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Mercury Nickel Silver Zinc

' Reference 11.

Figure 3. Solubility of mataJ sulfides as a function ol pH,

SOURCE; U.S. £P* Trnacur MM. v«um. m.EP*-soa»*WM2. jg* imd

Figure 3. Solubility of mataJ sulfides as a function ol pH,

SOURCE; U.S. £P* Trnacur MM. v«um. m.EP*-soa»*WM2. jg* imd

Because of operational problems and odor generation, the soluble sulfide process has shown little promise. With the recent development of ion-speclf1c probes, however, control of sodium sulfate addition to match demand is possible, which has sparked new interest in the process. Polyelectrolyte developments have eliminated previous separation difficulties by flocculating the fine metal sulfide particles generated.1'

Carbonate Precipitation-Carbonate precipitation with soda ash (sodium carbonate) or calcium carbonate has proven to be an effective process for removal of cadmium, lead, nickel, and zinc. Precipitation as insoluble carbonates tends to occur at more neutral pH conditions than with hydroxide precipitation. Carbonate solubilities tend to be less than those of the corresponding hydroxide. The carbonate-based reaction mechanism, however, proceeds at a slower pace than the hydroxide-based system.'1 The solubility of soda ash also limits its use because a chemical feed of only 20 percent by weight can be maintained at room temperature without recrystallization. An advantage of soda ash is low sludge generation; however, these sludges can be difficult to filter. Calcium carbonate sludges show much better filtration properties.

Carbon dioxide has also been used to treat metal-containing waste streams." Carbon dioxide gas is Injected into the wastewater, and upon hydration 1t will form carbonic acid. The carbonic acid will then react with the available hydroxides to form the less-soluble carbonates.

High reagent costs associated with COj systems make this treatment technique less attractive than conventional systems. Carbonate has proven to be more effective at removing lead than hydroxide, and It is the method of choice if nickel reclamation is deemed appropriate.

Sodium Borohydride Precipitation-*

Sodium borohydride (NaBH,), a strong reducing agent, can precipitate heavy metals in their elemental form from an alkaline solution (pH 3 to 11). This technology was pioneered In the late 1960's. Precipitation of single metal waste streams can produce metal sludges suitable for recycling or reclamation. Sodium borohydride offers the advantage of low sludge volumes compared with those produced by the conventional hydroxide precipitation method.10 This chemical also has the added benefit of removing metals to lower concentration levels than possible with conventional treatment. Sodlu« borohydride is commonly available as a stabilized water solution of 12 percent NaBH^ in caustic soda.10 Sodium borohydride is capable of reducing chromates to the trivalent state before deposition as chromium hydroxide. The high cost of the reagent currently limits its use In industrial metal-bearing wastewaters.

Phosphate Precipitation-Pilot studies have been conducted to evaluate phosphate precipitation as a treatment alternative for recovering trivalent metals such as chromium, iron, and aluminum from mixed metal solutions. Under low pH conditions, phosphate, in the form of phosphoric acid (HjP04) or sodium phosphate, can effectively strip trivalent cations from solution in preference to divalent cations. Phosphate products filter easily and can be compacted to a high solids content. Work conducted on metal sludge leachates has shown good results.

Differential Precipitation--

Dlfferential precipitation is an emerging wastewater treatment technique in which multistep titration is used to form and precipitate out specific metal salts at selected titration points. This process may be followed by a recovery process (e.g., electrowinning) for the metals remaining in solution. The process can be designed to precipitate specific metals targeted for recovery. Thermodynamic modeling is required for complex wastewaters to identify key points in the titration process. 3

Zinc Cementation-Powdered zinc can be used to precipitate elements from wastewaters that are more electronegative than zinc such as chrome or copper. This precipitation technique is termed cementation. The cementation process has been shown to be effective in precipitating lead and cadmium from wastewaters. Zinc has been used to treat mercury in sludges. These sludges were then retorted as a final treatment step.

Reduction (Sulfur Dioxide, Sodium Bisulfite, Sodium detabisulfite, and Ferrous Sulfate) —

Sulfur compound reduction of chromium from valence +6 to +3 is widely practiced by industry. Three such reduction reagents are sulfur dioxide (S02), sodium bisulfite (NaHSO,), and metabisulfite (NagSjO,). A reaction scheme for SOj reduction is as follows:

After reduction, the chromium (+3) can be removed from the wastewater by increasing the pH above 8.0. The reduction reaction proceeds rapidly at a wastewater pH between Z and 3." The reaction end point is more distinct at this lower pH. Atmospheric oxygen consumes a major portion of the reducing agent; therefore, excess chemical addition is required.

Because of its abundancy and low cost, ferrous sulfate (FeSOJ is frequently used for chromium reduction. For rapid reduction, FeSO^ requires the pti of the chromate waste stream to be between 2 and 3 as shown in the following:

Subsequent neutralization results in large volumes of iron hydroxide sludges, which makes disposal costly. An alkaline ferrous sulfate reduction process is currently being tried. Because this process requires maintaining the pH between 7 and 10, it can be accomplished in the neutralization/ precipitation tankage. Qnt» deficiency of this alkaline process is the difficulty encountered in accurately controlling ferrous sulfate additions to the wastewater.

Coagulation/Coprecipitation (Alum, Lime, and Polyelectrolytes)--

Coagulatlon/flocculation systems aid sedimentation times of precipitated metal particles. Inorganic coagulants are used primarily for waste streams having dilute concentrations of insoluble constituents. These coagulants, however, add to the overall sludge generation. Lime, alum, and ferric chloride are industry's inorganic coagulants 'of choice. Synthetic polyelectrolytes are used mainly with heavy metal precipitates. These chemicals use chemical bridging and physical entrapment to coalesce the precipitated metals. Anionic polymers are generally used because heavy metal precipitates tend to carry slightly positive charges. A natural anionic polymer, ISX, has also been developed as a coagulant. Because of handling difficulties and disposal problems, however, the natural anionic polymer has not been as widely accepted as the synthetic polyelectrolytes and inorganic coagulants.

Coprecipitation 1s the process of precipitating a given metal species in association with other metal species. For some metal ionic forms, such as arsenate (AsOt ), coprecipitation is the treatment method of choice. Coprecipitation involves both adsorption of the soluble ion onto a bulk solid and coagulation of fine solids by the bulk precipitate.

Waste Feed Characteristics

Wastes must be contained in a water matrix for treatment by precipitation, Hetals in sludges must be leached before treatment by precipitation.

Specific waste characteristics that affect the performance of chemical precipitation systems include the following: 1) the concentration and type of metals, 2) the concentration of total dissolved solids, 3) the concentration of complexing agents, and 4) the concentration of organics, oil and grease. In precipitation systems containing «ore than one metal ion species, optimal removals may not occur for a given m«ta) species when another has been treated for maximum removal. Because each metal has a unique minimum solubility pH, the pH will typically need to be adjusted to minimize the discharge of the problem species. Acidic wastewater streams should not be precipitated with ferrous sulfide because liberation of H2S gas can result. High concentrations of total dissolved solids can interfere with precipitation reactions as well as inhibit settling. Metal complexing agents such as cyanide, chlorides and ethylenediaminetetraacetlc acid (EDTA) may necessitate higher concentrations of precipitating agents, pretreatment, or other more applicable treatment technologies. High concentrations of oil and grease may result in a longer settling time for the precipitate due to formation of emulsions.

Pretreatment Requirements

Pretreatment of wastewaters prior to metals precipitation involves removal of large solids, flow equalization, cyanide destruction (if applicable), chrome reduction, oil separation, neutralization, and/or waste treatment of the individual process streams. In the presence of completing agents (e.g., metal cyanides or chelates), hydroxide precipitation may not yield wastewater effluents of acceptable quality; therefore, these complexes must undergo pretreatment (e.g., cyanide destruction). Organo-metal1ic undissolved matter (scum) may form in some processes; when this occurs, separation of the undissolved matter (scum removal) is required by skimming the surface,

Posttreatment Reoui reronts

Sand filtration is a common postprecipitation/sedimentation effluent treatment technique. If concentrations in the effluent do not meet discharge standards, other metal treatment technologies (e.g., ion exchange and reverse osmosis) may be used.

Metal sludges generated by chemical precipitation may be dewatered (by use of a pressure or vacuum filter), and processed for metals recovery. Various recovery technologies may be employed for this purpose (e.g, leaching/electrowinning, smelting/refining),

Performance Oata

Hydroxide precipitation has been shown to be effective in removing arsenic, cadmium, chromium (+3), copper, iron. Manganese, nickel, lead, and zinc.' Because this type of precipitation is widely used in industry, considerable data are available concerning its performance. Table 18 presents data on the effectiveness of hydroxide precipitation in the removal of soluble metal ions in coimterci al operations.

Sulfide precipitation is effective in the removal of cadmium, cobalt, copper, iron, mercury, manganese, nickel, silver; tin, and zinc.9 Table 19 presents full-scale performance data for sulfide precipitation.

Carbonate precipitation has been cited as being effective in the removal of cadmium, lead, nickel, and zinc. The literature contains only limited industry performance data, however. Residual lead levels of 0.2 to 3.6 mg/L are possible with wastewater feed levels of 10.2 to 70.0 mg/L (82 to 99+1

removal).

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