2012 Borowik Studies on the screw granulation of fertilizers manufactured on the basis of urea and calcium sulphate adduct
The completed investigations demonstrated that fertilisers classified to the type of calcium sulphate and urea can be produced with use of screw granulation. Fertilizers with the high degree of urea conversion into the form of adduct can be produced from such raw materials as crystallized urea and phosphogypsum with reduced total content of water (28-30% H2O w/w). For phosphogypsum grades with high water content (phosphogypsum from ZCh ‘POLICE’) it is first necessary to pre-dry the material or use of a feedback loop to supply dried and milled final product at the amount sufficient to achieve production of the final mixture with the total content of water ranging from 12% to 14%. It was revealed that reduction of the total water content in the reactive mixture to the level of 10% entailed considerable growth of pressure inside the extruder and, in turn, increased energy consumption of the manufacturing process. It is beneficial to run the extrusion process at the elevated temperature (ca. 40ºC) since it enables to reduce pressure inside the extruder. The granulated materials produced under the foregoing conditions demonstrated high degree of urea conversion into adduct as well as high mechanical (crushing) strength, exceeding 110 N/gran. It was found out that admixture of kieserite to the blend of urea and phosphogypsum led to increased plasticity of final granulates and drop of their crushing strength, similarly to the degree of urea conversion.