Frothing in Flotation II

Recent Advances in Coal Processing, Volume 2

E.T. Woodburn, E.T. Woodburn, Janusz Laskowski, E T Woodburn

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9789056996314
• eBook ISBN:
  9781351446952
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

Dr. J. S. Laskowski has written several papers on frother-collector interactions and the effect of such interactions on flotation kinetics, and on frothers chemistry and frothing. He is founder and Editor-in-Chief of the journal, Coal Preparation. Dr. E. T. Woodburn has published numerous papers on flotation froth and flotation kinetics. Frothing in Flotation, published in honor of Jan Leja, appeared in 1989. Many important contributions on various aspects of flotation froth properties and behavior and the relationship between froth appearance and flotation performance have appeared since, and this volume intends to summarize these achievements. Flotation kinetics involves a number of mass transfer processes with some of them being critically determined by the behavior of froth. Since froth is complex, and controlled experimentation is difficult, the froth phase was, until recently, either ignored or treated entirely empirically. With wide applications of flotation columns, the behavior of the froth is now often recognized as being dominant in determining flotation performance, and the research in this area is one of the most actively pursued.

Chapter 1

FROTHERS AND FROTHING

J.S. LASKOWSKI

1.1 INTRODUCTION

Frother molecules have uneven distribution of polar and nonpolar groups which enables them to preferentially orient at water/air interface. The polar groups in frother molecules, hydroxyl (—OH), carbonyl (—C=O) and ether linkages (—O—), do not, however, form stable bonds at mineral surfaces [1]. It is known from electrochemical research [2,3] that aliphatic alcohols adsorb reversibly at the mercury/aqueous solution interface at the potentials close to the zero-point-of-charge potential of the mercury electrode. Leja and Schulman’s [4] and Leja’s [5] experimental results have revealed that frother molecules can adsorb at collector-coated mineral surfaces, probably through van der Waals interactions with the preadsorbed collector molecules, and dipole—ion and dipole—dipole interactions of polar groups. More recent studies have confirmed adsorption of frothers onto hydrophobic solids such as coals and methylated silica [6–10]. Adsorption of phenol and cresols onto coals was well documented in the 50s by Allum and Whelan [11], Eveson et al. [12], Hindmarch and Waters [13], and Klassen et al. [7]. The adsorption of polyoxyethylene nonylphenol with 9 ethoxy units per molecule onto hydrophobic surfaces has been shown by Aston et al. [14] to be of a Langmuirian type, which indicates a strong surfactant—solid interaction. The adsorption isotherm for the same surfactant onto a hydrophilic silica has a distinct s-shaped character, indicating weaker affinity. The adsorption isotherms for MIBC on methylated hydrophobic silica and hydrophilic silica reveal that the interaction between MIBC and the hydrophobic silica is stronger than its interaction with the hydrophilic silica.

Interactions between frother and collector molecules are well documented in the works by Jan Leja [15,16]. Since frother molecules accumulate preferentially at the water/gas interface, they actively interact with collector molecules at the moment of the particle-to-bubble collision and attachment, if the frother forms a gaseous film at the liquid/gas interface.

This aspect of collector—frofher interaction has turned out to be extremely important for understanding flotation kinetics. It is well established that frothers reduce induction time and, hence, make the process more kinetically favorable [17,18]. But frothers also facilitate air dispersion into fine bubbles and stabilize the froth. Some authors [19,20] still see these latter effects as the most important frother characteristics. Flotation froth can, however, also be stabilized by water supplied to the top of the froth [21]. The froth in the flotation column seems to be stabilized mainly by wash water. In this case, frother still plays an important part in the particle-to-bubble attachment, but the subsequent secondary upgrading in the froth is mainly caused by wash water which flows down the column and removes hydrophilic particles.

1.2 FROTHER-CHEMISTRY AND CLASSIFICATION

There are three main groups of reagents employed by the mineral industry as frothers. As seen in Table 1.1, they include alcohols, alkoxy-substituted paraffins and polyglycol-type frothers (polyglycols and polyglycol ethers).

Aliphatic alcohols that contain a single —OH group are generally of limited solubility in water. The frothers which belong to this group have a chain length of 5 to 8 carbon atoms. These include iso-amyl alcohol, hexanol, cyclo-hexanol, heptanol; probably the best known examples in this group are MIBC (methyl-iso-butyl carbinol) and 2-ethyl hexanol. In some countries diacetone alcohol is commonly used as a good flotation frother [22]. This compound has a very limited surface activity and it reduces the surface tension only very slightly.

Table 1.1. Flotation frothers.

The best known compound that belongs to group 2 is triethoxybutane (TEB), developed in South Africa [23].

The chemistry of the third group, polyglycol type frothers, was first described by Leja and Nixon [24]. These are polymeric derivatives of ethylene or propylene oxide. This group includes well known frothers manufactured by Dow under the trade name Dowfroth, the products manufactured by Union Carbide (PPG frother), Cyanamid (Aerofroth), ICI Australia (Terric 400 series frothers), Huntsman (Unifroth 250), and Witco (Arosurf F-214 and F-215).

The frothers classified into this group range from completely miscible in water to partially soluble types. This is achieved by varying the ratio of hydrophobic to hydrophilic groups in the frother molecule. The relative length of its hydrophobic and hydrophilic ends can be modified by changing the number of —CH₂— groups in the alkyl ether, and ethylene oxide (EO), —CH₂CH₂O—, propylene oxide (PO), —CH₂—CH₂—CH₂—O—, or butylene oxide (BO), —CH₂—CH₂—CH₂—CH₂—O—, groups in the polyoxyethylene chain.

In general:

In PO- and BO-based frothers, the propylene and butylene groups are hydrophobic moieties, while the ether oxygens and hydroxyls represent the hydrophilic groups. Varying the relative length of the hydrophobic to hydrophilic group in the molecule changes its hydrophile—lipophile balance (HLB). This allows tailoring of not only the molecule with the designed hydrophile—lipophile balance (HLB number), but also the one with the desired molecular weight.

Several manufactures now offer MIBC-like commercial products (e.g. Allied Colloids Procol F937, Witco Arosurf F-139 and F-141).

1.3 SURFACE ACTIVITY OF FROTHERS

Figure 1.1 shows the surface tension—concentration curves for aliphatic alcohols [25]; surface tension of MIBC and α-terpineol solutions are given in Figure 1.2 [10].

As can be seen, the fall off of the surface tension is much steeper for aliphatic alcohols with longer hydrophobic chains; in other words, surface activity of an aliphatic alcohol increases with the number of carbon atoms in the molecule. In the homologous series of n-alcohols, the bulk concentration necessary to obtain a given surface coverage decreases by about three times for every additional —CH₂— group in the hydrocarbon chain of the molecule (Traube rule). Alcohols with branched radicals are less surface-active than the corresponding straight-chain alcohols [26]. The surface activity of MIBC is comparable with that of n-hexanol, while the surface activity of α-terpineol is a little higher t...

Table of contents

• Cover Page
• Halftitle Page
• Recent Advances in Coal Processing
• Title Page
• Copyright Page
• Contents
• Preface
• List of Contributors
• 1 Frothers and Frothing—J. S. Laskowski
• 2 Effect of Particle and Bubble Size on Flotation Kinetics—J. B. Rubinstein and V. D. Samygin
• 3 Water Contents and Distribution in Flotation Froths—K. Malysa
• 4 Mechanisms Operating in Flotation Froths—V. E. Ross
• 5 Characterization of Flotation Froth—J. B. Rubinstein and V. I. Melik-Gaikazyan
• 6 Simultaneous Determination of Collection Zone Rate Constant and Froth Zone Recovery Factor—M. A. Vera, J.-P. Franzidis and E. V. Manlapig
• 7 Modelling of Froth Dynamics with Implications for Feed-Back Control—D. G. Murphy, E. T. Woodburn and J. J. Cilliers
• 8 The Interrelationship Between Flotation Variables and Froth Appearance—D. W. Moolman, C. Aldrich and J. S. J. Van Deventer
• 9 Froth Image Analysis in a Flotation Control System—S. Lenczowski and J. Galas
• 10 Kinetic Flotation Modelling Using Froth Imaging Data—J. J. Cilliers, R. A. Asplin and E. T. Woodburn
• 11 Dependence of Froth Behaviour on Galvanic Interactions—J. S. J. Van Deventer
• Subject Index