| Dicyandiamide formaldehyde polycondensation is a kind of simple cationic polymer,but it has many disadvantages,such as low molecular weight and free formaldehyde in the polymer.And cationic grafting starch is a kind of efficient,environmental,stable and affordable Modified method of polymer.In this subject,dicyandiamide and formaldehyde were used as the main raw materials.With dicyandiamide and formaldehyde as the main raw materials,the starch acrylamide graft and melamine were used to modify the dicyandiamide and formaldehyde respectively.It was used as the retention and drainage agent in the wet part of paper making and the decolorization flocculant of the anionic colored wastewater.The MDDF-bentonite particle retention system was composed of modified products and bentonite,and its mechanism was explored in this paper.1.The effects of four factors,such as the ratio of starch to acrylamide,polymerization temperature,reaction time and initiator concentration,on the synthesis reaction were investigated.And the best process of polymer synthesis is judged by the retention rate of pulp.The optimum synthesis condition is starch: acrylamide:dicyandiamide: formaldehyde mole ratio is 1:2.5:5:10,the temperature is 80 ℃,reaction time is 2 hours,initiator is ammonium persulfate: Sodium Bisulfite =1:1.The molecular weight of polymer can be increased to about three million,which greatly improves pulp retention rate.2.MDDF is used as a unit retention system in papermaking wet end.The effects of three factors such as MDDF addition,slurry pH and mixing time on fine fraction retention and tensile index of paper were investigated.The amount of MDDF added is0.03% of the dry fiber,and when the contact time is 30 s,the retention effect is the best.While the amount of 0.03%,and the contact time of making slurry neutral 10 s condition,the tensile index of the paper reaches the maximum value.3.MDDF is the main raw material,combined with calcined bentonite to form MDDF-bentonite particle retention system.Under the neutral condition of MDDF addition of 0.08% and the amount of bentonite added to 0.3%,the initial floc was broken by 1500rpm/min high-speed shearing.After adding inorganic filler,the retention rate of fine fraction of MDDF-bentonite system can reach 79.6%,and the retention and drainage effect is very obvious.The mechanism of MDDF-bentonitewas deduced by measuring the charge density of the slurry and the particle size of the sub group.The initial floc was dispersed,and then the inorganic fillers with opposite charges were added to form a smaller,more tightly structured small floc,distributed on the wet paper,so that the filler and the fine fibers were left on the paper,and the retention rate was improved.Because the charge density of MDDF is higher than that of CPAM,after adding MDDF,the slurry system is closer to the isoelectric point,and the filtration effect is better than CPAM,so better than the unit retention system.4.The effects of melamine and dicyandiamide ratio,melamine and formaldehyde ratio,melamine and ammonium chloride ratio,reaction temperature,reaction time and other factors on the reaction were studied by orthogonal test.Subsequently,the purpose of formaldehyde removal were achieved by adding methanol and urea.The best preparation conditions for the preparation of low free formaldehyde flocculant are n(melamine): n(dicyandiamide): n(ammonium chloride): n(formaldehyde): n(methanol): n(urea)= 1:3:2:15:2.5:0.1.The optimum condition is that the dosage of decolorant is 0.28%,pH=8,the decolorization rate is 99.91%,the content of formaldehyde is 0.00151g/L,which not only reduces the content of formaldehyde,but the effect of decolorization and flocculation is very obvious. |
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