Recovering Dead Soldering Tip That's Not Tinned
Soldering iron tips at their core are copper, but are plated in nickel and tinned.

If you sand/scrape the tip then you remove this plating and the internal copper becomes exposed and oxidises over time. Since the plating is already removed, you could keep constantly sanding back the oxidised layer, but you will be forever doing so and, like constantly sharpening a pencil, will eventually be left with nothing.
I recommend that you get a new tip for now and keep up good maintenance on it. The top answer in this question here has some really good maintenance tips.

If you are attached to your old soldering iron tip and have some spare time/feel like experimenting, you could always try to sand your old tip back to the copper layer and electroplate it yourself. I've found a video on youtube that shows how to do it yourself, normally I would say there's no point to taking so much time/cost/effort to plate it yourself but if you want to preserve it as a keepsake you could always try it.

Electroplate your own soldering iron tips - Youtube