UDK 546.46:547.27:551.464 Izvirni znanstveni članek ISSN 1580-2949 MTAEC 9, 36(5)395(2002) V. MARTINAC ET AL.: THE EFFECT OF THE pH OF THE RINSING SOLUTION ... THE EFFECT OF THE pH OF THE RINSING SOLUTION ON THE PROPERTIES OF MAGNESIUM OXIDE FROM SEAWATER VPLIV pH RAZTOPINE ZA IZPIRANJE NA LASTNOSTI MAGNEZIJEVA OKSIDA IZ MORSKE VODE Vanja Martinac, Miroslav Labor, Nedjeljka Petric Faculty of Chemical Technology, Department of Thermodynamics, Teslina 10/V, 21000 Split, Croatia vanja.martinacŽktf-split.hr Prejem rokopisa - received: 2002-09-26; sprejem za objavo - accepted for publication: 2002-11-04 We have investigated the effect of the pH of the rinsing agent on the content of B2O3, CaO and MgO in magnesium oxide obtained from seawater by substoichiometric precipitation (where the precipitation of magnesium hydroxide took place with 80% of the stoichiometric quantity of dolomite lime as the precipitation agent). In such a case, i.e. when this precipitation method is employed, the boron content adsorbed onto the magnesium hydroxide during the precipitation process is somewhat higher than during stoichiometric precipitation, and should therefore be reduced. The purpose of the study was to ensure high-purity magnesium oxide, particularly with respect to the B2O3 content, because boron causes problems in refractory magnesia for specialized refractory applications where a high hot-strength is required. The rinsing agents were alkalized distilled water with pH = 11.0 and 12.5, which were alkalized by the addition of concentrated NaOH prior to each use, and non-alkalized distilled water with pH = 5.95. It was established that the content of B2O3 in magnesium oxide samples is significantly reduced when the pH value of the agent used for rinsing the magnesium hydroxide precipitate increases. The magnesium oxide samples obtained were then sintered with the addition of TiO2 (1, 2 and 5 mass %) in order to determine the properties of the sintered samples (density, porosity, the B2O3 content) relative to the method used for rinsing the magnesium hydroxide precipitate and to the quantity of added TiO2. The results indicate that all the samples, sintered at 1650 °C for 3 hours, had very low porosity, and densities up to 95 % of the theoretical density. Also, the B2O3 content in the products was much lower than in the samples prior to sintering. Key words: substoichiometric precipitation, rinsing solution, magnesium oxide, seawater, TiO2 addition, activated sintering Raziskovan je bil vpliv pH raztopine za izpiranje na vsebnost B2O3, CaO in MgO v magnezijevem oksidu, dobljenem iz morske vode z nestehiometričnim načinom usedanja (usedanje je napravljeno z dodatkom 80-odstotne stehiometrične količine dolomitnega apna kot usedalnega reagenta). Pri tem načinu usedanja je količina bora, ki se adsorbira na usedlino magnezijeva hidroksida med usedanjem, nekoliko večja v odnosu na stehiomatrično usedanje in jo je treba zmanjšati. Namen postopka je dobiti magnezijev oksid visoke čistote, predvsem glede na vsebnost B2O3, ker bor povzročatežave v ognjevarnem magnezijevem oksidu za posebne namene, ker je potrebna visoka čvrstota (raziskovanje na toplo). Sredstvo za izpiranje je lužnata destilirana voda pH = 11,0 in 12,5, ki je lužena z dodatkom koncentrirane NaOH pred vsako uporabo, kot tudi destilirana voda pH = 5,95. Dokazano je, da se vsebnost B2O3 v vzorcih magnezijevegaoksidaobčutno zmanjšuje s povečanjem pH-vrednosti sredstva za izpiranje usedline magnezijevega hidroksida. Dobljeni vzorci magnezijevega oksida so potem sintrani z dodatkom TiO2 (masni deleži: 1, 2 in 5 %) z namenom, da se določijo lastnosti sintranih vzorcev (gostota, poroznost, vsebnost B2O3), odvisno od načinaizpiranjausedline magnezijevegahidroksidain količine dodanega TiO2.Rezultati raziskovanja kažejo, da imajo vsi vzorci, sintrani pri 1650 °C v trajanju 3 ur, zelo majhno poroznost, a dosežene gostote so do 95 % teoretične. Tudi vsebnost B2O3 je v produktu precej manjša, kot je v vzorcih pred sintranjem. Ključne besede: nestehiometrično usedanje, sredstvo za izpiranje, magnezijev oksid, morska voda, dodatek TiO2, aktivirano sintranje 1 INTRODUCTION Magnesium oxide is one of the most important materials used in the production of high-temperature-resistant ceramics. Besides its high refractoriness, MgO ceramic is non-toxic and chemically inert in basic environments at elevated temperatures. Today, in large-scale technical processes, magnesia (MgO) for refractories is produced from three basic sources: natural magnesite, extraction from seawater and extraction from inland brine. The production of magnesium oxide from seawater is a well-know industrial process 1-8 and has been studied all over the world for a number of years. In principle it is chemically a very simple process, requiring only the addition of an alkaline base, such as calcined dolomite or calcined limestone, to precipitate the magnesium salts present in seawater as magnesium hydroxide. The precipitate is then washed and calcined to form caustic magnesia. The apparently simple chemistry of the process is unfortunately complicated in practice because seawater is not a pure solution of magnesium salts and dolomite or limestone, although abundant, are never found free of impurities. Taking into consideration that B2O3 is acommon impurity in seawater-derived magnesia, the purpose of this study was to examine the effect of the pH of the rinsing solution as well as the possibility of adding TiO2 in quantities of 1, 2 and 5 mass % for reducing the boron content in the product, i.e. magnesium oxide sintered at 1650 °C. MATERIALI IN TEHNOLOGIJE 36 (2002) 6 395 V. MARTINAC ET AL.: THE EFFECT OF THE pH OF THE RINSING SOLUTION 2 EXPERIMENTAL The composition of the dolomite lime used for precipitating the magnesium hydroxide from seawater was as follows (mass %): MgO = 40.90 % CaO = 57.89 % SiO2 = 0.102 % Al2O3 = 0.866 % Fe2O3 = 0.319 % and the composition of the seawater was as follows: MgO = 2.179 g dm-3 ; CaO = 0.556 g dm-3. In order to prevent lime contamination on the precipitate of magnesium hydroxide the seawater was pretreated to remove bicarbonate and carbonate ions. This was done by acidifying the seawater with sulfuric acid to approximately pH 4, followed by the removal of the liberated carbon dioxide by aeration in a desorption tower. The flow rate of the induced air was 120 dm3 h-1, and the volumetric flow rate of the seawater through the desorption tower was 6 dm3 h-1. The chemical reactions are: Ca2+ + CO32- + H+ + HSO4- = = Ca2+ + SO42- +H2O+CO2(aq) (1) Ca2+ + 2HCO3- + H+ + HSO4- = = Ca2+ + SO42- + 2H2O+CO2(aq) (2) The calcium sulfate formed remained in the solution. After the pretreatment of the seawater, a calculated amount of dolomite lime was added to precipitate the magnesium hydroxide. The experiments were carried out with substoichio-metric precipitation, with the addition of 80% of the stoichiometric quantity of dolomite lime. The experimental procedure was similar to that employed in our previous investigation 9. The rinsing agent was: – distilled water, pH = 5.95 – alkalized distilled water, pH = 11.00 and 12.50, which was alkalized by the addition of concentrated NaOH. The magnesium hydroxide thus obtained was dried at 105 °C and then calcined at 950 °C for 5 h to form caustic magnesia. Mixtures of magnesium oxide were then prepared with 1, 2 and 5 mass % of added TiO2. The dopant oxide used was analytical reagent grade titania (TiO2 p.a.) in the rutile form, produced by Merck. Its analysis is shown in Table 1. Samples were homogenized by manual stirring in absolute ethanol (C6H5O p.a.) for 30 min. The mixtures were then dried at 80 °C until all the alcohol had evaporated. The mixtures were cold pressed into compacts in a hydraulic press at a pressure of 625 MPa. The compacts were then sintered at 1650 °C for aduration of ? = 3 h. The sintering was carried out in a gas furnace, made by a French firm, Mecker (type 553), with azirconium (IV)-oxide lining. It took approximately 2 h to rea ch the ma ximum tempera-396 ture in the furnace. After sintering, the samples were left to cool in the furnace. The sample density after sintering (?) was determined from the volume of water displaced from a calibrated cylinder. The total (Pt), apparent (Pa) and closed (Pc) porosities in the samples examined were determined according to standard methods (HRN: B. D8. 302, B. D8. 312, B. D8. 313). The boron content in the samples examined was determined potentiometrically. The variation coefficient for the method applied was ±1 % 10. The results listed represent an average value of a number of measurements - an average of five analyses in each case. Table 1: Chemical analysis (mass %) of TiO2 p.a. (Merck) Tabela 1: Kemijska analiza (masni delež) TiO2 p. a. (Merck) TiO2 (99 %) Water-soluble matter 0.3 % Chloride (Cl) 0.01 % Sulphate (SO4) 0.05 % Heavy metals (such as Pb) 0.001 % Iron (Fe) 0.005 % Arsenic (As) 0.0002 % 3RESULTS AND DISCUSSION Table 2 shows the operating conditions during the rinsing of the magnesium hydroxide precipitate, as well as the experimental results for the determination of the chemical composition of the magnesium oxide obtained by precipitation with 80 % of the stoichiometric quantity of dolomite lime. The above results indicate that the method of rinsing the magnesium hydroxide precipitate significantly affects the chemical composition of these samples, primarily in terms of the CaO and B2O3 contents in the calcined magnesium oxide. Rinsing the magnesium hydroxide precipitate with alkalized water, especially at pH = 12.50, contributes to a noticeable reduction in the quantity of boron adsorbed in the final product, i.e. calcined magnesium oxide. Table 2: Chemical composition (mass %) of magnesium oxide (80 % precipitation) after calcining at 950 °C / 5 h Tabela 2: Kemijska sestava (masni delež) magnezijevega oksida (80-odstotno usedanje) po kalcinaciji na 950 °C / 5 h Rinsing agent pH of the rinsing water CaO MgO B2O3 mass % distilled water 5.95 0.85 98.62 0.1764 alkalized distilled water 11.00 0.95 97.89 0.1198 12.50 1.28 97.83 0.0518 Boron occurs in seawater, partly as non-dissociated orthoborate acid (H3BO3) and partly as borate ions (H2BO3-, HBO32- and BO33-), and during the magnesia-precipitation process boron is adsorbed onto the MATERIALI IN TEHNOLOGIJE 36 (2002) 6 V. MARTINAC ET AL.: THE EFFECT OF THE pH OF THE RINSING SOLUTION magnesia. H3BO3 is a weak acid with the following dissociation constants 11: H3BO3 = H+ + H2BO3- K1 = 5.8·10-10 (3) H2BO3=H++HBO32- K2 = 1.8·1013 (4) HBO32- = H+ + BO33- K3 = 1.6·10-14 (5) The increased pH in the rinsing agent increases the degree of dissociation of the orthoborate acid. At pH 12.50 the first-stage dissociation is complete, with about 1/3 dissociated in the second stage, while the BO33-concentration is very low. Owing to the alkalinity of the medium, H2BO3- ions are adsorbed to a much lesser degree. This can be explained by the fact that the increased pH of the medium (12.50) when rinsing the magnesium hydroxide precipitate with alkalized distilled water first affects the preferred adsorption of OH- ions, and consequently the reduction of B2O3 in the final product (in seawater-derived magnesia). However, the increased pH also increases the adsorption of Ca2+ ions onto the Mg(OH)2 precipitate, as the stability of Ca(OH)2 is higher in a very alkaline medium, i.e. at pH 12-13. In order to determine the properties of the samples examined relative to the precipitate-rinsing method and the B2O3 content in the magnesium oxide, samples of magnesium oxide containing additions of TiO2, and samples without TiO2, were sintered at 1650 °C for 3 hours. Table 3 presents the values of the density and the porosity for sintered magnesium oxide samples (80% precipitation) sintered at 1650 °C for 3 h with various quantities of sintering agent (in mass %). The experimentally obtained values for the density of the sintered samples indicate a favorable effect of the TiO2 addition on product densification during the isothermal sintering of the magnesium oxide from the seawater. The density amounts to 95-98% of the theoretical density (?t = 3.576 g cm-3). Magnesium oxide compacts with TiO2 were found to be denser than the undoped samples, after being fired under the same thermal conditions. However, this increase in density was not very significant. Data on the apparent porosity of the sintered samples indicate the presence of very few open pores in the system. The pores that are present are mainly of the closed type. As a result, the total porosity is almost identical to the closed porosity. The low values obtained for densification during isothermal heating in the samples examined indicate that densification occurred even before the maximum sintering temperature was reached, which may be attributed to the activity of the TiO2 at lower temperatures. It is evident that the addition of TiO2 promotes low-temperature densification of magnesium oxide, in proportion to the extent of solid-solution formation and vacancy formation. In this case the sintering was intensified in the presence of the liquid phase in the MgO-TiO2 system 12,13. Table 4 shows the results of an examination of the effect of TiO2 on the boron content in sintered magnesium oxide samples for the operating conditions described. Although the addition of TiO2 is known to reduce the quantity of boron adsorbed during isothermal sintering of the magnesium oxide obtained from seawater 14-17, the purpose of this work was to relate these two aspects, i.e. the effect of the precipitate-rinsing method and the effect of the addition of TiO2 on the B2O3 content in the samples of magnesium oxide from seawater that were sintered for 3 hours at 1650 °C. The results obtained for the effect of TiO2 addition on the B2O3 content in sintered samples show that the B2O3 content changes during sintering, depending on the method of magnesium oxide preparation for the operating conditions described. The effect of added TiO2 is higher in magnesium oxide samples obtained by rinsing the magnesium hydroxide precipitate with distilled water, pH 5.95, than in the samples obtained by rinsing the precipitate with alkalized distilled water, pH 12.50, which is related to the quantity of CaO in the examined samples. In our previous paper 18 the content of dicalcium borate (Ca2B2O5) was determined in sintered samples by means of X-ray diffraction, i.e. it was found that during sintering B2O3 reacts with CaO to form Ca2B2O5. Also, the studies 12,13,19 show that the methods of X-ray diffraction and EDAX analysis indicate that in the sintering process the added TiO2 Table 3: Density (?), apparent (Pa), total (Pt) and closed (Pc) porosity for sintered magnesium oxide samples (80% precipitation) with 1, 2 and 5 mass% TiO2 added, and no sint. aid, t = 1650 °C, ? = 3 h, p = 625 MPa Tabela 3: Gostota(?), odprta(Pa), ukupna(Pt) in zaprta (Pc) poroznost zasintrane vzorce magnezijevegaoksida(80-odstotno usedanje) z masnimi deleži TiO2 1, 2 in 5 % in brez dodatka TiO2, t = 1650 °C, ? = 3 h, p = 625 MPa No. of sample Rinsing water pH of the rinsing water TiO2 addition p Pa Pt Pc mass% gcm-3 % 1 distilled water 5.95 no sint. aid 3.392 0.09 5.42 5.33 2 1 3.433 0.08 4.16 4.08 3 2 3.450 0.07 3.65 3.53 4 5 3.481 0.02 2.73 2.71 5 alkalized distilled water 12.50 no sint. aid 3.358 0.09 6.49 6.40 6 1 3.388 0.09 5.55 5.46 7 2 3.399 0.08 5.21 5.13 8 5 3.497 0.05 2.26 2.21 MATERIALI IN TEHNOLOGIJE 36 (2002) 6 397 V. MARTINAC ET AL.: THE EFFECT OF THE pH OF THE RINSING SOLUTION reacts with the CaO from the MgO-CaO solid solution and transforms into calcium titanate, CaTiO3. Thus, two reactions cause the B2O3 content to reduce during sintering. These reactions are: 2 Ca O + B2O3 = Ca2B2O5 (6) CaO + TiO2 = CaTiO3 (7) Therefore, TiO2 binds a part of the CaO in the CaTiO3, and thus reduces the CaO content that reacts with B2O3. Thus, the higher the CaO content, the more B2O3 is retained in the sintered samples. In order to examine this effect of TiO2 on the reduction of B2O3 content in the sintered samples, experimental results were processed, i.e. the percentage of B2O3 liberated during sintering (J1) was calculated from the experimental data on the B2O3 content in the sintered samples and the B2O3 content in the calcined magnesium oxide, as well as the percentage of CaO that had reacted with the TiO2 (J2). Based on the experimental data for the boron determination by potentiometrical titration in magnesium oxide samples sintered at 1650 °C, the reaction yield degree, ?, was calculated for the dicalcium borate formation reaction in all the samples examined, relative to the quantity of boron present in the calcined magnesium oxide. Table 5 presents the results obtained. The results shown in Table 5 indicate that the dicalcium borate formation reaction yield degree decreases when the addition of TiO2 increases. Thus, an increased TiO2 addition leads to increased boron evaporation from the sample into the atmosphere. Also, the dicalcium borate formation reaction yield degree is higher in samples obtained by rinsing with alkalized distilled water, pH 12.50, which may be attributed to the increased CaO content in the sample (1.28 mass %). While with magnesium oxide obtained by rinsing with distilled water, pH 5.95, the addition of 1 mass % of TiO2 binds all of the CaO (J1 = 84.30 %; J2 = 80.63 %) with magnesium oxide obtained by rinsing with alkalized distilled water, pH 12.50, a higher addition of TiO2 is needed to bind excess CaO into CaTiO3, i.e. only the addition of 5 mass % TiO2 causes greater evaporation of B2O3 during sintering (J1 = 82.24 %; J2 = 96.41 %). Although the magnesium oxide samples obtained by rinsing the magnesium hydroxide precipitate with alkalized distilled water, pH 12.50, have a much lower B2O3 content (0.0518 mass %), owing to the increased CaO content in the specified sample (1.28 mass %) the sintered samples contain a higher quantity of B2O3 (J1 = 24.52% in samples with 1 % TiO2 added, J1 = 62.74 % in samples with 2 % TiO2 added, and J1 = 82.24 % in samples with 5 % TiO2 added). Therefore, one should take into account the CaO content in the initial sample in order to choose the optimum TiO2 addition that will bind the excess CaO during sintering and thereby significantly affect the quantity of B2O3 evaporated from the sample into the atmosphere. Table 4: Effect of TiO2 on the B2O3 content in the sintered magnesium oxide samples (80% precipitation) at 1650 °C, ? = 3 h, p = 625 MPa Tabela 4: Vpliv TiO2 navsebnost B2O3 v sintranih vzorcih magnezijevega oksida (80-odstotno usedanje) pri 1650 °C, ? = 3 h, p = 625 MPa No. of sample Rinsing water pH of the rinsing water TiO2 addition B2O3 mass% 1 distilled water 5.95 no sint.aid 0.0387 2 1 0.0277 3 2 0.0142 4 5 0.0028 5 alkalized distilled water 12.50 no sint. aid 0.0486 6 1 0.0391 7 2 0.0193 8 5 0.0092 Table 5: Dependence of J1, J2 and ? on the different quantities of sintering aid and the method employed for rinsing the magnesium hydroxide precipitate in the sintered magnesium oxide samples (80% precipitation) at 1650 °C, ? = 3 h, p = 625 MPa Tabela 5: Odvisnost J1, J2 in ? od količine dodanega TiO2 in načinaizpiranjausedline magnezijevegahidroksidav vzorcih MgO (80-odstotno usedanje) pri 1650 °C, ? = 3 h, p = 625 MPa No. of sample Rinsing water pH of the rinsing water TiO2 addition J 1 J 2 S mass % % 2 distilled water 5.95 1 84.30 80.63 15.70 3 2 91.95 95.02 8.05 4 5 98.41 97.13 1.59 6 alkalized distilled water 12.50 1 24.52 53.54 75.48 7 2 62.74 95.17 37.25 8 5 82.24 96.41 17.76 398 MATERIALI IN TEHNOLOGIJE 36 (2002) 6 V. MARTINAC ET AL.: THE EFFECT OF THE pH OF THE RINSING SOLUTION 4 CONCLUSIONS The B2O3 content is significantly reduced if the magnesium hydroxide precipitate is rinsed with a high-pH rinsing agent, i.e. with alkalized distilled water with apH value of 12.50. All samples sintered at 1650 °C for 3 hours had a very low porosity, and the densities were up to 95 % of the theoretical density. The optimal TiO2 quantity needed to bind CaO into calcium titanate, and thus cause greater evaporation of B2O3 from the sample into the atmosphere during sintering, depends on the method used for rinsing the magnesium hydroxide precipitate, i.e. the CaO content in the initial sample. 5 REFERENCES 1 C. Sims, Industr. Minerals, 7 (1997) 21 2 H. Tsuge, Y. Kotaki, S. Asano, 7th Symposium on Salt, Vol. II, Elsevier Science Publishers B. V., Amsterdam, 1993, 213 3 W. C. Gilpin, N. Heasman, Chem. Ind., 16 (1977) 567 4 N. Heasman, Gas Wärme International, 28 (1979) 329 J. C. Hicks, S. Tangney, Ceram. 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