| For alkaline pulping of non-wood materials, calcination of Causticizing calcium carbonate(CCC) is inefficient and costly due to the interference of silicate. The results showed when used CCC of non-wood as paper filler, many drawbacks have been produced, such as the particle size and crystal degree are difficult to control; the brightness of paper filled with CCC is lower than co mmercial PCC; in wet end system, microbial growth and biological film formation on the surface of equipment increase, finally speed up the formation of sediments; the dosage of AKD sizing agent will increase, etc. However, previous research mainly focused on removing the silicon and refining the CCC, and the mechanism of silicon affecting the CCC was hardly mentioned. This paper reports on the properties of CCC with different silica content, as well as its impact on paper quality when it was used as filler in papersheet, so as to creat a new way for the application of CCC fro m non-wood pulping.The results showed that CCC and calcium silicate were produced at the same time. The silica content had no significant effect on the CCC’s crystal shape, and that it was formed as calcite in all cases. Calcium silicate itself did not polymerize to for m crystals in the course of causticization, but the Si atoms instead of part C ato ms in CCC produced the isomorphous replacement. Since the bond of Si-O is different fro m the C-O, the CCC’s morphology changed grad ually fro m amorphous state to square with the decrease of the silica content, and specific surface area decreased fro m 18.64m2/g to 5.68m2/g. The CCC with different silic a content are all negatively charged, when the silica content decreased fro m 6.40% to 0.31%, the negative charge density decreased by more than 200%, fro m-1.94 umol/g to-0.60 umol/g, the CCC’s brightness increased slightly and the average particle size tended to grow after silica removal. The CCC’s sedimentation volume and oil absorption value decreased gradually with increasing desilication rate.When CCC was added to paper as filler, with the silica reduction in the CCC, the opacity of the paper almost re mained the same and the brightness increased slightly; but the Cobb value decreased significantly and the sizing efficiency obviously improved. When the CCC’s silica content decreased to 3.20%, the Cobb value was 27.7 g/m2. The tear strength increased gradually, while the tensile strength decreased with the silica reduction. When the CCC and commercial GCC are mixed as paper filler, for the total filler amout of 20%, with the increase of CCC proportion, the brightness and opacity of the paper increased slightly, sizing efficiency gradually beca me worser and worser, and tensile strength inc reased while the tear strength decreased.The negative impact of the CCC on AKD sizing was found to be associated with its silicate content. The higher the silicate contents of the CCC, the lower the AKD sizing efficiency. Scanning electro n microscopic(SEM) images revealed that the presence of silicate in the green liquor changed the morphology of the CCC particles significantly. When the silicate content of the CCC increased from 0.31% to 6.40%(as silica), the cubic crystal structure of the CCC calcite p articles was almost completely covered up with amorphous materials, most likely for med fro m the silicate in the causticizing process. The amorphous materials were porous and thus had larger surface area than the calcite crystals. It the negative charge den sity of the CCC particles increased by more than 200%. As a result, more AKD sizing agent was adsorbed onto the CCC particles, and less AKD was available for sizing the fibers.Under different causticizing process, the presence of silicate in the green liquor changed the morphology of the CCC particles significantly, and the cubic crystal structure of the CCC calcite particles was almost completely covered up with a morphous materials. It also made the average particle size of CCC irregular ly. |
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