Flotation separation of plastics using selective depressants

Int. J. Miner. Process. 48 (1996) 127-134

Flotation separation of plastics using selective depressants J. Shibata a, S. Matsumoto

a, H. Yamamoto Pradip ’

a, E. Kusaka b,*,

a Department of Chemical Engineering, Kansai University, Suita 564, Japan b Department of Mineral Science and Technology, Kyoto Unioersity, Kyoto 606-01. Japan ’ Tata Research Deuelopment and Design Centre, Pune 4/1001, India

Received 31 July 1995; accepted 24 May 1996

Abstract Four important plastics, namely polyvinyl chloride (PVC), polycarbonates (PC), poly acetal (POM) and polyphenylene ether (PPE) were successfully separated from their synthetic mixtures using common wetting agents like sodium ligninsulfonate, tannic acid, Aerosol OT and saponin. The efficient flotation separation amongst these naturally hydrophobic polymers could be attributed to the selective reduction in hydrophobicity (measured as the drop in contact angle) as a consequence of surfactant adsorption. The relative order of floatability measured with the help of column flotation experiments in the presence of various depressants selected for this study was found to be PPE > POM > PC > PVC, a trend consistent with the critical surface tension values determined for these solids except PPE. A flowsheet was developed on the basis of flotation results. It was possible to accomplish almost a perfect separation using the proposed flowsheet involving heavy media separation followed by flotation in two stages. Keywords: flotation; plastics; selective depressants: criticalsurface tension; contact angle

1. Intrjoduction The consumption of plastics, particularly engineering plastics has been steadily increasing. For example, in Japan, the annual production (1994) of four important plastic materials, namely polyvinyl chloride (PVC>, polycarbonates (PC>, poly acetal (POM) and polyphenylene ether (PPE) was 2.1, 0.13, 0.12 and 0.072 million tons respectively. -* Corresponding author. Fax: + 8 1-7-753-5428; E-mail: [email protected] 0301-75 16/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. PII SO301-7516(96)00021-X

In view of the problems associated with the safe disposal of plastics waste, the efforts are being made to find efficient ways of recycling this waste. The common mineral separation processes such as classification, heavy media separation and froth flotation can be utilized in the separation of plastics: this has been intensively reviewed in a recent publication by Buchan and Yarar (1995). Most of the patents obtained on the separation of plastics aim at flotation separation of polyvinyl chloride from other plastics (Saito et al., 1974; Hayashi, 1975; Saito, 1975; Saito and Izumi, 1976 Saito and Izumi, 1978, 1979; Grimm and Sehlmeyer, 1986; Sisson and Tompkin, 1992). Sodium lignin sulfonate is a common wetting agent used in this separation. The increasing use of engineering plastics in industrial applications such as automobile, electronic and precision machining industry has made it imperative to find ways to separate PVC, which on burning during waste processing leads to generation of environmentally hazardous hydrogen chloride gas. Furthermore, engineering plastics are relatively more expensive than other plastics. As compared to the cost of 300 yen/kg for PVC, PC’s cost 1000 yen/kg in Japan. There is thus a need to develop techniques for selective separation amongst various plastic materials. We present in this paper the results of our successful investigations on the flotation separation of certain plastics using some common surfactants as selective depressants. It was possible to achieve almost perfect separation of four plastics selected for this study from their synthetic mixtures.

2. Experimental

materials

and methods

2.1. Materials The samples of four different kinds of plastics, namely PVC, PC, POM, and PPE, were obtained from Mitsubishi Engineering Plastics Corporation, Japan. Some of the physical and economic properties of these samples are summarized in Table 1. Each of the plastic samples was crushed by a roll mill and screened. The - 4 + 2 mm (- 5 + 9 mesh) sieve size fraction was used for flotation experiments. These were mixed in the desired proportions and used for selective flotation. Since the samples were of different colors, and of sufficiently large size, the concentrate samples could be conveniently analyzed through manual sorting at the end of each experiment. For contact angle measurements, the plate sample made of the particular plastic without coloration was used.

Table I Properties

of. plastics materials used in this investigation

Property

PVC

PC

POM

PPE

Density (g/cm’) Color Cost (yen/kg)

1.3 red 300

1.2 colorless 1000

1.41 white 750

1.05 black 750

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J. Miner. Process. 48 (1996) 127-134

129

The surfactants used for selective depression included tannic acid, polyoxyethylene lauryl ether, sorbitan monolaurate, Aerosol OT, polyvinyl alcohol, and saponin from Wako Pure Chemical Industry, Japan and sodium lignin sulfonate from Tokyo Chemical Industry, Japan. Pine oil frother obtained from Nippon Koryo Yakuhin Kaisha Ltd., Japan, was used during all flotation experiments. Various solvents used for determining the critical surface tension for the plastics samples included water, glycerol, formamide, methylene iodide, ethylene glycol, triethylene glycol and tricresylphosphate with surface tension (dynes/cm) of 72.0, 63.4, 58.2, 50.8, 47.4, 45.0 and 40.9 respectively. 2.2. Flotation

tests

The :[lotation tests were carried out in a column of 3 litres volume (100 mm dia. and 400 mm height) fitted with a glass frit of size 3 for bubbling nitrogen gas. 10 g each of the plastics samples was taken for flotation. Thus the total sample weight during flotation of synthetic mixtures containing all the four plastics was 40 g unless otherwise noted. The nitrogen gas flow rate, the reagent conditioning time, and the flotation time were m,sintained at 12 ml/min, 10 min, and 3 min, respectively. Pine oil dosage was always 0.05 g/l; this was determined on the basis of a preliminary test. The recovery of individual component at the end of a flotation test was determined by counting the particles (easily distinguishable by color) of each plastics collected in the concentrate. Also, the percent recovery corresponds to the mean value of five tests. 2.3. Contact angles measurements The (contact angles for various liquids on a plastic were measured with the help of a Kyowa Kaimenkagaku contact angle goniometer. A drop of the liquid was placed on the plastic disc through a syringe and then the equilibrium contact angle was measured. The measures correspond to mean results obtained from five tests.

3. Results and discussion All the plastics were found to be naturally floatable. It was therefore necessary to use an appropriate depressant for achieving selective separation. The percent flotation recovery of individual plastics components from a synthetic mixture of all the four plastics in presence of three different depressants is shown in Fig. 1. The order of flotation response was found to be as follows: Tannic acid Lignin sulfonate

PPE z==POM > PVC = PC PPE = POM > PC >> PVC

POE

PPE > POM = PC B PVC

J. Shihntn et nl./Int.

“0

Em

400

200

800

,000

J. Miner. Process. 48 (19961 127-134

100

0

300

200

500

400

II

200

100

300

400

Concentration(mgl1) Fig. 1. Recoveries as a function of concentration for PVC (0 ), PC (O), POM ( ??) and PPE ( A ) in presence of tannic acid, sodium lignin sulfonate and polyoxyethelene lauryl ether, respectively.

10 O7 0

0.8

6 0

B

5 03

0.4 2 0.2

001

I,,

20

30

40

Surface

50

tension

60

70

80

01

__\_

0.0 / m 0.6

.\

20

30 Surface

(dyne/cm)

40

50

tension

b)

a) PVC

60

70

80

(dyne/cm)

PC

T I

6

08 ’ m

5

06

.3

B

2

04

1

-----.i;

02

I 20

30 Surface

40

50

tension c)

60

70

(dyne/cm)

POM

: 72.0dynelcm : 63.4dynelcm 3 Formamide : 58.2dynelcm 4 Methylene iodide : 50.8dyne/cm 1 Water

2 Glycerol

Fig. 2. Critical surface tension figure.

60

A

20

30 Surface

40

50

tension

60

70

80

(dyne/cm)

d) PPE

5 Ethylene glycol 6 Triethylene glycol 7 Tricresyl phosphate

: 47.4dyneIcm : 45.0dynelcm : 40.9dynelcm

-y, for (a) PVC, (b) PC, (c) POM and (d) PPE using solvents shown in the

131

J. Shibata et al./ Int. J. Miner. Process. 48 (1996) 127-134

The critical surface tension values, as determined by contact angle measurements with liquids of varying surface tension (Fig. 2) were found to decrease in the order POM (26 dyne/cm)

> PPE (30 dyne/cm)

> PC (33 dyne/cm)

> PVC (40.2 dyne/cm) Except for PPE, the order of flotation recoveries observed during these investigations was thus consistent with that predicted by the critical surface tension. There was no significant effect of pH observed in the range of 5 to 12 during

100 .

POM

Lignin

sulfonate

Lignin

20 -

PVC

sulfonate

‘c,

, 100

200

300

Dosage

400

500

“‘7’m’s’100

,J

(mgll)

L Lignin

.o

300

200

Dosage

400

500

(mgll)

Saponin200mgll

60

sulfonata 40

PVC

“L

y.

100

200

Dosage

300

*

0

200

400

Dosage

400

500

Aerosol

(mgll)

600

(mgli)

OT,Dosage

(mg/l)

PPE

800

,000

0

50

150

100

Dosage

Fig. 3. Flotation recovery as function of depressant dosage during experiments showing the selectivity of separation achieved with appropriate depressant.

200

(mgll) on binary synthetic

mixtures

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J. Miner. Process. 48 (19961 127-134

Table 2 Results obtained during flotation tests with binary mixtures using the most effective depressants depressant selected for each test) Binary mixture

Depressant/dosage

Results

PVC/PC

lignin sulfonate 500 mg/l

PC d PVC

74 99.6

99.5 79.3

PVC/POM

lignin sulfonate 500 mg/l

POM a PVC

96.7 90.2

90.8 96.5

PVC/PPE

lignin sulfonate 300 mg/l

PPE ’ PVC

87.4 95

94.9 97.3

PC/POM

saponin 200 mg/l AOT 50 mg/l

POM a PC

99.5 86.3

88 99.7

PC/PPE

tannic acid 500 mg/l

PPE a PC

85.5 99

98.8 87.2

POM/PPE

sorbitan 200 mg/l

PPE a POM

73.4 98

96.2 78.7

(appropriate

Grade (%)

Recovery (%)

’ Collected as flotation concentrate.

flotation of plastics with the reagents used in the study. All the subsequent flotation experiments were therefore carried out at natural pH between 5 and 6 of suspensions. All the surfactants selected for the study were investigated for their selective depression behavior during flotation tests with binary mixtures. The most effective depressants for each of these binary combinations were thus established. The flotation results indicating successful separation are summarized in Fig. 3 and the corresponding recoveries and concentrate grades are presented in Table 2. 3.1. Mechanism

of depression

The contact angles of plastics in aqueous solutions were measured to be around 90”, a characteristics of hydrophobic solids. In order to accomplish selective separation, appropriate depressants were selected so as to render one or more surface hydrophilic (Zisman, 1963; El-Shimi and Goddard, 1974). As illustrated in Fig. 4 for PVC/lignin sulfonate combination, the contact angle could be reduced to as low as 20” with the addition of lignin sulfonate. Consequently, as expected, the floatability of PVC also dropped precipitously with the lignin sulfonate addition. Selective flotation was thus possible at an appropriate dosage of the depressant. 3.2. Separation flowsheet Based on the flotation tests, a flowsheet was developed to produce separate clean concentrates of the four plastics from a synthetic mixture. Since PPE is the lightest and has a density significantly different from the other three plastics as shown in Table 1, it

133

J. Shibata et al. / Int. J. Miner. Process. 48 (1996) 127-134

-0

600

400

200

Dosage Fig. 4. Correlation between the drop in hydrophobicity measured as a function of depressant dosage.

600

1000

(mg/l) (contact

angle) and decrease

in floatability

of PVC,

might be more appropriate to separate PPE from the others using a media of density l.‘l (controlled with the help of sodium chloride) in the first separation step; this seems ‘to reduce the load of the following flotation process. Secondly, PVC, PC and POM can be

sYNTlETlc of

MlxTuRE

PLASTICS

NaCl Solution(Sp.Qr.l.1)

Sodium Lignh sldfonlte 5Oomg/l

Rinse & Dry

Fig. 5. Proposed flow sheet for separation of plastics from mixtures: the mass balance as well as the recovery and grade of concentrated products at each stage are shown in the figure.

134

.I. Shibatu et nl./Int.

.I. Miner. Procrss.

48 OF-61 127-134

separated by flotation processes in which a sigle depressant or a depressant mixture helps the separation. In Fig. 5, the proposed flowsheet in this study are presented. The first separation step involved heavy media separation. The PPE concentrate having 100% purity was obtained as a float product with 100% recovery. The PVC concentrate of 95.7% purity was separated next by the flotation using sodium lignin sulfonate depressant. Final flotation step involved depression of PC using saponin/Aerosol OT combination. The float with 87.6% POM and the sink with 90.3% PC were obtained in this step. These results of the heavy media and flotation separation are also summarized in Fig. 5.

4. Conclusions Four common plastics namely PVC, PC, POM, and PPE selected for this study were found to be strongly hydrophobic. It was however possible to separate all the four plastics with the help of appropriate wetting agents. Among a wide variety of surfactants tested during the course of this work, lignin sulfonate was found to be a good depressant for PVC. PPE, the most floatable plastics also happened to be the lightest and could be separated with the help of a heavy media of sp. gravity 1.1. Saponin/Aerosol OT combination was most effective in depressing PC. Interestingly, the order of floatability in the presence of different depressants was observed to be PPE > POM > PC > PVC, more or less consistent with the order of their corresponding critical surface tension values. A flowsheet was thus developed for separating the various plastics of the kind used in this investigation.

References Buchan, R. and Yarar, B., 1995. J. Miner. Met. Mater. Sot. (JOM), (February): 52-55. El-Shimi, A. and Goddard, E.D., 1974. J. Colloid Interface Sci., 48(2): 242-248. Grimm, M.J. and Sehlmeyer, T.R., 1986. US Patent No. 4617111. Hayashi, Y., 1975. Japan Patent No. 50-12677. Saito, K., Nagano, I. and Izumi, S., 1974. Plastics Age., (October): 99-103. Saito, K., 1975. Japan Patent Nos. 50-32274 and 50-37874. Saito, K. and Izumi, S., 1976. Japan Patent No. 51-30879. Saito, K. and Izumi, S., 1978. US Patent No. 4119533. Saito, K. and Izumi, S., 1979. US Patent No. 4132633. Sisson, E. and Tompkin, M., 1992. In: Davos Recycle ‘92, International Forum and Exposition, 1-13. Zisman, W.A., 1963. Ind. Eng. Chem., 55(10): 19-38.

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