 81.
3,761.2399/1973 Cooket aI.
3,763.04110/1973 Cooket aI.
Primary &amilt~r-Peter
Hruskoci
210/907
,
210/907
210/907
210/714
Attorney. Agent. or Firm-John
P. White
[57]
ABSTRACf
A method for dewatering phosphate slimes and recovering a liquid solution involves adding a calcium sulfate
hemihydrate to the slimes, mixing the resulting admixture to effect formation of calcium sulfate dihydrate
crystals in the admixture, and recovering a liqwd solution from the resulting crystal-containing admixture.
The calcium sulfate hemihydrate addition may be followed by addition of calcium sulfate dihydrate. The
calcium sulfate hemihydrate addition may be preceded
by acidification of the slimes to pH 1-3 with sulfuric
acid, which addition may be followed by the addition of
other calcium salts to the slimes.
A suff"lCientamount of calcium oxide or calcium carbonate or both may be added to the recovered liquid
solution to raise the pH of the liquid solution to a value
above about 6.0. Mixing permits calcium phosphate
salts to precipitate. allowing for separately recovering
the calci~m phosphate salts and a dephosphatedliquid
portion.
8 Claims, No Drawings
4,563,285
1
2
involves adding calcium sulfate hemihydrate; comMETHOD FOR DEWATERING PHOSPHATE
monly known as plaster of paris. to the slime. The reSLIMFS
suiting mixture is then mixed to effect the formation of
calcium sulfate dihydrate; commonly known as gypBACKGROUND OF THE INVENTION
S sum. crystals in the admixture. The liquid solution is
One of the most water-intensive industries in the
then recovered from the resulting crystal-containing
United States is the processingof phosphate rock minadmixture by filtration, decantation or centrifugation.
eral. A major part of this industry is located in Florida
In one embodiment the calcium sulfate hemihydrate
which processesover 80% of the mineral processedin
addition is preceded by addition of sulfuric acid to the
the whole country. During such processingextraneous, 10 slimes to reduce the pH of the slimes to a value from
undesirablematerial in the ore, e.g., clay and sIica, accuabout I to 3. The addition of sulfuric acid results in the
mulatesin the form of an unsettablevery fme suspension
chemical dissolution of the phosphatic contents of the
in th~ water. This ~ater is employed for washing and
slime into the aqueous medium. That is calcium phos~otatlo:~-conce~trahonof. t~e crushed ore. The fl°.ta- IS phate is converted into monocalcium phosphate which
tlon tailings typically.coDSlShngof ISOmeshsandeasily
is water soluble. This renders the remaining solids in the
~ewater. ~e .clay sllm.e,however, does not settl~ ~8;S- slime more readily separable from the liquid portion.
Ily. The slime ISsto.redm earthen dams where the mltial
This reaction of calcium phos hate being converted
4 to S percent solids becomes I S to 25 percent over.
.
. p.
I
severa
f
years
0
ttl
se
.
mg,
d
an
tuall
to
even
30
t
SS
mto
monocalclum
phosphate
yo.
IS
Illustrated
by
the
fol-
.
percent solids in the course of 20 to 30 years.The phos- 20 lowmg equation:
phate v~ues in the.sl!~e cannot ~ recovered .because
CaJ(PO.)2
+2HzSO.
= 2CaSO..2HzO
+ Ca(Hof the dIfficulty of mltlal dewatenng and the high cost
2PO.)2
of chemicals required for successfulflotation.
Thus, very large tonnages.of water drawn fr~m nat~A further embodiment involves filtering the slime
raI sourcesare held up as.sl~meand cannot be Immedl- 25 after the sulfuric acid and calcium sulfate hemihydrate
ately reused.Pho~phatemIning ~ already lowered the
treatment and adding a calcium salt such as calcium
fresh water table In some places like .the Polk Country.
oxide or calcium carbonate or both to the filtrate to
Several
Governmental
and
Industrial
Research
Orga.
. h.
ed . establ' bin
raise the pH of the filtrate to a value above 6.0, e.g., to
mza ons are engag m
IS g econolDl.caJmeth ..
.
ods to dewater the Florida phosphate industry's slimes. 30 a ,:,aIuebetw.een6.2 and 6.5..The addition o~ .calCIum
The techniques developed so far include flocculation
oXld.eor calcIum carbonate will produce ~reclpltates of
and consolidation of the particles by addition of various
calCIum phosp:hat~salts and othe~ metallic phosphates
electrolytes, flocculation and consolidation by techafter the.so!utlon 's: thoroughly mlX~.
.
niques involving freezing and thawing and electroosThe liquid portion should be mIXed or otherwIse
mosis,with or without filtration. So far it cannot be said 35 agitated during the addition of the calcium salt to ensure
that anyone of the methods investigated has met with
that the proper pH is reached. The calcium phosphate
complete success.
salts and the dephosphatedliquid portion may be sepaOther techniques under development include chemirately recovered by methods such as filtering, decanting
cally hardening phosphaticclays by the addition of lime
or centrifuging.
or gypsum to the clays. In addition, the use of accelera- 40
tors or surfactantsin combination with lime or'gypsum
DETAILED DESCRIPTION OF THE
are being tested. See Florida Institute of Phosphate
INVENTION
Research,Information Clearinghouse Newsletter; VolPhosphate slimes may be dewatered by adding calume V, Number 2; Summer 1984. C. C. Cooke et al.,
cium sulfate hemihydrate to the slime. Preferably, calU.~. Pat. Nos. 3,761,239(19:3)and 3,763,O41(1973~
~e- 45 cium sulfate hemihydrate is added in an amount equivascnbes .a metho? for makl.ng a land-~I.1compos~hon
lent to the total weight percent of solids in the slime.
fr~m S~I~es.ThIS method mvol,:,e:smIxing the slimes
More calcium sulfate hemihydrate may be added to the
with taIlIngs. Large amountsof tallmg~ are used,prefer-slime
but if more is added there will be a corresponding
abl,!,between 60 ~nd 99 pe~centb,!,weIght ba.sedon t.he
decreasein the water recovered from the slime. Likew.e~ghtof the slimes. This a~lnIXture of slimes Wlt.h SOWise, if less calcium sulfate hemihydrate is added the
talll.ngs dew.ater at a. substa~tlally gr~ter rate than IS
phosphate slime will not be completely dewatered.
ac:hle,:,edusmg gravlt~ settling. Optlonal.ly, after the
Therefore, the optimum amount of calcium sulfate
slime ISdewatered calcIum sulfate may be Incorporated
h .h d t t be dded t th I "
.
Into
.
the
reconstituted
.
landfill
composItIons
.
eml
to
produce
a.
y
ra
e
0
a
0
.
e
s
Ime
IS
an
amoun
t
.
lertile landfill having acceptable bearing strength. A 55 ~wvale.nt to the total weIght percent of solids present
method has now been discovered for dewatering phosIn the slime ~~ be dewat.ered.
.
phate slimes which involves adding calcium sulfate to
. Upon addlt.lon.of calcium sulf~te hemlhy~rate to the
the slimes, but which does not require the addition of
sl.lme and agItation of the ad~ture cal.clum sulfate
tailings. The method is surprisingly effective in permit~Ih,,:drate ~rystals are formed In the admixt,;,re and a
ting the separationof the liquid and solid componentsof ~ lI<l:uldsolution can be.recovered fr~m the adlnlXture. By
the slimes and allows for recovery of dissolved mineral
this method the calcIum sulfate dihydrate crystals are
values.
formed upon suspended particles. Such crystals may
enmesh more than one suspended particle. Thus the
SUMMARY OF THE INVENTION
nature of slimes which cannot be settled, or which can
The method of the invention enablesthe separationof 65 be settled only with difficulty, is altered, and the solids
water content from the slime in reusable form. The
become easily separable from the liquid pc;>rtionby
method of this invention also enables the phosphate
known methods of separation, e.g., filtration, centrifucontent to be separated from the slime. This method
gation or decantation.
3
4.563.285
4
In one embodiment the calcium sulfate hemihydrate
end of which time the slurry WitSfiltered through the
addition is followed by the addition of calcium sulfate
Buchner filter with a vacuum of 20 in. of Hg using a
dihydrate. The addition of calcium sulfate dihydrate
fresh filter.
increases th.e favora~le filter ~edium and .enmes~es
The dried cake of phosphate weighed 38 gm. Analysmaller partIcles miling separatIonof the solids easIer. .5 sis indicated the presenceof 35.7% P20S.Since the total
It.should be re~embered that th~ total the am°un.t of
PO ori inally present in the slime was 17.5 gm. the
calcIum sulfate dIhydrate and calcIum sulfate hemihy2 S .g
f 77
'
drate added to the slime is preferably equivalent to the
above YIeld representsa recovery 0
%. The filtrate
total weight percent of solids in the slime. The proporof. ~e clear water representsa recovery of 92% of the
tion of calcium sulfate dihydrate to calcium sulfate 10 ongtnal volume of water.
hemihydrate will vary depending on the type of partiEXAMPLE 2
cles to be separated.If smaller particles need to be enmeshed then a greater proportion of calcium sulfate
Five liters of another sample of phosphatic clay slime
dihydrate should be added. In a preferred embodiment,
was placed in a ten liter vesselfitted with variable speed
the calcium sulfate hemihydrate addition is precededby 15 stirrer.
addition of sulfuric acid to the slimes to reduce the pH
Thefollowing sequenceof operations was carried out
of th~ slimes to a value.from about 1 to 3. The ~ount
employing the indicated parameters:
of acId added to the sltme to reduce the pH will vary
I. Addition of sulfuric acid
from slimeto.slime.The add.itionof.sulf~ric acid con-
verts
phosphate
In the sltme
20
. the
h calcIum
h
h
I
I bl mto
al .monocalh
clum p osp ate, t e on y water so u e c clum p osphate. The equation for this reaction is Ca3(PO4)2+2H2SO~2CaSO..2H20+Ca(H2PO4)2. The addition of
calcium salt to this solution permits the precipitation of
calcium phosphate salts and allows for separately re- 25
covering the calcium phosphate salts and a dephosphated liquid portion. To accomplish the precipitation
of calcium phosphate salts another embodiment of this
invention involves adding to the liquid solution, aliter
t~e addi.tion of sul~uric acid, a calcium salt s~ch as cal- 30
clum ox.lde.or calc!um carbonate or both to raIsethe pH
of the liquId solutIon to a value above about 6.0, e.g.,
6.2-6.5. As with the addition of sulfuric acid the amount
of calcium salt neededto be added to raise the pH will
vary from slime to slime. Preferably, the liquid portion 35
should be agitated during the addition of calcium oxide
or carbonate to ensure mixing so the proper pH is
reached.As in the other embodimentsof this invention,
the liquid portion and the solids are separately recovered by known methods of separation including, e.g., 40
filtration, decantation at1dcentrifugation.
EXAMPLES
Stirrer speed:150rpm
'
R t f add.t ' . 1 II .
aeo
lIon. m mln
.
S~rengthof acId: 10
Flna~~H: 1.9 .
.
2. AddItIon of calcIum sulfate hemlhydrate slurry
Strength of slurry: 50%. Weight of CaSO..1/2H20=50 gm.
Rate of addition: All at once
Stirring rate: 50 rpm.
Time of stirring: 45 minutes.
3. Addition of gypsum: Phospho-gypsum,produced by
the action of sulfuric acid on rock phosphate during
the manufacture of wet process phosphoric acid was
d
~..h
added. 50
.elg t
..
gm.
4. F~ltered as In Exa~ple 1.
5. Filtrate treated wIth Ca? powder slurry
Strength of slurry: 15% m water
Stirring rate: 300 rpm.
Final pH: 6.2
Time of stirring: 1.5 hours
6. The slurry was filtered as in Example 1. No washing
was done.
7. Volume of water obtained as filtrate was 4700 ml,
representing 94% recovery.
Dry weight of phosphates:58.2
P20Sin the above: 37.2%
Percentage recovery: 76%
EXAMPLE I
Five liters of a sample of phosphatic clay slime pro- 45
cured from a working plant in Florida, was placed in a
ten liter vesselfitted with a variable speedstirrer. Analysis indicated that the slime contained 5.05% (wt/vol)
suspendedsolids and 0.35% P20S(wt/vol).
The slime was stirred at 100 rpm while sulfuric acid .50
EXAMPLE 3
(1N) was added dropwise at a rate of one drop per
Five liters of a phosphatic clay slime containing 3.5%
second. When the pH of the slime reached 2.6, the
suspendedsolids and containing 0.24% P20S (w/vol)
add.itionof acid was stopped and stirrin~ was cont!nued
were placed in a ten liter vessel fitted with a variable
untIl the pH was constant for fifteen minutes. ThIs pH
speed stirrer. While stirring the slurry at a rate of 75
was 2.86: .
55 rpm, a 20% water slurry of calcium sulfate hemihydrate
The stlrnng rate was ~hen decreased t.o 50 rpm.and a
slurry of 45 gm. of calcIum sulfate hemlhydrate
In 100
containing
50 gm was added
at a rate
of
10 mI/min.
Afit th add.t .
th I
.
.
. .
er e
1 Ion, e s urry was agltated at 50 rpm Iior
mI. water was added to the sltme. The stlrnng was
continued for one hour.
half an hour more and then I gm of phospho.-gypsum
The slurry was then filtered through a medium of ro were added. The mass was stirred for fiv~ rntnut~ at
drill cloth using a Buchner funnel and a vacuum of 20
300 rpm and was then filt~red throu~h dnll cloth m a
in. of Hg. The filter cake was washed with 100 mI. of
Buchner funnel of 15cm. dIameter, WIth a vacuum of 20
water in three lots and the filtrate and wash water were
inches of Hg. The filtration was completed in 12 minthen placed in a clean ten liter vessel. A slurry of 5%
utes. The original slime would not filter through the
Cao in water was then added to the stirred (300 rpm) 65 same medium.
solution at a rate of five ml/min. Addition of CaD was
The volume of liquid obtained by dewatering the
stoppd when the pH of the slurry reacheda value of 6.5.
slime was 4700 mI. i.e., 94% of the original volume of
Stirring was continued for fifteen minutes more, at the
the slime.
~
4,563,285
5
EXAMPLE
4
6
3. A method according to claim I, wherein the calcium sulfate hemihydrate addition is preceded by addi-
8 liters of slime containing 5.05% solids was treated
tion of sulfuric acid to the slimesto reduce the pH of the
with 300 gm of CaSO..l/2H20 added in small portions
slimesto a value from about 1 to 3 in order to solubilize
over the course of 30 minutes. Stirring was at 75 rpm. 5 phosphatesin the slime.
After the addition, the admixture was stirred for 30
4. A method according to claim I, wherein the dewamore minutes and filtered with a vacuum of 20 Hg
tering and recovering comprises filtering, decanting or
through a IS" diameter Buchner funnel using drilJ cloth.
centrifuging.
Filtration was completed in 12 minutes. The volume of
5. A method according to claim 3, which further
water recovered by dewatering the slime was 7360 mI. 10 comprisesadding a calcium salt to the liquid solution to
i.e., 92% of the original 8 liters.
raisethe pH of the liquid solution to a value above about
What is claimed:
6.0, mixing the liquid solution to permit calcium phos1. A method for dewatering phosphate slimes and
phate salts to precipitate and separately recovering the
recovering a water solution comprising adding calcium
calcium phosphatesalts and a dephosphatedliquid porsulfate hemihydrate to the slimes,in an amount equiva- 15 tion.
lent to the total weight percent of solids in the slime,
6. A method according to claim 5, wherein the caImixing the resulting admixture to effect formation of
cium salt is selected from the group consisting of calcalcium sulfate dihydrate crystals in the admixture, and
cium oxide, calcium carbonate and mixtures thereof.
dewatering the admixture to recover a water solution
7. A method according to claim 6, wherein the pH is
from the resulting crystaI-containing admixture.
20 raised to a value between about 6.2 and 6.S.
2. A method to claim I, wherein the calcium sulfate
8. A method according to claim 5, wherein the sepahemihydrate addition is folJowed by the addition of
rately recovering comprises filtering, decanting or cencalcium sulfate dihydrate in an amount sufficient to
trifuging.
enhance
theseparation
ofsolids
fromtheslime.
. . . . .
25
30
3S
40
.,
~
55
60
65