>I don't understand why it's so hard ... we have machines that separate tiny seeds Not hard, just expensive. And understanding how seeds are separated (not onl

>I don't understand why it's so hard ... we have machines that separate tiny seeds Not hard, just expensive. And understanding how seeds are separated (not only that they are separated) makes it clear why plastics require so many more steps. For seeds, it's a two-step process. First comes threshing[1], which is the act of physically breaking the seeds off the plant. Basically they solved this problem by hitting the stalks against a solid surface. This works because all the plants are the same type and age, so you can dial in a force that will break the seeds _off_ without breaking them _apart_. This won't work for plastics, because they're combined with all sorts of other materials that whacking won't dislodge. One shot of that "Plastic China" documentary[2] shows a worker using a box cutter to remove a plastic label that's shrink-wrapped around (not glued to) a crushed container. The second step is called winnowing[3], and it's basically just blowing air through the falling seeds to separate them aerodynamically from the straw and chaff. This works because seeds have a higher ballistic coefficient (mass/drag). Winnowing is also done with screens or slats, relying on the fact that seeds are of uniform size. But plastics have all different shapes and sizes. Wind winnowing can certainly be used as part of a processing chain (eg to separate films from bulk plastics), and so can screens, but they're nowhere near a complete solution. Because of these complexities, automatically processing plastic at scale requires a lot more than just those two steps (which nowadays are combined together with reaping in the cleverly-named "combine harvester" or combine[4]). That means it's expensive, and that means someone in China with cheap labor and "externalized" pollution costs (read: make your neighbors get sick and die, but don't even have the decency to pay their doctor bills or keep their windowed spouses/orphaned children off the street) can outbid you. [1] https://en.wikipedia.org/wiki/Threshing https://en.wikipedia.org/wiki/Threshing [2] https://news.ycombinator.com/item?id=17678513 https://news.ycombinator.com/item?id=17678513 [3] https://en.wikipedia.org/wiki/Winnowing https://en.wikipedia.org/wiki/Winnowing [4] https://en.wikipedia.org/wiki/Combine_harvester https://en.wikipedia.org/wiki/Combine_harvester