Abstract
Seawater treated with lime and sodium carbonate in different proportions to reduce magnesium and calcium contents is used in flocculation and sedimentation tests of artificial quartz and kaolin tailings. Solid complexes were separated from water by vacuum filtration, and factors such as lime/sodium carbonate ratio, kaolin content, flocculation time, and flocculant dose are evaluated. The growth of the aggregates was captured in situ by a focused beam reflectance measurement (FBRM) probe. Solid magnesium and calcium complexes are formed in raw seawater at pH 11, impairing the performance of flocculant polymers based on polyacrylamides. The results show that the settling rate improved when the treatment’s lime/sodium carbonate ratio increased. That is, when a greater removal of magnesium is prioritized over calcium. The amount of magnesium required to be removed depends on the mineralogy of the system: more clay will require more significant removal of magnesium. These results respond to the structural changes of the flocs, achieving that the more magnesium is removed, the greater the size and density of the aggregates. In contrast, calcium removal does not significantly influence flocculant performance. The study suggests the necessary conditions for each type of tailing to maximize water recovery, contributing to the effective closure of the water cycle in processes that use seawater with magnesium control.
Highlights
An important number of mineral deposits are located in arid or semi-arid regions, where any water consumption has significant economic and socio-environmental repercussions [1,2]
A common flocculant is a high molecular weight anionic polyacrylamide (A-PAM), which generates high sedimentation rates when applied at relatively low doses [8,9,10]
Seawater was treated with lime and sodium carbonate to precipitate magnesium, and filtered
Summary
An important number of mineral deposits are located in arid or semi-arid regions, where any water consumption has significant economic and socio-environmental repercussions [1,2]. It is typical for the copper minerals processing to operate at a pH higher than 10.5, with the aim of depressing non-valuable minerals such as pyrite, which tend to float due to their hydrophobic nature, contaminating the concentrate [19,20] This strategy cannot be implemented when using seawater since solid Ca/Mg complexes can arise, producing a buffering effect at a pH above 10 and a considerable reduction in the recovery of molybdenite [21,22]. Ramos et al [23] analyzed the flocculation of mine tailings using seawater in a wide range of pH in the pulp, and reported a severe detriment to the sedimentation rate once the pH of the slurry reaches the value at which solid precipitates (pH > 10.3) In their main results, Ramos et al found that magnesium causes the greatest detriment. Polymers 2021, 13, 4108 and sedimentation tests of synthetic quartz and kaolin tailings in terms of lime/sodium carbonate ratio, kaolin content, flocculation time, and flocculant dose
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