Study of the Swelling Process of Some Bentonites
Abstract Generated abstract
This study examines the swelling behavior of several bentonites used as binders in iron ore concentrate pelletizing, with attention to both equilibrium swelling and swelling kinetics under conditions closer to industrial practice. The authors compare swelling volume and swelling moisture in distilled and process water, relate the results to the exchange adsorption composition of the clays, and measure short term swelling in technical water. Swelling moisture generally increases with exchange capacity, but process water reduces swelling through exchange of monovalent cations with calcium and magnesium, with the effect depending on the bentonite’s cation composition. The results indicate that one hour of swelling does not bind enough water to account for pelletizing requirements, suggesting that coagulation structures in pellet pores also play an important role.
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INVESTIGATION OF THE SWELLING PROCESS OF CERTAIN BENTONITES
Communication 1
V. M. Vityugin, M. N. Lantsman, M. V. Kaynasova
(Presented by the scientific seminar of the Department of General Chemical Technology)
In the practice of pelletizing iron-ore concentrates, as a rule, small additions of bentonite are used to increase the efficiency of the pelletizing process. The role of bentonite in the pelletizing of iron-ore concentrates is reduced to binding excess water and forming coagulation structures in the pore space. When bentonites come into contact with water, swelling and an increase in volume occur, caused by the absorption of water by the crystal lattice of the minerals, the formation of crystal hydrates, and the hydration of exchangeable bases [1].
At the Sokolovsk-Sarbai Mining and Processing Combine (SSGOK), it was proposed to evaluate the suitability of bentonite for pelletizing by the swelling index [2]. The method for determining swelling consists in placing bentonite in cylinders with a capacity of 100 ml, mixing it with distilled water, and leaving it until complete sedimentation. The ratio of the volume of clay after mixing with water and sedimentation to the initial volume of dry clay gives the swelling value.
Under real conditions, bentonite comes into contact with process water containing divalent and trivalent cations, and the mixing time of the charge does not exceed one hour. Therefore, in addition to the equilibrium swelling value, the swelling rate will be of great importance.
Experiments to determine the swelling value were carried out in distilled and process water.
The chemical composition of distilled and process water is given in Table 1.
The bentonites taken for the work were characterized by the exchange-absorption complex, which was determined by the Babko–Askenazi method [3]. The results of the determination are given in Table 2.
Table 1
Results of chemical analysis of water, mg-eq/l
| Name of water | pH | Cation content Na | Cation content Ca | Cation content Mg | Anion content Cl | Anion content SO₄ | Anion content HCO₃ | Total hardness | Total mineralization |
|---|---|---|---|---|---|---|---|---|---|
| Distilled | 7.3 | 0.2 | 0.3 | — | 0.1 | — | 0.4 | 0.5 | 35 |
| Technical | 6.5 | 045 | 200 | 045 | 060 | 051 | 180 | 291 | 156.98 |
Table 2
Composition of the exchange-adsorption complex of the bentonites studied, mg-eq/100 g of dry clay
| No. | Name of bentonite | Ca | Mg | K | Na | Absorption capacity |
|---|---|---|---|---|---|---|
| 1 | Il’ichevsky | 32.50 | 14.75 | 1.99 | 0.068 | 58.74 |
| 2 | Cherkassky | 10.00 | 36.80 | 2.33 | 0.28 | 56.07 |
| 3 | Kushmurunsky | 41.00 | 39.38 | 1.51 | 12.10 | 98.96 |
| 4 | Oglanlinsky | 29.00 | 59.07 | 6.41 | 18.60 | 118.04 |
| 5 | Azkamarsky I | 25.00 | 42.96 | 2.15 | 55.65 | 129.16 |
| 6 | Makharadzensky | 25.95 | 25.95 | 6.05 | 14.20 | 89.63 |
| 7 | Sarigyukhsky | 27.50 | 16.11 | 1.59 | 52.17 | 128.00 |
| 8 | Azkamarsky II | 12.50 | 42.05 | 2.01 | 66.09 | 135.58 |
To determine the swelling capacity, the samples were recorded both by volume and by weight. This made it possible, in addition to the increase in volume, to determine the value of the swelling moisture. The results of the determinations are given in Table 3.
Table 3
Dependence of swelling capacity (H) and swelling moisture (\(W_{\mathrm{н}}\)) on the ionic composition of the water
| Name of bentonites | Sample, g | Distilled water: readings, ml | Distilled water: H | Distilled water: \(W_{\mathrm{н}}\) | Technical water: readings, ml | Technical water: H | Technical water: \(W'_{\mathrm{н}}\) | Decrease in technical water, %, \(\dfrac{W'_{\mathrm{н}}}{W_{\mathrm{н}}}\) |
|---|---|---|---|---|---|---|---|---|
| Il’ichevsky | 5.37 | 14.60 | 2.80 | 167.5 | 13.0 | 2.60 | 149.0 | 11.0 |
| Cherkassky | 4.63 | 15.2 | 3.04 | 220.0 | 14.0 | 2.80 | 194.0 | 2.7 |
| Kushmurunsky | 5.06 | 17.0 | 3.04 | 237.0 | 15.0 | 3.00 | 197.0 | 16.9 |
| Oglanlinsky | 4.33 | 21.2 | 4.24 | 374.0 | 16.0 | 3.20 | 254.0 | 32.1 |
| Azkamarsky I B | 4.36 | 24.0 | 4.80 | 436.0 | 21.0 | 4.20 | 367.0 | 15.8 |
| Makharadzensky | 5.08 | 26.0 | 5.20 | 413.0 | 14.0 | 2.80 | 177.0 | 57.0 |
| Sarigyukhsky | 5.70 | 32.0 | 6.40 | 473.0 | 30.0 | 6.00 | 440.0 | 7.0 |
| Azkamarsky II | 5.63 | 43.7 | 8.74 | 687.0 | 40.0 | 8.00 | 623.0 | 9.3 |
Analyzing the data obtained, one can say that the swelling moisture is higher, the higher the exchange capacity of the bentonite. However, this dependence is of a complex character. When distilled water is replaced by technical water, the swelling moisture becomes smaller, which is connected with the exchange of monovalent cations located on the surface of the clay for calcium and magnesium ions contained in the technical water. This tendency is expressed the more strongly, the greater the content of potassium and sodium ions in the bentonite relative to the total exchange capacity. The small change in swelling moisture in the Il’ichevsky, Cherkassky, and Kushmurunsky bentonites with an increase in the content of calcium and magnesium in the dispersion medium is due to the small content of monovalent cations in
exchange-absorbing complex. Bentonites from the Oglandinskii and Makharadze deposits, which have the highest content of potassium ion in the exchange-absorbing complex, change the swelling moisture most noticeably. For bentonites of the Saryg-Yukh, Azkamar I and II deposits, the exchange for the added divalent cations (0.245 mg-equiv/100 ml) proved small in comparison with the high content of monovalent cations in the absorbing complex.
It should be noted that the porosity in the dry state is different for different bentonites; therefore, only the value of the swelling moisture can be compared with one another.
In addition to the final results for determining the swelling moisture, which were taken after 24 hours, the change in swelling with time of actual contact under conditions close to production conditions (up to 1 hour) is of interest.
The experimental data of this series of investigations (Table 4) indicate that the maximum swelling rate is observed for a comparatively weakly swelling bentonite, while highly swelling bentonites swell slowly.
Table 4
Kinetic parameters of bentonite swelling
(technical water)
| No. | Name of bentonite | Indicator reading, mm | Swelling rate at $T = 1$ hour, mm/min | Swelling moisture $T = 1$ hour, % | % content $\dfrac{W_{\mathrm{н}}\ (1\ \text{hour})}{W_{\mathrm{н}}\ (24\ \text{hours})}$ |
|---|---|---|---|---|---|
| 1 | Oglandinskii | 0.46 | 0.0076 | 52.6 | 20.7 |
| 2 | Makharadze | 1.39 | 0.0232 | 43.5 | 24.5 |
| 3 | Cherkasskii | 4.24 | 0.0707 | 62.5 | 32.2 |
| 4 | Ilyichevskii | 4.91 | 0.0820 | 55.4 | 37.2 |
Under practical conditions, the magnetite cake entering pelletizing has a moisture content of 9.5–9.6%. To the iron-ore concentrate are added 8% dry limestone and 1% dry bentonite. The working moisture of pelletizing such a charge is 8.5–8.7%. The share of bentonite accounts for the binding of 1.7–1.8% moisture, i.e., 170–180% of its own weight. None of the bentonites investigated, during one-hour contact with water, binds such an amount of water through swelling. In all probability, the binding of water occurs, in addition, through the formation of coagulation structures in the pore space of the pellets.
Thus, when calculating the optimal amount of bentonite in an iron-ore charge, it is necessary to take strict account of the quality and quantity of ions capable of exchange in the liquid and solid phases, as well as the kinetics of swelling of bentonites and structure formation of pore suspensions. The latter question will be covered in the second communication.
LITERATURE
- Proceedings of the Soil Institute named after V. V. Dokuchaev, issue III, 1958.
- P. N. Dokuchaev. Dissertation. Tomsk, 1968.
- A. M. Vasil'ev. Fundamentals of the Modern Methodology and Technique of Laboratory Determinations of the Physical Properties of Soils. Moscow, Publishing House of Construction and Architecture, 1953.