Originally presented at Extraction 2018, August 26-29, 2018. Please visit Springer for more information about the conference or how to purchase the paper or proceedings.
Donald D. Downey
Purolite Corporation, Paris, ON Canada
Pilot Testing Ion Exchange Resin (IEX) can go from very simple beaker testing to complicated column testing. Space velocities, regenerant quantities, loading capacities, ion leakage, are all terms that get accumulated together to prove or disprove a resin’s (or resins’) ability to load a single ion (or specific set of ions). Not getting all the resin’s operating parameters correct will disqualify (or at least skew) the results.
Laboratory tests for capacity, moisture content, bead size, metals fouling, etc. – what does it all mean? At last count, there were over twenty-five separate ion exchange resin tests (analyses) that were available to end users. Standard testing procedures identify the resin properties but more specific procedures can be used to identify problems with equipment operation. Then, there are the costs to consider; simple cation resin testing (moisture content, total capacity and bead integrity) is in the $200/sample range. However, add to this Chatillon, Russian Ball Mill Test, HIAC particle size, metals and % regeneration test procedures and the lab work can increase to over $1000/sample.
In this paper, the author will explain the “must-do” pilot and test procedures and why they are important and the key to understanding how resin performs ion exchange.
Keywords: Ion exchange resin, pilot study, laboratory analysis
Introduction to Polymer Chemistry
Ion exchange resins (IEX) are synthetic resins having a chemical structure based on a cross-linked three-dimensional polymer molecule into which functional groups (sulfonic acid and quaternary ammonium) are introduced. Most of the polymer bases used for IEX are copolymers of styrene and divinylbenzene (DVB) and generally consist of spherical particles of 300 to 1200 µm diameter. The cross-linked copolymer is synthesized by mixing styrene (one vinyl group) with DVB (two vinyl groups) and carrying out a suspension polymerization in water.
The ion exchange resin is then manufactured by introducing functional groups into this copolymer matrix by means of chemical reactions. The ion exchange groups introduced are chemically bonded to the polymer, and as they cannot move freely, they are known as fixed ions. Mobile ions of opposite charge to the ion exchange groups (H+ ions in the case of R-SO3H) are known as counter ions. If the quantity of the bi-functional monomer DVB is increased in the polymerization, a tight, porous structure with considerable chain branching is obtained. If, on the other hand, the quantity of DVB is decreased, a loose, porous structure without much branching is obtained. As DVB determines the tightness of the porosity, it is known as a cross-linking agent. The proportion of DVB in % is referred to as the cross-linkage.
Since IEX contain pores, the pores are filled with liquid (for this paper we will assume water). Ions diffuse throughout these pores and give rise to ion exchanges. The more cross-linked the resin is, the more ion diffusion will be impeded. Conversely, a low degree of cross-linkage will facilitate a greater diffusion of ions. If the cross-linkage is too low, however, moisture content becomes excessive, so the resin will be softer and difficult to use, and its strength also decreases. Resins are, therefore, generally used with a cross-linkage of about 8%.
Ion exchange reactions are carried out with a diffusion of counter ions in the resin particles. The pore structure of the polymer matrix has a considerable effect on these reactions, and cross-linkage is therefore, an important factor when it comes to analyzing the properties of the resin.
The styrene-DVB copolymer does not absorb water until the functional groups are added. Water absorption is due to hydration of the fixed and counter ions. The chains of the polymer matrix elongate and swell until a balance is reached between the water absorbing capacity of the resin and the elastic forces of the polymer. This keeps the resin in its swollen state with a stable moisture content. The higher the cross-linkage, the less the polymer matrix can elongate resulting in a resin with a lower moisture content. Low cross-linkage results in a bead with a high moisture content as the swelling increases. As the resin swells during the functionalization phase, micro-pores are created in the beads.
The ion exchange resin manufactured by simple polymerization of styrene with DVB is transparent and has a gel structure; hence, it is known as a gel type ion exchange resin. Using special polymerization techniques however, IEX resins of higher porosity can be manufactured, creating a macroporous resin. The photos that follow are (1) a weak acid cation resin, (2) macro porous strong base anion resin and (3) a strong acid cation gel reins. The magnification of the photos is 100x.