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* Reference 3

TABLE 40. SOLUBILITY OF HETAU COMPOUNDS3

HydrotIds

Oxide

Sulfate

Sulfide

Carbonate

Phosphate

AlunlnuM

Arsenic Barium

Cadaiun

Chram\us Copper

Lead

Adds, alkali Slightly soluble In , adds, alkalIj

Acids, alkali, HC1a

Acids

Water, esp. hot

lotubl« in llttll

Water, add; Insoluble In

Slightly Water soluble In HC1,

Ac ids, nh4 salts; Water; Acids tnioWl* In Insoluble

Acids

Alkali, alkaline sulfur, NaHC03

Acids, alkalic

Acids, NH^Cl Acids, NH4Clc in NM,

Acids, KH4 Acids, NH4 salts, salts Insoluble .

Acids, alkali in NH,

Acids UNO

Water' Acids, HNO-j

4 1 tn HH, ds, alkali, HC1,jHNO-, alkali, HH, salts; Aci i. acid J slightly

:1ds soluble 1n acids

Acid, alkali, HN03, alkali*

insoluble in acids

Tabla TO (continuad)

Hydroxide

Sulfate

Sulfids

Carbonate

Phosphate

Mercury N icke 1 Selenium

Si (ver

NM^OH,acids, Zinc

HN03, acids Acids, Ntl4OH Acids, KH4OI<

Acids, NH^OH

Acids, alkali Acids, alkali, nh4ci

Water

HNO,

Acids, NH40H Acids, tone, mnoj, h,s04

Water Acids nh4ci

Acids

HH40H, Na,S2Oj

Acids, alkali, Acids, alkali, NH4 salts NH40H, nh4 salts

Reference 55,

Aluminum oxide trihydrates are soluble In hot acids, jAlualnun orthophosphate (AlPO^) is soluble In acids and alkali; aluminum metaphosphate [Al (PO^l Is not. Depends On for» of compound. ®Crs03 Is insoluble; CrO, is soluble.

Some chromic acid salts exist 1n both soluble and Insoluble forms. 9SeSQ3 Is soluble In H,S04-

Environmental Evaluation

Two products result from the leaching step: the liquid stream containing dissolved metals and a secondary sludge. The liquid stream may be reused in the process or further treated to recover valuable metals. Depending on the degree of metals recovery, the resulting wastewater may be further treated to adjust the pH and to remove residual metals.

Constituents that are not soluble 1n a strong leaching medium (such as sulfuric acid) may not be soluble under the conditions of the Toxicity Characteristic Leaching Procedure (TCLP) test standards. In any case, the volume of sludge to be disposed of will be reduced.

Costs

For a previous EPA study of metals recovery from metal hydroxide sludges, an order-of-magnitude cost estimate was performed on the leaching/ precipitation step and on the overall process train. A return-on-invest-ment (HOI) calculation for the leaching process alone is not useful because the metals are not immediately reusable and the only economic benefit would be the reduction in sludge. The overall RQI for the entire process train was 41 1 12 percent.

A first-order cost estimate was previously prepared for a theoretical composite of the sludges studied during the second phase of the previous EPA project, which had a solids concentration of 32.7 percent and metal concentrations of 5.0 percent copper, 5.1 percent iron, 4.3 percent chromium, 1.8 percent zinc, 10,4 percent nickel, 0.4 percent aluminum, 0.9 percent calcium, 15.0 percent silicon, and 3.0 percent phosphorus. The overall design of the process train consisted of sulfuric acid leaching, solvent extraction of copper, phosphate precipitation of iron and chromium, zinc sulfate crystallization, and solvent extraction of nickel. The total yearly cost for the leaching step alone ranged from $148,900 for a 10 tons/day plant, to $391,200 for a 50 tons/day plant. The ROI for the overall process depends on plant size; the ROIs are 11 t 8 percent for a 10 tons/day plant, 75 ± 23 percent for a 30 tons/day plant, and 106 t 32 percent for a SO tons/day plant.

Davy McKee estimates the installed capital cost of a plant to treat approximately 160 tons per day of electroplating sludge for recovery of copper-nickel carbonate and a cadmium-zinc sulfide to be approximately $3,5 million. Annual operating costs are estimated at $1.3 million per year for the two years of operation needed. The estimated operating costs include reagents (H;S0,, Na,COj, Ca(0H)j, and NaHS), utilities (electricity, water, steam and gasoline), maintenance costs, and operating labor (4 operators for 3 shifts). Revenues are based on the sale of the mixed copper nickel carbonate and do not take credit for the potential sale of the cadmium-zinc sulfide. The estimate indicates that the overall cost of the project can be recovered by revenue sales from the remediation of the plating sludge.

ADSORPTION Carbon Adsorption Process Description-Activated carbon has been widely used for the removal of organics present In low levels (usually less than 1,000 mg/l) in contaminated liquids; relatively little attention, however, has been given to the removal of inorganics (e.g., metals) by carbon adsorption. Studies conducted on the removal of metals indicate the applicability of activated carbon for treatment of wastewaters; however, this process is not directly applicable to the recovery of metallic compounds from contaminated waste streams.

Activated carbon is available in either powder (PAC) or granular form. Granular activated carbon (GAC) is more convenient for use in conventional unit processes and regeneration equipment, whereas the powdered form offers higher surface area and maximum rate for sorption of contaminants. Both types have large surface areas in the order of 600 to 2600 m/g, which result from a network of pores 20 to 100 angstroms in diameter.

Activated carbon has a fixed adsorption capacity for each type of metallic compound. Once this capacity is saturated, contaminants will no longer be adsorbed and the activated carbon must be regenerated or replaced. The carbon can be reactivated by using a strong acid or base to remove metal particles and bring them back into the solution.

The adsorption characteristics of activated carbon for metals removal are more complex than those for organic compounds because the charged nature of the metals affects their rate of removal from the solution. In general, the specific surface area, pore structure, and surface chemistry of the activated carbon significantly affect its adsorption characteristics for removal of contaminants.

Other parameters that influence the metals removal efficiency of activated carbon are pH, temperature, presence of chelating agents, ionic strength, carbon dose, and metal concentration. The pH of the solution affects contaminant removal by influencing the surface Charge of the activated carbon and affecting the distribution of the metal ions in the solution. As the pH decreases, the solubility of metal ions generally increases. Complexing the metal ions in the solution by using chelating agents considerably increases the adsorption of metallic compounds onto the activated carbon. Chelating agents such as ethylenediaminetetraacetic acid (EDTA) and nitrilotrlacetic acid (NTA) significantly increase the removal of mercury and cadmium by carbon material.

The process of metals removal by activated carbon often involves the use of multiple columns or tanks filled with carbon and operated in series or parallel configurations. Figure 13 is a schematic of column arrangements used to treat contaminated solutions. The carbon bed depth should be high enough to remove all the metals from the solution to the required concentra-

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