​Smelting Process of Closed Calcium Carbide Furnace (Submerged Arc Furnace)

Smelting Process of Closed Calcium Carbide Furnace (Submerged Arc Furnace)

1. Process of Smelting with Calcium Carbide Furnace

Under the influence of arc heat and resistance heat from the carbide calcium furnace (submerged arc furnace), a high-temperature reaction molten pool system with gradually changing temperature from top to bottom is formed inside the carbide furnace. As the reaction progresses, the generated liquid carbide moves downward and accumulates in the molten layer at the bottom. In the upper region of its high-temperature radiation layer, the generated high-temperature gas continuously agitates the molten pool system and rises to disturb the movement of the upper layer materials, promoting contact between the raw materials and enabling the reaction to proceed efficiently.

(1) Ingredients

It is automatically performed during normal working hours, and manually performed on-site during abnormal working hours.  Furnace material ratio:

The ratio of carbon raw materials or quicklime before the furnace can be adjusted according to the requirements of raw material quality, product specifications, and furnace conditions, by adjusting the computer program.

(2) Loading

The furnace charge is weighed and loaded into a four-layer circular silo by transportation equipment. It is then sent to the calcium carbide furnace through the silo and the feeding tube, and continuously replenished by gravity as the furnace charge is consumed.

(3) Operation

The operation of an electric carbide furnace can be conducted based on either electrode resistance or electrode current.  Optimal operation hinges on maintaining a suitable level of heat within the furnace. The optimal heat level is determined by the operating current and voltage conditions. It is advisable to maintain the temperature of the electric furnace's melting pool at 2000-2200℃.

The reaction within the furnace primarily occurs in the molten pool surrounding the electrode tip, with the charge material situated between the molten pool wall and the furnace lining serving as the primary heat-resistant and insulating component.

Regular inspections shall be conducted on the composition, particle size, humidity, and proportioning of the raw materials fed into the furnace in accordance with relevant regulations. If necessary, the frequency of inspections should be increased to avoid fluctuations in electrode current or resistance caused by variations in furnace materials, which could affect the stable operation of the calcium carbide furnace, excessive electrode consumption, and unstable calcium carbide quality.

The expected distance from the electrode tip to the furnace bottom is 1.3 to 1.5 meters, and the optimal distance should be determined based on specific process conditions and experience.

(4) Electrode Adjustment

Electrode adjustment is achieved by moving the electrodes with a lifting cylinder.  It should be combined with the selection of voltage levels for the transformer to obtain the ideal position of the electrode tip and the load of the calcium carbide furnace.  The calcium carbide furnace uses self-baked electrodes.  The electrode column consists of an electrode shell filled with electrode paste, and the electrode tip of a newly started furnace is welded with a starter motor shell.  The electrode shell can transmit most of the electrode current.

The electrode adjustment method is as follows: Using the electrode current or operating resistance as the standard, it is manually operated by jogging the operating computer and adjusting the knob on the operation cabinet.

(5) Main Components and Properties of Furnace Gas

During the operation of a sealed calcium carbide furnace, the chemical reaction process gas generated will not leak into the atmosphere, and at the same time, the gas in the external atmosphere of the sealed calcium carbide furnace will not enter the furnace.

After undergoing chemical reactions inside a sealed calcium carbide furnace, all that is produced are harmful gases, primarily including CO, CO2, H2, CH4, etc.

If exposed to the air, these harmful mixed gases generated after the operation of the calcium carbide furnace will explode.

Therefore, it is essential to properly maintain the sealed calcium carbide furnace.  The main component of the furnace gas is CO, which is toxic and explosive.  Good ventilation must be maintained within the factory building.

(6) Discharge of Calcium Carbide

For initial blow-in and blow-in after major maintenance, when the load is low, oxygen purging may be used as appropriate, and safety regulations for oxygen use must be adhered to.

During normal tapping, oxygen blowing should be minimized. Depending on the flow rate, steel chisels can be used to create holes. When using steel chisels, they should be inserted deeply and pulled out gently to facilitate the smooth flow of calcium carbide.

2. Basic Reaction Processes Occurring in the Calcium Carbide Furnace

The most fundamental reaction within the calcium carbide furnace is the main reaction for the preparation of calcium carbide, which involves the generation of calcium carbide and carbon monoxide from calcium oxide and a carbon source under high-temperature conditions.

In addition to this main reaction, there are also a series of side reactions.  The reactions within the molten bath are complex, making it impossible to analyze each reaction individually.

3. Analysis of the Material Layer Structure inside the Reaction Furnace

Under normal operating conditions, the system inside the calcium carbide furnace is in dynamic equilibrium. The calcium carbide furnace can be subdivided from top to bottom into a preheating layer, a diffusion layer, a reaction layer, and a melting layer, with the bottom layer being the slag layer.

A. Preheating Layer

The preheating layer, also known as the raw material layer, has a temperature range of 500 to 1200℃. Since the newly added materials first enter this area, the main substances are synthetic raw materials.

After passing through this area, the furnace gas preheats the furnace charge. The raw materials begin to react at a lower temperature, manifesting as green combustion decomposition, volatile decomposition, and water gas generation.

B. Diffusion Layer

The temperature range in this area is between 1200 and 1800℃. The main components at this location are furnace materials and molten red materials containing a small amount of calcium carbide. This area is mainly used for the mutual diffusion of calcium oxide and carbon sources.

The gas (mainly carbon monoxide) rising from below the molten pool agitates the material, promoting the combination of calcium vapor with low-temperature substances to form CaOC intermediate matter, which then further diffuses into the pores of calcium oxide and carbon.

In addition, the reduction reactions of substances such as carbon dioxide formed by the pyrolysis reaction and magnesium oxide in the raw materials also occur in this region.

C. Reaction Layer

The temperature range in this area is between 1800 and 2100℃, primarily for the reaction process between calcium oxide and carbon sources. The reaction layer is covered by a diffusion layer and located in the area beneath the electrode tip, where the temperature is relatively high.

In this section, the molten substances of quicklime, coke, and semi-finished products mostly exist in a loose state. The raw material state is such that coke is dispersed on the mixture of the eutectic, resulting in uneven composition.

D. Melted Layer

The temperature range in this area is between 2000 and 2100℃, primarily to maintain the continuous reaction between calcium oxide and carbon, ensuring a certain quality of calcium carbide amidst the constant changes of dilution and concentration.

E. Accumulated Slag Layer

The slag layer is located at the lowest part of the melting layer. Due to the presence of impurities in the raw materials, such as silicon carbide and iron silicide, the impurity content in the slag layer will accumulate after the calcium carbide furnace has operated for a period of time. Therefore, it needs to be cleaned after a certain period.