G.H. Luttrell

The Effect of Bubble Size on Fine Particle Flotation

Abstract Expressions for the probability of collision (Pc) and adhesion (Pa) have been derived for fine particle flotation by calculating the trajectory of particles as they flow past a bubble in streamline How. Three different flow regimes have been considered in the present work, i.e., Stokes, potential and intermediate. For the intermediate flow conditions in which most flotation operations are carried out, the particle trajectories have been determined using an empirical stream function derived in the present work. For the case of a very hydrophobic coal sample, the values of the probability of collection (P) determined experimentally have been found to be in close agreement with the theoretically predicted Pc values over a range of bubble and particle sizes The expression for Pa has been derived by determining the time it takes for a particle to slide along the surface of a bubble after collision. It has been assumed that the bubble-particte adhesion occurs when the sliding time is equal to or exceed...

A parametric study of froth stability and its effect on column flotation of fine particles

Abstract Laboratory flotation tests have been conducted to examine the effect of froth stability on the column flotation of finely pulverized coal. It has been demonstrated that the upgrading of coal in a flotation column can be significantly improved when froth stability is properly controlled through the manipulation of appropriate variables such as gas flow rate (Vg), wash water flow rate (Vw), froth height, wash water addition point, and feed solid concentration. Increases in wash water flow rate and gas flow rate promoted froth stability, resulting in higher combustible recovery, but possibly higher ash recovery as well. The optimum rates were determined to be 2 and 0.3 cm/s for Vg and Vw, respectively. The specific influence of wash water flow rate on water recovery was found to be closely related to how the frother dosage was maintained. Coal particles could stabilize or destabilize the froth, depending on their size and concentration in the cell. Those in −100 mesh fraction destabilized froth at lower concentrations and stabilized it at higher concentrations while micronized particles always showed froth-breaking power. The froth profiles of solid content and ash content were established at varying wash water flow rates and wash water addition points, from which it was concluded that froth cleaning occurred primarily at the pulp–froth interface and drainage above the wash water addition point.

A hydrodynamic model for bubble-particle attachment

Abstract The process of bubble—particle attachment has been modeled by simulating the trajectory of a small particle as it approaches a bubble. The particle trajectory has been determined using a dynamic force balance between the force pressing the particle against the bubble and the force resisting the viscous film thinning. From the simulated trajectories, the closest approach distance between the particle and the bubble has been determined. The model assumes that bubble—particle adhesion occurs when the approach distance becomes smaller than the critical thickness at which the disjoining liquid film ruptures spontaneously. The model predictions have been verified experimentally using different sizes of bubbles and particles.

Surface studies of the collectorless flotation of chalcopyrite

Abstract It has been shown recently that chalcopyrite can be floated without a collector when the mineral is brought to an oxidizing environment. Some investigators consider that elemental sulfur formed under oxidizing conditions is responsible for collectorless flotation, even at alkaline pH values where the sulfur is thermodynamically unstable. Therefore, a search for the presence of elemental sulfur has been made by means of ESCA, mass spectrometry and solvent extraction followed by UV-spectrophotometric analysis. On the basis of these results, the possible mechanisms of collectorless flotation are discussed.

An evaluation of coal preparation technologies for controlling trace element emissions

Abstract Recent studies suggest that low-cost coal preparation technologies can play an important role in reducing the emissions of air toxics at electric utilities. To evaluate this approach, a detailed study was conducted at Virginia Tech to quantify the capabilities of a variety of conventional and advanced precombustion cleaning processes for removing hazardous air pollutant precursors (HAPPs). Characterization data obtained from this study indicate that most HAPPs associate with mineral impurities commonly found in run-of-mine coals. Some elements (e.g., mercury) were found to associate with pyritic sulfur, while others (e.g., manganese) were more closely associated with ash-bearing minerals. Furthermore, pilot-scale tests conducted as part of this work demonstrated that conventional cleaning processes could achieve HAPPs rejections of 50–80%. The use of advanced processes further improved these rejections, particularly for HAPPs associated with pyrite. The data also showed that trace element rejections could be improved by pulverizing the feed coal to liberate mineral matter. However, mathematical simulations indicated that these apparent gains are often not realized in industrial operations due to the lower separation efficiencies of the finer coal cleaning circuits.

The Effect of Bubble Size on Fine Coal Flotation

To better understand the fundamental steps involved in the froth flotation process, the probability of collection has been determined for the flotation of a very hydrophobic, ash-free coal sample. The results show that probability of collection increases rapidly with decreasing bubble size, and the values are in good agreement with the theoretical collision probabilities predicted from Weber and Paddocks analysis. In the present work. Weber and Paddocks model has been expanded to predict the flotation rate constant as a function of bubble size and other operating variables. The flotation rate constants, determined experimentally using a run-of-mine coal, have been found to exhibit the same power dependence of bubble size as predicted by the model, but the values are lower than predicted by approximately 50%. This discrepancy may be attributed to the high ash content of the coal (15.5%) and, hence, a low probability of adhesion.

The collectorless flotation of chalcopyrite ores using sodium sulfide

Abstract The collectorless flotation process has been tested on six different chalcopyrite ores while monitoring the potentials ( E h ) of the pulp. The results show that collectorless flotation is effective only under oxidizing conditions. In addition, the flotation requires that the chalcopyrite surface be relatively free of hydrophilic oxidation products, which can be accomplished by treating the ore pulp with sodium sulfide (Na2S). On the basis of these findings, methods of improving the collectorless flotation process are discussed.

Upgrading Coal Using a Pneumatic Density-Based Separator

The potential of dry cleaning coal of varying ranks using a pneumatic table concentrator has been evaluated as part of an ongoing investigation. The evaluation has been performed at several sites throughout the United States where coal is extracted from surface open cast, highwall and underground operations as well as from coarse reject. The treated coals varied in feed ash content (i.e., 7–70%). Regardless of the mineral matter type, pure rock removal into the reject stream was achieved in all applications with little coal loss. Field data obtained when treating 50 × 6 mm run-of-mine bituminous coal indicate that 70–90% of the >2.0 Relative Density (RD) rock can be rejected. As a result, a clean product having acceptable market quality was generated from several coal sources including lignite, sub-bituminous, and bituminous coals. This article provides an overview of data from recent field testing of the dry air table technology and discusses the potential implementation strategy for the various sites evaluated.

A study of axial mixing in column flotation

Abstract Pulsed tracer tests have been conducted to quantify the effect of physical variables on axial mixing in column flotation. The tracer studies were used to determine the residence time distribution (RTD) for different column geometries and operating conditions. For each test, the Peclet number and mean residence time were determined from the RTD data. Based on these results and industrial test data, a dimensional analysis approach was taken to develop an expression that relates Peclet number to column geometry and operating variables. Using this expression, it is possible to estimate the scale-up factors, in terms of retention time, that are necessary to account for increased mixing in larger columns.

Electrochemical studies of pyrite oxidation and reduction using freshly-fractured electrodes and rotating ring-disc electrodes

Abstract Oxidation and reduction processes on coal- and mineral-pyrite surfaces have been investigated to better understand the reactions that control the hydrophobicity and flotation behavior of pyrite. The incipient oxidation and reduction reactions were studied using fresh surfaces of pyrite that were created by in situ fracturing electrodes potentiostated at a predetermined potential. Chronoamperometry immediately after fracture and subsequent cyclic voltammetry have established that fresh fracture surfaces of pyrite instantaneously assume a unique potential (referred to as the “stable” potential) at which neither oxidation nor reduction takes place. For Peruvian and Chinese pyrites, the stable potential is −0.28 V (standard hydrogen electrode, SHE) at pH 9.2 and 0 V at pH 4.6. The initial oxidation of pyrite begins at potentials slightly positive of the stable potential and is believed to produce a hydrophobic sulfur-rich species, most likely a polysulfide or metal-deficient sulfide. A rotating ring-disc electrode (RRDE) was employed to study the kinetics and mechanisms of surface reactions on pyrite over moderate potential ranges. Two distinct soluble reduction products (ferrous hydroxide and HS−) and one distinct soluble oxidation product (ferrous hydroxide) were observed on pyrite in alkaline solutions. It is concluded that the initial oxidation of pyrite and the oxidation of ferrous to ferric hydroxide occur in a similar potential range. When the electrode is oxidized, e.g. by polishing, prior to experiments, the initial oxidation of pyrite is masked by the oxidation of ferrous hydroxide, making it difficult to study the oxidation of pyrite itself.