To attain reduction in equipment cost and operating cost, inhibition of plugging or clogging of pipelines nd equipment/instruments due to the iron component of a spent hydrochloric acid to be treated, therefore inhibition of reduction in heat transfer coefficient, etc., improvement in efficiency of hydrochloric acid recovery and advanced reduction in volume of the concentrated residue, in a spent hydrochloric acid treatment process.

This treatment process using an evaporator comprises: (1) a stage for introducing a spent hydrochloric acid stock solution to be treated into the evaporator; (2) a stage for heating and evaporating the stock solution in the evaporator to generate steam; (3) a stage for cooling the generated steam and recovering a hydrochloric acid solution; (4) a stage for recovering condensed liquid from the used steam; (5) a stage for controlling the iron concentration of a liquid concentrate in the evaporator by adjusting the temperature and/or the ratio of the amount of liquid to be evaporated to the amount of the spent hydrochloric acid stock solution introduced; (6) a stage for withdrawing the liquid concentrate from the evaporator bottom correspondingly to the liquid level height in the evaporator and transferring the liquid concentrate to a liquid concentrate storage tank; and (7) a stage for heating the liquid concentrate withdrawn from the evaporator bottom to the outside to generate steam, cooling and recovering this generated steam together with the generated steam described in the stage (3) and also recovering iron chloride.