UDC/UDK 546.46:546.27:551.464 ISSN 1580-2949 Original scientific article/Izvirni znanstveni članek MTAEC9, 41(2)95(2007) ACTIVATED SINTERING OF MAGNESIUM OXIDE OBTAINED FROM SEAWATER AKTIVIRANO SINTRANJE MAGNEZIJEVEGA OKSIDA, DOBLJENEGA IZ MORSKE VODE Vanja Martinac, Miroslav Labor, Meri Miroševi}-Anzulovi}, Nedjeljka Petric Faculty of Chemical Technology, Department of Thermodynamics, Teslina 10/V, 21000 Split, Croatia martinacŽktf-split.hr Prejem rokopisa – received: 2006-09-18; sprejem za objavo - accepted for publication: 2006-11-19 The process of isothermally sintering magnesium oxide obtained from seawater by substoichiometric (where precipitation of magnesium hydroxide took place with 80 % of the stoichiometric quantity of dolomite lime) and by overstoichiometric precipitation (which took place with 120 % of the stoichiometric quantity of dolomite lime) was examined, with the addition of mass fractions (1, 2 and 5) % TiO2, at temperatures in the range 1550-1750 °C, with (1, 3 and 5) h of soaking at the maximum temperature. The process was followed by a determination of the product density, as well as of the B2O3 content in the sintered samples. The results indicate that, besides favorably affecting the density of the sintered samples, the addition of TiO2, with increasing temperature and soaking time, also significantly reduces the B2O3 content in the sintered magnesium oxide obtained from seawater. A statistical analysis of the obtained data was performed with the “Statistica” package in order to obtain a model of the density (p) of sintered samples of magnesium oxide relative to the temperature (t), time of isothermal heating (t), and the percentage of added TiO2 (w). Key words: substoichiometric precipitation, overstoichiometric precipitation, magnesium oxide from seawater, TiO2 addition, activated sintering Proučevan je proces izotermnega sintranja magnezijevega oksida, dobljenega iz morske vode z nestehiometrijskim (dodatek dolomitnega apna je bil 80 % od stehiometrijsko potrebne količine) in prestehiometrijskim usedanjem (z dodatkom 120 % od stehiometrijske količine dolomitnega apna) z dodatkom masnih deležev TiO2 (1, 2 in 5) %, v temperaturnem intervalu 1550-1750 °C, v trajanju izotermnega sintranja (1, 3 in 5) h. Proces je bil spremljan z določevanjem gostote in določevanjem vsebnosti B2O3 v sintranih vzorcih. Rezultati preučevanja kažejo, da dodatek TiO2 pozitivno deluje na gostoto sintranih vzorcev, s povečanjem temperature in obdobjem izotermnega sintranja pa občutno vpliva tudi na zmanjšanje B2O3 v sintranem magnezijevem oksidu, dobljenem iz morske vode. Da bi dobili model odvisnosti gostote (p) sintranih vzorcev magnezijevega oksida od temperature (t) in časa izotermnega sinteriranja (t), ter deleža dodanega TiO2 (w) je bila narejena statistična obdelava dobljenih podatkov z uporabo paketa “Statistica”. Ključne besede: nestehiometrijsko usedanje, prestehiometrijsko usedanje, magnezijev oksid iz morske vode, TiO2 dodatek, aktivirano sintranje 1 INTRODUCTION Magnesia1 (MgO) is a very important material for the refractory industry. Due to its high refractory properties (MgO melts at (2823 ± 40) °C), MgO ceramics are chemically inert, resistant to the effect of metal melts, acid gases, alkali slag, neutral salts, and react with carbon only above 1800 °C. Raw magnesites for refractory use are obtained from natural ore or are synthetically processed from seawater. Magnesium oxide obtained from seawater29 is a high-quality refractory material, and its advantages lie not only in the huge reserves of seawater10 (1 m3 contains 0.945 kg of magnesium), but in the higher purity of the sintered magnesium oxide (> 98 % MgO). The magnesium oxide used here was obtained from seawater either by substoichiometric (MgO 80 % pptn - where pptn stands for precipitation) or overstoichiometric (MgO 120 % pptn) precipitation of magnesium hydroxide in seawater using dolomite lime. The purpose of this work was to examine the effect of TiO2 additions on the properties (density and the B2O3 content) of sintered samples of magnesium oxide obtained from seawater at elevated temperatures. The examinations described were carried out in order to obtain a model of the density of sintered samples of magnesium oxide relative to the temperature and the time of isothermal heating and the percent of added TiO2. 2 EXPERIMENTAL The composition of the seawater used for the precipitation of magnesium hydroxide was: MgO = 2.124 g dm3; CaO = 0.5573 g dm–3 The composition of the dolomite lime used as the precipitation agent was as follows (mass fractions, w/%): CaO = 57.17 % MgO = 42.27 % SiO2 = 0.099 % Al2O3 = 0.051 % Fe2O3 = 0.079 % The experimental procedure used to obtain magnesium hydroxide from seawater was similar to that employed in our previous investigations8,9. The sedimentation rate was increased by the addition of the Materiali in tehnologije / Materials and technology 41 (2007) 2, 95-98 95 V. MARTINAC ET AL.: ACTIVATED SINTERING OF MAGNESIUM OXIDE OBTAINED FROM SEAWATER optimum amount of the anionic 818A flocculent (polyacrylamide), produced by the Dutch firm Hercules. The experimental procedure used to determine the optimum quantity of the anionic 818A flocculent has been described in a previous study11. The magnesium hydroxide obtained from seawater was dried at 105 °C and then calcined at 950 °C. The composition of the magnesium oxide obtained by precipitation with 80 % of the stoichiometric quantity of dolomite lime was (mass fractions, w/%): 98.76 % MgO, 0.88 % CaO and 0.1934 %B2O3. The composition of the magnesium oxide obtained by precipitation with 120 % of the stoichiometric quantity of dolomite lime was (mass fractions, w/%): 97.92 % MgO , 1.45 % CaO and 0.1055 % B2O3. Mixtures of magnesium oxide were then prepared with the addition of a mass fraction of 1 %, 2 % and 5 % TiO2. The doping oxide used was analytical reagent grade titanium (TiO2 p. a.) in the rutile form, produced by Merck. The samples were homogenized by manual stirring in absolute ethanol (C6H5O p. a.). The mixtures were cold pressed into compacts in a hydraulic press at a pressure of 625 MPa. The compacts were then sintered in a gas furnace, made by a French firm Mecker (type 553), with a zirconium (IV)-oxide lining, at 1550 °C, 1650 °C and 1750 °C with 1h, 3h and 5h of soaking at the maximum temperature. It took approximately 2 h to reach the maximum temperature in the furnace. After sintering, the samples were left to cool in the furnace. The sample density after sintering (p) was determined from the volume of water displaced from a calibrated cylinder. The boron content in the magnesium oxide samples examined was determined potentiometrically. The variation coefficient for the method used was ± 1 %12. The results shown represent the average of a number of measurements. 3 RESULTS AND DISCUSSION Tables 1 and 2 show the experimentally obtained values for the density under the operating conditions described for sintered magnesium oxide samples (80 % precipitation) and magnesium oxide samples (120 % precipitation). The results indicate that at higher temperatures, such as 1550 °C, 1650 °C and 1750 °C, the increase in the density of the samples of magnesium oxide obtained from seawater is not very significant when compared to the un-doped samples, even though they were sintered under the same thermal conditions. Therefore, at higher temperatures in the range 1550-1750 °C, the effect of the TiO2 addition is less evident because the concentration of the added ions is evenly distributed over the whole grain mass. One can assume that the mass transfer is the same as with pure MgO, determined by the diffusion of O2 ions through the MgO lattice as the slower diffusion species. The increased temperature leads to the increased mobility of the elements forming the crystal lattice, a contact surface 96 Table 1: Density (p) of sintered magnesium oxide samples (80 % precipitation) with the addition of a mass fraction of 1 %, 2 % and 5 % TiO2, and no sintering aid, t = 1550 °C, 1650 °C, 1750 °C, T = 1 h, 3 h, 5 h, p = 625 MPa Tabela 1: Gostota (p) za sintrane vzorce magnezijevega oksida (80-odstotno usedanje) z masnimi deleži TiO2 in 1, 2 in 5 %, in brez dodatka TiO2, t = 1550 °C, 1650 °C, 1750 °C, t = 1 h, 3 h, 5 h, p = 625 MPa sample t/°C T/h p/(g cm 3) TiO2 addition, w/% no sint. aid 1% 2% 5% 80% pptn 1550 1 3 5 3.2401 3.3181 3.3514 3.2691 3.3498 3.3643 3.3243 3.3439 3.3953 3.3361 3.3744 3.4017 1650 1 3 5 3.3463 3.3513 3.4230 3.3798 3.3803 3.4128 3.4115 3.4245 3.4385 3.4257 3.4351 3.4510 1750 1 3 5 3.3522 3.3660 3.4244 3.4007 3.4593 3.4881 3.4199 3.4513 3.4933 3.4391 3.4719 3.5289 Table 2: Density (p) of sintered magnesium oxide samples (120 % precipitation) with the addition of a mass fraction of 1 %, 2 % and 5 % TiO2, and no sintering aid, t = 1550 °C, 1650 °C, 1750 °C, T = 1 h, 3 h, 5 h, p = 625 MPa Tabela 2: Gostota (p) za sintrane vzorce magnezijevega oksida (120-odstotno usedanje) z masnimi deleži TiO2 in 1, 2 in 5 %, in brez dodatka TiO2, t = 1550 °C, 1650 °C, 1750 °C, t = 1 h, 3 h, 5 h, p = 625 MPa sample t/°C T/h p/(g cm 3) TiO2 addition, w/% no sint. aid 1% 2% 5% 120% pptn 1550 1 3 5 3.2880 3.3179 3.3739 3.3088 3.3557 3.3887 3.3980 3.4156 3.4409 3.4036 3.4383 3.4619 1650 1 3 5 3.3350 3.3570 3.4244 3.3726 3.4029 3.4412 3.4253 3.4378 3.4662 3.4357 3.4603 3.5047 1750 1 3 5 3.3711 3.4153 3.4591 3.4033 3.4516 3.4786 3.4292 3.4860 3.5018 3.4473 3.5180 3.5434 is formed between the particles of compacted powder, the porosity is eliminated, and the whole system shrinks1315. Tables 3 and 4 show the results of the examination of the effect of TiO2 on the boron content in the sintered magnesium oxide samples (80 % and 120 % precipitation) for the operating conditions described. The results indicate that the TiO2 addition significantly influences the reduction of the B2O3 content in the process of the isothermal sintering of magnesium oxide from seawater, even at higher temperatures. The different behavior of the magnesium oxide obtained by 80 % from that obtained by 120 % precipitation of magnesium hydroxide in seawater is due to the different contents of CaO in these samples. CaO retains the B2O3 in the samples during sintering. With MgO (120 % precipitation) the CaO content is much higher (w = 1.45 %) than with MgO (80 % precipitation) (w = 0.88 %), Materiali in tehnologije / Materials and technology 41 (2007) 1, 95-98 V. MARTINAC ET AL.: ACTIVATED SINTERING OF MAGNESIUM OXIDE OBTAINED FROM SEAWATER Table 3: Effect of TiO2 on B2O3 content in the sintered magnesium oxide samples (80 % precipitation) at t = 1550 °C, 1650 °C, 1750 °C r = 1 h, 3 h, 5 h, p = 625 MPa Tabela 3: Vpliv TiO2 na vsebnost B2O3 v sintranih vzorcih magnezijevega oksida (80-odstotno usedanje) pri t = 1550 °C, 1650 °C, 1750 °C, T = 1 h, 3 h, 5 h, p = 625 MPa sample t/°C T/h B2O3, w/% TiO2 addition, w/% no sint. aid 1% 2% 5% 80% pptn 1550 1 3 5 0.1894 0.1337 0.0759 0.1249 0.1075 0.0637 0.0913 0.0193 0.0145 0.0245 0.0174 0.0132 1650 1 3 5 0.1445 0.1020 0.0589 0.0750 0.0640 0.0257 0.0272 0,0161 0.0138 0.0216 0.0096 0.0085 1750 1 3 5 0.0862 0.0415 0.0319 0.0594 0.0261 0.0126 0.0193 0.0116 0.0090 0.0158 0.0074 0.0055 and favors the Ca2B2O5 formation reaction. In a previous paper16 the X-ray diffraction method was used to prove the content of di-calcium borate (Ca2B2O5) in sintered samples of magnesium oxide from seawater. Therefore, in the process of sintering, B2O3 reacts with CaO to form Ca2B2O5. In papers1720 the X-ray diffraction method and EDAX analysis have helped determine that during the sintering process the added TiO2 reacts with CaO from the solid MgO-CaO solution and forms calcium titanate CaTiO3. Thus, TiO2 binds a part of the CaO in the CaTiO3 and as a result reduces the content of CaO that reacts with B2O3. In this way a smaller quantity of Ca2B2O5 remains in the sintered sample, i.e., a higher quantity of B2O3 evaporates. With MgO (120 % precipitation) there is an excess of CaO, which favors the formation of Ca2B2O5, while with MgO (80 % precipitation) a larger part of the B2O3 evaporates from the sample into the air during sintering. A positive effect of the TiO2 addition on the reduction of the B2O3 content in the magnesium oxide obtained from seawater makes it possible to achieve a high-purity product, because the hot-strength properties of certain refractory products are significantly affected by their boron content. The results shown in Tables 1 and 2 have also been considered in a regression analysis. The "Statistica" package was used to analyze statistically the data obtained, in order to obtain a model of the density of the sintered samples of magnesium oxide relative to the temperature (t), the time of isothermal sintering (t), and the mass fraction of TiO2 added (w). A model of multiple regression has been proposed, p = o + 1 t + ß21 + 3 ln (w+1) + s for MgO samples (80 % precipitation) and for MgO samples (120 % precipitation), where p is the density (g cm–3), t is the temperature (°C), t is the time of isothermal sintering (h), w is the mass fraction (%) of added TiO2, o, 1, 2, 3 are Materiali in tehnologije / Materials and technology 41 (2007) 2, 95-98 unknown coefficients, and s is the random error in the model. Table 4: Effect of TiO2 on B2O3 content in the sintered magnesium oxide samples (120 % precipitation) at t = 1550 °C, 1650 °C, 1750 °C r = 1 h, 3 h, 5 h, p = 625 MPa Tabela 4: Vpliv TiO2 na vsebnost B2O3 v sintranih vzorcih magnezijevega oksida (120-odstotno usedanje) pri t = 1550 °C, 1650 °C, 1750 °C, t = 1 h, 3 h, 5 h, p = 625 MPa sample t/°C T/h B2O3, w/% TiO2 addition, w/% no sint. aid 1% 2% 5% 120% pptn 1550 1 3 5 0.0705 0.0582 0.0312 0.0618 0.0354 0.0270 0.0360 0.0264 0.0245 0.0222 0.0119 0.0058 1650 1 3 5 0.0592 0.0315 0.0180 0.0405 0.0286 0.0171 0.0331 0.0254 0.0106 0.0148 0.0068 0.0055 1750 1 3 5 0.0331 0.0177 0.0148 0.0312 0.0119 0.0100 0,0216 0,0106 0.0084 0.0093 0.0046 0.0032 Table 5: Correlation matrix for MgO (80 % precipitation) samples Tabela 5: Matrica korelacije za vzorce MgO (80 - odstotno usedanje) variable correlations (MgO (80 % pptn) samples) t/°C T/h TiO2 addition, w/% density, p/(g cm–3) t/oC 1.00 0.00 -0.00 0.69 T/h 0.00 1.00 -0.00 0.46 TiO2 addition, w/% -0.00 -0.00 1.00 0.43 density, p/(g cm-3) 0.69 0.46 0.43 1.00 Table 6: Correlation matrix for MgO (120 % precipitation) samples Tabela 6: Matrica korelacije za vzorce MgO (120 - odstotno usedanje) variable correlations (MgO (120 % p ptn) samples) t/°C T/h TiO2 addition, w/% density, p/(g cm-3) t/°C 1.00 0.00 -0.00 0.53 T/h 0.00 1.00 -0.00 0.51 TiO2addition, w/% -0.00 -0.00 1.00 0.60 density, P /(g cm-3) 0.53 0.51 0.60 1.00 Tables 5 and 6 show the correlation matrices for the examined samples. The results indicate that there is no correlation between the independent variables (temperature t, time t and percentage of addition of TiO2). The estimate for the regression function for MgO (80 % precipitation) is: p = 2.461 + 0.000512 T+ 0.0173 r + + 0.0434 ln (w + 1) where s = 0.01899; R2 = 0.9057; F(3, 104) = 333.2251; p = 0.00 The estimate for the regression function for MgO (120 % precipitation) is: p = 2.686 + 0.000382 T + + 0.0181 r + 0.0567 ln (w + 1) where s = 0.01413; R2 = 0.9433; F(3, 104) = 576.8233; p = 0.00 97 V. MARTINAC ET AL.: ACTIVATED SINTERING OF MAGNESIUM OXIDE OBTAINED FROM SEAWATER The regression functions and the regression coefficients are significant at the level of p = 0.00 in both models. The statistical analysis indicates that these models are useful for an estimation of the density of the samples examined. 4 CONCLUSIONS The statistical analysis using the Statistica package indicates that the proposed models are useful and acceptable for an estimation of the density of the sintered magnesium oxide samples relative to the temperature (t), the time of the isothermal heating (t), and the percent of added TiO2 (w). The favorable effect of the TiO2 addition on the reduction of the B2O3 content in the sintered magnesium oxide obtained from seawater is due to two interdependent reactions of the formation of Ca2B2O5 and CaTiO3, which lead to a reduction in the B2O3 content during sintering. 5 LITERATURE 1 Shand M. A., The Chemistry and Technology of Magnesia, Willey Interscience, N.York, 2006 2Frith M., Buttrey T., Strawbridge I., Brit. Ceram. Trans., 97 (1998), 29 3 Sims C, Industr. Mnerals, 7 (1997), 21 4Gilpin W. C, Heasman N., Chem. 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