Bagasse contains silica. When the bagasse is pulped, the silica dissolves into the black liquor by-product stream. If the black liquor is concentrated in a multiple-effect evaporation system, some of the silica precipitates and deposits onto the evaporator heating surfaces, rapidly decreasing their capacity. The presence of silica in black liquor also limits the solids concentration that can be achieved during evaporation.
Mills that do concentrate their liquor have traditionally used a "Copeland" reactor which has no energy recovery or a "Chemical Recovery Unit" (CRU), commonly called a recovery boiler, which recovers energy as steam and cooking chemicals.
In a Copeland reactor, the sodium is converted into solid sodium carbonate which needs to be landfilled, or in some cases can be sold. A recovery boiler produces green liquor, a solution of sodium carbonate in water which is causticized with fresh lime (CaO) to produce white liquor, a solution of sodium hydroxide in water. In the causticizing process, the fresh lime is converted to lime mud (CaCO3). The presence of silica in the green liquor causes "settling" problems within the causticizing plant.
The lime which has been converted to lime mud (CaCO3) would normally be reprocessed into fresh lime by "reburning" it in a lime kiln. The high silica content of the lime mud makes reburning difficult. If a lime kiln is employed, a large portion of the lime mud must be landfilled to control the accumulation of silica. If the lime mud is not reprocessed in a kiln, it is landfilled and fresh lime must be purchased.
At mills with a recovery boiler, the Siloxy desilication process can be used to remove the silica from the weak black liquor and CO2 capture technology is used to produce the CO2 for the desilication process. Removal of silica reduces scaling in the multiple-effect evaporators and decreases the amount of lime mud that must be purged to control silica.
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