Practical Exploration of Hermetically Sealed Calcium Carbide Furnaces

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With the shortage of petroleum resources and rising prices, acetylene-based chemical industry has become increasingly important in China’s industry. In recent years, calcium carbide-based PVC products have driven rapid development in the calcium carbide industry; the annual production capacity per furnace has increased from 10,000–20,000 tons to over 50,000 tons per furnace today. The capacity and type of calcium carbide furnaces are determined in accordance with the industry’s regulatory requirements. The closed furnaces that are widely used in China today are basically calcium carbide furnaces equipped with modular holders from Norwegian Elkem, with capacities ranging from 25,500 KVA to 30,000 KVA. Currently, many manufacturers are converting their calcium carbide furnaces from open or semi-enclosed types to enclosed types. The production results are not ideal; these enclosed furnaces are considered less easy to operate than open-type calcium carbide furnaces, and their performance and economic indicators are also unsatisfactory. I believe the fundamental problem lies in the fact that, over the past few years, we have continued to use the parameters and designs proposed by the Norwegian company Elkem, without making any fundamental changes to the equipment. We have not truly combined advanced foreign technologies with the local raw materials available in different regions of our country. Below, I would like to discuss various aspects of enclosed furnaces with everyone. I. Basic conditions and design flaws of the Elkem furnaces. In 1987, **in order to improve the overall technical level of the domestic calcium carbide industry, $14 million was spent to introduce, through joint efforts by the former Ministry of Chemical Industry and the Ministry of Machinery, the key technologies and equipment for 7 25,500 KVA hermetical calcium carbide furnaces from the Norwegian company Elkem (including one unit introduced by Xiaguanhua at a cost of $3 million). This was accompanied by a dry gas purification system for furnace gases, as well as sleeve-type gas-fired lime kilns from the German company VEMAG (in addition to lime kilns of Italian origin manufactured by Fluorspar, which were introduced by the former Guizhou Organic Chemical Factory). The calcium carbide plant in Xiaguanyuan, Hebei, began operations on September 6, 1991, and has now been in operation for 16 years. During the 1990–1992 period, the calcium carbide plants of Zhejiang Juhua Company and Qinghai Electrochemical Plant also introduced calcium carbide furnaces and gas-fired kilns. At that time, five key technologies were introduced: ① hermetically sealed calcium carbide furnaces and their modular holders ； ②Dry purification of furnace gas ； ③Hollow electrode ； ④Computer automatic control ； ⑤Gas-fired lime kiln ； They were regarded as the “five golden flowers” of the calcium carbide industry at that time. Through practice, closed-carbide furnace and modular gripper technology ； Computer automatic control ； The gas-fired lime kiln has been successfully introduced, but there are significant design issues regarding the parameters of the furnace. In particular, the dry purification of furnace gases and the hollow electrode technology are highly immature techniques that have resulted in substantial waste of financial, material, and human resources in our country. Design flaws in the Elkem furnace: ① The geometric parameters of the calcium carbide furnace itself are not properly designed. Elkem is a company that specializes in the production of ferroalloys. The parameters designed for its calcium carbide furnaces are influenced to a large extent by factors related to ferroalloys, which means they are not entirely suitable for calcium carbide production. The geometric parameters of its 25,500 KVA furnace are roughly similar to those of China’s traditional 30,000 KVA calcium carbide furnaces. The following table compares the parameters of China’s traditional calcium carbide furnaces with those of Elkem’s furnaces: Comparison of furnace parameters: Parameter – Elkem 25,500 KVA furnace; China’s traditional 25,500 KVA furnace; 30,000 KVA furnace. Electrode diameter: 1250 mm, 1170 mm, 1250 mm. Diameter of the circle centered around the electrodes: 3580 mm, 3160 mm, 3380 mm. Inner diameter of the furnace chamber: 7800 mm, 7110 mm, 7590 mm. Depth of the furnace chamber: 2700 mm, 2690 mm, 2880 mm. As can be seen from the table above, there are both similarities and differences between the geometric parameters of Elkem’s furnaces and those of China’s traditional calcium carbide furnaces. As is well known, the basic parameters of the geometric dimensions of calcium carbide furnaces are the electrode diameter and the diameter of the circle centered on the electrodes. The diameter of the circle centered on the electrodes in Elkem furnaces is much larger than that in traditional Chinese calcium carbide furnaces. Practice has shown that due to the excessive diameter of the center circle in Elkem furnaces, it often leads to a breakdown in the electrical connection between the three-phase electrodes; this is especially true during the startup phase and at low load levels. As a result, calcium carbide is frequently added back, which worsens the operating conditions of the furnace. There are two ways to resolve the above contradiction: one is to increase the capacity load ； Secondly, the diameter of the concentric circles should be reduced. Through practical testing, it has been found that when an Elkem 25500 KVA calcium carbide furnace is equipped with three 3×10000 KVA single-phase transformers, the depth of the furnace chamber increases by 200–300 mm; the inner diameter of the furnace is changed from using single-layer refractory bricks to double-layer refractory bricks, and the outer diameter of the furnace shell increases to 9100 mm – only under these conditions do the parameters of the calcium carbide furnace become more optimal. ②The discharge nozzle is short. Many manufacturers in China believe that the height of the lid on Elkem furnaces is too low, at only 945 mm, while the height of similar electric furnaces in China is generally between 1400 and 1500 mm. The distance between the lid of an Elkem furnace and the material layer inside it is too small; in some cases, there is even direct contact, which accelerates the wear and damage of the lid. This leads to increased moisture content in the coke, pulverization of lime, an increase in dust particles, and more severe dust accumulation inside the furnace. As a result, there is often insufficient space inside the furnace, and furnace pressure fluctuates greatly, posing a range of problems related to furnace gas emission and purification. For the above reasons, many manufacturers have requested design institutes to increase the height of the furnace lids during the design process. Currently, the height of the furnace lids in Elkem’s plants in China has been increased from 945 mm to 1200–1400 mm, in the belief that this will resolve the issue of furnace pressure. In fact, increasing the height of the furnace lid does not solve the aforementioned problems; it only leads to a series of issues such as easy damage to the ring at the bottom of the electrode. Because the space between the furnace lid and the material layer is controlled by the height of the feeding nozzle. Imagine that the feeding nozzle is very short or severely damaged; no matter how high the furnace lid is, the material layer will remain high as well, leaving no space between it and the furnace lid, and thus the furnace pressure still cannot be controlled. Therefore, the original Elkem furnace lid height of 945 mm is reasonable. However, the length of the material feeding nozzle was increased from 850 mm to 1050 mm, so there is sufficient space inside the furnace. Regarding the material choice for the feed nozzle, cast steel is sufficient. I cannot agree with some manufacturers who choose to use underwater feed nozzles. The reasons are as follows: (1) The use of water-passing feeding columns leads to an increase in leakage points; especially since the feeding nozzles are usually located around the molten pool, explosion accidents are highly likely to occur. ⑵The surface temperature of the material and the furnace conditions are the main factors determining the lifespan of the feeding nozzle. If the electrodes are properly inserted, the furnace conditions are good, and the surface temperature remains below 800°C, then a cast steel feeding nozzle is more than sufficient. ⑶If the electrode rises and the furnace operation is poor, even the water supply nozzles will be damaged, resulting in heavy water leakage and creating a significant safety hazard. ③In the originally designed electric furnace for the short-net Elkem furnace, the furnace transformer is located at some distance from the electrodes; at least 800–1000 mm of this distance needs to be reduced. This results in an increase of about 1000 mm in the length of the short mesh, which undoubtedly also increases power loss. Its short-net structure is also not entirely reasonable. The in-phase, end-to-end wiring method is employed; according to theoretical calculations, this configuration results in 2% higher power loss compared to the phase-skipping, cross-wiring method. If the transformer is moved forward by 1000 mm and phase skipping compensation is applied at the ends of the short circuit, it will **reduce the impedance of the short circuit, as well as its proximity effects and skin effects; this is expected to result in a power savings of around 2–3%. This can be verified by measuring the voltage drop across the short circuit. ④The hollow electrodes in Elkem furnaces have many problems; they simply won’t function, and significant modifications are required to enable them to work. The main problems with the hollow electrodes in Elkem furnaces are: (1) the flexible conveyance pipes designed for this purpose are unable to perform their conveying function; they wear out easily, leading to leaks, and they can also bend and cause blockages. ⑵Welding hollow tubes is difficult to operate and unsafe. The hollow electrode and feed pipe should be replaced with a rigid telescopic structure; by ensuring proper sealing in this telescopic design, the requirements of the hollow electrode can be met. As the balling technology for powders (coke dust, lime powder) in our country matures and improves, the hollow electrode technology will be phased out. ⑤Furnace gas purification: The process flow designed for Elkem’s furnace gas purification is reasonable, but in practice it cannot be put into operation. The main problems are as follows: (1) There are many equipment defects. ⑵Improper layout ⑶; insufficient integration with the conditions in the calcium carbide furnace. Since the furnace gas from calcium carbide furnaces is one of the most important aspects in terms of \"energy conservation and emission reduction\" in our industry, I will provide a detailed overview of this topic. The purification of calcium carbide furnace gas mainly employs two methods: wet purification and dry purification. ⑴The wet purification process primarily involves various washing methods to clean dust-containing gases, thereby achieving the purpose of purifying them. In the 1960s and 1970s, China mainly used wet purification methods for gas treatment, with processes that relied on scrubbers or Venturi scrubbing techniques. The purification system using the gas scrubber method comprises over 30 main pieces of equipment; it is a system capable of handling 3000 NM3/h of gas, and it consumes 200–300 tons of industrial water per hour, as well as around 110 kwh of electricity per hour. The Venturi scrubbing method requires nearly 20 main pieces of equipment for handling 3000 NM3/h of furnace gas; it consumes 90 tons of industrial water per hour and 60 kwh of electricity per hour. The aforementioned method only removes the dust from the furnace gas; however, the wastewater generated still needs to be further treated. This wastewater contains cyanide as well as a large amount of solid suspended particles. The levels and composition of these substances are shown in the table below:

**Wastewater Components**
CN-: 1000–2000 mg/L
Solid suspended particles: 1000–2000 mg/L (composition of suspended particles)
Mg/L: Free substances, acid-insoluble substances, dioxide, calcium oxide, magnesium oxide, others: %%%%%%%
**Composition**
CN-: 10–20 mg/L
Solid suspended particles: 5–15 mg/L
Dioxide: 20–30 mg/L
Calcium oxide: 5–15 mg/L
Magnesium oxide: 15–20 mg/L
Others: 20–30 mg/L

This wastewater must be treated to prevent secondary environmental pollution. Common treatment methods include chlorination to precipitate and remove toxic substances as well as to separate the solid suspended particles. As a result of this treatment, the level of CN- drops from 10–20 mg/L to less than 0.5 mg/L, while the level of solid suspended particles drops from 1000–2000 mg/L to 100 mg/L. Due to the addition of chlorine during water treatment, trace amounts of residual chlorine and cyanide remain after chemical reactions, which still have some impact on the natural environment. The aforementioned wet furnace gas purification production process essentially relies on the technologies used in Japan and Germany in the 1960s. It features a long process flow, numerous pieces of equipment, large floor space, high investment costs, and difficulties in operation, maintenance, and management. It also consumes a large amount of industrial water and energy. Dry purification methods can be classified into 3 types based on filters: (1) Microporous ceramic filters. At the end of the 1950s, the French company SKW was the first to develop microporous ceramic filters for use in the gas from calcium carbide furnaces. This technology is also employed by French companies UHDE and DEMAG. At the end of the 1970s, the American companies ENVIRUTECH and Minnesota Machinery Manufacturing developed ceramic fiber cloth filtering systems. The operating temperature for these filters ranges from 210 to 500°C; both the filters themselves, the filtering materials, as well as the corresponding fan equipment require materials with high heat resistance. As a result, these systems are expensive to manufacture and have high maintenance costs, and they are not used in China. ⑵Electrostatic dust removal filter. In the mid-1960s, Japanese diamond companies also developed electrostatic dust removal filters for use in the purification of gas from calcium carbide furnaces. However, due to the need to cool the gas first in order to prevent tar dust from accumulating on the electrodes of the dust collectors, as well as the lower efficiency of electrostatic dust removal compared to bag filters and its higher cost, these systems were not widely adopted. ⑶Bag filter. In the late 1980s, our country introduced the flue gas purification technology from Norway’s Elkem company, which used filter bags made of glass fiber cloth; however, their lifespan was only 3 to 4 months. Through continuous research by Qinghai Dongsheng Chemical Company, high-temperature resistant filter bags made primarily of PPS are now being used, with a lifespan of 12 to 18 months. In this method, the furnace gas is first cooled to 200–260°C, purified, and then sent directly to the user. It is a simple process that requires few pieces of equipment, has a short flow sequence, and consumes very little water and electrical power. It is now widely used in our country. The dust resulting from dry purification needs to be processed. One method is to send it to users such as cement plants and brick factories for combustion during production, while another method involves the calcium carbide plant itself burning it in the drying kilns used for drying coke (the heat generated by the combustion of carbon and volatile substances in the furnace dust can be recovered at a rate of 20,000–40,000 MJ per day); this approach allows for full recovery of both the latent heat and residual heat from the furnace dust). One method is to burn it in a converter or a fluidized bed furnace to reduce its CN- level to less than 10 ppm. II. Problems during the operation of closed calcium carbide furnaces 1. Environmental protection facilities: There are three major sources of emissions in the production process of calcium carbide furnaces – namely, the exhaust gases from the furnace, the carbon material drying process, and the outlet of the furnace. The rest are dust generation points during crushing, screening, and transportation processes. In today’s industrial production, **great emphasis is placed on environmental protection and pollution prevention, and it is the obligation and responsibility of enterprises to carry out this work properly. Among some manufacturers I have encountered, the dust removal process after product extraction is often not satisfactory. The main problem is likely that the wind volume and pressure parameters have not been chosen appropriately, which prevents the dust removal mechanism from functioning effectively. Regarding dust removal from carbon materials, it is recommended to use a water film for dust removal, as this method is highly effective; the water can be reused. Although dry dust removal methods for carbon materials can meet the **standards**, the appearance and color characteristics of the resulting product are not perfect. There is considerable concern regarding the issues of phosphorus and sulfur in the calcium carbide production process, especially among many newly established calcium carbide manufacturers; environmental protection agencies pay particular attention to these issues. According to relevant data, the transformation process of phosphorus and sulfur impurities is as follows: Lime – P=100%, Carbon – S=100%. In the furnace, PH3+P2O5 = 0, while H2S+SO2 ≈0. In the calcium carbide furnace, acetylene gas contains PH3 at 75% concentration and almost no H2S. The calcium carbide produced consists of Ca3P2 and CaS, with P=100% and S=100%. In the liquid residue resulting from the production of acetylene, H2S concentration is 100% while PH3 concentration is approximately 25%. Both phosphorus and sulfur in calcium carbide originate from lime and carbon; during the production process, all phosphorus and sulfur introduced from the raw materials end up in the calcium carbide in the form of Ca3P2 and CaS. When acetylene is produced from calcium carbide, almost all of the generated H2S dissolves in water containing slaked lime, while about 75% of the generated PH3 ends up in the acetylene gas, with 25% dissolving in the calcium carbide solution. 2. Requirements for raw materials in closed calcium carbide furnaces ⑴ Coke: In fact, the main difference between closed furnaces and semi-open furnaces regarding raw materials lies in the requirement for the moisture content of the coke fed into the furnace; in closed furnaces, this value must be less than one percent. So, can it be used with 2% bran carbohydrates? What if it’s 3%? Practice has shown that when the moisture content of coke is around 2%, slight fluctuations in the pressure of the calcium carbide furnace and a small amount of powder formation within the furnace do occur, but these have little impact on the operation of a hermetically sealed calcium carbide furnace; only minor changes occur in the quality of the calcium carbide. If the moisture content of coke is greater than 3%, there is often an excess of fine material in the furnace, resulting in unstable furnace pressure; in severe cases, calcium carbide may spill over, the product quality declines, and the molten pool around the three-phase electrodes becomes blocked. Currently, anthracite is most commonly used in calcium carbide production in China. Its advantages include a high specific resistivity of the furnace charge, which makes operation of the calcium carbide furnace easier; it also results in higher secondary voltage and active power, as well as a higher natural power factor. However, it has a high moisture content, which increases the difficulty of drying, and its low strength leads to the generation of more powder. In particular, the reactivity is poor, and large amounts of raw coke are often present in calcium carbide products. The lime used in closed furnaces should be the same as that in open furnaces; only its particle size needs to be controlled. Hermetically sealed furnaces have very strict requirements for electrode pastes, mainly with regard to their ash content, volatile matter, and strength. A large degree of compressive elasticity and a reasonable flow coefficient are required. In Elkem-type calcium carbide furnaces, due to the special design of the electrode cylinders, accidents involving softening or breaking of the electrodes occur rarely; of course, this is closely related to the quality of the electrode paste. 3. Furnace pressure control: Strictly speaking, the furnace pressure in a hermetically sealed calcium carbide furnace is generally kept between 0 and 5 mm of water column. The purpose of this is to: (1) prevent large amounts of air from entering due to inadequate sealing of the furnace ； ⑵There are no obvious large flames on the stove, preventing damage to equipment and environmental pollution. ⑶It is to prevent carbon monoxide in the furnace from escaping through the ring feeding system due to excessive furnace pressure, thereby avoiding poisoning of employees. Controlling the pressure in a sealed furnace is extremely important; if the negative pressure is too high, a large amount of air will enter, causing the temperature of the material to rise further. As the temperature increases in the upper part of the material layer, its electrical resistance decreases, which in turn increases the current flowing through the branch circuits. This affects the penetration depth of the electrodes, leading to a vicious cycle in which both the material layer and the electrodes rise. Therefore, furnace pressure control in a hermetically sealed furnace is extremely important. 4. Equipment: (1) Due to the large capacity of closed calcium carbide furnaces, on-load tap changers must be used. During operation, the voltage should be adjusted as little as possible in order to extend the lifespan of the tap changers. Larger-sized cables for the short circuits and water supply in these furnaces should be selected during design; this plays a key role in reducing the long-term energy consumption of the calcium carbide furnaces. ⑵During shutdown, it is necessary to regularly check for wear and damage to the water circulation equipment in order to prevent leaks. In actual operation, the water cooling jacket is the weakest part; its insulation should be checked frequently, as well as the depth to which the electrodes extend into the furnace. Shutdown for maintenance may be required when necessary. ⑶The oil pressure system of the calcium carbide furnace is located right on the top floor of the furnace. When the furnace pressure is high or when welding the electrode cylinders, flames and sparks often get into the lifting platform; therefore, all hydraulic systems must be leak-free to prevent fires. Fires resulting from leaks in the calcium carbide furnace’s oil pressure system have occurred in many manufacturers. ⑷The level signal of the circular silo must be accurate; in many manufacturers, distortion in this level signal often leads to leakage from the silo to the discharge pipe, allowing furnace gas to enter the empty silo and come into contact with air, thereby causing an explosion. 5. Hydrogen detection system: In closed calcium carbide furnaces, components such as the double-layer furnace lid, water cooling jackets, bottom ring, and contact elements are all equipped with water supply systems. The volume of water used in these systems is much greater than that in open furnaces, which also leads to numerous safety hazards. To determine whether there is any leakage in the equipment inside the furnace, hydrogen detectors are used; when the reading on these detectors reaches a certain level, it is necessary to stop the furnace for inspection and repair, otherwise serious safety explosions can occur. Sampling of hydrogen gas is carried out from the furnace gas; after the furnace gas has been dried and purified, a portion of it is taken for analysis, the results are displayed on a dashboard, and it is connected to a computer-based alarm system. 6. Discharge nozzle: The discharge nozzle should have a certain length when it is functioning properly, in order to control the level of the material surface. When it becomes damaged to a certain extent, it should be replaced promptly, to prevent the material surface from rising due to the short nozzle, which could affect the depth to which the electrode can penetrate as well as the insulation of the electrode itself, and may cause damage to the furnace lid and other equipment. 7. Operations: (1) Starting up the furnace: To start an Elkem furnace, an electric oven is generally used, serving the dual purpose of baking the electrodes and raising the temperature at the bottom of the furnace. Based on our experience, it is advisable to keep the oven heating time as long as possible; for a newly installed calcium carbide furnace, 7–10 days is an appropriate duration, while for a calcium carbide furnace that has been overhauled (where some of the electrodes are hardened), 3–4 days is sufficient. If the furnace start-up is not done properly, it can lead to the phenomenon of calcium carbide reversal, causing endless problems. Under normal conditions, it takes 8 to 12 days for a calcium carbide furnace that has been overhauled to reach full capacity after being brought online, while it generally takes 12 to 16 days for a newly installed calcium carbide furnace to reach full capacity and resume normal operation. ⑵Due to the presence of carbon monoxide gas in the sealed furnace, according to Elkem’s original operation manual, it is necessary to open the rough gas valve after shutting down the furnace, and then fill the furnace with nitrogen before opening the furnace lid. Based on our experience, it is safe to open the rough gas valve after shutting down the furnace and wait for 1 minute before opening the furnace lid. ⑶The electrodes do not descend. In a closed calcium carbide furnace, when the electrodes fail to descend, the same solutions as in an open furnace apply: first, it is necessary to reduce the load and voltage level in order to allow the electrodes to penetrate into the charge; second, the specific resistance of the charge is adjusted by modifying the ratios of the elements within and outside the charge – that is, the ratio of the elements inside is reduced while the ratio of those outside is increased; third, the surface of the charge needs to be managed by using intermittent feeding (care must be taken to prevent wear on the feeding nozzles); fourth, the furnace should be emptied frequently but in small amounts, in order to raise the furnace temperature. ⑷For calcium carbide production, the main issues are a small three-phase melt pool, low furnace temperature, poor connectivity within the three-phase melt pool, and difficulty in inserting the electrodes deep enough. It is necessary to reduce the load to allow the electrodes to penetrate deeper, increase the dosage of certain materials, and alternate between using the three different outlets for taking the product out of the furnace. As the furnace temperature rises, gradually increase the load to force communication among the three-phase molten pools, and take the furnace out of service frequently. III. Management: Ensuring safe production, reducing consumption, and saving energy are the most important aspects of management in calcium carbide production, and they represent the key tasks and challenges for those working in this field. The management of closed-type calcium carbide furnaces should focus on the following four areas: 1. Improving the quality of raw materials – Using qualified raw materials is essential to ensure high yields, high quality, low consumption, and safe production in calcium carbide manufacturing. The lime used in the furnace should have a full CaO content of over 92% and a moisture content of less than 7%. It is best to use a combination of anthracite and metallurgical coke as the carbon source, with a ratio of 30% metallurgical coke, 50% anthracite, and 20% bituminous coal; this combination yields better results. The carbon material fed into the furnace should have a fixed carbon content of over 84%, a moisture content of less than 1%, and its particle size should be adjusted according to the capacity requirements of the furnace, with a particle size compliance rate of over 85%. 2. Optimal electric furnace parameters: The parameters of the calcium carbide furnace should be carefully considered during design, by taking into account both theoretical principles and the characteristics of local raw materials, as well as input from experts with relevant experience. For electrical parameters, since multi-stage voltage regulation switches are used, it is important to accurately calculate the secondary current and to properly design the short-circuit network structure. 3. Strengthen equipment management. Equipment is a necessary condition for the effective and balanced operation as well as safe production of an organization. Equipment management should adhere to the principle of \"prevention first, with equal emphasis on maintenance and scheduled repairs.\" It is important to focus on these four aspects: proper use, careful maintenance, scientific repairs, and the production of replacement parts. Efforts should be made to improve the natural power factor of electric furnaces, extend their service life, and reduce the number of shutdowns, thereby increasing their utilization rate and operating frequency. 4. Operate with care and conduct regular inspections of the electrodes. Strengthen monitoring of the electric furnace, operate it carefully to ensure that the electrodes are properly embedded in the furnace charge, and keep the furnace operating in a normal and stable state at all times. The main manifestations are as follows: the depth to which the electrode penetrates into the material layer is approximately 0.6 to 1.0 times the diameter of the electrode; the damaged section of the electrode should not be too long; the material layer has good air permeability, with no red material on the surface; the material layer sinks evenly, and there is no spraying of material; each time the electrode is pressed in, the process proceeds smoothly, and the maximum safe pressing interval should not be exceeded. The baking and consumption of the electrodes are in balance; there is no over-baking or under-baking, and no electrode-related accidents occur. The electrodes are removed from the furnace at the specified time, with smooth unblocking of the furnace. IV. Discussion on large-capacity enclosed calcium carbide furnaces. The largest enclosed calcium carbide furnace in China at present is one that was introduced by the former Guizhou Organic Chemical Factory in 1969, a 35,000 KVA calcium carbide furnace from Japan. Due to its design parameters not being suitable for the conditions of China’s raw materials, this furnace has been in operation for over 30 years, and its economic and technical performance is unsatisfactory; in particular, its natural power factor is very low. Based on information from foreign electric arc furnaces, it can be seen that the larger the capacity of a furnace, the lower its power factor; this has also been confirmed by numerous practices in China. Internationally, there are few calcium carbide furnaces with a capacity of over 50,000 KVA, but there are many ferroalloy furnaces. The principle and furnace structure of these furnaces are essentially the same – they are all electric arc furnaces using electric resistance heating. Experience from large-scale ferroalloy production both domestically and internationally shows that due to differences in the charge used during the smelting process, electric furnaces of the same capacity exhibit significant variations in their natural power factor when smelting ores of different grades. I believe that to develop large-capacity calcium carbide furnaces, it is first necessary to address their natural power factor. To improve this natural power factor, in addition to reducing the impedance of the transformers and short circuits, it is crucial to increase the specific resistance of the furnace charge. If it is possible to enhance the specific resistance of the furnace charge, then developing large-capacity calcium carbide furnaces can be beneficial; however, if the issue of raw materials cannot be resolved, there is no point in developing such furnaces. Secondly, based on our experience, shifting a calcium carbide furnace from low load to high load is also a challenging operation issue that needs to be addressed in large-capacity calcium carbide furnaces. Thirdly, it remains to be determined through practice whether the calcium carbide furnace should discharge material continuously or intermittently. Therefore, I believe that the 30,000 KVA calcium carbide furnaces currently in use in China are relatively mature and reliable in all aspects. If the natural power factor of a 40,000 KVA calcium carbide furnace can reach above 0.7 (corresponding to a gas generation volume of 300), I believe it would represent a significant advancement for our calcium carbide industry.