Frothing Properties of Amine/Frother Combinations
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Methods to determine frother functions, control of bubble size and froth properties, are now widely used. Some collectors also exhibit frother functions which are less understood. Using a water-air system in a mini-mechanical flotation cell setup, this paper determines bubble size and water overflow rate for three amine collectors: one primary amine (dodecylamine, DDA) and two commercial ether amines (Flotigam® EDA and Flotigam® 2835-2L) and their combination with two common frothers, MIBC (methyl isobutyl carbinol) and PPG 425 (polypropylene glycol, molecule weight 425). Compared to the frothers, the amines were less effective in reducing bubble size, giving larger minimum size and the two commercial amines showed evidence of coalescence at low concentration. In blends, at fixed frother dosage, frother eliminated the coalescence but as amine concentration increased the amines dominated bubble size. Water overflow was a strong function of reagent type. For fixed 1-cm froth depth, PPG 425, Flotigam® 2835-2L and to a lesser extent DDA produced overflow while MIBC and Flotigam® EDA did not. In blends with frother overflow increased except with Flotigam® EDA. Mechanisms are briefly explored. The principal benefit of blending identified is the elimination of coalescence if residual concentration of the two commercial amines is below ca. 10 ppm.
KeywordsBubble size Water overflow rate Conventional frothers Amines Frother/amine systems
Flotation is the process of collecting hydrophobic particles on bubbles to separate from hydrophilic particles. Introduced in the early 1900s, flotation permitted the mining of previously uneconomic low-grade and complex ore bodies. Collectors and frothers are the two principal reagents used in mineral flotation: collectors are used to render selected minerals hydrophobic, and frothers to aid production of fine bubbles and stability of froth .
Evaluating the functions of frothers though measures of gas dispersion (such as bubble size and gas holdup), and froth stability (such as froth height and water recovery) are now common [2, 3]. Some collectors also exhibit frother functions, but such studies are comparatively few. Kerosene, diesel oil, and most sulfide mineral (sulfhydryl) collectors (e.g., xanthate and dithiophosphate) show limited frother functions [4, 5]. Non-sulfide mineral collectors appear more active, for example, oleate  and dodecylamine . Studies of frother-collector combinations reveal both enhanced (synergistic) and reduced frother functions. Espinosa-Gomez et al.  showed fatty acid collector (mainly oleic) caused bubble coalescence and froth collapse using TEB (tri-ethoxy-butane) frother. El-Shall et al.  found a synergistic effect with the combination fatty acid/kerosene collector and F507 (polyglycol) frother. Ravichandran et al.  found weak synergy between kerosene and MIBC/ethyl alcohol frother. Zhou et al.  reported sulfhydryl collectors blended with MIBC and F150 (polypropylene glycol) had little impact on bubble size but invariably reduced froth stability (in two phase air-water systems), some combinations showing marked time effects. Corona-Arroya et al.  found synergy in bubble size reduction with dodecylamine and both MIBC and F507 frothers.
After fatty acids, amines are the most widely used collectors for non-sulphides . A large-scale application is reverse flotation of silica in processing iron ores, where ether amines are commonly used acting as both collector and frother [11, 12]. Evaluating frother functions of amines is therefore relevant to practice and including combination with conventional frothers addresses whether relying on one reagent to deliver both collector and frother functions compromises the efficacy of either . Corona-Arroya et al.  used a small-scale Jameson Cell setup to measure bubble size and gas holdup to characterize dodecylamine (DDA) and the frothers MIBC and F507 (polyglycol) alone and blended with DDA. The purpose of this paper is to determine bubble size reduction and water overflow rate as measures of frother functions for a primary amine, dodecylamine (DDA), and two commercial ether amines, Flotigam® EDA and Flotigam® 2835-2L, alone and blended with two common frothers, MIBC and PPG 425 (polyglycol). A focusing question is whether blends confer any benefit.
Solutions were prepared using Montréal tap water at natural pH (pH 7.3–7.6) and room temperature. The commercial amines were readily soluble up to the maximum concentration tested (80 ppm). The DDA was first dissolved in an acetic acid solution at 1:1 M ratio agitated by magnetic stirrer set at 900 rpm in a 1-L beaker. At natural pH the amines are mostly present in ionic form (pKa ca. 10.6 for a range of amines  and which appears applicable to the ether amines ). Mixed solutions (blends) were prepared using a fixed dosage of frother and variable concentration of amine. About 50 L of solution was required for each test.
The solution was divided between the two tanks, distinguished as “conditioning” and “holding” tanks, both agitated by mechanical stirrers at 500 rpm. The test commenced with flow Regulator-1 open and Regulator-2 closed with solution pumped between the two tanks for 30 min to ensure homogeneity. Due to MIBC’s volatility , it was added only for the final 10 min. Regulator-1 was then closed and Regulator-2 opened to feed the cell. For all tests, solution level (solution/froth interface) was held 1 cm below the overflow lip, feed rate was 2.3 L/min (i.e., cell retention time ca. 2 min), air flow rate 4.2 L/min, and impeller speed 1250 rpm. At the solution level position gas superficial velocity (Jg) is 0.7 cm/s, in the range of industrial flotation cell operation . Overflow and underflow were pumped back to the holding tank to provide a closed continuous system. In the case of Flotigam® 2835-2L the froth was persistent, and a cap was used to restrain froth flow to the launder. After 5 min, the system was at steady state (solution level was steady), and measurements were started. At completion of an experiment, the pump was turned off, the circuit drained and cleaned thoroughly.
Bubble size, reported as Sauter mean diameter (D32), was determined using the McGill Bubble Size Analyzer (MBSA) (Fig. 1), as described in detail elsewhere . The sampling tube was 2.54 cm internal diameter and placed approximately 10 cm below the solution/froth interface. The MBSA was filled with the same solution under test. Typically, for each condition 400 images were taken with a digital CCD camera and processed using Empix Northern Eclipse v6.0 software.
Water Overflow Rate
Executed five times, at the end of the test overflow was collected for 1 min and weighed. The mass was converted to volume assuming solution density of 1 g/cm3 and divided by the cell cross sectional area at the solution/froth interface level to derive the water superficial overflow rate, Jwo (cm/s).
3.1 Bubble Size
3.1.1 Reagents Alone
3.1.2 Amine/Frother Blends
3.2 Water Overflow Rate
3.2.1 Reagents Alone
3.2.2 Amine/Frother Blends
4.1 Bubble Size—Concentration Relationship
For the two frothers, the D32-C trend follows the expected rapid decrease from the zero-concentration bubble size with CCC values concordant with published data. In contrast, the two commercial amines at low dosage (5 ppm) gave bubbles significantly larger than in water alone (Fig. 3), a finding indicating coalescence. Bubble coalescence is known to occur in the presence of oil droplets [23, 24] and these might be present as undissociated amine or impurities in the commercial amines. Whatever the source, increasing amine concentration offsets the coalescence and bubble size decreases below the water-only value. The fact that the minimum bubble size with the amines is about twice that given by the frothers again may reflect coalescence, or possibly an impact on the creation size .
The D32-C relationship for MIBC agrees with that determined by Corona-Arroyo et al.  in a quite different setup (a small-scale Jameson Cell), reporting minimum bubble size ca. 0.7 mm and CCC 8.1 ppm. Where results differ from Corona-Arroyo et al. is the trend for DDA. They report DDA gave higher rate of bubble-size reduction with concentration (on a ppm basis) than MIBC and reached the same minimum bubble size as MIBC whereas we find DDA is less effective in bubble size reduction. The reason for the difference is not clear but the current results are supported by those for the two commercial amines, all three amines giving a significantly larger minimum bubble size than MIBC.
Corona-Arroyo et al. also evaluated frother/DDA blends, finding the presence of a fixed amount of DDA (1, 2 ppm) intensified the effect of frother, lowering bubble size at frother concentrations below the frother CCC. Direct comparison is not possible as we chose fixed frother concentrations, close to the CCC, and varied the amine dosage. This choice was based on two arguments: one, CCC represents maximum bubble size reduction at lowest concentration; and two, since we do not know residual amine concentration in plant practice, a wide concentration range seemed justified.
The presence of frother did counter the coalescence effect of the two commercial amines at low concentration (5 ppm). At higher concentrations, the amines dominated bubble size, with frother only acting to lessen the tendency for bubble size to increase with DDA and Flotigam ® EDA. Whether adding frother confers any benefit, therefore, depends on the amine concentration: if it is below ca. 10 ppm, adding frother benefits reducing bubble size, but above ca. 15 ppm any impact would probably not be noticed in practice.
4.2 Water Overflow Rate
The relative water overflow rate for the two frothers is comparable to that reported previously . The higher flow with PPG 425 than MIBC can be attributed to the large number of hydrophilic sites, 2 OH and 6 ether –O–, which H-bond with water molecules and increase water transport capacity. Two of the amines, DDA and especially Flotigam® 2835-2L gave overflow, while Flotigam® EDA did not. In DDA there is one (charged) amine group which is hydrophilic and provides some water transport capacity. The two commercial ether amines in addition have the hydrophilic –O– site. With mono-amine Flotigam® EDA, there is now the possibility of intra-molecular H-bonding between the –O– and –NH2 group [26, 27, 28], which reduces H-bonding with water and water transport capacity, evidently very effectively. The di-amine Flotigam® 2835-2L having two amine groups means that while intra-molecular bonding occurs it is focussed between the –NH– closer to the –O– (see structure, Table 1) the –NH2 farther away is free to H-bond with water and thus permits this amine to carry more water. Another factor is that the charged amine groups provide froth stability through mutual repulsion between neighboring bubbles. The di-amine with two charged amine groups may confer additional stability to the froth compared with the other two amines further contributing to its high water transport capacity. The persistence of the Flotigam® 2835-2L froth was noted.
For the amines that produced overflow, the addition of frother increased it, which may be considered to benefit froth stability. Froth stability, however, is not just a property of the reagent(s) as floatable (hydrophobic) particles have a major impact . This has been demonstrated with MIBC [30, 31] and this particle effect likely applies to Flotigam® EDA, as its commercial use implies.
Bubble size reduction and water overflow rate have been determined in a mechanical cell setup for dodecylamine (DDA) and two commercial ether amines, Flotigam® 2835-2L and Flotigam® EDA alone and blended with frothers MIBC and PPG 425. Compared to the frothers, the amines were less effective in reducing bubble size, the commercial amines showing evidence of coalescence at low concentration and all amines giving a minimum bubble size about twice that produced by the frothers. Blending with frother eliminated the coalescence but did not alter the minimum bubble size given by the amines alone. Flotigam® 2835-2L and DDA produced overflow which increased on blending with frother; Flotigam® EDA gave no overflow, alone or blended. Mechanisms of water transport based on amine structure were explored. Based on the findings here whether blending with frother confers any benefit requires knowledge of the residual amine concentration: if it is below ca. 10 ppm, blending eliminates coalescence but there appears to be no practical benefit if it is above 15 ppm.
The work was conducted under the Chair in Mineral Processing funded through the NSERC (Natural Sciences and Engineering Research Council of Canada) CRD (Collaborative Research and Development) program sponsored by Vale, Teck, Xstrata Process Support, Barrick Gold, Shell Canada, Corem, SGS Lakefield Research and Flottec. Provision of the ether amine samples and discussions with Vale personnel are gratefully acknowledged.
Compliance with Ethical Standards
Conflict of Interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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