| With the improvement of coal mining mechanization and the deterioration of geological conditions in coalfields,the secondary slime raises resulting in a sharp increase in the content of coal fines in coal preparation process.This directly leads to a series of problems,such as the raised flotation pressure,the increase of drug consumption and the severe pollution for clean coal.These issues hinder the improvement of coal quality and clean-and-efficient of coal processing,leading to the increase of the cost of coal washing process,all of which are not in line with the current demand for a green energy economy.In order to improve coal separation efficiency and develop clean coal utilization,many scholars conducted theoretical and practical studies on flotation technology recovering coal fines in industrial applications.It is generally believed that flotation reagents improvement is the most convenient and efficient way to improve flotation efficiency.According to theoretical and practical studies,it is found that preparation of flotation agent into emulsion can not only reduce the drug consumption,but also increase the flotation rate and the yield of clean coal.In recent years,it is found that the addition of frother is non-uniform,and it is difficult to quantitatively control,resulting in the fluctuation of flotation effect and the waste of flotation chemicals.In the present study,to solve the above problems,a series of experiments were carried out on the development and utilization of frother emulsions,including the preparation,properties,and applications of the frother emulsions.The present paper focused on frother emulsion stability,foaming properties and coal flotation behavior,by studies of frother emulsion preparation,emulsion stability,frother and its emulsion foaming performance,flotation performance comparison,fractional flotation,flotation rate,laser particle size analysis,Zeta potential analysis,Turbiscan stability analysis,X-ray diffraction spectroscopy,X-ray fluorescence spectrometry,and combined with DLVO theory and classical flotation kinetics model.The research mainly includes the emulsification and foaming mechanism,stability of frother emulsion and foaming ability of the frother emulsion.In addition,the study investigated the proper particle size of slime for frother emulsion,established classical flotation kinetic model,and revealed the effect of frother emulsion on flotation behavior of the coal slime.The present paper chose typical frother for coal preparation,including isopentyl alcohol,methyl isobutyl methanol and 2-octanol,as emulsifying objects,and the common emulsifiers,including Span 80,Tween 80,Tween 20,and sodium oleate compound,as raw materials for emulsifier formulation.The average particle size of the emulsion was used as the evaluation index.A high-shear emulsification method was used to prepare the frother emulsion.The results of frother emulsion preparation showed that the three major factors for emulsion forming were the energy input,the oil-water two-phase and the emulsifier.The key factors for emulsion forming by high shear method were shearing time,shear rate,oil(water)content and the type and amount of the emulsifier.The present paper studied the storage stability,the acid-base stability and ion stability of the emulsion,to determine the storage time and operating environment of the emulsion,especially the dispersion in coal slime.The storage stability was measured by a Turbiscan stability analyzer.The results showed that the stability of the frother emulsion decreases with prolonged storage time.The stability of the frother emulsion is not only related to the inherent interfacial properties of the emulsion system,but also to the solubility of frother.The greater the frother solubility,the greater the increase in average particle size with longer the storage time,leading to weak stability.The result of acid-base stability test showed that both the acidic and strong alkaline environments are not conducive to frother emulsion stability,while the weak alkaline environment can enhance emulsion stability.Ionic stability study showed that the ions have a negative effect on the stability of the frother emulsion.The higher ion concentration and the greater valence leaded to more unstable frother emulsion.The 2-octanol emulsions prepared in this paper performed best in the three emulsions in terms of storage stability,acid-base stability,and ion stability.A foam testing system was established based on the improved air flow method in this paper to study the foaming performance of frother and its emulsion,by investigation of foaming capacity,foam stability,and foam average flow rate of the frother.The mechanism of the effects of frother and its emulsion on foam performance was studied by Laplace formula.The results of foaming performance test show that,with the increase of frother dosage,foaming capacity of frother enhanced,foam stability increased,foam average flow rate decreased,and foam viscosity increased.In addition,different types of frother have different foaming capabilities,foam stability,and foam average flow rates.For frother with same polar group,the longer the non-polar based carbon chain,the lower the solubility and foaming capacity are.However,the interaction between the frother molecules,and between the frother and water enhances the foam viscosity and stability,with longer carbon chain.By preparation frother into an emulsion,the emulsifier on the one hand can reduce the surface tension of the system and enhance the foaming capacity.On the other hand,it leaded to an increase in the non-polar carbon chain of the frother and enhances the intermolecular interaction,which increased water viscosity and enhanced foam stability.After emulsifying the three frothers,the foaming capacity of the octanol emulsion has the largest increase,the foam stability changes minimally,and the viscosity change range is the smallest.Therefore,the 2-octanol emulsion is more suitable for coal flotation.In addition,the effect of the slime water on the stability of the 2-octanol emulsion was studied.It was found that the particle size of the 2-octanol emulsion treated with different coal slimes with different pH values and comparable ionic strength changed little,which can maintain long-term stability.This shows that the 2-octanol emulsion can be better adapted to the coal slimes with slightly acidic,slightly alkaline and low electrolyte concentrations environment.In another word,the 2-octanol emulsion can be better dispersed in the slurry.The 2-octanol emulsion was selected as the flotation frother in the present paper.The effect of the emulsion on the flotation of coal slime was studied by flotation comparison test.The results showed that the frother emulsion can improve the flotation effect of coal slime.With the same frother dosage,by addition of frother emulsion,when the clean coal ash content is equivalent,the clean coal yield,the combustible recovery rate and the flotation improvement index increased by 1.12-1.44%,1.14-1.44%,and 0.11-0.33%,respectively.When flotation index is the same,20-30% of frother can be saved by using frother emulsion.In this paper,the flotation size suitable for the emulsion was investigated by comparative experiments of fractional flotation of octanol and its emulsion.It was found that the frother emulsion has good flotation effect on fine coal slime and poor flotation efficiency on coarse coal slime.For coarse coal slime with 0.5-0.25 mm grain size,the flotation effect of frother emulsion is worse.For coal slime with the 0.25-0.125 mm grain size,the floatation effect of the frother emulsion changed little,and for fine slime with 0.125-0.074 mm and-0.074 mm size,the flotation effect of frother emulsion was significantly improved.The difference in foaming performance between the frother emulsion and the frother leaded to difference in flotation efficiency.By preparation of frother into an emulsion,the foaming rate increased,a large number of bubbles with small diameters are generated,and the probability of colliding with fine particles is increased,which is favorable for the recovery of fine coal.It is not conducive to coarse particles,but the effect on fine particles has been more pronounced,so overall it showed better flotation effect.In this paper,the flotation behavior of coal slime was tested and theoretically studied.The classic kinetic model was established by fractional flotation speed test.The flotation process was deeply discussed through the flotation rate constant and the average flotation rate constant.Under the action of the octanol and its emulsions,the flotation rate constant of each particle size during the flotation process tends to decrease with the prolongation of the flotation time.At the beginning of the flotation,the coal slurry concentration and flotation agent dosage are sufficient,and the flotation rate constant is relatively large.As flotation progresses,the coal slurry concentration and the amount of flotation agent began to reduce,and the flotation rate constant also decreases.Compared to the 2-octanol,with the addition of 2-octanol emulsion,the average flotation rate constant for coal samples with a full-size(-0.5 mm)coal fraction increased,the average flotation rate constant for very coarse(0.5-0.25mm)coal fraction decreased,the average flotation rate constant for coarse(0.25-0.125mm)coal fraction changed little,the average flotation rate constant for fine(0.125-0.074mm)and very fine(-0.074mm)coal fraction increased.The conclusion of the model is consistent with the test result. |
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