TREATMENT OF WASTEWATER FROM THE MINING AND PROCESSING
OF DIATOMACEOUS EARTH
Libby Morris Lace, Graduate Student
Paul L. Bishop, Associate Professor
Department of Civil Engineering
University of New Hampshire
Durham, New Hampshire 03824
INTRODUCTION
Diatomaceous earth is being used in ever-increasing amounts by industry, municipalities
and the general public for filtration, insulation and as a filler. The U.S. Bureau of Mines
reports that over 60% is used in filtration as a filter aid [ 1 ]. The United States is the
world's largest producer of diatomaceous earth, currently producing about 500,000 tons
per year. This is nearly double the amount produced in the early 1950s [2]. Diatomaceous
earth deposits exist in all states, but most deposits are small and impure. Presently there
are eleven companies operating 15 mines and processing facilities in 5 states: California,
Kansas, Nevada, Oregon and Washington.
Diatomaceous earth is composed of the siliceous remains or fossils of diatoms, which
are the golden-brown algae of the division, Chrysophyta. These are unicellular, symmetrical
organisms ranging in size from 1 to 100 il. The importance of these tiny fossils is due to
the structure and composition of the cell wall, which is actually a rigid skeleton composed
of hydrated amorphous silica or silicic acid. The wall is ornate, having an intricate and
fragile appearance caused by variations in wall thickness and pore spacing. The wall ornamentation and pore structure create an extremely large surface area and are responsible
for its fine filter aid properties. The material has an average porosity of 90% and a specific surface area of 20 m2/g [3].
Deposits of diatomaceous earth accumulate as diatoms settle to the sediments in lake
bottoms. Rate of accumulation of diatomaceous earth is slow, estimated at 1.2 in./lOO yr
[4]. Common impurities often present in deposits are clay, sands, organic matter, lime
carbonates and iron [3,4]. These impurities must be removed during processing. Natural
purification occurs by leaching, chemical action, decomposition and oxidation. Periodic
earth movements raised many deposits and continuous erosion, good drainage, leaching
and reducing conditions removed most impurities [4,5]. Where natural purification is not
complete, processing is required for removal of these impurities.
Present mining operations are almost entirely by open pit or quarry methods [ 5,6].
Underground methods were used in the past but have been largely discontinued due to the
dangerous weakness of the sediments. Bog deposits have been mined in Germany and
Florida, but are not currently in operation. The only known lake dredging operation is
in Iceland.
Processing of diatomaceous earth involves size reduction, drying, classification and
optional heat treatment and calcination. Heat treatment is used to remove organics and
alter surface characteristics. Calcining is heat treatment at about 1800 F which shrinks,
hardens and fuses particles into clumps. Alkali salts, usually soda ash, are often used in
flux calcination to produce a pure white color and to make impurities insoluble [7].
Bog and lake deposits employ dewatering prior to processing.
Diatomaceous earth deposits in New Hampshire, particularly in Lake Umbagog, have
recently received much attention. Development of these deposits would greatly reduce
transportation costs on the East Coast, since all diatomaceous earth currently originates in
western states.
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