3,681,931 Treatment of phosphate rock slimes by freezing

Patent Number/Link:

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

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t--...

1------ r........

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e----!iN0'1 Leafhed Shines --- -- e- - ~ ,

5fafic Frdzen II

1------+~-1-~---- --j------'- 1------- _ .. - I_ I

---- - -Non 5faftc. Froz~n I f----+ - ---+-----_+__ r--- -~-~-

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- -Nof Frozen

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.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

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.

BACKGROUNDOFTHEINVENTION

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.

3,681,931

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

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

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.

. 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

* * * * *

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°

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

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: