The Relation Beween Flotation Froth Property And Clean Coal Ash Content

Ash content is a vital indicator for coal industry.In coal preparation plants,the performance of the gravity separation of bulk particles is usually better than that of the flotation of fine particles,so the ash content of flotation concentrate must be strictly controlled to maximize the financial profitability.Traditionally,operators of flotation could only adjust parameters timely according to their experience.The exact results of coal flotation performance could only be tested after the long-time processing of sampling,filtrating,drying,sample preparing and burning to ash.It is too late to adjust the flotation parameters after obtaining the result of concentrate ash content,which restricts the development of flotation control.Once the disturbances of flotation input appear,amounts of concentrate could not meet the requirement of ash content by the delayed adjustment.The timely,accurate and applicable measurement of the concentrate ash content is needed to be solved.In this research,the relation beween flotation froth property and clean coal ash content was studied to solve the problem above.The range of particle size and density of flotation feed is wide,and the extremely coarse(around 0.5 mm)and hydrophobic(contact angle > 65°)particles could cause abnormality in froth properties.The effect of particle size and density on froth properties was studied first.The feed was divided into coarse,medium,and fine size fractions and the flotation was conducted with particles of single and mixed size fractions.Then the yield and ash content of clean coal particles of different size fraction and the corresponding froth property were analyzed.The froth properties investigated include water recovery,homogeneity(bubble size),burst parameters,and froth velocity.The effects of fine coal particles with different density and hydrophobicity on the froth properties and the recovery of coarse coal were also investigated.By analyzing the coarse coal yield,water recovery,concentrate weight composition,froth gray value,and froth height over weir,two mechanisms of increasing the coarse coal recovery by fine coal particles were respectively put forward for hydrophobic and hydrophilic particles.The hydrophobic particles mainly affected the dynamic froth stability by hydrophobicity and the overflowing froth volume by the amount of floatable particles,while the hydrophilic particles affected water drainage by the entrained particles in the froth network.These critical froth properties influence the coarse coal recovery together from different aspects.Flotation operating parameters affacted froth properties and their relation with clean coal ash content.The ash content of clean coal increased linearly with the water recovery,gray value,and froth velocity and decreased with froth homogeneity at different frother dosage,gas velocity and froth height.The changing ranges of the froth properties and ash content were more profound with shorter froth height.Homogeneity correlated best with the clean coal ash content by polynomial fitting when different flotation operating parameters were changed together.The coefficient of determination(R2)was 0.9028,0.809,0.630 and 0.543 for homogeneity,water recovery,gray value,and froth velocity.It was found that froth variable based on foam geometry correlated best with the clean coal ash content.A model of clean coal ash content based on foam geometry was proposed by some assumptions and Kelvin cell was used to quantify the foam geometry to represent the foam liquid fraction and clean coal ash content.Foam drainage caused the change of foam structure and affected product ash content.In order to study the change of foam drainage regime from two phase to three phase reliably,the effect of experimental condition on power law relation index in forced drainage was first discussed in a foam.The particle behaviours during foam drainage were also studied.It was found that the effect of Plateau border viscous loss on foam drainage increased when the foam was wetter with shorter drying time.The large variation of ?(0.38 – 0.47)indicated a significant influence of foam instability on power law relation index in forced drainage experiments.Three regions(ascending,swirling and convective regions)of foam instability were divided with the increase of liquid fraction and two revised calculation methods for predicting the critical liquid fraction were proposed.It was sugguested that the traditional forced drainage experiments should be conducted at low liquid fractions where the analysis of drainage regime is credible.The Plateau border viscous loss increased and the foam permeability decreased in three phase forced drainage experiments with particles.In the drainage experiments with particles of different ash content,it was found that the remained particles mass of different hydrophobicity in froth with short drainage time decreased with particle ash content due to that the hydrophobic particle with low ash was easy to attach on the bubbles and difficult to detach with the drainage flow.