Flotation in Seawater
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A classification of flotation processes carried out in concentrated electrolyte solutions, e.g., seawater, is proposed using the most obvious features of these processes: low or high content of Mg2+ and Ca2+ ions, pulp ionic strength, and pH. The first distinguishable group is the processes carried out in NaCl/KCl solutions, about 0.5 M in the case of salt flotation of inherently hydrophobic minerals, and at concentrations about 10 times higher in the flotation of potash ores. In the flotation of sulfide ores, such as nickel or copper ores, with xanthate-like collectors, the xanthate collector is apparently not affected by pulp ionic strength and only adjustment of frother may be required. Content of Mg2+ and Ca2+ ions in seawater is the main difference between such systems and fresh water. The presence of these metallic ions can adversely affect flotation in the pH ranges over which these ions hydrolyse. The successful flotation of Cu-Mo ores typically requires depression of pyrite at high pH values achieved with the use of lime. However, in seawater, flotation of Cu-Mo ores requires removal of the hydrolysis products of the Mg2+ and Ca2+ ions or the use of a pyrite depressant that can be effective over the pH ranges that are much below the pH of hydrolysis. Mg2+ and Ca2+ ions also affect flotation of phosphate ores with fatty acids. In this case, the depression mechanism is not caused by precipitating magnesium hydroxides on the mineral surface but by precipitation of collector insoluble salts, and the same ions are responsible for depression in both cases. In the seawater flotation of Cu-Mo sulfide ores and phosphate ores, the practical solution involves either removal of Mg2+ and Ca2+ ions prior to the flotation or complexation with other reagents.
KeywordsSeawater Flotation Cu-Mo sulfide ores Phosphate ores Electrolyte concentration
According to Gleick , 97.5% of the total water resources on Earth is in the oceans and ice caps, and thus, only about 2.5% is fresh water. Fresh water is typically used by the mining industry, and this is a very important issue as it is generally believed that the use of fresh water in flotation critically determines the outcome of the process. It should also be noted that mining operations are often located in arid areas with very limited access to fresh water. The demand for and shortage of water in certain parts of the world (e.g., the Atacama Desert in Chile) makes the use of seawater by the mining industry the only sustainable solution.
Flotation in highly concentrated electrolyte solutions was studied more than 70 years ago by Klassen  who reported that hydrophobic, bituminous coals could float in salt solutions (e.g., 0.3–0.5 M NaCl) without addition of any other flotation agents. The process is referred to as salt flotation. Other inherently hyhrophobic minerals (e.g., talc, graphite, sulfur) were shown by Klassen and Mokrousov  to behave similarly.
2 Salt Flotation
In Klassen’s monograph , coal salt flotation was tested using various inorganic salts (Na2SO4, NaCl and NaNO3). While some differences between the tested salts (explained by different foaming properties of these solutions) were detected, all the tests indicated that coals that are hydrophobic float very well in concentrated electrolyte solutions. Such a flotation process in which the solid particles are inherently hydrophobic and do not have to be made hydrophobic by adsorbing collector provides a unique opportunity to study the effect of electrolyte concentration on flotation. This eliminates the effect of the electrolyte on the collector and frother and makes interpretation of the results much simpler.
To avoid the effect of foamability of electrolyte solutions, tests carried out 40 years ago by Fuerstenau et al.  at the University of California Berkeley were all conducted in 0.5 M NaCl using various coals from different mines.
Dispersion of gas into fine bubbles is a central component of the flotation process. In a conventional flotation process, the size of bubbles is determined by bubble coalescence, which can be entirely prevented by a frother [5, 6].
The solid particles are hydrophobic;
In the environment of high ionic strength, the energy barrier opposing attachment of the hydrophobic particles to bubbles is reduced making attachment possible;
At the same time, fine bubbles are generated under such conditions.
3 Effect of Electrolytes on Behavior of Flotation Reagents
The case of salt flotation is an example of a very “clean” system in which only the effect of electrolytes on surface properties of mineral particles, and on foaming, is to be studied. In all other cases in which collectors are also utilized, the effect of electrolyte concentration on properties of these agents has to be considered as well.
For the sake of discussion in this paper, flotation surfactants will be classified in three groups: (i) thio compounds (collectors for sulfides), (ii) non-thio ionizable surfactants (collectors), and (iii) weakly surface-active agents used as frothers.
Figure 7 summarizes the data for the three tested frothers in a general fashion and indicates that the surface properties of frother solutions are modified with increasing ionic strength of the solution. The plot allows for the determination of the intersection point termed “surface tension switched point” (STSP). In the area to the left side of this point, the solution properties are dominated by inorganic electrolyte, while to the right side, the properties are dominated by the frother. The STSP values were determined to be around 120 ppm for MIBC, 40 ppm for α-terpineol, and around 1.2 ppm for DF-250. This implies that the frothing properties of DF-250 are only affected by inorganic salts at extremely low concentrations of this frother in solution. This frother exhibits very powerful foaming properties in seawater.
4 Classification of Flotation in Concentrated Electrolyte Solutions
This paper is an attempt at classification of the flotation operations carried out in process waters that are concentrated electrolyte solutions. In the development of such a classification system, the first question that is to be considered is which parameters to select in classifying such processes. The classification of the flotation processes carried out in concentrated electrolyte solutions is based on the following observations: (i) in simple solutions (e.g., NaCl) inherently hydrophobic minerals float very well (salt flotation) and in NaCl-KCl saturated brine potash ores are floated. (These cases do not require any pH adjustment.) (ii) Flotation of sulfides with xanthate-like collectors in concentrated electrolyte solutions may require adjustment of frothers; (iii) Cu-Mo sulfide ores commonly contain pyrite and its depression is achieved with the use of lime. This increases pH values, and in seawater in the pH range of 9.5–10, magnesium hydrolysis products (magnesium hydroxy-complexes, magnesium hydroxide) start appearing and depress molybdenite flotation. This harmful effect can be alleviated either by removal of the Mg2+ ions , or by floating the ore at a pH value well below the magnesium ions hydrolysis range (this requires a different pyrite depressant). Alternatively, when lime is applied it might be possible to disperse the magnesium hydrolysis products from the molybdenite surface; (iv) Fatty acids used in the flotation of phosphate ores form insoluble salts with Ca2+ and Mg2+ ions. Improvement of flotation in seawater can be achieved either by precipitating Ca/Mg ions prior to the flotation or by complexation of these ions with dispersing agents (e.g., water glass, hexametaphosphate).
The graphical form of the classification of the flotation processes carried out in concentrated electrolyte solutions is shown in Fig. 9.
The zone on the left includes flotation pulps containing mostly NaCl and KCl salts, and natural pH ranges that are not specifically adjusted.
The zone on the right covers systems with considerable concentrations of Mg2+ and Ca2+ that may also require pH adjustment (this also includes seawater that will be treated here as 0.6 M NaCl solution with 1.3 g of Mg2+ and 0.4 g/L of Ca2+).
5 Effect of Electrolytes on Flotation
Short-chain alkali xanthates are highly soluble in water. Poling  reports the solubilities of sodium or potassium xanthates at 20 °C as varying from 8 M for ethyl xanthate to 2 M for hexyl xanthate. Due to their short hydrocarbon chains and to the high aqueous solubilities, these collectors do not adsorb at the air/liquid interface. That is, ethyl xanthate has a negligible effect on the surface tension of water . The most visible difference between commercially utilized xanthates and other flotation surfactants is that only short-chain xanthates are utilized by industry while the ionic collectors used in the flotation of a large variety of minerals have alkyl chains of at least C12. These differences result from the fact that xanthate ions interact with mineral surfaces by forming dixanthogen and heavy-metal xanthates, while for long-chain surfactants, the most likely mechanism of adsorption proceeds via hemi-micelle formation on the mineral surface . Thus, it is obvious that while concentration of NaCl/KCl does not affect adsorption of xanthates onto sulfides, it does affect very strongly micellization of long-chain surfactants and consequently their adsorption. At high electrolyte concentration, the Krafft point of the surfactant is also affected, as it is in potash ore flotation [19, 20].
The tests carried out with all three types of copper sulfide ores, processed in the KGHM plants, gave very similar results to those shown in Fig. 11. Lekki  concluded that NaCl concentration in the process water does not affect copper recovery in the flotation of copper sulfide ores. In the flotation with high content of salts in process water, frother dosage can be reduced; it was also found that at higher concentrations of α-terpineol and NaCl reduced foamability could affect Cu recovery.
The results reported by Quinn et al.  agree well with Lekki’s findings. While studying the flotation of Cu-Ni sulfide ore at the Raglan plant in Quebec carried out at 0.4 M NaCl, they concluded that the plant could be operated without a frother.
The effect of ionic strength on flotation of Cu-Ni sulfide ore (platinum bearing ore) was studied by Corin et al. . They concluded that increased ionic strength of the pulp resulted in increased froth stability and water recoveries and that the increase in ionic strength had no apparent effect on the recovery of sulfide minerals.
Of special interest to the proposed classification system described in this paper is that these three publications, which came from different countries and were conducted with different sulfide ores, provide similar conclusions. The studies conclude that the ionic strength of the pulp in the flotation of sulfide (Cu, Ni) ores with xanthate-like collectors does not affect recovery of the valuable metal, but that frothability of these systems is affected so that the dosage of frother has to be reduced. In the group of flotation processes of sulfide ores with xanthate-like collectors carried out in the plant waters with elevated salt concentration, it is important to note that these process waters do not contain considerable amounts of Mg2+ and Ca2+ ions and pulp pH is far from being close to the hydrolysis pH for these ions. With these limitations, the dominant factor in this group of processes is ionic strength of the pulp.
6 Flotation Processes in Plant Waters with Considerable Content of Mg2+ and Ca2+ Ions
In the diagram shown in Fig. 9, the diagonal line divides the whole figure into two areas. The zone on the left covers NaCl and KCl solutions at natural pH values that are not specifically adjusted. The zone on the right is applied to considerable concentrations of Mg2+ and Ca2+ ions and alkaline pH ranges.
Seawater conditions occupy a special position. Seawater will be treated here as a 0.6 M solution of NaCl that also contains about 1.3 g of Mg2+ per liter, and 0.4 g of Ca2+ per liter. For this content of magnesium in seawater (1.3 g/L, equivalent of 5.3 × 10−2 mol/L) and using the solubility product for magnesium hydroxide as Ksp = 1.2 × 10−11, the concentration of OH− ions at which magnesium hydroxide will start precipitating in seawater can be calculated as
6.1 Dispersion of the Colloidal Precipitates to Prevent Molybdenite Depression
6.2 Flotation without the Use of Lime
A highly alkaline pH achieved by lime addition is frequently used to depress pyrite in the flotation process. The depression of molybdenite caused by hydrolysis products of magnesium ions when the flotation process is carried out in seawater can be prevented by removal of Mg2+ and Ca2+ ions from seawater prior to flotation or by flotation of the ore at a pH that is much lower than the hydrolysis range of these ions. Lower pH values for flotation requires a new technology in which pyrite is depressed at natural pH values.
A patented process has been developed to replace lime and cyanide in the depression of pyrite and is called the AMBS (air-metabisulfite) flotation process . The key feature of this operation is an aeration process applied after regrinding of rougher concentrates along with recycled cleaning circuit streams followed by staged addition of sodium metabisulfite without the need for any pH adjustment. In this process, sodium metabisulfite, a reducing agent, is applied in a very unusual way after aeration. A most important aspect is that the process works well around neutral pH, which allows Mg2+ hydrolysis to be avoided.
6.3 Flotation with Fatty Acids
Solubility of calcium and magnesium salts of fatty acids is very low (the solubility product of magnesium oleate is about four orders of magnitude smaller than for magnesium hydroxide ) and thus “hard” water, that is, water with a high content of Ca2+ and Mg2+ is not a good medium for flotation with fatty acids as collectors.
Flotation of the phosphate ore at the Kapuskasing Phosphate Operation in Ontario was studied by Nanthakumar et al. . While the ore from their current operation was relatively easy to upgrade by flotation, the marginal ore, that is, the ore which was stockpiled at the site for about 2 years, turned out to be extremely difficult to process. In the flotation of the marginal ore, it was reported that the process water contained more than 1000 ppm Ca2+, and it was demonstrated that by using soda ash (1800 g/t) in the desliming circuit it was possible to drastically improve flotation recovery of P2O5 and concentrate grades.
The Atacama Desert in Chile is an obvious example of an arid area with a large concentration of mining industry. Use of seawater in flotation is believed to be a sustainable solution in such cases. Research studies have identified magnesium ion hydrolysis products as a main culprit responsible for depression of molybdenite flotation when the process is carried out in seawater. The presence of Mg2+ and Ca2+ ions in plant water is also responsible for poor flotation of phosphate ores. In general, problems in flotation separations appear when fatty acid collectors are applied in hard water, and seawater is just an example of extremely hard water. While the reasons for depression in these two cases are not identical, precipitating magnesium hydro-complexes and hydroxides depresses molybdenite by the “slime coating mechanism,” and in the flotation of apatite, depression results from the precipitation of the collector with Mg2+ and Ca2+ ions. Our attempt at analyzing these two systems, Cu-Mo sulfide ore flotation in seawater and phosphate ore flotation in seawater, is presented in this paper. The classification of such flotation processes allows for better understanding of the accompanying phenomena and better selection of the most promising process modifications.
This study received financial support from CRHIAM provided via CONICYT/FONDAP-15130015 project.
Compliance with Ethical Standards
Conflict of Interest
The authors declare that they have no conflict of interest.
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