Treatment and Pollution Control Treatment and Pollution Control TechnologiesTechnologies

ENVM 644: New Technologies in Environmental Management

Dr. Robert Beauchamp

CHEMICAL PRECIPITATIONCHEMICAL PRECIPITATION

A SOLUBLE SUBSTANCE IS CONVERTED TO AN INSOLUBLE SUBSTANCE.

CAN BE REMOVED BY SETTLING OR BY FILTERING.

METALS OF CONCERN: – ARSENIC,BARIUM,CADMIUM,CHROMIUM, COPPER, LEAD, MERCURY,

SILVER, ZINC.

SOURCES OF METALS: – METAL PLATING AND POLISHING, STEEL & ELECTRONICS INDUSTRY.

PROCESS: – A PRECIPITANT IS ADDED - HYDROXIDE (LIME) I.E. CALCIUM

HYDROXIDE.

– REMOVES METALS AS: METAL HYDROXIDE.

METALS PRECIPITATED AS SULFIDES & CARBONATESMETALS PRECIPITATED AS SULFIDES & CARBONATES

DEPENDS ON:

– CONCENTRATION, PH

– PRESENCE OF OTHER MATERIALS.

– PH OF 9 - 11 IS OPTIMAL

CAN REDUCE TO = OR < 1.0 MG/L

NOTE: METAL HYDROXIDES ARE AMPHOTERIC (SOLUBLE @ HIGH AND LOW PH).

SULFIDE PRECIPITATIONSULFIDE PRECIPITATION

SULFIDES ARE NOT AMPHOTERIC

THEY HAVE VERY LOW SOLUBILITIES

SOURCES OF SULFIDE INCLUDE IRON SULFIDE & SODIUM SULFIDE.

PROBLEM - PH NEEDS TO BE > 8 TO PREVENT FORMATION OF HYDROGEN SULFIDE (TOXIC GAS)

CARBONATE PRECIPITATIONCARBONATE PRECIPITATION

NEED A LOWER PH 7.5 - 8.5 (LOWER THAN THE HYDROXIDE METHOD)

SODIUM BOROHYDRIDE PRECIPITATION SODIUM BOROHYDRIDE PRECIPITATION

A REDUCING AGENT

PH - 8 - 11. GOOD FOR LEAD, MERCURY, NICKEL, COPPER, GOLD.

ELECTROLYTIC RECOVERY TECHNIQUEELECTROLYTIC RECOVERY TECHNIQUE

WORKS BEST FOR PROCESS STREAMS WITH HIGH CONCENTRATION OF METALS.

REQUIRES ANODE & CATHODE ELECTRODES

CATHODES ARE IMMERSED IN A CHEMICAL MEDIUM UNDER AN APPLIED POTENTIAL. METAL IONS ARE REDUCED TO ELEMENTAL FORM AT THE CATHODE.

ELECTROLYTIC RECOVERY TECHNIQUE (cont)ELECTROLYTIC RECOVERY TECHNIQUE (cont)

ADVANTAGES:– METALS RECOVERED FOR REUSE.

– TOXIC DISCHARGE IS REDUCED.

– INITIAL INVESTMENT IS LOW.

– OPERATION EXPENSES ARE LOW.

– MINIMAL MAINTENANCE.

DISADVANTAGES:– ONLY GOOD FOR STREAMS WITH HIGH CONCENTRATIONS.

– DEPLETION OF METALS IN WASTE STREAM OFFER RESISTANCE TO CONTINUOUS METAL MIGRATION TO THE CATHODE.

– GAS FORMATION @ CATHODES (H & CL).