The Sukpo Zinc Refinery of Young Poong Cooperation (YPC) has recovered zinc and lead from zinc residue by using top submerged lance (TSL) technology since 2007. This recycling operation, named the "TSL Plant", was designed to operate 2 furnaces each equipped with one top submerged lance. The operation performance shows good metal recoveries. The plant is currently measuring oxygen pressure in its slag bath to respond to the varying slag bath conditions instantly. This paper describes the behaviors of lead and zinc acquired by determining the oxygen pressure in the slag bath and the results of measurements are compared with those of the thermodynamic calculations.