## 1 Introduction  
Access to affordable raw materials and minerals is essential for our industrial, social, and technological progress. A recent European Union (EU) study titled Critical Raw Materials for Strategic Technologies and Sectors defined 30 materials as critical raw materials (CRM), including aluminum, lithium, titanium, and strontium for the first time in 2021 [1]. This report also highlights the vulnerable situation of the EU regarding its dependence on critical materials from other countries. The Monteview mine (Granada, Spain) contains the largest European deposit of celsestine, which is the main source of strontium. The celsestine ore consists of mineralized stromatolites, originally made of calcite, which have been partially replaced by celsestine [2, 3, 4]. In addition to celsestine and calcite, the celsestine mineral also contains other minority phases such as dolomite, strontium, quartz, clay minerals (kaolinite, paragonite, and illite), iron oxides, and hydroxides, which are part of a finer mineral fraction. Canteras Industriales S.L. (CI), the company that operates the mine, has traditionally processed only high-grade mineral (approximately 80% celsestine), while the lower-grade mineral has accumulated in mine tailings and dumps. To make mine operation more sustainable and decrease its impact, CI is exploring new ways to process and concentrate the medium-grade mineral. Celestine mineral concentration and recovery would reduce mine operation costs by avoiding blasting and clearings, ground displacement, generation of residues, and fuel consumption. Overall, it would minimize the impact of the mine operation, making it more sustainable and eco-friendlier. The most commonly used methods for celestine concentration are classification by particle size, which separates the minerals present according to their grain size [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15]; gravimetric concentration by dense media (in hydrocyclones, drums, or spirals), which separates minerals by density differences and hydrodynamic drag [10]; and flotation methods, which use differences in the surface properties of minerals for separation [16; 17]. Oil agglomeration methods have also been used to concentrate celestine [18; 19]. The relative high density of celestine (3.96 kg/L) makes it susceptible to separation from other associated lower density minerals (i.e., quartz 2.65 kg/L, calcite 2.71 kg/L, illite 2.80 kg/L, dolomite 2.84 kg/L) using a dense medium. Dense media separation (DMS) methods separate the mineral of interest from the gangue minerals using a suspension of dense medium minerals operated in a circuit [20]. However, the dense medium is a heterogeneous suspension of finely ground particles of a different nature (dense medium minerals and raw ore mineral) in water. Thus, mineral particle size and shape, as well as the properties of the dense medium minerals (i.e., ferrosilicon, magnetite) used to prepare the dense medium influence the separation process [11; 21; 22]. The DMS method is considered practical for large particle sizes; however, some studies show good separation for particles as small as 25 \(\upmu\)m [15; 23; 24]. Depending on the run-of-mine mineral's particle size, the DMS process uses cyclones, drums, or baths for processing circuits [20]. In this study, considering the characteristics of the run-of-mine celestine mineral (particle size from 100 mm to 6 mm), a hydrocyclone system was chosen. The run-of-mine mineral in the suspension (in this case, celestine ore) is fed, at high pressure, into the top of the hydrocyclone system, entering it tangentially [25]. Then, the run-of-mine mineral components are separated by their densities and drag. The lighter mineral particles float and emerge in the over stream and the heavier mineral particles (e.g., celestine and strontium) sink and emerge in the under stream (see Figure 1). However, in the hydrodynamic regime of the hydrocyclone, the mineral particles' behavior is also affected by other factors (i.e., particle size and shape, flow rate).