BORNITE FLOTATION
Bornite is in many porphyry copper operations that have a complex mineralogy and its complexity can be very high. However is possible observe zones with secondary sulphides whose main minerals are covellite and chalcocite. In primary zones, mineralogical association can include sulphides like pyrite and chalcopyrite. Also is possible find other minerals with some complexity during its concentration. Those kinds of minerals are molybdenite, bornite, enargite, tennantite and tetrahedrite.
In froth flotation only some minerals have been studied with intensity. The main minerals are pyrite, chalcopyrite, galena, sphalerite and molybdenite. It is difficult find studies about others minerals such as covellite, enargite, bornite, tennantite, tetrahedrite, and other ones.
The next lines will try to show some difficulties and solutions on bornite flotation.
Bornite MINERALOGICAL PROPERTIES.
According to Betejtin (1), Bornite with formula Cu5FeS4 can form solid solutions with chalcopyrite. Its chemical composition is Cu 63.3%, Fe 11.2%, S 25.5%. However, its composition can change in some intervals because the mineral has chalcopyrite and chalcocite in solid solution. Its main characteristics are:
- Color is dry red but its fresh surface can get a blue color (Fig. 1 and Fig. 2).
- Streak is gray black and luster is metallic.
- Bornite is weak and its cleavage is very poor.
- Specific gravity is 4.9-5.0.
- Best Field Indicators are lack of good crystals, tarnish, streak, color of fresh surfaces and associations.
- Some times can be confused with covellite but the true color can be identified scratching its surface.
- Smelted bornite forms a magnetic small ball. With sodium carbonate and carbon forms copper grains.
- Can be easily attacked by nitric acid producing sulfur to atmosphere.
- There are three polymorphic structures (2) that depend of thermic changes.
Bornite has a complex cubic structure. The crystalline structure has two variations of copper ions in different positions and its formula can be 2Cu2S.CuFeS4. There are four covalent ions and the fifth is bivalent. Iron is also bivalent.
Bornite can be found in zones where exist copper secondary sulphides and many times near to chalcopyrite forming irregular veins and compact masses. In relation with other sulphides is less stable allowing a structural change to chalcocite and covellite. During its degradation in oxidation zones can be possible obtain oxygenated components such as malachite, azurite, cuprite and others.
In Figure 3, Bornite (Bo, blue) is incrusted on the surface of chalcopyrite (Cc, gray). Bornite areas have been substituted by chalcocite. The rounded areas of many borders suggest that Bornite was formed by exsolution. Bornite and chalcocite form a continuous solid solution that can be deposited like a simple solid with high temperatures.
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