The Mechanism Associated with the Flotation of Some Important Ores

Sulfides - recovery of galena - effect of collectors

Most data are associated with the flotation of the lead ore galena PbS, which has been studied extensively. Although the tonnage of lead ore processed is only a tenth of the amount of the copper ores processed (Table 1), the fact that it exists in only one valence state simplifies data interpretation. The overall mechanism for copper sulfide ore processing is accepted as being similar to that of lead despite the copper valence change under oxidizing conditions. Both copper and lead are floated with sulfhydryl collectors and it is of interest to review the concepts associated with the collector action. It is often the case that, despite the high selectivity of the sulfhydryl collectors for sulfides in the presence of oxide and silicate gangues, that further selectivity between different sulfide minerals is required. It is consequently not necessarily sufficient to determine optimum recovery for a specific collector, but it is also necessary to investigate its effect on the simultaneous recovery of an undesired sulfide mineral.

The effect of oxygen on the ^-potential and KEtX adsorption on a galena surface In the absence of oxygen the adsorption density of ethyl xanthate is independent of pH and remains constant at a level of 2 x 10 ~6 g moles/g of galena, which corresponds to effectively a coverage of a monolayer assuming one xanthate ion adsorbed at one lead site on the surface. The area occupied by an anion of amyl xanthate disposed perpendicularly to the surface of a mineral is given by Klassen and Mokrousov to be 28 A2. This adsorption area should be independent of the length of the hydrocarbon chains of normal xanthates.

However, the adsorption density does fall as the length of the alkyl chain increases to about half the monolayer adsorption density with octyl xanthate. Additionally, in the absence of both oxygen and xan-thate, the (-potential vs pH curve for galena, falls from + 20 mV at pH 2 to — 40 mV at pH 12. The pH at which the (-potential is zero is called the point of zero charge (PZC) which is a useful characterizing parameter.

In the presence of 1 x 10"3 molar ethyl xanthate the C-potential remains steady at — 40 mV over the pH range of 4-12.

This seems to indicate that the attachment is due to the chemisorption of the alkyl part of the xanthate molecule on the galena surface, presumably after displacing H3O+ or OH" ions. The unchanged outer negatively charged compact surface layer is associated with the polar end of the molecule.

In the presence of oxygen, but without xanthate, the C-potential vs pH differs significantly from the previous case. Over the pH range from 4 to 6 the C-potential vs pH curve is convex upwards with a maximum of just under + 20 mV at pH 6. At increasing pHs the C-potential falls to 0 at pH 6.9, the PZC; from 6.9 to 12 it falls from 0 to - 40 mV. It should be noted that for the previous case of no oxygen and no xanthate the pH at the PZC is 2.6.

In the presence of 1x 10" 4 mol L" ethyl xanthate and oxygen, the C-potential-pH curve is again electronegative over the range 4-12, as with the case for xanthate without oxygen, but is no longer constant, falling from — 20mVatpH4to — 40mVatpH12.

There are conflicting explanations for the effect of oxygen on the galena surface. One plausible theory is that, in the presence of oxygen, the surface is oxidized to thiosulfate and sulfate. As air also contains CO2, lead carbonates may also form displacing the sulfates. Some of these surface compounds may move into the inner compact layer in the solution. In the presence of xanthates, a significant xanthate fraction may react directly at the surface, possibly with the sulfates, forming a strong bond with the lead mineral. The rest of the xanthate will displace the sulfate from the inner to the outer compact layer, causing the potential at the outer compact layer ^ to remain electronegative.

It is further postulated that with excess xanthate, metathetic replacement of the sulfate will occur producing uncharged hydrophobic, insoluble lead ethyl xanthate. This process is clearly kinetic requiring diffusion of the unreacted sulfate at the surface through the compact layer. Multiple layers of the insoluble lead ethyl xanthate will confer increased flotability on the galena. Finkelstein, Allison, Lovell and Stewart reported improved flotation recoveries for galena using ethyl xanthate in terms of the thickness of the lead ethyl xanthate layers expressed as monolayers. The recovery increased from zero with no ethyl xanthate to 70% for a single monolayer increasing to 95% with five monolayers.

Selective flotation of galena - effect of depressants

Reagents which, to a greater or lesser degree depress recoveries of sulfides (depressants), include OH", S2" and chromate.

Fuerstenau in 1982 reported 100% recovery of galena, using 1 x 10"5 mol L"1 ethyl xanthate in the presence of oxygen, over the pH range 2-10. However, if the lead content (grade) of the concentrate is required to be low in the presence of other sulfides, which are themselves being floated by the 1 x 10"5 molL"1 ethyl xanthate, this requires a reduction (depression) of the galena recovery simultaneously with a smaller reduction in the recovery of the desired sulfide.

Depression of galena using hydroxyl is effected at pH values greater than 11, due to the formation of HPbO2, which is the unhydrated form of Pb(OH)3", in the Stern compact layer, this causes the galena surface to become hydrophilic with consequent reduction in lead recovery.

The depression of galena using sodium sulfide is much greater than that of all other sulfide minerals. Additions of sodium sulfide to a galena suspension in xanthates will cause an insoluble hydrophilic lead sulfide rather than lead xanthate to form at the solid surface according to the reaction:

In the presence of chromate the adsorption of xan-thates does not decrease, but when lead chromate forms on the surface, the hydration of the chromate is so strong that the collector hydrophobicity is significantly reduced.

Grade-recovery of copper ores In view of the economic importance of chalcocite (Cu2S) and chal-copyrite (CuFeS2) their selective recovery from ores containing undesired pyrites will be briefly reviewed.

Depression of pyrite - Effect of pH. Flotation of pyrite will be possible with short-chain xanthates such as potassium ethyl xanthate at pHs below 11. At addition rates of 2 x 10"4 mol L"1 of KEtX, 100% recovery is possible. The recovery is thought to be due to the adsorption of dixanthogen on the surface. However, at high pH values xanthate oxidation to dixanthogen does not occur according to the reaction shown below, thus depressing the pyrite recovery:

Effect of CN". Pyrite is significantly depressed by KCN. In the absence of KCN pyrite recoveries of 100% are obtained at pH below 7, using 1x10"4molL"1 KEtX. Significant amounts of pyrite are still present in the concentrate even up to pH 10. However, with a KCN addition of 6.0 x 10~3 mol L-1, recovery is significantly depressed at pH 4 and completely depressed above pH 7.

Effect of pyrite depressants on copper recovery To assess the selective separation of copper ores from pyrite it is necessary to see how the copper floats under conditions identical to those reported for pyrite depression.

Chalcocite will float completely at up to pH 10 using 5.0 x 10-5 ethyl xanthate. The mechanism of flotation in the presence of amyl xanthate is similar to that of galena, with a chemisorbed cuprous amyl xanthate after which further xanthate additions form multilayers of cuprous xanthate in the compact layer, increasing the overall hydrophobicity.

At pHs between 8 and 10, the addition of sodium sulfide will depress pyrite without significantly altering the chalcocite recovery. The most effective depressant for this separation does however appear to be potassium cyanide which hardly affects the chalco-cite recovery between pH 8 and 12 while virtually completely suppressing the pyrite over this range.

Chalcopyrite will float at 100% recovery over the pH range 2-12 both with 1x10"5molL_1 ethyl xanthate and 1.3 x 10~5 mol L_1 dixanthogen. No depression in recovery is observed up to pH 13. From this it may be inferred that at these high pHs it is the stability of the cuprous ethyl xanthate which effects flotation.

As chalcopyrite contains iron it is to be expected that it will to some extent be affected by the pyrite depressants but will be less sensitive to NaCN additions than the pyrite. For example, in a system where 60.9% pyrite recovery was achieved simultaneously with 90.5% chalcopyrite recovery with no depressant at pH 8.8, the addition of 0.04kgton~1 NaCN changed these figures to 26.1% pyrite recovery and 84.2% chalcopyrite recovery. Further additions of cyanide reduced the pyrite recovery to 15.6%, but also cut the chalcopyrite recovery to 31.0%.

Finally, it should be noted that sodium sulfide will depress chalcopyrite more than pyrite. This is due to the preferential formation of cupric sulfide over that of cuprous ethyl xanthate.

Flotation of oxides and silicates Oxides and silicates are the most abundant minerals in the earth's crust. Their flotation behaviour is important both because of their inherent value and because they act as gangue in the flotation of other minerals. Once again only a sample of the separations of interest can be discussed. Consider firstly the separation of quartz from haematite in iron ore processing.

Fuerstenau and Healy have suggested six possible procedures, four of which have been successfully employed in industry. These are based on the (-potential for haematite being positive in the pH range 2-4 with a PZC in the region of pH 6, while for quartz over the same range the (-potential is negative with a PZC at pH 2. These procedures are:

• Flotation of haematite using an anionic sulfonate collector, for example sodium dodecyl benzene sul-fonate which will adsorb on the positively charged haematite surface but not on the negatively charged quartz.

• Flotation of the haematite by the chemisorption of a fatty acid collector at pH 6-8. Chemisorption on the quartz will not occur because of its large negative (-potential at this pH.

• Reverse flotation of quartz at pH 6-7 using a cationic collector, for example, amylamine which will adsorb on the highly negatively charged quartz but not on the essentially uncharged haematite.

• Reverse flotation of quartz activated with calcium ions, with a long-chain fatty acid collector and the haematic depressed through the addition of starch. The hydrophilic starch molecules will chemisorb on the haematite through their carboxyl groups. Without the starch the haematite would also float under these conditions.

The successful implementation of these separations is clearly consistent with the electrostatic theory of collector adsorption.

Further thoughts on the electrostatic model of flotation. For collectors to function through physical interactions they must be present as counterions in the compact layer. Under these conditions the net charge on the outer boundary of the diffuse layer is reduced and hydrophobic interactions with the nonpolar surfactants on the bubble surface can occur. Iwasaki et al. reported very interesting results relating to the flotation of goethite (FeOOH). Goethite has a PZC at pH 6.7 and they measured flotation recoveries, in a laboratory Hallimond tube, as a function of pH using 1.0 x 10~3 mol L-1 of the anionic collectors sodium dodecyl sulfate and sodium dodecyl sulfonate and the cationic collector dodecyl ammonium chloride. They achieved effectively 100% recoveries up to pH 6.0 using the anionic collectors with the recovery falling effectively to zero above pH 7.0.

The recovery of goethite vs pH using the 1.0 x 10~3 mol L_1 cationic dodecylammonium chloride was the inverse of this with virtually no recovery up to pH 6.0 and 100% recovery between pH 8 and 12.2 when the recovery fell to zero because the quaternary ammonium salt hydrolysed to the parent amine.

The increased flotation response at lower pHs with the anionic collectors is associated with an increased charge density oS in the compact layer which causes larger amounts of collector to be present as counter-ions. The increased concentration of their non-polar groups in the compact layer will increase the particle's hydrophobicity.

Modi and Fuerstenau reported tests on corundum (Al2O3), which has a PZC at pH 9, at four different pH values using an anionic collector, sodium dodecyl sulfate, at concentrations ranging between 10"7 and 10"1 mol L"1.

At pH 4.0, 100% recovery was achieved with 0.5 x 10"4 mol L"1, but it required 10 times that concentration for 100% recovery at pH 6.0, while at pH values of 9.3 and 11.0 the highest recovery achieved was only 30% at a reagent concentration of 10"3 mol L"1.

Effect of the length of the hydrocarbon chain on the collector performance. Data have been reported for the flotation of quartz (PZC at pH 2.0) at pH 6-7, using the cationic alkylammonium acetates, with the alkyl chain length varying between C4 and C18. Incipient flotation occurs from about 10"8molL"1 for C18 to 10 "1 for C4. Similarly high rates are observed at 10"6 mol L"1 for C18 to 0.5 mol L"1 for the C4. The onset of the high flotation rates are equated to the onset of hemimicelle formation.

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