HIGH PURITY WATER FOR ELECTROPLATING
OGO WATER TECHNOLOGY looks at the requirement for Pure Water in the Electroplating Process, considers some of the latest purification techniques and how they can help electroplaters to Reuse treated water within their processes.
Industrial processes are renowned for their high consumption of raw water. Raw water, however, drawn either from a dedicated source such as a borehole or taken direct from the mains supply, contains varying levels of chemical and biological contaminants that must be treated or removed before it can be used in a process stream.
This fact is especially true of the electroplating sector, where component parts or workpieces need to be thoroughly cleaned or rinsed using clean water in special baths as the parts move through a sequence of process stages. Cleaning is essential to prevent surface residues being transferred from stage to stage and depends on a consistent supply of water that has been purified to a known level of quality.
Large quantities of water are used within these multiple processing baths or tanks, with typically two or even three grades of pure water required to rinse away the residues of plating solutions that contain the metallic salts needed for plating processes.
Although metal finishers are used to dealing with a high level of water consumption, the marketplace in which they operate is now changing dramatically and is leading many companies to review their process strategies, for both economic and environmental reasons. For example, a number of the UK’s major water suppliers recently announced that the cost of mains water is likely to rise considerably between 2005 and 2010, while legislation governing the use and disposal of water is becoming increasingly restrictive. A classic example is legislation to be introduced in July 2006 which will restrict the use of lead solder in circuit boards; this means that companies are changing to alternative products such as silver based solder, which in turn requires circuit boards to be cleaned in purified water before the solder can be applied.
One area that is currently generating considerable attention is the potential for re-use of appropriately treated wastewater for non-critical rinsing stages, such as after the introduction of alkaline cleaners. In particular, there is growing interest in the latest advances in membrane separation technology, to provide effective and commercially viable levels of contaminant removal.
Before we consider this latest technology, it is first worthwhile examining the two principal methods of achieving the desired level of water quality for rinse bath processes: ion exchange and reverse osmosis. It should be noted that the type of technology specified will ultimately be driven by a number of factors, including the quality and quantity of water required, the feed water source in terms of chemistry, the availability of space, capital costs and effluent disposal.
The deionization process is a rapid and reversible technique in which the ionic contaminants present in the feed water are exchanged for ions released by the ion-exchange resin.
An ion is an atom or group of atoms with an electric charge. Positively charged ions are called cations and are usually metals; negatively charged ions are called anions and are usually non-metals.
During the ion exchange process, cation resins will exchange hydrogen ions for any cations they encounter, and, similarly, anion resins will exchange hydroxyl ions for any anions present. The ionic impurities remain bound to the resins and the hydrogen and hydroxyl ions combine to form water.
One of the simplest methods of providing pure water via ion exchange is through the use of standalone deionizing cylinders that can be connected directly to a mains or process water supply. These cylinders are easy to install, require little space and can be run until they are exhausted and are then simply exchanged for a new unit by the supplier, who subsequently regenerates each cylinder using specialized plant.
Water purification cylinders offer a quick, simple and cost effective method of producing reasonable volumes of high quality purified water of all qualities, including ultrapure water. In addition, there is no need for regeneration chemicals to be stored on site, or for the disposal of chemical effluent, as these issues are dealt with by the cylinder supplier.
An alternative to the use of regenerable cylinders is the installation of a twin bed deionizer, which comprises separate cation and anion chambers that are automatically regenerated in-situ using acid and caustic solutions. As the majority of plating establishments are already set up to handle acids and caustic wastes, and hold stock of the chemicals used to regenerate the ion-exchange resins, the option of regenerating the resins on site via a twin bed deionizer can be a cost-effective one. The installation simply requires suitable space and site conditions to allow for the treatment of effluent and the handling of chemicals.
Standard twin bed deionizers can typically provide deionized water in the range of 5-35 micro Siemens per centimeter and can offer a wide range of inter-regeneration capacities.
Typically, R O membranes are capable of operating at an efficiency of up to 75% recovery, dependant on the feed water quality. In addition to de-mineralizing the water, the R O membranes will also remove over 99% of micro-organisms.
Reverse osmosis provides a solution to a metal finisher looking for the option of managing water purification in-situ, but without the necessity of handling the chemicals used with twin bed deionisers. R O plants typically require less room on site and can play a role in water metal recovery.
Figure 1 refers to a typical water purification system that could provide purified water for electroplating rinse baths. Mains water passes through a pre-treatment stage that removes particulate matter from the mains water and removes any contamination that would impair the RO stage of the process. Water feeds the rinse baths after the RO stage of the process, or, if additional grades are required water passes through the ion exchange cylinders polish the water to a higher level of purity.
As we have already noted, the need to reduce water consumption, recycle process water and minimise the disposal of waste water is steadily increasing, both to reduce operating costs and comply with ever more stringent legislation.
Reverse osmosis is a possible method of water recovery for use within the electroplating sector as the technology provides a means of recovering plating chemicals from the rinse water by removing water molecules through the semi-permeable membrane of the reverse osmosis system.
In essence, the membrane allows water molecules to pass through, but blocks metallic salts and additives. Diluted or concentrated rinse waters are circulated through the membrane at pressures greater than aqueous osmotic pressure, resulting in the separation of water from the plating chemicals. The removed chemicals can then be handled in a number of different ways; for example, they can be reused or concentrated for easier handling and eventual disposal.
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