Impact of seawater salts on the viscoelastic behavior of flocculated mineral suspensions

• We study effects of salts on the viscoelastic behavior of flocculated suspensions.
• Small maker ions promote structured water layers around sites of solid–flocculant.
• Small counterions and similar water structure on surfaces lead to strong networks.
• Consequences are sediments with high yield stress.
• Compliance depends on applied stress revealing a non-linear viscoelastic behavior.

The impact of several electrolytes present in seawater on the rheological properties of flocculated alumina suspensions was assessed by using creep testing at pH conditions both above and below the isoelectric point. Flocculated suspensions were obtained by adding a high molecular weight anionic polymer flocculant to a cylinder of alumina suspension and aggregating the suspension using the plunger method. The suspension was allowed to sediment and the supernatant liquid decanted prior to tests of the viscoelastic properties of the resulting sediment. Salts analyzed were MgCl2, CaCl2 and NaCl, known as water structure makers, and KI and KCl, known as water structure breakers. A high salt concentration of 0.5 M was used to approach the ionic strength of seawater. For both pH's studied, the sediments demonstrated viscoelastic non-linear behavior observed as a non-linear relationship between yield strain and applied stress. In the absence of flocculant, it has been observed that the viscosity and yield stress of a suspension increase in the presence of structure maker salts for the case of both silica and alumina suspensions. In the presence of flocculant, new results are found: when the solid and flocculant were of opposite charge, the shear yield stress of the alumina suspensions increased with the breaker character of the salts, however when the solid and flocculant were of similar charge, then the yield stress is independent of the character of the salts. Similar water structures on the interacting surfaces lead to strong particle networks. The work demonstrates the importance of salt-surface interaction to the rheology of suspensions in the presence of polymer flocculants, as is typical of many mineral processing operations world-wide.

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