Study On The Technology Of Removing Silicon From Green Liquor By Seeding In The Chemical Recovery Process Of Wheat Straw Pulping

Because of the shortage of wood resources, nonwood materials such as straw raw materials are used for pulping and papermaking as important fiber raw materials. As is known to all, the high silicon content of straw raw materials causes a series of serious problems in the chemical recovery process of pulping and papermaking. Due to the problems caused by high silicon content of green liquor in the chemical recovery process of wheat straw pulping, in this paper, the green liquor was treated with carbon dioxide to precipitate silica by a one-step process and a seeding process, and the interrelated properties of silica crystal were discussed.When carbon dioxide was flowed into green liquor to remove silicon directly, the results showed that the particle size of silica decreased to a minimum firstly and then increased with increasing reaction temperature, when the reaction temperature was 40℃, the silica size of the smallest, was 70.89 nm. With increasing stirring speed,the particle size of silica increased to a maximum firstly and then decreased, so when the stirring speed was 100r/min, the silica size of the minimum, was 95.07 nm. The size of silica increased with the increasing amounts of carbon dioxide, so when the carbon dioxide was5 L, the small particle size of silica can be acquired, the size was101.99 nm. Then the silicon in green liquor was removed under the above-mentioned optimal conditions, and silica was analyzed by a scanning electron microscope, infrared spectrometer and X-ray diffraction. The results indicated that silica was granular and close to the spherical structure, meanwhile the silica was amorphous nanoscale,and the particle size distribution was 70.89nm~171nm.Seeding to remove silicon refers to adding the seed prepared inadvance to the green liquor, then the carbon dioxide continued flowing to the mixed green liquor to make the silicon precipitate on the surface of the primary silica particles and aggregate into secondary particles.When green liquor was treated with carbon dioxide by a seeding process,keep other reaction conditions fixed, when the amounts of seeding increased from 2% to 15%, the sedimentation performance of precipitate deteriorated, the average particle size decreased from61.90μm to 52.95μm, the drainage time increased from 83 s to 113 s, and the total silicon removal of green liquor declined by 3.99%, so the optimal amounts of seeding was 2%. With the reaction temperature increased from 20 ℃ to 80 ℃, the sedimentation performance and drainability of precipitation significantly were improved, but the total silicon removal rate dropt by 2.43%, the average particles size decreased from 56.75μm to 16.4μm, what is more, higher temperature increased the solubility of silicate and was not conducive to precipitate silicon, so the optimal reaction temperature was 60℃. When the stirring rate increased from 100r/min to 400r/min, the sedimentation performance of precipitate was improved, the average particle size increased by 12.13%, drainage time decreased from 161 s to 93 s, the total silicon removal of green liquor increased by 3.72%, when the stirring rate was 300r/min, the sedimentation performance and drainability of precipitate were already satisfactory. The sedimentation performance and drainability both changed better, the average particle size of precipitate and the total silicon removal of green liquor both increased with increasing crystal cultivating time, but considering that over-length time to cultivate crystal would reduce the work efficiency,so optimal time was 60 min. The p H of green liquor decreased with increasing amounts of carbon dioxide, and the p H value became stable at 8.2 after 15 L carbon dioxide was added. With increasing amounts of carbon dioxide, the average particle size of precipitate increased and drainage time decreased, sedimentation performance were improved,more silicon in green liquor was removed, when the p H value was 9.73,a total silica removal rate of 98.76% was obtained from the greenliquor.Green liquor of straw pulping was treated with carbon dioxide when the amount of seeding was 2%, p H of green liquor was 9.70, reaction temperature was 60 ℃, stirring rate was 300 rpm, crystal cultivating time was 60 min. The product was analyzed with SEM, FT-IR and XRD after the silica particles were washed and dried naturally, the results showed that the silica particles were nano-sized, amorphous and bonding together in different degrees, and the particle size distribution was 147.7nm~265.6nm, the specific surface area was 89.14 m2/g.