| Desilication of Wheat Straw Black Liquor | China | - | Pilot scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 17
Background
Silica disturbance is well known as a troublesome question in the recovery of alkaline straw pulping black liquor, because the large quantity of SiO2 contained in the black liquor precipitated and plugged the separating wire and scaled in the evaporators. Thus obviously decreased the separating and evaporating efficiencies of the recovery process. The principles of this case study are:
Cleaner Production Principle
Recovery, reuse and recycle
Cleaner Production Application
Pilot plant trial of desilication for alkaline wheat straw pulping black liquor by flue gas treatment to improve the evaporating and lime recovering efficiencies.
In normal alkali recovery process, the black liquor contained in the digester pulp is:
Due to silica disturbance as showed in 4.0, a desilication process for treating alkali wheat straw pulping black liquor by flue gas was investigated through pilot plant trials. The main operating procedures are:
The main parameters obtained by this study could be summarized as followed:
| Test capacity | 3 m3 extracted black liquor/h |
| SiO2 in original liquor | 6 -8 g/l |
| SiO2 in clarified liquor | 1,0 - 2,3 g/l |
| Dilute sludge volume | 20% |
| Desilication efficiency | 83 - 88% |
| Black liquor loss | < 1% |
| Bone dry sludge | 3,5 - 4,1 kg/m3 liquor |
| Power consumption | < 10 kWh/m3 liquor |
In discussion of the study, the authors pointed out: "The pH value of desilicated liquor is reduced, and also the evaporation efficiency, the ultimate concentration of the liquor. So white liquor or alkali must be added to increase the pH value of liquor as to satisfy the requirement of evaporation."
Pilot plant trial on a capacity of 1-3 m3 extracted black liquor/h, which roughly corresponding to 3 tons pulp/day.
This technology is in the pilot plant stage, the quantitative figures shown above are estimations based on the pilot test.
This technology is a long way from commercialization as it was only a small scale pilot trial.
Environmental and Economic Benefits
Theoretically, the desilication of straw pulp waste liquor can decrease the scaling problem and thus obviously improves the evaporation efficiency, and at the same time the silica problem caused in lime recovery may decreased. But it must be proved in productional scale.
It is difficult to estimate the actual economics from this pilot study. The key point is whether the evaporating efficiency would be improved, which could not be answered by the study.
Constraints
Adding white liquor or alkali to satisfy the requirement of evaporation would be a drawback of this process which calls for further investigation.
Many efforts have been made by numerous researchers by different ways, such as flue gas, biological or precipitating method (adding Ca, Mg, Al precipitants). None of these have been successful on a commercial scale. As for the CO2 flue gas method, the investment and maintenance cost would be high due to a large gas-liquor reactor, fan and separator. Furthermore, the flue gas absorbed by the liquor will be liberated in the evaporators and thus foam problems would occur which would influence the evaporating efficiency in a negative way.
A further productional scale test was published by the same first author of the pilot test with the title:"Production Scale Desilication Experiment from Alkaline Straw Pulp Black Liquor." The test scale was 250-500 m3 black liquor/day. The silica content in desilicated clean black liquor was about 0,37 g/l, silica removal rate about 90% and electric energy consumption was about 5 kWh/m3 black liquor, loss of black liquor < 4%. This report was published in "China Pulp and Paper" vol. 5, 1991, with English abstract. According to the discussion of this report, some shortcoming still existed as the foam problem of treated black liquor occurred in evaporation must be further overcome and the practical benefits of desilication for evaporation, combustion, as well as the character of lime mud etc. have to be proved in further practice yet.
Type and operating parameter of the evaporator used in the pilot test were not given, so it is difficult to evaluate the variation of the test data about evaporation.
Two consequences would be caused by the CO2 flue gas in contact and reaction with black liquor: (1) viscosity of straw pulp black liquor would change which do influence the evaporation efficiency, and (2) flue gas dissolved in black liquor would cause foam problem during evaporation. Both of these factors must be further studied for successful application.
The addition of white liquor or alkali to the desilicated black liquor could improve the evaporation efficiency by decreasing the viscosity of the liquor, but the recovery rate of alkali will decrease due to an incomplete extraction rate of alkali in the causticization process.
Contacts
Review Status
This case study was submitted by the UNEP Working Group on Cleaner Production in the Pulp and Paper Industries, based at the Technical Research Center of Finland (address above) in 1992, as part of a contract for UNEP IE. Before submission the case studies were reviewed at the Center. They were edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.