| الملخص | In this work light and dead-burned MgO were produced from sea bittern spent after NaCl production from sea water in Al-Basrah saltern. Dead burned MgO of pure and dense form (96.8% MgO and bulk density > 3.7) was prepared, using optimum conditions including; Sea bittern (26Be) pre-treatment with CaCl2 with a ratio (1.05:1) CaCl2:MgSO4 to remove CaSO4. Dolime, a dolomite calcined at 1000°C was used in molar ratio of (1:1) Dolime:MgCl2, at 70°C for 60 min. to precipitate Mg(OH)2. The Mg(OH)2, was subjected to calcination at 1000°C for 60 min. A milky soft powder of light-burned MgO was obtained; further burning at 1650°C for 60 min. produced dead-burned MgO. A byproduct of high purity CaSO4 (≥98) can be also produced.
Key: Magnesia, Sea bittern, Dolomite.
INTRODUCTION
Magnesia (MgO) is one of several materials that are vital for refractories; it is classified under the so called basic refractories that are stable to alkaline slugs, dust and fumes at elevated temperatures (Bathia, 2011). These characteristics together with its ubiquitousness, and moderate coast, make MgO the right choice for heat intensive metallurgical processes, such as the production of metals, cements, and glasses. Since magnesium oxide does not occur free in nature, therefore it has to be obtained from some natural sources that are available in commercial quantities. The first source is from the calcination and sintering of naturally occurring magensite. Other source comes from sea water, inland brines, salt lakes, which containing soluble MgCl2 (Landy, 2004). The main objective of this work is to produce a pure MgO compatible to the Iraqi standards of refractory magnesia brick [IQS (1977), 1995] shown in table (1), from sea-bittern spent after the production of NaCl in Al-Basrah Saltern.
MATERIALS AND METHODS
• Materials: Three materials were used in this research, including Limestone from Wadi-Ghadaf, Dolomite from Rutba and Sea water from Basrah Saltern with the chemical composition shown in table (2). Calcium Chloride Dihydrate CaCl2.2H2O (72% CaCl2), BDH, England.
• Methods: The production of light-burned MgO was passed through three stages:
A. Preparation of Lime, Dolime and Sea-Bittern: This stage includes the preparation of calcined limestone (Lime), calcined dolomite (Dolime) and the sea bittern from sea water. The raw limestone and dolomite were crushed to -10 cm size fraction, and then calcined in a muffle furnace at (1000 oC) for 1 hour. Sea bittern with a density of about 26 Baume (1.22 gm/cm3) was prepared by solar evaporation of sea water.
B. Precipitation of Magnesium Hydroxide: Consisted of two steps as shown below
MgSO4(aq.)+ CaCl2(aq.) → MgCl2(aq.) + CaSO4 ↓(ppt.) …………….. (1)
CaO(s)+MgCl2(aq.) + H2O→ CaCl2(aq.) + Mg(OH)2↓(ppt.) …………….. (2) Or
CaO.MgO(s)+ MgCl2(aq.)+2H2O → CaCl2(aq.) + 2Mg(OH)2↓(ppt.) ………….. (3)
C. Preparation of Magnesia (MgO): This was done by calcination of Mg(OH)2 at (1000°C) for 1 hour. The product obtained is a milky powder of light-burned magnesium oxide. Further burning at (1650 °C) lead to dead–burned magnesia.
RESULT AND DISCUSION
The preparation of light burned magnesia (MgO) passed along three stages.
A. Preparation of Lime, Dolime and Sea Bittern.
B. Precipitation of Magnesium Hydroxide Mg(OH)2: In this stage, six parameters were evaluated towards optimal purity of MgO product.
1. Effect of Precipitant Type: Three types of precipitants were used for the precipitation of Mg(OH)2; Lime (CaO), Milk of Lime Ca(OH)2 and Dolime (CaO.MgO). It is obvious from figure (1), that high purity of Mg(OH)2 produced with the addition of dolime lesser purity occurred with lime and the milk of lime. Moreover, using dolime bring advantages of Mg(OH)2double quantity production.
2. Effect of Mg2+ containing solution: An experiment was conducted to precipitate Mg(OH)2 from sea water to compare it with the precipitation of bittern at the same conditions. Figure (2) shows a comparison in purity expressed as Mg(OH)2. The results in figure (2) above, shows that Mg(OH)2 with much quantity and better purity can be obtaind from sea bittern compaired with that of sea water.
3. Effect of Precipitation Time: Three precipitation experiments were conducted for (30, 60 and 120) minutes to optimize the precipitation time. It is clear from Figure (3) that the purity of Mg(OH)2 is highly affected by the precipitation time, the results pointed out that a 60 minuts of time is considered as the best precipitation time.
4. Effect of Precipitation Temperature: To compare the heat effect on the precipitation of Mg(OH)2 with previous experiment conducted at room temperature; a further experiment was done at a temperature of (70 oC). From the results shown in figure (4) above, it can be stated that the increasing in precipitation temperature enhances the purity of the product.
5. Effect of Precipitant Molar Ratio (Dolime:MgCl2): Several experiments were done using various molar ratios (CaO.MgO : MgCl2) 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1 and 1:1 at (70 ˚C) for 60 minutes reaction time after pre-treated with CaCl2 (1:1). The results in figure (5) indicated that MgO product purity is directly proportional with the increasing in precipitant (dolime) molar ratio.
6. Effect of Pre-treatment Molar Ratio (CaCl2:MgSO4): For precipitation of Mg(OH)2 from sea-bittern containing MgCl2 and MgSO4, the MgSO4 should be converted to MgCl2, to do so, the bittern must be pre-treated with CaCl2. Therefore three ratios were selected 1:1, 1.05:1 and 1.1:1 (CaCl2:MgSO4). The results are shown in figure (6). According to the results shown in figure (6), one can notice that when the molar ratio increased from 1:1 to 1.05:1, the purity is markedly increased, but it decreases again when the molar ration is raised from 1.05:1 to 1.1:1.
C. Production of Magnesia MgO
- Production of Light Burned Magnesia: Light burned magnesia produced by calcination (1000oC for about 60 minutes) magnesium hydroxide prepared at the optimum conditions. The XRD pattern of this product is shown in figure (7).
- Production of Dead-Burned Magnesia: To obtain dead-burned Magnesia, the milky powder of the reactive magnesia produced at optimum conditions was briquetted and then burned at 1650°C. The product (Magnesia) was dark green in color, was ground to a powder of-75μ. The XRD pattern of this powder is shown in figure (8), indicating that no significant variation occurred after burning at 1650 C˚, than that at 1000˚C. As it can be seen from table (3), the MgO produced is more pure than all of the former reactive MgO products obtained through this study. The properties, of this Magnesia, however, fulfill the Iraqi Standard of Refractory Magnesia Brick (IQS, 1995).
According to the experimental work, the following points can be concluded:
• Product equivalent to that of Magnesia Refractory Brick can be prepared from sea bittern of raw NaCl salt production of Al-Basrah Saltern.
• Calcined Iraqi Dolomite (MgO.CaO), can be used as precipitant for Mg+2 present in sea bittern.
• Optimum conditions represented by; Pre-treatment of sea bittern with molar ratio (1.05:1.00) of CaCl2:MgSO4. Precipitating Mg(OH)2 by a stoichiometric molar ratio (1:1) of CaO.MgO:MgCl2. Precipitation temperature of 70 °C, for 60 minutes. Light-burned Magnesia obtained by burning the precipitate Mg(OH)2 at 1000 °C, while dead-burned Magnesia is produced by burning Mg(OH)2 at 1650°C for 60 minutes.
• High purity and dense Magnesia of >96% MgO with bulk density >3.7, can be produce by the aforementioned optimum conditions.
• Calcium Sulfate of high purity (≥98), can be obtained as a byproduct of the process.
Four major recommendations can be extracted from this work:
1- This work was done in a small laboratory scale (few tenths of grams) and hence, to highly assure the results obtained a bench-scale study is advisable to carry out.
2- Dolomite deposit of Khadary, located about 40 km south-west of Samawa city / Al-Muthana governorate can be used because it is close to Basrah saltern, to minimize the coast of transportation.
3- It will be possible achieving a preliminary feasibility study to clarify the economical aspect of this study in accordance to the result of proposed bench-scale study.
4- Establishment of a unit of magnesia production from sea bittern in Al-Basrah saltern.
REFERENCES
1- Bathia, A., 2011. Classification of refractories, PDH Course M158, www.PDHcenter.com , www.PDHonline.org .
2- Central Organization of Standardization and Quality Control, 1995. Iraqi Standard of Refractory Magnesia Brick, IQS, No. 1977.
3- Landy A. Richard, 2004. Magnesia refractories, Refractories Handbook, Marcel
Table 1: Iraqi standards specification of magnesia brick [IQS (1977), 1995].
Grade
Characteristic Grade (1) (Burned) Grade (2) (Unburned)
Class-1 Class-2 Class-3 Class-1 Class-2 Class-3
MgO Wt. % Min. 85 92 95 85 89 93
Apparent porosity % Max. 26 23 20 - - -
Bulk specific gravity Min. 2.70 2.75 2.80 2.75 2.80 2.80
Compressive strength (MPas) Min. 29.4 34.3 39.2 39.4 39.4 39.4
Resistance to heat under load (C°) Min. 1450 1550 1550 1350 1400 1450
Table 2: Chemical composition of sea water and sea bittern.
Composition (g/L) Na K Ca Mg SO4 CO3 Cl
Sea Water (4.9 Baume) 1.719 0.0511 0.028 0.090 0.368 0.012 2.276
Sea Bittern (26 Baume) 101.67 4.2468 0.600 15.68 21.57 0.580 191
Table 3: Chemical composition of the product obtained at the optimum conditions.
MgO % CaO % SO3 % L.O.I % Bulk Density gm/cm3
96.8 1.96 0.58 0.37 3.79*
Fig. 1: Purity expressed as Mg(OH)2% as a function to precipitant type. Fig. 2: Purity expressed as Mg(OH)2% as a function to containing solution.
Fig. 3: Effect of precipitation time on the purity of the products. Fig. 4: Effect of Precipitation temperature on the purity of the products.
Fig. 5: Effect of precipitant molar ratio on the purity of the products MgO. Fig. 6: Effect of pre-treatment ratio on the purity of the product. | en_US |
