Hydraulic and Sedimentation Efficiencies of
Tailings Clarification Basins
C. ROBERT BAILLOD. Assistant Professor
Department of Civil Engineering
Michigan Technological University
Houghton, Michigan
FINN B. CHRISTENSEN, Sanitary Engineer
A/S Hjellnes& Co.
Oslo, Norway
INTRODUCTION AND PURPOSE
Over the past 25 years, the iron mining industry has witnessed a trend toward the
exploitation of low grade ore formations. Formerly, nearly all ofthe ore shipped consisted
of richer, natural ores which required little or no processing (concentration) at the mine site.
As the high grade ore deposits became scarcer, technological improvements in
concentration processes made the exploitation of low grade ore deposits economically
attractive. The term taconite, although descriptive of a hard, dense minnesota rock with 25
to 35 percent iron content, has come to be frequently used to refer to low grade iron
formations in general. In 1945, only 23 percent of the ore shipped resulted from
concentration processes. However, in 1965, 77 percent of the useable ore had been
concentrated. The Lake Superior district accounted for three-fourths of this production.
In the concentrating process the low grade ore (25 to 35 percent Fe) is concentrated to
55 to 70 percent Fe. This results in a waste material containing 10 to 20 percent Fe. Thus, for
every four tons of ore at 35 percent iron, 1.8 tons of concentrate at 65 percent iron would be
produced. The remaining 2.2 tons at 10 percent Fe would be discharged to the tailings
deposit.
A variety of unit operations such as gravity separation, screening, cycloning, magnetic
separation and flotation are employed in the concentrating process. Nearly all of these
operations are wet processes. To produce one ton of concentrate from four tons of crude ore
can require from 600 to 6,000 gallons of water depending on the process. Upon leaving the
process, this water serves to transport the remaining waste materials to the tailings basins.
A system for handling tailings wastewater serves chiefly to separate the suspended load
from the liquid so that the water can be either discharged to a natural watershed or reused
directly. Since the tailings wastewater contains on the order of 70,000 to 500,000 mg/1
suspended solids (98 percent of which settles very rapidly), large volumes of solids are
deposited. As pointed out above, the volume of this deposit is roughly equivalent to 50 to 75
percent of the volume of ore mined. In any tailings disposal system, therefore, a certain
amount of land would be required for the permanent disposal of these solids. This has led to
the development of huge "tailings deltas" in the proximity of concentrating plants. In some
cases, these deltas extend into Lake Superior. In beneficiation operations located further
inland, these deposits may be better contained. Nevertheless, some supra-colloidal solids
may remain in suspension and settle very slowly. Since mostconcentrating operations are
located in areas of relatively low land values, the most economical solution has been to
create large impoundments to accomplish this secondary removal.
One advantage of the impoundment system is that the partially clarified wastewater
may be reused in the plant operation. However, in a particular situation, water reuse may be
limited by the build-up of dissolved substances which may interfere with the concentrating
process.
Some inherent problems associated with impoundment systems are caused by the areal
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