Alumina scale formation

Hi folks, I came across an interesting problem recently and am hoping one of you may be able to shed some light on the observed phenomenon. One of clients, uses
pneumatic conveying system to transfer alumina (<150 micron). Primary alumina is transported using lean phase conveying system (operating pressure is ~ 80 kPa) while secondary alumina is transported using dense phase conveying system (operating pressure is ~ 200 kPa).
Secondary alumina system experiences scaling and blocked pipe problems (regular occurrence), whilst primary alumina does not experience similar problems. Initially I thought that scaling problem could be attributed to moisture in the air (alumina is very hygroscopic). But the plant maintenance technician has shot down my theory by asking "Why is that Primary alumina system is not affected by moisture?"
My second guess is that the operating pressure might aid scale formation. I also gathered another piece of information i.e., Secondary alumina comes from the gas treatment centre (alumina used in scrubbing off gases and therefore contains hydrogen fluoride [HF] / traces of silica). Since I do not have a chemical engineering background, I am hoping that some of you may be able to point me in the right direction to understand 'How and why scale forms in secondary alumina conveying system and what measures can be implemented to prevent scale formation'.
Thanks in advance for any pointers / leads. Regards, Raj Sreenevasan
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On Tue, 19 Aug 2003 01:19:11 GMT, "Walter Driedger"

There won't be any free HF around in the presence of an excess of activated alumina. It will form aluminum fluoride.
Thiophilus
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The first question is: Did this ever work without blocking? If so, then this is an environment/maint. issue. If not, then its an engineering issue.
... I did an alumina transport system, where the material transporting pipes were lined with ceramic material.
Also, it was a dense phase (pressure pot) system, not a dilute phase system. Perhaps the velocity of the material is what is complicating things.
And, ditto to previous comments regarding air moisture.
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