MINERALS & METALLURGICAL PROCESSING Vol. 32 No. 2 • May 2015 69 Role of water chemistry in the selective flocculation and dispersion of iron ore H.J. Haselhuhn and S.K. Kawatra * Graduate researcher and professor, respectively, Department of Chemical Engineering, Michigan Technological University, Houghton, MI, USA * Corresponding author email: skkawatr@mtu.edu Abstract Fine-grained hematite ore can be concentrated by the process of selective flocculation and dispersion, which relies on proper reagent selection and control of water chemistry. While many previous studies have been performed analyzing the effects of different reagents on this process in a laboratory environment, this paper focuses on investigating the water chemistry within the process in a pilot-scale continuous deslime thickener. The pH, sodium concentration, calcium concentration and magnesium concentration were varied to determine their effects on the iron concentrate grade and recovery, and the phosphorus concentrate grade and rejection in the pilot- scale selective deslime thickener. The ideal pH for the iron grade and recovery of the process using a starch selective flocculant was found to be 10.5. Phosphorus rejection, however, was increased at lower pH values. Minimization of sodium concentration was shown to improve iron grade, iron recovery and phosphorus rejection. Calcium acted as a nonselective flocculant showing higher iron recovery, lower iron grade and lower phosphorus rejection with increasing concentration. Conclusions could not be drawn from the experiments that varied magnesium concentration. The zeta potential of the solid-liquid interface of particles in each sample taken was also analyzed to show relation- ships between zeta potential and process performance. In all cases, a maximization of the magnitude of zeta potential correlated with increased iron grade and recovery. This supports the hypothesis that a higher level of dispersion en- hances the selective flocculation and separation process. Paper number MMP-14-038. Original manuscript submitted April 2014. Revised manuscript accepted for publication September 2014. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to November 30, 2015. Copyright 2015, Society for Mining, Metallurgy & Exploration Inc. Introduction The United States produced 53.2 million metric tons out of the 3,000 million metric tons of concen- trated iron ore produced worldwide in 2012 (Tuck, 2013). With this in mind, there has been a large increase in iron ore processing research (Bolen, 2014; Carlson and Kawatra, 2008, 2013; Halt and Kawatra, 2014; Halt et al., 2014; Haselhuhn et al., 2012a,b, 2013a,b; Liu et al., 2014; Manouchehri, 2014; Sandvik and Larsen, 2014; Semberg et al., 2014). Of the 53.2 million metric tons produced, roughly 8 million metric tons of hematite (Fe 2 O 3 ) concentrate were produced using a process known as selective flocculation and dispersion (Cliffs Natural Resources, 2011). This process is the only economically viable method for concentrating low grade (< 40% Fe) fine grain (< 25µm liberation size) hematite ores. It is scarcely used due to difficulty in controlling the surface chemistry of the ore and high reagent costs (Department of Energy, 2001; Haselhuhn et al., 2013a). The process is highly dependent upon both reagent selection and precise control of the water chemistry during separation. This study shows the dependence of the process performance of a pilot-scale deslime thickener on the water chemistry in the feed slurry. The process used to concentrate hematite ore via selective flocculation and dispersion (Fig. 1) requires two concentration steps. The first and arguably most important step is a selec- tive thickening process known as selective flocculation and desliming (Siirak and Hancock, 1988). Selective flocculation and desliming is the focus of this research. During this process, a dispersed slurry of liberated ore is treated with a selective flocculant that acts by bridging hematite particles together. These flocs are settled in a thickening vessel while the dispersed gangue minerals leave the process through the overflow. This process requires three reagents to function effectively: caustic soda, cooked corn starch and sodium polyphosphate. Caustic soda (NaOH) is added during primary autogenous grinding as a pH modifier. The addition rate is adjusted to deliver a pH of between 10.5 and 11 at the feed of the deslime thickener. This pH is required to both ensure adequate dispersion and to facilitate maximum starch selectivity and adsorption (Green and Colombo, 1984; Haselhuhn et al., 2013a; Weissenborn, 1996). Key words: Iron ore, Water chemistry, Flocculation, Hematite, Slimes Minerals & Metallurgical Processing, 2015, Vol. 32, No. 2, pp. 69-77 An official publication of the Society for Mining, Metallurgy & Exploration Inc. SPECIAL FOCUS ON IRON ORE PROCESSING