Acid Activation of a Ferrous Saponite (Griffithite)

8/12/2019 Acid Activation of a Ferrous Saponite (Griffithite)

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Clays and Clay MineralsVol. 42, No. 6, 72 4--730, 1994.

A C I D A C T I VA T I O N O F A F E R R O U S S A P O N I T E G R I FF IT H I TE ):P H Y S IC O - C H E M I C A L C H A R A C T E R I Z AT I O N A N D

S U R F A C E A R E A O F T H E P R O D U C T S O B T A I N E D

M. A. VICENTE RODRIGUEZ 1 2 M. SUAREZ BARRIOS 3 J. D. LOPEZ GorZZALEZ, AND M. A. BA~ARES MUI~OZ

Departamento de Qulmica Inorganica, Facultad de CienciasUniversidad Nacional de Educacirn a Distancia, Senda del Rey, s/n. E-28040-Madrid, (Spain)

2 Departamento de Quimica Inorg~nica, Facultad de Qu imicaUniversidad de Salamanca, Plaza de la Mercer, s/n. E-37008-Salamanca, (Spain)

Area de Mineralogia y Cristalografm, Depar tamen to de Geologia, Facultad de CienciasUnivers idad de Salamanca, Plaza de la Merced, s/n. E-37008-Salamanca, (Spain)

Abs tract- -A ferrous saponite (griffithite) from Griffith Park (California, USA) was treated with solutionsof HC1 (0.62, 1 .25 and 2.5% by weight) at 25~ for 2, 6, 24 and 48 hours. The resulting solids werecharacterized by XRD, FT-IR spectroscopy, thermal analyses, SEM, TEM and nitrogen adsorption iso-therms at 77 K, showing the dest ruction of silicate structure by the treatments. The free silica generatedby these treatments was digested and determined in all samples. Several samples had specific surfaceareas up to 250 m 2 / g with maximum values which are 10 times higher than the surface area of naturalsaponite (35 m2/g). A sudden decrease in specific surface areas was observed when free silica was digested,which indicates that free silica makes a very impor tan t cont ribu tion o the surface area of leached samples.Key Words- -Acid activation, Ferrous saponite, Free silica, Surface area.

I N T R O D U C T I O N

The physico-chemical properties of clay minerals arestudied because of their influence on the pote ntial ad-sorbing and/or catalytic properties of these minerals.The extent and nature of their external surface areprobabl y the properties that most influence this be-haviour. The acid treatments of clay minerals whichmodify their surfaces are usually called activ ationtreatm ents if they produce an increase in the specificsurface area and/or in the number of acid centres bydisaggregation of clay particles, elim inat ion of minera limpurities, removal of metal-exchange cations andproton exchange. Appropriate acid and thermal treat-ments increase the catalytic and adsorbing activity ofsome clay minerals but further a nd stronger treatme ntsdecrease this activity.

Saponite is one of the mos t imp orta nt trioctahedralsmectites, originating as a product of the hydrot hermalalteration and weathering ofbasalts and ultramafic rocks(Velde 1985; de la CaUe and Suquet 1988). Saponitehas a negativel y charged tetrahedr al sheet, which re-sembles beidellite. This negat ive charge is partially bal-anced by the positive charge in the octahedral sheet;in this respect sapphire is more similar to vermiculi te.In its ferrous variety a q uant ity of Fe(II, III) substitutesMg(II) octahedric cations; this substitution acquiresimportance when the ratio Mg/Fe is greater than 5:1(de la Calle and Suquet 1988).

As indicated before, the structure of saponite is verysimilar to that of vermiculite. In a very pure Ca-sap-onite from Koz~ikov (Czechoslovakia), Suquet e t a lC o p y r i g h t 9 1 9 9 4 , T h e C l a y M i n e r a l s S o c i e ty

(1975) found a monoclin ic unit cell 5 x 9 A, 3 = 97 -100 ~ and a basal spacing of 15/~. Deposits o fsap onit ehave recently been described (Post 1984; Gal~n e t a l1986; Kod ama e t a l 1988; April and Kell er 1992), and

the adsorbent and catalytic properties of this silicatehave bee n studied in recent years, with the finding thatit has an excellent ability to form pillared clays (Wa-tanabe and Sato 1988; Chevalier e t a l 1992; Schoon-heydt and Leeman 1992; Usami e t a l 1992).

Although much research has been carried out on theacid and thermal activations of the different clay min-erals, certain aspects of their nature and mechanismremain unclear. Numerous papers about acid activa-tion of m ontm oril loni te (the principal silicate in thesmectite group), sepiolite (a fibrous magn esic silicate),vermiculite and other silicates can be found in the

literature (Lrpez Gonzfilez e t a l 1981; Mendior oz e t a l1987; Cetisli and Gedikbey 1990; Suquet e t a l 1991;Pesquera e t a l 1992). However, in spite o f the growinginterest in the study of saponite, no references to acidactivation of this silicate have b een found.

Accordingly, we have studied the e volut ion of someproperties of saponite when it is treated with HC1 overdifferent ranges of acid con centrati on and tim e o f treat-ment. These studies have been extended to the acti-vated samples after removal of the free silica formedduring the acid treatments.

EXPERIMENTAL

The clay miner al used in this work is a ferrous sap-onite of a var iety called griffithite, presen t in the b asic

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weathered rock from Griffith Park (California, USA),supplied by Minera ls Unl imi ted (Ridgecrest, Califor-nia). This rock is made up, mainly, of anortite, andabout 10% is saponite. The solid obtained by aqueousdecantati on of the original pulverized rock (


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726 Rodriguez et al Clays and Clay Minerals

Table 1. Cations dissolved by acid treatments, expressed aspercentage, in oxide form, of the original content of the sam-pies.

Tim e M g O A 1 2 0 3 F e 2 0 3 M n O C a O 1 N a 2 0 1 K 2 0 1

0.62% HCI

2 h t9.5 19.9 14.6 3.76 h 26.3 23.6 15.9 4.6 85 6 22

24 h 33.5 30.7 15.6 5.448 h 34.7 32.4 15.3 5.8

1.25% HC12 h 48.3 45.8 37.2 5.86 h 73.6 54.1 44.8 6.2 88 11 22

24 h 83.8 57.1 47.1 7.548 h 82.1 57.3 46.8 8.3

2.5% HC12 h 84.2 63.3 59.8 7.56 h 100 84.2 84.3 9.6 9 2 15 22

24 h 100 92.7 94.0 10.448 h 100 96.0 97.2 11.3

Average value on each series.

the min eralogi cal form ula of the silicate was calculated,obtai ning for the dry sample, on a basis of 12 oxygenat om s: [Si3.46 A10.54] T [Mgl.46 Fe3+o.v9 A10.14 Tio.02M n o . o 3 [] o .5 6 ] ~ O l o O H ) 2 [ C ao . 31Nao.~o Ko.o2], All ir onwas considered to be as Fe(III). The Mg/Fe ratio de-duced from this formula is 1.85. This formula impliesthat this saponite is a high charge smectite: -0.54 inthe tetrahedral sheet, - 0.15 in the octahedral and - 0.69in the layer, in good agreement with the charge + 0.7 4given by the exchangeable cations. The charge o f thelayer is higher than the value of - 0. 60 usually assumedfor trioctahedral ferromagnesian smectites; in the min -eralogical formula of this mineral referred by Ross(1960), the in terlay er charge was 0.61. The fact thatthe octahedral sheet has, in the formula we have ob-tained, a negative charge and, consequently, increasesthe interl ayer charge likens this sample to the ferroussaponite from Mont M6gantic (Kodama et al 1988)rather than to the griffithite referred to by Ross. Thishigh charge suggests that the sample would be a ver-miculite or a ferrous stevensite, instead of a ferrous

saponite. The nam e of ferrous saponite-griffithite ismai ntai ned because the sample of this deposit has beenalways known to be a saponite (Newman and Brown1987; R os s 1960).

The a moun ts of the cations of the solid that aredissolved in each acid tr eatme nt are given in Table 1.As observed, Ca(II), present in the solid as an extra-lattice exchangeable cation, is i mmedi ately dissolvedin a high percentage (about 85%), and the same be-havio ur is observed with 22% of the total small amoun tof K(I). The quantities of Ca(II), Na(I) and K(I) thatrema in are probably due to the presence in the solids

of a very small amount of the feldspars observed inthe basaltic rock. In the structural cations, Mg(II) un-dergoes the faster dissolution process, while the dis-

Table 2. f.w.h.m, index for natural and activated samplesof saponite in the (001) reflection peak.

f.w.h.m.Sample ~ de 20)

Natural saponite

Saponite 0.62% HC1 2 hSaponite 0.62% HC1 6 hSaponite 0.62% HC1 24 hSaponite 0.62% HCI 48 h

Saponite 1.25% HCI 2 hSaponite 1.25% HC1 6 hSaponite 1.25% HC1 24 hSaponite 1.25% HCI 48 h

Saponite 2.5% HC1 2 hSaponite 2.5% HC1 6 hSaponite 2.5% HC1 24 hSaponite 2.5% HC1 48 h

0.469

0.5570.6931 0 5 1

1.085

0.5280.9521.1881.192

0.717No peakNo peakNo peak

solution of Fe(III) is more difficult. Due to the insol-ubility of silica in acid solutions, a small qua ntity ofthis compound, less than a 5%, is removed in eachtreatment.

The removal of the Mg(II) and Fe(III) octahedralcations in griffithite is simi lar to that of Mg(II) durin gacid treatme nt of sepiolite, but is much easier than theremoval of Al(III) in montm orill onite , which can affectthe properties of the solids obtained after treat ment ofgriffithite. In this sense, we have observed that only0.1% of Al(III) and 2.5% of Mg(II) cations are re movedwhen treating Gador montmor il lonite with 0.34 tool.liter -~ HC1 for 48 hour s (Vicente Rodr iguez 1994). Onthe other hand, Suquet et al (1991) found an almostcomplete rem oval o f AI(III) and Mg(II) cations whentreating Llano vermiculit e with 2.0 mol.l iter -~ HCIsolutions at 80 C for two hours. All these results con-firm that the attackability of these silicates under acidleaching depends both on their structure and chemicalcomposition.

X ray diffraction

The X-ray diffractograms of the series treated with1.25% HC1 are given in Figure 1, in which the n atur al

saponite is include d for comparativ e purposes. W henthe time of treatment was increased, the crystallinityof the samples decreased, as can be qualitatively seenin the m ain peak corres ponding to the (001) reflectionof the silicate at about 15.0/~, which became broad erand less intense as the time of treatment progressed.This loss ofcrystall inity was evalu ated using the f.w.h.m.index (Table 2), which increases as acid treatm ent pro-gressed. Dur ing the tre atment there was an increase inthe quantity of free silica in the samples, as shown bythe appearance an d increase of the broad characteristicban d of this com pou nd situated between 20 < 20

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Acid Activation of a Ferrous Saponite (Griffithite)