The problem of cooling slag in boiling furnaces
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Recently, our plant processed a batch of imported zinc concentrate with a fine particle size, and cold slag formation occurred. The temperature in the boiling layer was at the required level (900–950°C), but the temperature at the bottom of the furnace wouldn’t increase, and this issue couldn’t be resolved even by using full-air stirring. During the first turnover, the bottom temperature of the furnace rose before dropping immediately; during the second turnover, the bottom temperature remained constant, while the temperature of the boiling layer stayed stable, allowing the furnace to operate in a semi-production state. What’s going on? Could an expert give some advice?
:Dizzy: I just can’t figure out why the sand, which is about forty to fifty centimeters thick at the bottom of the furnace, cannot be turned over, blown away, or removed. For now, I have to use the boiling layer above to roast the ore, and the resulting roasted ore is still of satisfactory quality. The bottom pressure of the furnace is also stable.
It’s possible that the particle size distribution of the ore is too wide; it is recommended to conduct a check. The smaller particles of ash should be blown out directly, while the larger particles settle at the bottom and cannot be removed. Production can be maintained by using pipes to remove the ash, and it would be best to adjust the ore mixture as well.
The particle size of the ore is too fine, and the gas velocity in the air cap is high; as a result, the gas passes straight through the sand without causing any boiling.
If it is due to fine-grained air currents passing through, increasing the pressure at the bottom of the furnace can help address the issue; however, if there is boiling at the upper part and the temperature there is normal, then the temperature in the upper area certainly will not be normal if air currents are passing through the bottom.
Is this acidized roasting? The temperature control is set too high! Based on the conditions you described, it can be concluded that the boiling inside the furnace is not adequate; it is not fluidized roasting. This can be confirmed by inspecting the furnace or touching it. It can also be determined that the moisture content of the zinc concentrate you are using is not high; it must be less than 9%. Otherwise, after two cycles of reversing the furnace, the roasting furnace will not be able to operate for more than a few days, and it will no longer be in a semi-production state. It can be inferred that the silicon content in the semi-finished roasted ore will exceed the limit. The treatment method is as follows: 1. Material preparation: Use some zinc concentrate with a larger particle size, or raw materials with a low sulfur content, but the sulfur content must not be lower than 26%. 2. Air compression: Reduce the height of the boiling layer, increase the thickness of the material in the furnace bed, and raise the pressure at the furnace bottom. 3 The moisture content of the raw materials should not be too low; it is best to keep it between 7% and 9%. The moisture level can be adjusted manually during the adjustment of the furnace. 4. Increase the height of the discharge port. It’s very simple: make a “hat” to fit over the furnace bricks at the discharge opening. This current state won’t last long, and the most to be avoided are: power outages to the system and major adjustments to the air volume.
At midnight yesterday, the temperatures at points 1 and 3 in the boiling layer also dropped; an inspection of the furnace revealed that it had formed deposits, so the operation was stopped and the furnace was cleaned. This morning, when I opened the furnace door and the fuel injection ports, I observed that the boiling inside the furnace was extremely uneven, with sand piles in the shape of stalactites visible. It has been decided to remove all the wind caps and drill again, expanding the diameter to 5.3 mm (originally 4.5 mm). As mentioned above, we use sulfurization roasting; we have not tested the silicon content in the roasted ore, and we will arrange for such testing as soon as possible. For imported ore, due to the long time it takes to store it, the moisture content of the raw ore is insufficient; therefore, we use leaching treatment when preparing the ore, requiring that the moisture content of the ore fed into the furnace be above 8%. Due to the raw materials, it is difficult to make adjustments to the ore mixture, but we have always used crushed sand as part of the ore mixture. We’ll clean the furnace today, and tomorrow we’ll check and fix the air caps and smoke pipes (the furnace throat has been very clogged these past few days; it’s tough to get this furnace working properly!) ) Thank you for your advice, friends. Feel free to discuss; I’ll come back tomorrow night……
In response to 2#, at first I thought it was a problem with the particle size of the ore, but after checking it by hand, I realized it wasn’t that. Now, I think it’s a problem with the air cap.
7# Bamboograss, I’ve encountered this situation too. Tell me: what is the size of the furnace? What is the air volume? What is the wind cap density? Sulfur-containing? Cylinder head area? Vent hood opening ratio? What is the structural design of the bellows? Drive only after a proper diagnosis; don’t act recklessly, otherwise the outcome will be the same!
Changing the size of the holes in the air cap may not be effective; reducing the air velocity through those holes increases the amount of air inside the furnace, which in turn raises the linear velocity there. As a result, the furnace temperature rises as well. The amount of ash produced will increase while trying to maintain the current output level, and the flue ducts will become clogged. It is necessary to reduce the load and adjust the air supply accordingly in order to increase the thickness of the boiling layer.
There is one furnace with a volume of 24 square meters; it will be started up today after the hole has been enlarged. Under normal operating conditions, the sandpaper at the bottom of the furnace becomes boiling due to the airflow volume used for starting up, and an airflow rate of 13,000 m³/h is required in order to start it up. 1,575 air caps, with six openings each; bucket-type blowers. The area of the water jackets is unknown (4 in total). The ore contains 28% sulfur. Based on this, the wind speed at the openings of the air caps would be 4.33 m/s – isn’t that incorrect? Another 33-square-meter furnace is still being cleaned of its furnace throat; it’s almost completely blocked! Since this situation occurs immediately after both furnaces burn the same material, I think it is more likely due to operational reasons. Note: I just took over these two furnaces; I don’t have any prior experience in operating boiling furnaces, so I still need to learn a lot*.