Discussion by Matthew Gould
As the authors point out, the only commercial processes currently in operation for kraft color
removal are variants of the lime precipitating process originally proposed by Berger et al
1 '
2 of the
National Council of Air and Stream Improvement.
Over the years laboratory work has been conducted by many researchers into the feasibility of an
aluminum sulphate based precipitation process akin to lime discolorization
3 '
4 »
5 . It is the opinion of
this discusser that, although the various proposals are technically feasible, none of them are
economically attractive unless a pulp mill is fortunate enough to be adjacent to an alum manufacturing
p l a n t
6 '
7 .
The caustic extract, which is usually the second stage effluent of a conventional bleached kraft
mill, is correctly identified as the main cause of color in the mill effluent. Caustic extract A.P.H.A.
color values from 5000 to 20000 Pt-Co units are common, varying with wood species and both
digester and bleach plant operating conditions. Treating caustic extract before dilution with less
colored streams achieves the maximum color reduction, combined with minimal chemical
consumption at lowest capital cost.
The paper discusses the status of color removal with lime. However, without the benefit of
acquaintance with the most recent studies, the information given is badly outdated. In particular,
reference is made to errors introduced in color measurements by prefiltration
8 . The assumption made
as to the mechanism of the lime precipitation process must surely be beyond a shadow of doubt, after
the classic research by C. Dence, et al
9 . It is also disappointing that only the early Berger work is
reported, invalidating the economic conclusions from the comparison of the alum and lime processes.
This will be examined in more detail later.
B.O.D. removal efficiences at around 40% are comparable for both lime and alum treatment.
Laboratory studies confirm however, that slightly higher color removal is possible with massive alum
doses in range of 2000 - 4000 p.p.m. This dosage in itself would represent a significant materials
handling problem for the 600 - 1000 t.p.d. kraft mill, which is more typical of current industry
practice, than the 300 t.p.d. example cited in the paper. At an effluent discharge of 13 m 3/T of pulp
even the tiny 300 t.p.d. mill would process 11 — 17 ton of alum, on a dry basis, each day and be faced
with dewatering four times this weight as dewatered sludge: assuming of course, that the notoriously
sticky hydrated sludge can be thickened and dewatered to 25% solids successfully on a continuing
basis. For the 1000 t.p.d. mill the problem would become correspondingly greater. The writer having
gone through the agonies of evaluating centrifuges, vacuum filters and presses for handling difficult
sludges, must emphasize once again that the transition from theory to practice is often fraught with
misadventure. The prospect of having to install and operate a rotary or fluidized bed kiln, specifically
for alum regeneration, will deter many mill managements, not to mention an additional sulphuric acid
handling and dissolving system.
This touches on a related aspect. Lime kiln operation and lime slaking and clarification are part of
standard kraft pulp mill practice and, human nature being frail as it is, lime color removal is less
frightening to most plant operators and management than the prospect of a new process that has
seemingly little in common with pulp production. The prospect of calcinating alum at lower
temperatures than lime is attractive, but the statement inferred in the middle of P.3 of the paper that
the B.T.U. value of the organic matter associated with lime would have a lower B.T.U. value than
essentially similar material associated with alum is unacceptable! One would also feel that the much
lower moisture content of dewatered lime (50 - 60% solids) would more than offset any benefit from
lower kiln operating temperatures for the alum sludge (18 - 23% solids).
The report of the anomalous behavior of alum floe exhibiting only partial precipitation on initial
pH depression to 3.8 and completing precipitation on restoring to pH 5.4 is of particular interest. One
wonders however, if the apparently critical pH values and the need for a dual clarifier will tend to
deter potential users from considering color removal with alum. Reliable and sophisticated control
systems would appear an absolute necessity.
Finally, I would refer to current practice with lime discolorization. There is one partial and three
complete systems in operation in the U.S.A. Operation of the partial process, C. Davis °, is on total
effluent of an unbleached mill and to date precipitated lime solids have been discarded. Dosage is in
the 1500 p.p.m. lime range as contrasted with 15000 p.p.m. values used in the original massive lime
process
l '
2 .
The remaining systems all selectively treat caustic extract in bleached kraft mills and use lime in
the 1500 - 3000 p.p.m. range
12 '
13 . The two Georgia-Pacific mills I am familiar with, use a patented
lime dewatering process and calcine the sludge through existing rotary kilns
π '
12 >
14 . A comparison of
the actual operating cost data of these two mills with the cost projections in the paper shift the
economics substantially in favor of lime and appear discouraging to future development of alum as a
decoloring means in kraft mill applications. It should be noted that recarbonization is not practiced at
these mills. The additional operating and capital costs of recarbonization are avoided as the lime lost in
solution in the decolorized effluent is needed for pH correction in the final waste water from these
mills.
603
As the authors point out, the only commercial processes currently in operation for kraft color
removal are variants of the lime precipitating process originally proposed by Berger et al
1 '
2 of the
National Council of Air and Stream Improvement.
Over the years laboratory work has been conducted by many researchers into the feasibility of an
aluminum sulphate based precipitation process akin to lime discolorization
3 '
4 »
5 . It is the opinion of
this discusser that, although the various proposals are technically feasible, none of them are
economically attractive unless a pulp mill is fortunate enough to be adjacent to an alum manufacturing
p l a n t
6 '
7 .
The caustic extract, which is usually the second stage effluent of a conventional bleached kraft
mill, is correctly identified as the main cause of color in the mill effluent. Caustic extract A.P.H.A.
color values from 5000 to 20000 Pt-Co units are common, varying with wood species and both
digester and bleach plant operating conditions. Treating caustic extract before dilution with less
colored streams achieves the maximum color reduction, combined with minimal chemical
consumption at lowest capital cost.
The paper discusses the status of color removal with lime. However, without the benefit of
acquaintance with the most recent studies, the information given is badly outdated. In particular,
reference is made to errors introduced in color measurements by prefiltration
8 . The assumption made
as to the mechanism of the lime precipitation process must surely be beyond a shadow of doubt, after
the classic research by C. Dence, et al
9 . It is also disappointing that only the early Berger work is
reported, invalidating the economic conclusions from the comparison of the alum and lime processes.
This will be examined in more detail later.
B.O.D. removal efficiences at around 40% are comparable for both lime and alum treatment.
Laboratory studies confirm however, that slightly higher color removal is possible with massive alum
doses in range of 2000 - 4000 p.p.m. This dosage in itself would represent a significant materials
handling problem for the 600 - 1000 t.p.d. kraft mill, which is more typical of current industry
practice, than the 300 t.p.d. example cited in the paper. At an effluent discharge of 13 m 3/T of pulp
even the tiny 300 t.p.d. mill would process 11 — 17 ton of alum, on a dry basis, each day and be faced
with dewatering four times this weight as dewatered sludge: assuming of course, that the notoriously
sticky hydrated sludge can be thickened and dewatered to 25% solids successfully on a continuing
basis. For the 1000 t.p.d. mill the problem would become correspondingly greater. The writer having
gone through the agonies of evaluating centrifuges, vacuum filters and presses for handling difficult
sludges, must emphasize once again that the transition from theory to practice is often fraught with
misadventure. The prospect of having to install and operate a rotary or fluidized bed kiln, specifically
for alum regeneration, will deter many mill managements, not to mention an additional sulphuric acid
handling and dissolving system.
This touches on a related aspect. Lime kiln operation and lime slaking and clarification are part of
standard kraft pulp mill practice and, human nature being frail as it is, lime color removal is less
frightening to most plant operators and management than the prospect of a new process that has
seemingly little in common with pulp production. The prospect of calcinating alum at lower
temperatures than lime is attractive, but the statement inferred in the middle of P.3 of the paper that
the B.T.U. value of the organic matter associated with lime would have a lower B.T.U. value than
essentially similar material associated with alum is unacceptable! One would also feel that the much
lower moisture content of dewatered lime (50 - 60% solids) would more than offset any benefit from
lower kiln operating temperatures for the alum sludge (18 - 23% solids).
The report of the anomalous behavior of alum floe exhibiting only partial precipitation on initial
pH depression to 3.8 and completing precipitation on restoring to pH 5.4 is of particular interest. One
wonders however, if the apparently critical pH values and the need for a dual clarifier will tend to
deter potential users from considering color removal with alum. Reliable and sophisticated control
systems would appear an absolute necessity.
Finally, I would refer to current practice with lime discolorization. There is one partial and three
complete systems in operation in the U.S.A. Operation of the partial process, C. Davis °, is on total
effluent of an unbleached mill and to date precipitated lime solids have been discarded. Dosage is in
the 1500 p.p.m. lime range as contrasted with 15000 p.p.m. values used in the original massive lime
process
l '
2 .
The remaining systems all selectively treat caustic extract in bleached kraft mills and use lime in
the 1500 - 3000 p.p.m. range
12 '
13 . The two Georgia-Pacific mills I am familiar with, use a patented
lime dewatering process and calcine the sludge through existing rotary kilns
π '
12 >
14 . A comparison of
the actual operating cost data of these two mills with the cost projections in the paper shift the
economics substantially in favor of lime and appear discouraging to future development of alum as a
decoloring means in kraft mill applications. It should be noted that recarbonization is not practiced at
these mills. The additional operating and capital costs of recarbonization are avoided as the lime lost in
solution in the decolorized effluent is needed for pH correction in the final waste water from these
mills.
603