Field of the invention
The present invention relates to propose a method for improving the causticizing
efficiency of carbonated spent caustic liquor. More particularly, the present
invention relates to a method of improving causticizing efficiency by reaction with
lime at specific temperature and retention time.
Background of the invention
A variety of processes utilize alkali based chemicals such as sodium hydroxide in
the pulping, bleaching or oxidation of weed or non-wood cellulose based
materials, bauxite digestion for alumina production in baler's process, including
silica extraction from rice husk ash. The spent chemicals are recovered by
filtering the slurry, concentrating the filtrate by evaporating excess water and re-
using the same in the beginning of the process. If the contaminants and
carbonates are high, the same solution is treated with lime (CaO) for converting
the sodium carbonates to sodium hydroxide for maintaining the high efficiency
across the digestion of feed materials. In conventional process, the causticization
is carried out at atmospheric or 80°C reaction temperature.
The prior art lime causticizing process is carried out through two major steps. 1)
slaking of lime 2) treating the carbonated spent liquor with slaked lime at
different reaction temperatures.
Lime slaking: CaO(s) + H2O(1)? Ca (OH)2 (s)
Causticizing: Na2CO3(aq) + Ca (OH)2(s)?NaOH (aq) + CaCO3(s)
There is an inherent problem in the causticization operation. The causticizing
reaction does not normally achieve 100% conversion, but it approaches to
equilibrium which can be described by the following equation:

where K is the apparent equilibrium constant, fOH- and fco2_3 are dimensionless
activity co-efficients of OH" and CO32- ions respectively.
The degree of conversion of causticizing efficiency (CE) is customarily defined as

Where [NaOH] and [Na2CO3] are concentrations of NaOH and Na2CO3
respectively in the liquor, in g/L of Na2O. The equilibrium causticizing efficiency is
strongly affected by the liquor strength (total titratable alkali), the hydroxide
concentration (active alkali), and to a smaller extent, the liquor temperature.
In practice, the conversion from Na2CO3 to NaOH is lower since it is also
controlled by reaction kinetics. The reaction usually proceeds rapidly in the
beginning, then becomes very sluggish after 20 to 30 minutes . The main
reasons for the sluggishness are a) with the increase in concentration of OH" ions
with time, the reaction equilibrium tends to shift to the left, b) with the progress
of the reaction process, the surface of Ca(OH)2 particles becomes covered with
precipitated CaCO3, which hinders the diffusion of CO32- ions from the solution to
the core of Ca(OH)2 particles. Due to equilibrium and kinetic effects, causticizing
efficiency is greatly affected by the liquor temperature, retention time, lime
dosage and quality of the lime.
OBJECT OF THE INVENTION
It is therefore, an object of the invention to propose a method for improving the
causticizing efficiency of carbonated spent caustic liquor.
Another object of the invention is to propose a method for improving the
causticizing efficiency of carbonated spent caustic liquor, which is enabled to
convert sodium carbonate to sodium hydroxide by reaction with lime at different
reaction temperatures of 60 and 85°C with high yield.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS,
Fig. 1 is a schematic of diagram of causticization process followed by filtration
according to the invention.
Fig. 2 is a graph of the effect of temperature on causticization efficiency at
different retention times according to the invention.
Detail description of the invention
According to the invention, the causticization process comprises reacting an
aqueous solution of carbonated spent caustic liquor (sodium carbonate) with lime
at different reaction temperatures to produce sodium hydroxide (causticization)
to improve the causticization reaction efficiency. In particular, the causticization
is conducted at 85°C to achieve improved efficiency.
The inventive process can be used in carbonated spent caustic liquor treatment,
alkaline pulping processes, soda process, and bleaching or oxidizing processes
involving either wood or non-wood cellulose based materials where conventional
lime causticizing is used to generate sodium hydroxide from sodium carbonate.
The efficiency of the reaction of carbonated spent caustic liquor (sodium
carbonate) with lime to regenerate sodium hydroxide is limited by chemical
equilibria and kinetics. The causticizing efficiency is in the range of about 76.5 to
80.5% . The regenerated caustic soda is recycled in the process which reduces
the fresh caustic consumption in the plant.
A series of laboratory experiments were conducted using carbonated spent
caustic liquor (62 g/L of Na2O), barium chloride solution (0.65M) and lime as
indicated in Fig. 1. The methods to evaluate the effect of retention time, reaction
temperature and lime dosing are detailed below.
50 ml of carbonated spent caustic liquor is mixed with calculated amount of lime
(Na2CO3: CaO molar ration 1:1) and the system is continuously stirred for
homogeneity, the slurry is maintained at specific temperature for a
predetermined retention time. After completion of retention time, the slurry is
filtered and residue is washed with 100 ml hot water. The wash water is
analysed for unreacted Na2CO3.
The wash water (100 ml) is mixed with 50 ml of barium chloride solution to
precipitate the unreacted CO32- ions as barium carbonate. The resultant solution
is filtered to separate the barium carbonate residue and is dried in an oven. And
the filtrate obtained from barium carbonate filtration is again treated with 10 ml
of barium chloride solution to ensure that all CO32- ions have been removed. The
unreacted sodium carbonate is calculated and the causticizing efficiency is
measured by using below equation.

From Fig. 2, it is clearly evident that the causttcization efficiency is high at 85°C
when compared with 60°C reaction temperature. It also reveals that the
retention time doesn't have significant effect on causticization efficiency. The
difference between the causticiation efficiencies at 30 and 60 min retention time
at 85°C is 2%.
We claim:
1. A method for improving causticization efficiency of carbonated spent
caustic liquor, comprising the steps of:
- mixing carbonated spent caustic liquor with lime with Na2O3: CaO molar
ratio of 1:1, with the mixture being continuously stirred;
- maintaining the formed slurry at specific reaction temperature for a
predetermined retention time;
- filtering the slurry and washing the residue with hot water; and
- analyzing the wash water for calculating, unreacted sodium carbonate and
causticizing efficiency,
wherein,

and wherein the specific reaction temperature and the predetermined retention
time is respectively 85° C, and 30°C to 60° C.
2. A method for improving causticization efficiency of carbonated spent caustic
liquor as substantially described and illustrated herein with reference to the
accompanying drawings.

ABSTRACT

The invention relates to a method for improving causticization efficiency of
carbonated spent caustic liquor, comprising the steps of mixing carbonated spent
caustic liquor with lime with Na2O3: CaO molar ratio of 1:1, with the mixture
being continuously stirred; maintaining the formed slurry at specific reaction
temperature for a predetermined retention time; filtering the slurry and washing
the residue with hot water; and analyzing the wash water for calculating,
unreacted sodium carbonate and causticizing efficiency, wherein,

and wherein the specific reaction temperature and the predetermined retention
time is respectively 85° C, and 30°C to 60° C.