Study On Characteristics And Reclamation Of Spent Liquor Of Total Chlorine Free Bleaching

Total chlorine fee (TOC) bleaching technique is a kind of advanced process that fully conforms to the rules of green chemistry. Compared to the traditional chlorine-based bleaching, it has remarkable advantages including simple operating process, lower cost of investment, lower energy consumption, environmentally friendly products and less even none pollution emission. In the past decades, medium consistency TOC obtained quite great development and has been widely used in domestic pulp and paper industry. It showed significant environmental and social benefits. As the global paper market steadily grows, the modern pulp and paper industry confront many challenges such as shortage of water, energy and fiber resources, exacerbation of environmental pollution, and the global warming. So, it is urging to reduce the reliance on fossil energy, renovate the idea about traditional pulp and paper industry, and enhance the ability of all-around use of fiber resources. Furthermore, the further study of potential of TCF which will lead to the innovative way of recovering process products, lower water consumption and the elimination of pollution, is the only way of making pulp and paper industry more eco-friendly and society-sustainable.Based on the achievement in TCF technology our research group has acquired, the present study focused on chemical reactions taking place in stages of oxygen delignification, pretreatment for hydrogen peroxide and hydrogen peroxide bleaching. The reaction products dissolving in spent liquor were determined by HPLC (High Performance Liquid Chromatography), and physicochemical and biological characteristics were then studied. Based on the above results, study on aromatic aldehyde/ketone in oxygen delignification liquor and regeneration of pretreatment spent liquor was carried out in the hope of reducing the consumption of water resource, eliminating pollution, developing potential of TOC technology in promoting the sustainability of pulp and paper industry. The study on chemical reactions, resultant of reaction and formation of aromatic aldehyde/ketone in oxygen delignification process was performed. The results showed the molecular oxygen transforms into free-oxygen radicals through electron transfer in alkaline condition, the formed radicals lead to degradation of enol lignin, and the resultants are then soluble in spent liquor. The composition of spent liquor was determined by GC/MS, the results showed aromatic aldehyde/ketone, carboxylic acids and phenols are majorities. Furthermore, vanillin, syringaldehyde, acetosyringone, hexadecane acid, p-hydroxy-acetophenone, acetovanillone are the first six resultants in concentration. The pathway of aromatic aldehyde/ketone was also investigated. It was found the oxidative elimination reaction induced by electrophilic attack occurs on side chain and methylene quinine of lignin with alkyl or carbonyl group. Such reaction leads to the formation of aromatic aldehyde/ketone which is of high recoverable value and of high concentration in oxygen delignification spent liquor, 24.32mg/L and 24.66mg/L respectively. The results of study also showed that the formation of aromatic aldehyde/ketone in oxygen delignification process was related to the structure of lignin in plant materials.Mechanism of pretreatment for peroxide bleaching, effects of conditions of the pretreatment on removal of metal ions and determination of metal ions in spent liquor were investigated. The results showed solubility of metal ions is dependent on pH value of pretreatment solution, and the removal of metal ions is in the order of Mn²⁺>Cu²⁺>Fe³⁺ in pretreatment condition. The concentrations of metal ions in pretreatment spent liquor are high because metal ions transfer from pulp to the spent liquor. So, the spent liquor is usually sent to effluents treatment mill. Furthermore, chemical reactions occur more or less to lignin and carbohydrates in pulp. However, it was worthy to point out some kinds of plant sterols in pulp dissolved into spent liquor in pretreatment stage.Based on the above study on OQP bleaching, we investigated the physicochemical and biological characterizations of OQP bleaching spent liquor。The results showed OQP spent liquor has a color of 450±10 Pt-Co with a very lower turbidity, pH of 10.8, 7.07g of solid residue at 103-105℃, 3.95g/L of salinity. The molecular weight distribution of dissolved lignin in spent liquor was 10% MW < 187, 30% MW< 284, 60% MW<467, 85% MW<860, 10% MW>1100,with an 315Da of average molecular weight. The results also show that organic loading of spent liquor of oxygen delignification was much higher than that of H2O2 bleaching, while there were just mental ions in spent liquor of pretreatment stage. The study on biological characterizations of OQP led to two conclusions. The first was that spent liquor of any stage in OQP bleaching process had a BOD5/CODcr vale below 0.4, which suggested the OQP spent liquor are really to degrade in environment. The second was that the 3h-EC50 of OQP spent liquor and CEH spent liquor were 3h-EC50>80% and 3h-EC50=31.3% respectively, which suggested that the OQP spent liquor have no biological inhibition on microorganisms, while spent liquor of CEH had obvious biological inhibition on microorganisms.The chemical mechanism of peroxide bleaching was investigated, and the reactive resultants were determined by GC/MS. The results indicated peroxide ionizes in alkaline condition and becomes converted into HOO- which bleaches pulp. These bleaching reactions include reaction between peroxide and benzene ring in lignin, peroxide and sidechains of lignin. Consequently, chromophoric group conjugated double bond in lignin break down and the lignin degrades into soluble compounds. Aromatic ketone and carboxylic acids were found in spent liquor, which verifies the mechanism of peroxide bleaching. Additionally, sterols derived from plants and cooking additive anthraquinone were also found in spent liquor of peroxide bleaching.Oxygen delignification spent liquor is rich in acrobatic aldehyde/ketone. The present work reported on an approach for separating vanillin and syringaldehyde (VSA) from oxygen delignification spent liquor using nonpolar macroporous resin. The effects of temperature and pH on the adsorption isotherms, adsorption model, and kinetic model were studied. The adsorption capacity and adsorption equilibrium constant were obtained using the Langmuir adsorption model at pH<4.5, where the ionization of the VSA to ionic forms was negligible. The standard enthalpy change was calculated using the Van't Hoff equation and clearly showed that the adsorption is an exothermic process. The effect of pH on the adsorption isotherm is well-described by a modified Langmuir model and shows the adsorption equilibrium constant decreases significantly with the increasing pH. The VSA adsorbed on macroporous resin was eluted by ethyl ether. The separated VSA contained 37.51% vanillin, 31.88% syringaldehyde. The recoveries of vanillin and syringaldehyde were 96.2% and 94.7%, respectively.The study on regeneration of peroxide bleaching spent liquor with a purpose of elimination metal ions Fe²⁺ and Mn²⁺was performed by using ion exchange resin. The effect factor, absorption model, kinetic model were discussed. The results showed agitating rate, resin dosage, temperature and pH value are the factors affecting the absorption process most. Both Langmuir model and Freundlich model can describe Fe²⁺/ Mn²⁺ absorption on ion exchange resin, but Langmuir is better in fitting experimental data. It is also found that saturated capacity of ion is 53.47mg/g and 64.10mg/g for Fe²⁺ and Mn²⁺ respectively, adsorption rate constant is 0.6. Furthermore, the adsorption complies with second-order dynamics.