THE POINT OF ZERO CHARGE (pzc)of KAOLINITE AND 8iO2+A1203 MIXTURES. M. TSCHAPEK, L. TCHEICHVILIt AND C. WASOWSKI

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1 Clay Minerals (1974) 10, 219. THE POINT OF ZERO CHARGE (pzc)of KAOLINITE AND 8iO2+A1203 MIXTURES M. TSCHAPEK*, L. TCHEICHVILIt AND C. WASOWSKI* Centro de Investigaciones para Industrias Minerales, I.N.T.L, Libertad 1235, Buenos Aires, Argentina (Received 6 November 1973; revised 20 May 1974) ABSTRACT: The pzc of H+-kaolins determined by titration method is nearly = 3. The pzc of SIO2+A1203 mixtures is not additive, and depends principally on A1203 content in mixture. The curve pzc--t~ (t~ = temperature of treatment of kaolins) has two maxima: 4"7 after treatment at ~ and 5 after treatment at 1300~ INTRODUCTION It is known that surfaces of oxides insoluble in water, are charged: positively or negatively. According to Verwey (1950) the H + and OH- are commonly considered as the potential--determining ions, although it is impossible to show experimentally the sorption of one and desorption of the other. The appearance of the charges on the oxide surface is generally explained by hydration, solubilization, etc. It must be emphasized that surfaces of insoluble hydroxides by interaction with alkali behave as proton donors and with acid as proton acceptors. ROH + MOH-* RO- M + + H20 ROH + HC1 ~ ROH ~ C1 - As may be seen, the OH- is not adsorbed but neutralizes the H + of the surface, increasing in this way the density of negative charges. In order to maintain electroneutrality, the charges attract the ion of opposed sign (counter-ion) electrostatically, therefore the electrolyte which serves as source of the counter-ions is called the indifferent or supporting electrolyte. For example, for KOH and HCI the KCI is the supporting electrolyte. As presented in the above equilibrium, we show in the first case that O- is the potential-determining ion and M + is the counter-ion, whereas in the second case H + is the potential-determining ion and CI- is the counter-ion. As explained above, the sign of charge depends on * Consejo Nacional Investigaciones Cientificas y Technicas. Chief of Department of Silicate Chemistry, Centro de Investigaciones para Industrias Minerales (I.N.T.I.).

2 220 M. Tschapek, L. Tcheichvili and C. Wasowski the of solution. At the pzc the surface could be uncharged or charged with equal densities of positive and negative charges. Due to the fact that the surface might contain simultaneously positive and negative charges, the density of charges is considered to be the difference between them: where: cr = F (F+Z+ - I~_Z_) C cm -z (1) o = density of charges (C cm- 2) ; F = Faraday constant (C mole-l); F = adsorbed amount of ions (mol cm-2); Z = valence, including sign. As is known, the negative surface charges of kaolinite are originated principally by isomorphic substitution of A13+ for Si 4+ in the silica tetrahedra layer and Mg z+ for A13+ in the aluminium (gibbsite) octahedral layer (Schofield & Samson, 1953; Cashen, 1959; Weiss, 1959). The counter-ions of these charges are localized on the OH- free face of the silica tetrahedral layer (Weiss, 1959), which is independent of : RO- H (in acid medium) RO- M (in alkali medium). As may be seen, the potential determining ion in fact does not originate by isomorphic substitution. In this sense the ionic layer on the kaolinite surface originated by isomorphic substitution is different to those originated by adsorption from solution. Nevertheless, it is commonly accepted that the laws established for the classical electric double layer are applicable also for the 'incomplete' electric double layer. Due to a great distance between surface charges (f.i. for aluminosilicates 7-10 A) the polyvalent counter-ions, in order to maintain electroneutrality, reverse g-potential. The reversal of g-potential of kaolinite by three and four valence cations was shown by Beavers & Marshall (1950). On the other hand, A1 may appear on the surface of kaolinite (especially in acid medium) in its different forms: AI(OH)~, AI(OH) 2 AI6(OH)~ ~ (Cashen, 1959.) Naturally, these A1 compounds are capable of reversing a negative g-potential into a positive one: 'RO':'i H+ + A1C13-~ i~o::'"ai(ofi'iz~:i C1- + H20 In addition to charges due to isomorphic substitution, there also exist other possibilities for the appearance of charges: hydroxyl groups of Si and A1 on the edges and on A1 octahedral layer might react with acid and alkali resulting in positive and negative charges: HC1 MOH SiOH~ C1- < SiOH z~ > SiO- M + + H20

$Pzc of kaolinite and Si02 + A1203 mixtures \!/ \!,/o \!$

3 Pzc of kaolinite and Si02 + A1203 mixtures \!/ \!,/o \!,/ 221 /,\ +,c, c, o\t / \,/ AI AI AI /1\ /i\ /i\ M + + H20 Thus, the change of the negative charges of kaolinite can be caused by aluminium complex ions as well as by the substitution of a metallic polyvalent ion for H +. The pzc of kaolinite is not known. Buchanan & Oppenheim (1958), on the basis of electrophoretic mobility studies, supposed that the pzc is probably in the region The lack of data about kaolinite is probably due to the fact that pure kaolinite is difficult to obtain, as it always is present in kaolins together with other minerals, commonly called 'impurities'. Although the principal mineral of kaolins is kaolinite (sometimes up to 99.9%), the influence of impurities on pzc is likely and the pzc of different kaolins should be different. On the contrary, when impurities are absent, different kaolins should have the same pzc. The composition of kaolinite indicates that the pzc may depend on the SiO2 and A1203. It is known that the dehydroxylation of kaolinite takes place at >500~ therefore the pzc of kaolinite pretreated at temperature > 500~ must be different. The aim of this work was to find the main pzc value for kaolinite and its dependence on the heating treatment. MATERIALS AND METHODS The origin and some properties of the kaolins used are given in Table 1, their potentiometric titration curves in Fig. 2, Starting from the mean exchangeable cation density, 2 X 1014 ions per cm 2, (Weiss, 1959) and the exchange capacity at = 10, the specific surface of kaolins used was calculated. The results are as follows (m2g-1): Higlow--14-5; Astra brite and Ultra white--12 each. As may be concluded from the foregoing, the above mentioned methods are applicable only if the surface is protonized, i.e. saturated with H +. In order to substitute H + for Ca 2+, the kaolins were treated with 0.05 N HC1 up to complete substitution of Ca 2+ and later washed with distilled water to completely eliminate free electrolyte. In this way the kaolins were converted almost entirely to their H + form. Inevitably a few exchange sites would be occupied by A13+. For purposes of comparison, the mechanical mixtures of SiO2 and A120~ were also studied. The first being of Degussa with specific surface 380 m~g -1 and the second of Fluka with specific surface 155 m2g -1. The specific surface values of AI~O3 and SiO2 were taken from manufacturers' data. The pzc can be determined by different methods: suspension effect; electrophoretic mobility; change of by adding of adsorbent to solution; titration with acid and alkali, etc. In this study the titration method was used. The 'addition' method was used only for A1203 to show its inaccuracy.

222 M. Tschapek, L. Tcheichvili and C. Wasowski TABLE 1. Properties of kaolins Astra brite Higlow Ultra brite Properties Georgia kaol. C ~ Georgia kaol. C ~ Enhelhard Miner. & Chem.

4 222 M. Tschapek, L. Tcheichvili and C. Wasowski TABLE 1. Properties of kaolins Astra brite Higlow Ultra brite Properties Georgia kaol. C ~ Georgia kaol. C ~ Enhelhard Miner. & Chem. Exchange capacity, at = 10 (meq.g- a) "040 Specific surface (m2g- 1) "0 SiO '85 A "93 39"55 39"77 Fe203 0' 32 1 "40 0"25 TiO2 0"95 0" CaO 1"45 tr. 1"20 MgO 0'67 0'10 0"75 K20 0"10 0"06 0"10 Na Loss on ignition 13" " "66 The 'addition' method has been used before with some variations by various investigators (Ahmed, 1966; Berube & De Bruyn, 1968; Tewari & McLean, 1972). The variation of Tewari and McLean, used in this study is as follows. To 30 cm 3 of water solution of KC1 of determined ionic strength and, 1 g of Al~O3 was added and the change is observed during 10 min, stirring with a Teflon coated stirrer. The quasiequilibrium takes place in a few minutes (Fig. 1). Owing to the fact that at pzc no change of occurs, the curve --t, as is shown in Fig. 1 can be used to find the pzc of Al~O3. Due to the fact that the pzc found by addition method depends on the concentration of supporting electrolyte (see Fig. 1) the usefulness or applicability of this method, as was emphasized by Breeuwsma (1973), is questionable. Therefore, in this study the "titration' method was preferred. The 'titration' method consists in obtaining the curves of surface charges-- at different ionic strength of supporting electrolyte (KC1) by the following method. 5 g of kaolin were added to 30 cm 3 of supporting electrolyte of different concentrations ( M), adjusted to determine, and in a N2 atmosphere and stirred with a Teflon coated stirrer. This suspension was titrated with 0.1 N NaOH above and with 0.1 N HC1 below this. The difference between amounts of alkali or acid added to the suspension (electrolyte solution + kaolin) and to a blank solution (without kaolin) gave the net amount of consumed OH- and H +. The point of intersection, of curves obtained at different ionic strengths of supporting electrolyte, corresponds to the pzc. The net amounts used of OH- and H + are expressed in meq.g -a or/,c cm-l For the purpose of calculating the net values of adsorbed OH- or H the titration, starting exactly at pzc, was repeated. The titrations were carried out in a Metrohm Universal Titration Vessel of 50 cm 3 volume without thermostatic jacket. The was measured with a Metrohm calomel electrode (saturated) E A 404. No correction was made for the suspension effect

5 Pzc of kaolinite and Si02 + A1203 mixtures 223 (< 1 m V). The titration was effected by adding a small amount of acid or alkali through a Metrohm manual piston burette E 274 at intervals of 15 rain. The calomel electrode during measurements was submerged for a short time in solution of low (< 3). Before using again (submersion) its potential was checked. 8- I0 F /x--x--x--x--x--x--x} / /x pzc - OOIM 80' X--X--X--X--X--X--X x/x/, X... X~ x-- X--X--X 9 7"90 7.8, pzc - / ~ I M // -r Q" 7-7q 7'6 7' x--x--x--x--x--x--x I I I I I I I I0 12 [4 l (rain) FIG. I. The curves -time for A , 0"01 M KC1; o, 1'0 M KC ~ ~ x 003,,/./ o o, ~x "~ ix/~ I0 FIG. 2. The potentiometric titration curve of H+-kaolins with 0-1 NNaOH. x, Astra brite (H+); o, Higlow (H+); *, Ultra white (H+). RESULTS AND DISCUSSION In Figs. 3 and 4 the titration curves: --quantites of adsorbed H + and OH- (in meq.g -1 or in I~C cm -2) are given for SiO2 and AIzO3.

6 i 224 M. Tschapek, L. Tcheichvili and C. Wasowski " up E ~ o,m / 2.o /"./o,. / to ::L 05 *' 0 I r-- 05 io ~ O-Q-- I ; i I /"/ ooo. I,,..-// I I I I FIG. 3. The titration curves for SiO2, o, 1.0 M KCI; x, 0'1 M KCI; o, 0"01 M KCI; A, 0"001 M KC1. E i,= E o o ::::L 'o ~ i ! O.Oh~ //O.O01M /_# / r-.-u 3" "4 M, LI I I I II I I I I0 II FIG. 4. The titration curves for A12Oa. o, 1"0 M KC1; e, 0"01 M KCI; 0"001 M KCI.

7 Pzc of kaolinite and SiOz + Al203 mixtures 225 The pzc of SiO2 lies between = 2.8 and 3.0 and of Al~O3 at , whereby no adsorption of H + and OH- is observed. The pzc for SiO2 coincides with values given by certain authors, whereas for AlzO~ the value is lower than that given by others (at ~ 9-0), although as is shown, in the review of Parks (1965), the pzc of A1203 varies very much from batch to batch, sample to sample. In Fig. 5 the curve: pzc--composition of mechanical mixture of SiO2 + A1._,O3 is given / AIz03 20 Si02 80 I I I I I O :FIG. 5. The curve pzc---composition of mixture SiOz + A1203, using KCI as supporting electrolyte. The pzc is not additive, but its dominant component in the mixture is A1203. Such behaviour is comprehensible because the Al~O3 is more sensitive to changes. The titration curve for the three studied kaolins are given in Figs. 6, 7 and 8. -o 02o - 12.o I I i:m d E E "~ ::L 2 4 pzr 9 ~X O"~O01M, i L,- 0- ~: 0 r B // I I I I ~IG. 6. The titration curves for H +-kaolin Astra brite, using KC1 as supporting electrolyte.

8 L 226 M. Tschapek, L. Tcheichvili and C. Wasowski " -008 E '~ 58 = 0./I. "//4 pzc / I o / I I./~ ~,, I 2 1 I i i FIG. 7. The titration curve for H+-kaolin Higlow, using KCI as supporting electrolyte o oo4 - E F- ~% o L i g- '5 2ql ol ~ l!l '1 I 1 1 I t, Flo. 8. The titrat ion curve for H +-kaolin Ultra white, using KCI as supporting electrolyte. Due to the fact that the pzc of kaolinite is nearly = 3 (the same as SiO,_,) and also to the smallness of their specific surface, the titration below = 3 presents difficulties. Nevertheless, there can be no doubt in all cases that the interception of the curve takes place, The pzc of both kaolins: Astra brite and Ultra white is nearly -= 2.8, whereas the pzc of Higlow kaolin is a little higher. This fact is probably due to the presence of Fe,,Oa in an amount quantity (1-20%) greater than in the other two kaolins ( ~

Pzc of kaolinite and Si02 + A1203 mixtures 227 Presenting the processes of interaction of kaolinite with acid and alkali as proton adsorption--desorption, it is easy to find the equilibrium constant

9 Pzc of kaolinite and Si02 + A1203 mixtures 227 Presenting the processes of interaction of kaolinite with acid and alkali as proton adsorption--desorption, it is easy to find the equilibrium constant at pzc: RO- + 2H + ~ ROH ~ (2) aroh (K)Pze -- aro- X (all+) 2 (3) At pzc: aaoul = ago_, therefore: (K)pzc = (all+) -z (4) The pzc of H+--kaolinite is = , therefore its (K)ozo = 1 X "5 X Thus, the pzc of kaolinite is a little below the as indicated by Buchanan & Oppenheim (1968). Knowledge of the equilibrium constant, K, permits the calculation of the standard free energy change, accompanying the charging of kaolinite by adsorption-- desorption of H+: AG ~ =-RT In K (5) In K = 2.3 log K = (2-3) X (--2) X (3) = hence: AG ~ =--8.3 K cal tool -1 The AG ~ value (--8.3 K cal mol-0 is very near to the hydrogen bonding energy (5-7 K cal mol-0 suggesting that the adsorption of the H + is due to the hydrogen bonding. As it is well known, the differential thermal curves of kaolinite have one endothermic peak at ~ and two exothermic peaks at ~ and at ~ (Grim, 1962). The endothermic peak is the result of the dehydroxylation of kaolinite. According to Eitel, Miiller & Radczewski (1939) dehydroxylation begins at 400~ but the principal part of OH is lost at ~ Between the endothermic peak and 800~ the metakaolinite (the transition phase between dehydroxylation and formation of mullite) is formed. The metakaolinite (Tcheichvili, Btissen & Weyl, 1939) decomposes at nearly 800~ and subsequently the y--a1203 and free SiOz appear. The formation of mullite takes place at nearly 950~ and up to 1200~ The chemical composition of kaolinite is (A12Oa. 2 SiO2) and mullite (3AlzO3.2 SiO3) and free SiO2 is released. At t ~ > 1200~ the free SiO2 crystallizes into cristobalite. The above transformation which occurs during calcination of kaolinite should change pzc. In general, the pz~ of kaolinite depends on its two principal components: SiO2 and A120~. It must be borne in mind that the specific surface, i.e. the adsorptive capacity of components of a mixture with respect to ions, plays an important role in the determination of its p~. We have no data about the specific surface of the components, nor the degree of their destruction or formation over the range of temperature of calcination and it is therefore difficult to forecast the p~o for every temperature treatment. It is interesting to present the study of dependence of the pz~ of kaolinite on the temperature of its calcination. The calcination at t~ > 500~ is effected during

10 228 M. Tschapek, L. Tcheichvili and C. Wasowski 2 hr. The calcined samples were used without any further treatment. The pzc--t~ curve is shown in Fig ~O-, --~O I I I I I I I I do toc FIG. 9. The curve pzc--t~ for kaolins. +, Astra brite (H+); A, Higlow (H+); e, Ultra white (H+); o, Ultra white (Ca+). Observing the curve of pzc--t ~ we can conclude that it responds to the transformations occurring in kaolinite during its calcination. Beginning at the value does not change up to 200~.. The smooth increase of ozc between 200~ and 500~ is probably due to the onset of dehydroxylation of the octahedral layer. With a further increase of temperature, the pzc increases up to 4.7 at ~ due to formation of metakaolinite and afterwards of free,/-al203. The formation of the mullite begins at 900~ accompanied by the appearance of free SiOz. The pzo of SiOz is near 3, therefore the pzo of the mixture (mullite + SiOz) should be less than the p~ of pure mullite. According to Smolik, Harman & Fuerstenau (1966) the ozo of mullite is 8, whereas that of the aluminosilicates of composition A1203. SiO2 is 6-7. At t ~ > 1200~ the crystallization of the free SiO,~ into cristobalite begins. This transformation must be accompanied by a decrease of the specific surface, thereby decreasing of the role of SiO2 in the mixture, and in consequence increasing the ~r of the mixture. The decrease of the p~c at t ~ > 900~ is not consistent with the appearance of v-alumina. If there is y-alumina present the p~ should be increased, although it is not impossible for ~,-Alumina to form a very limited specific surface. It is worthwhile mentioning the behaviour of kaolin not treated with acid. According to the manufacturer (Enhelhard) the Ultra white kaolin is only water-washed and it is presumably saturated with Ca 2+. As may be seen in Fig. 9 the circles, corresponding to this kaolin are well sited on the curve pzc-t~ Before temperature

11 Pzc of kaolinite and Si02 + Al203 mixtures 229 treatment it was difficult to find its pzc. The temperature treatment 'activates' the AlzO3 in kaolinite, facilitating its pzc determination without its protonization. SUMMARY (1) The point of zero charge (pzc) of kaolinite studied through the intersection of titration curves is nearly = 3. (2) The study of pzc of the mhecanical mixture of A SiO2 shows that the A1203 is the dominant component. (3) The pzc of temperature pretreated kaolins varies according to the transformation occurring in them. REFERENCES AHMED S.M. 0966) Can. J. Chem. 44, BEAVERS A.H. & MARSHALL E. (1950) Proc. Soil Sci. Soc. Am. 15, 142. BERUBE Y.G. & DE BRUYN P.L. (1968) J. Colloid Interface Sci. 27, BREEUWSMA A. (1973) Adsorption of ions on hematite (Fe203). Thesis, State Agricultural University, Wageningen. BUCHANAN A.S. & OPPENHE1M R.C. (1968) Aust. J. Chem. 21, CASHEN G.H. 0959) Trans. Faraday Soc. 55, 477. EITEL W., MOLLER H.O. RADCZEWSKI O.E. (1939) Ber. dt. keram. Ges. 20, 165. GRIM R. (1962) Applied Clay Minerology. McGraw-Hill, New York. PARKS G.A. (1965) Chem. Rev. 65, 177. SCHOFIELD R.K. & SAMSON H.R. (1953) Clay Min. Bull. 2, 45. SMOLIK T., HARMAN t~ FUERSTENAU O. (1966) Trans. Soc. Mining Eng. A.I.M.E. 235, 367. TCHEtCHVlLI L., B~)SSEN W. & WEYL W. (1939) Bet, dt. keram. Ges. 20, 249. TEWARI P.H. & McLEAN A.W. (1972) J. Colloid Interface Sci. 40, 267. VERWEY E.J. 0950) Colloid Chemistry 7, 47. WEISS A, (1959) Z. Anorg. Allg. Chem. 299, 92.

GENARAL INTRODUCTION TO METALLURGY :Std: XI-CHEMISTRY

GENARAL INTRODUCTION TO METALLURGY :Std: XI-CHEMISTRY 1. What is matrix? The ore is generally associated with rock impurities like clay, sand etc. called gangue or matrix 2. What is mineral? The natural