Scaling at the feed port of the roasting furnace and its treatment
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Since engaging in the production of sulfuric acid from pyrite, what I’ve been most concerned about is the roaster furnace, particularly issues related to scaling at the discharge port and the measures to address them. Scarring has occurred on multiple occasions, and dealing with it is also very difficult. In a plant’s 300,000-ton pyrite-based acid production system, since its commissioning, accumulation has occurred at the feeding ports of Furnace 2# and 3#, forcing shutdowns for maintenance. Accumulation also occurred at port 2# during the night shift. Through proactive measures, the accumulation did not spread; it was gradually eliminated after the feed material washed through, and the temperature returned to normal. After discussing and analyzing the scorching issues at these two feed port temperature points, the following operational recommendations are proposed: First, in the case of scorching due to high temperatures, take the following actions: 1. Stop feeding material and monitor the rise in temperature; 2. Use a stick to inspect the condition of the area around the feed port; 3. Prepare water pipes to deal with potential high temperatures; 4. If there are minor scorch marks, use a stick to clear them away and push the mineral powder accumulated around the thermocouple to make it loose. 5. Reduce the amount of ore fed in, and use water spraying to control the temperature at the discharge port. 6. Observe the mineral feeding situation at this discharge port, and use a stick to inspect the ore inside the furnace. 7. Once the temperature at this discharge port returns to normal, the ore feeding rate is gradually adjusted to normal levels. 8. If the scab is relatively large and difficult to pierce, the amount of ore fed should be reduced, and the machine should be stopped for treatment to prevent further deterioration of the situation. Once cleaning is complete, resume normal driving. II. In case of low-temperature scabbing, the following measures should be taken: 1. Halt feeding; use a rod to inspect the condition of the furnace in the charging area. 2. If there are minor scabs, use the rod to clear them, and loosen the accumulated ore powder around the thermocouple. Observe the temperature changes. 3. Observe the temperature at this discharge port, and use a stick to inspect the ore inside the furnace. 4. After the temperature at this discharge port returns to normal, resume ore feeding and gradually adjust the feeding rate to normal levels. 5. Prepare water pipes to deal with possible high temperatures. 6. If the scar masses are large and puncturing or throwing methods are not effective, stop the machine for treatment to prevent the situation from worsening. Once cleaning is complete, resume normal driving. By analyzing the scarring situations in both instances and drawing lessons from them, efforts are made to prevent similar problems from occurring. All team operators are required to abide by labor discipline during working hours, pay close attention to the temperatures at the three feeding points, ensure that the amount of ore fed and the ratio of air to ore are appropriate, regularly monitor the feeding process, and frequently check the condition of the furnace. In case of any issues, prompt and decisive judgments must be made to address them. The 300,000-ton roasting system consists of three pre-chambers and a furnace chamber. The pre-chambers serve as the areas for feeding the ore; if scale accumulation occurs and is not dealt with in a timely manner, it will prevent the ore from being fed, which will severely affect the proper operation of the system. In addition, it is necessary to regularly monitor the slag discharge situation and the particle size of the slag. If necessary, perform particle size analysis on the slag, and combine this with the roasting temperature to analyze and predict the conditions inside the roasting furnace.
If the boiling is proper, scarring actually does not occur easily. Compared to temperature, I personally think good boiling is more important.
This is closely related to the structure of the furnace body and the fixed layer during baking. When there are fluctuations in the moisture content of the raw materials and the fixed layer is thin, problems are likely to occur. In short, everything from the raw materials to the structure of the furnace is related to the operator’s habits
I haven’t worked with boiling furnaces for many years, so I feel a bit out of practice; still, thanks for sharing.
It is related to operational habits. Especially when the moisture content is high, it can easily lead to accumulation at the discharge port. If this accumulation is not cleared in time, attempting to clean it once a large amount has accumulated may result in poor boiling conditions at the discharge port due to an excessive amount of material being fed into it momentarily, which ultimately leads to scarring inside the furnace. Scarring inside the furnace is influenced by various factors; as you mentioned, raw materials and the structure of the furnace are among those factors. From my practical experience, the air velocity at the small holes in the wind cap is a crucial factor. If the gas velocity is too low, once the furnace collapses due to slag accumulation on its walls, it may be impossible to aerate the fixed layer, leading to scorching. There is a plant with a capacity of 200,000 tons per year. Previously, the gas velocity through the small holes was less than 38 meters per second, and on average, the furnace would stop operating due to collapse at least twice a year. At the end of 2014, I replaced all the air nozzles; as a result, the gas velocity through those holes increased to around 55 meters per second. Since then, there have been no more instances of furnace failure caused by collapse. In the event of a collapse, add some wind to maintain a boiling condition, and the bottom pressure will be adjusted back to normal quite quickly. Even during a planned shutdown for major maintenance after 14 months of operation, it was found upon normal shutdown and cooling of the boiling furnace that there was a scar-like mass with a diameter of about 1.5 meters; this mass was not attached to any air cap and was located in the upper middle part of the fixed layer. The surface of this mass had become relatively smooth due to being exposed to airflow, and it did not affect any operational parameters before the shutdown. Our analysis suggests that scarring occurred before parking (the exact time cannot be determined), but due to the relatively high gas velocity, it remained floating in the upper part of the boiling layer or on its surface, which prevented it from sticking to the wind cap and thus avoided fluctuations in performance or shutdowns caused by scarring. As for the thickness of the fixed layer, I believe that once a relatively stable bottom pressure value is established after the new furnace starts operating, as long as the bottom pressure remains within the specified range during normal production, the thickness of the fixed layer will remain fairly constant.