Sulfide precipitation process

U.S. Army Belvoir Research Development Center (1984), "Pilot Plant Demonstration of a Sulfide Precipitation Process for Metal Finishing Hastewater Treatment."... 

Since the solubilities of metal sulfides were much lower than those of the metal hydroxides, an investigation was made into the combination of lime, followed by a sulfide precipitation process (6,7). By adding a small dosage of sodium sulfide solution to the neutralized slurry, at pH of 8.7, bench scale tests showed the sulfide reaction to be fast and the soluble cadmium was preferentially precipitated. Although the sulfide precipitate was extremely fine, it was flocculated and settled easily. Typical bench-scale data, conducted in 1982, are depicted in Figure 5. In plant practice, the sodium sulfide solution was added to the neutralized slurry in the launder reporting to the 26 m thickener, prior to flocculant addition. 

The removal of copper from the pregnant nickel solution in the Sherritt-Gordon process is an example of purification by precipitation of a fairly insoluble compound. First, in the copper boil step, ammonia is driven off by heating the solution, and some copper sulfide precipitates. The residual copper is removed by a dding hydrogen sulfide for the chemical precipitation of mote copper sulfide. 

Precipitation is affected by pH, solubiUty product of the precipitant, ionic strength and temperature of the aqueous stream, and the presence of metal complexes. For each metal precipitant, there is an optimum pH where its solubiUty is lowest and hence, the highest removals may be achieved. When an aqueous stream contains various metals, the precipitation process caimot be optimized for each metal, sometimes making it difficult to achieve effluent targets for each. SolubiUty products depend on the form of the metal compound and ate lowest for metal sulfides, reflecting the relative insolubiUty of these compounds. For example, the solubiUty product for lead sulfide [1314-87-0] is on the order of compared to 10 for lead carbonate. Metal... 

The equation above is the basis of the curves shown in the hydroxide precipitation diagram (Figure 5.29). It is seen from the above equation that each line in Figure 5.29 has a gradient of- , where n is defined by the equation appeared earlier (M(OH) =f M2++ n OH-). The precipitation process is dependent on both the metal concentration and the solution pH. As in the case of the metal sulfides, hydroxide precipitation will occur in solutions whose compositions lie to the right of the lines on this diagram. The diagram demonstrates the... 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Wastewater treatment in the copper sulfate industry can further be improved, particularly the removal of the toxic metals, through sulfide precipitation, ion exchange, and xanthate processes. Addition of ferric chloride alongside alkaline precipitation can improve the removal of arsenic in the wastewater. 

At least three pesticide plants use priority pollutant metals separation systems in the United States [7]. One plant uses hydrogen sulfide precipitation to remove copper from its pesticide wastewater. The operating system consists of an agitated precipitator to which the H2S is added, a soak vessel to which sulfur dioxide is added, a neutralization step using ammonia, and a gravity separation and centrifuging process. Copper is removed from an influent level of 4500 mg/L to 2.2 mg/L. 

Two sulfide precipitation methods currently used are the soluble sulfide precipitation (SSP) and insoluble sulfide precipitation (ISP) processes. Soluble sulfides and insoluble sulfides are used to precipitate dissolved metals as metal sulfides. Metal sulfides have lower solubilities than the hydroxides, resulting in lower residual metal concentrations in the treated water (Figure 1). Also, sulfides can be used to precipitate complexed metals in the presence of complexing agents such as ammonia, citrate and EDTA and over a broader pH range than possible for the hydroxide precipitation.3 A typical sulfide precipitation reaction is as follows ... 

The Na2S solution is mixed with a cobalt-treated zinc salt solution under precisely controlled conditions. The resulting zinc sulfide precipitate is calcined and processed to give the finished product. 

The results of the Hg2 reaction experiments using particle-size-mediated synthesized metal sulfides are shown in Table 5. The powder XRD pattern of the resulting solids reveals amorphous material formation. Previous studies using Cu,S revealed the removal of Hg by a process other than dissolved sulfide precipitation [10], Other researchers have shown that MoS2 removes Fig2 from aqueous solutions via an intercalation mechanism [11]. 

The tribochemical reactions based on the hard and soft acids and bases (HSAB) principle linked to the friction coefficient (p) are summarized above. The effect of tribological processes consists of the formation of mixed short-chain phosphate glasses containing sulfide precipitates. 

Reductive Precipitation. Reductive precipitation involves the production of reduced species with limited solubility. An example of reductive precipitation in the environment involves the reduction of S04 to H2S and the precipitation of metals as metal-sulfides. In nature, the process of reductive precipitation is mostly microbiologically controlled. Production of H2S is the rate-controlling reaction of metal-sulfide precipitate formation. 

Cadmium pigments have been manufactured by both a direct calcination process and a precipitation-calcination process. In the first instance, a mixture of cadmium carbonate and sulfur (and zinc oxide and selenium if the hue to be produced requires their addition) is calcined at 520-600°C for 1-2 h. This direct calcination process is complicated by the volatility of cadmium oxide and selenium, both of which are toxic and require special handling. In the precipitation process, an alkali sulfide solution is added to a solution of cadmium and (in the case of green-shade yellows) zinc salts or to a solution of cadmium and (in the case of deep oranges, reds, and maroon) selenium metal to precipitate the appropriate compound. The precipitate is washed, dried, and calcined at 600-700°C in an inert or reducing atmosphere to convert the precipitated cubic structure to a more stable wurtzite crystal. The calcination conditions control particle size, which ranges from 0.2 to 1.0pm. 

Two basic manufacturing processes are distinguished the precipitation process and the powder process. Common to both of them is that the raw materials must be free of transition metal compounds, which form deeply colored sulfides (e.g. Cu, Fe, Ni, Co, Pb) [3.103]. 

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