The production of medium and low carbon ferrochrome is characterized by intermittent operations, with various electrical systems in different smelting periods.
During the melting period, the reduction reaction has begun in the high-temperature zone, but the surrounding furnace temperature is low and most of the solid materials are still melting. Therefore, a higher voltage can be used, and the power of the furnace is higher at this time.
After the reduction period of the furnace material is completed, do not use high power. If the long arc high-voltage operation continues, the heat loss will increase and the operating conditions will deteriorate, which will damage the furnace material. Therefore, a lower secondary voltage should be used. The general melting period uses 178V, and the refining period voltage is 156V.
 
The reducing agent used in the smelting of medium and low carbon ferrochrome by the electric silicon thermal method is silicon chromium alloy, and the chromium in the silicon chromium alloy enters the medium and low carbon ferrochrome during the smelting process.
 
In order to prevent arc light from eroding the furnace bottom and to be suitable for power transmission, iron retention operation is adopted, and the appropriate amount of iron retention at the furnace bottom is 150-200mm thick. The production of retained iron has the following advantages:
(1) Separate the slag from the bottom of the furnace to prevent slag from eroding the furnace temperature;
(2) Keep the furnace lining at a constant temperature to prevent damage caused by frequent rapid cooling of cold materials;
(3) Prevent high-temperature arc light from piercing the furnace bottom.
 
But leaving iron should be appropriate, too much or too little is not beneficial.
Leaving too much iron can cause slag flipping and material collapse, affecting product quality; If the amount of iron left is too small, the furnace bottom will be subjected to high temperature, mechanical erosion, and chemical erosion, thereby shortening the service life of the furnace lining.
 
The main steps in the smelting of medium and low carbon ferrochrome are furnace repair, blockage of iron outlet, feeding and melting, refining, etc.
After tapping, the tapping port should be immediately blocked with magnesium sand and the erosion of the furnace lining should be checked.
The furnace lining operates at high temperatures of 1650-1700 ℃, and is also subjected to strong chemical erosion from slag iron and mechanical erosion during agitation. In addition, abnormal furnace conditions and unreasonable use of the tapping hole can cause a shortened furnace life.
 
Production practice has proven that the following methods can extend the service life of the furnace:
Improve the quality of furnace construction, develop reasonable operating procedures, use reasonable secondary voltage, maintain stable control of temperature and alkalinity, maintain a constant amount of residual iron, and use and maintain the tapping port reasonably.
 
When severe erosion of the furnace wall (mainly slag line) is found, a portion of lime should be promptly extracted from the material batch or replaced with magnesium bricks. Only after repairing the furnace lining can arc ignition be used for power transmission. Fast load operation can be used for smelting medium carbon ferrochrome, and full load operation can only be carried out 15 minutes after power transmission. To promote the use of chemical materials, sufficient load should be provided during the melting period of smelting medium and low-carbon ferrochrome, in order to increase the furnace temperature. The load during the refining period can be slightly reduced.
 
There are two ways to add materials: one is to mix and add chromium ore, lime, and silicon chromium alloy into the furnace in one go;
Another feeding method is the batch feeding method, which means that after a furnace material is mixed, it is added to the furnace several times.
The former is currently a widely used method. Its characteristic is to slowly add the mixture into the furnace after power transmission, with a slightly bowl shaped distribution, more behind the electrode, and less on the large surface. If there is a lot of rain in summer and the raw materials are damp, the materials can be first placed around the furnace for baking. After drying, they can be slowly pushed into the furnace with a rake. To accelerate melting and make full use of heat, according to the material melting situation, consumables can be used to gradually push the surrounding materials into the furnace. To accelerate melting and fully utilize heat, a rake can be used to gradually push the surrounding materials into the high-temperature zone of the furnace according to the material melting situation. The period from power transmission to the completion of furnace material melting is called the melting period.
 
The refining period refers to the period from the completion of material transformation in the furnace to before iron tapping. This stage is a reduction reaction, and sufficient stirring should be carried out during this period to promote the progress of the reduction reaction. The refining period must be maintained for a certain amount of time. If it is too long, it will increase the carbon content of the metal and waste electrical energy; If it is too short, the reduction reaction will not be thorough and the metal recovery rate will be low.
 
Before casting, samples should be taken from the middle of the three electrodes to determine the silicon content. If the silicon content is low, silicon chromium should be added for silicon adjustment; Silicon high should be treated with chromium ore desilication as appropriate. Once the ingredients are qualified, iron can be produced. The method for determining silicon content is as follows:
(1) The cross-section of the sample is grayish white with slight vertical forks (columnar crystals). The iron sample is relatively tough, indicating that the metal contains less than 3% silicon and can be cast.
(2) The cross-section of the sample is gray black, with many vertical forks and is tough and not easy to break, indicating that the silicon content is too low. Reducing agents should be added for silicon adjustment. But after adding silicon chromium, stirring should be carried out and there should be a certain refining period before iron can be produced.
(3) The cross-section of the sample is white, with a brittle texture and easy to break, indicating that the silicon content is too high to produce iron. Refining should be carried out, and if necessary, a certain amount of chromium ore can be added for desilication. Iron can only be produced after the ingredients are qualified.
 
Before casting, the ladle, slag tank, trolley, ingot mold, etc. should be prepared, and the winch should operate normally. If the ingredients are qualified, immediately open the tapping port and cut off the power for tapping. Sometimes it is difficult to open the iron tap, so use a burner or oxygen burner to open the iron tap. Proper use and maintenance of the tapping hole is one of the key issues in extending the service life of the furnace. The height of the tapping hole and the depth of the blockage should be appropriate. If opened too high, the molten iron cannot come out, and a large amount of molten iron is stored in the furnace, causing slag flipping, which can seriously lead to furnace leakage accidents; If the tapping hole is opened too low, it will release all the molten iron from the furnace, prevent the steel from being packaged properly, cause accidents in front of the furnace, lower the furnace bottom, and significantly shorten the service life of the furnace. It is best to open according to the depth of the furnace bottom, discharge slag first, and then discharge molten iron, and evenly discharge iron from the furnace according to the size of the material batch. The depth of the iron outlet blockage should also be appropriate. If it is too deep, it will cause difficulty in opening the hole. If it is too shallow, the iron outlet will move outward, erode both sides of the iron outlet, shorten the service life of the furnace lining, and it is also easy to run the hole.
 
After tapping, separate the slag and iron in a specially designed slag iron separator (i.e. slag separation mold).
After the separation of slag and iron, the alloy is cast into the ingot mold. Or cast with a cast iron machine. After being lifted to the finishing machine for cooling, the surface dust and slag are removed. After crushing, each piece with a mass of less than 15kg is placed in the designated warehouse number according to the variety and grade. Products with unqualified chemical composition and physical form are returned to the furnace for remelting. When smelting low-carbon ferrochrome (C ≤ 0.25), appropriate attention should be paid to the issue of carbonization. Raw materials must be clean and free from carbon, especially silicon chromium alloys with low carbon content. Silicon chromium alloys are strictly prohibited from containing slag, as the carbon content in the slag is high.