To extract keywords from your corpora, you will need a reference wordlist. The 100 million word BNC World Edition is an ideal reference corpus. You can download a word list based on the BNC World Edition here:
Users of WordSmith version 3.0: http://www.ling.lancs.ac.uk/coprlang/cbls/zipfiles/bnc.zip
Users of WordSmith version 4.0: http://www.lexically.net/downloads/version4/downloading BNC.htm
Hope you find it useful.
Users of WordSmith version 3.0: http://www.ling.lancs.ac.uk/coprlang/cbls/zipfiles/bnc.zip
Users of WordSmith version 4.0: http://www.lexically.net/downloads/version4/downloading BNC.htm
Hope you find it useful.