The basic chemicals industry is responsible for producing thousands of raw materials at very large scales. The industries downstream rely upon a certain level of chemical purity to manufacture their own goods, as certain impurities can cause major issues in various processes. During the production of the basic chemicals sodium and potassium hydroxide (NaOH and KOH), electrolysis with membrane cells of saturated brine solutions yields the product which is further concentrated by evaporation. Impurities from the salts used in the brine will also be concentrated. Typically, this impurity analysis is performed offline using various hazardous chemicals with varying shelf-lives.

This Process Application Note is focused on monitoring anionic impurities in caustic soda and caustic potash. The 2060 IC Process Analyzer is the perfect solution to perform the measurement described in ASTM E1787 online, ensuring quality product without the need for time-consuming, hazardous laboratory experiments.

The production of caustic soda (sodium hydroxide, lye, NaOH) and caustic potash (potassium hydroxide, KOH) is extremely significant, as these are mainly used as precursors for many other chemicals used in all industries. For example, the pulp and paper industry is especially reliant on concentrated caustic soda for chemical pulping of wood in the Kraft process, and the agrochemical industry is heavily reliant on KOH.

Caustic soda and caustic potash are produced alongside chlorine in the chlor-alkali process, which is explained in more detail in AN-PAN-1005. In this process, chlorine and caustic soda (or potash) are produced via electrolysis of sodium chloride (or potassium chloride) brine, mainly with the membrane-cell technique [1]. Some production plants make both NaOH and KOH in the same cell room, though generally the brine circuits are kept separate to avoid time-consuming cleaning and purging processes between the different brines. In both situations, the caustic product is concentrated to about 50 wt-% by two- or three-step evaporations before it is stored. This concentrated product contains impurities from the salts used which are undesirable in certain chemical purity grades needed for the subsequent production processes.

Typically, anionic impurities in 50 wt-% caustic soda or potash are determined by gravimetric or titration methods which require a variety of reagents with diverse shelf lives and hazards. In 2016, the ASTM method E1787 was released, specifying ion chromatography (IC) to measure bromide (Br^-), chlorate (ClO₃^-), chloride (Cl^-), fluoride (F^-), nitrate (NO₃^-), phosphate (PO₄^3-), and sulfate (SO₄^2-) in concentrated NaOH or KOH solutions. Anions of primary interest are Cl^-, ClO₃^-, and SO₄^2-, as shown in Figure 1b.

(a)

(b)

The 2060 IC Process Analyzer from Metrohm Process Analytics (Figure 2) is ideal for fulfilling ASTM E1787. The analyzer is able to continuously measure and monitor anionic impurities in caustic soda and caustic potash in a robust housing suitable for such a process environment. Metrohm offers many fully automatic inline sample preparation techniques for ion chromatography, making analysis even more hands-off and flexible. Automated calibration guarantees excellent detection limits, high reproducibility, and excellent recovery rates.

The caustic stream is sampled frequently, giving up-to-date information about the status of the membrane cells. The 2060 IC Process Analyzer can provide an alarm if pre-set warning or intervention concentration limits are reached, helping to save costs by preventing irreparable damage due to membrane fouling and other problems. One 2060 IC Process Analyzer has the possibility to connect to up to 10 sample streams, meaning multiple cells with different end products could be monitored for impurities by a single instrument.

The 2060 IC Process Analyzer can run for extended periods in less-frequented areas as there is adequate space reserved for reagents, containers of ultrapure water and/or prepared eluent, and level sensors to alert users when liquid levels are low. By choosing a built-in eluent module and optional PURELAB® flex 5/6 from ELGA® for continuous pressureless ultrapure water supply, the 2060 IC Process Analyzer can be configured to run even trace analyses autonomously.

Concentrated KOH and NaOH samples can be analyzed according to ASTM E1787, with Metrohm Inline Sample Preparation techniques for extra application flexibility. Analyte detection is by conductivity.

For sulfate quantification, it is essential to use perchloric acid (HClO₄) for the inline neutralization. An anion trap (A Trap 1) should be used in line with any ultrapure transfer water to ensure the highest quality results. For high concentrations of chloride, potentiometric titration can be used. Metrohm Process Analytics offers potentiometric titration options for samples above 2 mg/L Cl^-.

The 2060 IC Process Analyzer from Metrohm Process Analytics can continuously measure and monitor anionic impurities in caustic streams according to ASTM E1787. Integrated with liquid handling modules and automated sample preparation, the 2060 IC Process Analyzer makes calibration and validation as simple as pressing a button.

[1] How Are Chlorine and Caustic Soda Made? Euro Chlor 17.

[2] Standard Test Method for Anions in Caustic Soda and Caustic Potash (Sodium Hydroxide and Potassium Hydroxide) by Ion Chromatography https://www.astm.org/e1787-16.html (accessed 2022-04-08).

• ­Increased longevity of valuable company assets
• Monitor multiple sample streams (up to 10) for more savings per measurement point and results ­
• Fully automated diagnostics – automatic alarms for when samples are out of specification parameters