Is there a mongodb-backed user authentication library for codeigniter?
Looks like https://github.com/benedmunds/CodeIgniter-Ion-Auth (Ion Auth) has now a MongoDB support. See the README section on GitHub.
I don't think any use it natively, but before you go converting things yourself you may want to check out this answer I made to a similar (non-dupe) question: oAuth and Codeigniter with MongoDB
It will get you started on a conversion with little work needed. I'd also recommend using ion auth, tank auth is huge and DX auth is outdated.