1. CHAPTER 8 FLOTATION OF NON SULPHIDE MINERALS

2. Contents Flotation of oxides & silicates
Flotation by physical adsorption
Modulation of flotation
Effect of hydroxyl ions
Activation by fluoride
Depression by inorganic ions
Flotation by chemical adsorption
Activation of quartz
Flotation of semi-soluble minerals
Calcite
Phosphate
Depressants

3. Flotation of Insoluble Oxides and Silicates:
Very large number of minerals of this type
Whether or not they float depends on
Electrical properties of the mineral surface
Charge of the collector
Molecular weight of the collector
Solubility of the mineral
Stability of the collector/metal salt
 Depending on these properties the adsorption may be electrostatic (physical adsorption) or chemical (chemical adsorption)

4. Flotation by Physical Adsorption
This is common with oxides and silicates
Controlled by the pzc, collector charge and pH
Hematite, alumina and silica closely studied
The behaviour of goethite (see Lecture 2) and corundum (Al2O3), below, show this behaviour clearly. The pzc of corundum is 9.1.

5. Flotation of goethite with 1 x 10-4 M additions of collector

6. Flotation recovery of corundum (Al2O3) with 4x10-5M additions of collector

7. Flotation by Physical Adsorption
Quartz has a pzc of 1.8
Anionic collectors such as sulphonates or alkyl sulphates are not adsorbed in sufficient amount below the pzc to promote flotation
Anions used for pH adjustment are already present at concentrations greater than 10-2 mole/l
The collector must compete with these ions
Above pzc cationic collectors used to float quartz

8. Quartz (continued)
 Extensive hydrogen bonding of water molecules occurs on oxide and silicate surfaces (ie they are very hydrophilic)
As a result long chain collectors (more hydrophobic in character) are very much more effective than short chains
The same flotation result can be achieved with about 10-6 as much collector when changing from C4 to C18.

9. Flotation of quartz in amonium acetate solutions at neutral pH

10. Modulation of flotation
Effect of basicity – the amines are weak bases so that the species present in solution are sensitive to pH. At high pH (pH > 10)
RNH OH- → RNH2 + H2O
The amine species is no longer charged so the collector is not adsorbed and not effective

11. Modulation of flotation
Activation by fluoride – some silicates are activated by the presence of fluoride
Minerals from the silicate groups were floated as follows:
Orthosilicates – eg andalusite, beryl, tourmaline, Sensitive to fluoride
Pyroxenes – eg augite, diopsite, spodumene, Not affected
Amphiboles – eg hornblende, tremolite, actinolite, Not affected
Sheet silicates – eg muscovite, biotite, chlorite, Sensitive to fluoride
Framework silicates – eg quartz, feldspar, nepheline ,Less sensitive & Quartz not affected
The mechanism is by HF attack on surface to give AlSiF6- sites on surface for cationic collectors

12. Activation of Feldspar by Flouride
Al-OHsurf + 2HF → Al-F2- : H+ + H2O
Al-F2-:H+ + RNH+3 → Al-F2- : RH3N+ + H+

13. Contact angle of feldspar (microcline) & quartz showing the effect of fluoride

14. Modulation of flotation
Depression with inorganic ions
Inorganic cations compete with RNH3+ for surface sites
The effect of potassium nitrate on quartz flotation by dodecylamine at different pHs is shown below
As pH is reduced the surface charge is reduced and the amount of potassium ion needed for depression is also reduced.

15. Flotation of quartz showing depression by inorganic ions (1
Flotation of quartz showing depression by inorganic ions (1.5x10-4 M dodecylamine)

16. Flotation by Chemisorption
Chemisorption of high mlecular weight collectors on oxides and silicates seems to involve the hydrolysis of cations comprising the minerals
The hydroxycomplexes formed are very surface active and adsorb strongly on the solid surfaces

17. Flotation of pyrolusite (MnO2)
The pzc for this mineral is 7.4.
The strong flotation response at pH 8.5 when the surface is negatively charged cannot be explained by physical adsorption of an anionic collector
It is thought to be due to formation of a hydroxy complex of the manganous ion MnOH+ 
The flotation response at lower pH is probably due to physical adsorption of an oleate species.

18. Flotation of pyrolusite (MnO2) with 1x10-4M oleate

19. Activation of quartz by metal ions
A further example of hydroxy-complex formation
The solubility of quartz is very limited and the only cation from the mineral is silicon
Thus quartz activation for flotation by anionic collectors requires metal ions in the pH ranges in which they form hydroxy complexes
Quartz flotation shown below and compared with the pHs of hydroxy complex formation
The pH range for flotation is attributed to the range overwhich sufficient hydroxy complex is present in the system.

20. Minimum pH for flotation of quartz with 1x10-4M sulphonate and 1x10-4M metal ion

21. Semisoluble Salt Flotation
 These materials include carbonate, phosphate, sulphate tungstate and some halide minerals
They are characterised by ionic bonding and moderate solubility in water.
Calcite (CaCO3) - very common mineral in ores
Oleate, and also shorter chain carboxylates, are found to chemisorb on the mineral
There is a strong effect of chain length with flotation occuring at much lower concentrations of collectors when the chain length is increased.

22. Flotation of calcite at pH 9.7

23. Semisoluble Salt Flotation
Apaptite (calcium phosphate) is one of the major non-metallic minerals produced by flotation
The pzc is 6.4 yet as shown below the flotation response with oleate extends well below this pH indicating chemisorption.

24. Flotation of apatite with 1x10-4M oleate with/without silicate addition

25. Semisoluble Salt Flotation
Rock phosphate contains phosphate as both apaptite and collophane – they are chemically similar, but collophane occurs as porous nodules
Flotation of phosphate to reject quartz requires conditioning at high pulp density with fuel oil
The oil appears to fill the voids in the collophane and to bridge between adsorbed collector molecules.

26. Depressants for semisoluble salts
Sodium carbonate. Surface carbonation is an important reaction caused by CO32- in solution from CO2 in the air or carbonate in the ore
For example fluorite (CaF2) surface is readily given a carbonate coating
There is a practical implication since soda ash is sometimes used for pH control.

27. Depressants for semisoluble salts
Sodium silicate: Composition of aqueous sodium silicates represented by mNa2O.nSiO2 where ratio n/m is called the modulus & ranges from
Some polymerisation occurs to give medium molecular weight units that adsorb as a function of pH and can compete with anionic collectors
They are very effective with calcite over the pH range 7 –10 (see plot).
 With fluorite and apatite there is a minor effect, but with scheelite very little effect
The effect with phosphate is shown above for the flotation of phosphate.

28. Flotation of calcite with 5x10-4 oleate and 5x10-4 sodium silicate

29. Flotation of apatite with 1x10-4M oleate with/without silicate addition

30. Depressants for semisoluble salts
Starches, etc. These natural products are used widely in industry as depressants
Their adsorption properties are strongly effected by molecular weight
They give hydrophillic character to the surface
The problem is selectivity as they tend to adsorb on all surfaces.

31. Na-Feldspar concentrate
Feed (-3.35 mm)
Grinding
Desliming
Mica flotation
Orthoclase flotation
Quartz concentrate
Slime (-25µm)
( µm)
Tailings1 (micas)
Metal oxides flotation
Tailings 2
(metal oxides)
K-Feldspar concentrate
pH:2.8-3, 200 g/t Tallow amine acetate
pH:5.5-6, 2500 g/t Na-oleat
pH:2.8-3, 20 g/lt NaCl,600 g/t HF,
250 g/t Tallow amine acetate
Plagioclase flotation
floated
Na-Feldspar concentrate
pH:2.8-3, 200 g/t HF +
100 g/t Tallow amine acetate
Dewatering
Feldspar/Quartz flotation
Feldspar concentrate
pH:2.8-3, 600 g/t HF +

32. Summary Many minerals in oxide/silicate grouping
Surface charging and pzc important
Collector adsorbs physically or chemically
Carbon chain length important
Separations with physical adsorption based on pzc control through pH
Activation by hydrolysis of metal ions
Sparingly soluble minerals means metal ions in solution