FI62810C - FOERFARANDE FOER RENING AV AVLOPPSVATTEN AV KROM - Google Patents

Description

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Patent- och registerstyreisen 'AmMcu utisgd och utUkrHUn public · »* (32) (33) (31) sey« uolk ··· —Begird priori * · * 23. Ol. 71 07.05.71 Italy-Italy (IT) 23519 A / 71, 2I205 A / 71 (71) Snam Progetti SpA, Corso Venezia, l6, Milan, Italy-Italy (lT) (72) Nunzio Mastrorilli, Milan, Italy-Italy (IT) (Tl) Antti Impola (5I) Method for the treatment of effluents from chromium -Förfarande för rening av avloppsvatten av krom

The present invention relates to a process for the purification of effluents from chromium, in which hexavalent chromium is electrolytically reduced to trivalent chromium, which is then precipitated as a hydroxide, and in which the effluent itself is used as an electrolyte. If a polyvalent metal ion participates in anion formation, the process can result in a salt of the same metal with a lower valence.

In this way, for example, chromates are reduced to salts of trivalent chromium.

The invention relates to a process for the purification of waste water from chromium, in which the liquid to be treated itself is used as an electrolyte and in which hexavalent chromium is reduced to trivalent chromium, which is precipitated as hydroxide.

According to the principle of the method, two different types of elements are used, e.g. iron and amalgamated copper, which are immersed in the liquid to be treated and which are electrically connected inside or outside the contaminated liquid phase.

The method according to the invention is characterized in that chemically different electrodes are used, which are connected to each other to provide an electric current with a pH of less than 3.5, one of the electrodes being electropositive for hydrogen and the other carbon or electrically negative.

The acidic dissolution of a less noble electrode with respect to hydrogen results in an electron current flowing to another electrode where their charge is discharged relative to the ions in the electrolyte.

Previously, according to FI patent application 1490/69, it has been proposed to treat wastewater by precipitating impurities in the water, e.g. phosphorus, by means of precipitated metal ions. The metal ions are then introduced into the liquid by means of direct current or by using a natural potential difference, in which case, for example, iron and copper or iron and silver are mentioned as metals.

According to the invention, the reactions take place without energy supply from the outside, or alternatively the systems implemented according to the invention supply electrical energy in the form of a direct current reaching the non-affected cathode from the anode and can be measured through an external conductor if two suitable elements to the liquid to be treated and short-circuited outside the liquid phase.

The connection between two different substances can also be achieved inside the solution, in particular by using different electrodes with very small dimensions, and thus a mixture of granules of the two substances: in this way the liquid can be continuously passed through the mixture and removed completely purified.

Redox reactions do not depend on the arrangement of the electrodes, on the contrary, their kinetics depend on the shape of the electrodes and sometimes on the distance.

Austrian patent 264,392 also discloses the electrolytic treatment of chromium-containing wastewaters, but states that an electrolyte oxidation-reduction process takes place in the electrodes to which an electric current is applied. According to our invention, the electrodes can also be in the form of granules or powders as a mixture of two substances, whereby a continuous supply of liquid and complete purification is possible.

The process according to the invention can further be used for the conversion of oxidizing salts into other salts in which the metal is in the lower valence, the reaction mechanics for the treatment of chromate and mercury salts being as follows. In the sodium chromate solution, when the anode is metallic zinc and the solution is acidified with nitric acid, the following reaction takes place: 3 6281 0 3Zn + Na2Cr 2 O? + 14 HNO3 = 3 Zn (NO3) 2 + 2 NaNc> 3 + 2 Cr (NO3) 2 + 7H2 ° which can be written in the more general form 3 Me + Cr20 ^ + 14H + = 3 Me ++ + 2CR +++ + 7H2O where Me is 2-valued a metal that is less noble than hydrogen.

If said metal (Me) has a higher valence, a further reaction takes place, e.g. as follows: 6 Me ++ + Cr 2 O + + 14H + = 6 Me +++ + 2Cr +++ 7H 2 O.

This is the case, for example, when Me is an iron that changes to a divalent state and then to a trivalent state.

The following describes the treatment of effluents containing chromates from the treatment of chromium or nickel-Kro steels. In this case, it is very easy for a person skilled in the art to apply the principles of the invention for the treatment of waste cattle containing chromates from different sources.

The electrochemical treatment of chromium or nickel-chromium steels essentially involves an anode oxidation treatment "in a bath of a 30% aqueous solution of sodium nitrate. During the treatment considerable amounts of soluble sodium chromate and also slurries containing iron and chromium hydroxide are formed. After centrifugation, such a suspension causes the precipitates to separate and the solution rich in alkaline nitrate or chromate to be recycled.

However, and for reasons related to centrifugation, and above all to avoid a continuous increase in the chromate concentration in the circulating electrolyte, the sludges are removed after they have partially settled; thus, the substance to be removed is a suspension of iron and chromium hydroxides and basic nitrates dispersed in a solution of sodium nitrate and chromate.

The concentration of sodium chromate present in the liquid phase of such a suspension depends on the chromium content of the mixtures to be treated and is generally greater than 250 ppm Cr, which corresponds to about 750 ppm NagCrO 2.

An example diagram of this electrochemical process is shown in the figure below.

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Referring to the figure, there is a tank 1 containing pure electrolyte, sodium nitrate fed through point 14. Through step 2, the electrolyte is added to step 3, which is an apparatus where electrolytic treatment is performed. Point 3 removes the dirty electrolyte consisting of soluble sodium chromate and sludges containing iron and chromium hydroxides, which are passed to the vessel via point 5 (4). Through point 6, the suspension is passed to a centrifuge 7, which is fed with water only in the washing phase through point 15. After centrifugation of the suspension, the precipitates are separated and the solution rich in alkaline nitrate and Chromate is returned to point 1 via points 8 and 9. The slurries passed to point 11 via point 10 form an iron suspension of chromium hydroxides and basic nitrates dispersed in a solution of sodium nitrate and chromate.

In step 11, the purification is carried out according to the above-mentioned method. The two electrodes as an iron anode and the amalgamated copper as a cathode are immersed in the liquid: they are short-circuited and the following reaction takes place: 3 Fe + Cr 2 O + 14 H + ^ 3 Fe 2+ + 2 Cr 3 + + 7 H 2 O.

whereby chromates are reduced to salts of trivalent chromium.

The divalent iron that has gone into the solution is oxidized to the divalent iron due to the presence of excess nitrate ions, and the nitrogen oxides obtained by this redox reaction dissolve in the system in which they remain. Nitrate ions are quantitatively reconstituted when the system is made alkaline to obtain 3-valent chromium and iron hydroxides and to separate them from the system. The acidity required to form iron and chromium salts may be such that no dissolution of the iron compounds already present occurs.

After the reaction, iron and chromium are removed, as mentioned above, by adding sodium hydroxide, which causes the precipitation of heavy metal hydroxides.

In this phase, all the nitric acid, previously treated, is present in the solution as sodium nitrate, which is returned via points 12 and 13 to point 1; thus, the alkaline nitrate content of the electrolyte is automatically restored: for this reason, the otherwise necessary additions of nitrate to the returned solution are avoided.

After separation of the heavy metal hydroxides, e.g. by means of the same centrifuges already present, the above-mentioned treatment gives a solution containing only sodium nitrate, which as such is suitable for reprocessing in a closed loop.

An interesting variant of the method according to the figure, which makes such a method completely general, comprises the application of the present invention also for the treatment of the suspension in step 5. In this way, after the reduction and neutralization, it is possible to carry out the separation of the sludges in step 7, where the sludges can be removed because they do not contain chromates or heavy metal salts in the remaining liquid phase.

In general, regardless of the element to be reduced, it is possible to use as a cathode a substance which acts as a mercury electrode, in order to obtain a high overvoltage compared to hydrogen and thus to affect the pH over a wider range without hydrogen evolution.

For this purpose, amalgamated metals, mercury-impregnated carbon or simply metallic mercury can be used. The cathode elements can also be obtained by coating or impregnating the support with a metallic element which can form Amalgae, and thus amalgamating the metallic element.

When using an apparatus consisting of packed columns in which the contact between the electrodes is effected inside the aqueous phase, it is suitable to use as the anode an element which does not form amalgam in order to maintain a constant potential difference between the electrodes: Iron is preferably used. However, the anode may preferably be zinc, nickel, tin, lead, iron, chromium, and any metal available from a non-oxidizing acid, i. which are less noble than hydrogen; sometimes they can be amalgamated to adjust the etching rate. The cathode can be formed by carbon, hydrogen-noble metals, their amalgams, substances impregnated with mercury and all substances which are not corrosive by acids and which are conductors.

The process according to the invention can be carried out using only an anode element having a metallic nature, according to the above-mentioned reaction schemes. However, the benefits are not very significant in this case. Above all, since no cathode is used, which acts more precisely than hydrogen on the electrode, a smaller potential difference is present compared to the anode material: therefore the reaction rates are lower.

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In addition, since a cathode having a significant overvoltage over hydrogen is not used, as is the case where the cathode acts as a mercury electrode, hydrogen can be released from the anode, which increases acid consumption and polarizes the anode itself.

When treating solutions containing oxidizing compounds in high concentrations, e.g. chromates at a concentration of 1000 ppm Cr, the above reactions take place and the consumption of acid is equivalent to the amount of reduced chromates, the concentration of hydrogen ion decreases, i. The pH increases, especially on the anode surface, if the metal is in its higher valence as it enters the solution. In the case of iron, Fe +++ ions are formed at such pH values, giving hydroxides or basic salts of trivalent iron. Because they are insoluble products formed near the metal surface where Fe +++ ions are formed, the dissolution process may be reduced due to the surface polymerization phenomenon.

In such a case, it is possible to add a complexing agent, e.g. a sodium salt on the EDTA day, to avoid the formation of insoluble iron hydroxides and basic salts.

A useful device that forms most of the cathode surface that is not in contact with the anodes involves the use of cathodes with special shapes, even when using columns filled with cathode and anode material mixed together.

For example, it is possible to use a cathode material in the form of small tube thick pieces, and if the anode material has such dimensions that it does not extend into the hollow small cylinders thus obtained, a cathode surface formed by the inner walls of tube thick pieces not in contact with the anodes can be used. , when they are electrical due to contact between the outer anodes and the outer walls of the hollow cylinders.

In the above assumption, the solution containing chromates is reduced when it is in contact with the inner wall of the cylinder, while the subsequent reduction reaction takes place with ferric ions already present in the solution, but not when they are formed.

The iron hydroxide and basic salts, if formed, are thus between the liquid phase and the anode-liquid state.

The advantages of the method according to the invention are described in more detail by means of the following examples, which are presented for illustrative purposes only.

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7 for trying.

Example 1

We refer to the process according to the figure, in which chromium and n.ik-chromium chromium steels were electrochemically treated and carried out with the intention of giving said steels the desired shape using anode dissolution: sodium chromate present in the liquid phase of the suspension to be removed (point 11 in the figure); the concentration was. about 750 parts per million, which corresponds to about 250 parts per million Cr. By the method previously described, the complete reduction of the chromates took place in about 2 hours: during this time it was possible to detect a current which indicated a strength of 100 mA for a potential difference of 1 V.

No reduction was observed with other ferric salts or nitrates.

Example 2 This experiment was performed solely for comparison and was related to the general case of chrome plating equipment. The resulting effluents were purified by adjusting the pH to less than 3 and adding a gaseous solution of SO 2 or sodium sulfite or bisulfite at a concentration of 50-100 g / l. The following reactions occurred:

Cr20 '+ 3 HS0 "+ 5 H + -> 2 Cr +++ + 3 SOj + 4 H2O

Cr 2 O + 3 SO 2 + 2 H + - * 2 Cr 3 + + SO 2 and H 2 O 17.35 g of hexavalent chromium as metal required 63.0 g of anhydrous sodium sulfite. In practice, the amount of reducing agent required had to be increased to about 25% for complete reduction of chromium to the trivalent state.

The reduction can be carried out in another way using ferrous sulphate: the following reaction took place:

Cr20 + 6 Fe ++ + 14 H + ~ 7 2 Cr +++ + 6 Fe +++ + 7 H? 0

When using a C / Zn pair according to the method of the invention, the consumption of zinc is proportional to the amount of chromium present in the removal solution and to the pH. At pH 3-3.5, 17.35 hexavalent chromium consumed 41 g of metallic zinc as the metal. Such wear is greatly increased if the pH is brought below 2.

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Instead, if an electrode pair Cu, Hg / Fe poi :: is used. to a solution pH of 3 or lower is an iron consumption to reduce 17.35 g of chromium to a 3-value state of 20 g. The 6-value chromium was less than 0.05 ppm in the extraction solution.

Example 3

The reduction to 3-value chromium was performed starting from G-valent chromium in concentrated sodium nitrate solution using a pair of electrodes Cu, Hg / Fe. The waste solution contained 30% by weight

The concentration of NaNOg and 6-value chromium was 250 ppm as the declared metal. In this case, 30 g of iron was used for 17.35 g of 6-valent chromium for complete reduction. The amount of 6-value chromium in the removal solution was less than 0.05 ppm.

Example 4 In the coating treatment, by feeding 1500 mg / h of 6-valent 3 chromium, the amount of chromium contained in 2 m of feed solution could be completely reduced to 3-valent chromium using 3 kg of iron and 100 g of H 2 SO 4 to acidify the system. An electrode pair Cu, Hg / Fe was used.

Example 5

100 m 2 / h of waste from cooling towers was obtained, which waste contained about 30 ppm of chromates (15 ppm of Cr, which is equivalent to 1.5 kg / h of chromium). Complete reduction of 3-valent chromium was performed using 1.5 kg / h of iron.

The pair used was Cu, Hg / Fe.

Claims (3)

6281 O 9 A process for purifying wastewater from chromium, in which hexavalent chromium is electrolytically reduced to trivalent chromium, which is then doped as a hydroxide, and in which the treated wastewater itself is used as an electrolyte, characterized in that chemically different electrodes are used which are connected to each other at lower pH than 3.5, with one of the electrodes being electropositive for hydrogen and the other carbon or electrically negative. Method according to Claim 1, characterized in that iron and amalgamated copper are used as electrodes. Method according to Claim 1 or 2, characterized in that the electrodes are in the form of a rod, granules or powder. Method according to one of the preceding claims, characterized in that the clean solution is recycled and the cleaning waste is recovered.