Table 2: Electrodialysis (ED) studies of industrial phosphoric purification (54% P2O5 or 74.5% H3PO4).

ED process characteristicsEnergyReference

Phosphoric acid was concentrated using a combination of apatite Ca5(PO4)3(F, Cl, OH) digestion by sulfuric acid and aluminum production from alunite KAl3(SO4)2(OH)6. The catholyte (H3PO4) was separated from the middle compartment (K2SO4) by a CEM, and the anolyte (water) was separated by AEM. The products were KH2PO4 (catholyte), a diluted solution of K2SO4, and H2SO4 (anolyte)A cell voltage of 6 V and a current of 4.5 A were used. The current efficiency was approximately 96%. Electric power consumption was 1.27 KWh/Kg of P2O5 at 2.5 weight % P2O5[19]

This method concentrated phosphoric acid by ED using membranes typically used to desalinate water. The stack assembly consisted of 20 cell pairs sandwiched between two electrode compartments. A CR-61 cation-transfer membrane and an AR-I03 anion-transfer membrane were used to concentrate the acid from 0.1 to 1.0 M by electrodialysis. The diluted compartment was fed with 0.10 M, 0.20 M, 0.29 M, and 0.45 M reagent-grade acid solutions and the compartment concentrate had 1 M of industrial-grade acidThe total effective membrane area was 2300 cm2. The electrical currents applied were 8, 10, 12, and 14 A. The voltages varied from 10 to 43 V. This method concentrated reagent-grade H3PO4 from 0.1 to 1.0 M. The energy requirements ranged from 1.73 to 2.50 kWh/Kg of P2O5[20]

A cell (13.3 cm2) was divided into two compartments by an AEM. The cathode was graphite and the anode was platinized titanium. The volume of the catholyte (4.11 M industrial-grade acid, supplemented by ammonium ions) was 100 cm3 and the volume of the anolyte (pure 1 M H3PO4) was 1000 cm3. The impurities were retained in the catholyte compartment. In the anolyte compartment, H3PO4 was formed with the anions crossing through the AEM and protons produced at the anode. AEM ARA 17/10 from Solvay (France) and RAI 5035 from Raipore (USA) were usedA constant current of 60 Am−2 was applied for 90 min. The acid concentration changed from 1 M to 4.11 M with a 63% of yield. A 4 M acid solution was obtained for a current efficiency of 74% in the anolyte compartment. Mg, Al, and Fe were removed at 95–100%. The efficiency was limited by the migration of protons across the anion membrane (proton leakage)[21]

Cell with two compartments was divided by a membrane. The catholyte was industrial phosphoric acid 10 to 55% percent P2O5 and anolyte was 1 to 40% P2O5The electrical current density ranges from 100 to 3000 (A/m2)[22]

An electrodeionization cell (36 cm2) with a central compartment (industrial 11% P2O5 or 15.2% H3PO4) was delimited by AEM/textile membrane and CEM/textile membrane. Catholyte and anolyte were 2 M H3PO4 solutions placed in the other compartments Very low current efficiencies for both cations and anions were observed after 5 h of treatment. This method produced 11% phosphoric acid with low concentrations of impurities: Mg (1.4 gdm−3), Fe (0.9 gdm−3), Zn (0.4 gdm−3), Cd (0.01 gdm−3), and (16.5 gdm−3). The highest current efficiency for cations was found for Mg2+[23]

Cell with liquid membrane of amyl alcohol with the addition of a trialkylamine. The catholyte was H3PO4 1.0 M and anolyte was H2O. The cathode was made of stainless steel and anode was made of platinum or titanium coated with ruthenium dioxideExtraction of phosphate ions from the starting solution was 95%[24]

The raw material was H3PO4 containing mainly calcium phosphate. A low concentration of acid was fed into alternate partition chambers formed cation transfer membranes. H3PO4 was introduced on the anode sides of the partition chambers. Process is carried out under acidic conditions, avoiding precipitates of the impurities such as iron and aluminumNo data[25]

Cells with a large number of anion and cation membranes were alternately arranged. Industrial phosphoric acid and diluted pure phosphoric acid (10–44% P2O5) were supplied to the anode and to the cathode compartments, respectivelyThe current density was 100–2000 A/m2. The energy requirement ranged from 3.95 to 15.78 kWh/Kg of P2O5[26]

Electro-electrodialysis with laboratory-scale cell. The cathode was graphite and the anode was platinized titanium. There were three compartments divided by anionic and cationic membranes. The industrial H3PO4 was introduced into the central compartmentThe cell voltage was 4 V and current of 10 A. A 28% H3PO4 was removed from central compartment and impurities decrease. Also, Food-grade H3PO4 at 25% was produced in anolyte after 21 h at 3-4 V and 11 A[27]

AEM: anion-exchange membranes; CEM: cation-exchange membranes.