On the Chemistry of Non-Process Elements in Systems with a Pressurized Black Liquor Gasifier, 1998 International Chemical Recovery Conference Proceedings

The behavior of non-process elements (NPE’s) and the formation of deposits in a pressurized chemical recovery system with a black liquor gasifier are in many ways different from these of a conventional system with a recovery boiler. The composition and temperature of the green liquor produced with a gasifier system depend on the configuration of the system and can vary to a large extent. An efficient process for the separation of dregs from the green liquor processing has to be arranged in order to purge NPE’s which form insoluble compounds in alkaline liquors just as in today’s system with a recovery boiler. The NPE’s of main concern are those that are more soluble and can accumulate in the liquor cycle, i.e. Al, Si, Cl and K, and in the lime cycle, e.g. P and Mg.

A high carbonate ion concentration in the green liquor can lead to the formation of deposits of double salts of calcium carbonate and sodium carbonate, i.e. CaCO3 •Na2 CO3 •2H2 O s), CaCO3 •Na2 CO3 (s) or 2CaCO3 •Na2 CO3 (s).

The solubility of these compounds in the Ca2+ - Na + - CO3 2- - OH - system increases when the temperature is raised in the 95 - 125°C range, and decreases when the temperature is raised in the 125 - 200°C range.

The solubility of hydrotalcite, Mg1-x Alx (CO3 )(OH)2 •nH2 O (0.1 < x="">< 0.34),="" increases="" when="" the="" temperature="" is="" increased="" in="" the="" 95="" -="" 150°c="">

The possibility of purging aluminium as hydrotalcite with the green liquor dregs is thus decreased when the temperature of the green liquor is raised. On the other hand, the possibility of purging phosphorus as calcium phosphate with the green liquor dregs is slightly increased when the temperature is raised, due to the fact that the precipitation of calcium phosphate is faster at temperatures greater than 150°C than at 90°C.