DETERMINATION OF THE OPTIMAL MOISTURE CONTENT OF THE CHARGE BEFORE PELLETIZING

V. M. Vityugin, E. N. Chulkova

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

The process of pelletizing dispersed materials in mechanical granulators is very sensitive to fluctuations in moisture content, and therefore forecasting the optimal moisture content of the raw-material mixture is of great importance. In wet granulation of dispersed materials, a pore suspension is formed in the forming aggregates, representing a structured system. The solid phase of the pore suspension is usually represented by colloidal and semicolloidal particles of plasticizing additives and by the clay fraction of the main finely dispersed material being pelletized. The particle size of the clay fraction, depending on density, is generally less than 5–20 μ, and its content in the main materials ranges from 1 to 5%. Thus, for example, in producing iron-ore pellets, the charge consists of a mixture of moist magnetite concentrate and small (1–2%) amounts of dry bentonite. During pelletizing of the mixture, moisture is redistributed, and part of it passes from the concentrate to the more moisture-capacious bentonite. Bentonite with water readily forms a thixotropic coagulation structure, which causes large plastic deformations [1]. A bentonite suspension exhibits maximum plasticity at a definite bentonite concentration. The optimal pelletizing moisture content \((W^p_{\text{opt}})\) and the moisture content of maximum plasticity of the pore suspension \((W_{\text{max.plast}})\) are interrelated. The main quantity of water contained in the initial mixture is concentrated in the pore suspension during the granulation process. The framework particles apparently retain on themselves only hygroscopic moisture, the quantity of which is constant at any concentrate moisture content. On this basis, the optimal moisture content of the charge before pelletizing is composed of the moisture of maximum hygroscopicity of the framework \((W^k_{\text{mg}})\) and the moisture of maximum plasticity of the latter.

\[ W^p_{\text{opt}} = W^k_{\text{mg}} + x \cdot W^c_{\text{max.plast}}, \tag{1} \]

where \(x\) is the fraction of the solid phase of the pore suspension in the entire solid phase of the charge.

The validity of calculation equation (1) was checked on samples of magnetite concentrate from SSGOK. The maximum hygroscopic moisture content of the framework grains of the concentrate was determined by saturating a weighed portion of washed iron-ore concentrate to constant moisture content over a saturated solution of potassium sulfate.

\[ W^k_{\text{mg}} = 0.3\%. \]

The content of slimes (20 μ fraction) in this sample of magnetite concentrate was 1.5%. The pink variety of bentonite from the Tagan deposit was used as the plasticizer.

Bentonite was added in an amount of 1% by weight of the concentrate; i.e., the value of \(x\) with bentonite additions of 1% was, accordingly, 2.5%.

The moisture content corresponding to the optimum plasticity of the pore suspension was found from the rheological parameters of the pore suspension. The latter were determined on an RV-8 rotational viscometer of M. P. Volarovich [2]. The results of the determinations are given in Table 1 and Fig. 1.

Table 1

Structural-mechanical properties of pore suspensions

In parallel, the optimum moisture content of the mixtures was determined by direct pelletizing experiments. The results of calculations and direct determinations

Fig. 1. Dependence of plasticity on the concentration of the pore suspension.

\(W_{\text{opt}}\) agree quite satisfactorily. Thus, for example, at an average actual moisture content of iron-ore pellets of 8.95%, the calculated value was 9.00%.

REFERENCES

1. P. A. Rehbinder. Physicochemical Mechanics of Disperse Structures. “Nauka,” 1966.

2. M. P. Volarovich. Study of the rheological properties of disperse systems. Colloid Journal, 1966, No. 6.