ES361138A1 - Process for producing potassium peroxydiphosphate - Google Patents

Abstract

Potassium peroxydiphosphate (K 4 P 2 O 8 ) is produced electrolytically by introducing an aqueous mixture containing potassium, phosphate and fluoride ions into the anode compartment of an electrolytic cell as the anolyte, the anolyte having a phosphate ion (i.e. PO 4 ---, HPO 4 -- or H 2 PO 4 -) concentration of from 1 to 4 molar, a fluoride ion concentration of 0À5 to 1À25 atoms of fluroide ion for each phosphate ion and a potassium ion concentration sufficient to have at least one potassium ion present for each fluoride ion and in addition from 2 to 3 potassium ions present for each phosphate ion in the anolyte solution, introducing an aqueous mixture containing phosphate ions into the cathode compartment of the electrolytic cell as the catholyte, the catholyte and anolyte of the electrolytic cell being contained by means (preferably diaphragm means) which permit potassium and/or phosphate ions to pass freely between the anolyte and catholyte but which prevent any substantial amounts of peroxydiphosphate values in the anolyte from mixing with the catholyte, passing an electric current through the catholyte and anolyte by means of a cathode in the catholyte and an anode in the anolyte, converting phosphate values to peroxydiphosphate values in the anolyte at a pH of 9À7 to 14À5, removing an anolyte enriched in peroxydiphosphate values from the electrolytic cell, and precipitating and recovering potassium peroxydiphosphate values from the anolyte. The anolyte may be prepared by mixing KOH, H 3 PO 4 and HF in an aqueous solution or may be prepared from KH 2 PO 4 , K 2 HPO 4 , K 3 PO 4 , KHF 2 and KF. The catholyte may be prepared from KOH and H 3 PO 4 or from KH 2 PO 4 or K 2 HPO 4 . A diaphragm between the anolyte and catholyte may comprise any porous membrane such as a porous porcelain sheet, asbestos membrane, plastic membrane or a cation exchange membrane. The anode may be of Pt, Au or Ta reinforced Pt. The cathode may be of Pb, graphite, stainless steel, Pt or Au. Cathodes may be in the form of plates, wire screens or spirals made of hollow tubing through which cooling solution can be passed. Specified electrolysing conditions are:- an e.m.f. of at least 2 volts, a temperature of 10-90 C. and an anode current density of 0À1-1 amp/cm2. The pH may be constantly adjusted throughout the electrolysis by the addition of KOH. The process may be operated batchwise or continuously. In a continuous process, anolyte removed from the cell is passed to an evaporator-crystallizer and, after removing K 4 P 2 O 8 crystals, the mother liquor is mixed with used catholyte and returned to the anode compartment. Fresh catholyte is supplied to the cathode compartment. The process may be performed with solutions passing from cell to cell in series. In the Examples a cell comprises a Pt mesh anode, a stainless steel tubular cathode and a porous ceramic plate diaphragm. A tubular glass coil in the anode compartment is connected with the tubular cathode by rubber tubing. Water is passed through the glass, rubber and stainless steel tubing to maintain the cell temperature constant.