Citric acid can be extracted from the juice of citrus fruits, by fermentation of glucose with the aid of the mold Aspergillus niger and synthetically from acetone or glycerol.
Today, essentially all of the commercial citric acid is produced by fermentation. Processes employed are surface or submerged fermentation by mold (Aspergillus niger) and submerged fermentation by yeast (Candida guilliermondii, C. lipolytica), using a variety of substrates including sucrose, molasses, corn syrup, enzyme-treated starch, and normal paraffin.
In this process, the cultures of Aspergillus niger are fed on molasses to produce citric acid. Molasses containing 54-60 percent sugar is taken into mixer and sulphuric acid is added to adjust the PH between 5-6. Phosphorous, potassium and nitrogen in the form of acids or salts are added as nutrients for proper mold. Then mixture is sterilized with live steam and finally diluted with water. Now take this medium to shallow aluminium trays which are arranged in tiersin sterile fermentation chambers.
After cooling the fermentation medium to about 30°C, it is inoculated with spores of organisms. As time passes more and more acid is produced and PH drops to 2 and acid content varies from 10-12% at the end of fermentation and the temperature is maintained at 28-30 °C. Purified air is passed in the fermentation chamber.
Chamber and trays are thoroughly sterilized after completion of each run. Run crude liquor into settling tanks and allow waste mould to drain. Send liquor to precipitating tank and heat up to 90°C. Remove oxalic acid by preferential precipitation by hydrated lime.
Now add one part of lime in two parts liquor and raise the temperature to 95°C. Precipitated calcium citrate is filtered on a vacuum filter and filtrate is drained. Acidulate the cake with H2SO4 in decomposition tank. New filter the slurry for giving cakes of calcium sulphate. Concentrate the solution in vacuum evaporator then send to crystallizer where it is cooled to give crystals of monohydrate citric acid.
Citric acid is a weak acid which has no serious harmful effect on environment. Liquid waste to be generated will be contained by a concrete vessel to be constructed and neutralized by using soda ash to a PH of 6-9 and disposed.
Boric acid production process
Sulfuric acid production process
Phosphoric acid production process