For manufacturing boric acid, borax is charged into an acidifier and dilute sulfuric acid is added slowly until the solution becomes strongly acidic. It is produced by acidifying a saturated solution of borax or orthoborate materials. The following reaction take place:-
Na2B4O7.10H2O + H2SO4 –H3P2O3+No2SO4+5H2O
Borax in hot water with concentrated sulfuric acid is mixed in a lead lined vessel. The hot solution is vacuum crystallized and is cooled. First crop comes of boric acid and on further cooling sodium sulphate.
The crude boric acid is purified by re crystallization from hot water to give the final pure boric acid. Boron is an essential micronutrient for healthy growth of plants. It can be harmful to boron sensitive plants in higher quantities, and care should be taken to minimize the amount of boric acid released to the environment to avoid ecological effects by adopting clean technology or closed system which avoids leakage of the raw material and finished product as well.
Reaction is carried out in a 4-stage cascade reaction system. Total retention time is 3–3.5 h in order to eliminate supersaturation of gypsum (2CaSO4·2H2O) and to avoid its precipitation during boric acid crystallization. Inert solids consist of side minerals in the colemanite ore (mostly clay and feldspate minerals) remain unreacted in the reaction. Following to the reaction step, precipitated gypsum and inert solids are filtered. In order to recover boric acid from the wet cake (stream 5), it is mixed with stream 9 and subjected to second filtration. Cake obtained in the second filtration is washed and the filtrate is recycled to the reaction step. The production liquor (stream 4) (boric acid content is approximately 18%) is crystallized in a 3-stage crystallization unit (in adiabatic vacuum crystallizers) where the final temperature is 35 °C. Crystal composition of stream 7 is enriched in a hydrocyclone up to 40% and it is filtered and washed. Consequently, moisture content is reduced approximately to 7% in a centrifuge. Stream 10 with 7% moisture content is dried in a fluidized bed drier until the crystals get bone dry. Fine crystals escaped from the fluidized bed dryer with air are recovered with a cyclone and gas washer, respectively. The solution taken from the gas washer is mixed with stream 13 and recycled to the reaction unit as mother liquor which involves approximately 10% boric acid.
Alternative processes flow sheet
Sulfuric acid production process
Phosphoric acid production process
Flow sheet of citric acid