Flocculation launders

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. 

Camp (1955) as well as the American Society of Civil Engineers (1977) modified this equation for launders, conduits, and flocculation basins by proposing that the power term Phe replaced by the power due to friction in the launder ... 

A taUing is flowing to a thickener. In line flocculation is applied to raise the viscosity to 20 mPa - s. The flow rate is 1800 m /hr. The density of the mixture is 1420 kg/m . The launder is rectangular with a slope of 1.2%. The chemical process requires that the G gradient be smaller than 100. Determine a suitable size for the launder. 

The actual feed to the thickener is through a launder to the center. A feed box leads the slurry to a depth lower than the relatively clear water. Some special processes use intermediary mixing chambers where flocculants are added to accelerate the precipitation. 

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