Black Liquor is the digester waste mixture of spent chemicals and lignin extracted from wood chips. When burned in a recovery boiler Black Liquor produces heat for steam and also releases digester chemicals called "smelt." Mixed with water, smelt becomes Green Liquor. This is treated with lime in the causticizers to produce White Liquor, the digester’s cooking chemical.
Stored in varying concentrations, liquors are corrosive solutions with high levels of organic compounds. Liquors can cause chemical burns or damage the lungs if inhaled. Level sensors contend with the chemicals’ harshness, variable density and dielectric, agitation, foaming, and media stickiness. Tank controls should activate the appropriate alarms or emergency shutdown systems.