Case Study

Quality problems due to contaminated rinse tanks
 

Problem

The hot summer months have created an ongoing problem for GALVANIZER ABC. The high temperatures require longer rinse times to avoid flash rusting and still there have been quality problems. Even with the longer 18-second summer rinses, the TDS of the final rinse is 1890 mg/l and heavily contaminated with iron. The 12 to 13 second standard off-season rinse obviously carries over greater than 50% more TDS (estimated at 3000 TDS), acid and iron, but the greatest and most frequent quality problems present themselves during the high temperature, high humidity summer months.

The galvanizer’s existing rinse evaporator is costly to operate and maintain and cannot keep up with the current waste rinse water generated. A decision is needed to either replace the existing evaporator with a larger unit or take another direction. 

Objectives

• Eliminate hauling of waste water by installing a system to provide a ZLD (Zero Liquid Discharge) for the plant. Evaporation and Membrane technologies will be considered.
• Improve rinse for the coils so that the carryover of contaminants does not create re-work of coils. Contaminants were measured in Fe, TDS, and pH (acidity).
• Lower the operating cost of rinse handling.

Solution

After consideration of the capital and projected operating costs of the two competing technologies, Beta and Galvanizer ABC worked together to install a membrane-based recovery system that provides a cleaner rinse while minimizing the volume of water usage. The system and the final pipe and utility installation were completed in January 2009. The system completed successful start up by the end of February 2009.

Description of Process

The Rinse Membrane Recovery System from Beta Control Systems creates a constant clean and reuse scheme that continuously removes contaminants from the first drag out rinse tank (Rinse #1). The system removes the iron and acid from the solution, generating a clean permeate stream. The permeate stream is used as water makeup for the tank and/or as an exit spray rinse. When the permeate stream is used as an exit spray rinse, a pump pushes it into the Booster Tank, at approximately 70 p.s.i.g (4.8 bar). This pressurized reservoir of recovered water is then used to spray the coils as they leave the first drag out rinse.
 

The clean, high pressure exit spray keeps the majority of the iron and residual acid in Rinse #1 as well as minimizes the carryover of contaminants (pH 2.1, 21K TDS) to the final rinse tank. It also keeps the coils wet to avoid flash drying and rusting. As the coils enter Rinse #2, they record very low iron and acidity and are ready for final cleaning.

The Rinse #2 station requires a two step process. The coils are first blasted with a programmed spray of permeate to further clean the coils and minimize any contaminants nested in the coils. After a 10 second, 70 p.s.i.g. spray, the automated process waits 10 seconds to allow the spray to drip from the coils. A final spray of city water cleans the coils before they proceed to the coating tank.

This process virtually eliminates iron carryover to the coating tanks. Analysis of the water collected from the coil as it leaves the final rinse shows the iron levels as not detectable using the <0.1% test range of the pickle house lab. The TDS level is below 1400; the pH averaged 2.9. There was no sign of orange staining of the Lime tank as the coils were immersed into the tank.
 

Energy & Labor Costs

At current energy costs in the USA, the steam consumption for an evaporator will cost about $300 per 24 hour day for a 170 coil production day ($1.75 per coil for steam)¹. Beta’s membrane-based system uses electricity to run the pump and no chemicals so the cost of operation is $15 per day ($0.09 per coil). Both systems require approximately the same number of man-hours of attention. Even with annual membrane replacement, it is clear that the membrane system is less expensive to operate.
 

Summary

• TDS drainage below 1400 after final 10 second spray rinse (>3000 at 12 sec.)
• Iron was not detectable in the pickle lab analysis
• pH 2.9 was the average drainage acidity
• No coils were rejected during the week of running with the outside temperatures in the 92 ºF (34 ºC) range. No sign of “flash rust” in the rinse cycle was reported.
• The coils were tested with a 10 second permeate spray exiting from Rinse #1, followed by a 10 second permeate spray in Rinse #2, a 10 second drain delay before a final 10 second fresh water spray.
• An average of 8.4 gallons of fresh city water was used for the final rinse of the coil during the test period. Using the 170 coils per 24 hour day, yields 1,428 gallons per day sprayed on the coils.
• Old Method: $1.75 per coil cost for evaporation. New Method: $0.09 per coil for membrane recovery. (90 gallons per hour)

The addition of the Beta Rinse Recovery System has proven to be the most cost effective solution to the production problem. After six months in service, the coil rinse quality has continued to surpass past performance and no rejects have been reported due to rinse failure.

The water usage has dropped proportionately. The pickle house crew currently has to add additional city water to the tanks since the consumption during rinse is less than the sum of crystal removal and pickle tank evaporation. There is currently ZLD (zero liquid discharge) due to the rinse process.

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¹ Evaporator Thermodynamics, as reported by Galvanizer ABC:
Flow rate: 1.5 gpm Mass flow rate: 747.3498 lb/hr
Fluid temperature: 90 degrees F Enthalpy: 57.68263 Btu/lb
Vapor temperature: 212 degrees F Enthalpy: 1151.002 Btu/lb
Power required to evaporate water at flow rate: 817,092 Btu/hr
Cost per kWh: $1.51 per therm
Total cost per hour: $12.34
Total cost per 24-hour day: $296.11
Assumptions:
Ambient pressure is close to seal level.
Fluid is an aqueous solution of mostly water.
BP elevation due to salts is minimal.
Specific Vol: 0.016099 ft3/lb