Behavioral characteristics and renewal mechanism of deadman coke in the hearth under low-carbon ironmaking blast furnace conditions

Abstract

As the sole solid material in the lower part of a blast furnace (BF), the multiphase reaction behavior of coke within the deadman region of the hearth is of significant theoretical and practical importance for carbon emission control and low-carbon production. The multidimensional characterization of the occurrence state, multiphase reaction behavior, and renewal mechanism of deadman coke in the hearth was performed through the dissection of a 3200-m3 BF, combining various methods such as rope-sawing residual iron removal, image processing techniques, microscopic analysis, and coke dissolution experiments. The results showed that the deadman root in the hearth exhibited a “curved” shape and a distinct “floating” state, with the floating height at the center approximately 0.45 m, increasing toward the hearth edge. Vertically, the deadman was divided into three regions: the “slag–coke zone,” the “iron–coke zone,” and the “coke–free zone.” The average deadman voidage was calculated to be 54.75% (±0.85%), and the coke particle size was 21.47 mm (± 0.53 mm), based on image processing techniques. A “slag + CaS isolation layer” was identified on the outer surface of the deadman coke during the dissolution and erosion process of hot metal analysis of the “slag–iron–coke” three-phase microstructure indicated that this layer suppressed the carburization reaction and represented the most limiting factor for deadman coke renewal. A coke renewal formula was established from the coke dissolution experiments, and the renewal time was calculated to be 14.74 d. The renewal mechanism of deadman coke was elucidated through comprehensive analysis, and recommendations for low-carbon operation were proposed. These findings provide a scientific basis and theoretical guidance for low-carbon production and fuel optimization in ironmaking blast furnaces.

1. Introduction

Coke is regarded as the most critical raw material and fuel in the blast furnace (BF) ironmaking process. It serves not only as an indispensable reducing agent and heat source but also as the core solid material forming the supporting skeleton of the lower burden column [1–4]. In the lower parts of the BF shaft, tuyere, and hearth, a space exists where coke accumulates densely and descends at an extremely slow rate. Moreover, the voids between the coke particles are filled with coke fines, unreduced ores, and flowing molten metal and slag, forming what is referred to as the “deadman” [5–7]. The operational state of the hearth, the renewal mechanism of coke, and the dissolution of internal coke are directly affected by the shape, size, permeability, and “sinking–floating” behavior of the deadman.

Consequently, the behavioral mechanism and renewal cycle of deadman coke in the hearth have long been the focus of research in ironmaking [8–9]. Dong et al. [10] investigated the behavior of deadman coke and its influence on temperature variation in the hearth based on production practices. Nogami et al. [11] employed a semi-three-dimensional BF model with a semi-circular cross-section to observe the movement trajectory of a single coke piece during the sinking and floating motions of the deadman, and proposed two renewal paths. Furthermore, Zhang’s group [12–14] and Niu et al. [15] conducted a detailed analysis of the multiphase behavior of deadman coke using various methods to clarify the interaction reactions and products among coke, slag, and hot metal.

Regarding the renewal cycle of deadman coke, renewal times vary among different BFs. The typical process of coke dissolution and renewal in hot metal lasts approximately four weeks, while extended renewal periods may reach up to 60 d [16]. Study of [17] determined the renewal cycles of deadman coke in Kawasaki No. 3 BF and Chiba No. 1 BF using a graphite tracer, and found that the coke descent process at the top of the deadman was intermittent, with a renewal cycle of approximately 1–3 weeks. Raipala [18] reported that the renewal cycle of coke surrounding the deadman was 2–3 d, and that of coke in the center ranged from 15 to 19 d, based on a Sc2O3 tracer study. Most of the above research methods rely on calculations and inferences from original design data or laboratory simulations of the BF environment, which cannot fully reproduce the actual behavior of deadman coke during BF smelting.

With the rapid development of BF dissection technology, the morphology of deadman coke can now be obtained more directly during furnace shutdown, making it the fastest and most accurate approach for characterizing deadman coke and its multiphase behavior in the hearth.