Caracterización de la flotación de PET y PVC en presencia de diferentes plastificantes

7/17/2019 Caracterización de la flotación de PET y PVC en presencia de diferentes plastificantes

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Flotation characterization of PET and PVC in the presence of differentplasticizers

Ali Güney, Cihan Özdilek, M. Olgaç Kangal, Fırat Burat ⇑

Istanbul Technical University, Faculty of Mines, Mineral Processing Engineering Department, 34469, Maslak, Istanbul, Turkey

a r t i c l e i n f o

Article history:

Received 1 April 2015Received in revised form 30 June 2015Accepted 15 July 2015Available online 15 July 2015

Keywords:

PlasticsLignin alkaliDiethylene glycol dibenzoateSelective froth flotation

a b s t r a c t

Plastic is one of the most widely used materials in our daily life and industry. Recycling is an importantactivity in the minimization of waste that results from human activities. Depending on plastics surfacecharacteristics, they can be separated from each other by flotation method which is useful mineralprocessing technique with its low cost and simplicity. To succeed in a separation of mixed polymersbased on polymer type using selective flotation, it is necessary to render one surface more hydrophilicwhile the others are still in a hydrophobic state. The separation could be achieved with the help of appro-priate plasticizer.

In this paper, the characteristics of plastics particles and flotation behaviors of polyethylene terephtha-late (PET) and polyvinyl chloride (PVC) were studied. The contact angle, liquid surface tension, zetapotential measurements and FITR analysis were performed in order to investigate the effect of plasticizerreagents and the success of froth flotation at plastic recycling. It was observed that contact angle values of PVC were smaller than PET in the presence of lignin alkali (LA) and diethylene glycol dibenzoate (DIB).Both plasticizer render PVC more hydrophilic and cause higher wettability. Therefore, PVC remains inthe floatation cell while air bubbles are attaching to PET. At the end of the selective flotation experiments,PET was separated from the mixture of PET and PVC with 97% content and more than 99% recovery whenusing 25 g/t LA as a plasticizer at pH 8. To succeed in a separation of PET from PVC by selective flotation,

250 g/t DIB was added into the solution at pH 5 and PET particles were separated from the mixture at 90%content and 95% recovery.

2015 Elsevier B.V. All rights reserved.

1. Introduction

Plastics are used in a wide variety of products and have dis-placed other materials, such as wood, metal, and glass. It can beformed into polyesters for use in fabrics and textiles, polyvinyli-dene chloride for food packaging, and polycarbonates for eye-glasses and compact disks, among thousands of other uses. Theproduction of plastic requires four basic steps: the acquirement

of raw material, synthesizing a basic polymer, compounding thepolymer into a usable fraction, and lastly, molding or shaping theplastic. In 2013, global production of plastics reached nearly300 million metric tons, with 57 million metric tons in Europealone (Fig. 1). China is one of the largest producers of plastics inthe world, accommodating almost 25 percent of the global share[1].

In recycling of plastics material, it is often necessary to separatemixtures of plastics into individual plastics in order to get a useful

recycling plastic product because some plastics are not compatibleduring re-melting [2]. The recycling procedure for plastic bottlesnormally involves three basic steps: the collection and sorting of plastic bottles, re-processing into reusable flakes or pellets, anddevelopment of market for recycled plastics [3,4].

A number of promising technologies for the separation of mixedthermoplastics are under investigation and include air classifica-tion, hydrocycloning, flotation/sedimentation, depolymeriza

tion/purification/repolymerization, selective dissolution, sortingbased on infrared analysis or laser scanning of polymers, and incor-poration of chemical markers into different polymers [5].

The idea to apply flotation to separate plastics was a logical stepas ore flotation research has demonstrated that the surface proper-ties of different materials can be altered selectively by surfactantadsorption [6]. The use of froth flotation for plastic separation isparticularly challenging because of the similarity in the surfaceproperties of the materials. All plastics have naturally hydrophobicsurfaces and will readily float with the aid of air bubbles. Theselected treatment system must render at least one plastichydrophilic, resulting in no air attachment. The wettability of

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⇑ Corresponding author.

E-mail address: [email protected] (F. Burat).

Separation and Purification Technology 151 (2015) 47–56

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hydrophobic (low energy) surfaces can be enhanced by depres-sants commonly used in ore flotation. A desired plastic can be sep-arated from a mixture by froth flotation after treating the plasticsin alkaline solution [5,7–10]. Additionally, separation of mixed

plastic based on plastic type is achieved with the help of appropri-ate wetting agents [4,6,9,11–13]. To success an adequate separa-tion of mixed plastic based on plastic type using selectiveflotation, it is necessary to render one more surface hydrophilicwhile the others are still in a hydrophobic state. The hydrophilicone remains in the flotation solution while air bubbles are attach-ing to hydrophobic one.

As well documented in literature, NaOH and lignosulfonateshave been shown to be effective for PVC/PET separation[8,10,14]. It was found that NaOH caused hydrolysis of the surfaceof PET making it hydrophilic, whereas PVC was relatively unaf-fected. Lignosulfonate molecules adsorb onto the surface by phys-ical adsorption. Depending on concentration/pH and metal ionconcentrations both plastics can be rendered hydrophilic, with

specific conditions allowing selectivity. Burat et al. [15] reportedflotation of mixed plastics utilizing selective wetting characteris-tics changing the surface of specific plastics from hydrophobic tohydrophilic. According to their results, PVC particles were sepa-rated from PET particles at 100% content and 98.8% recovery ratein virgin polymer separation whereas PET particles were obtainedwith 99.7% content and 57.0% recovery in post-consumer polymerseparation. Kounosu et al. [16] also studied flotation of plastics.Employing polyvinyl alcohol (PVA) of relatively low degree of poly-merization, they reported a successful separation of PP from PE.Shibata et al. [17] separated four different types of plastics, namelypolyvinyl chloride (PVC), polycarbonate (PC), polyacetal (POM) andpolyphenylene (PPE), using common wetting reagents like sodiumlignin sulfonate, tannic acid, Aerosol OT and saponin.

In this contribution, the selective separation of PET and PVCparticles with plasticizer conditioning process followed by selec-tive froth flotation were investigated. The mechanism of plasticizertype and amount on modification of plastic surfaces and the per-formance of selective flotation were examined through contactangle, surface tension, electro kinetic potential measurementsand FT-IR (Fourier transform infrared spectroscopy) analysis.

2. Experimental

2.1. Materials

Samples of virgin PET and PVC (Fig. 2) less than 3 mm werereceived from PETKIM (Turkish Petrochemical Company) and

employed to measure electro kinetic potential and FT-IR. These

plastics were selected according their similar surface propertiesas present for common beverage bottles and windows frames.

The samples of post-consumer plastics, PET and PVC, wereobtained from Fuxing-Usas, Plastic Company in Turkey. PET sam-ples were chosen from soft drink bottles and PVC samples wereselected from waste window frames (Fig. 3). Paper labels weremanually removed and each plastic sample was shredded andclassified into different size fractions. Coarse size fraction sampleswere well cleaned before performing the measurements of con-tact angle. Feed mixture of 25 g PET and 25 g PVC was preparedwithin the fraction of 3.36 + 2 mm and used for flotation testsas reported in our previous studies [12,15,18]. Because of differ-ence in color and shape, the PVC and PET particles were separatedfrom each other by hand sorting easily. The compounds of plasticsamples were identified by chemical analysis and presented inTable 1.

Chemicals used for the flotation tests were NaOH and HCl as pHadjuster; methyl isobutyl carbinol (MIBC) as a frother; diethyleneglycol dibenzoate (DIB) and lignin alkali (LA) (Aldrich, Munich,

Germany) as plasticizers (Fig. 4). Diethylene glycol dibenzoate(C18H15O5), a glycol benzoate ester, is a clear liquid with chemicallystable properties and high boiling point. It is slightly soluble inwater and very soluble in polymers. It has an excellent compatibil-ity with polyvinyl acetates and polyvinyl chloride. It is used as aplasticizer for PVA (polyvinyl alcohol) homopolymer and copoly-mer emulsions and as well as for PVC coatings [19]. Lignin alkaliis primary dispersing agent for dye stuffs and agricultural chemi-cals. It is compatible with anionic and nonionic surfactants andwetting agents [13].

Fig. 2. The virgin PET (dull) and PVC granules (transparent).

Fig. 3. Post-consumer PET (transparent-blue) and PVC (white) granules. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

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Fig. 1. World production of plastics (million tons) [1].

48 A. Güney et al. / Separation and Purification Technology 151 (2015) 47–56



2.2. Measurements

2.2.1. Contact angle measurements

The sessile-drop contact angle technique was used in this studyas well described in following publication [20]. The contact anglemeasurements were carried out using a NRL goniometer (Rame

Hart Inc.). Plates of PET and PVC plastics were crushed into15 15 mm and polished using abrasive papers and then washedwith deionized water. A drop containing various concentrationsof reagents was placed on the surface of plastics using a syringe.The drop size was made to increase by adding a small volume of water, and the water advancing contact angles were measuredwithin 30–60 s. The diameter of drop changed between 8 and10 mm and drops were monitored under a microscope with suffi-cient enlargement. For each plastic sample, the average value of three measurements was taken from different surface of plasticparticles.

2.2.2. Surface tension measurements

The liquid surface tension was measured using DuNouy ringmethod. In this method, platinum ring with standard perimeterat 60 mm was submerged below the liquid/air interface and thenlifted through the surface. The force kept increasing until the max-imum force was reached. Eventually, the meniscus tore from thering and returned to its original position. The maximum forcedirected to the ring was measured. All measurements were con-ducted at room temperature of 25 C.

2.2.3. Zeta potential measurements

The zeta potential of plastic samples was determined using anElectro Kinetic Zeta meter 3.0 Analyzer. The plastic samples werecrushed into very fine particles using a file and then mixed withthe solutions of electrolytes at various plasticizers concentrations

and pH values. The conditioning time was fixed to 15 min, thenplastic particles filtered and monitored. The point of zero charge(pzc) was estimated from these values. HCl and NaOH were usedas pH modifiers and all tests were accomplished at room

temperature (25 ± 2 C). The reported values for zeta-potential rep-resent the mean value of five measurements.

2.2.4. FT-IR analysis

FT-IR analysis was carried out using a Perkin ElmerSpectrometer in order to determine the adsorption characteristicsof PET and PVC particles in the presence of DIB and LA.

2.3. Selective flotation experiments

The conditioning of post-consumer plastics with the plasticizersolution was accomplished in a 0.5 L glass beaker using 50 g mixedPET and PVC particles at a solid concentration of 25 wt%. FollowingpH adjustment, technical grade plasticizers, DIB and LA were usedat between 0 and 1200 g/t concentration and conditioned with tapwater at about 10 min. Stirring speed was fixed to 300 rpm for allconditioning tests. At the end of the conditioning operation withthe plasticizers, the suspension of plastics was screened and trans-ferred to a flotation cell.

The flotation experiments were carried out in a short columnflotation cell with a volume of 5 5 25 cm3 at an ambient tem-

perature. The bottom of the flotation cell was fitted with a porousmaterial (glass frit) for bubbling air with 75 L/h airflow rate. A col-umn flotation cell generates small bubbles and plastic particleshaving hydrophobic surfaces attach and move upwards (Fig. 5).Tap water was used to prepare about 0.5 L of solution. MIBC wasused as a frother at constant concentration of 1000 g/t in all flota-tion experiments. The suspension of plastics was conditioned withthe frother about 2 min before the flotation experiments. The flota-tion time was fixed to 3 min. After the flotation tests, the productswere screened, rinsed and dried. All samples were weighted andflotation recovery was calculated based on the mass balance.

3. Results and discussion

3.1. Contact angle measurements

PET and PVC plastic strips cut from beverage bottles and win-dow frames were rinsed with deionized water and contact anglemeasurements were conducted in order to investigate the effectsof various concentrations of DIB and LA. The results of contactangle measurement tests are shown in Fig. 6.

It can be observed from Fig. 6 that contact angle variation of PETand PVC against plasticizer concentration is opposite. Both plasti-cizer increase the hydrophobicity of PET particles and make thesurface of PVC particles more hydrophilic. DIB and LA increasethe contact angle of PET up to 5. A reduction of PVC hydrophobic-ity is more pronounced with LA. LA also causes higher wettabilityfor PVC compared to DIB. The contact angle of PVC decreases with

increasing concentration, and it drops from 79 to 73 when LAconcentration is increased to 7.5 g/L. It is obvious that more differ-ence in contact angle of those plastic is obtained when applying LAas a plasticizer. Difference in contact angle attributes to differencein wettability and results in selective flotation behavior.

3.2. Surface tension measurements

According to Zisman [21], if a sufficient difference existsbetween the critical surface tensions of two hydrophobic materials,selective wetting can be achieved by reducing the liquid surfacetension. At appropriate values of liquid surface tension, air bubbleswill adhere onto particles with the lower critical surface tensionvalue, promoting flotation, whereas particles having the higher

critical surface tension value will be wetted sufficiently as to sup-press bubble attachment. Within this context, a series of surface

Table 1

The chemical analysis of plastics.

Compound (%) PET PVC

SiO2 0.36 0.84Al2O3 0.07 0.50Fe2O3 0.04 0.11CaO 0.04 19.84MgO 0.32 0.42

Na2O 0.04 0.08K2O 0.01 0.01Cr2O3 0.82 0.01TiO2 0.82 6.74MnO 0.01 0.01P2O5 0.20 0.04

Loss on Ignition (LOI) 98.09 71.4

Fig. 4. The chemical structure of plasticizers.

A. Güney et al. / Separation and Purification Technology 151 (2015) 47–56 49

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tension measurement were tested at various concentrations of plasticizers. At the beginning of tests, the surface tensions of dis-tilled water were measured and obtained as 72.2 mN / m. In Fig. 7,the results of surface tension measurements are shown as averagevalues of three measurements.

Lignin’s is one of the several cellulose derivatives which arecommonly used as an organic depressants and dispersants. Theyare used as depressants in both sulfide and non-metallic mineralflotation. When they are used in plastic flotation adsorbeddepressant molecules selectively influence the affinity of bubblesfor different types of plastic under appropriate conditions [22]. Itwas clear from Fig. 7 that different from DIB, LA can reduce liquidsurface tension to a minimum value of 55.3 mN/m and have thegreatest influence on the value of the surface tension compared

to DIB. The results from Figs. 6 and 7 depict that the adsorptionof LA as a depressant might negatively affect the floatability of

PVC while PET’s hydrophobicity was promoted. The variation of contact angle values of PET and PVC particles were found to besimilar in the presence of DIB and LA. One can conclude that thereduced liquid surface tension is not the main reason for depress-ing and activating of plastic’s but also the surface tension of solidsmust be identified.

3.3. Zeta potential measurements

The zeta potential of both PVC and PET was measured over thepH range 3–11. Fig. 8 clearly show that pH of solution has a signif-icant effect on the magnitude of zeta-potential. The point of zerocharge (pzc) of PET and PVC was obtained at pH 4.8 and 3.7,respectively.

Adsorption theories predict that using a flotation liquid of con-siderable ionic strength, e.g. tap water, would result in a stronger

depressant effect of all types of ionic surfactant. The cations havea profound influence on the adsorption of polyelectrolytes, mainly

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Fig. 7. Surface tension of solution as a function of plasticizer concentration.

Fig. 5. The column flotation unit.

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Fig. 6. Contact angle of PET and PVC as a function of plasticizer concentration.




because they reduce the repulsion between the charged segments,which facilitates adsorption [6].

From this point of view, the electro kinetic measurements of PET and PVC were performed against pH in the presence of mono-valent (K+ and Na+) and multivalent (Ca2+) ion concentrations with102, 103, 104 and 105 M of KCl, NaCl and CaCl2. The results of zeta potential measurement of PET and PVC solutions with variousconcentrations of KCl, NaCl and CaCl2 are shown in Figs. 9–11,respectively.

According to Figs. 9 and 10, pzc of PET and PVC in the presenceof KCl and NaCl were found to occur around pH 4.8 and 3.7, respec-tively. Indifferent electrolytes are not expected to change pzc;however, as concentration increases the change in the magnitude

of the zeta potential is attributed to the compression of the electri-cal double layer.

Zeta potential tests in the presence various CaCl2 concentra-tions were conducted to understand the effect of these ions onthe surface charge variation of PET and PVC. As it is seen fromFig. 11, when concentration of Ca2+ ions are increased above 105

M in alkaline pH values the surface charge of PET and PVC isreversed and led to positive. These results indicate that Ca 2+ ionsspecifically adsorb onto the surface of plastics and make their sur-face’s positively charged.

The change of electro kinetic properties of PET and PVC wereinvestigated against pH in the presence of plasticizer solutionswith various concentrations of LA and DIB and the results areshown in Fig. 12.

Alkali ions make the zeta potential of solution negative. It isclear from Fig. 12, increasing of lignin alkali concentration changethe magnitude of the zeta potential of PET and PVC more negative.Besides that higher DIB concentration causes zeta potential of PETand PVC to alter positive charge because of increased concentra-tion of cations in the solution.

3.4. FT-IR spectrum

Domination of lateral groups in polymer chain of plastics, whichcontains non-polar carbon and hydrogen, leads to a physicaladsorption between the plasticizer and plastic surfaces.Interaction forces herein include dispenser Van der Waals bondsas in hydrogen bonding. Chemical adsorption is extremely rare

between the molecules of the plasticizer and polymer surface.The underlying reason could be related to the low surface energy

of plastics. FTIR measurements in Figs. 13 and 14 clearly show thatthe peaks of plastics and plasticizers are available in a combinationof the curve for plastics and plasticizers. The results of FTIR mea-surements conducted with PET and PVC using LA and DIB as a plas-ticizer depicts that only physical adsorption takes place betweenmolecules of plastics and plasticizers.

3.5. Flotation experiments

To success a selective flotation separation of mixed plastic, it isnecessary to render one more hydrophilic while the other is still ina hydrophobic state. It is well known that PET and PVC are natu-rally floatable (hydrophobic). Therefore, it is necessary to use anappropriate agent to achieve selective separation. On this basis, atechnology involving treatment of post-consumer plastic particleswith plasticizer followed by froth flotation at various pH levels hasbeen tested in laboratory scale.

Previous studies showed that pH has an important role forchanging surface characteristics of polymers [8,15,22]. In the frameof this study, a series of flotation experiments were conductedusing post-consumed PET and PVC in the absence of plasticizers.The recovery was defined as a ratio of a component separated fromthe flotation cell to the original amount of this component at thebeginning of the experiment. The results were shown in Fig. 15.

The electrostatic charge of particles and the ionic character of reagents play an important role for selectivity in flotation. As seenin Fig. 15, pH has a great impact on selectivity and recovery forPET–PVC. As pH decreases to 2, the content of PET decreases dra-matically to 59% where the content and the recovery of PVC weregreatest. At the natural pH of the solution, PET particles gainhydrophobic property and the separation efficiency of PET andPVC flotation increases. It was also clear from Fig. 15 that strongalkaline solutions were able to destroy the hydrophobicity of plas-tics and pH values more than 10 did not give any considerableresults in terms of content and recovery for both plastics. From thispoint of view, the presence of OH has a significant role on flotation

performance.To investigate the effect of plasticizers concentration on plastic

separation, LA and DIB have been tested with selected conditions:particle size of 3.36 + 2 mm, 15 min of plasticizer condition timeand frother concentration of 1000 g/t MIBC. In these experiments,LA concentration was selected between 0–500 g/t and DIB was0–1500 g/t. Recovery and content of PET and PVC against LA andDIB concentration is presented in Figs. 16 and 17, respectively.

It is clear from Fig. 16 that increasing concentration of LA up to25 g/t improved the floatation performance of PET particles interms of content and recovery. Further addition of LA does notchange the recovery of PVC while recovery of PET decreases contin-uously. The wettability of plastic surfaces by water can beincreased by grafting hydrophilic functionalities, such as O, OH

and COOH onto the polymer chains located on the surface [6]. Atthe higher concentrations of LA, the potential determining ion(OH) is attained to be more effective on changing the flotationkinetics of plastics. Because PET is an ester-type polymer, alkalineconcentration breaks up the ester links located at the solid surfaceserves and wet the PET flakes for floatation separation. As previ-ously shown in Fig. 15, the floatability of plastics diminishes inalkaline media. When employing more than 250 g/t LA concentra-tion, the amount of floated product decreases (about 10% of feed)and high recovery rates could not be obtained. From Fig. 17, DIBalso significantly changes the floatability of polymer resins. Thebest results were obtained when plasticizer concentration waschosen as 250 g/t. At higher concentrations, the floatability of PET particles decreases and selectivity of flotation decreases.

The flotation performance of plastics was found to be in goodagreement with measurements of contact angle and surface

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Fig. 8. Zeta-potential of PET and PVC as a function of pH.


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Fig. 9. Variation of the zeta potential of PET and PVC in the presence of KCl.

Fig. 10. Variation of the zeta potential of PET and PVC in the presence of NaCl.

Fig. 11. Variation of the zeta potential of PET and PVC in the presence of CaCl 2.




tension. At lower concentrations the surface tension of solution

decreases and the frothing effect dominates. Conversely, at higherconcentrations, the presence of plasticizers increases the

wettability, which leads to insufficient flotation recovery. It was

observed from tests that both DIB and LA were able to make thesurface of PET particles hydrophobic whereas PVC particles were

Fig. 13. FTIR spectrums of PET in the presence of plasticizers.

Fig. 12. Variation of the zeta potential of PET and PVC in the presence of LA and DIB.


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affected differently. Contact angle measurements (Fig. 6) haveshown that LA was very effective to render the surface of PVC

hydrophilic compared to DIB. Selective flotation results depictedthat PVC particles were depressed successfully at higher

concentrations of LA whereas DIB has little effect on wettability.The depressing effect of LA and DIB on PVC is not attributed mainly

to the reduced surface tension of solution but also the adsorptionof plasticizers.

Fig. 14. FTIR spectrums of PVC in the presence of plasticizers.

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Fig. 15. Effect of pH on PET and PVC flotation.

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Fig. 16. Effect of Lignin Alkali concentration on PET and PVC flotation.




A series of experiments were carried out to investigate theeffect of pH and plasticizer type on selective separation of PETand PVC. The polymer mixture was subjected to plasticizers treat-ment with different pH values (4–12) for 15 min. The optimumplasticizer concentrations were chosen from previous experimentsas 25 g/t LA and 250 g/t DIB and frother concentration was1000 g/t. PET–PVC content and recovery against pH for each plasti-cizer are shown in Figs. 18 and 19.

Fig. 18 depicts that the amount of PET particles in float productis very low and the separation of plastics with LA at strong acidicand alkali solutions was inefficient. PET particles gain hydrophobicproperty around pH 8. The recovery of PET material was greaterthan 99%. As pH decreases to 6, the recovery of PET decreasesdramatically to 12%. The same situation is apparent at higher pHvalues. It is obvious from Fig. 19, DIB yielded better results in termsof recovery at low and high pH levels; however, the content of PETparticles in floated product could not be improved and selectiveflotation of plastics failed. The best results were obtained (90% con-tent and 95% recovery) when pH was around 5 which is close topzc of PET.

4. Conclusions

In the frame of this study, the separation of mixedpost-consumer plastics was enhanced with the help of appropriateplasticizers and by adjusting experimental conditions. The mecha-nism of plasticizer type and amount on modification of plastic sur-faces and the performance of selective flotation were investigatedthrough contact angle, surface tension, electro kinetic potentialmeasurements and FT-IR (Fourier transform infrared spectroscopy)analysis. According to the flotation results, PET was separated fromthe mixture of PET and PVC with 97% content and 99% recoverywhen using 25 g/t lignin alkali (LA) as a plasticizer at pH 8. Atthe end of the froth flotation experiments conducted withpost-consumer polymers using other plasticizer, diethylene glycol

dibenzoate (DIB), PET particles were separated from the mixture at90.0% content and 95% recovery rate around pH 5. Among twoplasticizers tested lignin alkali was found to be a good depressantfor PVC and selective separation could be enhanced when LA wasused as a plasticizer.

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Fig. 19. Effect of pH on PET and PVC flotation in the presence of DIB.

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Fig. 18. Effect of pH on PET and PVC flotation in the presence of LA.

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Fig. 17. Effect of DIB concentration on PET and PVC flotation.


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Caracterización de la flotación de PET y PVC en presencia de diferentes plastificantes