Anionic Agents

The principal anion-active resurfacing agents include sulfides, cyanides, carbonates, and phosphates as well as oxidizing and reducing agents. Ins this rationales, air might be considered the truly universal flotation agent. Its use is not only convenient but often essential for the flotation of certain minerals. The control of surface oxidation with heat, air, oxidants, or reducing agents can often cause considerable variation in the selectivity of the flotation separation.

Oxidizing agents other than air are not generally used but a number of interesting applications do exist. Sodium hypochlorite (NaOCI) and potassium permanganate (KMnO₄) have been found to be effective as depressants for copper, iron, and cobalt sulfides in lime alkaline pulps. For example, the separation of copper and molybdenum sulfides may be carried out by depressing the copper sulfide with sodium hypochlorite.

Reducing agents often are of greater importance than oxidizing agents in ore flotation, because the prevention of excess oxidation is so frequently a problem. In lead-zinc flotation, sodium sulfite (Na₂SO₃) or sodium bisulfite (NaHSO₃) is frequently used for this purpose. These compounds are especially effective on ores which also contain some copper minerals. Without a reducing grinding, thus becoming more soluble. The resultant copper or lead ions may then activate the sphalerite (ZnS), causing it to float into the lead concentrate. However, a reducing agent such as Na2SO3 or even metal powders such as magnesium or zinc, can prevent oxidation and therefore act as depressants for the sphalerite.

Sodium sulfide is used to resurface cerussite (PbCO3), cuprite (Cu₂O), azurite (2CuCO₃. Cu(OH)₂), or chrysocolla (CuSiO₃ . 2H₂O)with a sulfide coating to make them floatable with xanthate collectors. Sulfide ions also may be employed in small amounts to reactivate pyrite, pyrrhotite, and other sulfides which have been depressed with lime or cyanide. However, when sulfide s are used in excess, they will depress all sulfide minerals.

Cyanides are added usually as sodium cyanide (NaCN) or calcium cyanide (Ca(CN)2) and are among the most commonly used reagents for the depression of iron, copper, or zinc sulfide minerals. The principal disadvantage of cyanides is their ability to dissolve gold. Excess gold losses are therefore likely to result when gold-bearing ores are processed.

Sodium solicates (-SiO3) are frequently used as dispersants for acidic clay materials and the corresponding carbonates (- CO3) are used as dispersants for basic slimes. Such dispersion prevents both the formation of interfering slime coatings on minerals being recovered, and the mechanical entrainment of flocculated slime in the froth.