3.3.1 Fresh ash samples
Seven pristine volcanic ashes were used in this study. The volcano, eruption dates and compositional information are listed in Table 3.2. All ashes were stored in dry conditions within sealed polyethylene bags since collection. Non-essential movement was minimised to reduce modification of ash properties.
Freshly fallen volcanic ash loses its soluble content rapidly in the presence of moisture (such as rain or wet soil) so the collection of fresh volcanic ash must be done shortly after an eruption and adequately stored to avoid leaching of the soluble content. The low frequency of explosive eruptions and logistical difficulties of collecting pristine volcanic ashes meant we only had access to seven ashes (Table 3.2) collected following seven different eruptions. Whilst useful to establish the electrical properties of these particular ashes, it does not allow in-depth analysis of the influence of different grain sizes, soluble salt loads, soluble salt chemistries, bulk densities and grain morphologies which can vary significantly between and within explosive eruptions, on the electrical properties of the ash. It was therefore necessary to develop a pseudo ash which replicates the physical, chemical and electrical properties of freshly fallen volcanic ash, but could be manufactured to specific parameters.
Table 3.2: Sources of the seven fresh ash samples used in this study. Samples were obtained from a variety of locations and cover a vast time range.
Additionally, four different ash compositions were tested during the resistivity analysis.
Sample ID Volcano Country Duration of Eruption Collection Date of
# Days Between Deposition and
Sampling
Approx. Distance From
Source (km) Composition Magma
GRIM-11 Grímsvötn Iceland May-11 22-May-11 1 95 Basalt
EYJA-10 Eyjafjallajökull Iceland Apr-Oct 2010 15-Apr-10 <1 60 Trachyandesite
SHIL-09 Soufriere Hills Montserrat (UK) Jul 1995-Present 27-Nov-09 <1 7 Andesite
RDBT-09 Redoubt USA. Mar-Jul 2009 4-Apr-09 <1 110 Andesite
CHTN-08 Chaiten Chile May 2008-Present 28-May-08 6 90 Rhyolite
MRPI-06 Merapi Indonesia Apr-Jun 2006 27-Jun-06 5 5 Andesite
3.3.2 Pseudo Ash Samples
The replication of both soluble and non-soluble pollution for experimental use in the electrical industry is not uncommon. Kaolin, tonoko and bentonite (among others) have frequently been used in HV insulator contamination testing (e.g. Diesendorf and Parnell, 1974; IEEE Working Group, 1979; IEC 60507, 1991; Sundhar, 1994; Hernandez-Corona et al., 1999; Bennoch et al., 2002; Naderian et al., 2004; Gautum et al., 2006).
Unweathered Stoddart olivine basalt (from Halswell Quarry, Lyttelton volcano, New Zealand) (Guard, 1999) and rhyolite Kaharoa tephra (from the 1314 AD eruption of Tarawera volcano, New Zealand) (Nairn et al., 2004) were used in the creation of proxy ashes. Whole rock chemistry is provided in Table 3.3. Their low and high silica (SiO2) compositions allowed a simple
comparison of whether base rock chemistry influences conductivity. Bulk samples were crushed using a hydraulic press and subsequently milled with a ring pulveriser.
In order to investigate the influence of grain size on conductivity, eight pseudo ashes of different grain size were created. These were created by dry sieving the original crushed and pulverised product. Five grain sizes, <0.032, <0.1, <0.5, <1, and <1.4 mm, were produced to replicate ash deposits with wide particle size distributions. In order to analyse the influence of specific particle size fractions on ash conductivity, three pseudo ashes were sieved to 0.1<x<0.5, 0.5<x<1, and 1<x<1.4 mm.
To replicate the interactions at the ash-gas interface and other processes occurring between ash and volatiles within a volcanic plume, a simplified method of chemical dosing was used to produce soluble salts on the surfaces of the pseudo ash. Either sulphuric acid (H2SO4) or common salt
solution (NaCl) was added to the pseudo ash to replicate the volatiles found on fresh ash. These compounds were chosen because of their high abundance during explosive volcanic eruptions (Rose, 1977; Delmelle et al., 2005; Witham et al., 2005; Delmelle et al., 2007). Approximately 15 cm3
had been prepared to their respective molar concentrations, 5 ml of H2SO4
or NaCl solution was added to each vessel and subsequently stirred to ensure even distribution of the solution throughout the ash. Vessels were then placed in an oven at 85 C for a period of two days to evaporate the water from the slurry and expedite the formation of soluble salts. After one day of drying, a hard crust developed on the ash. To continue the drying process, it was necessary to gently break up and mix this crust using a plastic spatula to allow underlying moisture to evaporate.
Table 3.3: Whole rock chemistry for the Kaharoa tephra rhyolite (Nairn et al., 2004) and
the Stoddart olivine basalt (Guard,1999). Given the different constituents minerals present in both pseudo ash rock types, we were interested to see whether whole rock chemistry would influence ash resistivity.
Kaharoa rhyolite Stoddart olivine basalt Wt.% SiO2 77.89 47.75 TiO2 0.1 2.31 Al2O3 12.56 14.78 Fe2O3 T 1.04 12.28 MnO 0.06 0.16 MgO 0.07 7.36 CaO 0.71 10.32 Na2O 3.73 3.12 K2O 3.82 1.16 P2O5 0.002 0.56 ppm V 4 207 Cr <3 255 Ni <3 124 Zn 29 95 Zr 89 187 Nb 7 58 Ba 941 417 La 25 19 Ce 54 62 Nd <10 28 Ga 11 20 Pb 16 3 Rb 125 30 Sr 50 610 Th 12 4 Y 29 26
A range of H2SO4 and NaCl molar concentrations were used, as it was
unclear how much would be absorbed by the ash, react with the ash surface, or evaporate during the drying process. For the moisture content analysis, molar strengths 0.02, 0.18, 0.46, 1.81 and 9.19 M of both, NaCl and H2SO4,
were prepared for the ash dosing procedure. Early results obtained from ash dosed with these solutions suggested that molar strengths >0.46 M were excessively high, thus the resistivity analysis employs concentrations no greater than 0.46 M. Table 3.4 shows the volumes and pH levels of the prepared dosing agents relative to their molar concentrations.
Table 3.4: Volumes, Wt.% and pH levels of the prepared dosing
agents relative to their molar concentrations. The maximum molarity used during the resistivity analysis was 0.46 M while the resistance measurements taken during the moisture content analysis employed concentrations up to 9.19 M. Volumes Wt.% Molarity (M) pH H2SO4 0.20 ml H2SO4 + 199.8 ml H2O 0.1 0.02 1.74 2 ml H2SO4 + 198 ml H2O 1.0 0.18 0.74 5 ml H2SO4 + 195 ml H2O 2.5 0.46 0.34 19.70 ml H2SO4 + 180.3 ml H2O 9.9 1.81 -0.26 100 ml H2SO4 + 100 ml H2O 50 9.19 -0.96 NaCl 0.21 g NaCl + 200 ml H2O 0.1 0.02 7 2.15 g NaCl + 200 ml H2O 1.1 0.18 7 5.38 g NaCl + 200 ml H2O 2.7 0.46 7 21.16 g NaCl + 200 ml H2O 10.6 1.81 7 107.41 g NaCl + 200 ml H2O 53.7 9.19 7
In order to evaluate a more chemically complex acid dosing solution, waters from the crater lakes of Mt Ruapehu (central vent; pH 1.12) and White Island (pH -0.37) were also used. These waters have been enriched with soluble products from the active hydrothermal systems of each volcano and contain common volcanogenic elements which might be expected to leach from fresh volcanic ash (Table 3.5).