Unencrypt an SD card from a .key file

First of all, I'd like to apologise in case I asked this in the wrong place. I didn't think that this question was advanced enough for the Cryptography site, and not security-related enough for the Security site. Please feel free to correct me/migrate this question if I'm wrong.

Anyway, I'm currently trying to unencrypt an SD card so that I may enlarge a partition on it to fill a load of empty space. The SD card is used as an adopted SD card in an android phone ruing CM13 Marshmallow, and as such is a 128-bit AES encrypted EXT4 drive. I've extracted a .key file from data/misc/vold on my phone, but I'm not really sure how to use it.

Basically, how would I unencrypt this drive, make the necessary changes, and then reencrypt it using this .key file? I've got a Kali/Mint/Windows triple-boot on my laptop, so methods for any of them would be great. Someone recommended this page to look at, but I don't know what to make of it.

Cheers,
Jack