(66d) Dissolution of Aluminum in Sludges and Its Effect on the Settling Rates of the Residual Sludges

A study has been undertaken to evaluate the impact of the settling rates of residual sludges from prior dissolution in caustic solutions. This study is part of the sludge mass reduction program at Savannah River Site (SRS). The sludge mass reduction program seeks to dissolve the aluminum in sludges with the overall goal of reducing the amount of sludge sent to Defense Waste Processing Facility (DWPF) for vitrification. A reduction in the mass of sludge vitrified will in turn reduce the number of glass canisters produced.

One of the post-dissolution operations is sludge settling. Its rate affects the post-dissolution operations. For proper operational planning, it is important to know the settling rate of the residual sludge. The study involved batch kinetic dissolution tests followed by settling rate measurements of the residual sludges.

The study used two sludge simulants mimicking SRS Tanks 13 and 15. The two tanks were chosen because their aluminum contents are at the ends of the aluminum composition spectrum of the tanks (12, 13, 15, 32, 35, and 39) of interest in the sludge mass reduction program.

The dissolution tests demonstrated that for initial solution or supernate OH- concentrations of 4 and 6 M, 78-84 wt% and 76-78 wt% of aluminum is dissolved in 12 days at 80 oC from Tanks 13 and 15 sludge simulants, respectively. It is presumed that all the gibbsite [Al(OH)3] in both sludges dissolved while 67 wt% (Tank 13) and 12 wt% (Tank 15) of the boehmite [AlO(OH)]) dissolved. With the exception of chromium, copper (only Tank 15), phosphorus (only Tank 15), and zinc which dissolved to varying extents, most of the metals in the sludges remained virtually insoluble.

While the overall settling rates (or settling completion times) of Tank 13 residual sludge is primarily independent of prior dissolution periods that of Tank 15 residual sludge depends on prior dissolution periods. For Tank 15 sludge, longer prior dissolution periods give shorter settling completion times. Further, residual Tank 13 sludge settles faster overall than residual Tank 15 sludge. The main reason for all the above settling characteristics is that Tank 13 sludge is mainly composed of heavy iron compounds which remain essentially insoluble during the dissolution process. Tank 15 sludge, on the other hand, is predominantly aluminum (gibbsite and boehmite) which dissolves in the caustic supernate.

For Tank 13, the settling rate of pre-dissolution sludge and post-dissolution (residual) sludge are roughly the same. However, for Tank 15, the settling rate of pre-dissolution sludge is higher than the post-dissolution (residual) sludge. Both pre-dissolution sludges (Tanks 13 and 15) have nearly the same settling rates.

The particle size distribution of post-dissolution/settling sludge is greater than the respective pre-dissolution sludge. This is attributed to aging (i.e., transformation from amorphous to crystalline form, agglomeration, recrystallization, etc.) of the sludges during dissolution and subsequent settling. Besides, the cohesive nature of the residual sludges favors agglomeration. The increase in particle size is more marked in Tank 15 sludge than Tank 13 sludge because Tank 15 sludge has a larger fraction of soluble material than Tank 13 sludge and as result is expected to have more slime.

Compounds identified by X-ray diffraction in both pre-dissolution sludges were gibbsite and boehmite for Tank 13; and gibbsite, boehmite, and calcite for Tank 15. The prepared sludges for both Tank 13 and 15 (excluding the gibbsite and boehmite) were virtually amorphous.

The post-dissolution/settling residual Tank 13 sludge contained boehmite and hematite (Fe2O3). The post-dissolution/settling residual Tank 15 sludge contained silica (SiO2), hematite, boehmite, and possibly thermonatrite (Na2CO3? H2O).

Mass balance of aluminum for both sludges was 3% and 14% for Tank 13 and Tank 15, respectively.