Investigation of the Kinetics of Surface Dissolution in Aqueous Dispersions of Certain Materials
Abstract Generated abstract
The study examines surface dissolution of finely dispersed materials used in iron ore pelletizing, where leached ions affect pH, electrical double layer structure, colloidal stability, and wet granulation behavior. Using aqueous dispersions with contact times from 6 to 1500 minutes, the authors assessed leaching kinetics by measuring centrifugate pH and electrical conductivity for iron ore concentrate, sludges, sand, bentonite, and related materials. Materials with strong crystal lattices, including sand, magnetite concentrate, and sludges, reached practical dissolution equilibrium within about 1.5 to 2 hours, with similar kinetic curves that changed little with increased water content. Bentonite behaved differently, showing delayed but much stronger conductivity growth, apparently because dispersion and renewal of exchange surface developed over time and occurred faster at higher water content.
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STUDY OF THE KINETICS OF SURFACE DISSOLUTION IN AQUEOUS DISPERSIONS OF CERTAIN MATERIALS
V. M. Vityugin, E. N. Chulkova
(Presented at a scientific seminar of the Department of General Chemical Technology)
In the process of preparation for pelletizing, finely ground iron-ore concentrate and other materials included in the charge are in contact with water for a certain time. In this process, surface leaching of mineral components occurs. The ions of salts dissolving in water determine the pH of the medium and the structure of the electrical double layer, the magnitudes of which substantially affect the stability of the colloidal fraction of the materials and their structural-mechanical properties. The instability of sols, expressed in the appearance of large particles or crystals, manifests itself the faster, the greater the solubility of the dispersed phase. With a large value of the electrical double layer, the interlayer of medium between the particles is thicker and the structure is less strong, i.e., surface dissolution predetermines a number of properties of dispersed materials that play an essential role in the process of wet granulation.
The degree of leaching was determined from the change in pH of the centrifugate solutions and from the electrical conductivity \((\chi)\). The pH was measured on an LPU-01 pH meter; electrical conductivity was measured according to a circuit that is a Kohlrausch bridge [1]. The time of contact of the material with water ranged from 6 to 1500 minutes.
A number of dispersed materials were used as the objects of study; their characteristics are given in Tables 1, 2, 3. The averaged results of the studies are shown in the form of kinetic curves in Figs. 1, 2. As can be seen from Fig. 1, the smallest amount of soluble
Table 1
Distribution of particles by granulometric fractions
| Fraction, mm | Tashlinsky sand | Iron-ore concentrate | Bentonite of the SSGOK | Limestone |
|---|---|---|---|---|
| 1.0—0.5 | 1.90 | — | — | — |
| 0.5—0.25 | 76.85 | 0.02 | 0.02 | — |
| 0.25—0.20 | 15.82 | 0.04 | 0.13 | 0.02 |
| 0.20—0.16 | 4.81 | 0.12 | 0.29 | 0.01 |
| 0.16—0.125 | 0.50 | 0.71 | 1.01 | 0.17 |
| 0.125—0.10 | 0.04 | 0.45 | 0.54 | 0.30 |
| 0.10—0.04 | 0.08 | 38.45 | 6.14 | 17.25 |
| 0.04—0.02 | — | 47.19 | 6.12 | 40.10 |
| 0.02—0.015 | — | 9.23 | 12.47 | 27.18 |
| 0.015—0.010 | — | 3.79 | 30.12 | 10.60 |
| 0.010—0 | — | 3.79 | 43.11 | 4.37 |
Table 2
| Material | Fe total | S | FeO | SiO₂ | CaO | Al₂O₃ | MgO | Fe₂O₃ | Loss on ignition | Other components |
|---|---|---|---|---|---|---|---|---|---|---|
| Iron-ore concentrate | 67,4 | 0,40 | 28,67 | 2,68 | 0,82 | 1,28 | 0,85 | — | 0,78 | — |
| Enriched iron-ore concentrate | 67,4 | 0,37 | 28,80 | 2,69 | 0,68 | 1,22 | 0,80 | — | 0,88 | — |
| Taganskii pink bentonite | — | — | 0,04 | 58,75 | 3,58 | 25,03 | 0,00 | — | 3,41 | 4,19 |
| SSTONa bentonite | 2,87 | — | 4,10 | 57,44 | 1,34 | 17,86 | 3,87 | — | 8,24 | 4,28 |
| Tashlinskii sand | — | — | — | 99,04 | 0,11 | 0,44 | — | 0,14 | — | — |
| Brown iron ore | 35,84 | — | 10,50 | 26,87 | 0,84 | 5,48 | 1,02 | 40,19 | 13,20 | 6,25 |
| Limestone | 1,13 | — | — | 0,72 | 53,54 | 0,57 | 0,50 | — | 42,58 | 0,96 |
Table 3
Characteristics of the surface properties of materials
| Material | Particle size, mm | Specific surface, cm²/g | Specific weight, g/cm³ | MMV, % | MG, % |
|---|---|---|---|---|---|
| Tashlinsky sand | 1.0—0 | 234 | 3.08 | 0.95 | 0.350 |
| Iron-ore concentrate | 0.2—0 | 2100 | 4.27 | 4.07 | 0.7 |
| SSGOK bentonite | 0.2—0 | 562000 | 2.50 | 86.20 | 32.90 |
| Tagansky pink bentonite | 0.1—0 | — | 2.33 | — | 38.00 |
| Limestone | 0.25—0 | 4820 | 3.34 | 15.27 | 2.50 |
| Brown iron ore | 0.1—0 | — | 3.80 | 35.3 | 22.01 |
| Dolomite | 0.1—0 | — | 2.86 | — | 0.87 |
| Sludges | 0.125—0 | 9900 | 2.94 | 21 | 16.80 |
| Washed iron-ore concentrate | 0.1—0 | 1100 | 4.90 | 3.5 | 0.30 |
Tashlinsky sand contains the smallest amount of components, while bentonite contains the greatest. In materials such as magnetite concentrate, sludges, and sand, equilibrium is practically established within 1.5–2 hours. With a relatively small amount of water (s:l = 1:4), the surface dissolution of bentonite—
Fig. 1. Kinetics of the surface dissolution of materials (s:l = 1:4): 1 — sand, 2 — bentonite, 3 — sludges, 4 — iron-ore concentrate.
—in the first hour is significantly less than in the other materials studied (\(\chi = 4\text{–}6\)), and then increases sharply to \(\chi = 40\). Probably, at the initial moment dispersion has not yet occurred and the exchange surface is small. Subsequently, intensive dissolution is observed with renewed surface areas. At a higher water content, dispersion of bentonite occurs more rapidly, and already after one hour of contact between the material and water a significant increase in electrical conductivity is observed (\(\chi = 22\)). The electrical conductivity of sludges is greater than the electrical conductivity of magnetite concentrate; that is, the sludge fraction of the concentrate will have a greater influence on the structure of the electric double layer.
Conclusions
- For minerals with a strong crystal lattice, equilibrium in the process of surface dissolution is established within 1.5–2
Fig. 2. Kinetics of surface dissolution of materials (s:l = 1:10): 1—sand, 2—bentonite, 3—silts, 4—iron-ore concentrate.
hours. The character of the kinetic curves of surface dissolution for these materials is the same and practically does not change with an increase in the amount of water.
- The character of the kinetic curve for bentonite, the time required to establish equilibrium, and the quantitative comparison of the measured electrical conductivity of bentonite suspensions differ from those for the other materials studied.
Literature
- O. N. Grigorov et al. A Guide to Practical Work in Colloid Chemistry. Moscow–Leningrad, “Khimiya,” 1964, p. 217.