Polyaluminium Silicate Sulfate And Its Application In Papermaking Industry

A series of polyaluminum silicate sulfates with different basicities and Si / Al molar ratios were synthesized by the reactions of alum with sodium silicate and sodium aluminate. And their structure characteristics, aluminum species, stability of hydrolysis, electrokinetic properties and coagulation performance were studied. Their applications as coagulants in wastewater treatment and as microparticles in microparticle retention aid systems were investigated. Retention mechanism of amphoteric starch / PASS was proposed.The structure characteristics of polyaluminum silicate sulfate (PASS) were analyzed by means of infrared and X-ray diffraction spectra. It was found that the PASS had similar structures with polyaluminum sulfate (PAS), the silicon in PASS complexed with the other polyhydroxy aluminum at same behavior with aluminum. PASS was more stable against hydrolysis and had different crystallization behavior with PAS in environmental temperatures.The aluminum species in PASS aqueous solutions were identified with 27^A1-NMR and Al-Ferron complex timed colorimetric method. As expected that the degree of polymerization of PASS increased with the basicities but decreased with the Si/ Al molar ratios. No Al₁₃ was found in PASS and PAS, but a considerable amount of Al_b, which represents the polynuclear aluminum species, were detected. PASS tended to aggregate to form larger molecules with dilution. And, the higher the basicity, the larger the trend. At the pH of papermaking PASS existed mainly as the form of Al_c, which represents the large colloid molecules of aluminum in the aluminum consistency oftenused in papermaking. The hetero-ions in tap water lessened the discrepancy caused by pH, and increased the amount of Alb, consequently, Alb and Alc became the major aluminum species at the pH of papermaking.The hydrolysis destability of PASS was investigated by measuring the variation of the precipitate concentration with time caused by hydrolysis using spectrophotometric method. The results showed that the hydrolysis destability of PASS increased with basicity and pH, decreased with Si/Al molar ratio.The eletrokinetic characteristics of PASS were examined with microeletrophoretic method. The isoelectric point of PASS when titrated with NaOH was at approximately pH5, that is just agreement with alum at same conditions. Yet, its absolute values of Zeta potential either before or after the isoelectric point were larger than that of alum. The higher the basicity, the larger the absolute values of the Zeta potential of PASS. The Si/Al molar ratio only had negligible effect on the Zeta potential, especially for the positive value before isoelectric point. The absolute values of Zeta potential also increased with the stirring, but the isoelectric pH was hardly changed. The isoelectric pH for PASS was shown to be higher when it was diluted with tap water instead of deionized water, but the absolute values both before and after its isoelectric point became smaller.The coagulation - flocculation of PASS was investigated with kaolin suspension. The results showed that the dominated mechanism of coagulation - flocculation of PASS for removal of turbidity was not the charge neutralization. The most effective PASS for removal of turbidity had the basicity of 50% as well as Si/A! molar ratio of 0.05. The higher the basicity of PASS, The lower the most effective pH ofkaolin suspension for removal of turbidity.When treated the self-made deinked wastewater for removal of turbidity, chromaticity and COD with coagulation - flocculation method, PASS and PAS both were more effective coagulants than alum. However, PASS had a higher removal efficiency of chromaticity, PAS had a higher removal efficiency of COD. PASS with higher basicity (50%) had a higher removal efficiency of turbidity; PASS with lower basicity (30%) had a higher removal efficiency of COD and chromaticity . The higher the basicity , the lower the most effective pH for the removal of turbidity, chromaticity and COD. For the same PASS, the most effective pH for the removal of turbidity, chromaticity and COD were decreased according as above order. When the deinked wastewater was coagulated and flocculated by the synergism of PASS and CPAM, the floes produced were large and heavy, therefore, its settling velocity increased. Yet, CPAM had little effects on the removal efficiency of turbidity, chromaticity and COD.When the pulping and papermaking combined effluent and their medium-sector effluent that include screened, cleaned as well as bleaching effluents were treated with PASS, the similar results were obtained, but the most effective pH for the removal of turbidity was lower than that of chromaticity and COD for the combined effluent, and the medium-sector effluent was easier to obtain higher removal rates of turbidity, chromaticity and COD.The retention tests on wheat straw pulps were carried out by making and weighing the handsheets as well as measuring their ash contents with standard methods. The drainage tests were conducted by measuring the beating degree of furnish with straw pulps. The results showed that PASS/cationic or amphoteric starch were more effectiveretention and drainage systems than their counterpart starch at all pH scope from slightly acid to alkaline papermaking, but they had better retention and drainage effects on wheat straw pulps at neutral / alkaline pH. The PASS / amphoteric starch system was more effective on retention and drainage of wheat straw pulps than PASS/cationic starch because amphoteric starch itself was more effective than cationic starch. The PASS/cationic or amphoteric starch systems behaved more like starch itself in retention and drainage characteristics at low starch dosage. Yet, they behaved more like microparticle system at higher starch dosage.The retention effects of PASS/amphoteric starch on wheat straw pulp were tested further with dynamic drainage jar (DDJ) fitted with 70 mesh screen. Similar results.were obtained with above test. The discrepancy was that amphoteric starch/PASS behaved like microparticle system at all starch dosage. The dosage between starch and PASS for most effective retention on wheat straw pulp had a relationship of compensation. More information was obtained from DDJ test. The addition sequence, that is the addition of starch followed by PASS, the simultaneous addition of starch and PASS, the addition of PASS followed by starch, included different retention mechanism. The stirring time after PASS addition at 500rpm had great effect on the retention of wheat straw pulps. The longer the time, the lower the retention. However, the stirring time at 750rpm before PASS addition had little effect on the retention. The retention decreased only when the starch-formed floes were destroyed by long time stirring at higher speed. The shear forces after starch addition also had remarkable effect on the retention of wheat straw pulps. Either low (less than 900rpm) or high (more than 11 OOrpm) shear forces could be...