Example Milling Problem: Low Mesh/High Blaine Cement
At Plant A, the finish mill has been operating satisfactorily up until last weekend. During the weekend, mill started backspilling forcing the shift crews to reduce feed rates by as much as 15%. Curiously it was also reported that blaine rose from 370 to 410 (m2/kg) but the % passing 325 mesh fell from 90% to 86%. This alarmed the lab since it will have an effect on concrete performance. When approached by the Q.C. Manager for a solution, the shift supervisors were perplexed since they can understand that lowering the feed rate will increase blaine but they cannot explain why the 325 mesh dropped at the same time. All they could suggest was to shut it down or wait to see if the problem would go away.
The problem persisted to Thursday when the plant, frustrated, shut the mill down to inspect it inside. After the inspection, people were even more bewildered because they couldn’t find “anything unusual” in the mill and the separator was in good shape.
The plant manager approaches you and says, “….First the new coal stockpile screw-up! Then the kilns won’t burn worth a damn! Now the mill mysteriously won’t grind! Can you put off to one side whatever you’re working on and have a look at this finish mill? We need to find out what’s going on quickly and figure out what action we’re goin’ to take.”
Promptly, you drop the work you’ve just started on the kiln hard burning problem and run off to the mill for a quick inspection. Afterwards you do a little digging into old reports on this mill. Here are your findings:
- a) Inside the mill there isn’t anything remarkably unusual except the following which differed from the inspection performed a month ago:
- i) There were some clinker chips or spitzers in the second compartment charge, but not enough to cause any swelling.
- ii) There were some clinker spitzers partially plugging both the partition grate slots and the discharge grate slots, but not enough to impede flow (5 to 10%).
iii) There was a little bit of ball and liner coating in the second compartment. Normally the amount isn’t really significant except it wasn’t there a month ago.
- b) Interviewing the supervisors, you discover that everything else was operating normally on the circuit. Yesterday somebody did verify that grinding aid addition was correct.
- c) According to an mill audit report done 20 months ago the steel to clinker ratio then was about 12.5. Moreover operationally it appears that this mill has performed more or less the same way, up until last week.
By the way, on your desk is the last clinker analysis which shows free lime at 0.46% and literweight is at 1482g. “Darn”, you say to yourself, “the kiln’s still burning hard”.
What conclusions can you make? What will you report to the Plant Manager?
Here are some conclusions one can draw from this example:
- a) A steel to clinker ratio of 12.5 suggests that the mill is slightly underfed. This will make the mill more sensitive to feed material characteristic changes.
- b) Obviously, there has been a kiln fuel change which resulted in a burnability change. In this example, the clinker has become harder burning which tend to make clinker harder grinding as well.
- c) The situation can be reconstructed as follows:
- i) The harder grinding clinker will stay in the first compartment longer thus allowing it to fill quicker and backspill sooner. At the same time it will starve the second compartment (which was already slightly underfed).
- ii) The first compartment grinding efficiency is reduced. That fact that spitzers found in the second compartment is evidence since it supports the notion that the first compartment must be over fed (for this clinker).
iii) The fact that some ball and liner coating was detected, despite the correct amount of grinding aid also supports the notion that the second compartment is underfed. Recognize that the lack of material flow in the mill sharply reduces its ability to shed heat. Some overheating did occur resulting is ball and liner coating.
- iv) Most people agree that spitzers rubbing will generate a lot of superfines (this is the only mechanism by which spitzers will grind down). Yet at the same time the ball voids are largely empty which will permit coarse particles to pass through unground. This condition generally leads to the particle size distribution to migrate to the extremes. Hence the apparent contradiction of high blaine and low mesh can result.
- v) The presence of more coarse in the separator feed will cause a higher reject rate thus a higher circulating load. This will only compound our over filling problem in the first compartment. In addition the 325 mesh in the finish product can also be expected to drop.
Actions to be taken:
That afternoon you corner the Plant Manager and the Q.C. Manager long enough to explain your conclusions and make recommendations. In short this is what you recommend:
- a) The root problem is that the kiln is over burning the clinker. Therefore focus should be on curing this problem and the grinding difficulties ought to go away. There is unfortunately very little one can do about the clinker already made except perhaps pre-blend with good clinker, (depending on the plant this isn’t always practical).
- b) If for whatever reason the grindability hardness refuses to change then the only recourse is to adjust the ball charge coarser, especially in the feed end, to cope with harder grinding clinker.
- c) In general some study could be done such that the material filling ratio is closer to optimum (ie. steel to clinker ratio of say 10). Moving the mill from a slightly underfed condition could help you de-sensitize the mill to future occurences of this problem.