I have 2 partitions on my pc. One contains an old Linux Mint 17 installation I believe, which I have used for a long time. The second partition contains a new install of Linux Mint 18. Over the course of a couple years, I have added to the LibreOffice 4 (then 5) spell checker under Mint 17 a lot of custom words. I assume that they exist in a dictionary file somewhere, but searching the internet has not provided me any information on where this information exists or how I might copy the required files from Mint 17 to Mint 18. I obviously still have the files, I just cannot determine where they are and how I can move them to Mint 18.
Thanks. I see the standard.dic file in Mint 17. In Mint 18, no wordbook folder or standard.dic file. I had searched for it under Mint 18, but never found it. I will add the directory and copy the standard.dic over and see what happens.
Yes, that worked well. Copied the wordbook folder and the standard.dic file it contained to Mint 18 and everything appears to be working fine.
Thanks again.
Please accept the answer it with 
(here on the left). That will help other people with the same question.
Continues to work in Linux Mint 20 and LibreOffice 7. Just recently upgraded both, copied over my old standard.dic file from Linux Mint 19.3.