Limiting conditions in large flotation cells: Froth recovery and bubble loading

Abstract

In recent decades, the exponential increase in the cell volume of flotation cells has promoted significant advantages, such as those related to investment costs, footprints, and energy savings. In larger cells, the diameter/height ratios and froth transport distances increase. Thus, the use of froth crowders and internal launders becomes compulsory for maintaining froth transport distances while decreasing the ratio between the gas and collected mineral flow rates. Consequently, the bubble load will increase until reaching a critical bubble surface coverage.In this paper, the concentrate carrying rate (tph/m2) was described in terms of the bubble surface coverage (bubble loading) at the pulp-froth interface and the froth recovery (froth transport) and was evaluated as a function of the cell volume and operating variables. A sensitivity analysis based on industrial operating and design conditions, such as the superficial gas rate, particle size, froth cross-sectional area and cell volume, was developed to evaluate the metallurgical performance of cells with volumes ranging from 100 to 630 m3.The results show that carrying rate limitations can arise for the whole range of cell volumes depending on the critical operating conditions, particularly for the first rougher cells and in the cleaning stages of industrial flotation circuits. An increase in cell volume consistently increases the bubble loading (bubble surface coverage) under all conditions, approaching the limiting conditions more rapidly for cells without internal launders. Otherwise, the limiting condition was achieved when the particle size DPS decreased from 50 to 20 µm and for two separate conditions of superficial gas rate: high (much higher than 1 cm/s) and low (much lower than 1 cm/s). The minimum bubble surface coverage was observed around JG = 1 cm/s, which corresponds to the maximum bubble surface area flux, independent of the cell size.The use of internal launders increases the range of operating conditions without exceeding the maximum bubble loading.