A method is provided wherein the equilibrium HF concentration in the mixture of HF, sulfuric acid and water fed to an HF reaction furnace is greatly reduced by injecting a controlled amount of steam into an HF-removal process wherein the sulfuric acid mixture is also contacted with the hot HF gas leaving the HF reaction furnace. Reducing the HF concentration in this manner significantly improves furnace performance.The purpose of injecting steam is to adjust the water content of the mixture to the optimum value (about 12.5 weight percent) for HF removal, as well as to supply the heat required for HF removal without the need for an indirect heat exchanger.An additional advantage to this method is that since direct steam injection adds water to the process, in order to keep the furnace feed acid water concentration at the desirable level of less than 1%, oleum or SO.sub.3 must be added to react with the excess water. Since the reaction of water and SO.sub.3 is exothermic, this supplies additional heat to the process and no indirect heat exchanger is needed to achieve the desired high furnace feed acid temperature (>100.degree. C.).