(669f) Aluminum Solubility: Impact of Sodium Nitrate, Sodium Nitrite, and Sodium Carbonate in the Presence of Sodium Hydroxide

The stability of waste streams containing aluminate is a significant factor relating to unit operations (ion exchange) within the Waste Treatment and Immobilization Plant (WTP) under construction at the Hanford site. At an operational cost of $500M/yr for the WTP, a loss of one day of operation would amount to $1.37M. The overall recovery cost will be a function of the amount of gibbsite [Al(OH)₃] that forms. For small loadings where additional caustic leaching at elevated temperature will restore operation, the time delay may be five days. At significant precipitation where 50% or more of the interstitial volume within the ion exchange (IX) column is blocked, options will include re-insertion of new resin or extended leaching. A conservative estimate would then be from $6.6-41M/year.  Thus, it is imperative that aluminum solubility be reliably predicted by available process simulation software.

The Environmental Simulation Program (ESP, OLI Systems, Inc.) is used for prediction of the electrolyte chemistry in these high ionic strength solutions.  Since 1997, the Process Waste Chemistry team at the Institute for Clean Energy Technology (ICET, Mississippi State University) has undertaken to improve process waste chemistry predictions through measurement of the solubility of relevant systems under conditions of high ionic strength and at elevated temperatures.  The efforts described herein were carried out under this program.

A test plan was written, designed, and reviewed in 2009 to identify the influence of sodium, hydroxide, nitrate, nitrite and carbonate on aluminum solubility as a function of temperature.   Sodium hydroxide solutions were prepared over a range of 0.5 to 5.5 m.  Gibbsite was used as the aluminum source.  Two sets of samples were prepared at each nitrate, nitrite and carbonate loading.  One set was allowed to equilibrate undisturbed.  The other set was used to identify when equilibration had been achieved, through weekly sampling of the samples.  Solubility experiments were conducted at three temperatures (25, 40, 55 °C), encompassing the expected range of operating temperature to be realized in the WTP.

The measured data in the form of molality for aluminum, hydroxide, carbonate, nitrate and nitrite were regressed using the Environmental Simulation Program (ESP V8.2) and the Mixed Solvent Electrolyte (MSE) model.  Like-ion interactions not already present in the MSE Public (MSEPub) database were considered for data representation.  Regression results will be presented along with the experimental data.