What is a risk of water contacting smelt during black liquor processing?

The risk of water contacting smelt during black liquor processing relates to the rapid expansion of gas that can occur, leading to potential explosions. In the context of the pulp and paper industry, black liquor is a byproduct of the pulping process that contains dissolved organic material and inorganic chemicals. When water makes contact with smelt, which is often composed of molten inorganic chemicals including sodium and potassium salts, it can rapidly convert to steam due to the high temperatures involved. This rapid phase change generates a significant volume of steam in a very short period, creating high pressure. If this pressure builds up quickly enough, it can lead to an explosive situation, potentially causing damage to equipment, safety hazards, and loss of containment.

Understanding this hazard is crucial for ensuring safety protocols are in place during the processing of black liquor. The other options, such as slow thermal expansion or reduced efficiency of energy recovery, do not encapsulate the immediate danger posed by a violent reaction when water comes into contact with hot smelt, making the risk of rapid gas expansion the most pertinent concern.