EVALUATION OF HIGH-RATE CLARIFICATION PROCESSES FOR MINE WATER TREATMENT WITH CEMENT KILN DUST

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22 Φεβ 2014 (πριν από 3 χρόνια και 6 μήνες)

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EVALUATION OF HIGH
-
RATE CLARIFICATION PROCESSES FOR MINE WATER TREATMENT
WITH CEMENT KILN DUST


A.L.

Mackie and
*
M
.E.

Walsh


Department of Civil and Resource Engineering, Dalhousie University

1360 Barrington Street, Rm D215

PO Box 15

000, Halifax, NS, CA
B3H 4R2

(*Corresponding author
: MWalsh2@dal.ca)



ABSTRACT


Mine water from base metal mines is generally characterized as having high concentrations of heavy metals
and low pH, resulting in the need for treatment prior to discharge to receiving waters.


I
n this study, two high
-
rate
clarification processes were evaluated at bench
-
scale: the high density sludge (HDS) process and sand
-
ballasted
flocculation.

High
-
rate clarification processes generate larger, denser flocs which settle more quickly than those
formed in conventional flocculation processes.

The HDS process uses recirculated sludge in order to seed floc
formation, whereas ballasted flocculat
ion uses microsand.

Both
of these high
-
rate processes require the addition of
quicklime or hydrated lime t
o increase pH and allow for the formation of insoluble metal hydroxides, which can
then be removed in the clarification stage.

Previous research has shown that cement kiln dust

(CKD)

can effectively
replace quicklime in
conventional sedimentation processe
s

for metals and acidity reduction to meet discharge
guidelines (Mackie & Walsh 2012).

CKD

is an alkaline by
-
product of cement manufacture that has fine particle
sizes and varying free lime concentrations depending on the manufacturing process.

Th
e lower

solubility (i.e.
,

free
lime content) of CKD requires larger doses of material to be added to reach treatment pH ta
rgets for metals
precipitation.
The purpose of this study was to evaluate the treatment efficacy of high
-
rate
clarification
processes
for mi
ne water treatment
, compared with conventional sedimentation,

at bench
-
sca
le with conventional (i.e.,
lime)
and novel (i.e.
,

CKD) pH adjustment additives.


Results of the study showed that
all treatment

processes

(i.e., HDS,
ballasted flocculation, and con
ventional sedimentation)

were able to reduce target metal concentrations to well
below Canadian
guidelines (i.e., total zinc and total nickel
< 0.5 mg/L
, total copper < 0.3 mg/L) using either CKD or
lime.

Additionally, mine water samples treated with CKD
generated lower sludge volumes than those treated with
lime (i.e., 50 versus 100 mL for conventional sedimentation and 100 versus 200 mL for HDS)
even with increased
concentrations of solids introduced during treatment.

This is most likely due to slower n
eutralization rates when
using CKD, due to its lower solubility, as well as finer particle size.

The sand
-
ballasted flocculation process was
able to reduce final turbidity in CKD
-
treated samples while the HDS process did not when compared to
conventional
sedimentation, making the ballasted flocculation process preferable for reducing final solids
concentrations in CKD
-
treated mine water.