DE3635219A1 - Process for the oxidation of scrubbing waters of low thiosulphate concentration - Google Patents

Abstract

Aqueous solutions which contain 0.002 to 0.05 mol of thiosulphate/l of solution can be oxidised by H2O2 to give sulphate-containing solutions if the oxidation is catalysed with compounds of chromium, vanadium, tungsten or molybdenum, in particular with molybdate.

Description

The invention relates to the oxidation of thiosulfate H₂O₂ in wash water that has only a low concentration have thiosulfate.

At the end of photographic development processes fall Usually wash water containing silver ions and thiosulphate, which must be prepared so that it easily released into the environment or in a cycle can be reused. The follow up the desilvering diluted thiosulfate solutions particular difficulties.

It is known from DE-AS 20 09 826, with thiosulfate Oxidize H₂O₂ to sulfate; however, this only works with Concentrations of at least 0.5 moles of thiosulfate per Liter of solution because three oxidation reactions, which from pH of the solution and the temperature are dependent on each other compete, on the one hand, oxidation Tetrathionate, on the other hand oxidation to sulfate, the tetrathionate in turn under certain Conditions to sulfate is further oxidized.  

The following equations apply:

S₂O₃ ^2- + 4 H₂O₂ → 2 SO₄ ^2- + 3 H₂O + 2 H⁺ (1)

2 S₂O₃ ^2- + H₂O₂ → SO₄O₆ ^2- + 2 H₂O⁻ (2)

S₄O₆ ^2- + 7 H₂O₂ → 4 SO₄ ^2- + 4 H₂O + 6 H⁺ (3)

The reaction according to (1) is preferably alkaline Milieu and / or at temperatures above 80 ° C without additives any catalysts. Since the reaction is exothermic is heated with sufficient thiosulfate concentration the system so far that it was largely above 80 ° C remains. The protons released can be added be caught by alkali.

With dilute thiosulfate solutions, the temperature of 80 ° C is not reached unless you heat up what but is uneconomical. Then reaction starts (2) , whereby hydroxyl ions are released. In alkaline According to (3), the tetrathionate continues to become sulfate oxidized, the. because of the released protons Environment is becoming increasingly acidic. Since tetrathionate in acidic environment is relatively stable against H₂O₂, remains Reaction largely at the tetrathionate level stand.

The object of the invention was also dilute thiosulfate solutions to oxidize with H₂O₂ to sulfate without alkali need to add or let heat energy take effect.  

Another object of the invention was the wash water so that it can be easily recycled to the environment submitted or reinstated in the process can be.

It has been found that this is possible if the reaction mixture with certain heavy metal compounds catalyzed.

The invention therefore relates to a method for Oxidation of aqueous solutions containing 0.002 to 0.05 mol Contain thiosulfate / l solution, with at least the stoichiometric amount H₂O₂, characterized in that the reaction with compounds of chromium, vanadium, Catalyzes tungsten or molybdenum.

Preferably, the sulfate ions z. B. with Calcium ions precipitated and the water for washing more photographic Reinstalled materials.

The metals are preferably at their highest Oxidation stage, especially molybdate, for example Ammonium molybdate. Catalytic are enough Amounts of the metal compound from, in particular 0.002 to 0.02 mol / mol thiosulfate.

The oxidation can be homogeneous, for example in one Stirred kettle, or heterogeneous, for example in one with a carrier material on which the metal compound is noticeable or with which the metal compound is chemically bound is to be carried out, filled tube the second variant is particularly environmentally friendly because  no heavy metal ions are released. As a carrier activated carbon or aluminum oxide come in Question.

The stoichiometric amount of H₂O₂ required for the complete Oxidation used is according to the equation (1) 4 moles of H₂O₂ / mole of thiosulfate. A little surplus of H₂O₂ is appropriate, a larger excess should be avoided for economic reasons.

External heating is not excluded, preferably the procedure is done without external heating carried out.

The rate of the oxidation reaction can be according to equation (1) based on the formation of protons, that is, track by lowering the pH. For the experiments described below were each time required to reach pH 4.0 in minutes measured.

To 100 ml of an aqueous 0.02 mol / l thiosulfate Solution were below at room temperature (20 ° C) Stir 0.8 ml H₂O₂ (30 wt .-%) and in the following table given amounts of a 0.014 molar Ammonium molybdate solution added. A tangible one No warming was found.  

Claims (5)

1. A process for the oxidation of aqueous solutions containing 0.002 to 0.05 mol of thiosulfate / l solution, with at least the stoichiometric amount of H₂O₂, characterized in that the reaction is catalyzed with compounds of chromium, vanadium, tungsten or molybdenum. 2. The method according to claim 1, characterized in that that you catalyze with molybdate. 3. The method according to claim 1, characterized in that 0.002 to 0.02 mol of metal compound / mol Uses thiosulfate. 4. The method according to claim 1, characterized in that the solution obtained after the oxidation of the Free sulfate ions and more photographic for laundry Materials is reused. 5. The method according to claim 1, characterized in that it is done without external heating.