Nonwood Pulp Mills – Rice Straw



Background
Rice straw contains high levels of silica. When the straw is pulped, the silica dissolves into a by-product stream called black liquor. High silica levels in the black liquor make it impossible for the rice mill to concentrate the black liquor in a multiple-effect evaporation system. The silica precipitates onto the evaporator heating surface, causing severe fouling. The inability to concentrate the liquor to a level that can sustain combustion makes it impossible to use a traditional recovery boiler to recycle the cooking chemicals and produce steam.

Instead, the black liquor is sent to an anaerobic and/or an aerobic wastewater treatment system where a portion of both the biological oxygen demand (BOD) and chemical oxygen demand (COD) is removed. These systems are sizeable and consume large amounts of electrical energy. The effluent from the treatment system still has a high COD load which may require tertiary treatment with expensive chemicals. In addition, valuable sodium is discharged with the effluent.

Opportunity
Using a combination of the Siloxy desilication process soda alkali recovery technology makes the removal of silica from rice straw black liquor now possible. The Siloxy desilication process uses carbon dioxide (CO2) from a CO2 capture process to remove the silica from the weak black liquor. Multiple-effect evaporators can then be used to concentrate the liquor to a level that sustains combustion.

Benefits
Environmental
Eliminates the black liquor BOD and COD load to the wastewater treatment plant.
Significantly lowers the discharge of sodium into the effluent receiving water.
Lowers the release of volatile organic compounds into the atmosphere from the aerobic wastewater treatment system.
Substantially lowers the consumption of water and the amount of effluent by recycling condensate from the multiple-effect evaporator system.
Economic
Substantially decreases the purchase of costly sodium hydroxide.
Lowers the consumption of fossil fuel (e.g., coal).
Substantially lowers the power requirements of the wastewater treatment system.
Eliminates the need for tertiary treatment.
Reduces the capital cost (size) of the wastewater treatment plant (new installations).


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