A Silica/Fly Ash-Based technology for Controlling Pyrite Oxidation. Page: 2 of 22
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1. Overall Objective:
To develop methodologies by which sodium metasilicate or fly ash may produce an
effective coating on pyrite surfaces for inhibiting pyrite oxidation.
2. Broad Objective:
a. To evaluate the mechanisms by which a stable silica coating can be produced on the
surface of pyrite/pyritic coal waste using reagent grade sodium silicate.
b. To evaluate alternative oxidants (Na - hypochlorite or Ca - hypochlorite vs. hydrogen
peroxide) and pH buffers (acetate vs. bicarbonate) for the most cost effective silica coating
c. To evaluate the most effective method (based on results from "objective a and b") in
establishing silica coatings employing fly ash as silica source.
d. To evaluate durability of coating produced a) by sodium metasilicate, and b) by fly
ash; compare coating performance results with results obtained from control treatments and
traditional limestone treatments.
First six-month: Characterize pyrite surface reactions for understanding pyrite coating
establishment. Start a preliminary outdoor leaching - column experiment using 10 kg mine pyritic
spoil treated with silicates to evaluate potential application of coatings on a large scale.
Second six-month: Characterize silicate - iron reactions in solution and on pyrite surface for
understanding pyrite silica - coating formation.
Third six-month: Evaluate pyrite surface deposition of silicate having Na - silicate or fly
ash as source.
Fourth six-month: Evaluate silicate coating durability in large outdoor columns.
Pyrite in coal waste is initially oxidized by atmospheric 02, releasing acidity and Fe 2+ and
is the main source of acid mine drainage (AMD). At pH around 3, Fe2+ is rapidly oxidized by T.
ferrooxidans to Fe3+ which oxidize pyrite at a much faster rate than 02 (Singer and Strumm,
1970). Commonly, the approach used to control AMD in the field involves the use of limestone.
This approach, however, has a short span of effectiveness (Evangelou, 1995 and references
therein). The objective of a study carried out in our laboratory was to demonstrate that pyrite
oxidation could be controlled or inhibited through the formulation of an iron - oxide - silicate
coating on the surface of pyrite which would prevent either 02 or Fe3+ from further oxidizing
pyrite. Silica solubility is relatively constant for a wide pH range while relatively strong short-
range interactions between silicate ions and Fe3+ may make the latter (Fe3+) unavailable to
electron transfer processes (U. S. Gov., 1969). Ferric hydroxide-silica coating formation involved
leaching pyrite at room temperature with a solution containing H202,, NaOAc, and soluble
silicate at a flow rate of 0.43 ml min-I (Zang and Evangelou, 1998). It was postulated (based on
the results of our previous studies and on reported solubility behavior of silica in the presence of
Fe3+ (Iler, 1979)) that when H202 would reach the pyrite, its outermost layer would oxidize and
release Fe3+ which in the pH range of 4-6 would form an ferric hydroxide coating with some
form of polymerized silica. The ferric hydroxide-silica coating would be expected to significantly
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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/m1/2/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.