THE THERMAL DECOMPOSITION OF NH4-MONTMORILLONITES. PART II

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1 (?lay Min. Bull. (1964), 5, 401. THE THERMAL DECOMPOSITION OF NH4-MONTMORILLONITES. PART II J. L. MARTIN VIVALDI, F. GIRELA VILCHEZ AND P. FENOLL HACH-ALI Estaeion Experimental del Zaidin, Avenida de Cervantes, Granada, Spain Received 7th October 1963 ABSTRACT: Conflicting evidence has been published concerning the occurrence of an oxidation exothermic peak on differential thermal curves for NHr clay minerals. The present experiments show that this peak is affected by (a) the clay mineral concerned and (b) the nature of the thermocouple, Pt/PtRh thermocouples apparently acting as good catalysts for oxidation of released ammonia. INTRODUCTION In a previous paper Martin Vivaldi et al. (1959) described the thermal behaviour of NH4-montmorillonites and concluded that their decomposition occurs mainly at about 400~ with loss of ammonia and that some of the ammonia released may be oxidized during the process of decomposition. This oxidation is shown on differential thermal curves as a strong exothermic effect in the ~ region. When an inert atmosphere is used only a normal endothermic peak due to the decomposition of the NH4-complex is observed, the absence of the ex0thermic peak indicating that no oxidation of the NH4 + occurs under these conditions. There are records of differential thermal curves for NH4-saturated samples in which exothermic effects are absent; some of these refer to samples investigated in a ceramic specimen holder (Mackenzie & Bishui, 1958) and some to other, but closely related, clay minerals. For this reason different clay minerals treated in several ways and under various experimental conditions have been studied. As some of the early results are interesting, a brief account of these are given here. RESULTS AND DISCUSSION Specimens of vermiculite from Beni-Buxera (Hoyos de Castro, Gonzalez Garcia & Martin Vivaldi, 1950), sepiolite from Vallecas (Martin Vivaldi & Cane Ruiz, 1953) and montmorillonite from South Dakota ('Volclay') 401

2 402 J. L. Martin Vivaldi et al. were ground and sieved through a 270-mesh ASTM sieve. From their powder diffraction diagrams the sepiolite, montmorillonite and vermiculite are known to be very pure. In the previous experiments (Martin Vivaldi et al., 1959) samples were treated with ammonium acetate, so that some residual acetate could contribute to the exothermic effect on the thermal curves. In view of this the present samples were treated with 1 N ammonium chloride solution and washed with water and methyl alcohol until all CI- was removed. B63 (a) ~ " ",829 ~ r 732 Fro. 1. Differential thermal curves for untreated samples of: (a) vermiculite from Beni-Buxera; (b) sepiolite from Vallecas; (c) montmorillonite from South Dakota. The differential thermal equipment used a heating rate of about 10~ and a nickel sample holder with Pt/Ptl0% Rh thermocouples. The furnace used is similar to that described earlier (Martin Vivaldi, Girela Vilchez & Rodriguez Gallego, 1963), but the refractory tube is now made of quartz, 50 cm long, sealed at each end by a rubber stopper pierced by glass tubing with a stopcock. Thus, it is possible to examine samples in a controlled atmosphere or under vacuum. During the heating cycle an inert atmosphere is maintained by passing a current of 99% pure nitrogen at a rate of 8 l/hr. Fig. 1 shows the differential thermal curves for untreated samples and Fig. 2 shows the curves for NH4-saturated samples in normal and inert atmosphere. The NHa-montmorillonite curve shows the intense exothermic effect in the 500=600~ region which disappears in an inert

3 Decomposition of NH4-montmorillonites. H 403 atmosphere. Ammonium vermiculite shows a similar but less intense exothermic effect and this, too, disappears in an inert atmosphere. There is, however, no exothermic reaction with sepiolite. It may therefore be concluded that the nature of the clay mineral is highly significant in the oxidation of the ammonia evolved.,856 (b) 796, (b9, B ~ ~2 430 g '~ 6z FIG. 2. Differential thermal curve for NHa-saturated samples: (a) and (a') NH4-vermiculite in air and nitrogen, respectively; (b) and (b') NH4-sepiolite in air and nitrogen, respectively; (c) and (c') NH4-montmorillonite in air and nitrogen, respectively. In order to test the influence of the sample holder and the thermocouple on the thermal decomposition and oxidation of the ammonia, the following experiments were performed: (a) To prevent direct contact between the sample and the nickel of the holder each hole in the specimen block was lined with quartz tubing.

4 404 J. L. Martin Vivaldi et al. 569 '~)~ (b) FIG. 3. Differential thermal curves for NH4-montmorillonite: (a) with sample holder coated internally with quartz and Pt/Pt10%R_h thermocouple; (b) with normal sample holder and Fe-constantan thermocouple. The thermal curve for NH4-montmorillonite under these conditions is shown in Fig. 3a. As the exothermic effect still appears the nickel of the sample holder does not seem to affect the oxidation of the ammonia. (b) Secondly, an Fe-constantan thermocouple was substituted for the usual Pt/PtRh one, the galvanometer resistance being modified to get the same sensitivity. Fig. 3b shows the thermal curve obtained for NH4- montmorillonite. It is clearly shown that the thermocouple has a specific action, since the oxidation occurs at a lower temperature and more sharply than with the Pt/PtRh thermocouple. The Pt/PtRh thermocouple was then coated with a thin layer of aluminous cement, and now the exothermic effect no longer appears (Fig. 4a). Since the thermal conductivity of the cement is low, however, the sensitivity is very low. To get more conclusive results a new experiment was run using (a) (b) FIG. 4. Differential thermal curves for NH4-montmoriUonite with Pt[PtRh thermocouple: (a) coated with refractory cement; (b) coated with gold.

5 Decomposition of NH4-montmorillonites. 1I 405 a Pt/PtRh thermocouple coated electrolytically with a thin layer of gold which prevents the catalytic oxidation of ammonia (Berkman, Morrell & Egloff, 1940) but is a good thermal conductor. Curve 4b shows a typical curve for the decomposition of NH4-montmorillonite without any exothermic peak. This demonstrates that the thermocouple plays an important part in the oxidation of the ammonia evolved during decomposition of NH4-montmorillonite. The thermocouple also acts as a catalyst in the oxidation of ammonia evolved in the thermal decomposition of an ammonium salt, as is shown by Figs. 5a and 5b, which demonstrate how the oxidation produced by the Pt/PtRh thermocouple is suppressed when the thermocouple is coated with a refractory cement. (a) 44] (b) 486 FIG. 5. Differential thermal curves for ammonium sulphate (at k sensitivity) ' with (a) Pt/Ptl0 %Rh thermocouple; (b) Pt/Ptl0 %Rh thermocouple coated with refractory cement. CONCLUSIONS In the oxidation of the ammonia evolved when NH4-saturated clay minerals are thermally decomposed both the type of clay mineral and the kind of thermocouple are important. Thus, the oxidation peak on the differential thermal curve is very pronounced when NH4-montmoritlonite is used, bu~ is Iess obvious for vermiculite, and does not appear at all for sepiolite. Pt/Ptl0%Rh and Fe-constantan thermocouples both participate very actively in the oxidation of the ammonia evolved from NH4-montmorillonite and ammonium sulphate, but the catalytic effect is suppressed if these thermocouples are coated with a refractory cement or a thin layer of gold. The nickel of the sample holder does not seem to play any part in the oxidation.

6 406 J. L. Martin Vivaldi et al. REFERENCES BERKMAN S., MORRELL J.C. & EGLOFF G. (1940) Catalysis, Inorganic and Organic. Reinhold, New York. HoYos DE CASTRO A., GONZALEZ GARCIA F. & MARTIN VIVALDI JL. (1950) An. R. Soc. esp. Fis. Quire. 46, 715. MACKENZIE R.C. & BISr~UI B.M. (1958) Clay Min. Bull. 3, 276. MARa~N VIVALD~ J.L. & CANO Rtnz J. (1953) An. Edafol. Fisiol. veg. 12, 827. MARTIN VIVALDI J.L., GIRELA VILCHEZ F., HERNAIZ BERMIJDEZ DE CASTRO M. ~: ROD~Gt~Z GALLEGO M. (1959) Clay Min. Bull. 4, 81. MARTIN VIVALDI J.L., GIRELA V1LCHEZ F. & RODRIGUEZ GALLEGO M. (1963) Rev. Cienc. apl 17, 11 I.