Abstract: With the advancement of mineral processing technology, the diameter of tailings is getting smaller and smaller, even reaching the ultra−fine level. In order to realize the high concentration filling of ultrafine tailings in the concentrator, the flocculation settling and thickening of ultrafine tailings is the critical technology. Therefore, the flocculation of certain ultrafine iron tailings was studied with ferric chloride produced from iron and steel hydrochloric acid pickling waste liquor as a flocculant. The effects of ferric chloride dosage, stirring speed, and stirring time on ultrafine iron tailings' flocculation were investigated using an industrial CCD camera and image processing software (Image−Pro Plus). The experimental results show that when the amount of FeCl₃ was 2700 g / t, the speed of the magnetic stirrer was 800 r / min, the stirring time was 80 s, the particle size of the floc was 39.79 μm, and the fractal dimension was 1.92. Based on the Box−Behnken principle, response surface methodology was used to establish the multiple regression equation of the interaction among the amount of flocculant FeCl₃, stirring speed, and stirring time on the flocculation of ultra−fine tailings, and ANOVA analyzed the experimental results. The results show that the optimum conditions were 2763.74 g/t of ferric chloride, 832.76 r/min of stirring speed, 95.89 s of stirring time, and 40.28 μm of floc particle size predicted by the model, and 1.92 of fractal dimension, which was consistent with the experimental results. The effect law of superfine tailings is obtained by studying flocculation conditions, which provides theoretical support for superfine tailings' high−efficiency flocculation production practice. This study also realized the comprehensive utilization of waste acid, which is conducive to energy saving and emission reduction and reduces the cost of industrial production.