REMOVAL OF COLLOIDAL LIGNIN BIOMASS FROM PULP MILL BLACK LIQUOR AND LIGNIN CONTAMINATED WASH WATER

HASSAN EL-SHALL

ENGINEERING RESEARCH CENTER FOR PARTICLE SCIENCE AND TECHNOLOGY

MATERIALS SCIENCE AND ENGINEERING DEPARTMENT

UNIVERSITY OF FLORIDA

GAINESVILLE, FLORIDA


 

LIGNIN SEPARATION FROM PULPING LIQUOR

Colloidal lignin extracted into pulping liquor by alkali digestion of plant tissue can be precipitated when pH of liquor is made acidic.  Lignin coagulated at low pH does not agglomerate into readily collected flocs -- instead lignin appears as a gelatinous and inseparable mass.

By treating black liquor (or wash water where lignin is carried in lower concentrations) with selected polymeric flocculants, together with surfactant treatment in many applications, lignin solids agglomerate on reducing pH to 3 to 4.   Typically, dense biomass aggregates form immediately on polymer reaction followed by acidifying, and precipitated biomass accumulations are easily collected from liquor or wash water.

 

PILOT PLANT AT UNIVERSITY OF FLORIDA ENGINEERING RESEARCH CENTER

Black liquor treated with surface active polymeric reagent is continuously contacted with flow of dilute sulfuric acid for reducing pH to the level required to achieve lignin precipitation.  The mini pilot plant operates at approximately one liter per minute black liquor flow rate.  Lignin agglomerates in an upward flow reaction pipe chamber and separates after discharging over a segmented screen.  Clear liquor remaining after solids separation, including small amounts of fugitive solids carried through the screen, is collected in a clarifier tank.  Separated lignin biomass solids dewater during residence on the screen.  Further dewatering can take place in a filter press.   Typical water content of biomass solids is in the range of fifty per cent.  The mechanical system for lignin separation and handling of flows is illustrated by Figures 1 through 7 following.

Lignin separation pilot plant work at the Engineering Research Center was sponsored by an industrial affiliate, member company Bountiful Applied Research Corporation.  Pilot plant assembly and operation was carried out by John Henderson and Gary Sheiffele, ERC engineering staff.  Kevin Powers is Laboratory Manager supervising all ERC pilot plant projects.

 

TESTS CONDUCTED ON PULP WASH WATER AND LIGNIN CONTAMINATED PLANT EFFLUENT

Separation of lignin from pulping plant streams was evaluated for determining reduction of color (measured by transmittivity) and contained organic matter (by TOC and COD measurement).  Influence of process variables pH and polymer addition were investigated with three examples of pulping plant water flows selected to represent a range of approximate per cent solids content from 0.6 to 0.025.   Presentations of data showing reductions of transmittivity from polymer treatment, correlated with TOC and COD measurements, are given in following Figures 8 through 17.

 

CONCLUSIONS

Efficient separation of lignin biomass from black liquor is demonstrated in the continuous flow pilot plant.  The separation process is carried out using a simple and low cost system, with minimal chemical and operating requirements.  It can be concluded that practical means of resolving black liquor waste stream handling for meeting acceptable environmental standards are available by application of this process.

Treatment of low concentration water streams from pulping operations is seen as successfully reducing color and COD.  As illustrated in graphs representing data for three process streams provided as examples, degree of reduction can be variable according to process variables of acidity and polymer content as well as concentration of lignin in the stream.  However, it is seen generally that influence of polymer addition and pH on lignin separation becomes less significant on a relative basis beyond threshold values.  The important factor shown in this development is that lignin can be removed from waste water streams effectively by means of polymer treatment.  Feasibility of applying the separation process to a given set of waste stream conditions on a practical basis is obviously dependent on overall pulping plant economics.

 

ILLUSTRATIONS OF PILOT PLANT

FIGURE 1. FIGURE 1.  

SEPARATION SCREEN

FIGURE 2. FIGURE 2.  

LIGNIN FLOCCULATION IN PIPE REACTOR

FIGURE 3. FIGURE 3.  

REACTION AT MIXING TEE

FIGURE 4. FIGURE 4.  

PROCESS PUMPS FROM FEED TANKS

FIGURE 5. FIGURE 5.  

BIOMASS DEWATERING ON SCREEN

FIGURE 6. FIGURE 6.  

CLARIFIED LIQUOR COLLECTION TANK

FIGURE 7. FIGURE 7.  

BIOMASS PUMP AND DEWATERING FILTER

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TREATMENT OF DILUTE WASH WATER AND EFFLUENT SAMPLES

FIGURE 8. FIGURE 8.  

REDUCTION IN LIGHT TRANSMITIVITY WITH pH IN DILUTE BLACK LIQUOR

FIGURE 9. FIGURE 9.  


REDUCTION IN TOC IN DILUTE BLACK LIQUOR WITH pH FROM LIGNIN BIOMASS PRECIPITATION

FIGURE 10. FIGURE 10.  


REDUCTION OF COD IN DILUTE BLACK LIQUOR WITH pH FROM LIGNIN BIOMASS PRECIPITATION

FIGURE 11. FIGURE 11.  


CHANGE IN WASH WATER TRANSMITANCE WITH pH - 0.06 PER CENT SOLIDS (APPROXIMATE)

FIGURE 12. FIGURE 12.  


REDUCTION OF COD WITH pH FOR WASH WATER WITH APPROXIMATELY 0.06 PER CENT SOLIDS

FIGURE 13. FIGURE 13.  


REDUCTION OF COD WITH pH FOR WASH WATER OF ABOUT 0.06 PER CENT SOLIDS

FIGURE 14. FIGURE 14.  


EFFECT OF POLYMER ON TRANSMISSIVITY WITH pH FOR WASH WATER OF ABOUT 0.06 PER CENT SOLIDS

FIGURE 15. FIGURE 15.  


DECKER PIT WATER (ABOUT 0.025 PER CENT SOLIDS) TRANSMITIVITY VS. pH BY LIGNIN PRECIPITATION

FIGURE 16. FIGURE 16.  


DECKER PIT WATER (ABOUT 0.025 PER CENT SOLIDS) TOC REDUCTION WITH pH BY LIGNIN PRECIPITATION

FIGURE 17. FIGURE 17.  


DECKER PIT WATER (ABOUT 0.025 PER CENT SOLIDS) COD REDUCTION WITH pH BY LIGNIN PRECIPITATION

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