Very, very difficult in my opinion and not worth the effort. Can’t you get any other source of H3PO4? If you can't find it 85%, find the rust cleaners containing only diluted phosphoric acid, some with a little methanol added. See the thread: Post 512651 (https://www.thevespiary.org/talk/index.php?topic=10684.msg51265100#msg51265100)
(fnord: "isolating phosphoric acid", Newbee Forum).