On the Influence of Various Additives on the Thermal Stability of Green Iron Ore Pellets
Abstract Generated abstract
This study examines how bentonite, the polymeric structure-forming agent K-4, and lignin affect the pelletizing behavior and thermal stability of green iron ore pellets made from magnetite concentrate and related plant materials. Pellets prepared on a disk granulator from burdens with different additive compositions were evaluated by yield of usable pellet size, porosity, cracking temperature, and shock destruction temperature. Moisture-absorbing additives improved pellet formation, with K-4 giving the highest yield of pellets larger than 10 mm, but bentonite and K-4 lowered the initial cracking temperature while slightly increasing shock resistance. Lignin produced the strongest improvement in thermal stability, with no observed destruction up to 1200°C, suggesting that K-4 and especially lignin could serve as alternatives to bentonite in pellet production.
Full Text
On the Effect of Various Additives on the Thermal Stability of Green Iron Ore Pellets
T. G. Leont’eva, A. V. Vityugin
(Presented by the scientific seminar of the Department of General Chemical Technology)
Various finely dispersed moisture-absorbing additives are used as admixtures to the main production burden for iron ore pellets: clays, bentonites, polymeric structure-forming agents, lignin, and others. Additives are introduced in order to increase the plasticity of the pelletizing burden, reduce its moisture content, and increase the mechanical strength and thermal stability of the pellets.
In the present work, the effect of additions of bentonite, the structure-forming agent K-4, and lignin on the thermal stability of green iron ore pellets was investigated. The objects of study were production samples of materials from the pelletizing plant of the Sokolovsko-Sarbaiskii Mining and Beneficiation Combine: magnetite iron ore concentrate with a size of 0–0.1 mm, limestone from the Kzyl-Zhar deposit with a size of 0–0.15 mm, as well as additions of bentonite from the Makhadzhensk deposit, the structure-forming agent K-4, which is a product of incomplete saponification of polyacrylonitrile in an alkaline medium, and lignin from the Krasnoyarsk hydrolysis plant. Burdens with different ratios of the indicated components were studied. The burdens, moistened to the optimum working moisture for pelletizing and thoroughly mixed in the dry and wet states, were pelletized on a disk granulator with a diameter of 1 m. Pellets with a diameter of \(15 \pm 1\) mm were selected for the investigations.
The thermal stability of the green pellets was evaluated by the temperatures at which cracking began and “shock” destruction occurred. The experimental results are given in Table 1.
It was established that moisture-absorbing additives improve the pelletizability of the burdens, which leads to an increase in the yield of the suitable pellet fraction. The greatest effect is observed when the structure-forming agent K-4 is introduced into the burden (the yield of the pellet fraction with a diameter greater than 10 mm is 70%, whereas from a burden without additives it is only 18%).
The nature of the effect of the additives studied on the thermal stability of the pellets is different. The cracking temperature decreases when bentonite and K-4 are introduced into the burden. At the same time, the temperature of “shock” destruction increases somewhat. The decrease in the cracking temperature is caused by the fact that these additives increase the optimum moisture for pelletizing, the removal of which during heating of the pellets causes cracks to appear. The slight increase in the “shock” temperature can be explained by an increase in porosity, which promotes improved mass transfer during heating and a reduction in temperature gradients inside the pellet.
Table 1
Properties of green iron-ore pellets
| No. | Burden composition, % | Optimum pelletizing moisture, % | Amount of pellets over 10 mm, % | Pellet porosity, % | Crack-formation temperature, °C | Shock temperature, °C |
|---|---|---|---|---|---|---|
| 1 | Concentrate—92 limestone—8 |
8.2 | 18 | 25 | 580 | 640 |
| 2 | Concentrate—91 limestone—8 bentonite—1 |
8.9 | 51 | 26 | 560 | 700 |
| 3 | Concentrate—89 limestone—8 bentonite—3 |
9.6 | 63 | 28 | 520 | 680 |
| 4 | Concentrate—92 limestone—8 K-4—400 g/t |
8.86 | 70 | 30 | 560 | 680 |
| 5 | Concentrate—95 lignin—5 |
10.4 | 60 | 30 | no destruction is observed | no destruction is observed |
When lignin is used as an additive, the thermal stability of the pellets increases sharply. The pellets did not break down when heated to 1200°C. The additional heat released during the combustion of lignin promotes uniform and, at the same time, intense heating of the pellets, which leads to an increase in their thermal stability and also to a reduction in energy expenditures during heat treatment of the pellets.
Thus, additions of the structure-forming agent K-4 and especially lignin may be recommended as substitutes for the ballast bentonite now widely used in the production of iron-ore pellets.