Study On Pretreatment Process Of Vintage Papermaking Black Liquor

In this thesis, acid precipitation and flocculation-sedimentation methods were applied respectively in the pretreatment of Vintage black liquor from papermaking. In the experiment of acid precipitation, firstly through the initial acid precipitation, it was found that this process produced large amounts of foam and lignin particles were small and caused liquid difficult to be separated. Three methods of acid precipitation and antifoam were applied in foam problem and the results showed that "antifoam-acid precipitation" and "antifoam+acid precipitation" could solve the foam problem effectively. For the latter problem, the results of PAM coagulation experiment showed that:PAM coagulant could improve the filter performance of black liquor and the recovery of lignin effectively by means of making lignin flocs cohered quickly and liquid stratified effectually.Under the premise of the above problems solved basically, the way of sulfuric acid+antifoam (synergy acid precipitation and defoamer)+PAM coagulant was used in order to research the effect of pH on treatment of black liquor by acid precipitation. The experimental results showed that large amounts of lignin was precipitated and black liquor CODcr and chroma were also removed significantly when pH was less than or equal to4. The minimum of black liquor chroma was620degree when pH was equal to2, while the minimum of black liquor CODcr was6648mg/L when pH was equal to3. Besides, different acids which were used in the process of acid precipitation would cause differences in treatment effect of black liquor. As to lignin precipitation, the effect of phosphoric acid was best and nitric acid ranked second, while formic acid was worst. For chroma removal, sulfuric acid was best and chroma removal rate was97.9%. As to CODcr removal, adding acid except formic acid played an important role in CODcr removal. CODcr removal rate of hydrochloric acid was58.1%and sulfuric acid, nitric acid, phosphoric acid and oxalic acid were up to65%. Considering the treatment effect and operating costs, it was a wise choice to apply sulfuric acid to acid precipitation.For the conclusion that the treatment mechanism of black liquor by adding flocculant PAC or PAM when pH was less than or equal to4, was mainly attributed to the flocculant or coagulant effect, the effect of PAC flocculation and acid precipitation in the treatment of black liquor was contrasted.The results showed that the effect of PAC on CODcr and chroma removal of black liquor was better than the pure acid precipitation, regardless of pH value. However, the effect of the pure PAC flocculation was not obvious except when pH was more than or equal to5. In summary, the treatment mechanism of black liquor by adding flocculant when pH was less than or equal to4, was mainly attributed to the acid precipitation and flocculant removal only played a supporting role.In the experiment of flocculation-sedimentation, four kinds of coagulant mentioned in the paper were mixed gradually after optimization of pH and dosage flocculation experiments of a single coagulant. Besides, the effect of complex coagulant on the treatment of black liquor was studied, under the premise of optimum pH and original pH. The results showed that the optimum mixture ratio of coagulants was19(Fe2(SO4)3):8(Al2(SO4)3):27(PAFC):13(KAl(SO4)2), and the preferred pH was6. Complex coagulant could remove CODcr and chroma of black liquor effectively when pH was equal to6and dosage was4mL. At this moment, black liquor chroma was680degree and CODcr was533mg/L. Under the premise of original pH, black liquor CODcr reduced continuously with the dosage of complex coagulant increase, while the CODcr was still more than10584mg/L. During the whole experiment, black liquor chroma didn’t change significantly, remaining at about4500degree. This showed that pH had a greater impact on the treatment effect of complex coagulant.The results of economic and technical indicators analysis of acid precipitation and flocculation-sedimentation on the treatment of black liquor, showed that the drug costs of acid precipitation was19.46yuan per ton and CODcr and chroma removal rate were72.35%and97.69%respectively. While the drug costs of flocculation-sedimentation was10.53yuan per ton and CODcr and chroma removal rate were97.78%and97.74%respectively. Therefore, it was a wise choice to apply flocculation-sedimentation to treatment of black liquor.At the same time, in order to use the lignin extractive reasonably, lignin sludge was recovered and made into adsorbent. The experimental results showed that this two kinds of lignin sludge could removal Cr6+and p-nitrophenol effectively. For Cr6+removal, the lignin sludge from acid precipitation was superior to the lignin sludge from flocculation-sedimentation, while it was contrary to p-nitrophenol removal.