During the procurement and use of graphitized petroleum coke, particle size is one of the key parameters affecting its performance and application effectiveness. Failure to meet particle size standards can negatively impact usage in multiple ways, with specific manifestations as follows:
1. Significant Decline in Carbon Absorption Rate
Imbalance Between Dissolution and Oxidation Loss:
The carbon-enhancing process of recarburizers is a dynamic balance between dissolution-diffusion and oxidation loss. If the particle size is too small (e.g., below the recommended lower limit), dissolution accelerates, but oxidation loss also increases simultaneously, leading to reduced carbon absorption. For furnaces under 1 ton, the recommended recarburizer particle size is 0.5–2.5 mm. If particles are too fine (<0.5 mm), they are prone to oxidation and burning at high temperatures, potentially reducing absorption rates from over 80% to below 60%.
Insufficient Dissolution of Large Particles: If particle size exceeds the furnace capacity's recommended upper limit, dissolution slows down, and undissolved particles may be discharged with slag, resulting in waste. For a 3-ton furnace, if 5–20 mm particles are used (where the standard is 2.5–5 mm), carbon absorption may drop by 20%–30%.
2. Unstable Calcination Quality
Fluctuations in True Density and Resistivity:
Petroleum coke of different particle sizes exhibits significant differences in true density and resistivity after calcination.
Small Particles (0.25–3 mm): Higher true density after calcination (e.g., ≥2.0 g/cm³) and lower resistivity, suitable for high-conductivity applications.
Large Particles (3–30 mm): If insufficiently calcined, true density may drop to 1.8 g/cm³, increasing resistivity and affecting the conductivity and thermal stability of electrode materials.
Poor Process Adaptability: Uneven particle sizes lead to uneven temperature distribution in calcination equipment (e.g., rotary kilns), requiring frequent adjustments to process parameters, increasing energy consumption and production costs.
3. Process Efficiency and Cost Issues
Extended Melting Time: Non-compliant particle sizes prolong dissolution-diffusion time. The carbon dissolution limit formula for cast iron indicates that excessively large particles require extended melting time to meet carbon concentration requirements, increasing energy consumption.
Higher Impurity Risks: Mixed particle sizes may introduce insufficiently calcined impurities (e.g., sulfur, ash), exacerbating issues like gas expansion and cracking. Particles with excessive sulfur (>0.05%) release gases during graphitization, causing product cracking.
4. Performance Defects in Final Products
Casting Defects: In foundry applications, excessively large or small particles lead to uneven carbon distribution, affecting mechanical properties. In ductile iron, non-compliant recarburizer particle sizes reduce carbon absorption, potentially causing poor nodularization and casting porosity.
Degraded Electrode Material Performance: When petroleum coke with non-compliant particle sizes is used in graphite electrode production, it may result in low bulk density (<1.6 g/cm³) and a high thermal expansion coefficient (>3×10⁻⁶/°C), impairing thermal shock resistance and service life.
5. Adaptability Issues in Industry Applications
Variable Coverage Effectiveness: For recarburizers used in ladles, the recommended particle size is 0.5–1 mm. If particles are too coarse, they float on molten iron, failing to form a uniform protective layer and increasing oxidation loss.
Restrictions in Specific Applications: High-purity graphite production requires ash content ≤0.15%. Mixed particle sizes introducing ash impurities directly disqualify the product for high-end applications.
Non-compliant particle sizes significantly increase production costs and reduce product competitiveness by affecting dissolution efficiency, calcination quality, process stability, and final performance. During procurement, the appropriate particle size range should be selected based on specific application scenarios (e.g., furnace type, process temperature). Additionally, incoming material inspection must be strengthened, particularly in controlling related indicators such as ash and sulfur content.
