A novel type of organic binder, modified humic acid (MHA), has been developed and patented in China. Humic acid (HA) is one of the main active fractions in the MHA binder. In this study, adsorption of HA onto natural magnetite, hematite and quartz surfaces as a function of initial HA concentration was comparatively investigated. It was found that the initial HA concentration has a positive effect on the adsorbance of HA onto magnetite and hematite surfaces but HA was hardly adsorbed by quartz particles even if the initial HA concentration was very high. When increasing the initial HA concentration, the zeta potential of magnetite/hematite particles became more negative because the negatively charged HA molecules entered into the electric double layer of the iron ore particles, while there was nearly no change in the zeta potential of quartz particles. There was less change in the zeta potential of magnetite compared to that of hematite. DRIFTS studies showed that there was a stronger chemisorption between the HA molecules and the iron ore surfaces under alkaline conditions. ESEM images further confirmed that magnetite particles coated by HA easily clustered to form agglomerates via chemisorption and physisorption, and the aggregates bonded together with HA by chemisorption were more resistant to breakage, which would be beneficial for the strength of green pellets. (C) 2013 Elsevier B.V. All rights reserved.