DK142049B - PROCEDURE FOR ELECTROCHEMICAL CLEANING OF WASTE CONTAINING HEAVY METAL COMPOUNDS - Google Patents

Description

(11) PRESENTATION 142049 (one in gpHf y V ^ Ra / DENMARK ("i) intci.3 i / 62" (21) Appendix No. I975 / 72 (22) Filed on Apr 21, 1972 (23) Running Day 21 · Apr · 1972 (44) The application submitted and also

the petition published on 18 Aug. 1 SoO

DIRECTORATE OF, n, D. _. _. .

PATENT AND TRADE MARKET (30> Priority requested from it

Apr 23 1971s 235W71, IT

May 7, 1971a 24205/71, IT

__________ _ _______ (71) SNAM PROGETTI S.P.A., Corso Venezia, 16, Milan, IT.

(72) Inventor: Nunzio Maetrorilli, Via Rlsorgimento, 10, Milan, IT. (74) Plenipotentiary in the proceedings:

International Patent Bureau._ ________________ (54) Process for the electrochemical treatment of wastewater containing heavy metal blend is.

The present invention relates to a process for the electrochemical purification of wastewater containing heavy metal compounds, in particular chromates and mercury salts, in which process the content of heavy metal is reduced by providing a redox process.

It is known to provide a redox process in solutions containing heavy metals such as e.g. copper salts, chromates or mercury salts or complexes, by the addition of a less noble metal as such, e.g. zinc or iron, or in the form of ions that can be oxidized to a higher valence state, e.g. ferrous ions, or the addition of another reducing agent.

In this way, various forms of waste water have been sought for heavy metals, either for the purpose of avoiding environmental poisoning or for the recovery of valuable metals or both. Eg. For example, 2 14 2 Ο Λ 9 chromates have been reduced to trivalent chromium salts, copper salts to metallic copper and anionic mercury complexes in equilibrium with the cation to metallic mercury.

However, no completely satisfactory discharge of wastewater for heavy metals has been achieved by the known methods, and other disadvantages often occur. Thus, by reducing mercury compounds with a metal such as e.g. iron, an amalgam is formed which requires further treatment both for the separation of the amalgam and for the recovery of the two components. Furthermore, a relatively large amount of reducing agent is often used. weight unit heavy metal. A number of other disadvantages will become apparent from the following description in connection with the discussion of the present invention.

In the method according to the invention, these disadvantages are substantially reduced. The process is characterized in that at acidic pH two, electrode materials, one of which is electronegative and the other electropositive with hydrogen, are contacted with the wastewater. The two electrode materials can e.g. be zinc and carbon or iron and mercury.

The acid-effected solution of the less noble electrode relative to hydrogen elicits a flow of electrons toward the other electrode, and the reactions elicited in the electrolyte proceed without outside energy supply, producing, on the contrary, electrical energy in the form of a direct current which passes from the anode to the intangible cathode and can be measured through the outer conductor if two appropriate elements are used, which are separately immersed in the liquid (electrolyte) to be treated and which are short-circuited outside it.

The connection between the two different materials can also be provided in the solution itself, especially using multiple electrodes of very small dimensions. For this purpose, electrode materials in the form of rods, grains or powders may conveniently be used. When used in admixture with one another, the liquid can continuously pass through the mixture and come out completely purified.

The redox reactions do not depend on the setup chosen. In contrast, the kinetics depend on the shape and possibly the distance between the electrodes.

The process of the invention is quite general, that is, it can be used both in the conversion of oxidative salts to other salts in which the metal has a lower valence state, and to obtain noble metals from the corresponding salts. The reaction mechanism is described below with reference to the treatment of chromates and the treatment of mercury salts, respectively. In the presence of a sodium chromate solution, the following reaction takes place as the anode consists of metallic zinc and the solution is acidified by nitric acid 3 Zn + Na 2 Cr 2 0 7 + 14 HNO 3 = 3 Zn (NO 2 + 2 NaNO 2 + 2 Cr (NO + 7 H 2 O and in a more general form 3 Me + Cr 2 O 7 "+ 14 H + * 3 Me 2 + 2 Cr 2 + 7 H 2 O, Me being a divalent metal electropositive to hydrogen.

If the metal (Me) has higher valences, a further reaction occurs, e.g. as follows 6 Me "1" 1 · + Cr207'_ + 14 H + <= 6 Me444 "+ 2 Cr +++ + 7 H20

This occurs, for example, when Me is iron, being first divalent and then trivalent.

Illustrated by way of example, the following describes the treatment of chromate-containing wastewater from the chromium or nickel-1 chromium steel reprocessing. It will then be very easy for a person skilled in the art to apply the principle of the invention in the treatment of wastewater containing chromates from another source.

The electrochemical work-up of chromium or nickel-1 chromium steel consists essentially of an anodic oxidation in an electrolytic bath which is made up of an aqueous 30% sodium nitrate solution.

During the work-up, considerable amounts of soluble sodium chromate are formed, and also mud containing iron and chromium hydroxides as well as basic salts is formed.

Upon subsequent centrifugation of such a suspension, the precipitates can be separated and the alkaline nitrate-rich or -chromatic-rich solution recycled.

However, the sludge is discharged after a partial thickening, both for the sake of centrifugation and, above all, to avoid a continuous increase of the chromate concentration of the recycled electrolyte. The effluent to be derived is therefore a suspension of iron and chromium hydroxides and basic nitrates dispersed in a solution of sodium nitrate and chromate.

The concentration of the sodium chromate present in the liquid phase of such a suspension depends on the chromium content of the treated alloys, and is usually greater than 250 ppm Cr, corresponding to approx. 750 ppm NajCrO ^.

This electrochemical work-up is carried out, for example, as schematically shown in the drawing of FIG. First

In FIG. 1, a container 1 contains the pure electrolyte, and sodium hydroxide nitrate is passed to the container through a conduit 14. Through a conduit 2, the electrolyte is sent to an apparatus 3 in which the electrolytic reprocessing takes place.

The impure electrolyte, composed of soluble sodium chromate and sludge containing iron and chromium hydroxides, exits the apparatus 3 and is passed through a conduit 4 to a container 5. Through the conduit 6, the suspension is sent to a centrifuge 7 which is fed with water in the wash phase. through a conduit 15. At the subsequent centrifugation of the suspension, the precipitates are separated and the alkaline nitrate-rich and chromatically rich solution is recycled through conduits 8 and 9 to the container 1. The sludge sent through a conduit 10 to a container 11 is constituted by a suspension of iron and chromium hydroxides and basic nitrates dispersed in a solution of sodium nitrate and chromate.

The cleaning in the container 11 is carried out in the manner indicated above. Two electrodes are immersed in the liquid, iron as anode and amalgamated copper as cathode, and by shorting the electrodes, the following reaction occurs 3 Fe + Cr207 ~ '+ 14 H + ^ => · 3 Fe ** + 2 Cr +++ + 7 H20 thus chromates are reduced to trivalent chromium salts.

The divalent iron present in the solution is oxidized to trivalent iron as a result of an excess of nitrate rations, and the nitrogen oxides formed by this redox reaction dissolve in the system and remain therein. The nitrations are quantitatively restored when the system is made alkaline in the presence of air for the purpose of obtaining the trivalent chromium and iron hydroxides and separating them from the system.

The acidification necessary for the formation of iron and chromium salts may be such that no solution of the iron compounds already present occurs.

After the reaction, as mentioned above, iron and chromium are removed by addition of sodium hydroxide, to precipitate in the form of metal hydroxides.

During this phase, the entire amount of nitric acid, before use, is present in the solution in the form of sodium nitrate which is recycled through conduits 12 and 13 to the container 1. The electrolyte's alkaline nitrate amount is therefore automatically restored and this causes the otherwise necessary nitrate additions to the solution to be used. recirculated, avoided.

After the separation of the metal hydroxides, e.g. by means of the centrifuges already present, a solution containing only sodium nitrate can be obtained in the aforementioned treatment and the solution as such is suitable for recycling in a closed cycle.

One for the one shown in FIG. In the schematic procedure, an interesting variant which makes such a procedure quite general consists in also using the method according to the treatment of the suspension in the container 5. After the reduction and neutralization it is possible in this way to make a separation in the centrifuge 7 of sludge. which can be deduced since in the residual liquid phase there are no chromates or heavy metal salts.

By applying the method of the present invention to solutions or suspensions of mercury salts, metallic mercury is formed. The reactions apparently occur without the use of acid, e.g. according to

HgSO4 + Zn. + H2SO4 - ZnSO4 + H2SO4 + Hg but, in fact, the amount of metallic mercury formed is stoichiometric equivalent to zinc, the amount of acid reacted with zinc, as shown in

Zn + H2SO4 = ZnSO4 + 2H + 2H ++ HgSO4 = H2SO4 + Hg

This relationship can be illustrated by means of the drawing of FIG. 2nd

In FIG. 2, an outer container 1 contains the mercury salt solution, while a container 2 of a porous material contains an acid. The first container contains an inaccessible electrode 3, while the second container contains one of the acids attackable metal electrode 4. The arrows indicate the direction of the electron current.

Provided that the mercury salt is difficult to hydrolyze, the solution in the container 2 at the beginning of the experiment is acidic, while the solution in the container 1 is neutral.

During the experiment, a migration of electrons from the electrode 4 to the electrode 3 and a passage of hydrogen ions from the container 2 to the container 1. takes place if the original solutions in the containers 2 and 1 have the same normality and if the ratio of the volumes of the solutions in the containers 2 and 1 is l: n, with n being much greater than 1, the solution in the container 2 is neutral after 1 / n of the mercury initially present in the container 1 is in reduced form, while the amount of acid from the container 2 is recovered in the container 1.

Therefore, without mercury, metallic mercury can be obtained from mercury salt solutions using another metal attackable by acids and an amount of acid equivalent to the amount of mercury to be separated in metallic state.

An acidity equivalent to the introduced acidity is constantly present in the system and it can be attributed, without regard to the degree of porosity of the inner container, which theoretically can be derived from the combination of the hydrogen ions of the added acid with the anions of the originally present one (s). mercury salts or mercury salts. The free acid is part of the aforementioned process, but the real acidity of the system is kept constant, so an acid addition is usually not required.

In the case of water containing mercury salts, it will be appreciated that the advantage of using the process of the present invention is considerable, because without the addition of energy it is possible to isolate mercury in metallic state.

According to the aforementioned system, the metallic mercury is obtained directly in the pure state in the zone in which the carbon electrodes are immersed, especially by the insertion of a porous wall between the zones in which the zinc and carbon electrodes are placed.

No such result could be obtained in any of the known methods, nor could the metallic mercury be precipitated by the precipitation process, although it appears to be similar to the above described method with respect to some of the parameters stated, since it consists in reduction. of mercury salts using a less noble metal. As mentioned earlier, an amalgam is formed which necessitates further treatments. In addition, the reprocessing rate decreases as the amalgamation proceeds.

In addition to extracting the precious metal directly in the metallic state, in the present process, these metals in solution can be replaced with the ions of a metal which may be pre-selected, taking into account the following in the further purification of the methods used.

Prior purification according to the prior art involves a local treatment of the effluents, it contains in advance of the precious metals purified waste an equivalent amount of ions of a less noble metal, e.g. zinc. This is precipitated during flocculation in the form of insoluble hydroxide.

If calcium hydroxide is used in the flocculation, after the complete treatment of the effluent, the precious metals are replaced with calcium ions, and zinc is converted into zinc hydroxide which can be recovered as an oxide from the smoke from a combustion furnace used in the sludge.

Below is a diagram of the possible reaction in which copper is the precious metal

CuS04 + H2S04 + -Zn = ZnS04 + H2S (> 4 + Cu

ZnSO4 + H2SO4 + 2 Ca (OH) 2 = 2 CaSO4 + Zn (OH> 2 + 2 H2 O

Zn (OH) 2 = ZnO + Η2 <3

The relationship is as follows:

CuSO4 + Zn + Ca (OH) 2 = CaSC> 4 in the liquid effluent + ZnO in the solid state + Gu in the solid state + HjO'i in the vapor state

Thus, the salinity of the treated effluent is virtually unchanged since the cation of a noble metal is simply replaced by calcium ions.

This makes it possible to purify the derived material, also for any biological purification that would not be possible in the presence of salts of metals, such as Cu, Hg, Ag, etc., which are toxic to the bacteria.

A third significant advantage of the present process is that no purification apparatus is required, simply introducing into the effluent a number of different electrodes.

The same results can be obtained by using as an anode a metal which does not form amalgam with mercury, e.g. iron. Just such an application makes it easy to use simple columns filled with small grains of the two materials of anodic and cathodic nature (iron and other non-corrosive materials, respectively, which act as a mercury electrode). , as a cleaning device: the materials are brought into very intimate contact with one another. Since iron does not form amalgam, mercury can be extracted in metallic state.

Regardless of the pollutant element to be reduced, it is usually advantageous as a cathode to use a material which acts as a mercury electrode, for the purpose of obtaining a high voltage over hydrogen, thereby allowing it to operate in a wider pH range without development of hydrogen.

For this purpose, amalgamated metals, carbon there, can be used. is impregnated with mercury, or simply metallic mercury. The cathode components may also be prepared by coating or impregnating a support with a metal berry tooth portion capable of forming amalgam, and then by amalgamating the metal component.

If apparatus made up of filled columns are used in which contact between the electrodes is provided in the aqueous phase itself, it is appropriate as an anode to use a non-amalgam constituent for the purpose of maintaining a constant potential difference between the electrodes, and advantageously, iron can be used.

In any case, the anode may advantageously consist of zinc, nickel, tin, lead, iron, chromium and all the metals which can be attacked by non-oxidative acids, that is, the metals which are electropositive to hydrogen. They may optionally be amalgamated for the purpose of controlling the corrosion rate.

The cathode may be carbon, metals that are electronegative to hydrogen, their amalgams, mercury-impregnated materials, and all materials that are not corroded by acids and conductors.

If electrochemical purification alone was performed using the anode component of metallic nature according to the above reaction schemes, the advantages would not be particularly noteworthy. First of all, the fact is that when a cathode acting as an electrode that is electronegative to hydrogen is not used, there are lower potential differences with respect to the anode component, and as a result, the reaction rates are lower. Further, when a cathode having a significant voltage over hydrogen is not used, as is the case with a cathode which acts as a mercury electrode, hydrogen can be released from the anode, thereby increasing the acid consumption and polarizing the anode. In the case of water containing precious metal salts, a precipitate or amalgam could be formed, whereby all the disadvantages mentioned above could occur. If solutions containing oxidative compounds at high concentrations, e.g. chromates at a concentration of 1000 ppm Cr so that the above reactions proceeded and the acid consumption was equivalent to the amount of reduced chromates, the hydrogen ion concentration would be reduced, that is, the pH would be increased, especially on the anode surface if the metal assumes higher • «· +++ valences when dissolved. In the case of iron, Fe ions are formed at such pH values at which trivalent iron hydroxides or basic salts can be formed. Since these are insoluble substances formed near the metal surface on which Fe +++ ions have been formed, the dissolution process could be slowed down due to surface polymerization phenomena. In this case, a complexing agent, e.g. the post-salt salt of ethylene diamine tetraacetic acid, with the aim of avoiding the formation of insoluble iron hydroxides and basic salts.

In order to avoid contacting most of the cathode surface with the anodes in the method according to the invention, it is advisable to use specially designed cathodes, even if columns which are arbitrarily filled with cathodic and anodic materials are used.

It is e.g. it is possible to use a cathodic material in the form of small tubes, and if the anodic material has such dimensions that it cannot fall into the small hollow cylinders thus provided, a cathodic surface made up of the inner walls of the tubes can be used. that are not in contact with the anodes, even if they are applied to a voltage due to the contact between these anodes and the outer walls of the hollow cylinders. In such an arrangement, the chromate-containing solution is reduced by contact with the inner wall of the cylinder, and then chemistries already present in the solution are reduced while not reduced during formation.

The optionally formed iron hydroxide and the optionally formed basic salts then assemble between the liquid phase and the anode liquid compartment.

The process according to the invention is explained in more detail in the following examples.

Example 1.

The process outlined in FIG. 1, dealing with electrochemical treatment of chromium and nickel-chromium steels, was carried out with the aim of giving the steel a desired contour by means of an anodic solution and the concentration of the sodium chromate present in the liquid phase of the suspension to be discharged (11 in the figure) was approx. 750 ppm, corresponding to approx. 250 ppm Cr.

Proceeding as previously described, the chromates were completely reduced in about 2 hours, and it was possible to detect a passage of a current of 100 mA at a potential difference of 1 V.

No reduction of ferric salts or nitrates was observed.

9 142049

Example 2 In systems for preparing acetaldehyde from acetylene, the catalyst compartments discontinuously deliver mercury sulfate-rich solutions which may contain up to 200 mg Hg / l.

The example relates to the treatment of a mercury sulfate solution containing 200 mg Hg / l. If the mercury sulfate is dissolved, the solution is acidic due to hydrolysis.

The minimum amount of acid needed to avoid the hydrolysis is one which gives the solution a pH of approx. 2, i.e. an acidity at which the process can be performed.

2

It was therefore necessary to use a number of zinc plates (5 m / m solution) and a number of carbon bars. Zn and C are electrically connected outside the solution.

The Zn-C system, in the presence of the solution, allowed metallic Hg to be recovered on the carbon surface, first in the form of a gray powder and then in the form of small droplets, after which it was separated as metallic Hg and collected in the space under carbon. The pH remained constant without acid addition. After 8 hours, the reaction was complete (99.5%). At least 65 g of Zn was dissolved and 200 g of metallic mercury was formed per ml. m added effluent.

Example 3 In plants for the production of vinyl chloride from acetylene, the tubes connecting the reactors and the washing tower have significant HgClg coatings as mercury salts are used as catalysts. During the repair stops, HgCl 2 is washed out with water, and thus a waste which is considerably contaminated with Hg is obtained, as the aforementioned coatings can; be 10 to 15 mm thick.

The apparatus of the preceding example was used and a better result could be obtained if the carbon rods were introduced into a porous container.

The first reduction product is the insoluble mercuric chloride formed within the porous container which is then converted to metallic mercury. The stirring of the liquid in the vicinity of the carbon electrodes increased the rate of the reactions which took place here in a heterogeneous system due to the presence of the insoluble mercurochloride.

If the contaminated solution contained 1000 ppm Hg, at least 310 g of Zn was used per day. ra treated effluent, and approx. 1000 g of mercury.

Example 4

This experiment is purely a comparative experiment and shows the ordinary course of a chromium plating plant.

10 142049

In the most commonly used process for purifying the corresponding wastewater, the pH is adjusted to a value lower than 3, and a gaseous SO 2 or a solution of sodium sulfite or hydrogen sulfite is added at a concentration of 50 to 100 g / l.

The reactions are as follows:

Cr20 "+ 3 HS03" + 5 H + ->. 2 Cr ^ 4 + 3 SO ^ „+ 4 H2 O

Cr207 ~~ + 3 SO2 + 2 H + -K 2 Cr444 + 3 SO4 "+ R20 17.35 g hexavalent chromium, as metal, needs 63.02 g anhydrous sodium sulfite.

In reality, the amount of reducing agent must be increased by approx. 257 «for the complete reduction of chromium to the trivalent state.

The reduction can also be carried out in another way, namely using ferro-sulfate, and the reaction is as follows:

Cr20? + 6 Fe + 14 H -fc. 2 Crm + 6 Fe ^ + 7 H20 In the freffl method of the invention, the zinc consumption using the electrode pair C / Zn is dependent on the amount of chromium contained in the delivered solution and on the pH.

At a pH of 3 to 3.5, 17.35 g of hexavalent chromium, like metal, consumes 41 g of metallic zinc. Consumption is significantly increased when the pH is lowered below 2.

By contrast, if the electrode pair Cu, Hg / Fe is used, the iron consumption for reducing 17.35 g of chromium to the trivalent state is 20 g at pH values of the delivered solution of 3 or less.

The content of hexavalent chromium in the delivered solution was less than 0.05 ppm.

Example 5

Using the electrode pair Cu, Hg / Fg, hexavalent chromium contained in a concentrated solution of sodium nitrate was reduced to trivalent chromium. The waste solution contained 30% by weight sodium nitrate and the concentration of hexavalent chromium 250 ppm expressed as metal. Here, 30 g of iron was used to completely reduce 17.35 g of hexavalent chromium.

The content of hexavalent chromium in the delivered solution was less than 0.05 ppm.

Example 6

The example relates to the removal of mercury salts from wastewater containing this in an amount of 25 ppm. Using the Cu / Fe electrode pair, it was noted that during the experiment, the copper cathode was amalgamated while the dissolved solution became rich in divalent iron. The consumption of metallic iron to reduce 100.30 g of divalent mercury was approx. 30 g.