Experimental Study on Bubble Size Measurement for Development of Seafloor Massive Sulfides

Abstract

Abstract Seafloor Massive Sulfides have been expected to be future mineral resources. To promote the development of Seafloor Massive Sulfides, Seafloor Mineral Processing, a method of extracting valuable minerals from the ores on deep seafloor using flotation to reduce the lifting cost of ores from the seafloor to the sea surface, was proposed. To apply flotation for the seafloor mineral processing, a measurement method of bubble size applicable to deep-sea conditions has been desired because it is necessary to generate fine air bubbles suitable to flotation under pressure conditions on deep seafloor. Then, the authors have studied on bubble size measurement by image analysis, which is expected to be applicable to deep-sea conditions. At the first phase of this study, photographic conditions suitable to image analysis of air bubbles were distinguished. Air bubbles were generated by using a porous nozzle at some air flow rates in a bubble column with a rectangular cross-section. Video images of air bubbles were taken by using both high-speed camera and video camera with a low frame rate. Bubble size was measured by binarizing the video images of bubbles. Under optimal photographic conditions, bubble size was obtained from not only the high-speed camera but also the video camera; and both size data agreed relatively well, which implies that bubble size measurement by using image analysis would be applicable to deep-sea conditions. At the second phase, experiments were carried out under high-pressure conditions up to 2.4 MPa. Single bubble generation by using a capillary nozzle in a small pressure chamber with a sight glass was observed by using a digital microscope. Bubble size measurement by image analysis was carried out by the procedure established at the first phase. While the process of bubble generation at the high pressures was similar to that at the atmospheric pressure, the bubble size was decreased as the pressure rose. The result implies there is a strong correlation between the pressure and the bubble size.