Process and apparatus for the anodic oxidation of cyanide in aqueous solutions

The present invention relates to a cell for the electrochemical oxidation of cyanides in aqueous solutions and a method for carrying out the oxidation.

Cyanides in water, especially in industrial wastewater, are a major problem. Cyanides are highly toxic and highly ecotoxic. This applies to varying degrees to cyanides that are present in the solutions as free ions or are bound as metal complexes.

A particular problem is the exposure to cyanides, particularly in industrial wastewater. These occur in the areas of surface technology, in particular in metal deposition, demetallization processes, and in the production of metal coatings. Other areas are the hardening of steels by bath nitriding as well as the general area of ​​special waste treatment.

A frequently requested limit for the exposure to cyanides in waste water is 1 mg / L. Due to national or regional legislation, the permitted limit values ​​for cyanide pollution in wastewater can still be below this value.

There has been no lack of attempts in the past to reduce the cyanide load in aqueous solutions, in particular waste water. A frequently chosen approach to reduce the cyanide concentration in aqueous solutions is to oxidize the cyanide ions to carbon dioxide and ammonia.

The oxidation basically takes place in two reaction steps, the first reaction step in the decomposition of the cyanide ion in alkaline solution via the oxidation to the cyanate anion according to the following redox half-reaction (1): <maths id="math0001" num="(1)"><math display="block"><mrow><msup><mrow><mtext>CN</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msup><mrow><mtext> + 2OH</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msub><mrow><mtext> ⇒ H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext>O + OCN</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msup><mrow><mtext> + 2e</mtext></mrow><mrow><mtext>-</mtext></mrow></msup></mrow></math><img file="EP1518828A1_D0001.tif" /></maths>

The cyanate anion OCN formed^- is then broken down hydrolytically in the alkaline solution according to formula (2): <maths id="math0002" num="(2)"><math display="block"><mrow><msup><mrow><mtext>OCN</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msup><mrow><mtext> + OH</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msub><mrow><mtext> + H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msub><mrow><mtext>O ⇒ CO</mtext></mrow><mrow><mtext>3</mtext></mrow></msub><msup><mrow><mtext>​</mtext></mrow><mrow><mtext>2-</mtext></mrow></msup><msub><mrow><mtext> + NH</mtext></mrow><mrow><mtext>3</mtext></mrow></msub></mrow></math><img file="EP1518828A1_D0002.tif" /></maths>

The transfer of the two electrons per conversion of the formula can take place by oxidizing agents or electrochemical ways.

Suitable oxidizing agents are, for example, peroxides. However, processes using peroxides are generally uneconomical, since the catalytic effects of the metal cations usually present in the wastewater cause a strong decomposition of the peroxide to oxygen. Frequently, a 10-fold excess of hydrogen peroxide must be used to ensure reliable oxidation of the cyanides. The Kinetically Stable Cyano Complex [Ni (CN)₄]^2- is often insufficiently destroyed.

Another method is the use of UV radiation in conjunction with hydrogen peroxide. Such methods usually require high investment costs. They also show the disadvantage that UV radiation is only effective in thin layers due to the strong absorption by the metal cations present in the waste water. So-called charge transfer transitions occur here. As a result, the performance of such reactors is severely limited.

Another known method is the use of ozone. These processes are also characterized by high investment costs. In addition, the safe handling of ozone-generating equipment in operation with high ozone concentrations is extremely problematic. Ozone has a high toxicity, a short shelf life and in particular a high level of aggressiveness towards the apparatus components. This significantly reduces the service life of the corresponding equipment.

It is also known to use salt-like peroxides, such as carot, as an oxidizing agent. In addition to the disadvantages mentioned above, such as when using hydrogen peroxide, there is also the fact that undesired salting of the waste water can occur. This applies in particular to sulfate enrichment. This in turn has the consequences that, for example the concrete pipes of sewage systems take place due to a conversion of calcium compounds into calcium sulfate dihydrate, which has a destructive effect on the concrete pipes.

The likewise known use of sodium hypochlorite for cyanide oxidation also has disadvantages. The organic substances present in the wastewater load, such as surfactants, can be converted into organic halogen compounds by using hypochlorites. Such organic halogen compounds are also often toxic and ecotoxic.

The sum parameter for such organic halogen compounds is the so-called AOX value, defined as organic halogen compounds adsorbable on activated carbon. A typical tolerable limit value for the AOX value is 1 mg / L for waste water.

The methods described above are known, for example, from the monograph: Graef, R .; Paperback of wastewater, environmental technology in surface finishing; Karl Hansa publishing house; Munich (1998, pages 135-146).

Electrochemical processes for cyanide oxidation are also known. DE 23 16 124 describes a process for the oxidation of cyanides using two-dimensional electrodes made of graphite or titanium anodes coated with platinum. However, the device described there has the disadvantage that graphite is oxidized in alkaline media to graphite oxides and / or graphite acids and the electrodes are therefore not stable to corrosion and are passivated after a short electrolysis time. In addition, the oxygen overvoltage is low, so that the current yields are low even at high cyanide concentrations. The latter in particular must be rejected from an economic point of view.

The platinum coatings on platinum-coated titanium anodes slowly dissolve to form the tetracyano-platinate (II) complex. Therefore, even these cannot prove themselves in practice. In addition, the complexed platinum (II) cation is toxic and difficult to degrade.

A method for the anodic oxidation of cyanides is also known from DE 40 470 766. Chlorides are converted to hypochlorites at the anode in parallel to the cyanide oxidation. However, hypochlorite formation leads to the further formation of AOX compounds and should therefore be rejected.

The object of the present invention is to provide methods and electrochemical cells for the efficient and cost-effective oxidation of cyanides in aqueous solutions and to overcome the disadvantages of the prior art.

The problem is solved according to the technical features of the independent claims. Preferred embodiments are presented in the subclaims.

General description of the inventive concept.

The following invention provides an electrochemical cell comprising a cathode and an anode in the form of a fixed bed electrode. Fixed bed electrodes are known in principle.

Another feature of the invention is that the bed of the fixed bed electrode consists of high-purity manganese particles and / or particles consisting of oxides of titanium, the titanium oxide particles being in the Magneli configuration.

The present invention is based on the following known facts:

The current density for the electrochemical oxidation of CN^- is limited by the mass transport at the electrode. The diffusion limiting current density has an imitative effect, ie the convective mass transfer, which is defined as follows:         i_G = k_szFc_{(x = ∞)}     (3)

The parameters can be seen in the definition list.

From formula (3) it can be estimated which limit current densities and which space-time yields can be expected in p if the cyanide oxidation is carried out with conventional electrodes, ie planes with 2-dimensional electrodes. With such 2-dimensional electrodes, the microkinetic is equal to the macrokinetic current density. In contrast, the macrokinetic current density is much greater with 3-dimensional electrodes. It turns out that the factor 10 - 100 is reached, depending on the depth of penetration of the electrical current into the fixed bed.

In other words, with 3-dimensional electrodes under the same operating conditions it is possible to achieve about 20 times higher sales speeds than when using 2-dimensional electrodes. Accordingly, the space-time yield increases by this value, so that 3-dimensional electrodes can advantageously be operated extremely economically even in the range of low cyanide concentrations.

Based on the basic advantages of a 3-dimensional electrode, ie a fixed bed electrode, the choice of the anode material for the fixed bed electrode for the oxidation of cyanides is of crucial importance for the solution of the above tasks.

Surprisingly, it was found that a fixed bed composed of particles of high-purity manganese with a purity of at least 95% and / or particles of the oxides of titanium, the particles being in the Magneli phase, are particularly suitable.

The Magneli phases are unstoichiometric titanium oxides of the general formula Ti_nO_2n-1, with (n = 4 -10). Sandwich structures occur in the Magneli phases. These can be of the following type , for example : .... TiO₂/ TiO₂/ TiO₂/ TiO / TiO₂/ TiO₂/ TiO_2....

The TiO unit is formed by neighboring TiO₂-Units protected against oxidation. The redox couple Ti⁴⁺/ Ti²⁺ represents a semiconductor system, which gives the material good electrical conductivity. This is of the same order of magnitude as that of graphite. Such phases also have a high chemical resistance.

For the purposes of the present invention, the Magneli phases are Ti₄O₇ as well as Ti₅O₉ particularly suitable. These have outstanding electrical semiconductor properties. Experiments on this invention have shown that the oxygen overvoltage was sufficiently high in alkaline aqueous solutions, so that cyanides, including complex cyanides, can be used according to reaction equation (1) with high efficiency in the range of low concentrations, ie with low reaction inhibitions and thus with low Surges for the cyanides on the TiO_x-Magneli phases can be oxidized. Acknowledging this behavior of the TiO_x-Magneli phases compared to the alkaline solutions containing cyanide is new and was implemented in this invention to a technical system. Surprisingly, pure manganese behaves analogously.

Basically, Magneli phases show high oxygen overvoltages in alkaline solutions. It was found that such fixed bed anodes in an electrochemical cell according to the invention were stable for months with the alkaline and cyanide electrolytes used in the temperature range from 20 to 40 degrees Celsius and high anodic current loads.

Methods for producing titanium oxide particles in the Magneli phase are described, for example, in EP 0 047 595.

In a further embodiment of the present invention, the fixed bed can be formed from particles of manganese. The particles are in principle not subject to any restriction, with the exception that the purity is at least 95%, preferably 99%, particularly preferably 99.5%.

Such particles of high-purity manganese are generally known.

Pure manganese is very brittle. The desired particle fractions can be obtained from commercially available high-purity manganese flakes by light mechanical pressure and subsequent sieving.

During operation of the device according to the invention, the manganese particles coat with MnO in a short start-up phase with anodic polarization₂. The MnO₂ forms the electrochemically effective surface for cyanide oxidation. It should be noted that the electrical fixed bed resistance of a fixed bed made of manganese particles is higher than that of a fixed bed made of pure TiO_x-Mangneli particles. The cell voltage should be increased by around 2 to 5 V here.

Comparative example:

A fixed bed anode with anodically polarized lead particles was used for the oxidation of cyanides in aqueous solutions. It was shown that the electrochemically active PbO₂Layer was slowly converted to lead (II) oxides, hydroxides and / or carbonate. The fixed bed became inactive and was not dimensionally stable. The voluminous Pb corrosion products clogged the fixed bed and the flow resistance increased strongly. At the same time, the water was treated with highly toxic Pb²⁺-Contaminated compounds.

Fixed bed anodes made of glassy carbon, mesocarbon and crystalline graphite were also used. Under the harsh oxidizing conditions, these converted to graphite compounds, such as graphite oxides, graphite acids. The fixed bed material dissolved. The electricity yields and sales levels dropped to almost zero.

Brief description of the figures:

<ul id="ul0001" list-style="none"><li>Figure 1 shows a preferred embodiment of the basic structure of a device according to the invention for the anodic oxidation of cyanides in aqueous solutions.</li><li>FIG. 2 shows the mean current yield as a function of the cyanide concentration in a fixed bed with TiO_x-Magneli particle fill.</li><li>FIG. 3 shows the degree of conversion as a function of the cyanide concentration with a fixed bed with TiO_x-Magneli particle fill.</li><li>Figure 4 shows the degree of conversion depending on the number of passes through the cell with a TiO_x-Magneli particle fill.</li><li>FIG. 5 shows the mean current yield as a function of the cyanide concentration in a fixed bed with Mn particles.</li><li>FIG. 6 shows the degree of conversion as a function of the cyanide concentration in a fixed bed with Mn particles.</li><li>FIG. 7 is a summary illustration of the mean current yield as a function of the cyanide concentration with an Mn particle bed and a TiO_x-Magneli particle fill.</li><li>FIG. 8 is a summary of the mean space-time yield as a function of the mean cyanide concentration and an Mn particle bed.</li></ul>

Further advantages of the invention are clarified on the basis of the detailed description of the figures.

Figure 1 shows an embodiment of a cell according to the invention. The cell body consists of a lower tub body 1, a central ring 2 arranged above it, and a closing cover body 3. A suitable material for the cell body is polypropylene. Between the individual cell body parts 1, 2 and 3 there are sealing rings 5 ​​for sealing the cell body.

The anode 10 and the cathode 8 are arranged in the interior of the cell body. In the present exemplary embodiment, the cathode is in the form of a copper sheet with a circular geometry of approximately 280 mm in diameter. The cathode sheet is also provided with bores to allow the electrolyte, ie the solution containing cyanide to be cleaned, to pass through. For the electrical connection of the cathode 8 to the voltage source 9, the cathode 8 is provided with a web which passes through the cover body 3 and is held by means of a crimp connection 4, the crimp connection 4 simultaneously producing a gas-tight seal. Another suitable material for the cathode 8 is graphite.

In the illustrated embodiment, the electrochemical cell is designed for the batch process. The solution to be cleaned is temporarily stored in the reservoir 6 and flushed through the cell by means of a pump 7. The solution to be cleaned enters the tub body 1 below the anode 10, penetrates the anode and cathode and is pumped off in the upper region. The volume flow v in the illustrated embodiment is 100-300 L / h, for the illustrated embodiment with a cell cross section of 30 cm.

The arrow v shown inside the cell shows the direction of flow. The arrow I shown inside the cell indicates the cell current direction.

The voltage supply 9 is designed in the form of a rectifier system for direct current with a galvanostatic circuit, ie current I = constant. Typical values ​​for the operation of the cell are up to 15 volts and up to 30 amps of current, for the embodiment shown with a cell cross section of 30 cm.

The anode 10 is designed as a particle bed 12 with a defined layer height and a defined degree of gap. The layer height, hereinafter referred to as the fixed bed depth h_F referred to, is 2-8 cm, preferably 3-6 cm, particularly preferably 5-6 cm. The average gap degree ε is 0.4-0.65, preferably 0.45-0.6, particularly preferably 0.5-0.55.

The fixed bed anode 10 rests on the feeder electrode 11, through which the electrical contact to the voltage source 9 is provided. In the illustrated embodiment, the feeder electrode is formed by a perforated Pb sheet with around 7000 1 mm holes based on the 30 cm circle diameter. In the embodiment shown, the feeder electrode 11 is squeezed in a sealing manner between the sealing rings 5. The feeder electrode 11 can also be made from other suitable materials, such as graphite, glassy carbon or so-called mesographite. The following criteria must be observed when selecting the material for the feeder electrode. The material must be sufficiently corrosion-resistant and mechanically resilient to bear the load of the anode 10; the material must be sufficiently electrically conductive and sufficiently machinable to accommodate passages for the solutions to be refurbished, which in turn ensure the volume flow.

It was shown that the feeder electrode is not subject to corrosion due to the electrochemical interaction. Materials which are not suitable for forming the anode 10 can thus also advantageously be used for the feeder electrode 11. Without being bound by any theory, it is assumed that the feeder electrode 11 is not or only slightly polarized, as a result of which electrochemical corrosion of the feeder electrode 11 does not occur. The polarization takes place exclusively or almost exclusively in the fixed bed 12 of the anode 10.

In the embodiment shown, the cathode 8 is a copper sheet in circular geometry with a diameter of around 280 mm, which is designed as a perforated sheet. The entire cathode can be moved in the axial direction. Depending on the solution to be worked up, metals may separate out in sponge form during operation. These can be easily removed from the cathode. For this purpose, the cathode 8 is removed by opening the lid body.

During operation, hydrogen is primarily formed on the cathode 8 according to formula 4: <maths id="math0003" num="(4)"><math display="block"><mrow><msub><mrow><mtext>2H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext>O + 2e</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><msub><mrow><mtext>→ H</mtext></mrow><mrow><mtext>2</mtext></mrow></msub><msup><mrow><mtext>(g) + 2OH</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><mtext>.</mtext></mrow></math><img file="EP1518828A1_D0003.tif" /></maths>

The hydroxide ion is also advantageously formed here, which leads to an increase in the pH of the solution.

In a further reaction of the cathode in the treatment of waste water containing metal ions, as occurs in surface technology / electroplating, the corresponding metals are separated from their cyanide complexes. The deposition takes place according to equation 5:<maths id="math0004" num="(5)"><math display="block"><mrow><msup><mrow><mtext>[Me</mtext></mrow><mrow><mtext>Z +</mtext></mrow></msup><msup><mrow><mtext>(CN</mtext></mrow><mrow><mtext>-</mtext></mrow></msup><msub><mrow><mtext>)</mtext></mrow><mrow><mtext>4</mtext></mrow></msub><msup><mrow><mtext>] + ze</mtext></mrow><mrow><mtext>-1</mtext></mrow></msup><msup><mrow><mtext> → Me + 4CN</mtext></mrow><mrow><mtext>-</mtext></mrow></msup></mrow></math><img file="EP1518828A1_D0004.tif" /></maths>

The cyanide ions released on the cathode according to equation 6 are then oxidized on the fixed bed anode. In the illustrated batch process, this can happen, for example, when the solution to be processed is run through the cell a second time.

Basically, the deposition of metal cations from their complexes on the cathodes of electrochemical cells is known. According to the prior art, the breaking up of such complexes is limited to less stable metal complexes.

It was shown that the thermodynamically and kinetically very stable nickel (II) cyano complex, which is considered to be particularly ecotoxic, is almost completely broken down in the cell according to the invention. This is particularly advantageous since the nickel (II) cyano complex is not effectively broken down by strong oxidizing agents such as peroxides or hypochorites. The same applies to the known electrochemical methods. In the illustrated embodiment of the present invention, the complex was found to be below 0.05 mg-Ni²⁺/ L lower.

According to the present invention, the nickel (II) cyano complex is destroyed on the anode side. Due to the high selective oxidation ability of the cell according to the invention with regard to cyanide ions, it is possible to oxidize the cyanide ions bound in the complex. The metal cations in question are thereby freed from their cyanide complexes and are accessible for cathodic deposition.

Table 1 shows the results of the processing of electroplating, cyanide and heavy metal waste solutions using the cell according to the invention. The total volume of the solution to be worked up was 14 liters. The fixed bed was made of manganese particles or TiO_x-Magneli particles formed. The purity of the manganese particle bed was 99.5%. The geometry of the particles was platelet-shaped with a diameter of 1.5 mm and a thickness of 1.5 to 3 mm. The TiO_x-Magneli particles had a cylindrical particle shape with a diameter of 1.6 mm and a height of 1.6 mm. The cell is undivided. The cathode is in the form of a copper sheet. The feeder electrode is a Pb perforated disc.

The solution to be cleaned is worked up in a batch process, with the solution being simultaneously conveyed through the cell in a cyclic process.

The other operating conditions can be found in the table.

FIG. 2 is a graphical representation of the results of working up the solution to be purified, the mean current yield as a function of the cyanide concentration being shown in the range of low cyanide concentrations.

The parameters are as follows:

The solution to be cleaned flowed continuously through the fixed bed. Titanium oxide Magneli particles (cylindrical particle shape with d = 1.6 mm, h = 1.6 mm) served as the bed of particles; H_F= 3 cm, pH = 13.5, ε = 0.54, V '= 1.6 L / (hdm²), 20 ° C, U_Z.= 10 V ', Δ≈10 mm, parameter i_b. The cell was undivided and fitted with a graphite electrode. A Pb perforated disc served as the feeder electrode. The concentrations were measured after a single pass through the cell.

FIG. 3 shows the degree of conversion as a function of the cyanide concentration according to FIG. 2.

It can be seen from FIGS. 2 and 3 that effective cyanide oxidation is possible by the invention even in the lowest concentration ranges.

Figure 4 shows the results of a long-term test. The degree of conversion is plotted as a function of the number of passes (n) of the solution to be cleaned through the cell. Also in the range of low cyanide concentrations. The parameters are identical to those according to FIG. 1, i_b= 0.94-0.85 A / dm² and β = 0.2-0.3.

FIG. 5 shows the average current yield as a function of the cyanide concentration in the range of low cyanide concentrations when using a fixed bed with Mn particles. The following parameters applied: The fixed bed of Mn particles was continuously flowed through. The purity of the manganese was 99.5%, the particles were designed as platelets with a diameter of 1.5 mm and a thickness of 1.5 to 3 mm; H_F= 4 cm, pH = 13.5, ε = 0.52, V. = 1.6 L / (hdm²), 20 ° C, U_Z.= 15 V, Δ≃10 mm, parameter i_b. The cell was undivided and provided with a graphite electrode; a Pb perforated disk was used as the feeder electrode.

FIG. 6 shows the results according to FIG. 5 plotted as the degree of conversion as a function of the cyanide concentration.

FIGS. 5 and 6 show the outstanding suitability of the cell according to the invention with manganese particles for removing cyanides from solutions with low concentrations.

FIG. 7 is a summary representation of the average current yield as a function of the average cyanide concentration when working up electroplating, cyanide and heavy metal-containing waste solutions using a cell according to the invention in accordance with the results in Table 1.

FIG. 8 is a summary of the mean space and time yield as a function of the mean cyanide concentration according to the data in Table 1.

FIGS. 7 and 8 show outstanding current efficiency and space and time efficiency of the cell according to the invention, refer. of the method according to the invention can be seen. This minimizes the operating costs and the duration of the treatment when cleaning aqueous solutions containing cyanide.

Industrial applicability.

From the above it can be seen that the invention provides a cell and a method for the effective and economical of aqueous solutions containing cyanide.

Definitions:

<ul id="ul0002" list-style="none"><li>A_p= Particle surface (= reaction surface)</li><li>A_L= Empty pipe cross-sectional surface</li><li>A_S= specific surface (based on A_L)</li><li>c_O(CN^-) = Initial cyanide concentration (cell entrance)</li><li>c (CN^-) / t = cyanide concentration after the electrolysis time t</li><li>c (Me^{n +}) = Concentration of heavy metals Cu²⁺, Ni²⁺, Zn²⁺ and Cr³⁺ and change due to cathodic reduction after the electrolysis time t</li><li>c_{(x = ∞)}= Concentration of the CN to be converted^-- Quantity in front of the reaction surface, outside the diffusion boundary layer (= "bulk concentration")</li><li>c (CN^-) = mean cyanide concentration in the cell between the initial concentration c_O and the concentration at the outflow of the cell c_A : c (CN^-) = (c_O + c_A)/2</li><li>d, h = characteristic particle geometry (d = diameter, h = height)</li><li>H_F= Fixed bed depth</li><li>F = Faraday constant</li><li>I._Z.= Cell current of the 14 L cell</li><li>i_b= Current density related to the empty pipe cross-section ("bulk current density")</li><li>i_G= microkinetic limit current density (based on the particle surface)</li><li>k_S= Mass transfer coefficient</li><li>M (CN^-) = Molar mass of the cyanide ion</li><li>t = electrolysis time</li><li>U_Z.= Cell voltage</li><li>u = degree of conversion after the electrolysis time t</li><li>v = volume flow rate</li><li>V_O= total volume of waste water to be processed (L = liter)</li><li>V '= volume flow density, based on the empty pipe cross-section of the cell</li><li>V_Z.= Volume of the electrochemical cell</li><li>z = number of charges</li><li>β = average current yield based on c (CN^-)</li><li>ϑ = electrolyte temperature</li><li>Δ = distance between fixed bed anode (upper edge) and cathode</li><li>ε = gap degree</li><li>κ = specific electrical conductivity of the electrolyte</li><li>ρ = mean space-time yield for the cyanide degradation, here based on the 14 L cell; cyanide mass Δm (CN^-) based on the volume of cell V_Z. : ρ = {Δm (CN^-) / Δt} · 1 / V_Z.<img file="EP1518828A1_D0005.tif" /></li></ul>