LIME SLAKING
I Slakers and movers
by Roman Stoiber,
STT Enviro Corp Systems &
Solutions, Canada
Poorly-slaked lime can have a major negative impact on lime consumption
and the operating cost at an end-user’s site. The amount of lime used in
the process is determined by a combination of slaking parameters. The
optimisation of the slaking process will help drive operating costs down
and also reduce CO2 emissions.
W
hen the slaking of lime (ie,
the conversion to calcium
hydroxide – Ca(OH)2) takes
place on site it is often carried out by
operators who don’t have specialised
slaking training. Optimising the slaking
step can result in significant savings in
lime consumption. Studies have shown
that anywhere between 10-40 per cent
can be saved in chemical consumption by
ensuring lime slaking is carried out under
optimum slaking conditions. This is true
whether the process application is water
treatment, exhaust gas scrubbing, mining
beneficiation or downstream chemical
processing.
Quicklime is provided by calcining
limestone. Limestone properties can
vary greatly depending on the precise
location from which it originates and its
mineralisation. Thus, quicklime properties
produced from limestone of different
qualities can vary greatly. Most quicklime
suppliers understand the importance of
tightly controlling the calcining parameters
in the conversion of their particular
limestone (CaCO3) to quicklime (CaO)
Lime production, conversion and
what slaking efficiency really
means to the end user
to produce the best-quality quicklime
possible for the end-user. In many cases
quicklime received by an end-user is slaked
prior to application in the process.
Dual silo make down system for
hydrated lime and magnesium oxide
INTERNATIONAL CEMENT REVIEW JULY 2015
Importance of optimum
conditions
The importance of optimum
conditions during slaking taking
place at the point of use are not
always apparent to most quicklime
users.
The ‘quality’ of calcium hydroxide
produced on site from quicklime is
often measured by the following key
parameters:
• 30 seconds, three minutes, and
final temperature rise during lime
slaking process
• total active slaking time (time taken
to complete the slaking reaction)
• total and available lime (in per cent
terms).
Key slaking parameters
The quality of the hydrate produced
is determined by the following slaking
parameters:
• final slaking temperature achieved
• water-to-quicklime ratio used (by
weight)
• grit production and composition,
generally reports to a waste stream
• hydrated lime particle size (as it relates to
specific surface area)
• slaking water composition
• neutralisation capacity of calcium
hydroxide produced
• neutralisation kinetics (how fast does it
react?).
Final slaking temperature achieved
In general, it is best to target a slaking
temperature as high as practical without
allowing the lime slurry to boil over.
Different types of slaking equipment
can have slightly different maximum
©Copyright Tradeship Publications Ltd 2015
LIME SLAKING
Lime slaking apparatus as per
ASTM C-110-09a Section 11
Acid neutralisation capacity
and kinetics apparatus
losses. Some sites use a vertical ball mill as
a slaker to alleviate some of this problem.
Slaking water composition
If high-quality quicklime was slaked
with cold winter temperature ground- or
lake water as is often the case (thus not
allowing the final slaking temperature to
reach the target value), the amount of
lime residue or grit production multiplies.
The amount of quicklime wasted therefore
multiplies.
If a site is using recycled process water
containing impurities such as sulphates/
sulphites, carbonates and phosphates/
phosphites, research has shown that the
slaking efficiency can suffer dramatically.
Hydrated lime particle size
The overall goal of efficient slaking is
to make the smallest-possible calcium
hydroxide particles which yield the highest
specific surface area (SSA) and will be the
most chemically reactive. SSAs of greater
than 200,000cm2/g are considered to
Sulphates: Sulphate/sulphite anions can
coat quicklime particles when they are
mixed together and dramatically slow
the slaking reaction. This coating blocks
the pores of the quicklime particle. It
effectively prevents water from getting
inside the particles and inhibits the
recommended operating temperatures.
For detention slakers a target temperature
of 85˚C ±3˚C (185˚F ±5˚F) is typically
recommended and for paste slakers a
target temperature of 88˚C ±3˚C (190˚F
±5˚F) is often recommended.
Water-to-quicklime ratio used
In the case of detention slakers, it is
normal to operate at a water:lime ratio of
between 3.5-6.0:1 by weight. This ratio
keeps the lime slurry in a manageable
viscosity range as well as allows for higher
final slaking temperatures. In the case of
paste slakers it is normal to operate in the
range of 2.0-2.5:1 by weight.
Grit production and composition
Grit is considered a byproduct or waste
stream of the slaking process. It is often
thought of as being an inert waste
material that comes delivered with any
quicklime, but this is only a part of the
story. A very high-quality quicklime might
have 96 per cent total CaO and only 1-2
per cent inert grit which can be comprised
of silica, alumina or ferric oxide. If this
quicklime was slaked with warm, potable
water under ideal conditions such that the
target slaking temperature is achieved,
grit produced might be in the 2-5 per cent
range. (The ASTM standards report lime
residue (grit) as being over 30 mesh or
600µm in size). If the same quicklime was
slaked with cold recycled or process water
containing impurities research has shown
that lime residue production can be as high
as 42 per cent. If a grit screen or grit screw
was used, this would often contribute to
waste resulting in unnecessary quicklime
be very high quality. This corresponds to
an average d50 particle size of 4-6µm
(surface area weighted mean diameter).
Keep in mind that SSA is a combination of
particle size and porosity of the material.
Slaking at a lower target temperature or
with cold initial water will shift the particle
size curve towards larger particles. Larger
particles will have a lower combined SSA
and will be less reactive. More of this lessreactive calcium hydroxide will be needed
to do the same job. It is not uncommon
that an end user consumes 20 per cent
more quicklime in the winter when their
slaking water is cold than in the summer
when the incoming water temperature is
higher.
If a site is using recycled process
water containing impurities such as
sulphates/sulphites, carbonates and
phosphates/phosphites, research has
shown that the slaking efficiency
can suffer dramatically.
Pebbled quicklime sampling – cone and quartering
©Copyright Tradeship Publications Ltd 2015
JULY 2015 INTERNATIONAL CEMENT REVIEW
LIME SLAKING
test the efficiency of what an end user
is currently producing with their lime,
their water, at their site and under their
operating conditions and compare that
result to what they could produce at their
site (see Figure 1).
Figure 1: neutralisation capacity of equivalent dry weights of calcium hydroxide
reaction with quicklime which is critical
to efficient slaking. Slaking efficiency will
be progressively worsened by elevated
sulphate levels.
It is generally accepted that
concentrations of <100ppm SO42- have
a negligible effect on slaking while
concentrations between 200-500ppm
SO42- have a moderate negative effect
on slaking. A severe negative impact on
slaking lime is observed at concentrations
>500ppm SO42-. It is accepted by some
that ball mills used as slakers can typically
handle up to 500ppm SO42- because the
inherent grinding action wears away this
inhibiting coating.
Carbonates: Recycled plant or process
water typically has elevated carbonate
hardness which is reported as CaCO3
concentration in common water analyses.
The level of CO32- has a direct impact on
scale formation in the slaking equipment
as well as in the pumps and slurry pipelines
often associated with these systems.
Concentrations higher than 1000ppm
CO32- can cause catastrophic scaling
issues. Various strategies are used to try
to combat scale formation including lime
slurry additives, running with very high
lime slurry per cent solids and treating the
water hardness or better yet switching the
slaking water source.
Saving lime reagents costs, although
significant on an annual basis, are
only part of the complete story.
Those same savings of 7050tpa of
quicklime also led to a reduction in
CO2 emissions of 9150t, offering a
benefit to the environment.
The various standards that exist to test
the efficiency of hydrated lime slurry,
whether from ASTM, EPRI, AWWA
or another organisation, compare the
efficiencies of laboratory-produced
samples only.
Although these can be somewhat useful
benchmarks, it is far more important to
Neutralisation capacity
Ultimately the majority of quicklime is
made into hydrated lime slurry which is
used to vary the pH of a process.
INTERNATIONAL CEMENT REVIEW JULY 2015
Neutralisation kinetics
End-users vary wildly in terms of how
long of a retention time their process
has to effect this pH change. It can vary
from a low of just a few seconds to
several hours in some cases. As a result,
the neutralisation kinetics trend is very
important but often overlooked. In Figure
2 a properly-slaked site sample (blue line)
is fully reacted in 7-9min.
Even adding an extra 17 per cent
mass of poorly-slaked lime (red line)
did not compensate for the reaction
kinetics. In this case, the larger amount of
poorly-slaked lime did not complete the
neutralisation reaction until 22 minutes
had elapsed.
Slaking optimisation in practice
In one example alone, where a single
flue gas desulphurisation end-user had
been using 44,000tpa of quicklime, minor
process optimisation has saved 16 per cent
or 7050tpa to remove the same amount
of SO2. In financial terms, this offered cost
savings of US$700,000 per year.
Saving lime reagents costs, although
significant on an annual basis, are only
part of the complete story. Those same
savings of 7050tpa of quicklime also led
to a reduction in CO2 emissions of 9150t,
offering a benefit to the environment.
_______________________________I
Figure 2: equivalent neutralisation capacity for two different milk of limes
©Copyright Tradeship Publications Ltd 2015