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The interaction of partially weathered sulphides in the Mt McRae shale formation with ammonium nitrate

Rumball, John Alfred (1991) The interaction of partially weathered sulphides in the Mt McRae shale formation with ammonium nitrate. PhD thesis, Murdoch University.

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Spontaneous explosion of ANFO charged in partially-weathered sulphides and the spontaneous combustion of these sulphides are serious hazards at the Mt Whaleback mine in the Pilbara region of Western Australia. The potential exists for the problems to become equally serious at Tom Price, Paraburdoo and other iron ore mines operating in the Brockman Iron Formation.

The partially weathered sulphides occur in the Mt McRae Shale Formation, which is stratigraphically below the iron orebody in these mines. Folding and slope stability considerations necessitate the mining of large volumes of this shale to access the stratigraphically younger ore.

The only sulphide mineral of significance in the shales is pyrite. Pyrite occurs as micron sized pyritohedra, nodules, veins and sub-hedral clusters. Graphite exists as a fine-grained, poorly crystalline phase that forms a continuous network imparting high electrical conductivity to the shale. Chert and 10 A sheet silicate minerals are abundant, but appear to be relatively inert.

The shales are strongly acidic (0.6 moles of H+/kg) and contain ferrous sulphates (0.07 moles of FeSO4/kg). These salts are the result of partial oxidation of pyrite deep below the zone of obvious weathering, a process made possible by the electrical conductivity of the shales. The acidic iron sulphates are retained within the shale by adsorption on the graphite.

The spontaneous explosion of ANFO results when AN comes into contact with partially weathered sulphides. A detonation will occur if there is sufficient ANFO confined in an environment such as a stemmed blast-hole.

Spontaneous combustion results from the same series of reactions that initiate an explosion. In this instance, however, the heat from the interaction between the partially weathered sulphides and AN ignites the shale rather than initiating the explosive decomposition of AN. The amount of AN required to initiate spontaneous combustion is dependent on conditions, but may be less than 25 g. Such a small amount of AN may readily accumulate in a mining environment due to the quantity used in blasting and the high mobility of AN.

The reaction between partially weathered sulphides and AN consistently shows four stages:
Initial Stage
Sulphuric acid derived from partial oxidation of pyrite during incipient weathering, interacts with the AN to generate nitric acid, which is reduced to nitrous acid by a slow reduction reaction: NO3- (aq) + 3H+ (aq) + 2e- → HNO2 (aq) +HNO2 (aq) + H2O(1)(Exothermic, ∆H° = -200 kJ mol-1)
Oxidation of ferrous ions adsorbed on graphite completes the redox couple.
This reaction may take place as a galvanic process with reduction and oxidation occurring on cathodes and anodes of graphite.
Intermediate Stage
When nitrous acid reaches a concentration that is sufficient for it to act as a catalyst the rate of the Initial Stage reaction increases dramatically. The nitrous acid formed rapidly oxidises the ferrous ions and pyrite forming nitric oxide, ferric, ferrous and sulphate ions:
HNO2 (aq) + H+(aq) + e- → NO(g) + H2O (1) (Exothermic, ∆H° = -76 kJ mol-1)
Fe2+ (aq) → Fe3+ (aq) + e- (Endothermic, ∆H° = +41 kJ mol-1)
FeS2 (s) + 8H2O(1) → Fe2+ (aq) + 2SO42-(aq) + 16H+(aq) + 14 e- (Endothermic, ∆H° = +557 kJ mol-1)
Although the oxidation reactions are mildly endothermic, the reduction reaction is strongly exothermic, hence the redox pairs generate heat.
Ignition Stage
Nitrogen dioxide, produced from the atmospheric oxidation of nitric oxide, together with nitric oxide and heat initiate the violent exothermic decomposition of AN at temperatures below 100°C:
NH4N03 (s) → N2O (g) + 2H2O (g) (Exothermic, ∆H° = -36 kJ mol-1)
2NH4NO3 (s) → 2N2(g) + O2 (g) + 4H2O (g)
(Exothermic, ∆H° = -236 kJ mol-1)
Combustion Stage
The high temperatures attained by the Ignition Stage cause the pyrite and graphite in the shale to combust generating sulphur dioxide and carbon dioxide respectively:
2FeS2 (s) + 11/2O2(g) → Fe2O3(s) + 4SO2(g)
(Exothermic, ∆H° = -1655 kJ mol-1)
C(s) + O2(g) → CO2 (g)
(Exothermic, ∆H° = -394 kJ mol-1)

Alkali and surfactant chemicals may be used to inhibit the interaction between partially weathered sulphides and AN.

Before they may be used to prevent spontaneous explosions further development is required to blend the inhibitor within prilled, emulsified or slurried AN explosives. Simply mixing the inhibitor with coarsely prilled AN does not reliably inhibit potentially reactive pockets of AN that may be smaller than 25 g. In the interim, tough and durable liners are used at Mt Whaleback to separate physically the partially weathered sulphides and AN.

To minimise spontaneous combustion a slurry of limestone in a short chained anionic sulphonate and water may be applied. The limestone is cheap and easy to handle while the short tail group on the surfactant is water soluble, the anionic head group adsorbs to active cathodic surfaces and the sulphonate is acid resistant.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Thornber, M.R. and Davidson, Lawrence
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