On the chemistry of non-process elements in systems with a pressurized black liquor gasifier

The behaviour of non-process elements (NPEs) and the formation of deposits in a pressurized chemical recovery system with a black liquor gasifier are in many ways different from those 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 to separate dregs from the green liquo r processing must be arranged in order to purge NPEs which form insoluble c ompounds in alkaline liquors just as in today's system with a recovery boil er. The NPEs of main concern are those that are more soluble and can accumu late 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 sodi um carbonate, i.e. CaCO3. Na2CO3. 2H(2)O(s), CaCO3. Na2CO3(s) or 2CaCO(3). Na2CO3(s) The solubility of these compounds in the Ca2+ - Na+ - CO32- - OH- system increases when the temperature is raised in the 95-125 degrees C ra nge, and decreases when the temperature is raised in the 125-200 degrees C range. The solubility of hydrotalcite, Mg1-xAlx(CO3)(x/2)(OH)(2). nH(2)O (0 .1 < x < 0.34), increases when the temperature is increased in the 95-150 d egrees C range. The possibility of purging aluminium as hydrotalcite with t he green liquor dregs is thus reduced when the temperature of the green liq uor is raised. On the other hand, the possibility of purging phosphorus as calcium phosphate with the green liquor dregs is slightly increased when th e temperature is raised, due to the fact that the speed of precipitation of calcium phosphate increases when the temperature is raised.