The Effect of Water Chemistry on Adsorption of Reagents on Minerals

Chemical equilibria in aqueous solutions containing both the minerals and the surfactants can be expected to be much more complex than in either of the individual systems discussed above. In addition to surfactant adsorption at the solid-liquid interface, interactions between dissolved mineral species with various surfactant species can be expected. All these interactions can affect the surfactant adsorption and the subsequent flotation.

As indicated earlier, oleic acid has a very low solubility and adsorption of oleate, in some cases, is in fact precipitation of the surfactant in the interfacial region. In Figure 3, the activity of Ca2+ in equilibrium with various solid phases is plotted. If, at any stage, activity of Ca2# in solution is greater than that in equilibrium with Ca2#-oleate, Ca2#-oleate can be expected to precipitate.

Depletion isotherms of oleic acid on both francolite and dolomite has been observed to be a two-region linear isotherm with a change of slope at about 10"4kmolm"3 (Figure 7). Simultaneous analysis of the dissolved mineral species in the supernatants of the samples used in the adsorption experiments (Figure 8) shows a sharp decrease in the concentrations of both Mg and Ca species when oleate concentration exceeds 1.0 x 10"5 kmol m"3 in the case of francolite and 3.0 x 10"5 kmol m"3 in the case of dolomite. This suggests that bulk precipitation of calcium and magnesium species can occur under such conditions.

Major chemical equilibria for the precipitation of Ca and Mg species by oleate can be given as follows:

Ca2+ # 2Ol" = Ca(Ol)2 Koa(Ol)2 = 3.81 x 10"13

Mg2 ++ 2 Ol" = Mg(Ol)2 KMg(Ol)2 = 1.58 x 10"11

The onset of the precipitation of Ca(Ol)2 and Mg(Ol)2 is calculated from the solubility products given above and marked in Figure 8. The calculated oleate concentrations at the onset of precipitation are in good agreement with experimental observations.

It is postulated that, in the case of oleate adsorption on dolomite and francolite, different mechanisms govern the adsorption process. In the low concentration range ( < 10"4 kmol m"3), the adsorption

10~5 10"4 10~3 10"2 Initial oleate concentration (kmol rrf3)

Figure 7 Depletion isotherms of 14C-labelled oleic acid on francolite (squares: pH = 8.2) and dolomite (circles: pH = 9.2). Temperature, 25°C; S/L = 0.3; I = 3 x 10~2 kmol m~3 KNO3. (From Somasundaran P, Xiao L and Wang D (1991) Solution chemistry of flotation of sparingly soluble minerals. Mineral and Metallurgical Processing 8: 115-121.)

(A) Initial K oleate concentration (kmol m 3) Q (B) Initial K oleate concentration (kmol m 3)

Figure 8 Dissolved Ca (squares) and Mg (circles) levels from (A) francolite (pH = 8.2) and (B) dolomite (pH = 9.2) suspensions as a function of oleate concentration. (From Somasundaran P, Xiao L and Wang D (1991) Solution chemistry of flotation of sparingly soluble minerals. Mineral andMetallurgical Processing 8: 115.)

(A) Initial K oleate concentration (kmol m 3) Q (B) Initial K oleate concentration (kmol m 3)

Figure 8 Dissolved Ca (squares) and Mg (circles) levels from (A) francolite (pH = 8.2) and (B) dolomite (pH = 9.2) suspensions as a function of oleate concentration. (From Somasundaran P, Xiao L and Wang D (1991) Solution chemistry of flotation of sparingly soluble minerals. Mineral andMetallurgical Processing 8: 115.)

of oleate on both minerals occurs mainly due to chemical bonding on surfaces without any precipitation. At an intermediate concentration of about 10~4kmolm~3, the solubility limit of Ca and Mg oleate can be reached in the interfacial region but not in the bulk solution, suggesting surface precipitation of oleate on both minerals. In the high concentration range (>5 x 10"4 kmol m~3), oleate depletion may be dominated in the case of both minerals by the precipitation of Ca and Mg species with oleate, on the mineral surface and in the bulk solution.

From the above discussion on apatite-calcite conversion, it is clear that a Sotation separation scheme designed on the basis of the surface properties of a single mineral is not likely to perform satisfactorily. The effect of dissolved species of calcite and apatite on fatty acid Sotation of both minerals has in fact been studied using mineral supernatant solutions containing various dissolved species. The Sotation results are shown in Figure 9. Both supernatants of calcite and apatite are found to depress the calcite Sotation by oleic acid in the tested pH range, with apatite supernatant exhibiting a greater depressing effect. Similar results have also been obtained for apatite Sotation. The supernatants of calcite and apatite depress the apatite Sotation under all tested pH conditions.

Studies on the dissolved species responsible for the observed effect revealed that, for calcite Sota-tion, the depression role of apatite supernatant results from the combined effects of calcium species and the phosphate species in solution, while the depression role of calcite supernatant is mostly that of the calcium ion and possibly some carbonate ions. The depression due to calcium ion is caused by the de pletion of oleate owing to the precipitation of calcium oleate. In the case of apatite Sotation, the depression is due to phosphate and carbonate species in solution. The adsorption of these ions on the surface calcium sites reduces the sites available for oleate adsorption which, in turn, lowers the hydrophobicity of the surface and so depresses the apatite Sotation. Calcium oleate precipitation, in this case, does not occur to a significant extent due to the low concentration of oleic acid used in Sotation. The above observations clearly show that water chemistry plays a crucial role in the Sotation of apatite-calcite systems.

In addition to reagent complexation and precipitation, other reactions that occur in the bulk solution can take place in the interfacial region. For example, hemimicellization at a solid-liquid interface is a phenomenon that drastically affects the adsorption of collector reagents on solids.

Flotation is a dynamic process. In addition to the equilibrium effects associated with the water chemistry, it can also inSuence the adsorption kinetics of surfactants on the solid surfaces. Anionic conditioning is a unit operation that precedes rougher Sotation and skin Sotation of phosphates in Florida Sotation plants. The effect of water chemistry on oleic acid adsorption on francolite during anionic conditioning has recently been studied in detail. In order to identify the effect of process variables on the adsorption, the experiment was carried out under both laboratory and plant conditions (Table 2).

The kinetics of oleic acid adsorption on francolite under both laboratory and plant conditions, using distilled water and plant water, is shown in Figure 10. The adsorption density and kinetics are quite different depending on the conditions and the

Figure 9 (A) Effect of apatite supernatant (squares) on calcite flotation. K oleate 10~4kmolm~3; I = 3x 10~2 kmol mT3 KNO3. Circles, water. (B) Effect of calcite supernatant (squares) on calcite flotation. K oleate 10~4 kmol mT3; I = 3 x 10~2 kmol mT3 KNO3. Circles, water. (C) Effect of calcite supernatant (squares) on apatite flotation. K oleate = 2 x 10~6 kmol mT3; I = 3 x 10~2 kmol mT3 KNO3. Circles, water. (D) Effect of apatite supernatant (squares) on apatite flotation. K oleate = 2 x 10~6 kmol m~3; I = 3x 10~2kmolm~3 KNO3. Circles, water. (From Ananthapadmanabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral and Metallurgical Processing 1: 36.)

Figure 9 (A) Effect of apatite supernatant (squares) on calcite flotation. K oleate 10~4kmolm~3; I = 3x 10~2 kmol mT3 KNO3. Circles, water. (B) Effect of calcite supernatant (squares) on calcite flotation. K oleate 10~4 kmol mT3; I = 3 x 10~2 kmol mT3 KNO3. Circles, water. (C) Effect of calcite supernatant (squares) on apatite flotation. K oleate = 2 x 10~6 kmol mT3; I = 3 x 10~2 kmol mT3 KNO3. Circles, water. (D) Effect of apatite supernatant (squares) on apatite flotation. K oleate = 2 x 10~6 kmol m~3; I = 3x 10~2kmolm~3 KNO3. Circles, water. (From Ananthapadmanabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral and Metallurgical Processing 1: 36.)

water. Under laboratory conditions, the adsorption in plant water is significantly lower than that in the distilled water. It is proposed that this is due to reagent loss resulting from the dissolved species in plant water precipitating the oleic acid. In contrast, under plant conditions, the adsorption behaviour of oleic acid in plant water and distilled water is similar and adsorption densities are lower than those under laboratory conditions. The high solid/liquid ratio under plant conditions will reduce the adsorption density on the solids because of the much greater solid surface on to which the reduced total amount of

Table 2 Comparison of laboratory and plant conditions

Laboratory conditions

Plant conditions

Conditioner

Wrist-action shaker

Lightnin Labmaster L1U08, four-bladed

cruciform propeller operating at 350 rpm

pH

9.1-9.5

9.1-9.5

Water

Distilled and plant water

Plant water

Solid (%)

10 (2 g sample)

72 (1000 g sample)

Time (min)

120 (except for kinetics)

3 (except for kinetics)

Figure 10 Kinetics of oleic acid adsorption on francolite in distilled water and plant water under laboratory and plant conditions. Open squares, distilled water in laboratory conditions; filled squares, plant water in laboratory conditions; open circles, distilled water in plant conditions; filled circles, plant water in plant conditions. Oleic acid concentration: 8.1 x 10~3 mol L~1; pH 9.1-9.5. (From Maltesh C, Somasundaran P and GruberGA (1996) Fundamentals of oleic acid adsorption on phosphate flotation feed during anionic conditioning. Mineral and Metallurgical Processing 13: 157.)

Figure 10 Kinetics of oleic acid adsorption on francolite in distilled water and plant water under laboratory and plant conditions. Open squares, distilled water in laboratory conditions; filled squares, plant water in laboratory conditions; open circles, distilled water in plant conditions; filled circles, plant water in plant conditions. Oleic acid concentration: 8.1 x 10~3 mol L~1; pH 9.1-9.5. (From Maltesh C, Somasundaran P and GruberGA (1996) Fundamentals of oleic acid adsorption on phosphate flotation feed during anionic conditioning. Mineral and Metallurgical Processing 13: 157.)

reagent in the water adsorbs. This will also result in a lower reagent concentration in solution reducing the precipitation eject. The intense agitation in the plant conditioner may also remove some of the bound reagent from the surface.

The adsorption isotherms of oleic acid on fran-colite under laboratory and plant conditions are compared in Figure 11. Adsorption is markedly higher under laboratory conditions than under plant conditions. On the other hand, under plant conditions the adsorption is similar in distilled water and plant water. This suggests that the effect of dissolved species is reduced under plant conditions.

From the above discussion, it can be seen that the adsorption of surfactant on a mineral is a complicated process involving interactions such as surfactant self-association, mineral dissolution, bulk precipitation, adsorption and surface precipitation. The interactions are further complicated by the kinetic effects of the various reactions.

Understanding the effect of the water chemistry on reagent adsorption offers opportunities to manipulate such processes by optimizing the contributing factors such as alteration of the surface properties, complexation of ions which cause precipitation of the surfactant, prevention or enhancement of collector adsorption and changes in the adsorption kinetics to achieve the desired selectivity in flotation.

In the anionic flotation of phosphate, Ca2 + affects the grade of phosphate by activating the quartz through formation of calcium-bearing precipitates at high pH. This detrimental effect can be prevented by adding sodium silicate, which can interact with Ca2+ and form calcium silicate. Since calcium silicate and quartz are negatively charged, detachment of calcium silicate from quartz can occur and thus quartz flotation can be depressed.

It has been found that in carbonate/phosphate systems, with fatty acid as collector, apatite is depressed in the acid medium (pH 5.5-6.0) while carbonate is floated. The depression of phosphate at this pH is possibly due to the adsorption (or formation) of aqueous CaHPO4 on its surface, preventing surfactant ions from approaching the surface of the phosphate particles. Free Ca2+ in solution can affect the formation of aqueous CaHPO4. From thermody-namic considerations it can be predicted that the selective flotation of carbonates from phosphates in acid media can be enhanced by minimizing free Ca2 + in solution and by increasing HPO4~ in the system. This can be done by (1) decreasing free Ca2 + concentration in the system to low values by adding suitable chemical reagents such as sulfuric acid or chelating agents such as oxalic acid, and (2) adding soluble phosphate salts to enhance the depression of the phosphate minerals. Results from experiments with

Figure 11 Adsorption isotherms of oleic acid adsorption on francolite in distilled water and plant water under laboratory and plant conditions. Squares, distilled water in laboratory conditions; open circles, distilled water in plant conditions; filled circles, plant water in plant conditions. (From Maltesh C, Somasundaran P and Gruber GA (1996) Fundamentals of oleic acid adsorption on phosphate flotation feed during anionic conditioning. Mineral andMetallurgical Processing 13: 157.)

Figure 11 Adsorption isotherms of oleic acid adsorption on francolite in distilled water and plant water under laboratory and plant conditions. Squares, distilled water in laboratory conditions; open circles, distilled water in plant conditions; filled circles, plant water in plant conditions. (From Maltesh C, Somasundaran P and Gruber GA (1996) Fundamentals of oleic acid adsorption on phosphate flotation feed during anionic conditioning. Mineral andMetallurgical Processing 13: 157.)

natural phosphate ores are in agreement with the theoretical predictions.

Based on the oleic acid solution chemistry, a two-stage conditioning process for the flotation of dolomite from apatite has been proposed. The mixed minerals are first conditioned at pH 10 with oleic acid collector. The system is then reconditioned below pH 4.5 where dolomite is floated. The selectivity of dolomite from apatite is attributed to two factors in this process.

1. High adsorption of oleate on dolomite during the first stage at pH 10, which is maintained after reconditioning at lower pH.

2. Oleate to oleic acid transformation upon reconditioning, reducing its efficiency, and this reduction being more severe for apatite than for dolomite.

In the high pH range, oleate adsorbs on to apatite and calcite through specific interactions, while at low pH, when oleic acid is the major species, the adsorption is through weaker physical interaction. Thus, oleic acid is a poor collector compared to oleate.

Modification of collector adsorption on minerals can be used to control their flotation response. In one study, Alizarin Red S, a dye that stains calcite, was tested as a modifying agent in calcite-apatite system due to its preferential adsorption on these minerals. Even though Alizarin Red S adsorbs more on apatite than on calcite, it depresses the flotation of apatite using oleate as collector more than that of calcite (Figure 12). In the absence of the dye, both calcite and apatite float with oleate at pH 10.5. When the dye concentration increases to 5 x 10"6 mol m"3, the flotation of calcite is very little affected with a recovery of about 90%, while apatite flotation is depressed to 5-10%. Calcite flotation is only affected at higher concentrations of dye. Alizarin Red S or its derivatives are hence promising reagents for the beneficiation of phosphate with carbonaceous gangues.

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