Influence of alkaline (Na, K) vapors on carbon and mineral behavior in blast furnace cokes
Introduction
A reduction in coke consumption is one of the key approaches being used towards minimizing greenhouse gas emissions and energy consumption in the current blast furnace (BF) ironmaking process [1]. Due to the twin pressures of resource shortage of coking coals and continuous requirements for improved coke quality, an in-depth understanding of coke behavior in the BF has become increasingly important [2], [3]. As coke traverses through the blast furnace, various changes occurring in coke is poorly understood [3], [4]. While the alkalization of quartz has been investigated through studies on mineral grains in tuyere cokes exposed to the blast furnace gas [4] and the catalytic effect of iron on coke reactivity is well known [5], there are several knowledge gaps in our understanding regarding the influence of alkali-bearing minerals on various coke characteristics [6]. Because of the volatilization and condensation of alkalis in different thermal zones, these tend to cycle and remain within the BF leading to their accumulation and interactions with other feed materials. Even small amounts of alkalis in the charge could have a significant impact on the overall process [2], [7]. In the 1970s, over ten blast furnaces were successfully dissected in Japan [8], [9]. These studies were focused on the variation of physical properties (mean size, drum index, coke-strength) and chemical properties (reactivity with CO2) of coke along with temperature or blast furnace height. From the Japanese dissection results [10], the distribution of alkalis in a blast furnace became better understood, and it was confirmed that the high temperature zones (above 1000 °C) in a blast furnace contained highest levels of alkalis. While industrial dissection results clarified the degradation of cokes by alkalis, detailed mechanism was still not well understood. Since 1980’s, a number of researchers have sought to establish the mechanism of coke degradation by alkalis through controlled laboratory investigations [11], [12].
The degradation of raw material quality is an unavoidable new trend for the iron and steel industry. The concentrations of harmful elements especially alkalis in raw materials has kept on increasing in recent years affecting the efficiency of blast furnace operation. Large-scale, medium and small scale enterprises are facing the challenge of harmful elements especially alkalis poor quality raw materials. This previously hot topic of 1980s has once again become a hot topic of research towards the utilization of low quality raw materials and deceasing associated production costs. A recent study has shown that the apparent reaction rate of tuyere level cokes can be up to ten times that of the feed coke reaction rate, and was strongly related to the total amount of potassium species present [13], [14]. The presence of recirculating potassium was found to be one of the most distinctive features of tuyere cokes especially outside the raceway [13], [15], [16].
Previous research [7] in this area usually soaked cokes in solutions with different concentration of alkalis carbonates for a range of times, and then the soaked cokes were dried and tested to analyze the variation of properties. This method can only investigate the influence of alkalis as carbonates on the coke properties. As alkalis are primarily present as a gaseous phase in the high temperature zone of a BF, the influence of alkali vapors on the alkalization of coke minerals and coke characteristics is a subject of key interest and further research. Few results have been reported regarding the interaction of alkali vapors with coke. In this article, we report an in-depth investigation at high temperature (1300 °C) on the alkalization behavior of both carbon and various mineral phases present in two blast furnace cokes and their corresponding influence on the coke behavior. In order to understand the true alkalization process of coke minerals by alkalis vapor in a BF, this reaction needs to be isolated from other reactions that could occur simultaneously in an industrial BF, e.g., coke gasification and/or the reduction of minerals by carbon in coke. The final goal of this research is to establish a detailed mechanism about the alkalization process of coke by alkalis vapor and its corresponding influence on coke properties.
Based on a novel approach, this study investigates the adsorption and the alkalization of two BF cokes by Na and K vapor at 1300 °C. The interaction of alkalis vapor with coke carbon and coke minerals were analyzed with scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS) examinations, X-ray diffraction and Raman spectroscopy. Key coke characteristics, namely the coke reactivity index (CRI) and the coke strength after reaction (CSR), were measured according to Chinese National Standards (GB/T 4000–2008; equivalent to ASTM Standard D 5341); the correlation between CRI and CSR values and the amount of alkalis incorporated into the coke structure was also established along with possible reaction mechanisms.
Section snippets
Experimental
Two types of feed cokes with different reactivities, labeled here as Coke 1 and Coke 2, were obtained from a large (bigger than 4000 m3) blast furnace. CRI and CSR values, proximate and ultimate analysis, and ash compositions of original cokes are shown in Table 1. The feed coke samples were crushed with a jaw crusher and sieved using a 25.0 mm sieve placed on top of a 23 mm sieve to yield sufficient quantities of 23–25 mm size specimens. Discarding flake-like and strip-like pieces, the edges of
The influence of alkalis on coke texture
A visual examination of potassium alkalized coke specimens showed expanded cokes with a number of layers peeled off from the coke lump. This phenomenon, referred to as “pealing effect” in this study, was observed in both Coke 1 and Coke 2 (see Fig. 1(a and b)). The extent of these peels was seen to increase with increasing K/Coke ratios. A plot of the mass percentage of the peels against K/Coke ratio is shown in Fig. 1(c), which shows a stronger dependence below a ratio 3.0/100. Another feature
Conclusions
An experimental approach has been developed to investigate the influence of potassium and sodium vapors on the structure mineral matter and high temperature behavior of blast furnace cokes. Key findings of this study are:
- (1)
Coke textures were observed to peel off layer by layer after alkalization by potassium vapor; macro fissures were observed for K/Coke ratios above 3/100. Coke samples alkalized by sodium vapor did not show such peeling behavior.
- (2)
In a significant contrast to original coke
Acknowledgements
This work was financially supported by the Open Foundation of the State Key Laboratory of Advanced Metallurgy (41603007), the National Natural Science Foundation of China and Baosteel Group Co., LTD of Shanghai for the Key Joint Project (U1260202), and the National Science Foundation for Young Scientists of China (51304014).
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