Froth Flotation in Saline Water†

Froth Flotation in Saline Water† S. Castro* Department of Metallurgical Engineering, University of Concepcion1 J. S. Laskowski Department on Mining Engineering, University of British Columbia2 Abstract The use of seawater in mining/metallurgical operations seems to be the only sustainable solution in many zones with limited resources of fresh water. This requires new flotation technologies for processes which are to be carried out in highly concentrated electrolyte solutions. This paper reviews fundamental aspects of flotation in aqueous solutions with high concentration of inorganic electrolytes. Salt flotation, the process of flotation of inherently hydrophobic solids in concentrated electrolyte solutions, is especially suitable for theoretical analysis since no other organic agents are used in it. Starting from this example, the case of flotation of sulfide ores (chalcocite, chalcopyrite, pyrite and molybdenite) is discussed. The flotation of Cu-Mo sulfide ores requires the use of flotation agents, which are different for the inherently hydrophobic molybdenite and hydrophilic copper sulfides. The process is commonly carried out in alkaline pH adjusted with lime to depress pyrite, but in seawater depressing effect of Ca ions on molybdenite flotation is augmented, and different pyrite depressants are needed. Keywords: Froth flotation, Salt flotation, Cu sulfides flotation; saline water; seawater flotation Closed water circuits in flotation plants result in a 1. Introduction high electrolyte concentration in the process water. Water is a medium in which flotation takes place Hence, the question arises how the ionic strength and flotation efficiency is highly dependent on wa- of the process water affects flotation. Many differ- ter quality. In general, water is becoming a scarce ent chemical additives (e.g. collectors which may resource for mineral processing plants, and in arid be weak or strong electrolytes, either low molecular regions the need of saving freshwater for communi- weight polymers used as dispersants or high mo- ties is imperative. Rivers and groundwater are being lecular weight polymers used as flocculants, etc.) increasingly depleted at an alarming rate in many dry are utilized in flotation processes. The properties of places. Hence, the use of water with a high concentra- aqueous solutions of some of these compounds are tion of inorganic electrolytes in flotation plants is be- strongly affected by ionic strength. At the same time ing increasingly important. The use of seawater could ionic strength affects directly particle-particle (co- be a sustainable solution for many dry zones located agulation/flocculation) and particle-bubble (flotation) close to sea. The oceans represent the earth’s major interactions. The simplest flotation system in which water reservoir. About 96.5-97% of the earth’s water is only inorganic compounds, for instance NaCl, are uti- seawater, while another 1.7%-2% is locked in icecaps lized as flotation agent is so-called salt flotation. and glaciers. Fresh water accounts for only around The aim of this paper is to review fundamental 0.5%-0.8% of the earth’s total water supply1). aspects of flotation in aqueous solutions with substan- tial concentration of inorganic salts, and to discuss † Accepted: July 8th, 2011 available information on the use of seawater in com- 1 Concepción, Chile mercial flotation operations. We limit the scope of 2 Vancouver, B.C., Canada this paper to the range of electrolyte concentrations * Corresponding author Ｅ-mail: [email protected] comparable with concentration of seawater that is TEL: (+56) 41-2204956 FAX: (+56) 41-2243418 to the range up to 1 M NaCl. This eliminates from ⓒ 2011 Hosokawa Powder Technology Foundation 4 KONA Powder and Particle Journal No.29 (2011) our discussion the case of potash ore flotation, the results cannot be ascribed to a changing coalescence flotation process which is carried out in saturated of bubbles and is clearly a function of hydrophobicity NaCl-KCl brine (at 20 ℃，1,450 kg of the NaCl-KCl of the floated particles. But since small inorganic ions saturated aqueous solution contains about 0.300 kg cannot change solid wettability these results show of NaCl, 0.150 kg of KCl and 1 kg of water2); thus, the what could be expected, namely that only very hydro- saturated brine is about 6-7 mole/L solution of NaCl phobic particles can float under such conditions. and KCl). In order to study these effects further, a model was needed for which electrical charge and hydrophobic- 2. Salt Flotation Process ity could be independently maintained, and meth- 2.1 Flotation of inherently hydrophobic miner- ylated silica was used as a model of hydrophobic als in salty water surface6,7). Surface properties of this model system Klassen and Mokrousov3) in their monograph on are characterized in Fig. 2 6). The surface of silica fundamentals of flotation dedicated one chapter to the is completely hydrophilic but it can be made hydro- phenomenon of “salt flotation”; the term coined to de- phobic by reaction with trimethyl chlorosilane. The scribe the flotation of inherently hydrophobic miner- hydrophobicity depends on the number of surface als in concentrated electrolyte solutions without any hydroxyls that actually reacts with silane. Since quite organic agents. As demonstrated by Klassen4), this a large number of the surface hydroxyls do not react process may be quite efficient if the floated mineral with silane, the zeta potential values for both methyl- is highly hydrophobic; very hydrophobic bituminous ated hydrophobic silica and hydrophilic silica – as coals were shown to float in 0.3-0.5 M NaCl solutions demonstrated by the bottom (b) part of Fig. 2 - are quite well, while less hydrophobic low rank coals did the same. not. Fig. 1 taken from the publication that appeared Fig. 3 shows the results of the flotation tests in in 19835) confirms such a relationship quite clearly. which methylated quartz particles were floated in Fig. 1 shows the flotation rate constants obtained aqueous solutions of KCl at a constant pH of 6.1-6.5 8). from batch flotation tests in which coals varying in Flotation rate does not only depend on hydrophobic- rank were floated in 0.5 MNaCl. Moisture content in ity of the particles but also - since these particles coal is a function of its rank; it is very low for very hydrophobic bituminous coals, and is much higher for lower-rank coals which are much more hydrophilic. Since all these experiments were carried out at the same electrolyte concentration (0.5 MNaCl) these (a) Fig. 2 Effect of pH and pre-treatment on contact angle of methylated silica. (A) Silica coated in 0.04 M trimethyl chlorosilane solution; (B) Silica coated in 0.001 M solution; (C) Silica heated at 450℃ for 20 hrs before coating in 0.001 M solution. Fig. 1 Maximum flotation rate constants (salt flotation Bottom part: (o) Methylated hydrophobic silica; in 0.5M NaCl) versus moisture content for U.S. (+) Pure hydrophilic silica [after Laskowski and western coals (after Fuertenau et al., 1983)5). Kitchener (1969)6)]. (b) KONA Powder and Particle Journal No.29 (2011) 5 carry electrical charge - the particle-to-bubble attach- enon. ment which depends on the energy barrier opposing the attachment (equivalent of activation energy in 2.2 Effect of electrolytes on bubble coalescence chemical reactions)8,9). The particles used in these ex- Flotation requires small bubbles and the flotation periments were hydrophobic (θ = 53 deg.), however rate constant is proportional to the bubble surface the tests were carried out over the pH range (6.1- area flux, Sb; (Sb depends not only on the amount of 6.5) where the zeta potential of the methylated quartz air pumped into a cell, it increases with decreasing particles is in the range of -35 – -40 mV. As seen from the size of bubbles). Dispersion of gas into bubbles Fig. 3, the rate of the flotation process carried out is the heart of the flotation process. In conventional under such conditions clearly depends on electrolyte flotation process the size of bubbles is determined by concentration and the correlation of the flotation rate bubble coalescence which can be entirely prevented and the energy barrier is quite good (Fig. 4)8). These by a frother10,11). findings explain very well the salt flotation phenom- Frothers are best characterized by their critical co- 100 80 60 40 Recovery, % Recovery, KCl, 10-3 M 20 KCl, 10-2 M KCl, 10-1 M KCl, 10º M 0 0 50 100 150 200 250 Time, sec Fig. 3 The effect of KCl concentration on flotation of the methylated quartz particles (θ=53°) at pH 6.1 to 6.5 (after Laskowski et al., 1991)8). 0.1 30 25 2 1 - Rate constant Energy barrier 20 15 0.01 10 5 Rate constant (k), sec 0 Energy barrier, erg/cm 0.001 0.0001 0.001 0.01 0.1 1 KCl concentration, mole/l Fig. 4 The effect of KCl concentration on the flotation rate constant and the energy barrier; θ=53° at pH 6.1 to 6.5 (after Laskowski et al., 1991)8). 6 KONA Powder and Particle Journal No.29 (2011) alescence concentration (Cho and Laskowski10,11). As cess requirements: Fig. 5 shows, the critical coalescence concentration of MIBC in water is about 10 p.p.m. At the concen- (i) In the environment of high ionic strength, the trations higher then that the bubbles generated in energy barrier opposing attachment of the MIBC solutions are stable and do not coalesce. Bub- hydrophobic particles to bubbles is reduced ble coalescence can also be prevented by increasing making attachment possible; electrolyte concentration. As Fig. 5 shows, in con- (ii) At the same time, fine bubbles are generated centrated electrolyte systems the bubbles are stable under such conditions.

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