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