2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P91 ThE bORON IN KRAFT PuLP MILL AND
INFLuENCE IN wASTE wATER
eVA GEMZICKá and MILAn VRŠKA
Department of Chemical Technology of Wood, Pulp and
Paper, Slovak University of Technology, Radlinského 9,
812 37 Bratislava, Slovak Republic,
milan.vrska@stuba.sk
Introduction
The elemental boron almost is not occurred in nature
and it usually is component only its different compounds.
The boron forms boranes with hydrogen. It forms acids of
these types: (HBO 2 ) n and H 3 BO 3 . The borides are boron compounds
with metals (Me) and boron forms BX 3 with halides
(X 3 ). The oxycompounds are formed with boron and oxygen,
i.e. B 2 O 3 and (BO) n . B 2 O 3 (l) [molten] dissolves oxides of
metals (borax pearls). Wide range of boron compounds exists
with different stoichiometry and structure (see Fig. 1.). The
boron more often exists as borax in nature. The molten borax
covers molten other metal and so it is protective compound
against oxidation. The mixture boron and na 2 CO 3 is used for
dissociation of geological and hard dissolving samples 1,2 .
Fig. 1. Chart for boron reaction
Borax is used in recovery line for process improvement
in kraft pulp mill too. The conventional recovery cycle
with addition of borates; this produces 10 % of the formed
naOH (partial autocaustification). The borates react in two
places in the recovery cycle: the recovery boiler and the
smelt dissolver. In the former sodium carbonate reacts with
a sodium-lean borate, forming a sodium-rich borate formed:
s523
naBO 2 (l) + na 2 CO 3 (l) = na 3 BO 3 (l) + CO 2 (g). In this reaction,
the borates and the sodium carbonates are in a smelt
phase, which implies that the reaction rate is high. In the dissolver,
na 3 BO 3 reacts with water: na 3 BO 3 (l) + H 2 O = 2naO
H(aq) + naBO 2 (aq)(ref. 3 ).
Fig. 2. borax Na 2 b 4 O 7 . 10h2 O i.e. Sodium tetraborate. It is applied
in different industry: metallurgy, glass industry, ceramic
industry and in some pulp mill
Boron compounds are occurred in fresh/salt water and
in waste water too. Concentration of boron compounds is
app. 5 mg dm –3 in salt water 4 . Boron occurs naturally in fresh
water at an average of 1 mg dm –3 or less many times. High
boron concentrations may be toxic to freshwater fish regarding
concentration of 10–300 mg dm –3 (e.g. for Rainbow trout
24-day LC50 is 88 mg dm –3 ) in flowing water. Mainly borate
is hazardous for water plants (e.g. for Green algae 96-hr IC10
is 24 mg dm –3 ). Any definitive ecotoxicological conclusions
don’t exist about genotoxic, carcinogenic or mutagenic effects
to water organisms, but still under investigation 5 .
Experimental
The autocaustification was realized in kraft pulp mill.
Trial was started on December 1, 2007 till December 14, 2007
and restarted again on January 4, 2008. This trial was finished
on February 25, 2008. Waste water was monitored for
boron concentrations in two different laboratories. The boron
concentration was determined in the first laboratory (spectrophotometry
with azomethin at 415 nm) as calculation from
borates. The boron also was analyzed using OES ICP method
(in 2 nd laboratory) the first time in different samples of waste
water, which were from different pH and COD. Alkali waste
water, acid waste water, waste water of sewage water treatment
plant, water from river and output to river were analyzed
for boron in two steps, i.e. during trial and after trial in
2 nd laboratory with OES ICP.
Borax was added into dissolvent tank for green liquor
step by step. Boron concentration was increased in waste
water in February, when borax was added more in recovery
line.
Results
Boron analysis was difficult because of small particulates,
which were filtrated from samples, but boron concentration
was estimated from samples, which were not filtrated.