A simple method of evaluating economics of partial borate
autocausticizing is to compare it with conventional lime causticizing.
A typical comparison shows that one ton of
Neobor makeup could replace 10-30 tons of lime for the same
causticizing power. The replacement ratio will vary from one mill
to another depending specifically on the soda loss rate (from the liquor
cycle) and the autocausticizing reaction efficiency in the recovery
boiler.
Below you will find a simple approach to determine the
replacement ratio of Neobor makeup to lime. Once this ratio is
known, the economic comparison can be performed using the price of Neobor
versus the cost of lime purchase or lime production. This
provides a reasonable initial estimation of economics. We could provide
you with a more detailed analysis if you complete the mill-trial
Questionnaire.
The actual benefit of this technology would be
evaluated during a trial stage.
The estimation is based on the stoichiometry of borate
autocausticizing and lime causticizing reactions:
-
Borate Autocausticizing Reactions
Na2CO3 +
NaBO2
→
Na3BO3 +
CO2 (Autocausticizing)
Na3BO3 +
H2O
→
NaBO2 +
2
NaOH (Hydrolysis)
-
Lime Causticizing Reactions
CaO
+ H2O
→
Ca(OH)2 (Slaking)
Na2CO3 +
Ca(OH)2
→
CaCO3 +
2 NaOH (Lime
Causticizing)
CaCO3
→
CaO + CO2 (Lime
Reburning)
Comparison of the two causticizing reactions suggests
that stoichiometrically one mole of NaBO2 (sodium metaborate)
is equivalent to one mole of CaO (lime) in its causticizing power.
However, CaO is consumed during causticizing, i.e. converted to CaCO3
which requires reburning or recalcining, while NaBO2 is
automatically regenerated when the Na3BO3 is
hydrolyzed in the smelt dissolving tank.
Autocausticizing only requires small amounts of Neobor
makeup to compensate for the natural process losses. A material balance
approach can be used to compare the
Neobor
makeup requirement with the reduction in lime
usage achievable by borate autocausticizing.
The following equation shows the result of this comparison, which
provides a simple basis to calculate the economics of borate
autocausticizing:
where: A = Lime
Availability, wt% of CaO in lime
B = White
Liquor Causticity
,
mole%
C = Borate
Loss Rate, % of Borate Used
Borate loss rate can be assumed equal to Na loss rate from
the liquor cycle. However, the Na loss rate has to be
expressed as a percentage of the total amount of Na passed
through digester (or in circulation).
For example: If
4 m3/ADT of white liquor is used at a TTA of 120 kg Na2O/m3,
the total Na usage is
4x120/62x46=356.1
kg Na/ADT. Therefore, a soda loss of 10 kg Na/ADT is
equivalent to the loss rate of C=10/356.1=2.8
% of Na used.
D = Borate AC
Reaction Efficiency*, %
Borate reaction
efficiency can be assumed 80% in these calculations. The
more accurate value is obtained during the trial stage.
After calculating the ratio of Neobor makeup to
lime, you can calculate the economic benefits to your mill if you know
how much lime usage you intend to replace by autocausticizing and the
cost of lime and Neobor. If increased pulp production is the
target, you need to determine the increase in white liquor and lime
usage required to accommodate the production increase.
