Samples for SPFT were prepared using the HIP as described in Section 3.3 and the relevant technical chapters. For each composition under investigation, samples were synthesised and prepared in triplicate using an identical methodology. After processing in the HIP the welds on the canister lids were removed using a Buehler Abrasimet cutting saw. This allowed for the removal of the sample end caps. The sample was then placed in a bench vice and pressure applied to the canister walls to produce a set of fragments/powder from the monolith via fracturing.
The retrieved sample from each canister was powdered using a hardened steel percussion mortar and sieved to obtain the 106-180 μm powder fraction. The following cleaning regime was utilised to remove residual fines from the powders. The powders were washed three
times with ethanol from a squirt bottle, using three times the volume of powder. The ethanol was then decanted after each wash. The powders were then covered with ethanol and placed in an ultrasonic bath for 2 minutes. This process was repeated in triplicate prior to drying the powders in a 110 °C oven for 24 h. A BET surface area was recorded using a Coulter SA 3100 BET surface area analyser, to allow comparison of the BET and geometric surface areas.
The reactor vessels utilised in the SPFT experiments were 60 ml PFA jars fitted with a 53 mm PFA closure containing two 3.175 mm tube ports. PFA tubing (3.175 mm OD) was used to connect the reactor vessels to both the pump and effluent input/outputs. All PFA based lab wear was provided by Savillex. A Watson Marlow 205S pericyclic pump fitted with an eight station pumping cassette was used to pump the eluent solutions, this allowed for up to 8 solution compositions to be utilised using a single pump.
Prior to beginning the experiment all vessels were cleaned and prepared according to the ASTM PCT standard, as described in [202]. A solution of 1 x 10-4 M HNO3 was used to flush the reactor vessels and tubing for four days to determine if any contamination was present in the vessels or lines and to stabilise the pump flow rate. Three samples were taken during this period and analysed via ICP-MS for contamination prior to insertion of the samples into the reactor vessels. When reactor vessels were not in use, a continuous flow of 1 x10-4 M HNO3 eluent was maintained.
Prior to the insertion of samples in the reactor vessels, 10 L of eluent was prepared for each reactor vessel to be used. Five eluents were used in this study to provide a pH range from 2-10 at 90 °C. These eluents are described in Table 3.1. Un-buffered solutions were selected for pH 2, pH 4 and pH 7 to limit the introduction of solubilising organic materials in the eluent.
The pH change of all eluents was modelled using the PhreeqC geochemical modelling code with the LLNL database to provide the pH at 90°C and is provided in Table 3.1.
A 30 ml sample of the eluent solution was taken for analysis immediately after preparation and 24 hours later. These samples were used to measure the input pH and to prepare ICP-MS samples in order to provide the background concentration of the eluent for blank subtraction.
To start each experiment the reactor vessels were removed from the oil bath, taken apart and the existing eluent discarded. Care was taken to avoid contamination of the vessel interior during sample insertion. The vessel was rinsed with the new eluent and 10 ml of eluent was added to the vessel before the powder sample. This eluent helped to eliminate
static charging in the powders and subsequent ‘jumping’. The samples were inserted into each reactor vessel to obtain the desired log(Q/S) range and resealed. The pump was turned to maximum speed until the first flow from the test vessel was observed, then returned to the desired pumping rate. The pump rate (Q) was set to 2.5 x 10-4 m3 day-1 and the log(Q/S) was varied by alteration of sample mass in the reaction vessel.
The flow rates used and log(Q/S) were determined with reference to data obtained by static experiments and the available literature, to ensure the reaction lay in the forward rate regime, where log(Q/S) < -7.1 and the concentrations in solution would be measurable via ICP-MS. The NRi from the PCT static batch experiments after three days were used with Equation 3.8 to estimate the expected change in concentration during testing.
From this information it was determined that measurable concentrations would be best determined near the point where log(Q/S) = -7.1. The exact experimental conditions used are presented in Chapters 6 and 7.
Buffer solution Reagents to make 10L of solution
Deionised water Deionised water 5.7 - 6.8
(CO2 dependant) geochemical program- database LLNL. **assuming degassing of solution at 90 °C.
During the experiment, the effluent was collected in storage vessels. These were weighed during sampling to determine the flow rate during the experimental period. Samples of ~35 ml of effluent solution were collected. 13 ml of each solution was filtered to determine whether colloids or solids were present in the solution. Both the filtered and unfiltered
samples were prepared and submitted for ICP-MS analysis as undiluted, 1:10 and 1:100 solutions to provide a range of analysis possibilities for the various elements using ICP-MS.
The remaining solution was used to record a pH reading for the sample.
The SPFT set up which was utilised is shown in Figure 3.5.
Figure 3.5 - Images illustrating a) the main stages of solvent flow during SPFT b) the experimental set up utilised and c) the reaction vessels.