A Silica/Fly Ash-Based technology for Controlling Pyrite Oxidation. Page: 3 of 22
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reduce the rate of pyrite oxidation most likely because of the coating's ability to serve as a sink for
Fe3+, and/or the coating's ability to act as a barrier to H202 and other oxidants. The bench-scale
study showed that formation of a ferric hydroxide-silicate coating on the surface of pyrite was
induced and inhibited pyrite oxidation (Zang and Evangelou, 1998).
The objective of this study was to test the durability of the silica pyrite coatings under
natural environmental conditions in large leaching columns for possible application in the field.
Materials and Methods
Two types of pyritic waste material were used for this field - leaching column study. One
type represented a fresh, salt free pyritic waste obtained from a metal ore processing plant located
in Dester, Canada, which contained approximately 80% pyrite. Prior to use, this material was
stored under water saturated conditions to create a reduced environment. The pyritic waste was
mixed with sharp sand at a waste to sand ratio of 1:20, giving 4.0% pyrite based on weight. The
second type of pyritic waste represented a highly weathered Western Kentucky coal spoil with
21.4% pyrite content, which was obtained from a site close to Central City in Muhlenberg
County, Kentucky. This spoil was mixed with sharp sand at a 10:1 waste to sand ratio, giving a
19% pyrite content on a weight bases. The percent pyrite present in both of these waste materials
was determined by oxidizing the pyrite in the presence of excess 30% hydrogen peroxide (H202)
and measuring the acidity produced from the pyrite by titrating with NaOH, under nitrogen gas, to
pH 7 (O'Shay et al., 1990). We used the Canadian mine tailings because it was free of salts that
may have interfered with the coating process. The Muhlenberg spoil, in contrast, had a high salt
content (see results and discussion) and was selected as a good representation of what may be
encountered at a reclamation site. The total soluble salt concentration was determined by carrying
out water saturated paste extracts on the spoil and using the extracts for chemical analyses (U.S.
Salinity Lab. Staff, 1954). In addition, successive extractions by water and 1M KCl were carried
out and these extracts used to determine solution and exchangeable acidity, water extractable
chemical species, and cation exchange capacity (CEC) of the spoil (Thomas, 1982).
The field - leaching study consisted of three groups of leaching columns. One group
included enough agricultural limestone to neutralize approximately 113.5% of the potential acidity
in the pyritic material while the other two groups used only enough limestone to neutralize
approximately 9% of the potential pyritic acidity. Under field conditions, enough limestone is
used to neutralize one hundred percent of the potential pyritic acidity; therefore, we used close to
this amount in one group of our field columns to approximate those conditions. Only enough
limestone to neutralize 9% of the potential pyritic acidity was used in the other two column
groups so that we could quickly distinguish a coating effect from a limestone neutralization effect
on pyrite oxidation. The 113.5% neutralization group and one of the 9% groups used the
Canadian mine tailings as their pyritic material while the other 9% group used the Western
Kentucky mine spoil as the pyritic material source. Each of these field - leaching column groups
consisted of a control, silica coating treatments, and limestone treatment. In addition, columns
that combined limestone with each individual component of the coating process (calcium
hypochlorite (Ca(OCl)2), sodium acetate (NaOAc), and silica (Na2SiO3)) were included so that
these components could be evaluated separately. All of these columns were in duplicates.
Calcium hypochlorite was substituted for the H202 used in earlier studies because it was
considered to be a more stable oxidizer in the open environment than H202.
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A Silica/Fly Ash-Based technology for Controlling Pyrite Oxidation., report, September 21, 1997; United States. (digital.library.unt.edu/ark:/67531/metadc691633/m1/3/: accessed September 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.