Comparison of organic solvent-based ethylcellulose coatings of KCl crystals applied by top and bottom spraying in fluidized-bed equipment

Differences between the efficiency of organic solvent-based membranes applied by countercurrent coating and by Wurster-based, concurrent coating were examined by coating KCl crystals with an ethylcellulose-based membrane, The dissolution from crystals coated by the concurrent process was a factor 2.6 – 2.8 slower than that achieved by the countercurrent process as assessed at the time for dissolution of 63.2%. The factor was shown to be similar on both the laboratory and production scales. The cause of this higher efficiency was examined. The coated crystals were examined by sieve analysis, by measuring the specific surface area, the film thickness, the amount of coating dry matter applied, by scanning electron microscopy of surfaces and cross-sections, and by determining the porosity of the membranes. It was shown that only the membrane porosity can explain the differences in coating efficiency.