C.A. Strydom

DTA and FT-IR analysis of the rehydration of basic magnesium carbonate

The rehydration characteristics of a commercially produced hydromagnesite and two basic magnesium carbonates synthetically produced from Mg(OH)2, are presented. The products were dehydrated and dehydroxylated at 325°C before rehydration was attempted. DTA and FT-IR were used to follow the structural changes that occurred during the rehydration processes. The results obtained for the commercially and synthetically produced hydromagnesite products indicated that the original symmetry of the groups was reclaimed during rehydration. This was not observed for the synthetically produced unidentified basic magnesium carbonate product. This investigation provides insight into the rehydration characteristics of a select group of basic magnesium carbonates.

Thermogravimetric analysis of the reaction between carbon and CaSO4·2H2O, gypsum and phosphogypsum in an inert atmosphere

Abstract Heating stoichiometric amounts of carbon and pure CaSO 4 , synthetic gypsum or phosphogypsum in a nitrogen atmosphere, results in the formation of CaS between 700°C and 1100°C. Different heating rates were used to investigate the reaction, and the amount of CaS formed depends on the heating rate used. A quantitative XRD method was used to determine the amounts of CaSO 4 , CaS, CaO and C in the samples. More CaS formed with increasing heating rate. Addition of 5% Fe 2 O 3 and 5% ZnO as catalysts lowers the temperature range, as well as the activation energy of the reaction. The relationship between the activation energy values and degree of conversion ( α ) for the reaction between carbon and CaSO 4 indicates that it is a complex reaction, and that simultaneous competitive reactions are taking place.

X-ray photoelectron spectroscopy studies of some cobalt(II) nitrate complexes

X-ray photoelectron spectra of Co(NO3)2·2L with L = tdpo, tppo and tpyrp, were recorded and interpreted. It seems that in these complexes tpyrp is the strongest electron donor, followed by tdpo and then tppo. In these complexes a stronger electron donor ligand results in a more ionic Conitrate bond, and it could thus result in a less stable complex. Paramagnetic CoII satellite structures were observed. The modified Auger parameters (α′) were calculated and used to compare the polarizability of the compounds to each other. Co(NO3)2·2tpyrp seems to be the most polarizable of the three complexes, and Co(NO3)2·2tppo the least polarizable.

An investigation into the effect of various chemical and physical treatments of a South African phosphogypsum to render it suitable as a set retarder for cement

The work describes various physical and chemical treatments to eliminate the deleterious effects of impurities in phosphogypsum on the delayed setting time and impaired strength development behaviour of cement to which it was added as a set regulator. The physical treatments included washing, milling, and ultrasonic treatment of the material, while the chemical treatments dealt with acidic and basic additions to the phosphogypsum during the washing stage. It was found that chemical treatment with a milk of lime solution, which is often recommended in literature, was ineffective in reducing set retardation. Treatment with ammonium hydroxide or sulphuric acid was more effective in this regard. Intergrinding phosphogypsum with slaked lime improved its effectiveness in reducing set retardation, but the use of unslaked lime was less effective and also resulted in marked reductions in compressive strengths. A combined treatment of wet milling phosphogypsum with a lime slurry in a ball mall was derived from these experiments and is recommended for full-scale plant applications.

The thermal decomposition of cerium(III) nitrate

The thermal decomposition of anhydrous Ce(NO3)3 has been studied. The thermal decomposition reaction is described by the second order kinetic equation, [1/(1−α)]−1=kt. The apparent activation energy was determined asEa=104 kJ mol−1 while the enthalpy of the reaction was estimated asδHr=111.1 kJ mol−1. The decomposition reaction differs from that observed for Nd(NO3)3.ZusammenfassungDie thermische Zersetzung von wasserfreiem Ce(NO3)3 wurde untersucht. Die thermische Zersetzung wird durch die Geschwindigkeitsgleichung zweiter Ordnung[1/(1−α)]−1=kt, beschrieben. Für die scheinbare Aktivierungsenergie wurde ein Wert von 104 kJ mol−1 und für die Enthalpie der Reaktion ein Wert von 111,1 kJ mol−1 ermittelt. Die Zersetzungsreaktion unterscheidet sich von der für Nd(NO3)3.РЕжУМЕИжУЧЕНО тЕРМИЧЕскОЕ РАжлОжЕНИЕ БЕжВОДНО гО НИтРАтА цЕРИь. РЕАкцИ ь тЕРМИЧЕскОгО РАжлОжЕНИь ОпИсыВАЕ тсь кИНЕтИЧЕскИМ УРА ВНЕНИЕМ ВтОРОгО пОРьДкА: [1/(1−А)]− 1=kt. ОпРЕДЕлЕНы кАжУЩАьсь ЁНЕРгИь Ак тИНАцИИЕa, РАВНАь 104 кД ж · МОль−1, И ЁНтАльпИь РЕ АкцИИδHr, РАВНАь 111.1 кДж · МОль−1. Р ЕАкцИь РАжлОжЕНИь От лИЧАЕтсь От НАБлУДАЕМОИ Дль НИ тРАтА НЕОДИМА.

Thermogravimetric studies of the synthesis of cas from gypsum, CaSo4·2H2O and phosphogypsum

Using a heating rate of 2°C min−1, CaS reacts with oxygen in air from 700°C to form CaSO4, with a complete conversion at 1100°C. Synthesis of CaS from the reaction between CaSO4 containing compounds and carbon compounds in air would not be possible, as the carbon reacts from 600°C with oxygen in the air to give CO2. Heating stoichiometric amounts of carbon and pure CaSO4, synthetic gypsum or phosphogypsum in a nitrogen atmosphere, results in the formation of CaS from 850°C. Using a heating rate of 10°C min−1, the formation of CaS is completed at 1080°C. Addition of 5% Fe2O3 as a catalyst lowers the starting temperature of the reaction to 750°C. Activation energy values at different fraction reaction values (α) differ between 340 and 400 kJ mol−1. The relationship between the activation energy values and conversion (α) indicates that the reaction proceeds via multiple steps.

Kinetics of the thermal dehydrations and decompositions of some mixed metal oxalates

Both isothermal and programmed temperature experiments have been used to obtain kinetic parameters for the dehydrations and the decompositions in nitrogen of the mixed metal oxalates: FeCu(ox)2·3H2O, CoCu(ox)2·3H2O and NiCu(ox)2·3.5H2O, [ox=C2O4]. Results are compared with those reported for the thermal decompositions of the individual metal oxalates, Cuox, Coox·2H2O, Niox·2H2O and Feox·2H2O. X-ray photoelectron spectroscopy (XPS) was also used to examinee the individual and the mixed oxalates.Dehydrations of the mixed oxalates were mainly deceleratory processes with activation energies (80 to 90 kJ·mol−1), similar to those reported for the individual hydrated oxalates. Temperature ranges for dehydration were broadly similar for all the hydrates studied here (130 to 180°C).Decompositions of the mixed oxalates were all complex endothermic processes with no obvious resemblance to the exothermic reaction of Cuox, or the reactions of physical mixtures of the corresponding individual oxalates.The order of decreasing stability, as indicated by the temperature ranges giving comparable decomposition rates, was NiCu(ox)2>CoCu(ox)2>FeCu(ox)2, which also corresponds to the order of increasing covalency of the Cu−O bonds as shown by XPS.ZusammenfassungSowohl isotherme als auch programmierte Temperaturexperimente wurden zur Ermittlung der kinetischen Parameter der in Stickstoff erfolgenden Dehydratation und Zersetzung folgender Metallmischoxalate eingesetzt: FeCu(ox)2·3H2O, CoCu(ox)2·3H2O und NiCu(ox)2·3.5H2O [ox=C2O4]. Die Ergebnisse wurden mit denen verglichen, die für die thermische Zersetzung der einzelnen Metalloxalate Cuox, Coox·2H2O, Niox·2H2O und Feox·2H2O beschrieben wurden. Zur Untersuchung der Einzel- und Mischoxalate wurde auch Röntgenfotoelektronspektroskopie eingesetzt.Die Dehydratationen der Mischoxalate sind hauptsächlich negativ beschleunigte Prozesse mit einer ähnlichen Aktivierungsenergie wie die individuellen hydratierten Oxalate (80–90 kJ/mol). Das Temperaturintervall für die Dehydratation ist für alle untersuchten Hydrate ähnlich (130 bis 180°C).Die Zersetzungen der Mischoxalate waren alle komplexe endotherme Prozesse ohne sichtbare Ähnlichkeit mit der exothermen Reaktion von Cuox oder mit den Reaktionen der physikalischen Gemische der entsprechenden Einzeloxalate.Die Stabilität, wie sie durch Temperaturintervalle zur Erzielung vergleichbarer Zersetzungsgeschwindigkeit gezeigt wird nimmt in der Reihenfolge NiCu(ox)2>CoCu(ox)2>FeCu(ox)2 ab, was auch mit der steigenden Kovalenz der Cu−O-Bindung in dieser Reihenfolge übereinstimmt.

The thermal decomposition of zirconium sulphate hydrate

Zr(SO4)2 · 5.5H2O decomposes in five steps to tetragonal ZrO2 at 800°C. Dehydration occurs with the loss of 0.5, then 3, then 1 and, in the last dehydration step, 1 molecule of water of hydration. Using a heating rate of 5°C min−1, Zr(SO4)2 decomposes to zirconia from 540°C. This decomposition step consists of at least three intermediate steps that can be derived from the determination of the activation energy values at different transformation degrees (α). Between 800 and 900°C tetragonal zirconia is transformed slowly into monoclinic zirconia which, on cooling to room temperature, remains in the monoclinic structural phase. DTA and DSC measurements showed that 2505 J per g Zr(SO4)2 · 5.5H2O is needed to obtain ZrO2 in the monoclinic structure at 900°C. To obtain tetragonal zirconia at 800°C, 2306 J per g of the initial compound is needed.

The thermal decomposition of (NH4)[VO(O2)2(NH3)]

The compound, (NH4)[VO(O2)2(NH3)], thermally decomposes to ammonium metavanadate, which then decomposes to vanadium pentoxide. Using a heating rate of 5 deg·min−1, the first decomposition step occurs between 74° and 102°C. The transformation degree dependence of the activation energy (α-E) is shown to follow a decreasing convex form, indicating that the first decomposition step is a complex reaction with a change in the limiting stage of the reaction. Infrared spectra indicated that the decomposition proceeds via the gradual reduction of the ratio of the ‘(NH4)2O’ to ‘V2O5’ units from the original 1∶1 ratio in ammonium metavanadate, which may be written as (NH4)2O·V2O5, to V2O5.ZusammenfassungDie Verbindung (NH4)[VO(O2)2(NH3)] wird thermisch zuerst zu Ammoniummetavanadat und im Anschluß zu Vanadiumpentoxid zersetzt. Bei einer Aufheizgeschwindigkeit von 5 grad·min−1 setzt der erste Zersetzungsschritt zwischen 74° und 102°C ein. Die Abhängigkeit der Aktivierungsenergie von der Konversionsrate (α-E) zeigt als Funktion einen abnehmenden konvexen Verlauf, was zeigt, daß der erste Zersetzungsschritt eine komplexe Reaktion ist. IR-Spektren zeigen, daß die Zersetzung über eine allmähliche Abnahme des Verhältnisses “(NH4)2O”/V2O5 verläuft, beginnend bei einem anfänglichen Verhältnis von 1∶1 in Ammoniummetavanadat, was auch als (NH4)2OV2O5 geschrieben werden kann, bis hin zu V2O5.

The thermal decomposition of lanthanum(III), praseodymium(III) and europium(III) nitrates

Abstract The thermal decomposition kinetics of some anhydrous lanthanide nitrates were investigated and compared with those of neodymium nitrate. The kinetics of the decomposition reaction of La(NO 3 ) 3 and Pr(NO 3 ) 3 are described by the contracting area and contracting volume mechanism, respectively. The enthalpy of decomposition amounts to 123.4 and 102.6 kJ mol −1 , respectively. No reversible changes were observed for these two nitrates. The decomposition reaction of Eu(NO 3 ) 3 is similar to that of Nd(NO 3 ) 3 in so far as a reversible change occurs simultaneously with the decomposition reaction, causing a change in the temperature dependence of the rate constant. The enthalpy of decomposition was estimated as 119.6 kJ mol −1 . Decreasing ionic size of the metal ions appears to decrease the thermal stability of the nitrate, as manifested by the values of the temperature of initiation of decomposition.