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Fluid Journal : Fluid Journal 2008-2009
As detailed above, purchasers should monitor the product they receive and ask questions about the quality of the UAN being purchased. If suppliers can't tell buyers about the quality of the UAN being offered, they may not be monitoring it themselves. Chemical Fluid fertilizer dealers should ask what type and how much corrosion inhibitor is being used in the UAN they purchase. It might be possible to request the treatment rate and brand of corrosion inhibitor be included on the Certificate of Analysis. If purchasing UAN from multiple suppliers or sites, it is possible to end up with UAN containing multiple inhibitors in your storage tanks. Although compatibility among different inhibitors is not an issue, these different inhibitors (now diluted) may not be as effective together as they were by themselves at full strength. Fluid fertilizer dealers should consider purchasing and applying their own corrosion inhibitor programs. Corrosion inhibitor chemicals are inexpensive, compared to the cost of UAN. Corrosion inhibitor programs typically cost $0.60- $0.70 per ton of treated UAN when purchasing in the small amounts likely to be needed by fluid fertilizer dealers. This cost could be much less if the inhibitor choice is matched to the one used by the UAN supplier. Most UAN corrosion inhibitors are easy to apply and safe to use. There are some products, however, which themselves are quite acidic and are inherently hazardous to handle and store. Be sure to discuss this question with potential inhibitor vendors. There are two basic types of UAN corrosion inhibitors. "Filmer" type inhibitors form a protective film on steel. "Passivator" type weight percent of ammonium nitrate divided by the weight percent of urea. In UAN 32, this ratio is typically something like 45%/35% = 1.28 AN/Urea Ratio. The AN/Urea Ratio is important because it greatly affects the temperature at which UAN starts to salt out. UAN 32 typically has a salt-out temperature of about 30o F. But this is only true at the optimal AN/Urea ratio range of about 1.20 to 1.40. At ratios above 1.6 or below 1.0, UAN 32 could start to salt out at temperatures above 45o F or higher. This is important because salt-out means lost nitrogen content. In addition, both AN and urea salts form very aggressive corrosion cells, so they are not a good thing to have sitting at the bottom of a tank (it's like leaving rock salt on the driveway at home). Lastly, both AN and urea salts have a negative heat of solution. This means that large amounts of heat are needed to get these salts back into solution. This may not be a problem when purchasing UAN in the spring, when ambient temperatures are already well above 50o F, and the UAN is to be applied right away. But the AN/Urea Ratio should be considered if one plans to store UAN 32 during cold winter months. There is no easy way for UAN purchasers to measure this parameter, but local suppliers should be able to give customers some typical values, or possibly include this measurement on the Certificate of Analysis, if requested. Try to store only UAN 32. Contrary to what many believe, UAN 28 is more corrosive than UAN 32. Consider purchasing and storing only UAN 32, and then if needed, dilute it with water to 28% just prior to application. Lastly, purchasing quality UAN from a trusted source is probably worth a higher price. inhibitors actually combine with the corrosion products when corrosion is initiated and stop corrosion by "sealing off" the corrosion cell. Both types have advantages and disadvantages and when applied correctly either type will greatly reduce UAN corrosion. Some inhibitor formulations offer additional features such as sludge dispersants, metal chelants, anti-foam additives and tracer compounds. A sales engineer, educated in UAN corrosion inhibitors, can help you decide which type is best for your UAN. Phil Bureman is Nalco's technical consultant for Chemicals and Biofuels and lives in Olathe, KS; pebureman@ nalco.com and Dr. Craig Myers is a research scientist at Nalco's headquarters in Naperville, IL; firstname.lastname@example.org
Fluid Journal 2005-2007