Density Adjustment In Gas-Solid Fluidized Bed For Beneficiation Using Geldart A Dense Medium

Coal,as the primary energy resource,plays the vital influence on the economic development in the world.Coal beneficiation is an important part to achieve sustainable and healthy energy development.Due to shortage of coal distribution,investment cost and coal properties,wet beneficiation methods still has some deficiency.It is urgent to study efficient dry coal beneficiation technology.Gas-solid fluidized bed beneficiation technology is one of the representative dry technologies,which has been successfully applied for coal beneficiation in industry.However,when the bed height is high,bubble size is relatively bigger,influencing the bed density stability in the fluidized bed using Geldart B dense medium.Therefore,Geldart A particles is selected to increase proportion of emulsion phase and improve the stability of bed density and increase the bed height.Based on above analysis,Geldart C ultrafine pulverized coal is selected as additive to decrease the average size and density of dense medium firstly.In addition,the mixture of A particles and additive can improve the expansion ratio of emulsion phase and whole bed.As a result,the adjustment range of bed density is widened,meeting the industrial requirements.Combined with the correlation curve between operational gas velocity and bed pressure drop,the influence of bed height on the minimum fluidization velocity of Geldart A particles was studied.The difference in the variation of minimum fluidization velocity of Geldart A and Geldart B was then compared.The results showed that the influence of bed height on the minimum fluidization velocity of Geldart A particles was relatively slight.By summarizing the minimum fluidization velocity data of Geldart A particles in the literatures,the prediction errors of different minimum fluidization velocity models were compared,and the best prediction model was then selected for predicting the minimum gas velocity of Geldart A particles.The bubble behavior in Geldart A particles has been carried on the detailed study.The bubble distribution and size were then analyzed.In addition,bubble size in Geldart A and Geldart B particles was compared.The results showed that bubble size is small in Geldart A particles.With the increasing bed height,bubble size increases first,then keeps stable.In order to improve the prediction of bubble size in the fluidized bed using Geldart A particles,the bubble growth model was modified based on Darton's bubble size model.Combined with the bubble growth model,the bubble velocity model is further established.The results show that the model can accurately predict bubble size and velocity,which provides a theoretical basis for understanding bubble motion behavior.Combination with the above studies on minimum fluidization velocity and bubble motion behavior,the expansion characteristics of Geldart A particles were explored,and the variation of emulsion phase and bubble phase were analyzed by using the bed collapse curve.Combination of the two-phase theory in gas-solid fluidized bed,prediction model of bed expansion was further established for Geldart A particles.The results showed that the experimental and the theoretical data have small difference.The difference of the error can be controlled at 10%,showed that the prediction precision is higher.Thus,the model can accurately predict bed expansion behavior to understand the two-phase distribution in the gas-solid fluidized bed using Geldart A particles.Meanwhile,the model is beneficial to establish the model of prediction bed density.Based on the above studies,the bed density distribution and density stability in Geldart A particles were systematically studied in the gas-solid fluidized bed.The radial and axial variation of bed density was analyzed in the fluidized bed,and the difference of bed density distribution between Geldart A and Geldart B particles was compared.Combined with the above study on bed expansion behavior,the prediction model of bed density was established.Based on the comparison with the experimental data of bed density,the prediction model of bed density was further modified to improve the accuracy of the model prediction.Detailed research was then carried out about the bed density stability.The time domain characteristics and square deviation of bed density was analyzed.Probability distribution and cumulative distribution of bed density were then put forward for further studying on the bed density stability,the results showed that density distribution is uniform in fluidized bed using Geldart A particles,providing theoretical basis to achieve the higher separation efficiency.The beneficiation density was studied to verify the prediction model of bed density in the fluidized bed using Geldart A particles.Through the force analysis of simulated minerals in fluidized bed,the theoretical separation density model for minerals was established,and the difference of theoretical separation density between Geldart A and Geldart B particles was compared under different operation factors.The resultant force of simulated minerals was studied by dynamometer.Through the beneficiation experiments in gas-solid fluidized bed,the predicted and experimental data of separation density were compared,which further indicated that the separation density model had higher reliability and could accurately predict the separation density in the fluidized bed using Geldart A particlesGeldart C ultrafine pulverized coal was selected as additives to widen the range of density adjustment.The influence of ultrafine pulverized coal on the expansion emulsion phase and the whole bed were analyzed.The effect of ultrafine pulverized coal on bed density and separation density were then investigated.The results showed that the ultrafine pulverized coal can improve the range of density adjustment between1.5~2.0 g/cm~3,meeting the industrial requirements.It further verified the possibility to use the Geldart A particles for coal beneficiation.