Surface Energy and Induction Time of Fine Coals Treated with Various Levels of Dispersed Collector and Their Correlation to Flotation Responses

Abstract

Separation of fine coal in froth flotation relies upon the wettability difference between the coal-rich and mineral-rich particles in the aqueous solution. Two methods were used to measure the wettability of six ranks of coal as well as coal samples treated with various levels of dispersed collector in aqueous solution. Wettability was determined by measuring the distribution of critical wetting surface tension, i.e., surface energy, using the film flotation technique and by measuring the induction time, i.e., bubble−particle attachment time, of the material. The wetting of coal particles is strongly dependent upon the coalification processes and can be affected by ash content. For anthracite coal with a high ash content, very high surface energy and intermediate induction time were measured, but intermediate floatability with a very good ash rejection was obtained. Sub-bituminous coal with low mineral inclusion was found to have a relatively low surface energy, and thus a high floatability, but very poor selectivity was observed, which was reflected in lengthened induction time. When dynamics of flotation behaviors are involved, flotation results can be better interpreted by induction time. For dispersed collector-treated coal samples, an increase in collector dispersion (i.e., a decrease in kerosene droplet size), by direct liquid collector mechanical agitation, ultrasonic energy emulsification, or atomization, caused decreases in surface energy and induction time and closely matched the increase of flotation recovery and selectivity. With similar particle density, mineral liberation conditions, and particle size, the induction time was found to be closely related to the mean critical surface tension for untreated coal samples and for HV-bituminous coal samples treated by various levels of dispersed collector.

A review of induction and attachment times of wetting thin films between air bubbles and particles and its relevance in the separation of particles by flotation

“…7, the flotation recovery was inversely proportional to the attachment time, and the maximal flotation recovery matched the (Fig. 6a) and Peng [28] (Fig. 6b).…”

“…6a. Peng [28] also observed the similar trend for attachment time versus coal rank and the close correlation between attachment time and flotation recovery for coal samples having similar ash content (Fig. 6b).…”

“…For example, the attachment time is a more sensitive indicator for predicting mineral floatability than the equilibrium contact angle and the dynamic surface tension [25][26][27][28]. Moreover, the contact angle measurements of some minerals show that the contact angle of the minerals is very small but the minerals can be floated [29].…”

Beneficiation of an Indian non-coking coal by column flotation

“…The efficiency in separating the impurities from coal by flotation is basically determined by the relative hydrophobicity of coal and the associated gangue (Laskowski 2001;S Dey et al 2013). In froth flotation, coal particles are often subjected to a suitable hydrocarbon oil treatment (Boylu and Laskowski 2007) to alter their hydrophobicity, enhance recovery, and/ or improve selectivity (Peng 1996). Conventionally, the collector used in most of the Indian coal washeries is diesel oil in combination with different frothers.…”

Measurement of sliding velocity and induction time of a single micro‐bubble under an inclined collector surface

“…Attachment efficiency, however, is a function of 'induction time' largely dependent on interactions between bubble and solid surfaces, which makes it more complicated to analyse Finch, 1986, 1987;Yoon et al, 1989;Ralston et al, 1999). Flotation efficiency is also strongly dependent on bubble size, bitumen or particle size, surface and physiochemical properties, process temperature, and hydrodynamic conditions of flotation systems.There are four diverse definitions of induction time in the literature: (1) the time required for the intervening liquid film thinning to its critical thickness (Schulze and Birzer, 1987;Schulze, 1992); (2) the time required for the film depletion up to gas nucleation and formation of three-phase contact line (tpcl) (Paulsen et al, 1996); (3) the time required for the film depletion to a minimum rim diameter (the rim diameter required to have stable attachment; Peng, 1996;Gu et al, 2003); and (4) the time required for all three steps of film thinning, rupture, and development of stable attachment (Gu et al, 2003(Gu et al, , 2004. In this study, the last definition of induction time is used.…”