Water Quality Regulationof Coal Slime WaterClarification And Coal Flotation

Coal is the most important source of energy in China, which accounts for72.85%of total consumption of primary energy, national coal production has reached3.5billion tons in2011. Three tons of water is consumed to separate one ton of raw coal, and two-thirds of China’s coal is located in the northwest dry region, producers have to reclaim water from coal slime water. Therefore, a kind of high efficient, low consumption and green treatment method ofcoal slime water should be proposed to improve ecomomic returns, save water resources, and reduce emission.According to the properties of solution chemistry and colloidal chemistry of coal slime water, a water quality regulation technology of coal slime water clarification and coal flotation was proposed, and the effect of water quality on coal slime water clarification, coal flotation andinteraction behaviorsof mineral particles were studied respectively, then, clarified recycling of coal slime water and high efficient coal flotation can be realized by using water quality regulation technology.Circulating coal slime water system is a mixed dispersion system with various dispersion states, which contains water, ions and fine particles. From the studies of mineral compostion and mineral properties, solution chemistrycharacteristics and colloidal chemistry characteristics of circulating coal slime water system, adsorption characteristics of clay minerals, we can known that kaolinite, montmorillonite and illite are the main gangue minerals in raw coal, and calcium element is existed in the form of sediment of CaCO3or Ca (OH)2(s) in clay mineral surface. And then, a prediction model of equilibrium adsorption amount was developed, and the conception of original hardness of circulating coal slime water system was proposed, which depends on the hardness of additional water (base hardness) and mineral compostion in raw coal.Settling tests were conducted to study the effect of water quality on coal slime water clarification. It was demonstrated that coal slime water became easy to clarify with increasing water hardness, meanwhile, it can save consumption of polymeric flocculants with high water hardness. DLVO theory was calculated to determine the lowest water hardness of mineral particles coagulation. It proved that critical hardness is inversely proportional to the square of Hamaker constant and ion valence of the symmetric type electrolyte, while proportional tobiquadrate of the surface potential of particles.Considering the clarification process of the coal preparation plant, particles were moving in the form of interference settlement in vertical rise flow in the settling zone of thickener, and the effective surfaceweight was proposed to be the critical condition of particles sedimentation.Laboratory flotation tests, industrial scale flotation tests and artificial mixture mineral flotation tests were designed to study the effect of water hardness on coal flotation. The results of laboratory tests showed that ash content of flotation clean coal was increased by1-2%with the increasing of water hardness, the ash content of clean coal became stable when the water hardness was between6mmol/L and10mmol/L, and there was no obvious change for the yield of flotation clean coal. The results of industrial scale tests demonstrated that ash content of flotation clean coalwas increased by1.1%when the water hardness increased from3.1mmol/L to6.4mmol/L, the ash content of clean coal leveled off when the water hardness was between6.4mmol/L and7.4mmol/L. Single mineral microflotation of coal, artificial mixture mineral microflotation of coal-kaolinite and coal-montmorillonite were carried out to further study the effect of water quality and clay minerals on coal flotation, it can be concluded that the ash content of microflotation concentrate increased by about3%in the presence of clay minerals, and the ash content would further increased by another3%in co-presence of clay minerals and high concentration of Ca2+.Zeta potential distribution measurement of single minerals of coal, kaolinite, montmorillonite, illite and mixed minerals of coal-kaolinite, coal-montmorillonite, coal-illite were carried out, it can be concluded that the presence of Ca2+can promoted homogeneous and heterogeneous coagulation, and Na+had no obvious effection on coagulation. Optical microscope and laser particle size analysis was proposed to evaluate the coagulation of fine particles, the results showed that the higher concentration of Ca2+, the easier coagulation of fine paritcles. It can be known that the effect of water quality on coal slime water clarification and coal flotation can be boiled down to the effect of water quality on dispersed-coagulated state of fine particles. The higher concentration of Ca2+, the easier coagulation of fine paritcles, and it was benefit for coal slime water clarification. The lower concentration of Ca2+, the easier dispersion of fine paritcles, and it was helpful for coal flotation.Atomic force microscope (AFM) was used for interaction force measurements between coal and clay mineral particles. It can be concluded that the influence trend of water quality on coal slime water clarification and coal flotation can be explained by the influence trend of water quality on interaction force. In the settling tests and flotation tests, when the concentration of Ca2+less than5mmol/L, the higher concentration of Ca2+, the weaker electrostatic repulsive force between particles, and the easier coagulation of fine paritcles. So the initial settling velocity and ash content of flotation clean coal increased gradually with increasing concentration of Ca+. When the concentration of Ca2+above5mmol/L, further increasing the concentration of Ca2+didn’t further greatly increased the initial settling velocity and ash content of flotation clean coal. And there is almost no effect on interaction forcein the presence of Na+.The research on the effect of water quality on coal slime water clarification and coal flotation showed that it is benefit for coal slime water clarification with higher water hardness, and it is helpful for coal flotation with lower water hardness. A water quality regulation technology which used MC as water quality regulator was proposed, and the soft-sensing technique by using electrical conductivity as the auxiliary variable was used for water hardness online monitoring, the amount of water quanlity regulator was depended on the water hardness of coal slime water system and the turbidity of recycle water.According to the properties of circulating coal slime water system of Xingtai mine coal preparation plant, the turbidity of recycle water could be less than0.3g/L when the water hardness of thickener feed was about3.5mmol/L by controlling the amount of MC, and then the water hardness of flotation slurry can be controlled below2mmol/L, so the negative effects of high water hardness on coal flotation can be avoided.