## 1 Introduction  
The increasing demand for critical raw materials, driven by their essential role in clean energy technologies, has raised concerns about potential supply shortages. Developing countries, particularly in Africa, which possess 30% of the world's mineral reserves, are pivotal sources of these materials [1; 2]. Critical minerals such as cobalt (Co), lithium (Li), nickel (Ni), and manganese (Mn) are essential to produce clean energy technologies [3; 4; 5]. The demand for these minerals is particularly significant in the production of electric vehicle batteries, with each battery requiring up to 200 kg of essential minerals. Notably, the battery sector accounts for 70% of the global demand for cobalt [6; 7; 8]. For this reason, these minerals are classified as 'critical raw materials' due to their immense importance and the limited availability of reserves on the Earth's surface [9]. Consequently, a steady increase in their demand is expected in the medium to long term (see Figure 1). It is imperative to generate fresh momentum to address the stagnation in the growth capacity of the terrestrial mining industry. Large-scale mining, which plays a critical role in surplus production, faces significant challenges, chief among them being the decline in ore grades. To compensate for this deterioration and maintain profitability, overexploitation becomes prevalent, leading to the substantial generation of polluting waste, such as tailings [7; 10; 11; 12; 13]. Among the minerals found on the seafloor, ferromanganese crusts offer a promising alternative for the recovery of metals required in contemporary clean technologies, particularly due to their high Co and Mn content [14; 15; 16]. These resources are distributed across the global oceanic expanse, occurring at depths ranging from 400 to 7000 m along the slopes of submarine mountains and ridges [17], and plateaus. There are few studies on the co-processing of ferromanganese crusts for Co and Mn extraction, a process that generally involves leaching with a reducing agent and, in some cases, preliminary roasting steps. With respect to studies that specifically utilize steel slag as a reducing agent and sulfuric acid to recover Co and Mn from underwater mineral resources, two recent investigations stand out. In the study by [PERSON] et al. [12], it was demonstrated that Co and Mn could be jointly dissolved from marine nodules, with the best results achieved using a reducing agent ratio of 1:2 (nodule/steel slag) at 60 \({}^{\circ}\)C for a duration of 15 min. This process resulted in extraction efficiencies of 98% for Mn and 55% for Co. Subsequently, [PERSON] et al. [12] replicated these conditions to recover Co and Mn from ferromanganese crusts. The researchers found that the optimal results were obtained with a reducing agent ratio of 1:2 (Fe-Mn crust/steel slag) at 60 \({}^{\circ}\)C for 10 min, achieving joint extraction efficiencies of approximately 80% for Co and -40% for Mn. The proposed reactions (Reactions (1)-(4)) for the dissolution of Co and Mn from ferromanganese crusts using foundry slags and H\({}_{2}\)SO\({}_{4}\) are outlined in Table 1.  
Figure 1: Projection of the value of critical metals for the year 2050 (modified from [PERSON] et al. [1]). In this study, a novel acid-reductive leaching mechanism is proposed, repurposing steel residue (slag) from the Chilean steel industry for the joint recovery of Co and Mn from ferromanganese crusts. This approach offers a sustainable solution to resource recovery, minimizing waste and the environmental impact associated with traditional mining methods. The response surface methodology (RSM) and design of experiments (DOE) were employed to optimize the leaching parameters, ensuring maximum recovery efficiency and showcasing the potential for industrial applications within circular economic practices. The utilization of RSM and DOE to model the leaching parameters for Co and Mn is pivotal in optimizing and understanding the complex interactions between various process variables in laboratory experiments.