Effects Of Extra Hard Water On Coal Slime Flotation

In order to reveal the influence laws of water quality on coal slime flotation, extra hard water obtained from Wuhai Coal Mine Area and Da Wukou Coal Mine Area was selected as research object. Then, influences of water hardness on coal wettability, coal zeta potential and bubble size distribution were studied. Besides, the effect of extra hard water on bubble mineralization was analyzed. Furthermore, contrast tests in flotation were carried out by using three different metamorphic degree coal samples.Contact angle of different particle size fraction of three types of coal samples under different hardness levels was tested respectively by contact angle measurement. Results show that the wettability of each grade coal sample increases with the increasing hardness of extra hard water as inorganic salt ions can promote the content of-OH which is a part of coal molecule and shows strong hydrophilicity. Moreover, the coal wettability is stable at 210 ° DH. In addition, the wettability of TX coal is mostly affected by extra hard water because of its highest oxygen content and largest oxygen group number among the three samples tested.Micro electrophoresis apparatus was used to testify the zeta potential of each grade of three different metamorphic degree coal samples. As electrolyte ion existing in extra hard water can compress diffusion layer and reduce its thickness, the zeta potential of coal samples decreases with the increasing hardness of extra hard water and finally achieves its stability at 210°DH. Three kinds of particle size fraction were tested, including 0.5-0.25 mm, 0.25-0.125 mm and below 0.125 mm. It is found that coal slime of particle size below 0.125 mm is most heavily affected by extra hard water on account of its highest hydrophilic inorganic mineral content, biggest oxygen group quantity and strongest electronegative. While, the result of the experiment on LXJ coal is reverse.Using self-designed pneumatic device, properties of bubble generated by extra hard water with and without frother were measured under different quantity of gas flow. The extra hard water can polarize water molecules to form stable hydrated layer as well as reduce the bubble rate of merger and acquisition, thus it is inclined to promote the formation of tiny air bubbles, which results in its foamability. The experimental results indicate that the foamability of extra hard water increases with the rising of water hardness. Meanwhile, extra hard water can strengthen the performance of frother. On one hand, it can promote absorption of salt ion on the gas-liquid interface and form hydration layer which may hinder the bubbles of merger and acquisition; on the other hand, electrolyte ions in high hardness water may interact with nonpolar group of frother, which enables to improve frother solubility. In addition, extra hard water can promote probability of bubble-particle mineralization and flotation rate. Three reasons are as follows. To begin with, extra hard water could enhance the bubble stability and make bubble size distribution move toward the direction in which the bubble diameter decreases. Besides, it could facilitate the contact between bubbles and particles by reducing the rate of air bubbles and increasing the number of air bubbles. What’s more, it could also lower the energy barrier of collision and attachment occurred between bubbles and particles through decreasing bubble zeta potential and particle zeta potential.Floatability test results of three types of coal samples show that clean coal yields and combustible recovery increases with the rising hardness of extra hard water which is mostly affected in the hardness of 210 ° DH. It is also found that both the flotation rate and combustible recovery rate increase with the increasing hardness of extra hard water, and the fine particles are more affected compared with the coarse ones. To some extent, extra hard water would decrease the selectivity of coal slime though it can improve coal slime flotation. On one side, with electric double layer around coal particle being compressed by mineral ions, extra hard water could decrease electrostatic repulsion between coarse particles and fine particles, facilitate electric double layers both around coarse particles and fine particles to interact with each other, ultimately resulting in the fine particles more easily adhering to coarse ones. On the other side, extra hard water could increase the volume of liquid carried by bubbles through enhancing foaming agent performance, which consequently makes high ash fine particles mixed into concentrates with the force of water. With bigger mass and stronger secondary enrichment, the selectivity of coarse particles is less affected than that of the fine ones.