EE AND CO-WORKER WERE REMOVING SLIDE GATE ON NOVAC CRUDE TANK AT P-1 HOPPERS MILL. THE GATE WAS NOT ADEQUATELY SECURED AND FELL WHEN THE LAST BOLT WAS REMOVED, PINNING EE'S RT. HAND BETWEEN THE GATE AND HOPPER, CAUSING A SEVERE LACERATION IN THUMB, WHICH REQUIRED SUTURES.