1,359,911
2,174,873
2,922,761
3,019,611
3,248,890
United States Patent
Hadzeriga
[54] TREATMENT OF PHOSPHATE ROCK
SLIMES BY FREEZING
[72] Inventor: Pablo Hadzeriga, Arvada, Colo.
[73] Assignee: Hazen Research, Inc., Golden, Colo.
[22] Filed: March 11, 1970
[21] AppI. No.: 18,490
[52] U.S. CI. 62/58, 23/165, 252/346,
23/312 P
[51] Hnt. CI BOld 9/04
[58] Field of Search ..62/58; 252/319, 347, 349, 346,
252/348; 23/165 B, 293 R, 165,312 P
[56] References Cited
UNITED STATES PATENTS
11/1920 Oman 62/58
10/1939 Downes et al 62/58
1/1960 Davenport 252/349
2/1962 Toulmin 62/58
5/1966 Oman 62/58
[IS] 3,681,931
[45] Aug. 8, 1972
OTHER PUBLICAnONS
Burton, The Phys. Properties of Colloidal Solutions,
3rd Ed., 1938, pp. 216- 219.
Primary Examiner-Norman Yudkoff
Assistant Examiner-R. T. Foster .
Attorney- Harris, Kern, Walker and Tinsley
[57] ABSTRACT
This process is a method of treating an aqueous inorganic
colloidal suspension to make it amenable to
separation by decantation, filtration, and centrifu~ation
and comprises statically freezing the suspensIOn
and thawing the thus frozen suspension prior to
separation by decantation, filtration, and centrifugation.
In the preferred embodiment of the process, the
suspension is kept in a frozen state. for a predete~mined
length of time after the static freezmg. ThIS
process is particularly adept at making phosphate rock
slimes amenable to separation by the above-named
methods.
14 Claims, 2 Drawing Figures
{}O./ .2 .4.G.8 /.0 2.0
T/me (minllfes) (logarithmic scale, base /0)
PATENTEbAUG 8197Z 3,681,931
~ ""- I
~ I I
........
I'-.... I :
........1'-.... : I
I
t--...
1------ r........
r-...
e----!iN0'1 Leafhed Shines --- -- e- - ~ ,
5fafic Frdzen II
1------+~-1-~---- --j------'- 1------- _ .. - I_ I
---- - -Non 5faftc. Froz~n I f----+ - ---+-----_+__ r--- -~-~-
__• _____ ·w
- -Nof Frozen
,
i
--- --
~ i
1:::":::::..:::::r::::::::: ..:::::..:::t--- --
f--~t- '-- I'-:::::-
i
--.::: f-::::...-_ : i
.2 .4.6.8 1.0 2.0
T/me (mintJfes) (Iogarlfhmic :5Cale, base /0)
EXG.1..
Mg(oH)2. Frozen
at-ZO°C
Mg(OH)Z Frozen
af- DOC
tt Felo&~ Frozen af-20"c} Filtered foo rapidly fo
A/(O/1J.3 Frozen al-20"C measure fi'ltmf/ol7 rafe
Fe (OH)3
UnFrozen
INVENTOR
~-- ~__-L ~~..!_ ____L.I- PABLO HADZERIGA
/.0 2.0 3.0
Time (mlnufes) BY flI5 ATTORNEVS
HARRIS? f(;£cH, RusseLL 8: kERN
3,681,931
2
DETAILED DESCRIPTION
BRIEF DESCRIPTIONOFDRAWINGS
tion of a substantial pbrtion ofthe water and suspended
material, during the static freezing step. Upon thawing,
the water .and suspended material remain separated,
thus permitting the removal of the water from the
5 remaining. SuspensiOn·by decantation. or other known
methods. The method also makes the suspension readily
susceptible to separation by filtration or centrifugation.
An. opjeQt Ofthis invention is. to provide a relatively
10 rapid method of separating aqueous suspensions
without the addition ofsalts, adds or bases. In particular,
it is an object to provide a method of treating
phosphate rock slimes in order to makethem amenable
to dewatering by decantation.
15 A further object·of the. present invention is· to •providea
methodoftreating aqueous inorganic suspensions
that are not normally susceptible to filtration. or
centrifugation amenable to separation by such
methods.
20
BACKGROUNDOFTHEINVENTION
1
TREATMENTOF PHOSPHATE ROCKSLIMES BY
FREEZING
l. Field of the Invention
This invention. is directed to the field of art relating
to· methods ·of.separating colloidal· suspensions; more
particularly, to freeze-melt processes for the treatment
of aqueous inorganic colloidal suspensions to make
them amenable to separation by decantation, filtration
and centrifugation.
2.· Description of Prior Art
The prior art methods of separating colloidal suspensions
are difficult, time-consuming, often expensive,
freq uently unsuccessful and nonuniversal,and often
require expensive equipment,and/ora large area oftlat
land for settling ponds. The prior art methods include
settling, electrolytic salting with polar salts, such as
alum, pH alteration with acid or base,. filtration, centrifugation,
cooling, freezing, and heating. The separation
· methods of settling, electrolytic. salting and pH al- FIG. 1 isa graph illustrating the change in filtration
teration require holding or settling tanks or ponds rate of variously treated metal hydroxide suspensions
which are either relatively expensive or •require rela- 25 with respect to time; and
tively large areas of valuabletlat land. Electrolytic salt- . •FIG. 2 is a graph (logarithmic scale) illustrating the
ing and· pH alteration add unwanted contaminants.to change in filtration· rate of variously. treated·HNOa
the suspension. Settling, filtration,cooling and heating leached phosphate slimes with respect to time.
are frequently unsuccessful or require inordinate
periods oftime. 30
In the sewage •treatment. art,. freezing processes.for The present invention comprises statically freezing· a
the thickening of sewage sludge have been patented substantial portion of the liquid in. an aqueous inor-
(see U.S. Pat No.2, 174,873 to J. R.. Downeset al. and ganicsuspension and then thawing the frozen suspen-
U.S. Pat No. 2,703,782 to C. J. Reganet al.). Sewage sion. Static freezing consists of subjecting the suspensludge
is an.aqueous organic colloidal suspension con- 35 sion to a freezing temperature in a quiescent,
taining relatively large volumes of bacteria which are nonagitated state during the freezing step. The adcontinually
metabolizing the sewage sludge to liberate vantages of the present process are nullified if the
heat, and metabolites such as ammonia, amines, car- suspension is agitated during the freezing, Le., if the
boxylic acids, acetaldehyde, ethanol, CO2, H2S, suspension is agitated upon freezing, little, if any,
methane, water and the like. This liberation of heat and 40 separation of the water and suspended material will
chemicals keeps the sludge in a sufficient state of agita- occur and the thawed suspension will filter no more
tion to prevent settling. The sludge cannot effectively rapidly than an untreated or unfrozen suspension (FIG.
be filtered because the bacterial cells which are 2). The suspension is frozen to at leastthe freezing temgelatinous
readily clog and seal the pores of the filter perature of the liquid in the suspension; however, immedia.
However, upon freezing sewage sludge, the 45 proved separation results are obtained when the
majority of the bacterial cells are lysed or ruptured and suspension is frozen to even lower temperatures. The
the metabolic processes of the surviving bacterial cells suspension can be frozen down to any freezing temare
effectively stopped. When the sludge is thawed, it is perature; however, the actual freezing temperature emin
a nonagitated state and it settles at a relatively fast ployed wiIl be governed by economic factors and the
rate. Furthermore, the sludge can be filtered without 50 available equipment. For purposes of the present inrapidly
clogging the filter medium because of the great vention, a practical temperature for the freezing step
reduction in bacterial cells. If, after thawing, the has been found to be a temperature between about -10
sewage sludge is allowed to attain room temperature C. and about -1000 c., preferably between about -200
over a 24-hour period or longer, the sludge again enters and -800 C.
an agitated state and will not settle and will not filter 55 The thawing step is generally conducted at ambient
because of the metabolism of the new bacterial cells or room temperature, although it can be conducted at
resulting from normal cell division, of the surviving any temperature above the freezing temperature of the
bacterial cells. suspension. In the preferred embodiment of the inven-
BRIEF SUMMARY OF INVENTION 60 tion, the thawing is conducted as a static thawing, that
is, the frozen suspension is allowed to melt in a
The present method renders aqueous inorganic quiescent state without agitation.
suspensions which are not normally separable by settle- In the preferred mode of the present invention, the
ment and decantation, filtration or centrifugation frozen suspension is allowed to remain in a frozen state
amenable to such separation techniques. The present 65 for a predetermined length of time after the static
method comprises statically freezing a substantial por- freezing step. The frozen suspension is generally kept in
tion of the liquid in the suspension and then thawing the frozen state for at least about one hour, preferably
the frozen suspension. This method causes the separa- for about 24 hours.
3
3,681,931
4
EXAMPLE 2
Aluminum, magnesium, and ferric hydroxides were
prepared by precipitating the hydroxides from aqueous
solutions of aluminum chloride, magnesium chloride,
and ferric chloride by the addition of ammonia. The
concentration ofsolids in the suspensions were 1.8, 2.6,
and 3.6 percent for aluminum hydroxide, magnesium
hydroxide, and ferric hydroxide, respectively.
The cooling and freezing of aqueous suspensions by
refrigeration equipment requires a substantial energy
expenditure. In order to minimize the energy expenditure,
the present process is preferably conducted so
5 that the heat energy of a batch of unfrozen suspension
is used to thaw a batch offrozen suspension resulting in
the cooling of the unfrozen batch. Additional cooling
can be accomplished by passing a batch of thawed
suspension through a heat exchanger also being fed the
10 cool decant or filtrate of a batch ofthawed suspension.
The following examples are included to further illustrate
the practice of this invention and are not intended
as limitations of the present invention.
After the frozen suspension has statically thawed,
there remains a two-phase or layer mixture. The top
layer, which often represents between about 10 percent
and about 60 percent of the original volume of the
suspension, is generally a clear or slightly turbid liquid
which contains dissolved material and which is substantially
free of suspended matter. The bottom layer contains
the remaining liquid and solid matter of the
original suspension. After the frozen suspension has
thawed, the top liquid layer or decant is removed by decantation
or by another known method from the
thawed suspension, and the remaining bottom layer or
concentrated suspension is filtered or centrifuged to
further dewater the solids contained therein. Optionally,
the thawed suspension can be directly filtered 15 EXAMPLE I
or centrifuged without removing the top liquid phase. Phosphate slimes were leached with nitric acid and
The present method is effective in the treatment of a subjected to different freezing techniques. In one case
large number of aqueous inorganic colloidal suspen- the leached slimes were introduced into a household
sions that are not normally amenable to separation, 20 freezer at a temperature of -200 C. Another sample
such as suspensions of colloidal metal hydroxides (see
FIG. 1), colloidal suspensions of titanium oxide, col- was frozen using an agitated refrigerated drum at a
loidal suspensions of silicon dioxide, colloidal suspen- temperature of -200 C. Upon thawing, the filtration
sions of clays like montmorillonite, illite, Fuller's earth rates were measured using a 0.019 square foot filter
and limonitic laterite, and phosphate rock slimes. leaf and compared with filtration rates obtained from
The present method is particularly adept for the 25 leached slimes which were not frozen. The results
treatment of phosphate rock slimes, unleached or acid- plotted against cake formation tim~ are p~esented in
leached, which are the waste tailings from the FIG. 2. It ~an be seen th?t the static. freezl~g method
beneficiation process of phosphate rock ores. In ga~e filtration rates on thiS co~paratlve basiS o.f about
Florida, as well as in Tennessee, this process consists of 30 7 times (f~r 15 sec011:d formatl~:m) those obtained.by
washing and removing the slime-clay fraction followed d~um freezing. Th.e agitated refngerated drum.freezing
by flotation to further upgrade the phosphate rock did ~ot sho~ any Improvement over the filtration rates
product. During the desliming, from one-third to one- of slImes which were not frozen.
half of the contained phosphate values are discharged
with the slimes. These slimes are removed, not only 35
because they are refractory toward presently known Comparative filtration tests were performed using
upgrading processes, but they also interfere with the untreated Florida phosphate rock slimes containing
operation of the flotation on the remainder of the 13.1,12.7, 10.0, and 9.4 percent solids in suspension.
material. These slimes represent a tremendous loss of Using a filter leaf with an area of 0.1 square foot and
raw material as well as constituting an expensive 40 vacuum of between 15 and 18 inches of mercury
nuisance since they must be impounded to prevent proved to be unsuccessful in obtaining filtration rates
stream pollution. These slimes are a colloidal water on untreated slimes. The filtrate which was obtained
suspension of micron and submicron size clay-sand- was very cloudy and at best the filtered cake was never
phosphate rock containing between 5 and 10 percent more than one-sixteenth inch thick regardless of how
total solids which contain between 10 and 18 percent 45 long the leaf was left in the slurry. The thin slime coat-
P20 S' These solids are very difficult to dewater by filtra- ing on the fIlter cloth could not be blown off with air
tion, settlement or centrifugation. It has been found pressure as it had effectively impregnated the filter
that several years are required before the solids will set- cloth.
tie to a density of 20 percent solids upon standing in the 50 Duplicate samples of the above were frozen in a
disposal ponds. Since the volume of slimes produced is kerosene bath held at -200 C. After allowing to thaw at
1.2-1.5 the volume of phosphate rock mined, constant room temperature, each sample separated into two
additional new areas of land are needed to maintain layers: a clear top liquid and a bottom slurry. After denormal
mining and production operation, and prevent canting the clear liquid, the slurry was filtered using the
pollution. The slimes are practically impossible to de- 55 same procedure described above. The measured filtrawater
economically by filtration or other solid-liquid tion rate varied from 3,000 to 4,300 pounds of wet
separation techniques.
If a phosphatic slime containing 5 to 10 percent cake (containing betv.:een 30 and 40 percent solids)
solids is subjected to static freezing and thawing, it may per square foot offiltenng area per 24 hours.
be concentrated to 25-30 percent solids after separa- 60 EXAMPLE 3
tion of the clear top layer of water. Further dewatering
of the solids to a cake containing 40-45 percent solids
can be easily accomplished by filtration. In the case of
leaching the slimes with nitric acid to dissolve the P20 S
values, the filtration rates of the insoluble residue after 65
static freezing and thawing are increased by a factor of
4 to 10 times compared with unfrozen or nonstatically
frozen slimes (see FIG. 2).
3,681,931
EXAMPLE 6
6
original suspen- Filtration
method sion volume rate
No 18 (cloudy)
No 3 minutes
Yes 80 (clear)
Yes Thawed suspension
filtered
as rapidly as
it was transferred
to the
filter funnel
EXAMPLE 5
First
Second
Third
Fourth
no.
Decant as Percent
of Total Colloidal
Suspension
8.0
4.3
15.5
30.6
50.5
46.2
EXAMPLE 4
Subjected to
Freeze-Melt
Method
No
No
No
Yes
Yes
Yes
Subjected
to the Percent volume of
Fraction freeze-melt decanted liquid
Metal Hydroxide
Suspension
Aluminum hydroxide
Magnesium hydroxide
Ferric hydroxide
Aluminum hydroxide
Magnesium hydroxide
Ferric hydroxide
An aqueous sample of-200 mesh hydrolyzed titani15
urn dioxide, which had not settled at all upon several
weeks of standing, was frozen in a kerosene bath (-20°
C) for a period of 3 hours and then allowed to thaw at
ambient temperature. The thawed suspension had two
Further samples of aluminum, magnesium and ferric phases; a turbid supernatant, which was decanted and
hydroxides were prepared as described abov~ and each 20 represented 6~ percent of the original volume of colhydroxide
sample was divided into two fractions; one 100dal sl;iSpenslon, and an opaque concentrated closed
fraction was subjected to the freeze-melt method of the suspensIon phase at the bottom.
present invention and the other fraction was left untreated.
The filtration rates for the metal hydroxide
fractions were measured using a Buchner funnel and an 25 Aqueous suspensions of montmorillonite, illite, Ful-
Eimco ~Y319F filter cloth. The results are graphically I~r's earth and limonitic laterite clays, which are very
shown 10 FIG. 1. The ferric hydroxide and aluminum dIfficult to dewater by filtration or settling, were
hydroxide fractions subjected to the freeze-melt prepared.
method filtered too rapidly to obtain data and are After being allowed to settle for 40 minutes, an aquerepresented
by two arrows in the left portion of FIG. 1. 30 ous suspension of montmorillonite (9 percent solids)
FIG. 1 shows that unfrozen colloidal suspensions filter produced no clear decant. The suspension was
at a much slower rate than one subjected to the freeze- removed from the sands and an attempt was made to
melt method. filter it on a leaf filter which proved unsuccessful. The
Further samples of magnesium hydroxide were suspension was then frozen in a kerosene bath (-20°
prepared as described above and divided into two frac- 35 C) for one hour. The frozen suspension was then
tions. Both fractions were frozen in a kerosene bath thawed at ambient temperature to produce a clear de-
(-20° C) for 90 minutes; one fraction was removed cant which constituted 83 percent of the original
and thawed and filtered as described above; the other volume of the suspension. The decant was removed and
sample was immersed in a dry ice-acetone bath (-80° the lower ~oncentrated suspension phase was filtered at
C.) for an additional 30 minutes. This latter fraction 40 a very rapId rate.
was then removed from the bath, allowed. to melt at An aqueous suspension of illite was allowed to settle
ambient temperature, and filtered. The filtration rates for 20 minutes to produce a decant which constituted
of the two fractions are graphically illustrated in FIG. 1 3.2 percent of the original volume of the illite suspenwhich
shows that the employment of a lower freezing slon. An0tb:er aqueous suspension ofillite was prepared
temperature in the static freezing step of the freeze- 45 and frozen 10 a.kerosene bath (-20° C.) for I hour; the
melt method improved the resulting filtration rate of frozen suspensIon was then allowed to thaw in a hot
the colloidal suspension. water bath ~or a 20(P) -minute period. Upon thawing,
the suspensIon produced a clear decant which constituted
55 percent of the original volume of the
50 suspension.
An aqueous suspension of 2.5 percent Cab-Q-Sil silicon
dioxide was prepared. The sample was divided One liter ofan aqueous 10 percent suspension of Fulinto
four fractions; two samples were subjected to the ler's earth was prepared and divided into two 500 ml.
freeze-melt technique of the present invention and the fractions. One fraction was allowed to settle for 2 hours
remaining two fractions were untreated. The first un- 55 to produce a cloudy decant of 25 mI. The other fraction
treated fraction was allowed to settle for 60 minutes was frozen in the kerosene bath (-20° C) for 2 hours.
and the resulting turbid supernatant was decanted The frozen suspension was then allowed to thaw to
therefrom. The second untreated fraction was filtered produce a les~ ~Ioudy decant which constituted 47 perthrough
a No.3 Whatman filter. The third treated frac- cent ofthe ongmal vol~me oft~e s~~ension.
tion (i.e., the fraction subjected to the freeze-melt 60 An aqueous suspensIon of hmomtlc laterite was altechnique)
was subject to decantation upon thawing to lowed to settle for 16 hours without producing any
remove the clear supernatant. The fourth treated frac- measurable decant. The suspension was then removed
tion was filtered through a No.3 Whatman filter. The from the settled sands and allowed to further settle for
following table presents the results obtained: several days without any results. The suspension was
65 then frozen in a kerosene bath (-20° C.) and then allowed
to thaw. Upon thawing, 99 percent of the total
solids of the suspension were flocculated and settled to
the bottom of the container.
10
5
. Eac~ of the metal hydroxide suspensions were divIded
1Oto two fractions. One fraction was frozen to
-20° C. and then allowed to melt at ambient temperature
over a period of 60 minutes. The unfrozen remaining
fraction was allowed to settle for 60 minutes. The 5
clear liquid phase that developed at the top of each
fraction after 60 minutes was decanted off and measured
and compared with the total colloidal suspension
volume. The results are shown in the following table:
3,681,931
* * * * *
20
8
freezing temperature is between -I° and -100° C
4. The process as defined in claim 1 wherein the
freezing temperature is between about -20° and -80°
C
5. The method as defined in claim 1 wherein the total
solids content of the phosphate rock slime is between
about 5 percent and about 10 percent.
6. The method as defined in claim 1 wherein the
p~osphate rock slime is an acid leached phosphate
10 slime.
7. The method as defined in claim 1 wherein the
freezing temperature is about -20° C
8. The method as defined in claim 7 wherein the
phosphate rock slime is kept in a frozen state for at
15 least about 24 hours.
9. A process for the treatment of phosphate rock
slimes. whjch comptises:
sUbJectmg the phosphate rock slimes to a freezing
temperature, without agitation of the suspension,
for a period sufficient to separate and freeze a substantial
portion of the water therein;
subjecting the phosphate rock slimes to a thawing
temperature without agitation for a period sufficient
to melt the frozen water therein to yield a
bottom layer of phosphate rock slimes concentrated
suspension and an upper layer of liquid containing
essentially water and dissolved matter; and
filtering off the upper thawed liquid layer from the
thawed concentrated suspension.
30 10. The process defined in claim 9 including the additional
step of keeping the phosphate rock slimes in a
frozen state for a predetermined length of time after
the freezing step.
11. The process defined in claim 9 including the ad35
ditional step of decanting the upper layer of liquid from
the thawed phosphate rock slimes prior to the filtration
step.
12. The process defined in claim 9 including the additional
step of centrifuging the thawed phosphate rock
40 slimes to effect further concentration of the concentrated
suspension prior to the filtering step.
13. The process defined in claim 9 wherein the
phosphate rock slime is an acid leached phosphate rock
slime.
14. The process defined in claim 9 wherein the freezing
temperature is between about -20° C and about
-80°C
7
EXAMPLE 6
Time of Freezing ofHN03 Leached Slimes
Volumes of Weight of cake
Time in Temp. of decanted (dry) obtained
freezer frozen slimes liquid phase in 30 seconds
(hours) (degrees C. ) (mI.) forming (grams)
3.0 -6 25 1.60
5.5 -18 28 1.64
7.5 -18 28 1.60
24.0 -20 35 2.03
It can be seen that as the time was extended the
frozen slimes approached the working temperatu~e of
the freezer (-20° C). However, it can be noticed that 25
as this happens, more effective decantation and filtra~
tion are obtained. Comparing the tests of 7.5 and 24
hours having a difference of only 2° C, it can be conclu~
ed that. by substantially increasing the time during
static freezmg, the filtration rates will be increased.
However, the difference in filtration rates between the
3.0 and 7.5 hour tests is negligible.
I claim:
1. A method of making a phosphate rock slime
amenable to separation by decantation, filtration and
centrifugation which comprises:
statically freezing a substantial portion of liquid in
the phosphate rock slime at a freezing temperature;
maintaining the phosphate rock slime in a frozen
state for a predetermined length of time; and
thawing the frozen phosphate rock slime without
agitation prior to the separation by decantation,
filtration, or centrifugation.
2. The method as defined in claim 1 wherein the 45
frozen suspension is statically thawed in the thawing
step.
3. The method as defined in claim 1 wherein the
Four samples of 100 ml. each of nitric acid leached
phosphate rock slimes were put in the household
freezer. At different intervals of time, they were taken
~ut .and allowed to melt. Since there is always a clear 5
liqUid at the top upon melting, this was carefully decanted
and measured. Filtration tests, using the 0.019
squcu:e. foot leaf were performed under comparable
conditIOns (30 seconds forming time). The following
results were obtained:
50
55
60
65