Study On Effect Of Silicon On The Structure And Properties Of Causticizing Calcium Carbonate

Lack of fiber raw materials has become a major problem worldwide paper production in recent years. China is rich in non-wood fiber, but silicon content of non-wood fiber raw material is much more, which will affect the alkali recovery process. Causticizing calcium carbonate（CCC） is difficult to handle, and pollutes the environment. Many scholars have conducted in-depth research, mainly including straw green liquor desilication and refining calcium carbonate filler from straw pulp white mud. The report that based on the "simulation green liquor" introduces the research on effect of production of green liquor in different reaction conditions on the straw CCC. The green liquid straw CCC influcned by crystal control agent on particle morphology and various performance of paper was researched.By comparing the "simulated green liquid" and green liquor, it was found that simulated green liquor and green liquor had the same properties, and simulated green liquid can be used to research various properties of green liquid. The silica content had no significant effect on the CCC’s crystal shape. As silicon content increased, morphology of calcium carbonate changed from a neat square or needle gradually into flake, more and more holes appeared on calcium carbonate surface, whiteness degree of calcium carbonate slightly increased, the average particle size reduced, specific surface area increased, the adsorption rate of AKD increased. Silicon is conducive to the formation of smaller pores on calcium carbonate particles. In the paper filling experiment influenced by different silicon content, the paper optical and mechanical properties changed little, but the degree of sizing decreased with the silicon content increased. As the silicon content was greater than 3g/L in the green liquid, paper sizing effect lost totally.Research on different causticizing conditions for the undesilicated and desilicated straw CCC, considering various conditions, indicated that for desilicated straw CCC, the optimal reaction conditions were as following:75℃,2h,200rpm,6mL/min; for undesilicated straw CCC, the optimal reaction conditions were as following:75℃,2h,200rpm, 20mL/min. Under optimal conditions it was found that by varying the causticizing reaction conditions the performance of desilicated straw CCC can be adjusted and controlled, but straw CCC performance can not be controlled. Silicon had a great influence on the straw CCC.Through modification by a single crystal control agent on undesilicated straw CCC, as STP increased, calcium carbonate particle size decreased, the morphology structure gradually became the amorphous structure; With the increase of CMC, calcium carbonate particle size decreased, the morphology of calcium carbonate with cube structure gradually became grain-like particles. When CCC was added to paper as filler, compared with the unmodified undesilicated straw CCC, paper whiteness, tensile tear index and sizing effect had a larger increase. Overall, when STP content is 0.007wt% and the CMC content was 0.25 wt%, the paper filling performance is best.By modification of undesilicated straw CCC with combining crystal control agent, the modified calcium carbonate particle size was greater than the unmodified, the surface area was smaller; particle size for the modified calcium carbonate with STP was greater than SPP, specific surface area was smaller; modifying with STP, calcium carbonate particle size of ZnCl₂, MgCl₂ and BaCl₂ modified was decreased gradually, specific surface area increased gradually. When modified CCC was added to paper as filler, with the use of SPP, maximum paper whiteness was obtained by modified with ZnCl₂; the highest opacity and tear index and minimum paper Cobb value were obtained by modified with MgCl₂; the highest paper tensile index was obtained by modified with BaCl₂. With the use of STP, the highest whiteness and opacity and minimum paper Cobb value were obtained by modified with MgCl₂; the highest paper tensile index was obtained by modified with ZnCl₂; the highest tearing index was obrained by modified with BaCl₂. In general, STP and MgCl₂ combination is suitable, the paper filling performance is best.