EP0368767B1 - Continuous process for manufacturing alkali metal perchlorate - Google Patents

Abstract

Continuous manufacture of alkali metal perchlorate by electrolysis of an aqueous solution of chlorate of the said metal in a single electrolytic stage with a uniform electrolyte of stationary composition, characterised in that the said composition is that of an aqueous solution of perchlorate from which the latter is capable of being isolated directly by crystallisation, which is maintained thus by continuously introducing chlorate and water simultaneously into the electrolysis stage, each being in a quantity equal to that of the chlorate and of the water respectively, which, in this form or in combined form, leave the said stage continuously and definitively.

Description

The present invention relates to a continuous process for the manufacture of sodium perchlorate by electrolysis of an aqueous chlorate solution of said metal.

In what follows, unless indicated or indicated, sodium chlorate and perchlorate of said metal will be designated by chlorate and perchlorate respectively.

The advantage of operating continuously is mentioned, for example, in French patent No. 1402590. This patent and, for example, United States patents No. 3518173, No. 3518180, No. 3475301 and English patent No. 125608, illustrate the known technique.

This consists of electrolyzing the chlorate in a succession of individual electrolytic steps, each step being different from another, dependent on another, and ensuring only a partial electrolysis result compared to the final industrial result. targeted.

This is in fact how an aqueous solution of perchlorate leaving the chlorate electrolysis is produced until today, which is such that the perchlorate can be separated therefrom directly by crystallization, for example by cooling or by evaporation. of water.

Indeed, it was known that an electrolysis of chlorate in a single step did not lead to such a solution under the practical conditions for carrying out the single step described for example in US Patent No. 2512973.

Operating successively in a large number of individual steps is however recommended, for example in the patent of the United States of America No. 3475301 already cited.

In a multi-step process, commonly called "cascade", the overall electrolyte balance is disturbed by the electrolyte imbalance of a single step and does not recover by simply decommissioning the failed step.

A continuous process has now been found in a single electrolytic step which does not have the above drawback and which delivers a perchlorate solution leading directly by crystallization to solid perchlorate of high degree of purity.

• étape électrolytique ou étape d'électrolyse, l'ensemble formé de l'électrolyse et de ce qui en sort pour y entrer à nouveau.
• électrolyte, le liquide auquel sont appliquées dans l'électrolyse les conditions électriques permettant de transformer du chlorate en perchlorate, et qui contient ces deux composés à l'état dissous,
• solution de perchlorate de laquelle celui-ci est isolable directement par cristallisation : solution qui dépose, par évaporation d'eau ou par refroidissement, le perchlorate solide sous forme monohydrate, dihydrate ou anhydre ; on peut se reporter en la matière à L'ouvrage publié sous la direction de Paul PASCAL, Nouveau traité de Chimie Minérale, 1966, Tome II, fascicule 1, p.353 et figure 37 qui montre le diagramme ternaire NaClO₄-NaClO₃-H₂O.

In everything that precedes and in everything that follows, we mean by:

• electrolytic step or electrolysis step, the assembly formed by electrolysis and what comes out of it to re-enter it.
• electrolyte, the liquid to which the electrical conditions used to transform chlorate into perchlorate are applied in electrolysis, and which contains these two compounds in the dissolved state,
• perchlorate solution from which the latter can be isolated directly by crystallization: solution which deposits, by evaporation of water or by cooling, the solid perchlorate in monohydrate, dihydrate or anhydrous form; one can refer on the matter to the work published under the direction of Paul PASCAL, New treaty of Mineral Chemistry, 1966, Volume II, fascicule 1, p.353 and figure 37 which shows the ternary diagram NaClO ₄ -NaClO ₃ - H ₂ O.

The present invention consists of a continuous process for the manufacture of perchlorate by continuous electrolysis in a single electrolytic step of an electrolyte, this electrolyte being kept uniform with a stationary composition by continuously bringing chlorate and water into this step. , each in an amount equal to that which disappears from this step by electrolytic transformation and by a continuous and final exit from this step of an electrolyte current forming a sample having the same stationary composition, said composition being chosen such that the sample can be deposited directly by cooling perchlorate crystals in monohydrate, dihydrate or anhydrous form, followed from said sample by crystallization of sodium perchlorate by cooling or evaporation of water.

• électrolyte uniforme, un électrolyte qui est le même en tout point de l'espace qu'il occupe, pour ce qui concerne en particulier sa composition, son pH, sa température,
• composition stationnaire, une composition stable, constante, dans le temps.

In the definition thus given of the invention, as in all that follows, by:

• uniform electrolyte, an electrolyte which is the same at every point in the space it occupies, as regards in particular its composition, its pH, its temperature,
• stationary composition, a stable, constant composition over time.

The electrolyte is uniform thanks to its agitation due for example to the evolution of gas in the electrolysis, associated if necessary with recirculation external to the latter, for example using a pump.

The electrolyte, the composition of which, according to the invention, is the same as that of the aqueous perchlorate solution which leaves the single electrolysis step, contains, in the case of the electrolysis of sodium chlorate into perchlorate sodium, preferably at least 100 g. of chlorate per liter to obtain a FARADAY yield exceeding 90%.

Maintaining the chlorate and perchlorate concentration of the electrolyte, respectively, at a constant value over time, makes it possible to avoid an increase in the voltage across the electrodes.

The energy consumption per ton of perchlorate ultimately produced is lower than that conceded by operating according to known methods.

The electrolysis is carried out in known equipment, such as for example a non-compartmentalized cell with monopolar electrodes, anode based on platinum, such as for example a sheet of solid platinum or platinum deposited on a conductive substrate, cathode for example of mild steel. or bronze.

The electrical conditions adopted are those allowing the transformation of chlorate into perchlorate, for example, for sodium perchlorate, an anodic current density ranging for example between about 10 and 70 A / dm ² and often of the order of 40 A / dm ² .

The pH of the electrolyte can be between fairly distant limits, for example between about 6 to 10. It is reached using for example perchloric acid or an alkali metal hydroxide such as hydroxide. sodium in the case of electrolysis of sodium chlorate.

Water which enters the single electrolysis step with, for example, the above compounds or with other possible ingredients of the electrolyte such as sodium dichromate, most often used at a rate of approximately 1 g . at 5 g. per liter of electrolyte in the case of the electrolysis of sodium chlorate, must be taken into account in the conduct of the process according to the invention.

The same applies, where appropriate, to the water supplied to the single electrolysis stage which originates from the crystallization of the aqueous solution as it leaves said stage: condensate of water evaporated from said solution, water -mothers and washing water of the solid perchlorate produced.

The temperature of the electrolyte is generally between approximately 40 ° C and 90 ° C. Heat exchange means which can be both internal and external to the electrolyte make it possible to maintain it at the chosen value.

The simultaneous and continuous addition of the chlorate and of the water which enter the single electrolysis stage can be carried out by bringing into this stage an aqueous chlorate solution containing all the chlorate and all the water necessary for the invention. . The concentration of this chlorate solution can be very high, for example 900 g. sodium chlorate per liter, forming the solution at a temperature which is itself high, for example 80 ° C.

Relative amounts of chlorate and water such as those indicated for example above can also be achieved by adding chlorate and water separately, the chlorate being used in solid form. In this case, the external recirculation current in the single electrolysis step can serve as a vector for the chlorate.

Part of it can be supplied in the solid state and the additional part supplied in the form of an aqueous solution, for example in the form of a solution containing 700 g. of chlorate per liter, formed at 20 ° C.

The method according to the invention makes it possible to retain the advantage relating to a reduced consumption of platinum found in US Patent No. 3,475,301.

The perchlorate which constitutes the ultimately targeted production is separated in practically pure solid form directly by crystallization from the aqueous perchlorate solution as it leaves the single electrolysis step according to the invention. In the case of the manufacture of perchlorate sodium, the product particularly targeted by the industry is sodium perchlorate monohydrate rather than anhydrous perchlorate or that perchlorate dihydrate, the manufacture of which, depending on the composition of the electrolyte required, is also feasible according to the invention.

The following examples, given as an indication but not limiting, illustrate the invention.

Example 1 :

In this example, sodium perchlorate is produced by electrolysis of sodium chlorate in equipment essentially comprising an electrolysis cell with an external recirculation loop, an assembly in which the single electrolysis step is carried out, exchange means calorific, measurement and control of temperature and pH. The electrolysis cell is not compartmentalized and is equipped with monopolar electrodes, platinum anodes and mild steel cathodes, traversed by an electric current such that the anodic current density is equal to 40 A / dm ² . The gas evolution in the cell and the sufficiently significant recirculation ensure the uniformity of the electrolyte in said cell.

It is initially formed in the latter, either directly from its constituents, or already by progressive electrolysis of sodium chlorate, an electrolyte which is an aqueous solution of sodium chlorate and sodium perchlorate in the presence of a small amount of dichromate. sodium, from which sodium perchlorate can be isolated directly by crystallization.

In the present case the electrolyte contains, for 100 g. of water, 26 g. sodium chlorate, 180 g. sodium perchlorate and 0.3 g. sodium dichromate.

The composition of the electrolyte thus fixed is kept stable over time by continuously entering the single electrolysis step 96 cm ^{3 /} h.dm ² anodic of a sodium chlorate solution at 80 ° C. containing, per liter, 900 g. sodium chlorate, 1.5 g. sodium dichromate and the amount of perchloric acid necessary for the pH of the electrolyte in the electrolytic cell, temperature is 65 ° C, equal to 6.5. 85 cm ^{3 /} h.dm ² anodic of an aqueous solution which, according to the invention, has the composition of the electrolyte, continuously leaves the single electrolysis stage in order to isolate it directly by crystallization sodium perchlorate monohydrate which represents the targeted production.

Example 2 :

This example is carried out in the equipment and according to the operating process of Example 1. The electrolysis is carried out in particular at the same temperature and at the same pH as in Example 1. The electrolyte this time contains, for 100 g. of water 36 g. sodium chlorate, 220 g. sodium perchlorate and 0.3 g. sodium dichromate. This composition is kept stable over time by continuously entering into the single electrolysis step 46 g / h.dm ² anodic solid sodium chlorate via the recirculation current, and 84 cm ³ / h. dm ² anodic of an aqueous solution at 20 ° C containing, per liter, 500 g. sodium chlorate, 1.5 g. sodium dichromate and the amount of perchloric acid required to reach a pH of 6.5 in the electrolyte. 76 cm ^{3 /} h.dm ² anodic aqueous solution of perchlorate leaves the single electrolysis stage from which sodium perchlorate monohydrate can be collected directly by crystallization.

Example 3 :

This example is also carried out in the equipment and according to the operating process of Example 1. The electrolysis is carried out at the same temperature and at the same pH as in Example 1.

The electrolyte, the composition of which is that of the aqueous sodium perchlorate solution from which the sodium perchlorate produced can be isolated directly by crystallization, contains, per 100 g. of water, 30 g. sodium chlorate and 290 g. sodium perchlorate next to 0.3 g. sodium dichromate.

The electrolyte is maintained at this stable composition over time by continuously entering the step single electrolysis 45 g / h.dm ² anodic of solid sodium chlorate via the recirculation current and 74 cm ^{3 /} h.dm ² anodic of an aqueous solution of sodium chlorate of Example 2, while leaving the single electrolysis step 66 cm ^{3 /} h.dm ² anodic aqueous solution of the same composition as the electrolyte and from which the perchlorate produced can be isolated directly in anhydrous form by crystallization.

The FARADAY yield, expressed as the ratio of the amount of electricity actually used for the conversion of chlorate to perchlorate, in a given time, to the total amount of electricity consumed at the same time, is greater than 90% for the three examples above. It is more than 93%, even in the absence of sodium dichromate by repeating Example 1 with an electrolysis temperature equal to 55 ° C instead of 65 ° C.

Claims (8)

1. Continuous process for the manufacture of sodium perchlorate by continuous electrolysis in a single electrolysis stage of an electrolyte consisting of an aqueous solution of sodium chlorate, sodium perchlorate and optionally other electrolysis ingredients, this electrolyte being kept uniform with a stationary composition by continuously introducing into this stage sodium chlorate, water and optionally the said ingredients, each in a quantity equal to that which disappears from this stage by electrolytic conversion and by a continuous and definitive output from this stage of an electrolyte stream forming a withdrawal stream which has the same stationary composition, the said composition being chosen so that the withdrawal stream can deposit sodium perchlorate crystals, anhydrous or monohydrate or dihydrate, directly on cooling, followed, starting with the said withdrawal stream, by a crystallization of sodium perchlorate by cooling or water evaporation.
2. Process in accordance with Claim 1, characterized in that the electrolyte contains at least 100 g of sodium chlorate per litre.
3. Process in accordance with either of Claims 1 and 2, characterized in that all the sodium chlorate and all the water which enter the single electrolysis stage are contained in an aqueous solution of sodium chlorate.
4. Process in accordance with either of Claims 1 and 2, characterized in that all the sodium chlorate enters in solid form into the single electrolysis stage.
5. Process in accordance with either of Claims 1 and 2, characterized in that a part of the sodium chlorate enters in solid form into the single electrolysis stage, the remaining sodium chlorate entering in the form of aqueous solution into the said stage.
6. Process in accordance with one of Claims 1 to 5, characterized in that the electrolysis is carried out in an uncompartmented electrolysis cell equipped with monopolar electrodes.
7. Process in accordance with Claim 6, characterized in that the anode material is based on platinum and the cathode material is mild steel or bronze.
8. Process in accordance with Claim 6 or 7, characterized in that the electrolysis is carried out with an anode current density equal to a value of between 10 and 70 A/dm², at a temperature equal to a value of between 40°C and 90°C and at a pH equal to a value of between 6 and 10.