Sulfuric acid (H2SO4) is widely used as a depressant in the flotation of phosphorus minerals, usually in large dosages. This study used microflotation, flotation of the actual ore, X-ray photoelectron spectroscopy (XPS) analyses, reaction thermodynamic analyses and adsorption experiments, to examine the effect of citric acid (H(3)Cit) and a combined depressant (H2SO4/H(3)Cit) on collophanite ore flotation. H2SO4 and H(3)Cit both exhibit selective depressive effects on apatite when they are used individually as depressants. When combined, the added H(3)Cit greatly reduced H2SO4 dosages above 30% and produced better flotation results. Closed-circuit reverse flotation results for tests on the actual ore also confirmed this. Mechanism analyses demonstrated that H(3)Cit adsorbs onto apatite surface via a chemical chelating reaction with Ca2+center dot H(3)Cit is selectively and largely adsorbed onto apatite, thus selectively depressing it. Compared with SO42- the reactions of Cit(3-) and Ca2+ are thermodynamically favored at pH 5.5-13.0. Therefore H(3)Cit manifested a stronger inhibitive effect than H2SO4 and significantly reduced the required H2SO4 dosage. A rough economic evaluation suggests that the combined depressant exhibits economic advantages and shows potential for industrial application.