Title: Removal of Molybdate from Cooling Loops using Ion Exchange Resin
Learner Objectives:
This presentation illustrates how molybdate can be removed from cooling loops or wastewater using ion exchange resin.
Content/Topic Outline:
1. Ion Exchange Theory
Concentration effects
Chromatographic effects
pH effects
Choice of counter ion
2. Sizing a system
Batch process
Recycle process
Loop volume
Molybdate concentration
3. Regeneration strategies
Regenerate for repeated use
Resin disposal
Presenter:
Francis J. (Frank) DeSilva has been employed in the water treatment industry for over 40 years.
Frank has a Master of Science in Environmental Engineering from New Jersey Institute of Technology and a Bachelor of Science in Environmental Science Technology from Florida Institute of Technology.
Frank is a “semi-retired” employee of ResinTech Inc., a supplier of ion exchange resins to the O.E.M. market in the United States. He has been with ResinTech, Inc. for more than 33 years. Prior to that, he was employed by Belco Pollution Control, a division of Foster Wheeler, serving as Manager of the Process Department. He has authored numerous articles on ion exchange and related topics.
Frank is Vice President of the Pacific Water Quality Association and was the Director of the Liberty Bell Corrosion Course in the 1990’s. He currently serves on the AWWA Ion Exchange Standards Committee, the Legislative and Regulatory Committee of the Association of Water Technologies, and as the former chair of the Federal Governmental Relations Committee of the Water Quality Association.
Presentation Description:
Molybdate has been used in water treatment for many years as a corrosion inhibitor. It has become necessary to remove molybdate from most cooling loop blowdown or wastewater prior to discharge. Ion exchange is an effective technology to accomplish this. This presentation will illustrate two practical process schemes used to accomplish molybdate removal in a safe and simple manner.
The element molybdenum (Mo) appears directly underneath chromium on the periodic table which means the two elements have similar properties and react in a similar fashion.
When the use and discharge of chromate was banned due to its very high toxicity, molybdate was initially thought to be a promising substitute. Although soluble molybdate is a good corrosion inhibitor, it requires application at relatively high concentrations, and exhibits moderate toxicity. It is generally too expensive to use in open (evaporative) cooling water systems. Despite drawbacks in open cooling systems, molybdate chemistry has been widely used in smaller closed-loop high temperature cooling systems.
Discharge of molybdate to the municipal sewer is not federally regulated, but its presence in sludges used in Land Applications is limited to 75 mg/Kg (40 CFR 503 – subpart B). Molybdate contaminated soil-applied sewage sludge may result in the production of molybdenum-rich plants, a cause of metabolic disorders in animals. The federal sludge limit has caused some states to prohibit sanitary sewer discharge of molybdate-containing water from cooling towers. Other states have established voluntary limits.
In places where it is no longer permissible to discharge molybdates, it is necessary to remove molybdate from the cooling water prior to discharge or possible reuse. Like chromium, molybdenum forms a divalent “oxy” anion. Because it is a somewhat larger ion than chromate, molybdate is somewhat more preferred by strong base anion resins, although both are preferred over most other anions. The high preference for molybdate makes anion exchange (IX) resin one of the best available technologies for molybdate removal.
The equipment used for this process is most commonly portable fiberglass tanks that are used on an exchangeable basis. The tanks can hold one or several cubic feet of resin, or in the case of large volume batch applications, bigger tanks are used which contain 30 or 40 cubic feet of resin.
Presenter Bio:
I have been involved in a number of molybdate and chromate removal systems that have utilized anion exchange resin in portable fiberglass tanks. Aside from the direct involvement with molybdate and chromate removal, I have studied, written, and presented on the removal of dissolved ions in water for many years.