Method for treatment of mixed electroplating wasterwater without cyanide and phosphorus-containing reductant

Abstract

A method for treatment of mixed electroplating wastewater without a cyanide and a phosphorus-containing reductant without a cyanide and a phosphorus-containing reductant. A ferrous chloride solution is added into electroplating wastewater without a cyanide and a phosphorus-containing reductant. The pH of wastewater is adjusted to 10.5-12. Pollutants such as sodium cyanide and hydroxyl-containing organic amine complexants are oxidized with sodium hypochlorite. Carboxyl-containing organic acid complexants are precipitated. Hexavalent chromium is reduced to trivalent chromium to form chromium hydroxide precipitate. Precipitate is removed by filtering and wastewater is adjusted to pH 4.5-5.5. Heavy metal ions are precipitated with sodium dimethyldithiocarbamate or sodium diethyldithiocarbamate. Precipitate and heavy metal capturing agents are adsorbed with activated carbon followed by removal of precipitate. Wastewater is adjusted to pH 6-8. Aliphatic polyamine complexants are destroyed using an available biological degradation technique to reduce chemical oxygen demand.

Claims

1. A method for treatment of electroplating wastewater without a cyanide and a phosphorus-containing reductant, comprising the following steps: (a) Adding a ferrous chloride solution into the electroplating wastewater without a cyanide and a phosphorous-containing reductant under mechanical stirring; (b) Adjusting the electroplating wastewater treated in step (a) with lime milk to pH of 10.5-12; wherein a synergistic effect of ferrous ions and calcium ions enables a complete precipitation of carboxyl-containing organic acid complexants; metal ions releases from complexes react with hydroxyl ions to form hydroxide precipitate; and ferrous ions reduce hexavalent chromium to trivalent chromium to form a chromium hydroxide precipitate; (c) Flocculating the precipitate in the electroplating wastewater treated in step (b) into particles of large size to settle with a flocculant; (d) Filtering the electroplating wastewater treated in step (c) to remove the precipitate; (e) Adjusting and maintaining pH of the electroplating wastewater treated in step (d) at 4.5-5.5 with dilute hydrochloric acid, and introducing a heavy metal capturing agent to precipitate heavy metal ions; (f) Adding activated carbon to the electroplating wastewater treated in step (e) to absorb the resulting precipitate and the remaining heavy metal capturing agent so as to settle the precipitate; (g) Filtering the electroplating wastewater treated in step (f) to remove the precipitate; and (h) Adjusting the electroplating wastewater treated in step (g) to pH of 6-8 with a sodium hydroxide solution, and further processing the resulting electroplating wastewater using a biochemical method.

2. The method of claim 1, wherein the lime milk contains calcium oxide at a concentration of 50-100 g/L.

3. The method of claim 1, wherein the ferrous chloride solution in step (a) contains ferrous chloride tetrahydrate at a concentration of 150-250 g/L; and the ferrous chloride solution is added until a green precipitate of ferrous hydroxide appears, and then the ferrous chloride solution is further added to the electroplating wastewater at a volume ratio of 1-10:1000.

4. The method of claim 1, wherein the heavy metal capturing agent in step (e) is a sodium dimethyldithiocarbamate solution or a sodium diethyldithiocarbamate trihydrate solution at a concentration of 80-120 g/L, and volume ratios of the added sodium dimethyldithiocarbamate solution and sodium diethyldithiocarbamate trihydrate solution to the electroplating wastewater are 0.5-3:1000 and 0.3-5:1000.

5. The method of claim 1, wherein the flocculant in step (c) is a polyacrylamide (PAM) aqueous solution at a concentration of 3-8 g/L; the dilute hydrochloric acid in step (e) has a concentration of 2%-8% by weight; and the sodium hydroxide solution in step (h) has a concentration of 50-100 g/L.

6. The method of claim 1, wherein the activated carbon in step (f) is a powdered activated carbon for sewage treatment, and the activated carbon is added into the electroplating wastewater at an amount of 50-300 g per ton electroplating wastewater.

7. The method of claim 1, wherein the biochemical method in step (h) employs a microbial degradation technique to perform a reaction in a biochemical reactor for 8-24 hours according to chemical oxygen demand (COD).

Description

EXAMPLE 1

Treatment of Cyanide-Containing Electroplating Wastewater

(1) The electroplating wastewater included: cyanide copper plating wastewater, cyanide copper-zinc alloy electroplating wastewater, gun-color tin-nickel alloy electroplating wastewater, pyrophosphate copper plating wastewater, alkaline zinc-nickel alloy electroplating wastewater, trivalent chromium plating wastewater, trivalent chromium passivation wastewater, hexavalent chromium passivation wastewater, acid copper plating wastewater, bright nickel plating wastewater, potassium chloride zinc plating wastewater, alkaline non-cyanide zinc plating wastewater and hexavalent chromium plating wastewater, and pretreatment wastewater for degreasing and pickling, but did not include electroless nickel plating wastewater and electroless copper plating wastewater.

Step (1) Cyanide Breaking

(2) The electroplating wastewater was delivered from an electroplating wastewater regulating tank to a primary oxidation tank and was then stirred using a mixer. The electroplating wastewater was adjusted to pH of 10.5-12 with lime milk, and then an oxidant was added to break the cyanides for 60 minutes.

(3) The pyrophosphate in the wastewater reacted with metal ions such as calcium ions to form a precipitate, and the phosphate reacted with metal ions such as copper, zinc, nickel and calcium ions to form precipitate.

(4) A large amount of precipitate was produced in the primary oxidation tank, and especially calcium sulfate was easy to scale, so that it was not suitable to use a potentiometer in the primary oxidation tank to control the ORP value, avoiding the scaling of electrode.

(5) Subsequently, the wastewater flowed from the primary oxidation tank into a secondary oxidation tank and was continuously stirred by a mixer. The lime milk was used to maintain the pH of the wastewater at 10.5-12, and controlling the ORP value at 300 mV with a potentiometer, the oxidant was continuously added to oxidize for 60 minutes, thereby oxidizing the cyanides to carbon dioxide. The ratio of the amount of the oxidant added to the primary oxidation tank to the amount of the oxidant added to the secondary oxidation tank was adjusted to 1:1.

(6) After the wastewater flowed from the secondary oxidation tank into a tertiary oxidation tank, the reactions such as cyanide breaking and degradation of the oxidant were continued for 60 minutes.

Step (2) Precipitation of Carboxyl-Containing Organic Acid Complexants

(7) Carboxyl-containing organic acids such as citric acid generally have higher oxidation resistance than cyanides, so that the wastewater after oxidation still contained complexants such as citric acid. The wastewater flowed from the tertiary oxidation tank into a complexant precipitation tank and was then stirred by a mixer. Lime milk was used to adjust the wastewater to pH of 10.5-12 followed by addition of a ferrous chloride solution into the wastewater until a green precipitate of ferrous hydroxide appeared. Hexavalent chromium was reduced to trivalent chromium to form a chromium hydroxide precipitate. Then, the ferrous chloride solution was further added into the wastewater at an amount of 3 L per ton wastewater to precipitate the carboxyl-containing organic acid complexants, and the heavy metal ions released from the complex reacted with hydroxyl ions to form a hydroxide precipitate.

Step (3) Separation of Precipitate

(8) After the wastewater flowed from the complexant precipitation tank into a flocculation tank, a flocculant was added to flocculate the precipitate until the precipitate was agglomerated into particles of large size. The wastewater then flowed from the flocculation tank into an inclined-tube sedimentation tank A, and the precipitate settled to the bottom of the inclined-tube sedimentation tank A. The precipitate was pumped into a plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (4) Chelation Precipitate

(9) The supernatant in the inclined-tube sedimentation tank A flowed into a chelation tank and was stirred by a mixer. The pH of the wastewater was adjusted and maintained at 4.5-5.5 with dilute hydrochloric acid and then a heavy metal capturing agent was added into the wastewater at an amount of 2 L per ton wastewater to precipitate the remaining heavy metal ions such as copper ions. Afterwards, activated carbon was added into the wastewater at an amount of 150 g per ton wastewater to adsorb the precipitate and the remaining heavy metal capturing agent.

Step (5) Separation of Precipitate

(10) After the wastewater flowed from the chelation tank into an inclined-tube sedimentation tank B, the precipitate settled to the bottom of the inclined-tube sedimentation tank B. The precipitate was pumped into the plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (6) Biochemical Treatment

(11) The supernatant in the inclined-tube sedimentation tank B flowed into a biodegradation tank followed by addition of sodium hydroxide solution to adjust the pH to 6-8 to perform a biodegradation treatment. When the COD reached the standards, the wastewater was allowed to be discharged.

EXAMPLE 2

Treatment of Electroplating Wastewater Containing Electroless Plating Wastewater

(12) The electroplating wastewater included: cyanide copper plating wastewater, cyanide copper-zinc alloy electroplating wastewater, gun-color tin-nickel alloy electroplating wastewater, pyrophosphate copper plating wastewater, alkaline zinc-nickel alloy electroplating wastewater, trivalent chromium plating wastewater, trivalent chromium passivation wastewater, hexavalent chromium passivation wastewater, acid copper plating wastewater, bright nickel-plating wastewater, potassium chloride zinc plating wastewater, alkaline cyanide-free zinc-plating wastewater and hexavalent chromium plating wastewater, electroless copper plating wastewater, electroless nickel plating wastewater and pretreatment wastewater for degreasing and pickling.

Step (1) Oxidation of Cyanides, Electroless Plating Complexants and Reductants

(13) The electroplating wastewater was delivered from an electroplating wastewater regulating tank to a primary oxidation tank and was then stirred using a mixer. The electroplating wastewater was adjusted to pH of 10.5-12 with lime milk, and then an oxidant was added to oxidize the cyanides, electroless plating complexants and reductants for 60 minutes.

(14) Meanwhile, the pyrophosphate and the phosphate in the electroplating wastewater reacted with free heavy metal ions to form a precipitate.

(15) Subsequently, the electroplating wastewater flowed from the primary oxidation tank into a secondary oxidation tank and was continuously stirred by a mixer. The lime milk was used to maintain the pH of the wastewater at 10.5-12, and controlling the ORP value at 350 mV with a potentiometer, the oxidant was continuously added to oxidize for 60 minutes, thereby oxidizing the cyanides, the hydroxyl-containing organic amine complexants and sodium hypophosphite to carbon dioxide, carboxyl-containing organic acid salt and sodium phosphate, respectively. The ratio of the amount of the oxidant added to the primary oxidation tank to the amount of the oxidant added to the secondary oxidation tank was adjusted to 1:1.

(16) After the wastewater flowed from the secondary oxidation tank into a tertiary oxidation tank, the reactions such as cyanide breaking and degradation of the oxidant were continued for 60 min.

Step (2) Precipitation of Carboxyl-Containing Organic Acid Complexants

(17) The wastewater flowed from the tertiary oxidation tank into a complexant precipitation tank and was then stirred using a mixer. Lime milk was used to adjust the pH of the wastewater to 10.5-12 followed by addition of a ferrous chloride solution into the wastewater until a green precipitate of ferrous hydroxide appeared. Then, the ferrous chloride solution was further added into the wastewater at an amount of 4 L per ton wastewater to precipitate those carboxyl-containing organic acid complexants, and the heavy metal ions released from the complex reacted with hydroxyl ions to form a hydroxide precipitate. Moreover, hexavalent chromium was reduced to trivalent chromium to form a chromium hydroxide precipitate.

Step (3) Separation of Precipitate

(18) After the wastewater flowed from the complexant precipitation tank into a flocculation tank, a flocculant was added to flocculate the precipitate until the precipitate was agglomerated into particles of large size. The wastewater then flowed from the flocculation tank into an inclined-tube sedimentation tank A, and the precipitate settled to the bottom of the inclined-tube sedimentation tank A. The precipitate was pumped into a plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (4) Chelation Precipitate

(19) The supernatant in the inclined-tube sedimentation tank A flowed into a chelation tank and was stirred using a mixer. The pH of the wastewater was adjusted and maintained at 4.5-5.5 using dilute hydrochloric acid and then a heavy metal capturing agent was added into the wastewater at an amount of 2 L per ton wastewater to precipitate the remaining heavy metal ions such as copper ions. Afterwards, activated carbon was added into the wastewater at an amount of 150 g per ton wastewater to adsorb the precipitate and the remaining heavy metal capturing agent.

Step (5) Separation of Precipitate

(20) After the wastewater flowed from the chelation tank into an inclined-tube sedimentation tank B, the precipitate settled to the bottom of the inclined-tube sedimentation tank B. The precipitate was pumped into the plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (6) Biochemical Treatment

(21) The supernatant in the inclined-tube sedimentation tank B flowed into a biodegradation tank followed by addition of sodium hydroxide solution to adjust the pH to 6-8 to perform a biodegradation treatment. When the COD reached the standards, the wastewater was allowed to be discharged.

EXAMPLE 3

Treatment of Electroplating Wastewater Without Cyanide and Electroless Plating Wastewater

(22) The electroplating wastewater included: pyrophosphate copper plating wastewater, alkaline zinc-nickel alloy electroplating wastewater, trivalent chromium plating wastewater, trivalent chromium passivation wastewater, hexavalent chromium passivation wastewater, acid copper plating wastewater, bright nickel-plating wastewater, potassium chloride zinc plating wastewater, alkaline non-cyanide zinc-plating wastewater and hexavalent chromium plating wastewater, and pretreatment wastewater for degreasing and pickling.

Step (1) Oxidation of Organic Substances Such as Electroplating Additives

(23) The electroplating wastewater was delivered from an electroplating wastewater regulating tank to a primary oxidation tank and was then stirred using a mixer. The electroplating wastewater was adjusted to pH of 10.5-12 with lime milk, and then an oxidant was added to oxidize for 60 minutes.

(24) The pyrophosphate and the phosphate in the wastewater reacted with the free heavy metal ions to form a precipitate.

(25) Subsequently, the wastewater flowed from the primary oxidation tank into a secondary oxidation tank and was continuously stirred by a mixer. The lime milk was used to maintain the pH of the wastewater at 10.5-12, and controlling the ORP value at 150-200 mV with a potentiometer, the oxidant was continuously added to oxidize for 60 minutes. The ratio of the amount of the oxidant added to the primary oxidation tank to the amount of the oxidant added to the secondary oxidation tank was adjusted to 1:1.

(26) Strong oxidation was not required for the absence of the cyanide in the wastewater, so that the ORP value may be controlled at a lower level, thereby reducing the amount of oxidant to achieve to a lower cost. Some organic compounds with a strong reducibility can be destroyed through the oxidation, thereby alleviating the burden of the subsequent biochemical degradation.

(27) After the wastewater flowed from the secondary oxidation tank into a tertiary oxidation tank, the reactions such as cyanide breaking and degradation of the oxidant were continued for 60 minutes.

Step (2) Precipitation of Carboxyl-Containing Organic Acid Complexants

(28) The wastewater flowed from the tertiary oxidation tank into a complexant precipitation tank and was then stirred using a mixer. Lime milk was used to adjust the wastewater to pH of 10.5-12 followed by addition of a ferrous chloride solution into the wastewater until a green precipitate of ferrous hydroxide appeared. Then, the ferrous chloride solution was further added into the wastewater at an amount of 4 L per ton wastewater to precipitate the carboxyl-containing organic acid complexants, and the heavy metal ions released from the complex reacted with hydroxyl ions to form a precipitate. Moreover, hexavalent chromium was reduced to trivalent chromium to form a chromium hydroxide precipitate.

Step (3) Separation of Precipitate

(29) After the wastewater flowed from the complexant precipitation tank into a flocculation tank, a flocculant was added to flocculate the precipitate until the precipitate was agglomerated into particles of large size. The wastewater then flowed from the flocculation tank into an inclined-tube sedimentation tank A, and the precipitate settled to the bottom of the inclined-tube sedimentation tank A. The precipitate was pumped into a plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (4) Chelation Precipitate

(30) The supernatant in the inclined-tube sedimentation tank A flowed into a chelation tank and was stirred using a mixer. The pH of the wastewater was adjusted and maintained at 4.5-5.5 with dilute hydrochloric acid and then a heavy metal capturing agent was added into the wastewater at an amount of 2 L per ton wastewater to precipitate the remaining heavy metal ions such as copper ions. Afterwards, activated carbon was added into the wastewater at an amount of 150 g per ton wastewater to adsorb the precipitate and the remaining heavy metal capturing agent.

Step (5) Separation of Precipitate

(31) After the wastewater flowed from the chelation tank into an inclined-tube sedimentation tank B, the precipitate settled to the bottom of the inclined-tube sedimentation tank B. Then the precipitate was pumped into the plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (6) Biochemical Treatment

(32) The supernatant in the inclined-tube sedimentation tank B flowed into a biodegradation tank followed by addition of sodium hydroxide solution to adjust pH to 6-8 to perform a biodegradation treatment. When the COD reached the standards, the wastewater was allowed to be discharged.

EXAMPLE 4

Another Method for Treatment of Electroplating Wastewater without Cyanide and Electroless Plating Wastewater

(33) The electroplating wastewater included: pyrophosphate copper plating wastewater, alkaline zinc-nickel alloy electroplating wastewater, trivalent chromium chromium-plating wastewater, trivalent chromium passivation wastewater, hexavalent chromium passivation wastewater, acid copper plating wastewater, bright nickel-plating wastewater, potassium chloride zinc-plating wastewater, alkaline cyanide-free zinc-plating wastewater and hexavalent chromium plating wastewater, and pretreatment wastewater for degreasing and pickling.

(34) Carboxyl-containing organic acid complexants in the electroplating mixed water without cyanide and electroless plating wastewater can be directly precipitated with no requirement of the oxidation, thereby reducing the cost for treating the wastewater. And organic substances such as electroplating additives in the wastewater were remained to be treated in the subsequent step of biochemical degradation.

Step (1) Precipitation of Carboxyl-Containing Organic Acid Complexants

(35) The wastewater flowed from an electroplating wastewater regulating tank into a complexant precipitation tank and was then stirred using a mixer. A ferrous chloride solution was added into the wastewater at an amount of 5 L per ton wastewater followed by adjusting the wastewater to pH of 10.5-12 with lime milk. Ferrous ions and calcium ions co-precipitated the carboxyl-containing organic acid complexants and the pyrophosphate and the phosphate reacted with the free heavy metal ions to form precipitate. Moreover, hexavalent chromium was reduced to trivalent chromium to form chromium hydroxide precipitate.

Step (2) Separation of Precipitate

(36) After the wastewater flowed from the complexant precipitation tank into a flocculation tank, a flocculant was added to flocculate the precipitate until the precipitate was agglomerated into particles of large size. The wastewater then flowed from the flocculation tank into an inclined-tube sedimentation tank A, and the precipitate settled to the bottom of the inclined-tube sedimentation tank A. The precipitate was pumped into a plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (3) Chelation of the Precipitate

(37) The supernatant in the inclined-tube sedimentation tank A flowed into a chelation tank and was stirred using a mixer. The pH of the wastewater was adjusted and maintained at 4.5-5.5 with dilute hydrochloric acid and then a heavy metal capturing agent was added into the wastewater at an amount of 2 L per ton wastewater to precipitate the remaining heavy metal ions such as copper ions. Afterwards, activated carbon was added into the wastewater at an amount of 200 g per ton wastewater to adsorb the precipitate and the remaining heavy metal capturing agent.

Step (4) Separation of Precipitate

(38) After the wastewater flowed from the chelation tank into an inclined-tube sedimentation tank B, the precipitate settled to the bottom of the inclined-tube sedimentation tank B. The precipitate was pumped into the plate and frame filter press using a sludge pump for a pressure filtration, and the resulting filtrate flowed back to the electroplating wastewater regulating tank and the filter residue was treated by a qualified and professional factory.

Step (5) Biochemical Treatment

(39) The supernatant in the inclined-tube sedimentation tank B flowed into a biodegradation tank followed by addition of sodium hydroxide solution to adjust pH to 6-8 to perform a biodegradation treatment. When the COD reached the standards, the wastewater was allowed to be discharged.

Experimental Example 1 Synergistic Effect of Ferrous Ions and Calcium Ions

(40) A solution containing 200 mg/L of nickel sulfate hexahydrate and 400 mg/L of nitrilotriacetic acid was prepared and then adjusted to pH of 7 with 50 g/L of sodium hydroxide solution to produce a mixed solution.

(41) Three portions of the mixed solution were added to three vessels separately labeled as Nos. 1, 2 and 3 at 1 L each. 3 g of anhydrous calcium chloride was added to No. 1 vessel and dissolved under stirring to obtain a mixture. The mixture was adjusted to pH of 11 with lime milk under stirring to produce a suspension. And 30 min later, the suspension was filtered with a quantitative filter paper to obtain a filtrate 1.

(42) 15 mL of 200 g/L ferrous chloride tetrahydrate solution was added to No. 2 vessel to obtain a mixture. The mixture was adjusted to pH of 11 with lime milk under stirring to produce a suspension. And 30 min later, the suspension was filtered with a quantitative filter paper to obtain a filtrate 2.

(43) 30 mL of 200 g/L ferrous chloride tetrahydrate solution was added to No. 3 vessel to obtain a mixture. The mixture was adjusted to pH of 11 with 50 g/L of sodium hydroxide solution under stirring to produce a suspension. And 30 min later, the suspension was filtered with a quantitative filter paper to obtain a filtrate 3.

(44) The nickel content in the three filtrates was measured using atomic absorption spectrometry, and the results were presented in Table 1. As the results demonstrated, the single use of calcium ions to precipitate the nitrilotriacetic acid complexant under alkaline conditions can not remove the nickel ions effectively and the single use of ferrous ions to precipitate the nitrilotriacetic acid complexant under alkaline conditions also can not remove the nickel ions effectively, but the simultaneous use of calcium ions and ferrous ions to precipitate the nitrilotriacetic acid complexant under alkaline conditions can remove the nickel ions effectively.

(45) TABLE-US-00001 TABLE 1 Results of precipitating nitrilotriacetic acid complexant with ferrous ions and calcium ions Amount of Amount Nickel ion ferrous of Reagent content GB chloride calcium to after 21900- tetrahydrate chloride adjust treatment 2008 Samples (g/L) (g/L) pH (mg/L) Standard 1 0 3 calcium 1.64 substandard hydroxide 2 3 0 calcium 0.41 up to standard hydroxide of Table 2 3 6 0 sodium 1.72 substandard hydroxide

Experimental Example 2 Effect of pH on Precipitating Zinc Ions

(46) A solution containing 100 mg/L of zinc sulfate and 200 mg/L of malic acid was prepared.

(47) Five portions of the solution were added in five vessels at 1 L each. Each of the five solutions was added with 15 mL of 200 g/L ferrous chloride tetrahydrate solution to obtain a mixed solution. The five mixed solutions was adjusted to pH of 10.0, 10.5, 11.0, 11.5 and 12.0, respectively, with lime milk to produce five suspensions. After a standing for 30 min, the five suspensions were filtered independently with quantitative filter papers to obtain five filtrates. The zinc content in each of the filtrates was measured using atomic absorption spectrometry, and the results were presented in Table 2.

(48) TABLE-US-00002 TABLE 2 Results of effect of pH on treating zinc ions Zinc content after GB 21900-2008 pH treatment (mg/L) Standard 10.0 0.07 up to standard of Table 3 10.5 0.18 up to standard of Table 3 11.0 0.41 up to standard of Table 3 11.5 1.13 up to standard of Table 2 12.0 2.18 sub standard

(49) As the results demonstrated, the zinc content with the treatment did not meet the requirements of GB 21900-2008 at a pH of 12, nevertheless, pH of 10.5-12 was selected in the present invention. Though a trace amount of zinc remained in the wastewater at a pH greater than 11.5, the zinc can be removed with a heavy metal capturing agent in the subsequent treatment.

Experimental Example 3 Reduction of Hexavalent Chromium With Ferrous Chloride Under Alkaline Conditions

(50) 1 L of 200 mg/L chromium trioxide solution containing 104 mg/L of chromium was prepared.

(51) 1 L of the chromium trioxide solution was added with 20 mL of 200 g/L ferrous chloride tetrahydrate solution to obtain a mixed solution. The mixed solution was adjusted to pH of 11 with lime milk under stirring to produce a suspension. The hexavalent chromium was reduced to trivalent chromium with ferrous ions and then formed chromium hydroxide precipitate and the remaining ferrous chloride was precipitated in the form of ferrous hydroxide. After a reaction for 30 min, the suspension was filtered with a quantitative filter paper to obtain a filtrate.

(52) The hexavalent chromium content in the filtrate was determined to be 0.032 mg/L using diphenylformylhydrazine spectrophotometry, and the removal rate was 99.97%. The results demonstrated that the hexavalent chromium can be effectively removed using the method of the present invention for integrated treatment of the electroplating wastewater.

Experimental Example 4 Treatment of Copper Ions

(53) A solution containing 300 mg/L of copper sulfate pentahydrate and 600 mg/L of triethylenetetramine was prepared.

(54) L of the solution was added with 20 mL of 100 g/L sodium dimethyldithiocarbamate solution and stirred uniformly to obtain a mixed solution. The mixed solution was adjusted to pH of 5 with dilute hydrochloric acid to produce a suspension. After a reaction for 30 min, the suspension was filtered with a quantitative filter paper to obtain a filtrate.

(55) The content of the copper ions was determined using atomic absorption spectrometry and the obtained concentration of copper in the filtrate was 0.11 mg/L. It can be seen that precipitation of copper ions with sodium dimethyldithiocarbamate can effectively remove copper ions in the electroplating wastewater containing aliphatic polyamine complexants.

Experimental Example 5 Treatment of Electroplating Wastewater

(56) Electroplating wastewater was collected from an electroplating sewage treatment plant of an electroplating industrial park, and the electroplating wastewater contained cyanides and complexants such as aliphatic polyamine.

(57) 1 L of the electroplating wastewater was adjusted to pH of 11 with lime milk to produce a mixed solution. The mixed solution was added with 15 mL of a sodium hypochlorite solution having an active chlorine concentration of 3% for an oxidation for 180 min. Then the oxidized solution was added with a ferrous chloride solution under stirring to reduce hexavalent chromium, and when a green precipitate of ferrous hydroxide was observed, 3 mL of the ferrous chloride was further added to produce a mixture. After that, the mixture was adjusted again to pH of 11 with lime milk and added with 1 mL of a flocculant for a flocculation for 30 min to produce a suspension. The suspension was subsequently filtered with a quantitative filter paper to obtain a filtrate. The filtrate was adjusted and maintained at pH of 5 with dilute hydrochloric acid and then the adjusted filtrate was added with 2 mL of 100 g/L sodium dimethyldithiocarbamate solution and stirred uniformly to produce a blend. The blend was added with 0.3 g of activated carbon and stirring for 10 min to produce a turbid liquid. The turbid liquid was filtered with a quantitative filter paper to obtain a filtrate.

(58) Total chromium, copper, nickel and zinc were determined using atomic absorption spectrometry, and the cyanide and hexavalent chromium were determined using spectrophotometry. The results of the determination of the filtrate were shown in Table 3. The cyanide, hexavalent chromium, trivalent chromium, copper, nickel, and zinc in the treated wastewater met the requirements in Table 3 of GB 21900-2008.

(59) TABLE-US-00003 TABLE 3 Results of treatment for electroplating wastewater Items Results (mg/L) GB 21900-2008 Standard CN.sup.− 0.13 up to standard of Table 3 Cr.sup.6+ 0.04 up to standard of Table 3 total Cr 0.38 up to standard of Table 3 Cu.sup.2+ 0.06 up to standard of Table 3 Ni.sup.2+ 0.08 up to standard of Table 3 Zn.sup.2+ 0.43 up to standard of Table 3

(60) The technical features of the above-described embodiments may be combined in any combination. For a concise description, only a part of the possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction between the combinations of these technical features, all combinations should be considered to be within the scope of this description.

(61) The above examples merely describes several embodiments of the present invention, and the specific and detailed description thereof is not intended to limit the scope of the invention. It should be noted that various variations and modifications of the invention made by those skilled in the art without departing from the spirit and scope of the invention should be within the scope of the present invention. Therefore, the scope of the invention is defined by the appended claims.