The fatty acid flotation of a difficult-to-float igneous phosphate ore (26.2% P(2)O(5), 15.4% Fe(2)O(3)) was investigated through batch flotation tests in the presence of soda ash. The poor flotation of this otherwise high quality ore was attributed to very high concentrations of calcium and sulfate ions (about 1000 and 2000 mg/L, respectively) found in process water, which indicated that the flotation pulp was saturated with respect to calcium sulfate. Despite the very high concentrations of calcium, the flotation results and pulp titration tests as a function of soda ash dosage showed that high concentrate grades (>36% P(2)O(5)) and phosphate recoveries (similar to 80%) could readily be achieved straight from such a high-calcium environment using pulp pH and conductivity as parameters for controlled addition of soda ash. especially during a continuous plant process. The main role of soda ash was to precipitate calcium and other interfering cations from the pulp thus allowing the tall oil collector to interact with the phosphate components. A comparative study of corn starch and guar gum as iron depressants for two phosphate ores varying in mineralogy of iron minerals (high-siderite and low-siderite,and various oxides) showed that guar gum was a more effective iron depressant than starch in terms Of P(2)O(5) grades of the concentrates. However longer flotation times were necessary in the presence of higher doses of guar gum in order to achieve high P(2)O(5) recoveries. The depressant performance of guar gum was superior to that of starch especially for the high-siderite ore. (C) 2009 Elsevier B.V. All rights reserved.