The mechanism of fatty acid adsorption in the presence of fluorite, calcite and barite

International Journal of Mineral Processing, 14 (1985) 161-176

161

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE MECHANISM OF FATTY ACID ADSORPTION IN THE PRESENCE OF F L U O R I T E , CALCITE AND BARITE

K.I. MARINAKIS and H.L. SHERGOLD

Department of Mineral Resources Engineering, Imperial College of Science and Technology, London SW7 2BP, United Kingdom (Received February 13, 1984; revised and accepted July 10, 1984)

ABSTRACT Marinakis, K.I. and Shergold, H.L., 1985. The mechanism of fatty acid adsorption in the presence of fluorite, calcite and barite. Int. J. Miner. Process., 14: 161--176. The interaction of oleic acid with fluorite, calcite and barite has been studied using solubility, oleate abstraction, electrophoretic mobility and Hallimond-tube flotation measurements. Abstraction of oleate from aqueous solution corresponds to the precipitation of the metal oleate. Multilayers of metal oleate inhibits the dissolution of the minerals and prevents true equilibrium from being obtained. Flotation is not only dependent on the amount of oleate abstracted but also on the strength of adhesion of the precipitated metal oleate to the minerals. Selectivity between the flotation of calcite, fluorite and barite is unlikely to be obtained by varying the pH because similar responses are observed.

INTRODUCTION

Salt-type minerals are characterized by solubilities which are higher than most oxides and silicates but lower than the simple salt minerals like halite and sylvite. Minerals in this category include fluorite (CaF2), barite (BaSO4), calcite (CaCO3), scheelite (CaWO4) etc. Many salt-type minerals often occur together and their separation by flotation is difficult because of their similar surface properties. F a t t y acids and their soaps are c o m m o n l y used as collectors but without the use of modifying agents little or no selectivity is obtained. The surface chemistry of the salt-type minerals and the conditions used in their flotation is the subject of a review by Hanna and Somasundaran (1976). The nature of the interaction between fatty-acid-based collectors and salttype minerals is not certain. Coulombic attraction between the carboxylate anions and cationic surface sites followed by the formation of hydrophobic associates, physical adsorption of carboxylate species on a chemisorbed layer of metal soap and the precipitation of multilayers of metal soap on the mineral surfaces have all been suggested as possible adsorption mechanisms. This paper describes an attempt to clarify the mechanism of adsorption of 0301-7516/85/$03.30

© 1985 Elsevier Science Publishers B.V.

162

oleic acid by fluorite, barite and calcite by correlating the results of solubility, electrokinetic mobility, and adsorption studies. EXPERIMENTAL

Materials

Selected samples of high purity minerals were supplied by R.F.D. Parkin son and Co., Shepton Mallet, Somerset. The fluorite was from Weardale, Co. Durham, the calcite, described as fluorescent calcite, was from Minsterley, Salop, and barite was from Sandgate Beds, Fullers Earth, Nutfield Surrey. Hand-picked samples were ground in an agate mortar. The products were dry-sieved and the - 0 . 3 0 0 + 0.150 mm and - 0 . 1 5 0 mm fractions were collected. The - 0 . 3 0 0 + 0.150 mm fraction was used in flotation tests while the - 0 . 1 5 0 mm fraction, after grinding to - 4 5 #m, was used in adsorption studies. A semi-quantitative X R F analysis of the samples showed that all but the barite were more than 99.5% pure. Barite contained aluminium (-~ 2%) and minor amounts (< 0.5%) of Sr, Ca and Ti. Impurities (< 0.5%) in the other samples included A1, Si, Mn, Fe for calcite and A1, Si for fluorite. Fluka " p u r u m " , I> 95% oleic acid was used throughout. A stock sodium oleate solution was prepared by saponification of the oleic acid for several hours at 60°C with a slight stoichiometric excess of sodium hydroxide. The resulting solution had a sodium oleate concentration of 5.0 × 10 -3 mol 1-1 and pH approximately 11.0. The solution was kept in an amber glass bottle and fresh solutions were made up every 5 days. All other chemicals were of analytical grade except otherwise stated. CO2-free double-distilled water was used in all experiments. Flotation tests

Flotation tests were carried out in a modified Hallimond tube containing a long sample arm to minimize the errors caused by mechanical "carry-over". Conditioning and flotation times of 3.0 and 1.5 min were used, respectively. 0.1 mol 1-1 HCL and NaOH were used to modify the pH. A sample of 1.0 g of the mineral to be tested was conditioned with 200-ml solution containing sodium oleate at the required pH in a beaker. At the end of the conditioning the suspension was transferred to the Hallimond tube and agitated with a magnetic stirrer. Nitrogen at a constant flow rate was passed through the suspension. The pH values quoted in the results are those obtained at the end of the conditioning period. Solubility measurements

Solubility measurements were carried o u t b y equilibrating 1.0 g o f mineral (--0.150 mm) with 100 ml of the appropriate solution for 6 days in Erlen-

163

meyer flasks. Preliminary experiments showed that equilibrium was established at the end of this period. Plastic containers were used in all tests involving fluorite. The solutions were kept under nitrogen atmosphere during the equilibration time at the end of which samples were taken, centrifuged and analyzed for mineral cation and, where appropriate, anion.

Electrokinetic measurements The electrophoretic mobility of all three minerals was measured with a Rank Bros MK II microelectrophoresis instrument. Small amounts of the mineral under investigation were added to the required solutions and conditioned for 10 min prior to each measurement.

Oleate abstraction measurements Preliminary adsorption tests showed that the abstraction of oleate from solution was a rapid process. In further tests, therefore, 0.1 g of the appropriate mineral was shaken with 100 ml of the reagent solution in 100-ml Erlenmeyer flasks for 30 min. After agitation, the pH of the solution was measured and an aliquot was taken for oleate analysis. The amount of oleate abstracted was determined from the difference between the initial and equilibrium oleate concentrations. The specific surface areas of the mineral samples, obtained by a single-point technique using a Quantochrome Monosorb apparatus, were 3.37, 2.92 and 0.84 m 2 g-i for barite, calcite and fluorite, respectively.

Analytical methods Calcium was determined by atomic-absorption spectrophotometry. Fluoride was determined with a fluoride-ion selective electrode as described by Frank and Ross (1968) and Shergold and Selfe (1974). For oleate, the spectrophotometric method of Gregory (1966) was used. RESULTS AND DISCUSSION

The effect of oleic acid on the solubility o f calcite, fluorite and barite The sulphate concentration (3.0 + 0.4 × 10 -s mol 1-1) in equilibrium with a saturated barite suspension was independent of pH in the range 2 to 12 and was in close agreement with that determined from the solubility product (1.1 × 10 -s m o p 1-2) (Butler, 1964) assuming that the activity coefficients are unity and that barium sulphate behaves as the salt o f a strong acid and strong base. Barium concentrations were not determined because of the difficulty of determining barium at low concentrations. The solubility of calcite in water at different pH values was determined

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by measuring the total calcium concentration. The results obtained in a closed vessel with no CO2 present are shown in Fig. 1. At high pH values ( > 1 1 . 5 ) the solubility was low but as the pH decreased hydrolysis of the carbonate ion occurs and the solubility increased markedly. The experimental curve is in excellent agreement with theoretical predictions using the equilibria constants quoted in Garrels and Christ (1965). Equilibrium pH values below 9.8 were not obtained because this represents the buffering pH for the system. Both the fluoride and calcium concentrations were determined in saturated solutions of fluorite at different pH values. At almost all pH values tested the fluoride concentration was approximately double that of calcium (Fig. 2). The difference is, however, of the same order as the experimental error and is probably not significant. The value of the solubility product calculated from the experimental data is 7 ± 1 X 10 -11 mol 31.3 which is in good agreement with quoted literature values (Stumm and Morgan, 1970). The effect of oleate concentration on the solubility of calcite and fluorite was determined at pH 10.0 and 9.6, respectively. These pH values were used because an alkaline circuit is generally used to float fluorite and calcite and calcite buffers at pH 9.8. The results presented in Fig. 3 show that at high oleate additions the concentration of calcium in both suspensions decreas-

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ed. Surprisingly, the concentration of fluoride also decreased under these conditions. Measurements were not made of the effect of oleate addition on the carbonate concentration in calcite suspensions. Values quoted in the literature for the solubility product of calcium oleate are contradictory (Yoke, 1958; Irani and Callas, 1960; Fuerstenau and Elgillani, 1966; Clarke et al., 1976). An apparent solubility product, based on the concentrations of calcium and oleate in solution, was therefore determined by a nephelometric technique. A value of 7.4 × 10 -is mol 31-3 was obtained which is an order of magnitude higher than t h a t quoted by Du Rietz (1975). Whichever value is, however, correct does not alter the conclusion that at oleate concentrations in excess of about 1 X 10 -s mol 1-1 calcium oleate should form in solution. Precipitation of calcium oleate in solution should produce a decrease in the calcium concentration and an increase in the concentrations of lattice anions i.e. fluoride and carbonate as the oleate concentration is increased. That the latter was not the case suggests that equilibrium was not attained. A possible explanation could be that a layer of calcium oleate formed on the calcite and fluorite surfaces and inhibited further dissolution.

The effect of oleic acid on the electrophoretic mobility o f calcite, barite and fluorite Calcite was negatively charged and became increasingly more so as the pH was increased from the buffering pH. Ageing the calcite for 11 days had no significant effect on the electrophoretic mobility and this is demonstrated by the results presented in Fig. 4. Shown in the same figure is the effect of pH on the mobility of barite. An i.e.p, was obtained at pH 4.5 but between pH 6 and 11.5 the negative mobility was essentially independent of pH. A similar i.e.p, has been reported by Plitt and Kim (1974). Similar to calcite, ageing the barite for 11 days did not produce marked changes in the mobilities.

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Fluorite exhibited an i.e.p, at pH 9.5 and this was not affected by ageing for 11 days (Fig. 5). The shape of the electrophoretic mobility vs. pH curve agrees well with those of Miller and Hiskey (1973) and Fuerstenau et al. (1972) who obtained i.e.p, values at pH 10.2 and 10, respectively. The effect of sodium oleate on the electrophoretic mobilities of calcite, fluorite and barite at pH 10.0 + 0.2 is shown in Fig. 6. All three minerals had a negative electrophoretic mobility at this pH value which became more negative on addition of the sodium oleate. Although the same value of approximately 2.0 X 10 -4 cm 2 V -1 s -1 was obtained in 1.0 X 10 -3 mol 1-1 oleate, there were some differences in the intermediate region. Thus, whereas the electrophoretic mobility of barite and fluorite decreased continuously with an increase in oleate concentration from 2.5 X 10 -6 mol 1-1 the electrophoretic mobility of calcite only changed markedly above 1.0 X 10 -4 mol 1-1 . Coulombic adsorption followed by the formation of hydrophobic associa-

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tions has been suggested by several authors (Purcell and Sun, 1963) as the mechanism of interaction between oleate ions and salt-type minerals. This has been supported by electrokinetic data which show that the electrophoretic mobility of the minerals decreases from a positive value as the oleate concentration is increased and shows a marked dependence on the oleate concentration over a narrow concentration range in which charge reversal occurs. The present results suggest that such a mechanism is unlikely even if, despite the overall negative surface charge, there are some positive sites. Layers of calcium oleate on the surfaces of calcite and fluorite would be expected to produce the observed dependency of the mobilities on the oleate concentration. Calcium oleate has a high negative electrophoretic mobility at alkaline pH values (Matijevi~ et al., 1966) and it is to be expected that barium oleate will behave similarly. Furthermore, the oleate ion will be potential determining for a metal oleate and therefore an increase in negative surface potential would be expected with an increase in oleate concentration. The dependency of the electrophoretic mobilities of calcite, fluorite and barite on the oleate concentration is therefore not inconsistent with the hypothesis that layers of metal oleate are formed on the surfaces of the minerals.

Abstraction o f oleic acid by fluorite, calcite and barite The amount of oleate abstracted from aqueous solution by calcite, barite and fluorite was determined at pH 10 as a function of oleate concentration. The abstraction curves obtained (Figs. 7--9) are characterized b y two welldefined regions. In the first region, at low oleate concentrations, the oleate abstracted displayed a mark dependence on the oleate concentration whereas in the second region the amount abstracted increased slowly (Fig. 8) or remained constant (Figs. 7 and 9}. In all cases, however, the amount abstract-

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ed was in excess of that required for the formation of a close-packed vertically oriented monolayer (assuming surface area of the carboxylic group is 20.5 X 10 -20 m 2 (Lovell et al., 1974). Abstraction curves obtained at different solid to liquid ratios were not coincident suggesting that the abstraction process was not reversible. An explanation of the results can be obtained by considering the equilibrium conditions required for the precipitation of calcium oleate in a solution saturated with respect to fluorite and in which sodium oleate is added

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(in the following c a l c u l a t i o n s a c t i v i t y c o e f f i c i e n t s are t a k e n equal t o u n i t y ) : CaF2 (s) ~- Ca2+ + 2 F -

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where R C O O - represents the oleate ion. C o m b i n i n g eqs. ( 1 ) - - ( 6 ) and t h e e q u a t i o n o f m a s s b a l a n c e f o r fluoride gives:

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170 In eq. (12) it is assumed that compared to the concentrations of Ca :÷ and Fall other calcium and fluoride species can be neglected (Shergold, 1972; Marinakis, 1980). At pH 10.0 eq. (12) holds and [RCOO-] >> [RCOOH] (up to 1.0 × 10 -3 mol 1-1 total oleate concentration). Therefore, if calcium oleate precipitates in solution a mass balance on oleate and fluoride ions will give: [RCOO-]

e =

[RCOO-] i - [RCOO-]

[F-] = gs~pl + [RCOO-]

(13)

a

(14)

a

where the subscripts a, i and e indicate abstracted, initial and equilibrium oleate concentrations, respectively. Dividing eqs. (1) and ( 1 0 ) a n d substituting the concentration of fluoride from eq. (14) produces: 1

[RCOO-]e = ~ Ks p ~ 2 (K~p, + C)

(15)

\Ksp, / where C represents the initial (total) oleate concentration. Substitution of eq. (15) in eqs. (13) and (14) gives:

!

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(18)

Equation (16) shows that the amount of oleate abstracted should display a linear dependence on the initial concentration. Alternatively when C > 1.08 × 10 -6 mol 1-1 a logarithmic plot should give a straight line with a slope of unity. Following the same procedure as above, similar equations to eqs. (16)--(18) can also be derived for calcite and barite. Comparisons between the theoretical and experimental results are shown in Figs. 10--12. The agreement between the measured abstraction and that calculated from solubility considerations is good especially in the case o f calcite and fluorite. With barite, however, at high oleate concentrations the calculated abstraction was about ten times higher than that measured. Despite this descrepancy the overall comparison clearly demonstrates the dependency of the abstraction on the precipitation of metal soap.

171

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Comparison of the experimental and theoretical curves o f equilibrium oleate c o n c e n t r a t i o n against the initial oleate c o n c e n t r a t i o n for calcite and fluorite shows g o o d agreement at l o w initial oleate concentrations. A t higher c o n c e n t r a t i o n s the experimental equilibrium c o n c e n t r a t i o n is higher t h a n that measured. This observation is consistent w i t h the f o r m a t i o n o f the

172

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metal oleate on the surface of the minerals which then inhibits further reaction between the lattice cations and oleate anions. In the case of fluorite the inhibiting effect is so great that it prevents the measured abstraction from reaching the calculated value even at relatively low oleate concentrations. The presence of such a layer would also prevent dissolution of the minerals. It would, therefore, appear that true equilibrium conditions were not obtained in either the solubility or abstraction studies.

Hallimond-tube flotation with oleic acid Since oleate precipitates as the metal soap and does not chemisorb on the surface of barite, calcite and fluorite its effectiveness as a collector will be dependent on the strength of adhesion o f the precipitate to the mineral surface and not on the relative amount of oleate abstracted by each mineral. The flotation of these minerals with oleic acid, therefore, will be influenced by any parameter which affects the formation and stability of the precipitate on the surface. The pH will be one of these parameters because of its effect on solubility of all three minerals and the hydrolysis o f oleate anion in solution. The Hallimond tube recovery of barite, calcite and fluorite as a function of pH at two different levels of sodium oleate concentration is presented in Figs. 13, 14 and 15, respectively. In the same figures, the amount o f oleate, abstracted by each mineral is also included. Although the two sets of experi. ments were not carried out on the same particle size fraction some useful comparisons can still be made.

173

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Abstraction of oleate by all three minerals was high at alkaline pH values which is consistent with a chemical reaction between oleate ion and the lattice cation. At pH values above 8.0 and at a total oleate concentration of 5.0 X 10 -s mol 1-1 almost all the oleate is in the form of oleate ions and, therefore, available to react with the cations on the surface. Below pH 8.0, the abstraction of oleate decreased in case of barite and fluorite but it increased again below pH 6.5 in case of barite. Determinations were not made with calcite below pH 9.0 because of its dissolution. At pH values below 8 oleate exists in solution mainly in the form of neutral oleic acid molecules and precipitated oleic acid. Several authors (Zimmels et al., 1975; Hanna and Somasundaran, 1976) have proposed that fatty acid adsorption at low pH

174

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values involves some physical process. If this is the case then the process is apparently selective between fluorite and barite. The recovery of barite in 1.0 X 10 -4 mol 1-1 sodium oleate solution was constant at pH values above 7.0. Reducing the concentration by a factor of ten had little effect on the recovery in the pH range 7--10 but at higher pH values a small decrease was obtained. The decrease in recovery at pH values below 7.0 corresponds to a rapid decrease in the oleate abstraction. However, whereas the abstraction increased again at pH values below 6.0 the recovery continued to decrease. This shows that the abstraction of oleate at these pH values is due to its precipitation in solution in the form of neutral oleic acid molecules rather than on the mineral surface. The flotation of fluorite in 1.0 X 10 -s mol 1-1 sodium oleate was complete throughout the pH range. In 1.0 X 10 -6 mol 1-1 sodium oleate the recovery decreased at pH values above 9.0 and below 7.0, respectively. Comparison of the fluorite recovery and oleate abstraction curves suggests that the flotation is independent of the amount of oleate abstracted. Obviously this cannot be the case and the results must reflect the degree to which the calcium oleate adheres to the fluorite surface. Thus, in the pH range below 9.0 proportionally more of the calcium oleate adheres to the fluorite surface than at higher pH values. It is perhaps significant that the i.e.p, of fluorite occurs at pH 9.5 so that if the calcium oleate is negatively charged the calcium oleate will coulombically adhere to the surface at pH values below 9.0. At high oleate concentrations some of the calcium oleate will adhere to the mineral surface despite the presence of charges of similar signs on the calcium oleate and mineral surfaces. The favourable coincidence of calcium oleate formation and unlike surface charges could explain why less oleate is required to float fluorite than barite and calcite. Comparison of the flotation recovery curves for calcite, fluorite and barite

175

shows that selectivity is unlikely to be obtained between these minerals by varying the pH. Fluorite apparently floats less readily than calcite and barite at high pH values, b u t at the oleate concentrations required to float these two minerals, fluorite also floats. SUMMARY AND CONCLUSIONS

A study has been made of the interaction of oleic acid with fluorite, calcite and barite and its effect on the flotation of these minerals. The results show that the abstraction of oleate from aqueous solution corresponds to the precipitation of metal soap. It is suggested that the layers of metal soap on the minerals inhibits the dissolution of the minerals and that true equilibrium is never obtained. Flotation of the minerals is not just dependent on the amount of oleate abstracted but also on the strength of adhesion of the precipitated metal soaps to the minerals. Selectivity between the flotation of calcite, fluorite, and barite is unlikely to be obtained b y varying the pH because similar responses are obtained. Although the studies were only carried o u t on fluorite, barite and calcite, it is reasonable to assume that the interaction of oleic acid with other salttype minerals will involve a similar mechanism.

REFERENCES Butler, J.N., 1964. Ionic Equilibrium: A Mathematical Approach. Addison-Wesley, Reading, Mass., 547 pp. Clarke, D.E., Lee, R.S. and Robb, I.D., 1976. Precipitation of calcium salts of surfactants. Disc. Faraday Soc., 61: 165--174. Du Rietz, C., 1975. Chemisorption of collectors in flotation. Int. Miner. Process. Congr., l l t h , Cagliari, S-13, pp. 1--29. Frank, M.S. and Ross, J.W. Jr., 1968. Use of total ionic strength adjustment buffer for electrode determination of fluoride in water supplies. Anal. Chem., 40(7): 1169-1171. Fuerstenau, M.C. and Elgillani, D.A., 1966. Calcium activation in sulphonate and oleate flotation of quartz. Trans. AIME, 235: 405--413. Fuerstenau, M.C., Guttierez, G. and Elgillani, D.A., 1972. The influence of sodium silicate in non-metallic flotation systems. Trans. AIME, 241: 348--352. Garrels, R.M. and Christ, C.L., 1965. Solutions, Minerals and Equilibria. Harper and Row, New York, N.Y., 74, 450 pp. Gregory, G.R.E.C., 1966. The determination of residual anionic surface-active reagents in mineral flotation liquors. Analyst, 91 : 251--257. Hanna, H.S. and Somasundaran, P., 1976. Flotation of salt-type minerals. In: M.C. Fuerstenau (Editor), Flotation, A.M. Gaudin Memorial Volume. AIME, New York, N.Y., 1: 197--272. Irani, R.R. and Callas, C.F., 1960. Metal complexing by phosphorus compounds. II. Solubilities of calcium soap of linear carboxylic acids. J. Phys. Chem., 64: 1741-1743. Lovell, V.M., Good, L.A. and Finkelstein, N.P., 1974. Infrared studies of the absorption of oleate species on calcium fluoride. Int. J. Miner. Process., 1: 183--192.

176 Marinakis, K.I., 1980. The action of sodium silicate on the flotation of salt-type minerals with oleic acid. Ph.D. Thesis, University of London. Matijevic, E., Leja, J. and Nemeth, R., 1966. Precipitation phenomena of heavy metal soaps in aqueous solutions. I. Calcium oleate. J. Colloid Interface Sci., 22: 419--429. Miller, J.D. and Hiskey, J.B., 1973. Electrokinetic behaviour of fluorite as influenced by surface carbonation. J. Colloid Interface Sci., 41: 567--573. Plitt, L.R. and Kim, M.K., 1974. Adsorption mechanism of fatty acid collectors on barite. Trans. AIME, 256: 188--193. Purcell, G. and Sun, S.C., 1963. Significance of double bonds in fatty acid flotation -- an electrokinetic study. Trans. AIME, 226: 6--12. Shergold, H.L., 1972. Infrared study of adsorption of sodium dodecyl sulphate by cab cium fluoride (fluorite). Trans. IMM, 81: C147--156. Shergold, H.L. and Selfe, F.L., 1974. Determination of fluorine content of ores with the fluoride-ion selective electrode. Trans. IMM, 83: C256--257. Stumm, W. and Morgan, J.J., 1970. Aquatic chemistry: An introduction emphasizing chemical equilibria in natural waters. Wiley Interscience, New York, N.Y., 445 pp. Yoke, J.T., 1958. The solubility of calcium soaps. J. Phys, Chem., 62: 753--755. Zimmels, Y., Lin, I.J. and Friend, J.P., 1975. The relation between stepwise bulk association and interfacial phenomena for some aqueous surfactant solutions. Colloid Polym. Sci., 45: 601--607.