Citric Acid Production Process
Classical bioprocess from Aspergillus niger — mycelial removal, calcium citrate precipitation, acidification, carbon treatment, crystallization, and drying
Process Overview
Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) is the world’s most produced organic acid, with over 2.5 million tonnes annually. The classical downstream process from Aspergillus niger submerged fermentation uses calcium citrate precipitation to selectively recover citric acid from the complex broth. This century-old process remains economically competitive due to its high selectivity and simplicity. Overall yield: 70–80% from fermentation broth to crystalline citric acid.
Process Steps
1
Solid–Liquid Separation
Mycelial Removal
Fermentation broth (80–150 g/L citric acid) is heated to 70–80°C to kill the Aspergillus niger mycelia and reduce viscosity. The broth is filtered through a rotary vacuum filter (RVF) with diatomaceous earth as filter aid. The mycelial cake (containing oxalic acid co-product) is washed and discarded or used as fertilizer. The clear filtrate contains citric acid, residual sugars, and salts.
Yield: 95–98%
Purity: 60–70%
2
Precipitation
Calcium Citrate Formation
Hot filtrate (70–90°C) is treated with slaked lime (Ca(OH)2) at stoichiometric ratio. Citric acid reacts with calcium hydroxide to form insoluble calcium citrate tetrahydrate: 2 C6H8O7 + 3 Ca(OH)2 → Ca3(C6H5O7)2·4H2O. The precipitate is filtered and washed to remove soluble impurities (sugars, oxalic acid, salts). This step provides exceptional selectivity for citric acid.
Yield: 90–95%
Selectivity: Very High
3
Acidification
Sulfuric Acid Treatment
Washed calcium citrate cake is resuspended in water and treated with dilute sulfuric acid (H2SO4) at 60–70°C: Ca3(C6H5O7)2 + 3 H2SO4 → 2 C6H8O7 + 3 CaSO4. Insoluble calcium sulfate (gypsum) precipitates and is removed by filtration. The filtrate is a crude citric acid solution. Gypsum is washed to maximize citric acid recovery and disposed as waste or used in construction.
Yield: 92–96%
Purity: 85–90%
4
Adsorption
Activated Carbon Treatment
Crude citric acid solution is passed through activated carbon columns or treated batchwise with powdered activated carbon (1–3% w/v). Carbon adsorbs color bodies, residual proteins, and organic impurities. The decolorized solution is filtered and polished through ion exchange resins (optional) to remove trace heavy metals (Fe, Ca) that catalyze degradation during storage.
Yield: 97–99%
Purity: 95–98%
5
Evaporation
Concentration
Purified citric acid solution is concentrated in multi-effect vacuum evaporators from 15–25% to 70–75% w/w. Vacuum operation (60–70°C) prevents thermal degradation and caramelization. Triple-effect evaporators provide energy efficiency (steam economy ~2.5 kg water evaporated per kg steam). The concentrated liquor is supersaturated and ready for crystallization.
Yield: >99%
Concentration: 70–75% w/w
6
Crystallization
Cooling Crystallization
Concentrated citric acid is fed to vacuum crystallizers and cooled from 70°C to 20–25°C over 2–3 days. Citric acid crystallizes as the monohydrate (C6H8O7·H2O) below 36.6°C or anhydrous form above. Seeding and controlled cooling rates produce uniform crystals (200–800 μm). Mother liquor is recycled to the evaporator. Crystals are centrifuged and washed.
Yield: 85–90%
Purity: >99.5%
7
Drying
Fluidized Bed Drying
Wet citric acid crystals (5–10% moisture) are dried in a fluidized bed dryer at 50–60°C with dehumidified air. For anhydrous citric acid, drying temperature is maintained above 36.6°C to convert monohydrate to anhydrous form. Product is screened, graded by particle size, and packaged. Final product meets USP/FCC food-grade specifications.
Yield: >99%
Moisture: <0.5%
Target Molecule: Citric Acid
| Molecular Weight | 192.12 Da (C6H8O7) |
| pKa Values | 3.13, 4.76, 6.40 (triprotic acid) |
| Solubility in Water | 730 g/L at 20°C (highly soluble) |
| Melting Point | 153°C (anhydrous), 100°C (monohydrate) |
| Crystal Forms | Monohydrate (<36.6°C) or anhydrous (>36.6°C) |
View full Citric Acid molecule page →
Cost Considerations
| Step | Key Cost Driver | Relative Cost |
|---|
| Mycelial Removal | Filter aid, RVF maintenance | Low |
| Ca(OH)2 Precipitation | Lime consumption | Medium |
| H2SO4 Acidification | Sulfuric acid, gypsum disposal | Medium |
| Carbon Treatment | Activated carbon replacement | Low |
| Evaporation | Steam energy | High |
| Crystallization | Cooling energy, cycle time | Medium |
| Drying | Hot air energy | Low |
Lime and sulfuric acid are the major chemical costs; evaporation dominates energy costs. Each tonne of citric acid generates ~1.7 tonnes of waste gypsum. Modern alternatives (solvent extraction, electrodialysis, simulated moving bed chromatography) can eliminate the precipitation–acidification loop, reducing waste by 90%. Use
untangle.bio to compare classical vs. modern citric acid recovery routes.
Frequently Asked Questions
Why is calcium citrate precipitation still used despite generating waste gypsum?
The Ca(OH)2 precipitation step provides exceptional selectivity: calcium citrate is nearly insoluble while sugars, oxalic acid, and other organic acids remain in solution. This single step achieves both purification and concentration. Despite generating ~1.7 tonnes of gypsum per tonne of citric acid, the method remains cost-effective at large scale because lime and sulfuric acid are inexpensive commodity chemicals.
What fermentation conditions favor high citric acid titers?
High citric acid production by A. niger requires: (1) excess carbon source (sucrose or glucose at 150–200 g/L), (2) phosphate and manganese limitation (Mn²+ < 2 ppb), (3) low pH (pH 2–3 during production phase), (4) high dissolved oxygen (>25% saturation). Under these conditions, the TCA cycle is redirected toward citrate accumulation. Titers reach 150–200 g/L in 6–10 days.
What is the difference between citric acid monohydrate and anhydrous forms?
Below 36.6°C, citric acid crystallizes as the monohydrate (C6H8O7·H2O, MW 210.14). Above 36.6°C, the anhydrous form crystallizes (MW 192.12). The monohydrate contains 8.58% water of crystallization. Both forms are commercially available; anhydrous is preferred for pharmaceutical applications while monohydrate is common in food. The transition temperature of 36.6°C is critical for crystallizer operation.
Are there alternatives to the classical precipitation process?
Yes. Modern alternatives include: (1) Liquid–liquid extraction with tri-n-butyl phosphate (TBP) or tertiary amines, which eliminates gypsum waste; (2) Electrodialysis with bipolar membranes for direct acid recovery; (3) Simulated moving bed (SMB) chromatography on ion exchange resins; (4) Reactive extraction with supercritical CO2. However, the classical process remains dominant due to proven reliability and low capital cost at >50,000 tonne/year scale.
Design Your Citric Acid Recovery Process
Drag-and-drop the full citric acid downstream train, simulate mass balance, and compare classical vs. modern recovery routes.
Open untangle.bio