flotation

Flotation is a separation process that exploits differences in how minerals interact with water and air. A slurry of finely ground ore (typically 20 to 30 micrometers) is mixed with water, chemical reagents, and air. Some minerals preferentially attach to air bubbles and rise to the surface as a froth; others sink. The floating minerals are skimmed off as concentrate, while the heavier material settles as tailings. This process handles mixed ores containing multiple valuable minerals that cannot be separated by gravity alone.

The chemistry is deliberate. Collectors are hydrophobic compounds that coat the surface of target minerals, making them water-repellent and bubble-prone. Frothers are organic compounds (often pine oil or synthetic equivalents) that stabilize the air bubbles themselves, preventing them from coalescing too quickly. Depressants selectively inhibit certain minerals from floating, keeping them in the sink. The pH, temperature, and reagent dosages are adjusted for each ore type; a copper-molybdenum circuit requires different chemistry than a lead-zinc circuit.

Flotation cells are mechanized vessels ranging from 5 cubic meters to 100 cubic meters or larger in industrial plants. A mechanical impeller at the bottom draws in air through a pipe and simultaneously stirs the slurry, creating fine bubbles and turbulence. The residence time is typically 5 to 15 minutes per cell. Multiple cells are arranged in series: rougher cells produce a bulk concentrate that is then reground and cleaned in scavenger and cleaner cells to remove unwanted minerals and improve grade.

The process works because the attachment is selective but imperfect. A well-operated flotation plant might recover 85 to 95 percent of the target mineral while rejecting 70 to 90 percent of the gangue (waste rock). However, this selectivity depends heavily on particle size, liberation (the degree to which ore minerals are physically separated from waste), and reagent balance. Fine particles float poorly and may be lost; very coarse particles settle too quickly.

Flotation became dominant in the early 1900s because it solved the problem of processing low-grade, finely disseminated ores that gravity separation and magnetic separation could not handle economically. Today it is the standard for copper, lead, zinc, molybdenum, nickel sulfides, and many precious metal ores. The term comes from the simple observation that treated mineral particles literally float to the surface, a visual and mechanical principle that remains unchanged from the earliest practice to modern circuit design.