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How to produce paper coating grade calcium carbonate with high brightness

04/22/2026

To produce high-brightness paper-coating grade calcium carbonate (CaCO₃), the core targets are whiteness ≥95% ISO, purity ≥98%, low chromophoric impurities (Fe₂O₃ ≤0.01%, MnO ≤0.006%), and a tailored particle size distribution (1–3 μm for coating). The process splits into two mainstream routes: Ground Calcium Carbonate (GCC, mechanical grinding) and Precipitated Calcium Carbonate (PCC, chemical precipitation). Below is a step-by-step technical guide for both.

Core Targets for High-Brightness Paper-Coating Grade CaCO₃

Parameter	Requirement	Purpose
Whiteness	≥95% ISO	Maximize light reflection for paper whiteness
Purity	CaCO₃ ≥98%	Minimize non-CaCO₃ impurities
Impurities	Fe₂O₃ ≤0.01%, MnO ≤0.006%, HCl-insoluble matter ≤0.3%	Prevent discoloration
Particle Size	d50 1–3 μm, d97 ≤10 μm	Ensure uniform dispersion in coating
Brightness Stability	No yellowing after storage	Maintain long-term application performance

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

GCC is made by grinding high-purity limestone/marble. Brightness hinges on raw material purification + ultra-fine grinding + impurity removal .

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)

Implement a multi-stage purification process to remove chromophoric impurities:

Magnetic Separation: Use high-gradient magnetic separators to remove iron-bearing minerals (e.g., hematite, magnetite). Target Fe₂O₃ reduction to ≤0.01% .
Flotation: Remove silica, mica, and other non-magnetic impurities via reverse flotation with collectors (e.g., fatty acid salts) .
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

Choose the right process based on target fineness:

Process	Equipment	Target Fineness	Brightness Advantage
Dry Grinding	Ultra-fine vertical mill	d97 3–10 μm	Low cost, simple operation
Wet Grinding	Stirred mill + ball mill	d50 1–3 μm	Finer, more uniform particles; higher whiteness

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

Step 5: Classification & Surface Modification

Air Classification: Separate oversize particles to ensure strict PSD control .
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)

PCC offers superior brightness and optical performance (refractive index 1.59) for high-end coating paper. Brightness depends on calcination + carbonation control + crystal form regulation .

Step 1: Raw Material & Calcination

Limestone Preparation: Select high-purity limestone (CaCO₃ ≥98.5%, Fe₂O₃ ≤0.1%) .
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):

$CaO + H_{2} O \to Ca(OH)_{2}$
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)

Introduce CO₂ (from calcination or industrial sources) into the Ca (OH)₂ slurry to precipitate CaCO₃. Crystal form (纺锤状 /spindle, cubic, chain-like) directly impacts brightness and coating performance .

Critical Carbonation Parameters for High Brightness

Parameter	Optimal Range	Effect
CO₂ Concentration	30–40%	Ensure complete reaction
Reaction Temperature	20–30°C (low temp)	Promote spindle-shaped crystals (highest brightness)
pH	9.5–11.5	Prevent premature precipitation
Stirring Speed	200–300 rpm	Uniform mixing, uniform particle size

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

Aging: Age the slurry for 1–2 hours to stabilize crystal structure .
Filtering & Washing: Remove mother liquor; wash with clean water to reduce soluble salt content .
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

Raw Material Control: Strictly limit Fe₂O₃, MnO, and organic impurities; select ore with natural whiteness ≥90% .
Equipment Material: Use ceramic, plastic, or stainless steel instead of carbon steel to avoid metal contamination .
Process Cleanliness: Maintain dust-free production environments; clean equipment regularly to prevent cross-contamination .
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

Standard	Requirement
Whiteness (ISO)	≥95%
CaCO₃ Purity	≥98%
Moisture Content	≤0.3%
Oil Absorption	60–90 g/100 g
Dispersibility	≥98% in water

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.

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