Leaching is a process in which a solid material is contacted with a liquid solvent for selectively dissolving some components of the solid into the liquid phase. Leaching can sometimes be used to extract various metals from sludges. The goals of this process are as follows: 1) to dissolve the metals in a liquid phase to produce a solution that can be reused directly in a process or from which the metal can be recovered by other techniques, such as electrowinning; and Z) to produce a secondary sludge (leach residue) that is nonhazardous or from which additional metals can be reclaimed by other processes. A leaching process selected for metals recovery should be sufficiently flexible to remove a mixture of elements from sludge with a variety of characteristics (e.g., extent of aging, solids content).
Several leaching agents can potentially be used, including sulfuric acid, ferric sulfate, amnonia or ammonium carbonate, hydrochloric acid, sulfur dioxide, ferric chloride, nitric acid, or a caustic solution. Selection of a suitable solvent and unit processes depend on the chemical state and physical environment of the metals. Each reagent has its advantages and disadvantages. Sulfuric acid, the most commonly used and developed reagent, has the advantages of low cost, minor corrosion problems, and the ability to dissolve many metal compounds. Acid leaching has been used to recover metals such as copper, nickel, Silver and cadmium from inorganic wastes generated in the primary metals and inorganic chemicals industries. The acid leaching process is most effective with wastes having high concentrations (greater than 1,000 ppm) of metal constituents. Ferric sulfate can be obtained from spent pickle liquors and used to provide a sulfate solution for metal removal.
Ammonia and aranoniura carbonate offer better selectivity for dissolving metals, but these reagents are expensive and must be recovered for the process to be economical. In a«onia leaching, ammonium carbonate is used to convert nickel, copper, zinc, and cadmium to water-soluble amines.*5 Iron, chromium, and calcium remain in the sludge as insoluble hydroxides. The remaining chromium hydroxide in the sludge can then be oxidized by air to the soluble dichromate species in the presence of added caustic. The leach solution of sodium dichromate can be crystallized for recovery of chromium. Sulfur dioxide must be added to the leached sludge to reduce chromium to the trivalent state, and the residual sludge must be adjusted to a pH of 7 or 8 to minimize chromium solubility. The sludge resulting after ammonia leaching can be leached with sulfuric acid to produce the hydrated trivalent form of chromium, which can then be oxidized to dichromate and recovered.
Leaching of hydroxide sludges with caustic has also been evaluated.'5 Because of the low solubility of trivalent chromium in caustic and the amphoteric character of zinc, proper control of pH and sludge conditions could allow for dissolution of zinc and separation from the insoluble chromium. In particular, calcination can be used to convert the hydroxide to more inert oxides to avoid solubilization of chromium; however, the calcination step would add significantly to process costs. After zinc is extracted, oxidation of the remaining sludge would produce dichromates that are readily soluble in an additional caustic leach. Remaining metals could be recovered with an acid leach process as described here.
A sulfuric acid leach procedure has been the most widely investigated because it has the following advantages:
o Sulfuric acid yields the highest extraction of heavy metal species .
After leaching, the sludge is apt to be inert to the conditions of the Extraction Procedure (EP) Toxicity tests.
o Sulfuric acid 1s inexpensive and can be recovered by distillation if desired, o Sulfuric acid will not leach calcium sulfate (gypsum) in the sludge.
° The heat of dilution for concentrated sulfuric acid and the heat of reaction between the acid and the hydroxide sludge accelerate the leaching kinetics.
" The acidic pH usually required for resulting treatment schemes is already established.
o Add addition can be minimized and washing of the gypsum cake can be maximized by stagewise leaching with countercurrent flow of sol ids and acid.
Sludges that contain only one metal often can be sent directly to a refiner for reclamation; however, in some operations (e.g., electroplating), all metals are precipitated from solution in the same wastewater treatment plant, usually as hydroxides. The resulting sludge will therefore contain a variety of metals that must be redissolved into a solution and then segregated so the metals can be reused. A sludge may also contain impurities (e.g., iron) that may prevent direct recycling to a reclaimer. These complexities may necessitate a process train with numerous unit operations.
The efficiency of acidification for the removal of metals from wastewater treatment sludges depends on several waste characteristics and process variables. The acid leaching process is more economically feasible with wastes having high levels (over 1000 ppm) of metal constituents. Wastes with low metal concentrations require longer contact times in the leaching process.
In an EPA study conducted in 1982, hydrochloric acid was selected as a representative acid to investigate the effect of several variables on sludge leachabi1ity. Metal removal from sludges was determined to be dependent on pH, solids concentration, specific types of metals, and length of acidification times. Table 38 summarizes the data obtained during this study.
Dissolution rates (kinetics) and ultimate solubilization (thermodynamics) for all metals increased with time and decreasing pH and solids concentrations. For many metals, the ultimate solubilization was limited regardless of the length of time the sludge was exposed to the leaching agent; the maximum solubility of a metal decreased with increasing pH and sludge solids concentrât ion.
The most rapid leaching kinetics occurred with chromium and nickel. Over half of the total leachable metal was solubilized immediately upon the addition of acid. This finding may indicate that a large amount of these metals exist as inorganic precipitates or as weakly chelated forms within or on the sludge solids. The initial removal of other metals was usually less than 10 percent of the total leachable metals, which may indicate that these metals exist as organically chelated metallic forms.
The quantity of acid used in a leaching process may significantly affect process costs, both in terms of the reagent cost and equipment size. The amount of acid required in the leach process depended on sludge solids content, the pH, and the length of leaching time. Acid usage increased with time and sludge solids content, which indicates an ongoing chemical reaction. Although pH values of 1,5 and 2 resulted in comparable metal removals, significantly more acid was required at the lower pH; thus a pH of 2 may be the optimum operating condition. Although acid usage requirements increased with increasing solids content, the acidification efficiency, as measured by the quantity of acid needed to treat a unit weight of sludge (dry basis), also increased with solids content. The researchers recommended a solids concentration of I percent for optimum metal removal and acid savings.1*
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