Powder Technology. 31(1982) 115 - 116 0 Elsevier Sequoia S-A.. Iallann e -Printed in The Netherlands

115

Short Communication

The Coagulation of Red Mud Suspensions and Its Application

$I= exp(ze</lBkZ”) - 1

exp(zeU2kT) + 1

HIDEHARU HIROSUE, NORJIYUKI YAMADA and EIICHI ABE

Government Industrial Research Institute. Kyushu, Shuku-machi, Tosu. Saga 841 (Japan)

(Received May 16.1981)

The coagulation and dispersion of red mud suspensions have been studied in a previous paper Cl], in which the coagulation and dis- persion of alkaline red mud suspensions, the pH values of which were controlled by NaOH, have been found to be explained fairly well by using the measured zeta potential and the DLVO theory. Similarly, it seems possible to analyse the coagulation and dispersion of ac- idic red mud suspensions on the basis of the same method as before. In the analysis, SO:- must be taken into account as counter-ions because H2S04 was used to prepare the pH values of the suspensions and the zeta poten- tial or red mud was positive in the pH range less than 6.3 [l] _ Apart from this analysis,

this paper sets out to estimate whether or not rapid coagulation takes place at each pH value of 3 to 12 [1] on the basis of the concept of flocculation value.

Furthermore, this paper deals with the re- moval of red mud particles in suspensions by means of deep bed filtration utilizing the coag- ulation phenomena of red mud suspensions as an application of the coagulation.

FZocculation value The repulsive energy V,(E) acting on two

spherical particles is given by the following

appro ximate equation under the condition of Kl-s 1 [2]-

8ek2T’ rQt2 VIZ(E) = e2 - z eXp (-K &)

fi = 2h,/r

(1)

in which V,(h) is the repulsive energy (erg), E the dielectric constant (x80), k the Boltz- mann constant (=1.38 X lo-l6 erg K-l), T the absolute temperature (K), e the elementary charge (=4.803 X lo-” esu), r the sphere ra- dius (cm), z the ion valency, K the reciprocal of electrical double layer thickness (cm-‘), ho half the shortest distance between surfaces of two spherical particles (cm) and < the zeta potential (mV).

The attractive energy V,(h) acting on the two particles is approximately expressed by the following equation under the condition ofZe 1 [2]_

V,(E) = -A/(12h) (2)

in which A is the Hamaker constant; a value of A = lo-l2 erg was used.

It is well known that when the following relations are established, rapid coagulation takes place.

V(E) = V,(Z) + V*(h) = 0, dV(h)/dE = 0

(3)

Combination of eqns. (l), (2) and (3) leads to the following equation expressing the floc- culation value Cc (mol/cm3).

cc = 144 X;rrxp(-2) ( 8,tzT2)2( >;G) (4)

in which F is the Faraday constant and R the gas constant (=8.314 X 10’ erg K-l mol-l).

The theoretical relation between Cc and c is obtained by determining T and z. In this study, the relations between the two were ob- tained in the case of z = 1 and z = 2, respec- tively and the theoretical relation between each pH value and < was obtained by using both a value of Cc at each pH value calculated horn Cc = 10pH-14 in the case of alkaline solu- tions and horn Cc = 0.5 X 10mPH in the case of acidic solutions_

Table 1 shows the comparison of the theor- etical value of the zeta potential, real, where rapid coagulation takes place at eact pH value

116

TABLE 1

Comparison of experimental and theoretical values of zeta potential at each pH value

pH value

3 4 5 6 6.3 8 9 10 11 12

L, 1 (mW 5 13 14 7.5 0 18 24 30 36 37

k,~l (mv) 30 16 8-7 4.9 4.1 4.1 7.3 13 23 43

Fiwculation rate R R R R

R = rapid.

of 3 to 12, with the experimental value, r,,,. It is recognized from the table that rapid co- agulation occurs theoretically below pH 4, in the vicinity of the isoelectric point (about pH 6.3) and above pH 12, since rapid coag- ulation takes place in the case of lr,,l i IS_,,,I at the same pH value and does not occur in the reverse case. This conclusion coincides with the experimental results which have been presented in Fig. 3 in the previous paper [l] _

In particular, the fact that rapid coagula- tion takes place above pH 12 will be apph- cable to deep bed filtration aiming at the cap- ture of red mud particles in dilute suspensions which may flow out of ‘the red mud reservoir. This matter will be described in the following_

Figure 1 shows that the packed bed of sand, particle size O-5 to 1.3 mm, 4.0 cm in diam- eter and 10.5 or 11.5 cm in height, could cap- ture red mud particles when a dilute red mud suspension of pH 12 was supplied to the bed and that the bed could not capture red mud in a suspension having an initial value of pH 8 because the suspension was in a too dispersed state [l j _ Solids concentration in the figure was measured by means of a spectrophoto- meter and the relationship between the solids concentration and the transmittance of light by the spectrophotometer was previously de- termined. However, even in the case of the suspension at an initial p_H value of 8, red mud particles could be captured when the packed bed was composed of crushed Portland cement mortar having a particle size of 0.5 to 2.0 mm instead of the sand and the pH value of the 5hxat-e was increased above 12 with the disso- lution of alkaline components in the cement mortar. The pH value of the solution saturated

Time (main1

Fig. 1. Effect of pH on solids concentration in filtrate (filtration velocity 100 m/day, initial solids concen- tration 10 p.p.m.).

with the crushed mortar was 13.5. Figure 1 also indicates that the red mud concentition in the filtrate was increased when the pH value of the filtrate decreased below 12 with the lapse of time. These results are qualitatively in accordance with the experimental [l] and theoretical results concerning the coagulation and dispersion of red mud suspensions pre- sented in Table 1.

REFERENCES

1 H. Hirosue, E. Abe, N_ Yamada and H. Ihara. Pow- der Technoi. 27 (1980) 197_

2 A Kitahara and A Watanabe, Kaimen Denki Gen- she. Kyoritsu Shuppon. 1972. Chaps. 1 - 3.