Chemistry: physical processes – Physical processes – Agglomerating
Patent
1988-12-15
1991-07-16
Russel, Jeffrey E.
Chemistry: physical processes
Physical processes
Agglomerating
55 69, 75444, 422145, 423210, 4232155, 423DIG16, B01D 500, B01D 700, C22B 514, F27B 1510
Patent
active
050321439
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for treating process gases by cooling the process gas in the presence of solid particles.
Different high-temperature processes such as melting of ores or metal concentrates and processes with melting, reduction and fuming of metallurgical slags, calcination of cements, high-temperature processes in the chemical industry, etc. generate high-temperature gases containing components that tend to stick to the heat transfer surfaces, thus making the heat recovery from said gases as well as cooling of them difficult. Sticky compounds may also be generated in ordinary gasifying processes. Such compounds that foul heat transfer surfaces are, for example cooling, than 1 micron, and by a tendency to stick to other fume particles and surfaces met with,
Depending on the case, a process gas may contain one or more of the components mentioned above. Their common feature is a tendency to stick to the heat transfer surfaces of the heat exchanger or the boiler when the gas flows through them.
As a result of this, the heat exchanger gradually becomes clogged thus losing its effect, which usually results in running down the process.
The harmful effects of fouling can, in many cases, be reduced by different kinds of blow sweepers or mechanical sweepers such as shakers or blow hammers. The blow sweepers have a disadvantage of consuming high-pressure steam and their sweep gas affecting the composition of the gas to be treated. For reducing gases, for example, air cannot normally be used.
Shakers and blow hammers have proved to be an effective sweeping method under various conditions. Their disadvantage is the restrictions set by them on the boiler structure. Furthermore, shakers are ineffective on superheaters in operation.
Experience has shown that, usually the fouling problem is greatest at a certain temperature range typical to the process where the sintering tendency of "dust" is highest. The reasons affecting such temperature range are explained more in detail in the following.
The following factors, among other things, affecting sintering are well known in the field of powder metallurgy and ceramics combustion technology: in the beginning of sintering, multi-component system, there will be melt formed in the system, such melt filling the pores between the particles, thereby causing highly effective sintering at a temperature range that can be very narrow indeed.
The components evaporated in the process, such as heavy metals and alkalis, tend to condense or sublimate at a certain temperature characteristic of them. In connection with cooling, there is formed either melt which condenses on the heat transfer surfaces or on the surfaces of dust particles, thereby making them more sticky, or the evaporated components sublimate direct on the heat transfer surfaces. Phenomena of this kind occur, for example, in the alkali by-pass system in the cement kiln, which is why heat recovery is not usually successful in this connection. Corresponding phenomena appear in gasifying processes if the product gas contains alkalis and/or residual tar.
When cooling down close to the solidification temperature, the melt drops in the process gas either easily stick to the existing process particles, thereby contributing to the sticking of dust to the heat transfer surfaces, or solidify direct on the heat transfer surfaces and sinter to them.
In fuming operations, metals are intentionally evaporated from molten slag for recovery. For example, Zn, Pb and Sn are separated from the gas phase after evaporation by changing the oxygen potential i.e. by reburning. Especially fine particles or drops are thus formed in the flue gas. The size of particles in this kind of fume is typically at the range of 0.1 to 1 micron or even smaller. The fume is characterized by an especially large surface activity and tendency to stick to the heat transfer surfaces, thereby impeding the boiler operation. Therefore, a great deal of fuming operations are still carried out without heat recovery.
A phenomenon, much like
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A. Ahlstrom Corporation
Russel Jeffrey E.
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