Recycling Technology Of Papermaking Sludge By Preparing Flocculant And Its Application

Papermaking sludge is the byproduct of papermaking industry, of which disposal impedes the development of the whole industry, and which has serious side effect on environment. Based on the current status, papermaking sludge was recycled through preparing flocculant in this study. Coagulation performances and properties of flocs, including floc size, growth rate, strength and recoverability as well as fractal dimension, were investigated when the flocculant was used in combination with aluminum sulfate (AS), polyaluminum chloride (PAC) and polyferric chloride (PFC). Membrane fouling in coagulation-ultrafiltration hybrid process was also studied. The main research contents and conclusions are as follows:1. Papermaking sludge was used to prepare a novel flocculant LA (lignin-acrylamide polymer). Physicochemical properties of LA were measured by FTIR, SEM, XPS and zeta potential as well as molecular weight. Results indicated that acrylamide was successfully grafted onto lignin that contained in papermaking sludge through graft copolymerization reaction. Through this, papermaking sludge was recycled.2. LA was used in combination with AS or PAC in humic acid-kaolin simulatedwater. Different dosing methods, including Al-LA (AS or PAC was dosed first, and then LA was dosed), LA-A1(LA was dosed first) and single use of Al, were investigated in terms of coagulation performance and floc properties. Results showed that LA could improve turbidity removal and DOC removal under raw water pH value, and flocs size and growth rate were also significantly enlarged. Floc recoverability was enhanced due to charge neutralization and adsorption of LA. However, bridging of long LA chain did not increase floc strength dramatically.3. At the whole pH range of4-9, LA improved coagulation performances of AS and PAC. Floc properties, such as floc size, growth rate and recoverability, were in the order of Al-LA> LA-A1> Al. Generally, flocs got better recoverability but looser structure under acid condition. For Al-LA, LA-A1and Al, floc strength under pH7was the weakest, and flocs tended to break when shear force increased. Flocs were the largest under pH5and8. Larger flocs had smaller fractal dimension, and fractal dimension was in the order of Al> LA-A1> Al-LA.4. Dual-coagulant PFC-LA (PFC was dosed first), composite coagulant PFCLA (PFC and LA were mixed before dosed) and PFC were investigated in terms of turbidity and DOC removals and membrane fouling when they were used in coagulation-ultrafiltration hybrid process in reservoir water treatment. Under pH condition of5-8, LA enhanced coagulation performance of PFC when LA was used in the forms of PFC-LA and PFCLA. Under the optimal coagulation pH condition (pH=6), PFC-LA, followed by PFCLA and PFC respectively, generated flocs of best characteristics:floc size was the biggest; growth rate was the rapidest; floc strength was the strongest; recoverability was the best and structure was the densest.5. Membrane flux reduction of PFC-LA was the smallest (i.e., membrane fouling was the smallest), while membrane foulings of PFCLA and PFC were similar. Membrane flux of second filtration was smaller than first filtration, which indicated that membrane cannot fully recover after backwash after first filtration was conducted. In the case of PFC, compared with coagulation suspension without breakage period, suspension with breakage and regrowth periods aggregated membrane fouling, and longer breakage period generated smaller flux. For PFC-LA and PFCLA, breakage on coagulation suspension had little effect on permeate flux.6. Single dose of PFC, dual-coagulant PFC-LA and second dose of LA after breakage PFC+LA were comparatively investigated in terms of color removal and floc properties in disperse yellow and reactive blue wastewater treatment. Results showed that PFC gave excellent color removal for both wastewaters, while LA addition enhanced removal efficiencies. Second dose of LA after breakage period based on PFC had evidently positive effect on coagulation performance. 7. Compared with single use of PFC, PFC-LA generated larger flocs at rapider growth rate. However, PFC-LA flocs had opener structure due to bridging of LA chain. For disperse yellow water, flocs formed by PFC-LA were weaker than those of PFC, while the case was opposite for reactive blue water. Floc strength weakened with increasing hydraulic shear. LA could improve floc recoverability and regrowth rate, and the three coagulants in terms of them were in the order of PFC+LA> PFC-LA> PFC. LA played significant roles in charge neutralization, adsorption and bridging.