**1. Introduction**

The gas-injection technique has been widely adopted in the pyrometallurgy smelting processes of ferrous and nonferrous metals and in the recovery of secondary resources, as well as in chemical engineering processes, such as extraction processes. All the above processes generally involve a complex gas–liquid–liquid multiphase system in the vessel. The flow, mixing, transfer, and reaction among the components of the multicomponent fluid play important roles in increasing smelting efficiency and improving product quality. One phenomenon involves the gas bubbles crossing the liquid–liquid interface. Experimentally investigating bubble motion and interfacial phenomena during the blowing–smelting process at high temperatures is difficult [1–3], except in a few simple cases [4,5]. Most research on this subject has been carried out using cold-water model experiments, theoretical analysis, and numerical simulation techniques.

Reiter et al. [6,7] studied the interaction between single bubbles and a liquid–liquid interface system. The bubble motion at the interface (e.g., residence time and velocity), the interfacial phenomena (e.g., liquid "jet" and interfacial area), and the phase entrainment (e.g., number and size of droplets) were measured using high-speed photography. Dietrich et al. [8] used the Particle Image Velocimetry (PIV) technique to describe the flow fields around a bubble crossing the interface. Dayal [9] analyzed slag specimens collected from the slag–metal interface in an industrial 65-ton ladle furnace and explained the slag–metal interface phenomenon on the basis of cold-water experiment results. Kobyas [10] established a model to explain iron droplet formation and behavior in slag when gas bubbles pass through the molten iron–slag interface. Ueda [11] and Kochi [12] used a CFD model based on the finite volume method to predict the flow field and the penetration stage when a bubble rises through

a water–oil interface. Some other simulation methods, including smoothed particle hydrodynamics (SPH) [13] and the multiphase particle method [14], have also been used to model gas bubbles passing through liquid–liquid interfaces.

In the current study, we simulated a slag–metal system using a water–oil system under cold experimental conditions and investigated the movement of bubbles and their behavior when they passed at various flow rates through a water–oil interface. The effects of different oil viscosities on bubble behavior were studied. We found that bubble movement is greatly influenced by the viscosity of the oil at low gas flow rates, whereas the movement of bubbles is more complex at high flow rates.

#### **2. Experimental**

A cold-water model was established for investigating both bubble motion (e.g., the path of movement, rising velocity, breakage, and coalescence) for a bubble crossing the liquid–liquid interface and the variation of the interfacial phenomena with bubble motion. Water and silicone oil were selected to investigate the liquid–liquid movement. Air was injected from a bottom nozzle to the lower phase (water). The nozzle diameter was 2 mm. The vessel was 500 mm in length (*L*) and 100 mm in width (*W*). The liquid height was 210 mm, and the height ratio between the water and silicone oil was 2:1. Silicone oils with different viscosities (shown in Table 1) were used [15,16], and the water was colored red to obtain a clear interface. The bubble and interface movements were recorded with a high-speed camera (HiSpec 5). The gas dispersion process was recorded from the moment of injection and 500 frames per second were captured until the flow field was stable for a duration of 30 s. The experimental system is shown in Figure 1.

**Figure 1.** Experimental setup.

**Table 1.** Properties of the silicone oils used in the experiments.

