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close this book Tools for Mining
close this folder Technical Chapter 16: 0ther Sorting and Separating Techniques
View the document 16.1 Roasting oven, calcining furnace
View the document 16.2 Salt gardens, salt works, salterns
View the document 16.3 Sulfur production in heap smelting or chamber ovens
View the document 16.4 Autoclaves for extracting sulfur
View the document 16.5 Copper sulfate plant
View the document 16.6 Electrostatic sorting
View the document 16.7 Flotation

16.7 Flotation

Mining General (Gold, Ore, Coal,Industrial Minerals)

Beneficiation, Sorting

germ.:

Flotation

span.:

flotacion

Manufacturers:

Aker, Booth, Denver, Galigher Comp., KHD, Krupp, Machinoexport, Minemet Ind., Hoechst (reagents), Outokumpu, Sala, Wemco, Maxwell, INCOMEC, Volcan, Eg. Ind. Astecnia, IAA, COMESA, FAHENA, FINA, Famia, Fund Callao, MAGENSA, MAEPSA, Met. Mec. Soriano, PROPER, IMPROCON, MILAG

TECHNICAL DATA:

Dimensions:

flotation cells approx. 1 x 1 x 0.8 m up to 5 x 5 x 2.5 m and larger

Weight:

approx. 1 - 20 t

Extent of Mechanization:

fully mechanized

Power:

2.2 kW to 100 kW, approx. 1.5 - 5 kW/m volume of flotation cells

Form of Driving Energy:

electromechanical

Mode of Operation:

continuous

Operating Materials:

 

Type:

compressed air reagents bubbles < 2 mm in diameter

Quantity:

0.3 - 2 m /min m of slurry

ECONOMIC DATA:

Operating Costs:

high grinding costs

Related Costs:

dosing mechanism for reagents, grinding facility, classifying facility, settling pond for tailings

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Personnel Requirements:

for good separation results, precise control of slurry density, alimentary quantities and concentration of reagents is necessary

Grain Size of Feed:

50 200 ym

Recovery:

with preliminary flotation and subsequent cleaning of sulfidic ores, considerably higher than with gravimetric methods

Replaces other Equipment:

wet-mechanical sorting processes

Regional Distribution:

worldwide, flotation is the most widely used sorting process for mineral raw materials, partially also used in small~scale mining; approx. 2 billion tons of raw material is floated annually

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very bigh

 

high environmental impact through discharge of reagents with the tailings. The use of tailing ponds, neutralization basins, etc. and precise dosages of reagents are absolutely necessary.

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

flotation cells can be locally produced, e.g. from wood, iron, ferro-concrete or plastic materials; remaining components from imports

Lifespan:

very long |————|————| very short

 

when components subject to wear are made of elastomers

Bibliography, Source: Stewart, Priester, Taggert, Schubert, Gerth, Manufacturers information

OPERATING PRINCIPLE:

The flotation process utilizes the differences in surface wettability of various minerals, which can be artifically influenced, to achieve a separation. The completely-liberated feed material is suspended in a slurry containing approx. 30 % solids (by volume) and the valuable mineral selectively hydrophobed through the addition of collector reagents, which are mostly long-chained hydrocarbons of specifically regulated pH-values. This conditioned slurry then flows into the flotation cell' where it is brought into contact with injected, dispersed air bubbles; the electively-hydrophobed valuable-mineral particles adhere to the bubbles and travel upwards as a foam-mineral mixture (possibly stabilized through the addition of foam reagents or 'frothers') to the slurry surface where this "float" is then skimmed off. To suppress the unwanted hydrophobing of accompanying minerals and to enhance their removal with the "non-float", depressant reagents are added to the slurry. In the indirect flotation process, the valuable mineral is concentrated in the hydropilic non-float.

AREAS OF APPLICATION:

For the selective extraction of valuable minerals from raw ore feed:

- sulfide minerals

non-ferrous metal minerals (sometimes following sulfidizing)

precious metals

- fluorite, apatite, phosphorite, sulfur

- wolframite, scheelite, cassiterite, industrial minerals (sand and gravel)

- coal, graphite

- potassium (potash) salts

- quartz, keolin, feldspar, mica

SPECIAL AREAS OF APPLICATION:

To separate impurities and accompanying minerals from mineral-material mixtures

e.g.

reversed iron-ore flotation

 

reversed magnesite or calcite flotation

 

cleaning of glass sands

REMARKS:

For small-scale mining needs, flotation cells with external air supply can be recommended. This process requires more equipment and therefore higher investment costs, however permits regulation of the air supply to accommodate fluctuations in feed-quantity, feed contents, slurry density, etc. Self-aspirating cells allow a narrow range of variation only by changing the rpm of agitation.

Of importance for successful flotation is freshly-exposed surfaces. Especially sulfidic ores, which are easily subject to surface corrosion, require wet grinding prior to flotation.

Oil-flotation: W. Haynes/England/1860

Flotation with reagents for the separation of graphite, 1877 by Gebr. (brothers) Bessel/Germany

Foam-flotation: since the mid-twenties, important for very fine feed: agglomeration flotation (not economically significant)

REAGENTS:

Collectors

For sulfide minerals, anionic sulfhydryl collectors such as xanthate and dialkyldithiophosphate (for example, aerofloat, phosokresol) at concentrations of 10 - 200 g/t feed are applied,for non-sulfidic minerals: use of anionic oxhydryl or cationic collector, for example, long-chained, non-saturated (as much as possible) fatty acids or their soaps, which have previously been dissolved in hot oil, in concentrations of between 100 and 1000 g/t feed; by these quantities, the cost of reagents substantially affects operating costs. Silicates, halides and oxidic zinc ores are floated with organic amines as collector. To strengthen natural hydrophobia, for example in sulfur and coal or through the addition of an artificial hydrophobia, saturated hydrocarbons such as petroleum and oils are suitable.

Foaming agents/Frothers

Terpene and cresol or synthetic foaming agents added in quantities of around 5 - 50 g/t during sulfide flotation reduces the size of the bubbles and stabilizes the foam by lowering the surface tension.

Depressing agents/Depressants

Examples: zinc sulfate to depress zinc blende (sphalerite) in Pb-Zn-ores, cyanide to depress gold and silver, copper minerals, etc. by complexing.

Activators

Examples: addition of small quantities (1 - 10 g/t) of cyanide to clean mineral surfaces; sodium sulfide to convert oxide layers in sulfides; copper sulfate to activate zinc blende.

pH-reagents

to establish basic conditions: hydrated (slaked) lime, soda or caustic soda; to establish acidic conditions: sulfuric acids.

For small-scale mining, of special interest are individual components such as stator/impeller units from Aker which can be installed into existing, or possibly locally-manufactured, cells. In addition, these parts, being highly subject to wear, are normally made today of elastomers (for example polyurethane) which are extremely wear-resistant.

In order to assure the quality of the end products of flotation, precise control of the process is crucial. It is essential that the quantities of reagents added during flotation remain constant. Whereas this is performed today in large mechanized plants via dosing pumps, in small-scale mining, bucket-wheel proportioners have proven to be extremely effective. By altering the volume and/or number of buckets, or by modifiying the rpm of the bucket-wheel disk, they can be adjusted to cover a wide range of dosages. Furthermore, they are very sturdy, simple, accurate and suitable for local manufacture.

In addition to the process control, flotation also requires continuous monitoring of product quality. A simple periodic product sampling with the batea or gold pan assists many plants in quickly detecting possible deviations from the standard values. Small pan-shaped or inverted roof-shaped wooden troughs are used for this purpose.

Local products are sometimes used as reagents for the flotation, for example, natural oils, wastes from wood processing and from paper plants, etc. In this way the costs for imported reagents can be decreased substantially.

Tailings from flotation also provide a good aggregate or filler for lean mixed concrete backfill consisting of approx. 10 % cement, 60 % mine waste and 30 % flotation tailings.

EXPERIENCE FROM APPLICATION IN DEVELOPING COUNTRIES:

Representing the simplest forms of foam flotation, pipe flotation, in addition to flotation in sluices and settling basins (buddies see 14.10), is also being used.

The slurry, preconditioned with reagents, is allowed to fall into an open vertical standpipe, whereby air is drawn down along with it (after the principle of the water-jet vacuum pump). The aerated slurry is directed through the pipe into the flotation cell; perforations in the pipe allow the bubbles to escape and the flotation to take place. The float is subsequently scooped or skimmed off.

The quality of the flotation can be assessed simply by visual inspection of the bubbles on the slurry surface. A thick, fine-bubbled, and especially dark-colored foam indicate a correct reagent dosage and good mineral loading on the bubbles.

A foam with big bubbles and a transparent appearance removes only low quantities of "float" minerals and indicates an insufficient addition of reagents or an incorrect pH-value.

SUITABILITY FOR SMALL-SCALE MINING:

Flotation of sulfides is a suitable technique for small-scale mining, particularly if local manufacturers build the flotation cells and are dependent only on a few imported components. The selective sulfide flotation can also be considered appropriate for supplementing gravimetric beneficiation in small-scale processing operations.


Fig.: Designs of standard commercially-sold flotation cells. Source: Young.


Fig.: Types of impellers for standard, commercially-sold flotation cells. Source: Young.


Fig.: Operating principle of a flotation cell. Source: Otero.