Cutting the cost of deionised water

Deionised water is a vital resource for the surface finishing industry but securing its supply can require significant capital expenditure. The Sales Director of Veolia Water Purification Systems Ltd, Steve Mines, outlines the options available to manufacturers.

Water is a vital resource for all sorts of industrial processes, including surface preparation, coating, anodising, painting, laser cutting, ultrasonic baths, the cleaning of printed circuit boards (PCBs), chrome plating and electroplating. However, water from the mains normally contains ions that have come from the ground, such as sodium and calcium, and from water pipes, such as iron and copper. In order to avoid corrosion, the build-up of minerals and contamination, water used for these processes must first be treated or purified to remove these ions. This process is called deionisation.

Through the process of ion exchange, deionisation removes all ionised minerals and salts (both organic and inorganic) from a solution. Because most non-particulate water impurities are dissolved salts, deionisation produces a high-purity water. Compared with the distillation process, deionisation is faster, less energy-intensive and more cost-effective.

There are numerous routes a manufacturer can take to source deionised water. It can be bought off-the-shelf, but this is only economical for those that need negligible amounts of deionised water for their operations, and the purity of deionised water can degrade quickly when exposed to air.
A cheaper and more efficient approach in the long run is for the manufacturer to produce deionised water as and when it is required.

This can be achieved using deionisation cylinders, which feature two ionised resin beds that are oppositely charged. These operate by exchanging positive hydrogen and negative hydroxyl molecules for the positive and negative contaminant molecules in the water. The hydrogen and hydroxyl ions introduced in this process then unite to form pure water molecules.

Cationic resin is typically made from styrene-containing negatively charged sulfonic acid groups, pre-charged with hydrogen ions, while anionic resin is also made from styrene and contains positively charged quaternary ammonium groups. It is pre-charged with hydroxide ions.

Over time, positive and negative contaminants in the water displace all the active hydrogen and hydroxyl molecules on the ion exchange resin, and the cylinder loses its ability to remove contaminants. The lifecycle of a such a filter can vary greatly with the quality of the feed water, quantity of water purified, and the type of resins used. When resin is exhausted, it must be removed from the cylinder and regenerated. This is a much more cost effective, and environmentally friendly process than disposing and refilling with new resin.

Deionisation can also be carried out using an electro-deionisation (EDI) process, through which electrical current forces the continuous migration of contaminant ions out of the feedwater while continuously regenerating the ion exchange resins. This is achieved with hydrogen and hydroxyl ions that are derived from water splitting. A single EDI unit may operate for a few years before a replacement is required, but the up-front costs associated with the purchase and installation of such a system can be significant, as are the costs of running it. EDI cells are prone to problems such as scaling and CO2 damage. These issues with the EDI process can lead to periods of costly downtime.

For many manufacturers, the most cost-effective means of getting access to deionised water will be a subscription-based service. Leading water purification companies including Veolia have invested in a central cylinder processing facility, taking on all the capital costs associated with the supply and regeneration of ion-exchange resins, so that customers can obtain pure water with minimal outlay.

The way these services work is very simple. Taking Veolia’s Cylinder Exchange Service as an example, a customer can purchase the cylinder it needs to fulfil its deionised water needs. When the resin in this cylinder is exhausted, the customer sends its purchase order or pre-paid voucher to Veolia. The company then organises the collection of the exhausted cylinder, regenerates the resin it contains at its facility in Peterborough, UK, and then returns it—all within seven working days.

Regenerating the resin in this manner is more environmentally friendly than replacing the resin wholesale. As the customer can own the cylinder, it is a highly traceable and transparent system.
Operating cylinders in tandem, so one is in use while the other is being regenerated, ensures that any production downtime is kept to a minimum.

Two types are of deionising cylinder are available: a general deionising cylinder (DC) containing exchangeable regenerated resin and a nuclear-grade deionising cylinder (NC).

The DC cylinder is a single-pass deioniser filled with mixed-bed ion-exchange resin and is capable of removing up to 99% of dissolved mineral impurities from a mains or process water. This cylinder is ideal for surface finishing, the de-fluxing and cleaning of printed circuit boards (PCBs), ultrasonic cleaning, humidifier supply, laboratory glass rinsing, laboratory reagent make-up, boiler feed and printing solutions, or wherever water of up to 10 MΩ.cm is required.

The NC cylinder is a deioniser filled with nuclear grade resin and is able to provide ultra-pure water up to 18.2 MΩ.cm. It is principally used in semiconductor, cosmetics and pharmaceutical production. For maximum performance NC cylinders are typically pre-fed with water that has already been pre-purified by reverse osmosis or passed through a DC deionising cylinder.

When looking for the right service, efficiency and reliability are obviously important factors. But additional things to look for are features within the cylinders themselves. Cylinders can be supplied with all the fittings necessary for connection to a mains or process water supply. They should also be designed for ease of use. Look for features such as colour-coded cylinder heads for easy identification and a quick and easy release system that simplifies the job of connecting the cylinders. Transit plugs will ensure that cylinder ports are kept clean and that here is no spillage during transport, while an air bleed valve will purge cylinders prior to use. Cylinders should also feature high-efficiency internal pipework for maximum capacity.

Given the importance of deionised water to all manner of industrial processes, it is vital that manufacturers choose the right technologies for its production and trust their supplier. As we have seen, a subscription-based system may prove to be the most cost-effective option to secure this vital resource.