ball mill heating and cooling
DESCRIPTION
Ball Mill Heating and CoolingTRANSCRIPT
3C Moisture
3C.1 Heating and Cooling
Many of us have encountered moisture and moisture related
problems, whether we are working with raw, coal or finish grinding
mills. When we talk about moisture, we also talk about heating
(drying) and cooling mill circuits.
Mills themselves generate a lot of heat. For ball mills, 90% of
the mill motor power gets converted into heat. For bowl mills, it's
about 75%.
Often when we need to dry materials, heat from the motor is not
enough and we are then forced to bring in an additional heat source
(furnace, hot gases from the kiln or cooler). Sometimes this extra
heat is added to the separator and the fresh feed is sent there first.
This arrangement is called flash drying. Sometimes the mill itself is
designed to receive the hot gases. Vertical roller mills and airswept
ball mills with drying chambers are common examples.
In finish grinding, usually we seek to shed the excess heat,
(some heat is actually useful). So here we often use water to cool the
system. We'll also examine alternatives to water cooling.
With moisture our problem revolves around the inability for
some reason to handle the water without causing operational or
quality problems. Most of it is associated with condensation of water
somewhere in the system.
3C.2 Dew Point Defined
Being able to measure wet and dry bulb temperatures and
determining the dewpoint is a very handy skill for troubleshooting
moisture related problems.
The dew point of any vapor is that temperature at which it will
condense. The higher the temperature of a gas, the more water vapor
it can contain on a mass basis without condensation of the water. If
air contains a specified mass of water, expressed in lbs. of water per
pound of dry air (or kg/kg air), and the temperature of the gas/water
mixture drops below the dew point or saturation temperature for that
water concentration in air, then the water vapor will begin to
condense from the air.
Symptoms of operating a ventilation and dust collection system
too near its dew point are:
1) Sweating and buildups in duct work.
2) Increased dust collector pressure drop.
3) System ventilation capacity decrease.
4) Mill puffing and/or backspilling.
5) Dust collector bag plugging.
A good general rule of thumb is that the dry bulb temperature in
a gas stream should be at least 50° F, (28° C) above the dew point
temperature. The dry bulb temperature is the actual gas stream
temperature. The wet bulb temperature is that temperature at which
the heat being taken away from the wet bulb of a wet bulb
thermometer by the evaporation of water is equal to the heat being
input to the wet bulb by the force convection effect of the gas stream
at the bulb temperature.
The evaporation rate from the wet bulb is affected by the
moisture content of the gas stream vs. the moisture capacity of the
gas. If the gas is dry the evaporation rate from the wet bulb will be
higher at the same dry bulb temperature than if the gas is moisture
laden. The higher the evaporation rate, the more heat is being taken
away from the wet bulb thermometer, and the lower the equilibrium
wet bulb temperature.
Therefore, a heavily moisture laden gas stream will have a
higher wet bulb temperature because of the lower evaporation rate,
than a gas stream at the same dry bulb temperature but with a lower
moisture content; and, of course, a higher wet bulb temperature at a
constant dry bulb temperature means a higher dew point
temperature. As the dew point temperature approaches the dry bulb
temperature, dew point problems become more likely.
How to read the psychrometric chart
(Psychrometric Charts in both U.S. and Metric units can be found in
the Appendices)
The psychrometric chart can be utilized to determine the dew
point temperature of a gas stream from the wet and dry bulb
temperature measurements. The dry bulb temperatures are shown
along the horizontal axis of the chart. Wet bulb temperatures run
diagonally across the chart and increase from the lower left and to the
upper right hand corner of the chart.
To determine the dew point temperature, locate the measured
dry bulb temperature on the horizontal axis. Follow this line up the
chart until it intersects the diagonal line representing the measured
wet bulb temperature. From the point of intersection of the wet and
dry bulb temperature lines, follow the nearest horizontal line to the
left and read the dew point temperature (always lower than the wet
bulb temperature).
Examples: A B
Dry Bulb Temperature = 160° F = 140° F
Wet Bulb Temperature = 100° F = 120° F
Dewpoint Temperature = 87° F = 118° F
It's not very easy to read psychrometric charts. For those who want
to be very accurate, there are computer programs that will calculate
psychrometric chart values.
If you have a continuous problem, then the plant may wish to invest in
a dewpoint meter or monitor and install the instrument in the problem
area. (WARNING: very few of these instruments will work well in
dusty gases. Check closely with supplier - get references! This is
fairly new technology. Early models were not very reliable.)
3C.3 Internal Water Sprays in Ball Mills
Most cement finish mills employ some type of internal water
spray to control mill discharge temperatures. Through proper control
of the mill internal temperatures the operator influences:
a) Gypsum dehydration which impacts false set performance (or in
isolated cases other types of setting).
b) Ball coating which influences grinding efficiencies and in the
worst cases grinding rates.
c) Mill sweep dust collector efficiencies (Bag coating for baghouses
or gas conditioning for ESP's).
Depending on the mill circuit, different locations can be used for
effective water sprays, but in all cases some atomization is required as
well flowrates must be regulated to some temperature setpoint.
3C.3.1 Feed End Water Sprays
Mechanically this is the simplest arrangement but in many cases
the most difficult to control. Usually the spray nozzle is located
beside or is an integral part of the feed spout assembly and sprays
atomized water into the first compartment. This type of water spray
is usually very sensitive to abrupt changes in feed material
temperatures and characteristics.
Figure 1 illustrates what a cement mill shell temperature profile
might look like. (Note how the diaphragm and discharge grates act as
heat sinks. In addition this diagnostic technique has good resolution
on high L/D ratio mills, but gets poorer with shorter mills.) Generally
the first compartment does not generate as much heat as the second.
Coupled with the influx of relatively cooler fresh feed, causes the first
compartment to grind at a much lower temperature. This results in
the first compartment being highly sensitive to:
a) Too much water input, (from either the spray or weathered
clinker or wet gypsum).
b) Over atomization.
c) Abrupt changes in clinker temperatures.
Usually this leads to backspillage and/or reduced production
stemming from either a plugged partition and/or excessive ball and
liner coating. Hence, good control is critical to feed end water sprays
working well. Typically a fast response thermocouple or RTD is
inserted in between the diaphragm walls to measure the product
temperature leaving the first compartment. However, this
arrangement is often accompanied by the usual headaches associated
with slip rings.
Depending on the mill configuration, the temperature sensitivity
may limit the amount of water that can be injected at this end and
thereby limit its temperature control capabilities. High L/D ratio mills
will have a greater problem than mills with low L/D ratios.
3C.3.2 Discharge End Water Sprays
Mechanically, this type of spray is located in the center of the
end compartment discharge grates and sprays water against the flow
of material. It generally works well in mills with relatively short
compartments and low internal mill sweep velocities. However this
arrangement is particularly sensitive to over atomization.
The finer spray droplets will have a greater tendency to be
captured by the mill sweep and be pushed up against the discharge
grates. In the worst case, the grates will plug. Moreover work done
in Exshaw and at Woodstock suggests that a discharge end water
spray does not penetrate deeply into the compartment itself. This is
also illustrated in Fig. 1 where it shows that the discharge end water
spray localizes its cooling effect at one end of the mill, thus
aggravating the tendency to plug the mill discharge grates. Some
plants use the smallest amount of air possible but others will find that
due to inadequate water pump pressures compressed air is needed to
ensure a high enough injection pressure.
3C.3.3 Partition Water Sprays
Mechanically this type employs water lines that are run along
the mill shell and then down in between the partition walls. The
nozzle itself is located at or near the center of the diaphragm,
spraying water, with the material and air flow into the end
compartment. As shown in the same Figure, this type of water spray
tends to produce a flatter thermal profile suggesting a superior
cooling effect. Although to a far lesser degree, partition water sprays
can still be over atomized leading to plugging problems. In most
cases though, it would lead to just reduced cooling efficiencies.
Partition water sprays are however more susceptible to
mechanical difficulties than the other methods. For example, if there
is some movement or shifting of the partition (which in itself is
trouble) could cause pipeline joints to rupture leading to some great
disasters.
3C.3.4 Control and Atomization
Water spray atomization is important to ensure adequate
dispersion into the load and to minimize the risk of localized cement
hydration. Usually compressed air is intermixed with the water flow
to accomplish this. As mentioned in the previous sections over
atomization usually leads to plugged mills. Thus some care is
required in determining the appropriate air/water ratio.
Figure 2 is a plot of air/water ratios gathered empirically on an
outdoor, full scale test rig done at the Woodstock plant in 1966.
During the tests, air flows were adjusted until the same water spray
plume was approximated for each new water flowrate. Note how non-
linear the relationship is. Recognize also that the above curve is
highly dependent on many physical characteristics such as nozzle
size, pump size, air pressure, etc. Repeating this test on a installed
system would be very impractical, but clearly the correct air/water
ratio is important.
In most plants the main control point is the mill discharge
product temperature and generally this is adequate. Other plants also
monitor mill sweep temperature as well. With this extra
thermocouple operators can fine tune the water spray. In theory the
mill sweep and discharge product temperature should be close to the
same temperature (except in high sweep mills). If too much air is
used, the water spray will become too fine which has a tendency to
cool the mill sweep first. With some experimentation the optimum
settings can be determined.
3C.4 Other Temperature Control Methods
There are a variety of other methods for controlling the milling
temperature. Some are obscure, others are not that effective and
others still are gaining new importance. In no particular order:
a) High mill sweep air flows (not practical on most mills built
before 1980)
b) Fresh air intake and high separator venting (see "Influence of
Circulating Loads")
c) Water spray on the feed belt (risks prehydration)
d) Cooling jacket on the separator (not very good)
e) Water spray on the mill shell (great for leaky liner bolts, but
very messy.)
Recognize that not all of the methods mentioned will work on
any given mill. Some are more suitable than others and the degree of
effectiveness is dependent on many different factors. If changes are
being contemplated, some care must be taken in selecting the right
one.
3C.4.1 Influence of Circulating Loads
With the advent of highly swept mills and highly swept (high
efficiency) separators, it has become possible is to do away with
internal water sprays. We often overlook the fact the circulating load
brings back a tremendous amount of heat with it, into the system.
Therefore:
1) Cooling the rejects.
2) Lowering the circulating load.
Will make the circuit operate cooler. Of the reverse is also true.
3) Hot rejects.
4) Increasing the circulating load.
Will make the circuit operate hotter.
These principles have been used on high efficiency separator
installations. Mills with HES units tend to run with low circulating
loads and lot's of air through the separator. They were so cool that:
a) They did not need cement coolers.
b) They did not need water sprays.
c) They ran into quality control problems from having too low of a
milling temperature.
In response, the circuits now recirculated hot back through the
separator to keep the rejects temperature hot enough, to maintain a
milling temperature setpoint.
Raw mills that do not have enough drying capacity should take
heed. Lot's of hot rejects will easily maintain system up to
temperature and even improve on drying capacity.
3C.5 Flash Set and False Set
Gypsum and anhydrite - calcium sulfate is added to cement
chiefly as a set retarders. Calcium sulfate reacts with the quick setting component of cement - C3A.
During the milling process, some or all of the gypsum will
dehydrate to hemihydrate. Hemihydrate reacts much faster than
gypsum or anhydrite because of its higher solubility. Typically,
some hemihydrate is needed for the desired setting process.
However, too much hemihydrate causes false set, while not enough
hemihydrate can cause flash set.
solubility
(CaSO4 g/litre)
gypsum CaSO4·2H2O 2.4
hemihydrate (plaster of Paris) CaSO4·0.5H2O ≈6.0
insoluble (natural) anhydrite CaSO4 2.1
The conversion of gypsum to hemihydrate is a time,
temperature relationship and takes place between 80 -120 °C. This
also happens to be the same range that the mill outlet temperature
operates in.
Tightly controlling the mill temperature would help one to
control the formation of hemihydrate. However, it is not easy to
predict what the correct amount of hemihydrate and gypsum should be. This is a function of the amount and reactivity of C3A and the SO3
in the clinker.
The use of water spray has its risks, excessive water use can
cause preliminary hydration of clinker. This can cause setting
problems, and adversely affect strength development. It is
recommended that the dew point never exceed 70 °C in the mill.
The substitution of anhydrite for gypsum is often suggested
when there is too much hemihydrate, but delayed false set problems
have been reported because of incompatibility between anhydrite and
chemical admixtures added to concrete. As well some plants use
gypsum/anhydrite pre-blends , but have experienced irregularities due
to segregation and just plain poor blending.
Premature setting is usually divided into one of two general
categories:
False set: Early development of stiffness without the evolution of
much heat; can be dispelled and plasticity regained by further
mixing without the addition of water [also called "grab set",
"premature stiffening", "hesitation set", "rubber set"]. In
laymen's terms, too much hemihydrate leads to a weak plaster
set in the concrete which is easily broken.
Flash set: Early development of stiffness usually with considerable
evolution of heat; cannot be dispelled nor can plasticity be
regained by further mixing without adding water [also called
"quick set"].
Severe false set may cause difficulty, from a placing/handling
standpoint, but it is not likely to cause difficulties in transit mixing
(trucks) or remixing (pumping). It is most apt to be noticeable for
mixing for a short period of time in stationary mixers (small jobs &
some paving jobs). False set, per sec, has no deleterious effects on
quality. Additional mixing water may result in slightly lower
strengths.
Flash set severe enough to cause placing/handling difficulties
and will fail ASTM/CSA specifications.
Testing
The tests usually involves making a cement paste and measuring
how deeply a specially shaped needle penetrates. After a set time this
repeated. If the second try reaches 50% or less of the depth, it's
considered false set. Above 50% is usually OK. Then afterwards the
paste is remixed and the test is repeated for the third time. If you get
100% of the first penetration depth there is no flash set occurring. If
you do not, you may have some premature set. It may or may not be
flash set.
Unfortunately the ASTM/CSA tests are not very reliable in that
results in concrete differ widely. Sometimes at the plant we find no
false set but in the field they might run into it. The reverse has also
been known to happen. Research is ongoing to try and develop a
much better test to predict real results in concrete.
3D: Ball Mill Control
3D.1 Basic Instrumentation for Closed Circuit Dry Ball Mills
Minimum Recommended:
a) Total mill feed rate (production)
b) Feed rate of individual components
c) Mill motor kW
d) Gas and material temperatures at mill exit
e) First compartment sound
f) Static pressure at mill exit
g) Discharge bucket elevator motor kW
Additional Instrumentation for better troubleshooting:
a) Fan damper positions (and rpm for variable speed applications)
b) Separator diaphragm position (for Sturtevants)
c) Separator rotational speed (for H.E. or modified 1st generation
separators) and motor kW.
d) Separator rejects flow rate.
e) Finish product temperature.
f) Injected water flowrate and compressed air flow (if used).
g) Grinding aid addition rate
h) Second compartment sound
i) Finish product bucket elevator or F.K. pump motor kW.
3D.2 Basic Ball Mill Control Theory
From the plant Production department's point of view, we are
interested in keeping the mill running as smooth as possible without
overloading the mill. Recognize that an overloaded system generally makes
a mess of the plant and increases the likelihood of something breaking.
Essentially, plants strive to maintain the total throughput rate (fresh
feed plus separator rejects) at an "ideal constant". Of course the ideal
throughput will change as conditions alter, such as circulating load, feed
characteristics, etc.- to suit quality targets and requirements.
For many years, technology allowed us only to approximate this
indirectly using mill motor kW, mill sound or elevator kW. Today, with the
advent of newer weighfeeder technologies we can now directly measure
total throughput with much better accuracy and consistency. This has led
to better mill control but not all plants can justify installing an impact
flowmeter. Thus some plants using existing instrumentation have resorted
to rule based control programs (fuzzy logic) to respond to system changes
caused by feed material changes for example. Many plants also have
developed their own schemes by blending two or more control methods
together, and have operated this way successfully. In all cases though mill
control requires a lot of study and persistence to perfect.
3D.3 Mill Motor kW Control
In general, the mill motor kW will vary with mill feed, but it is non-
linear. It varies in the following way:
1) With the mill at rest and completely run out of feed material, imagine
the total weight of the ball charge (W) to be concentrated at one point. This
point is called the center of gravity and
is located a certain distance from the
mill center. As the mill turns, the
center of gravity becomes slightly
offset from the mill's vertical centerline
by a distance "m", sometimes called
the moment arm. W X m = the torque
required (excluding friction) to turn the
mill.
To illustrate, let's say that we've
installed a badly designed mill liner
which causes the charge to climb
higher, then the moment arm "m" gets
longer and torque and therefore mill
power increases. On the other hand let's say that the liners are badly worn
and the charge slips down. In this case "m" becomes shorter and torque
and hence mill power decreases.
2) Going back to the original case, and let's begin to add feed. Under
these circumstances, the voids in between the balls that started out empty
now begin to fill. "W" increases since we are adding more mass to the ball
charge without increasing its volume. "m" doesn't change therefore torque
and mill motor kW increases.
3) However, at a certain point as we continue to add feed, the voids
become completely full and the ball charge starts to expand. At a critical
point the balls are pushed far enough apart that they lose contact or "grip"
with one another. Consequently the whole ball charge has a tendency to
slide down. This shortens the moment arm "m" which reduces torque and
mill power despite the fact that we are continuing to add feed. (Actually
one must also remember that as one adds feed, mill retention time
decreases which technically will hold "W" approx. constant.)
From the graph we can see that there is one major problem or flaw
with mill motor kW to control mill feed. For a given mill motor kW setpoint
there are potentially two feed rates which can cause a single loop controller
to hunt or oscillate. In addition, the kW value will change over a relatively
narrow range with a corresponding large change in feed. This coupled with
the difficulty in calibrating kW meters for large HP motors accurately makes
this control scheme very difficult to tune. This type of control is never used
by itself but occasionally it is used with another type of control loop.
3D.4 Mill Sound Control
The basic principle of mill sound control is simple. A directional
microphone is used to pick-up sound generated by the grinding media
tumbling inside a mill chamber. When the mill is empty of feed, metal to
metal contact is at its highest and therefore the microphone will record the
loudest decibels. As the mill fills, the cushioning effect deadens the noise
levels. In theory a mill that's completely plugged such that grinding media
cannot tumble will produce no noise at all. Some plants report this value as
decibels. Others reverse the signal and express it as % level or % full.
In general, mill sound is useful in determining whether a given
compartment is plugged or plugging or to show that the compartment is
emptying. With each compartment equipped with microphones, mill sound
is very useful in monitoring and troubleshooting a cycling mill. Some plants
successfully use the 1st compartment sound to control mill throughput.
However mill sound is very imprecise and is not considered to be very
repeatable (for example, 65% level which corresponded to a backspilling
condition one shift may not repeat itself the next shift). Microphones can
pick up noise from other mills which can impair it's reliability. Furthermore
the microphones are easily damaged as well dust can affect its
performance. All of these things generally makes mill sound microphones
difficult to calibrate. Despite these problems, a well isolated and
maintained microphone can be made to work well.
3D.5 Discharge Bucket Elevator Motor kW
On most mill circuits, mill product is fed into a bucket elevator which
transports the product to the separator feed. As the mass flow rate
increases so does the motor load on the bucket elevator, since it now has to
move more material. Many plants use this fact to control their mill since, in
most cases, elevator kW's fairly reflects the mill's total throughput. In fact
total throughput can be estimated using this formula:
M = (Ka-Ke) x 3600 x E / 9.81 x H
where:
M is material flow in mtph
Ka is actual power measured in kW
Ke is power measured with elevator empty in kW
E is elevator efficiency
H is the inter axis height in meters
(Note that this give an approximate answer since the above values will
change with mechanical wear and the amount of recirculation and/or boot
digging that occurs. Efficiency should be rechecked after each major
overhaul.)
For the most part elevator control works well. However there are two
major difficulties with this type of control:
a) Elevators are volumetric devices and are therefore very susceptible to
changes in bulk density or flowability which leads to the boot overfilling.
Many mill bucket elevators operate with the boot full or overfilled. (In such
cases the elevator was probably designed for mill product whose bulk
density was estimated at 90 to 100 lbs/ft3 but whose real bulk density can
be as low as 30 to 40 lbs/ft3.) Consequently any fluctuations in grinding
aid, internal water spray or airslide aeration can dramatically change the
product bulk density causing the boot to overfill. The resulting digging and
possible recirculation that occurs alters the elevator kW's but with no real
change in mass flow. For example in a few plants whose bucket elevators
are fed with airslides, reducing the under canvas pressure from 20 inwc to
10 inwc reduced motor kW by 8 to 10 % with no change in flow.
b) Bucket elevators are located after the mill and therefore controls will
always experience a lag time. Elevator controls should work well in short
mills or those with very short retention times, but will have less success
with long mills or long retention times. Moreover since these are feedback
loops often they cannot detect 1st compartment plugging problems. For
example, suppose a mill is beginning to plug somewhat in the mill's 1st
compartment due to a bin segregation problem. In such cases the second
compartment will start to empty which is detected by the elevator kW
control. However it assumes that the mill is not getting enough feed
therefore starts adding more feed to maintain a setpoint. If the operator
fails to notice this then the mill will either backspill and/or begin to cycle.
Most plants over come this difficulties by using more than one type of
control method together.
3D.6 Rejects Flowrate
Ideally to know the true mill throughput one should be measuring all
streams entering the mill. By summing the fresh feed rate and the rejects
flowrate from the dynamic separator one will know the throughput with no
guesswork. A flowmeter on the separator rejects will allow you to
determine immediately the circuit's circulating load, the effects the
separator adjustments has on the mill circuit and evaluate the whole circuit
retention time(s). However it should be used in conjunction with other
instruments previously mentioned since total flow will tell you very little
about internal mill problems.
This is perhaps the most expensive control device to install since in
most cases it requires mechanical adaptations to the system.
3D.7 Rule Base Mill Control
As we have seen in the previous sections, each control type has its
own set of difficulties - to some degree depending on the mill. Already
many plants utilize more than one type of control together, depending on
the limitations of the electronic equipment used. The most elaborate
combination would be to install a Rule Base control scheme, which is a table
of decisions a computer can take for each possible combinations of
conditions that could occur. Such a table properly set-up will mimic the
responses a human operator should make when confronted with the same
set of conditions. Fuzzy logic type systems operate on a similar principle.
An example for m Demopolis is shown in the chart.
Notes:
a) Since for each parameter there are 3 conditions (high, low, ok) then 3
to power equal to the number of control parameters will equal the number
of possible combinations. For example, for 3 parameters, 3 conditions
cubed equals 27 combinations; for 4 parameters - 34 equals 81; for 5
parameters - 35 equals 243; and so on. In the Demopolis example, there
should be 81 combinations where only 45 are shown. Recognize that for the
remaining 36 combinations, there are no actions to be taken.
b) Each combination must be studied in detail to determine whether feed
should be added or subtracted (or no action's to be taken) and by how
much. Clearly certain combinations will call for a stronger action than
others. As well one must also define what is high, what is low and what is
OK.
c) Rule base controls work on a regular intervals. In other words the
rules are evaluated once each interval and picks one action to take
(including "do nothing"). In between evaluations the program does not
make any moves. Intervals can be adjustable to determine the appropriate
frequency (ie. every 30 minutes or 25 minutes, etc.)
d) For each parameter there should be an adjustable deadband. For
example on paper strip chart a pen will be erratic within a certain band but
the average continues to trend up or down. The width of this band varies
depending on mechanical wear, instrument fatigue, etc. The deadband
takes this into account, thus only when the signal is outside of the deadband
is the parameter considered to be high or low.
e) Fuzzy logic or AI (artificial intelligence) systems work on a similar
principles.
f) WARNING: a considerable amount of study and tuning is required to
get the program just right. In other words you must teach it how to operate
the mill.
DEMOPOLIS RULE BASE CONTROL CHART
ElevatorKW
MillKW
#1 ComptLevel
#2 ComptLevel
FeedChange
1 low ok ok low add2 low ok ok ok add3 low ok low ok add4 low ok low low add5 low high ok ok add6 low high ok low add7 low high low low add8 low high low ok add9 ok high ok ok add
10 ok high low ok add11 ok high ok low add12 ok high low low add13 ok ok low low add14 ok ok ok low add15 ok ok low ok add16 high ok ok high sub17 high ok ok low sub18 high ok ok ok sub19 high ok low ok sub20 high ok low high sub21 high ok low low sub22 high ok high high sub23 high ok high low sub24 high ok high ok sub25 high low ok ok sub26 high low ok low sub27 high low ok high sub28 high low low ok sub29 high low low low sub30 high low low high sub31 high low high high sub32 high low high ok sub33 high low high low sub34 ok low ok high sub35 ok low ok ok sub36 ok low ok low sub37 ok low low ok sub38 ok low low high sub39 ok low low low sub40 ok low high high sub41 ok low high low sub42 ok low high ok sub43 ok ok high ok sub44 ok ok high low sub45 ok ok high high sub