CLARIFICATION OF BLACKWATER WASTES BY FLOTATION
AND BY COAGULATION
.Alan J. Rubin, Professor
Paul R. Schroeder, Research Associate
Ralph J. Kramer, III, Research Associate
Water Resources Center
The Ohio State University
Columbus, Ohio 43210
INTRODUCTION
Virtually all of the coals cleaned in the United States are wet cleaned. Consequently,
bUlions of gallons of black wash waters are discharged annually to the nation's streams
[ 1 ]. The fine coal particles carried away by wash waters represent a waste of energy and a
significant source of pollution. The successful recovery of this waste coal could provide the
economic incentive needed to resolve many present environmental problems associated with
coal preparation.
Waters laden with suspended coal particles are generated in increasing amounts as a result of modern methods of mechanized mining, the expansion of the coal mining industry
and the greatly increased demand for ash-free coal with a minimum sulfur content. Because,
frequently, very little more than plain settling is applied, especially in older preparation
plants, much -48 mesh coal is lost [2]. Clarification should be employed in order to recover
the coal and to meet the suspended solids effluent discharge limit of 0.07 g/1 imposed on
the industry [3].
There has been little research reported in the literature which deals specifically with the
removal or recovery of colloidal coal. Since it is the most difficult to remove by standard
coal beneficiation methods, this ultrafine coal ends up as a major constituent of plant
blackwaters. Accordingly, this study was initiated to investigate the feasibility of coal recovery by flotation and by coagulation under laboratory conditions. The major parameters
controlling both processes were also examined in the study.
EXPERIMENTAL METHODS AND MATERIALS
Coal Suspensions
Coal suspensions were prepared from a highly volatile, low sulfur, low ash bituminous
coal from the Elkhorn seam in Kentucky. In order to prepare stable and reproducible
suspensions a lengthy procedure was developed which is described in detail elsewhere [4].
In summary, the coal was ground, ball mUled and passed through a 400-mesh standard U.S.
Series sieve. Batches were oxidized with dilute hydrogen peroxide at 80 C for several hours.
The resultant slurry was washed, filtered and resuspended in distilled water. Typically, the
pH ranged between 5.5 and 6.0; any suspensions having an unusually low pH were discarded.
The surface areas of the coal particles were measured with a Quantasorb BET multipoint analyzer manufactured by Quantachrome Corporation. The ground, sieved coal had
a surface area of 7.2 m2/g prior to oxidation and 9.3 m2/g afterward.
Suspension turbidities were estimated by absorbance measurements taken with a Coleman
model 14 spectrophotometer at a wavelength of 400 nm. Absorbance was linear with suspension concentration. A suspension absorbance of 0.6 corresponded to approximately 28 mg/l
of coal. A Sargent-Welch model LS or model NX pH meter and combination electrode were
used to measure pH. Suspension mobilities were measured using a Zeta-Meter microelectrophoresis apparatus.
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