Comparison of the differences between different extraction processes for citric acid

Mature citric acid fermentation mash contains, in addition to the main product citric acid, insoluble solids such as mycelium, cellulose and raw material residues; miscellaneous acids such as oxalic acid, isocitric acid and gluconic acid; soluble compounds such as polysaccharides, non-fermentable sugars, protein colloidal substances, pigments, minerals and other metabolites. They are either derived from fermentation raw materials or produced during the fermentation process, and they are either dissolved or suspended in the fermentation mash.

To prepare pure citric acid, these impurities need to be removed, which usually involves four steps: ① removal of mycelium and other solids to obtain a filtrate; ② preliminary purification of citric acid solution from the filtrate by calcium salt method or chromatography; ③ preliminary purification of citric acid solution using ion exchange to remove ions, and then decolourisation to obtain refined citric acid solution; ④ refined citric acid solution after evaporation, crystallisation and drying to obtain finished citric acid.

Among them, the method of extracting citric acid from filtrate is the core step of the whole citric acid extraction process, which determines the flow of extraction process, equipment input, yield of finished product, material consumption, energy consumption and product quality. The reported methods of citric acid extraction mainly include calcium salt method, direct extraction method, solvent extraction method, ion exchange method and chromatography method, etc.

1、Calcium salt method extraction process

At present, the purification process is basically the calcium salt precipitation method, including the following steps: the fermentation solution is filtered to remove the bacterium → adding calcium carbonate to generate calcium salt (CO2 discharge) → calcium salt filtration and separation (fermentation wastewater discharge) → decomposition of calcium salt with sulfuric acid to generate citric acid and calcium sulfate → gypsum filtration and separation (gypsum discharge) → cation exchange to remove impurities → anion exchange to remove impurities → activated carbon decolourisation → concentration, crystallisation → citric acid product. Citric acid product.

This process has been in use for decades, since the industrialisation of citric acid production. Although it has been improved many times, its process route is long and costly, requiring the consumption of a large amount of sulfuric acid and calcium carbonate (stone powder), and generating a large amount of waste gas, waste water, waste residue, becoming a heavy burden for enterprises, "three waste" treatment costs account for 10% to 15% of the production costs, has become a problem plaguing the development of the citric acid industry.

2、Solvent extraction method extraction process

Compared with other methods, solvent extraction method has good separation effect and can be operated continuously, while the extractant can be recycled for the extraction process after regeneration, reducing production operation costs, not producing waste acid, waste, easy to reverse extraction and other characteristics. Therefore, solvent extraction technology has a wide range of applications, such as extraction of petroleum hydrocarbons; extraction of metals in wet metallurgy, especially the extraction of rare metals; treatment of wastewater and other areas.

Citric acid is Lewis acid, which can form complexes with extractants with Lewis base groups, so that citric acid can be transferred from the aqueous phase to the organic phase and complete the chemical complexation extraction process, so that it can be separated from sugar, protein and other impurities in the fermentation broth to achieve the purpose of selective separation of citric acid.

Here is the method of extracting and separating citric acid. The extraction method is simple and effective, and at the same time can achieve environmental protection separation and purification of citric acid. Its process flow is mainly as follows.

(1) Preparation of extractant. n-Dodecyl tetradecylamine and methyl isobutyl ketone are mixed to prepare the extraction solvent, and sec-octanol is added in proportion to obtain the extraction solvent of citric acid.

(2) Extraction. The organic phase of the prepared extractant was used to carry out counter-current extraction with the citric acid feed solution. The CWL-M series centrifugal extractor could be selected as the core extraction equipment.

(3) Reverse extraction. The organic phase of loaded citric acid and water were mixed and separated rapidly in CWL-M series centrifugal extractor in proportion to each other. 78.63% of the primary reverse extraction was achieved, and the yield of the reverse extraction solution was 99.92% after counter-current reverse extraction.

The above citric acid purification and extraction process is simple and effective, and at the same time can achieve environmentally friendly separation and purification of citric acid.