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STUDYIING THE EFFECTS OF VARIOUS FACTORS ON

GRINDING OF COAL USING BALL MILL

A project report submitted in partial fulfillment for the award of degree of

BACHELOR OF TECHNOLOGY

IN

CHEMICAL ENGINEERING

SUBMITTED BY

B. SANTHOSH DHARMAN (07131A0810)

E. SRINIVAS (07131A0816)

K. KALYAN (07131A0823)

V. SAI SURENDRA (07131A0834)

Under the Esteemed guidance of

Sri .SIVA RAMA KRISHNA, M.Tech

Associate Professor

Department of Chemical Engineering

Gayatri Vidya Parishad College of Engineering

(Affiliated to Jawaharlal Nehru Technological University, Kakinada, A.P.)

Madhurawada, Visakhapatnam – 530 048

2007-2011



GAYATRI VIDYA PARISHAD COLLEGE OF ENGINEERING

DEPARTMENT OF CHEMICAL ENGINEERING

CERTIFICATE

This is to certify that the project work titled STUDYIING THE EFFECTS OF

VARIOUS FACTORS ON GRINDING OF COAL USING BALL MILL beingsubmitted by B. SANTHOSH DHARMAN, E. SRINIVAS, K. KALYAN, and V.

SAI SURENDRA as partial fulfillment of the requirement of Main Project is a bonafied work done by them under my guidance and supervision.

GUIDE: HEAD OF THE

Sri. SIVARAMAKRISHNA, M.Tech Dr. B.SRINIVAS Associate Professor. Head of the Department.

Viva voice held on

INTERNAL EXAMINER

EXTERNAL EXAMINER



ACKNOWLEDGEMENT

It is our privilege to express our profound gratitude to our project guideSri. SIVARAMAKRISHNA, Associate Professor , Department of ChemicalEngineering , G.V.P. College of Engineering for his incessant co-operationand suggestions towards the completion of the technical report .

Heartfelt thanks to Prof. B.Srinivas, HOD and for the support provided by all the faculty members of the department.

We are glad to express our heartfelt thanks to all the personnel whohave kindly assisted us with our project.

B. SANTHOSH DHARMAN

E. SRINIVAS

K. KALYAN

V. SAI SURENDRA



ABSTRACT

Grinding is very important unit operation. Grinding can be done usingdifferent equipments. The grinding of material depends upon various factors.The objective of the project is to study the various factors that affect the

grinding. Coal was taken as the grinding material and the equipment of grinding

is the ball mill. The factors that affect the grinding characteristics of coal are

• Amount of grinding media

• Amount of material

• Time of Grinding

The design strategy of experiments involves a series of tests in which purposeful changes are made to the input variable of the process so as toidentify the effects of the input parameter on the two output responses i.e.,Power consumption and Product passing through ISS 18mesh.



CONTENTS

s.no title page no

1. INTRODUCTION

1.1 strategy of experiments1.2 ballmill

1.3 working of ball mill1.4coal1.5 dry grinding

2. GUIDELINES FOR DESIGNING EXPERIMENTS

2.1 recognition of and statement of the problem

2.2choice of factors, levels and range

2.3 Selection of the response variable

2.4 Choice of experimental design

2.5 Performing the experiment

2.6 Statistical analysis of the data

2.7 Conclusions and recommendations

3. EXPERIMENTAL SETUP

3.1 TYPES OF GRINDING MEDIA

3.2 SIZE OF GRINDING MEDIA

3.3 AMOUNT OF GRINDING MEDIA



3.4 TIME OF GRINDING

4. MATERIAL PREPARATION

4.1 Preparation of material

4.2 Selection of grinding media

4.3 Factors to be considered

4.4Output Parameters considered

5. PROCEDURE

5.1 PARAMETERS TAKEN

5.2 OBSERVATIONS

5.3 EXPERIMENTAL PROCESS

6. CALCULATIONS

6.1 POWER CONSUMPTION

6.2 PRODUCT SIZE THROUGH ISS 18

6.3 MODEL FITTING

6.3.a Fitting the model for POWER CONSUMPTION

6.3.b Fitting the model for fixed size through 18-

7. RESULTS AND CONCLUSIONS

7.1 RESULTS

7.2 conclusion



8. FUTURE SCOPE OF THE EXPERIMENT

9.BIBLIOGRAPHY

1. INTRODUCTION

1.1 STRATEGY OF EXPERIMENTATION

Experiments are performed by investigators in virtually all fields of inquiry, usually todiscover something about a particular process or system. Literally, an experiment is atest. More formally, we can define an experiment as a test or series of tests in which

purposeful changes are made to the input variables of a process or system so that we mayobserve and identify the reasons for changes that may be observed in the output or response.

Planning and conducting the experiments and analyzing the resulting data so that validand objective conclusions are obtained is our goal. In engineering, experimentation playsan important role in new product design, manufacturing process development, and process improvement. The objective in many cases may be to develop a robust process,that is, a process affected minimally by external sources of variability.

1.2 Ball mill

The ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.

A ball mill, a type of grinder , is a cylindrical device used in grinding (or mixing)materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate arounda horizontal axis, partially filled with the material to be ground plus the grinding medium.Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously fed at one end and discharged at the other end. Largeto medium-sized ball mills are mechanically rotated on their axis, but small ones

normally consist of a cylindrical capped container that sits on two drive shafts ( pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder , but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensiveand can grind mixture particles to as small as 5 nm, enormously increasing surface areaand reaction rates. The grinding works on the principle of critical speed. The criticalspeed can be understood as that speed after which the steel balls (which are responsible

http://en.wikipedia.org/wiki/Grinder_(milling)


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http://en.wikipedia.org/wiki/Black_powder

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for the grinding of particles) start rotating along the direction of the cylindrical device;thus causing no further grinding.

Ball mills are used extensively in the Mechanical alloying process in which they are notonly used for grinding but for cold welding as well, with the purpose of producing alloysfrom powders.

The ball mill is a key piece of equipment for grinding crushed materials, and it is widelyused in production lines for powders such as including cement, silicates, refractorymaterial, fertilizer, glass ceramics, etc. as well as for ore dressing of both ferrous non-ferrous metals. The ball mill can grind various ores and other materials either wet or dry.There are two kinds of ball mill, grate type and overfall type due to different ways of discharging material. There are many types of grinding media suitable for use in a ballmill, each material having its own specific properties and advantages. Key properties of

grinding media are size, density, hardness, and composition.

• Size: The smaller the media particles, the smaller the particle size of the final product. At the same time, the grinding media particles should be substantiallylarger than the largest pieces of material to be ground.

• Density: The media should be denser than the material being ground. It becomesa problem if the grinding media floats on top of the material to be ground.

• Hardness: The grinding media needs to be durable enough to grind the material, but where possible should not be so tough that it also wears down the tumbler at afast pace.

• Composition: Various grinding applications have special requirements. Some of

these requirements are based on the fact that some of the grinding media will bein the finished product. Others are based in how the media will react with thematerial being ground.

o Where the color of the finished product is important, the color of thegrinding media must be considered.

o Where low contamination is important, the grinding media may beselected for ease of separation from the finished product (i.e.: steel dust produced from stainless steel media can be magnetically separated fromnon-ferrous products). An alternative to separation is to use media of thesame material as the product being ground.

o Flammable products have a tendency to become explosive in powder

form. Steel media may spark, becoming an ignition source for these products. Either wet-grinding or non-sparking media such as ceramic or lead must be selected.

o Some media, such as iron, may react with corrosive materials. For thisreason, stainless steel, ceramic, and flint grinding media may each be usedwhen corrosive substances are present during grinding.

1.3 WORKING OF THE BALL MILL

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When the mill is rotated, the balls are picked up by the mill wall and carried nearly to thetop, where they break contact with the wall and fall to the bottom to be picked up again.Centrifugal force keeps the balls in contact with the wall and with each other as they are

carried upward. While in contact with the wall, the balls do some grinding by slippingand rolling over each other, but most of the grinding occurs at the zone of impact, wherethe free-falling balls strike the bottom of the mill.

The faster the mill is rotated, the farther the balls are carried up inside the mill and greater the power consumption. The added power is profitably used because the higher the ballsare when they are released, the greater the impact at the bottom and the larger the productive capacity of the mill. If the speed is too high, however, the balls are carriedover and the mill is said to be centrifuging. The speed at which centrifuging occurs iscalled the critical speed . Little or no grinding occurs when the mill is centrifuging, andoperating speed must be less than the critical.

The speed at which the outermost balls lose contact with the wall of the mill depends onthe balance between centrifugal and gravitational forces. This is illustrated in the diagramgiven below.

Consider the ball at point A on the periphery of the mill. Let the radii of the mill and the ball be R and r, respectively. The centre of the ball is, then R-r meters (or feet) from theaxis of the mill. Let the radius AO form an angle with the vertical. Two forces act on the



ball. The first is the force of gravity mg, where m is the mass of the ball. The second isthe centrifugal force (R-r)ω2/g, where ω = 2∏n and n is the rotational speed. Thecentripetal component of the force of gravity is (mg)cosα , and this force opposes thecentrifugal force. As long as the centrifugal force exceeds the centripetal, the particle willnot break contact with the wall. As the angle α decreases, however, the centripetal forceincrease, and unless the speed exceeds the critical, a point is reached where the opposingforces are equal and the particle is ready to fall away. The angle at which this occurs isfound by equating the two forces, giving

(mg)cosα = m[4∏2n2(R-r)]/g

cosα = 4∏2n2(R-r)/g

At the critical speed, α = 0, cosα = 1. and n becomes the critical speed nc. Then,

nc = 1/2∏√g/(R-r)

There is a specific operating speed for most efficient grinding. At a certain point,controlled by the Mill speed, the load nearest the wall of the cylinder breaks free and it isso quickly followed by other sections in the top curves as to form a cascading, slidingstream containing several layers of balls separated by material of varying thickness. Thetop layers in the stream travel at a faster speed than the lower layers thus causing a

grinding action between them. There is also some action caused by the gyration of individual balls or pebbles and secondary movements having the nature of rubbing or rolling contacts occur inside the main contact line.

It is important to fix the point where the charge, as it is carried upward, breaks away fromthe periphery of the Mill. We call this the “break point” or “angle of break” because wemeasure it in degrees. It is measured up the periphery of the Mill from the horizontal.

There are four factors affecting the angle of break:

1. Speed of Mill

2. Amount of grinding media3. Amount of material

4. Time of Grinding



1.4 COAL

Coal is a combustible black or brownish-black sedimentary rock normally occurring inrock strata in layers or veins called coal beds or coal seams. The harder forms, such as

anthracite coal, can be regarded as metamorphic rock because of later exposure toelevated temperature and pressure. Coal is composed primarily of carbon along withvariable quantities of other elements, chiefly sulfur , hydrogen, oxygen and nitrogen.

Coal begins as layers of plant matter accumulate at the bottom of a body of water. For the process to continue the plant matter must be protected from biodegradation andoxidization, usually by mud or acidic water. The wide shallow seas of the Carboniferous period provided such conditions. This trapped atmospheric carbon in the ground inimmense peat bogs that eventually were covered over and deeply buried by sedimentsunder which they metamorphosed into coal. Over time, the chemical and physical properties of the plant remains (believed to mainly have been fern-like species antedating

more modern plant and tree species) were changed by geological action to create a solidmaterial.

1.5 DRY GRINDING

Whenever there is a choice between grinding a product wet and grinding it dry, wetgrinding will generally prove better. However, in many cases, it is impractical to grindwet due to the nature of the process or product.

The void volume between the grinding media, with the mill half charged, representsapproximately 20% of the total volume of the mill-and with a one-third charge of

grinding media 13 1/3%.

We usually try to limit the size of the batch to 25% of the total Mill volume which issufficient to fill all voids and slightly cover the grinding media. Any larger batches causethe pebbles to spread out through the mass of solids so they cannot make effectivecontact with each other, because of the layers of material between them. This greatlyreduces the grinding efficiency of the mill and, in some cases, makes it impossible toattain the desired results. The only occasion for larger batches than 25% of total volume,

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http://en.wikipedia.org/wiki/Metamorphic_rock

http://en.wikipedia.org/wiki/Carbon

http://en.wikipedia.org/wiki/Sulfur

http://en.wikipedia.org/wiki/Hydrogen

http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Nitrogen

http://en.wikipedia.org/wiki/Biodegradation

http://en.wikipedia.org/wiki/Oxidization

http://en.wikipedia.org/wiki/Carboniferous

http://en.wikipedia.org/wiki/Era_(geology)

http://en.wikipedia.org/wiki/Peat

http://en.wikipedia.org/wiki/Bog

http://en.wikipedia.org/wiki/Metamorphosed



http://en.wikipedia.org/wiki/Fern

http://en.wikipedia.org/wiki/Tree



is on products requiring a good mix rather than a grinding action or on products that aresoft and easy to grind and the grinding media do not necessarily have to make closecontact with each other.

The feed material should preferably be about 8 mesh or smaller, although many operatorsstart with much larger pieces. Having the feed material as fine as possible enables the useof smaller sizes of grinding media, which are always best for fine Uniform grinding anddispersions. For hard material, it is especially advantageous to start with fairly fine products.

Clogging of material in the Mill makes further operation harmful. This is generallycaused by moisture of fat, as in oily seeds. Possible remedies include:

1. Taking the material out and thoroughly drying it.

2. Adding a dry filler to absorb the excessive moisture while the batch is beingground.

3. Adding a few pieces of steel angle, bar, or chain which can slide along the Millsurface and scrape off any materials starting to pack.

4. If the material is packing due to particle size alone, grinding should be stopped prior to this point. The material should then be screened and tailings returned tothe mill.



2 GUIDELINES FOR DESIGNING EXPERIMENTS

To use the statistical approach in designing and analyzing an experiment, it is necessaryfor everyone involved in the experiment to have a clear idea in advance of exactly what isto be studied, how the data are to be collected, and at least a qualitative understanding of how these data are to be analyzed.

2.1 Recognition of and statement of the problem. This may seem to be a rather obvious point, but in practice it is often not simple to realize that a problem requiringexperimentation exists, nor is it simple to develop a clear and generally acceptedstatement of this problem. It is necessary to develop all ideas about the objectives of theexperiment.

Usually, it is important to solicit input from all concerned parties: engineering, qualityassurance, manufacturing, marketing, management, the customer, and operating personnel (who usually have much insight and who are too often ignored). For this reasona team approach to designing experiments is recommended.

It is usually helpful to prepare a list of specific problems or questions that are to beaddressed by the experiment. A clear statement of the problem often contributessubstantially to better understanding of the phenomenon being studied and the finalsolution of the problem. It is also important to keep the overall objective in mind; for example, is this a new process or system--in which case the initial objective is likely to becharacterization or factor screening--or is it a mature or reasonably well-understoodsystem that has been previously characterized--in which case the objective may beoptimization? There are many possible objectives of an experiment, includingconfirmation (is the system performing the same way now that it did in the past?),discovery (what happens if we explore new materials, variables, operating conditions,etc.?), and stability (under what conditions do the response variables of interest seriouslydegrade?). Obviously, the specific questions to be addressed in the experiment relatedirectly to the overall objectives. Often at this stage of problem formulation manyengineers and scientists realize that one large comprehensive experiment is unlikely toanswer the key questions and that a sequential approach using a series of smaller experiments is a better strategy.

Guidelines for Designing an Experiment

1. Recognition of and statement of the problem. Pre-experimental planning

2. Choice of factors, levels, and ranges

3. Selection of the response variable.

4. Choice of experimental design.



5. Performing the experiment.

6. Statistical analysis of the data.

7. Conclusions and recommendations.

“In practice, steps 2 and 3 are often done simultaneously or in reverse order.”

2.2 Choice of factors levels and range. (As noted in the Table, steps 2 and 3 are oftendone simultaneously, or in the reverse order.) When considering the factors that mayinfluence the performance of a process or system, the experimenter usually discovers thatthese factors can be classified as either potential design factors or nuisance factors. The potential design factors are those factors that the experimenter may wish to vary in theexperiment. Often we find that there are a lot of potential design factors, and some further

classification of them is helpful. Some useful classifications are design factors, held-constant factors, and allowed-to-vary factors. The design factors are the factors actuallyselected for study in the experiment. Held-constant factors are variables that may exertsome effect on the response, but for purposes of the present experiment these factors arenot of interest, so they will be held at a specific level. As an example of allowed-to-varyfactors, the experimental units or the "materials" to which the design factors are appliedare usually non homogeneous, yet we often ignore this unit-to-unit variability and rely onrandomization to balance out any material or experimental unit effect. We often assumethat the effects of held-constant factors and allowed-to-vary factors are relatively small.

Nuisance factors, on the other hand, may have large effects that must be accounted for,

yet we may not be interested in them in the context of the present experiment.

Nuisance factors are often classified as controllable, uncontrollable, or noise factors.

A controllable nuisance factor is one whose levels may be set by the experimenter. For example, the experimenter can select different batches of raw material or different daysof the week when conducting the experiment. The blocking principal, discussed in the previous section, is often useful in dealing with controllable nuisance factors. If anuisance factor is uncontrollable in the experiment, but it can be measured, an analysis procedure called the analysis of covariance can often be used to compensate for its effect.For example, the relative humidity in the process environment may affect process

performance, and if the humidity cannot be controlled, it probably can be measured andtreated as a covariate. When a factor that varies naturally and uncontrollably in the process can be controlled for purposes of an experiment, we often call it a noise factor.

In such situations, our objective is usually to find the settings of the controllable designfactors that minimize the variability transmitted from the noise factors. This is sometimescalled a process robustness study or a robust design problem. Once the experimenter hasselected the design factors, he or she must choose the ranges over which these factors will



be varied, and the specific levels at which runs will be made. Thought must also be givento how these factors are to be controlled at the desired values and how they are to bemeasured. For instance, in the flow solder experiment, the engineer has defined 12

variables that may affect the occurrence of solder defects. The engineer will also have todecide on a region of interest for each variable (that is, the range over which each factor will be varied) and on how many levels of each variable to use. Process knowledge isrequired to do this. This process knowledge is usually a combination of practicalexperience and theoretical understanding. It is important to investigate all factors thatmay be of importance and to not be overly influenced by past experience, particularlywhen we are in the early stages of experimentation or when the process is not verymature.

When the objective of the experiment is factor screening or process characterization, it isusually best to keep the number of factor levels low. Generally, two levels work verywell in factor screening studies. Choosing the region of interest is also important.

In factor screening, the region of interest should be relatively large--that is, the range over which the factors are varied should be broad. As we learn more about which variables areimportant and which levels produce the best results, the region of interest will usually become narrower.

2.3 Selection of the response variable. In selecting the response variable, theexperimenter should be certain that this variable really provides useful information aboutthe process under study. Most often, the average or standard deviation (or both) of themeasured characteristic will be the response variable. Multiple responses are not unusual.

Gauge capability (or measurement error) is also an important factor. If gauge capability isinadequate, only relatively large factor effects will be detected by the experiment or perhaps additional replication will be required. In some situations where gauge capabilityis poor, the experimenter may decide to measure each experimental unit several times anduse the average of the repeated measurements as the observed response. It is usuallycritically important to identify issues related to defining the responses of interest and how

they are to be measured before conducting the experiment. Sometimes designedexperiments are employed to study and improve the performance of measurementsystems.

2.4 Choice of experimental design. If the pre-experimental planning activities above aredone correctly, this step is relatively easy. Choice of design involves the consideration of



sample size (number of replicates), the selection of a suitable run order for theexperimental trials, and the determination of whether or not blocking or other randomization restrictions are involved. This book discusses some of the more important

types of experimental designs, and it can ultimately be used as a catalog for selecting anappropriate experimental design for a wide variety of problems.

There are also several interactive statistical software packages that support this phase of experimental design. The experimenter can enter information about the number of factors, levels, and ranges, and these programs will either present a selection of designsfor consideration or recommend a particular design. (We prefer to see several alternativesinstead of relying on a computer recommendation in most cases.) These programs willusually also provide a worksheet (with the order of the runs randomized) for use inconducting the experiment.

In selecting the design, it is important to keep the experimental objectives in mind.

In many engineering experiments, we already know at the outset that some of the factor levels will result in different values for the response. Consequently, we are interested inidentifying which factors cause this difference and in estimating the magnitude of theresponse change. In other situations, we may be more interested in verifying uniformity.

For example, two production conditions A and B may be compared, A being the standardand B being a more cost-effective alternative. The experimenter will then be interested indemonstrating that, say; there is no difference in yield between the two conditions.

2.5 Performing the experiment. When running the experiment, it is vital to monitor the process carefully to ensure that everything is being done according to plan. Errors inexperimental procedure at this stage will usually destroy experimental validity. Up-front planning is crucial to success. It is easy to underestimate the logistical and planningaspects of running a designed experiment in a complex manufacturing or research anddevelopment environment.

2.6 Statistical analysis of the data. Statistical methods should be used to analyze thedata so that results and conclusions are objective rather than judgmental in nature. If theexperiment has been designed correctly and if it has been performed according to thedesign, the statistical methods required are not elaborate. There are many excellent

software packages designed to assist in data analysis, and many of the programs used instep 4 to select the design provide a seamless, direct interface to the statistical analysis.

Often we find that simple graphical methods play an important role in data analysis andinterpretation. Because many of the questions that the experimenter wants to answer can be cast into a hypothesis-testing framework, hypothesis testing and confidence interval



estimation procedures are very useful in analyzing data from a designed experiment. It isalso usually very helpful to present the results of many experiments in terms of anempirical model, that is, an equation derived from the data that expresses the relationship

between the response and the important design factors. Residual analysis and modeladequacy checking are also important analysis techniques. We will discuss these issues indetail later.

It is worth remembering that statistical methods cannot prove that a factor (or factors) hasa particular effect. They only provide guidelines as to the reliability and validity of results.

Properly applied, statistical methods do not allow anything to be proved experimentally, but they do allow us to measure the likely error in a conclusion or to attach a level of confidence to a statement. The primary advantage of statistical methods is that they add

objectivity to the decision-making process. Statistical techniques coupled with goodengineering or process knowledge and common sense will usually lead to soundconclusions.

2.7 Conclusions and recommendations. Once the data has been analyzed, theexperimenter must draw practical conclusions about the results and recommend a courseof action. Graphical methods are often useful in this stage, particularly in presenting theresults to others. Follow-up runs and confirmation testing should also be performed tovalidate the conclusions from the experiment.

Throughout this entire process, it is important to keep in mind that experimentation isimportant parts of the learning process, where we tentatively formulate hypotheses abouta system, perform experiments to investigate these hypotheses, and on the basis of theresults formulate new hypotheses, and so on. This suggests that experimentation isiterative. It is usually a major mistake to design a single, large, comprehensiveexperiment at the start of a study. A successful experiment requires knowledge of theimportant factors, the ranges over which these factors should be varied, the appropriatenumber of levels to use, and the proper units of measurement for these variables.Generally, we do not perfectly know the answers to these questions, but we learn aboutthem as we go along. As an experimental program progresses, we often drop some input

variables, add others, change the region of exploration for some factors, or add newresponse variables. Consequently, we usually experiment sequentially, and as a generalrole, no more than about 25 percent of the available resources should be invested in thefirst experiment. This will ensure that sufficient resources are available to performconfirmation runs and ultimately accomplish the final objective of the experiment.



3. EXPERIMENTAL SETUP

3.1TYPES OF GRINDING MEDIA

STEEL AND OTHER METAL BALLS – Steel balls are unquestionably doing a faster grinding job than any of the other commercially available media. They have provenespecially valuable in the paint industry. This has not always been the case, however. Inthe early days Mill operators were insistent upon large steel balls, comparable in size tothe flint pebbles or porcelain balls in use at the time. Contamination was excessive andthey did not appear to grind much faster than the other grinding media. It was not untilmuch smaller sizes were put into use and correct operating techniques were developedthat such outstanding results were obtained, in some cases reducing grinding time to one-third that required for other grinding media.

The following types of metal balls are commonly used in Ball Mills:

1. High Carbon – High Manganese Steel with alloying elements or molybdenum,chromium or nickel. These balls especially made for Ball Mills are uniformlythrough hardened to 60 - 65 Rockwell C. While they are almost perfect spheresthey should not be confused with case hardened ball bearings. They represent thehighest quality of all metal balls and most operators insist on using them.

2. Cast Nickel Alloy – This is also very popular and, as it is basically a white metal ball, it causes less metallic staining than the others. Principal objection is its rough

outer surface and projecting nubs typical of cast balls. It requires longconditioning periods before being placed into general use.

3. Stainless Steel – because of their high cost they are only being used on specialwork requiring an acid resistant and non- magnetic ball.

4. Chilled Iron

5. Forged Low Carbon Steel – both 4 and 5 are the cheapest metal balls obtainable.They are only recommended for rough grinding, where metallic contamination isnot objectionable.

6. Other, more special types include bronze or brass, aluminum, tungsten carbide,etc.

Special note: No matter how good the metal ball might be, care must beexercised in the operation of the Mill if excessive wear with its resultantcontamination is to be avoided.

3.2 SIZE OF GRINDING MEDIA



Probably the most common cause for faulty operation and complaints has been due to thesize of grinding media. It is strongly recommended that the smallest feasible grindingmedia be used in all cases. The optimum size of media should not change with Mill size.

If the laboratory Pebble or small Ball successfully grinds a sample batch in a lab Mill, thesame size grinding media will do the best job in a production Mill whether the Mill is onefoot or eight feet in diameter.

Small grinding media are recommended because:

1. They provide many more grinding contacts per revolution than larger media. Thisresults in much quicker grinding action.

2. They provide smaller voids, limiting the size of particles or agglomerates whichcan exist there.

3. They do not create excessive energy which cannot be utilized. Oversized grindingmedia frequently develop more grinding energy than is needed for the job. Thisexcess merely builds up heat and wears down the media and lining, introducingcontamination in the batch. Using an extremely large grinding media is somewhatlike using a sledgehammer to drive in a carpet tack.

The chief disadvantage of the smallest size grinding media is that discharging takessomewhat longer due to increased surface tension in the smaller voids. Almost invariably,however, the reduced grinding time realized by smaller media more than offsets thisdisadvantage. Slight air pressure may be used to assist in more rapid discharge.

Using extremely small media, with their greater surface area for the material to adhere to,may yield a smaller initial batch. Subsequent batches will be of normal size, however.

When steel balls are used, the optimum sizes we have usually been recommending have been ½ and 5/8”. However, many operators are now using media as small as ¼” in production mills and find these extremely advantageous where exceptionally fine grindsare required. Generally, the viscosities must be slightly lower for the small size balls thanwe would recommend for the more popular ½ and 5/8” sizes.

3.3 AMOUNT OF GRINDING MEDIA

For the most efficient results, the Mill should be at least half filled with grinding media.Some operators prefer to go a little beyond the halfway mark to compensate for wear.There is no objection to this and we have been suggesting a limit of about 5 percent.

In steel ball grinding, many operators, especially in the paint industry, are satisfied to runwith a smaller ball charge ranging as low as one-third the volume of the Mill. They find



the smaller charge gives them the required grind within allowable limits of grinding timeand the extra space gives them more loading room.

There is no objection to this practice when the grinding cycle falls within the desiredworking limits. Where speed of grind is of utmost importance, larger ball charges rangingup to the recommended 50% for other types of grinding media are advisable. The logic inthis system is best illustrated as follows:

5/8” steel balls are one of the most popular sizes, and there are 36 of these per pound. In a54” x 60” Steel Ball Mill, for example, the difference between the weight of a one-thirdand one-half ball charge is 3,970 pounds, or 103,220 balls. The ½” steel ball is another very popular size and, as there 53 of these per pound, the difference would amount to200,410 balls. It s therefore, reasonable to expect (and experience has proven this to betrue) that any addition above the minimum limits prescribed can only result in increased

grinding efficiency. This improvement is usually related to the surface area of the mediainvolved.

It is not true that a one-half ball charge consumes proportionately more power than a one-third ball charge. The difference in weight between the tow charges is about 50% but thecenter of gravity of the larger is nearer the center of rotation of the Mill. Consequently,the power required to turn the larger charge only runs between 15 and 20% more.

The grinding efficiency of the one-half charge is considerably greater than for the one-third and, therefore it can be expected that power consumption per gallon output willactually be less than with the smaller charge.

Grinding media should be periodically checked. Reduction in the quantity and size of thegrinding media will result in poor grinding. We suggest a maximum schedule of onceevery six months, but any established procedure should be decided by individualexperience. In some cases, where abrasive materials are involved, once a month is not toooften and, in a few cases, even shorter intervals are indicated.

A simple method for checking is to have a rod cut indication the distance from the top of the grinding media to the underside of the manhole opening and use this for checking thedepth of the charge.

When grinding enamel frit, wear to the porcelain balls is quite excessive because to theabrasive nature of the frit. Consequently, many operators have been able to closelydetermine the ball wear per batch and, when a batch of frit is loaded for grinding, aquantity of new balls is added equaling the weight lost during the previous grind.However, even with this system, we still advise an occasional check with the measuringrod because there is no positive guarantee that all balls will wear the same.



Dumping the charge once a year or as often as experience indicates, and removing anygrinding media found to be excessively worn or damaged is necessary and advocated.

The following general rules should be carefully adhered to regardless of the type mediaused.

1. There should be enough material in the batch to cover the grinding media.

2. Grinding time must be watched carefully to avoid excessive grinding.

3. Excessive buildup of heat should be avoided. In paint grinding, this may lower theoperating viscosity beyond the critical point. A reduction in Mill speed may helpto avoid overheating, but it is more desirable to circulate a cooling mediumaround the cylinder. If the Mill is not jacketed, a water spray can be used withsatisfaction.

4. The smallest grinding media should be employed. These not only reduce thedanger of overheating but, as is well known, the smaller grinding media providefaster and better results.

5. When using extenders, their abrasive nature may cause excessive wear. To avoidthis, some operators are able to hold out the extenders until the grinding isalmost .completed and then add them for the final operation.

3.4 TIME OF GRINDING

Time is an important parameter for the grindability of the material. Hence, the time takenfor running the ball mill should be considered as a major factor. In the experimentsconducted, time is taken as one of the factors of experimentation.



4. MATERIAL PREPARATION

4.1 (A) Preparation of material

The material being considered for grinding is coal which is available naturally in lumps.The lumps are reduced to a smaller size by hammering them. These smaller lumps arethen fed into the Jaw Crusher to reduce them to an even smaller size. The crushed piecesare then sieved through 480- and 340+, and 200- and 100+ sieves.

4.2 (B) Selection of grinding media

The most common cause for faulty operation and complaints is due to the size of grindingmedia. It is strongly recommended that the smallest feasible grinding media be used in all

cases. The optimum size of media should not change with Mill size. If the laboratoryPebble or small Ball successfully grinds a sample batch in a lab Mill, the same sizegrinding media will do the best job in a production Mill whether the Mill is one foot or eight feet in diameter.

Small grinding media are recommended because:

1. They provide many more grinding contacts per revolution than larger media. Thisresults in much quicker grinding action.

2. They provide smaller voids, limiting the size of particles or agglomerates which can

exist there.3. They do not create excessive energy which cannot be utilized.

Oversized grinding media frequently develop more grinding energy than is needed for the job. This excess merely builds up heat and wears down the media and lining, introducingcontamination in the batch. Using an extremely large grinding media is somewhat likeusing a sledgehammer to drive in a carpet tack.

The chief disadvantage of the smallest size grinding media is that discharging takessomewhat longer due to increased surface tension in the smaller voids. Almost invariably,

however, the reduced grinding time realized by smaller media more than offsets thisdisadvantage. Slight air pressure may be used to assist in more rapid discharge.

Hence, the average size of balls with a specific ratio Large: Small: Medium = 1:2:2 has been used.

4.3 (C) Factors to be considered



1. Feed size

Two feed sizes are taken into consideration. The larger feed size is the material passedthrough 480 and retained on 340 ISS, and the smaller feed size is the material passedthrough 200 and retained on 100 ISS.

2. No. of balls

The number of balls used in each run was 10 or 20.

3. Time of grinding

Each run was carried out for a time span of 30 or 60 minutes.

4.4 (D) Output Parameters considered

1. Power consumed

Power consumption is an important response to consider as it directly affects theeconomy.

2. Product passed through18-

The 18 ISS is taken as the basis and the Grindability is measured.



5. PROCEDURE

5.1 PARAMETERS TAKEN:

1. Size of the feed:

Feed size large= -480, +340

Feed size small= -200, +100

2. Average size of the balls ratio is taken as

Large: Small: Medium=1:2:2

Average size of large balls is = 4:8:8;

Average size of small balls is = 2:4:4;

3. Time of grinding:

Time larger = 60 sec;

Time small = 30 sec;

5.2 OBSERVATIONS:

Ball mill thickness = 5mm

Ball mill perimeter (inner perimeter) = 947.55mm

Time taken for the empty ball mill running with 10 balls for one revolution = 55.30 sec

Time taken for the empty ball mill running with 20 balls for one revolution = 51 sec

Revolutions for one unit power consumed in the meter = 240



5.3 EXPERIMENTAL PROCESS

[Note: The strategy in the design of experimentation is to run the experiments withchange in the levels of the factors for each run and to study the interaction effects.

As this experiment is a 3 factor model, and each parameter takes 2 levels, the number

of runs is 23. The 8 runs so obtained are performed randomly with two replicates.]

1. The ball mill is emptied and the grinding media (metal balls) is added without anymedium.

2. The mill is run for 60 seconds and the corresponding electric meter reading isnoted.

3. The prepared coal is added to the mill and it is run for a given time (either 30 or 60 minutes).

4. The time taken for the experiment and the electric meter reading are noted.

5. The ground material is taken out and sieved through ISS 18.

6. The material retained and passed through ISS 18 is weighed and their valuesnoted down. The error in weight is also calculated.



FACTOR LEVEL

REPLICATE 1

REPLICATE 2

6. CALCULATIONS

FACTORS HIGH LOW

FEED SIZE (A) Between 4.75 and 3.35 mm Between 2.0 and 1.0 mm NO OF BALLS(B) 20 BALLS 10 BALLS

TIME OF GRINDING(C) 60 MINUTES 30 MINUTES

Expt.no

Feedsizeof

balls

Noof

balls

Time Sieving result on ISS18

Percentage of grinding

Wastagein grams

Time for one

revolution

Power consumed

Retainedon 18+

Passedthrough

18-1 - - - 397.7 96.5 70 5.8 47 0.1592 + - - 414 83 47 3 46 0.1633 - + - 309.6 190.2 45.2 0.2 42 0.1784 + + - 259 235 60.16 6 47 0.1595 - - + 192.6 279.8 19.3 27.6 47 0.1396 + - + 260 226 55.96 14 46 0.3267 - + + 192.1 300.8 38.04 7.1 47 0.3198 + + + 146 350 16.6 4 38 0.394

Expt.no

Feedsizeof

balls

Noof

balls

Time Sieving result on ISS18

Percentage of grinding

Wastagein grams

Time for one

revolution

Power consumed

Retainedon 18+

Passedthrough

18-1 - - - 407.6 9102 55.5 1.2 45 0.1502 + - - 383.3 133.3 37.72 3.4 50 0.3403 - + - 292.5 192.9 38.30 14.6 46 0.2834 + + - 297.5 188.6 67.36 13.9 41 0.1665 - - + 274.5 219.9 18.24 5.6 53 0.1636 + - + 299.3 191.5 43.98 9.2 45 0.3657 - + + 152.9 336.8 38.58 10.3 44 0.1828 + + + 192.3 277.8 22.66 29.9 41 0.333



6.1 POWER CONSUMPTION

RUN TREATMENTS A B C REP1 REP2 SUM1 1 - - - 0.159 0.150 0.309

2 a + - - 0.163 0.340 0.503

3 b - + - 0.178 0.283 0.461

4 ab + + - 0.159 0.166 0.325

5 c - - + 0.319 0.163 0.482

6 ac + - + 0.326 0.365 0.691

7 bc - + + 0.319 0.182 0.501

8 abc + + + 0.394 0.333 0.727

Effect of A: 2(Contrast)/n2k

In this case, n=2, k=3

Hence,

Effect of A:

2(a+ab+abc+ac-1-b-c-bc)/2(8) =

1/8(0.503+0.325+0.691+0.727-0.309-0.461-0.482-0.501) = 0.061

Effect of B: 1/8(b+ab+bc+abc-1-a-c-ac) = 0.003

Effect of C: 1/8(c+ac+bc+abc-1-b-a-ab) = 0.100

Effect of AB: -0.039

Effect of BC: 0.010

Effect of AC: 0.003

Effect of ABC: 0.043



SSA = (contrast)2/nk 2 = (0.488)2/16 = 0.015

SSB = 5.25+10-5

SSC = 0.04

SSAB = 6.084X10-3

SSAC = 8.836X10-3

SSBC = 4X10-4

SSABC = 7.396X10-3

SST = 0.155

SSE = ∑yi2 – (∑yi)2/n

MSA = SSA/a-1 = 0.015

MSB = SSB /b-1 = 5.25X10-5

MSC = SSC /c-1 = 0.04

MSAB = SSAB/(a-1)(b-1) = 6.034X10-3

MSAC = SSAC/(a-1)(c-1) = 8.856X10-3

MSBC = SSBC/(b-1)(c-1) = 4x10-4

MSABC = SSABC/(a-1)(b-1)(c-1) =7.396X10-3

MSE = SSE/abc(n-1) =9.775X10-3

FA = mSA/mse = 1.534

FB = mSB/mse = 0.537

FC = mSC/mse = 4.092



FAB = mSAB/mse = 0.622

FBC = mSBC/mse = 0.040

FAC = mSAC/mse = 0.903

FABC = msabc/mse = 0.756

POWER CONSUMPTION TABLE

SOURCE OFVARIATION

SUM OFSQUARES

DEGREES OFFREEDOM

MEANSQUARE

F0

FEED SIZE(A) 0.015 1 0.015 1.539

NO.OF BALLS(B) 5.25x10

-5

1 5.25x10

-5

0.537TIME(C) 0.04 1 0.04 4.092

AB 6.084x10-3 1 6.084x10-3 0.622AC 8.836x10-3 1 8.836x10-3 0.903BC 4x10-4 1 4x10-4 0.040

ABC 7.396x10-3 1 7.396x10-3 0.756ERROR 0.0782 8 9.775X10-3 ---TOTAL 0.155 15 --- ---

6.2 PRODUCT SIZE THROUGH ISS 18:

RUN TREATMENTS A B C REP1 REP2 AVERAGE

1 1 - - - 96.5 91.2 187.7

2 a + - - 83 113.3 196.3

3 b - + - 190.2 192.9 383.1

4 ab + + - 235 188.6 423.6

5 c - - + 279.3 219.9 499.7

6 ac + - + 226 191.5 417.5

7 bc - + + 300.8 336.8 637.6

8 abc + + + 350 277.8 627.8

Effect of A: 2(Contrast)/n2^k



In this case, n=2, k=3

Hence,

Effect of A:

2(a+ab+abc+ac-1-b-c-bc)/2(8) = -5.362

Effect of B: 1/8(b+ab+bc+abc-1-a-c-ac) = 96.36

Effect of C: 1/8(c+ac+bc+abc-1-b-a-ab) = 123.98

Effect of AB: 13.03

Effect of BC: -9.31

Effect of AC: -17.63

Effect of ABC: 5.06

SSA = 115

SSB = 37140.9

SSC = 61484.16

SSAB = 679.12

SSAC = 1243.26

SSBC = 346.704

SSABC = 102.41

SST = 202246.37

SSE = ∑yi2 – (∑yi)2/n = 101134.82

MSA = SSA/a-1 = 115

MSB = SSB/b-1 = 37140.9

MSC = SSC/c-1 = 61484.16



MSAB = SSAB/(a-1)(b-1) = 679.12

MSAC = SSAC/(a-1)(c-1) = 1243.26

MSBC = SSBC/(b-1)(c-1) = 346.740

MSABC = SSABC/(a-1)(b-1)(c-1) = 102.41

MSE = SSE/abc(n-1) = 12641.

FA = mSA/ mse = 9.09X10-5

FB = mSB/ mse = 2.937

FC = mSC/ mse = 4.803

FAB = mSAB/ mse = 0.053

FBC = mSBC/ mse = 0.027

FAC = mSAC/ mse = 0.098

FABC = msabc/ mse = 8.1X10-3

PRODUCT SIZE ON ISS 18 TABLE

SOURCE OFVARIATION

SUM OFSQUARES

DEGREES OFFREEDOM

MEANSQUARE

F0

FEED SIZE(A) 115 1 115 0.009 NO.OF BALLS

(B)37140.9 1 37140.9 2.937

TIME(C) 61484.16 1 61484.16 4.863AB 679.12 1 679.12 0.053

AC 1243.26 1 1243.26 0.098BC 346.704 1 346.704 0.027ABC 102.412 1 102.412 0.008

ERROR 101134.82 8 12641.852 ---TOTAL 202246.37 15 --- ---



6.3 MODEL FITTING

6.3 (a) Fitting the model for POWER CONSUMPTION

A B C REP1 REP2 Y-1 -1 -1 0.159 0.150 0.309+1 -1 -1 0.163 0.340 0.503-1 +1 -1 0.178 0.283 0.461+1 +1 -1 0.159 0.166 0.325-1 -1 +1 0.139 0.163 0.432+1 -1 +1 0.326 0.365 0.691-1 +1 +1 0.319 0.182 0.301+1 +1 +1 0.394 0.333 0.725

X =

XT=

1 -1 -1 -11 +1 -1 -11 -1 1 -11 +1 1 -11 -1 -1 11 +1 -1 11 -1 1 1

1 1 1 1

1 1 1 1 1 1 1 11 1 1 1 1 1 1 11 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1



Y=

XTX=

XT

Y=

β =XTX -1XTY

0.3090.5030.4610.3250.4820.6910.5010.725

8 0 0 00 4 0 00 0 4 0

0 0 0 4

3.9970.4910.0270.801



=

β=

The fitted regression model is

Y=0.499-0.122x1+0.006x2+0.2x3

6.3 (b) Fitting the model for fixed size through 18-

A B C REP1 REP2 Y-1 -1 -1 96.5 91.2 187.7+1 -1 -1 83 113.3 196.3-1 +1 -1 190.2 192.9 383.1+1 +1 -1 235 188.6 423.6-1 -1 +1 279.8 219.9 499.7

1/8 0 0 00 1/4 0 0

0 0 1/4 00 0 0 1/4

3.9970.491

0.0270.801

0.4990.1220.0060.200



+1 -1 +1 226 191.5 417.5-1 +1 +1 300.8 336,8 637.6+1 +1 +1 350 277.8 627.8

X=Y=

XT=

XTX=

XTY=

1 -1 -1 -11 +1 -1 -11 -1 1 -11 +1 1 -11 -1 -1 11 +1 -1 11 -1 1 1

1 +1 1 1

187.7196.3333.1423.6499.7417.5637.6

627.8

1 1 1 1 1 1 1 1

-1 1 -1 1 -1 1 -1 1

-1 -1 1 1 -1 -1 1 1

-1 -1 -1 -1 1 1 1 1

8 0 0 0

0 4 0 0

0 0 4 0

0 0 0 4

3373.3-42.9770.9991.9



β= XTX -1XTY

=

=

The fitted regression model is

Y=421.66-10.72x1+192.72x2+247.9x3

1/8 0 0 0 3373.30 1/4 0 0 -42.90 0 ¼ 0 770.90 0 0 1/4 991.9

421.66-10.72192.72247.9



7. RESULTS & CONCLUSION

According to the power consumption table:

Main Effect of Feed Size: 0.061

Main Effect of number of Balls: 0.003

Main Effect of Time of Grinding: 0.100

Interaction Effect of Feed Size, No. of Balls: -0.039

Interaction Effect of No. of Balls, Time of Grinding: 0.010

Interaction Effect of Feed Size, Time of Grinding: 0.003

Interaction Effect of ABC: 0.043



Hence we can conclude that only the time of grinding has significant effect on the power consumption and both the feed size and number of balls do not show much effect on the power consumption

The interaction effect of the three factors is not showing any significant effect on power consumption, but the interaction effect of feed size and number of balls is showingnegative effect on power consumption

According to the Product size table:

Main Effect of A: 5.362

Main Effect of B: 96.36

Main Effect of C: 123.98

Interaction Effect of AB: 13.03

Interaction Effect of BC: -9.31

Interaction Effect of AC: -17.63

Interaction Effect of ABC: 5.06

Hence in this case the main of the factors number of balls and grinding time are showingmuch influence on the product size through iss 18mesh. But the main effect of the feedsize less when compared to that of the other two effects

the interaction effects are not showing any influence except the the interacton effect of the fed size and the no of the balls used

hence we can conclude that the for better product size we need to concentrate on time of grinding and the no.of balls used for grinding



Effect of C increase cheste output power consumption will be low.

A,b nullified values.

Iss 18Factor a negativeEffect o a inc prod size on iss 18- wioll come out low

B or c increase output response will be bettyer

Interaction effect ab is +Bc ac –veAbc +ve 5.06

Speed

The effects of various factors on grinding of coal using ball mil.



8. FUTURE SCOPE OF THE EXPERIMENT

The experiment was conducted by taking into consideration 3 parameters. The

speed of the mill was not considered as a factor. Hence the interaction effects were shown



9. BIBLIOGRAPHY

1. Wikipedia

2. Design and Analysis of Experiments.

3.

surendra project

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