Bell Helicopter Textron Inc.
Bell Helicopter Textron Inc. has replaced lime with magnesium hydroxide as a neutralization agent at our wastewater treatment plants. Bell is saving money, providing a safer working environment for our employees and reducing our annual hazardous waste sludge.
Metal finishing processes at Bell generate a variety of acidic waste streams containing dissolved toxic metals. Before the wastes may be discharged to a sewer or receiving stream, the pH of the solution must be adjusted to an acceptable level and the metals removed. These acidic waste streams are commonly neutralized by adding caustic soda or lime, both are strong bases. Magnesium hydroxide, a fairly weak base, has been used for years as a neutralizing agent for acids. It is the major ingredient in such medicines as "Milk of Magnesia", but has rarely been used in wastewater treatment. Magnesium hydroxide can be easily purchased as an aqueous white slurry with a density between 12.3 and 12.5 pounds per gallon. The slurry is 55-60% magnesium hydroxide by weight and is similar to latex paint in consistency and appearance. A convenient way of comparing magnesium hydroxide to lime or caustic soda is to look at neutralization strength on a weight basis. Magnesium hydroxide has 27% more hydroxyl ions than lime and 37% more than caustic soda. Thus, it takes less magnesium hydroxide to neutralize a given amount of acid or to remove a given amount of metals.
Slower reaction time of magnesium hydroxide produces denser solid particles of metal hydroxides than with lime or caustic soda. Magnesium hydroxide is a weak base and forms solutions that are buffered or resistant to changes in pH. Therefore, acids neutralized with magnesium hydroxide form solutions that are buffered within a pH range of 9.0 to 9.4. This means that adding too much magnesium hydroxide will not push the pH above the limits allowed for discharge to most sanitary sewers. The by-products produced when magnesium hydroxide neutralizes sulfuric acid is magnesium sulfate (epsom salts) and is soluble and may be discharged safely to the sanitary sewer. Bell Helicopter's operators found magnesium hydroxide was much easier to use and more predictable than lime. Besides saving time while neutralizing tanks, the lessened volume of hazardous waste greatly reduced the time required to condition and dry the sludge. The only disadvantage noted was a slight increase in the amount of flocculating polymer required.
Details of Reductions
Additional Information :
Bell Helicopter has realized a real reduction in the volume of greater than 50% or 500 tons of hazardous waste sludge per year. This translates to a net annual savings of over $220,000 and greatly reduces our liability due to less sludge being disposed of per year. Bell Helicopter has realized a real reduction in the volume of greater than 50% or 500 tons of hazardous waste sludge per year. This translates to a net annual savings of over $220,000 and greatly reduces our liability due to less sludge being disposed of per year.