Table of Contents
It is envisioned that the utilization of nonpathogenic microorganism and/or extracellular surfactant derived from microorganism as a flotation collector will replace some of the environmentally toxic flotation collectors and at the sometime improve flotation efficiency. The selection screening, and judicious selection of appropriate microorganism as flotation collector has been a challenge to process engineers.
Surface Chemistry of Mycobacterium Phlei
M. phlei is a gram-positive procaryotic cell. It has a rod like shape with 1-1.5 µm in diameter and 5 µm in length. However, the shape can be altered depending on the culture method and rate of growth. Figure 1a shows the Scanning Electron Microphotograph of M. phlei at pH 3.
From the foregoing it can be realized that the surface composition and surface chemistry of M. phlei, in a microscopic scale, resemble to conventional flotation collectors. In this paper, particular attention is focused to elucidate the role of M. phlei as a flotation collector of hematite and to correlate flotation results with bacterial adhesion, hydrophobicity and electrokinetics behavior.
Experimental Materials and Procedure
Hematite samples were hand ground to minus 10 mesh followed by ceramic ball mill grinding to achieve required size range. For contact angle measurement, large crystals of hematite were cut into cubes of 2 cm x 2 cm (height 1.5 cm) and were polished on polishing machine using distilled water and 0.5 micron alumina abrasive.
The strain of Mycobacterium phlei was obtained from Carolina Biological Supply Company. M. phlei was grown in culture medium containing the following (per liter of distilled water): 10 g D-(+)-glucose, 2 g enzymatic hydrolysate casein , 1 g beef extract power and 1 g yeast extract which was sterilized at 121 °c for 30 minutes.
A Contact Angle Goniometer manufactured by Rame’ Hart Inc. was used in contact angle experiments. Freshly prepared M. phlei suspensions were adjusted to different pH with NaOH or HCl and then were incubated together with mineral samples on a rotary shaker at 25 °C.
Flotation experiments were conducted in a Hallimond tube with 2 grams of sized hematite.
Bacterial Adhesion Experiments
Initially adhesion of M. phlei on the hematite was conducted as a function of concentration. A 50 ml of M. phlei solution containing 2 grams of sized hematite was used in each experiment. The adhesion of M. phlei on hematite for different pH was conducted at a fixed initial concentration of M. phlei, i. e. 400 ppm. In selected experiments, hematite were rinsed with water and were examined with Scanning Electron Microscope to observe the attachment of M. phlei onto hematite.
Results and Discussion
Contact angles on the polished surface of hematite as a function of M. phlei concentration is given in Figure 6. The equilibrium contact angle increased with an increase in M. phlei concentration and it levels of at M. phlei concentration of 50 ppm. A contact angle of 46 ± 2 degrees at the surface of hematite indicates a moderately hydrophobic character of microorganism treated surface.
Flotation recovery of narrow size hematite (53×20 µm) particles as a function of pH is given in Figure 10. More than 77% hematite can be recovered in the flotation concentrate around pH 3. Further, flotation recovery decreases with an increase in pH. At around pH 8.5, there is an increase in flotation recovery. The relative insensitivity of flotation recovery with pH at pH values greater than pH 3.0 is indicative of insensitivity of bacterial adhesion to hematite with pH (as discussed latter). Perhaps, the phenomenon is due to a competition between charge interaction and hydrophobic interaction as a function of pH.
Adhesion of Microorganism
The adhesion of organism on hematite decreases with an increase in pH and remain constant beyond pH 8.0. Calculation show that beyond pH 8.0, only 0.03 mg M. phlei adsorbed/g of hematite exhibiting low flotation recovery. The Scanning Electron Microphotograph of microorganism on hematite conditioned at pH 3 is given in
Potential Energy of Interaction
DLVO theory considered interaction between particles to be determined totally by the repulsive forces resulting from the presence of electrical double layers and due to London-van der Waals attractive force.
VT = VA + VR…………………………………………………………………………………(1)
where a1 and a2 are the radius of dissimilar spheres hematite and M. phlei respectively; H is the separation distance between two spheres surfaces; A is a Hamaker constant.
where ε is the dielectric constant, a1 and a2 are the radii of respectively spherical particles, Ψ01 and Ψ02 are their surface potentials and H0 is the minimum separation of particle surface.