Long-range hydrophobic forces in the amine flotation of quartz S.A. Ravi shankar and R.-H. Yoon Abstract - A Mark IV surface force appara tus (SFA) was used to conduct directforce measurements between mica surfaces in dodecylamine hydrogen chloride collector solu- tions at varying pH levels. It was found that long-range hydrophobic fo rces reach a maximum at pH 9.5, which also corresponds to the maxima attained by the contact angle and flo tation recovery ofquartz. The charge densities are compa - rable on both mica and quartz surfaces (0.40 to 0.48 nm 2 ), making it possible to draw conclusions on the flotation of quartz fr om the direct fo rce measurements conducted on mica. Therefore, it may be suggested that, in the pH range of 8.0 to 10.3, the maximum fl otation recovery ofquartz may be attributed to the appearance of a long-range hydrophobic force, which, in turn, can be related to the formation of a close-packed monolayer (or hemimicelle) in the presence of neutral amines fo rmed as a result of hydrolysis. Introduction Amine flotation is one of the most widely practiced processes for floating oxides and aluminosilicates (de Bruyn, 1955; Gaudin and Fuerstenau, 1955; Fuerstenau, 1957). One of the most interesting aspects of amine flotati on is that it is strongly pH-dependent. It has been shown that maximum flotation occurs over a narrow pH range of from 8.0 to 10.3 (de Bruyn , 1955; Gaud in and Fuerstenau, 1955; Fuerstenau, 1957; S om a sund aran , 1968; S om a sundar an and Ananthapadmanabhan, 1979; Novich and Ring, 1985; Castro et al., 1986;Laskowski, 1989). Gaudin and Fuerstenau (1955) showed that the sharp increase in flotation is manifested by the increase in surface hydroph obicity (water contact angle). Because the formation of a hydrophob ic body by the adsorp- tion of a cationic surfactant is the precursor to their subse- quent levitation by means of a carrier gas, it is envi saged that the driving force for the flotation process is the hydrophobic force. However, no one has been able to show a correlation between the hydrophobic force and maximum flotation. Recently, Rutland et al. (1992) conducted force measure- ments between mica surfaces in the presence of 10-4 M s.A. Ravishankar , member SME , IS scientist with Nord Kaolin Co., Jeffersonville, GA; A.-H. Yoon , member SME, is professor with the Center for Coal and Mineral Process ing , Virginia Polytechn ic Insti- tute and State University, Blacksburg, VA. Preprint 96-56, SME Annual Meeting, March 11-14, 1996, Phoenix, AZ. Original manu- script February 1996; revised manuscript December 1996. M&MP paper 96-617. Discussion of this peer -reviewed and approved paper is invited and must be submitted to SME prior to August 31, 1997. 10 MAY 1997 dodecylamine hydrogenchloride (DAHCI) solutions at dif- ferent pH levels. They showed that, compared to lower pH levels, the hydrophobic forces become significantly stronger at pH 8 to 9. These authors attributed this phenomenon to the coadsorption of dodecylamin e (DA) and dodecyl ammonium ions (DAH+) on mica. However, the hydrophobic force measured under this condition was not as long ranged as those measured with insoluble and double-chain surfactants such as dioctadecyldimethyl a mmo nium bromide (DDOAB) (Pashley et al., 1985; Parker et al., 1989; Tsao et al., 1993; Ts ao eta l., 1991) and fluorocarb on (C laesso n and Chri sten son , 1988; Christenson et al., 1989) or even those measured with DAHCI in the presence of dodecanol as the coadsorbing agent. This is evident from the jump distances (12 nm, corrected for the thickne ss of the adsorbed layer) reported by these authors, which are typical characteristics of a short- range forces measured with short, single-chain surfactants such as DAHCI and CTAB at pH 5.7 (Israelachvili and Pashley, 1984; Herder, 1990). A possible reason why Rutland et al. (1992) did not detect long-range hydrophobic forces is that the measurements were conducted above the point of charge reversal of mica (p.c .r. = 10- 5 M), which is defined as the DAHCI concentration at which the surface charge of the mica surface is zero. Above this concentration, DAH+ ions adsorb on mica with "flip-flop" orientation (Chen et al., 1992), exposing the hydrophilic polar groups toward the aqueous phase. It is, theref ore, the purpose of the present investigation to conduct direct force measurements with mica in DAHCI solutions of various concentrations and pH levels and to elucidate the role of hydrophobic forces in amine flotation. Experimental Materials. Dodecyl amine hydro chloride (DAHCI), ob- tained from Eastman Kodak , was recrystallized from ethanol before use. Conductivity water was prepared by passing double-distilled water through a Nanopure II water-treat- ment unit and, subsequently,deaerating it under a vacuum for 3 hrs. Potassium hydroxide (KOH) and hydrochloric acid (HCI) solutions were used for pH adjustments. Muscovite mica (Grade II) was obtained from UniMica Corp. in New York. Surface-force measurements. Direct force measurements were carried out using a Mark IV surface force apparatus from Anutech, Pty. (Parker et al., 1989).Two thin (3 to Sum) , MINERALS AND METALLURGICAL PROCESSING