How to prepare citric acid monohydrate

1. Juice extraction method

This method is to squeeze out the juice from unripe lemons, tangerines and other fruits containing citric acid, then mix the juice with lime milk to generate calcium citrate, and then treat it with sulfuric acid to obtain a purer citric acid solution, and then The product is obtained after concentration and crystallization. Since the Swedish chemist Scheele first extracted citric acid from lemon juice and crystallized solid citric acid in 1784, the natural juice extraction method of citric acid became the main method of early citric acid production, which lasted for about 50 years. The yield of citric acid produced by this method is low. It was not until the large-scale promotion and application of microbial fermentation in the mid-20th century that the method of extracting citric acid from natural fruit juice was gradually eliminated.

2. Chemical synthesis method

From 1880, when citric acid was first synthesized in the laboratory, there have been many different technical routes and methods. Among them, using acetone as raw material to synthesize citric acid is a common synthesis method. After acetone is hydrated, reacts with hydrocyanic acid, and then hydrolyzed, citric acid can be obtained. In addition, there are methods such as synthesis using quinic acid methyl ester aqueous solution and reaction synthesis between formaldehyde and isobutylene. However, the above-mentioned synthesis methods are complex and most of the chemicals used are toxic, and industrial production has not been achieved so far.

3. Microbial fermentation method

1. Culture of bacteria

Microbial fermentation is used in the industrial production of citric acid, and only a few Aspergillus and yeast species are valuable. Among them, Aspergillus niger is the most competitive strain in the industry, and the most competitive among yeasts is Pseudomonas lipolytica. Saccharomyces cerevisiae and Trichoderma spp. Aspergillus niger is cultured on agar and forms limited colonies on agar. After culturing at room temperature for 10 to 14 days, it becomes rich and dense sporophores. The colonies are black, sometimes dark brown and black. Considering that citric acid-producing bacteria should have strong acid-producing ability and high resistance to citric acid concentration, the acid filter paper method, color change circle method and single spore transplantation method can be used to isolate Aspergillus niger to avoid interference from other bacteria and make it Become Aspergillus niger for the production of citric acid. Yeast culture There are two types of yeast that can be used for citric acid production: Candida lipolytica and Candida quaymondo. The former has a strong ability to decompose fat, and a better carbon source is n-alkane. The latter can produce citric acid by fermenting alkanes or fermenting sugars. The acidity pH value of yeast fermentation is 3.5~4.0.

2. Fermentation

The fermentation process is divided into surface fermentation and solid fermentation. Culture media of different raw materials are prepared according to different processes, and then steamed. The purpose of steaming the material is to gelatinize the starch and sterilize it. When steaming the ingredients, the ingredients should be heated evenly and the steam should be smooth. Add the ingredients while steaming. Add the ingredients to the steaming place and add them layer by layer. The steamed materials should be spread and cooled. When the temperature drops below 37°C, water can be replenished and inoculated, plated for fermentation. The end point of fermentation is determined by acidity. The acidity should be measured regularly to ensure that the material is discharged when the acidity is highest to avoid citric acid being absorbed. Bacterial decomposition.

3. Extraction

Fermentation acid - filtration (removal of bacteria and residues) - neutralization, filtration - calcium citrate salt - acidolysis, filtration - crude acid solution - purification - concentration - crystallization - centrifugation - drying - packaging - finished product. After the fermentation is completed, the fermented aldehydes must be treated. For surface fermentation, the caps and fermentation liquid should be separated immediately, then the caps and shallow plates should be washed with a small amount of water, and the fermentation liquid and washing water should be combined; the citric acid in the solid fermentation solution should be leached with water, the water temperature should be 80°C, and leached 2 to 3 times , soaked in water and merged. The fermented acid is filtered with a filter press, and the filtrate and washing water are combined and pumped into the filtrate tank. Citric acid reacts with calcium salt and calcium alkali to form calcium citrate, which precipitates from the liquid phase and is separated from soluble impurities. If the acid solution contains a lot of oxalic acid, it can precipitate out in a hot neutralizing solution below a pH value of 3, so that the oxalate can be separated first. The neutralization end point is tested with precision test paper and the pH value is maintained at 6.0~6.8. Stir at around 85°C for 30 minutes to fully precipitate calcium sulfate and filter. Calcium citrate is acidolyzed with sulfuric acid. The amount of sulfuric acid is determined according to the citric acid content in the solution. Generally, the excess sulfuric acid does not exceed 0.2%. After acidolysis, the acid solution is filtered and the citric acid solution is purified to remove pigments, colloids and metal cations such as iron ions, calcium ions, copper ions, magnesium ions and anionic impurities such as sulfate ions in the solution through adsorption decolorization and ion exchange. Purification is mostly carried out on columns. The decolorizing carbon is GH-15 granular carbon, and the ionic resin is anion and cation resin. The concentration of citric acid purification solution is only 20%~25%, and crystallization can only occur when it is concentrated to more than 70%. The temperature during concentration should not be too high to avoid decomposition of citric acid. The concentration of purification liquid can be carried out under negative pressure. In order to save energy, a double-effect or triple-effect evaporator can be used. The concentration is carried out in two stages. After the first concentration, it is placed in a settling tank for heat preservation and sedimentation, and then most of the gypsum is removed. The second concentration contains about 80% citric acid and is released for crystallization in time. For the second concentration, a lift-type or bracket-type evaporator can be used to reduce the contact time between the feed liquid and the heat medium and improve product quality. Different crystallization methods can produce different products. For the crystallization of citric acid monohydrate, 80% solution is stirred in a crystallizer at a temperature of 55°C and then cooled. When the temperature drops to 0°C, seed crystals are added to start crystallization. Control the temperature not to exceed 36°C. At this time, the product is citric acid monohydrate; if the solution is concentrated to 83% at 60°C, cool it to 46°C and add seed crystals. Maintain the temperature at 40~60°C and slowly crystallize until it finally drops to 38℃, the product is anhydrous citric acid. The crystal paste is separated and centrifuged to obtain crystalline commercial citric acid.