ZINC ACTIVATION

Sphalerite is usually floated by short-chain xanthates after activation by copper ions [5]. Also sphalerite can be floated in high pH such as happen in some circumstances on the aerators [6]. On contact with iron and pyrite, a significant uptake of xanthate is taken by sphalerite. This can have been due to the activation of ZnS by Fe²⁺ and Fe³⁺ formed as a result of the oxidation of iron or pyrite, or to the precipitation of iron xanthates on the ZnS sulphide surfaces [7]. This reaction may occur when pyrite starts to diminish, and consequently the iron in the ore. In this way, only exist the iron from the grinding media like the main ferric agent. Thus, the sphalerite is able to take ferric and ferrous compounds on its surface which originate its floatability. At the moment that pyrite starts to diminish, the pH of pulp can increase its value making easy the zinc activation. According to M. Xu et. al. [8], the flotation mechanism has three steps, adsorption of ferrous species, oxidation to ferric species, and interaction with xanthate. The ferric ion catalyzes xanthate oxidation and the resultant dixanthogen is adsorbed on sphalerite surface.

In contact with iron, the quantity of sulfur formed on sphalerite can be increased significantly when the oxidation is carrying out in the presence of xanthate. The formation of an increased amount of sulfur would do sphalerite more floatable and diminish flotation selectivity.

When the supply of oxygen is limited, the demand for oxygen by metallic iron competes with that by the valuable sulphide. If excessive amounts of pyrite are present, the conditions become so reducing that there would be little adsorption of xanthate on the valuable sulphides to make them hydrophobic and floatable [9].

In a test done with an ore from Polaris (Pb 3.2%, Zn 13.8%, Fe 6.9%, Cu 0.25, [7]), it was observed that without add copper sulphate, at the moment that the pH of pulp started to increase its value, grade and zinc recovery increased its value too (Fig. 2).

In the same work is mentioned microflotation tests for examining the role of Fe⁺². In the table is shown the results obtained. They indicate that the combination of Fe⁺² and alkaline pH produce sphalerite flotation.

Ions added	pH	Recovery(%)
None	7.2	33
None	10.0	27
Fe +2, 1 ppm	7.2	35
Fe +2, 1 ppm	10.0	63

Results over the range 6.6 to 12 in the presence of 1 ppm Fe ⁺² is shown in the figure 3. The trend is similar to the above table. The proportion of Fe ⁺² and Fe(OH)₂ is very important. Thus, iron can cause high changes in electrochemical conditions and complicate the stability of a flotation system [10].

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