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Fifty-four samples of sodium sulfate-bearing soil excavated from mines in the Salt Desert (Dasht-e-Namak) of Iran were examined to investigate difficulties encountered during sodium sulfate extraction by the partial dissolution of the raw materials in water, using pressure filtration. From min-eralogical and chemical composition and particle-size distribution of the samples, the chief causes of the extreme decrease in filtration rate were found to be: (1) water adsorption and enhanced dispersion of clay minerals, particularly smectites, in a Na2SO4-rich slurry, which caused the formation of colloidal particles and hence decreased the filter cake porosity and permeability; and (2) disintegration of aggregates of insoluble particles during agitation of the slurry, which caused a reduction of the average particle size and, hence, of the cake porosity.

The filtration rate was increased by the addition of lime to the slurry and by the reduction of particle disintegration through reducing the agitation time of the slurry from 30 to 3 min. This treatment caused only a small decrease in dissolution efficiency of sodium sulfate (from 91 to 85%).

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