Effects Of DCS On Deinking And Bleaching Of ONP And Its Mechanism

High-yield pulp and deinked pulp were important raw materials for producing newsprint. In the process of their pulping and bleaching, a large amount of dissolved and colloidal substances (DCS) were released, and with the recycling of white water, the DCS in pulp suspension were accumulated. The DCS in pulp suspension would influence the operation and the quality of the products.The DCS from aspen chemi-thermo-mechanical pulp(CTMP), bleached deinked pulp(BDIP), aspen and softwood bleached chemi-thermo-mechanical pulp(BCTMP) were characterized, and their composition was analyzed. Then the effects of the DCS on the flotation deinking of old newsprint(ONP) and bleaching of deinked pulp were studied, and some model substances were used as the main components in DCS and their effects on deinking and bleaching efficiency were investigated, and the mechanism of the effect of model substances on deinking and bleaching was discussed.The results showed that, DS substance was the main substances in the DCS. The solid content of the DS accounted for more than 60% of the total solid content of the DCS, and the ash content of the DS was also above more than 50% of the total ash content of the DCS. The characteristic of the DCS in the four kinds of pulp were different. The concentration of the DCS from aspen BCTMP was much more than that of from other pulps. By GC-MS analysis, it was found that the main components of lipophilic extracts in DCS from the pulps were benzoic acid, dodecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, etc., and the carbohydrates in DCS were mainly glucose, arabinose and xylose etc.The flotation efficiency was improved with addition of DCS prepared from different pulps, and the residual ink of the deinked pulp decreased, the brightness of the pulp was increased. With the increased of DCS addition in flotation, the flotation efficiency increased. Compared to the DCS from other pulps, the DCS from aspen CTMP was more helpful to the decrease of residual ink of deinked pulp, and the DCS from softwood BCTMP was more helpful to the improvement of the brightness of the deinked pulp. The effect of DCS added in the flotation on the bleachability of the deinked pulp was related to the source of the DCS. The DCS from aspen BCTMP and softwood BCTMP could improve the bleachability of the deinked pulp, but DCS from aspen CTMP led to the decrease of bleachability of the deinked pulp, which was perhaps due to the high concentration of transition ions in it. The breaking length of the deinked pulp decreased with addition of DCS in flotation, which was more serious with addition of DCS from aspen CTMP and BCTMP compared to the DCS from other pulps.Lipophilic extractives, sodium lignosulfonate, calcium ions and pectic acid were used as the model substances of DCS to study the effect of these substances on the flotation efficiency of ONP deinking and bleaching. The results showed that, the flotation efficiency was improved in the presence of low concentration of lipophilic extractives, but the flotation efficiency was deteriorated at high concentration of lipophilic extractives. Both pectic acid and calcium ions positively influenced the flotation efficiency. The removal of ink particles and brightness of deinked pulp were all improved in the presence of pectic acid or calcium, but the pulp yield after flotation decreased. The effect of sodium lignosulfonate on the flotation efficiency was less than the other three substances. When the mixture of pectin acid and calcium ions added in flotation, the flotation efficiency was further improved than any one of them was added in flotation. The addition of pectin acid, calcium ions and high concentration of lipophilic extracts in flotation could cause decrease of breaking length of deinked pulp, but the addition of sodium lignosulfonate in flotation could slightly improve the the breaking length of the deinked pulp.Pectic acid could be degraded to oligosaccharide by pectinase. The cationic demand of DCS or pectic acid decreased after pectinase hydrolysis. Compared to the pectic acid without pectinase treatment, the degraded pectic acid added in flotation could further improve the flotation efficiency.