Comparative Assessment of Mechanical Forces in the Process of Pressing and Pelletizing Fine-Grained Materials
Abstract Generated abstract
This study evaluates whether the mechanical forces acting during wet pelletizing of fine grained materials can be treated as equivalent to briquetting pressures required to obtain similar density. Quartz sand and iron ore concentrate were formed into pellets and pressed briquettes, then compared by density, strength, moisture, and post crushing sieve composition, with additional measurements of pellet contact area during formation. Pressing at high specific pressures produced noticeable grain comminution, especially in coarser sand, while pelletizing produced no detectable change in granulometric composition. The results suggest that pelletizing involves much lower specific pressures than those assumed from briquetting analogies, and that pellet density and strength arise mainly from more ordered grain packing rather than grain crushing.
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COMPARATIVE EVALUATION OF MECHANICAL FORCES IN THE PROCESSES OF PRESSING AND PELLETIZING FINE-GRAINED MATERIALS
V. M. Vityugin, V. A. Trofimov
(Presented at the final scientific conference of the Faculty of Chemical Technology)
In evaluating the magnitude of mechanical forces in the processes of pelletizing wet materials, a number of researchers [1, 2] have assumed these forces to be equivalent to the pressing pressure necessary to obtain briquettes with the density of a pellet; that is, the specific pressures experienced by grains during pelletizing are equal to 300–800 kg/cm².
However, a comparative evaluation of the briquetting and pelletizing process from the energy standpoint calls into question the possibility that such high pressures arise during pelletizing. In the present work, the question of changes in the granulometric composition of fine-grained materials was investigated in order to evaluate mechanical forces.
As the materials under study, Tashlinsk quartz sand of particle size 0.5–0 mm and Sokolovsko-Sarbaisk iron-ore concentrate of particle size 0.5–0 mm were used.
Research Methodology
To obtain pellets, 2 ml of water was introduced by syringe over 5–7 sec into a layer of preliminarily dried sand or concentrate. The spherical aggregate thus obtained was pelletized on a laboratory pan granulator of 300 mm diameter with additions of water and dry material over a specified period of time. The average pellet diameter, its weight, and its crushing strength were determined. The density of the pellets was determined by the paraffin method. After crushing the pellets, wet screening of the material into fractions was carried out: −0.5 + 0.2 mm, −0.2 + 0.1 mm, −0.1 + 0.05 mm, −0.05 + 0.0 mm.
Briquettes were prepared on a laboratory press in a split mold of 300 mm in the following manner.
A weighed portion of preliminarily dried material, equal to the weight of the dried pellet, was uniformly moistened to the specified moisture content and pressed at various values of specific pressure (\(p\) kg/cm²). The briquettes were evaluated by moisture content, density, strength, and the sieve composition of the material.
Comparative data on the density and strength of pellets and briquettes are presented in Table 1.
Analysis of the granulometric composition of the starting materials and of the raw pellets and briquettes obtained shows that, during the pressing of materials, a noticeable change in granulometric
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Table 1
| Fraction | Sand, initial, % | Quartzite, initial, % | Quartzite concentrate, % | Quartzite sand, % | Briquette sand, \(p = 1000\), % | Briquette concentrate, \(p = 1000\), % |
|---|---|---|---|---|---|---|
| \(-0.5 + 0.2\) | 93.6 | 93.5 | 9.1 | 9.2 | 61.4 | 3.2 |
| \(-0.2 + 0.1\) | 6.4 | 6.5 | 19.0 | 18.9 | 12.8 | 14.2 |
| \(-0.1 + 0.05\) | trace | trace | 33.4 | 33.7 | 17.9 | 30.6 |
| \(-0.05\) | trace | trace | 38.5 | 38.2 | 7.9 | 52.0 |
| \(\alpha\) | 0 | 0 | 0 | 0 | 32.2 | 13.5 |
Comparative strength and density of quartzites and briquettes
| Material | Oxidation time, min | Density, g/cm\(^3\) | Strength, kg/cm\(^2\) | Material | Pressure, kg/cm\(^2\) | Density, g/cm\(^3\) | Strength, kg/cm\(^2\) |
|---|---|---|---|---|---|---|---|
| Yellow concentrate | 3 | 2.71 | 910 | Yellow concentrate | 50 | 2.66 | 760 |
| Yellow concentrate | 5 | 2.84 | 1020 | Yellow concentrate | 250 | 2.77 | 920 |
| Yellow concentrate | 10 | 3.08 | 1190 | Yellow concentrate | 500 | 2.86 | 1070 |
| Yellow concentrate | 15 | 3.33 | 1670 | Yellow concentrate | 750 | 2.99 | 1310 |
| Yellow concentrate | Yellow concentrate | 1000 | 3.13 | 1540 | |||
| Quartzite sand | 3 | 2.29 | 61 | Quartzite sand | 750 | 2.08 | 19 |
| Quartzite sand | 3 | 2.27 | 57 | Quartzite sand | 1000 | 2.19 | 27 |
composition of the materials. With approximately the same density of pellets and briquettes, in the latter the yield of the newly formed class \((\alpha)\) varies over wide limits. Thus, quartz sand at a specific pressing pressure of \(1000\ \text{kg}/\text{cm}^2\) has a yield of the newly formed class of 32.2%, and iron-ore concentrate, 13.5%. In the processes of pelletizing materials, no change in the sieve composition was found. The character of the change in the sieve composition of materials at different values of the specific pressing pressure is shown in Fig. 1.
As can be seen from the figure, with an increase in the specific pressing pressure, the yield of the newly formed class increases to a greater extent for samples of relatively coarse-grained Tashla sand and to a lesser extent for iron-ore concentrate. Curves 2 show that a change in the moisture content of materials within the range 2.5–10% has no substantial effect on their granulometric composition. On the basis of the data obtained, it may be concluded that the energy expenditures in the processes of pressing and pelletizing are different.
Fig. 1. Effect of material moisture and specific pressing pressure on the degree of grain comminution during briquette formation.
1 — Dependence between \(p\) and \(\alpha\); \(\alpha\) — yield of class, %. 2 — Dependence between \(\alpha\) and \(W\); \(W\) — moisture, %.
In order to check the magnitudes of the specific pressures developed during the pelletizing process, experiments were carried out to determine the contact area of a pellet during its formation. Water with dye additions was used as the binder in making the pellet. The contact area was determined from the trace of the pellet on a sheet of thick paper during pelletizing. The results of these experiments show that the contact area of a pellet is not limited to a single grain. Thus, for example, in the initial period of pelletizing, granules 30 mm in diameter and weighing 31 g had a contact area equal to \(4\ \text{cm}^2\), i.e., the specific pressures experienced by the aggregate grains at the point of contact were \(0.02\ \text{kg}/\text{cm}^2\). With an increase in the pelletizing time with additions of dry material, as the granule was compacted, the contact area decreased and in the final period amounted to \(0.1\ \text{mm}^2\), which corresponds to a specific pressure of \(31\ \text{kg}/\text{cm}^2\).
During pressing, the external application of pressure leads to crushing of individual grains that have a disordered, random contact with a small area. Conditions for the removal of air from the material being pressed are hindered. Thin films of water, in the case of relatively coarse-grained materials, help increase the efficiency of crushing (see Fig. 1). A considerable share of the energy expenditures goes to overcoming friction both against the walls of the press mold and internal friction. The comparatively small mechanical forces during pelletizing of fine-grained materials merely promote a more ordered packing of grains in the granule, with a maximum contact area, and this determines the density and strength of the pellets.
REFERENCES
- I. I. Rovenskii. Production of Fluxed Pellets. Promin Publishing House, 1965.
- C. V. Firth, Amer. Inst. Min. Eng. Proceedings of the Blast Furnace and Coke Ovens. Raw Materials Conference, v. 4, pp. 46–69, 1944.