Adsorption of surfactants and polymers on iron oxides : implications for flotation and agglomeration of iron ore

Sammanfattning: Iron ore pellets are an important refined product used as a raw material in the production of steel. In order to meet the requirements of the processes for iron production, the iron ore is upgraded in a number of steps including, among others, reverse flotation. Under certain circumstances the flotation collector may inadvertently adsorb on the iron ore particles increasing the hydrophobicity of the iron ore concentrate, which in turn has been shown to have an adverse effect on pellet strength. To minimize the influence of the collector on pellet properties, it is important to understand the mechanism of collector adsorption on iron oxides and how different factors may affect the extent of adsorption. In Papers I-III, the adsorption of a commercial anionic carboxylate collector Atrac 1563 and a number of model compounds on synthetic iron oxides was studied in-situ using attenuated total reflectance Fourier transforms infrared (ATR-FTIR) spectroscopy. The effect of surfactant concentration, pH, ionic strength, calcium ions and sodium silicate on surfactant adsorption was investigated. The adsorption mechanism of anionic surfactants on iron oxides at pH 8.5 in the absence and presence of other ions was elucidated. Whereas silicate species were shown to reduce surfactant adsorption, calcium ions were found to facilitate the adsorption and precipitation of the surfactant on magnetite even in the presence of sodium silicate. This implies that a high concentration of calcium in the process water could possibly enhance the contamination of the iron ore with the flotation collector. In Paper III, the effect of calcium, silicate and a carboxylate surfactant on the zeta-potential and wetting properties of magnetite was investigated. It was concluded that a high content of calcium ions in the process water could reduce the dispersing effect of silicate in flotation of apatite from magnetite. Whereas treatment with calcium chloride and sodium silicate made magnetite more hydrophilic, subsequent adsorption of the anionic surfactant increased the water contact angle of magnetite. The hydrophobic areas on the magnetite surface could result in incorporation of air bubbles inside the iron ore pellets produced by wet agglomeration, lowering pellet strength.Based on the adsorption studies, it was concluded that calcium ions could be detrimental for both flotation and agglomeration. Since water softening could result in further dissolution of calcium-containing minerals, an alternative method of handling surfactant coatings on magnetite surfaces was proposed in Paper IV. It was shown that the wettability of the magnetite surface after surfactant adsorption could be restored by modifying the surface with polyacrylate or sodium silicate. In Paper V, the results obtained using synthetic magnetite were verified for natural magnetite. It was illustrated that the conclusions made for the model system regarding the detrimental effect of calcium ions were applicable to the natural magnetite particles and commercial flotation reagents. It was confirmed that polyacrylate and soluble silicate could be successfully used to improve the wettability of the flotated magnetite concentrate. The fact that polyacrylate improved the wettability of magnetite more efficiently at the increased concentration of calcium ions indicates that this polymer is a good candidate for applications in hard water.Finally, it was concluded that in-situ ATR-FTIR spectroscopy in combination with zeta-potential and contact angle measurements could be successfully applied for studying surface phenomena related to mineral processing.