Borate autocausticizing equilibria in recovery boiler smelt

Partial borate autocausticizing is an interesting option for eliminating bottlenecks at the recaust plant and the lime kiln of a kraft pulp mill chem. recovery process.  The autocausticizing reactions may to some extent occur already in the early stages of the black liquor droplet combustion, but they can also occur in the recovery boiler smelt, mainly as a reaction between sodium carbonate and sodium metaborate.  It is of vital importance to understand the equil. of borate compds. in the smelt to obtain optimum conditions for the autocausticizing reactions in the recovery boiler.  The effect of the potassium and boron concn. in the salt mixt. and CO₂ in the gas on the autocausticizing reactions in the recovery boiler smelt was studied by thermogravimetry and thermal anal.  Results show that the autocausticizing reaction is readily reversible with 1 % CO₂ or higher in the gas atm. in pure sodium systems.  Higher CO₂ and potassium concn. lead to lower autocausticizing.  In pure potassium systems, the autocausticizing is low and the wt. loss is irreversible.  In potassium-​rich systems volatilization of alkali- and​/or boron-​components becomes more prominent than in sodium systems.