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  • [Intro Music]

  • I don't know about you, but I take recycling for granted.

  • You finish your drink, and you just toss the plastic bottle into a bin marked "Recycling".

  • Then, something happens to it, and you're told it's good for the environment,

  • and then you feel good because you're saving the planet

  • in your own small way by not throwing that bottle into the garbage.

  • But how does recycling really work?

  • What happens to that plastic, or that can,

  • or that piece of paper I toss into that little bin with the arrows on it?

  • Recycling is, basically, the process of collecting waste materials and

  • breaking them down into building blocks that can be turned into new products.

  • Since each material is made of different things, it needs to be broken down in its own way.

  • Paper, for instance, becomes wood fibres, but glass is just crushed into tiny pieces.

  • And, since 1973, scientists and engineers have been working on the best ways

  • to separate clean and processed recyclables at "Material Recovery Facilities".

  • They're also known as "MRFs", for short, which is a much more awesome and fun name that we will be using from now on.

  • There are, typically, two kinds of MRFs:

  • "Single Stream Recycling Plants" and "Dual Stream Recycling Plants".

  • Dual Stream Recycling means that the curb-side bins are split into two categories:

  • "Mixed Paper" and "Everything Else".

  • These two categories are kept separate in the truck,

  • dumped into two separate piles, and offloaded onto two separate conveyor belts.

  • Single Stream Recycling, on the other hand, means exactly that.

  • Everything is thrown into the same recycling bin

  • and sorted later by a combination of people and high-tech machines.

  • Less than half of all Material Recovery Facilities use this method,

  • but that number is growing.

  • So here's how it works: almost anything can be recycled,

  • but some materials, like computers, batteries and light bulbs are too complex,

  • too large, or contain too many toxins, to properly recycle at any given MRF.

  • If they show up in the recycling pile, they're either thrown away or

  • taken to different, specialised facilities.

  • Single Stream Recycling focuses on five different kinds of waste:

  • Paper, steel, glass, aluminium, and plastic.

  • As they make their way through the plant, each of these materials is separated from the mixture,

  • and is processed.

  • The paper and the cardboard come first,

  • thanks to a series of rubber, star-shaped wheels

  • called "Rotary Screen Separators".

  • With the help of blown air pushing them along,

  • the cardboard and paper ride the wheels at a 45° incline,

  • up to a higher conveyor belt,

  • while the more three-dimensional, heavier objects,

  • like containers and bottles, fall through the gaps in the wheels

  • and land back on the main conveyor belt.

  • That cardboard and paper is sorted by workers

  • who remove any remaining contaminants, or stray plastics,

  • and separated the materials into bins for

  • newspaper, mixed paper, and corrugated cardboard.

  • Paper is made out of two basic ingredients:

  • cellulose fibres from wood, and water.

  • So, to recycle it, you need to break it down to those two component parts and let it reform.

  • Plus, there are contaminants, like ink and dirt, that need to be filtered out.

  • So, first, the paper is compacted, baled, sent to a mill and placed in a hot water bath.

  • This bath quickly breaks the paper down into tiny strands of cellulose fibre,

  • creating a mushy substance called "pulp",

  • basically, wet, lumpy paper.

  • That pulp is still dirty though,

  • so it's forced through screens that filter out any remaining residues,

  • like glue or bits of plastic.

  • It's then sent to the "De-inker",

  • which is another bath containing air bubbles and soap-like chemicals called surfactants

  • that separate the paper from the ink.

  • Air bubbles carry the ink up to surface, while the pulp, which is heavier,

  • sinks to the bottom.

  • That pulp, now clean, can be formed into new paper products.

  • You might have even eaten from a recycled cereal box this morning.

  • Meanwhile, back at the MRF,

  • anything that didn't make it up the rotary separator

  • enters another sorting line.

  • What's left on the conveyor belt is moved through a "cross-belt magnet",

  • which is a high-powered magnet that attracts iron-containing metals,

  • mostly steel.

  • Aluminium gets left behind because aluminium isn't usually magnetic.

  • Steel isn't all that hard to recycle,

  • it's just crushed and baled and sent to foundries where it's melted down.

  • After that, it's ready to be mixed with new steel

  • and refashioned into cans, or cars, or beams, or anything else made of steel.

  • So, now all we have left at the MRF are glass, aluminium, and plastic.

  • They're separated using an "Air Classifier",

  • which is a fan that pushes lighter goods, like aluminium and plastic,

  • toward a higher conveyor belt,

  • while allowing glass, which is heavier, to fall down to a lower conveyor belt.

  • Then the glass takes a ride through a "Rolling Drum".

  • It's shattered into pieces

  • and filtered through sieve-like screens

  • that ensure that no glass piece is larger than 5cm wide.

  • Those glass pieces are then sorted by colour:

  • clear, brown, and green.

  • The different colours are important.

  • They make recycling tricky because they're permanent.

  • Glass is made of silica, plus a bunch of other ingredients, depending on the colour,

  • that's been melted by high heat and then rapidly cooled.

  • Around 60% of all the glass in the U.S. is clear,

  • it's the easiest glass to make because it's just silica, limestone and soda ash.

  • But, clear glass can sometimes cause the substance inside to degrade

  • due to light exposure from the sun.

  • Beer, for example, has a light-sensitive chemical called "isohumulone" inside of it

  • that breaks down into free radicals when struck by ultraviolet light.

  • The side effect of this reaction is a skunky smell and some carbon monoxide.

  • Generally, not what you're looking for in a beer.

  • That's why we have brown and green glass,

  • which act like sunglasses and protect the contents from UV rays.

  • But they also have ingredients in them to make them that colour,

  • like nickel, iron, and sulphur.

  • This means that the brown and green colours can't be removed from glass,

  • so the need to be recycled separately.

  • Once the glass is all separated, it's crushed into tiny pieces,

  • called "cullet".

  • These cullets melt at a lower temperature than new glass,

  • because the ingredients and have already been mixed and fused together when that glass was initially made.

  • Back at the MRF, we've only got aluminium and plastic left.

  • We are almost done.

  • Aluminium can be separated from the plastic using a machine called an "Eddy Current Separator".

  • It's a big drum with a spinning rotor that contains magnetic poles,

  • which creates a really strong, magnetic field called an "induction field".

  • Remember how I said that aluminium isn't magnetic?

  • Well, that is true, most of the time.

  • This induction field is so powerful

  • that it causes electrons in a conductor, like aluminium,

  • to create their own magnetic field.

  • The two fields repel each other and the aluminium is pushed away to a different conveyor belt,

  • while the plastic continues on.

  • Like steel, there isn't much to do when it comes to making the aluminium reusable.

  • It's shredded, washed, and turned into chips,

  • which are melted in a large furnace, and poured into moulds.

  • These moulds are shipped to manufacturers,

  • where they're melted again, and rolled out into thin sheets

  • that can be cut, and bent, and shaped into new products,

  • like cans, and licence plates, and aluminium foil.

  • So now all we have left is the plastic.

  • Kinda.

  • It turns out the plastics are made of one of six different kinds of chemicals,

  • which correspond to the numbers 1 through 6, that you'll see on the bottom

  • of that soda bottle, or yoghurt container.

  • Sometimes, you'll also see a "7",

  • but that just means it's made of any one of the less popular types.

  • Each kind of plastic has a different molecular structure,

  • which determines the physical properties of the plastic.

  • It also means that some plastics are a lot easier to recycle than others.

  • Plastic is made out of long, carbon chains.

  • Usually, the hydrocarbon molecules are extracted from fuels,

  • like crude oil, or natural gas,

  • then linked up, forming big, repeating structures called "polymers".

  • Take PET, for example,

  • which corresponds with code number 1.

  • It's made of a polymer called "polyethylene terephthalate",

  • which has rings of carbons separated by chains of carbons,

  • some of which have single and double bonds, to oxygen atoms.

  • Those rings and double bonds don't give PET much flexibility,

  • so the polymers tend to pack closely together, and are harder to melt.

  • Since PET will soften at temperatures more than a hundred degrees below that high melting point,

  • it's easy to reshape, without damaging its molecular structure.

  • That's pretty helpful when you're trying to recycle it.

  • Then there's code number 3, PVC,

  • which is made of a polymer called "polyvinyl chloride".

  • It's a strong and durable plastic

  • often used in piping or in bottles of shampoo.

  • It's also known as "the poison plastic"

  • because, when it's melted, it can release a corrosive and toxic gas called,

  • yeah, hydrochloric acid.

  • Most of the time, recycling PVC involves

  • grinding into a powder at a specialised plant,

  • and then mixing it with additives so that it can be reused.

  • But you definitely do not want it contaminating the rest of your plastic,

  • which is just one reason why all the different kinds of plastic are separated at the MRF.

  • More importantly, separating plastics by types preserves their special properties.

  • For example, PET is harder to shatter, while PVC is more resistant to harsh chemicals.

  • In most plants, the separation is done using human sorters

  • but, in newer plants, there are infrared sensors

  • that identify the plastics based on the spectrum of light they reflect.

  • Since each type of polymer reflects light differently,

  • the sensors can identify which plastic is which.

  • Once identified, little puffs of air blast those plastics

  • onto different conveyor belts.

  • Depending on the MRF's capabilities, plus other factors, like market demands,

  • some of these plastics are recycled, while others are thrown away.

  • Take "expanded polystyrene", for example,

  • which you know as "Styrofoam".

  • While it can technically be recycled, it's not particularly practical,

  • because a truck full of lightweight, air-filled Styrofoam

  • doesn't melt down into a lot of polystyrene.

  • But soda bottles, which are made of PET plastic, are recycled at almost all MRFs.

  • They can be melted down, mixed with new plastic,

  • and used to create things like

  • clothing, and carpet, or even to fill pillows.

  • So that soda bottle you just recycled?

  • Some day, you might end up wearing it.

  • Thanks for watching this SciShow Infusion,

  • which was brought to you by our patrons on Patreon,

  • who make SciShow possible for free for everybody.

  • If you want to help us keep making videos like this,

  • you can go to Patreon.com/SciShow,

  • and don't forget to go to YouTube.com/SciShow, and subscribe.

  • [Outro Music]

[Intro Music]

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B1 中級 美國腔

循環利用的方式 (How Recycling Works)

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    Victor 發佈於 2021 年 01 月 14 日
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