**3.2 Mixed plastics**

In 2017 a proposal on friendly management of waste/contaminated polymeric materials from differentiated municipal solid waste collection through microwaveassisted pyrolysis [29] was reported to enlighten the properties and possible use of the main products formed.

Zou et al. [30] in 2021 reported the pyrolysis of plastic wastes with SiC as an MW absorber and it is a new, continuous MAP process for fuel production. Higher pyrolysis temperatures promoted the cracking of wax and lighter and more stable hydrocarbons were formed. Talc as a filler in commercial polypropylene showed high cracking activity. Incorporating ZSM-5 catalysts in the waste using a space velocity of 10 h−1 and a pyrolysis temperature of 620°C a yield of liquid of 48.9 % was obtained and this product consisted of 73.5 % of gasoline-range hydrocarbons rich in aromatics (45.0 %) and isomerized aliphatic hydrocarbons (24.6 %). However, the catalyst rapidly lost its activity using a feedstock/catalyst ratio of 5:1.

Microwave-assisted catalytic pyrolysis of municipal solid waste was reported by Yu et al. in 2020 [31] using a catalyst able to be regenerated. The catalyst was

employed in a fluidized bed pyrolysis furnace with a microwave generator. Data were not reported on the summary of the Chinese paper.

An artificial mixture of cellulose, paraffin oil, kitchen waste, and garden waste that closely mimic municipal solid wastes (MSW) was pyrolyzed at different reaction conditions [32] using MAP. Ten different MW absorbing materials such as aluminum, activated carbon, garnet, iron, silica beads, cement, SiC, TiO2, fly ash, and graphite was tested. MAP was run up to 600°C, and the effects of MSW/MW absorber ratio and composition of the model MSW mixture were reported. The MW absorber affected the yields of oil, gas, and char, and played a catalytic role in altering the selectivity towards the various components present in the liquid. The oil contained oxygenated compounds (furans, phenolics, cyclic-oxygenates) from the biomass present in MSW, aliphatic, and aromatic hydrocarbons (mono- and polycyclics). Aromatic hydrocarbons were mainly derived from lignin decomposition while aliphatic hydrocarbons were derived from cellulose and plastic pyrolysis. The highest yield of oil (53 wt. %) was achieved with a 1/1 wt./wt. ratio of MSW/graphite. The energy in the oil corresponded to energy recovery of nearly 95 % with an 85 % deoxygenation. Using a high ratio of MW absorber/MSW monoaromatics such as benzene, toluene, xylene and styrene, and C8-C20 aliphatic hydrocarbons were formed with high selectivity, while polycyclic aromatics were obtained with low selectivity. Methane, ethene, propene, isobutene, and hydrogen were the major products in the gaseous phase, whose selectivities varied with MSW composition.

MSW containing about 40 % of food was treated through MAP at different MW power [33]. The maximum oil yield of 30.2 wt. % was obtained under the optimized pyrolysis conditions: 400°C, residence time 30 min and a nitrogen flow rate of 50 mL/min at the microwave power of 450 W. Surprisingly the liquid contained oxygen, sulfur, nitrogen, and phosphorous containing compounds: methylphosphine [CH3PH2] (the main compound), 2-ethoxyethylenamine (CH3CH2-O-CH2CH2-NH2], 2-methoxyethylenamine [CH3CH2-O-CH2CH2-NH2], 2-fluoropropane [C3H7F], (2-hydroxyethyl) (trimethylsilylmethylen)sulfide [Me3Si-CH2-S-CH2CH2OH], and 1,3-bis(2-hydroxymethylen)urea [OC(NH-CH2OH)2] were identified and quantified by GC–MS analysis. The heating value of the oil was 23.94 MJ/kg.

### **3.3 Plastic composites**

The MAP of some plastic composites such as end cycle tyres [34–39], multilayer packaging beverages, corn derived plastic bags, biomasses and so on have also been reported.

### *3.3.1 Tyre*

Waste tyres are well known to have relevant disposal or reprocessing problems under environmental and economic sustainable conditions and their disposal is a challenge for industrial and academic research. Mechanical recycling to produce renewed tyres or granular tyres (employed in several fields such as athletics track, road paving, and so on) are followed but these processes use only a fraction of tyres to be disposed of. In this contest, pyrolysis represents a modern valid alternative to generate value-added products. Anyway improvements in the heat transfer technology are crucial to optimize the efficiency of the process itself. The use of MAP is one of the most promising heating technologies for their pyrolysis, due to MAP's ability to heat quickly and directly any MW absorbing material. Tyre contains a high amount of MW absorbing materials such as metal wires,

### *Mixed or Contaminated Waste Plastic Recycling through Microwave - Assisted Pyrolysis DOI: http://dx.doi.org/10.5772/intechopen.100179*

metal oxides, and carbon, and they quickly absorb MW and turn it into heat [12–19]. The process was performed in a short time comparing with traditional heating techniques [2]. Typical products were a char (yield 40.6-65.0 wt. %), a liquid (yield 20.7-44.0 wt. %) and a gas (yield 9.0-27.4 wt. %). MAP variables such as MW power, reaction time (15-100 min) and tyre mass, may strongly affect the properties of the products. The char was characterized through chemical (ultimate analysis and ion coupled plasma-mass spectroscopy (ICP-MS)), morphological (BET surface area, scanning electron microscopy), and X-ray diffraction (XRD) analyses. It contains a large amount of amorphous carbon (up to 92.0%) and inorganic compounds formed from additives employed in tyres formulation. XRD analyses of crystalline phases of char showed a marked MW effect: different crystalline ZnS forms, spharelite or wurtzite were identified. The presence of these compounds suggested that tyres were heated to a temperature higher than that one usually accounted. The liquid was a low viscosity oil (<2.9 cP, with a large amount of single-ring aromatic hydrocarbons) while the gas contains light hydrocarbons, hydrogen and only traces of N2. The three products collected had a high calorific value, respectively 34 MJ/kg for solid, 45 MJ/kg for liquid, and 46 MJ/kg for the gas fraction. The most performing conditions were achieved using an MW power of 3 kW per 0.2 kg of tyres even if these conditions were not optimized.

The char from MAP of waste tyres contains iron and carbon. The iron may be separated using an electromagnet and sold as armonic steel (iron) while the carbon can be directly reused to produce new tyres, or used in many other production processes: as a pigment for the production of plastics, or textile printing, for printer toners, etc. It was also evaluated for Oxygen Reduction Reaction (ORR) in electrocatalysis [40]. The presence of different metals together with high carbon in char may produce synergic catalytic effects which are necessary for ORR. The char obtained from microwave-assisted pyrolysis of waste tyres represents a low-cost and friendly source of metals need for the preparation of the cell cathode for ORR.

The liquid is a fuel oil with low sulfur content (< 1%), formed in about 35% of the tyre treated. It contains aliphatic and aromatic hydrocarbons and may be used as marine diesel fuel or sent to a refining treatment to obtain any kind of fuel.

The gas contains hydrogen and low molecular weight hydrocarbons and may be employed as fuel gas or directly used to produce the electricity required for the process [41].

## *3.3.2 Multilayer packaging beverage*

Multilayer packaging beverages are containers of composite material usually manufactured using five layers of materials: LDPE, Ink, Board, Aluminiun (Al), LDPE [42]. The most famous product is Tetrapack©. At the end of their life cycle multilayer packing beverages contaminated by the liquid present in the container, may be displaced through MAP using different MW absorbers (none, chopped tyre, carbon, and iron powder) and apparatus set-ups. Board was pyrolyzed forming water containing bio-oil where alcohols, aldehydes, acids, and anhydrosugars were present, according to pyrolysis conditions. LDPE was converted as reported in chapter 3.1.1, into a high viscosity liquid (wax), solid at room temperature, except when a fractionating system was directly connected to the pyrolysis oven. In these last conditions linear alkanes, alkenes, cyclic, and aromatic hydrocarbons were formed. Al was always recovered as unscratched samples.

### *3.3.3 Corn derived plastic bags*

Europe introduced measures on the management of waste packaging in 2005 through the revision of Directive 94/62/EC and the introduction of the norm EN13432:2000 where it was specified the characteristics of bio-materials to be employed for plastic bags. These directives were adopted in Italy in 2011 and, as a consequence, the polyethylene base shopping bags were forbidden and replaced by bio-degradable materials. Bio-plastics are materials obtained from a vegetable source such as corn and they are largely used, especially those derived from corn starch with the aim to reduce world pollution. Their production requires starch destructuring, complexation, blending with specific synthetic and/or natural polymers, and the addition of compatibilizers, plasticizers, and other additives [43, 44]. This great variability of processing methods brings different types and grades of bio-polymers with a wide range of properties which allows their use in various fields [45].

Corn-derived plastic bags (CDP) are biodegradable and they may be disposed of through anaerobic digestion, however their energy and chemical content, required for their production, will be largely lost, only biogas was sometimes recovered. Furthermore, the biodegradation process is too long with respect to the time required for the biodegradation of the biomass present inside the container, usually derived from a waste collection. For this reason CDPs must be initially separated and then separately bio-degraded. In a greener process, these wastes may be employed for energy production with the recovery of their thermal content, but the atom economy of the process is very poor because the chemical content of CDP is lost. MAP of CDP was performed to evaluate the possibility to efficiently dispose of this waste obtaining a liquid useful as a source of chemicals and/or fuels. MAP was performed using different MW power, MW absorbers (carbon or Fe), and apparatus obtaining a liquid, a char, and a gas in amount depending on the pyrolysis conditions. Liquids were always separated into three phases: upper, middle, and bottom and deeply characterized. They contained a large amount of aromatic acid, phthalates, and their derivatives in the upper fraction; water (70 wt. %), organic acids, alcohols, anhydrosugars, and their pyrolysis products were present in this fraction; bottom fraction showed close properties and composition than the upper fractions. The same classes of compounds were present in upper and bottom phases but in the last ones, these compounds were oligomers of those present in the upper fractions. Working with more drastic conditions bottom fraction was reduced while char yield was increased. MAP converted a waste CDP, even contaminated, obtaining some chemicals or even fuel thus avoiding to send them to anaerobic digestion for their transformation into carbon dioxide, methane, water, and a residue with a very low recovery of chemicals or energy. MAP process may represent a green solution to dispose of CDP recovering a liquid useful as a source of chemicals or employed to produce fuels. Furthermore, this process avoids the contamination of the soil improvement obtained by an anaerobic digestion with the residue of plasticizers present in CDP.

### *3.3.4 Waste electric and electronic equipment (WEEE)*

The collection of waste electric and electronic equipment (WEEE) is largely and rapidly increasing, and even if WEEE is recycled today there are still valuable residues left after recycling, ending up in landfills. MAP may be a valuable way to recycle these components. Six different fractions (from light dust to particles sized of 7-12 mm) were pyrolyzed, producing an oil, a gas, and a solid residue [46]. A mass reduction was observed as a function of process time, independently on the reaction temperature for all of the WEEE treated.

### *Mixed or Contaminated Waste Plastic Recycling through Microwave - Assisted Pyrolysis DOI: http://dx.doi.org/10.5772/intechopen.100179*

WEEE from the envelope of end-life computers was pyrolyzed using different absorbers and set-ups in a multimode batch reactor [47]. A large amount of the liquid fraction (yield 76.6 wt. %) was obtained together with a strong reduction of the solid residue (yield 14.2 wt. %). The liquid fraction was characterized using Fourier Transform-Infrared Spectroscopy-Attenuated Total Reflection (FT-IR ATR), Nuclear Magnetic Resonance Spectroscopy (1 H NMR), and quantitative GC–MS analysis. The liquid showed a low density, viscosity and contained a high concentration of useful chemicals such as styrene (up to 117.7 mg/mL), xylenes (up to 25.6 mg/mL of p-xylene) while halogenated compounds were absent or present in undetectable amounts.

Waste printed circuit boards (a fraction of WEEE) were also pyrolyzed using carbonaceous absorbers such as graphite or activated carbon [48]. Char was the main product (58 wt. %) and the presence of a high amount of absorber reduces the yield of tar and gas. Char contained phenolic compounds and phenylphosphates in significant amounts. CO2 was the major component in the gas fraction, while the concentration of H2 in the gas was increased when activated carbon was used as an absorber. The major metal-containing compounds present in char were Cu, Pb, Ti, while a lower amount of Bi, Fe, and Ca were present.

MAP of WEEE with bromine-containing compounds as flame retardant was also performed with special attention to the fate of these compounds. Conversion is increased with increasing temperature, reaching 93.3 wt. % at 650°C [49]. High pyrolysis temperature enhanced the transfer of bromine containing compounds to pyrolysis gas while the presence of K2CO3, Na2CO3 and NaOH mixed with the WEEE formed KBr and NaBr reducing the presence of HBr in the gas. Increasing heating time did not exhibit a remarkable influence on pyrolysis conversion: Working at 350°C, the main compounds in the liquid were phenols (91.1 %) while at 650°C, polycyclic and monocyclic aromatic hydrocarbons (except phenols) increased to 20.5 % and 19.0 %, respectively. Meanwhile, the non-condensable gases were composed of CO2, CO, CH4, and H2 and they improved significantly with increasing temperature. Working in the presence of ZSM-5 and kaolin as catalysts, the monocyclic aromatic hydrocarbons (except phenols) and C11-C20 compounds in the oil was increased while non-condensable gas was reduced.

A review of existing classical pyrolysis techniques for recycling of plastics in WEEE with special attention to the recovery of the products obtained (monomers, hydrocarbons, phenols, etc.) was reported [50]. Special attention was devoted to the processes to remove bromine containing compounds in the starting materials such as solvent extraction, supercritical fluid technology, and so on. Co-pyrolysis was also investigated and plastic wastes were displaced without the use of solvents or catalysts. The catalysts were employed to affect the distribution of the products and to enhance the removal of bromine containing compounds from pyrolysis oils.
