Operating Your DI System Economically
Ryonex develops the world’s first fully closed loop resin system and reveals the best ways of minimising the cost of running your DI system.
The purity of the final rinse in Powder Coating Pre-treatment is crucial to the corrosion performance of the powder coated layer and is an important criterion in all quality standards.
The widely followed international architectural standard for Powder Coated Aluminium, Qualicoat, has their standard for the conductivity of run-off water of standard open section (not hollow) from the final rinse which is a maximum of 30µS (micro Siemens). This means the conductivity of the final dip needs to be lower and typically around 20 µS is required to achieve this standard of run-off.
The major Powder suppliers are tending to follow these standards for their accredited Powder Coating Applicator programmes. While the Australian Standard 3715 still calls for a higher run-off reading, this is under review and will likely lower to reflect the Qualicoat standard and that of the Applicator programmes when it is updated.
Water quality standards of the rinse preceding the conversion coating has now also come under scrutiny and is particularly important with the industry trend to move away from Hexavalent Chrome based treatments.
There has also been a trend towards zero discharge systems, where serviced DI Systems are used to continuously treat the water which results in supplying consistent DI water to the plant and when the exchangeable columns are exhausted, these are either sent for regeneration off site or the resin is transferred on site with regenerated resin.
Purifying water
Tap water has quite varying conductivities, typically around 90 µS (Melbourne) to 280 (Sydney) to 400 (Brisbane). Distilled water is pure and has a conductivity of 0 µS.
In order to achieve the water standards required in the accreditation programmes even tap water needs to be treated.
There are 2 most widely used treatment options in obtaining highly pure water; RO (Reverse Osmosis) or DI (De-Ionisation).
RO is generated by pumping water at high pressure across a RO membrane generating a continuous stream of pure water and another of a concentrated brine waste stream containing all the impurities (salts). Typically, with RO plants the water needs to be filtered and softened prior to being processed through the membrane. Depending on the efficiency of the membrane, the brine (30-50% of water treated) might need to be processed through a waste facility. The conductivity of RO water is dependent on the feed water quality with permeate quality expected to be less than 10 µS.
A DI or Demin System is a resin-based system that removes ions from the water to generate a highly pure water. Typically, systems will have anion and cation resins which remove all the cations (negatively charged ions) and anions (positively charged ions) in the water stream. When the resins are saturated with impurities (ions), the resin is regenerated by chemically unbinding the impurities (ions) it has exchanged from the feed water, leaving the resin clean (regenerated) to start the process again. The waste generated by the regeneration process must be treated in a waste facility. The water quality from a DI system is dependent on the feed water quality but treated DI water is expected to be less than 5 µS.
Mixed beds are mixtures of these resins that can be used in some circumstances to produce higher quality DI water (0. 5 µS). These resins cannot regenerate easily but are often used to polish RO and separate bed DI systems water.
Serviced DI Systems
Serviced DI Systems (SDI) have become quite commonplace in the Powder Coating industry. SDI systems allow users to operate a total zero discharge system where there is no discharge to sewer, no effluent treatment (plant and chemicals), no requirement for a discharge permit and no responsibility to the EPA and Water Authorities. The user also gets DI rinsing water in all their rinse tanks which is required by the Powder Coating Applicator programmes as well as extended life of their chemical processing solutions. This is due to the reduction of chemical drag in from preceding processes.
This system will remove all the chemical species that are introduced into the water rinses.
With these SDI systems the user pays for the regeneration of the resin and it will be in their best interest to keep the drag out into the rinses to a minimum. Drag out costs are two-fold, which includes the cost of replacing the wasted chemical plus the cost of removing it from the water.
Minimising the cost of your waste
There are numerous practical methods that will help minimise the running cost of your SDI system.
1. Reduce Drag Out Volume - this is the most recognised method. Most chemical suppliers will concede that the volume of chemicals dragged out of a process bath often exceeds the volume consumed by the treatment of the work being processed. This is usually the case in the acid etching of aluminium extrusions in a dip line. That means you are wasting more than half the chemicals you are using. Often this etch equates to around 80% of the chemical cost of operating these types of lines before its waste treatment cost is calculated.
Drain times are critical in reducing drag out. From a process cost point of view, the maximum drain time possible should be utilised without allowing the solution to dry on the components being processed. This available time will change depending on the prevailing climatic conditions. In an extrusion dip line, the drain time should exceed 6 minutes from the acid etch and static rinse if you are running a SDI system. It is also important to remember that solution viscosity greatly impacts the drag out volume. Acid etches with a high Aluminium content can easily double the drag out volume over a fresh acid because of its greater viscosity. Drain time eventually becomes a compromise between the production demands and cost savings.
2. Minimise your process solution concentration - by minimising the concentration of the solutions, the effective drag out that requires treatment is reduced.
Acid Etch - In Aluminium extrusion lines the acid etch concentration could be run as low as 2% if the Aluminium content in the solution was being controlled. An etch rate of 1g/m2 can be achieved at this concentration with a dip time of under 6 minutes if the Aluminium content is managed to around 1g/L. It is not uncommon to see an acid concentration being run at around 8-10% generating the required etch in a much longer period 10-15 minutes when the Al concentration is approximately 5 g/L. The impact of this high solution concentration coupled with the high Aluminium content impacts dramatically on the waste loading of the line. The Aluminium content can be controlled by dilution (i.e. bleeding off at a constant rate). In order to control the solution in this manner, you continuously need to remove the Al you are dissolving in the solution daily (i.e. if you are dissolving 100g or treating 100m2/day with a 1g/m2 etch rate). And if you are controlling the solution at 2 g/L, you will need to bleed off 50L of solution per day. In this fashion you will control the Al content and wasting 1L of your acid per day.
Ryonex offers an Aluminium removal system for acid etches. With this system, the Aluminium is constantly being removed and the recovered acid gets returned to the solution. This system can treat both the acid etch as well as the acid drag out solution. This means that most of the acid can be recovered which results in you wasting minimal acid and importantly, reducing the impact on your waste recycling system.
Chromate treatment tanks - Chromate treatment tanks can operate at very dilute (0.5% Cr) concentrations when made up new. The required chromate layer of 600mg/m2 can be deposited in about 3 mins dip time. As impurities (Al, Cr3 and other metallic oxides) build up in the solution, the deposition rate decreases with the age of the solution. Either the dip time or the solution concentration, or both, gets increased. Commonly the Chromate then gets doubled and sometimes tripled to reach the required deposition, which can often lead to unexpected costly impact on waste management.
Again, bleeding the solution can be an effective control measure to keep the solution within the optimum range with the least financial impact on waste management costs.
Ryonex is currently testing a Chromate recovery system that is designed to recover the chromate from rinse tanks in order to allow the Chromate to be returned to the treatment tank while effectively purifying the rinse tank continuously. The impact of the loading into the DI system is greatly reduced, saving considerable regeneration costs.
Non-Hex Chrome Systems - Non chrome systems are generally Zirconium based or either a mixture with Trivalent Chrome or Titanium. These systems have the advantage of operating at much lower concentrations when compared with the Hexavalent Chromates and therefore, the concentration of the drag out to the rinse tanks is greatly reduced. Often the drag out from the conversion tank (i.e. little or no drain times of processing load) is encouraged by the suppliers as a form of solution bleed to control the build-up of accumulated impurities and reactionary by-products. These solutions are deemed more sensitive to impurities because of their low operating concentrations. Bleeding is often used as a control measure and is preferable to periodic dumping.
Due to the need to control the transfer of impurities into the conversion tank, the preceding rinse tank must be kept very clean (100uS is a typical recommendation). It is highly recommended to operate the rinse tank immediately following the acid etch as a static drag out rinse before the flowing rinse tank. As stated earlier, the Aluminium removal system could be used to control the static water conductivity and recover clean acid as make up solution to the etch tank.
In response to the complex nature of the chemistry, Ryonex has developed the world’s first fully closed loop resin system for the rinse waters and bleed off solution. The new system was successfully commercialised in 2018 with many new customers transitioning to Non-Chrome chemistry systems.
Contact sales@ryonex.com.au for assistance in optimising your chemistry and pre-treatment line operation to maximise your cycle times.