A method for removing thiocyanate from cyanide-containing wastewater

Abstract

The invention discloses a method for removing thiocyanate from cyanide-containing wastewater, and belongs to the technical field of metal refining. Concentrated sulfuric acid is added to adjust the pH of the cyanide-containing wastewater to 2-3, and the mixture is filtered after stirring for reaction. Sodium dithionite is added to the filtrate, and concentrated sulfuric acid is added to maintain the pH at 3-5, and the mixture is stirred for reaction. The overflowed H2S and HCN gases are first introduced into an acidic sodium sulfite solution to absorb the H2S gas, and then introduced into a strong alkaline sodium hydroxide solution to absorb the HCN gas. The invention is directed to the cyanide-containing wastewater after acidification, and firstly a strong reducing agent sodium dithionite is added to absorb the SCN ‑ Reduction generates H2S and HCN with recycling value, then absorbs H2S and recovers elemental sulfur, and finally absorbs HCN and recovers CN ‑ , used for cyanide gold extraction in gold leaching system. It realizes the absorption and separation of H2S and HCN gas, and further realizes the separation of sulfur and CN ‑ recovery without introducing other ions into the water system.

Description

Technical Field

[0001] The invention relates to a method for removing thiocyanate from cyanide-containing wastewater, belonging to the technical field of metal refining. Background Art

[0002] A large amount of cyanide-containing wastewater is produced during the cyanide leaching process of gold mines. The cyanide in the cyanide-containing wastewater mainly includes CN - and Fe(CN)6 4- 、Cu(CN)3 2- 、Zn(CN)4 2- In addition, during the cyanide leaching process, the sulfur element in the sulfide ore in the gold ore reacts with the CN in the leaching solution. - , O2 undergoes chemical reaction to generate SCN - Under alkaline conditions, SCN - It has no gold leaching effect and will react with O2, consuming dissolved oxygen and affecting the leaching indicators of gold and silver.

[0003] SCN - ) is composed of a sulfur atom and a cyanide group. It is an ion that is widely present in the field of chemistry and has many important applications. Thiocyanate ions can form complexes with some metal ions. These complexes are often used as color developers or indicators for metal ions in analytical chemistry. Thiocyanate ions are reducing and can react with oxidants to generate N2, CO2, and NH4. + 、SO4 2- In addition, thiocyanate ion also has a certain oxidizing property and can be reduced by zinc to H2S and HCN (Zn+SCN - +3H + →Zn 2+ +H2S↑+HCN↑). Therefore, SCN - CN - It is a resource with recycling value. Its enrichment and accumulation in the cyanide leaching water system is a waste of resources, and measures should be taken to recycle it.

[0004] Add sulfuric acid to the cyanide-containing wastewater to make the wastewater acidic (pH 2-3), and the Cu(CN)3 2- 、Zn(CN)4 2- The complex cyanide in the cyanide complex ion is converted into HCN, CuCN precipitation and Zn 2+ , taking advantage of the low boiling point (26°C) and volatile characteristics of HCN, the HCN is blown off from the liquid phase with the help of air, and then the volatile HCN is absorbed by NaOH and recycled. 4- With SCN - Cannot be decomposed, although Fe(CN)64- Will be with Zn 2+ The reaction generates Zn2Fe(CN)6 precipitate, SCN - It will react with CuCN to form CuSCN precipitate, but due to Zn 2+ , Cu + The content is relatively low and is not enough to convert Fe(CN)6 4- and SCN - Therefore, the acidified liquid of cyanide wastewater still contains a certain amount of Fe(CN)6 4- and SCN - .

[0005] At present, the SCN in wastewater or waste residue - , mainly using oxidation method to "destroy" decomposition method for treatment. For example, aeration, electrocatalytic oxidation, adding potassium permanganate and other oxidants. The Chinese invention patent application with publication number CN113683246A discloses "a purification and disposal method for wastewater containing thiocyanate". Its main technical measures are: wet catalytic oxidation treatment of wastewater containing thiocyanate, reaction temperature of 150℃~300℃, reaction pressure of 2~4MPa, reaction time of 2~5h; stripping of wet catalytic oxidation effluent to recover ammonia nitrogen; mixing the stripping effluent with sodium formate wastewater containing copper ions for complex precipitation to remove residual thiocyanate ions; electrocatalytic oxidation treatment of complex precipitation effluent; adsorption treatment of electrocatalytic oxidation effluent with adsorbent; distillation treatment of adsorbed effluent, and reflux of distillation residue to wet catalytic oxidation treatment. The Chinese invention patent application with publication number CN116251325A discloses "a method for degradation of thiocyanate in cyanide tailings". The main technical measures are: (1) slurrying the cyanide tailings with water and adjusting the pH value to 2-4; (2) adding a thiocyanate stripping agent to the slurry obtained in step (1) and aerating for 1-3 hours; the thiocyanate stripping agent is one or more of potassium permanganate, manganese ore powder, and 2-ethylhexanoic acid manganese; (3) adding a thiocyanate degradation agent to the slurry obtained in step (2) to remove the thiocyanate; (4) separating the slurry obtained in step (3) into solid and liquid.

[0006] There are few reports on the removal of thiocyanate from the acidified liquid of cyanide-containing wastewater. The Chinese invention patent application with publication number CN110980916A discloses "a method for degrading thiocyanate in cyanide-containing lean liquid under acidic conditions". Its main technical measures are: (1) controlling the pH value of the cyanide-containing lean liquid to <2, adding manganese oxide, or manganese oxide and hydrogen peroxide to oxidize and decompose the thiocyanate in the cyanide-containing lean liquid; (2) filtering the cyanide-containing lean liquid after oxidation and decomposition to obtain precipitate and filtrate, adjusting the pH value of the obtained filtrate to 6-12, and then adding oxidizing substances to remove manganese in the filtrate; (3) filtering the liquid after manganese removal in step (2) to obtain purified lean liquid and precipitate containing manganese oxide, and the precipitate containing manganese oxide is directly dried or dried by blast drying and returned to step (1) for oxidation and decomposition of thiocyanate in the cyanide-containing lean liquid, so as to realize the recycling of manganese oxide; the purified lean liquid is returned to cyanide leaching or deep oxidation to meet the standards and then discharged. The Chinese invention patent application with publication number CN109502811A discloses "a flocculation precipitation purification method for thiocyanate ions in cyanide wastewater". Its main technical measures are: 1) adding concentrated sulfuric acid to the cyanide wastewater to adjust the pH value to 2-3; 2) after the acidified liquid is filtered, adding excess soluble iron salt to the filtrate for deep removal of free cyanide, stirring thoroughly and filtering; 3) adding lime milk to the filtrate to adjust the pH value to 5-9, stirring, adding polyferric sulfate or polyferric chloride to adsorb thiocyanate ions; 4) adding waste ore powder and stirring for further flocculation and sedimentation; 5) filtering after sedimentation.

[0007] The "method for purifying and treating wastewater containing thiocyanate" (CN113683246A), "method for degrading thiocyanate in cyanide tailings" (CN116251325A), "method for degrading thiocyanate in cyanide-poor liquid under acidic conditions" (CN110980916A) and "method for flocculation and precipitation purification of thiocyanate ions in cyanide-containing wastewater" (CN109502811A) all belong to the method of "destruction" decomposition by oxidation. They degrade SCN - Oxidative decomposition to generate NH4 + 、SO4 2- etc. products that are not worth recycling, or adding soluble iron salts and SCN - The reaction produces ferric thiocyanate (a red complex, not a precipitate), which is a waste of resources. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a method for removing thiocyanate from cyanide-containing wastewater without introducing impurity ions such as metal ions into the production water system and without destroying SCN. - CN - Under the premise of achieving the absorption and separation of H2S and HCN gases, and further achieving the separation of sulfur and CN - of recycling.

[0009] The technical solution of the present invention is as follows:

[0010] A method for removing thiocyanate from cyanide-containing wastewater is carried out according to the following steps:

[0011] Step (1): adding concentrated sulfuric acid to the cyanide-containing wastewater to maintain a pH of 2 to 3, stirring to react fully, and then performing press filtration or filtration to achieve solid-liquid separation to obtain a filtrate and a filter residue;

[0012] Step (2): Place the filtrate obtained in step (1) in a reaction container, add concentrated sulfuric acid to maintain the pH at 3-5, and add sodium dithionite intermittently, stirring the reaction to release the gas;

[0013] The overflow gas is discharged into the 1# gas absorption container filled with soluble sulfite solution, and H2S and SO3 are mixed under stirring conditions. 2- The reaction is complete, achieving the purpose of fully absorbing H2S gas; after the H2S gas is completely absorbed, the elemental sulfur in the 1# gas absorption container is filtered and recovered;

[0014] Step (3): discharge the remaining gas into the 2# gas absorption container filled with sodium hydroxide solution, and make HCN and NaOH react completely under stirring conditions to fully absorb HCN and recover CN. - Purpose of solution.

[0015] Preferably, the stirring reaction in step (1) refers to stirring the reaction for 30-60 minutes.

[0016] Preferably, the temperature of the reaction system in the 1# gas absorption container is 26°C to 30°C; and the pH is 6.0 to 6.5.

[0017] Preferably, the mass concentration of the sodium hydroxide solution in the 2# gas absorption container is 10% to 30%.

[0018] Preferably, the SCN in the filtrate obtained in step (1) is detected - The theoretical addition amount of sodium dithionite and soluble sulfite in step (2) is determined based on the content.

[0019] Preferably, the intermittent addition in step (2) refers to adding sodium dithionite once every 10 to 30 minutes.

[0020] Preferably, in step (2), the soluble sulfite in the 1# gas absorption container is added in batches, each time with an interval of 10 to 30 minutes; and the overflow of H2S gas is detected. When no overflow of H2S gas is detected, the addition of soluble sulfite is stopped.

[0021] Preferably, during the process of absorbing H2S gas in step (2), the gas above the liquid surface is extracted from the No. 1 gas absorption container by a gas circulation pump, and then pumped into the liquid in the No. 1 gas absorption container, and the circulation is continued so that the H2S gas is fully absorbed; during the process of absorbing HCN gas in step (3), the gas above the liquid surface is extracted from the No. 2 gas absorption container by a gas circulation pump, and then pumped into the liquid in the No. 2 gas absorption container, and the circulation is continued so that the HCN gas is fully absorbed.

[0022] Preferably, in step (2), the H2S gas alarm is used to detect the content of H2S in the mixed gas. When the content is equal to zero, it is considered that H2S and SO3 2- The reaction is complete; in step (3), the HCN gas alarm is used to detect the HCN content in the mixed gas. When the content is equal to zero, it is considered that the reaction of HCN and NaOH is complete.

[0023] The present invention SCN - The theoretical basis for absorption and elemental sulfur recovery is: Sodium dithionite converts SCN - Reduction generates hydrogen sulfide (H2S) and HCN (S2O4 2- +2SCN - +6H + +2H2O→

[0024] 2HCN↑+2H2S↑+2H2SO3), H2S gas and SO3 2- A redox reaction occurs to produce elemental sulfur and water (2H2S+H2SO3→3S↓+3H2O).

[0025] The present invention is hydrogen cyanide (HCN) absorption and CN - The theoretical basis for recovery is that hydrogen cyanide is highly soluble in water, has a low boiling point (26°C), is volatile, is a gas under standard conditions, and is easily reacted with sodium hydroxide to produce an acid-base neutralization reaction (HCN+OH - →CN - +H2O), thereby completing the absorption of HCN and CN - of recycling.

[0026] The positive effects of the present invention are:

[0027] First, according to the existing oxidation treatment technology, SCN - Will be destroyed and generate N2, CO2, NH4 + 、SO4 2- or reacts with soluble iron salts to form ferric thiocyanate complex ions ([Fe(SCN)6] 3-). The present invention is directed to the cyanide-containing wastewater after acidification treatment, firstly adding a strong reducing agent sodium dithionite to SCN - Reduction generates H2S and HCN with recycling value, then absorbs H2S and recovers elemental sulfur, and finally absorbs HCN and recovers CN - , used for cyanide gold extraction in gold leaching system. It realizes the absorption and separation of H2S and HCN gas, and further realizes the separation of sulfur and CN - of recycling.

[0028] Second, the present invention effectively avoids the introduction of metal ions and CN in the process of removing thiocyanate. - Complexation reaction occurs and CN is consumed - .

[0029] Third, the aqueous solution of sodium dithionite (Na2S2O4) is unstable. Like zinc powder, it is a strong reducing agent under acidic conditions and can convert SCN - Reduction, generating S 2- With CN - , and then overflow in the form of H2S and HCN gas. Strong acidity is conducive to the reduction reaction of sodium dithionite with oxidizing substances, but too high acidity will lead to an increase in side reactions such as self-decomposition and reaction with water. Based on the above facts, the present invention selects a suitable acidity range of pH 3 to 5 for the sodium dithionite reaction system. This pH range is conducive to the normal reduction reaction of sodium dithionite with oxidizing substances and will not lead to an increase in side reactions such as self-decomposition and reaction with water. And it can ensure that the reduction reaction of sodium dithionite with oxidizing substances has a faster rate and the side reactions are effectively suppressed.

[0030] Fourth, in the soluble sulfite reaction system, temperature and pH control are also key technologies of the present invention. If the temperature is too high, sulfurous acid will decompose to release SO2, which is not conducive to the dissolution of H2S; if the temperature is too low, HCN gas will not overflow from the soluble sulfite solution to enter the next stage of absorption; if the pH is too high, H + The lower the concentration, the less favorable the CN - With H + The reaction generates HCN and overflows in the form of HCN gas; if the pH is too low, H + Too high a concentration will promote S 2- With H + The reaction generates H2S, which is not conducive to the dissolution of H2S. Based on this, the present invention selects the soluble sulfite reaction system (reaction system in 1# gas absorption container) with an appropriate temperature range of 26°C to 30°C and an appropriate acidity range of pH 6.0 to 6.5. In the process of soluble sulfite solution and sodium hydroxide solution absorbing H2S gas and HCN gas, the selected temperature range and acidity range. DETAILED DESCRIPTION

[0031] The present invention is further described below in conjunction with embodiments and experimental data. Embodiment 1

[0032] Step (1): Add concentrated sulfuric acid to the cyanide-containing wastewater, maintain the pH at about 2.5, and stir the reaction for 60 minutes to obtain a filter residue and a cyanide-containing wastewater acidified liquid as a filtrate. Some of the valuable metals in the filter residue have a high grade and can be sold as a product. Detection of SCN in cyanide-containing wastewater acidified liquid - The content is 8.6g / L. Based on this calculation (S2O4 2- +2SCN - +6H + +2H2O→2HCN↑+2H2S↑+2H2SO3), theoretically, at least 12.9g / L of sodium dithionite should be added, and the maximum amount of H2S gas produced by decomposition is 5.04g / L, and the maximum amount of HCN gas produced by decomposition is 4g / L.

[0033] Step (2): 100 ml of the acidified cyanide wastewater obtained in step (1) is placed in a reaction container, 1.0 g of sodium dithionite powder is added every 20 minutes, and concentrated sulfuric acid is added to maintain the pH of the reaction system at 4.0, and the reaction is stirred to release the mixed gas.

[0034] According to the chemical reaction equation 2H2S+H2SO3→3S↓+3H2O, theoretically 5.04g / L of H2S gas needs to consume at least 18.68g / L of sodium sulfite (Na2SO3·7H2O).

[0035] The overflow gas is discharged into the 1# gas absorption bottle filled with 500ml of water. Under stirring conditions, 1g of sodium sulfite is added every 20min, and the overflow of H2S gas is detected. When no H2S gas overflow is detected, stop adding sodium sulfite. After the H2S gas is absorbed, filter and recover the elemental sulfur in the 1# gas absorption bottle, adjust the pH of the filtrate to above 11, and return it to the cyanide production water system.

[0036] During the process of absorbing H2S gas in this step, the gas above the liquid surface is extracted from the 1# gas absorption bottle by the first gas circulation pump and then pumped into the gas storage tank, and the gas in the gas storage tank is continuously pumped into the liquid in the 1# gas absorption bottle by the second gas circulation pump, and the cycle is carried out so that the H2S gas is fully absorbed.

[0037] The gas storage tank is equipped with an H2S gas alarm, which is used to detect the content of H2S in the mixed gas. When the H2S gas is absorbed and reacted for 3 hours, the content is equal to zero. H2S and SO3 2- The reaction was complete.

[0038] Among them, the temperature of the reaction system in the 1# gas absorption bottle is controlled at 26°C~30°C, and the pH is controlled at 6.0~6.5.

[0039] Step (3): After the H2S gas is fully absorbed, the gas stored in the gas storage tank is pumped into a 2# gas absorption bottle containing 500 mL of sodium hydroxide solution (mass concentration 20%) by means of a second gas circulation pump and by switching the gas line valve. Under stirring conditions in the 2# gas absorption bottle, the gas above the liquid surface in the 2# gas absorption bottle is continuously extracted by a third gas circulation pump and then continuously pumped into the liquid in the 2# gas absorption bottle. The cycle is repeated to allow the HCN to react completely with the NaOH, thereby achieving full absorption of the HCN and recovery of the CN. - Purpose of solution.

[0040] In this step, the HCN gas alarm is used to detect the HCN content in the air inlet or outlet pipeline of the third gas circulation pump. When the HCN absorption reaction lasts for 3 hours, the CN is detected. - The content is 522.36 mg / L.

[0041] The following is the test process and results for verifying the effect of the first embodiment:

[0042] (1) After the reaction is completed, take part of the filtrate from the 1# gas absorption bottle to detect SCN - The content is 1.6g / L, calculate SCN - The removal rate was 81.40%.

[0043] (2) After the reaction is completed, filter the solution in the 1# gas absorption bottle and detect CN in the filtrate. - The content is zero, indicating that the 1# gas absorption bottle has not absorbed HCN gas. The presence of elemental sulfur on the filter paper indicates that H2S gas reacts with sulfurous acid. The present invention can separate H2S from HCN, absorb H2S gas, and then recover elemental sulfur.

[0044] (3) After the reaction is completed, take the sodium hydroxide solution in the 2# gas absorption bottle to detect S 2- With CN - CN in solution - The content is 522.36 mg / L, indicating that the 2# gas bottle can absorb HCN gas. Embodiment 2

[0045] The only difference between this embodiment and the first embodiment is that potassium sulfite is used instead of sodium sulfite.

[0046] The test results used to verify the effect of the second embodiment show that SCN - The removal rate was 80.40%. Embodiment 3

[0047] The only difference between this embodiment and embodiment 1 is that concentrated sulfuric acid is added in step (2) to maintain the pH of the reaction system at 2.5.

[0048] The test results used to verify the effect of the third embodiment show that SCN - The removal rate was 81.52%. When the reaction time was extended to 4 h, the consumption of sodium dithionite increased significantly. Embodiment 4

[0049] The only difference between this embodiment and the first embodiment is that concentrated sulfuric acid is added in step (2) to maintain the pH of the reaction system at 5.5.

[0050] The test results used to verify the effect of the fourth embodiment show that SCN - The removal rate was 77.40%. When the reaction time was extended to 8 h, the consumption of sodium dithionite increased significantly. Embodiment 5

[0051] The only difference between this embodiment and the first embodiment is that the temperature of the reaction system in the 1# gas absorption bottle is controlled at 25°C. The test results used to verify the effect of the fifth embodiment show that SCN - The removal rate was 78.52%. The solution in the 1# gas absorption bottle was filtered, and the CN in the filtrate was - The concentration is 32.20 mg / L, indicating that at this temperature, the 1# gas absorption bottle will absorb part of the HCN gas. Embodiment 6

[0052] The difference between this embodiment and the first embodiment is that the temperature of the reaction system in the 1# gas absorption bottle is controlled at 32°C. The test results used to verify the effect of the sixth embodiment show that SCN - The removal rate was 80.22%. The solution in the 1# gas absorption bottle was filtered and there was no CN in the filtrate. - , indicating that the 1# gas absorption bottle did not absorb HCN gas at this temperature. Embodiment 7

[0053] The only difference between this embodiment and the first embodiment is that the pH of the reaction system in the 1# gas absorption bottle is controlled at 5.5. The test results used to verify the effect of the seventh embodiment show that SCN - The removal rate was 81.15%. The solution in the 1# gas absorption bottle was filtered and the CN in the filtrate was - The concentration is zero, indicating that under this pH condition, the 1# gas absorption bottle did not absorb HCN gas, but the amount of SO2 gas released by the decomposition of sulfurous acid increased, resulting in an increase in sulfite consumption. 2- 、SO4 2- Content increased. Embodiment 8

[0054] The only difference between this embodiment and the first embodiment is that the pH of the reaction system in the 1# gas absorption bottle is controlled at 7.0. The test results used to verify the effect of the sixth embodiment show that SCN - The removal rate was 81.56%. The solution in the 1# gas absorption bottle was filtered, and the CN in the filtrate was - The concentration is 212.38 mg / L, indicating that under this pH condition, the 1# gas absorption bottle will absorb a large amount of HCN gas, deteriorating the HCN gas separation effect.

Claims

1. A method for removing thiocyanate from cyanide-containing wastewater, characterized in that Follow these steps: Step (1): adding concentrated sulfuric acid to the cyanide-containing wastewater to maintain a pH of 2 to 3, stirring to react fully, and then performing press filtration or filtration to achieve solid-liquid separation to obtain a filtrate and a filter residue; Step (2): Place the filtrate obtained in step (1) in a reaction container, add concentrated sulfuric acid to maintain the pH at 3-5, and add sodium dithionite intermittently, stirring the reaction to release the gas; The overflow gas is discharged into the 1# gas absorption container filled with soluble sulfite solution, and H2S and SO3 are mixed under stirring conditions. 2- The reaction is complete, achieving the purpose of fully absorbing H2S gas; after the H2S gas is completely absorbed, the elemental sulfur in the 1# gas absorption container is filtered and recovered; Step (3): discharge the remaining gas into the 2# gas absorption container filled with sodium hydroxide solution, and make HCN and NaOH react completely under stirring conditions to fully absorb HCN and recover CN. - Purpose of solution.

2. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: The stirring reaction in step (1) refers to stirring the reaction for 30-60 minutes.

3. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: The temperature of the reaction system in the 1# gas absorption container is 26°C~30°C; the pH is 6.0~6.

5.

4. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: The mass concentration of sodium hydroxide solution in the 2# gas absorption container is 10% to 30%.

5. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: Detection of SCN in the filtrate obtained in step (1) - The theoretical addition amount of sodium dithionite and soluble sulfite in step (2) is determined based on the content.

6. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: The intermittent addition in step (2) refers to adding sodium dithionite once every 10 to 30 minutes.

7. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: In step (2), the soluble sulfite in the 1# gas absorption container is added in batches, each time with an interval of 10 to 30 minutes; and the overflow of H2S gas is detected. When no overflow of H2S gas is detected, the addition of soluble sulfite is stopped.

8. The method for removing thiocyanate from cyanide-containing wastewater according to claim 1, characterized in that: During the process of absorbing H2S gas in step (2), the gas above the liquid surface is extracted from the No. 1 gas absorption container by using a gas circulation pump, and then pumped into the liquid in the No. 1 gas absorption container, and the circulation is continued so that the H2S gas is fully absorbed. During the process of absorbing HCN gas in step (3), the gas above the liquid surface is extracted from the No. 2 gas absorption container by using a gas circulation pump, and then pumped into the liquid in the No. 2 gas absorption container, and the circulation is continued so that the HCN gas is fully absorbed.

9. The method for removing thiocyanate from cyanide-containing wastewater according to any one of claims 1 to 8, characterized in that: In step (2), the H2S gas alarm is used to detect the H2S content in the mixed gas. When the content is equal to zero, it is considered that H2S and SO3 2- The reaction is complete; in step (3), the HCN gas alarm is used to detect the HCN content in the mixed gas. When the content is equal to zero, it is considered that the reaction of HCN and NaOH is complete.