The elemental compositions of the salt mixtures were analysed with EDX before and after the tests and have been gathered in tables for comparison purposes.
The tables with the atomic percentage for each salt element (namely K, Na, Cl, S and O) are enclosed in Appendix B.
Composition (in atomic percentage) of the salts before Test 1 was calculated according to their molar ratio and six salts were chosen to be analysed with EDX in order to compare the accuracy of the calculations. The difference between the calculated and measured by EDX values was in the range of 2.5 – 25.7 at% for K, 0 – 14.2 at% for Na, 2.5 – 21.4 at% for Cl, 0 – 5.3 at% for S and 4.9 – 39.8 at% for O. For three other tests (Test 2 – 4), the elemental compositions before the exposures were analysed only with EDX. For Test 3 (where 12 mixtures were analysed in two sets at the same time), the elemental composition before the test was analysed for only one set of mixtures
(assuming that the other set would have the same composition). However, 3 salts from the second set were also analysed with EDX in order to confirm the credibility of the analysis. Similarly, for Test 4 (two sets of 6 chloride-based mixtures) only one set was characterised with EDX before the test. The values for set no 2 were assumed to be the same.
The ratio between Cl and S was calculated for each salt before and after Tests 1 - 3 and these data are presented in Figures 4.9 - 4.12. The ratio was not specified for Test 4 because only chlorides were analysed in that exposure.
The below formula was used to calculate the error for the Cl and S ratios:
𝑎 = 𝑐+𝑑𝑏 (4.1)
∆𝑎2 = 𝑎2((∆𝑏𝑏)2+(∆𝑐)(𝑐+𝑑)2+(∆𝑑)2 2) (4.2) Where a is either the Cl or S ratio, b – 0.5Cl (at%) or S (at%), c – 0.5Cl (at%), d – S (at%) and Δ – the EDX error.
The accuracy of the EDX analysis (described in section 3.4.6) depends on the chemical element. For instance, in the case of nickel (50 at%) the error is number of Cl and S). As an example, the errors for two salts exposed in Test 1 are presented. In the case of mixture 2 (80% KCl + 20% NaCl) after the exposure, the errors for the Cl and S ratios were calculated and are as follows:
0.01 ± 3.2×10-3 and 0.99 ± 0.09 respectively. In the case of mixture 7 (80% KCl + 20% K2SO4) before the exposure, the errors for the Cl and S ratios were calculated to be: 0.8 ± 0.04 and 0.2 ± 0.05 respectively.
Figure 4.9 presents the ratio S/Cl+S as a function of Cl/Cl+S for the salts exposed in Test 1 (at 600ºC). Blue dots indicate the atomic ratio before the
exposure (calculated values), whereas the red crosses indicate the ratio afterwards (EDX measured values). The formula used to calculate the ratios was S/(0.5Cl+S) and 0.5Cl/(0.5Cl+S) in order to recompense the content of chloride and sulphur. For chlorides before the test the ratio of Cl to (Cl+S) was 1:0 (first bottom point on the graph) and then gradually increased towards the left top side of the graph to reach 0:1 point at the very top of the graph (100%
sulphates). The red crosses located at the top left side of the graph represent the Cl and S contents remained after the exposure. It can be seen that the amount of Cl decreased a lot leaving about 0.1 - 0.2 value, whereas the S content was greater than 0.8 with its highest number at 1.
Figure 4.9 S/Cl+S and Cl/Cl+S ratios for the salts exposed in Test 1
Figure 4.10 shows the function between the S/Cl+S and S/Cl+S ratios before and after Test 2 (at 550ºC). Similarly to Figure 4.9, blue dots represent the atomic ratio before the exposure (calculated values), whereas the red crosses indicate the ratio afterwards (EDX measured values). In this test only two salts showed the residues of chloride left. This was for deposits that were initially 40% KCl + 60% NaCl (0.06 Cl:0.94 S) and 100% NaCl (0.77 Cl:0.23 S). These two salts were only partially converted to sulphates, whereas the rest of the 20
mixtures had the ratio 0 Cl:1 S, which means that they were fully converted to sulphates.
Figure 4.10 S/Cl+S and Cl/Cl+S ratios for the salts exposed in Test 2
Figure 4.11 and Figure 4.12 present the Cl and S ratios for the salts exposed in Test 3. In this test two sets of 12 salt mixtures were exposed at the same time (Table 3.1, section 3.1.2). A significant difference can be seen between this test and two previous ones. The chlorine content for the salts in this test was higher than for their equivalents exposed in Test 1 and 2 (where SO2 was present in the gas atmosphere).
Only two salts from set 1 (Figure 4.11) were fully converted into sulphates (these with initial compositions of 80% KCl + 20% NaCl and 20% KCl + 80%
K2SO4) and 90% of another one was fully converted (originally 60% KCl + 40%
NaCl). For the rest of the salts from set 1, the Cl:S ratio was between 0.1:0.9 and 1:0. One salt (initially 20% KCl + 80% NaCl) did not change its ratio (1:0), moreover, for three other salts (initially 100% NaCl, 80% KCl + 20% K2SO4, 100% KCl) the Cl:S ratio remained quite high after the test and it was 0.87:0.13, 0.72:0.28 and 0.6:0.4 respectively.
Figure 4.11 S/Cl+S and Cl/Cl+S ratios for the salts exposed in Test 3 (12 salts from set 1)
Figure 4.12 S/Cl+S and Cl/Cl+S ratios for the salts exposed in Test 3 (12 salts from set 2)
In set 2 (Figure 4.12) none of the salts was fully converted into sulphates. The Cl:S ratio was higher than before the test for four salts (60% KCl + 40% K2SO4,
40% KCl + 60% K2SO4, 20% KCl + 80% K2SO4 and 100% K2SO4), whereas for three other salts (initially 100% KCl, 20% KCl + 80% NaCl, 100% NaCl and 80% KCl + 20% K2SO4) the ratio between Cl and S remained the same and it was 0.99:0.01, 0.98:0.02, 1:0 and 0.87:0.13 respectively. For one salt (initially 40% KCl + 60% NaCl) the Cl:S ratio was a bit lower than before the test (0.82:0.18), whereas for two mixtures (originally 80% KCl + 20% NaCl and 60%
KCl + 40% NaCl) the ratio was much lower (0.12:0.88 and 0.49:0.51 respectively).
Scanning electron microscope was used to get images of the salt crystals before and after the exposures. Figure 4.13 below depicts typical salt crystals being exposed in Test 2. Salts that were chosen to present are those with initial compositions of 80% KCl + 20% NaCl (two top pictures) and 25% KCl + 10%
NaCl + 25% Na2SO4 + 40% K2SO4 (two bottom pictures). Left column represents crystals before the test, right column after the test. The pictures were aimed to be taken in the centre of the disc for comparison purposes.
For the first mixture, 80% KCl + 20% NaCl, the crystals, before being exposed in the furnace (first top left image), had clearly visible edges, each crystal can be seen separately as a cube with the average size of ~48 - 67 µm. However, there are also clustered crystals visible in the picture. After the exposure (top right image), the salt crystals are no longer cube-shaped and are smaller. It looks like they clustered together to a greater extent than before being exposed.
For the second mixture of KCl, NaCl, K2SO4 and Na2SO4, crystal shape before the test was not cubic. The size of the majority of crystals was about 20 µm, although there are some bigger crystals visible around the corners of the image.
The shape of the crystals changed after the test completely. Their surface looked melted (streamlined shape), clustered into dome-shaped surfaces.
However, this only applied for small crystals (between ~20 – 96 µm), it can be seen that a big crystal (seen in the right bottom corner of the picture and probably larger than ~100 µm) did not look as melted as the small crystals did.
Figure 4.13 Electron images of typical salt crystals. Left column shows pictures before exposure, right column shows images after exposure