Soil Wastes

Tests on the soil samples were conducted simultaneously with the research on the battery casings. Since upgrading by gravity separation, flotation, etc. were not effective, the hydromet-allurgical approach used for the casings was followed for the soils (see Figure 3). First, the as-received soil samples were characterized and chemical analyses were performed. The amount of lead in the soils varied widely, depending on the specific site and the area within that site. Partial analyses on one shipment of six samples are listed in Table 9. In addition to the analyses, sample 3 was identified as vegetation, and samples 1, 2, 4, 5, and 6 were identified

Table 8 TCLP Test Results After Leaching with H2SiF6

Test H2SiF6 (g/L) 30% H202 (mL) Residue Filtrate

Table 8 TCLP Test Results After Leaching with H2SiF6

Test H2SiF6 (g/L) 30% H202 (mL) Residue Filtrate

Test No. 1

First leach

80

2

ND

ND

Second leach

80

2

116

< 1

Test No. 2, single leach

80

2

468

2.3

Test No. 3, single leach

200

2

129

1.3

ND = not determined.

ND = not determined.

CONTAMINATED SOIL

CONTAMINATED SOIL

CLEANED SOIL, TO SITE

Figure 3 Flow diagram for decontamination of soil wastes.

CLEANED SOIL, TO SITE

Figure 3 Flow diagram for decontamination of soil wastes.

Table 9 Partial Soil Analyses

Sample No.

Sample No.

Table 9 Partial Soil Analyses

1

2

3

4

5

6

Elements, ppm

Ag

< 5

< 5

< 5

< 5

< 5

< 5

As

6.6

11

< 5

8.7

8.1

< 5

Ba

442

492

109

725

698

419

Cd

< 5

< 10

< 5

< 5

< 10

< 5

Cr

40

43

< 20

44

34

21

Hg

< 5

< 5

< 5

< 5

< 5

< 5

Pb

47

87

517

477

2200

4067

Sb

< 5

< 5

< 5

< 5

11

221

Se

< 5

< 5

< 5

< 5

< 5

< 5

Elements, pet

Al

5.7

5.6

.8

6.2

6.2

3.5

Ca

2.2

2.1

2.1

.92

1

5.2

Fe

3.5

3

.41

3.9

4.3

1.5

Mg

1.5

1.1

.26

.82

.82

1.4

Si

28.3

27.7

2.5

30.6

29.5

25.6

Table 10 Results of EP Toxicity Tests

Filtrate, ppm

Element

Soil 1

Soil 2

As

< 0.5

< 0.5

Ba

1.1

< 0.5

Cd

< 0.1

< 0.1

Cr

< 0.5

0.21

Pb

3.2

1.0

Hg

< 0.2

< 0.2

Sb

< 0.5

1.5

Se

< 0.5

< 0.5

as soil. One significant feature observed in samples 1,2, and 6 was the presence of dolomite [CaMg(C03)2] and, in sample 6 only, bassanite (CaS04-'/2 H20). Sample 6 also had more rocks than the other samples. Since lead in samples 1 and 2 analyzed below the standard of 500 ppm, only an EP toxicity test was completed to verify that lead content would not exceed the 5 ppm extraction standard. Table 10 outlines the results.

Approximately 100 g of the vegetation sample (No. 3) was cut into smaller pieces, washed with 1.6 L of room temperature tap water for 0.5 hr, drained, and then rewashed for a total of five times. The EP toxicity test results were <1.0 ppm Pb in the filtrate and <110 ppm in the residue. This indicated that the majority of the lead contamination was in the soil clinging to the plants and not assimilated into the plant.

In the fourth and fifth soil samples, <5% of the material was larger than 18 mesh. The sixth sample contained ~28% rocks, wood, and battery casings. The distribution of this last sample is shown in Table 11. Lead analyses of the minus 18 mesh material in this sample varied from 7100 to 8500 ppm.

Table 11 Characterization of Soil by Size, Weight, and Material Distribution

Rocks 94.3

Casings 2.7

Wood 3.0

Metallic Pb 0

Rocks 99.0

Wood and casings 1.0

Metallic Pb 0.0 Minus 18 mesh, 71.7 wt %

Soil 100.0

Note: The minus 18 mesh soil included ~ 15-20% moisture.

1. Soil Carbonat ion

The following procedure was used with soil samples 4-6. The sample was wet-screened, and the plus fraction, which was mostly rock, was set aside and studied with the subsurface samples. The minus 18 mesh fraction of the sample was mixed with tap water and a carbonate in a ball mill to give a pulp density of 25 wt %. Since preliminary testing proved that PbS04 in the soil could not be leached to pass the standard and had to be converted to PbC03 before acid leaching, all subsequent testing was on carbonated materials. The soil slurry was tumbled without a grinding medium at room temperature for 1 hr with more than the stoichiometric amount of the chosen carbonate. Initially, the carbonate used was (NH4)2C03; however, as with the chips, there were objections to the cost and odor. Of the other carbonates used, K2C03 and MgC03 were too expensive and CaC03 was ineffective; therefore, Na2C03 was chosen. Sodium carbonate at room temperature for 1 hr in concentrations of 8-100 g/L was an effective carbonating agent. The amount of bisulfite added depended upon the amount of Pb02, usually about 30% of the lead analysis. When H2SiF6 was used as the lixiviate, bisulfite was omitted and, instead, H202 was added with the acid to reduce the Pb02.

2. Soil Dewatering—Trommel and Flocculant

After carbonation, the minus 18 mesh fraction of the soil sample was dewatered and rinsed. However, with the soils, extreme difficulties were encountered with the solid-liquid separations. Pressure filtration at 60 psi, vacuum filtration, settling and decanting, and centrifuging were all tried with little or no success. The filtration problems may have been caused by the formation of a sodium silicate or the extremely fine particle size of the soils. This problem was finally solved by using a flocculant, polyethylene oxide (PEO) [10]. PEO is a nonionic, water-soluble polymer with a molecular weight of 5 million. Published studies have confirmed that PEO and its degradation products are nontoxic and do not have an adverse effect on the environment [II]. The dewatering process using PEO was developed at the Bureau's Tuscaloosa Research Center, Tuscaloosa, Alabama, where it was used for dewatering phosphate slimes and other fine-grained wastes. The process consists of mixing a small amount of PEO with the carbonate or leachate solutions, allowing the floes to form, and dewatering the floes on a trommel.

The trommel (Figure 4) consisted of an 8-in.-diameter pipe, 32 in. long, fitted on the inside with a slightly smaller diameter 35 mesh screen that was 4 in. longer than the outer pipe. A !/s-hp motor supplied the power to rotate the trommel. A variable controller was used to

motor with variable speed control

Figure 4 Schematic diagram of trommel.

motor with variable speed control r-&

Figure 4 Schematic diagram of trommel.

UJ Inj

Uj change the rotation speed, and the trommel was mounted on a frame so the angle of inclination could also be altered. The dewatering efficiency of the system is affected by the slope and speed of rotation and the feed rate of the flocculant. The soil slurry (12 kg soil/60 L lixiviate or rinse) was pumped into a small tank, mixed with sufficient PEO solution to create floes, and then overflowed into the rotating trommel. The PEO solution concentration used was 2.5 g PEO/L H20. In the 12-kg test, ~3.5 L of PEO solution was needed in each solid-liquid separation. This created a product that dewatered rapidly, forming loose rolls containing 50 wt % solids. A drier solid could be generated by increasing the quantity of PEO added [10]. The amount of PEO needed depended upon the variables in the process, including soil characteristics, amount of carbonate used, acid concentration of the leachate, temperature, and solid/ liquid ratio.

3. Acid Leach—Nitric Acid

The soil (0.8% Pb) was carbonated with Na2C03 and NaHS03 and leached twice with HN03. Studies were conducted at ambient temperature, 70°C, and 90°C. The first leach used 50.4 g/L HN03, followed by a solid-liquid separation and a second 20.2 g/L acid leach. There was no rinse between the two acid leaches. After the last acid leach and solid-liquid separation, the slurry was rinsed with water until neutrality was reached. The conditions and results of tests on minus 18 mesh soil are given in Table 12.

As the cost of heating is high, lower temperatures were preferred for all steps. The leaches at ambient temperature had to have twice the leach volumes and greatly increased leach times (2-3 hr per leach) to achieve the objective of <500 ppm Pb in the residue. The increase in leach solutions required a corresponding increase in rinse volume.

The volumes of leachate and rinse water used require recycling to make this a viable process. In cleaning the chips, there was no difficulty in recycling, but in the soil, HN03 solubi-lizes Ca, Fe, Mg, organics, etc., leaving no free acid to be recycled. Fresh acid equal to the original concentration had to be added before any new soils could be leached. This procedure quickly built up the N03~ ion concentration in the leachate and in the rinse. Since nitrogen levels have to be reduced to < 10 ppm before discharge of the waste stream, reducing the nitrate level in the wastewater would have a major impact on the cost of site cleanup.

4. Acid Leach—Fluosilicic Acid

Similar acid leaching tests were performed using H2SiF6 based on its successful use in the recycling of scrap batteries [2]. The samples were carbonated using established conditions [8] and leached with various combinations of time and acid concentration. For each test, the soils were treated in two stages. First a leach, using the given parameters, then a solid-liquid separation and a releach. Tests were conducted at 50°C, 70°C, and ambient temperature. Results of tests using minus 18 mesh carbonated soil (0.8% Pb), 80 g/L H2SiF6, and varying time,

Table 12 Leaching with HN03

Temp, (°C)

First leach (hr)"

Second leach (hr)b

Residual Pb (ppm)

25

1

1

1058

25°

2

2

166

70

1

1

229

90

1

1

133

"Acid concentration, 50.4 g/L. bAcid concentration, 20.2 g/L.

Tests used l L leach solution/125 g soil instead of 0.5 L/125 g.

"Acid concentration, 50.4 g/L. bAcid concentration, 20.2 g/L.

Tests used l L leach solution/125 g soil instead of 0.5 L/125 g.

Table 13 Effect of Temperature, Time, and H202 on Soil Cleaning Using 80 g/L H2SiF6, Two-Stage Leach

Time (hr)

Temp. (°C)

H202 (mL/L)

Residual Pb (ppm)

4a

room

0

5332

4

room

0

1210

lb

70

0

ND

1

70

4

180

lb

50

0

ND

1

50

4

168

lb

room

0

ND

1

room

4

337

ND = not determined not determined on the first stage. "125 g soil/0.5 L leach solution. bl25 g soil/L leach solution.

ND = not determined not determined on the first stage. "125 g soil/0.5 L leach solution. bl25 g soil/L leach solution.

Table 14 Effect of H2SiF6 Concentration on Soil Leaching, Two-Stage Leach

H2SiF6 (g/L)

H202 (mL/L)

Residual Pb (ppm)

10"

1

ND

10

1

3580

40"

1

ND

40

1

983

100"

2

ND

100

2

387

200b

4

ND

200

4

317

ND = not determined on first stage. "125 g soil/L leach solution. b 125 g soil/0.5 L leach solution.

ND = not determined on first stage. "125 g soil/L leach solution. b 125 g soil/0.5 L leach solution.

temperature, and amount of 30% H202 are given in Table 13. Tests using ambient temperature, 1 hr leach time, and varying concentrations of H2SiF6 and amount of 30% H202 are shown in Table 14.

Using H2SiF6 as the lixiviate, the conditions adopted as a standard with 125-g soil samples were (1) a two-step leach using 1 L of 80-100 g/L H2SiF6 each leach, (2) ambient temperature, and (3) 2-5 mL/L H202. The amount of H202 required for cleaning the soil is more than the stoichiometric amount for Pb02 and had to be determined experimentally for each type of soil waste. Organic materials as well as iron and magnesium in the soil catalyze the decomposition of the H202.

All wash, rinse, and acid solutions used in the process are recycled in a closed loop. Before recycling the leachate, the lead was removed from the acid by electrowinning, and any acid loss was replenished.

5. Lead Stripping by Electrowinning

The acid leachates, containing 600-1200 ppm lead, were stripped to <100 ppm using a Pb02-coated titanium anode and a stainless steel cathode. Figure 5 shows the rate of lead removal from 1 L of PbSiF6 (80 g/L H2SiF6) by electrowinning at ambient temperature for 2 hr at a

Figure 5 Lead stripping by electrowinning.

Time, h

Figure 5 Lead stripping by electrowinning.

Figure 6 Effect of current density on the rate of lead stripping. Current density: (■) 150; (+) 200; (*) 400.

Figure 6 Effect of current density on the rate of lead stripping. Current density: (■) 150; (+) 200; (*) 400.

current density of 300 A/m2. After 2 hr, the lead level was reduced from 1240 ppm to 11 ppm. The effect on stripping rate of varying the current density from 150 to 400 A/m2 was studied (Figure 6). In these tests the acid was ~95 g/L and the leachate had been recycled four times. As impurity levels increased with the continued recycling of the leachate, the rate of lead stripping was degraded. After 2 hr, the lead level was reduced from 1100 ppm to 220 ppm. In the studies shown (see Figure 6), the current efficiency was about 15-20% at all current densities. During these tests, no attempt was made to optimize current efficiency. For most of the recycling tests, a current density of 300 A/m2 was used for stripping the lead.

Table 15 Results of Using Recycled H2SiF6 to Clean Soils

Test No.

H2SiF6 (g/L)

No. of recycles"

1

2

3

4

1

80

720

1000"

1086b

2600"

2

80

520

640

585

593

3

100

369

393

476

489

"Lead in cleaned soil residue, ppm.

bNo makeup acid added during recycling leach.

"Lead in cleaned soil residue, ppm.

bNo makeup acid added during recycling leach.

6. Soil Cleaning with Recycled Fluosilicic Leachate

Results from soil cleaning tests using recycled H2SiF6 with concentrations adjusted to 80 and 100 g/L are shown in Table 15. As indicated in tests 1 and 2, 80 g/L acid did not decontaminate the soil to meet the criteria. All tests were at ambient temperature on minus 18 mesh carbonated soil containing 0.8% lead. The acid was replenished as necessary before each recycling leach.

Test 3 (see Table 15) was continued by recycling the 100 g/L H2SiF6 solution to clean eight carbonated soil samples. After the eighth recycle, the soil still met the residual lead standard by containing only 448 ppm Pb. The test procedure was (1) acid leaching a carbonated soil sample, a solid-liquid separation; (2) electrowinning to remove lead from the acid leachate, followed by makeup additions to replenish the acid to 100 g/L, the desired concentration; and (3) a second leaching of the same soil sample using the replenished acid. These three steps were repeated to continuously leach carbonated soil samples. Experiments indicated that the most important criterion for acid recycling was to reduce the lead level in the acid leachate to <150 ppm before each recycle. Preliminary screening tests indicated that the clays in the soil acted like ion-exchange resins and reabsorbed any lead remaining in the acid from the previous leach [12].

It was necessary to replenish the acid prior to the second leach of the soil sample because of acid dilution by the water retained from the carbonation step and depletion by reactions with other ions in the soil. Makeup acid additions were about 8 vol % on average. A schematic of the procedure is given in Figure 7.

The impurity buildup in the first and second acid leaches during the eight recycling tests is given in Tables 16 and 17. After eight recycles, the impurity levels began to reach an equilibrium value. The impurity buildup during the current tests did not adversely affect the soil cleaning as long as the lead level was reduced to <150 ppm prior to the second leach.

One problem encountered in the leach-electrowin-recycle process was dilution of the acid. After each solid-liquid separation, the soil retained about 50% moisture; therefore, a bleed stream was necessary to maintain the concentration of lixivant and to control the volume. About 8 vol % was bled off after the electrowinning step, and additions of fresh acid were made to bring the concentration back to the working level. This bleed also helped control the buildup in impurity levels.

7. Recycling of Acid Rinse Water

After the final leach step, the rinse water had a pH of ~1.3 and contained about 5 g/L H2SiF6 and 50 ppm Pb. When the lead levels were above 20 ppm, the rinse water was cleaned before recycling. Initially, this was achieved by adding pure Na2C03 to the rinse to adjust the pH to ~5. A flocculant was added to aid in coagulation of the hydroxide precipitates and improve the filtration. By this treatment the lead was reduced to <5 ppm, and the rinse water was recycled successfully. The relationship between the pH, adjusted with pure Na2C03, and lead content in the rinse water is shown in Table 18.

Figure 7 H2SiF6 recycling test: Procedure and results. EW = electrowinning; F = filtering; R = rinsing.
Table 16 First Leachate: Impurity Buildup and H2SiF6 Concentration (g/L)

Number of recycles

Ca

Fe

Mg

Al

H2SiF6

1

1.34

1.10

.72

1.04

84.5

2

2.34

2.10

1.27

2.13

86.9

3

3.02

3.08

1.79

3.04

84.5

4

3.78

3.47

2.08

3.48

77.8

5

4.51

3.89

2.34

3.87

76.8

6

4.44

3.79

2.32

3.74

77.3

7

4.87

3.87

2.56

4.22

ND

8

5.24

4.21

2.91

4.82

ND

ND = not determined.

ND = not determined.

In later tests, instead of using pure Na2C03, the spent 10 g/L Na2C03 solution from the carbonation step was used to adjust the pH in the rinse water. The results were comparable and furnished an opportunity to utilize this waste stream. However, the addition of the impure Na2C03 solution to the rinse water formed a gelatinous compound that would not filter. The parameters investigated to improve the lead removal and filterability included pH, additions of alum and flocculants, and reduction of the metals with sodium borohydride. Optimum filtration was achieved by adjusting the pH to 5.0 and using 0.5 mL of Superfloc 320 per liter of solution. Maximum lead removal was achieved by adjusting the pH from ~5 to 8, and adding 0.75 g of alum and 0.3 mL of Superfloc 320 per liter of solution.

Table 17 Second Leachate: Impurity Buildup and H2SiF6 Concentration (g/L)

Number of

Table 17 Second Leachate: Impurity Buildup and H2SiF6 Concentration (g/L)

Number of

recycles

Ca

Fe

Mg

Al

H2SiF6

1

1.25

1.69

0.77

1.57

107.0

2

2.15

2.90

1.35

2.73

104.0

3

2.86

3.43

1.73

3.32

96.0

4

3.30

3.76

1.93

3.47

93.1

5

3.54

3.81

2.06

3.66

96.0

6

3.83

3.86

2.19

3.74

91.4

7

4.41

4.33

2.64

4.61

ND

8

4.88

4.90

2.64

4.61

ND

ND = not determined.

ND = not determined.

Table 18 Lead Content Versus pH

PH

Pb content (ppm)

1.35

50

2.0

21

3.0

14

4.0

9

5.0

5

5.8

0.8

Table 19 Screen Analyses of Subsurface Samples (wt %)

Mesh size

Sample no.

A

B

C

Plus 3/8 in.

46.0

70.3

0.5

Minus 3/8 plus 4

10.1

6.5

26.6

Minus 4 plus 8

7.7

4.2

13.4

Minus 8 plus 18

7.8

4.5

23.2

Minus 18

28.4

14.5

36.3

0 0

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