1992 Lazaridis Daf Metal Ions

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SEPARATION SCIENCE AND TECHNOLOGY, 27 13), pp. 1743-1758, 1992

Dissolved-Air Flotation of Metal Ions

N . K .

L A Z A R I D I S ,

K . A.

MATIS, G. A. STALIDIS,

and P. M A V R O S *

LABORATORY

OF

GENERAL

&

INORGANIC CHEMICAL TECHNOLOGY

DEPARTMENT OF CHEMISTRY (BOX 114)

ARISTOTLE UNIVERSITY

THESSALONIKI

54006

GREECE

Abstract

Metal ions (copper, nickel, zinc, and ferric ions) were separated from dilute

aqueous solutions by dissolved-air flotation. The ions were either precipitated as

sulfides

or

floated (as ions) by xanthates. Copper and nickel were selectively sep-

arated; promising results were obtained with single, binary, and ternary mixtures.

The effect of several parameters (solution pH, addition

of

chemical reagents at

varying concentrations, and the presence of oth er ions) on the rem oval

of

ions was

studied. The collectorless flotation of copper ions was also investigated.

INTRODUCTION

Metals ar e often c ontain ed in industrial effluent, and since several to ns

of them are lost each year, there is considerable interest in recovering

them. This has the additional benefit of complying with environmental

conservation policies which are already being implemented.

Similar proble m s are faced in the hydrom etallurgical recovery

of

metals

(e.g. , copper) from ores, where great volumes

of

aqueous solutions are

often prod uc ed, and which have to be treated to recover the valuable ion.

Flotation , a rathe r unique case of a thre e phase (gas/liqu id/solid) pro-

cess, is one of the most significant unit operations in mineral processing,

but i t may also be applied to water and wastewater treatment ( I ) a nd t o

hydrometallurgy. Flotation is envisaged at present to replace both the

concentration and separation processes, if feasible. In a recent review (2)

it was suggested that one of the m ain advantages of flotation may be the

selective recovery of ions from complex solutions with sev eral coexisting

metals.

*To

whom correspondence should be addressed.

1743

Copyright 992 by Marcel Dekker, Ine.


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1744

LAZARIDIS

ET AL.

Previous work-in the broad field-involves, among others, the flotation

as hydroxide precipitates of metal-bearing wastes mainly by amines

(3)

or

by fatty acids

4 )

and as sulfide precipitates

5 ,

6), a comparative study

for a column technique and a flotation machine on copper industry waste-

waters

7 ) ,

and the application of LIX reagents (known from liquid-liquid

extraction) in flotation (8). Adsorbing colloid flotation of metals, simul-

taneously for ferric or manganese hydroxides, has also been applied to

deep-sea nodules 9 ) .A short-chain xanthate was used as the collector for

copper ions during a study of the selective separation of copper, zinc, and

arsenic ions from solution by flotation 10 ) . Copper and zinc ions were

separated by precipitate flotation as sulfides in batch 2 2 ) and continuous

flow

(12)

conditions.

In this work the removal of single

or

multiple metal ions from aqueous

solutions is investigated, and conditions are determined for the removal

of the ions, with or without collectors, in order to achieve total removal

or selective separation.

EXPERIMENTAL

A dissolved-air flotation unit, previously described l o ) ,was used for

the experiments. Chemical analysis of the solution samples was carried out

by AAS, and the results are expressed as the percentage of initial ion

content removed (or recovered) by flotation.

The reagents used were copper nitrate, nickel sulfate, zinc nitrate, and

ferric sulfate p r o analysi grade). The pH of the solution (deionized water)

was adjusted by nitric acid

or

sodium hydroxide solutions.

Sodium sulfide was applied for the precipitation at a stoichiometric

amount (unless otherwise stated) at a pH of approximately 3.

In the first part of this work-precipitate flotation-laurylamine (0.1%

ethanolic solution), cetyl pyridinium chloride, sodium lauryl sulfate, and

cetyl trimethyl ammonium bromide were used as collectors. In the second

part-ion flotation-potassium 0-ethyl-dithiocarbonate (xanthate) was

used.

Several flocculants were also tested: Percol-173 (anionic), Magnafloc-

351 (nonionic), and Zetag-92 (cationic), kindly supplied by Allied Colloids

(UK) .

When these were used, the solution was agitated at 3.33 Hz for

600

s

during the initial flocculation stage. The stirring was later slowed

down during flotation and stopped completely when pressurized water was

introduced into the cell.

Finally, microfiltration tests were conducted using membrane filters hav-

ing pore sizes of 8.0 0.45, and 0.05 pm.


3/16



DISSOLVED-AIR

FLOTATION OF METAL IONS 1745

RESULTS AND DISCUSSION

Precipitate flotation generally includes all processes in which an ionic

species is concen trated from an aque ous solution by forming a precip itate,

which is subsequently removed by flotation.

Dissolved air is ofte n used in conjunction w ith precipitate flo tatio n. Air

is dissolved into wate r unde r pressure an d, when th e pressu re is released,

fine air bubbles with diameters of less than

120

pm a re p roduced . The

rising bubbles constitute the process transp ort m edium . The se have b een

fou nd to float fine particles effectively (13).From the solubili ty isoth erm s

of air in water, it has been calculated that the quantity

of

air released as

fine bubbles from

100

cm3 of pressurized w ate r is roughly 7 cm3, which we

introduced into a

1-L

solution of metal ions.

Collectorless Flotation

of Copper Ions

Ex per im ent s were initially conducted with on ly cop per ions in solution;

sodium sulfide was u sed to precipitate a colloidal C uS (artificial covellite).

Figure 1 i l lustrates th e results obtained without any additional re agen t. C u

removal drops to about 5 in the pH range from 3 to 6; a bove pH 6 it

increases back to almost 100 for doses of N azS less or equa l to the

Copper removal ( )

100

8

60

4 0

20

0

. .

I

1 I

a2SMCul

0 80

1-00

-I

16

1.80

2

4

6

8 10 12

PH

FIG.

1.

Effect of sodium sulfide and pH on the removal of copper ions by dissolved-air

precipitate flotation.

[Cu2+

: 50 rng/L. Sodium sulfide expressed as percentage equivalent

to

copper. Dissolvcd-air flotation recycle 10 .


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1746

LAZARIDIS

ET

AL.

Comer

removal 1 1

00

40

20

0 2 4 e 8 10 12

PH

FIG.2 . Use of flocculants in th e dissolved-air precipitate flotation of copper

ions.

A ) 5 m g/

L; ( B )

10

m g l L . [Cu+]: SO m g / L . Stoichiometr ic amoun t

of

sodium sulfide.

(a )

Percol 173;

(b)

Magnafloc

351;

( c ) Ze tag 92.

[NazS]/[Cuz+]

1.0.

of

CuS

passing filter

[ I

100

-

80

-

60

4 0

20

0 .

0

2

4

0 8

10

12

PH

FIG.

3 . Microfiltration tests; percentage

of

copper sulfide passing through filter. [Cu]:

S

mglL .

[Na2S]/[Cuz+]

1.0.


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DISSOLVED-AIR FLOTATION

OF

METAL IONS

1747

stoichiometric amo un t required. This seems to contradict previous results,

where a 30% excess of sodium sulf ide was found to be adeq uate If), but

a possible explanation can be traced to froth flotation 1 4 ) where sodium

sulfide constitutes a depressing agent for sulfide ores.

Th ree different polymers [a cationic (Z etag 92), an anionic (Percol 173),

and a nonionic (Magnafloc 351)] were then tested as flocculants; Fig. 2

presents the results obtained with 5 and 10 mg/L. The same flotat ion

behavior was noticed, more or less, with the anionic and the nonionic

flocculant, but with slightly higher removals. On the contrary, copper re-

moval was always ov er 80 with the cationic polyelectrolyte (concentration

at 10 mg /L) .

As

a further test

of

th e natu ral floatability of the copper sulfide precip-

itate, t he dispersion (afte r precipitation) was filtered thro ug h several filters

having various pore sizes (from 8.00 down to 0.05 Fm). Figure

3

shows

that in the

3-6

pH range the precipitate particles are very fine, and this

may explain why collectorless flotation is

so

inefficient in this

pH

range.

Flotation of Copper Precipitates Using Collectors

Several anionic and cationic surfactants w ere tested as collectors for the

removal

of

cop per precipitate: cetyl pyridinium chloride ( CP Cl) , cetyl tri-

methyl-ammonium bromide (CTMABr), sodium laurylsulfate (SLS), and

laurylamine (LA). The results are illustrated in Figs. 4 and

5 .

100

80

5

40

20

0

Comer

removal [ I

0 (a)

SLS

+

( c )

CTMABr

( b ) CPCl

0 2 4 6 8 1 0 1 2

PH

FIG.

4. Use of various collectors in copper flotation: sodium lauryl sulfate (SLS), cetyl

pyridinium chloride (CPCI), and cetyl trimethyl-ammonium bromide (CTMACI). [Cu

1:

50

mg/L. Surfactant concentration:

10

mg/L. [Na2S] / ICu2+] 1 .0 .


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1748

Cower removal [ I

20-

100

-.

80

8

4 0

20

x stoich. Na2S

4

30%

exc ss

Na2S

+

0.5

40

20

0

I

-

- +

1 mg/L

LA

I I

LAZARIDIS

ET

AL.

Cower removal [ I

0 2 4

6

8

10

12 0 2 4 6 8 1 0 1 2

PH PH

Flti. 5 .

(A ) Effect of laurylamine ( L A ) on copper ion flotation. (B) Excess

(30%)

of

Na,S.

[Cu?']: 50 mg/L. [NazS] / [Cu?+] 1.0.

Comer removal [ I


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DISSOLVED-AIR FLOTATION OF METAL IONS 1749

The first three surfactants display a similar behavior; they were unable

to float the copp er precipitate in the 3-6 p H range. Laurylamine, however,

prov ed m ore efficient; i t successfully rem oved t he cop per precip itate when

it was add ed at a dosage of 10m g/L (Fig . 5A ) . An excess of 30% of Na,S

further enhanced its efficiency to approximately 100% across almost the

whole pH range (Fig. 5B). Laurylamine also proved to be as efficient at

higher copp er concentrat ions

(250

m g/L , Fig. 6) . I t should be no ted, how-

ever, that at this high Cu concentration, even the collectorless removal

was significantly higher.

Ge nerally, i t seems most pro bable that flotation will be requ ired

to

solve

problems conn ected with dilute rather than highly conc entrated solutions.

The refore, the observed reduction in copper ion removal in the 3-6 p H

range may be corrected by adding some suitable cationic reagent

(surfactant).

Col lector less F loatat ion of Mult i-Ion Solut io ns

The removal of the copper precipitate in the presence of oth er ions was

also investigated. The analysis of an actual scrubber wastewater from a

copp er smelting plant

6)

was used as a guide for the ionic species and th e

respective co ncen trations that m ay be foun d in an industrial effluen t.

Zinc ions often coexist with copper. Figure 7 illustrates the effect of

adding

50

m g / L

of

zinc ions to the cop per ion solution (at p H 5 ) . I t seems

Ion

removal

[ I

0 0

1.0

2 0

3 O 4.0

[Na2Sl/[Cul ratio

FIG.

7 .

Effect of sodium sulfide and zinc ions

50

mg/L) on copper ion flotation

(pH

ap-

proximately

5 ) .

[ C U ~ ' ] : 0 m g / L .


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1750

Copper removal [ I

80

60

4 0

20

l o o t - -

-

O L

I

1

I I

0 2 4 0 8 1 0 1 2

PH

LAZARIDIS ET AL.

Sulfate i ons

1

', , ,:;:

FIG.8. Effect ofSOi

ions on copper ion flotation. [Cu ]:

50

m g / L . (Na,S]/[Cu''] = 1.0.

that the presence of Zn2+ nhances the collectorless removal of the copper

ion.

It should be remembered (from Fig. 1) that, at pH 5 , there is very little

removal of Cu2+, ven with an excess

of

30% of Na2S. When Zn is added,

a 60% removal is achieved for a stoichiometric amount of Na2S.A selective

separation of the two species is achieved at this dosage. A similar selective

separation has been achieved at pH 2 by using laurylamine

11).

If, on the other hand, a total removal of the ionic species is required,

then an excess of over 150% of NazS is required to remove both Cu2+and

Zn?+.

The addition of sulfate ions (200 or

800

mg/L SO:-) had no effect on

the (collectorless) flotation of copper (Fig. 8).

The addition of Fe3+had, however, a noticeable effect (Fig.

9) .

The

removal of Cu2+was enhanced in the 3-6 pH region (compared with the

single ion behavior-Fig. l , but then dropped dramatically. At pH over

6,

heavy flocs appeared in the cell due to the precipitation of Fe(OH),

which could not be floated under these collectorless conditions. However,

at pH

1992 Lazaridis Daf Metal Ions

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