For more than 60 years, the coating industry has studied the effects of rinsing on product quality. The increasing present-day focus on environmental impact and resource conservation adds additional restraints to an already heavily regulated industry. Today’s metal finishers must operate efficiently to compete financially. They must provide high quality products to satisfy the customer, while minimizing resources and generating the least amount of effluent. A combination of good management and tight process control is necessary to accomplish these goals. Providing efficient rinse control has progressed from a production engineering goal to a necessity in order to achieve quality, profitability, and sustainability.
Many engineering studies have identified the most effective rinse practices as well as the optimum use of water. In the galvanizing industry, the large tank size and the diverse racks associated with zinc coating present difficulties that have been successfully addressed and documented in several papers. We will address the most effective rinse practices, define the efficiency of each practice, and introduce a new method of achieving the best product quality while minimizing effluent.
1.0 Use of a Single, Stagnant Rinse Tank Since waste water treatment and sludge disposal is expensive, most galvanizers minimize their water consumption. One common practice is to use one rinse tank and only replace part of it when a pickling tank is dumped. The effluent rinse water is used as mixing water for the acid when the fresh acid bath is re-made. This practice causes the iron and acid concentration in the stagnant “drag out” tank to quickly reach contamination levels as high as 80% of the concentration of the pickling bath.
The “carryover” of contaminants from the stagnant rinse into the flux and then the kettle, poisons the process and causes many adverse effects. The negative impacts include: • Product spotting • Dross and ash increase • Thicker coatings • Steel etching
This attempt to minimize waste by using the single, stagnant rinse method actually costs much more in contamination of flux, product quality, and zinc consumption than it saves in treatment and disposal costs. Studies show that stagnant rinses create excessive carryover of ferrous ions into the flux and ferric hydroxide sludge generation in the tanks. Both ferrous and ferric salts adhere to the product and are carried into the kettle where they cause spotting as well as increase generation of dross and ash. Without adequate rinsing to eliminate the carryover of contaminants, both profitability and product quality suffer.
2.0 Use of Proper Racking and Drainage to Minimize Carryover Contamination Inappropriate rinsing systems are just a portion of the problem. Efficient salt removal from the parts is a crucial step. As the racks with parts are raised from the pickling tank, the iron contaminated acid drains from them. To minimize the carryover of contaminants, the parts must be suspended long enough at an offset angle to drain acid back into tank. Avoid horizontal surfaces and insure that no collection points of acid are carried with the parts into the next tank.
The general rule is to wait 60 seconds or until the last drips have fallen from the parts. However, to avoid “flash rusting,” the parts should not be allowed to dry in the air.
The parts will have a boundary film of acid and iron salts as they are transferred from the acid tank and immersed in the rinse tank. This film can be removed by using agitation in the tank to cause mass transfer across the surface of the part. Although vigorous circulation by pumping is used in many coating industries, the galvanizing industry typically uses compressed air agitation generated by an anchored plastic pipe with small holes placed along the bottom of the tank under the parts.
In addition to the air agitation, most picklers also apply the “double dipping” technique. The “double dip” rinse technique1 simply requires lowering the parts into the rinse tank, removing them, and then re-immersing. The movement of rinse water across the surface removes the boundary film of acid and salt. Studies have shown that the parts should then be removed slowly from the rinse tank for the best result 2.
When the parts are removed from the rinse, a similar drainage technique should be followed. The goal is to remove the acid and salt film carried into the rinse tank and to minimize the contaminated rinse carried to the flux. • Proper racking of parts for best drainage • Tank agitation for best removal of film • Double dipping for best part scrubbing • Slow removal of parts from the bath • No puddles of acid or rinse on the parts • At least 60 seconds (no drips) hang time over the tank
A study performed on a metal finishing line in the U.S.A.3 reveals the effect of proper racking and drainage on carryover from the pickling tank into the rinse tanks as well as from the rinse tank to the pre-flux tank. This study clearly shows the dramatic impact of proper racking and drain-time on cross contamination of subsequent process tanks.
3.0 Proper Rinse Techniques Minimize Contamination Three rinsing techniques are commonly used in galvanizing operations around the world: single dunk rinse, overflow rinse with exit spray, and double counter-flow rinse.
While all three techniques have varying degrees of effectiveness, eventually the contaminated rinse will require treatment, disposal, and replacement.
The single, stagnant dunk rinse method is the most widely used process in the galvanizing industry. It is also the most detrimental to product quality and company profit. The constant carryover of iron salts from the rinse impacts the performance of the flux and leads to iron salt contamination of the kettle. The flux must be treated frequently to remove the iron salt and the ferric hydroxide sludge that floats in the tank and deposits on the parts with the flux. If the flux is left to become excessively contaminated, it is often discarded and replaced with new flux solution.
Overflow rinse with exit spray Many metal finishers incorporate a spray rinse on the parts as they exit the dunk rinse. Considered cheaper than installing a second rinse tank, it is only about 50% as effective as a second counter-flow rinse tank. The fresh water dilutes the acid and salts that remain on the parts and dilutes the rinse tank as the parts drip into the tank. Since the spray cannot be as efficient at removing the boundary film as total immersion with agitation, the spray is less effective as a rinse method. Ultimately, some salt will be carried to the flux, eventually requiring chemical treatment or disposal of both the rinse and the flux.
Double Counter-Flow Rinse The most effective rinse method uses two connected counter-flow rinse tanks with agitation . When good drainage of the racks is observed, the carry-over of contaminants to the flux and kettle can be minimized. Most plants do not use a second rinse because of: a) the capital cost of a second rinse tank, b) limited area for tank installation, and c) loss of production time.
The graph below shows the projected rate of rinse tank contamination for one month starting with a clean rinse tank. The galvanizing plant coats 1,000 tons per month. The graph assumes that the process begins with a 30,000 liter fresh water rinse tank. The pickle tank carryover contamination level is 100 grams per liter. One cubic meter of fresh water is used each day for rinsing. The drainage time is estimated at 30 seconds per rack.
As the logarithmic scaled graph shows, a stagnant single rinse (#1) quickly begins to contaminate the flux. A flowing rinse (#2) uses the cubic meter of fresh water available through spent acid disposal to provide better protection of the flux; however, without adequate flow the flux will eventually require treatment or replacement. The fresh water spray method (#3) uses additional fresh water to reduce the contamination, but the tank will require water treatment since the volume is greater than the amount necessary to mix with the fresh acid. The cascade method (#4) uses two connected rinse tanks flowing countercurrent to the work flow. The countercurrent cascade method provides the best protection of the flux and maintains a stable and lower contaminant level. All four methods require waste treatment and disposal of hazardous wastes.
4.0 Financial and Environmental Impacts of Poor Rinse Practice The factors considered when identifying the cost of adequate rinse include: • Product quality • Zinc consumption (coating thickness, dross, ash) • Chemical replacement cost • Environmental - treatment and sludge disposal cost
4.1 Product Quality It has long been documented that the carryover of iron into the flux and kettle causes salt spotting and black spots on the surface of the parts. Degraded flux also damages the cosmetic appearance.
4.2 Zinc Consumption Most galvanizers try to maintain a total zinc usage of between 4% and 5% per ton of steel galvanized. Data has been gathered that shows the relationship between highly contaminated rinse, high iron flux, and an increase of coating thickness. Over the years, data has also shown the correlation between the iron salt in the flux and the generation of dross and ash in the kettle. Individual reports4,5 reveal that highly contaminated rinse and flux lead to zinc consumption figures between 6% and 10%.
1 kg Fe = 40 kg Zn (dross) 1% Zn / 1000 tons galvanized = $ 20,000 USD = 640,400 TWD
5.0 A Sustainable Alternative: Recycle and Recovery Options for processing waste rinse water have been limited to pH neutralization and disposal. A few plants have installed evaporators, but the capital and energy costs are prohibitive to small and medium-sized galvanizing operations if the evaporator is used only for waste water. Ion exchange systems have been installed in many metal finishing plants, but the capital cost of the equipment, chemicals required, and need for a technically proficient operator often limit the use of ion exchange to specialty metals and electronics applications.
Although reverse osmosis has been applied worldwide for purifying water, pH and salt limitations have stifled efforts to use reverse osmosis membrane technology for rinse water recycle. In the past few years, however, advances in acid-resistant membranes have made it possible to cost effectively recycle rinse water from the metal finishing industry.
5.1 Zero Discharge Rinse with Acid-Resistant Membrane Recovery This membrane-based water recovery technology maintains a very low contaminant concentration in the rinse tank by continuously filtering iron salts. The filtered water, called “permeate,” returns to the rinse. Only a small stream of high concentration iron and acid-rich salts are rejected and returned to the pickling tanks to replace the volume used to dilute fresh replacement acid.
This system provides plenty of clean water to protect the flux and kettle from iron contamination. When this iron-free recovered water is used as an exit spray on the parts, the effect is equivalent to countercurrent cascade double rinsing with an exit spray tank.
Figure above depicts a typical galvanizing operation with a membrane recovery system.
4.1 Zero Discharge Rinse with Membrane Recovery This membrane based water recovery technology maintains a very low contaminant concentration in the rinse tank by continuously filtering iron salt from the rinse tank. The filtered water, called “permeate,” returns to the rinse. Only a small stream of high concentration iron and acid-rich salts are rejected and returned to the pickling tanks to replace the volume used to dilute fresh replacement acid. This system eliminates the need for rinse water treatment while making plenty of clean water available to protect the flux and kettle from iron contamination. When this iron-free recovered water is used as an exit spray on the parts (as shown in the example), the effect would be equivalent to countercurrent cascade double rinsing with an exit spray tank. This process eliminates the need for further iron removal from the flux tank.
A typical rinse recovery system will only use 3 KW/hr of electricity to continuously process the rinse. This process uses 15 to 30 times less energy than evaporation or conventional treatment technologies and requires no chemicals. The capital cost of the equipment is about the same as the cost of installing a second rinse tank and is less expensive to own and operate than waste treatment and evaporation.
The process can be scaled to the needs of the individual plant to provide excellent rinse. The membranes can be cleaned in place and have a life expectancy of between one and three years. The process uses no chemicals but requires some pre-filtration to keep the membranes from clogging with solids.
The elimination of iron carryover and contamination to the flux and kettle quickly pays for the cost of the system through savings in zinc. The sustainable practice of re-using the water and minimizing waste generation meets all future environmental restrictions.
For Galvanizers: Increase profitability with sustainable production practices
Hydrochloric acid recovery
Rinse membrane recovery
Ground water remediation
The “carryover” of contaminants from the
stagnant rinse into the flux and then the kettle,
poisons the process and causes adverse effects.
The negative impacts include:
• Product spotting
• Dross and ash increase
• Thicker coatings
• Steel etching
Use of Proper Racking and Drainage to
Minimize Carryover Contamination
• Proper racking of parts for best drainage
• Tank agitation for best removal of film
• Double dipping for best part scrubbing
• Slow removal of parts from the bath
• No puddles of acid or rinse on the parts
• At least 60 seconds (no drips) hang time over the tank
Drag Out per Unit Area
Vertical parts, good drainage 1.63 0.34
Vertical parts, poor drainage 8.15 1.73
Vertical parts, very bad drainage 16.3 3.46
Horizontal parts, good drainage 3.26 0.70
Horizontal parts, very bad drainage 40.75 8.67
Cup shape parts, very poor drainage 32-100 6.8-21.2
Financial and Environmental Impacts
of Poor Rinse Practice
The factors considered when identifying the
cost of adequate rinse include:
• Product quality
• Zinc consumption (coating thickness, dross, ash)
• Chemical replacement cost
• Environmental (treatment & sludge disposal cost)
1 kg Fe = 40 kg Zn (dross)
1% Zn / 1000 tons galvanized = $ 20,000 USD
Iron Carry Over to Pre-Flux
Series1 43.8 11.7 5.4 3.4 1.7
Cascade Rinse Beta System
4% Zn by weight on product
4% = 71.35% of purchased Zinc
The lost Zinc (1.6%) was found in these places:
40.4% skimmings (ash)
14.4% in disposed acid
Data collected from 10 USA galvanizing plants, averaged
Zinc Consumption: History & Targets
1000 tons Galvanized Rinse Zn Use
10 USA galvanizers Double dip 5.6% $ 14,000 2 $ 350
Captive hardware Single dip 7.5% $ 52,000 2 $ 350
Centrifuge Single dip 9.3% $ 60,000 2 $ 350
General galvanizer 2 rinse/2 dip 4.9% $ 0 Zero $ 2,702
Membrane recovery 1 rinse/2 dip 4.9% $ 0 Zero $ 663
Zinc Consumption: Comparison