MINERAL PROCESSING SITE

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SULFIDE FLOTATION

The ores with sulfide minerals often consists of metal sulfide minerals and gangue. However, rarely, only the separation of a sulfide from the gangue can be requested. This is the simplest application of flotation which is called selective flotation. Besides, the separation of all of the sulfides from the gangue is called bulk flotation. Sulfide minerals such as those of copper (CuFeS₂), lead (PbS), and zinc (ZnS) are commonly found in the same ore. Xanthates (dithiocarbonates) and dithiophosphates are used as collectors in sulfide mineral flotation; frothers include pine oil and synthetic commercial compounds such as Dowfroth 250. Some qualified generalizations apply to sulfide mineral flotation:

1. Xanthates and dithiophosphates are collectors for sulfides.
2. The chemisorption of xanthates and the presence of oxygen are essential for the flotation of sulfide minerals such as galena. This correlates well with the electrochemistry of the system and the stability of the species that collect at the solid surface.
3. Other variables being constant, an increase in chain length for a homologous series of reagents strengthens them as collectors; this rule also applies to other minerals.
4. At a given constant collector concentration, a critical pH for each sulfide mineral determines the boundary of flotation - no flotation conditions. The ratio of concentration of xanthate to hydroxide for such a critical transition usually conforms to the Barsky relationship; [ X– ] / [ OH– ] = constant.
5. The ions OH– , S^2– , CN– , Cr₂O₇^2– are common sulfide mineral depressants for selectivity control in flotation; Cu²⁺ is an activator ZnS in flotation by xanthates.

Flotation of Complex Sulphide Ores

In the flotation of an ore consisting of chalcopyrite (CuFeS₂), galena (PbS), sphalerite (ZnS), pyrite (FeS₂), and gangue minerals following procedures are in use:

• Chalcopyrite: When the solubility of hydrophobic salts in sulphide collectors (xanthate, thiophosphate, thiocarbonate, etc.) is taken in consideration, it is observed that decrease in solubility results in increase in floatability. As a result, by using relatively small amounts of collectors and frothers, first copper minerals (chalcocite [ Cu₂S] , chalcopyrite [ CuFeS_2] ) can be floated.
• Galena: This mineral can be floated in slightly alkaline medium (pH = 9-10), with short-chain xanthates (potassium etyl xanthate, isopropyl xanthate). In addition to this, as the critical pH of galena is 10.4, when etyl xanthate is used, usage of lime will behave like a depressant. So, in the adjustment of pH, sodium carbonate is used.
• Sphalerite: If there is no activator ions in the medium, sphalerite does not float with xanthate, dithiophosphate collectors. Activation and flotation of sphalerite in acidic medium (pH = 4-5) is performed with As, Sb and Pt ions and in neutral conditions Ce, Pb, Cd, Cu, Ag, Hg, Bi and Au ions become effective.
• Pyrite: In the selective flotation of copper, lead, zinc, etc. to activate the depressed pyrite the most effective way is to decrease the pH to 5-6 by sulfuric acid. The second method to activate pyrite is addition of sodium carbonate. In this way, hydroxide and sulfate leaves the surface of pyrite and the lime in the medium turns into CaCO₃ and loses its effectiveness.

Sulfide Selectivity

Selectivity between sulfide minerals is possible when one can adsorb collector, and the others cannot.

Figure 1 shows contact angle data for three sulfide minerals. In this figure, flotation is possible to the left of each curve, but not to the right. Clearly, as the pH is increased, depression occurs. For pyrite when the pH is raised above about 6,the rest potential no longer exceeds the reversible potential for the oxidation of the collector ion; and also, the Fe-collector compound is not as stable as competing Fe-hydroxyl species. With galena, the Pb-collector compound prevails until the pH reaches about 8; above this value, it is Pb hydroxyl species that predominate. From inspection of Fig. 1, it is clear that selective flotation is possible based on the depressant action of the hydroxyl ion alone. However, although these results give an indication of possible flotation operating conditions, it must be kept in mind that they were obtained by contact angle tests on clean specimens under ideal conditions. In many sulfide systems, pH regulation alone is insufficient for acceptable selectivity.

The principle of using competing ions, however, can be used to improve selectivity. In Fig. 2, the depressant effect of cyanide ions is shown by contact angle curves, flotation being possible to the left of each curve. The mechanisms of depression are more complex than was thought in the past.

In the following table, the data obtained from rougher flotation cells of copper, is given as a relation of time with cumulative percent recovery for different average particle sizes. Evaluate the data with graphs and make a comment.

According to the graphical data, it is observed that the recovery of copper is highest at 180 µ m size. Besides, the low recovery at 300 µ m is a result of the mineral’s not adsorbing to the air bubbles as they are too large and heavy to float. The reason of the low recovery at smaller sizes like 53 µ m is that they are too small and the mineral has a slimy character. These small particles can come together and form larger particles which would be larger than the bubbles can float.

Eren Caner ORHAN
Hacettepe University
Ankara,Turkey

References:

1. Ullmann’s Encyclopedia of Industrial Chemistry, 5^th Edition, (1988).
2. Cevher Hazırlama El Kitabı, Editors: Prof. Dr. Güven Önal, Prof Dr. Gündüz Ateşok (June 1994).
3. Kelly, E.G., Spottiswood, D.J., Introduction to Mineral Processing, John Wiley & Sons Inc. (1982)
4. Atak, S., Flotasyon İlkeleri ve Uygulaması, İstanbul Teknik Üniversitesi Matbaası, No: 101, (1974).

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