Frothers in Flotation

Frothers in Flotation

Introduction

Flotation is a method patented in 1905 that is widely used in mining to separate minerals based on their hydrophilic and hydrophobic properties. In this method, air is introduced into a system containing a pulp phase composed of water and minerals mixed in a specific ratio, creating air bubbles. Minerals forming the solid phase adhere to air bubbles according to their surface properties and are transported to the foam phase. A simplified schematic view of the flotation system is shown in Figure 1. Frothers are used in flotation to produce stable foam and reduce bubble size. Frothers facilitate the dispersion of air into fine bubbles (Gupta et al., 2007). Since the structure and stability of frothers directly affect foam characteristics and variables related to entrainment of minerals that should not float, proper frother selection is very important for flotation efficiency. Frother type and dosage, particle size, process water quality, gas distribution and particle contact angle are parameters affecting foam stability (Farrokhpay, 2011; Schwarz and Grano, 2005). Frother reagents are nonionic organic compounds consisting of polar and nonpolar components. The hydrophilic/polar groups of frothers are directed toward the water phase; the hydrophobic/nonpolar groups are directed toward the air phase. The structure of frothers is shown in Figure 2, and the appearance of frother molecules on bubble surfaces is shown in Figure 3. The frother concentrates at the interface between water and air bubbles, forming a membrane around the bubbles that prevents them from colliding. Thus, the forces created around the air bubble in the presence of frother prevent bubble coalescence and rupture. When frothers are added to water, it is known that the surface tension of the solution decreases due to their heteropolar structure. As solution surface tension decreases, more stable foams are formed and bubble coalescence in the pulp phase is prevented.    

Classification of Frothers

In the literature, several different classifications of frothers have been made based on their characteristics and behavior in the pulp. The four commonly used classification methods are: 1. pH sensitivity, 2. Solubility, 3. Frother/collector ability, 4. Selectivity/frother power relationship. Among these, the most commonly used frother classification is pH sensitivity. Dudenkov and Galikov (1969) classified frothers according to their behavior and different pH values. This classification is shown in Table 1 (Khoshdast and Sam, 2011).  

Acidic and Basic Frothers

Acidic frothers have been in use since the 1960s and show good activity at acidic pH. Their use is gradually decreasing due to environmental factors. Phenols and alkyl sulfonates fall into this group (Bulatovic, 2007). Phenols are obtained as byproducts during the gasification of coal tar and/or distillation of crude oil. The main problem with phenol use as a frother is that its composition varies depending on the source from which the frother is derived. Alkyl sulfonate is an anionic frother with a structure composed of an aromatic hydrocarbon and an aliphatic radical. Although these frothers have good foaming properties, their use is quite limited because they contain trace amounts of sulfur. Basic frothers, on the other hand, are mostly used in the flotation of base metal ores and are represented by pyridine and its homologs recovered as byproducts from coal tar distillation (Bulatovic, 2007).  

Neutral Frothers

Neutral frothers are the most important frother group, widely used in flotation of base metal ores, oxidized minerals and industrial minerals in both acidic and basic environments. In this frother group, the most preferred frother groups in flotation are aliphatic alcohols, cyclic alcohols and polyglycol ethers.

Aliphatic Alcohols

Aliphatic alcohol type frothers are mixtures of alcohols containing 6-8 carbon atoms. The most well-known frother type in this group is methyl isobutyl carbinol (MIBC), and its chemical structure is as shown in Figure 4. Methyl isobutyl carbinol (MIBC), an organic chemical compound, is a liquid acetone derivative with a sharp alcohol odor. It has limited solubility in water but is soluble in most organic solvents (Phan et al., 2012). MIBC is a frother type that demonstrates superior performance in flotation and is therefore frequently used. It is cheaper compared to many other frothers in its field of use (Tan et al., 2005). It is a frother capable of providing effective performance with many different ore types. It is the most effective among aliphatic alcohol varieties. Foams formed when MIBC is used typically consist of large bubbles (Sam et al., 2011).

Cyclic Alcohols

Chemically, pine oil consists essentially of α-terpineol and other cyclic terpene alcohols. Terpene hydrocarbons, ethers and esters may also be present. The precise composition depends on various factors such as the pine species from which it is produced and the parts of the tree used. Terpene hydrocarbons, although lacking frother properties, improve the quality of the resulting foam. Pine oil produces foam with small bubbles closely bonded together that easily ruptures when discharged from the machine. This foam structure prevents mineral particles from settling easily. For this reason, it cannot be obtained with high-content concentrated pine oil to the same extent as with other frothers, but metal recovery is very high. Additionally, excessive use of pine oil reduces foam volume (Atak, 2017). In the early days of flotation, eucalyptus oil was a popular frother, but pine oil later became popular as a natural oil frother because it was more readily available in the west. In pine-growing countries such as the United States, China and Finland, different grades of pine oil were obtained, which led to many difficulties in flotation. For this reason, it has become less popular alongside MIBC (Crozier, 1992).  

Polyglycol Ethers

These types of frothers are methoxy polypropylene glycol or polypropylene glycol methyl ethers. Polyglycol ether type frothers are produced by different manufacturers such as Dow Kimyasal Şirketi under the brand name Dowfroth and Solvay under the brand name Aerofroth. MIBC and polyglycol ethers comprise 90% of frother use in flotation of metallic ores. Dowfroth 250 is one of the most used and preferred Dowfroth type frothers, and its chemical structure is shown in Figure 5 (Crozier, 1992). Molecular weight and polyglycol ether carbon chain length affect its strength and performance. Higher molecular weights produce more durable foams while lower molecular weights show less selectivity. The physical and chemical properties of MIBC, pine oil and Dowfroth 250, which are the most used frothers in the neutral frother group in flotation, are given in Table 2.   References • Atak, S. 2017. Flotation in Ore Preparation 100 Years. Istanbul: ITU Foundation Publications. • Bulatovic, S. M., 2007. Handbook of Flotation Reagents (Chemistry, Theory and Practice: Flotation of Sulfide Ores), vol.1. Amsterdam: Elsevier Science & Technology Books. • Crozier, R.D., 1992. Chemical Properties of Frothers: Flotation—Theory, Reagent and Ore Testing; Pergamon: New York, NY, USA. • Dudenkov, S.V., A.A. Galikov, 1969. Theory and Practice of Application of Flotation Reagents. Nedra: Russia. • Farrokhpay, S., 2011. The significance of froth stability in mineral flotation — A review. Advances in Colloid and Interface Science, 166, 1-7. • Gupta, A. K., Banerjee, P. K., Mishra, A., Satish, P., Pradip, 2007. Effect of alcohol and polyglycol ether frothers on foam stability, bubble size and coal flotation. International Journal of Mineral Processing, 82, 126-137. • Phan, C., M., Nakahara H., Shibata, O., Moroi, Y., Le, T., N., and Ang, H., M., 2012. Surface Potential of Methyl Isobutyl Carbinol Adsorption Layer at the Air/Water Interface. The Journal of Physical Chemistry B, 980-986. • Sam, A., Khoshdast, H., 2011. Flotation Frothers: Review of Their Classifications, Properties and Preparation. The open mineral processing journal, 4, 25-44. • Schwarz, S., Grano, S., 2005. Effect of particle hydrophobicity on particle and water transport across a flotation froth. Colloids and Surfaces A, 256, 157-164. • Tan, S. N., Pugh, R. J., Fornasiero, D., Sedev, R., Ralston, J., 2005. Foaming of polypropylene glycols and glycol/ MIBC mixtures. Mineral Engineering, 18(2), 179-188.   Dr. Ş. Beste Aydın Ore Preparation Engineering Department Istanbul Technical University Prof. Dr. Gülay Bulut Ore Preparation Engineering Department Istanbul Technical University