**Effect of TiO2 Addition on the Sintering Process of Magnesium Oxide from Seawater**

Vanja Martinac

*University of Split / Faculty of Chemistry and Technology Croatia* 

### **1. Introduction**

Magnesium oxide is one of the most important materials used in the production of hightemperature-resistant ceramics. Due to its high refractory properties (MgO melts at (2823 ± 40) oC), MgO ceramic is non-toxic and chemically inert in basic environments et elevated temperatures, resistant to the effect of metal melts, acid gases, alkali slag, neutral salts, and react with carbon only above 1800 oC. Today, in large-scale technicall processes, magnesia (MgO) for refractories is produced from two sources: natural and synthetic. Magnesia from natural sources constitutes 82 % of the world's magnesia installed capacity. The dominant source is magnesite (MgCO3) which occurs in both a macro and a cryptocrystalline forms. Less significant are dolomite (CaCO3·MgCO3), hydromagnesite (3MgCO3·Mg(OH)2·3H2O), brucite (Mg(OH)2) and serpentine (Mg3(Si2O5)(OH)4). Synthetic materials are manufactured either from seawater or from magnesia rich brines. Magnesium oxide obtained from sea water is a high-quality refractory material, and its advantages lie not only in the huge reserves of seawater (1 m3 contains 0.945 kg of magnesium), but in the higher purity of the sintered magnesium oxide (≥ 98 % MgO). The production of magnesium oxide from seawater is a well-know industrial process (Bocanegra-Bernal, 2008; Bonney, 1982; Gilpin & Heasman, 1977; Heasman, 1979; Maddan, 2001; Martinac, 1994; Petric & Petric, 1980, Rabadžhieva et al., 1997) and has been studied all over the world for a number of years. For most of the second half of the twentieth century, seawater provided almost 50 % of the magnesium produced in the western world, and today it still remains a major source of magnesium oxide in many countries. The process involves the extraction of dissolved magnesium, which has a concentration of around 1.3 g dm-3 in seawater (Brown et al., 1997), and 3 to 40 times this values for brines, and the reaction of magnesium salts (chloride and sulphate) with lime or dolomite lime to produce a magnesium hydroxide precipitate. The precipitate is 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. Boron is a particulary problematic impurity for the magnesia used as a high quality refractory material. Thus, boron can be a problem in refractory magnesia for specialized refractory applications where a high hot strength is required. Taking into consideration that B2O3 is common impurity in seawater derived magnesia, the aim of this study was to examine the possibility of adding TiO2 in quantities of 1, 2 and 5 wt.-% for

Effect of TiO2 Addition on the Sintering Process of Magnesium Oxide from Seawater 311

decanted and rinsed. The rinsing and decantation procedure was repeted five times with approximately 1 dm3 of distilled water as rinsing agent. After that, the magnesium hydroxide precipitate was filtered through a number of funnels. The rinsing agent used with the Mg(OH)2 precipitate on the filter paper was the same as the one used for rinsing by decantation. This procedure was also repeted five times, i.e. until rinsing was completely carried out. The magnesium hydroxide thus obtained was dried at 105 oC and then calcined at 950 oC for 5 h to form caustic magnesia. The boron content was determined potentiometrically. The variation coefficient for the potentiometric method employed in boron determination is ± 1% (Culkin, 1975). The results listed represent an average value of a number of measurements (an average of five analyses in each case). Table 1 shows the chemical composition of magnesium oxide obtained from seawater with regard to

Sample MgO / wt.-% CaO / wt.-% B2O3 / wt.-%

MgO (80% precipitation) 99.20 0.59 0.193 MgO (120% precipitation) 98.25 1.32 0.056

The substoichiometric precipitation of magnesium hydroxide from seawater is a very convenient precipitation method in the so-called «wet phase» (Petric & Petric, 1980), as it significantly increases the thickener capacity, i.e. the magnesium hydroxide settling rate which is the «bottleneck» of this technology. This is very important for the design of the thickener as its construction is the time-controlling factor in plants of this type. At precipitation of 80% the capacity of the thickener (calculated according to Kynch) increases by 71% in relation to complete precipitation (Martinac et al., 1997). Substoichiometric precipitation significantly increases the sedimentation rate of the magnesium hydroxide precipitate formed, due to the decreased thickeness of the double electrical layer around the magnesium hydroxide particle. A consequence of the increased adsorption of Mg2+ ions onto Mg(OH)2 particles is a decrease in the zeta-potential. Therefore, substoichiometric precipitation increases the coagulation stability of the given Mg(OH)2-seawater system. Also, one of the advantages of substoichiometric (80%) precipitation lies in the reduced quantity of concentrated HCl needed to neutralize waste seawater after sedimentation. This quantity amounts to only 1.1 g of concentrated HCl per kg of magnesium oxide, while it is 210.5 of concentrated HCl per kg of magnesium oxide with the overstoichiometric (120%) precipitation (Petric et al., 1991). 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 overstoichiometric precipitation, and should therefore be reduced. Boron oxide is a common impurity in magnesia obtained from seawater; it is capable of acting as a powerful fluxing agent for the calcium silicate phases

The use of sintering aids – small additions of various compounds that enhance densification, or allow it to occur at a lower temperature during sintering – is quite common in the

Table 1. Chemical composition of magnesium oxide obtained from seawater.

magnesium oxide, calcium oxide, and boron(III) oxide.

which can be present in refractory grades of magnesia.

**2.1 TiO2 addition as sintering aid** 

reducing the boron content in the product, i.e. sintered magnesium oxide obtained from seawater. The purpose of this paper was, first, to reduce the B2O3 content in magnesium oxide from seawater as much as possible in ensure a high-purity product, because the hotstrength properties of certain refractory products are significantly affected by their boron content, and, second to sinter the individual products and determine the properties of samples sintered depending on the precipitation method and the boron content in the magnesium oxide.

### **2. MgO from seawater**

Processing of seawater magnesium involves precipitation of magnesium hydroxide in seawater reacting with an alkaline base, such as calcined dolomite or calcined limestone. If dolomite lime is used as precipitation agent, the chemical reaction is as follows:

$$2\text{ CaO}\,\text{MgO}(\text{s}) + 2\,\text{Mg}^{2+} + \text{SO}\_4^{2-} + 2\,\text{Cl} + 4\,\text{H}\_2\text{O} = 4\,\text{Mg(OH)}\_2(\text{s}) + \text{CaSO}\_4(\text{s}) + \text{Ca}^{2+} + 2\,\text{Cl} \quad \text{(1)}$$

The composition of the dolomite lime (from the location Đipalo near the town of Sinj, Croatia) used for precipitating the magnesium hydroxide from seawater was as follows (wt.-%): MgO = 42.27%, CaO = 57.55 %, SiO2 = 0.076%, Al2O3 = 0.042%, Fe2O3 = 0.064%, and the composition of the seawater (from the location at the promontory of the hill Marjan near the the Oceanographic Institute in Split, Croatia) was as follows: MgO = 2.423 g dm-3 and CaO = 0.604 g dm-3.

Impurities from seawater and from precipitation agent get into the magnesium hydroxide precipitate, so that special attention has to be paid to precipitate purity, depending on the product application. Thus, seawater is pretreated by acidifying with H2SO4 to lower its pH from the normal 8.2 to 4.0, in order to remove bicarbonate (HCO3-) and carbonate (CO32-) ions. The chemical reaction are:

$$\rm Ca^{2+} + CO\_3^{2-} + H^+ + HSO\_4^- = Ca^{2+} + SO\_4^{2-} + H\_2O + CO\_2(aq) \tag{2}$$

$$\mathrm{Ca^{2+}} + 2\,\mathrm{HCO\_3^{-}} + \mathrm{H^{+}} + \mathrm{HSO\_4^{-}} = \mathrm{Ca^{2+}} + \mathrm{SO\_4^{2-}} + 2\,\mathrm{H\_2O} + \mathrm{CO\_2(aq)}\tag{3}$$

The calcium sulphate formed remained in the solution. Seawater was then passed through the desorption tower packed with Rasching rings where it flowed downward against a rinsing stream of air. The liberated carbon dioxide (CO2) gas was removed from falling water drops by the ascending airflow. In this way seawater derived lime contamination of the magnesia can be minimised. 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. After the pretreatment of the seawatwer, a calculated amount of dolomite lime was added to precipitate the magnesium hydroxide. The magnesium oxide used was obtained from seawater by substoichiometric precipitation (where precipitation of magnesium hydroxide took place with 80% of the stoichiometric quantity of the dolomite lime) and by overstoichiometric precipitation (which took place with 120% of the stoichiometric quantity of the dolomite lime). The precipitation reaction lasted for 30 min; a magnetic stirrer was used. After magnesium hydroxide precipitation, settling took place. The sedimentation rate was increased by addition of the optimum amount of the anionic Flokal-B flocculent (polyacrilamide) (produced by Župa-Kruševac, Serbia). The precipitate obtained was then

reducing the boron content in the product, i.e. sintered magnesium oxide obtained from seawater. The purpose of this paper was, first, to reduce the B2O3 content in magnesium oxide from seawater as much as possible in ensure a high-purity product, because the hotstrength properties of certain refractory products are significantly affected by their boron content, and, second to sinter the individual products and determine the properties of samples sintered depending on the precipitation method and the boron content in the

Processing of seawater magnesium involves precipitation of magnesium hydroxide in seawater reacting with an alkaline base, such as calcined dolomite or calcined limestone. If

The composition of the dolomite lime (from the location Đipalo near the town of Sinj, Croatia) used for precipitating the magnesium hydroxide from seawater was as follows (wt.-%): MgO = 42.27%, CaO = 57.55 %, SiO2 = 0.076%, Al2O3 = 0.042%, Fe2O3 = 0.064%, and the composition of the seawater (from the location at the promontory of the hill Marjan near the the Oceanographic Institute in Split, Croatia) was as follows: MgO = 2.423 g dm-3 and

Impurities from seawater and from precipitation agent get into the magnesium hydroxide precipitate, so that special attention has to be paid to precipitate purity, depending on the product application. Thus, seawater is pretreated by acidifying with H2SO4 to lower its pH from the normal 8.2 to 4.0, in order to remove bicarbonate (HCO3-) and carbonate (CO32-)


The calcium sulphate formed remained in the solution. Seawater was then passed through the desorption tower packed with Rasching rings where it flowed downward against a rinsing stream of air. The liberated carbon dioxide (CO2) gas was removed from falling water drops by the ascending airflow. In this way seawater derived lime contamination of the magnesia can be minimised. 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. After the pretreatment of the seawatwer, a calculated amount of dolomite lime was added to precipitate the magnesium hydroxide. The magnesium oxide used was obtained from seawater by substoichiometric precipitation (where precipitation of magnesium hydroxide took place with 80% of the stoichiometric quantity of the dolomite lime) and by overstoichiometric precipitation (which took place with 120% of the stoichiometric quantity of the dolomite lime). The precipitation reaction lasted for 30 min; a magnetic stirrer was used. After magnesium hydroxide precipitation, settling took place. The sedimentation rate was increased by addition of the optimum amount of the anionic Flokal-B flocculent (polyacrilamide) (produced by Župa-Kruševac, Serbia). The precipitate obtained was then

+ 4 H2O = 4 Mg(OH)2(s) + CaSO4(s) + Ca2+ + 2 Cl-


= Ca2+ + SO42- + 2 H2O + CO2(aq) (3)

(1)

dolomite lime is used as precipitation agent, the chemical reaction is as follows:

2- + 2 Cl-

magnesium oxide.

CaO = 0.604 g dm-3.

**2. MgO from seawater** 

2 CaO·MgO(s) + 2 Mg2+ + SO4

ions. The chemical reaction are:

Ca2+ + CO32- + H+ + HSO4

Ca2+ + 2 HCO3- + H+ + HSO4

decanted and rinsed. The rinsing and decantation procedure was repeted five times with approximately 1 dm3 of distilled water as rinsing agent. After that, the magnesium hydroxide precipitate was filtered through a number of funnels. The rinsing agent used with the Mg(OH)2 precipitate on the filter paper was the same as the one used for rinsing by decantation. This procedure was also repeted five times, i.e. until rinsing was completely carried out. The magnesium hydroxide thus obtained was dried at 105 oC and then calcined at 950 oC for 5 h to form caustic magnesia. The boron content was determined potentiometrically. The variation coefficient for the potentiometric method employed in boron determination is ± 1% (Culkin, 1975). The results listed represent an average value of a number of measurements (an average of five analyses in each case). Table 1 shows the chemical composition of magnesium oxide obtained from seawater with regard to magnesium oxide, calcium oxide, and boron(III) oxide.



The substoichiometric precipitation of magnesium hydroxide from seawater is a very convenient precipitation method in the so-called «wet phase» (Petric & Petric, 1980), as it significantly increases the thickener capacity, i.e. the magnesium hydroxide settling rate which is the «bottleneck» of this technology. This is very important for the design of the thickener as its construction is the time-controlling factor in plants of this type. At precipitation of 80% the capacity of the thickener (calculated according to Kynch) increases by 71% in relation to complete precipitation (Martinac et al., 1997). Substoichiometric precipitation significantly increases the sedimentation rate of the magnesium hydroxide precipitate formed, due to the decreased thickeness of the double electrical layer around the magnesium hydroxide particle. A consequence of the increased adsorption of Mg2+ ions onto Mg(OH)2 particles is a decrease in the zeta-potential. Therefore, substoichiometric precipitation increases the coagulation stability of the given Mg(OH)2-seawater system. Also, one of the advantages of substoichiometric (80%) precipitation lies in the reduced quantity of concentrated HCl needed to neutralize waste seawater after sedimentation. This quantity amounts to only 1.1 g of concentrated HCl per kg of magnesium oxide, while it is 210.5 of concentrated HCl per kg of magnesium oxide with the overstoichiometric (120%) precipitation (Petric et al., 1991). 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 overstoichiometric precipitation, and should therefore be reduced. Boron oxide is a common impurity in magnesia obtained from seawater; it is capable of acting as a powerful fluxing agent for the calcium silicate phases which can be present in refractory grades of magnesia.
