Operation of Industrial Silicon Smelting (2)

Operation of Industrial Silicon Smelting (2)

As we know from the above article Operation of Industrial Silicon Smelting (1).  The operation method of industrial silicon smelting mainly includes high-temperature smelting, correct charging, sinking and tamping, charge shape and simmering temperature raising, as well as furnace condition judgment and treatment. By accurately controlling the furnace temperature, charge ratio and electrode operation, the smelting process is ensured to be stable to obtain high-quality industrial silicon products. The operation method the Sixth and Seventh point of industrial silicon smelting and the detailed steps of furnace discharge, industrial silicon submerged arc furnace condition and pouring are as follows:

6. Signs of normal furnace conditions and treatment of abnormal furnace conditions

In the production of industrial silicon in electric furnaces, the furnace conditions are prone to fluctuations and are difficult to control. Therefore, the furnace conditions must be correctly judged and handled in a timely manner. Like the production of 75% ferrosilicon, there are many factors that affect the furnace conditions, but in actual production, the most important factor affecting the furnace conditions is the amount of reducing agent used. Improper use of reducing agents can cause drastic changes in the furnace conditions. Generally speaking, changes in furnace conditions are usually reflected in the electrode insertion depth, current stability, furnace surface flaming, iron tapping, and product quality fluctuations.

1) The sign of normal furnace conditions is that the electrode is deeply and steadily inserted into the charge, the current and voltage are stable, the arc sound in the furnace is stable and low, and the charge surface flaming area is wide and uniform; the charge has good air permeability, the charge surface is soft and has a certain sintering property, the sintering degree of the charge in each place is not much different, the simmering time is stable, and there is basically no piercing fire and collapse of the charge; the furnace eye is easy to open when tapping iron, the flow head is large at the beginning, and then it becomes uniformly smaller, and the product quality is stable.

2) Treatment of abnormal furnace conditions. Various factors such as fluctuations in raw material moisture content, changes in reducing agent quality, poor weighing accuracy, improper operation, etc. will affect the practical carbon content, resulting in insufficient or excessive reducing agents in the furnace.

The characteristics of excess reducing agent in the furnace are loose material layer, longer flames, mostly concentrated fire heads around the electrode, fast material feeding around the electrode, no sintering of the furnace charge, serious "fire stab" material collapse, slow electrode consumption, significant generation of SiC in the furnace, hardening of the cone edge, rising current, and lifting of the electrode. When the reducing agent is seriously excessive, "fire stab" material collapses frequently in the narrow area around the electrode, the material layer in other areas becomes hard and does not eat material, the crucible is greatly reduced, the heat is highly concentrated around the electrode, the electrode is lifted high, the heat loss is serious, the arc sound is very loud, the furnace bottom temperature drops seriously, the false furnace bottom rises quickly, the molten iron temperature is low, the furnace eye is reduced, and sometimes it cannot even burn the furnace eye, and the furnace is forced to stop.

In order to eliminate the phenomenon of excess reducing agent and reverse the furnace condition in time, when the excess reducing agent is not serious, a part of the reducing agent can be reduced in the batch, and at the same time, active furnace condition maintenance can be carried out to restore the furnace to normal. When the excess reducing agent is serious, the degree of excess reducing agent should be estimated, and then the method of adding silica in a centralized manner or adding silica in the charge should be adopted. As a temporary measure, the amount of silica added must be strictly controlled to prevent the formation of a large amount of slag. The concentrated addition of silica can destroy SiC and increase the resistance of the charge in a short time, promote the stable insertion of the electrode, gradually expand the "crucible", and reverse the furnace condition.

The phenomenon of insufficient reducing agent in the furnace is that the material surface is severely sintered, the air permeability is poor, the material is eaten slowly, the flame is short and weak, and the fire is serious. In the early stage of carbon deficiency, the electrode insertion depth increases, the temperature in the furnace increases, and the amount of molten iron increases instead. When the furnace eye is opened, white fire comes out of the furnace eye, and the molten iron is overheated. When carbon deficiency is serious, the material surface turns red and hard, the current fluctuates, the electrode is difficult to insert, the piercing fire becomes a bright white flame, the sound is whistling, and it is difficult to eliminate. The electrode consumption increases significantly, the furnace eye becomes sticky and difficult to open, and the amount of molten iron decreases significantly.

In order to eliminate the phenomenon of insufficient reducing agent, additional reducing agent should generally be added. When the insufficient reducing agent is not serious, in order to quickly improve the permeability of the material layer, a part of charcoal can be added to the material batch. When the reducing agent is seriously insufficient, in addition to adding charcoal to the material batch, petroleum coke that is not easy to burn should be added when sinking and ramming the furnace to effectively eliminate the phenomenon of insufficient reducing agent. In addition, in order to maintain the stability of the furnace condition, it is necessary to reduce the hot shutdown.

7. Silicon discharge and pouring

1) Silicon discharge. After a certain amount of silicon liquid accumulates in the furnace, in order to prevent affecting the furnace condition, the eye should be opened in time to discharge silicon. The heat loss is large when discharging silicon, so the number of silicon discharges should not be too many. For an electric furnace of about 6000KV•A, the furnace is opened every 2~3 hours; for an electric furnace of about 2000KV•A, the furnace is opened every 3~4 hours.

The furnace eye is burned with a graphite rod. Under normal circumstances, the silicon discharge process can be completed in about 20 minutes. In a double-electrode single-phase electric furnace, when the furnace eye is burned through with a graphite rod, the first thing that flows out is the silicon liquid accumulated in the cavity near the iron discharge door. The silicon liquid at the bottom of the "crucible" is far away from the furnace eye, so it is easy to seal the furnace mouth during the outflow process. In order to successfully complete the silicon discharge task, the graphite rod burner must be burned while discharging silicon. Or use a wooden pole or bamboo pole to expand the furnace eye so that the silicon liquid can flow out smoothly.

2) After the silicon is discharged, use a burner or a wooden pole to burn the furnace eye into a circle, and then quickly block the eye. The blocking material is a conical mud ball made of coke powder, water and a small amount of graphite powder. The plugging should be quick and accurate, the mud ball should be filled, and the plugging should be maintained at a certain depth, generally reaching the inner wall of the furnace wall. After the furnace eye is plugged, the outer opening is 200~300mm or the furnace eye cavity is filled with block industrial silicon and slag silicon, and then the furnace eye is sealed with crushed industrial silicon piled into a slope.

3) Pouring. The density of slag silicon of industrial silicon is not much different. In order to prevent the slag inclusion of silicon liquid from affecting the quality and causing waste, a slag blocking frame made of graphite rods should be placed at the impact of silicon liquid to facilitate the separation of silicon liquid and slag.