Reconciling material balances with laboratory test results: The case of the inorganic-to-organic ratio in black liquor

Abstract

When evaluating the composition of black liquor using material balance techniques that estimate the organics from the wood and the inorganics from the cooking liquor, the solids content of the black liquor is determined to be roughly 1/3 inorganic matter and 2/3 organic matter. When actual laboratory testing using simplified ashing methodologies are performed, the results typically suggest that black liquor solids contain roughly 43%-49% organics and 51%-57% inorganics. We determined that the applied hydroxide and sulfide mineralize carbon and oxygen from the organic portion of the black liquor and convert it to carbonate and sulfate during the cooking and ashing process, thus increasing the apparent inorganic content of the black liquor. This becomes problematic when attempting to estimate total black liquor production using the organics removed during cooking and the inorganic content using results from the ashed black liquor sample. We used Microsoft Excel material balances to develop a conversion factor to correct the laboratory test results. Multiplying the percent ash in the laboratory test by 0.65 to 0.70 provided a more realistic estimate of the initial inorganics to be used along with organics removed in the digester to estimate black liquor production. This conversion factor was determined to be independent of pulp yield and white liquor charge in the digester. The initial white liquor composition, as reported by the ABC test, does have a slight effect on this value.