What’s wrong with the sulfuric acid boiling furnace?

What raw materials are used? Is cold sand caused by reasons such as high moisture content in the raw materials? Or are they lumps that have fallen from the furnace?

Using sulfur concentrate with a moisture content of around 7%, this issue occurs whenever the machine is restarted after a power outage. I believe it’s caused by debris falling from the upper wall of the furnace, but I’m not sure. What measures should be taken?

How long was it stopped? What is the temperature difference before and after driving?

It was shut down for 2 hours; there was little difference in temperature before and after driving the machine, and the pressure remained constant. A few days later, accumulation occurred at the feed inlet of the furnace, causing the pressure to drop slightly. When the furnace was stopped and inspected, many large particles were found in the lower part of the material layer. What caused this? Was it due to insufficient air flow, overly wet raw materials, or damage to the furnace walls?

It is recommended to check whether they are coarse particles or sintered particles.

The moisture content is around 7% – is it too high?

What type of furnace is used in your plant’s fluidized bed furnace? Is it a sand furnace for tailings or a furnace for lump ore? What is the designed roasting intensity? What is the actual intensity in use currently? What is the average particle size of the ore fed into the furnace? What are the temperatures of the fluidized layer, the middle part, and the top of the furnace? Are the resulting lumps granular in shape or large in size? What is their color? A moisture content of 7–8% is considered normal.

The furnace in our plant is a straight-tube furnace; it was previously used for burning zinc concentrate, and now it is used for burning sulfur concentrate. The original design parameters regarding the roasting intensity are not clear, but currently the value is 9 T (of standard ore) per M2·day. The average particle size of the ore fed into the furnace is around 3 mm. The temperature at the bottom of the furnace is 865°C, while at the top it is 1020°C (there is no middle section). The output consists of both granular and large lump-shaped particles.

Personally, I think the moisture content is not high; sometimes in our factory it can reach 11%. I wonder if it’s because the storage location for the raw materials is at a mine dam with rocks, and during the process of loading the materials, some of those rocks are also transported to the factory. Finally, the crushed raw material enters the furnace, where it transforms into cold sand over time. If this is the reason, considering changing the sieve mesh pores might be an option. If it’s not for this reason, I think the most likely explanation is lumps falling inside the furnace.

The particle size should be 3 mm, and the baking strength must be no less than 15 t/m2·d; in some cases it can exceed 20 t/m2·d. First, carry out cold boiling to observe how the furnace behaves during this process. The baking strength of zinc furnaces is generally around 6. The specific gravity of the ore particles varies; among domestic sulfur concentrates, those from the Yongping mine, which have the finest particles, have a baking strength of 12–15 t/m2·d. Try cold boiling first; if everything works properly, note down the air volume supplied by the fan. In batch ore processing furnaces, the temperature at the bottom of the furnace is usually higher than the temperature at the furnace outlet. Feel free to contact me if you encounter any problems.

Our company currently uses a method for roasting zinc concentrate, with air nozzles of diameter ￠6.5–4 being employed. Due to the lack of flow meters, the data regarding the pore velocity is incomplete; it seems that the pore velocity is too low, which results in poor boiling. To increase the pore velocity, it is necessary to increase the air volume, but over time this can lead to the formation of cold sand and a drop in the temperature at the bottom of the furnace, ultimately causing the furnace to stop working. We have doubts regarding the actual procedures for producing sulfuric acid, and we are also concerned about making mistakes in our judgments. In order to improve our skills, I have a few questions for qingzz1965: 1. Does cold boiling refer to starting the fan before lighting the furnace? Can it be done during normal production? (Our company has never done this before.) 2. What is the typical temperature at the top of the furnace when roasting sulfur concentrate? I think the temperature of over 1,000 degrees in our company is too high; however, boiling with a lower air volume also doesn’t work well. Is this caused by the air nozzles? 3. Can the furnace be stopped immediately when there are problems with the system? Our company has encountered difficulties when restarting the furnace after stopping it too quickly, and there have also been instances where the furnace stopped working entirely as a result of such stops.

The reason may be the high moisture content of the feed material; there are large lumps in the slag due to this high moisture content, as well as low pressure at the bottom of the furnace, which prevents the feed material from reaching a fluidized state in the furnace bed. If the pressure is normal, large lumps will not form immediately; however, due to excessively high moisture content in the feed material over time, some mineral dust rises to the upper part of the furnace where there is a lot of water vapor. There, it forms lumps on the furnace top or walls. Over time, more and more such lumps accumulate, and when they reach a certain size, they fall down and are discharged along with the slag.

1. Generally, a micro-boiling and cold-boiling test is conducted before starting the furnace. This is done to check whether the air flow distribution across the air caps is even, as well as to determine the amount of air required for normal operation (or the opening degree of the fan valves). During normal production, it is only possible to observe the boiling condition through observation holes or the feed inlet area. 2. Under normal circumstances, the temperature at the top of the furnace is lower than that of the boiling layer when roasting lump ore, whereas it must be higher than the boiling layer temperature when roasting sulfur concentrate. It’s around 50 to 100 degrees, so tailings furnaces generally require secondary air ducts. Some even have three sets of air ducts ； However, the operating temperature at the furnace top generally should not exceed 1000 degrees ； Block ore furnaces generally do not have secondary air ducts ； 3. The gas velocity at the small holes of a typical zinc furnace’s air cap is 25–40; there are 4 such holes. In the case of pyrite furnaces, this value is generally 50–70, with 6 holes designed for that purpose ； This may be related to the specific gravity of the mineral particles. Therefore, your company’s fluidized bed furnace does not function well when operated at a low air flow rate, while at a high air flow rate the temperature on the furnace top becomes extremely high, leading to scorching. 4. Care must be taken when shutting down zinc concentrate furnaces; otherwise, it is very easy for the furnace to become damaged. Therefore, a comprehensive renovation is required when switching a zinc furnace to use pyrite as fuel. Regarding the matter of water vapor mentioned on floor 13, the mine contains 7% moisture, so it shouldn’t be that severe; the normal moisture content in zinc-based acid production is 8%. The key is still the grain size and specific gravity. This post was last edited by qingzz1965 on 2008-1-5 04:37.]

Thank you! ----QINGZZ1965, your guidance has helped me understand a lot. It also proves that my judgment was correct; I will turn to you for advice if I have any more questions in the future. Thank you once again!

Everyone should raise more questions related to actual production practices and discuss them like in this post, so that newcomers like me can learn a lot: lol

I just saw it and would like to say a few words as well. 1. Cold boiling is a very important procedure after building a new system or carrying out maintenance; it allows for the determination of various basic parameters of the furnace, providing a basis for future operations. It cannot be carried out during normal operation. 2. Regarding the pore velocity for baking sulfur concentrate using air nozzles, based on my many years of experience, a value of around 50 to 60 meters per second is appropriate. If this value is too low, poor boiling may occur or cold ash might form; if it’s too high, dust will tend to accumulate more, which is detrimental to subsequent processes. 3. When there are problems with the system, it’s not advisable to stop the furnace immediately! For example, at high temperatures—over 1000 degrees—the temperature is close to the melting point, and stopping the furnace at such times can easily cause scarring of the slag inside the furnace. This is especially true when the furnace temperature is still rising; it’s better to first bring the furnace temperature under control before stopping it. 4. A raw material moisture content of 7% is definitely fine. Regarding the issue you mentioned initially, I suggest checking the following aspects: 1. The uniformity of the raw material particles – usually, there’s a problem with the screening system, allowing coarse particles to enter the furnace. If these particles aren’t removed from the furnace in time, they can lead to the formation of cold ash. 2. Whether the raw material contains high levels of lead; raw materials with high lead content tend to cause deposits on the walls of the furnace during baking. Once enough deposits accumulate, the ash will fall off, and in severe cases, it can even cause the furnace to stop functioning. Of course, deposits can sometimes also be related to the design of the furnace itself, such as whether the angle of expansion is appropriate

I’m still not quite clear about this. Under normal conditions, the layer of material is about 800 MM thick, but the slag laid before ignition is only around 500 MM thick. How can one determine the appropriate air volume under normal conditions in such a situation? Also, when the fan is turned on, there is a lot of dust inside the furnace, making it difficult to observe the boiling process there. Could you provide more specific instructions on how to proceed? I don’t have any experience in this area. Thank you!

All boiling furnaces are equipped with manholes. When operating in cold boiling mode, these manholes should not be closed. If their location is too low, build about half a layer of bricks below them, with the height determined by the position of the discharge outlet. Start with mild boiling until full bubbling occurs, then stop the airflow and check whether the material layer is even. Repeat the mild boiling process, noting down the opening degree of the air valve for use when starting the furnace. Gradually increase the air flow; the level of bubbling should be 200 millimeters above the material layer. Remember this as the minimum air flow required to keep the boiling furnace operating properly. During cold boiling mode, a large amount of ash is produced, so install lights with higher brightness inside the furnace. Also, wear a mask and glasses – this should prevent any problems. Of course, the vent on the top of the furnace must be kept open.

All of this is understandable, but when slag is applied, its height is different from the normal level, so the amount of air required should also be different. Is it necessary to apply the slag to the normal layer height?