Borate Autocausticization of Black Liquor—a Potential Carbon Dioxide Mitigation Practice for the Pulping Industry

Forest products industry based on renewable wood resources is intrinsically more environmentally benign and emits less greenhouse gases than most other industries. There are still areas where further improvement can be made. One fossil fuel intensive unit operation in pulping is the lime kiln, which provides slaked lime to recausticize the cooking liquor. In this study, we used liquors and wood chips from a local pulp mill to examine the feasibility of autocausticization by adding borate to the liquor cycle in the lab. Despite numerous difficulties encountered in trying to simulate the chemical recovery process in the pulp mill, autocausticizaiton was achieved with calcined black liquor having 2 wt% to liquor solids reached more than 50% causticity, and notably reduced sodium carbonate concentrations. Reductive transformation of sodium sulfate to sodium sulfide was not achieved in the simulation, however. Based on the results, substantial carbon dioxide emission reduction was deemed practical. Furthermore, the borate helped to increase pulp yield and improved pulp properties slightly.