Nano-silica and SiO 2 /CaCO 3 nanocomposite prepared from semi-burned rice straw ash
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icate Na 2 SiO 3 solution was obtained from SRBRA by using alkali leaching process. All the leaching experiments were car- ried out in covered Teflon beaker (250 ml). Each experiment was performed by dissolving a calculated amount of NaOH in distilled water and then 10 gm of SBRSA was added grad- ually to attain stoichiometry (NaOH:SiO 2 ) 3:1 and liquid/solid ratio 10/1. The mixture was heated at 100 C for 4 h under vigorous constant stirring. After the reaction time completed, the slurry was filtered then washed with distilled water. The concentration of the silica in the obtained solution was mea- sured by ICP and the result showed that the dissolved silica equals 0.735 M with 94.5% extraction efficiency. Silica nano- particles were obtained by adding gradually 12 wt.% H 2 SO 4 solution to the prepared Na 2 SiO 3 solution with constant stir- ring until a pH 10 is reached. The formed silica gel was aged for 24 h and then it was disintegrated, washed with water and ethanol then dried at 120 C in a drying oven for at least 24 h to produce silica xerogel. The dried xerogel was washed with distilled water to make sure it is free of Na 2 SO 4 and calcinated at 300 C to produce SiO 2 nano-particles (Zaky et al., 2008). 2.2.2. Preparation of nano-calcium carbonate CaCO 3 nano-particles were prepared by using the reaction sys- tem Ca(OH) 2 –H 2 O–CO 2 as described in our previous work (El-Sheikh et al., 2013; Barhoum et al., 2014). Bulk CaO was firstly calcined at 1000 C for 2 h then slaked into lime milk in mono-distilled water at 80 C. The lime milk was cooled to room temperature (20 C). At this temperature, the CO 2 gas was injected into the lime milk (1000 mL min 1 flow rate and 1 M CaO concentration) with vigorous stirring. The pH value and electric conductivity of the reaction solution were recorded on-line with a pH-meter (Jenway 3305) and a conduc- tometer (Jenway 4510). When the pH value decreased from 14 to 9 and the electric conductivity showed a sharp decrease, this was an indication that the reaction was completed, the CO 2 flow was stopped. CaCO 3 particles were obtained and dried at 120 C in a drying oven for at least 24 h. 2.2.3. Preparation of silica/calcium carbonate nanocomposite SiO 2 /CaCO 3 nanocomposites were prepared via a step-by-step method to produce a core–shell structure (Bala et al., 2007). The core CaCO 3 nano-particles were obtained in the first step of a carbonation method and SiO 2 layer was coated on the core in the second step of sol-precipitation method from pre- cursor of Na 2 SiO 3 Æ 9H 2 O (Gamelas et al., 2011). The prepared CaCO 3 nano-particles were dispersed in 150 mL distilled water using high shear mixer of 3000 rpm for 30 min. The prepared slurry was exposed to high ultrasound waves using the W- 385 Sonicator for 30 min. to completely break the agglomer- ates. The prepared Na 2 SiO 3 solution was gradually added to the suspension. The molar ratio of SiO 2 /CaCO 3 nanocompos- ites was designed and controlled to be 1:15, 1:10 and 1:5 of the samples CS1, CS2 and CS3, respectively. The mixture was heated up to 80 C with vigorous constant stirring. The pH value was adjusted to 9 by gradual addition of 6 wt.% H 2 SO 4 solution for one hour. Then the mixture was aged for 3 h. The produced slurry was cooled, filtered and rinsed with distilled water to be sure it is free of Na 2 SO 4 . The SiO 2 /CaCO 3 nanocomposite powder dried at 120 C for 24 h then at 300 C for 3h.
and crevices between the fibres; thus improving the properties of the sheet. The common papermaking fillers are clay (kao- lin), calcium carbonate, talc (magnesium silicate) and titanium oxide (Smook, 1997). Modification of calcium carbonate filler with silica-contain- ing substances has been proposed to obtain modified filler sus- pension with high degree of stability and acid-resistant properties (Snowden et al., 2000; Kim and Lee, 2002; Shen et al., 2008, 2010). Various surface coating approaches have been introduced to improve the light scattering of pigments. The light scattering from calcium carbonate particles can be increased by coating the granules with a thin continuous layer of higher refractive index material (Lattaud et al., 2006). Silica has good properties such as a high specific surface area, high gas absorbability and high oil absorption. These properties are added to the coated pigment specifications (Park et al., 2007). It is also one of the most effective chemicals for paper coating due to its high brightness, opacity, porosity, hydrophilicity. The application of these materials, for instance, as fillers in the paper industry may contribute to the fibre-to- fibre bonding, when coated with silica thus improving paper strength (Gamelas et al., 2011; Lourenc¸ o et al., 2013). Silica is introduced as nano-particles with core–shell structure repre- senting a new type of constructional units; such materials enhance physical and chemical properties, thus allowing a broader range of applications compared to the single compo- nents (Hall et al., 2000; Vestal and Zhang, 2002; Bala et al., 2007). The objective of this work is to prepare fillers in a nano- size from waste material to reduce disposal and pollution problems in addition to improving paper filler performance. Also to avoid the reverse effect of filler in reducing the paper strength. The main goal is to prepare (a) nano-silica from the semi- burned rice straw ash (b) silica/calcium carbonate nanocom- posite. The prepared fillers were characterized, and therefore the effect on handsheet properties was investigated.
2. Experimental
2.1. Materials
A semi-burned rice straw ash (SBRSA) was obtained from gas production unit of rice straw located at El-Azazy village, Zagazig, Egypt. An analytical grade calcium oxide (CaO, 97% on a dry basis, Acros Organics) and pure CO 2 gas (99.8% supplied by Industrial Gases Company, El-Hawamdia, Giza, Egypt) were used in the preparation of CaCO 3 nano-par- ticles. Bleached kraft hardwood and softwood pulps, poly- acrylamide suspension (47% solid content) as a retention aid, commercial PCC (1.5 l m length and 250–300 nm in diameter) as reference filler for comparison (supplied by Rakta Company, Alexandria, Egypt) were used for paper handsheet preparation.
2.2. Filler preparation
2.2.1. Preparation of sodium silicate and silica nano-particles A SBRSA sample was burned at 800 C for 2 h to remove all the residual hydrocarbons. The produced powder was then sieved. Fraction size less than 0.71 mm (about 54.77%) was selected to perform the experiments. Concentrated sodium sil-
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