Environmental Engineering Reference
In-Depth Information
FIGURE 17.1
Typical Weathering Profi le with
Corresponding Ore Types
Ore
Boundary
Transition
Ore
Fault Zone
Maximum Water
Table Level
Oxide Ore
Minimum Water
Table Level
Base of Main
Oxidation
Sulphide
Ore
Base of Partial
Oxidation
Fault Zone
Natural groundwater in contact with sulphidic mineralization is commonly acidic,
particularly in the unsaturated zone where oxygen is also present; such water may lead to
acidii cation of surface streams. For example, near the discovery outcrop of the Batu Hijau
copper porphyry ore body on the island of Sumbawa in Indonesia, there was a perennial,
spring-fed stream known as Air Merah (meaning 'red water') with a pH of 4 and anoma-
lously high concentrations of manganese and copper.
More commonly, the rate of natural oxidation is low, and the acidic groundwater may
become diluted and/or neutralized by the time it reaches the surface drainage system.
In such cases, the only evidence that sulphide oxidation has taken place will be the rela-
tively high concentration of dissolved sulphate.
Development of ARD is a three stage process involving oxidation; leaching; and drainage.
Development of ARD is a three
stage process involving oxidation;
leaching; and drainage.
Oxidation
Pyrite, pyrrhotite, and some other sulpide minerals react with oxygen in the presence of
water, according to the following reaction (for pyrite):
→
FeS
2
15/4 O
2
7/2 H
2
O
Fe(OH)
3
2 H
2
SO
4
(17.1)
The reaction is complex, involving chemical, electro-chemical and, frequently, micro-
biological processes. The oxidation rate is highest at low pH where it is catalyzed by the
bacterium
Thiobacillus ferro-oxidans
. Commonly, oxidation proceeds slowly for months or
years with steady decline in pH until the pH falls declines below about 3.6, when there is
a sudden acceleration in the rate of oxidation. The sulphuric acid formed is itself highly
reactive and will react with adjacent minerals, dissolving metals and forming a variety of
sulphate salts. For example, if calcite is present, gypsum (Ca
2
SO
4
2H
2
O) may form. A
variety of more or less acidic compounds such as jarosites are commonly formed, which
may crystallize to i ll cracks and pores in the rock or form coatings, encrustations or efl o-
rescences on rock surfaces. These hydrated sulphates of iron usually combine with varying
proportions of other cations, and are usually readily soluble in water.
Leaching
Oxidation of sulphide minerals in itself, does not necessarily pose a problem. It is only
after the acidic oxidation products are l ushed from their source or dissolved in water
(the process known as 'leaching') - with the resulting leachate solution discharged to the
environment - that adverse effects may occur. Most leaching occurs in response to rain-
fall, although the appearance of ARD may occur some time after the responsible event.
Leaching of acid and salts in an open pit occurs as rainfall runoff l ows over pit slopes.
Oxidation of sulpide minerals in
itself, does not necessarily pose
a problem.
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