Qiang Wang1

1, Worcester Polytechnic Institute, Worcester, Massachusetts, United States

Antimony is a widely used element in human’s daily life. For example, Sb2O3 plays role as flame retardant in plastics, coatings, and electronics; functioning as a hardener, 2wt % to 7wt % of antimony metal is added into lead metal to fabricate electrode for lead-acid battery; and antimony can work as catalyst for synthesizing chemicals. In 2016, antimony mine production in the word was 130,000 t, and China, the leading producer, accounted for about 76.9 % of the production. The strong dependence of antimony industry on the import of Chinese antimony drives USA and Europe to seek solution of reclaiming and recovering antimony from secondary resource. Herein, an innovative method composed of electrochemical and chemical reactions is presented to purify antimony from lead alloy, which is treated as anode in an “H” shape electrolytic cell. Stainless steel sheet works as cathode, and potassium hydroxide solution is applied as electrolyte. O2 is purged into the cathode at the beginning, and the electrolytic cell works in the principle of metal-O2 battery, when voltage is applied on the two electrodes. Lead, antimony, and other metals dissolve into the electrolyte as ions at the first stage. When the ions inside the electrolyte accumulated to a certain amount, O2 is switched off without interference of the electrolysis. At this second stage, lead alloy of the anode dissolve into electrolyte successively, but lead metal deposits on cathode side simultaneously. Lead ions concentration stops increasing, and antimony ions concentration keeps on rising. When antimony concentration is high enough, the electrolysis is stopped, and Na2S is added into the electrolyte to precipitate lead ions in the stage 3. Almost all the lead can be removed from electrolyte, leaving antimony inside, by taking advantage of the low value of Ksp (PbS). At the last step, antimony can be extracted from the electrolyte at low potential of -1.25 V (vs Hg/HgO), leaving other impurity, such as As, Sn, Al inside with low concentration under 50 ppm. The electrolyte can be re-used for the next circle. This flow sheet can purity lead and antimony metals from lead alloy, with lead purity of 99.2 wt% and antimony purity of 92 wt%, and antimony recovery efficiency is ~ 82%. Because the electrolyte can be used repeatedly, no waste is generated in the whole process.
Stage 1. Cathode: O2 + 4e- + 2H2O → 4OH- (1)
Anode: Pb + 4OH- - 2e- → PbO22- + 2H2O (the main oxidation reaction) (2)
Sb + 4OH- - 3e- → SbO2- + 2H2O (3)
Stage 2. Cathode: PbO22- + 2H2O + 2e- → Pb + 4OH- (4)
Anode: Pb + 4OH- - 2e- → PbO22- + 2H2O (the main oxidation reaction) (5)
Sb + 4OH- - 3e- → SbO2- + 2H2O (6)
Stage 3. Pb2+ + S2- → PbS↓ (7)
Stage 4. Cathode: 4OH- - 4e-→ O2 + 2H2O (8)
Anode: SbO2- + 2H2O + 3e- → Sb + 4OH- (9)