Editing Kraft process (section)

===Recovery process===
The excess black liquor contains about 15% solids and is concentrated in a [[Multiple-effect evaporator|multiple effect evaporator]]. After the first step the black liquor has about 20–30% solids. At this concentration the [[rosin]] soap rises to the surface and is [[Skimmer (machine)|skimmed]] off. The collected soap is further processed to [[tall oil]]. Removal of the soap improves the evaporation operation of the later effects.

The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor"<ref>{{cite web|url=http://www.sealspecialists.net/black%20liquor.htm |archive-url=https://web.archive.org/web/20050420032421/http://www.sealspecialists.net/black%20liquor.htm |url-status=dead |archive-date=2005-04-20 |title=Equipment to handle heavy black liquor |access-date=2007-10-09 }}</ref>) and burned in the [[recovery boiler]] to recover the inorganic chemicals for reuse in the pulping process. Higher solids in the concentrated black liquor increases the energy and chemical efficiency of the recovery cycle, but also gives higher viscosity and precipitation of solids (plugging and fouling of equipment).<ref>{{cite web |url=http://thor.lib.chalmers.se/inst_fack/kurser/keml/TK/materials/dc220.pdf |title=Second Critical Solids Black Liquor Scaling |access-date=2007-10-09 |last=Hsieh |first=Jeffery S. |author2=Smith, Jason B. |publisher=Pulp and Paper Engineering, School of Chemical Engineering, Georgia Institute of Technology |url-status=dead |archive-url=https://web.archive.org/web/20110831033707/http://thor.lib.chalmers.se/inst_fack/kurser/keml/TK/materials/dc220.pdf |archive-date=2011-08-31 }}</ref><ref>{{Cite patent
 |country=US |number=5527427 |status=granted |title=High solids black liquor of reduced viscosity and viscosity reduction method for high solids black liquor |gdate=1996-06-18 |invent1=Mualla Berksoy |invent2=Yaman Boluk |assign1=Optima Specialty Chemicals & Technology Inc}}</ref> During combustion, sodium sulfate is [[redox|reduced]] to sodium sulfide by the organic carbon in the mixture:

:1. Na<sub>2</sub>SO<sub>4</sub> + 2 C → Na<sub>2</sub>S + 2 CO<sub>2</sub>

This reaction is similar to [[thermochemical sulfate reduction]] in geochemistry.

The molten salts ("smelt") from the recovery boiler are dissolved in a process water known as "weak wash". This process water, also known as "weak white liquor" is composed of all liquors used to wash lime mud and [[green liquor]] precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as "green liquor". The green liquor's eponymous green colour arises from the presence of colloidal iron sulfide.<ref name="Pump Handbook">{{cite book|title=Pump Handbook|year=2008|publisher=McGraw Hill|location=New York|isbn=9780071460446|url=http://accessengineeringlibrary.com/browse/pump-handbook-fourth-edition|edition=4th|author1=Giddings, J.F.|author2=Roll, D.R.|author3=Cappellino, C.A.|author4=Day, M.|author5=Nardone, R.A.|editor=Karassik, I.|editor-link=Igor Karassik|editor2=Messina, J.|editor3=Cooper, P.|editor4=Heald, C.|accessdate=February 21, 2013|chapter=12.12.4.|archive-date=March 4, 2013|archive-url=https://web.archive.org/web/20130304154803/http://accessengineeringlibrary.com/browse/pump-handbook-fourth-edition|url-status=dead}}</ref> This liquid is then mixed with [[calcium oxide]], which becomes [[calcium hydroxide]] in solution, to regenerate the white liquor used in the pulping process through an equilibrium reaction (Na<sub>2</sub>S is shown since it is part of the green liquor, but does not participate in the reaction):

:2. Na<sub>2</sub>CO<sub>3</sub> + Ca(OH)<sub>2</sub> ←→ 2 NaOH + CaCO<sub>3</sub>

[[Calcium carbonate]] precipitates from the white liquor and is recovered and heated in a [[lime kiln]] where it is converted to [[calcium oxide]] (lime).

:3. CaCO<sub>3</sub> → CaO + CO<sub>2</sub>

Calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used in Reaction 2:

:4. CaO + H<sub>2</sub>O → Ca(OH)<sub>2</sub>

The combination of reactions 1 through 4 form a closed cycle with respect to sodium, sulfur and calcium and is the main concept of the so-called recausticizing process where [[sodium carbonate]] is reacted to regenerate [[sodium hydroxide]].

The recovery boiler also generates high pressure steam which is fed to turbogenerators, reducing the steam pressure for the mill use and generating [[electricity]]. A modern kraft pulp mill is more than self-sufficient in its electrical generation and normally will provide a net flow of energy which can be used by an associated paper mill or sold to neighboring industries or communities through to the local electrical grid.<ref>{{cite web
 |url=http://www2.biotech.wisc.edu/jeffries/bioprocessing/pulping.html 
 |title=Kraft pulping: Energy consumption and production 
 |access-date=2007-10-21 
 |last=Jeffries 
 |first=Tom 
 |date=March 27, 1997 
 |publisher=University of Wisconsin Biotech Center
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20110928021005/http://www2.biotech.wisc.edu/jeffries/bioprocessing/pulping.html 
 |archive-date=September 28, 2011 
}}</ref> Additionally, bark and wood residues are often burned in a separate power boiler to generate steam.

Although recovery boilers using G.H. Tomlinson's invention have been in general use since the early 1930s, attempts have been made to find a more efficient process for the recovery of cooking chemicals. [[Weyerhaeuser]] has operated a [[Chemrec]] first generation [[black liquor]] [[gasification|entrained flow gasifier]] successfully at its [[New Bern]] plant in [[North Carolina]], while a second generation plant is run in pilot scale at [[Smurfit Kappa|Smurfit Kappa's]] plant in [[Piteå]], [[Sweden]].<ref>{{Cite web |url=http://chemrec.se/Chemrec-plants.aspx |title=Chemrec web site |access-date=2011-01-07 |archive-url=https://archive.today/20120709130105/http://chemrec.se/Chemrec-plants.aspx |archive-date=2012-07-09 |url-status=dead }}</ref>

An additional technology is employed to lower the use of lime.  In "partial borate autocausticizing" (PBAC), boric acid is added which produces sodium borate in place of sodium carbonate.<ref name=UllBO>{{cite book |doi=10.1002/14356007.a04_263.pub2 |chapter=Boric Oxide, Boric Acid, and Borates |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2015 |last1=Schubert |first1=David M. |pages=1–32 |isbn=978-3-527-30385-4 }}</ref>