united biates august
Environmental Protection 1984
Agency
• •
Lime
Handling
Systems
Problems
and
Remedies
905R84121
Z30 South Dearbbrti Street
-------
Lime Handling Systems - Probims
Introduction
Lime is one of the most widely-used chemicals for
wastewater treatment. It is marketed either as quick-
lime (CaO) or hydrated lime (Ca(OH)2). Quicklime is
hydrated in a slaking process before it is used as a
process chemical. Lime is generally added to treat-
ment processes as a slurry.
Lime is used in wastewater treatment for chemical
coagulation of solids, phosphate precipitation, pH
adjustment, and chemical stabilization of sludge.
Numerous problems have been experienced with
lime handling systems. These problems affect the
performance of treatment processes, resulting in
poor effluent quality, and increased operation and
maintenance costs.
A comprehensive evaluation of selected advanced
treatment (AT) facilities utilizing lime treatment
processes was recently completed with the
objective of identifying common problems
associated with lime handling systems. This
brochure i -ily summarizes the causes and
impacts of these problems, and offers suggestions
on how to solve them or at least mitigate their
impact.
The Lime Handling System
The lime handling system consists of a series of
operations that process raw lime (i.e. quicklime or
hydrated lime) into a slurry which is then applied to
wastewater in measured portions. A schematic pf a
typical lime handling system is shown in Figure 1.
Lime handling systems typically include the
following components: (1) loading/unloading facility,
(2) storage and dry feeder, (3) lime slaker, (4) slurry
tanks, (5) slurry transport lines, and (6) slurry feed
system.
The following discussion of problems and remedies
addresses each of these components. The major
problem areas for lime handling systems are
identified in Figure 1.
Vent
* Major "Problem" Areas.
• Recirculation Loop May Be Found In
Intermittent Type Lime Feed Systems
Lime
Loading -I.
pH Indicator/
Recorder
../-, .-:=- :>:c7i '?- •v.-«-:!'v.-"/ VJ
Agency
Figure! Schem^ipga MHnajHao^g4*»Jet»' ......
f;.--• South Dearborn Street
On >;w, fllfnois
-------
Lime Loading/unloading
Problems associated with the lime
loading/unloading operation include generation of
lime dust, severe damage of dry lime transport
piping due to abrasion at sharp elbows and bends,
and rupture of the lime bin due to buildup of
excessive pressure inside the bin. To minimize
these problems, dust collection baghouses should
be installed on lime storage bins and checked after
each loading operation; pressure relief valves
should be installed on lime bins; and long-radius
piping should be used for lime transport.
Lime Storage
In lime storage bins, the flow of lime to the dry lime
feeders is often interrupted by "arching" or
"bridging" of the dry lime above the hopper opening.
Moisture in the storage bin can cause the lime
powder to "cake," thereby preventing it from flowing
freely. The lime storage bin should have a minimum
60° bottom slope to facilitate the flow of material into
the feed hopper. A vibrating "live bin" bottom helps
to minimize "arching" and "bridging" problems. To
prevent the entry of moisture into the bin, a rotary
ball valve should be installed at the bottom of the bin.
In cases where bagged lime is used, the bulk
storage area should be adequately ventilated and
the humidity controlled to avoid "caking" problems.
Lime Slaking
Incorrect slaking temperature and water to lime
ratios in the slaking operation can result in poor
quality hydrated lime. If the slaker is not operated
under negative pressure, steam and moisture travels
back into the feeder and lime bin and causes
"caking" problems. Instrumentation panels located
on or adjacent to the slaker are often covered with
lime dust and grit resulting in frequent breakdown of
equipment.
Lime slakers should be airtight and operated under
negative pressure. A fan may be used to draw out
vapors to help maintain negative pressure in the
slaker. Instrumentation panels should be separately
housed and located away from the slaker.
Lime Slurrying
A severe problem of abrasion occurs if fiber rein-
forced plastic (FRP) tanks are used for slurry pre-
paration. Tanks made of corrosion resistant metal
should be used to avoid abrasion problems.
Probes used for level control in the slurry tank are
often coated with lime solids and rendered
ineffective. Sonic-type slurry level sensor systems
can be used to mitigate this problem.
Lime blurry Transport
The clogging of lime slurry lines is one of the most
difficult problems in a lime handling system. Scaling
in lime slurry piping is caused by the leakage of air
into pipes (which leads to formation of calcium
carbonate) and settling of solids during off cycles.
Use of small diameter slurry lines aggravates this
problem Cleaning of scale in pipes can be very
labor intensive, particularly in metal pipes where
"cleanouts" are not provided.
Problems with lime slurry transport systems can be
minimized by: continuously recirculating the lime
slurry through a recirculation loop (see Figure 1);
using flexible hoses for slurry transport lines; mini-
mizing the number of sharp bends; using piping at
least one and a half to two inches in diameter, and
periodically flushing the lines with water.
Slaked
Lime
c
D
Figure 2 Lime Feed System With Rotary Cup Type Feeder
Lime Slurry Feed
Common problems of lime slurry feed systems in-
clude clogging of metering pumps and valves, and
encrustation of pH probes. It is important to locate
the take-off point from the slurry line as close as
possible to the point of lime slurry application.
Rotary cup type feeders or similar slurry feed
systems should be used whenever possible. Figure
2 presents a typical lime slurry feed system using a
rotary cup type feeder. Feed control valves which
are operated in a fully opened or fully closed mode
should be used rather than variable rate type valves.
The problem of malfunctioning pH probes can only
be solved by routinely checking the probe, and
cleaning it when needed. One method of preventing
probe failure due to fouling is to rotate the use of
alternate probes on a regular basis.
i
-------
Conclusions
Based on the performance evaluation of lime han-
dling systems, it is clear that these systems are
prone to problems of equipment malfunction, and, in
general, require frequent maintenance and operator
attention. Deficiencies in design and inadequate
operation and maintenance of lime handling
systems are the major causes of the problems. The
inherent nature of lime and its limited solubility in
water are also responsible for some of these
problems.
The performance of existing lime handling systems
can be significantly improved by incorporating rem-
edial measures at a reasonable cost. A summary of
the major problems with lime handling systems and
suggested improvements is presented in Table 1.
Problem
• Lime dust generation
• Abrasion of dry lime transport lines
• "Arching" of dry lime in feed hopper
• "Caking" of lime in feed hopper
* Poor quality slaked lime.
• Malfunctioning of instrumentation located
near slaker.
• Abrasion of lime slurrying tanks
• Encrustation of probes used for slurry level
control
• Clogging of lime slurry transport lines
Clogging in lime slurry feed system
• Encrustation of pH probes
Remedy
• Install baghouse on lime silos.
• Use long radius bends and heavy duty
elbows.
• Install "live" bin systems.
• Operate slaker under negative pressure and
install rotary valve under the feeder.
• Maintain proper temperature and water
content in slaker.
• Locate instrumentation away from slaker.
• Use corrosion resistant metal tanks (not FRP).
• Use sonic-type level sensor systems.
• Install lime slurry recirculation loops; flush
transport lines with water; use flexible
hoses; avoid sharp bends; use minimum "P/2
to 2 inches diameter piping.
• Minimize distance between recirculation loop
and feeder; use open/close type valves; use
rotary cup type feeder.
• Rotate the use of alternate pH probes.
Table 1 Lime Handling Systems: Summary of Problems and Sug<
ad Remedies
------- |