How to Operate a Grinding Circuit

How to Operate a Grinding Circuit

How hard a ball mill operator has to work depends partly on himself, and partly on the kind of muck the mine sends over to the mill. In some plants, the ore may change two or three times a shift, and a ball mill operator has to keep on his toes.

That’s why it would be just as well for you, as a ball mill operator, to study out a few ways of doing your job easier and better, because there will be times, even in the best of mills, when you’ll run into a lot of trouble. Collected here you will find some practical suggestions, contributed by a number of good mill men, that might give you an idea or two that would help get around some of that trouble.

To be sure we understand each other, let’s begin with the equipment. In a simple grinding circuit there will be a ball mill and a classifier. Some circuits, especially in large mills, have more units or two or three stages of grinding, but whatever is said here will apply to the complicated circuits as well as the simple ones.

The two types of ball mill in general use are the grate mill and the open-end mill. Most manufacturers make both kinds; the difference between them is that the grate mill has a steel grid clear across the discharge end, but the open-end mill has only an open trunnion at the discharge end, through which pulp flows freely. If you don’t already know all about the inside of your ball mill and what it is supposed to do, it would be a good idea to ask the shift boss, the metallurgist, or the superintendent to tell you about it.

Mechanical classifiers make use of rakes, spirals, or a simple drag belt. For our purposes it doesn’t matter which type you are working with, because you would handle them all in pretty much the same way.

grinding circuit ball mill

In operation, you add water to the ball mill along with the ore. Flowing out of the ball mill, the ground ore pulp pours into the classifier pool, where the finished material is separated from the coarse sand. You do that by adding a lot more water to the pool, so that the finer sand overflows the weir and goes on to the next step (flotation perhaps), and the coarse sand settles to the classifier bottom and is raked back into the mill to be ground finer. You, the operator, are supposed to control these actions in order to send on to the machines below you the right amount of ore, ground just fine enough, and with just the right amount of water with it.

To help you do this, and to make a. record of how things are going, you will have to take samples of the pulp regularly. Different mills have different ideas on sampling, but all of them take at least hourly samples of the classifier overflow. What it amounts to is weighing a certain volume of the pulp to determine its density. Higher density means thicker pulp and usually coarser sand. Lower density means thinner pulp and finer sand. The shift boss will tell you what the density ought to be, and it will be up to you to hold it there.Grinding Circuit

You may also have to take density samples of the ball mill discharge, which runs a lot thicker than the classifier overflow, and some mills also expect you to take measured samples of the ball-mill feed and weigh them.

Another sample you may take is one for pH, which is a term that takes a little explaining. You can find out exactly what pH means from a chemistry book if you want to; but for all practical purposes, it is enough to know that pH is a number that tells you how much acid or alkali there is in the pulp. A pH of 7 is alkaline.

Table I 

Controls for Grinding Circuit
ball mill grinding circuit operating and troubleshooting parameter table

Feed Rate Increase from too low. Coarser grind, more circulating load. Effect on classifier overflow depends on fineness of ore feed and amount of increase in feed rate.
Decrease from too high. Finer grind, less circulating load. Probably more fine sand in classifier overflow.
Ball Mill feed water Increase Coarser grind; may wash out mill if add too much water. May increase circulating load and cause thinner classifier overflow.
Decrease Finer grind, but may choke mill if cut too far. May lower circulating load and cause thicker classifier overflow.
Water to classifier Increase Thinner and finer overflow; higher circulating load.
Decrease Thicker and coarser overflow; less circulating load.
In addition a few mills use a baffle board across the classifier pool. Shifting the baffle toward the tail end throws out a coarser overflow; moving it ahead makes the overflow finer and pulls out more sand into the circulating load. In a few mills the classifier overflow weir can be raised or lowered. Raising it makes the overflow finer; dropping the weir makes the overflow coarser. Variable rake speed is another control used in few mills. Most operators think rake speed is better left alone.

If you add acid, the pH goes down below 7; if you add an alkali like lime, the pH will go up, say to 9 or 10, depending on how much lime you add. In any case, you can bank on it that if the brass hats want you to watch the pH at all, they have good reasons for wanting you to hold it steady.

You may also have the job of adding balls to the mill each shift. The shift boss tells you how many or what weight, and you put them in. Drop them into the scoop if you have a grate mill, or put them in through the discharge trunnion if it is an open-end mill.

The controls you will have to work with are given in Table I, and are also indicated in the drawing. As to which one of these controls is most important, mill men don’t all agree. Probably it depends on what kind of ore you are grinding. Most good operators, though, say that the classifier water valve should be the first one to adjust, because it controls directly the kind of finished material you send on down the line to the next man.

The most important point is this: You cannot adjust any one of these controls without paying some attention to the other two. For example, if you change the feed rate, you will probably have to reset the two water valves. They all work together. In fact, the whole grinding circuit acts like a team of horses, and as the time at first.

What to Look For in a grinding circuit

In Table II you will find some suggestions on what to look for to help you decide how to use these controls. In the column headed “if you find,” there are set down the things you‘ll run into if something is wrong with the circuit. That is, if the ball-mill feed gets finer than it usually is, the top line tells you what to expect and what to do. But don’t think you have to do all these things all the time. Do only as much as you are sure you have to do.

The classifier overflow is really the most important spot in the circuit, because whatever comes over that weir is out of your hands, and your work will be judged by how good a product it is. Most operators believe that if there is any change in setting to be made, the density of the classifier overflow is where you make it first. Remember, more water to the classifier means thinner pulp and finer overflow; less water means thicker pulp and coarser overflow.

Watch the Mill Feed

The matter of feed to the ball mill brings up a point that is important in keeping you out of trouble. You can find out by asking the old- timers how each kind of ore is going to act when it hits the mill, and if you for each change as it comes along.

For example, suppose you are working in a lead-zinc flotation mill where there are two kinds of ore— one that is coarse and low grade, and another that is finer and higher grade. Keep the feed to the ball mill lower when the coarse stuff comes along, because it takes longer to grind and you don’t want to overload the mill. Then when the fine muck shows up, increase the feed and also run the classifier density higher. That will throw the high grade over into the flotation cells where it belongs.

You see, the high-grade mineral is heavier than the low grade, and it takes a little higher density in the classifier to lift it out. If you carry a low density, too much lead and zinc mineral keeps going back into the ball mill, and eventually may be ground into slime and lost altogether. Doing extra little things like this is what marks a really good operator, and you can learn these things only by study and asking questions.

Looking for Trouble

“Keep ahead of trouble” is good advice for flotation operators, and it is just as good for bail-mill men. A good operator can take care of even big changes in muck so smoothly and easily that if you were watching him, you’d never know anything was running differently.

On the other hand, consider Joe Blow, the Wonder Boy. That’s him down there sitting on the rail near the ball mill, swinging his heels and probably wondering whatever happened to that little blonde hasher over at the Greasy Spoon. Suddenly Joe looks up. He has heard a splashing sound that doesn’t belong there. The ball mill is strangely quiet. Joe looks at the feed box, and finds pulp pouring out on the floor.

Joe can tell right away what has happened. The mill has been overloaded and the grate has plugged. Quick as a flash, Joe races around and shuts the feed off, then whips open the valve pouring water into the mill. He’s fast; he wants action.

He gets it. The mill comes unstuck with a vengeance and belches sand into the classifier like a tidal wave. Joe, the dope, flushes water into the classifier, too, and you can almost hear it groan as the rakes get buried. The flotation man down below is tearing his hair and spinning valves. What he says about Joe blisters the paint on the concentrate launders, but Joe can’t hear him. Joe is up under the mill shovelling cleaned up before the shifter comes.

Table II 

Handling the Grinding Circuit



Ball-Mill Feed

Finer Mill will grind faster, classifier overflow will thicken and get coarser. Increase classifier water to raise circulating load; perhaps raise mill feed.
Coarser Mill will grind slower, classifier overflow will thin out, get finer. If fed too fast, a grate mill may plug. Cut down classifier water, and may need to cut mill feed and water.

Ball-Mill Discharge

Thinner Feed too slow or too coarse; too much water in feed. Mill will be underloaded and classifier too dilute. Increase feed if too slow; cut feed if too coarse Cut feed water. Check density in classifier and adjust with water.
Thicker Feed water too low; feed too fast or too fine. Mill may be overloaded, classifier overflow may get too coarse. Increase feed water carefully. Check on feed and other water conditions, and adjust where necessary.
Stops or slackens Grate plugged or trunnion blocked. Pulp backs up, spills from feed box. Shut off feed and classifier, leave head water on. Muck will start running in a minute. How did you get into this mess?


Sound of Mill

Loud, metallic rattle Mill is nearly empty of muck; balls banging each other and wearing out liners. Raise feed, cut head water if necessary. Increase classifier water to build load.
Thick mushy sound. Quiet Mill choked with muck, not grinding. Cut down feed, increase head water carefully. Cut classifier water a little.


Ball Mill Drive Amps

Power dropping Trouble coming. Mill either overloaded or underloaded. Can tell by sound, feed, or appearance of discharge. Checking ammeter especially important on grate mills. Check all other indicators and make required adjustments. Try to head off whatever is going wrong.
Power rising All OK, if no mechanical trouble. Looks like you’re doing all right. But don’t let power go above normal without your finding out why.


Classifier Overflow

Density rising Muck getting finer; perhaps too little water to feed; classifier water too low. More water to classifier (but slowly) and to mill feed if necessary.
Density falling Muck coarser or coming too fast; too much water to classifier; too much water in feed. Cut down classifier water and mill feed water. Reduce feed if necessary.


Classifier Overflow

Finer Feed to mill is getting coarser. See “Ball- Mill Feed.” Cut down classifier water. Check feed and feed water.
Coarser Feed to mill is getting finer. See “Ball-Mill Feed.” Add more water to classifier. Check feed and feed water.


Circulating Load

Rising If circulating load too high it means classifier pool is too thin. Mill feed may be coarser, or coming too fast Remember, though, that circulating load should be kept up good and high. Don’t do anything unless indications are that serious overload of classifier is on way. Then, cut down classifier water, reduce feed rate, or reduce head water. If classifier gets overloaded, don’t raise rakes. Things would only get worse. Keep rakes digging and adjust circuit as indicated.
Falling Classifier pool too thick, may throw too much sand into overflow. Muck is coming finer or too slowly. Do something right away, because it is a good idea to keep rakes loaded up unless mill is choked. Add more classifier water, increase feed rate and head water.

Watching the mill discharge (2) will tell you what goes on inside the mill. Some operators note how high on the side of the discharge flange the wave of pulp is carried when the mill is running right. Then if the wave runs higher or lower than that, they know something is wrong.

If the mill is low on muck, (3) it rattles and bangs like a boiler factory, and a lot of good steel goes to waste. But if the mill is too full of muck, you can hardly hear it. Keep your ear peeled for the sound of the mill that you know is right.

Many operators feel the classifier overflow (6) by nibbling their fingers together with their hands in the stream, and with a little experience, you can tell pretty accurately whether or not the overflow is fine enough.

The amp-meter (4) is really as good a guide to the condition of the mill as the sound or the discharge. It tells you how much power the mill drive motor is drawing, but remember that if you overload the mill, or if you underload it, it draws less power.

You check on the circulating load (7) by watching the height of the sand on the rakes or spiral flights as they push it back to the mill feed launder. The shift boss will tell you about how high the sand ought to come.

The funny part of it is that if Joe thinks about this at all, he probably says to himself, “It’s a lucky thing for the company it was me that was on shift when that happened.”

What was wrong? He shouldn’t have let the mill plug in the first place. But suppose it plugged anyway, he should have cut off the feed all right, but he should also have shut down the classifier, and increased the head water only a little. Then he should have cut down on the classifier water and then increased it, little by little, when the mill opened up. He should have done his best to keep things balanced instead of slamming everything out of adjustment at once. Well, he’ll learn. He will, or the boss will murder him some dark night.

Using the Table

Now, just because all these things to look for and to do have been put down in a table, don’t think you ought to walk your shift carrying this operating manual in one hand and a density sample can in the other. It is no use trying to run a mill out of a cookbook. But what we did want to do was to set these things down here so you could think about them, and keep thinking about them, while you are working.

Just go at the problem the way things are arranged in the table. When something in the circuit begins to change, make sure you know exactly what is happening; then ask yourself what is causing it. Then, when you have answered that question, decide what to do about it. Think out each thing you do, and don’t do things in a rush or without knowing why you are doing them. Don’t be a Joe Blow, in other words.

One thing more, and a very important thing: When you do make a change, allow a little time, say 15 or 20 minutes, for the effect to show up before you make another change. Don’t over-control. For instance, if the density in the classifier is up a little and you add more water, don’t expect the density to change right away, and don’t go back and open the valve even wider just because nothing seems to have happened. It will; just wait a while. A super¬operator who can’t let well enough alone gets on everybody’s nerves.

Starting and Stopping

In starting the grinding circuit after anything but a very short shut spitars enough to clear the same packed on the tank bottom, start the classifier overflow pump, then start the classifier, and after that, the ball mill. But don’t throw in all these switches at once. You’ll get the electricians down on you if you do. Keep the water fed to the circuit down low until the load builds up a little; then set the valve hand wheels at about the point they should be for normal operation. You can check on this setting by marking one spot on the rim of the wheel and counting turns, or by counting exposed threads on the valve stem. Don’t forget to lower the classifier rakes again as the load builds up.

In shutting down the mill, cut off the feed a few minutes before the shutdown is due. That will give time to grind out some of the circulating load and will make starting easier. Then when you are ready to stop, shut down the mill, then the classifier (raise the rakes), then the pump for the classifier overflow, if there is one.

If the power fails suddenly, shut off the water valves and raise the classifier rakes. And for goodness sake, don’t forget to shut off any drip cans or siphon feeders of pine oil or other reagent you may have running somewhere in the circuit.

So far as mechanical trouble goes, there will probably be little of that if the equipment is reasonably good. Ball mills spring leaks from time to time because the bolts holding in the liner plates work loose. If a leak develops near the discharge end of the mill, shut down right away and fix it. This is especially true of an open-end mill. The point is that you don’t want sand getting into the out in short order.



Now a word about safety, a subject that I am putting last because I want to leave it first in your minds. Whatever else you do, don’t go poking aimlessly around the mill or the classifier, sticking your nose or your fingers in here and there to see how the machinery works. I wouldn’t make that statement if I hadn’t seen a man or two doing just that. Nor have I forgotten the time I was routed out of bed at two a.m. to help bandage a man whose right-hand fingers had just been taken off by the ball-mill scoop as effectively, though not as neatly, as a surgeon could have done it. He had been “just poking around,” too. Remember your company, and your country, need you on that ball-mill floor, and you wouldn’t be happy holding down a hospital bed these days. So just be careful.

How to Operate a Flotation Circuit

How to Operate a Thickener

This Public Domain Robert Ramsey article is based in large part on experiences and opinions generously supplied by the following mill men: Clyde Simpson, Bagdad Copper Co., Hillside, Ariz.; E. J. Duggan and M. E. Kennedy, Climax Molybdenum Co., Climax, Colo.; John Palecek, Keystone Copper Corp., Copperopolis, Calif.; Frank M. McKinley, Bunker Hill & Sullivan M. & C. Co., Kellogg, Idaho; Malcolm Black, Wright-Hargreaves Mines, Ltd., Kirkland Lake, Ont.; and the concentrator staffs at Hudson Bay Mining & Smelting Co., Flin Flon, Manitoba, and Sherritt Gordon Mines, Ltd., Sherridon, Manitoba.