Bubble Size Effects in Coal Flotation and Phosphate Reverse Flotation using a Pico-nano Bubble Generator

Abstract

Froth flotation is widely used in minerals industry. However, it is effective in a narrow size range, nominally 45-250 μm for coal and 10-150 μm for minerals. And the flotation performance of coal and minerals beyond this size range is not quite satisfactory. It is now recognized that the low-flotation efficiency of ultrafine particles is mainly due to the low probability of bubble-particle collision while the main reason for poor flotation recovery of coarse particles is the high probability of detachment of particles from the bubble surface. Pico-nano bubbles refer to tiny bubbles smaller than 0.5 μm, which can be produced using ultrasonic or hydrodynamic cavitation method. A fundamental analysis has shown that pico-nano bubbles will serve as a secondary collector, bridging between hydrophobic particles and conventional sized bubbles, which can significantly improve the flotation recovery of hydrophobic particles by increasing the probability of collision and attachment and reducing the probability of detachment. In order to obtain an optimal designed cavitation venturi tube for pico-nano bubbles' generation, different designs of cavitation venturi tubes were fabricated and tested in this study. The test results showed that maximum volume of pico-nano bubbles, 65% to 75%, and minimum mean pico-nano bubble size, 150 to 240 nm, were achieved at the medium ratio of the diameter of throat and outlet of the venturi tube (3 to 4), medium outlet angle (11°to 13°), high inlet angle (26° to 27°) and high ratio of the length of the throat and the diameter of throat (2.3 to 3). The laboratory-scale coal flotation tests showed that, the combustible material recoveries for coarse fractions,-700+355 μm, and ultrafines fraction,-75 μm, were remarkably increased in the presence of pico-nano bubbles in coal flotation. The maximum combustible recovery of 85 to 90% and minimum clean coal ash of 9.5 to 11% were achieved from 29.6% ash content of Pittsburgh No. 8 seam coal. At a given flotation combustible material recovery, more than half of the collector can be saved by using pico-nano bubbles in the column flotation. The frother dosage was also reduced to half. The laboratory-scale phosphate reverse flotation tests revealed that both the P 2 O 5 grade and P 2 O 5 recovery had been greatly improved in the presence of pico-nano bubbles. The column flotation using pico-nano bubble was capable of extending to float minus 75 m and 355-700 m particle size ranges much more effectively than column flotation with conventional sparger devices. The economic analysis performed in this study demonstrated that the application of pico-nano bubbles in both phosphate reverse flotation and coal flotation were economically feasible.

Effect of Hydrodynamic Cavitation Assistance on Different Stages of Coal Flotation

“…Some researchers have used HC as a pulp pretreatment method before flotation [8][9][10][11][12][13]. Other studies have used HC in flotation separation processes; the tests are usually carried out in flotation columns [14][15][16][17]. Both of the HC-assisted flotation methods have been proven to improve concentrate recovery and flotation efficiency.…”

“…On the other hand, nanobubbles, which preferentially grow on the surface of hydrophobic material, improve the hydrophobicity of the target minerals and function as a "secondary collector" [19]. Due to the nanobubble-bridging effect, fine particles form aggregates with the nanobubbles and increase in size, thus enhancing flotation efficiency [14,20].…”