The Effect of Water Chemistry on Mineral Solution Equilibrium

When mineral particles are in contact with water, they undergo dissolution, the extent of which is dependent on the type and concentration of chemicals in solution. The dissolved mineral species can undergo further reactions such as hydrolysis, complexa-tion, adsorption and precipitation. The complex equilibria involving all such reactions can be expected to determine the interfacial properties of the minerals and their flotation behaviour. The equilibria that con trol the dissolution of calcite and apatite in water are given in Table 1.

In the case of carbonaceous phosphate minerals, apatite, calcite and dolomite will dissolve in water, followed by pH-dependent hydrolysis and complexa-tion of the dissolved species. Since these minerals are sparing soluble, the dissolved species have a marked effect on their interfacial properties.

It should be noted that, from theoretical considerations, depending on the solution conditions, the surface of apatite can be converted to calcite and vice versa through surface reactions or bulk precipitation of the more stable phase. The stoichiometry of the equilibrium governing the conversion of apatite to calcite can be written as:

Caw(PO4MOH)2(S) + 10CO2" = 10CaCO3(S) + 6PO2" + 2OH"

It can be seen from this equation that, depending on the pH of the solution, apatite can be converted to calcite if the total carbonate in solution exceeds a certain value. In fact, the amount of dissolved carbonate from atmospheric CO2 does exceed that required to convert apatite to calcite under high pH conditions.

Surface conversion due to the reaction of the dissolved species with the mineral surface can be predicted using stability diagrams for heterogeneous mineral systems. This is illustrated in Figure 3 for the calcite-apatite system. The activity of Ca2 + in equilibrium with various solid phases shows that the point of interception for calcite and apatite is pH 9.3. Above this pH, apatite is less stable than calcite and hence conversion of apatite to that of calcite can be expected in the calcite-apatite system. Similarly, apatite is more stable than calcite below pH 9.3. It is to be noted that Ca2# in equilibrium with calcite in an open system (open to atmospheric CO2) is significantly different from that in a closed system. Also,

Table 1 Equilibria controlling the dissolution of calcite and apatite in water

Calcite CaCO3 (S) CO3" + H + HCO3- + H + CO2(g) + H2O

Apatite

10-84 10103 106.3

10-15

Ca2##HCO33 Ca2##CO233

100.8 103.3

10-129

Apatite

10-84 10103 106.3

10-15

Ca2##HCO33 Ca2##CO233

100.8 103.3

10-129

Ca,0(PO4)6(F,OH)2(S) 8

± 10 Ca2+ +6PO4-

+2 (F, OH)3

10"118

po4' + H +

8 HPO4-

10123

Ca2+ + HPO4-

8 CaHPO4(aq)

1027

HPO2' + H +

8 H2PO3

1072

CaHPO4(aq)

8 CaHPO4(s)

1043

HPO' + H +

8 H3PO4

1022

Ca2 + + H2PO'

8 CaH2PO++

101.1

Ca2+ + H2O

8CaOH ++ H +

10'129

Ca2+ + 2F"

8 CaF2(s)

10104

Ca2+ +2H2O

8 Ca(OH)2 + 2H +

10 "228

Ca2+ + F"

8 CaF +

101.0

F3 +H+

8 HF

1031

Figure 3 pH dependence of activity of Ca2 # in equilibrium with calcium oleate (dotted line: OlT = 10~4 kmol mT3), calcite (open (closed lines) and closed (dots and dashes) systems) and apatite (dashed lines). (From Ananthapadmanabhan KP and Somasun-daran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral andMetallurgical Processing 1: 36.)

in the absence of atmospheric CO2, apatite has a wider stability region than in the open system. Atmospheric CO2 can thus be expected to play an important role in these types of mineral-solution equilibria and in operations dependent on interfacial properties.

60 r

Figure 4 Illustration of the effect of supernatants on the zeta potential and isoelectric point of calcite and apatite:

2 x 10~3 kmol mT3 KNO3. Open circles, calcite in water; open triangles, apatite in water; filled triangles, apatite in calcite super natant; filled circles, calcite in apatite supernatant. (From Anan-thapadmanabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineraland

Metallurgical Processing 1: 36.)

The surface conversions in the calcite-apatite system have been proved experimentally; electrokinetic data obtained for the calcite-apatite system in water and in the supernatant of each other are shown in Figure 4. When apatite is in contact with calcite supernatant, its zeta potential is seen to shift to that of calcite and vice versa, suggesting surface conversion of apatite to cal-cite and calcite to apatite, respectively.

The zeta potential data obtained in mixed super-natants of calcite and apatite also show the effect of dissolved mineral species. If supernatants of calcite and apatite are combined as a 1 : 1 mixture, the two minerals have almost identical surface charge characteristics in the basic pH range (Figure 5).

The surface conversion of apatite and calcite is further supported by the result of electron spectroscopy for chemical analysis (ESCA) measurements. The results in Figure 6 show that, when apatite is conditioned in the supernatant of calcite at pH & 12, its surface exhibits spectroscopic properties characteristic of both calcite and apatite. This behaviour is attributed to the precipitation of calcite on the apatite.

Dissolution equilibria of sparingly soluble minerals play a major role in determining the surface properties of these minerals and in turn, adsorption of reagents on them.

Figure 5 Illustration of the similarity in zeta potentials of calcite (circles) and apatite (triangles) in mixed supernatants. (From Ananthapadmanabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral and Metallurgical Processing 1: 36.)

295 290 285 280 275

Figure 6 ESCA spectra of C(1s) peak of apatite conditioned in calcite supernatant at pH &12. (A) Apatite in water; (B) calcite in water; (C) apatite in calcite supernatant. (From Ananthapad-manabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral and Metallurgical Processing 1: 36.)

295 290 285 280 275

Figure 6 ESCA spectra of C(1s) peak of apatite conditioned in calcite supernatant at pH &12. (A) Apatite in water; (B) calcite in water; (C) apatite in calcite supernatant. (From Ananthapad-manabhan KP and Somasundaran P (1984) The role of dissolved mineral species in calcite-apatite flotation. Mineral and Metallurgical Processing 1: 36.)

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