Deep-sea mining (DSM) focuses on extracting valuable resources, such as polymetallic nodules, from seabeds at depths of up to 5000 meters. These nodules contain critical metals but pose technical challenges due to extreme conditions like high pressure and long transport distances.

A critical component of deep-sea mining (DSM) is the vertical transport system (VTS), which lifts nodules from the seabed to the surface using a riser system with two-phase (solid-liquid) or three-phase (solid-liquid-gas) flow. The VTS plays a vital role in determining throughput, energy consumption, and the overall stability of the mining operation. Multistage centrifugal pumps are promising for powering the VTS due to their ability to handle high pressures and coarse solids. While the risk of blockages is minimal under normal operating conditions, flow assurance within the pump becomes a significant concern during reflux situations. In such cases, gravity causes the nodule-water mixture to reverse direction, substantially increasing the risk of blockages and clustering. This highlights the importance of analysing reflux scenarios in greater detail to ensure reliable system performance.

To address these challenges during reflux, a test setup is developed to investigate operational and design variables as well as pump geometry. Two model pumps were constructed: one without impeller and diffuser blades, focusing on the effects of internal spacing, and one with blades, aimed at evaluating overall reflux performance. The key vulnerabilities identified in this thesis provide a clear understanding of where issues may arise in the pump design and the underlying mechanisms causing them. This knowledge enables potential users of the multistage centrifugal pump to make informed adjustments, preventing flow assurance challenges in their systems.

Both the pump geometry and mixture variables were thoroughly analysed. For the geometry, the curvature of the impeller blades was found to cause solid accumulation at the entry, posing a significant risk of blockages. This research identified the issue and proposed modifications to the blade edges, which proved highly effective in eliminating accumulation and significantly improving solid flow. Narrowing diffuser vanes and shallow blade inclination were found to promote contact-dominated flow, velocity reductions, and obstructions. At the diffuser-impeller transition, solids entering from multiple directions caused bridging and blockages. Mixture variables also had a notable impact. Solids within the pump's design specifications passed through unobstructed when processed in single-solid batches. However, increased solid concentration at the pump inlet led to accelerating blockage formation. Higher-density solids demonstrated better performance by maintaining higher velocities and reducing the risk of obstructions.

To enhance future testing, it is recommended to use transparent materials, as implemented in this study. This approach provided valuable insights into the flow dynamics and greatly improved the ability to observe and analyse potential blockages. Additionally, optimizing the geometry of the impeller and diffuser blades is critical for improving the pump's overall performance. Adjustments should focus on reducing blockages and promoting smooth solid flow, while carefully balancing throughput efficiency and pump capacity. These design improvements are essential to ensure reliable and efficient operation, particularly in demanding applications as deep-sea mining.