Production of sodium hypochlorite by ground water electrolysis

Summary

Possible using groundwater to prepare a disinfecting chemical – sodium hypochlorite – has been considered. The use of natural water containing chlorides provides for eliminating the purchase of table salt and reducing the cost of the finished product by up to 40%. However, due to significant differences in the composition of natural waters, particularly, related to the concentration of chloride ions, the electrolysis of mineralized water can occur in an inefficient zone with high power consumption and low yield of active chlorine. On the territory of the Municipal Enterprise «Azovvodokanal» (Azov, Rostov region) an electrolysis plant is located with a capacity of 500 kg/day as active chlorine. The raw material for the production of sodium hypochlorite is mineralized groundwater. In the course of studies, the optimal conditions for the electrolysis of groundwater were established in order to determine the maximum current efficiency by chlorine with the minimum energy consumption. The studies were carried out using direct electric current in a flow-through mode at a current density of 10, 50, 100, 200, 500, 800 and 1000 A/m², and an initial solution temperature of 18–22°С. The working areas of the anode and cathode were 0.0022 m² each. The range of flow rates studied varied from 0.36 to 4.8 l/h, which corresponded to an electrolysis time of 3.5–0.26 min. The pH value during electrolysis ranged from 8.8 to 9.5. As a result of the studies, dependencies were obtained that characterize the parameters of the direct electrolysis of ground mineralized water.

Key words

water disinfection , membraneless electrolysis , low-concentrated sodium hypochlorite , direct electrolysis of ground water

Direct electrolysis of underground water in production

Summary

The results of production tests of flow-through electrolyzers operating in the mode of direct electrolysis of natural surface water are presented. In the process of electrolysis, sodium hypochlorite for disinfecting water is synthesized from the chlorides present in the source water. Electrolyzers are a part of the water treatment plant. Krinitsa-45 block-modular water treatment plant with a capacity of 45 m³/day has been in operation since 2020. Flow-through electrolyzers are used for the disinfection of raw water (primary disinfection) and drinking water (secondary disinfection) by direct electrolysis; they operate with current reversal in automatic mode. The oxide coating of the electrodes is OIRTA (oxide-iridium-ruthenium-titanium anode) with the addition of tantalum. In the course of studies, the process parameters of the operation of the electrolyzers were determined. The current density was changed from 25 to 75 A/m², the concentration of active chlorine was determined, and the change in the voltage on the electrolyzer terminals was recorded. The measurements were taken every fortnight during past three years. Current density 40–50 A/m² was considered as optimal in terms of efficiency since it provided for 10–12% current efficiency by chlorine and 2–2.8 mg/dm³ active chlorine concentration. Using current reversal (polarity reversal time +1 hour and –1 hour) provided for operating a flow-through electrolyzer with natural water with a total hardness of about 9–11 meq/dm³ eliminating the use of hydrochloric acid for regular washing of the electrodes.

Key words

water disinfection , membraneless electrolysis , low-concentrated sodium hypochlorite , direct electrolysis of surface water

Engineering-economic analysis and study of acid washing of electrolyzer cathodes with solutions of hydrochloric and sulfamic acids

Summary

Experience in operating electrolysis plants shows that the main problem with the reliability of their operation is associated with insoluble compounds – CaCO₃, CaSO₄, Mg (OH)₂ – buildup on the surface of the cathodes that cause abnormal electrolysis mode, overheating and warping of the electrode system, destruction of the oxide coating of the anodes, sudden reduction in the plant efficiency etc. The greatest effect on the liquid salts and gypsum buildup during the electrolysis of salt solutions is produced by the chemical composition of water that is associated with the presence of Ca²⁺, Mg²⁺, HCO^-₃, SO₄ and dissolved CO₂ ions. The method used in practice to remove carbonate deposits by regular washing the electrode system with 3–5% solution of hydrochloric acid is not always economically justified, particularly, for the water sources characterized by increased carbonate hardness. An engineering-economic comparison of two methods of acid washing of electrode plates of an electrolysis plant with solutions of hydrochloric and sulfamic acids while dissolving in them the accumulated buildup resulted from the preparation of sodium hypochlorite from a 3% solution of sodium chloride is presented. With a slightly higher cost of 1 liter of 5% sulfamic acid (6.9 rubles/l) compared to 5% hydrochloric acid (5.36 rubles/l), the cost of one washing with sulfamic acid (2.9 rubles) is 6 times higher than the cost of one washing with hydrochloric acid (0.48 rubles), which is associated with different possibilities for reusing acids for the next washing of the electrolysis plant. In other words, if 5% hydrochloric acid (HCl) can be used 11.1 times before its neutralization, then 5% sulfamic acid (NH₂SO₃H) can be used only 2.4 times, or 11.1/2.4 = 4.6 times less, which is illustrative of the obvious economic preference of hydrochloric acid over sulfamic acid.

Key words

water disinfection , membraneless electrolysis , acid-washing , low-concentrated sodium hypochlorite