Improvement in the permeability of sintering beds by drying treatment after granulating sinter raw materials containing concentrates

Highlights

• Granulation and packing tests of iron ore blend containing concentrates are conducted.

• The drying technique can improve bed permeability significantly.

• Adding hydrated lime can effectively suppress granules’ degradation caused by drying.

• The typical sintering process is optimized to meet the energy needs of drying.

• The optimization facilitates grade recovery and cascade utilization of waste heat.

Abstract

The permeability of the sinter green beds will remarkably deteriorate when magnetite concentrate powder is introduced into the raw blend. To address this problem, a technique of drying after high-moisture granulation is proposed. Using this technique, the permeability can be raised from 40.45 JPU to 46.05 JPU at 7.7 wt% granulation moisture. At the same time, the results show that drying may cause the degradation of granules when the granulation moisture is excessive. Therefore, the final permeability of the stacked bed depends on the combined effect of increasing the bed stiffness and decreasing the average particle size. Fortunately, the addition of hydrated lime (HL) can significantly inhibit granule degradation and improve granulation efficiency, which can further improve the bed permeability to 61.41 JPU with a 2% HL addition level (at 7.7 wt% granulation moisture). Finally, an optimized process is proposed that uses low-grade waste heat during sinter cooling to dry the wet granules. This optimization not only meets the energy needs of drying but also facilitates the grade recovery and cascade utilization of waste heat during the sintering process.

Introduction

The typical iron ore sintering process supplies the main ferrous charge for blast furnaces in most steel mills. During sintering, first the sinter raw materials are intimately mixed and granulated to produce granules with a narrow size distribution, large mean size and sufficient strength. The resulting granules are then charged onto a moving strand to form a stacked bed with an ideal permeability [1]. The top of the bed is ignited by a gas burner. The heat transfer through the sintering bed depends on the air resistance provided by the bed or the ability of air to penetrate through the bed [2]. Hence, the permeability of the bed plays a very important role in sintering. An enhanced permeability of the sinter bed is desirable for transferring heat more quickly from the top to the bottom of the bed and for obtaining better productivity and quality from the sintering plant.

Due to the recent rapid development of China’s steel industry and the depletion of high-grade iron ore resources worldwide, concentrates have gradually been used as an alternative raw material for iron ore sintering [1], [2], [3], [4], [5]. These concentrates have demonstrated a notable impact on the sintering performance and sinter quality. On the one hand, magnetite concentrates are generally known for their high Fe grade and low impurity content and will release extra heat when oxidized to hematite, which means reducing the solid fuel consumption during the sintering process [5]. On the other hand, the concentrate powder is typically extremely fine. Therefore, more fine-sized concentrate powder can lead to additional challenges for sintering, for example, the worsening of the bed permeability [1], [2], [3], [4]. As a result, it is very important to improve the bed permeability when sintering with the introduced magnetite concentrate powder.

To take full advantage of magnetite concentrates, various additional countermeasures have been developed to improve bed permeability. Zhou et al. [1], [6] and Lu et al. [7] used hydrated lime (HL) as a binder to increase the permeability of the packed bed. Zhu et al. [8] improved the granulation performance of Brazilian specularite concentrate by damp milling pretreatment. After damp milling, the average particle size, granulation efficiency and permeability index of the material layer were improved. Yasuhide Yamaguchi et al. [9], [10] improved the permeability of the sintering bed by adding dry particles to granulated raw materials. At the same moisture content, the −0.25 mm ratio of pseudo-particle size was decreased by this technique. The bulk density decreased when dry particles were included in the mixture. These two reasons led to an improvement in the permeability of the packed bed. Zhu et al. [11] employed the pre-briquetting technique to improve the granulation of a sintering mixture comprising specularite concentrate. Compared with traditional granulation, granulation with pre-briquetted specularite concentrate increased the permeability of the sinter bed and average granule size by 12.69% and 11.17%, respectively, when the sinter blend contained 36% specularite concentrate. Huang et al. [12] studied the effect of pre-wetting treatment on the granulation behavior of iron ore fines. The granulation process reduced the ratio of small particles and increased the ratio of large particles, thus increasing the permeability of the stacked bed. Masaru Matsumura et al. [13] proposed that drying treatment after granulating the sinter raw materials can make the −0.25 mm ratio of pseudo-particle size decrease and ultimately improve the permeability of the stacked bed. The experimental results showed that the sintering permeability is related to the proportion of −0.25 mm quasi-particles. In addition, separated granulation [14], pre-balling, pre-granulation [15] and other technical measures have been used to increase the permeability of packed beds [5].

From the above, few studies have used drying techniques to improve the permeability of stacked beds [13]. The drying treatment method mentioned above has not been used in the granulation and packing tests of sintered materials comprising magnetic concentrate powder. Therefore, we conducted this study to verify whether the drying technique can improve the permeability of the stacked bed when the mixture contains magnetic concentrate powder. Considering that drying may cause the collapse of granules [13], [16], [17], we also tested the cases of adding a binder (HL).

In the present study, we conducted granulation and packing tests and studied the effects of the drying process on the granule characteristics and packed bed properties. Additionally, we tested the drying cases with HL addition to verify whether this process can further improve the permeability. Then, the mechanism affecting permeability was explained. Finally, the original sintering process was optimized.