In Conversation with Roe-Hoan Yoon

Prof. Roe-Hoan Yoon is one of the world’s most distinguished flotation scientists, and the holder of many coveted awards, including the IMPC’s Lifetime Achievement Award. We are privileged to have him give a keynote lecture at this year’s Flotation ’17 in Cape Town, and I was honoured when he accepted my invitation to take part in one of the MEI Interviews. In the event he made my task very easy: I asked him many questions and he did not merely reply, but put together a fascinating mini-autobiography of his life from humble origins in South Korea to his position now as one of the world’s top scientists in his field, and his story should be an inspiration to all young scientists embarking on their careers. I publish it below as received.

Prof. Yoon with the Lifetime Achievement Award at IMPC 2014

 "I recall a high school chemistry class, in which a teacher drew a micelle on a blackboard to explain how detergency works. It fascinated a young mind. As is well known, micellization is a hydrophobic interaction in molecular scale. In college, I was fascinated again to learn how air bubbles selectively collect hydrophobic particles from water, which I now teach students as a hydrophobic interaction in macroscopic scale. Despite the large difference length scales, both are driven by the water molecules striving to maximize H-bonding in the vicinity of hydrophobic surfaces.  

Prof. Yunshik Kim of Seoul National University was my first flotation teacher. After completing his Master’s degree program under Iwao Iwasaki at the University of Minnesota, he returned to his alma mater to teach. Upon graduation in 1967, I worked briefly at the Korea Institute of Science and Technology (KIST), where I learned how to measure ζ-potentials to determine the points of zero charge (pzc) of minerals. Drs. Jae-Hyun Oh and Hyung-Sup Choi were my supervisors. After I left Korea for my graduate training at McGll University, the latter became the Minister of Science and Technology, who is credited for laying the foundation for R&D and economic development.

With Tal Salman in a flotation laboratory
at McGill in 1968

At McGill, I studied under Prof. Talat Salman to recover copper and cobalt ions from solution by ion and precipitate flotation. He earned his Ph.D. in gas-phase adsorption and was an expert in gold extraction and mineral flotation. After receiving my MS degree, I continued to work essentially on the same project for my dissertation. With a fellowship from the National Research Council (NRC) of Canada, I had a degree of freedom to do more fundamental research. One aspect of my work was to study the thermodynamics of adsorption, which included building a micro-calorimeter to measure enthalpy changes. It was a frustrating experience to build a major piece of equipment; however, it gave me an opportunity to learn instrumentation and thermodynamics, which was helpful later when I studied hydrophobic interactions. Both my Master’s and Ph.D. theses work were rated ‘excellent,’ for which I graduated with Dean’s Honor. Tal Salman was a nice person, and I got along with him well. His wife Alba occasionally visited our home in Ottawa, Ontario, when I was working at CANMET as a Research Scientist.



With Maurice Fuerstenau after my Richard Award lecture
at the 2007 SME meeting

McGill used to offer short courses annually for industry personnel, which created opportunities for students like me to visit with famous speakers such as George Pauling, who used infrared spectroscopy to identify the xanthate species adsorbing on surfaces; Vern Plitt who developed an excellent hydrocyclone model; and Maurie Fuerstenau, who was one of the most productive researchers at the time. I particularly enjoyed a seminar by Vern on the first bitumen extraction plant built in Alberta. I also discussed with Maurice my fractional charge model, which served as a basis for my points of zero charge (pzc) model. I got to know him better when I took a job at Virginia Tech. I also visited his department at University of Nevada to give a seminar on hydrophobic interactions. Shortly after my visit, he nominated me for some major national awards, for which I am grateful to this date. One day, he confided to me about him becoming 80 years old in the next year. Not long after that conversation, I heard the sad news that he died of pneumonia, which started as a common cold. I was saddened by his loss and continue to miss him a great deal.

From McGill, I went to work for CANMET, Ottawa, in 1976. I took this job over another in the U.S. in order to gain a working experience in sulfide flotation. Having studied the chemistry of oxide flotation at McGill, I was anxious to learn something different. CANMET had a long history of base metals flotation research, including extensive pilot-scale testing and field trips. I thought that sulfide flotation was both dynamic, in the sense that its chemistry changes continuously due to oxidation, and complex, as the collector adsorption mechanisms are controlled by multiple variables, e.g., Eh, pH, galvanic contacts, semiconducting properties, etc. My first project there was to construct mass-balanced Eh-pH diagrams for common sulfide minerals in the presence of xanthate collectors so that I could predict flotation from thermodynamic data readily available in literature.

I had planned to validate my thermodynamic predictions against a set of micro-flotation data conducted on pure minerals, but I immediately ran into a problem. The pure mineral samples I prepared were hydrophobic before any xanthate treatment. I had treated the samples using sodium sulfide and pyridine to remove surface oxidation products. I observed the same phenomenon with actual ore samples in a flotation cell. As soon as I reported these observations under the heading ‘collectorless flotation,’ it attracted a lot of attention and the subject matter became a controversy. Another project I started at CANMET was fine particle flotation, which was probably the most popular research topic at the time. I recall reading a paper written by Graeme Jameson on the hydrodynamics of bubble-particle collision, which inspired me to do something on it.

With Prof. Graeme Jameson at Flotation '11 in Cape Town

I took-up a faculty position at Virginia Tech in 1979, so that I could do more fundamental research. During the first year, I submitted four research proposals, all of which were funded – a feat I have since never repeated. So, I had a good start, which I would attribute to Dick Lucas and Paul Torgersen, who hired me and nurtured my career. I am thankful to them to this date. Knowing that I came from a minerals school, Dick suggested I develop a coal project to serve the local mining industry and introduced me to some of his friends in industry. One day, a U.S. Congressman, Rick Boucher, walked into to my lab without warning with a TV crew following behind him. I saw myself on a local evening news that night.

One day the Congressman invited me to a dinner meeting with coal company executives. After the meal, he stood at a corner of the room and gave a speech on what he did in Washington, D.C., and asked what he should be doing for the next three months. For a young oriental man who grew up under dictatorships all his life, it was a revelation. For the first time I witnessed at close range how democracy works for the first time. In later years, Rep. Boucher gave me opportunities to testify at Congressional hearings in a panel of experts whom I had seen only on TV. It was a humbling experience indeed. I do not think I did a good job, as I was nervous.  

Following Dick’s advice, I soon developed two coal projects: one was on the salt flotation of coal and the other was on fine coal flotation using small air bubbles (or microbubbles). Although I did not realize its significance at the time, the Kitchener’s group at Imperial College, London, invoked the term hydrophobic force for the first time in 1972 to explain the salt flotation phenomenon, which may be referred to as collectorless flotation of naturally hydrophobic materials. The graduate student who worked on the project (John Sabey) went on to work for Vern Degner of Wemco – a well-known flotation expert.

Bbuilding a pilot-scale microbubble flotation column
with Jerry Luttrell

Knowing that smaller air bubbles can give higher collision frequencies and hence higher flotation rates, I approached Al Deurbrouck, Director of Coal Preparation, DOE, who gave me an Oakridge Summer Faculty fellowship. I worked with Ken Miller, in-house flotation specialist, to demonstrate that the concept of microbubble flotation works well for fine coal. In fact, it worked so well with usual flotation feeds finer than 100 mesh that Ken and I ball mill-ground coal samples to obtain micron-size feeds. We then found that the product coal became much cleaner with finer coal, which was of course due to improved liberation. After my return to Virginia Tech, I wrote a proposal to DOE and received a small grant from the University Coal Research Program, which continues to this day.

This project led us to successive research projects involving scale-up, pilot-plant testing, and eventual commercialization under the trade name Microcel. During the course of this successful project, eight graduate students were trained on flotation, many becoming leaders in industry and academia.

Some years later, I competed for a large ($16 million) DOE project and lost. Its objective was to produce premium fuels, defined as the coal-water slurries prepared from super-clean coals with < 2-3% ash. Despite the loss, we received a subcontract for fine coal dewatering, which led us to the development of a series of advanced technologies such as dewatering aids, hyperbaric centrifuge, and dewatering by displacement (DbD). The first two have been commercialized, with the third one being in the process of commercialization. The DbD process has been developed further to a new process known as hydrophobic-hydrophilic separation (HHS), which is capable of recovering and simultaneously dewatering ultrafine particles. Both of these processes appear to be independent of particle size.

When I first arrived in Blacksburg from Canada, I was unsure if I could survive as a tenure-track faculty without a single degree received in the U.S. One phone call helped me overcome this fear. It was probably during the  first quarter of my teaching job at Virginia Tech, when Prof. Doug Fuerstenau of Berkeley called to inform me that Prof. George Parks of Stanford University was coming to give a departmental seminar on my work just published. It was my model for predicting pzc’s of minerals from crystallographic information, and was an improved version of George Park’s original model. I simply incorporated the charge neutrality principle of Linus Pauling into Park’s model and achieved a better fit between model predictions and experimental data.

Doug helped me in many other ways during my career at Virginia Tech. He came to visit with us in Blacksburg a couple of times. His first visit was in June, 1982, when I organized a flotation symposium as part of the 56th Colloid and Surface Science Symposium. It was very nice of Prof. Jim Wightman, Conference Chair, to ask me to organize the symposium. It gave me an opportunity to bring many famous flotation scientists to Blacksburg and show my laboratories and ongoing research.

Attendees for the flotation symposium in Blacksburg:
Fuerstenau, Yen, Wakamatsu, name unknown, Yoon, Mukerjie of NSF, Iwasaki

Of the many flotation scientists who attended the flotation symposium in Blacksburg were Bill Trahar and Ron Woods both from CSIRO. Bill was famous for identifying the upper and lower particle limits of flotation. He took much of his data from operating plants, which made his work particularly meaningful. Based on his basic training in electrochemistry, Ron consolidated the mixed potential theory for xanthate adsorption, for which he received the 2016 A.M. Gaudin Award and the 2017 Victoria Order. Bill had received the Gaudin award earlier in 1989. I was happy to see them attending the symposium I had organized. On the other hand, I was scared to see them as two of the world’s foremost leaders in sulfide flotation opposed my view on the origin of the collectorless flotation. I thought that sulfide minerals become hydrophobic when the sulfoxy oxidation products are removed, while both Bill and Ron suggested that it was the elemental sulfur formed during the initial stages of oxidation. I contended that elemental sulfur is unstable in alkaline media where I did all of my experiments, and that we could not detect the elemental sulfur by mass spectroscopy. For these reasons, we proposed that the hydrophobic species responsible for the collectorless flotation may be polysilfides rather than the elemental sulfur. The debate went on for more than a decade involving many other scientists, which I enjoyed. Despite the opposing views, we kept our friendships unspoiled for a long time.

With Ron Woods

Following his first visit, Ron Woods came to Blacksburg to do cooperative research for 14 consecutive years. He always came with his wife Elspeth. We had a great time together including my wife Myungshin. In the laboratory, we went beyond the controversies on collectorless flotation and worked together to better understand the mechanisms of xanthate adsorption on sulfide minerals and precious metals. Courtney Young and Mark Pritzker constructed mass-balanced Eh-pH diagrams in the presence of xanthate, while Ron helped us validate the thermodynamic predictions. Cesar Basilio and Dongsoo Kim carried out electrochemical experiments, while Jersey Mielczarski and Jaakko Leppinen conducted spectroscopic analyses using XPS and in-situ FTIR spectroscopic methods. In general, we were pleased to see the results obtained from the electrochemistry, spectroscopy, thermodynamics corroborate well with each other. We were using mainframe computers to handle the overflow and underflow problems associated with solving high-order polynomial equations. Nowadays, the same job can be done using laptop computers. Looking back, it was probably the most productive period of my career, and all of us in my group appreciated the teachings from Ron on electrochemistry.

Ron’s regular visit to our group attracted some of the best-known sulfide flotation chemists such as Paul Richardson and Norm Finkelstein to Blacksburg. We also attracted significant funding from industry (Cytec, Cominco, Inco, Phosphate Research Center, etc.) and government agencies (USBM and DOE). Companies came to us for help with problems concerning poor selectivity and the difficulties with fine particle recoveries. We helped the former by minimizing the inadvertent activation of sphalerite by potential control and using complexing agents. I was intrigued with the role of DETA as a pyrrhotite depressant. We suggested that the reagent desorbs heavy metal cations by forming water-soluble complexes. The problem of fines recovery was solved by installing better bubble generators.

With some of the best names in one place, I used to joke amongst ourselves that we ought to come up with a major new discovery. In retrospect, it is difficult to say what it was. If nothing at all, we trained many young talents, who became leaders in industry and academia.

In flotation, particles collide with air bubbles and form wetting films in between the two macroscopic surfaces. The thin liquid films (TLF) of water formed on hydrophobic surfaces drain and thin fast and eventually rupture, forming contact angles. The TLFs formed on hydrophilic particles, on the other hand, thin more slowly and never rupture. These differences serve as the basis for flotation separation.

In 1969, Janus Laskowski and Joseph Kitchener analyzed the process of contact angle formation using the Frumkin-Derjaguin isotherm and concluded that one must consider the role of “hydrophobic influence” to explain the contact angle formation. Three years later, Blake and Kitchener used the term “hydrophobic force” instead to explain the phenomenon of film rupture on a methylated silica surface at a high concentration of inorganic electrolyte solution. They thought that the hydrophobic force, which was considered a short-range attractive force, was masked under the influence of the long-range repulsive double-layer force. When the double-layer was compressed at a high concentration of inorganic salt, however, the hydrophobic force emerged as a surface force not considered previously in the classical DLVO theory. The authors thought that this mechanism had a bearing on the salt flotation of inherently hydrophobic materials such as bituminous coal.

With Janus and Barbara Laskowski, and Myungshin at a dinner

I read Janus’s paper when I was a graduate student at McGill and was fascinated. However, I did not quite comprehend its significance until I dug into it recently when we started measuring the hydrophobic forces in wetting films. I was also intrigued by Janus’ other work showing that the ζ-potentials of silica particles do not diminish significantly by methylation. I attributed this observation as a supporting evidence for my fractional charge model discussed above.

 In 1982, Jacob Israelachvili and William Pashley of Australian National University reported the first direct measurement of the hydrophobic force, confirming the suggestion made by Kitchener’s group during late 1960s and early 70s. The measurement was conducted using the surface force apparatus (SFA) by approaching curved mica surfaces to each other in a cationic surfactant solution. Many follow-up papers confirmed Jacob and Bill’s measurement; however, many others were skeptical. The controversy went on for more than a generation. My research group at Virginia Tech has been actively involved in the debate for over 25 years, which I enjoyed immensely. My background in flotation helped me a great deal in the debate. 

Some years ago, I met Jan Christer Eriksson, a thermodynamicist retired from the Royal Institute of Technology, at a surface force symposium in Stockholm. We hit it off with each other instantly as both of us believed in hydrophobic force and thought that it had something to do with water structure. Since the DLVO theory was derived by treating water as a continuum, it cannot address the structural changes associated with film thinning. Derjaguin wrote several papers addressing this issue and called the hydrophobic force a ‘structural force.’



With Kristina and Jan C. Eriksson in Blacksburg

I invited Prof. Eriksson to Blacksburg to work with us to study the thermodynamics of macroscopic hydrophobic interaction. We used an atomic force microscope (AFM) to measure the surface forces between thiol-coated gold surfaces. The measurements were conducted at several different temperatures to determine thermodynamic functions. We were surprised with the results; the interaction was enthalpic, that is, the free energy changes were dominated by enthalpy rather than entropy. This new finding was contrary to what had been known for the hydrophobic interactions at molecular-scale such as self-assembly of hydrocarbon chains.

Our thermodynamic data indicated that the water confined between hydrophobic surfaces becomes increasingly structured with decreasing film thickness. This conclusion was supported by the recent sum frequency generation (SFG) spectroscopic studies showing that the water at the hydrophobic surface/water interfaces forms strongly H-bonded structures, which are often referred to as “ice-like.”

The difference between the macroscopic- and molecular-scale hydrophobic interactions arises from the difference in the curvatures of the hydrophobic surfaces involved, which in turn affect the vicinal water structure. 

We also measured attractive surface forces in ethanol, which we called “solvophobic forces.” Both ethanol and water are H-bonded liquids and hence behave similarly in the TLFs confined between hydrophobic surfaces. In effect, hydrophobic force is a solvophobic force, which arises from the antipathy between the H-bonding molecules in the vicinity of surfaces that cannot support H-bonds.

On a little more practical side, we developed a theoretical model for hydrophobic coagulation by adding a hydrophobic force term to the classical DLVO theory. Gaudin in his textbook on flotation showed that the flotation rate of galena decreased with decreasing particle size but stayed constant below around 5 microns, which may be attributed to the hydrophobic coagulation. Scientists considered this work, which was carried out by Zhenghe Xu as part of his thesis work, provided an indirect evidence for the presence of hydrophobic force in colloid films. After many years of his successful career in Alberta, Zhenghe has accepted the deanship at the Southern University of Science and Technology in China.

With Zhenghe Xu

Encouraged by Zhenghe’s work, I decided to get involved in direct force measurement and bought an SFA from ANU. I was pleased to learn how well the results corroborate with the information available in flotation literature. We found also that hydrophobic force increased with water contact angle, which convinced me of its existence and role in flotation. We also used the SFA to measure the forces between bitumen-coated mica surfaces. At the time, most people thought that the surface chemistry of bitumen droplets in water was controlled by the naturally occurring surfactant, e.g., fatty acids, exposed on the surface. Our SFA data showed for the first time that it was asphaltene, rather than fatty acids, controlling the colloid chemistry of bitumen, which has far-reaching implications in bitumen extraction from oil sands. We then started using the atomic force apparatus (AFM) to measure surface forces, mainly because we were interested in force measurement with opaque minerals such as copper sulfide and precious metals.

We also used the extended DLVO theory that was used to model hydrophobic coagulation to explain the stability of foams and froth. However, our work drew criticisms from some well-known foam specialists in Europe, who had been measuring surface forces in foam films using the thin-film pressure balance (TFPB) technique of Scheludko. If the Hamaker constants are known, one can use the DLVO theory to back-calculate the ζ-potentials at the air/water interface. We found that this approach worked rather well at high surfactant concentrations but not so at lower concentrations. The back-calculated ζ-potentials were substantially lower than calculated using the Gibbs adsorption isotherm or measured experimentally. We suggested that the hydrophobic force not considered in the DLVO theory may account for the discrepancy.

Thus, the idea of air bubbles being hydrophobic was born. If one accepts that air bubbles are hydrophobic, flotation may then be considered a hydrophobic interaction. That air bubbles in water are most hydrophobic in pristine water is consistent with the high interfacial tensions at the air/water interface. When I wrote a book chapter summarizing our work, a letter-to-the-editor apposing our views appeared in Langmuir, to which I responded.

Having spent more than 25 years trying to convince myself of the existence of hydrophobic forces in both colloid and foam films, my next target is the flotation (or wetting) films. I knew that it would be a challenge, as many investigators had troubles coping with bubble deformation, which made it difficult to determine the exact separation distances between the two macroscopic surfaces, i.e., mineral and air bubble. To my surprise, however, it did not take too long for a Master’s degree student (Lei Pan, who now teaches at Michigan Tech) to quickly modify the TFPB that I had for foam film studies, so that it could also be used for studying wetting films. One thing we realized was that wetting films thinned much faster than foam films. Therefore, we used a high-speed camera to capture the fast-evolving optical fringes, which can then be analyzed offline to construct spatiotemporal film profiles. By analyzing the spatiotemporal film profiles, we were able to determine the kinetics of bubble deformation, which in turn could be analyzed to determine the hydrophobic disjoining pressure using the Reynolds lubrication theory and the extended DLVO theory. We found that hydrophobic forces increased with increasing xanthate concentration as reported in Faraday Discussions in 2010.

As part of his thesis work, Lei Pan carried out more theoretical studies, in which both hydrodynamic and surface forces were determined by analyzing the spatiotemporal film profiles with the help of a fluid mechanist (Dr. Sungwhan Jung) at Virginia Tech. We had no problems detecting the presence of the long-range hydrophobic force on a xanthate-coated gold surface. Analysis of the data using the Frumkin-Derjagiun isotherm suggested, however, that a short-range hydrophobic force must also be present in the film to account for the faster (nearly invisible) film thinning and de-wetting steps during the last stages of a bubble-particle interaction. We, therefore, wrote that the long-range hydrophobic force was responsible for film thinning, while the short-range force was responsible for film rupture in a JCIS paper published in 2011. In effect, we developed a method of using an air bubble as a sensor for the measurement of both the hydrodynamic and surface forces involved in bubble-particle interactions.

Lei Pan with the FADS he designed and constructed

I then challenged Lei Pan to validate the forces calculated by analyzing the spatiotemporal film profiles using direct force measurements. He met the challenge by designing and constructing a new instrument named the “force apparatus for deformable surfaces (FADS).” This new apparatus allows an air bubble to move toward the undersurface of a cantilever spring by means of a piezo crystal, while monitoring spring deflection using a fiber optic sensor. Before the measurement, the spring had been treated by gold and subsequently by xanthate coatings. Detailed methods of determining both the short- and long-range hydrophobic forces and validating them by direct force measurement have been described in our 2016 Minerals Engineering paper.

It seems that we have come full circle since 1969, when my good friend Janus Laskowski suggested that contact angle formation cannot be explained without considering the presence of the hydrophobic force (or influence) in a flotation film. He and Kitchener also wrote: “There is no theory leading to even approximate calculation of negative disjoining pressures on hydrophobic surfaces.” Of course, the negative disjoining pressure arises from the hydrophobic force in wetting films, which in turn arises from a collector coating. By virtue of many researchers’ hard work and vision, we now know how to determine the hydrophobic force using an air bubble as a sensor.

I was given a special honor to present a plenary lecture at the XVII IMPC meeting in Dresden, Germany, in 1991. The title of my lecture was “Hydrodynamic and Surface Forces in Bubble-Particle Interactions.” I was humbled to be at the same plenary panel as Nicolay Churaev, who was the world leader in wetting films. I had been reading his papers, but it was the first time I met him in person. I met him again in the Frumkin Institute in Moscow some years later. At the IMPC, both of us addressed the importance of hydrophobic force in flotation, which was a coincidence but was not surprising. 

At the plenary lecture panel, 1991 XVII IMPC meeting in Dresden:
Schubert, Yoon, Churaev, and Schoenert

It was very nice of Professor Heinrich Schubert, who gave a relatively young unknown investigator an opportunity to present a plenary lecture at the most prestigious meeting in minerals processing. I knew that he liked our work on microbubble flotation, which was intended to improve collision efficiency by reducing bubble size. In my Dresden lecture, I proposed a bubble-particle attachment model, which was in the same form as the Arrhenius equation. In effect, the model suggested that the efficiency of attachment should be a function of both energy barrier, which is determined by the surface forces in wetting films and the kinetic energy of attachment, which should be a function of hydrodynamic forces. As such, the attachment model was the first to link the surface and hydrodynamic forces in one equation, which served as a basis for my flotation model. Since the model has been derived from first principles, it has predictive and diagnostic capabilities, as shown in the special issue of IJMP published to honor Professor Schubert for his 90th birthday. It should be noted here also that of the various surface forces, hydrophobic force is the driving force for bubble-particle attachment and hence flotation.

In retrospect, I advocated the control of bubble size to improve flotation during the early part of my career. During the later stages, I advocated the control of hydrophobic force. I am not certain if I have a proper training to explore the origin of the hydrophobic force. Nevertheless, I will do my best with my graduate students and other colleagues.

I came a long way from a humble origin. I was lucky to have an opportunity to study at McGill, which has grown to become one of the best-known minerals processing schools largely due to the leadership of James Finch – my classmate. I was lucky also that my adviser allowed me to carry out fundamental research while teaching me to do something useful for the industry. I was also lucky that CANMET and Virginia Tech gave me the opportunities to do what I believed was important. This is my 39th year at the university, which is a long time. I was fortunate to have so many good people pass through my laboratory. The most fun part of my job has been to stand around a whiteboard and discuss problems with students, which is a learning experience. I feel that I have not left school because I still have so much to learn. I say to my students that teaching is the best job in the world. And both of our children became teachers like many of my former graduate students".

Once again I thank Prof. Yoon for taking some considerable time out of his very active life to provide for MEI the story of his journey through life, and I look forward to seeing him in Cape Town in November for what will be his second MEI Conference. I am sure that all who read this account will agree that he was a very worthy recipient of the IMPC's Lifetime Achievement Award.

References to all the research projects reported above can be found by contacting Prof. Yoon at ryoon@vt.edu

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