The metal finishing industry, in particular, is now being forced to become more efficient in water use in order to stay in business. Facilities that discharge pollutants have to decide whether to concentrate wastes and risk exceeding concentration limits, or treat the waste before discharge.

The old method of just diluting the discharge is now too expensive to consider.

The simplest methods to achieve reductions in waste and water consumption require analytical devices such as electrical conductivity meters to optimise water use.These meters are able to automatically adapt to changing conditions and verify both the cleanliness of water that can be considered for reuse, and the required rinse flows needed to clean the metalwork pieces.

Such conductivity meters have been shown to reduce water consumption by up to 40percent without additional labour cost or extensive process modification.

Throughout the metal finishing process stages, work pieces carry over small amounts of unwanted chemicals. Such carry-over can quickly cause problems downstream.
Rinse water between stages is the largest category of water consumption for metal finishers and has the largest potential for use reduction and water reclaim.

The first step in improving rinsing requirements is minimising carry-over from the process baths. Process and procedure modification can be very helpful here, typically by lowering bath concentrations, increasing bath temperature to lower the solution viscosity, using wetting agents to reduce the solution surface tension, removing work pieces slowly and allowing them to drain on racks, using spray rinses or air knives above the process baths, installing drainage boards after process baths to return carry-over to the baths.

Fresh water is traditionally used for rinsing purposes, but examination of the entire process can reveal water streams that can be re-used in this role to improve efficiency.

Counter-current rinsing, where the freshest water does not contact the work piece until the last rinse stage, is a technique that can effectively use the same rinse water twice or more, though it is a little more complex to install.

An excellent way to control rinse water flows and purity under dynamic conditions is to monitor the solution electrical conductivity.

Conductivity is a non-specific measurement of the ionic contamination in the rinse water. As salts accumulate in the rinse water, the solution will have higher conductivity. Conductivity meters minimise the use of makeup water by monitoring the rinse water bath and by only allowing fresh water addition when the conductivity reaches a certain set-point.

For a cost range of E1000–1700 per measurement point, a conductivity meter can reduce water consumption by 40percent and have a rapid economic payback (Fig.1).

The actual set-point chosen depends on the chemical being rinsed and the desired cleanliness of the work piece. For example, fresh water may have a conductivity of 100–500microS/cm while initial rinse water of 1000–3000microS/cm may be considered adequate. At the final rinse stage de-ionised (DI) water may be used.

Typically a dip DI rinse will operate at 30microS/cm. For the best possible rinsing, the work will be sprayed with virgin DI water on the exit from the dip DI rinse: the conductivity of the virgin DI will be <3microS/cm.

There are three major types of conductivity sensors: two-electrode, four-electrode, and toroidal. The simplest design involves two metal electrodes that are separated by a plastic insulator. These sensors are very accurate, but are designed for measurement over specific, relatively narrow ranges of conductivity (contaminant level).

Two-electrode sensors are ideal for measuring clean water solutions such as occur in reverse osmosis (RO) or deioniser applications. RO water is being recommended more and more for improved make-up water, so two-electrode sensors are frequently found on such rinse tanks.

The four-electrode sensors use a pair of counter electrodes to compensate for the coating and polarising interference effects common at higher contaminant levels. They are used on concentrated solutions of acids, bases, or salts that require measurement of higher conductivity levels in solutions that do not have large amounts of suspended solids.

Toroidal (also known as inductive or electrodeless) conductivity sensors do not have any exposed metal parts and consist of two wire-wrapped rings (toroids) that are encased in plastic.

An electrical signal is passed through the transmitting toroid and produces current flow in the surrounding solution, which is measured by the other (receiver) toroid and converted to a conductivity reading based on the signal received.

Toroidal sensors are very resistant to coating and are ideal for applications in both strong plating solutions and spent rinse waters. Conductivity measurement with toroidal sensors is nearly maintenance free and can provide many years of service if the equipment is properly installed. The sensor should be totally submersed in the liquid and kept at least one inch away from the tank walls. Gas bubbles will cause the reading to drop, so locate the sensor away from any air agitation.

Understanding measurements

The measured conductivity of a solution is dependant on temperature. To compensate for the temperature effect, the conductivity sensor also measures the temperature, and the meter compensates the reading to a reference temperature (25°C is standard) with an adjustable setting.

A simple twopercent/°C adjustment is sufficient for rinse baths, though measurements of concentrated solutions may benefit from a more customised setting.

Correlating the conductivity reading to a ppm (mg/l) concentration in a rinse bath can be a little challenging since each type of ion in solution has a different characteristic conductivity.

In natural water, the most common ions are sodium and chloride, so total dissolved solids (TDS) are usually assumed to be salt, sodium chloride. Rinse waters, however, represent dilute versions of the process baths, and may have a different relationship between conductivity and ppm.

These differences can usually be worked out by using adjustment factors that allow the meter to display customisable concentration values on the display. In addition to monitoring rinse water, conductivity meters can be useful in controlling the concentration in the process baths themselves.

Many process baths involve contacting the work piece with an ionic solution of a known concentration. A good example would be the acid dip or pickling stage. The acid concentration must be maintained to keep the process operating efficiently.

The need to add make-up acid into the tank can be monitored with conductivity: a net drop in conductivity can be used as the trigger for make-up acid, since the acid has much higher conductivity than the salt residues resulting from the process.

On-line control of the make-up acid can be construed as another way to save on rinsing costs since the acid tank can now be confidently run at a lower concentration, involving less carry-over and a reduced requirement to rinse.

Metal finishing processes require a high level of water consumption, but in this age of water conservation for both economic and social responsibility reasons, it is important that metal finishers find ways to reduce water usage.

By using conductivity analysis effectively, plants can obtain a significant reduction in water usage and costs. 

Alan Brackenbury is European Manager for Rosemount Analytical products in Emerson Process Management, and Peter Astles is md of Astles Control Systems.