Colloids and Surfaces A: Physicochemical and Engineering Aspects, 74 (1993) 31 l-312 0927-7757/93/$06.00 0 1993 ~ Elsevier Science Publishers B.V. All rights reserved.

Brief Note

311

Selective flocculation of coarse particles

Brij M. Moudgil*, Sanjay Behl, T.S. Prakash Mineral Resources Research Center, Department of Materials Science and Engineering, University of Florida, 161 Rhines Hall, Gainesville, FL32611, USA

(Received 5 October 1992; accepted 23 November 1992)

Key words: Coarse particles; Selective flocculation

Selective flocculation using polymeric flocculants

has shown promise in the benefication of mineral

fines. However it has been limited in practice

because the single mineral selectivity often does

not translate to the mixed mineral system. A

number of reasons have been suggested for this

loss in selectivity namely heterocoagulation,

entrapment and entrainment, and hetero-

flocculation.

The difficulty in flocculating submicron size par-

ticles has been reported previously [l]. However

the effect of particle size on selective flocculation

efficiency has not been systematically examined. In

preliminary experiments conducted in our labora-

tory, it was observed that the coarser size fraction

of apatite could be flocculated with polyacrylic

acid rather easily. Apatite and silica were ground

separately in a ceramic ball mill and various size

fractions such as -65 + 100, -100 + 150, - 150

+200, -200 +270, -270 +325, -325 +400 and

- 400 mesh were collected. A standard mixing unit

consisting of 150-ml beakers fitted with removable

plexiglass baffles of appropriate dimensions was

employed for flocculation in this investigation. The

sample was agitated by a stainless steel turbine

impeller driven by a variable speed motor at a

speed of 100 rev min- ’ such that the entire slurry

was in suspension.

A 2 wt.% suspension of synthetic mixture (50:50

apatite and silica) of various size fractions was

prepared in beakers. Sodium silicate (2 kg t ~’ of

solids) was added as a dispersant and the slurry

was aged for about 1 h. The pH was adjusted to

9.5 and the slurry was sonicated for 30 s. At this

stage the flocculant was added and the suspension

was agitated for 2 min at 1100 rev min-‘. The

floes were separated form the suspended fines by

sieving. It can be seen from Fig. 1 that at a constant

flocculant dosage of 1 kg t ’ higher selectivity is

observed for coarser size fractions.

The possible reasons for improved selectivity in

coarser particles are as follows:

(i) less physical entrapment of coarser size

particles;

(ii) relatively higher surface coverage of coarse

100.0 f p 90.0..

g

2 60.0~-

g ‘O.O ii 60.0 -.

A’ RECOVERY

O-O mixed mjnsrol

A 0-O single tfiffl

A-A mixed mineral

90.0 o

g 60.0 .”

: 70.0 3

=i m

60.0

40.0 60.0 120.0 160.0 PARTICLE SIZE. pM

*Corresponding author. Fig. 1. Recovery and grade as a function of particle size.

particles affecting the floe characteristics, e.g. higher

floe density;

(iii) less adsorption of flocculant on coarser silica

particles, hence less heteroflocculation.

The results obtained so far prove that not only

is it possible to flocculate coarser particles, but

that it may also lead to higher selectivity.

References

I S. Yu and Y.A. Attia, Review of selective flocculation in

mineral processing. in Y.A. Attia (Eds.) Flocculation in Biotechnology and Separation Systems, Elsevier,

Amsterdam, 1988, pp. 60 I-638.

Acknowledgements

Financial support for this work by the Mineral

Resources Research Center, University of Florida

is acknowledged.