I don't know for everything, but the recycling company I used a few years back when I ditched about 1500 HDD's, a bunch of computers and other equipment said they did the following:-
1) Electronic equipment such as PC's were resold to schools in third world companies. He explained that much of the UK aid budget goes into vouchers that can be spent on stuff from UK companies, so they sell on semi decent computers to them with new hard drives and images.
2) HDD's he receives gets pulled from the machine and melted down. In our case we'd already done this for wiping & deguassing, but he put them through his volumentric disintergratior on site (provided free of charge because of the quantity of drives I was destroying) because we were equally as determined that our names would not be attached to any data escaping.
While converting the drives to scrap metal, we discussed his next step which was to put the HDD metal in a furnace. Apparently when a few thousand drives get melted it seperates out into layers of the different metals, so you just carefully remove layers of aluminium, steel, gold and small amounts of more valuable metals like palladium.