Fires - Smelting Process

First we must be careful to find out if the fires are a result of ignitions by a spark. All these acids mixed with carbon can spontaneously ignite, especially if the precious metals include catalysts, such as vanadium, like they use in catalytic converters. Or, they get fires when they are in the ten hour mode? My calculations indicate that during the ten hour precious metal mode they are basically diluting the exhaust to meet discharge requirements into the atmosphere.  To discharge from the hood at 125 degrees they either need a lot of dilution air or water vapor. For 30 hours a week, they use it as a trash burner. Here again they seem to be trying to hide the true nature of the operation. In the trash burning 30 hours they probably have wild swings in emissions which might ignite poorly combusted components and would be ignited by sparks in the baghouse. I would hate to live downwind during either operation. You need to find out the following.

1.  Do the fires occur while operating?
2. Do fires occur when they are shut down?
3. What are the temperatures in the ducts over a 60 hour period?
4. If they run 8 hours a day do they run 2 hours on smelting mode and six hours trash mode?
5. Do they insulate the duct?
6. What is the charge to the smelter? Coins? scrap gold, scrap silver? perhaps catalytic converters?.
7. It seems like they would have a severe corrosion problem and preheating of ducts should be considered.

We can put out sparks with Quencher^tm spark arrester but we cannot remedy poor control.

The ten hour mode is the ten hours per week when they are smelting. The 30 hour mode is when they are trash burning. Get a piece of bag to examine. It will be obvious if it has been chemically attacked. You should be able to tear or burst a piece with pliers and a vise. 

For more on spark arresters, see  QUENCHER spark arrestor

Wood Fired Boilers

There are many conditions that lead to fires in wood fired boilers. The engineering solution requires recognizing the factors that contribute or cause these problems. Most are related to the combustion process and that the wood composition varies widely:

1. The emissions from the boiler consist of dust and gases that are not completely burned. These can be ignited by sparks coming from the boiler. The loading of these pollutants can vary widely. Gases may continue burning with a flame, and, sparks may be in the process gas stream. 
2. If we install a spark arrestor, such as a QUENCHER, before the dust collector, it will cause the gas pollutants to burn producing heat, carbon dioxide and some small fraction of water vapor. The QUENCHER will also prevent any sparks from entering the dust collector as the hot ciders will be immediately cooled to the gas temperature, as the air goes from laminar flow to turbulent flow and back to laminar flow. We now have cooled dust and ash entering the dust collector. The pulsing action of the cleaning system will fan any red-hot cinders if they are present. For reasons beyond the scope of this report, the dust cake will be dense and subject to ignition. Fortunately, conditions must be in a narrow LEL/UEL range to start a fire. With conventional pulse cleaning systems only a fraction of the filter cake on the bags is functional and the rest of the bags are plugged. 
3. We would modify the cleaning system with modifications that would enable the full surface of the filter media to be active. This would mean that the inventory of flammable dust would be reduced by over 95% so there would be no combustible dust between cleaning pulses. Surprisingly, the cleaning frequency would also be reduced. To accomplish this, we would need information regarding the design of the existing dust collector.

When completed this would be the best design, and, barring unforeseen circumstances such as power failures at inopportune times, the system should perform flawlessly for years. The only risk is that the filter bags could be attacked chemically. 

More information on... retrofitting existing dust collectors