Process Implementation

The cyanide streams contained nickel stemming from the nickel stripping operation. Since Ni(CN)42- cannot be destroyed through chlorination, this bath was substituted by a bath containing noncyanide chemicals. The feasibility of the process arrangement shown in Figure 7 had to be tested in the laboratory. Flow rates and stream compositions were needed. The flow rates were estimated from the individual rinse water streams to be approximately 5 gal/min for the cyanide-containing streams and 15-20 gal/min for the remaining streams (general rinse). The cyanide-based baths were operated intermittently whereas the chromium and nickel lines were in continuous use.

The concentrations expected in the cyanide and general rinse streams after stream segregation were estimated from the bath dragout rates. A dilution of 1:250 to 1:1000 is expected in the rinse tanks. With these data, a synthetic cyanide and synthetic general rinse effluent was composited from bath concentrates, which were mixed in the same ratios as the estimated bath dragout rates. The resulting composite was finally diluted 1:250 to obtain simulated cyanide and general rinse effluent streams. These synthetic effluents contained all the chemicals, additives, and brighteners used in the process in their approximate ratios and concentrations and were used in bench-scale testing to prove system feasibility. Analyses of these composites are shown in Table 6.

The cyanide was treated for cyanide destruction using hypochlorite at pH values greater than 11. The hypochlorite addition was monitored through measurement of the redox potential. The cyanide concentration decreased from 250 ppm to <0.02 ppm in this step.

Following cyanide destruction, the treated cyanide stream was combined with the general rinse streams in a ratio of 1:4 to simulate the reactor 2 influent. This solution was treated with FeS04 reagent at pH 10. The metal concentrations before and after the bench-scale treatment are summarized in Table 7. The discharge limits are also shown for comparison. The treatment strategy results in an effluent quality well below the discharge standards.

The data obtained in the bench-scale study were used to build a fully automated system as shown in Figure 8. It incorporates the following features.

1. The cyanide stream is collected in a holding tank and is uniformly fed to the cyanide destruction reactor.

Table 7 Treatment Efficiency and Discharge Standards (mg/L)

Treated effluent

Discharge standards

Inorganics

Reactor influent

(pH 10)

(pH 5.5-10.5)

Copper

18

0.20

2.02

Nickel

82

0.12

2.33

Zinc

7.4

0.02

1.45

Silver

4.1

0.03

0.24

Chromium

55

<0.05

1.67

Selenium

0.18

<0.008

0.02

Cyanide

<0.02

<0.02

0.64

Arsenic

0.70

Cadmium

0.07

<0.009

0.07

Lead

0.15

<0.04

0.42

Manganese

0.04

<0.016

1.00

Gold

0.16

0.17

NA

NA = not available.

2. This reactor is well stirred. The pH and the ORP are continuously monitored by electrodes. NaOH and hypochlorite are automatically fed into the reactor to maintain preset pH and ORP values determined during the bench-scale phase of the program.

3. The effluent of the cyanide reactor is fed by gravity feed into the main reactor.

4. The daily rinse is collected in a holding tank and is uniformly fed into the main reactor. This vessel is well stirred. The pH and the redox potentials are monitored by electrodes. FeS04 reagent and sodium hydroxide are automatically metered into the reactor to maintain preset ORP and pH values. This allows for chromium reduction and heavy metal precipitation in one reactor and at one pH:

Chromium reduction:

Cr6+ + 3 Fe2+ -* Cr3* + 3 Fe3+ Metals precipitation: Cr3* + 3 OH" Cr(OH)3 Fe3+ + 3 OH- -> Fe(OH)3 Cu2+ + 2 OH- -* Cu(OH)2 Ni2+ + 2 OH" — Ni(OH)2 Cd2+ + 2 OH" -> Cd(OH)2 Pb2+ + 2 OH" Pb(OH)2 Zn2+ + 2 OH" Zn(OH)2

5. Precipitated metal hydroxides usually form fine particles, which settle slowly. Addition of a polymer achieves agglomeration of the precipitate, greatly improving the solid-liquid separation in a gravity clarifier.

6. The main reactor effluent solution is transferred to a lamella clarifier with built-in floc-culator. The flocculator provides gentle, shear-free mixing for about 3 min of retention

Meter

Figure 8 Flow schematic of a two-step system for cyanide oxidation, chromate reduction, and metals precipitation.

Meter

Figure 8 Flow schematic of a two-step system for cyanide oxidation, chromate reduction, and metals precipitation.

O"

Table 8 System Performance, Values in mg/L

Grab samples 24-hr Composite startup period compliance monitoring

Grab samples 24-hr Composite startup period compliance monitoring

Table 8 System Performance, Values in mg/L

Parameter

8/26/90

8/27/90

8/30/90

8/31/90

9/1/90

10/9/90

12/3/90

2/25/91

4/2/91

As

<0.01

<0.01

<0.01

<0.005

<0.005

<0.005

<0.005

Ba

<0.7

<0.7

<0.7-

0.02

<0.02

<0.02

<0.02

Cd

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

Cr

<0.06

<0.06

<0.06

<0.02

<0.02

<0.02

<0.02

Cu

0.40

0.03

0.03

0.17

0.12

0.07

0.05

Pb

<0.06

<0.06

<0.06

<0.10

<0.10

<0.10

0.190

Mn

<0.04

<0.04

<0.04

<0.02

<0.02

<0.02

<0.02

Hg

<0.02

<0.02

<0.02

<0.001

<0.001

<0.001

<0.001

Ni

<0.04

<0.04

<0.04

0.20

0.13

0.14

<0.10

Se

<0.008

<0.008

<0.008

0.005

<0.005

<0.005

<0.005

Ag

<0.02

<0.02

<0.02

<0.02

0.02

<0.02

<0.02

Zn

0.06

<0.01

<0.01

0.11

0.14

0.10

0.12

Fe

0.4

0.2

0.3

CN

0.30

0.04

<0.02

0.04

0.06

0.08

<0.02

COD

43

19

28

16

time, allowing the particles to grow. The clarifier overflow is discharged to a holding tank. A final filtration step can be added if needed.

7. The sludge collected in the clarifier is discharged from the bottom via a diaphragm pump for which the frequency and duration of operation can be adjusted by a timer located on the central control panel. Sludge from the clarifier is collected in the sludge holding tank, from which it is pumped to the filter press. A level indicator will alert operators if the sludge exceeds a preset level.

8. Sludge that has been separated and prethickened by the clarifier is further thickened to 25-35% solids using a filter press equipped with gasketed, recessed polypropylene filter plates and a high pressure feed pump. Thickened sludge is collected in a self-dumping sludge hopper or in 55-gal drums.

9. A final pH adjustment is not necessary, since the system is operated at a pH 9.5-10, which is well within the discharge limits of 5.5-10.5.

C. System Performance

The system described was started up during July and August of 1990. Table 8 summarizes the system performance during the startup period as well as the monitoring results established using 24-hr composites.

SYMBOLS

aFem Activity of the Fe3+ ion a^ii Activity of the Fe2+ ion

CH+ Concentration of the hydrogen ion H +

COH- Concentration of the hydroxyl ion OH

CM2 + Concentration of the Me2 + ion

Concentration of the sulfide ion S

Normal potential Potential sp

Faraday constant Dissociation constant Solubility product tfsp(Me(OH)2) Solubility product of Me(OH)2

Me ORP

Ion product of water Metal

Oxidation reduction potential

Gas constant

Absolute temperature, °K

0 0

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