[stewie griffin]

Has anyone had experience with calcining material in a rotary tube kiln/furnace? A process of mine went swimmingly in the experimental runs on production scale. We began our first full scale run this week, and, as expected, the unexpected has happened 
The process ran for about 36hrs without problem and then we noted the material began to build up a coating around the inside of the tube (it's a quartz tube). The coating is inhibiting heat transfer. After stopping/cleaning/restarting we had observed the coating problem in about 6 hrs. We did not observe this coating during the experimental runs, though to be fair the experiments never went this long.
I should note that the the kiln is not one of the giant ones like you'd see for portland cement but rather an 8-10 foot quartz tube that passes through a hot zone and then into a collection container. Moisture does come off of the product during calcining, and  in fact I want that to happen. Actually, it's the combo of the high heat and the water vapor that gives the desired characteristics of the final product.
My question is, what could be causing the coating to build up? Has anyone observed coating formation in a calcining process? Any ideas on how to prevent it? I unfortunately cannot reveal specifics other than the material being calcined is an inorganic salt.

[curiouscat]

Have you tried scraping and analyzing a bit of the coating? Is it the salt itself or some side product?

How thick is the coating? How hard / adherent is it? Could you provide a stationary scraper near the wall?
 
Does it build all through the length or only in the hot zone?

[stewie griffin]

My next step was to get an analysis of the coating to see if I can learn anything from that.
The coating is about a half inch thick. It's soft, just as soft as the starting solid going in, and is very easy to push off of the tube. It builds up on the entire tube, which is why I don't think we could install of scrapping knife.
We'll see what Monday's analysis brings...

[curiouscat]

My next step was to get an analysis of the coating to see if I can learn anything from that.
The coating is about a half inch thick. It's soft, just as soft as the starting solid going in, and is very easy to push off of the tube. It builds up on the entire tube, which is why I don't think we could install of scrapping knife.
We'll see what Monday's analysis brings...

If it is that soft, could you interrupt the process for say a few minutes every so many hours and add a blast of air to drive it off?

Alternatively, always have a tiny air blast into the interior to prevent sticking? Or a few loose  ceramic balls or rods to keep tumbling in the tube?

[stewie griffin]

I like the idea of an occasional short air blast. I'll recommend that to my engineer teammate.

[curiouscat]

I like the idea of an occasional short air blast. I'll recommend that to my engineer teammate.

Would be interesting to know if it works. If not, how you eventually solve the problem.

[stewie griffin]

Well the tube broke. Not my fault though 
I'll try to post the solution when we figure it out. In the meantime we are looking at different tube materials simply b/c we can't be breaking tubes.

[curiouscat]

Well the tube broke. Not my fault though 
I'll try to post the solution when we figure it out. In the meantime we are looking at different tube materials simply b/c we can't be breaking tubes.

Stainless Steel  won't work?Temp.'s too high? The austentic / martensitic grades perhaps?

[stewie griffin]

We tried alloy 800 and it worked very well today. That material is made for operation at temps above 600C. Plus all data so far shows the metal is not leaching into the product. Coating is not nearly as bad. Plus the small coating that does develop can be readily removed with puffs of air. Looks like that will most likely be our solution 

[curiouscat]

We tried alloy 800 and it worked very well today. That material is made for operation at temps above 600C. Plus all data so far shows the metal is not leaching into the product. Coating is not nearly as bad. Plus the small coating that does develop can be readily removed with puffs of air. Looks like that will most likely be our solution 

Interesting that it coats quartz but not metal. I wouldn't have expected that. Glad the air puffs work.

As an aside, how long of a residence time is provided?

[stewie griffin]

Residence time is only minutes.