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.
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.
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