INTRODUCTION

Reagent methods are used for wastewater treatment in domestic electroplating. Pollutants are removed from the water by calcium hydroxide in a form of sparingly soluble compounds. Removing of suspended solids is made in the press filters, thin-sumps, clarifiers [1].

Removed solids are a galvanic sludge. In most cases, it doesn't find wide application industry, and therefore it is being disposed in specially designated areas.

High content of nickel, chromium, zinc, copper in the sludge means that at the present level of industrial development galvanic sludge can not be regarded as waste. It should be considered as secondary raw material.

Modern technology development in Ukraine mainly involves the use of electroplating sludge as an inert filler in building materials.

This work is devoted to seek ways of selective extraction of metals from the sludge.

1 PURPOSE, OBJECTIVES, OBJECT OF research

The object of research was the galvanic sludge of PJSC AZOTsM. This waste is produced during the wastewater treatment. The objective was to determine the possibility of recycling the sludge and separating the mixture of metal hydroxides with the possibility of being used as a raw material in electroplating.

Objectives of research have been:

• to study the composition and properties of electroplating sludge;
• to investigate the possibility of extracting metals from electroplating sludge;
• selection and justification of the method for electroplating sludge processing.

2 THE NEGATIVE IMPACT OF THE SLUDGE ON THE ENVIRONMENT

The calcium hydroxide is used for the treatment of water from metal ions. This produces insoluble substances which are suspended in the water. Galvanic sludges are formed when they are removed from the water by filtration. In most cases it is not widely used neither in production, nor in other industry sectors, so it is dumped into sludge pits.

Galvanic sludge (Fig. 1) has a hazard class III, non-flammable. In its composition contains heavy metal hydroxides, and the sludge has alkaline properties.

Figure 1 — Storage of electroplating sludge in the nopen area

Processing of such industrial waste is an important and actual task for today. Currently, the enterprises of Donetsk region Private JSC Donetskstal — Metallurgical Plant, PJSC Novokramatorsky engineering plant, Snezhnjansky Machinery Plant, PJSC Motor Sich, PJSC Donetsk plant ProdMash use electroplating in their technologies.

Using the methods of sludge processing would free the area used for sludge storage and the impact of waste on the environment.

3 COMPOSITION AND PROPERTIES OF ELECTROPLATING SLUDGE

Sludge is a pasty mass with dark green color (Fig. 2, a) insoluble in. Sludge pH varies from 6 to 9. Moisture content has been determined in a sample of the sludge by drying at an oven at temperature of 100–110 ^°C for 2 hours. It had a typical for chromium hydroxide green color after drying (Fig. 2, b).

a — initial sludge; b — sludge after drying in an oven

Figure 2 — Example of electroplating sludge

After calcination in a furnace at 600 ^°C for one hour metal hydroxides in sludge have been transformed into oxides.

To determine the chemical composition of the test sample of galvanic sludge it was been dissolved in sulfuric acid with a concentration of 15 wt%. Metal hydroxides contained in the sludge have transferred to water soluble sulfates. Part of the sludge has not been soluble in acid, the insolubles have been removed by filtration. In the filtrate the concentration of chromium, nickel, and copper have been defined. Determination of the concentration of chromium, nickel was performed using the photometric method. Determination of copper content was performed by iodometric method [2]. Analysis of sludge showed the following results:

• moisture content 65.4 %;
• the content of Cr(OH)₃ 6.2 %
• the content of Ni(OH)₂ 1.24 %
• the content of Cu(OH)₂ 1.77 %;
• acid insoluble 14.9 %.

The presence of sulfate ions in the original sludge have been indicated by infrared spectroscopy. The characteristic absorption peaks for the ions SO₄^2– are: 1105, 983, 611, 450 cm^–1. Figure 3 is an IR spectrum of the sludge washed with distilled water and dried at 110 ^°C, wherein the content of sulfate ions is much lower and the IR spectrum of the starting sludge.

a — the washed sludge; b — the original sludge

Figure 3 — IR spectrum of galvanic sludge

The presence of SO₄² is due to containing CaSO₄, which is used for the deposition of heavy metals.

4 METHODS OF METALS EXTRACTION FROM SLUDGE

4.1 Acid-alkaline metal selection

The dependence of the pH of the deposition of differernt insoluble metal compounds can be used for the selective extraction of metals from the sludge [3].

pH precipitations of the deposition of differernt insoluble metal compounds is given in Table 1 [3]. Starting deposition of metals deposited corresponds to the ion concentration of 0.01 mol/dm³. The entire deposition — metal ion concentration of 10^–5 mol/dm³.

Table 1 — The pH during the precipitation of metal hydroxides

Cation	pH
	start of precipitation	complete precipitation
Iron Fe3+	7.5	9.7
Iron Fe2+	2.3	4.1
Zinc Zn2+	6.4	8.0
Chrome Cr3+	4.9	6.8
Nickel Ni2+	7.7	9.5
Aluminum Al3+	4.0	5.2
Cadmium Cd2+	8.2	9.7

As seen from Table. 1, it is theoretically possible to separate the hydroxides of iron, nickel, zinc, chromium by precipitation at different pH.

4.2 Extraction by organic substances

Solvent extraction with organic solvents used in the technology of rare, non-ferrous and precious metals in chemical analysis.

As the fusible organic reagents commonly used 8–hydroxyquinoline, salitsilaldoksim, benzoylacetone, dibenzoylmethane, naphthol, aliphatic carboxylic acid [4].

Extraction is a cation exchange reaction. It has a strong dependence from the activity of hydrogen ions in the aqueous phase. During the extraction the metal salt with an anion of an organic acid is formed [8]:

2C₁₇H₃₅COOH + Me²⁺ → (C₁₇H₃₅COO)₂Me + 2H⁺.

Proceeding from the lowest to the ability of hydration salts, non-toxic and fire-safety for the experiment with the sludge as an extractant the stearic acid has been chosen — monobasic aliphatic carboxylic acid [5–6].

Scheme of cation exchange in the hydrogen molecule of the carboxylic acid is shown in Figure 5.

Figure 4 — The extraction of nickel (II) with stearic acid

(Animation, 14 frames, 164 KB, 5 cycles)

The acidity of the aqueous solution is one of the main factors that have a significant impact on the selectivity of the extraction.

5 EXPERIMENTAL part

To determine the possibility of selective separation of metals the sample of sludge has been dissolved in sulfuric acid. The insolubles were separated by the filtration.

5.1 Acid-alkaline metals separation

The pH level in filtrate has been changing by 1 N KOH solution. Investigated the possibility of separation at the following values: pH = 3; pH = 4.5; pH = 8.5. Found that at pH = 3 and pH = 4.5 respectively 5 % of nickel (II) hydroxide and 10 % of chromium (III) hydroxide have been precipitated. At pH = 8.5 85 % of chromium (III) and 100 % of nickel (II) have been precipitated.

Found that selective precipitation at pH values ​​is impossible because almost all the metals are coprecipitating.

5.2 Selective extraction by stearic acid

Stearic acid was used as an extractant in experiment. The aim of the study was to determine the possibility of selective separation of nickel and chromium from the solution by varying the pH level. Nickel concentration in the solution was 0.120 g/dm³, chromium 0.475 g/dm³. Volume ratio: filtrate phase to extractant as 3 : 1. Extraction was carried out at a temperature of 70–90 ^°C. At the pH = 1,75–2,2 and pH = 3,0–4,0 value of the difference amount of recovered nickel and chromium was maximum (Fig. 7).

Figure 5 — Dependence from pH of the degree of extraction of metals by molten stearic acid

Quantities of extractable solution was for chromium 0–10 % at pH = 1,75 – 2,2 and 20–35 % at pH = 3,0–4,0. Amount of nickel extracted at pH = 1,75–2,2 was 40–45 % and at pH = 3,0–4,0 it was 50–65 %.

Thus, the separation of metals in solution is possible with the use of extractant at varying pH. Stearic acid as an extractant exhibits selective properties for the metals at different pH values.

Conclusion

Following results and conclusions have been made from the performed work:

• the chemical composition of electroplating sludge has been studied, the negative effect on the environment and human health from such types of waste has been shown;


• it was concluded that liquidation methods of the sludge neutralization are not rational and safe for the environment. It is impossible to eliminate contact with air and surface waters the waste in the sludge pits.


• it was shown that for solving the problem of waste management, the question was to choose the method of processing with the lowest economic cost;


• the extraction methods by an organic extractant and acid-alkaline metals separation method were selected for studying;


• it was shown that for in the range of pH 4.5–8.5 metal hydroxides have not been separated by acid–alkaline separation method, that means the inability to use the method for the separation of metals;


• it was concluded that the separation of metals in solution is possible with an extractant at varying pH. It was proved that stearic acid as an extractant exhibits selective properties in terms of nickel and copper, which may allow to extract metals selectively from the sludge.

List of references

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