Methods for treating wastewater

Abstract

A method for treating wastewater containing cyanide compounds and metal compounds, wherein the wastewater is subjected to a single oxidation step during which the cyanide compounds are converted into carbon dioxide and nitrogen, the oxidation step comprising mixing the wastewater with a chlorine solution and an alkaline reagent to obtain a mixture, the alkaline reagent being added in an amount such that the pH of the mixture is maintained between 8.8 and 9.5, and the chlorine solution being added in an amount such that the redox potential of the mixture is maintained between 150 mV and 450 mV.

Description

[0001] This invention patent application is a divisional application of the invention patent application filed on December 13, 2018, with application number 201880074282.6 and invention title "Method for Treating Wastewater". Technical Field

[0002] This invention relates to a method for treating wastewater containing cyanide compounds and metal compounds. Background Technology

[0003] Steel mills emit large quantities of dust-laden gases. These gases need to be purified, typically using water and thus generating wastewater that must be discharged. This wastewater contains pollutants present in the dust, which may significantly contain cyanides, ammonium, fluorides, and metals harmful to health and the environment.

[0004] Cyanides are highly toxic compounds harmful to the environment, and they need to be converted into non-toxic components before water can be discharged and / or recycled. These cyanides exist in different forms: they can be simple cyanide compounds (NaCN, KCN, etc.) composed of cyanide polyatomic anions and alkaline earth metals, but they can also be weak acid dissociable (WAD) cyanides, namely coordinated metal cyanides (Zn(CN)). -2 4. Cd(CN) -1 3. Cd(CN) -2 4…), when exposed to weakly acidic environments (pH 4, 5 to 6), tend to decompose into free cyanide and transition metals. Free cyanide is the biologically available form of cyanide known for its toxic effects on organisms. In addition to cyanide, some thiocyanates (SCN) may also exist; these are not cyanide substances, but their effective treatment may be of concern in some cases.

[0005] As an example, target emission limits could be 0.4 mg / L for cyanide, 2 mg / L for zinc, 5 mg / L for iron, 0.5 mg / L for lead, and 30 mg / L for ammonia nitrogen.

[0006] One known method uses hydrogen peroxide as an oxidant to convert cyanide (CN-) into cyanate (OCN-) (1), which can then be rapidly hydrolyzed into carbonate and ammonia (2):

[0007]

[0008] As disclosed in several documents (US 3,970,554, US 4,416,786, US 5,246,598), this method requires the use of a catalyst, such as a copper- or silver-based catalyst, which also needs to be removed. Furthermore, this method allows for the removal of WAD cyanide, but cannot remove all cyanide present in the wastewater.

[0009] Another known method is the alkaline chlorination method, as shown in document GB 759 109. This method uses hypochlorite and is carried out in two steps. First, cyanide (CN) is... - ) oxidized to cyanate (OCN) - Hypochlorite (ClO) is then oxidized into carbon dioxide and nitrogen gas. - This is produced by contacting chlorine (Cl₂) with sodium hydroxide (NaOH) (Equations 3 and 3'). The reaction is reversible, and some free chlorine remains in the solution. In cyanide conversion, hypochlorite (ClO₂) is produced. - ) and cyanide (CN - The reaction produces cyanogen chloride (CNCl) (Equation 4). Cyanogen chloride (CNCl) reacts with available hydroxides (OH-) to form cyanogen chloride (CNCl) (Equation 4). - The reaction forms cyanate (CNO). - (Equation 5). Then cyanate (CNO) - It is converted into the more harmless carbon dioxide and nitrogen (Equation 6).

[0010]

[0011] Cyanide (CNCl(g)) is a highly toxic compound; it must be rapidly degraded to prevent release into the atmosphere. The first step (Equations 3 to 5) takes place in a first reactor where the pH is maintained between 10 and 12 to optimize the conversion of cyanide to cyanate and to immediately convert CNCl to cyanate, preventing its release from solution. This high pH also oxidizes the metal compound. This process typically lasts 40 to 60 minutes, up to 12 hours, when certain metal cyanide complexes are present. The second step takes place in a second reactor where the pH is lowered to 7.5 to 8.5. It must never be lowered below pH 7, as this could produce highly toxic hydrogen cyanide if the first stage reaction is incomplete. The second step requires a reaction time of 30 to 60 minutes at pH 7.5 to 8.5. Lime (Ca(OH)₂) is typically used to generate hydroxide ions (OH⁻). - This method requires the use of multiple tanks to perform different steps at different pH values. Furthermore, it consumes large quantities of reactive substances, namely sodium hypochlorite (NaClO) and lime (Ca(OH)2). Summary of the Invention

[0012] There is indeed a need for an improved method for treating wastewater containing cyanide and metal compounds, a method capable of converting all types of cyanide compounds into non-toxic compounds with greater efficiency (particularly in terms of reactive material consumption and treatment time). In a preferred embodiment, such a method can also treat thiocyanate compounds to reduce their concentration.

[0013] The problem is addressed by a method for treating wastewater containing cyanide compounds and metal compounds, wherein the wastewater is subjected to a single oxidation step during which the cyanide compounds are converted into carbon dioxide and nitrogen. This oxidation step involves mixing the wastewater with a chlorine solution and an alkaline reagent to obtain a mixture, wherein the alkaline reagent is added in an amount such that the pH of the mixture is maintained between 8.8 and 9.5, and the chlorine solution is added in an amount such that the redox potential of the mixture is maintained between 150 mV and 450 mV.

[0014] Those specific operating conditions allow for the oxidation of several cyanide substances and metal compounds present in the wastewater in a single step and therefore in a single device.

[0015] The method of the present invention may also include the following optional features, either individually or according to all possible combinations of techniques:

[0016] - The chlorine solution is a sodium hypochlorite solution.

[0017] - The alkaline reagent is lime.

[0018] - The pH of the mixture was maintained between 8.9 and 9.1.

[0019] - The redox potential of the mixture remained between 350 mV and 400 mV.

[0020] - The redox potential of the mixture remained between 150 mV and 200 mV.

[0021] - The redox potential of the mixture remained between 180 mV and 230 mV.

[0022] - The wastewater initially contains: 1.5 ppm to 15 ppm of cyanide by weight, which includes 1 ppm to 10 ppm of weakly acid-dissociable cyanide by weight; 0.8 ppm to 3 ppm of zinc by weight; up to 8 ppm of iron by weight; and 0.05 ppm to 0.5 ppm of lead by weight.

[0023] - Wastewater originates from blast furnace gas purification.

[0024] - Following the oxidation step, the mixture is further subjected to a clarification step, wherein the mixture is separated into clarified water and residue.

[0025] - Clean water contains less than 0.4 mg / L of cyanide, less than 2 mg / L of zinc, less than 5 mg / L of iron, less than 0.5 mg / L of lead, and less than 30 mg / L of ammonia nitrogen.

[0026] - Used to process 1 m 3 The volume of chlorine solution in the wastewater is less than or equal to 6 liters.

[0027] - Used to process 1 m 3 The amount of alkaline reagent in the wastewater is less than or equal to 10 liters. Attached Figure Description

[0028] The invention will be better understood after reading the following description given with reference to the accompanying drawings:

[0029] - Figure 1 An embodiment of an apparatus for performing the processing method according to the present invention is shown. Detailed Implementation

[0030] exist Figure 1 The diagram shows an apparatus 1 for carrying out the method according to the invention. Wastewater WW containing cyanide compounds and metal compounds is fed into a tank 2 equipped with a mixer 3. Additionally, a chlorine solution CS and at least one alkaline reagent AA are injected into the tank and mixed with the wastewater WW to form a mixture 4.

[0031] The chlorine solution CS can be sodium hypochlorite (NaClO) or calcium hypochlorite (CaClO). The chlorine solution is added in an amount that maintains the redox potential (ORP) of the solution between 150 mV and 400 mV. CS can be added periodically during treatment to maintain the ORP within a given range. The redox potential of a solution is a measure of its tendency to gain or lose electrons when subjected to a change by the introduction of a new substance. Solutions with a higher (more positive) reduction potential than the new substance tend to gain electrons from the new substance (i.e., be reduced by oxidizing the new substance), while solutions with a lower (more negative) reduction potential tend to lose electrons to the new substance (i.e., be oxidized by reducing the new substance). Just as the transfer of hydrogen ions between substances determines the pH of an aqueous solution, the transfer of electrons between substances determines the reduction potential of an aqueous solution. Like pH, the reduction potential indicates the intensity of electron transfer to or from a substance in solution. In a preferred embodiment, the ORP is 150 mV to 250 mV, and in a most preferred embodiment, it is 180 mV to 200 mV. In another embodiment, the ORP is 350 mV to 400 mV. This last specific range of ORP allows for the removal of ammonia nitrogen (N-NH3) from the mixture. Ammonia nitrogen (N-NH3) is a compound that, if present in excessive amounts, can disrupt the ecosystem balance; depending on its initial amount in the wastewater, its concentration may need to be reduced. The ORP can be continuously measured by a first sensor 11, preferably a gold-plated ORP sensor with specificity to avoid interference with cyanide compounds.

[0032] The alkaline reagent AA is, for example, lime (Ca(OH)2) milk (lime in water) or sodium hydroxide (NaOH). The alkaline reagent AA is added in an amount that maintains the pH at 8.5 to 9.5, more preferably 8.9 to 9.1. AA can be added periodically during treatment to maintain the ORP within a given range. The pH can be continuously measured by a second sensor 12, which can be a standard commercial pH sensor.

[0033] Wastewater WW containing cyanide compounds and metal compounds can be wastewater from a steel plant, such as wastewater transferred from the purification of blast furnace exhaust gas. Prior to treatment, the wastewater contains, for example, 1.5 ppm to 15 ppm of cyanide by weight, said cyanide comprising 1 ppm to 10 ppm of WAD by weight; 0.8 ppm to 3 ppm of zinc by weight; up to 8 ppm of iron by weight; and 0.05 ppm to 0.5 ppm of lead by weight.

[0034] This method can be carried out either by treating a given volume of wastewater one after another or by having a continuous wastewater input stream and a continuous treated wastewater output stream. In both cases, alkaline reagent AA and chlorine solution CS must be added to mixture 4 in the required amounts to achieve the pH and ORP conditions described above.

[0035] Following treatment, the mixture undergoes a clarification step to remove solid particles. For this purpose, the treated wastewater can be sent to a decanter (not shown), where a flocculant (e.g., TeCol from TRIENXIS Company) is added to improve the settling of colloidal particles (e.g., metal compounds) and suspended solids present in the water. The aim is to recover clean water. The residue containing solid particles is a byproduct of such a clarification process.

[0036] result

[0037] Wastewater originating from blast furnace gas purification is subjected to treatment methods according to the prior art (Method 1), methods according to a first embodiment of the invention (Method 2), and methods according to a second embodiment of the invention (Method 3). The wastewater initially contains: 1.5 ppm to 15 ppm of cyanide by weight, including 1 ppm to 10 ppm of WAD by weight; 0.8 ppm to 3 ppm of zinc by weight; up to 8 ppm of iron by weight; and 0.05 ppm to 0.5 ppm of lead by weight. The results are shown in Table 1.

[0038] The following contents have been measured in the final treated water:

[0039] - WAD content, measured using spectrophotometry (according to standard EN ISO 14403).

[0040] - Total cyanide content, measured using spectrophotometry (according to standard EN ISO 14403:2002).

[0041] - SCN content, measured using spectrophotometry (standard method 4500-CN-M)

[0042] - N-NH3 content, measured using potentiometric method (standard method 4500-NH3-D).

[0043] - Zn, Pb, and Fe content, determined using inductively coupled plasma optical emission spectrometry (ICP-OES) (standard EN ISO 11885:2010).

[0044] In Method 1, wastewater is mixed with a solution of lime slurry and NaClO in a first oxidation tank to achieve a pH of approximately 10.5. The ORP is measured and is between 325 mV and 400 mV. In this tank, the aforementioned reactions 3 to 5 and the oxidation of metal compounds, such as zinc, occur according to the following reaction:

[0045]

[0046] Hydrochloric acid (HCl) is then added to lower the pH to 7.5 for a second oxidation step (reaction 6 mentioned earlier) in a second oxidation tank, where NaClO is mixed with the solution. The ORP is measured and is between 600 mV and 800 mV. The treated water is then sent to a flocculation tank, where it is mixed with a flocculant (TeCol from TRIENXIS), and then sent to a clarification tank where solid particles are separated from the residue.

[0047] In Method 2, the wastewater is fed into a tank where it is mixed with NaClO and lime slurry. The pH is maintained at 9 by adding an appropriate amount of lime slurry, and the ORP is maintained at 150 mV by adding an appropriate amount of NaClO. The treated water is then fed into a flocculation tank where it is mixed with a flocculant (TeCol from TRIENXIS Company), and then sent to a clarification tank where solid particles are separated from the water.

[0048] In Method 3, the same steps as in Method 2 are performed at the same pH, but the ORP is maintained at 350 mV by adding NaClO appropriately.

[0049]

[0050] Table 1

[0051] As can be seen from Table 1, the method according to the invention allows for a reduction in the consumption of reactive substances (currently NaClO and lime slurry) while allowing for efficient removal of contaminants. Furthermore, the method according to the invention allows for a reduction in the generation of residues (residues requiring further recycling or landfill). The embodiment of the invention according to method 3 allows for the treatment of ammonia nitrogen. The treatment method according to the invention also shortens the treatment time.

[0052] In the second phase of the experiment, the continuous water flow rate was approximately 1.5 m. 3 / hour to 5 m 3Blast furnace wastewater per hour was fed into a reaction tank, where it was mixed with lime slurry and chlorine. The amounts of these two reactants were selected to achieve the ORP and pH shown in Table 2. The treated water was then sent to a flocculation tank, where it was mixed with a flocculant (TeCol from TRIENXIS Company), and subsequently to a clarifier tank, where solid particles were separated from the water. The results of these experiments are shown in Table 2. When using industrial wastewater, their compositions differ from each other, which may explain some of the differences in the results obtained.

[0053]

[0054] Table 2

[0055] As can be seen from Table 2, wastewater can be treated by using the method according to the present invention, while limiting reactant consumption and residue generation.

[0056] According to embodiments of the present invention, the following notes are also disclosed:

[0057] Appendix 1. A method for treating wastewater containing cyanide compounds and metal compounds, wherein the wastewater is subjected to a single oxidation step during which the cyanide compounds are converted into carbon dioxide and nitrogen, the oxidation step comprising mixing the wastewater with a chlorine solution and an alkaline reagent to obtain a mixture, the alkaline reagent being added in an amount such that the pH of the mixture is maintained at 8.8 to 9.5, and the chlorine solution being added in an amount such that the redox potential of the mixture is maintained at 150 mV to 450 mV.

[0058] Note 2. According to the method described in Note 1, the chlorine solution is a sodium hypochlorite solution.

[0059] Note 3. The alkaline reagent described in accordance with Note 1 or 2 is lime.

[0060] Note 4. The method according to any one of Notes 1 to 3, wherein the pH of the mixture is maintained at 8.9 to 9.1.

[0061] Note 5. The method according to any one of Notes 1 to 4, wherein the redox potential of the mixture is maintained at 350 mV to 400 mV.

[0062] Note 6. The method according to any one of Notes 1 to 4, wherein the redox potential of the mixture is maintained at 180 mV to 230 mV.

[0063] Note 7. The method according to any one of Notes 1 to 6, wherein the wastewater initially comprises:

[0064] • 1.5 ppm to 15 ppm of cyanide by weight, wherein the cyanide comprises 1 ppm to 10 ppm of cyanide that can be dissociated by a weak acid by weight;

[0065] • Zinc, ranging from 0.8 ppm to 3 ppm by weight;

[0066] • Iron up to 8 ppm by weight; and

[0067] • Lead content of 0.05 ppm to 0.5 ppm by weight.

[0068] Note 8. The method according to any one of Notes 1 to 7, wherein the wastewater is derived from blast furnace gas purification.

[0069] Note 9. The method according to any one of Notes 1 to 8, wherein, after the oxidation step, the mixture is further subjected to a clarification step, wherein the mixture is separated into clarified water and residue.

Claims

1. A method for treating wastewater containing cyanide compounds and metal compounds, wherein the wastewater is subjected to a single oxidation step during which the cyanide compounds are converted into carbon dioxide and nitrogen, the oxidation step comprising mixing the wastewater with a chlorine solution and an alkaline reagent to obtain a mixture, the alkaline reagent being added in an amount such that the pH of the mixture is maintained at 8.8 to 9.5, and the chlorine solution being added in an amount such that the redox potential of the mixture is maintained at 150 mV to 450 mV.

2. The method according to claim 1, wherein the chlorine solution is a sodium hypochlorite solution.

3. The method according to claim 1 or 2, wherein the alkaline reagent is lime.

4. The method according to any one of claims 1 to 3, wherein the pH of the mixture is maintained at 8.9 to 9.

1.

5. The method according to any one of claims 1 to 4, wherein the redox potential of the mixture is maintained at 350 mV to 400 mV.

6. The method according to any one of claims 1 to 4, wherein the redox potential of the mixture is maintained at 180 mV to 230 mV.

7. The method according to any one of claims 1 to 6, wherein the wastewater initially comprises: • 1.5 ppm to 15 ppm of cyanide by weight, wherein the cyanide comprises 1 ppm to 10 ppm of cyanide that can be dissociated by a weak acid by weight; • Zinc, ranging from 0.8 ppm to 3 ppm by weight; • Iron up to 8 ppm by weight; and • Lead content of 0.05 ppm to 0.5 ppm by weight.

8. The method according to any one of claims 1 to 7, wherein the wastewater originates from blast furnace gas purification.

9. The method according to any one of claims 1 to 8, wherein, Following the oxidation step, the mixture is further subjected to a clarification step, wherein the mixture is separated into clarified water and residue.

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