Field Test of Platinized Titanium Anodes for Hypochlorite Cells

Abstract

The demand for chlorine for water and waste treatment is increasing because of the increase of water consumption, a large amount of waste water discharge, and governmental regulations. Transportation and handling of chlorine cylinders and containers are strictly controlled to avoid hazard. As a result, on-site electrolytic production of hypochlorite becomes important for disinfection of drinking water, oxidation of sewage, chlorination of cooling water in process plants, and other uses. There are now a number of publications and patents on hypochlorite cells and the electrodes to be used (1-13).A hypochlorite cell must be simple in operation with minimum maintenance for a year or more. The energy consumption is also an important factor.In an electric power station located by the sea, saline water containing some 3% NaC1 is fed to the hypochlorite cell and is chlorinated prior to being sent to the heat exchangers in the plant. Because of the low salt concentration, oxygen evolution occurs, and this reduces chlorine current efficiency. Also, sea water contains Mg and Ca ions, which deposit at the cathode, resulting in high cell voltage. Periodic acid cleaning removes the scale, but affects the electrodes, especially the anode coating, and in some cases the electrode activity is not restored.Pretreatment of sea water prior to electrolysis is preferable, but it is expensive and complicated. Therefore, durable anodes having such characteristics as low chlorine overvoltage and high oxygen overvoltage in sea water must be developed to improve the operating performance of hypochlorite cells. Experimental ProcedureTwo types of the platinized Ti anodes were used as controls. One was Pt-plated Ti sheet prepared by conventional electroplating (hereafter referred to as "regular Pt/Ti"). The other was also a Pt-plated Ti sheet and had a large surface area ("modified Pt/Ti") (14). The average ~hickness of the Pt coating was 3 ~m.The modified Pt/Ti sheet was painted with a solution containing a metal chloride, such as RuC13 or IrCl.~, depending on the oxide catalyst proposed, dried in air, and then fired at 500~ for few minutes to deposit the oxide on the Pt layer (MODE| In this work, the Ir oxideloaded material was used mostly.Full-scale experiments were conducted in an electric power plant located at Tokyo Bay. The flowsheet is illustrated in Fig. 1, Eight cells were operated in series and parallel connections; the sea water was fed to the top end of the cell series, electrolyzed, and mixed together at the outlet. The chlorinated sea water was sent to the heat exchangers in the plant. Each hypochlorite cell employed was equipped with five anodes and six cathodes (carbon steel plate) in parallel. There were no separators. The design capacity of the cell is as follows. The effective area of * Electrochemical Society Active Member. electrode was 80 cm wide and 45 cm high each. The anode-to-cathode gap was 5 mm. The current density was 15 A/din 2 at 5.4 kA total current load. The cell voltage was 5.5 z 0.5V. The solution flow rate...