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Illustration: Illustration: FIGURE 1. Calcium binding capability of chelants for common precipitates and scales at various pH levels
Illustration: Illustration: FIGURE 2. Metal ion-control chart for selected APCAs
Illustration: Illustration: FIGURE 3. Effect of pH on EDTA's chelation strength
In chemical process industries (CPI) plants, metal ions pose a constant threat to equipment and processes. They may come from the water supply, from trace contaminants in raw materials, or from erosion or corrosion of process equipment surfaces. Metal ions may also be introduced in one form, and then transformed into another by changes in pH, concentration, oxidation or by reacting with other ingredients in the process stream.
Whatever their source, these ions spell trouble. Multivalent cations, such as calcium, magnesium, and barium form insoluble salts with carbonate, sulfate or phosphate, forming precipitates that foul surfaces and plug equipment. Transition metal cations of iron, copper, zinc and manganese can promote corrosion and catalytic chemical degradation, inhibit polymerization, result in unwanted oxidation and reduction reactions and cause product discoloration.
Chelating agents (from the Greek word chelos, for crab, suggesting their function) are critical allies in the war against metal-ion-induced equipment and process problems. Used in applications ranging from pulp and paper processing to water treatment and gas scrubbing, chelating agents bind with metal cations (Box, p. 87), preventing them from forming insoluble salts or causing other process problems.
The most widely used chelating agents for CPI applications are aminopolycarboxylic acids (APCAs), a group of compounds that includes ethylenediaminetetraacetic acid (EDTA), N-hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA) and nitrilotriacetic acid (NTA).
Phosphonic acids such as ethylenediaminetetramethylene phosphonic acid (EDTMP), diethylenetriaminepentamethylene-phosphonic acid (DTPMP) and nitrilotrimethylenephosphonic acid (ATMP) are also used. The other major chelating-agent category is polycarboxylic acids, which include gluconates, citrates, polyacrylates and polyaspartates.
This article will review very basic chelation chemistry. It will also present simple, effective ways to help you decide whether a chelating agent is the best solution for your process problems, and which agent to select.
Selection and use guidelines
The first step is evaluating your process. Ask the following questions.
1. What is the nature of the metal ion problem? Is it precipitation or scale deposition? Are trace metals affecting process performance or the quality of the final product?
2....