TREATMENT OF AQUEOUS SOLUTION CONTAINING BOROFLUORIDE ION

PURPOSE:To reduce the production of sludge considerably by controlling the pH of an aq. soln. contg. borofluoride ions with potassium hydroxide and subjecting the same to a reverse osmosis treatment. CONSTITUTION:For example, in a treatment of waste liquid of a borofluoride plating both, the waste plating soln. contg. BF4 (about 2-2.7pH) of an Sn-Pb alloy plating bath is introduced into a pH controlling tank 1. Here, an aq. KOH soln. is added, by which pH of the waste liquid is controlled to >=4.0. At this time, Sn, Pb settle as hydroxide and BF4 converts to KBF4 and part thereof settles when it attains a satd. state. The supernatant in the tank is transferred into a storage tank 2, after which it is treated in a reverse osmosis device 3. The resultant permeated water is discharged to the outside of the system and the concd. liquid is returned to the tank 2. If such operation is carried out continuously, the KBF4 in the tank 2 increases its concn. until it attains a satd. state, when it crystallizes and settles. The reason for using KOH lies in that it has low solubility in water of KBF4 and that it is possible to execute the reverse osmosis treatment in that low concn. state with eventually high treatment efficiency.