EFFECT OF FINELY DISPERSED MINERAL IMPURITIES ON CRACK FORMATION DURING THE DRYING OF PEAT MOLDINGS

S. I. Smolyaninov, G. G. Krinitsyn

(Presented by the scientific-methodological seminar of the organic departments of the Chemical-Technological Faculty)

According to A. V. Lykov [1, 2], the shrinkage of a moist material under a uniform distribution of moisture content and temperature within the material is a physical property of a body when liquid is removed from it and does not give rise to any dangerous stresses. Under real conditions, a greater or lesser nonuniformity in the distribution of moisture and temperature over the volume is always observed; this is the cause of the appearance of a volumetric-stress state and, if it develops beyond the limiting permissible value, leads to the appearance of cracks (local failure), as well as to complete destruction of the body.

Peat undergoes shrinkage during the entire drying period [3]. Figure 1 gives the shrinkage coefficients of peat (lowland sedge-hypnum peat, degree of decomposition 25–30%, working moisture content 87.0%, ash content on dry matter 11.52%, degree of processing 3) and of its mixtures with magnetite concentrate from the Abagur concentrating plant (iron content 62.7%, degree of grinding—through a 0.08 mm sieve) and reactive iron oxide (“chemically pure,” degree of grinding—through a 0.05 mm sieve), dried in the form of cylindrical moldings (40 × 60 mm) under three drying regimes. Drying was carried out: a) in open air at 21–28°C and a relative humidity of 84–98% (conventionally, a severe regime); b) in a climatic chamber at 40°C and a relative air humidity of 80% (conventionally, a mild regime); c) in a laboratory room at 20–24°C and a relative air humidity of 62–68% (conventionally, an intermediate drying regime).

Proceeding from S. S. Korchunov’s definition [4] of the maximum (limiting, in the case of a two-phase system) coefficient of volumetric shrinkage, equal to the ratio of the density of the solid phase to the density of water, it may be shown that the ratio of the actual shrinkage coefficient to the limiting one is the content of the solid phase in an absolutely dry sample, characterizing the packing density of the material. The data of Fig. 1 indicate a noticeable effect of iron oxides on the structure of peat, which, at a certain amount of filler, creates the possibility of denser packing of the particles of the solid phase. The influence of the drying regime on the shrinkage coefficient is also reflected here.

Figure 2 shows the change in the average moisture gradient (from the difference between the center and the surface) across the section of cylindrical samples during drying. Among the features of the behavior of this parameter, attention is drawn to its sharp increase at a certain average moisture content of the material and to its smaller value throughout the entire drying period for peat-ore mixtures. As was to be expected, under the mild drying regime the moisture gradient is noticeably smaller.

For a severe drying regime, an undoubted connection is observed between this index and crack formation (the ratio of the total length of surface cracks to the total surface area was determined). The onset of increased crack formation coincides with the maximum moisture gradient. However, under a mild drying regime, crack formation decreases much more strongly than would be expected at the given moisture gradient, which, incidentally, is fully consistent with notions of the limiting permissible stressed state.

Fig. 1. Effect of the content of iron oxides in mixtures with peat on the shrinkage coefficient during drying: 1 — severe regime, 2 — medium regime, 3 — mild regime (magnetite additions), 4 — medium regime (iron oxide addition)

Fig. 2. Dependence of the moisture gradient and surface crack formation of specimens of formed peat (light points) and its mixture with 5% magnetite (dark points) on moisture content: a — severe, b — mild drying regimes

Conclusions

The phenomena of shrinkage and crack formation during the drying of formed peat and its mixtures with iron oxides were investigated.

The possibility of controlling the rate of shrinkage by changing the amount and density of the mineral filler has been demonstrated.

Drying of mixtures of peat with iron oxides proceeds at a lower moisture gradient across the cross section of the specimens, and a smaller number of cracks is thereby formed.

LITERATURE

1. A. V. Lykov. Theory of Drying. Energiya Publishing House, Moscow, 1968.
2. A. V. Lykov. The Phenomenon of Transfer in Capillary-Porous Bodies. Gostekhteorizdat, Moscow, 1954.
3. N. N. Kulakov. Introduction to the Physics of Peat. Gosenergoizdat, Moscow–Leningrad, 1947.
4. S. S. Korchunov. Investigation of the Physicomechanical Properties of Peat. Gosenergoizdat, Moscow–Leningrad, 1953.