Page 184 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 173

process wastewater from phosphoric acid production, basic oxygen furnace and
open hearth furnace air pollution control dust/sludge from carbon steel production,
basic oxygen furnace and open hearth furnace slag from carbon steel production,
chloride process waste solids from titanium tetrachloride production, and slag from
primary zinc processing.
Six groups of wastes are classified hazardous: acid-generating tailings from

processing of sulfide ore, other tailings containing dangerous substances, other

wastes containing dangerous substances from physical and chemical processing of
metalliferous minerals, wastes containing dangerous substances from physical and
chemical processing of non-metalliferous minerals, oil-containing drilling muds
and wastes, and drilling muds and other drilling wastes containing dangerous
substances. The majority of hazardous waste (90 %) are produced by 18 high

productivity industrial units (metallurgy, petroleum refining, chemical industry,
fertilizer industry) while the rest of them (10 %) by about 500 smaller industries.
There are two principal types of solid mine waste: waste rock and tailings.

6.2.8.1 Waste Rock

A mining operation needs to move and dispose of a large amount of blasted rock
that does not have useful concentrations of minerals – this is called “waste rock.”
Waste rock is generated by both open pit mines and underground mines, but the
volume is higher with open pit mines. The amount of waste rock, compared with the
amount of ore is called the strip ratio. A strip ratio of 1 (volume of ore = volume of
waste rock) is considered low for an open pit mine. The largest open pit gold mines
have a strip ratio around 6. Waste rock is typically dumped into large piles within
the mines waste rock storage area, which can spread over an area of several square
kilometers. Both the physical and chemical characteristics of the waste rock must be
considered if it is to be properly disposed of. The coarse texture of waste rock
allows air and water to easily move through the pile. When exposed, waste rock can
be very reactive with the air, water, and microorganisms and may cause acid mine
drainage and release metals to surface and groundwater.

6.2.8.2 Tailings

Modern mines process huge quantities of ore, tens of thousands to hundreds of
thousands of tons a day. After blasting and hauling from the mine shaft of pit, the
ore is crushed and processed using massive volumes of water and a variety of chemical
and physical processes. The mineral content of an ore can be in the 5 % range for
base metals or as low as 0.00005 % for precious metals like gold. This means that
95–99.9995 % of the mined and processed ore becomes a waste product – known as
tailings. Tailings are usually deposited as a slurry – a thick liquid made up of water,
the finely ground ore, and any residual chemicals from the processing stages.

Because the rock has been finely ground, tailings can be very chemically reactive