(Mine company) is examining electrocoagulation for waste treatment at a copper mine in South America. A sample was provided for bench-scale testing. The sample is likely a waste from ore processing.
The sample showed an extremely high conductivity of 75,300μS (microsiemens) and a pH of 6.39. It had a brown color and contained cadmium at 0.335mg/l, copper at 22.6mg/l, chromium at 0.122mg/l, nickel at 8.452mg/l, zinc at 12.31mg/l, iron at 0.671mg/l, and fluorides at 100.5mg/l, and arsenic at 785 mg/l and selenium at 67 mg/l.
CONCLUSION
This waste would be a good candidate for treatment by the KASELCO EC using a two-stage (two-pass) system. The waste stream requires two passes. The residual cadmium and chromium may require two reactor passes. The arsenic and selenium would also require two passes and would certainly require pH adjustment (adding acid). While this may be practical for larger flows, it is not for smaller volumes, particularly in light of the enormous amount of sludge generated.
If this sample is truly representative of a real waste stream then one would need to pay particular attention to the sizing of solids separation equipment. If the sample has been doctored by addition of concentrated contaminants, then a sample truly representative of the expected waters should be provided. If dilution would restore the sample to its expected character, then dilution values should be provided to us so we can run the sample again. Dilution may affect the concentration but not the total volume of the solids. Solids separation with this waste would be best effected using a settling basin.
BENCH SCALE KASELCO ELECTROCOAGULATION TREATMENT
Test #1: Test as received:
The sample was tested without modification. It drew the optimum amperage but a very low voltage in a reactor configured for seawater. The conductivity fell from 75,300μS to 71,200μS after a single pass through the reactor (1B). The pH rose from 6.39 to 8.55 at mid-reactor (1A) and to 9.23 at the reactor’s exit (1B). This sharp rise in pH was unexpected and is uncommon for wastes with such high conductivity.
The water became clear, and a settling floc was developed. The floc volume was extremely high since the water was saturated with contaminants. The solids settled to 54% of the total volume after one hour. The dry sludge rate was 95.6 pounds dry sludge per 1,000 gallons of wastewater. Solids separation with this waste will be a real challenge, and a settling basin would be most appropriate.
The metals levels fell for the most part, but fluorides did not. A calcium ion, necessary for fluoride removal, may not have been present or may have combined with other metals at the low voltage. The fluoride concentration was basically unchanged at 100mg/l.
Cadmium was reduced from 0.335 to 0.120mg/l, which is still typically too high for discharge. Copper was reduced from 22.6 to 0.077mg/l, nickel from 8.452 to 0.320mg/l, and zinc from 12.31 to 0.059mg/l. Chromium was basically unaffected, beginning and ending near 0.12mg.l.
The iron concentration increased from 0.671 to 51.1 at mid-reactor despite the pH exceeding 8.1, and then fell to 2.80mg/l at the reactor’s exit when the pH had reached 9.23. Iron is usually not present in significant concentration at a pH above 8.1. The form of the iron in this waste could bear study, but is beyond the capabilities of our laboratory.
Several tests, including arsenic and selenium, were performed by a contract laboratory. Neither metal showed good removal. The pH rise was probably too steep and prompt for their treatment.
The sludge generation rate was high at 96 pounds of dry sludge per 1,000 gallons of wastewater. The wet sludge rate could be three times as high. Solids separation will be a challenge and will require over-sized equipment.
LAB DATA - abbreviated
TEST # 1 (as is)
PROFILE:. Appearance is brown water. Run in # 11 steel reactor.
COMMENTS: 1-A cloudy water, sinking green floc. 1-B clear water, sinking green floc.
1,000ml Wet sludge test, 950ml @ 15 min. 840ml @ 30 min. 540ml @ 60 min.
100ml Dry sludge test, 1.146g
* OUTSIDE LAB TESTING 08-06-03
GLOSSARY
• Profile: The characteristics of the waste being tested.
• 1A: Equivalent of half a pass through a full-scale production reactor.
• 1B: Equivalent of one full pass through a full-scale production reactor.
• Type: KASELCO EC reactors come in three electrical configurations and is chosen based on the wastewater’s conductivity.
