DE19546644A1 - Redn. of chromium-(VI) cpds. to chromium-(III) cpds. in aq. soln. - Google Patents

Abstract

Process for reducing Cr (VI) cpds. to Cr (III) cpds. in aq. soln. comprises reaction with Ca sulphite. Pref. the reaction is carried out at pH 1-5. The process is carried out e.g. by laying the Cr (VI) cpd. and/or the sulphite cpd. into a bed and washing through with an aq. soln., opt. with acidification of the reaction. Pref. the stoichiometric ratio of sulphite cpd. to Cr (VI) cpd. is 1:1 to 100:1. The Cr (III) cpd. is separated e.g. by pptn.

Description

The invention relates to a process for the reduction of chromium (VI) compounds to chromium (III) - compounds in aqueous solution.

Chromium (VI) compounds, hereinafter referred to as chromates, are often used industrially. However, due to their high toxicity, the chromium (VI) compounds are present in wastewater and in Waste problematic. Chromate wastewaters and eluates can be removed by reducing the Chromates detoxifies to chromium (III) compounds and also by precipitating the Chromium (III) compounds of the total chromium content can be reduced.

Object of the present invention is a most cost-effective method for detoxification of chromates.

This object is achieved according to the invention in the method described above with the measures of the characterizing part of claim 1 or 3. The invention sulfite used reduces the chromate to the substantially non-toxic chromium (III) compound fertilize. Due to the linkage of the redox potentials with the pH, the reaction is strongly dependent on this, so that the reduction of chromate especially in acidic Solution and particularly advantageous at a pH <5 is performed. To a hyperacidity In particular, to avoid effluents, the reaction is advantageously not below pH1 executed. The reaction according to the invention proceeds as follows

normal potentials

For a cost-effective reduction calcium sulfite is used according to the invention, the ins special in flue gas desulphurisation plants, especially dry flue gas desulphurisation is incurred. Advantageously, this filter dust can be used immediately, without that the calcium sulfite is previously isolated from the filter dust.

Advantageously, the sulfite compound and / or the chromate is set in a bed, and ent No water or an aqueous solution of the reactants sent through this bed. before partly sulfite-containing filter dust is placed in the bed through which then let chromate-containing wastewater seep through. The chromate wastewater can already before acidified in the reaction, but advantageously only the eluate is acidified. this has the advantage that flocculation of the chromium (III) compounds takes place only in the eluate and this not be retained in the filter cake.

When using chromate-containing solids such as granular solids or from sludge men or the like, these are placed in the bed and optionally with the sulfite compound mixed or layered. Then the bed is washed with water, the acidification advantageously takes place again in the eluate. When used acidic sewage be, then the inventive method has the advantage that the acid of the Sewage is neutralized in the sulfite-containing fly ash, so that a hyperacidity of What it is avoided.

The filter dust originating from dry flue gas desulphurisation plants has, for example following composition:

% By weight CaSO₃ · 2H₂O 9-11 CaSO₄ · 2H₂O 1-3 Flue dust (including CaO) 86-90

The ratio between the sulfite-containing filter dust and the treated Substance essentially depends on the chromate concentration that is to be reduced. Übli The sulphite compound is in an over stoichiometric ratio to the chromate from 1: 1 to 100: 1 used, advantageously in a 3 to 20-fold excess. For a sulphite content of 10% in the filter dust is advantageously about 150 g of filter dust per gram of chromate used.

The seepage of water or waste water through the filter dust bed, the calcium dissolved sulfite. Because of the relatively low solubility of the calcium sulfite will be advantageous  worked with dilute solutions, that is, chromium-containing wastewater usually need not to be concentrated. Advantageously, the acidification of the chromium compounds takes place here complete reaction in the eluate.

The inventive method has the advantage that the carzinogenic chromate from Abwäs and eluates in relatively non-toxic chromium (III) compounds, in particular Chromium (III) oxide is converted by filtration, centrifugation or sedimentation from the Wastewater is separable. Advantageously, with the inventive method also Chromate contents are washed out of granular solids or sludges and detoxified become.

The invention will be described in more detail below with reference to drawings and examples.

Show it

Fig. 1 shows a treatment of acid chromate wastewater;

FIG. 2 shows a treatment of neutral or alkaline chromate wastewater; FIG. and

Fig. 3 is a chromate treatment solids.

In the treatment of acidic, chromate-containing wastewater or eluates ( FIG. 1), a bed 1 containing the sulphite-containing filter dust, an acidic (pH 1 to 5) chromate wastewater 2 (or an eluate) abandoned. From the bed 1 leaking eluate 3 is collected in a collecting container 4 as wastewater charge 5 . The batch 5 is checked for its chromate content, optionally acidified or sent again on a sulfite-containing filter dust bed. To precipitate the chromium (III), the batch 5 is made weakly alkaline with ammonia and the precipitated chromium (III) hydroxide separated.

The cleaning of neutral or alkaline wastewater 12 ( FIG. 2) takes place in principle as described in FIG. 1 by passing the waste water 12 through a filter bed 11 and collecting the eluate 13 in a container 14 . However, unlike the treatment of acidic wastewater, the collected charge 15 still contains high levels of chromate and sulfite which react to form chromium (III) and sulfate by adding acid 16 according to the equation described above. The further processing takes place analogously to FIG. 1.

In Fig. 3, a chromate-containing, granular solid or sludge with sulfite-containing filter dust is mixed or layered as bed 21 presented. Elution with water 22 gives an eluate 23 , which is collected as a charge 25 in a container 24 . The sulfite and chromate compounds still present in significant amounts in batch 25 are treated analogously to FIG. 2 with acid 26 and further worked up.

Following the described procedure the following examples were carried out:

example 1

Chromate-containing fluidized bed ash is precipitated by a sulfite-containing filter dust (stabilizer) a dry flue gas desulphurisation plant to chromium (III) reduced. To the capacity of Filter Dust to determine were the following mixing ratios of chromate vortex layered ash and stabilizer and the chromate content of the eluate from the mixture determined according to DIN 38414 Part 4.

Example 2

Leach test: The chromium-containing fluidized bed ash was filled into a chromatography column and eluted with water pH7. The resulting solution was collected and acidified to pH3 brought. Subsequently, the chromate content was determined. Then the same amount Fluidized bed ash mixed with stabilizer or both substances stacked and also eluted with water to determine the chromate content after acidification of the template. The following measured values were obtained as mean values.

Series 1: 80 g fluidized bed ash
Series 2: 80 g fluidized bed ash + 20 g stabilizer mixed
Series 3: 80 g fluidized bed ash + 20 g stabilizer layered
Series 4: 80 g fluidized bed ash + 30 g stabilizer mixed

Chromate contents in the acidified eluate [mg / l] depending on the amount of eluate

Example 3

To each 200 ml of sodium chromate solution (concentration = 3.030 mg / l) stabilizer is added and mixed. The suspension is allowed to stand for 30 minutes and then acidified to one brought pH 4 and then determined the chromate concentration.

Claims (11)

1. A process for the reduction of chromium (VI) - to chromium (III) compounds in aqueous solution by reaction with a sulfite compound, characterized in that is used as the sulfite compound calcium sulfite. 2. The method according to claim 1, characterized in that a sulfite-containing filter dust is used. 3. Process for the reduction of chromium (VI) - to chromium (III) compounds in aqueous Solution by reaction with a sulfite compound, characterized in that a sulfite-containing filter dust is used. 4. The method according to claim 2 or 3, characterized in that the filter dust from a desulfurization plant comes. 5. The method according to any one of the preceding claims, characterized in that the Reaction in an acidic environment, especially at a pH between 1 and 5 takes place. 6. The method according to any one of the preceding claims, characterized in that the Chromium (VI) compound and / or the sulfite compound in a bed and submitted by Wash with an aqueous solution and optionally acidify the reaction is carried out. 7. The method according to any one of the preceding claims, characterized in that the Chromium (VI) compound is used from a sewage, sludge or ash. 8. The method according to any one of the preceding claims, characterized in that the Chromium (III) compound is separated by precipitation.   9. The method according to any one of the preceding claims, characterized in that the Sulfite compound to chromium (VI) compound in a stoichiometric ratio of 1: 1 up to 100: 1 is used. 10. Use of sulfite-containing filter dusts for the reduction of chromium (VI) compounds Chromium (III) compounds. 11. Use according to claim 10, characterized in that the filter dust from a Flue gas system, in particular a flue gas desulfurization comes.