Page 314 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

292 CORROSION CONTROL AND PREVENTION

4.26 CORROSION AND ITS PREVENTION IN THE MINING INDUSTRY

The corrosive environment of the mining industry limits the life span of the
processing equipment and as a result decreases production and endangers lives of
personnel. Some examples of mining equipment prone to corrosion are (i) wire rope;
(ii) roof bolts; (iii) pump and piping systems; (iv) mining electronics; and (v) acid
mine drainage.
Wire ropes are used extensively in the mining industry to help hoist equipment.
Mine workers also depend on the rope for safety. Wire rope undergoes both corrosion
and abrasion, which degrade the mechanical properties of the wire and reduce its
load-carrying strength and cause its failure. About 66% of the ropes lost strength in
the portion of the rope in contact with the shaft environment. Wire ropes need to be
periodically examined for structural damage, corrosion, and improper lubrication or
dressing. Nondestructive testing of the ropes must be done every 6 months. These
ropes are replaced every 18–36 months. These are made of carbon steel but are being
replaced with stainless steel and synthetic fiber ropes as carbon steel ropes are prone
to corrosion.
Roof bolts provide support in underground mines by tying the lower layer to a
stronger layer located above the main roof. These are low-carbon steel bolts and about
120 million/year, and corrosion failure of the bolts is hazardous and can result in loss
of lives. In sulfide mines, roof bolts fail within a year because of sulfide SCC.
Corrosion within pump and piping systems is another problem, and general uni-
form attack is common. Pitting, crevice corrosion, intergranular corrosion, dealloy-
ing, galvanic corrosion, and cavitation corrosion are also possible depending on the
environment.
Erosion–corrosion in milling is another problem. Particulates in a corrosive
medium go through pipes, tanks, and pumps. The particulates erode and remove
the protective film on the metal and expose the metal surface to high velocity
impingement, thus accelerating corrosion.
Harsh environments in the mines can cause electrical equipment to fail after a short
time and it can be avoided by a suitable enclosure.
When pyrite (FeS ) and other sulfide minerals are oxidized by exposure to oxygen
2
and water, ferrous ions and sulfuric acid are produced. The ferrous ions in turn are
converted to hydrated iron oxide and more acid. The pH of the water drops and leads
to corrosion of the metal. Thus the pipes, well screens, dams, bridges, water intakes
are attacked by acid mine water.
Material selection is the most important general procedure for corrosion preven-
tion. Material selection, protective coatings, corrosion inhibitors, and electrochemical
techniques such as CP are some of the corrosion control methods that can be used to
alleviate corrosion problems.
Cost–benefit analysis associated with four organic coatings used in the mining
industry with surface preparation such as surface blasting or no surface preparation
led to the finding of the data included in Table 4.44.