Study On Making Calicum Carbonate With Wheat Straw Alkali Recovered White Mud

The forest resource of our country is very limited, so pulp millswill still use rice straw and some other straw raw materials forpapermaking. The method to deal with black liquor of Wheat StrawAlkali Pulping is mainly the alkali recovery, which will lead to thegeneration of large amount of white mud. Wheat straw alkali recoveredwhite mud has some problems such as high dust impurities, high silica,low calcium carbonate whiteness and uneven particle sizes. Thus, it isnot easy to recover the white mud which can cause the secondarypollution of the environment. This paper focuses on the study of usingrecovered white mud to make packing calcium carbonate. The mainworks are summarized as below:1. analysis of straw pulping greenliquid components,2. analysis of traditional straw pulp alkalicarbonization processing,3. the influence of lime concentration,causticizing reaction temperature, dripping speed of green liquid andstirring rate on the morphology of calcium carbonate during thecausticization process,4. physical properties testing on Refined calciumcarbonate and papers which are filled with the calcium carbonate madefrom white mud. After the study, this dissertation draws conclusions asfollow：The essential components of straw pulping green liquid are sodiumcarbonate and sodium hydroxide, with a content of132.33g/Land30.73g/L. Respectively the major components of the impurities are silicondioxide and colloidal material, and the silicon content can reach to9.26g/L. In addition, it also contains some metallic and nonmetallic ion.The high content of silicon content is also the reason that recovered white mud is unsuited for papermaking filler. So, by naturalclarification pretreatment to green liquid, we conclude that it has acertain effect on purification and depuration. It can effectively wipe offmagnesium and partly remove aluminium and silicon. After silicaremoval from green liquid, silicon content is reduced from5.24%to1.67%. Then the petreated green liquid will meet the needs of refiningCaCO3for its use as papermaking fillers.In the study on making calcium carbonate from wheat straw alkalirecovered white mud, we discover that conventional straw pulp alkalicarbonization processing couldn’t control the crystal forms and particlesize of calcium carbonate efficiently, and the resultant calciumcarbonate is amorphous and uneven. Then, we improve the process byseparating the nitration reaction and causticizing reaction: calciumoxide powder was added into water first, and then the generated calciumhydroxide causticized with green liquid. When calcium oxideconcentration is20%,the temperature is50℃, the dropping rate ofgreen liquid is0.073ml/s, the mixing speed is400rpm, uniformneedle-like calcium carbonate are obtained. The final product is evenand has high aspect ratio and good particle dispersion. Through XRDmonitoring, it’s turned out to be needle-like calcium carbonate.Physical properties testing for needle-like calcium carbonate andcommercial PCC demonstrates that the whiteness of commercial pcc is92.81%ISO, while the needle-like calcium carbonate is90.89%ISO,which have increased whiteness by10percent compared withtraditional straw pulp white mud. The average size of commercial pcc is5.31μm, while the needle-like calcium carbonate is6.64μm, deceasedby47.6%compared with conventional straw pulp white mud. Thespecific area of commercial PCC is520.65m~2/kg, while the needle-likecalcium carbonate is818.25m~2/kg, which was3.35times higher thanconventional straw pulp white mud. The oil adsorption value ofcommercial PCC is0.890g/g, while the needle-like calcium carbonate is1.308g/g, which is deceased by63.6%compared with conventionalstraw pulp white mud. The comparison to refined straw pulp white mud calcium carbonate,commercial PCC and conventional straw pulp white mud filled in papershows that, the Cobb value of paper made with conventional white mudcan reach to180g/m~2, which has no resistance to water, while the sizingeffect of paper made with refined white mud is almost the same withcommercial PCC. The optical properties of paper filled with refinedneedle-like calcium carbonate have a clear advantage. When filled withthe same quantity of filler, the whiteness of paper filled with needle-likecalcium carbonate is6.0%higher than traditional white mud and asmuch as commercial PCC. The opacity of paper filled with needle-likecalcium carbonate is3.8%higher than traditional white mud and1.4%higher than commercial PCC. The strength properties such as while thetensile strength, bursting strength and tearing strength of paper filledwith needle-like white mud are equal to commercial pcc, while thoseproperties are increased with different degree compared withconventional white mud.