Cleaning Procedure Tests And Results A Battery Casing Wastes

The bench-scale cleaning process developed (see Figure 1) consisted of prewashing, gravity separation, granulation and sizing, carbonation, and acid leaching.

1. Prewashing

Prewashing the battery casings removed most of the adherent sludge and fine metallic lead. This prevented the sludge and metallic lead from becoming embedded in the casings during subsequent size reduction. The as-received battery wastes were mixed with water, tumbled in a ball mill without a grinding medium for 1 hr, and then screened through an 18 mesh screen to remove the freed sludge and small pieces of metallic lead. This sludge, containing 20-36 wt % lead compounds and metallic lead, was set aside without further treatment since it was assumed that it could be sent to a secondary smelter for lead recovery.

2. Gravity Separation

The second step entailed separation of the casing material from the metallic lead, rocks, and foreign matter by screening and elutriation. Since metallic lead has a much higher density than the rocks and casings, a gravity separation is possible. The separation technique used in this work was water elutriation, but air or other density separation devices would be effective. The feed material was screened through 3/4- and 3/8-in. screens, and in both plus fractions the metallic lead and rocks were separated from the pieces of battery casings by elutriation. The minus 3/8-in. material was rescreened through 4 and 8 mesh screens. Because the suspension of the different materials is a function of the surface area as well as density, it was preferable to screen the material into similar size fractions to provide a consistent feed for the laboratory-scale water elutriation system. A schematic diagram of the equipment used is shown in Figure 2.

SPRAYS

SPRAYS

SIZE REDUCTION

WATER WASH

CARBONATE

SEPARATION

>

f + 3/V

SIZE REDUCTION

>

r

CARBONATION

>

ROCKS, Pb METAL, ETC.

MAKE-UP ACID

ACID WASH

ELECTROWINNING

Pb METAL

CLEANED CASING WASTES Figure 1 Flow diagram for decontamination of casing wastes.

3. Granulation and Sizing

All the separated casings over 3/8 in. in size were reduced to less than 3/8 in. by granulation to facilitate carbonation and leaching of the lead compounds entrained in the cracks. Other size reduction equipment, such as a hammer mill or shredder, could also be used.

4. Carbonation

The prewashed minus 3/8-in. casings from the separation and size reduction steps were combined, and the residual PbS04 was carbonated at room temperature for 30 min to 1 hr.

The carbonate solution contained 4 g/L carbonate, to ensure excess above the stoichiometric amount of (NH4)2C03 needed to convert the PbS04 to PbC03. A reducing agent, am-

MATERIAL FEED

MATERIAL FEED

Figure 2 Schematic diagram of water elutriator.

monium bisulfite (47% NH4HS03), was added to reduce any Pb02 to PbS04 and then to PbC03 according to the equations

The carbonate solution was recycled five times and then used to neutralize the rinse water before discarding. Although it is an effective carbonating agent, there are drawbacks in using (NH4)2C03, such as odor, disposal problems, and higher cost. Sodium carbonate and NaHS03 were substituted without any problem in the carbonation of the chips. Any carbonate should be equally effective; however, CaC03 would be the least desirable because the CaS04 produced has little value and is difficult to dewater and discard.

5. Acid Leach with Nitric Acid

After a solid-liquid separation, the chips were rinsed and then leached with 5.0 g/L HN03 at ambient temperature for 1 hr. The reaction occurring during the leaching operation can be represented as

The cleaned battery casings were well below the EPA requirement of <500 ppm residual lead and ^5 ppm in the EP toxicity extract. When the TCLP test was designated as the standard, the HN03 acid concentration of the leachate had to be increased fourfold to pass the 5-ppm extraction portion of the test. The conditions and results of several tests are shown in Table 7.

To minimize the volumes of leachate and rinse, recycling of all waste streams is critical. The Na2C03 solution, water rinse, and HN03 leach solutions were each recycled five times. Makeup reagents were added to the carbonate and leach solutions. There was no reduction in cleaning efficiency even by the fifth recycle. Since there are few contaminants except lead in the chips, this recycling should be feasible until the lead levels are high enough to warrant precipitation as a lead sludge.

6. Disposal of Nitric Acid Solution and Lead Removal by Lead Sulfate Precipitation

Attempts were made to remove the lead by precipitating as PbS04 with H2S04. The solubility of PbS04 in water is 38 ppm, with solubility increasing as pH decreases. The drinking water standard is 0.05 ppm; therefore, before discarding the HN03 it had to be treated to remove the residual lead. After an H2S04 precipitation, the lead remaining in solution was 250 ppm. By raising the pH to ~8 with a hydroxide, adding sodium borohydride and a flocculant, and mixing for ~1 hr before filtering, the lead content was reduced to <0.2 ppm. The small amount of sludge produced by this procedure would have to be sent to a hazardous waste landfill. The spent HN03 is more troublesome to discard. Nitrates must meet drinking water standards of 10 ppm or less before being discarded to a waterway; possibilities include regeneration by a bipolar membrane water-splitting system. This process may be too expensive for the volume of acid to be cleaned.

7. Acid Leach using Fluosilicic Acid

An alternative to HN03 for the leach is fluosilicic acid (H2SiF6), which is produced as a byproduct of the fertilizer industry. The major advantage of using H2SiF6 is that the lead can be recovered directly as pure metallic lead by the Bureau's patented electrowinning process [3]. Additionally, the acid is regenerated during electrowinning and thereby made available for recycling. The problems associated with the disposal of nitrates are also eliminated. The pre-

Table 7 Battery Casings Leached with HN03—Results of TCLP Tests

Leach residue

TCLP filtrate

213 160

128 125 137

Note: All tests were on carbonated chips at room temperature for 1 hr; leach solution was I L/125 g of chips.

liminary steps are the same as with the HN03; the battery casings are prewashed, separated from the sludge and metallic lead, granulated, and carbonated. The NaHSOj is omitted in the carbonation step. Instead, the Pb02 is reduced to PbO by additions of hydrogen peroxide in the acid leach.

The process reactions for using H2SiF6 as the leach reagent are described as follows:

Carbonation:

PbS04 + Na2C03 PbC03 + Na2S04 (3) Acid leach:

Electrowinning:

2 PbSiF6 + 4 H+ + 4 e" -> 2 Pb° + 2 H2SiF6, 02 + 4 H+ + 4e"->2 H20, 2 PbSiF6 + 2 H20 ->• 2 Pb + 2 H2SiF6 + 02, ec: cathode cell potential ea: anode cell potential

£,: total cell potential

The cathodic and anodic reactions occurring during electrowinning are given in Equations (5) and (6), respectively, and the overall reaction is given in Equation (7).

As an alternative to electrowinning for cleaning and regenerating the acid, H2S04 was used to precipitate PbS04 from the lead-rich H2SiF6 leachate. At ambient temperature, the solubility of PbS04 is high enough to leave ~1000 ppm Pb in the leachate. This lead level is excessive, and, in addition, the sulfate ions remaining in solution will form PbS04 when the solution is recycled. This option was not pursued further.

8. Results of Decontamination of Casing Wastes Using Fluosilicic Acid Several tests using H2SiF6 were completed on the battery casings. After leaching, TCLP tests were done on the residues. For each test, enough fresh acid was diluted to the noted concentration to make 0.5 L lixivant/125 g granulated, carbonated chips. Leaches were at ambient temperature for 1 hr. The acid concentration, amount of 30% H202, and TCLP results are shown in Table 8. All residues met the <500 ppm Pb standard. However, using 80 g/L acid was considered only marginally successful.

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