DE3735903A1 - Process for the oxidation of washings having a low thiosulphate concentration - Google Patents

Abstract

Aqueous solutions containing from 0.002 to 0.05 mol of thiosulphate per litre of solution which are oxidised using H2O2 in the presence of compounds of chromium, vanadium, tungsten or molybdenum to give sulphate-containing solutions contain, after the oxidation, no traces of heavy metals if this oxidation is preceeded by a heavy-metal-free oxidation of the thiosulphate to the tetrathionate using H2O2.

Description

The invention relates to the oxidation of thiosulfate with H₂O₂ in wash water, the only low concentration have thiosulfate.

At the end of photographic development process fall usually silver ion and thiosulfate-containing wash water, which must be processed so that they Easily released to the environment or in circulation can be reused. They are preparing for this the desilvering dilute Thiosulfatlö particular difficulties.

Although it is known from DE-AS 20 09 826, thiosulfate with To oxidise H₂O₂ to sulphate; this succeeds only at Concentrations of at least 0.5 moles of thiosulfate per Liter of solution, because of three oxidation reactions by the pH of the solution and are dependent on the temperature, with on the one hand the oxidation to the other Tetrathionate, on the other hand, the oxidation to sulfate, the tetrathionate in turn under certain Conditions is further oxidized to sulfate.  

The following equations apply:

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

2 S₂O₃ ^2- + H₂O₂ → S₄O₆ ^2- + 2 OH ^- (2)

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

The reaction according to (1) is preferably carried out in alkaline Environment and / or at temperatures above 80 ° C without additive any catalysts. Because the reaction is exothermic is heated with sufficient Thiosulfatkonzentra tion the system so far that it is well above 80 ° C remains. The released protons can be added by adding be caught by alkali.

For dilute solutions of thiosulphate, the temperature of 80 ° C is not reached, unless you heat up what but it is uneconomical. It then runs reaction (2) from, wherein hydroxyl ions are released. In alkaline Milieu, the tetrathionate according to (3) on to sulfate oxidized, due to the released protons the Milieu increasingly acidic. Since tetrathionate in acid environment against H₂O₂ is relatively stable, the remains Reaction largely at the level of tetrathionate stand.

From DE-OS 36 35 219 is also known that the Oxidation of the thiosulfate to sulfate without the addition of Alkali or the action of heat then succeeds when  the reaction with compounds of chromium, vanadium, Tungsten or molybdenum catalyzes.

This reaction has the disadvantage that the so treated Wash water contains traces of these heavy metals, whether equal as catalyst a per se water-insoluble Heavy metal compound was used.

The object of the invention was dilute thiosulfate solutions with H₂O₂ using certain Schwermetallverbin oxidize to sulfates without the aqueous Lö after the reaction significant traces of heavy metals having.

Another object of the invention was to wash to treat water so that it can be easily transferred to the Um delivered or used in the process again can be.

It was found that this succeeds when the heavy metal catalyzed oxidation with H₂O₂ a Oxidation of thiosulfate to tetrathionate before on.

The invention therefore relates to a method for Oxidation of aqueous solutions containing 0.002 to 0.05 mol Thiosulphate / l solution containing, with at least the stoichiometric amount H₂O₂, characterized that is a heavy metal-free oxidation of the thiosulfate with H₂O₂ to tetrathionate and performs the  Tetrathionate solution in the presence of compounds of the Chromium, vanadium, tungsten or molybdenum to a sulfate solution oxidized.

Preferably, the generated sulfate ions z. B. with Calcium ions precipitated and the water washes photo used again in graphic materials.

The metals are preferably in their highest Oxidation stage, in particular molybdate, for example Ammonium molybdate used. It is enough catalytic Amounts of metal compounds, in particular 0.002 to 0.02 mole / mole of thiosulfate.

The heavy metal-free oxidation of thiosulfate to Tetrathionate with H₂O₂ is particularly by activated carbon catalyzed.

The reaction is preferably carried out so that the diluted thiosulfate solution together with H₂O₂ by a Pillar is passed, which is initially a sufficiently thick Layer of activated carbon and then a sufficient thick layer of the precipitated onto a substrate Contains metal compound. As a carrier come for example Activated carbon or alumina in question.

A buffering, for example with calcium carbonate advantageous.

The stoichiometric amount of hydrogen peroxide, which is for the completeness is used according to the equation  (1) 4 moles H₂O₂ / mole of thiosulfate. A small excess At H₂O₂ is appropriate, a larger excess should be avoided for economic reasons.

External warming is not excluded The procedure is preferably without external heating carried out.

Examples Example 1 (comparative example) Preparation of the catalyst

180 g of granular activated carbon is mixed with a solution of 21 g Ammonium molybdate and about 500 ml of distilled water ge soaked and dried. Then there is a renewed Impregnation of the catalyst with a solution of 1.3 g of sodium dihydrogen phosphate and 550 ml of water instead, then it is dried to constant weight. This Catalyst contains molybdate phosphate in a Ge Weight ratio to the carrier of 1:10.

Structure of the reaction space

A column to be fed from below is initially with Glass wool designed to clog the inlet slit ze to avoid.

Over this the catalyst is layered: 200 g molyb dato-phosphate on activated carbon 1: 10. This filling has a Volume of 0.8 l or a height in the column of 11.5 cm. Another thin glass wool layer separates the Catalyst from the following layers: 100 g of slurry chalk and then 40 g of granular activated carbon for filtering the solution of whiting. A subsequent one Glass wool layer weighted by marble dry should be a flooding of the topmost activated carbon layer prevent.  

Through the column is a 0.02 molar sodium thiosulfate solution, the per liter 8.1 ml of hydrogen peroxide solution (30% strength by weight). The throughput is 1.6 l / hour.

After 8 liters of dissolved solution have become stationary Ratios set. The expiring solution has following composition:

thiosulfate: not detectable tetrathionate: about 0.001 m Sulfide: about 0.002 m Sulfate: 0.016 m molybdate: 4 · 10 ^-4 m

Example 2 (according to the invention) Example 1 is repeated with the following change:

Between the first glass wool layer and the catalyst layer comes following layers:

350 g of granular activated carbon (1.6 l = 23 cm column height) and another 1 cm thick glass wool layer.

After setting stationary conditions has the ablau fende solution the following composition:

thiosulfate: not detectable tetrathionate: not detectable Sulfide: not detectable Sulfate: 0.033 m molybdate: 3 · 10 ^-6 m

Example 3 (comparative example)

Example 2 is without the catalyst layer and the Glass wool interlayer repeated.

thiosulfate: 0.005 tetrathionate: 0.08 Sulfide: 0.005 Sulfate: 0,002

The examples show that only in a pre-oxidation of the Thiosulfate to tetrathionate a washout of the heavy Metal catalyst from the carrier largely avoided who that can.

Claims (6)

1. A process for the oxidation of aqueous solutions containing 0.002 to 0.05 mol of thiosulfate / l solution containing at least the stoichiometric amount H₂O₂ using a compound of chromium, vanadium, tungsten or molybdenum, characterized in that a heavy metal free oxidation the thiosulfate with H₂O₂ leads to the tetrathionate and the tetrathionate solution in the presence of compounds of chromium, vanadium, tungsten or molybdenum oxidized to a sulfate solution. 2. The method according to claim 1, characterized that catalyzes with molybdate. 3. The method according to claim 1, characterized that 0.002 to 0.02 mol of metal compound / mol Thiosulphate. 4. The method according to claim 1, characterized that the solution obtained after the oxidation of the Sulfate ions freed and for washing photographic Materials is reused. 5. The method according to claim 1, characterized that it is carried out without external heating. 6. The method according to claim 1, characterized that the heavy metal-free oxidation in the presence is carried out by activated carbon.