Electrochemical modular cell for treating aqueous solutions, device for obtaining products of anodic oxidation of alkaline or alkaline-earth metal chloride solution

RF Patent # 2176989. Filed 01.11.2000, published 20.12.2001.

Bakhir V.M., Zadorozhny Yu.G., Leonov B.I., Panicheva S.A.

The principal component of AQUACHLOR device meant for producing an oxidant mixture from sodium chloride solution is electrochemical reactor.

The given patent for the first time in patent literature describes the reactor of AQUACHLOR device consisting of previously unknown FEM-7 flow-through electrochemical modular cells, each of which is a tiny diaphragm-type electrolyzer with co-axially mounted electrodes and a diaphragm. The outer electrode of FEM-7 module serves as cathode and is made of 35-cm-long titanium pipe 40 mm in diameter. The inner electrode (anode) is made of 29-cm-long titanium pipe 16 mm in diameter. The anode surface is coated with ruthenium and iridium oxides. As compared to routinely used ORTA electrodes, the anodic coating, all other working conditions being equal, has an 800-1500 times longer service life. The anodes in AQUACHLOR device reactor work under conditions very favorable for oxide coating: the рН value of the medium in anodic chamber is always below 2.5. Experience of continuous five-year employment of anodes in AQUACHLOR device reactors demonstrated complete absence of coating wear signs. FEM-7 module’s ceramic diaphragm is made of mixed zirconium, aluminum and yttrium oxides, is exclusively resistant to the action of acids, alkali, oxidants and reductants; its tensile strength is up to 5 atmospheres and it has an unlimited service life. No diaphragm cleansing is required in the process of AQUACHLOR device operation, if softened saline solution is supplied to the devices. When used saline solution contains hardness salts, it is necessary to periodically clean the diaphragm with a 3% hydrochloric acid solution. The cleaning is performed by rinsing AQUACHLOR device reactor without its disassembly for 15 minutes.

Unlike all other well-known processes of chlorine production from sodium chloride solution (diaphragm electrolysis, electrolysis with ion-exchange membrane, electrolysis with mercury cathode), in FEM-7 module of AQUACHLOR devices the process of sodium chloride decomposition into end products – a gaseous mixture of oxidants, sodium hydroxide solution of 150-170 g/l concentration (depending on initial saline solution concentration) and hydrogen – occurs during one cycle, i.е. without any return of anodic or cathodic products for repeated treatment into the reactor, and adding no water into the cathodic chamber. In other words, FEM-7 modules realize decomposition technology, the gist of which is that the entire volume of salt solution of 200-250 g/l concentration that enters the anodic chamber is completely (during one cycle) decomposed into humid gas (chlorine, chlorine dioxide, ozone), and in the cathodic chamber, with no extra water added, also during one cycle, sodium hydroxide solution with 150-170 g/l concentration (approximately, in the volume of incoming saline solution) and gaseous humid hydrogen are formed.

The main technological characteristic of AQUACHLOR device is that synthesis of oxidants in the anodic chamber occurs under pressure considerably exceeding the pressure in the cathodic chamber (drop of about 1 kgf/cm2).

Superposition and interaction of pressure gradients, electric field voltage, electrolyte concentration and density of current in porous space of relatively thick-walled ceramic diaphragm ensures removal of selective sodium ions together with excessive water from anodic chamber through the diaphragm into cathodic chamber; preservation of all chlorine ions in anodic chamber; and complete impossibility of hydroxyl anions penetrating from cathodic to anodic chamber.

AQUACHLOR device can be easily adapted to a dc power supply of required wattage with practically any output current and voltage parameters, since the reactor’s design provides for a possibility of changing the electric circuit of FEM-7 connection. When FEM-7 modules are consecutively connected, the device’s reactor turns into a bipolar electrolyzer of original design, as electrochemical cells (FEM-7 modules) it consists of are spatially and galvanically separated from each other. When FEM-7 modules are connected in parallel, the reactor becomes a sort of monopolar electrolyzer. Mixed (consecutive-parallel) connection of FEM-7 modules in the reactor of AQUACHLOR device is also possible.

The voltage on a single FEM-7 module in the process of work can be from 2.8 to 4.0 V, at current strength from 20 to 30 A, and initial saline solution mineralization varying from 200 to 250-300 g/l. The degree of salt decomposition is 99.3-99.8%. Therefore, actual specific consumption of salt for oxidant production in AQUACHLOR device is approximately 1.7 g per 1 g of oxidants.

At current strength of 23-24 A, a FEM-7 module produces 30 g/h of oxidants (as molecular chlorine), voltage on it approaching 2.8-2.9 V, and specific energy consumption being about 2 kW per 1 kg of oxidants (chlorine). In this regime the share of molecular chlorine in liberated gas is 98 - 99%.

To increase the share of chlorine dioxide and ozone in a mixture of synthesized oxidants to 3-7%, AQUACHLOR devices are used in more high-power conditions, that is, at current strength over 25 A on a single FEM-7 module, and voltage above 3 V. Specific energy consumption is, respectively, between 2.5 and 3.0, and can achieve 3.5 kW-h/kg in conditions of correspondingly increasing output of a single FEM-3 module to 40 and more grams of oxidants per hour. However, specific salt consumption remains the same – about 1.7 g/g in all regimes. Another way of increasing chlorine dioxide and ozone content is alkalization of initial saline solution up to рН = 9.5-10.5.

Three-year experience of AQUACHLOR devices operation in various conditions (climatic, technical, technological) in Russia and abroad demonstrated their high efficiency, cost-effectiveness, and practically complete absence of chlorination by-products even during treatment with oxidant solution of wastewaters with high organic compound content.