Separation and Purification Technology 118 (2013) 305–312

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Separation and Purification Technology

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Enrichment implication of froth flotation kinetics in the separationand recovery of metal values from printed circuit boards

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.07.027

⇑ Corresponding author. Tel.: +91 657 2349008; fax: +91 657 2345213.E-mail address: ari@nmlindia.org (A. Vidyadhar).

A. Vidyadhar ⇑, A. DasMNP Division, CSIR – National Metallurgical Laboratory, Jamshedpur 831 007, India

a r t i c l e i n f o

Article history:Received 13 May 2013Received in revised form 21 June 2013Accepted 18 July 2013Available online 27 July 2013

Keywords:Printed circuit boardsRecyclingFroth flotationKineticsMetal recovery

a b s t r a c t

The e-waste printed circuit boards (PCBs) are rich in metal content and processing these wastes forextracting the metal values and removing the non-metallic constituents is a prospective proposition.Froth flotation methodology was observed to be a promising technique for rejecting plastics from thecomminution product. It has been shown that nearly reagent-free flotation of relatively coarse size(�1.0 mm) pulverized e-waste is feasible with a reasonably good product at a high yield and excellentrecovery. In the present research work, enrichment of ground 1.0 mm PCB powder was investigatedthrough flotation route by varying the operating variables such as frother dosage, pulp density, air flowrate and rotational speed of impeller. The liberation studies accomplish that liberation of metal valuefrom non-metallic constituents at �1.0 mm size is excellent and the particulate system is significantlyrich in metal value, containing around 23% metal. In-depth study of froth flotation kinetics is primarilyfocused on high rejection of plastics and also identification of optimum operating conditions for the same.Single-stage flotation enhances metal content from 23% to over 37%, contributing a mass yield of around75% with recovery of nearly 95% metal values, suffering nominal loss of around 4% metal value only, whileeffectively rejecting 32% of the materials in feed through float fraction. The interdependence of kineticsand process variables has been discussed and it has been concluded that a high rotor speed aids efficientrejection of the plastics. However, addition of frother is essential to help stabilize the froth and enhancethe kinetics, while efficient pre-concentration is facilitated through a combination of moderate air flowwith low pulp density. Generation of pre-concentration through flotation route from the entire�1.0 mm comminution product stands accomplished.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

The vital issue of electronic waste associated grave concernfactually happens to be a potential risk for the populace and notsimply limited to the government bodies, taking into account thehazards of toxic environmental pollution endangering the safeexistence of inhabitants in the region. The incremental consump-tion trend of electrical and electronic goods as lifestyle productsacross the globe is contributing to the rapidly worsening scenario,which has been recognized world-over as a potential threat to thewell-being of the inhabitants on account of the increasing air andground-water pollution. The escalating environmental risk phe-nomena cannot be addressed through incineration unlike otherwaste, thus, evolving scientifically focused safe methodologies isimperative in effectively tackling the damaging consequences tothe region, particularly with respect to the growing concern, ofthe society at large, about the likelihood of formation of e-wastedump in the region. The disposal of these requires special treat-

ment to prevent the leakage and dissipation of toxics into theenvironment [1–5]. Physical processing is admittedly the mostenvironment-friendly approach among other prevalent techniquessuch as pyrometallurgical and hydrometallurgical processes formaterials recovery from end-of-life printed circuit boards (PCBs)[6,7]. The printed circuit board is a major constituent of obsoleteand discarded electronic scraps containing valuable metals suchas Cu, Ni, Au, Ag, Pd, Fe, Sn, and Pb. The incentives drawing theunorganized recyclers in the business is best explained by theenormity of volume and market valuation of the materials recov-ered from e-waste. The volume and cost of the metals recoveredfrom 1000 kg of PCBs has been presented by Chatterjee and Kumar[8].

It has been inferred that froth flotation happens to be an effec-tive enrichment technique in recovery of values from �75 lmcomminution fines [9]. Perceiving this as an applied minerals pro-cessing constraint, froth flotation has been scientifically advancedas a promising beneficiation technique for fine fraction [10,11]and the same technique has also been applied in processing muni-cipal solid waste [12–14]. PCB fines consist of mixture of particlesof metals, alloys, ceramics and plastics, each with distinct surface

Table 1Size analysis and microscopic liberation data for the powdered PCB scrap.

Size (lm) Weight (%) Liberation analysis (number %)

Free metal Interlocked Free gangue

1000 26.64 18.3 5.3 76.4500 15.07 17.4 3.2 79.4300 10.79 15.6 1.3 83.1150 6.89 13.4 0 86.6100 4.42 11.2 0 88.8

75 7.40 9.3 0 90.750 2.86 – – –35 25.93 – – –

306 A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312

properties that should enable selective wetting and make frothflotation separation possible. This is the overall hypothesis uponwhich froth flotation can be expected to be applicable for the ben-eficiation of this material mixture. Although the reports on flota-tion of metals have deliberated more on those that occur innative forms which are real situations in flotation operations, manymetals and alloys in the PCB fines that do not naturally occur innative forms can also be expected to respond to collectors. The fineplastic particles are expected to float under their natural hydro-phobicity, thus making it possible to achieve an initial bulk metal-lic enrichment of the sink prior to attempting any surfaceconditioning, contributes selective floats, thereby connoting a re-verse flotation in respect of metallic values.

The breakage characteristics of the metals and the non-metalsin the PCBs are strikingly different. Good liberation entails sizereduction in a fine range and in the process, albeit undesirable, sig-nificant amount of ultrafines are generated. The �75 lm fractionposes a potential challenge in the recovery of metal values, how-ever, removal of the ultrafines in a large scale operation could beyet another impeding factor considering the low density of theplastics. Relatively larger size of the feed was considered, takinginto account the lower density of plastic particles, for assistingits float. Lack of adequate momentum in the fine plastic particlesto aid rupturing the bubble film for effective attachment, floatprobability of these particles are likely to be less. However, elimi-nating such low density ultrafines from the feed is a formidabletask in industrial-scale processing, thus, it was decided to investi-gate the flotation of the whole �1.0 mm feed in the experimentalstudy.

In view of the stated hindrances, the present study was under-taken to investigate the possibility of using flotation as a pre-con-centration step for the entire �1.0 mm material and the impact ofthe process variables such as frother dosage, pulp density, air flowrate and rotational speed of the impeller on the effectiveness ofplastic rejection were analyzed in-depth, investigating the depen-dence of the overall flotation rate on the operating variables aswell. Systematic quantification of e-waste flotation performanceaccentuated on kinetics, supported by statistically designed exper-imental campaign, comprising integration of features ranging fromliberation to optimization, statistical analysis and modelling ef-forts, evidence novelty per se in this research work.

Table 2Chemical analysis of the powdered PCB scrap.

Size (lm) Cu(%)

Pb(%)

Sn(%)

Fe(%)

Al(%)

Ni(%)

Total metal(%)

Head sample 6.3 2.73 2.67 3.08 7.93 0.113 22.94

2. Materials and characterization methods

2.1. Raw material preparation

One hundred kilograms of waste PCB was procured from differ-ent sources and snipped into smaller pieces through mechanicalshearing, followed by trimming the same in shredding mill to a sizeof around 3–5 mm and thereafter, dry ground in a batch-type ballmill to a top-size of 1.0 mm. To prevent generation of ultra fines,the ball mill was intermittently halted for drawing the finesthrough screening and the grinding process was terminated afterreduction of material size to �1.0 mm. The pulverized PCB powderwas used as the feed material for further characterization and flo-tation studies.

(analyzed)1000 8.89 3.96 3.54 4.22 7.92 0.112 28.78500 8.04 3.02 3.04 3.94 7.41 0.122 25.67300 7.26 2.75 2.64 3.14 7.18 0.134 23.24150 6.28 2.27 2.04 3.01 7.6 0.126 21.44100 5.29 1.87 1.81 2.89 7.55 0.131 19.6475 4.22 1.49 2.17 2.83 7.51 0.156 18.4450 3.78 1.29 2.03 2.56 7.78 0.103 17.6535 2.67 1.23 1.63 1.83 7.02 0.093 14.59Head (calculated) 6.14 2.51 2.55 3.15 7.46 0.116 22.04

2.2. Size distribution

Pulverized PCBs were subjected to size analysis by wet sievingand the results were illustrated in Table 1. It was observed that sig-nificant amount of fines (�35 lm) were generated in pulverizationprocess and notably substantial quantity of the processed materi-als remained in the 1000 � 500 lm fraction while around half ofthe materials were in the size range of 500 � 35 lm.

2.3. Liberation studies

Liberation analysis was undertaken in a stereo zoom micro-scope, using grain count method to unveil the degree of interlock-ing in each size class, wherein, 400 frames were counted for eachsize class and counting in each frame the numbers of free metals,free plastic and interlocked metal-plastic pieces. The number per-centages of each type were computed from the overall countswhich exhibited the liberation pattern of the starting material.The liberation data of the ground powder are also illustrated inTable 1, from which it may be seen that the coarsest(1000 � 500 lm) size class contains only about 5% (by number)interlocking of metallic and non-metallic constituents. Interlockingwas not observed below 150 lm size and the metal values werefound to be adequately liberated in the pulverized mass, hence, itwas concluded that the grind size for effective liberation occursat the size of 1.0 mm.

2.4. Chemical analysis

Chemical analysis of head sample of the powder and of eachsize fraction as well was carried out and the data are shown inTable 2. It may be seen from Table 2, that the total metal contentin the powdered sample is around 23% which mainly contains6.3% Cu, 2.7% Pb, 2.7% Sn, 7.9% Al and about 0.1% Ni from whichit is evident that the metal content decreased significantly in thefiner size fractions. Al is mainly obtained from the capacitorswhere it is in the form of thin foil. In the process of grinding thesize reduction for these are mainly due to shear which is reflectedin relatively uniform distribution of Al over the size classes. How-ever, for other metals attrition is the primary mode of size reduc-tion which is reflected in decreasing concentration of the same inlower size classes, while Ni concentration is too low to have anypractical significance from recycling standpoint.

Table 3Sink-float analysis of the powdered PCB scrap.

Sample Head �1000 +500 �500 +300 �300 +150 �150 +100 �100 +75 �75 +50 �50 +35 �35

Sink (%) 31.4 40.0 36.6 32.2 29.7 26.1 23.5 19.7 12.2Float (%) 68.6 60.0 53.4 67.8 70.3 73.9 76.5 80.3 87.8

Table 4The flotation experimental conditions according to Box–Behnken design.

S. no. Stirrer speed(rpm)

Frother dosage(kg/t)

Pulp density(%)

Air flowrate (lph)

Yield (%) Grade (%) Recovery (%) Rate constantK (/min)

1 1100 4 9 5 70.1 31.8 97.2 0.34952 1100 2 12 35 68.9 31.4 94.3 0.35853 1000 2 12 20 73.9 28.7 92.5 0.28654 1100 4 12 20 73.7 27.9 89.6 0.27395 1000 2 9 5 75.0 27.7 90.6 0.31436 1100 2 9 20 74.1 28.7 93.2 0.28227 1200 2 9 5 72.4 38.7 94.7 0.31068 1000 2 6 20 70.1 31.0 94.7 0.33329 1100 0 9 5 91.3 27.0 93.6 0.2319

10 1000 4 9 20 73.2 29.5 94.1 0.308811 1200 2 6 20 68.9 31.1 93.5 0.338812 1200 4 9 20 67.5 32.3 95.0 0.394413 1100 2 9 20 74.1 29.0 93.2 0.284314 1000 2 9 35 75.2 27.3 89.5 0.283515 1200 0 9 20 72.2 29.5 92.8 0.328716 1100 2 12 5 76.8 27.7 92.7 0.282817 1100 2 6 35 70.6 30.8 94.9 0.336618 1100 0 12 20 76.6 28.1 93.8 0.258519 1100 0 6 20 72.2 29.7 93.5 0.315020 1200 2 9 35 75.5 27.7 91.2 0.268021 1100 4 6 20 69.7 30.7 93.4 0.322722 1100 2 9 20 74.0 29.2 93.2 0.283623 1200 2 12 20 68.3 32.3 96.2 0.397924 1100 4 9 35 72.4 29.5 93.1 0.308625 1100 2 6 5 79.9 26.8 93.3 0.226426 1100 2 9 20 73.8 28.8 93.2 0.288827 1000 0 9 20 75.5 28.4 93.5 0.271228 1100 2 9 20 73.4 29.1 93.2 0.277829 1100 0 9 35 75.3 27.7 91.3 0.2633

A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312 307

2.5. Sink-float analysis

In order to have an understanding of metal content in the pow-der, it was subjected to sink-float analysis, using a mixture ofbromoform and benzene with 2.0 specific gravity as the heavy li-quid. The specific gravity of the plastics was lower than 2.0 whileall metals were having specific gravity more than 2.0. Hence, liber-ated metals sank in the liquid while liberated plastic particlesfloated. However, depending upon the proportion of plastics inthe interlocked particles they sank or floated. The sink-float dataare shown in Table 3, from which it is evident that the finer frac-tions contained lower quantity of metals. In the larger size classes,the metals embedded in plastic also reported in the sink. Gravityseparation of metals from plastics in pulverized electronic wasteusing flowing film concentration in a shaking table has been inves-tigated [15].

2.6. Experimental design

Processing of �1.0 mm comminution product of e-waste wasinvestigated with a view to separate the metals from the non-metallic constituents. The differences in specific gravity betweenmetals and non-metals were exploited in froth flotation methodol-ogy. In order to optimize the performance of the flotation a total of29 experiments were performed according to the Box–Behnken de-sign of experiment. The four operating variables were stirrer speed,frother dosage, pulp density and air flow rate. The conditions forthese experiments and the concentration response in the form of

yield and grade are shown in Table 4. The product samples werecollected, dried and analyzed for total metal content (grade), whichhave also been presented in Table 4.

3. Experimental

Bench scale flotation tests were performed with 500 g of�1.0 mm PCB powder for each experiment in a WEMCO laboratorycell of the Fagergren type with a cell volume of 2.7 L and the pulpwas conditioned for about 15 min. The differential degree of hydro-phobicity between non-metals and metals is exploited using frothflotation wherein, the hydrophobic plastic particles are separatedfrom the hydrophilic metal particles by reverse flotation, usingonly a froth stabilizer. The hydrophobic plastic particles are madeto attach with the air bubbles to aid them float to the top while, themetal particles were retained in the pulp. Relatively narrower feedsize range augurs well for flotation performance. In order to havean idea of the flotation rate, kinetic studies were undertaken. Foreach experiment, float samples were collected at an interval of0.5, 1.0 and 2.0 min. The float fractions were dried, weighed andanalyzed for total metal. For proper understanding of the kinetics,these data were processed assuming a first order rate equation. Therate equation is presented as follows:

kt ¼ ln½1=1� R� ð1Þ

where R is the recovery of metals at time t (min) and k is the rateconstant. A plot of the right hand side of the above equation againsttime gives a straight line with a slope ‘k’, the rate constant. The rate

0.8

308 A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312

constants were estimated from the plots for all experiments. Table 4also lists the rate constants obtained under various operatingconditions.

0.0 0.5 1.0 1.5 2.0 2.50.0

0.2

0.4

0.6

Expt. No. 23 24 25

ln[(

1/1-

R)]

Time (min)

Fig. 1. First order fitting of flotation responses under various process conditions.

4. Results and discussion

4.1. Beneficiation by froth flotation

The loss of metal values in the float fraction was of primary con-cern in this study and a few exploratory tests unveils that enrich-ment in grade is achievable at a much lower yield of the valuablestream while suffering significant metal loss in the froth. Therefore,incremented recovery rather than grade enrichment was targetedin this investigation, which is considered to be a pre-concentrationstep.

Notably, characterization data exhibits 77% of the feed to beplastic and the same has been targeted for flotation. Since, the den-sity of plastic particle is lower in terms of percentage but number-wise higher in comparison, therefore, low pulp density in combina-tion with high aeration, bodes well. Higher impeller speed triggersproduction of finer bubbles and increased turbulence, which trans-lates into incremented particle-bubble collision frequency [16,17],a precursor to attachment. While fines recovery to float is aug-mented by smaller bubbles [18], higher turbulence can also leadto rise in entrainment and metal losses.

According to Box–Behnken experimental design for responsesurface methodology, 29 experiments were undertaken to estab-lish the influence of four operating variables on the separation per-formance wherein, the impact of four variables studied were stirrerspeed, frother dosage, pulp density and air flow rate. The condi-tions for these experiments and the concentration response inthe form of yield and grade are illustrated in Table 4. The productsamples were collected, dried, weighed and analyzed to ascertaintotal metal content (grade). The flotation response is quantifiedin the form of yield of the metal-rich tailings and the total metalcontent (grade) in it. The recovery of the valuables was also esti-mated. It can be seen from this table that the yield varies between67.5% and 91.3% while the grade obtained varies from 26.8% to38.6% total metal and recovery, under all circumstances, exceeds90%.

The grades of the valuables stream are indeed low. Evidently,not a great deal of enrichment has taken place. However, it shouldbe noted that nearly 20–30% of the material is rejected in the floatfraction with very little loss of metal values in the float fraction.Above 90% recovery under all circumstances corroborates this. Effi-cient flotation performance has been observed with moderate pulpdensity and air flow rate, at 1200 rpm stirrer speed preferably withlow frother dosage. However, no qualitative deterioration in frothstability adversely impacting flotation, has been observed withno frother dosage. To facilitate flotation, adequate momentum tolighter plastic particles is attributable to rise in stirrer speed whileon the flip-side, no improvement in product grade has been wit-nessed in experiments conducted at 1000 rpm stirrer speed, thus,better flotation performance is achievable with moderate to highstirrer speed.

Improvement in product grade has been achieved with higherfrother dosage in combination with high stirrer speed. It was estab-lished experimentally that no enhancement in float performance isobtainable by increase in frother dosage beyond a certain limit,however, maintaining minimum froth stability is imperative andfrother dosage of about 2 kg/t is considered adequate. The detri-mental impact of high pulp density, as illustrated by the data in Ta-ble 4 notwithstanding, excessive low pulp density is not desirablefrom commercial perspective, thus, good flotation performance interms of mass yield and grade of concentrate along with associated

recovery of metal values is obtainable with a moderate pulp den-sity of around 10% solids concentration.

Air flow rate being a crucial process variable, excessive high airflow rate accelerating the degree of turbulence triggers detach-ment of bubble particle aggregate, thus, better flotation perfor-mance is achievable maintaining a low to moderate air flow rate,as observed from data. Therefore, it has been inferred that a com-bination of high stirrer speed with low frother dosage, moderatepulp density and low to moderate air flow rate is ideal for better-ment of flotation performance.

4.2. Kinetic response

The values of the rate constant suggest only moderate flotationrates. The extents of fit of the kinetic responses to Eq. (1) can beseen in Fig. 1 which depicts a high, intermediate and low flotationrate as observed in this study. All the conditions resulted in goodcorrelation, implying that the observed response conforms to firstorder flotation kinetics. The reasons for the attainment of suchrates in these experimental tests are discussed below along withother experiments.

The highest rate constant values are obtained under high stirrerspeed (1200 rpm) and moderate air flow rate (12 lph) conditionswith over 95% recovery of the metal values. The rejection of plas-tics from the pulp is efficient under these conditions. A lower yieldwith good product grade along with a high recovery of metals ismost desirable. From this standpoint, it is not necessary that ahighest rate constant would fulfil all the criteria. The loss of metalsin the reject stream (float fraction) must be examined carefully be-fore deciding the best suitable conditions. However, a faster rate iscertainly desirable from an application angle.

4.3. System response

The system response is shown in Fig. 2 in the form of variationof yield and recovery along with grade of the valuable fraction. Thefigure indicates that the yield of the valuable fraction sharply de-clines as better grade is targeted. However, the recovery values re-main similar under the conditions tested. There is a small increasein the recovery at high target grades under specific conditions,which of course, should be the preferred conditions for the pre-concentration targeted in this experimental study. It indicates thatExpt. 23 gives the best conditions, under which less than 4% metalsare lost in the float fraction. A low yield of the sink indicates veryeffective rejection of plastics into the froth wherein, nearly onethird of the feed reports to the float fraction with negligible metalvalue in it. As discussed above, effective rejection of plastics underthese conditions is attributed to good stability of the froth, high

26 28 30 32 3460

65

70

75

80

85

90

95

100

PreferredOperating

Regime

Rec

over

y / Y

ield

(%

)

Grade (%)

Recovery Yield

Fig. 2. Grade vs. recovery/yield curve for all the experimental conditions.

A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312 309

concentration of air bubbles, appropriate turbulence level, highcollision frequency and adequate momentum of plastic particles.

The retrofitted model of the experimental data displayed that areduced cubic model correlates well to the yield data with a corre-lation coefficient (R2) of 0.9995. The model expression for the yieldis projected in Eq. (2) and in the same degree, a reduced cubic mod-el is well suited to the grade data with an R2 value of 0.9957, asprojected in Eq. (3). With the help of the model equations, optimi-zation exercise was undertaken to maximize the yield, at maxi-mum possible grade, at highest possible feed rate wherein, theoptimum conditions were inferred to be at 1198 rpm stirrer speed,0.61 kg/t frother dosage, pulp density 9.02% solids and at an airflowrate of 5.00 lph with a desirability value of 0.841. At the optimumconditions, the yield is predicted to be 76% with a grade of 37% andit was also established that removal of substantial plastics from thepulp could be accomplished with a residence time of 1 min, where-in, the metal content increases from 23% to over 37%, with a yieldof 76%, resulting in almost 95% recovery of metal values throughsingle-stage roughing.

In terms of coded factors A, B, C and D, the equations are:

0.00

1.00

2.00

3.00

4.0

62

69.5

77

84.5

92

Yie

ld

B: Frother Dosage

Fig. 3. Variation in yield with frother dosage and stirrer s

Yield ¼ 73:88� 0:57 A� 6:06 B� 2:62 C � 5:72 D

� 0:63 AB� 1:10 AC þ 0:72 AD� 0:075 BC

þ 4:56 BDþ 1:78 CD� 2:27 A2 þ 0:46 B2 þ 1:32 C2

þ 2:91 D2 þ 4:34 A2Bþ 3:42 A2C þ 6:55 A2D

� 1:65 AB2 � 1:13 AC2 þ 4:70 B2C þ 2:26 B2D

þ 4:74 BC2 ð2Þ

Grade ¼ 29:00þ 2:84 Aþ 1:65 Bþ 0:37 C þ 1:93 D

þ 0:42 ABþ 0:87 AC � 2:64 AD� 0:30 BC

� 0:75 BDþ 1:31 A2 � 0:20 B2 þ 0:32 C2 � 0:67 A2B

� 0:65 A2C � 4:76 A2D� 1:86 AB2 � 1:91 AC2

� 1:47 B2C � 2:33 B2D� 1:45 BC2 ð3Þ

where A, B, C and D corresponds to stirrer speed, frother dosage,pulp density, air flow rate respectively, varying from �1 to +1. In ac-tual, for all the process variables the �1 level corresponds to theminimum value and the +1 level corresponds to the highest valueas considered in the experimental design.

Figs. 3–6 illustrate the response surfaces generated from theexperimental data. Figs. 3 and 4 are plotted with the optimum con-ditions of pulp density 9.0% and at an air flow rate of 5.0 lph. FromFig. 3, it is evident that the maximum yield is obtained at lowerfrother dosage with intermediate stirrer speed. The yield increasesand levels off with a decrease in stirrer speed at high frother dos-age while at low frother dosage, the yield initially increases andthen decreases, with fall in stirrer speed and a linear drop in yieldis observed at low stirring speed with increase in frother dosage,however, at higher stirring speed the drop is nearly parabolic. Itis observed that the maximum grade is obtained at a high frotherdosage in combination with high stirrer speed (Fig. 4). The gradeof the concentrate was observed to have an inverse relationshipwith yield wherein, fall in yield improved the grade and vice versa.

1000.00

1050.00

1100.00

1150.00

1200.000

A: stirrer speed

peed at 9.0% pulp density and air flow rate of 5.0 lph.

1000.00

1050.00

1100.00

1150.00

1200.00

0.00

1.00

2.00

3.00

4.00

25

29.25

33.5

37.75

42

Gra

de

A: stirrer speed

B: Frother Dosage

Fig. 4. Variation in grade with frother dosage and stirrer speed at 9.0% pulp density and air flow rate of 5.0 lph.

0.00

1.00

2.00

3.00

4.00

5.00

12.50

20.00

27.50

35.00

62

66.25

70.5

74.75

79

Yie

ld

B: Frother Dosage D: Air flow rate

Fig. 5. Variation in yield with air flow rate and frother dosage at stirrer speed 1200 rpm and 9.0% pulp density.

310 A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312

Figs. 5 and 6 are generated for the optimum conditions at stirrerspeed 1200 rpm and 9.0% pulp density. It is seen from Fig. 5 thatthe yield is maximum at lower frother dosage with high air flow

rate and conversely decreases with an increase in frother dosage.However, beyond a threshold pulp density, the yield suffers a sharpfall. The grade was found to drop linearly with an increase in air

0.00

1.00

2.00

3.00

4.00

5.00

12.50

20.00

27.50

35.00

22

26.75

31.5

36.25

41

Gra

de

B: Frother Dosage D: Air flow rate

Fig. 6. Variation in grade with air flow rate and frother dosage at stirrer speed 1200 rpm and 9.0% pulp density.

A. Vidyadhar, A. Das / Separation and Purification Technology 118 (2013) 305–312 311

flow rate (Fig. 6). The grade increases with the increase of frotherdosage at low air flow rate, however, it increases and levels off withrise in frother dosage at a high air flow rate.

5. Conclusions

The �1.0 mm comminution product of e-waste can be treatedusing flotation method, provided the ultrafines are removed fromthe feed to facilitate adequate and effective enhancement of grade,as otherwise the utility of the same will remain confined to being apre-concentrate.

5.1. Flotation features

Notably, specific conditions with respect to maintaining highrotor speed in combination with moderate air flow, low pulp den-sity and addition of frother for froth stabilization contributes toeffective flotation performance and enhanced rejection of plasticsin the process. However, flotation performance is adversely im-pacted by excessively high or low aeration rate. As observed, col-lector less flotation serves the purpose of effective pre-concentration since removal of light ultrafines poses severe con-straints in fine PCB processing.

In view of the low density of the plastic particles, a high stirrerspeed is necessary to impart adequate momentum to these parti-cles for capture. However, when a high air flow is also introduced,the turbulence is excessive and detachment starts taking place. Amoderate airflow rate is required to maintain adequate concentra-tion of air bubble. Froth stability must be enhanced by adding onlymoderate amount of frother. Pulp density has to be maintained at alow level for good dispersion. However, from an applied angle, a

moderate pulp density is desired and is found to be acceptablefrom the process angle. Thus, a single pass flotation of the e-wasteis recommended for good pre-concentration of the pulverized masswith excellent rejection of plastic particles.

5.2. Model development

Model equations for the two primary responses, namely, massyield and product grade were obtained with excellent accuracy.The R2 values of over 0.99 in both cases stand testimony to that.

5.3. Optimum operating regime

The optimum conditions were identified as a stirrer speed of1198 rpm, a frother dosage of 0.61 kg/ton, a pulp density of9.02% solids and an air flow of 5.00 lph. Under these conditions aproduct grade of 37% metal is achievable at 76% mass yield.

Acknowledgement

The financial support from the Department of InformationTechnology, Govt. of India for carrying out the research is gratefullyacknowledged.

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