How to produce paper coating grade calcium carbonate with high brightness

Core Targets for High-Brightness Paper-Coating Grade CaCO₃

Route 1: High-Brightness GCC Production (Mechanical Grinding)

Step 1: Raw Material Selection & Pre-Treatment

• Ore Choice: Select high-purity marble/limestone (CaCO₃ ≥98.5%, Fe₂O₃ ≤0.12%, MnO ≤0.006%) . Avoid ores with visible iron/manganese oxides or organic impurities.
• Washing & Sorting: Use high-pressure water washing to remove clay, silt, and surface impurities; hand-sort to eliminate colored gangue .

Step 2: Coarse Crushing

• Use jaw crusher + cone crusher to reduce ore to 50–70 mm, ensuring uniform feed for subsequent grinding .

Step 3: Purification (Critical for Brightness)

1. Magnetic Separation: Use high-gradient magnetic separators to remove iron-bearing minerals (e.g., hematite, magnetite). Target Fe₂O₃ reduction to ≤0.01% .
2. Flotation: Remove silica, mica, and other non-magnetic impurities via reverse flotation with collectors (e.g., fatty acid salts) .
3. Acid Leaching (Optional): For ultra-high brightness, use dilute hydrochloric acid (5–10%) to dissolve residual iron/manganese compounds; neutralize with lime water post-leaching .

Step 4: Ultra-Fine Grinding

• Key Control: Use ceramic lining/grinding media to avoid metal contamination (which darkens CaCO₃) .

Step 5: Classification & Surface Modification

1. Air Classification: Separate oversize particles to ensure strict PSD control .
2. Surface Modification: Treat with stearic acid (1–3% by mass) or its salts to improve dispersion in water-based coatings; this also enhances brightness stability by preventing particle agglomeration .

Step 6: Drying & Packaging

• Dry at 105–120°C to reduce moisture to ≤0.3%; pack in moisture-proof bags to avoid caking .

Route 2: High-Brightness PCC Production (Chemical Precipitation)

Step 1: Raw Material & Calcination

1. Limestone Preparation: Select high-purity limestone (CaCO₃ ≥98.5%, Fe₂O₃ ≤0.1%) .
2. Calcination: Fire in a vertical/rotary kiln at 900–1100°C for 2–3 hours to decompose CaCO₃ into CaO and CO₂.
   • Key Control: Avoid over-calcination (causing CaO sintering) or under-calcination (unreacted CaCO₃); ensure complete decomposition .
   • Remove volatile organic impurities during calcination to boost whiteness .

Step 2: Slaking (Digestion)

• React quicklime (CaO) with hot water (80–90°C) to form calcium hydroxide slurry (Ca(OH)₂):

  CaO+H2​O→Ca(OH)2​

• Key Control: Maintain a slurry concentration of 10–15%; use clean water to avoid metal contamination. Higher slaking temperatures yield finer Ca(OH)₂ particles .

Step 3: Purification

• Filter the Ca(OH)₂ slurry to remove unreacted limestone and impurities; perform acid washing if needed to reduce Fe/Mn content to target levels .

Step 4: Carbonation (Core for Brightness & Crystal Form)

Critical Carbonation Parameters for High Brightness

• Crystal Form Regulation: Low-temperature carbonation (20–30°C) produces spindle-shaped PCC (d50 0.5–2 μm), which delivers whiteness ≥95% ISO and excellent light scattering .

Step 5: Post-Carbonation Treatment

1. Aging: Age the slurry for 1–2 hours to stabilize crystal structure .
2. Filtering & Washing: Remove mother liquor; wash with clean water to reduce soluble salt content .
3. Surface Modification: Treat with titanate coupling agents or stearic acid to improve compatibility with paper coatings .

Step 6: Drying & Milling

• Spray-dry at 120–150°C to produce fine powder; mill lightly to break up agglomerates without damaging crystal form .

Universal Brightness Enhancement Measures

1. Raw Material Control: Strictly limit Fe₂O₃, MnO, and organic impurities; select ore with natural whiteness ≥90% .
2. Equipment Material: Use ceramic, plastic, or stainless steel instead of carbon steel to avoid metal contamination .
3. Process Cleanliness: Maintain dust-free production environments; clean equipment regularly to prevent cross-contamination .
4. Quality Monitoring:
   • Real-time test whiteness (whiteness meter), particle size (laser particle size analyzer), and impurity content .
   • Adjust process parameters promptly if whiteness drops below 95% ISO .

Application-Specific Tuning

• High-Gloss Coating Paper: Use spindle-shaped PCC (d50 0.8–1.5 μm) for maximum light scattering .
• Offset Printing Paper: Mix GCC (d50 2–3 μm) with PCC (1:1) to balance cost and brightness .
• Low-Gloss Paper: Use cubic PCC (d50 1–2 μm) for better opacity .

Key Quality Standards

By following these processes and controls, you can consistently produce high-brightness paper-coating grade calcium carbonate that meets the strict requirements of high-end paper production.