In this paper, the minimum fluidization velocity and bed expansion of polydisperse sand mixtures fluidized with water were investigated. The fluidization experiments were carried out using 19 different fractions of quartz filtration sand with sieve diameters in the interval d(m) = 0.359 to 2.596 mm. The intercept fluid velocity U-E was determined from the plot of fluid superficial velocity vs. porosity. The results obtained indicated that U-E was about 11% higher than particle terminal velocity U-t, for all of the sand fractions investigated. Based on the experimental data, the new model for bed expansion of quartz filtration sand was proposed. Using the proposed model, the mean absolute deviation in predicting the bed porosity epsilon, for a given superficial velocity U, was 2.66%, while the mean absolute deviation in predicting U at a given epsilon was 8.75%. For the prediction of the minimum fluidization velocity, U-mf, several literature models were tested. The best results were obtained using the Ergun model, with mean absolute deviation in predicting U-mf of 12.42%. A new simple correlation for the prediction of the minimum fluidization velocity U-mf was proposed. The proposed correlation is the function only of the mean sieve diameter of the filtration sand fraction. The mean absolute deviation between the experimental and the predicted values of U-mf was 7.65%. (C) 2015 Elsevier B.V. All rights reserved.