Interaction of magnetite with soluble silicates and bentonite : implications for wet agglomeration of magnetite concentrate

Sammanfattning: Iron ore green pellets are produced by agglomeration of moist magnetite concentrates. The quality of green pellets is essential for the transportability and metallurgical benefits of the final product. The agglomeration behavior of magnetite concentrate particles is strongly influenced by its surface properties which are affected by the interactions with flotation reagents (i.e. water glass and collector) and species in process water. However, the mechanisms of these interactions and the influence on the following agglomeration process are still not completely understood. The present work has been focused on the interaction of magnetite with water glass (sodium silicate) and bentonite clay (silicate mineral) aiming for better fundamental knowledge of the magnetite surface properties in order to improve the agglomeration behavior of the magnetite concentrate. Water glass is used as a dispersing or depressing agent in flotation of magnetite. The former function is to improve the separation of mineral particles in the pulp, while the latter function is to protect the magnetite surfaces from attachment of the collector which is known to cause problems in the subsequent agglomeration process. Sodium activated bentonite clay is commonly used as an external binder in iron ore agglomeration owing to its swelling properties. The particle interaction between magnetite and bentonite platelets, which is expected to be affected by the surface properties of magnetite, is of importance for the wet and dry strength of the pellets. Sorption mechanisms of sodium silicate onto the magnetite surface at the molecular scale were studied at various pH and silicate concentrations using in-situ ATR-FTIR spectroscopy. Silicate concentration, pH, and conditioning time are the most important factors for silicate sorption and speciation at the surface of magnetite. A maximum sorption was observed in the pH range 8.5 - 9.5. Oligomeric or polymeric silicate species are formed and dominate at high surface loading of silicate. These oligomeric or polymerized species have stronger affinity for the magnetite as compared to monomeric species, resulting in a slower and less extent of desorption and implying a higher depressing efficiency in flotation. Sodium silicate makes the magnetite surface more negatively charged, while adsorbed calcium ions on the surface compensate this negative surface charge. Calcium ions promote the oligomerization of silicate on the surface of magnetite at high pH (pH > 10) possibly due to the increased local silicate concentration by additional sorption of silicate on adsorbed calcium at the surface. The competitive sorption between sodium silicate and collector (sodium oleate) for the magnetite surface was studied using the in situ ATR-FTIR technique. It was confirmed that oleate could still be adsorbed onto a sodium silicate modified magnetite surface but the amount of adsorbed oleate decreased with increasing concentration of silicate (0.1 - 5 mmol•L-1). This depression effect became much more significant when the concentration of sodium silicate was higher than 0.4 mmol•L-1 above which more dimers or oligomeric silicate species were formed at the magnetite surface. However, when the magnetite surface was pre-treated firstly with sodium oleate, the addition of sodium silicate only slightly reduced the adsorption of oleate. Meanwhile, the sorption of silicate anions was depressed and resulted in a lower degree of oligomerization or polymerization at high silicate dosage. Sessile drop method and Wilhelmy method were used to measure the water contact angle of synthesized magnetite, and Washburn method was used for contact angle measurements of magnetite concentrate. The synthesized magnetite has lower contact angle compared to mineral particles. Nevertheless, similar tendency of changes in wettability upon interaction with calcium, sodium silicate and collector was observed for synthesized magnetite and magnetite concentrate. Sorption of calcium and silicate increased the wettability of the surface, while adsorption of collector made the surface more hydrophobic. Further exposure of the collector modified magnetite surface to sodium silicate could restore the surface wettability. Therefore, an interesting implication for magnetite flotation and agglomeration is that the decreased hydrophilicity of magnetite by attachment of collector could be improved by further chemical conditioning with sodium silicate (water glass). The interaction of bentonite with magnetite was investigated by means of settling of bentonite platelets onto a layer of magnetite nano-particles. The magnetite layer was deposited on a horizontal ZnSe crystal to be able to examine the orientation of the platelets by polarized ATR-FTIR. The measured dichroic ratio of the bentonite platelets decreased with time in wet film and approached a minimum value in the dry bentonite film regardless of the pH of the bentonite suspension. This tendency is in good agreement with the results observed for settling on bare crystal, indicating a more ordered structure of platelets upon evaporation of water. The bentonite platelets in dry film were evidently tending to orient with their basal plane surfaces to the magnetite layer, whilst wet films adopted a much more disordered structure. Similarly, a rather disordered wet bentonite film was formed on the calcium and sodium silicate modified magnetite layer. These bentonite platelets became more ordered when the wet film was dried.