Sulfidic spent caustics present a more difficult disposal problem. Recovered products have little or no market value, especially if contaminated with undesirable materials. A summary of the available treatment and disposal processes is given in Table 8.4.2. One popular disposal method is to oxidize the sulfides to thiosulfate and the mer-captans to disulfides (API 1969), reducing toxicity and oxygen demand significantly. The disulfides are decanted and the thiosulfate solution is released to the plant sewer system. However, this increases dissolved solids concentrations in refinery effluent waters.
For a metallic surface to receive a high quality protective coating, it must be cleaned. Steel surface cleaning usually involves a detergent wash, a rinse and/or acid wash, followed by a rinse. In this process, called pickling, the acid wash cuts through surface oxide layers to expose bright base metal. Pickling, continuous or batch prepares a surface suitable for plating, galvanizing, and other surface treatments. Since about 50% of integrated steel mill products may be acid pickled, and because most plating lines utilize acid pickling, the steel industry accounts for most acid consumption in the United States.
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