Cooling water treatments containing zinc are being more strictly regulated. Fines are being imposed in some instances where zinc discharge concentrations are as low as 0.5 ppm (mg/L). With regulatory concerns looming, many plants are deciding to eliminate zinc from their cooling loops, Laboratory work was conducted to identify potential zinc replacement technology for use under mill supply conditions typical of those found in pulp and paper applications. This industry segment is one of the largest users of once-through cooling water. An organic replacement able to provide zinc-like performance under mill supply conditions was identified. This paper describes the laboratory protocols utilized to identify this organic material and further develop its use in once through cooling water applications. Case histories are also presented.
Once-through cooling water is prevalent throughout industry. These streams are known in the pulp and paper industry as “mill supply waters,” as “service water” in the power industry, and also as “once-through waters” in the chemical and hydrocarbon process industries. Large volumes of water are typically drawn from a nearby well, river, or lake for most of these applications and then discharged directly to an aqueous stream. Because evaporation in these systems is minimal, the influent water characteristics closely match the effluent (ie., uncycled raw water). Outside of mechanical screening to remove large contaminants, only relatively low treatment concentrations can be applied to these streams because of the costs associated with treating large volumes of water.
A mill supply treatment program typically consists of corrosion and deposit control agents as well as some type of microbiological (MB) control protocol. Threshold levels of inorganic phosphate, zinc, and/or polymer are commonly utilized for corrosion and deposit control. Protection of carbon steel is the main aim of these programs because it is the predominant metallurgy for most mill supply transfer piping. While copper-based alloys maybe a minor component in these loops, it is cost prohibitive to incorporate azoles for protection of these alloys, Typically, 0.5 to 3 ppm (mg/L) inorganic phosphate, 0.1 to 1 ppm (mg/L) zine, and/or 0.5 to 2 ppm (mg/L) polymer are applied to mill supply process waters. While untreated streams can yield carbon steel corrosion rates in excess of 50 mpy (1.27 mm/y), low level zinc treatments provide corrosion rates in the 5 to 10 mpy (0.127 to 0.254 rnm/y) range, with minimal pitting corrosion. Zine-free programs typically provide carbon steel corrosion rates in the 10 to 15 mpy (0.254 to 0.381 mm/y) range, with moderate pitting. A small amount of zinc provides superior pitting protection relative to the inorganic phosphates, even when these are used at elevated dosages. Therefore, there has been ongoing concern with regard to legislation more strictly regulating zinc discharge to rivers and streams1, 2.
The most common corrosion mechanism found in mill supply applications can be depicted by the classic corrosion cell as illustrated in Figure 1. Metal loss occurs at the anode with dissolution of the base metal as electrons flow from anode to cathode.
