Flotation separation

Flotation separation

Valuable minerals in an ore can be separated from each other, and from worthless gangue minerals, by the froth flotation process. This process was developed in Australia at the start of the 20th century to treat the primary sulfidic silver/lead/zinc ore at Broken Hill, NSW. Some of the laboratory observations upon which the process depends were made during the nineteenth century in England, Germany, and the USA, but it was at Broken Hill that the first sustained efforts were made to exploit them, and that commercial scale operations became firmly established. The need for a new process to treat Broken Hill ore arose from the depletion of the secondary oxide ore that overlay the major sulfide mineralization. Natural oxidation to form the secondary zone had resulted in a concentration of silver and lead, and ore from this section of the ore-body could be smelted directly. The underlying sulfide zone, which comprised the bulk of the resource, was less amenable to treatment in the smelters of that era, and a method was required to recover the metal values if the potentiality of the primary ore was to be realized.

Many approaches were pursued to solve the "sulfide problem" at Broken Hill before selective flotation was developed. The successful technology involves first crushing the ore, typically to a particle size of about 5 to 50 micrometers, to liberate separate grains of the various valuable minerals and worthless gangue components. Then the particles are pulped with water, and the surface of the mineral of interest selectively made hydrophobic through the addition of an organic species, which is termed a collector. The term hydrophobic denotes "having little or no affinity for water" and means that the surface rejects water, as does an oily or waxy one. Following this procedure, a stream of air bubbles is passed through the pulp; the bubbles attach to, and levitate, the hydrophobic particles, which collect in a froth layer that disengages from the flotation cell by flowing over the weir of the cell. A frother, such as a long chain alkyl alcohol, is added to create a stable froth layer in the cell. The collector used at Broken Hill in the early days was eucalyptus oil derived from the leaves of the ubiquitous Australian "gum" tree. This led the President of the Mining Institute in Broken Hill at the time to proclaim, "Thus Nature, in close proximity to the vast bodies of complex ore, has provided the means for the concentration of such ores".

Following the success at Broken Hill, mining companies throughout the world rapidly adopted the flotation process. A similar situation to that at Broken Hill existed at the massive copper ore-body in Bingham Canyon in Utah, USA, where mining had exhausted the overlaying high-grade oxide ore by the end of late 19th century. The introduction of flotation in the early 20th century allowed the huge underlying low-grade copper sulfide mineralization to be processed economically.

A major advance in flotation practice was made in 1925 by the introduction of alkyl xanthates (dithiocarbonates) as collectors to replace compounds such as the eucalyptus oil initially used at Broken Hill. The xanthates proved to be much more selective than the compounds used previously and different sulfides could be sequentially floated by adjustments to the pulp chemistry. Xanthates are most efficient for the flotation of sulfides but quite inert for common gangue minerals. The flotation process has been continuously developed throughout the 20th century and into the 21st century to increase the efficiency of separation and to treat ever more complex ores. More effective and selective collector compounds than the xanthates have been developed and are used in practice, although xanthates are still applied widely. Most of these collectors are thiol compounds and derive their efficacy from chemical bonding between the metal atom in the sulfide and a sulfur atom in the collector.

The flotation cell contains an impeller to mix the pulp and inject air bubbles. The size of the cell has progressed over the years from a couple of cubic meters to up to 300 m3 today. Plants consist of multiple banks of cells and the process for each stage usually involves three steps. In the first group of cells, termed the rougher circuit, most of the mineral to be floated is recovered and this concentrate is then refloated in a cleaner circuit to make a more pure product. The sink fraction from the roughing stage is sent to a scavenger circuit and the subsequent float fraction is returned, together with the sink fraction from the cleaners, to the roughers to avoid losing valuable minerals.


Flotation is a key unit process in the recovery of most of the world copper, lead, molybdenum, nickel, platinum group elements, silver, and zinc, and in the treatment of certain gold and tin ores. And what is the electrochemical aspect of flotation? The interaction of the organic collector with the sulfide mineral surface occurs by a corrosion-type mechanism in which an anodic oxidation reaction attaches the organic collector to the mineral surface and the electrons transferred to the mineral by this reaction are returned to the solution phase by the reduction of oxygen. It thus follows the same type of mechanism as does ore oxidation discussed above.

Not only does the flotation process enable valuable sulfide minerals to be concentrated from worthless gangue minerals that make up the ore matrix, but it also allows different sulfide minerals to be separated from each other. For example, Broken Hill ore contains lead sulfide (galena), and zinc sulfide (sphalerite). In the first flotation stage, galena is rendered hydrophobic ("water rejecting") with an organic collector such as xanthate, while sphalerite is kept from floating by the addition of zinc sulfate, which deposits a hydrophilic ("water-loving"), wetting layer on its surface. Chemicals used to impede the flotation of a mineral, such as zinc sulfate in this case, are known in the industry as depressants. In the second flotation stage, copper sulfate is added and this compound is an activator. It reacts with the sphalerite surface to incorporate copper atoms and displace zinc atoms. Now the mineral surface can interact with collector molecules by the electrochemical process discussed above to make it hydrophobic and be incorporated into a froth layer, while leaving behind in the sink fraction oxides, carbonates, silicates, etc. Silver sulfide minerals in the ore follow the lead sulfide stream and cadmium sulfide follows the zinc. Silver and cadmium are separated from lead and zinc, respectively, in the subsequent metal extraction processes. The Broken Hill mines contain 6% lead and 10% zinc and flotation concentrates the ore into two streams, a lead concentrate containing 70% lead and a zinc concentrate containing 50% zinc. These concentrates are then sent to lead and zinc smelters, respectively.

Another example is the treatment of ores containing copper sulfide (chalcocite) and copper iron sulfide (chalcopyrite), together with molybdenum disulfide (molybdenite). The usual practice is to float both copper and molybdenum minerals in the first stage, leaving iron sulfides and other gangue minerals behind. Then the concentrate is sent to a second flotation circuit and sodium hydrosulfide added to depress the chalcopyrite by removing collector from its surface and the molybdenite floated alone, exploiting the fact that this mineral is naturally hydrophobic. Copper ores generally contain between 0.5% and 2% copper and the flotation process increases this to above 30%. The flotation concentrate contains sufficient copper for it to be suitable for smelting to recover the metal. Silver, gold and platinum group metals present in copper ores follow the copper in both the flotation circuit and the smelter, and are recovered in the final, electrorefining stage. Electrorefining is discussed in the final Section.

Flotation is also used to split copper and nickel sulfides and separate them from iron sulfides and gangue minerals. Cobalt sulfides appear in the nickel concentrate and the cobalt is subsequently recovered.

Flotation has developed from the treatment of simple ores to the ever more complex ones that are being found today. More selective collectors are used and the pulp chemistry adjusted to achieve optimum separations and recoveries. Thus, for example, the three metals in copper/lead/zinc ores can be floated into three concentrates, each containing one of the metals.

The process of flotation is sometimes also used in electrochemical waste treatment .