FI68427B - RELEASE CHARACTERISTICS OF ELECTRIC DICHROMATERS WITH ELECTRICAL CHLORATHALTES - Google Patents

Description

TRI m1 / bulletin publication 68427

LJ 'ΊΊ' UTLÄG G NI N GSSKRIFT

C (45) Patent granted on 10 09 1935 Patent meddelat, (51) Kv.ik. * / Lnt.ci. 'C 25 B 1/26 // C 02 F 1/70 t (21) Patent application - Patentansökning 802629 (22) ) Date of application - Ansökningsdag 20.08.80 (23) Date of application - Giltighetsdag 20.08.80 (41) Has become public - Blivit offentlig Q1 gi

National Board of Patents and Registration Date of publication and hearing publication ηςβς

Patent-och registerstyrelsen '' Ansökan utlagd oeh utl.skriften publicerad 5l.u5.05 (32) (33) (31) Privilege claimed - Begärd priority 31 .08.79 Canada (CA) 33 * »909 (71) Chemetics International Ltd. , 1770 Burrard Street, Vancouver, BC,

Canada (CA) (72) Kenneth Raymond Maycock, White Rock, BC, Norbert Kin-Ming Tam, Richmond, BC, lan Harry Warren, Vancouver, BC, Canada (CA) (7 * 0 Oy Koi ster Ab (5 * 0 The present invention relates to the removal of dichromates from electrolytic chlorate solutions prepared electrolytically from alkali metal chloride chloride solutions. The present invention relates to the removal of dichromates from electrolytic chlorate-containing solutions prepared electrolytically from alkali metal chloride solutions.

A small amount of chromium, usually in the form of a dichromate, must be added to the electrolyte salt solution to prevent power loss at the cathode. As the added dichromate leaves the electrolytic process chemically unchanged, it has been found to remain entirely in the chlorate-rich solution formed in the electrolysis. Dichromate länsäolo said kloraattirikkaassa in the solution to form the environment of serious concern and should not be let in rivers and streams contaminated with eg. As part of the removal of chlorine-dioxide in an amount of water-generating plants and pulp mills in the future.

A well-known method for removing chromate from solution involves the chemical reduction of hexavalent chromium to 68427 trivalent chromium with a chemical reducing agent zinc hydrosulfide in an acidic medium, followed by pH adjustment to the alkaline range and precipitation of trivalent chromium as chromium hydroxide. However, this method, although it effectively removes chromium from solution, has serious drawbacks in that it sometimes produces toxic gases, the development of which is not environmentally acceptable. Another method previously used has been to precipitate a chromate ion by adding a suitable cation to the solution. For example, barium chloride has been used to precipitate barium chromate, followed by the addition of alkali metal carbonate to precipitate any excess barium. This method is not attractive as it requires a separate filtration step and requires the use of barium salts, the presence of which is particularly detrimental in the chlorate road electrolysis process. Several other methods have also been proposed for this purpose, such as those described in U.S. Patent Nos. 3,835,001 and 3,980,751. These patents describe, among many others, methods of using ion exchange resins to remove chromate from aqueous solutions. Although this is effective, such use of an ion exchange resin has proven to be so expensive that it is not economically attractive.

It is therefore an object of the present invention to provide an improved process for the removal of dichromates from chlorate-rich alkali metal chlorate chloride solutions formed in electrolysis, which process does not suffer from the significant disadvantages associated with said prior art processes.

The invention thus relates to an improved process for removing dichromates from chlorate-rich solutions, which process comprises reducing hexavalent chromium of dichromates to trivalent chromium with a chemical reducing agent and separating the trivalent chromium in the form of a chromium hydroxide precipitate. The process is characterized in that hydrazine is used as the reducing agent at a concentration of at least 3 moles per mole of dichromate, the reaction temperature is kept above about 0 ° C and the pH is adjusted to 7-8 at the end of the reaction.

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Chlorate-rich solutions suitable for treatment by the process of this invention are, as mentioned above, aqueous solutions of sodium chlorate, which are usually formed by the electrolysis of brine in electrolytic cells. In addition to chlorate, said solutions also contain substantial amounts of sodium chloride as well as minor amounts of dichromate and sodium hypochlorite and their composition is usually as follows: sodium chlorate 300-660 g / l sodium chloride 100-200 g / l sodium dichromate 2-8 g / l sodium hypochlorite 0.1 -5 g / l

When hydrazine is added to a solution, such as, e.g., to the extent suggested above in this invention, the dichromate in the solution reacts with it as follows:

Na2Cr20? + 3N2H4.H2O * 2 Cr (OH) 3 + 2 H2O + 2 NaOH + 2 N2 + 2 NH3.

As can be seen from the above reaction, the hexavalent chromium of the dichromate is reduced to trivalent chromium, which separates from the solution in the form of a chromium hydroxide precipitate if, as described later herein, the reaction conditions are carefully controlled. It should be noted that hydrazine forms a "pure" reducing agent in that it does not leave a significant harmful residue in the chlorate solution. This is an important advantage over the previous process in which zinc hydrosulfide is used as the reducing agent.

In order to reduce the amount of dichromate to a sufficient amount, hydrazine must be added to the chlorate solution to such an extent that at least 3 moles of dichromate per mole of dichromate are available. With such a ratio, the initial sodium di-chromate concentration has been reduced from 2.5 g / l to less than 0.015 g / l. The significance of this detail is clear from the following example and the accompanying drawing.

The process must be carried out at a temperature above 40 ° C, which is the temperature at which the chlorate and chloride in the solution to be treated begin to crystallize. Preferably, the working temperature is between 60 ° C and 80 ° C, which is the normal temperature range of the chlorate-rich solution as it enters the electrolyte cell.

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In order to facilitate the precipitation of trivalent chromium as chromium hydroxide and thus to obtain the eliminated dichromate as completely as possible from the chlorate solution, it has been found that the pH of the reaction mixture must be adjusted between 7-8 at the end of the reaction. If left unchecked, the pH can rise to as high as 9 or greater, and trivalent chromium will not precipitate or precipitate only partially. The final pH is thus adjusted to the correct level by adding a suitable acid such as hydrochloric acid.

By strong sodium hypochlorite solution is meant a solution containing at least 2 g / l of sodium hypochlorite. As such a solution, a fresh chlorate-rich solution from an electrolyte cell can be used if it contains the amount of sodium hypochlorite required above.

The oxidized chromium compound formed in the reaction between chromium hydroxide and sodium hypochlorite is an effective sodium dichromate. Said dichromate in solution can be recovered and stored or preferably recycled to the brine fed to the electrolyte cell to produce chlorate.

The present invention is further illustrated by the following non-limiting example in connection with the accompanying drawing, which is a graph depicting the amount of dichromate remaining in treated solutions as a function of the number of moles of hydrazine hydrate added to the solutions per mole of dichromate.

Example

Two experiments were performed, one with solution A containing 100 g / l sodium chloride and 2.7 g / l sodium dichromate and the other with solution B containing the same substances as solution A and 550 g / l sodium chlorate. Four samples of each solution were treated at 60 ° C with amounts of hydrazine to give hydrazine to dichromate ratios of 1, 2, 3 and 4. Treatment of solution A, which had an initial pH of 6.0, left the pH uncontrolled and rose to about 9 with the consequence that the reduction of dichromate did not substantially subside. When treating solution B, which had an initial pH of 6.0, the pH was initially left uncontrolled, but at the end of the reaction it was adjusted and 68427 was maintained at about 8.0. By thus controlling the pH and keeping the ratio of hydrazine to dichromate at least 3, the amount of dichromate in the treated solution could be reduced to as low as 0.012 g / l. The results obtained with solutions A and B are shown as curves in the accompanying drawing, in which they are shown as curves 1 and 2, respectively.

The conditions and results of the above experiments are summarized in the following table, which must be considered at the same time as the drawing.

Table

Reduction of Na2Cr207 to N2H4. Η 0,,?, 11a

Conditions Sample A Sample B

Na2Cr20? 2.7 g / l 2.7 g / l

NaCl 100 g / l 100 g / l

NaClO3 0 550 g / l

Temperature 60 ° C 50 ° C

pH (at the beginning) 6.0 6.0 pH (at the end) '9.0 8.0 (not controlled) (controlled with HCl)

Score

Molar ratio Remaining Na2Cr2O7 ^ N2H | + .H2O2 g / liter ^ Na2Cr2O7 ^

Sample A Sample B

1: 1 1.60 1.00 2: 1 1.35 0.24 3: 1 1.20 0.012 4: 1 1.10 0.012