BACK IN PRISONSANTIAGO (Reuter) — Luis, Corvalan. leader of Chile’s banned Communist parly, was: sent back to prison Thursday; from a hospital where he un-; ' dcrwcnl surgery last week for acute appendicitis, official . sources said. Corvalan. 60, had 'recovered well from the opera-. !tilt;m and was returned to the, ’ maximum camp at Riioquc. 90 . miles north of here, the. ; sources added.it