Quarry tile is a hard tile and would not normally absorb grout; it should be on the surface. It is so thick that you can sand it? Sanding the tile itself will ruin the surface, so that is not the thing to do.

Try chipping it off with a razor scraper, or if it is really thick, use a masonry chisel and hammer, cautiously. Once you get it down to just a thin layer or haze, use an acid to etch it off. Normally a tile pro would use phosphoric acid or sulfamic acid; you can get these in the tile dept of a home center. If they don’t work then use muratic acid; be very careful with that since it is potent and dangerous. Any acid will etch the grout itself, of course.