Influence of sodium silicate addition on the adsorption of oleic acid by fluorite, calcite and barite

International Journal of Mineral Processing, 14 (1985) 177--193

177

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

INFLUENCE OF SODIUM SILICATE ADDITION ON THE ADSORPTION OF OLEIC ACID BY FLUORITE, 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. Influence of sodium silicate addition on the adsorption of oleic acid by fluorite, calcite and barite. Int. J. Miner. Process., 14: 177-193. A study has been made of the adsorption of sodium silicate by calcite, fluorite and barite and the effect that this adsorption has on the flotation of these minerals with oleic acid. The results show that sodium silicate depresses these minerals by preventing oleate species from reacting with surface sites. This effect is independent of the total silica concentration. Aged sodium silicate solutions do not produce markedly different results to those obtained with fresh solutions. The concentration of the sodium silicate solutions was generally such that the total silica concentration was below that in equilibrium with amorphous silica. Under these conditions the solutions contained no polymeric silica species and the mechanism of silica adsorption can be generally attributed to interactions of monosilicate ion and monosilicic acid with surface sites.

INTRODUCTION

Flotation methods have been developed and are used commercially for the separation of salt-type minerals such as calcite, fluorite, barite, apatite and scheelite from oxides and silicates. Separation of the salt-type minerals from each other is, however, difficult because the surface chemistry of the minerals is dominated by the presence in the lattice of the same or a similar alkaline earth cation. Anionic collectors such as fatty acids are c o m m o n l y used to float silicates at high pH values. Selectivity between the salt-type and siliceous minerals and also between the salt-type minerals is often improved by the addition of sodium silicate. The mechanism by which sodium silicate acts, however, has remained obscure, mainly because of the lack of systematic data and inadequate descriptions of the sodium silicate and the experimental conditions used. The term sodium silicate refers to a whole family of chemicals which consist mainly of sodium oxide and silicon dioxide in various proportions, and 0301-7516/85/$03.30

© 1985 Elsevier Science Publishers B.V.

178 whose composition can be represented by the general formula Na20 r SiO2, where r is called the " m o d u l u s " or ratio of the sodium silicate. Most commercial silicates are not true chemical compounds but complexes of definable sodium silicate, and either Na20 or SiO2. There would appear to be only t w o definable sodium silicates Na2SiO3 (and its hydrates Na2SiO3" 6H20 and Na2SiO3 • 5H20) and N a 2 0 - 2SIO2 which correspond to the metasilicate (r = 1) and disilicate (r = 2), respectively. Several mechanisms of sodium silicate adsorption have been suggested, including the physical adsorption of silica gel and water glass (Cheng et al., 1963; Solnyshkin and Cheng, 1963), silicate ions (Glembotskii and Uvarov, 1964; Fuerstenau et al., 1972; Glembotskii et al., 1972), colloidal silica and polymeric silicic acid (Nikiforov and Skobeev, 1968) and chemisorption (Fuerstenau et al., 1972). Furthermore, several authors (Sollenberger and Greenwalt, 1958; J o y and Robinson, 1964; Berlinskii and Kluyeva, 1972) have suggested that the efficiency of sodium silicate as a depressant increases with the silica to soda ratio or the age of the sodium silicate solution (Berlinskii, 1962). Interpretation of the literature is, however, difficult because many investigators have not defined the sodium silicate used, its initial concentration and the method of addition. Colloidal silica has often been assumed present when solution equilibria considerations suggest that this would not be the case. This paper describes a detailed investigation of the effect of sodium silicate on the adsorption of oleic acid by fluorite, calcite and barite and the Hallimond-tube flotation of these minerals under well-defined conditions. The investigation included consideration of the solution equilibria of sodium silicate solutions, measurement of the effect o f sodium silicate on the solubilities of the minerals and their electrokinetic properties and measurements of the amounts of sodium silicate and oleic acid adsorbed in the presence and absence of each other. The effects of silica to soda ratio and age of the sodium silicate solution have also been determined. EXPERIMENTAL Materials The minerals used and their methods of preparation have been described previously (Marinakis and Shergold, 1985). Material used in the Hallimondtube flotation tests was - 3 0 0 + 150 pm and that used to determine oleate adsorption was - 4 5 #m. For the silicate adsorption measurements the - 1 5 0 ~m material was ground finer in a stainless steel " T e e m a " mill. Typical specific surface areas of these materials as determined b y BET nitrogen adsorption were 3.75, 2.40 and 0.82 m 2 g-1 for calcite fluorite and barite, respectively. All materials were stored in a vacuum dessicator until required. The sodium silicates used had silica to soda ratios varying from 1.00 : 1 to 3.41 : 1. The silicates with ratios 1.65 : 1, 2.07 : 1, 2.94 : 1 and 3.41 : 1

179 were provided by Crosfield Chemical Co. and those with ratios of 2.56 : I and 1.00 : 1 were obtained from Hopkin-Williams and BDH, respectively. With the exception of the silicate with a ratio 1.00 : 1 all were supplied as concentrated viscous liquids readily soluble in water. The silicate with ratio 1.00 : 1 was in the form of hydrated soluble pellets with the given formula of Na2SiO3" 5H20. All sodium silicates were commercial products and were used without further purification. Aged sodium silicate solutions were prepared by making up 1.0--13.6 × 10 -3 mol 1-1 SiO2 solutions at the required pH value and storing them in plastic containers for up to 34 days. Fresh solutions were prepared immediately prior to their use. Fluka " p u r u m " , />95% oleic acid was used. A stock sodium oleate solution was prepared by saponification of the oleic acid at 60°C with an excess of NaOH. The solution was kept in the dark and a fresh solution was made up every 5 days. All solutions were prepared with double-distilled CO2-free water.

Experimental methods and techniques The experimental methods used in the Hallimond tube flotation tests, electrokinetic and solubility studies and oleate adsorption measurements have been described previously (Marinakis and Shergold, 1985). The procedure used in the silica adsorption tests consisted of preparing 100 ml sodium silicate solution at the required pH and then adding 50 ml of this solution to 10 g of mineral (30 g in the case of barite) in a 50-ml Erlenmeyer flask. After equilibration by shaking for 15 h the suspension was centrifuged in 100-ml Teflon tubes for 30 min at 3000 rpm. The clear solution was transferred into a polypropylene beaker, the pH measured and the silicon content determined together with that of the remaining initial sodium silicate solution. The amount of silica adsorbed was deduced from the difference in silica content of these two solutions. Soluble silica was determined by the spectrophotometric methods of Garrett and Walker (1964) and Kato (1976). The latter was sometimes used to determine silica at high concentrations. Calcium, fluorite and oleate were determined as described elsewhere (Marinakis and Shergold, 1985). RESULTS AND DISCUSSION

Aqueous chemistry of sodium silicate solutions The species present in the aqueous sodium silicate solutions depends to a large extent on the concentration of the solution in relation to the solubility of amorphous silica. Dilute solutions are defined as those with less silica than the solubility of amorphous silica (1.99--2.33 × 10 -3 mol 1-1 SiO2 at 298°K) (Alexander et al., 1954; Kitahara, 1960). Solutions of this t y p e have been extensively studied by Ingri (1959), Bilinskii and Ingri (1967) and

180

Lagerstr6m (1959) who have determined equilibrium constants for the following equilibria: SiO2{s) + H20 ~ Si(OH)4(aq)

Ksp = 2.3 X 10 -3 mol 1-~

Si(OH)4 + OH- ~ SiO(OH)~ + H20

K~ = 1.95 X 104 mol -~ 1

SiO(OH)~ + OH- ~ SiO2(OH)]- + H20

K2 = 9.77 mo1-1 1

Si(OH)4 + 2OH- ~ SiO2(OH)~- + 2H20

f12 = 1.90 X l 0 s mo1-2 12

4Si(OH)4 + 2OH- ~ Si40~(OH)~- + 6H20

/324 = 1.07 X 10 is mol -s 1s

Using these equilibria, the solubility diagram shown in Fig. 1 can be constructed (Stumm and Morgan, 1970). The solubility of amorphous silica is independent of pH between pH 4 to 9 and in this pH range the predominant soluble silica species is monosilicic acid (Si(OH)4). At pH values above 9 the solubility increases because of the formation of monosilicate, disilicate and other polynuclear silicate ions. A typical silica distribution diagram is shown in Fig. 2 for a 1 X 10 -4 mol 1-1 SiO~ solution. This shows that Si(OH)4 predominates at pH values below 9.0 SiO(OH)~ between 9.5 and 12.5 and SiO2(OH)]- at pH values above 12.5. Si406(OH)~- does not predominate at any pH but its concentration does reach a maximum between pH 10.0 and 12.0. In solutions saturated with respect to soluble silica, polymerization of silica takes place. The rate of polymerization is very dependent on the pH, silica concentration and the temperature. Very slow rates are obtained at low and high pH values (Tarutani, 1970; Iler, 1979). Thus, in acidic media '

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the polymerization rate is so slow that supersaturated solutions of monosilicic acid can exist for a very long time. The fastest rate would appear to occur at weakly alkaline pH values. The structure of the various polymeric species is not well defined, but it is known that under certain conditions the polymers or colloidal sized particles exhibit a considerable negative charge. Polymerization/depolymerization is reversible, b u t depolymerization from colloidal silica to monosilicic acid is very slow. Some authors (Chernyi, 1962; Berlinskii and Kluyeva, 1972; Fuerstenau et al., 1972) have assumed that colloidal or polymeric silica is present even in dilute silicate solutions and that the concentration and form of the polymeric species is dependent on the silica-to-soda ratio and age of the solution. This assumption, based mainly on the results of Harman (1928), appears to be thermodynamically incorrect especially since Harman studied concentrated solutions rather than dilute ones. However, it is possible that dilute solutions, not at equilibrium, will contain polymeric species if they have been prepared by dilution of a concentrated solution. To see if this was the case an attempt was made to determine the degree of polymerization in the dilute sodium silicate solutions used in the present study. Fresh sodium silicate solutions were prepared by diluting a stock solution of I × 10 -2 mol 1-~ sodium silicate. The stock solutions were prepared every day by dilution of the supplied, concentrated solutions in which most of the silica was present in polymeric form. The pH values of the stock solutions varied between pH 10.5 and 11.5 depending on the ratio of the sodium silicate used. It was not possible to determine the degree of polymerization in the stock solutions because the concentration of rrionosilicic acid was above the range for determination by the molybdate method (Chow and Robinson, 1953). Instead, the degree of polymerization in diluted stock solutions was determined by comparing the monosilicic acid concentration 5 min after dilution with the total sodium silicate concentration. The value

182

obtained was, of course, probably less than the degree of polymerization in the stock solution because some depolymerization would occur on dilution; but it was useful in indicating whether or not polymeric species were present in the solutions used in the adsorption and flotation studies. The results showed that with the exception of sodium silicates with ratios higher than 2 . 5 6 : 1 and at concentrations above 1.5 × 10 -3 mol 1-1 SiO2, where up to 10% polymeric silica was obtained, the solutions contained only monomeric silica. Thus, at the sodium silicate concentrations (~ 1.5 × 10 -3 mol 1-1 SiO2) used in subsequent studies it can be assumed that the fresh solutions contained no polymeric silica species. Sodium silicate solutions with an SiO2 content less than that of the solubility of amorphous silica were unaffected by ageing. This was not the case with more concentrated solutions which contained 4 × 10 -1 mol 1-' sodium silicate or between 4 × 10 -3 and 13.6 × 10 -3 mol 1-1 SiO2 depending on the ratio of sodium silicate. Determination of the monosilicic acid concentration after 34 days gave results consistent with that in equilibrium with amorphous silica at pH values above 4.5. However, no solid silica was detected and some of the solutions must therefore have contained polymeric silica. Some adsorption and flotation studies were conducted with these aged solutions but only after they had been diluted. Large differences between the results obtained with fresh and diluted aged solutions would not be expected because in both cases depolymerization would have occurred during the preparation of the solutions. Small differences were, however, observed and these will be discussed later.

Adsorption of silica by calcite, fluorite and barite Preliminary tests with fluorite and calcite showed that the adsorption was almost complete within 4 h although minor changes were obtained over a prolonged period. Equilibration for 15 h was used in all subsequent tests. Temperature in the range 15--30°C had no significant effect on silica adsorption and therefore all other tests were conducted at room temperature. The adsorption isotherms of sodium silicates of ratios 1.00 : 1, 2.07 : 1 and 3.41 : 1 on calcite and fluorite at an initial pH of 9.3 are shown in Figs. 3 and 4, respectively. All the isotherms have the same shape with the change in adsorption density with concentration increasing with increasing silica concentration. In general, the most marked increase in the adsorption density was obtained at concentrations above 2 X 1 0 -4 mol 1-1 SiO2. The shape of the isotherms is consistent with the $2 type of Giles et al. (1960) which indicates a tendency of the adsorbed solute to condense on the surface, at high surface coverages, rather than remain as isolated units. The adsorption was irreversible as illustrated by the dependency of the adsorption on the solid/liquid ratio and adsorption tests in which half the supernatant solution after adsorption was replaced by water at the requisite pH and then reequilibrated with the solids. Increasing the silica-to-soda ratio

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184

apparently increased the adsorption density of silica on calcite. The effect is probably due, however, to the different equilibrium pH values obtained. With fluorite the isotherms obtained with 2 . 0 7 : 1 and 3 . 4 1 : 1 sodium silicates were coincident at an equilibrium pH o f 6.5 whereas that with 1.00 : 1 sodium silicate at pH 7.1 interacted the other isotherm at a concentration of about 3 X 1 0 -4 mol 1-1 SiO2. The adsorption densities obtained were much less than a monolayer independent of whether the area per adsorbed ion was 12.3 × 10 -20 m 2 for SiO2 (Holt and King, 1955) or 25 X 10 -20 m 2 for a close packed area of monosilicic acid (Hingston and Raupach, 1967). Adsorption densities in excess of 1.3 X 10 -s tool 1-1 or twice the value would be required to give monolayers of SiO~ or silicic acid. The effect of pH on the adsorption of silica by calcite and fluorite is shown in Figs. 5 and 6. Maximum adsorption on calcite was obtained at pH 9 and 10 and this was independent of the ratio of the sodium silicate. Adsorption on fluorite increased markedly at pH values above 6.5 and assumed a constant value over a wide pH range. Adsorption against pH curves with sodium silicates of different silica to soda ratios, but the same SiO2 content, were coincident. The adsorption of silica by barite (Fig. 7) was very much lower than that by calcite and fluorite and was only significant at high pH values. Adsorption isotherms of silica on barite were not obtained because, in general, the adsorption was low and the change in silica concentration was smaller than the experimental error. Silica adsorption on fluorite reduced the aqueous calcium concentrations I

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at silica concentrations above 5 X 10 -s mol 1-1 and the effect was independent of the silica to soda ratio (Fig. 8). The fluoride concentration was also reduced b u t not by as much as that of calcium. The reduction in calcium concentration was accompanied b y an increase in the negative electrophoretic mobility (Fig. 9). Comparison of Figs. 6, 8 and 9 shows a good correlation between increased silica adsorption, decreased soluble calcium concentration and increased negative electrophoretic mobility. Similar results were obtained for calcite and barite except that in the latter case no adsorption isotherm was obtained. The solubility and electrokinetic data indicates that the mechanism of

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silica adsorption does not involve coulombic interaction between monosilicic acid, monosilicate ions and cationic surface sites. Furthermore, the irreversibility of the adsorption suggests that it might be chemical in origin. The system CaO-SiO2-H20 has been extensively studied (Steinour, 1947; Taylor, 1953; Greenberg et al., 1960; Greenberg, 1961; Greenberg and Chang, 1965a, b; Ka~hik, 1965; Santschi and Schindler, 1974) and it is known that CaO and SiO2 can form a series of colloidal, hydrated compounds in relatively concentrated solutions and at high pH values. In the present work, it was found by nephelometry that, more than 2.4 × 10 -3 mol 1-1 SiO2 was required to form a precipitate in a solution containing 5 × 10 -3 gr-atoms 1-1 Ca2÷ at pH 12.0. This precipitate rapidly disappeared when the pH was reduced. It would therefore appear that the decrease in calcium concentration as silica is adsorbed cannot be attributed to the bulk precipitation of calcium silicate. The species present in a saturated solution of calcite will include Ca~÷, CaliCOS, C a O H ÷, CO~-, H C O ~ and CO2¢aq) at concentrations dependent on the p H and partial pressure of CO2. Fig. 10 shows the concentration of these species as a function of p H superimposed on a concentration diagram for solutions containing 2 X 10 -4 and 5 X I0 -s tool l-I SiO2. Assuming that the surface of calcite contains cationic sitesanalogous to those in solution then from comparison with Fig. 5 it would appear that the most likely species involved in the adsorption process are Ca~surf> and SiO(OH)~. M a x i m u m silica adsorption is obtained at a p H where SiO(OH)~ is the predominant silica species. At higher p H values the concentration of this species decreases in favour of the formation of the divalent silicatespecies and this corresponds to a decrease in the adsorption. At lower p H values the decrease in adsorp-

187 tion corresponds to a reduction in the monosilicic acid concentration. A suggested surface reaction is therefore: Ca2 + + SiO(OH)~ ~ Ca2+-SiO(OH)3 and possibly: Ca2+-OH + Si(OH)4 ~ Ca2+-OSi(OH)3 + H20 Reaction between Ca 2+ and Si(OH)4 is unlikely because both species are acidic in character. A similar comparison for fluorite and barite suggests that some interaction between the divalent silicate ion and CaOH + or BaOH ÷ might occur at very high pH values. However, at pH 9 where the adsorption by barite becomes significant the most likely reaction would appear to be: Ba2+ + S i O ( O H ) 3 "~ B a 2+ -S iO (OH)3 Reaction between Ba2+-OH and Si(OH)4 at lower p H values is unlikely to be m a r k e d because of the low concentration of Ba2+-OH.

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8 9 10 11 12 13 1/* pH

Fig. I0. Comparison of the concentration of the silicatespeciesin 2 x I0 -4 and 5 x I0 -4 mol l-I sodium silicateat differentpH values with the concentration of calcium species in saturated solutionsof calcite.

189

tions reduced the depression slightly at high pH values (results not shown) but had little effect at low pH values. Reducing the sodium silicate concentration to 5 X 10 -4 mol 1-1 produced depression in the same pH ranges as that obtained with more concentrated solutions, thus demonstrating that the depression was not attributable to the presence of metastable polysilicate species. Sodium silicate depressed the flotation of calcite over the pH range studied (Fig. 12). The degree of depression obtained was independent of the ratio o f the sodium silicate b u t increased with the SiO2 concentration. Ageing the sodium silicate solutions produced slightly less depression at alkaline pH values than fresh solutions but had a similar effect at other pH values (results not shown). Barite was strongly depressed b y sodium silicate at alkaline pH values (Fig. 13) but was little affected at neutral pH values. Similar to fluorite and calcite flotation ageing the sodium silicate reduced the degree of depression obtained. i

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Oleic acid is abstracted from solution by calcite, fluorite and barite by the formation of multilayers of calcium and barium oleates on the surfaces of the minerals and in bulk solution. Chemical reaction rather than chemisorption occurs between the minerals and the collector (Marinakis and Shergold, 1985). The amount of oleic acid abstracted from solution in the presence of sodium silicate correlated well with the flotation results. This is typified by the abstraction data obtained with fluorite and illustrated in Fig. 14. In the absence of sodium silicate, maximum oleate abstraction occurred at pH 10.5 but in the presence of soluble silica it was obtained at lower pH values as the SiO2 concentration was increased. In general, a reduction in the amount of

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10 1 12 pH

Fig. 1 4 . A b s t r a c t i o n o f o l e a t e b y f l u o r i t e as a f u n c t i o n o f p H in t h e p r e s e n c e o f f r e s h sod i u m silicates w i t h silica t o s o d a r a t i o s o f : (a) • 1 . 6 5 : 1, * 2 . 5 6 : 1, a n d ~ 3 . 4 1 : 1 ; ( b ) c 1 . 0 0 : 1, o 2 . 0 7 : 1, a n d • 2 . 9 4 : 1 ( i n i t i a l s o d i u m o l e a t e c o n c . 5 . 0 X 10 +s m o l l - ' ) .

oleate abstracted produced a decrease in the flotation recovery. Ageing the sodium silicate solutions did not produce significantly different oleate abstractions at neutral and acid pH values to those obtained with fresh solutions. At alkaline values, however, the oleate abstraction in aged silicate solutions was higher than that obtained with fresh solutions. This is consistent with the reduced depression observed with these solutions. Although the correlation between the amount of oleate abstracted and the flotation recovery is good it must be emphasized that the abstraction does not necessarily correspond to that on the mineral surface. Some is accounted for by precipitation of the metal oleate in the bulk solution and the extent of this process is not known. Small differences between the behaviour of fresh and aged sodium silicate solutions can probably be attributed to the lack of attainment of equilibrium in the fresh solutions after dilution from more concentrated solutions. Indeed, the ratio of Si(OH)+ to SiO(OH)~ concentrations in fresh and aged sodium silicate solutions will differ and will be higher in the fresh sodium silicate solutions than in the aged ones at alkaline pH values. This would favour silica adsorption through the reaction: Ca2+-OH + Si(OH)+ ~ Ca2÷-OSi(OH), + H20 at high p H values so that higher adsorption is obtained from fresh than from aged sodium silicate solutions. Assuming that similar sites are involved in the abstraction of oleate and silicafrom solution then an increase in the latter should result in a decrease in the former.

191

The adsorption of silica in the presence of oleic acid was not determined because oleic acid interfered with the analytical technique for silica. It is not known therefore whether silica coadsorbs with oleate or if the adsorption of oleate prevents the adsorption of silica. Certainly, silica adsorption reduces abstraction of oleate and the main mechanism of depression of fluorite, calcite and barite flotation by sodium silicate can therefore be attributed to the reduction in oleate abstraction. Metasilicate ions or metasilicic acid molecules adsorb at cationic surface sites and prevent these sites from reacting with oleate species. Furthermore, the silica adsorption inhibits the dissolution of the minerals and increases their negative surface potential. Both effects are likely to reduce interaction with oleate ions. Differences in the degree of silica adsorption and hence depression obtained with the different minerals can be attributed to the availability of suitable surface sites and the influence of pH on the concentration of these sites. Flotation depression with sodium silicate is not attributable to the adsorption of polymeric or colloidal silica provided that the total silica concentration does not exceed that in equilibrium with amorphous silica. Generally, the degree of depression obtained is independent of the silica to soda ratio of the sodium silicate. SUMMARY AND CONCLUSIONS A study has been made of the adsorption of silica by fluorite, barite and calcite from dilute sodium silicate solutions of different silica-to-soda ratios and the effect that this adsorption has on flotation o f the minerals with oleic acid. Mineral solubility and electrokinetic mobility measurements have been used to help interpret the adsorption and flotation data obtained. From the results it can be concluded that the adsorption of silica b y the three minerals is chemical in origin and is consistent with interactions between Si(OH)4 and SiO(OH)~ in solution and cationic surface sites of the 2+ type Ca and Ca2+-OH. The latter species are assumed to exist by analogy with their concentration in bulk solution. The amount of silica adsorbed on the three minerals decreases in the order fluorite > calcite > barite and is independent of the ratio of the sodium silicate used and the age of the solution provided that equilibrium is obtained. Hydrogen bonding between silanol groups and fluoride is suggested as a possible reason for some of the adsorption on fluorite. Silica adsorption reduces the solubility of fluorite, calcite and barite and increases the negative electrophoretic mobilities of the minerals. Oleate abstraction by fluorite, barite and calcite is reduced in the presence of sodium silicate to an extent dependent on the total SiO2 concentration rather than the silica to soda ratio. Flotation closely follows the oleate abstraction so that depression by sodium silicate can be attributed to the inhibition of oleate abstraction. Polymeric silica adsorption is not involved in the depression of the three minerals provided that the concentration of the soluble silica is lower than

192

that in equilibrium with amorphous silica. Furthermore, ageing the sodium silicate solutions does not produce a significantly different depression effect provided that equilibrium has been obtained.

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