Mycobacterium phlei

Received 1 November 1995; accepted 1 May 1996
Abstract Separation of fine coal particles from the associated mineral matter is achieved either by flotation or flocculation. Such a separation process relies on the surface modification of the components involved. Flotation is mainly accomplished with the addition of a suitable collector molecule to the coal slurry and flocculation makes use of synthetic flocculants such as polyacrylamide and polyethylene oxides. In the latter case, the interaction between coal particles and the polymer flocculant is considered to be non-specific and often results in inefficient flocculation. In the present study a microorganism, Mycobacterium phlei, has been used as a bioflocculant in the separation of combustibles from Illinois #6 and Kentucky #9 coals. This bacterium was reported to be negatively charged and highly hydrophobic. The flocculation phenomenon was explained based on the interactions between the coal particles and the bacteria. The results suggested that the hydrophobic interactions play a dominant role. The aggregated coal flocs were found to contain some pyrite and mineral matter due to similar settling velocities. Howes was accomplished by column flotation using a frother and nitrogen gas.
C O L L O I D S A N D
ELSEVIER Colloids and Surfaces B: Biointerfaces 8 (1996) 13-24
S U R F A C E S Flocculation and flotation of coal by adhesion of hydrophobic
Keywords: Adhesion; Coal; Flocculation; Flotation; Hydrophobicity; Mycobacterium phlei
1. Introduction One of the important processing steps in coal preparation is the separation of mineral matter from coal. A great deal of effort has been spent in exploring novel methods of cleaning coal to produce super-clean coal of high Btu values for advanced coal applications [1]. This goal can be accomplished in several ways with the incorporation of either flotation [ 2 - 4 ] or through other methods such as flocculation [5,6], agglomeration [7], and coagulation [8]. All these strategies of separation rely on the differences in the surface * Corresponding author. 0927-7765/96/$15.00 © 1996Elsevier Science B.V. All rights reserved PII S0927-7765 (96)0t 312-4
properties of coal and its constituent mineral matter. Coal, consisting principally of hydrocarbons with the presence of some hetero atoms, is hydrophobic in nature, whereas the majority of other associated minerals are hydrophilic. Separation of fine coal particles from the associated mineral matter is achieved mainly by flotation or flocculation. Flotation is accomplished with the modification of coal surface properties using such reagents as collector molecules, promoters, and frothers. Conventional flocculation of fine coal particles from slurry is carried out with the use of synthetic polymers such as polyethylene oxides, polyacrylamide and partially hydrolyzed versions thereof. The phenomenon of flocculation is the result of electrostatic and dipole interactions, and
B
Mycobacterium phlei
A. M. Raichur *, M. Misra, K. Bukka, R.W. Smith
Department of Chemical and Metallurgical Engineering, MS 170, Mackay School of Mines, University of Nevada, Reno, Reno, N V 89557, USA
14
A.M. Raichur et al./Colloids Surfaces B: Biointerfaces, 8 (1996) 1 3 ~ 4
occurs on the backbone of the polymer chain through hydrophilic interactions. Synthetic polymers are believed to be non-specific with regard to coal, which is hydrophobic, and the mineral matter from which the coal is being separated is hydrophilic. In recent years, in addition to the synthetic polymers as flocculants, a new class of biological agents has been explored in the task of separating coal from the mineral matter. In general, there are a variety of bacteria which have been evaluated for their utilization in such wide ranging applications as coal liquefaction [9] to other beneficiation efforts [10]. These bacteria differ in their nature, properties, and functions, hence differ in their mode of action. Attia et al. [-11] have used the bacterium Thiobacillus ferrooxidans to modify the mineral surface and thereby depress pyrite in the flotation of fine coal particles. Although the exact mechanism of biomodification of the pyrite surface was not clear, the interaction between the bacteria and the mineral was termed hydrophilic. In this paper the selective separation of coal particles from ashforming minerals using Mycobacterium phlei (M. phlei), a novel organism, as a bioflocculant is presented. Mycobacterium phlei is a ubiquitous bacterium found in soils and on the leaves of plants. It is also, sometimes, known as Hay or Timothy bacillus [12,13]. It has a negatively charged surface with a contact angle of 68°, which means it is highly hydrophobic. However, the surface chemistry of the cultured organism is dependent upon the culture medium and the growth rate [-14,15]. During the past few years, M. phlei has been successfully used as a flocculant and flotation collector for a wide variety of minerals and fossil fuels [16-19]. In a recent communication the differential adhesion of the same bacterium towards coal and its associated minerals was reported [20]. The present study was undertaken in order to develop some fundamental understanding of the mechanism involved in the interactions between the bacteria and coal. Two samples of coal, i.e. Illinois #6 and Kentucky #9, which differ significantly in their hydrophobicities, were chosen. A combination of flocculation and flotation techniques was used to accomplish separation of coal from the constituent mineral matter. The results