ARSENIC SLUDGE DEWATERING
R. W. Wilson, Associate
K. L. Murphy, Principal
IEC Beak Consultants Ltd.
Mississauga, Ontario
Canada L4V 1P1
J. C. Ramshaw, Project Manager
Utility Operations Section
Ontario Ministry of the Environment
Kingston, Ontario
Canada K7L 4X6
INTRODUCTION
The Ontario Ministry of the Environment (MOE) operates a water pollution control plant to
remove arsenic from contaminated leachate at the site of an abandoned gold mining and refining complex at Deloro, Ontario. Currently chemical sludge produced during the treatment process is pumped
to storage basins.
This paper describes bench and pilot scale work completed to develop an optimum process for the
dewatering and the ultimate disposal of the arsenic sludge. Conventional mechanical dewatering,
chemical fixation/solidification and dewatering by freeze-thaw techniques were investigated. The test
results and cost implications of the mechanical dewatering and the freeze-thaw trials are discussed in
detail.
SITE HISTORY
Deloro was the site for the mining and/or refining of gold ores and ore residues for almost 100
years. The opening of at least 25 shafts and the construction of a milling and smelting operation
developed quickly after gold was discovered in 1866 [1]. Substantial quantities of arsenic were produced as a by-product of the smelting process for the arseno-pyrite ore. Between 1904 and 1961,when
the smelter was closed permanently, the Deloro Mining and Smelting Company refined silver, arsenic,
cobalt oxide, metallic cobalt, nickel oxide, and stellite.
The mine and mill property straddles the Moira River which flows east and south through a mainly
rural area and eventually discharges into Lake Ontario at Belleville, 180 kilometers east of Toronto.
Yearly average arsenic concentrations in the river varied from 1.5 to 4.0 mg/1 in the 1950's while the
smelter was still operating. This decreased to 0.3 mg/1 after the commissioning of the first treatment
process at the site in 1962 [1]. After the MOE took over the operation in 1979, it was concluded that a
completely new system for collecting, storing, and treating the leachates was required if the provincial
objective of 0.05 mg/1 of arsenic as an annual average for the Moira River was to be achieved. This
was commissioned in January 1983.
TREATMENT PROCESS
The collection system consists of buried leachate collection piping, a concrete membrane wall
along an 85 metre portion of the river bank and several small lift stations pumping to the 11,400 m3
leachate storage basin. The leachate is pumped from the storage basin to the treatment plant which is
housed in the old research building. Ferric chloride, hydrated lime, and polymer are metered into the
leachate in successive mixing tanks before the flow enters the final clarifier. The 300 to 600 mg/1 of
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