Microstructural Characteristics of Low-Temperature ( 1400 C ) Sintered MgO Obtained from Seawater

The purpose of this study was to investigate the influence of a rinsing of Mg(OH)2 precipitated from seawater by substoichiometric precipitation (80% precipitation) and the addition of TiO2 on microstructural characteristics of the MgO obtained by sintering at low temperature (1400 C). The results of examination indicate that the method of rinsing of the magnesium hydroxide precipitate in the technological process of obtaining MgO from seawater significantly affects the chemical composition of samples, primarily with regard to the CaO and B2O3 content. The samples were doped with TiO2 to improve the evaporation of B2O3 and sintering of MgO samples that were characterized by XRD and SEM/EDS. These techniques confirmed the high purity of MgO samples obtained and the formation of secondary compounds in very small quantities that have a positive effect on the densification.


Introduction
Refractory materials based on magnesium oxide are widely used in the manufacture of refractory bricks, in steel, cement and glass industries.Magnesium oxide has high temperature strength, mechanical stability and chemical inertness at high temperatures.Important sources of magnesium oxide for refractories are natural magnesite, seawater and underground deposits of brine [1].High-quality refractory magnesium oxide of high purity (≥ 98 % MgO), very high melting point and excellent resistance to attack by iron oxide, alkalis and high lime fluxing agents can be produced from seawater [2,3].An additional advantage of this technological process lies in huge reserves of seawater (1 m 3 contains 0.945 kg of magnesium) [4].
The main impurities present in magnesium oxide from seawater are CaO and BB 2 O 3 .The content of these oxides needs to be reduced because they form compounds (CaO•B 2 O 3 , 2CaO•B 2 O 3 , 3CaO•B 2 O 3 ) with a low melting point at elevated temperatures, leading to poor refractory properties [5].Even a few tenths of a percent of B 2 O 3 can be detrimental to the good quality of magnesium oxide such as high strength and slag action resistance.
B 2 The content of B O 3 should be ≤ 0.05 mass % in the sintered magnesium oxide, according to N. Heasman [6].Therefore, it is important to reduce the content of CaO and B 2 3 . The content of B precipitate before sintering 2 O 3 in samples of can be reduced by adding TiO magnesium oxide from seawater 2 .In previous studies it has been found that the addition of TiO 2 reduces the B 2 O 3 content in sintered samples during sintering at high temperatures (> 1500 C) [7,8].Namely, during the sintering process TiO The addition of TiO 2 also promotes densification and grain growth at relatively low temperatures [9,10].
The magnesium oxide used in this study was obtained by substoichiometric precipitation, with the addition of 80 % of dolomite lime as the precipitation agent.The substoichometric precipitation of magnesium hydroxide from seawater is a very convenient precipitation method in the so-called "wet phase", as it significantly increases the thickener capacity, i.e. the precipitation rate.At precipitation of 80 % the capacity of thickener (calculated according to Kynch) increases by 86.5 % in relation to complete precipitation [11].With this precipitation method, the boron content in the form of B 2 O 3 is somewhat increased in the final product, i.e. magnesium oxide, and should therefore be reduced.The aim of this study was to investigate the influence of rinsing of Mg(OH) 2 precipitated from seawater and addition of TiO 2 on microstructural characteristics and chemical composition of MgO obtained by sintering at low temperature (1400 o C).The influence of this sintering temperature on the microstructure properties of MgO from seawater has not been investigated yet.Usually applied temperatures for MgO sintering was above 1500 o C [7][8][9][10][12][13][14][15][16][17].This study was carried out to improve the technological process in order to obtain magnesium oxide with as low B 2 O 3 and CaO content as possible and with good microstructure properties at a low sintering temperature.

Experimental
The main raw materials in the present study are magnesium oxide, obtained from seawater, and dolomite lime.The composition of seawater used for precipitation of magnesium hydroxide [18] and dolomite lime used as the precipitation agent are described in Tab.I. ) ions, followed by the removal of the liberated carbon dioxide (CO 2 ) by aeration in a desorption tower.After the pre-treatment of the seawater, a calculated amount of dolomite lime was added.The experiments were carried out with substoichiometric precipitation, with the addition of 80 % of the stoichiometric quantity of dolomite lime.The precipitation reaction took 30 min with stirring of the magnetic stirrer.Sedimentation was improved by the addition of the optimum amount of the anionic 818 A flocculent (polyacrylamide) produced by the Dutch firm Hercules.The optimum quantity of the anionic 818 A flocculent has already been described in a previous study [19].

Tab. I
After sedimentation, the magnesium hydroxide precipitate was decanted and rinsed with distilled water (pH = 5.86) and alkalized distilled water (pH = 12.50).The rinsing by decanting used the combined rinsing method (2+3), i.e. the magnesium hydroxide precipitate was rinsed 2 times by distilled water (pH = 5.86) and 3 times by alkalized distilled water (pH = 12.50).Rinsing by decanting was performed with about 1 dm 3 of the rinsing agent per 10.56 g of Mg(OH) 2 .The duration of contact with the rinsing agent was about 30 min, i.e. until the precipitate settled again before the following decanting.After rinsing by decanting, the samples were rinsed in the process of filtering on multiple funnels using distilled water 5 times (pH = 5.86).After completion of filtration, the rinsed precipitate was dried in an oven at 105 °C, and then calcined in a muffle furnace at 950 °C for 5 hours (sample MgO-R).For comparison, a MgO sample was prepared without rinsing (MgO-NoR).Mixtures of MgO-R were then prepared with the addition of mass fractions of 1 %, 2 %, and 5 % TiO 2 .The doping oxide used was the analytical reagent grade titanium (TiO 2 p.a.) in the rutile formproduced by Carlo Erba Reagent.Samples were mixed together by stirring in absolute ethanol for 30 min.The mixtures were dried at 80 ºC until all the alcohol evaporated.Mixtures were cold pressed into compacts at a pressure of 625 MPa.Pressing was performed in a "Maschinenfabrik Herzog Osnabrück" hydraulic press, model TP 40.All compacts were sintered at a temperature of 1400 ºC for durations of 1, 2 and 4 h.The time needed to reach that temperature in the furnace was about 2 hours.After sintering, samples were left to cool in the furnace.A gas furnace by the French company "Mecker", type 553, with a coating of zirconium (IV) oxide, was used for sintering.Sintered samples were labeled with codes shown in Tab.II.

Tab. II Sample codes used in the experiment
The sintered magnesium oxide samples were characterized by determing the content of B 2 O 3 (mass%), bulk density, porosity, lattice parameters, and microstructure analyses.The magnesium oxide samples were chemically analyzed to determine concentrations of B 2 O 3 using the potentiometric method, in the following way.The magnesium oxide obtained was dissolved in HCl (1:1), distilled water was added and the solution boiled to remove carbon dioxide.During titration, the solution was stirred using a magnetic stirrer.Sodium hydroxide (0.1 moldm -3 ) was used to obtain neutralisation of up to pH 5.0, and then 0.0231 moldm -3 sodium hydroxide neutralized the solution up to pH 7.0.The neutralization process was traced by changes on a pH-meter.At the starting point of titration (pH = 7.0) 5± 0.1 g D(-) mannitol was added.In the presence of boron, pH-meter values lower than 7.0 were read.Sodium hydroxide (0.1 moldm -3 ) was titrated until the pH of 7.0, i.e. until the starting point of titration was reached again.The variation coefficient for the method applied is ± 1 % [20].The results represent the average number of measurements (5 analyses in each case).
The bulk density was measured from the volume of water displaced from a calibrated cylinder.The porosity in samples was determined according to standard methods (HRN B.D8.302, B.D8.312, B.D8.313).
Lattice parameters and phase compositions were analyzed by X-ray powder diffraction using an ItalStructures diffractometer APD2000 with monochromatized Cu Kα radiation (graphite monochromator).Lattice parameters of sintered magnesium oxide samples were determined from the results of Rietveld refinements [21] (program MAUD [22]) of powder diffraction patterns with the weighted residual error indexes (R wp ) less than 10 %.
Silicon (Koch-Light Lab.Ltd.) was used as an internal standard (space group m 3 m, a = 5.43088 Ǻ).Rietveld refinements of power diffraction patterns were also used for quantitative crystal phase analysis of sintered magnesium oxide samples.
The microstructure of the samples was observed by scanning electron microscopy (JSM-6510 LV, JEOL) equipped with an attached energy dispersive X-ray spectrometer (Oxford INCA X-act.) for semiquantitative elemental analysis.

Results and discussion
The chemical analyses of calcined MgO samples obtained from seawater (80 % precipitation) are presented in Tab.III.MgO-R and MgO-NoR samples were prepared in order to demonstrate the effectiveness of the combined rinsing method on reducing the content of impurities in the sample.The results indicate that contents of B 2 O 3 , CaO and MgO have been significantly changed.The application of the combined method of rinsing significantly reduces the B 2 O 3 content in the examined samples.The repeated application of alkalized distilled water with a high pH value of 12.50 very favourably affects the reduction of B 2 O 3 in the calcined magnesium oxide.The comparison of the results indicates that the B 2 O 3 content in samples MgO-R is by 31 % lower than the B 2 O 3 content in the samples MgO-NoR (B 2 O 3 = 0.2735 mass%).The application of distilled water with the pH value of 5.86 in the first phases of rinsing in the combined method (2+3) in decanting and (5) on filter paper, significantly reduces the CaO content in the MgO sample (80 % precipitation) obtained from seawater.Specifically, increased pH favours greater adsorption of Ca 2+ ions onto the magnesium hydroxide precipitate, due to the increased stability of Ca(OH) 2 in a highly alkaline medium (pH = 12-13).The CaO content in samples MgO-R is by 70 % lower than the CaO content in samples MgO-NoR (CaO = 2.79 mass %).The results also indicate that the content of MgO is increased for 2 % by reducing impurities in the magnesium oxide sample.MgO-R samples were sintered at 1400 ºC for 1, 2 or 4 h and with the addition of 1, 2 and 5 mass% TiO 2 .Tab. IV shows the effect of the TiO addition on the B O content in sintered samples relative to the operating conditions indicated.It is evident that the mass fraction of B O is reduced in the samples examined with increased TiO addition and duration of isothermal sintering.The samples without the TiO addition show that the B O content in the sample after sintering is slightly lower than the B 2 3  Samples without TiO 2 show a slight increase in bulk density with increasing sintering time.These samples have a maximum bulk density of 3.36 gcm -3 .It has been observed that bulk density of samples with additions of 1 %, 2 % and 5 % TiO 2 increases slightly after 1 and 2 h and then increases significantly after 4 h of sintering.The apparent porosity increases significantly with a 1 % TiO 2 addition and then decreases with further increase of the TiO 2 addition.The M2T-4-14 sample has a minimum apparent porosity (0.28 %) with bulk density of 3.438 gcm -3 .This result indicates that the addition of TiO 2 promotes low-temperature densification of magnesium oxide, proportional to the extent of solid solution formation and vacancy formation.In that case the sintering was intensified in the presence of the liquid phase in the MgO-TiO 2 system, which was confirmed by SEM and XRD.Fig. 2 shows the XRD patterns of magnesium oxide samples with 1 %, 2 %, 5 % TiO 2 and samples without addition, sintered at 1400 o C for 4 h.The results of phase analysis obtained using the XRD analysis show that periclase is the main crystalline phase in all samples.In the sample without additions no other phase was detected.It is assumed that the amounts of CaO and B 2 3

Tab. III
O in this sample are be The presence of CaO and B too small to detected by the XRD analysis.
2 O 3 impurities were detected by the chemical analysis (Tab.III).
[13] It has been proven in a previous study that the presence of CaO and B 2 3 2 O .The addition of TiO causes the formation of low melting titanium compounds in the samples of MgO.They form reactive liquid phases which control the grain growth and the secondary phase distribution that promotes the densification of MgO samples O in samples of magnesium oxide during sintering causes only the formation of Ca B 2 B 5 2 [14][15][16][17].In samples with 1 % and 2 % TiO the main crystalline phases were MgO and CaTiO .Other compounds were not detected which indicates that all added TiO reacted with CaO and formed only CaTiO .This is also confirmed (Tab.V) by no changes of lattice parameter of the MgO phase when TiO was added which indicates that TiO did not form a solid solution with MgO but react with CaO and form CaTiO .The refined unit-cell parameters of the periclase (MgO) type phase were determined using the results of Rietveld refinements on powder diffraction patterns with the weighted residual error indexes (R wp ) [21,22], and are shown in Tab.V.
Tab. V Refined values of unit-cell parameters of the periclase (MgO) type phase as determined from the results of Rietveld refinements on powder diffraction patterns with the weighted residual error indexes (R wp ) given in the last column.O , was reduced and that a higher quantity of B O evaporated.The XRD analysis showed the presence of Mg TiO only in samples prepared with the addition of 5 % TiO which indicates small changes of lattice parameter of the MgO phase (Tab.V).It is assumed that the changes of the MgO lattice will increase proportionally to the amount of Mg TiO formation, caused by increased TiO addition.The quantitative phase analysis of crystal (Tab.VI) determined from the results of Rietveld refinements shows that these secondary compounds in samples of MgO obtained from seawater are present in small amounts.SEM images of MgO samples confirmed the results obtained by the XRD analysis.After 4 h of sintering at 1400 o C it can be seen that the microstructure of MgO samples without the addition of TiO 2 consists of periclase grains (M-dark phase) surrounded by CaO (C-bright phase) with pores along the grain boundary (Fig. 3c).The morphology of MgO samples changed with the TiO 2 addition (Fig. 4).The secondary compounds in the M5T-4-14 sample were identified by EDS (Fig. 5).The bright white phase represented CaTiO (P) and dark grains MgO (M).The MgO grains (M) are mostly directly bonded and surrounded by the liquid phase (Fig. 4f).CaTiO is mostly located along the grain boundary of MgO, which explains the increased formation of direct bonds and grain growth of MgO.

Conclusion
In order to reduce impurities of MgO obtained from seawater (80 % precipitation), MgO samples were prepared with the combined method of rinsing.This method significantly reduces the B 2 O 3 content in samples (by 31 %) compared to the B 2 O 3 content in samples MgO-NoR (B 2 O 3 = 0.2735 mass%).Also, the CaO content is by 70 % lower than the CaO content in samples MgO-NoR (CaO = 2.79 mass%).The addition of TiO 2 also reduces the BB 2 O 3 content in samples by forming low melting titanium compounds (CaTiO 3 2 4 and Mg TiO ) and significantly promotes the densification of MgO samples at relatively low temperatures (1400 C), which was confirmed by the XRD and SEM/EDS analysis.This study shows that MgO o obtained from seawater (80 % precipitation), could be sintered at low temperature (1400 C) for 4h with the addition of 2 % TiO 2 o and used to obtain good refractory material of high purity, minimum apparent porosity (0.28 %) and bulk density of 3.438 gcm .

o 2 added
reacts with CaO from the MgO-CaO solid solution and transforms into CaTiO 3 .This reduces the fraction of CaO that reacts with B 2 O 3 , i.e. a higher quantity of B 2 O 3 evaporates.

B
O content in the sample before sintering.The B O content in the sample with 1 % TiO is lower with the time of sintering than the B O content in the sample without the addition, but the B O content was still above the limit of maximum pollution according to N. Heasman [6].It is necessary to increase the content of TiO 2 sample at the sintering temperature (1400 C) for a certain period of time for the B o O content to get below 0.05 mass%.Two hours of sintering of samples with 2 % TiO , at the temperature of 1400 C, has proved to be sufficient to satisfy the requirement for the content of B o O in the sample.The addition of 5 % TiO in comparison to samples with 2 % TiO has no significant effect on the reduction of the B 2 B O 3 content in the samples sintered at the temperature of 1400 o C. Bulk density and apparent porosity in sintered samples of magnesium oxide (80 % precipitation) are shown in Fig. 1.
Chemical composition of seawater and dolomite lime.
Chemical composition of MgO samples obtained from seawater (80 % precipitation) after calcination at 950 o C for 5 hours (in mass%).