Research On Rice Straw Simultaneous Desilication During Cooking Process

Straw was the main raw material of paper industry in our country, and rice straw was important sources among these raw materials. The epidermal cells of straw were found to contain large amounts of silicon. In the traditional alkaline pulping process, silicon was dissolved out in the form of sodium silicate, resulting in the black liquor of high viscosity, low extraction rate, difficult to evaporation, recovery and recycling difficulties and so on. The rice straw simultaneous desilication technology was as follows:metallic oxides such as Al2O3, MgO and CaO were added to the soda-AQ cooking process as simultaneous desilication agent, respectively, reacted with silica and generated chemicals similar to talcum powder, kaoline etc., which cover the fiber surface, with the result that inhibited the silicon dissolution. Thus the negative effect of silicon interference was improved, and established the foundation of recovering and reusing the black liquor.In this study, practical straw cooking process was imitated under the laboratory condition, simultaneous desilication agent was seen as the research route. Firstly, the fiber morphology and chemical constituents of rice straw were analyzed, mill production technological parameter was simulated, the effect of adding desilication agent on the properties of black liquor and the viscosity were mainly detected and discussed, and the bleachabillity and pulping property of the resulted pulp were also investigated. Secondly, various strength parameters of the paper sheet were studied. As last, the mechanism was studied through element analysis by using XPS.Through the analysis of fiber morphology and chemical constituents of rice straw, we can concluded that this experimental used material was character of fine fiber, high ash content and nonunifirm distribution of silicon, which in coincidence with straw material, and possessed typical pattern. The parameters were as follows:liquid ratio of3.5, the added percent of LPA130and LCP89were0.0225%and0.0150%, respectively. The highest cooking temperature was160℃, alkaline dosage was9.0%, and the time of rising and keeping temperature were70min and10min, respectively. Under this condition, the study results and discussions were as follows:1. The addition of simultaneous desilication agent can improved the extraction rate of black liquor, even up to11.52%. Among the metallic oxides, the addition of CaO lead to the increase of solid substance content, whereas Al2O3and MgO could effectively reduced the solid substance content, and the largest decline was6.33%, this effect was most obvious when the dosage of desilication agent was3-5%, but when using more desilication agent under this condition, solid substance content increased. Simultaneous desilication agent can effectively reduce the content of silicon in the black liquor, the decrease rate up to78.86%and the viscosity of black liquor was reduced at the meantime. It can be inferred that the desilication agent can effectively restrain the silicon dissolution, thus the black liquor viscosity was decreased, and the optimum dosage of desilication agent was3%.2. The addition of desilication agent can effectively restrain the silicon dissolution, and kept silicon in the pulp, so the pulp yield was improved, and the good pulp yield was increased by6.94%. The beating degrees of these pulps were similar for the same amount of time, which indicated the influence of desilication agent on pulp beatability was negligible. Utilization of desilication agent effect the bleachability of the pulp to some extent, the loss of whiteness was9.1%at most. When using MgO as the desilication agent, the whiteness loss was small, adding3%MgO, the whiteness loss was only1.5%, and the absolute value was0.81%. The kappa number of pulp was declined when adding desilication agent in cooking process, which revealed that the loss of bleachability was not due to the increase of reducing substances (like lignin), but because of the interference of desilication agent and silicide during bleaching process. The addition of desilication agent led to the paper tensile strength and bursting strength were slightly increased, while tearing strength and folding strength were reduced somewhat. It was due to the fine fiber proportion was increased, the fiber average length was shorter and specific surface area was bigger, so the pulp bonding strength was improved, while tearing strength and folding strength were mainly affected by fiber length were reduced. Desilication agent reacted with silica and generated chemicals similar to talcum powder, kaoline etc., which cover the fiber surface, with the result that inhibited the silicon dissolution, and black liquor viscosity was reduced.