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Calcium Carbide Impurities: Where Do They Come From?
The main component of calcium carbide is calcium carbide (CaC₂), but from the perspective of the production site, industrial calcium carbide cannot contain CaC₂ alone. Associated substances introduced by lime and carbonaceous raw materials, high-temperature reactions inside the electric furnace, and the subsequent crushing, screening, conveying, and packaging processes may all leave traces in the finished product.
Gas yield is important, but it is not the only basis for evaluating the quality of calcium carbide. If the gas yield meets the standard, does that necessarily mean there are few impurities? Not necessarily. Ferrosilicon, sulfur- and phosphorus-containing compounds, carbon rod fragments, and dust are formed through different pathways, so naturally they cannot all be handled in the same way.
How Does Ferrosilicon Get into Calcium Carbide?
The production of calcium carbide requires lime and carbonaceous raw materials. After silicon dioxide, iron oxide, and other mineral components in the raw materials enter the electric furnace, they form silicon- and iron-containing substances in the high-temperature reducing environment. Some of these become ferrosilicon inclusions and are mixed into the furnace charge or finished product.
Ferrosilicon does not produce acetylene, but it adds to the weight of the finished product and may also increase the reaction residue left after the customer uses it. As the material passes through the production line, TYWH uses a 1,500 W electromagnet to attract and remove ferrosilicon. At the production site, the material can be seen moving continuously forward while magnetic inclusions are separated from the material flow. This is more direct than dealing with them after the product has been packed into drums.
However, it is easier said than done, and achieving 100% removal is not realistic. Ferrosilicon varies in size, shape, and magnetism, and particles that are too fine or embedded in calcium carbide lumps are more difficult to separate. We are still looking for more effective treatment methods. If production or quality engineers have proven experience in this area, they are also welcome to exchange ideas and discuss it with TYWH.
H₂S and PH₃ Are Not Gases "Hidden" inside Dry Calcium Carbide
After sulfur- and phosphorus-containing components in the raw materials react in the electric furnace, they may form compounds such as calcium sulfide and calcium phosphide. In a dry state, these compounds do not directly appear as H₂S and PH₃. Only when calcium carbide comes into contact with water to produce acetylene do these compounds participate in the reaction and release hydrogen sulfide and phosphine.
This also explains why control measures must be extended upstream. The composition of the raw materials and furnace conditions determine the baseline levels, while acetylene gas testing verifies the final results. During continuous production, TYWH conducts tests twice a week, with the following control limits:
- H₂S: ≤600 ppm
- PH₃: ≤400 ppm
This frequency is higher than the minimum testing frequency required by the current Chinese national standard for calcium carbide, allowing data deviations caused by fluctuations in raw materials or changes in furnace conditions to be detected earlier. TYWH's current standards are stricter than Chinese industry standards.
Why Do Carbon Rod Fragments Need to Be Picked Out Manually?
During electric furnace operation, carbon electrodes are continuously exposed to high temperatures, arc impacts, and mechanical forces, which may occasionally produce carbon rod or electrode fragments. Unlike ferrosilicon, these fragments cannot be removed by magnetic separation, so electromagnets alone cannot handle them effectively.
TYWH manually removes visible carbon rod fragments. Manual sorting may sound somewhat traditional, but human judgment remains direct and effective when dealing with non-magnetic foreign matter that is irregular in shape and varies significantly in size. The electromagnet attracts ferrosilicon, while operators identify carbon rod fragments. Combining these two methods within the same control chain is actually more consistent with the realities of the production site.
Dust Reflects the Condition of Crushing and Screening
Particle-size control during crushing, screening, conveying, and drum filling. Excessive fines add to the product weight and may also cause production problems.
TYWH conducts one spot check of dust on the surface of the drums for each batch, controlling the powder content at ≤1%. The same batch also undergoes one inspection for visible impurities inside the drums to confirm the control of ferrosilicon, carbon rod fragments, and other foreign matter, ensuring stable quality throughout the batch.
Gas Yield Testing Is Also an Essential Step
Gas yield directly reflects the level of effective CaC₂ in calcium carbide. TYWH conducts testing in accordance with the method specified in GB/T 10665—2004. The in-house sampling standard is to take 15 kg for inspection for every metric ton of product and for each finished-product particle-size specification.
The main specifications and current gas yield control values are:
- 15–25 mm: ≥290 L/kg
- 25–50 mm: ≥300 L/kg
- 50–80 mm: ≥300 L/kg
All the above gas yield values are based on reference conditions of 20°C and 101.3 kPa. Testing personnel record the temperature, atmospheric pressure, and gas volume at the same time to prevent environmental changes from interfering with the results and to allow data from different batches to be compared.
Even after passing inspection, the product cannot be loaded directly. TYWH carries out packaging drum appearance inspections at three points: in the production area, during production, and before loading. Staff inspect the drum body, drum lid, sealing condition, and any visible damage. After all, having a qualified product inside the drum is only the foundation. If the packaging loses its seal during transportation, even the best calcium carbide will have difficulty maintaining its original condition.