The interesting materials you see on I-94

I was driving home several days ago. I saw this semi with a trailer with a large metal cylinder. My first thought was, oh no, someone's got their own nuclear reactor!

As I passed it, I noted it was clearly labeled "Molten Aluminum". Why would you want to transport a molten metal on a highway? I know some dangerous chemicals are routinely transported over our roads, but can you imagine a liquid metal spill?