Problem Definition

Table 5 shows the bath types and chemicals used in the custom plating shop. The effluent concentrations vary because some baths are used intermittently. The following ranges were established through effluent monitoring: Crvl, 1-4 ppm; Ni, 7.5-107 ppm; Cu, 1.5-9 ppm; Zn, 1.4-19 ppm; CN-, 1.9-18 ppm. In addition, small amounts of Cd, Pb, Mn, and Se were found to be present. The amounts of Au dragged out from the gold bath as well as the Ag and Sb from the silver bath were small since the precious metal baths are used sparingly. The streams listed in Table 5 can be grouped into four categories:

Pump Stroke Frequency

100%

Low Alarm

High Alarm

-100

Electrode Potential [mV]

Low Set High Set

Figure 5 Chemical dosage by proportional addition using an ORP controller and a millivolt-to-frequency converter in combination with a proportional feed pump. The set points for hypochlorite addition to KCN solution are shown.

Cyanide streams containing Cu, Zn, Ni Chromate streams containing Crvl Metal streams containing Zn, Ni, Se Other: containing Pb, Sn, Fe, Cu, Zn, Ni

After elimination of the cyanide-based Ni-stripping operation, Cu and Zn are the primary heavy metals in the cyanide stream. Cyanide can be destroyed at pH > 11. Cu(OH)2 and Zn(OH)2 require a precipitation pH range of 10-11.

The conventional procedure for CrVI removal using sulfite operates at pH 2-3 in the reduction tank and at pH 8.5 in the Cr(OH)3 precipitation reactor.

The last two streams contain Pb, Zn, Sn, Cu, and Ni, with optimum hydroxide precipitation pH ranging from 8.5 (Pb) to 11 (Ni).

An additional complication arises when chlorine is used to treat cyanide-containing streams. Excess chlorine used in the cyanide oxidation step must be destroyed before discharge. The pH of the discharge stream must lie within a pH range of 6-10.

Conventional pretreatment technology for treating copper cyanide, chromium (VI), and nickel wastewater streams requires the system arrangement shown in Figure 6. In the situation at hand, a fourth treatment line may have to be installed to treat the lead-, zinc-, and tin-containing streams, which require a lower pH environment than nickel for quantitative precipitation of metal hydroxides. The expense of a continuous system would be excessive due to the multiple reactors, clarifiers, and controls. Batch treatment would be complex and very laborintensive.

A great simplification can be achieved if the treatment scheme shown in Figure 7 is implemented. The cyanides are destroyed in reactor 1. The effluent of the cyanide reactor is fed together with the chromate and general rinse into reactor 2, which operates at pH 9-10. Here chromate and excess chlorine from reactor 1 are reduced with Fe", and all metals are copre-cipitated with Fe"1 generated in the reduction of Crvl and hypochlorite. The effluent from reactor 2 can then be further processed in the conventional way.

Table 4 Process Performance Data for the Control of Cd, Cr, Cu, Pb, Ni, and Zn

Average

Metal

Plant

Process

treatment (mg/L)

Hydroxide precipitation

Cadmium

Lead battery mfg.

NaOH

0.055

Lead battery mfg.

NaOH

0.08

Zinc battery mfg.

Lime precip.

0.067

Metal finishing

Lime

0.02

Copper

Metal finishing

NaOH

2.63

Commercial TSD

Lime precip.

0.58

Lead

Lead battery mfg.

NaOH

0.11

Electr. components

NaOH

0.14

Nickel

Electr. components

NaOH

0.52

Commercial TSD

Lime

1.8

Battery mfg.

Lime

0.5

Zinc

Commercial TSD

Lime

1.05

Hydroxide precipitation in the presence of of Fe!Alum

Cadmium

Metal finishing

FeCl3 + base

0.07

Metal finishing

FeS04 + base

0.579

Lead battery mfg.

Ferrite coprecip.

0.008

Copper

Electr. components

NaOH + FeS04

0.17

Electr. components

NaOH + FeS04

0.59

Metal finishing

FeS04 + hydroxide

0.505

Commercial TSD

NaOH + FeCl3

1.2

Lead

Commercial TSD

NaOH + FeCl3

0.07

Lead battery mfg.

Ferrite coprecip.

0.01

Nickel

Electr. components

NaOH + FeS04

0.165

Electr. components

NaOH + FeS04

0.63

Metal finishing

FeS04 + hydroxide

0.018

Commercial TSD

NaOH + FeCl3

1.1

Battery mfg.

Ferrite coprecip.

0.2

Zinc

Metal finishing

FeS04 + hydroxide

0.005

Commercial TSD

NaOH + A1 coprec.

4.1

Sulfide precipitation

Cadmium

Copper smelter

Na2S + lime

0.01

Commercial TSD

Lime + sulfide

0.5

Army report

Na2S + FeS04 coprec.

0.06

Copper

Commercial TSD

Lime + sulfide

0.15

EPA test run

Sulfide + ion exchange

2.3

Lead

Commercial TSD

Lime + sulfide

0.01

Copper smelter

Na2S + lime

0.05;0.2

EPA test run

FeS + ion exchange

0.5

Nickel

Commercial TSD

Lime + sulfide

0.34

EPA test run

Sulfide precip. + ion exchange

13.4

Zinc

Metal finishing

FeS + ion exchange

0.5

Foundry

Sulfide precip.

0.88

Chromium treatment

Average treatment (mg/L)

Plant

Process

Hex. Cr

Total Cr

Metal finishing

Reduct. precip.

2.14

Metal finishing

Reduct. precip.

2.36

Metal finishing

FeS04 + hydroxide

0.151

Commercial TSD

NaHS03 reduct.

0.1

0.12

Commercial TSD

NaHS03 reduct.

0.63

Lead battery

FeS precip.

0.014

0.04

Lead battery

Na2S precip.

0.005

0.005

Source: Data from Ref. 7.

Source: Data from Ref. 7.

Table 5 Bath Types in Custom Plating Shop, Estimated Dragout and Bath Composition

Bath type

Dragout (gal/day)

Hazardous ingredients

Aluminum cleaner

1/4

Sodium tetraborate pentahydrate, zinc sulfate, sodium nitrate,

tetrasodium pyrophosphate

Steel cleaner

1/2

Sodium carbonate, tetrasodium pyrophosphate, sodium hy

droxide, ethtoxylated amphoteric sodium salt, sodium meta-

silicate

Mild acid

1/2

Sodium bisulfate, sodium fluoride, sodium chloride

Die cast boil

1/8

Not given

Nickel strike

1/8

Nickel sulfate solution

Copper solution

1/4

Copper cyanide, potassium hydroxide, sodium cyanide, zinc

cyanide, sodium hydroxide, thallium carbonate

Chrome solution

1

Chromic acid, magnesium silica fluoride, sulfuric acid

Nitric acid

1/4

Nitric acid

Zincate

1/4

Sodium hydroxide, zinc oxide

Nickel solution

3.0

Nickel carbonate, nickel chloride, nickel sulfate, sulfuric acid,

boric acid, saccharin (Besniflec 812)

Gold solution

1/10

Potassium cyanoaurate, cobalt complex, nickel complex, po

tassium citrate

Brass solution

1/4

Copper cyanide, zinc oxide, sodium cyanide

Silver strike

1/10

Sodium cyanide, potassium cyanide, silver cyanide

Silver solution

1/4

Potassium cyanide, sodium cyanide, silver cyanide, potassium

antimonyl tartrate

Antiquing

1/4

Selenous acid, cupric sulfate, phosphoric acid, zinc sulfate,

ammonium molybdate

Antiquing

1/4

Cupric sulfate, selenous acid, nitric acid

Cleaner

1/4

Sodium hydroxide, sodium metasilicate pentahydrate, sodium

carbonate, tetrasodium pyrophosphate

Paint stripping

1/2

Sodium hydroxide, phenyl diethanolamine, sodium meta

silicate

Silver stripping

1/4

Sodium cyanide

Nickel stripping

1/2

Sulfuric acid

Nickel stripping

1/2

Sodium cyanide, sodium hydroxide

Hydrochloric acid

1/2

Hydrochloric acid 50%

Figure 6 Conventional pretreatment system arrangement for treating wastewater containing copper cyanide, chromate, and nickel.

chromate chromate

Figure 7 Two-step system arrangement for treatment of cyanide, chromate, and heavy metals. Chromate is reduced by Fe", and all metals are coprecipitated with Fe1" at pH 9.5-10 in one reactor.

Table 6 Synthetic Stream Composition (mg/L)

Cyanide Noncyanide

Table 6 Synthetic Stream Composition (mg/L)

Cyanide Noncyanide

Inorganic

(pH 11.7), stream 1

(pH 2.7), stream 2

Copper

90

1.4

Nickel

8.6

99

Zinc

14

5.9

Silver

22

Chromium

67

Cyanide

250

Selenium

0.02

0.22

Cadmium

0.087

Lead

0.18

Manganese

0.05

Gold

0.87

0 0

Post a comment