The thickness of adsorbed water layers required to cause changes in the surface properties of urea was determined by comparing the mesured values of surface area, surface conductivity and X-ray diffraction peak intensity on the urea particles, in varying humidity with the water vapor adsorption isotherm. An adhesive force among the particles by water layers adsorbed and a change in the caking strength of the shaped urea with increasing water vapor pressure were also estimated.
The caking tendency and a change in the surface area were recognized at humidities above ca. 20%rH, at which water molecules with thickness 1.2 layer were adsorbed. This fact suggests that the adsorbed water layers dissolve partially the surface of the salt and some deposits are formed in the neighborhood of contacting points among the particles, making salt-bridges. On the other hand, a cohesive force could appeared above 30%rH, at that humidity the two layer adsorption was completed. At water vapor pressures above 60%rH, such the bridge formation occurred in a macro-scale, therefore the caking cecame remarkable.

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Dimensions of apparatuses in the closed circuit cement grinding system have increased remarkably in recent years. However, the efficiency of comminution has not yet been improved so much.
In this paper, the authors investigated the relations between output and grinding conditions in the closed circuit cement grinding system, by combining the rate equation of comminution in a tube mill with the relations between the classification characteristic values and the particle size distribution of materials after classification with air separator, as well as with the experimental results obtained with the test separator, so as to increase the efficiency of comminution or further to optimize the grinding system. Dimensions of mill and separator, fineness of the finished products and circulating load were considered as the grinding conditions. The results of numerical computation were summarized as nomographs.
Estimated values from the nomographs considerably agreed with actual values. The rate of finished products increases with increasing size of separator, when the same mill is used. Meanwhile, an optimum circulating load seemed to be expressed as a function of the ratio of the dimensions of separator to mill as well as the fineness of products.

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