Modelling of a Slot-Type Porous Plug with a New Design ^†

1. Introduction

The basic secondary processing procedure is the process of blowing liquid steel with inert gasses. Despite the simple technique involved, the hydrodynamic phenomena occurring in liquid metal are very complex in nature. Therefore, they have been the subject of many studies, as evidenced by the literature [1,2,3]. The purpose of the process is to accelerate the outflow of non-metallic inclusions from the volume of liquid steel and promote their effective transport to the slag phase. Liquid steel is also blown with inert gasses to homogenize it, both in terms of chemical composition and temperature [1,2]. The effectiveness of the process mainly depends on the conditions present for the formation of a hydrodynamic structure in the volume of liquid steel as a result of the influence of the injected gas stream, the design of the porous plug shape, the materials used for its construction, and the location of the shape at the bottom of the steel ladle [4,5,6]. The above-mentioned factors directly affect the formation and nature of the gas column and, consequently, the movement of liquid steel in the steel ladle [4,5]. It is assumed that, in order to obtain the required effectiveness of the process, gas bubbles flowing from the surface of the porous gas should be as fragmented as possible. The gas column should, therefore, contain as many gas bubbles of very small sizes as possible. This is also important from the point of view of the impact of the gas column on the metal–slag interface and the formation of the slag eye [3,4].

Modern solutions designed for porous plugs are characterized by a high complexity to meet the above functionalities [7,8]. Many years of practice and continuous advancements in fields such as materials engineering, materials shaping technologies, fluid mechanics research methodology, etc., have resulted in achieving such a high level of advancement that further increasing their effectiveness in its current form is difficult and ineffective [4,5,8]. Most often, this concerns certain details resulting from the needs of specific producers of high-quality and special-purpose steel. The solution to this problem may be to change the design of the porous plug so that it occupies a larger area of the bottom of the steel ladle (active surface of the plug) than before and, consequently, is a source of a larger number of very small inert gas bubbles while the degree of its consumption is maintained. One such innovative solution is the proposed design of a slot-type module (KS diffuser) [9,10]. The design concept of the module is fundamentally different from the one used so far. In principle, it is easy to install in the bottom of the ladle due to its cube-like shape. It does not require replacement during the ladle campaign, has a large active surface with a series of slots of specified width, and generates a large number of finely divided gas bubbles, which are conducive to achieving a high degree of dispersion in liquid steel [9,10]. The presented test results are intended to confirm the functionality of the proposed design solution of the KS diffuser compared to the previously used slot-type porous plug [3,8]. The confirmation of the validity of the adopted assumptions in new technological solutions not previously used in production conditions is most often obtained through model tests. Physical and numerical modeling are commonly used research tools in a very wide range of metallurgical processes, from the blast furnace [10,11,12,13,14], through steelmaking processes [15,16,17], and to continuous steel casting [18,19,20,21].

The research carried out was a part of the ongoing project “Development of a new technology for deep refining of steel in the process of secondary treatment and casting in a tundish enabling an increase in the degree of steel purity”—no. 1/POIR/08/2018 beneficiary Cognor SA Ferrostal Łabędy Gliwice.

A commonly used research tool to solve similar problems is water-based physical modeling, which allows for the identification of the mechanism of formation and the evolution of the hydrodynamic conditions that prevail in liquid steel as a result of the impact of the blown gas stream.

2. Subject of the Research and the Research Methodology

Laboratory tests were carried out using a water model of a steel ladle made in accordance with the requirements of the similarity theory on a linear reducing scale of S_L = 1:3.4. To build the model, an industrial steel ladle with a nominal capacity of 65 t was used as a real object [9].

The steel ladle model was equipped with appropriately prepared models of the porous plug and the KS diffuser, also made on a linear scale of S_L = 1:3.4. These models were placed on the radius of the bottom of the ladle at a distance of 2/3 of its length from its axis. The gas flow was regulated using a precise regulator system equipped with an additional rotameter. A special system was designed to inject the required portion of the tracer. The system has been designed in such a way that it is possible to introduce the tracer into the model liquid along with the stream of injected gas. Figure 1 shows a diagram of the test stand.

The modified Froude criterion [6,22] was adopted as the dominant criterion of dynamic similarity. The research analyzed two of inert gas flow rates—3.7 dm³/min (in the industry this value is 200 dm³/min) and 4.2 dm³/min (in the industry this value is 225 dm³/min).

The porous plug used in the existing technology was characterized by 12 slots, while the KS diffuser has 8 of them.

The active surface of the diffuser resembles a traditional slotted shape, but the slots are arranged in a rectangular shape, not as in traditional circular shapes. Additionally, it does not have steel armor and does not require shell fitting for installation. The module is covered by patent no. PL 229475 [10].

Due to the results obtained, the research was carried out using a water model of a steel ladle was of a double nature—qualitative and quantitative. The result of the qualitative research was the visualization of the process of mixing the model liquid and, based on it, the assessment of the mechanism of formation of a cone of gas bubbles (gas column). During these tests, the degree of gas dispersion in the model liquid was also determined. As a result of quantitative tests, RTD mixing curves and, based on them, the minimum values for the mixing time were determined. The minimum mixing time was adopted as a criterion for assessing the ability of the tested system to homogenize the model liquid. Figure 2 shows a schematic diagram of the research program.

3. Preliminary Results and Discussion

Based on the results of the model tests, it was found that the newly designed solution for the KS diffuser meets the assumed requirements. During the visualization of the gas column formation process, we observed a favorable phenomenon: a large number of small gas bubbles detached from the diffuser surface and flowed steadily towards the model liquid mirror. The growth of bubbles along their flow path was relatively small. This mechanism of gas column formation predicts an increased refining capacity when using the KS diffuser compared to the previous solution, where bubbles tended to merge into larger conglomerates and flow out in a swirling manner.

When using the KS diffuser, an increase in the degree of gas dispersion in the model liquid was also observed compared to a classic porous plug. The consequence of this phenomenon is a larger volume of the mixed gas–liquid phase in the steel ladle model, which favors the process of homogenization of the model liquid in its entire volume. Moreover, this structure of hydrodynamic conditions significantly reduces the possibility of creating dead zones. The increase in the system’s ability to effectively homogenize the model liquid when using the KS diffuser compared to the classic solution was also confirmed by the results of the analysis of the obtained RTD curves. The minimum mixing times determined on their basis were significantly shorter for the KS diffuser than for the porous plug.

To sum up, it should be stated that, based on research conducted using a physical water model of a steel ladle, the proposed innovative design solution for the KS diffuser meets the assumed expectations and may constitute an effective alternative to the previously used solution.