Flotation of Mineral and Dyes: A Laboratory Experiment for Separation

Laboratory Experiment pubs.acs.org/jchemeduc

Flotation of Mineral and Dyes: A Laboratory Experiment for Separation Method Molecular Hitchhikers Tim Rappon,§ Jarrett A. Sylvestre, and Manit Rappon* Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada S Supporting Information *

ABSTRACT: Flotation as a method of separation is widely researched and is applied in many industries. It has been used to address a wide range of environmental issues including treatment of wastewater, recovery of heavy metals for recycling, extraction of minerals in mining, and so forth. This laboratory attempts to show how such a simple method can be used to separate chemicals and minerals. With flotation, particles (mineral or dye molecules) hitch a ride with air bubbles to the surface of the liquid. Students learn how the basic principles that they learn from theory can be applied to a significant method of separation that they can connect with their daily lives. This laboratory is easy to set up and requires only simple pieces of apparatus that can be purchased from household stores. Therefore, it can be prepared for teaching in virtually any chemistry laboratory in the world. KEYWORDS: First-Year Undergraduate, Second-Year Undergraduate, Analytical Chemistry, Environmental Chemistry, Hands-On Learning/Manipulative, Colloids, Dyes/Pigments, Separation Science, Laboratory Instruction, Chemical Engineering

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• purifying minerals from ore6 • recovering metal ions from waste for environmental protection and reuse7 • enhancing oil recovery8 • treating waste in mining9 • removing dyes in the textile industry10 • separating plastics from municipal waste11 • removing pesticides12 • harvesting algae13 • treatment of byproducts from cosmetics manufacturing14 Other important factors of flotation such as the roles of wetting and the contact angle15 as well as the stability of the froth phase16 have also been reviewed.

INTRODUCTION In this laboratory experiment, students are introduced to one of the most well-known methods for separation, flotation. It has been used in various industries and in the treatment of wastewater for economic and ecological reasons. The experiment should serve to show students how some pollutants may be removed from wastewater. In some cases, recaptured pollutants may even be reused and recycled, increasing the efficiency of an upstream process. We have used this experiment as part of our teaching laboratories for our introductory analytical chemistry course for the past four years and each year there were 50−60 students in the class. The students’ feedback showed that they enjoyed performing this experiment. The experimental setup makes use of simple household goods that can be purchased from local stores. Students worked in pairs and the experiment requires one 3-h laboratory period. Students learned how to separate an ore from a mixture with sand and how to separate a dye from a dye mixture. Flotation is an established and well-studied process. The patent for this method was filed as far back as 18771 and a paper was published in this Journal in 1928;2 since then, many papers have appeared in the same journal including more recent ones.3,4 Numerous publications can be found in several journals across various disciplines. The method has been modified and refined over the years and it is one of the most widely used separation methods. The following lists some of the many applications of flotation: • deinking paper for recycling5 © XXXX American Chemical Society and Division of Chemical Education, Inc.

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OVERVIEW The basic flotation method used in these experiments consisted of an aqueous mixture of materials to be separated and an ionic surfactant (also known as collector). Air bubbles generated from the bottom of the reactor rise to the surface of the solution. Surfactant molecules migrate to the air bubbles, minimizing their interfacial energy by lining up their charged heads on the surface of the bubbles with their hydrophobic tails away from the water. Counter ions form another layer around the ionic heads. The arrangement of the ionic heads and the counterions forms an electrical double layer and is well studied Received: August 17, 2015 Revised: February 8, 2016

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DOI: 10.1021/acs.jchemed.5b00514 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

by the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory.17−19 The forces considered in this theory are the electrostatic and the van der Waals forces: they play important roles in the stability of a colloidal solution. The charged ions (or corrigend) to be separated can undergo ion-exchange with the counterions and are carried with the bubbles to the surface of the solution forming the froth. The froth is rich in the material to be separated and is removed from the surface. It should be mentioned that this froth flotation is only one of the several methods of flotation.

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EXPERIMENTAL SECTION The experimental setup and procedure are briefly described in two sections; the detailed procedure of each section is given in the Supporting Information. List of equipment (brief): • 1 × 1-L of cylindrical glass container • 2 × 1-L beakers • 1 × 1-ft glass rod • 1 × 4-ft Tygon tubing (good quality rubber tubing may also be used as a replacement) • 1 Pasteur pipet • air pump and bubble stone (pet shop supplies), the pump rated at 37.9 L/min. • absorbance meter or UV−vis spectrometer • 1 × vial of finely ground goethite • 1 × vial of sand • methylene blue (Baker) • Allura Red-a red food dye (food dye vendor) • Cetyltrimethylammonium bromide, CTAB (Aldrich)

Figure 1. Experimental setup for the separation of goethite from sand.

Separation of an Iron Ore (Goethite) from the Mixture

Goethite is an iron oxyhydroxide [FeO(OH)], and in hydrated form [FeO(OH)·nH2O], it is the major composition found in rust and iron bog mineral. Each student was given a vial of paleyellow, finely ground goethite. This was mixed with sand (to simulate another component found at a mine) and placed into a glass measuring cylinder containing 500 mL of water mixed with 1 mL of 10% dish soap solution. The mixture pH was adjusted to approximately 5 using small amounts of acid or base. The bubbles were generated from an air stone connected via Tygon tubing to a small aquarium air pump. The tubing and the air stone were lowered down the cylinder such that the air stone was closest to the bottom of the cylinder. The pump was kept away from the liquid. The cylinder and its contents was placed in a large aluminum tray (available in household goods store) to collect the overflow of the froth from the reactor (Figure 1). A picture of the reactor was taken with a cell phone or a digital camera. The pump was turned on and allowed to work for 10−15 min. Another picture was taken at the end of the period. The froth was filtered and the pale yellow solid content left on the filter paper was captured by a camera (Figure 2).

Figure 2. Goethite on filter paper after filtration of the froth.

Information). Air bubbles were generated the same way as described for the separation of goethite from sand. The pump was activated to generate air bubbles. A small amount of the mixture was withdrawn at 2 min intervals with a long Pasteur pipet and its absorbance at 625 nm was measured. The procedure was repeated for about 10 min. When the rate of bubble formation slowed, 1 mL of 10% dish soap was added and bubbling continued for another 5 min. The last absorbance was measured and a picture of the foam was taken. A typical picture is shown in Figure 3. Separation of Red Food Dye from the Mixture with Methylene Blue

The procedure was similar to when the MB was separated from the dye mixture, except cetyltrimethylammonium bromide (CTAB) was used as surfactant instead of dish soap and the absorbance at the beginning of the experiment was adjusted to be approximately 1 at λmax = 525 nm of red dye. For this part of the experiment, the absorbance of the red dye at 525 nm was monitored. The absorbance of the methylene blue at λmax of 625 nm should also be measured at the beginning and at the end of the experiment (see details in Supporting Information). A typical picture taken at the end of the experiment is shown in Figure 4.

Separation of Methylene Blue (MB) in the Presence of Red Dye

A mixture of 25 drops of methylene blue (0.5%), 3 drops of red food dye (Allura Red), and approximately 1 mL of 10% dish soap solution was placed into a 1-L beaker containing 500 mL of water. The solution was mixed thoroughly. The absorbance of methylene blue was measured at its λmax of 625 nm and adjusted to be approximately 1. The absorbance of the red dye at λmax of 525 nm should also be measured at the beginning and at the end of the experiment (see details in Supporting B




Figure 5. Plot of absorbance of methylene blue in the beaker with respect to the flotation time (min) as labeled; a similar plot for the case of red food dye as indicated.

Figure 3. Methylene blue (MB) separated from the dye mixture at the end of the flotation. Here, the MB molecules hitch a ride with rising air bubbles (conditioned by dish soap) and are separated from the mixture, leaving mainly the Allura Red in the reactor.

Figure 6. Model 1 shows the positive surface of goethite surrounded by the negative heads of the anionic surfactant, making it hydrophobic. The newly hydrophobic particle seeks refuge with the hydrophobic tails of surfactant that surrounds an air bubble; negative surfactant heads are at the surface of the bubble. A similar mechanism can be used to explain how a positive dye is separated when it is used instead of goethite particles. Model 2 shows a negative dye attached to a positive surfactant.

To facilitate the discussion, it is helpful to know the molecular structures of the surfactants and the dyes used in the experiments, which are shown in Figure 7. Linear alkyl(benzene) sulfonate (LAS) is a surfactant commonly found in dish soap. The structure shown in Figure 7 (i), sodium dodecyl benzenesulfonate, is a LAS. LAS normally consists of a mixture of linear alkyls of various chain lengths (e.g., C10−C13), depending upon the sources used to prepare

Figure 4. Red food dye separated from the dye mixture at the end of the flotation. The separation shows the Allura Red molecules hitch a ride with rising air bubbles (conditioned by CTAB) and they are separated from the mixture, leaving mainly MB in the reactor.

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HAZARDS It is important to keep the pump away from the solution or water to prevent any electrical shock. MB and the red dye can stain the skin for several days, so avoid direct contact with them by using a pair of gloves.

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RESULTS AND DISCUSSION The use of dyes in the experiments allows the progress of the separation to be easily monitored by measuring absorbance at each dye’s respective λmax. A representative plot of the change of absorbance of methylene blue (MB) in the reactor with time is shown in Figure 5. Likewise, the change of absorbance of red dye with time in the reactor is also shown in Figure 5.

Figure 7. Related molecular structures of chemicals used: (i) for anionic dye separation, linear alkyl sulfonate (LAS); (ii) for cationic dye separation, cetyltrimethylammonium bromide (CTAB), (iii) methylene blue (MB), and (iv) red dye.

Basic models

There are two basic models proposed to explain the mechanism of separation by bubble flotation20 as shown in Figure 6. C



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SUMMARY This laboratory experiment offers students some basic knowledge of colloid and interfacial science; it serves as a good introduction to the basic method of flotation. It has been run in our teaching laboratory for four years. The feedback from the students indicated that they liked learning from and enjoyed doing the experiment. They were excited by the sight of the colorful bubbles overflowing from the reactor into the collecting tray. Students learned that some basic chemistry could be used for environmental and economic benefits. This experiment is easy to set up and makes use of simple components that may be acquired from household goods, and it can be setup in most chemistry laboratories.

the surfactant. It is usually an anionic surfactant whose counterion can be Na+ (not shown). Cetyltrimethylammonium bromide (CTAB) [Figure 7ii] is a cationic surfactant having Br− as its counterion (not shown). The separation of goethite from sand is made possible by the assembly of anionic surfactant molecules at the air/water interface of air bubbles to form an electric double layer of negatively charged heads balanced by positively charged metal ions such as Na+. At a pH of 5, FeO(OH) is protonated which makes the goethite positively charged. The latter can then undergo an ionexchange with Na+ at the electric double layer, replacing Na+ with goethite, which is carried by rising bubbles to the surface of the reactor. Alternatively, in the presence of anionic surfactant, the positive surface of goethite is surrounded by the negative heads of the anionic surfactant, making it hydrophobic. The newly formed hydrophobic particle seeks refuge with the hydrophobic tails of surfactant that surround an air bubble; negative surfactant heads are at the surface of the bubble. This mechanism follows Model 1 as shown in Figure 6. The separation of MB in the presence of dish soap follows a similar mechanism. MB is positively charged [(Figure 7iii], so it too can undergo an ion-exchange at the electric double layer generated from the negative surfactant in dish soap; thus, MB particles are carried to the surface of the reactor. The negatively charged red dye shown in Figure 7iv, having the same charge as the heads of the surfactant at the double layer is not picked up by the bubbles due to the repulsion of like charges. In order to separate red dye from the mixture, a cationic surfactant such as CTAB (Figure 7ii] is needed. The red dye is an anionic dye, so its charge is opposite to the charge on the cationic surfactant heads of the double layer of the bubbles. The attraction of opposite charges allows the red dye to be picked up by the rising air bubbles via ion-exchange, leaving MB in the reactor as shown in model 2 of Figure 6. There is one more interesting point related to the relative ease in the removal of the dyes. It can be seen in Figure 5 that MB with dish soap follows exponential-like depletion, whereas the Allura Red with CTAB decays rather gradually. These results merit further discussion. Because of the differences in the monitoring wavelengths and the molar extinction coefficients corresponding to these wavelengths, the absorbance difference between the two curves alone cannot be used to determine which of the two dyes is more efficiently removed. However, it is meaningful to look at the time-rate of change of the absorbance for each dye. From the molecular structures of the positively charged MB, the dye can readily approach the negatively charged layer formed from the heads of the dish soap surfactant (Model 1). This, however, is not the case for the negatively charged Allura Red to be attracted to the positively charged layer of the CTAB heads. There are at least two plausible reasons for the latter point. First, each of the ammonium head of the CTAB molecule is surrounded by three CH3 groups; thereby sterically hinders the approach of the Allura Red molecule. Second, there is a plausible intermolecular hydrogen-bonding between the two −OH groups of the dye molecules, resulting in the formation of a dimer-like structure, making the associated dye bulkier and sterically hinders its approach to the CTAB positive layer (Model 2). These are the two plausible contributing factors leading to the observed difference.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.5b00514.

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Detailed experimental procedures. (PDF) Detailed experimental procedures. (DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Faculty of Medicine, University of Toronto, Canada.

Notes

The authors declare no competing financial interest

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ACKNOWLEDGMENTS We would like to thank our assistants, Cassandra Ostrom and Rhiannon Kamstra, for their help in collecting some information from students, and our students who performed the experiments with great enthusiasm.

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REFERENCES

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Flotation of Mineral and Dyes: A Laboratory Experiment for Separation

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