There may be a better way, but here's a first thought:
0. Back up your library.
1. Import everything into a new library.
2. In a Panes View, add the column "Bit Depth" (Panes is a good way to see what is going on).
3. Select for all files except 24-bit.
4. Delete all those selections from the library (but not from the disc!), leaving only your 24-bit files in the new library.
Going forward, you would have to manually maintain the library to only import new 24-bit files. Not great.
You could of course copy or move all your 24-bit album folders to a single directory and just point your new library there. Tools > Library Tools > Rename, Move & Copy Files... might help with this.
In general, I think most people would advise against creating a new library. Instead, the "MC way" is to create a filtered view of your existing library. Use Panes, or enter [bit depth]=24 in Search Bar for any view. You can also modify any view to show only 24-bit files:
Hover mouse over tab or RMB view in the tree, Customize View > ... > Settings > Included Files > Set rules for file display... > Add new rule > Bit Depth is 24.