Separation of PET from other plastics by flotation combined with alkaline pretreatment

Pita, Fernando; Castilho, Ana

doi:10.1590/0104-1428.03320

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ORIGINAL ARTICLE • Polímeros 30 (3) • 2020 • https://doi.org/10.1590/0104-1428.03320 copy

Separation of PET from other plastics by flotation combined with alkaline pretreatment

Authorship SCIMAGO INSTITUTIONS RANKINGS

Abstract

Plastics are naturally hydrophobic, so the selective flotation of plastic mixtures is impossible without changing their surface properties. The objective of this research is flotation separation of PET from PS, PMMA and PVC combined with sodium hydroxide pretreatment. NaOH pretreatment had a strong effect on PET’s floatability, medium effect on PMMA and PVC and limited effect on PS. The reduction of PET floatability is ascribed to a sharp decline of contact angle. The optimal pretreatment conditions were: 10% NaOH concentration, 70 °C (PET/PMMA and PET/PVC) to 80 °C (PET/PS) and plastic treatment times in the alkaline solution of 20 min (PET/PMMA and PET/PVC) to 30 min (PET/PS). Flotation separations were achieved efficiently and the best results were obtained for the PET/PS mixture, with a floated PS grade of 98% and a sunk PET grade of 100%. PET and PS was separated effectively, implying that sodium hydroxide treatment possesses superior applicability.

Keywords:
alkaline treatment; flotation; particle size; plastic

Parameters	Symbol	Range values and coded level ()
NaOH concentration (%)	A	2 (-1)	6 (0)	10 (+1)
Temperature (°C)	B	20 (-1)	40 (-0.333)	70 (+0.666)	80 (+1)
Treatment time (min)	C	2.5 (-1)	5 (-0.818)	10 (-0.455)	20 (+0.273)	30 (+1)

		model	A	B	C	AB	AC	BC	A²	B²	C²
+2-2.8 mm	p-value Prob>F	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	0.9635	0.1169	0.0588	0.4583	0.1557
	Sum of Square=79831; Mean of square= 8870; degree of freedom=9; F_model=44.3; R²=0.8885; Ajusted R²=0.8680
+2.8-4 mm	p-value Prob>F	<0.0001	<0.0001	<0.0001	0.0002	<0.0001	0.8789	0.1242	0.0667	0.4283	0.2593
	Sum of Square=79999; Mean of square=8889; degree of freedom=9; F_model=44.1; R²=0.8881; Ajusted R²=0.8678

		model	A	B	C	AB	AC	BC	A²	B²	C²
+2-2.8 mm	p-value Prob>F	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	0.2060	0.0001	0.3750	<0.0001	0.086
	Sum of Square=4316; Mean of square= 480; degree of freedom=9; F_model=102.4; R²=0.9485; Ajusted R²=0.9393
+2.8-4 mm	p-value Prob>F	<0.0001	<0.0001	<0.0001	0.0002	0.0088	0.1793	0.3509	0.5321	<0.0001	0.088
	Sum of Square=6645; Mean of square= 738; degree of freedom=9; F_model=128.0; R²=0.9584; Ajusted R²=0.9509

		model	A	B	C	AB	AC	BC	A²	B²	C²
+2-2.8 mm	p-value Prob>F	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	0.1006	<0.0001	0.8151	<0.0001	0.0651
	Sum of Square=406; Mean of square= 45.1; degree of freedom=9; F_model=71.8; R²=0.9282; Ajusted R²=0.9153
+2.8-4 mm	p-value Prob>F	<0.0001	0.0005	<0.0001	<0.0001	0.0088	0.7543	0.0051	0.8912	<0.0001	0.3082
	Sum of Square=1695; Mean of square= 188; degree of freedom=9; F_model=38.0; R²=0.8720; Ajusted R²=0.8495

Conditions			Contac angle (°)
NaO concentration (%)	Temperature (°)	Treatment time (min)	PET	PS	PMMA	PVC
2	20	2.5	71.6	93.3	75.5	82.7
2	20	30	68.5	91.5	73.0	81.1
2	40	2.5	69.9	93.0	74.7	81.6
2	40	30	68.1	91.0	68.4	80.6
2	70	2.5	70.1	91.0	72.1	80.8
2	70	30	58.2	89.0	66.5	78.4
2	80	2.5	68.4	88.0	67.6	77.7
2	80	30	53.4	85.7	59.7	73.9
10	20	2.5	70.8	91.8	74.8	80.6
10	20	30	66.2	89.5	70.2	79.5
10	40	2.5	64.7	90.1	72.9	78.9
10	40	30	51.5	88.6	63.8	77.6
10	70	2.5	52.6	88.5	65.1	76.5
10	70	30	42.3	86.7	59.2	73.1
10	80	2.5	45.9	85.2	59.4	71.4
10	80	30	40.1	83.3	55.7	68.2

Plastic Mixtures	Fraction (mm)	Products	Recovery (%)		Grade (%)		Separation Efficiency (SE) (%)
Plastic Mixtures	Fraction (mm)	Products	PET	OP*	PET	OP^*	Separation Efficiency (SE) (%)
PET/PS	+2-2.8	Floated	1.9	100	1.9	98.1	98.1
		Sunk	98.1	0	100	0
	+2.8-4	Floated	2.2	100	2.2	97.8	97.8
		Sunk	97.8	0	100	0
PET/PMMA	+2-2.8	Floated	6.2	80.5	7.2	92.8	74.3
		Sunk	93.8	19.5	82.8	17.2
	+2.8-4	Floated	5.5	73.8	6.9	93.1	68.3
		Sunk	94.5	26.2	78.3	21.7
PET/PVC	+2-2.8	Floated	5.7	97.1	5.5	94.5	91.4
		Sunk	94.3	2.9	97.0	3.0
	+2.8-4	Floated	5.2	92.6	5.3	94.7	87.4
		Sunk	94.8	7.4	92.8	7.2

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[1] * fpita@ci.uc.pt