Objective: Provide a low-maintenance, reliable means of generating bubbles and controlling bubble size for column flotation.
Approach: Commercial sparger systems used in flotation columns to process minerals provide only limited means of controlling the size of bubbles generated and the rate of air injection. Generally, these limitations are a detriment to flotation performance, particularly in recovering coarse or fine mineral particles. In addition, conventional bubble generation systems require frequent maintenance, often requiring shut-down of the column itself for repairs. To permit greater control of the bubble size generated in the flotation column and to eliminate the counter-productive problems associated with the operation of current air sparger systems, the Bureau of Mines investigated alternative methods for generating bubbles. Based on the investigation, an external bubble generation system was designed, developed, and laboratory and commercially tested.
External System
The improved gas sparger system is a pressurized mixing chamber (bubble generator) equipped with a gas injection mechanism that can be used to control bubble size from less than 0.2 mm to 4 mm average bubble diameter with surfactant dosages of less than 13 ppm. It is the first external bubble generation device that allows control of bubble size. Lower surfactant dosages and much higher column air holdups (volume pct air) are possible with this system than are possible with conventional systems. Air and water are added under pressure to a mixing chamber packed with a media such as glass beads or ground quartz, and are released into the reaction vessel through fine injection orifices. Bubble size can be controlled by adjusting the air and water flowrates, and by adding a small amount of surfactant.
System maintenance can be performed without shut-
ting down the column. Since the gas sparger system is external to the column, components can be easily isolated from each other, including the injectors which can be removed from the column with minimal disturbance to the column flotation process.
Test Results
The improved gas sparger system, when used with a column flotation process to recover chromite from a deslimed Montana chromite ore, produced a 44.7 pct Cr2O3 rougher concentrate at 87.1 pct recovery, whereas conventional flotation produced a 40.4 pct Cr2O3 concentrate with 85.4 pct recovery. Flotation of undeslimed chromite produced a 35.6 pct Cr2O3 concentrate with 86.5 pct recovery using conventional flotation, while column flotation with fine bubbles produced a 41.5 Cr2O3 concentrate with 94.9 pct recovery. The bubble generator also was used to generate bubbles for column flotation of a Nevada fluorite ore. The improved system yielded a 14 pct increase in CaF2 concentrate grade, and an 18 pct increase in CaF2 recovery over conventional flotation results. Other successful tests using the gas sparger system in association with column flotation were conducted on manganese, copper, molybdenum and precious metal ores from Utah and Nevada, phosphates from Florida and Idaho, and ocean crust minerals from the Exclusive Economic Zone.
Commercial applications of the gas sparger system have demonstrated an improvement in grade and recovery on a molybdenum ore, and an increase in grades and a reduction of product contaminants at a zinc-lead facility.
Benefits
The improved gas sparger system offers the following benefits over conventional systems:
- increased control of bubble size from coarse to fine
- low dosages of surfactant
- simple construction and operation
- lower manufacturing cost
- external bubble generation capability
- adaptability to any system requiring bubbles or solution aeration such as conventional or column flotation, wastewater treatment, and solution aeration.
These benefits, plus simplicity and favorable economics, make the external gas sparger system a reliable and cost-effective alternative to conventional air sparger systems used in flotation.
Patent Status
The U.S. Department of the Interior has applied for a patent on this development. For information on the availability of licenses, please contact the Bureau of Mines, Branch of Technology Transfer, 2401 E Street N.W., Washington, D.C. 20241.
