Indian Journal of Chemica l Technology Vol. 7, January 2000, pp. 23-29

Flocculation studies on hematite-silica system using polymeric flocculants

F F 0 Orumwense• & 1 C Nwachukwub

•Mechanical Engineering Department, "f>roduction Engineering Department, University o f Benin, P. M. B. 1154, Benin City, Edo State, Ni geria

Received 17 September / 998; accepted 12 October 1999

The dispcrsion-nocculation behaviour of synthetic mineral mi xture using polymeric tlocculants was inves ti gated . Results reveal that nocculation of the separate mineral s with 50 ppm causticised cassava starch occu rred within the acid ic pH region (pH 3-6) with maximum nocculation occu rring at pH 5 and the opt imum fl occul at ion occurring within a narrow causticised starch concentration range of 20-40 ppm (hematite suspension), while the noccul atio n of silica suspension was fou nd to be independent of starch concentration above 25 ppm. Fo r the e ffecti ve noccul ation o f the separate minerals, 75 ppm of polyacrylamide was found to be needed . It was estab li shed that for the hematite/silica mixture suspension, 50 ppm causticised starch concentration in the acidic pH region (pH 3-6) gave the best result. Electrokinetic studies conducted to study the mechani sms involved provided indication for the shift of shear plane. Based on the data gathered in the study, it is concluded that causticised cassava starch is a better nocculant for the selective nocculation of hematite/silica mixture suspension than polyacrylamide.

A number of valuable minerals that exist today are more finely di spersed in ores than in the past. In thi s form they are not of much use to the industrial users. So the separation of the des ired mineral species from the unwanted rock minerals (gangue) becomes essential. In order to separate the wanted mineral s from the gangue, the ore has to be ground to liberate the minera l grains from each other. Grinding processes produce suspension material in the sub­sieve size range which is too small to be efficiently recovered by conventi onal beneficiation technique, hence to develop a new or modified processes . Among the techniques that are being considered at present for fine partic le beneficiation, selective fl occulation appears to be one of the most promi sing. Separation of the unfloccul ated material by processes such as fl otation or elutriati on results in the desired beneficiati on .

Flocculation induced by dissolved polymer molecules has been attributed to charge neutralization and/or inter particle bridgi ng 1

• Selectivity in flocculation requires preferential adsorption of the polymer molecules on the desired mineral particles. Many of the commercia ll y avai lab le poly mers are however, bulk flocculants with insufficient selectivi ty, and selecti vity can be achi eved ei ther by a ltering the interfac ial potential of the mineral to create the desired electrical interaction between the polymer molecu les and the mineral surface or by incorporating suitable functional groups onto the polymers, which

by formation of complexes can make fl occul ati on selective2

. A typical example is the selecti ve flocculation of copper mineral s by cellulose xanthate ' . Similarly , the successful use of modified starches for the clarification of coal-washery effl uent slurries , selective flocculation of hematite from low grade iron ore, and the use of commercial polymers containing various ionic groups for the selective flocculat ion of mineral mixture have been reported in the past3

.4 .

Hematite and goethite each as uncalcined mineral in aqueous suspension have iron oxy/hydroxyl groups for reaction with the active COOH groups of starch . So, the di spersion-flocculation studies on a goethite­c lay system reported by Orumwense6

, the selec ti ve fl occulation of hematite from Barsua iron or by modified com and potato starches reported by Rao and Narasimhan7

, and the results of this preseRt work are likely to be simjlar.

The present study aimed to determine how such factors as agitation time, pH, polymer dosage, zeta potential, and the presence of di spersant affect adsorption-fl occulati on phenomena in the hematite­silica system.

Experimental Procedure

Minerals

Natural minerals (pure hematite and silica) used in this study were procured from the Geological Survey Division of the Federal Ministry of Petroleum an t.!

24 INDIAN J CHEM TECI-fNOL, JAN UA RY 2000

> E

60~----------------------~

20

Htmotitt

h!O-lm NaCI

I -No rtogtnl

[a)

2 Polyacrylamido 40ppm l S1arch

1 - 20 "­. ~

! ]

~ ~ ~ N

-40

10 12

pH

-4011'-....-----------------------.-

-50

-to

[b)

1- No r•agtnt

Silica

lw!O-)m NaCI

2 Starch 40ppm

l Polyaerylomidt

10

pH

12

Fig. !--Effect of pH on Zeta potentials in the absence and presence of polymeric flocculants (a) Hematite, (b) Silica.

Natural Resources, Benin City. The hematite sample was crushed, ground dry in a porcelain mill and wet screened using distilled wHter to obtain enough -38 J.lm (410 mesh Tyler Sieve) size materiaL The material was oven dried to a constant weight at 60°C and the resultant lump reground in a mortar. The grinding procedure of silica was similar to that of hematite . The ground silica was then transferred to a 500 cm3 beaker where the silica was repeatedly washed to remove organ~c impurities . The silica was then washed with deionised water and wet screened as in the case of hematite to obtain -38 J.lm size particles. The resultant material was oven dried to a constant weight at 60°C.

Synthetic flocculant, superfloc 16 (a non-ionic polyacrylamide, manufactured by the American

40r-------------------------------~

e Htmatitt

A Silica

0 oL--~40L----IL0--~12-0---1~60----2~00----2L40~~2~80

Initial Starch Conctntrotion. ppm

Fig. 2--Characteristic adsorption isotherms of caustici sed cassava starch on hematite and silica at pH 5.6.

Cyanamid Company and causticised cassava starch were used in this study. The starch was extracted from cassava tubers , dried to a constant weight in an oven between 60 and 70°C and finally ground to a powder. In preparing the flocculant starch solution, a I 0% (w/v) starch suspension was treated with an equal volume of 0.5 % (w/v) reagent grade NAOH solution.

The mixture was rapidly heated to 80°C, then quickly cooled to room temperature. The heating and cooling cycles were completed in less than 5 min8

,

and the resulting caustic starch solution was used in subsequent experiments. Causticising the starch at elevated temperature causes the starch to pass into colloidal solution or gel (depending on the concentration of the starch) and uncoil the chains resulting in increased flocculating power of the starch. This method of starch preparation was found to give high efficient starch flocculating reagent9

.

Other reagents such as H2S04, NaCI, HCl, phenol , and sodium silicate made use of in this study were reagent grade chemicals supplied by Fisher Scientific Company. Distilled water was used in the preparation of the reagents . Solution of NaOH and HCl were used for pH adjustment, while ionic strength by NaCI solution.

Method

Adsorption experiments- A series of suspensions of the different minerals (hematite and silica) were prepared by the dispersion of I g of each mineral in

ORUMWNSE & NWACHUKWU : FLOCCULATION STUDI ES OF HEMATITE-SILICA SYSTEM 25

41,----------------. Initial Starch Cone : (a)

• I.Oppm 81!l ·'

40 • 120 ..

0 140 ..

6 • pH Of pulp

40r-~~~~-~---~~ ln~ial starch con~

40PPJII ( b)

0 -z2D

i 0

"' ao ..

Fig. 3---Equilibrium amount of starch adsorbed as a function of pH (a) hematite (b) si lica

50 mL of distilled water. Thi s was followed by the addition of the desired amount of starch to give concentrations ranging from 40-1 80 ppm of the mineral slurry . The pH was then adjusted to the desired value ranging from pH 3- 11 .4 and the

solutions were left overnight to attain equilibrium pH values . While the polymer was be ing added, the solutions were mechanically ag itated .

During the adsorption equilibrium, continuous agi tation was stopped to avoid degradation of the long chain polymer molecules. After equilibration, a 2 cmJ sample of the supernatant liquid was taken and analysed for res idual starch by the calorimetric method of Dubois et al. 10

. The adsorpt ion dens ity of the starch was then calculated by the d iffe rence

10

1 ¥ 40

20

0 Htma tit•

Cou•t ids.d •torch eonc: SO ppm

pH S.6

Timt of ogitotiof'l. min.

Fig. 4-Effect of agitation time on the fl occul ation response of the minerals.

between the initial starch concentration in the pulp and that left in the supernatant di vided by the total weight of the starch in mglg.

Flocculation Experiments-Flocculation tests on separate pure minerals were performed in 250 cm3

graduated cylinders at about 5% solid pulp density. The flocculant was added and the suspension stirred mechanically at 500 rpm. The pH of the pulp was adju sted to the desired value by the additi on of standard solutions of NaOH or HCl and ionic strength by the addition of NaCl solution. The suspension was allowed to settle for 2 min (after inverti ng the cy linder fi ve times) after which the supernatant was decanted and the res idue (fl oes) was air dried and weighed to obtain the amqunt settled. The fl occul ation performance of the minerals is expressed as the per centage of suspended material settling into the bottom one-third ( 1/3) volume fraction in the cy linder duri ng a settling time of 2 min . The effects of time of agitati on, di spersant dosage, p H of slurry and polymer concentration on the amount of mineral settled were also studi ed.

T he selecti ve fl occulation tests were conducted us ing syntheti c mineral mi xture of hematite-s ilica prepared by the addit ion of equal amounts of each mi neral. The tes ts were performed in 250 em·' measuring cy linders at 5% so lid pulp density. 200 ppm sodium silicate concentration (basis suspended

26 INDIAN J CIIEM TECHNOL, JAN UA RY 2000

10or---------------------,

~ . . .

80

20

0 Htmatite

6 Sitic:a

Storch cone · SO ppm

pH 5.6

so 100

Oisptrsanl ConC't'ntra.tion. ppm 150 200

Fig. 5---Effect of di spersant dosage on the amount of hematite and sili ca settled .

solid) was added to the suspension and dispersed for 5 min. At 1500 rpm. Polymer solution of the desired pH value was added and agitated for I min. at 500 rpm. The pulp was allowed to settle for 2 min. After which the supernatant was decanted and the residue dried and weighed and, analysed for the individual minerals (Fe20 J and Si02).

Zeta potential measurements-Electrokinetic measurements were taken with an electrophoresis zeta meter. The apparatus and technique used are similar to those described elsewhere 1 1

'12

• From the electrophoretic mobility data, zeta potential values

were calculated by the use of the Helrnholtz­Smolouchowski's equation.

Results and Discussion Effect of pH on zeta potential-A plot of zeta

potential of hematite and silica lx i0-3 M NaCI as a function of pH in the absence and in the presence of polymeric flocculant s is presented in Fig. I. Since both starch and polyacrylamide are both nonionic polymers, the electrical double layer at the interface should not have any significant influence on adsorption, yet the results given in Fig. I clearly show that polymer adsorption does affect the zeta potential of hematite and silica. The point of zero charge of hematite is observed at about pH 5 and that of silica at

about pH 2.7. And with increasing pH of the solution, tlhe zeta potentials become more negative and reduce to an absolute value in the alkaline pH region. In the presence of flocculants, no significant shift is observed at the point of zero charge though more negative zeta potentials are observed between pH 6-8 (!hematite/starch) and between pH 7-9 (silica/starch). Since the polymer appears to reduce the zeta potential in absolute magnitude under all pH conditions studied, the primary effect of the large polymer molecules seems merely to shift the electrokinetic slipping plane farther from the: surface. This is in support of some of the various theories of surface

· · I 1 14 reagent mteract10n -· .

Adsorption Equilibrium adsorption isotherms for the uptake of

causticised cassava starch by hematite and silica are shown in Fig. 2'. These seem to fi t the Langmuirian­type plot which is essentially L-shaped. It can be seen from the figure that adsorption increased with increas ing starch dosage for all the pH values tested and then flatten s after complete surface coverage. The increase in the amount of starch adsorbed with increasing starch dosage is an indication that the adsorption process is largely due to hydrogen bridging between the hydroxyl groups of the hydrated oxides and the carboxylic acid group of starch.

Generally, starches possess polar groups such as - COOI-I, - 01-1 , - CO I-l and other minor function polar groups. Ionization of these groups in aqueous solutions (especially in the alkaline pH range) imparts a net negative surface charge on starch 15

. Al so, ionization of the phosphate and pyrophosphate constituents of amylopectin contributes towards the development of the surface charge. The slight decrease noticed in the adsorption density at higher polymer dosage probably reflects repulsion between the negatively charged solids and the high negative surface charge on starch which is a result of increased ionization of the functional groups.

The equilibrium amount of starch adsorbed on hematite and silica as a function of pH is shown in Fig. 3. The results indicate that maximum adsorption density occurs at around pH 6. As the pH is increased from this level , the amount of starch adsorbed reduces sharply, thus confirming that in the alkaline pH region wlhere there is ionization of the functional groups, electrostatic repul sion between the starch molecules and the mineral particles is quite strong 16

.

Flocculation

Effect of time of agitation-The effect of agitation

ORUMWNSE & NWACHUKWU: FLOCCULATION STUDIES OF HEMATITE-SILICA SYSTEM 27

100,..------------------,

'}

• "' .. :~ 0 40 .. ~

20

Polymtr Conctntration, ppm

Fig. 6---Effect of polymer concentration on the Oocculation response of the minerals.

time on the flocculation of hematite and silica suspensions at 50 ppm starch concentration and at pH 5.6 are shown in Fig. 4. As can be observed, flocculation increases with time of agitati'on until an 'optimum' time ~ .., -nin and 3.5 min for hematite and silica, respectively) was attained. Agitating above this optimum time, the flocculation of both minerals decreased due to deaggregation of some of the floes . At the times of agitation studied, the per cent hematite settled was higher than that of silica. This suggests that separation of hematite from quartz by selective flocculation should be possible unless there are strong interactions between these two minerals.

Effect of dispersant on the flocculation behaviour £~{ both minerals using causticised starch~esults presented in Fig. 5 show that silica is effectively di spersed at dispersant dosage of about 200 ppm, while hematite showed constant flocculation . This infers that the presence of sodium silicate has no influence on the adsorption of starch on hematite but inhibits the starch adsorption on silica. The constant flocculation shown by hematite is as a result of the incorporation of the metal ion (Na+) from the dispersant into the hematite particles resulting in strong affinity between hematite particles and the

., ~

• .. .. .'!? 0 .. ;,:

100

90

eo

70

60

50

40 0 Htmat i ~ /Starch

1::. Silica I .,

)() • Htmatitt I Polyacrylamidt

• Silica I

20 Polymtr cone : 50ppm

10~-~---~--L--~--~--~ 2 8

pH of pulp

10 12 14

Fig. 7-Effect of dispersant on the amount of mineral settl ed.

active functional groups of the polymer molecules . As the sodium silicate concentration increases, the silicate ions increase the total anionic silicate particles in the solution, resulting in high repul sive forces between the silica particles and the polymer molecules. This is responsible for the complete dispersion of the silica particle by sodium silicate at 200 ppm. At very high pH values (pH > 9 where ionization of the functional groups in starch is almost complete, electrostatic repul sion between the starch molecules and the silica particles is quite strong) may, however, over-disperse silica and reduce adsorption of the flocculant on the silica surface. Thus, over­dispersion may jeopardise selectivity in separation7

.

This is in agreement with previous work by the author6

.

Effect of polymer concentration on the setting behaviour of both minerals-The flocculation response of hematite and s ilica as a function of polymer concentration at pH 5.6 is presented in Fig. 6. Results for causticised starch flocculation of both mineral s show that good flocculation of hematite occurred between 25-50 ppm starch concentration with maximum value occurring at about 40 ppm starch concentration. Above 50 ppm starch concentration, flocculation of hematite decreases. For silica, flocculation increases gradually up to a

28 INDIAN J CHEM TEC IINOL, JANUA RY 2000

~or-----------------------------~

ao

60

20

0 Starch

0 Polyaerylamido

F loeculanl Cone : 50 ppm

o ;sporsant ·cone : 200ppm

0~--~----~--~----~--~~--~ a

pH of pulp

10 12 14

Fig. 8--Effect of pH on the fl occu lation of mineral mixture with and without polymer.

maximum value at 25 ppm starch concentration . Above 25 ppm starch concentrati on, flocculation of s ilica is independent of po lymer dosage. Thi s constant flocculation shown by silica is an indication of the saturation state of the reagent sorpti on.

It can also be observed from the same fi gure that the floccul ation of bot h minerals with polyacrylamide increases with polymer dosage up to a maximum value at 75 ppm. Above thi s value, flocculation gradually decreases. Polyacrylamide was found to flocculate hematite more than si lica. Al so, the difference between the flocculation response of the two minerals in the presence of causticised cassava starch is greater than that in the presence of polyacrylamide sugges ting better selectivity with starch than with the latte r.

Effect of pH on the flocculation of hematite and silica using causticised starch and polyacrylamide--Fig. 7 shows the effect of pH on the floccu lation behaviour of hematite and silica suspension at 50 ppm polymer concentration . Wi th causticised starch, results show th at per cent sol ids settled increases with pH up to about pH 5. Above th is pH value, per cent solids settled dropped sharp ly .

As can be observed for all pH investigation, the per cent hematite settled in the presence of causticised starch was found to be higher than per cent silica settled . Whereas, results of flocculation tests performed with polyacrylamide show that flocculation of both minerals is almost independent of pH (i.e. between pH 3-9). Above pH 9 or so, flocculation of both minerals decreases . From the figure, it can be seen that larger flocculability of both minerals was shown by ionic caustic starch than by polyacrylamide. This could be attributed to the bridging action of the long starch chains on the mjneral s 18

.

Effect of pH on solids settled of mineral mixture with and without jlocculant in the presence of dispersant-With reference to Fig. 8, it can be observed that selective flocculation of the mjneral mjxture in the' presence of sodium silicate is influenced by pH with maximum per cent solids settled occurring at pH 5. Beyond thi s pH value, the amount of solids settled fall s gradually indicating that in the alkaline pH region flocculat ion is very poor. It is also observed that the highest per cent solids se ttled occurred with causticised starch flocculant followed by the polyacrylamide while the least per cent so lid settled was recorded for mixture without any flocculant. From the figure , it is inferred that causticised starch is more selective since the ri chest he matite/silica mixture settled (88 % he matite, 11 .5% si lica) is comparatively equal to the amount ex pected on the basis of separate mineral fl occulation .

The results obtained confirm the feasi bility of the separation of hematite from synthetic hematite/s ilica mixture using causticised cassava starch . Selecti ve fl occulati on has successfully been used for the separation of he matite from both synthetic and natural mixtures containing quartz and other sili cates using po lyacrylamide; and hematite from Barsuam iron ore using caustic ised com and potato starch fl occul ants7

·19

-21

.

Conclusions

The investi gation s carried out have led to a number of findin gs :

(i ) Separa ti on of hematite from silica is achievable using causticised . Cassava starc h should b withi n a narrow concentration range of 20-40 ppm.

(ii) Flocculat ion of the mineral mixture with the polymers occurred wi thi n the acidic pH region with max imum floccul ation occurri ng at pH 5.

ORUMWNSE & NWACHUKWU : FLOCCULATION STUDI ES OF HEMATITE-S ILI CA SYSTEM 29

(iii) Silica was found to be a better dispersed mineral than hematite.

(iv) The single mineral flocculation tests showed that causticised cassava starch is a more effective flocculant for hematite than polyacrylamide.

(v) The pH of pulp affects the zeta potential, chain configuration of starch and hence flocculation of the minerals.

(vi) The presence of flocculants does not shift the point of zero charge.

(vii) The zeta potentials of the minerals were reduced to an absolute value with the flocculants due to movement of the slipping plane farther out into the solution by the adsorption of large nonionic polymer molecules.

Acknowledgements The authors are grateful to Petroleum Training

Institute (PTI), Warri, Geological Survey Division, Federal Ministry of Petroleum and Natural Resources, Benin City, Delta Steel Company, Aladja, Warri for providing both materials and facilities for the completion of this work.

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