Sizing Efficiency of AKD in Causticizing Calcium Carbonate Filled Paper

Causticizing calcium carbonate (CCC) is produced as a by-product in the causticization step of the kraft pulping process. It is often calcined in a rotary lime kiln after being dewatered and reused in the causticizing process. But for the China mill, the conventional recycled way is difficult because the CCC is mainly obtained from non-wood pulping materials, which higher silicon content led to serious silicon obstacle. So it is often discarded as solid waste or used in landfill after dewatering and secondary pollution is brought. In order to prevent its secondary pollution, recent years, the CCC is used as a filler in China papermaking industry. In mill trials, the CCC can be used to replace an amount of precipitated calcium carbonate (PCC). Unfortunately, the application scope and dosage of CCC have been limited due to its lower sizing efficiency than PCC. In this study, the reason for the lower sizing efficiency of alkyl ketene dimer (AKD) when CCC was used as a filler was investigated. The results showed that the materials in green liquid, such as insoluble matter in green liquid, silicon and metal ions, were a little influence on the sizing efficiency of AKD. The higher BET and BJH pore volume of the CCC were the main reason for lower sizing efficiency of AKD when it was used as filler.


Introduction
In the alkali recovery process, it must be to do that the sodium carbonate (Na2CO3) in green liquor is converted into active sodium hydroxide Slaking: CaO+H2O→Ca(OH)2 Causticizing: Na2CO3+Ca(OH)2→2NaOH+CaCO3 The causticizing process has two targets, which are obtained from separating the causticized liquor.   16) are shown in Table 2.

Preparation of CCC
Quicklime was added to a flask, which was filled with water and at 300 rpm to achieve slaking. Then the green liquor that was filtered by slow filter paper

Determination of physical properties of CCC
The determination of particle size was carried out using a BT-9300H laser particle size analyzer. The determination of BET and BJH was carried out according to an Autosorb-iQ.

Adsorption of AKD
The experiment designed in two steps. In the first step, using an ultraviolet spectrophotometer, the absorbance of different concentrations of AKD was determined at a wavelength of 238 nm, 17)

Sheet forming
The 0.2% AKD( to dry stock ) was added to the dispersed stock after 60 s stirring, and 30% PCC (to total paper ) or CCC was added. Then, 0.03% CPAM (to total paper) was added after 30 s stirring, the stock was poured into the sheet form and handsheets were prepared with a basis weight of 80g/m 2 . The wet paper had pressed for 3 min under the pressure of 4kgf/cm 2 and dried 5 min at 95℃.

Results and Discussion
3.1 Sizing efficiency of AKD in CCC filled paper The sizing efficiency of AKD in CCCs or PCC filled paper is shown in Table 3.
Compared with that of PCC filler, the Cobb60 values of paper filled with CCC fillers were higher. That displayed the lower sizing efficiency of AKD. Three kinds of CCC displayed the approximate result. The results showed that there must be differences in the properties of CCC and PCC, which caused the lower sizing efficiency of AKD in the CCC-filled paper.

The influence of insoluble matter
The green liquor and clarified green liquor were used to prepare the CCC with the same conditions. The basic properties of CCC and the Cobb60 values of paper filled with CCC are shown in Table 4.
We can know that the insoluble matter in green liquor just affected the brightness of CCC which had a the brightness of paper, but had no obviously influence on the sizing efficiency of AKD when it was used as filler.

The influence of silicon
The clarified green liquor was treated with sol-gel method treatment and obtained the treated green liquor. 18) Two kinds of green liquor were tested the Tyndall   effect, as Fig. 1 and Fig. 2. From the Fig. 1, there is colloid in clarified green liquor due to the Tyndall effect showing. When the clarified green liquor was treated with sol-gel method, the Tyndall effect is disappeared (as Fig. 2). The experiment was illuminated that the colloid in clarified green liquor was removed by solgel method treatment. Measured the silicon content of two kinds of green liquor, we found, along with the silicon dioxide content of sol-gel treated clarified green liquor was decreased from 6.45 g/L to 1.06 g/L.
Meantime, The main component of above sol-gel derived powders was silicon. So the silicon in green liquor had been existed as a colloidal state. The two kinds of green liquor were used to prepare the CCC with the same conditions. When it was used as filler, the Cobb60 values of paper filled CCC are shown as Table 5.
Although the silicon dioxide content contained in two kinds of green liquor were obviously different

The influence of other element in clarified green liquor
The clarified green liquor was dried at 60℃, and and ground into powder. The powder was tested with EDX as Fig.3.

The influence of CCC
By precisely controlling the causticizing reaction as Yasunori 19) and Wang's research, 20) the needle-like CCCs were prepared as Fig. 5. When it was used as filler, we surprised to find that the Cobb60 value of paper filled CCC was approximately that of PCC, as shown in Table 6.
There is no obviously different ave. particle size between PCC and needle-like CCC. But the sizing efficiency of AKD in the CCC-filled paper was enhanced significantly. The results showed that there must be some changes occurred in the properties of CCC with the change of its micromorphology. The physical properties of fillers are shown in Table 7.
From the Table 7, the ave. particle size of mill CCC 2# or irregular CCC is a little bigger than that of PCC or needle-like CCC, but, compared with PCC or needle-like CCC, both its BET and BJH pore volume are increased to about 3 times. The higher BET of filler, the more AKD is adsorbed. The AKD that is adsorbed by the filler will become spreading AKD molecules and migrate into the void structure of the 펄프․종이기술 46(2) 2014 pore of CCC in drying process. Meanwhile, a portion of the AKD molecules in contact with surface of CCC can also migrate into the void structure in the drying process. Those AKD are then unavailable for sizing. 21) That provides an explanation why there has lower sizing efficiency when conventional CCC is used to fill the paper. So controlling the BET and the BJH pore volume of the CCC are effective way to improve the sizing efficiency of the CCC filled paper.

Conclusions
In this study, the results showed as following; -The materials in green liquid, such as insoluble matter in green liquid, silicon and metal ions, were a little influence on the sizing efficiency of AKD.
-The higher BET and BJH pore volume of the CCC were the main reason for lower sizing efficiency of AKD when it was used as filler.