On the Determination of Molecularly Bound Water in Fine-Grained Materials

A. S. Boida, V. M. Vygodin

(Presented by the scientific seminar of the Department of General Chemical Technology)

The rate of the granulation process of fine-grained materials is determined primarily by their surface properties. The nature and surface properties of materials, as is known, are reflected rather accurately by the amount of molecularly bound water. However, the determination of this form of moisture in materials presents considerable difficulties.

One of the methods for the quantitative assessment of molecularly bound water is the determination of the maximum molecular moisture capacity (MMM) of materials, approximately corresponding to the amount of molecularly bound water.

The MMM index is usually determined by the method of moisture-intensive media [1]. In this method, a sample of moist material in the form of a pellet is placed between two packs of filter paper and compressed under a press at a pressure of 65–70 atm for a certain time, after which the residual moisture content corresponding to the MMM is determined.

Studies by S. I. Dolgov [2] and other authors have established that the moisture content of a material decreases as the load and the duration of pressing increase. Therefore, the MMM index determined by the method considered is somewhat conventional. For comparison, in addition to the method of moisture-intensive media, we determined molecularly bound water by the method of continuous drying.

In accordance with the theory of drying developed by A. V. Lykov [3], the regularities of drying porous bodies depend not only on the drying regime and on the physicochemical properties and structure of the body, but also on differences in the strength of the bond between water and the solid phase. According to the nature of its bond with the solid phase, as is known, four basic forms of water are distinguished: firmly bound (adsorption), loosely bound, capillary, and gravitational [4]. Adsorption water and loosely bound water together constitute molecularly bound water. These differences in the form of moisture bonding largely determine the character of the drying process of porous bodies, the regularities of which are most clearly reflected in drying-rate curves.

Natural dusty quartz-marshalite was used as the object of the study. The material was moistened to a slurry state and loaded in a layer of 2–3 mm into a mesh cuvette 10 mm in diameter. The cuvette with the sample was suspended by means of a thread from the pan of an analytical balance and placed in a thermostat that ensured constancy of the specified temperature within ±0.1°C.

The loss in weight of the specimen was determined by periodic weighing, and the percentage of moisture was calculated relative to the weight of the absolutely dry material. From these data, drying curves were constructed. From the drying curves obtained, drying-rate curves were constructed by the method of graphical differentiation [3] (Fig. 1).

It is evident from Fig. 1 that the curves have a typical form, expressed in the presence of a horizontal segment corresponding to a constant drying rate. In the portion of decreasing drying rate, the two breaks present (at a temperature of 50°C) and the three breaks (at a temperature of 70°C)—the critical points—also correspond to the transition of one form of moisture into another. Thus, on the upper curve, segment 1—2 corresponds to the evaporation of capillary moisture, while the segment below critical point 2 corresponds to the evaporation of molecularly bound water. At a temperature of 50°C, the segment of evaporation of capillary water is not clearly distinguished on the drying curve. The MMW of marshalite, determined by the method of moisture-capacity media, is 11.3—12.1%. According to the continuous-drying method, as is evident from Fig. 1, the interval of values for molecularly bound water is broader, amounting to up to 17% at a temperature of 70°C and up to 19.8% at a temperature of 50°C.

Fig. 1. Drying-kinetics curves of marshalite.

Thus, the continuous-drying method makes it possible only to establish the limits of the content of molecularly bound water, the value of which depends substantially on the drying conditions. The use of this method for determining the forms of water in a moist material requires additional refinement.

LITERATURE

1. A. M. Vasil’ev. Fundamentals of Modern Methods and Techniques for Laboratory Determinations of the Physical Properties of Soil. State Publishing House for Literature on Construction and Architecture, Moscow, 116, 1953.

2. S. I. Dolgov. Investigation of the Mobility of Soil Moisture and Its Availability to Plants. Academy of Sciences of the USSR, 1948.

3. A. V. Lykov. Theory of Drying. Gosenergoizdat, Moscow—Leningrad, 209, 212, 1950.

4. E. M. Sergeev. Soil Science. Moscow University Press, Moscow, 161, 1959.