On the Granulation of Ash from Tomsk GRES-2
Abstract Generated abstract
This paper examines wet granulation of finely dispersed ash from the Tomsk GRES-2 hydraulic ash dump for use as fired aggregate in concrete. It evaluates the water-physical properties and lump-forming ability of the ash, limestone, loam, and ash mixtures containing 20 percent limestone or loam, then tests pelletizing on a laboratory pan granulator followed by firing at 1170°C. The results indicate that both additives improve pelletizability, with limestone producing the highest lump-forming index and somewhat stronger fired granules, while loam-containing granules also remain suitable as aggregate. Fired granule strength and water absorption are reported to depend mainly on the firing regime, especially final temperature, with temperatures above 1200°C causing sintering into clusters.
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On the Granulation of Ash from Tomsk GRES-2
V. M. VITYUTIN, G. G. VERGUN, N. S. DUBOVSKAYA, I. S. KOROLYUK
(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)
Granulation of finely dispersed ash obtained in the combustion of pulverized fuel is one of the promising methods for utilizing this waste [1]. The fired granulate is used as an aggregate in the manufacture of concretes.
The physicomechanical properties of granules can be regulated by using various additives. Thus, when easily swelling clays and loams are introduced into ash, the granules can serve as a light porous aggregate for heat-insulating concretes. On the other hand, the introduction of lime additives makes it possible to obtain an aggregate with a high adhesive capacity with respect to cement paste and, consequently, such granules can be used for the production of high-grade concretes.
The quality of granules is determined mainly by the water-physical and structural-mechanical properties of the dispersed materials being pelletized, and therefore it is important to establish the influence of clay and lime components on these properties.
In the present work, ash from the hydraulic ash dump of Tomsk GRES-2 was used as the main object of study. Chemical analysis of an averaged ash sample showed the following composition:
SiO₂—62.4%, Al₂O₃—18.33%, Fe₂O₃—4.47%, CaO—3.90%, MgO—1.81%, SO₃—traces, loss on ignition—7.28%, including SiO₂ (active)—4.58%, CaO (free)—0.38%. The granulometric composition of the ash was as follows: content of fractions larger than 200 μm—10.5%, from 70 to 200 μm—8%, and less than 70 μm—81.5%. The specific surface of the ash was 2200 cm²/g.
As the lime component, limestone from the Kzyl-Zhar deposit, ground to 100 μm, was used. The total content of CaO and MgO in the limestone sample was 54%. The specific surface was 4820 cm²/g.
The clay material used in the work was represented by loam from the Beloyarsk deposit in Tomsk Oblast. The granulometric composition of the loam is characterized by the content of fractions in %:
0.25 mm—3.61; 0.05 mm—47.92; 0.01 mm—10.56; 0.005 mm—13.96; 0.001 mm—4.52.
Refractoriness No.—1280°C, third class of plasticity.
The evaluation of the pelletizability of the dispersed materials studied was carried out according to a method based on taking into account water-physical parameters [2]. In this case, the maximum molecular moisture capacity ($W_{\mathrm{MMB}}$)
was determined by the moisture-capacity method [3], and the maximum capillary moisture capacity \((W_{\mathrm{m.c.m.}})\)—by capillary saturation with water of columns of dry materials. The lump-forming index (K) was calculated from the relation
\[ K=\frac{W_{\mathrm{m.m.c.}}}{W_{\mathrm{m.c.m.}}-W_{\mathrm{m.m.c.}}}. \]
Both individual materials and mixtures of ash with 20% limestone and 20% loam were investigated. The results of the study are given in Table 1.
Table 1
Water-physical properties and lump-forming ability of the materials investigated
| No. | Material | \(W_{\mathrm{m.m.c.}}\), % | \(W_{\mathrm{m.c.m.}}\), % | Lump-forming index, fractions of a unit |
|---|---|---|---|---|
| 1 | Ash | 17.1 | 58.4 | 0.41 |
| 2 | Limestone | 11.0 | 25.8 | 0.74 |
| 3 | Loam | 12.2 | 34.4 | 0.53 |
| 4 | Mixture 1 ash—80% limestone—20% |
15.9 | 45.5 | 0.54 |
| 5 | Mixture 2 ash—80% loam—20% |
16.2 | 53.0 | 0.44 |
As can be seen from the table, limestone has the greatest lump-forming ability, and ash from Tomsk GRES-2 the least. Naturally, the introduction into ash of additives, both loam and especially limestone, increases the lump-forming ability of the mixtures. Thus, the introduction of additives helps not only to improve the quality of the artificial ash aggregate, but also substantially intensifies the process of wet granulation.
Table 2
Characteristics of granulated mixtures
| Mixture No. | Composition of mixture, % | Granule moisture content, % | Bulk density of dry granules, g/cm³ | Granulometric composition of the granulate, %: fr. 2–10 mm | Granulometric composition of the granulate, %: fr. 10–7 mm | Granulometric composition of the granulate, %: fr. 7–5 mm | Strength of granules (16 mm), kg/gran.: wet | Strength of granules (16 mm), kg/gran.: dry | Strength of granules (16 mm), kg/gran.: fired | Water absorption of fired granules, % |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Ash—80% Limestone—20% |
21–21.5 | 1.5–1.55 | 83.5 | 16.0 | 0.5 | 0.7–0.8 | 7–8 | 70–75 | 3.5 |
| 2 | Ash—80% Loam—20% |
20–20.5 | 1.6–1.7 | 80.7 | 15.4 | 3.9 | 0.3–0.4 | 6–7 | 60–65 | 2.7 |
Direct experiments on the granulation of mixtures of ash with limestone and loam were carried out on a laboratory pan granulator 1 m in diameter with a rim height of 0.1 m. The optimum pelletizing regime was estab-
obtained at a pan inclination angle of 45° and at a circumferential rotation speed of 1 m/sec. The granules were fired in a laboratory electric furnace at a temperature of 1170° C for 2 hours.
The results of pelletizing are given in Table 2. As was to be expected, the quality of granules made from a mixture of ash with limestone is somewhat higher. However, ash granules with an addition of clay are also quite suitable for use as an aggregate.
The water absorption and strength of the fired pellets, as the experiments showed, depend mainly on the temperature regime of firing and, chiefly, on the final firing temperature.
Exceeding this temperature above 1200° C leads to sintering of the granules into cluster-like aggregates.
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