The prevention of anode passivation is one of the major technical hurdles to stabilize high current density operation in copper electrorefining. Its mechanism is considered that the dissolution rate of Cu2+ from the anode increases under high current density and the Cu2+ concentration near the anode reaches its saturated concentration, causing slime deposits on the anode surface. Impurities in the anode are considered for their contribution to the anode passivation phenomenon. Arsenic has a strong influence on passivation. A low arsenic content leads to an increase of the passivation risk. Dissolved arsenic as As3+ in the electrolyte is oxidized to As5+ by dissolved oxygen under the electrolyte temperature, which is considered to contribute to the decrease of the slime.