Synthesis Of Water-soluble Starch Graft Copolymer And Its Application In Red-mud Separation

A binary starch graft polyacrylamide copolymer and a ternary starch-acrylamide-acrylate acid (sodium acrylate) copolymer were synthesized by aqueous solution polymerization and inverse emulsion polymerization. The two starch graft copolymers were employed in the red-mud separation of alumina industrial processing, and both of them showed the excellent separation ability.One starch graft acrylamide copolymer was synthesized by graft copolymerization of starch and acrylamide in solution, and the polymerization kinetics and its mechanism was discussed. The influence of temperature, mass ratio of raw materials, types of starch and gelatinization on monomer conversion rate, grafting ratio, graft efficiency and intrinsic viscosity of products, when copolymerization was conducted in solution, with starch and acrylamide as raw materials and ammonium persulfate as initiator. The results showed that the grafting ratio is 71%, the graft efficiency is 84%, the monomer conversion rate is 97%, and the molecular weight of polyacrylamide grafted on starch is 7 million, when the reaction temperature was 50℃, the mass ratio of acrylamide to starch was 2:1, the gelatinization temperature was 70℃, and the gelatinization time was 30 min. The correlation equation of initial reaction stage was derived at low initiator concentration, shown as follows:Therefore, it can be concluded that the graft polymerization of starch and acrylamide is a free radical polymerization. The free radical termination undergoes between initial free radicals, as well as free radicals initiated by initial free radicals.Starch graft acrylamide copolymer was also synthesized by inverse emulsion polymerization, and the polymerization process conditions and the polymerization kinetics were studied. The types, composition and amounts of emulsifiers and emulsification methods affected the monomer conversion rate, grafting ratio, latex particle size and the intrinsic viscosity of the product. It showed that the emulsion is of good stability and solubility at low temperature with fatty acid (salt) as emulsifier, and the mixing emulsifier of surfactants reveals better emulsification property than a single emulsifier. So we synthesized the starch graft acrylamide by inverse emulsion polymerization using the mixture of oleic acid and sodium oleate as emulsifier, with the monomer conversion rate 99%, the graft ratio 82.5%, and the intrinsic viscosity 1300 ml/g. The effect of three kinds of initiators, including ammonium persulfate, ammonium persulfate-urea and benzoyl peroxide-sodium sulfite, on the monomer conversion rate and the intrinsic viscosity was investigated. The results show that the correlation equation of the inverse emulsion polymerization of starch graft acrylamide was derived at low initiator concentration for the initial reaction stage, shown as follows:It is according with the reaction regularity of inverse emulsion polymerization.The ternary starch-acrylamide-acrylate acid (sodium acrylate) copolymer with different electricity distributing and electricity consistency was synthesized by adding acrylate acid or sodium acrylate into the emulsion of binary starch graft polyacrylamide copolymer. With the polymerization initiated by ammonium persulfate-urea, the effect of pH and monomer mass ratio on monomer conversion rate and grafting ratio was studied. A novel ternary starch graft copolymer was prepared at around pH 6-7. The catalysis of complexon, EDTA2Na and their mixture (E-N) in the initiation reaction was studied. The results showed that the three catalysts can reduce the initiation reacting temperature, and can improve the monomer conversion rate and the intrinsic viscosity. At last, the ternary starch-acrylamide-acrylate acid (sodium acrylate) copolymer was synthesized by inverse emulsion polymerization using the mixture of ammonium persulfate-urea-(E-N) as catalyst and initiator. When the mass ratio of acrylamide to sodium acrylate was 7:3, the mass ratio of vinyl monomer and starch was 2:1, the reaction temperature was 30℃and the reaction time was 6 h, the monomer conversion rate reaches 99%, the graft ratio is 80.5%, and the intrinsic viscosity is 1650 ml/g.The chemical structure, morphology and the latex particle size of binary and ternary starch graft copolymers were characterized by FT-IR, polarizing microscope and SEM. The chemical structure of the products was proved by FT-IR. After the graft reaction of ungelatinized starch, the morphology of the copolymers kept the aggregate structure of starch, and vinyl copolymer is grafted on its surface; while the gelatinized starch forms homogeneous polymer which is totally different with ungelatinized products.The starch graft copolymers and commercial synthetic polymeric flocculants were applied in the red-mud separation of alumina production. The effects of the types and amounts of flocculants on settling velocity, the amount of supernatants and underflow were investigated. The results showed that the amount of the feed and the flocculant of 820 m~3/h and 300 mg/Kg, respectively, resulted in settling rate higher than 16 cm/min, the amount of supernatants and underflow of 0.15 g/L and 470 g/L, which totally fulfilled the separation of red-mud requirements.