Laboratory Setup for the Treatment of Acidic Wastewater
Abstract Generated abstract
This paper describes the development and testing of a laboratory-scale continuous unit for treating acidic wastewater from electroplating and pickling operations containing heavy-metal ions and chromium compounds. Based on prior evaluation of treatment options, the proposed scheme mixes spent pickling solution with chromium-containing wastewater, reduces chromium, neutralizes the combined stream with lime milk, and treats rinse waters so that metals precipitate as hydroxides, aided by the sorptive properties of freshly formed ferric hydroxide. A 1:2000 scale setup was constructed and operated in an enlarged two-hour experiment, processing about 8 liters of wastewater and producing clarified water and filterable sludge. Analysis of the treated water showed impurity concentrations within permissible limits, with several metals undetected and copper and cadmium below standards, indicating that the unit can be used to test acidic wastewater treatment schemes in continuous mode.
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LABORATORY SETUP FOR THE TREATMENT OF ACIDIC WASTEWATER
S. A. BABENKO, R. I. TASOEV, S. M. OLENEV
(Presented by the scientific seminar of the Department of PMKhTP KhTF)
Wastewater formed during the washing of parts after copper plating, nickel plating, zinc plating, and cadmium plating from acidic electrolytes, as well as pickling wastewater, contains acids and heavy-metal ions. Wastewater from washing parts after degreasing contains alkalis. In the wastewater balance of electroplating-coating shops, as a rule, acidic waters predominate, favoring the presence of metals in the dissolved state in amounts exceeding standards. Therefore, such waters must be treated before discharge into a water body.
This paper presents the results of treatment of acidic wastewater from one of the plants.
The plant’s wastewater contains a pickling solution of practically constant composition: \( \mathrm{Fe}^{3+} \)—70 g/l, \( \mathrm{Fe}^{2+} \)—96 g/l, \( \mathrm{Cu}^{2+} \)—60 g/l; chromium-containing water, in which the content of \( \mathrm{Cr}^{6+} \) is 1 g/l; and rinse waters with a variable amount of heavy-metal ions (Table 1). In the plant’s combined effluent, organic matter was found in an amount from 1.6 to 9.2 mg/l.
The results of investigations carried out on the treatment of wastewater of the specified composition by various methods, and their comprehensive evaluation, made it possible to recommend the following technological scheme for treating the plant’s wastewater. The spent pickling solution and chromium-containing water are fed into a reactor. The resulting mixture, containing ions \( \mathrm{Cr}^{3+} \), \( \mathrm{Fe}^{2+} \), \( \mathrm{Fe}^{3+} \), \( \mathrm{Cu}^{2+} \), and lime milk, is fed into a mixer, into which all the plant’s rinse waters also enter. From the mixer, the neutralized wastewater flows by gravity into settling tanks, where metal ions precipitate as hydroxides. Freshly formed ferric hydroxide has enhanced sorption properties with respect to metal ions and organic matter and therefore ensures the required degree of treatment of all the plant’s wastewater. The clarified water from the settling tank enters the sewer system, while the thickened product, after filtration, is transported to a dump.
On the basis of the adopted technological scheme, a laboratory setup was designed and manufactured for treating wastewater at a scale of 1:2000 of the actual size (Fig. 1). The setup includes vessels 1 and 2 for the chromium-containing water and the pickling solution. Their mixing, owing to three-way valves, can be carried out in reactors 4 or 5. The reactors are equipped with stirrers 7. After reduction of the chromium, the mixture from reactor 4, lime milk
Table 1
Chemical composition of rinse waters from the electroplating shop
| Date of sample collection | NaOH | H₂SO₄ | Cu | Ni | Cr⁺⁶ | Cr⁺³ | Pb | Zn | Cd |
|---|---|---|---|---|---|---|---|---|---|
| 2.10.70 | 80 | 50 | 78 | 375 | 0.4 | 1.9 | 15 | 40 | 58 |
| 22.10.70 | 60 | 1200 | 107 | 30 | 0.4 | 0.3 | 400 | 32 | 12 |
| 27.10.70 | 40 | 900 | 172 | 2.8 | 0.8 | — | 8 | 34 | 0.09 |
| 4.11.70 | 40 | 800 | 31 | 14 | 3.0 | 1.7 | 288 | 0.1 | 0.24 |
| 11.11.70 | 50 | 1900 | 82 | 8.4 | 5.9 | 3.7 | 100 | 3.4 | 8.7 |
| 19.11.70 | 160 | 1800 | 16.4 | 0 | 1.4 | 0 | 0.6 | 2.4 | 5.5 |
| 23.11.70 | 100 | 60 | — | 15.6 | 0.9 | 0.3 | 13.6 | 14.0 | 3.3 |
| 2.12.70 | 120 | 60 | 3.6 | 19.0 | 0.4 | 0.1 | 0 | 0.6 | 2 |
| 8.12.70 | 120 | — | 31.6 | — | 0.08 | 0.03 | 10 | — | 4.3 |
6 and rinse waters 3 enter mixer 8. The amount of mixture from reactor 4 and of lime milk is limited by the amount of rinse waters that can be regularly added to vessel 3 as the unit operates.
The studies carried out to determine the dependence of clarification of the total effluent treated with lime showed that, in 20 min, \(2/3\) of the volume of the water being purified is clarified, while \(1/3\) is occupied by sludge.
After adjustment of all systems of the unit, an enlarged experiment was carried out, in which during 2 hours of continuous operation about 8 l of wastewater was processed (7.845 l of rinse waters, 0.064 l of chromium-containing water, 0.016 l of pickling solution, 0.095 l of lime milk). In the course of the enlarged experiment, the purified clarified water was continuously withdrawn as overflow from the settling tank, while the settled sludge, because of the small dimensions of the unit and its small quantity, was removed periodically.
Fig. 1. Diagram of a laboratory unit for treating wastewater by neutralization. (Explanations in the text)
A total of 5.2 l of clarified water was withdrawn, and the total volume of thickened product was 2.8 l. After filtration, the volume of precipitate was 90 cm³. The clarified water was mixed with the filtrate, and the resulting mixture was analyzed for the content of metal ions.
The results of analysis of the purified water obtained as a result of the enlarged experiment indicate that the content of impurities in it does not exceed the permissible standards. Chromium of both valences, nickel, lead, and zinc were not detected. The copper content was 0.05 mg/l, and cadmium 0.01 mg/l, which is below the standard.
Thus, the proposed unit makes it possible, in a continuous mode, to test technological schemes for purifying acidic wastewater.