What is Borate Autocausticizing?

Sodium borates expel carbon dioxide during the combustion of black liquor and form intermediate borates (mainly Na3BO3 and some Na4B2O5) in the recovery boiler smelt (Figure 1). 

 

Figure 1: Borate Autocausticizing Concept 
 

The hydration of borate-smelt produces caustic and sodium metaborate (NaBO2).  Sodium metaborate stays in solution and circulates through the chemical recovery cycle to continue forming caustic in the recovery boiler.  The process is called “Borate Autocausticizing” since it automatically causticizes the smelt during combustion in the recovery boiler without using additional causticizing stages.
 

How Does Borate Autocausticizing Work? 
 

Neobor^® borax penthydrate is added to the liquor cycle.  At the alkaline pH prevailing in the liquor cycle, Neobor is converted to sodium metaborate (NaBO2) and remains in this form in almost the entire liquor cycle.
 

The borate autocausticizing reaction occurs in the recovery boiler and caustic forms in the smelt dissolving tank: 
 

-        Autocausticizing in the Recovery Boiler

                        Na₂CO₃ + NaBO2  →  Na₃BO3 + CO₂ 

-        Hydrolysis in Smelt Dissolving Tank

                        Na₃BO3 + H₂O  →  NaBO₂+ 2 NaOH 
 

Borates are water soluble; they are easily washed from the pulp and stay dissolved in the green, white and black liquors.  They remain circulating in the liquor cycle to repeat their autocausticizing function time and time again.  The only requirement is to add make-up Neobor to compensate for the normal process losses.

What Is Partial Borate Autocausticizing? 
 

Partial borate autocausticizing (AC) is a commercially available technology to increase the causticizing capacity of kraft or soda pulp mills. It works by causticizing some of Na2CO3 content of smelt to NaOH in smelt dissolving tank. The remainder of Na2CO3 will continue to be causticized by the conventional lime causticizing process.
 

Figure 2 shows the chemical changes introduced by partial borate AC (in red) along with the major chemical components of a typical kraft recovery process (in black). The changes are related to the addition of borates to the liquor and the conversion of carbonate to caustic in smelt dissolving tank.

 

 

Figure 2: Schematic showing how Borate Autocausticizing works with Kraft pulping 

 

Partial autocausticizing is normally specified by an autocausticizing level, which is defined based on the borate content of the liquor. For example: 10% autocausticizing is used to describe a process in which the borate content of liquor is sufficient for causticizing up to 10% of Na₂CO₃ in smelt.