metals

Article Thermochemical Modelling and Experimental Validation of In Situ Volatilization by Released during Pyrolysis of Smartphone Displays

Benedikt Flerus 1,2,*, Thomas Swiontek 1, Katrin Bokelmann 2, Rudolf Stauber 2 and Bernd Friedrich 1

1 Institute of Process Metallurgy and Metal Recycling IME, RWTH Aachen University, Intzestraße 3, 52056 Aachen, Germany; [email protected] (T.S.); [email protected] (B.F.) 2 Project Group, Materials Recycling and Resource Strategies IWKS, Fraunhofer Institute for Silicate Research ISC, Brentanostraße 2, 63755 Alzenau, Germany; [email protected] (K.B.); [email protected] (R.S.) * Correspondence: bfl[email protected]; Tel.: +49-(0)241-80-95856

 Received: 22 November 2018; Accepted: 5 December 2018; Published: 8 December 2018 

Abstract: The present study focuses on the pyrolysis of discarded smartphone displays in order to investigate if a halogenation and volatilization of indium is possible without a supplementary halogenation agent. After the conduction of several pyrolysis experiments it was found that the indium evaporation is highly temperature-dependent. At temperatures of 750 ◦C or higher the indium concentration in the pyrolysis residue was pushed below the detection limit of 20 ppm, which proved that a complete indium volatilization by using only the halides originating from the plastic fraction of the displays is possible. A continuous analysis of the pyrolysis gas via FTIR showed that the amounts of HBr, HCl and CO increase strongly at elevated temperatures. The subsequent thermodynamic consideration by means of FactSage confirmed the synergetic effect of CO on the halogenation of indium oxide. Furthermore, HBr is predicted to be a stronger halogenation agent compared to HCl.

Keywords: pyrolysis; smartphone; displays; halogenation; indium; volatilization; thermodynamics; recycling

1. Introduction During the last decade, the role of waste electric and electronic equipment (WEEE) as a feedstock has become increasingly important for European metal refineries. Both its significant domestic supply and its high metal content compared to primary resources have made it an attractive raw material for the recovery of valuable metals—especially copper and precious metals (Au, Ag). Moreover, WEEE contains a broad variety of several other metals, ranging from base metals (Fe, Zn, Sn, Al, Pb) to special metals (Ga, Ge, In, Ta, rare earth elements), whereby the content and the actual occurrence of the individual metals depend on the particular kind of WEEE. The metal content of a smartphone, for instance, differs strongly from that of a washing machine, resulting in a high heterogeneity of the total WEEE stream. Following the example of the smartphone, which represents a contemporary, widely distributed type of electronic consumer product, it can be stated that this kind of device exhibits a high complexity—not only in terms of various metals, but also regarding other sorts of materials, like glass, ceramics and plastics. This complexity makes holistic recycling and metal recovery a big challenge. Currently, WEEE with a high content of copper and precious metals is introduced into

Metals 2018, 8, 1040; doi:10.3390/met8121040 www.mdpi.com/journal/metals Metals 2018, 8, 1040 2 of 12 the pyrometallurgical copper route which is able to handle larger amounts of feedstock. However, the chance of an easy recovery of precious metals is hindered by the high diversity of additional materials and elements, which leads to an increased input of impurities into the copper phase and requires an extensive slag design during the smelting process. Simultaneously, less noble trace metals, such as indium, tantalum and gallium, are lost in the slag and cannot be recovered [1,2]. From the environmental point of view, the processing of WEEE in copper smelters makes high demands of the off-gas treatment because the combustion of adhering plastics releases harmful substances such as dioxins and halides. The regard of all these aspects induces the consideration of appropriate techniques that can be applied prior to the smelting process in order to remove unwanted substances or separate particular elements which cannot be recovered in the smelter. Existing mechanical dissembling and sorting processes reach their limit when they are faced with composite materials and miniaturized components, i.e., smartphones and printed circuit boards. At this time, a thermal pretreatment of the electronic scrap via pyrolysis is a promising way to overcome these barriers. In this context, the work of Diaz et al. [3] shows that the pyrolysis of adhering plastics has several benefits:

• breakup of plastic-metal composites, so that a mechanical separation of a concentrated metal fraction is possible; • removal of harmful organic substances and corrosive halides; • production of a high-caloric pyrolysis gas that can be used as fuel or reduction agent.

Additional to these aspects, the process of pyrolysis enables not only the volatilization of hydrocarbons and halides, but also specific metals because of their affinity to form volatile metal halides. Thus, this work focusses on the case of indium in terms of a complete indium separation during the pyrolysis of indium-containing WEEE.

1.1. ITO Displays As soon as electronic devices equipped with a flat panel display are discarded, indium is introduced into the stream of WEEE. Incorporated into the structure of indium tin oxide (ITO), indium is an indispensable compound to realize the functionality of different types of flat displays which are used for televisions, PCs, tablets and smartphones [4]. In 2017, the European Commission confirmed indium’s status as critical because of its insecure import reliance and a recycling rate of 0% [5]. Regarding the latter, and keeping in mind that there is a domestic indium supply by way of discarded flat panel displays, an uncomplicated and cost-efficient way to include a pyrolytical indium separation step into the pyrometallurgical recycling route for WEEE by keeping all the other mentioned benefits of pyrolysis should be considered. The most widespread technology for flat panel displays in electronic consumer products (i.e., televisions) is LCD technology (liquid crystal display). However, with respect to other more modern devices such as smartphones, OLED technology (organic light emitting diode) is becoming more established as a standard. In both cases, the displays are composed of several glass and polymer layers: lid (glass), polarizer film (polymer), active layer (LCD or OLED), ITO film (glass or polymer substrate), thin film transistors (glass substrate) and optical layers (polymer) [6]. A visualization of this sandwich construction is shown in Figure1. The exact number and combination of layers depend on the technology, as well as the manufacturer, and will not be examined further. Nevertheless, the organic polymer films, and especially the brominated flame retardants (BFR) enclosed in the polymer structure, play a fundamental role for this work, which is why their chemical composition is taken into account. Due to their high transparency and their good mechanical and electrical properties, polyethylene terephthalate (PET), polycarbonate (PC), polyethylene naphthalate (PEN) and polymethyl methacrylate (PMMA) are appropriate materials to be used in flat displays [7,8]. For the polarizer film, a polymer based on cellulose triacetate (CTA), is state of the art [9]. With regard to PC, PET and PEN, it has been reported that their structure allows the application of BFR [10]. Coming back to the idea of Metals 2018, 8, 1040 3 of 12 the pyrolytic pretreatment of WEEE, the question of using bromine, originating from the BFR, as a Metalshalogenation 2018, 8, x FOR agent PEER for REVIEW the formation of volatile indium bromide remains. 3 of 13

Figure 1. Sandwich construction of a liquid crystal display (LCD) [9]. Figure 1. Sandwich construction of a liquid crystal display (LCD) [9]. 1.2. Halogenation of Metals 1.2. Halogenation of Metals Generally, there are several types of halogenation reactions, depending on the number and nature of reactantsGenerally, in the there system. are several Nonetheless, types allof havehalogena the transformationtion reactions, ofdepending a metal oxide on the to anumber metal and naturein common, of reactants which in can the be system. seen in Nonetheless, the following all equations, have the transformation where the chlorination of a metal of oxide a bivalent to a metal metal halideoxide isin carriedcommon, out which [11]: can be seen in the following equations, where the chlorination of a bivalent metal oxide is carried out [11]: 1 MeO + Cl2 ⇐⇒ MeCl2 + O2 (1) 1 2 MeO + Cl ⟺ MeCl + O (1) MeO + C+ Cl2 ⇐⇒ MeCl2 2+ CO (2)

MeO + 2HCl ⇐⇒ 2MeCl2 + H2O (3) MeO + C + Cl ⟺ MeCl + CO (2) As can be seen from Equations (1)–(3), both the use of pure chlorine (halogenation) or gaseous HCl (hydrohalogenation) is possible.MeO + 2 HCl Furthermore, ⟺ 2 MeCl chlorine + HO can be replaced by bromine without(3) changing the stoichiometry of the reaction agents, however, it has to be taken into account that the As can be seen from Equations (1)–(3), both the use of pure chlorine (halogenation) or gaseous thermodynamics of the reactions are slightly different. As it will be explained later, the halogenation HCl (hydrohalogenation) is possible. Furthermore, chlorine can be replaced by bromine without reactions are strongly affected by temperature and other gas components occurring in the system. In changing the stoichiometry of the reaction agents, however, it has to be taken into account that the this context, the formation of so-called subhalides plays an important role. thermodynamics of the reactions are slightly different. As it will be explained later, the halogenation reactions1.3. Current are Research strongly on affected Indium by Volatilization temperature from and Displays other gas through components Chlorination occurring in the system. In this context, the formation of so-called subhalides plays an important role. So far, there have been several scientific works on the halogenation and volatilization of indium 1.3.from Current flat screens. Research All on of Indi theseum wereVolatilization conducted from by Displays using HCl through as the Chlorination halogenation agent. Ma et al. [12] examined the vacuum chlorination of LCD glass powder, whereby HCl was generated by the thermal So far, there have been several scientific works on the halogenation and volatilization of indium decomposition of NH4Cl, which was blended with the glass powder. Prior to the chlorination process, from flat screens. All of these were conducted by using HCl as the halogenation agent. Ma et al. [12] the polymers from the displays were removed via a pyrolysis step. The results of the chlorination examined the vacuum chlorination of LCD glass powder, whereby HCl was generated by the thermal showed that the indium recovery increased with the applied temperature due to the higher vapor decomposition of NH4Cl, which was blended with the glass powder. Prior to the chlorination process, pressure of InCl3. As a second significant influencing factor, a smaller particle size of the LCD powder the polymers from the displays were removed via a pyrolysis step. The results of the chlorination was identified to increase the indium recovery. showed that the indium recovery increased with the applied temperature due to the higher vapor Similar investigations were carried out by Terakado et al. [13], who also used NH4Cl and pressure of InCl3. As a second significant influencing factor, a smaller particle size of the LCD powder concluded that high temperatures support the chlorination/volatilization. For this work, the input was identified to increase the indium recovery. material was synthesized by covering soda glass with ITO, so that no polymers were involved. The Similar investigations were carried out by Terakado et al. [13], who also used NH4Cl and temperature was adjusted between 400 and 800 ◦C under normal pressure. Additionally, Terakado et concluded that high temperatures support the chlorination/volatilization. For this work, the input al. found that the addition of small amounts of carbon powder had a beneficial effect on the indium material was synthesized by covering soda glass with ITO, so that no polymers were involved. The recovery. In contrast to Ma et al., a longer milling time of the glass and thus a smaller particle size temperature was adjusted between 400 and 800 °C under normal pressure. Additionally, Terakado lowered the rate of indium recovery. et al. found that the addition of small amounts of carbon powder had a beneficial effect on the indium Another concept of HCl generation for the chlorination was followed by Kameda et al. [14], who recovery. In contrast to Ma et al., a longer milling time of the glass and thus a smaller particle size used the products of the thermal decomposition of polyvinyl (PVC) to create a HCl-rich lowered the rate of indium recovery. atmosphere. The experiments of Kameda et al. were conducted under air as well as under an Another concept of HCl generation for the chlorination was followed by Kameda et al. [14], who used the products of the thermal decomposition of polyvinyl chloride (PVC) to create a HCl-rich atmosphere. The experiments of Kameda et al. were conducted under air as well as under an inert nitrogen atmosphere, resulting in a higher indium volatilization when nitrogen was applied. Furthermore, the In recovery from pure In2O3 was lower than using LCD powder as input material.

Metals 2018, 8, 1040 4 of 12 inert nitrogen atmosphere, resulting in a higher indium volatilization when nitrogen was applied. Furthermore, the In recovery from pure In2O3 was lower than using LCD powder as input material. Similar to Me et al., Takahashi et al. [15] removed the polymer fraction from the milled LCD displays via incineration. Afterwards, the LCD powder was moistened with aqueous HCl and dried so that water was removed and HCl remained in the solid material. The thermal process was executed between 400 and 700 ◦C. Like the previous research, high temperatures and a nitrogen-rich atmosphere promoted the formation and evaporation of InCl3.

1.4. Motivation and Innovative Approach of this Work In each of the papers presented above, a supplementary chlorination agent was used without consideration of the polymer fraction from the LCD displays. However, this polymer fraction probably contains BFR, which can influence the whole process and act as a halogenation agent in situ unless the polymers are not removed. In one of our preceding papers, we investigated the temperature dependence of the gaseous and solid pyrolysis products during the thermal decomposition of printed circuit boards (PCB) [16]. It was found that elevated temperatures and a high heating rate have a considerable influence on the decomposition mechanism of the organics and thus on the composition of the pyrolysis gas. One important aspect was the formation of HBr at 700 ◦C during the decomposition of BFR in the epoxy resin. Hence, it is expected that this HBr generation will also occur during the pyrolysis of LCDs. Therefore, this work focuses on the possibility of an in situ halogenation of indium oxide by HBr. A special point of interest is the influence of other gaseous pyrolysis products on the halogenation reaction. At this point, carbon monoxide (CO) is a matter of particular interest because its formation increases with higher temperatures. According to Peek [11] and Grabda et al. [17], the presence of CO has a synergetic effect on the thermodynamics of the halogenation reaction.

2. Materials and Methods

2.1. Materials The input materials for the experimental work were displays from 20 discarded smartphones of different brands. First, the devices were dismantled manually so that the multi-layer displays, having an overall mass of 458 g, could be removed. To obtain a bulk material with a homogenous chemical composition, the displays were processed in a Fritsch Pulverisette 25/19 cutting mill, which resulted in a maximum particle size of 4 mm. The final comminution was completed via cryogenic grinding in a SPEX 6870D Freezer/Mill. Concerning the chemical analysis, the concentrations of most elements were measured by means of an ICP–OES (Perkin Elmer Optima 8300, Waltham, MA, USA), whereas Br and Cl were analyzed via RFA (PANalytical Axios, Malvern Panalytical, Almelo, The Netherlands). The total carbon (TC) was measured in a LECO R612 (LECO, St. Joseph, CT, USA) multiphase carbon determinator. Table1 contains the results of the elemental analysis. Concerning the results for bromine and chlorine, a certain discrepancy has to be accepted because no appropriate standard material is available to fulfil an appropriate calibration and thus a precise analysis. Due to the high share of polymer films, the comminuted product was not a flowable powder but a dense, rubbery fluff. In order to ensure smooth feeding, the material was compressed to pellets with a diameter of 6 mm and a length of 20 mm. For the second test series, pure ammonium chloride was mixed with the display fluff, adjusting to a ratio of 1:100, which means that 10 g display fluff was mixed with 0.1 g NH4Cl. Metals 2018, 8, 1040 5 of 12

Table 1. Chemical analysis of milled smartphone displays.

Element conc./ppm conc./wt.-% Method In 130 0.013 ICP–OES Metals 2018, 8, x FOR PEER REVIEW 5 of 13 Sn 470 0.047 ICP–OES Ba 2000 0.2 ICP–OES CaBr 18,20050 0.005 1.82 ICP–OESRFA KCl 5500100 0.010 0.55 ICP–OESRFA MgC 231,000 11,800 23.100 1.18 ICP–OES TC Na 35,500 3.55 ICP–OES Sr 1400 0.14 ICP–OES Due to the high share ofAl polymer films, 41,100 the comminuted 4.11 product ICP–OES was not a flowable powder but a dense, rubbery fluff. In orderSi to ensure 17,400smooth feeding, 17.4 the material ICP–OES was compressed to pellets with a diameter of 6 mm and a lengthBr of 20 mm. 50 For the second 0.005 test series, RFA pure ammonium chloride was mixed with the display fluff,Cl adjusting to 100 a ratio of 1:100, 0.010 which means RFA that 10 g display fluff was C 231,000 23.100 TC mixed with 0.1 g NH4Cl.

2.2.2.2. ExperimentalExperimental SetupSetup andand ProcedureProcedure TheThe pyrolysispyrolysis experimentsexperiments werewere performedperformed inin aa closedclosed 1.51.5 LL stainlessstainless steelsteel reactor,reactor, whichwhich waswas placedplaced inin anan electricelectric resistanceresistance furnace,furnace, asas cancan bebe seenseen inin FigureFigure2 .2. ToTo prepare prepare for for the the insertion insertion of of furtherfurther equipment (thermocouple, (thermocouple, charging charging tube tube an andd probe probe head head for for the the off-gas off-gas analyzer) analyzer) the lid the was lid wasequipped equipped with with four four gastight gastight lead-throughs. lead-throughs. Additionally, Additionally, the the lid lid was was water-cooled water-cooled to to avoid anyany damagedamage ofof thethe rubberrubber sealsseals byby thermalthermal impact.impact.

FigureFigure 2.2. SchematicSchematic figurefigure ((leftleft)) andand photophoto ((rightright)) ofof thethe experimentalexperimental setup.setup.

ForFor thethe temperaturetemperature measurement,measurement, aa typetype KK thermocouplethermocouple waswas chosen.chosen. InIn orderorder toto maintainmaintain anan oxygen-freeoxygen-free atmosphere,atmosphere, thethe reactorreactor waswas purgedpurged withwith aa constantconstant argonargonstream streamof of 3 3 L/min. L/min. TheThe testtest ◦ runsruns werewere conducted conducted at at various various temperatures temperatures between between 300 300 and and 800 800C, raising°C, raising the temperature the temperature in steps in ◦ ofsteps 50 ofC with50 °C each with trial. each After trial. heatingAfter heating the reactor the re upactor to the up desiredto the desired temperature, temperature, 10 g of display10 g of display pellets werepellets charged were charged into the hotinto reactor the hot using reactor a densely using a sintered densely alumina sintered pipe alumina and apipe funnel. and On a funnel. the bottom On the of thebottom reactor, of the an aluminareactor, an crucible alumina was crucible placed towas collect placed the to pellets. collect Subsequent the pellets. to Subsequent feeding, the to pipe feeding, and thethe funnelpipe and were the removed funnel were and theremoved lead-through and the was lead-t closedhrough with was a steel closed plug with to avoida steel any plug leakage to avoid of pyrolysisany leakage gas. of For pyrolysis each temperature gas. For each step, temperatur the abovee outlined step, the procedure above outlined was repeated procedure twice. was Following repeated thetwice. pyrolysis, Following it was the necessarypyrolysis, toit letwas the necessary reactor coolto let down the reactor slowly, cool before down the slowly, solid residue before couldthe solid be removedresidue could from thebe crucibleremoved and from homogenized the crucible in aand ball homogenized mill. Finally, thein concentrationsa ball mill. Finally, of indium the andconcentrations tin were measured of indium via and ICP–OES. tin were One measured separate via series ICP–OES. of trials One was separate performed series to measureof trials was the compositionperformed to of measure the pyrolysis the composition gas at 300, 500of the and pyrolysis 700 ◦C. Forgas thisat 300, purpose, 500 and a gas700 pump, °C. For manufactured this purpose, bya gas Ansyco, pump, was manufactured used to extract by Ansyco, an off-gas was volume used to of extract 2.0 L/min an off-gas from the volume reactor. of Downstream2.0 L/min from from the thereactor. pump, Downstream the gas flew from through the a Gasmetpump, the DX4000 gas FTIRflew (Gasmetthrough Technologiesa Gasmet DX4000 Oy, Helsinki, FTIR Finland)(Gasmet (FourierTechnologies transform Oy, Helsinki, infrared) gasFinland) analyzer (Fourier which transform allowed the infrared) continuous gas analyzer detection which and quantification allowed the continuous detection and quantification of various compounds in the pyrolysis gas. Compounds of special interest were HBr, HCl and CO. The overrun gas flow was released from the reactor through a bypass.

3. Results and Discussion In contrast to earlier studies on the pyrolytic volatilization of indium, the experimental setup used in this work did not allow the characterization of volatilized indium compounds. The water

Metals 2018, 8, 1040 6 of 12 of various compounds in the pyrolysis gas. Compounds of special interest were HBr, HCl and CO. The overrun gas flow was released from the reactor through a bypass.

3. Results and Discussion

MetalsIn 2018 contrast, 8, x FOR to PEER earlier REVIEW studies on the pyrolytic volatilization of indium, the experimental setup6 of 13 used in this work did not allow the characterization of volatilized indium compounds. The water cooling of the lid resulted in the condensation of pyrolysispyrolysis oil on its underside. Hence, in any case of indium halide vaporization, indium must occur in the condensed condensed oil. However, However, there currently exists no successful procedure for an elemental analysis of the oil. Regarding a structural analysis of the solid pyrolysis residue via XRD, no satisfying resultsresults were obtained due to the high amount of carbon and glass, as well as the low content of indium compounds, in the powder. Therefore, the following results refer to the elemental analysisanalysis ofof thethe solidsolid residueresidue (via(via ICP–OES)ICP–OES) andand thethe off-gasoff-gas analysis.analysis.

3.1. Mass Loss and Volatilization of ITO The mass loss of the pyrolyzed materials can be seen in Figure 3 3.. ApartApart fromfrom slightslight discrepanciesdiscrepancies at the beginning andand thethe end,end, bothboth graphs graphs show show an an almost almost identical identical trend, trend, reaching reaching their their maximum maximum at 650at 650◦C °C (35% (35% for for pure pure display display powder) powder) and and 800 800◦C °C (39% (39% for for NH4Cl-addition). NH4Cl-addition).

40

30

20

10 without additive Mass loss / wt.-% with NH4Cl 0 300 400 500 600 700 800 T / °C

Figure 3. Mass loss during the pyrolysis of ground smartphone displays. Figure 3. Mass loss during the pyrolysis of ground smartphone displays. Figure4 shows the concentration of indium and tin in the solid residue at several pyrolysis temperatures.Figure 4 shows The first the value concentration on the abscissa, of indium which and is labelled tin in “NP”,the solid represents residue the at indiumseveral contentpyrolysis of thetemperatures. non-pyrolyzed The first material. value It on should the abscissa, also be takenwhich into is labelled account “NP”, that the represents detection the limit indium of indium content in theof the ICP–OES non-pyrolyzed was 20 ppm. material. In the It caseshould of fivealso samplesbe taken (between into account 700 andthat 800the detection◦C), the analysis limit of resultedindium in indiumthe ICP–OES concentrations was 20 ppm. below In this the limit. case To of illustrate five samples these points(between in the 700 diagram, and 800 the °C), values the analysis were set toresulted zero but in itindium must be concentrations assumed that thebelow actual this indium limit. concentrationsTo illustrate these are locatedpoints somewherein the diagram, between the 0values and 19were ppm. set Apart to zero from but that it must aspect, be itassumed can be seenthat clearlythe actual that indium the processing concentrations temperature are located has a strongsomewhere influence between on the 0 and volatilization 19 ppm. Apart of indium from that from aspe thect, material it can be whether seen clearly or not that NH 4theCl wasprocessing added astemperature a supportive has chlorinationa strong influence agent. on Looking the volatilization at both graphs, of indium no decrease from the of material indium whether in the material or not canNH4 beCl observedwas added up as to a 350supportive◦C. After chlorination the application agent. of Looking higher temperatures, at both graphs, an no increasing decrease amountof indium of indiumin the material left the material.can be observed The increase up to before 350 °C. can After be due the to application the mass loss of referringhigher temperatures, to the pyrolysis an ofincreasing polymers. amount Both graphsof indium exhibit left a the similar material. course, The where increase the graphbefore showing can be due the experimentsto the mass withloss NHreferring4Cl addition to the pyrolysis keeps an of average polymers. distance Both of graphs 57 ppm exhibit below a the similar graph course, without where any additive.the graph Therefore, showing 4 ◦ the experiments detection limit with of NH 20 ppmCl addition was already keeps reached an average at 700 distanceC, whereas of 57 ppm the below samples the without graph without NH4Cl hadany additive. to be processed Therefore, at least the atdetection 750 ◦C limit to bring of 20 the ppm indium was contentalready toreached the same at 700 level. °C, Obviously,whereas the a completesamples without volatilization NH4Cl ofhad indium to be processed without anyat least extra at halide-providing750 °C to bring the substance indium content is possible to the and same it level. Obviously, a complete volatilization of indium without any extra halide-providing substance is possible and it follows that there must be sufficient halides in the polymer fraction of the displays. According to Figure 4, a significant indium vaporization starts between 350 and 400 °C because the enrichment of indium by the act of pyrolysis of polymers is balanced.

Metals 2018, 8, 1040 7 of 12

followsMetals 2018 that, 8, x there FOR PEER must REVIEW be sufficient halides in the polymer fraction of the displays. According7 of to13 Figure4, a significant indium vaporization starts between 350 and 400 ◦C because the enrichment of indium by the act of pyrolysis of polymers is balanced.

200 withoutwithout additive additive

withwith NH4Cl NH4Cl 150

100 In / ppm In /

50

0 200NP 300 400 500 600 700 800

0.12

0.10

0.08 Sn / wt-%Sn /

0.06 withwithout NH4Cl additive

withoutwith NH additive4Cl 0.04 200NP 300 400 500 600 700 800

T / °C

Figure 4. Concentration of In and Sn in the solid residue at different pyrolysis temperatures.

ConcerningFigure 4. theConcentration analysis results of In and of Sn tin, in thethe solid graphs residue in the at different second diagrampyrolysis temperatures. of Figure3 show an unsteady performance which is contrary to the results of indium. All in all, tin tends to be enriched Concerning the analysis results of tin, the graphs in the second diagram of Figure 3 show an in the material and no volatilization seems to happen. Comparing the tin concentration of the unsteady performance which is contrary to the results of indium. All in all, tin tends to be enriched non-pyrolyzed material and the residue from the 800-◦C trial, there is an increase of more than 90%, in the material and no volatilization seems to happen. Comparing the tin concentration of the non- which is consistent with the mass loss of 35% (pure display powder) and 39% (NH4Cl-addition) as can pyrolyzed material and the residue from the 800-°C trial, there is an increase of more than 90%, which be obtained from Figure2. is consistent with the mass loss of 35% (pure display powder) and 39% (NH4Cl-addition) as can be 3.2.obtained Off-Gas from Analysis Figure 2.

3.2. Off-gasThe results Analysis of the off-gas analysis for CO, HBr and HCl during the pyrolysis of display powder at 300, 500 and 700 ◦C are depicted in Figure5. While the off gas contained numerous other gaseous species—especiallyThe results of the organic off-gas substances—for analysis for CO, this HBr work, and the HCl three during named the compoundspyrolysis of appeardisplay to powder be the mostat 300, important 500 and 700 ones. °C Theare concentrationsdepicted in Figure of each 5. While component the off aregas normalizedcontained numerous to one gram other of chargedgaseous material.species—especially By comparing organic the substances—for three diagrams, this the wo changingrk, the three scale named of both compounds axes has toappear be respected. to be the Basically,most important the course ones. of The the curvesconcentrations resembles of theeach results component from our are previous normalized work to whereone gram printed of charged circuit boardsmaterial. were By pyrolyzedcomparing (Diaz the three et al. [di2]).agrams, At 300 the◦C, changing little gas formationscale of bo couldth axes be has measured, to be respected. at which noBasically, detection the of course HCl andof the HBr curves happened. resembles The the oscillating results from CO concentration our previous work around where 8 ppm printed proves circuit that boards were pyrolyzed (Diaz et al. [2]). At 300 °C, little gas formation could be measured, at which no detection of HCl and HBr happened. The oscillating CO concentration around 8 ppm proves that there is a slow thermal decomposition which lasts for at least 50 min. With increasing the temperature to 500 °C and 700 °C, the amount of all gas components rose rapidly, particularly in the case of CO.

Metals 2018, 8, 1040 8 of 12

thereMetals is 2018 a slow, 8, x FOR thermal PEER decomposition REVIEW which lasts for at least 50 min. With increasing the temperature8 of 13 to 500 ◦C and 700 ◦C, the amount of all gas components rose rapidly, particularly in the case of CO. DifferentDifferent to to Diaz Diaz et et al., al., a a significant significant release release of of HBr HBr was was already already observed observed at at 500 500◦ C°C but but did did not not increase increase atat 700 700◦ C.°C. This This is is different different for for HCl, HCl, which which exhibits exhibits a a strong strong increase increase from from 500 500◦ C°C to to 700 700◦ C.°C. Concerning Concerning thisthis temperature temperature increase, increase, the the period period from from the the first first time time of of gas gas evolution evolution to to the the last last point point of of significant significant concentrationsconcentrations was was reduced reduced to ato third. a third. However, However, the graphs the forgraphs HBr revealfor HBr some reveal challenges some regardingchallenges itsregarding detection its in detection the gas phase. in the It gas must phase. be kept It must in mind be kept that in the mind detection that the of detection HBr using of FTIR HBr isusing possible FTIR butis possible is strongly but affectedis strongly by affected the sampling. by the sampling. From practice From it practice is known it is that known HBr that tends HBr to tends adhere to toadhere the insideto the surface inside surface of the sampling of the sampling equipment equipment so that the so that molecules the molecules reach the reach FTIR the detector FTIR detector with a certain with a delay.certain Thus, delay. its Thus, concentration–time its concentration–time record, record, as it is as shown it is shown in Figure in Figure5, is drawn-out 5, is drawn-out as against as against the recordsthe records for CO for andCO and HCl. HCl. It was It was also also observed observed that th theat the concentration concentration of of HBr HBr continued continued to to oscillate oscillate betweenbetween 0 and0 and 2 ppm2 ppm although although the the formation formation of all of other all other compounds, compounds, and therefore and therefore the pyrolysis the pyrolysis itself, wereitself, finished. were finished.

16 CO 12

300 °C 8

CO / ppm/g CO / 4

0 0 500 1000 1500 2000 2500 3000 t / s 1400 8 CO 1200 HBr 1000 6 HCl 800 500 °C 4 600

CO / ppm/g CO / 400 2 200 HCl / ppm/g HBr, 0 0 0 100 200 300 400 500 600 t / s 14000 8 CO 12000 HBr 10000 6 HCl 700 °C 8000 4 6000 CO /ppm/g 4000

2 HCl / ppm/g HBr, 2000 0 0 0 50 100 150 200 t / s

Figure 5. Concentrations of CO, HBr and HCl during the pyrolysis of display powder at 300, 500 and Figure 5. Concentrations of CO, HBr and HCl during the pyrolysis of display powder at 300, 500 and 700 ◦C. 700 °C.

Despite the limited HBr measurement, it can be stated that higher pyrolysis temperatures favor the formation of CO, HBr and HCl. On the one hand, this aspect confirms the results of the indium decline with increasing temperature (see Figure 4), but on the other hand, it is not known yet which

Metals 2018, 8, 1040 9 of 12

Despite the limited HBr measurement, it can be stated that higher pyrolysis temperatures favor theMetals formation 2018, 8, x FOR of CO,PEER HBr REVIEW and HCl. On the one hand, this aspect confirms the results of the indium9 of 13 decline with increasing temperature (see Figure4), but on the other hand, it is not known yet which compound—HBr or HCl—acted as the major halogenation agent. Although the pyrolysis reaction at compound—HBr or HCl—acted as the major halogenation agent. Although the pyrolysis reaction at 700 °C is three times faster compared to 500 °C, there seems to be no limitation in terms of kinetics of 700 ◦C is three times faster compared to 500 ◦C, there seems to be no limitation in terms of kinetics of the volatilization. In other words, the halogenation reaction and evaporation are fast enough and do the volatilization. In other words, the halogenation reaction and evaporation are fast enough and do not require residence times of HBr/HCl and CO longer than 100 s. not require residence times of HBr/HCl and CO longer than 100 s.

3.3.3.3. Thermochemical Modelling ForFor aa moremore detaileddetailed investigation investigation on on the the halogenation halogenation reactions reactions occurring occurring during during the the pyrolysis pyrolysis of powderedof powdered smartphone smartphone displays, displays, several several thermochemical thermochemical calculations calculations were executed were executed using the using software the TM FactSagesoftwareTM FactSage7.0 [18], which7.0 [18], is provided which is by provided GTT Technologies. by GTT Technologies. The background The of background these calculations of these are considerationscalculations are of considerations Ma et al. [3] concerning of Ma et theal. [3] formation concerning of indium the fo subrmation , of indium which sub may chlorides, depend onwhich the presencemay depend of a on reducing the presence agent. of a reducing agent. TM AllAll calculationscalculations inin FactSageFactSageTM werewere done by assuming ideal conditions, which means that there areare no molecular molecular interactions interactions in in the the gas gas phase. phase. The The required required thermodynamic thermodynamic data datawas received was received from fromthe SGPS the SGPS database database for pure for pure substa substances.nces. As Asa result, a result, two two series series of of Gibbs Gibbs free free energy energy lines werewere obtained:obtained: one series for the hydrobrominationhydrobromination and the other for thethe hydrochlorinationhydrochlorination of oneone molemole In2O3 (see Figure 6). In fact, a calculation involving ITO would represent the real system rather than In2O3 (see Figure6 ). In fact, a calculation involving ITO would represent the real system rather than only In2O3. However, the available data bases of FactSageTM do not provide any data for ITO so a only In2O3. However, the available data bases of FactSage do not provide any data for ITO so a compromisecompromise hadhad toto bebe made.made.

100 100

0 0 In2O3 -100 -100 In2O3

-200 -200 G / kJ/mol G / G / kJ/mol Δ -300 -300 Δ

-400 -400 0 200 400 600 800 1000 0 200 400 600 800 1000 T / °C T / °C

In2O3In2O3 + + 4 4 HBr HBr+ + CO CO = 22 InBr2 InBr2 ++ 2 2 H2O H2O + CO2CO2 InIn2O32O3 + + 2 2 HCl HCl + + 2 2 CO = 2 2InCl InCl + +H2O H2O + + 2 2 CO2 CO2

In2O3In2O3 + + 2 2 HBr HBr+ + 2 2 CO CO = 22 InBr InBr ++ H2O H2O + + 2 2 CO2 CO2 InIn2O32O3 + + 6 6 HCl HCl = In2Cl6In2Cl6 ++3 3 HH2O2O

In2O3In2O3 + + 6 6 HBr HBr = In2Br6In2Br6 +3 H2OH2O InIn2O32O3 + + 4 4 HCl HCl + + CO CO = 2 InCl2InCl2 ++ 22 HH2O2O + + CO CO22

In2O3In2O3 + + 6 6 HBr HBr = 22 InBr3 InBr3 ++ 3 3 H2O H2O In2O3In2O3 + + 6 6 HCl HCl = 22 InCl3 InCl3 ++ 33 H2OH2O

Figure 6. Gibbs free energy for hydrobromination (left) and hydrochlorination (right) of indium oxide. Figure 6. Gibbs free energy for hydrobromination (left) and hydrochlorination (right) of indium Alloxide. the graphs exhibit kinks at 100 ◦C, which indicates a phase change due to the evaporation of water at this temperature. Inflection points at higher temperatures represent the formation of halides All the graphs exhibit kinks at 100 °C, which indicates a phase change due to the evaporation of in the gaseous state. In the cases of InCl and InBr, the melting point at 210 and 285 ◦C can be observed. water at this temperature. Inflection points at higher temperatures represent the formation of halides Comparing the curves of the reactions, there is only a small difference between HBr and HCl, leading in the gaseous state. In the cases of InCl and InBr, the melting point at 210 and 285 °C can be observed. to the conclusion that HBr might be a slightly better halogenation agent due to the lower Gibbs free Comparing the curves of the reactions, there is only a small difference between HBr and HCl, leading energy. As temperature increases, there are only three halogenation reactions, which become more to the conclusion that HBr might be a slightly better halogenation agent due to the lower Gibbs free favorable from the thermochemical point of view—as long as CO is added to the system in order to act energy. As temperature increases, there are only three halogenation reactions, which become more as a reducing agent. Thus, if temperatures above 400 ◦C and sufficient CO is present, the generation of favorable from the thermochemical point of view—as long as CO is added to the system in order to indium in the form of InBr and InCl seems to be the predominant mechanism. However, at 750 ◦C act as a reducing agent. Thus, if temperatures above 400 °C and sufficient CO is present,◦ the the formation of InBr3 and In2Br6 is predicted to be also possible. As seen in Figure3, 750 C is generation of indium in the form of InBr and InCl seems to be the predominant mechanism. However, the temperature where a complete volatilization of indium from the display powder without any at 750 °C the formation of InBr3 and In2Br6 is predicted to be also possible. As seen in Figure 3, 750 °C is the temperature where a complete volatilization of indium from the display powder without any additional NH4Cl was achieved. The different courses of the solid grey lines, which represent the formation of InBr2 and InCl2, create doubt that the quality of the thermodynamic data is appropriate

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Metals 2018, 8, x FOR PEER REVIEW 10 of 13 additional NH4Cl was achieved. The different courses of the solid grey lines, which represent the toformation reach a of InBrconclusion2 and InCl regarding2, create doubtthe temperat that the qualityure-dependent of the thermodynamic formation of datathese is appropriateparticular compounds.to reach a conclusion regarding the temperature-dependent formation of these particular compounds. AsAs a a further further step, step, the the temperature temperature dependence dependence of of the the evaporation evaporation of of the the formed formed halides halides from from FigureFigure 66 will will be be investigated, investigated, because because all halidesall halides can occurcan occur in different in different aggregate aggregate states andstates the and success the successof the practical of the practical work is wo basedrk is on based their on evaporation. their evaporation. Therefore, Ther theefore, logarithmic the logarithmic vapor pressurevapor pressure lines of linesthe different of the different gaseous gaseous indium indium bromides bromides and chlorides and chlorides are shown are inshown Figure in7 .Figure 7.

1 1 0 0 -1 -1 / bar / bar

vap -2

-2 vap

log p -3 -3 log p log -4 -4 -5 -5 0 200 400 600 800 1000 0 200 400 600 800 1000 T / °C T / °C

InBrInBr33 InBrInBr InBrInBr22 InIn2Br62Br6 InCl3InCl3 InCl InCl2InCl2 In2Cl6In2Cl6

Figure 7. Vapor pressure curves of pure indium bromides (left) and indium chlorides (right). Figure 7. Vapor pressure curves of pure indium bromides (left) and indium chlorides (right). If the curves are compared to the Gibbs free energy lines (see Figure6), it becomes clear that there If the curves are compared to the Gibbs free energy lines (see Figure 6), it becomes clear that is a competition between the Gibbs free energy and the vapor pressures of the single components, there is a competition between the Gibbs free energy and the vapor pressures of the single which means that a halogenation reaction with a low Gibbs free energy forms a halide with a low components, which means that a halogenation reaction with a low Gibbs free energy forms a halide vapor pressure (except InBr2 and InCl2). Therefore, the process of evaporation is another barrier which with a low vapor pressure (except InBr2 and InCl2). Therefore, the process of evaporation is another is strongly influenced by temperature. Assuming that mostly InBr is generated (according to Figure6) barrier which is strongly influenced by temperature. Assuming that mostly InBr is generated it still requires sufficiently high temperatures to bring it to the gas phase. (according to Figure 6) it still requires sufficiently high temperatures to bring it to the gas phase. To complete the contemplation of thermodynamics, we will have a closer look at some equilibrium To complete the contemplation of thermodynamics, we will have a closer look at some calculations—especially with respect to the amount of supplied CO and the resulting proportion of equilibrium calculations—especially with respect to the amount of supplied CO and the resulting the gaseous . In other words, the results from Figures6 and7 are combined in one proportion of the gaseous indium halides. In other words, the results from Figure 6 and Figure 7 are diagram. For this purpose, a system was defined consisting of 1 mole of In2O3 and an excess amount combined in one diagram. For this purpose, a system was defined consisting of 1 mole of In2O3 and of 6 moles each of HBr and HCl. The quantity of CO was varied from 0 to 5 moles in steps of 0.1 mole an excess amount of 6 moles each of HBr and HCl. The quantity of CO was varied from 0 to 5 moles and the whole procedure was executed for 700 ◦C and 800 ◦C. Finally, Figure8 illustrates the resulting in steps of 0.1 mole and the whole procedure was executed for 700 °C and 800 °C. Finally, Figure 8 data from the equilibrium calculations. Also, here attention must be paid to the different scale of the illustrates the resulting data from the equilibrium calculations. Also, here attention must be paid to axes. Due to the great difference in their amount, bromides and chlorides are shown in two different the different scale of the axes. Due to the great difference in their amount, bromides and chlorides are diagrams, although they exist in the same system. shown in two different diagrams, although they exist in the same system. As already predicted by the Gibbs free enthalpy lines in Figure6, the occurrence of indium chlorides in the gas phase is inhibited by the preferred formation of indium bromide and for all amounts of CO, both In2Br6 and In2Cl6 play only a minor role. In contrast, the proportion of InBr/InBr3 shifts to higher values as the supply of CO increases. Raising the temperature from 700 ◦C (broken lines) to 800 ◦C (solid lines) intensifies this effect. This fact can be also observed concerning the formation of chlorides, but in a much lower scale as was previously mentioned. Again, it is theoretically proven that a CO-rich (reducing) atmosphere supports the formation of gaseous subhalides as well as the halogenation process. Referring to the results from the off-gas measurement (see Figure5), a preferred formation of InBr seems to be the most probable mechanism at 700 ◦C because there is an enormous excess of CO compared to HBr and HCl.

Metals 2018, 8, 1040 11 of 12 Metals 2018, 8, x FOR PEER REVIEW 11 of 13

2.0 0.4

1.5 0.3

/ mole 1.0 0.2 / mole

compound 0.5 0.1 compound n n

0.0 0.0 01234500.511.52

nCO / mole nCO / mole

n (InBr), 700 °C n (InBr), 800 °C n (InCl), 700 °C n (InCl), 800 °C

n(Inn (In2Br6),2Br6), 700 700 °C °C n(Inn (In2Br6),2Br6), 800 800 °C °C n(Inn (In2Cl6),2Cl6), 700 700 °C °C n(Inn (In2Cl6),2Cl6), 800 800 °C °C

n(InBrn (InBr3),3), 700700 °C°C nn(InBr (InBr3),3), 800800 °C°C n(InCln (InCl3),3), 700 700 °C °C nn(InCl (InCl3),3), 800800 °C°C

Figure 8. Proportion of formed indium halides by HBr (left) and HCl (right) depending on the amount Figureof CO. 8. Proportion of formed indium halides by HBr (left) and HCl (right) depending on the 4. Conclusionsamount of CO.

AsThe already aim of thispredicted work wasby the to assessGibbs whetherfree enthalpy it is possible lines in to Figure volatilize 6, the indium occurrence from theof indium display chloridesmaterial of in discarded the gas phase smartphones is inhibited without by usingthe preferred an additional formation halogenation of indium agent. bromide This requirementand for all ◦ amountswas accomplished of CO, both at In a2 minimumBr6 and In2 processingCl6 play only temperature a minor role. of In 750 contrast,C. At thisthe proportion temperature, of significantInBr/InBr3 shiftsamounts to higher of HBr/HCl values as and the CO supply were of released CO incr fromeases. the Raising plastic the fraction temperat to reacture from with 700 indium °C (broken oxide. lines)The theoretical to 800 °C study(solid onlines) thermodynamics intensifies this confirmed effect. This the fact idea can of be a synergeticalso observed effect concerning of CO on the formationhalogenation of atchlorides, which In but tends in toa bemuch evaporated lower scale in the as form was of InBr/InClpreviously rathermentioned. than InBr Again,3/InCl it3 atis theoreticallylow CO concentrations. proven that Compared a CO-rich to HCl,(reducing) HBr is atmosphere slightly preferred supports in order the toformation act as a halogenation of gaseous subhalidesagent from as the well thermochemical as the halogenation point process. of view. Re Dueferring to to the the successful results from decrease the off-gas of indium measurement and the (seerecorded Figure concentrations 5), a preferred of HClformation and HBr of inInBr the seems off gas to it canbe the be concludedmost probable that the mechanism amount of at halide 700 °C in becausethe displays’ there plasticsis an enormous is sufficient excess for of a completeCO compared indium to halogenationHBr and HCl. so that no additive is required. Of course, the indium volatilization can also be realized at lower temperatures by adding an excess 4.of Conclusions NH4Cl, as was demonstrated in other papers [12–15], as well as in this work. However, both thermochemical calculations and experiments have shown that even though a successful halogenation The aim of this work was to assess whether it is possible to volatilize indium from the display would have happened, at least 700 ◦C is necessary to achieve a complete evaporation of the indium material of discarded smartphones without using an additional halogenation agent. This requirement halides from the display material. As a final result, it can be stated that the success of a complete indium was accomplished at a minimum processing temperature of 750 °C. At this temperature, significant volatilization during pyrolysis depends on a chain of several mechanisms which are all promoted by amounts of HBr/HCl and CO were released from the plastic fraction to react with indium oxide. The high temperatures: theoretical study on thermodynamics confirmed the idea of a synergetic effect of CO on the • halogenationthermal decompositionat which In tends of plasticsto be evaporat and theed supply in the of form sufficient of InBr/InCl HBr/HCl rather and than CO; InBr3/InCl3 at low• COhalogenation concentrations. of indium Compared oxide andto HCl, the formationHBr is slightly of (sub) preferred halides in depending order to act on as temperature a halogenation and agentsupply from the of CO; thermochemical point of view. Due to the successful decrease of indium and the recorded• evaporation concentrations of formed of HCl (sub) and halides. HBr in the off gas it can be concluded that the amount of halide in theAlthough displays’ no plastics product is materialsufficient was for collected,a complete the indium process halogenation appears to be so selective that no becauseadditive the is required.concentration Of course, of tin inthe the indium pyrolysis volatilization residue increased can also (due be realized to of the at mass lower loss temperatures of the material) by adding which anmeans excess that of tinNH was4Cl, notas was volatilized demonstrated significantly. in other In papers this paper, [12–15], the basicas well idea as isin to this include work. the However, process bothof indium thermochemical volatilization calculations into a current and recyclingexperiment concepts have for shown WEEE that with even as little though effort a assuccessful possible. halogenationThe main requirement would have for happened, this, indeed, at isleast the application700 °C is necessary of a pyrolysis to achieve step ina complete the WEEE evaporation processing ofroute the whichindium provides halides severalfrom the other—probably display material. more As substantial—advantagesa final result, it can be stated [3,16]. that Based the on success this fact, of ahowever, complete a indium simultaneous volatilization indium during separation pyrolysis would depends be a great on feature. a chain of several mechanisms which are all promoted by high temperatures: •Author Contributions: B.F. (Benedikt Flerus) and T.S. conceived and designed the experiments; T.S. performed the experiments;thermal decomposition B.F. (Benedikt Flerus)of plastics analyzed and thethe data supply and wroteof sufficient the paper HBr/HCl with contributions and CO; of K.B.; R.S. and •B.F. (Berndhalogenation Friedrich) of supervised indium oxide the work and andthe formation contributed of reagents, (sub) halides materials depending and analysis on tools.temperature and Conflictssupply of Interest: of CO; The authors declare no conflict of interest. • evaporation of formed (sub) halides.

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