| High copper content, high salt content and high acidity in printed circuit board micro etching waste was a problem for electroplating wastewater treatment. The methods were used including electrolytic method and precipitation method. High acidity, high salt content and small amounts of copper in were found after using. It was difficult to subsequent biochemical treatment on electroplating plant wastewater and resulted in non-compliance of emissions. The costs of treatment waste after electrolysis recycling copper was up to 250 yuan/m3 according to one electroplating plant in GZ. Since this process does not consider the recovery of sulfate ions, so that the sulfate ion concentration is too high after neutralization, then inhibition of microorganisms and produce toxic effects, which seriously affects the purification effect of biological treatment systems, so that the water quality cannot stable discharge.In this paper, stepwise precipitation method was used to treatment wastewater after electrolytic recovery of copper, then less base was used to neutralize waste water and recycle small amount of metal copper. The advantages of process were consumption less alkaline, easy to operate, recovering sulfate for preparation of gypsum and recovery of copper metal.The factory emissions 1000m3 monthly micro-etching wastewater after electrolytic recovery of copper, with wastewater pH 0.5, sulfate content up to 72g/L, copper ion concentration (787mg/L). Stepwise precipitation method was used in experiments and research, obtained results as follows:1. The first-order precipitation of calcium hydroxide. 100mL waste water of a electroplating factory was took, the dosage of calcium hydroxide was 3.55g and the output of gypsum was 7.5g. The purity of calcium sulfate was 80.68% and the removal rate of SO42-was 57.48%. The pH was up to 2.0 and the concentration of Cu2+ was reduced to 748mg/L from 787mg/L due to the mechanical entrainment of calcium sulfate precipitation.2. The secondary precipitation of calcium chloride. 100mL filtrate after the first step of reaction was took, the dosage of calcium chloride was 3.5g and the output of gypsum was 4.13g. The purity of calcium sulfate was 75.87% and the total removal rate of SO42- of the primary and secondary precipitation was 94.33%. The pH was down to 1.3 and the concentration of copper ion changed little.3. The tertiary precipitation of copper ion.The treatments of 100mL filtrate after the primary and secondary precipitation were as follows:(1) The dosage of NaOH was 0.5g when adding NaOH only and the removal rate of copper was 99.37%. The concentration of copper of effluent did not meet the emission standard in Table 3.(2) The dosage of sodium sulfide was 0.45g when adding sodium sulfide only and the removal rate of copper was 99.90%. The concentration of copper in effluent did not meet the emission standard in Table 3. Although the method of sodium sulfide precipitation had a higher removal rate for copper, the pH of the effluent was low that it impacted the subsequent biological wastewater treatment process and the hydrogen sulfide gas was produced that it caused secondary pollution.(3) In order to make the concentration of copper in effluent reach the emission concentration limits of 0.3mg/L of "electroplating pollutant emission standards" (GB 21900-2008) (Table 3) after tertiary precipitation,0.4g NaOH was added in the wastewater after the secondary precipitation to adjust the pH and then added 0.55g Na2S to produce precipitation. The concentration of copper ion dropped below 0.3mg/L and the pH was 8.5. The effluent reached the emission standard in Table 3, the irritant hydrogen sulfide gas was not produced and the copper content in copper slag was 21.23% which had great economic efficiency.4. Preparation of high-purity gypsum. The gypsum building materials after primary precipitation was washed with 5% sulfuric acid, the purity of gypsum was 98.1% and the operating rate was 84%. The gypsum building materials after secondary precipitation was washed with 5% sulfuric acid, the purity of gypsum was 94.22% and the operating rate was 82%.5. In the process of treating micro-etching wastewater which was electrolyzed for recovering copper with the method of step-by-step precipitation (Ca(OH)2+CaCl2+NaOH ). The pharmaceutical cost of treating one cubic meter wastewater was 114.7 yuan, the value of gypsum building materials generated was 12 yuan and the value of copper byproduct generated was 19.25 yuan. Thus the total cost of treating such wastewater was 83 yuan/m3 and it saved 167 yuan/m3 compared to the current cost which was 250 yuan/m3.6. The design of technology process was scientific and rigorous. The method of effective stepwise precipitation was designed to treat the wastewater according to the characteristics of various substances in wastewater. The wastewater was processed in keeping with the standards with better economic and environment benefit. There were no relevant reports about this technology at home and abroad, this technology had great theoretical and practical significance and had important reference value for wastewater treatment of similar enterprises. |
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